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Electrochemical Energy Reviews

, Volume 1, Issue 3, pp 294–323 | Cite as

Recent Advances in Sodium-Ion Battery Materials

  • Yongjin Fang
  • Lifen Xiao
  • Zhongxue Chen
  • Xinping Ai
  • Yuliang CaoEmail author
  • Hanxi Yang
Review article

Abstract

Grid-scale energy storage systems with low-cost and high-performance electrodes are needed to meet the requirements of sustainable energy systems. Due to the wide abundance and low cost of sodium resources and their similar electrochemistry to the established lithium-ion batteries, sodium-ion batteries (SIBs) have attracted considerable interest as ideal candidates for grid-scale energy storage systems. In the past decade, though tremendous efforts have been made to promote the development of SIBs, and significant advances have been achieved, further improvements are still required in terms of energy/power density and long cyclic stability for commercialization. In this review, the latest progress in electrode materials for SIBs, including a variety of promising cathodes and anodes, is briefly summarized. Besides, the sodium storage mechanisms, endeavors on electrochemical property enhancements, structural and compositional optimizations, challenges and perspectives of the electrode materials for SIBs are discussed. Though enormous challenges may lie ahead, we believe that through intensive research efforts, sodium-ion batteries with low operation cost and longevity will be commercialized for large-scale energy storage application in the near future.

Graphical Abstract

Keywords

Cathode materials Anode materials Sodium-ion batteries Energy storage 

PACS

82.45.Fk Electrodes 88.80.F- Energy storage technologies 88.80.ff Batteries 

1 Introduction

The growing demand for energy storage in intermittent renewable energy, transportation and the myriad portable electronic devices has continuously promoted the development of effective and economical energy storage technologies for constructing a sustainable “energy internet” (Fig. 1). Lithium-ion batteries (LIBs) have already dominated the portable electronics market and are expanding to the field of large-scale electric energy storage (EES) application. However, great concerns arise about the widespread availability and rising price of the lithium resources [1]. Room-temperature sodium storage technology attracts much attention in recent years and goes into the spotted light as a supplementary technology to LIBs for EES due to the wide abundance and low cost of sodium resources and similar chemical/electrochemical characteristic to established LIBs [2, 3, 4, 5]. Tremendous efforts have been made on the developments of cathode materials (such as, transition metal oxides [6], polyanionic compounds [7], ferrocyanides [8]), anode materials (such as, carbonaceous materials [9], alloys [10], sulfides [11]), and electrolytes (such as, electrolyte design [12, 13, 14], functional additives [15]). In some respects, the sodium-ion batteries (SIBs) exhibit nontrivial characteristic that differs to the established lithium storage technology, showing some interesting and fascinating phenomena. For example, the transition metal oxides exhibit multiple-phase transition during sodium uptake, and are very sensitive to moist atmosphere and the adding proportion of sodium resources during synthesis that plays a critical role on the phase of the final product. The graphite anode, which is widely used in commercial LIBs, has been proved to be unable to accommodate Na+ ions. And the Si material is considered as anode material for next-generation LIBs, but its alloying reaction with sodium has not been achieved yet. All the differences indicate that there may be a significant knowledge gap between SIBs and LIBs technology, but it provides numerous opportunities to better understand the battery reaction and explore suitable materials for high-performance sodium storage in return.
Fig. 1

Schematic illustration of the “energy internet”

To date, a large variety of electrode materials have been reported and some of them have exhibited superior sodium storage performance. As most anode electrode materials show low voltage and large capacity, exploring high-performance cathode materials with high voltage and large reversible capacity is the key point to achieve high energy density for SIBs. In this review, we will discuss the latest progress in electrode materials for SIBs, considering their fundamental properties, structural and compositional optimizations, Na storage mechanism, and challenges and perspectives.

2 Cathode Materials

Since the cathode materials dominantly determine the energy density of the SIBs, developing novel cathode materials with abundant active sites and unobstructed ionic channels for Na+ ions is the major challenge for constructing high-energy SIBs. A lot of materials have shown outstanding sodium storage performance due to the suitable structure. Among the cathode materials, transition metal oxides, polyanionic compounds and ferrocyanides have attracted much more attention due to their decent electrochemical performances.

2.1 Transition Metal Oxides

2.1.1 Classification and Structures

Due to the abundant active element centers, adjustable compositions and decent electrochemical activity, transition metal oxides exhibit to be a class of promising cathode materials for SIBs [6, 16]. Based on different structures, transition metal oxides can be classified into two categories: the tunnel-type oxides and layered oxides. The tunnel-type oxides exhibit an orthorhombic structure with large S-shaped tunnels and small pentagon tunnels (Fig. 2a). Sodium ions in the large S-shaped tunnels can be reversibly extracted, while the sodium ions in the small tunnels are not electrochemically active [17].
Fig. 2

Typical crystal structure of the metal oxides: a tunnel structure, b O3 structure, c P2 structure

Layered oxides with general formula NaxMeO2 (Me = transition metal) are composed of repeated sheets of MeO6 layers with Na ions being sandwiched between the oxide layers. According to Delmas’ classification, the layered oxides can be categorized into two main groups: O3 and P2 types, in which Na+ ions tend to locate at octahedral and prismatic sites (Fig. 2b, c), respectively [18]. Since the ionic radius of 3d-transition-metal ions (< 0.7 Å) is much smaller than that of sodium ion (1.02 Å), the sodium ions and 3d-transition-metal ions occupy distinct octahedral sites. When a sodium off-stoichiometry condition (typically 0.6 < x < 0.7 in NaxMeO2) is applied, the P2-type phase is empirically known to be structurally stabilized [19]. The charge/discharge curves of layered oxides often exhibit multiple potential plateaus, corresponding to a series of phase transitions and solid-solution reactions during sodium ions insertion/extraction.

2.1.2 Tunnel-Type Oxides

Na0.44MnO2 is a typical tunnel-type oxide with orthorhombic structure. Doeff et al. first reported the sodium storage performance of Na0.44MnO2 in a polymer electrolyte battery. The electrode demonstrated a high reversible capacity of 180 mAh g−1, but the cycling performance was not satisfying [17]. Cao et al. [20] reported the fabrication of single crystalline Na0.44MnO2 nanowires through a polymer-pyrolysis method. The obtained electrodes demonstrated a high reversible capacity of 128 mAh g−1 with long cycling life over 1000 cycles (Fig. 3a, b). The excellent electrochemical performance of Na0.44MnO2 nanowires benefits from shortened diffusion path of Na ions and stable tunnel structure. After that, a lot of Na0.44MnO2 materials with different nanostructures have been realized [21, 22, 23]. The sodium storage mechanism has also been studied. Sauvage et al. [24] reported the presence of six biphasic transitions within the potential range of 2–3.8 V by using potentiostatic intermittent titration technique and in situ XRD measurements. Kim et al. [25] conducted DFT calculations to study the sodium insertion and extraction mechanisms and found that seven intermediate phases identified with the calculated voltage profile were in agreement with experiments. They suggested that the relatively unstable intermediate phases led to the capacity fading, and the evolution of the lattice parameters in an asymmetric manner in Na0.44MnO2 mainly resulted from the Jahn–Teller distortion [25]. Their calculations further suggested the Cr-substitution could reduce the volume change in Na0.44MnO2 significantly by about 50%.
Fig. 3

a Typical discharge profile of Na4Mn9O18 samples calcined at 600, 750 and 900 °C between 2.0 and 4.0 V at a current density of 12 mA g−1; b Corresponding cycle performance of Na4Mn9O18 samples at a current density of 60 mA g−1; c In situ XRD patterns collected during the first charge/discharge of the Na0.61[Mn0.27Fe0.34Ti0.39]O2 electrode cycled between 2.6 and 4.2 V under a current rate of C/15. Black asterisks represent peaks from Al window. a, b Reproduced with permission [20]. Copyright 2011, Wiley–VCH. c Reproduced with permission [29]. Copyright 2015, Wiley–VCH

Ti-substitution can help to increase the ionic conductivity of the Na0.44MnO2 materials and control the tunnel length and size [26, 27, 28]. Qian’s group reported carbon-coated Na0.54Mn0.50Ti0.51O2 nanorods, which exhibited smooth charge/discharge curves with a reversible capacity of 122 mAh g−1 and stable cycle life [27]. By partially substituting Mn with Fe and Ti ions, Hu’s group reported a series of Na-rich Na0.61[Mn0.61−xFexTi0.39]O2 compounds [29]. In situ XRD analysis indicated a solid-solution reaction mechanism during charge/discharge process (Fig. 3c), implying that the tunnel structure was electrochemically stable and was well maintained even being charged to a high voltage of 4.2 V. This work also demonstrated that the Fe3+/Fe4+ redox reaction could be realized in nonlayered oxides, enabling a high storage voltage of 3.56 V.

2.1.3 Layered Oxides

Layered oxides with general composition of NaxMeO2 (Me = transition metal, such as Fe, Mn, Co, Ni, V, Cr, Cu, etc.) belong to a large family with various phases and abundant compositions [6, 30]. The electrochemical behaviors of layered oxides are heavily influenced by the active redox centers and the structure of the phases, including the amount of Na in the pristine state and the stability of each layer and kinetics affected by the surrounding environment of sodium [6]. To improve the structural stability of the layered oxides, an efficient structural regulation approach is cation substitution of the transition metals with Li+, Mg2+, Cu2+, Al3+, Ti4+, etc., which can help to suppress the phase transition of layered oxides during sodium insertion/extraction process.

Due to the superiority of their environment-friendly and low-cost properties, the iron-based materials have been widely investigated as electrode materials for sodium storage [8]. It should be emphasized that the O3-type NaFeO2 exhibits stable layered structure with surprisingly good electrochemical activity [31]. However, for the LiFeO2 analogue, it has a cation-disordered rock-salt phase with inert electrochemical behavior. The NaFeO2 electrode displayed a reversible capacity of 85 mAh g−1 with a flat voltage plateau at 3.3 V, and the electrochemical behavior depended heavily on the cutoff voltage [32]. The degenerative reversible capacity results from the phase transitions and some iron ions migration during electrochemical reactions [33]. Metal substitution has been widely adopted to improve the electrochemical performance of O3-type NaFeO2. For example, the P2-Na2/3Fe1/2Mn1/2O2 electrode delivers a reversible capacity of 190 mAh g−1 with a stable cycle life [34, 35]. And the NaFe1/2Co1/2O2 electrode shows good stability with a reversible capacity of 160 mAh g−1 [36, 37]. Hu and co-workers reported Cu-doped materials, O3-Na0.9[Cu0.22Fe0.30Mn0.48]O2 and P2-Na7/9Cu2/9Fe1/9Mn2/3O2 [38, 39], and the electrochemical Cu2+/3+ redox was also discovered during sodium insertion/extraction process.

Manganese-based layered oxides also have been widely studied [40]. The different oxidation states and multiple electron reactions could allow the manipulation of electrodes within different voltage ranges. However, it is worth mentioning that the structural distortions usually accompany with the manganese-based layered oxides, due to the Jahn–Teller distortion of Mn3+ species in the electrodes. It is generally believed that the Jahn–Teller distortion can lead to the structural degradation on cycling [40]. The NaxMnO2 systems exhibit complex compositions and phases with adjustment of the Na/Mn ratio and calcination temperature [41, 42]. Among these phases, the P2-Na2/3MnO2 material attracts more attention with high reversible capacity and stable cycling. And a lot of elements, such as Mg, Li, have been doped in the structure to improve the electrochemical performance [43, 44]. For example, P2-Na5/6[Mn0.75Li0.25]O2 electrode has been reported with a large reversible capacity of 190 mAh g−1, good rate capability and increased operating voltage [44]. The improved electrochemical performance may result from the redox activity of oxide ions in the structure and suppression of the phase transitions. Due to the synergetic redox activities and suppressed structural transitions, a lot of binary, ternary and quaternary manganese-based layered oxides have been reported with improved electrochemical performance [40, 45, 46, 47, 48]. For example, Yuan et al. reported the doping of an electrochemical inactive element to improve the electrochemical performance of layered oxides, and a Al3+ doped P2 phase Na0.67[Mn0.65Ni0.15Co0.15Al0.05]O2 was reported with stable cycling life and excellent rate capability [49].

O3-NaNiO2 with a typical O3-type structure also exhibits good performance. The cycling performance depends heavily on the cutoff voltage of the electrodes. When cycled between 1.25 and 3.75 V, the electrode delivered a reversible capacity of 123 mAh g−1 with good stability. A larger reversible capacity can be delivered among 2–4.5 V, but the electrode degraded rapidly [50]. The structural instability may result from the Jahn–Teller distortion of Ni3+ species, phase transitions and sodium rearrangement. Metal substitution has also been reported to suppress these evolutions. For example, Yuan et al. [51] have synthesized a series of O3-NaFex(Ni0.5Mn0.5)1−xO2 (x = 0, 0.1, 0.2, 0.3,0.4 and 1) materials, and found that the electrochemical performance and the structural stability of the electrodes could be greatly improved when partial Ni and Mn were substituted with Fe. The structural characterizations demonstrated that pristine NaMn0.5Ni0.5O2 and Fe-substituted NaFe0.2Mn0.4Ni0.4O2 lattices underwent different phase transformations from P3 to P3″ and from P3 to OP2 phases at high voltage interval, respectively [51]. Guo and co-workers reported a series of Ti-substituted O3-type NaNi0.5Mn0.5−xTixO2 (0 ≤ x ≤ 0.5) cathodes, and the substitution of Ti for Mn enlarged interslab distance and restrained the unfavorable and irreversible multiphase transformation in the high voltage regions (Fig. 4b) [52]. By introducing high-valent inert metal ions, Yuan et al. [53] first reported an O3-Na3Ni6SbO6 material with honeycomb layered structure. Due to the high Na+ ion diffusion coefficient and unique honeycomb-ordered phases, the O3-Na3Ni6SbO6 material exhibited a high capacity of 120 mAh g−1, superior rate capability and stable cycling life, and also the phase transition has been confirmed by ex situ XRD and solid-state NMR measurements (Fig. 4a). The ex situ XRD indicated reversible three-phase transformations of the compound (O3-Na3Ni2SbO6 ↔ P3-Na2Ni2SbO6 ↔ O1-NaNi2SbO6) during sodiation/desodiation, and solid-state NMR revealed a reversible variation of the Na+ coordination. This structural reversibility may provide a good account for the excellent cyclability of the layered Na3Ni2SbO6.
Fig. 4

a Structure evolution of Na3Ni2SbO6 during charge/discharge process: (i) charge/discharge profiles of Na3Ni2SbO6, the marks A–I indicate the depths of charge/discharge at which the samples were taken for ex situ XRD and ex situ NMR test; (ii) XRD patterns of the Na3Ni2SbO6 electrode at various charge/discharge states, diffraction peaks intrinsic to the Al foils are denoted by asterisk; (iii) 23Na MAS NMR related to single phase regions of O3 (A and I), P3 (C and G) and O1 (F), respectively. The inset chemical structures show the 23Na sites, the Na atoms are represented in yellow, the Ni atoms in blue, the Sb atom in green and the O atoms in red. b In situ XRD patterns collected during the first charge/discharge of the Na/NaNi0.5Mn0.2Ti0.3O2 cell under a current rate of 0.05C at voltage range between 2 and 4 V. Black asterisks represent peaks from Al window. c Charge compensation mechanisms in P2-Na0.78Ni0.23Mn0.69O2. d Comparison of X-ray and neutron PDF of Na0.6 [Li0.2Mn0.8]O2 collected at pristine and charged states (4.5 V). a Reproduced with permission [53]. Copyright 2014, Wiley–VCH. b Reproduced with permission [52]. Copyright 2017, Wiley–VCH. c Reproduced with permission [60]. Copyright 2017, American Chemical Society. d Reproduced with permission [61]. Copyright 2017, Elsevier

Other layered oxides with various redox centers (such as, Co, V, Cr) also have been studied [54, 55, 56, 57]. For example, Fang et al. [58] have synthesized microspherical P2-Na0.7CoO2 with improved reversible capacity and long cycle life. However, considering the high cost or toxic elements, and the uncompetitive electrochemical performance, this kind of materials may be hard to be applied for grid-scale application.

As mentioned above, apart from phase transition mechanism, some researchers have put forward the charge compensation mechanism of redox of oxygen anion [44, 59], which is similar to that observed in Li2MnO3-based electrodes. Meng and co-workers used electron energy loss spectroscopy and soft X-ray absorption spectroscopy to study the Na0.78Ni0.23Mn0.69O2 electrode, and proposed that part of the charge compensation mechanism during the first cycle took place at the lattice oxygen site, resulting in a surface to bulk transition metal gradient (Fig. 4c) [60]. Recently, Rong et al. reported that the reversible capacity observed in P3-type layered Na0.6 [Li0.2Mn0.8]O2 cycled in the voltage range of 3.5–4.5 V (vs. Na/Na+) originates exclusively from the redox activities on oxygen anions, and the shortening of O–O distance resulting from oxidation of oxygen anions was detected directly by nPDF (Fig. 4d) [61].

2.2 Polyanionic Compounds

Due to the 3D frameworks constructed from strong covalent bonds, polyanionic compounds exhibit high structural stability. The stable framework structure endows the electrode with high Na+ ion diffusion coefficient, low volumetric expansion and less phase transition during Na+ insertion/extraction, thus ensuring long-term cycling and high safety SIBs [7]. Nevertheless, the large polyanionic groups also lead to relatively low capacity and low electron conductivity. Polyanionic compounds are full of variety, exhibiting versatile and adjustable structure, for instance, phosphates, pyrophosphates, sulfates and mixed anions, as well as good electrochemical performance.

2.2.1 Phosphates

In a previous review, we have summarized in detail the phosphate framework electrode materials for SIBs [7]. In this section, we will focus on the major development of electrodes and mechanism study of phosphate materials.

2.2.1.1 Crystalline Phases
Olivine NaFePO4 has attracted great attention as cathode materials for SIBs, as its analogue LiFePO4 has been successfully commercialized for lithium-ion batteries application. However, the direct synthesis of olivine NaFePO4 through high-temperature calcination is infeasible. The olivine NaFePO4 is usually synthesized through cation exchange with chemical/electrochemical method. The obtained NaFePO4 electrode exhibits a reversible capacity of ~ 125 mAh g−1 with a average potential of 2.75 V [62]. Fang et al. have synthesized the olivine NaFePO4 microsphere through aqueous electrochemical displacement method from olivine LiFePO4 precursors (Fig. 5a). The NaFePO4 microsphere electrode showed excellent cycling stability with 90% capacity retention over 240 cycles. And a Na2/3FePO4 intermediate phase during discharge process was disclosed through conventional electrochemical techniques [63]. The detail reaction mechanism has been well studied by spectrum technique [64, 65, 66]. The sodium insertion/extraction occurred through different mechanisms due to the huge volumetric mismatch between FePO4 and NaFePO4 [65].
Fig. 5

a Constant current charge–discharge illustration of the aqueous electrochemical displacement process from olivine LiFePO4 to FePO4 in 1 mol L−1 Li2SO4 solution and then to NaFePO4 in 1 mol L−1 Na2SO4 solution. b Typical charge–discharge profiles of the NVP and HCF-NVP electrodes at a current rate of 0.2 C. c Galvanostatic voltage–composition curve of NaVOPO4 at a rate of C/20. d Galvanostatic discharging/charging profiles of amorphous FePO4 performed at a current density of 20 mA g−1. e Long-term cycling performance of the HCF-NVP electrode at a high current rate of 30 C over 20,000 cycles, voltage window is 2–3.9 V. a Reproduced with permission [63]. Copyright 2015, American Chemical Society. b, e Reproduced with permission [89]. Copyright 2015, Wiley–VCH. c Reproduced with permission [95]. Copyright 2018, Elsevier. d Reproduced with permission [101]. Copyright 2014, American Chemical Society

Apart from olivine NaFePO4 material, another promising iron-based phosphate is the Na2FeP2O7. The Na2FeP2O7 phase can be directly synthesized with high-temperature solid-state reactions. The Na2FeP2O7 compound generally consists of transition metal octahedral FeO6 and tetrahedral P2O7 units connected to form a robust framework. The Na2FeP2O7 electrode exhibited a reversible capacity of 82 mAh g−1 with two redox potentials at around 2.5 and 3 V [67]. The quasi-equilibrium measurements and first-principle calculations consistently indicated that Na2FeP2O7 experiences a single-phase reaction and a series of two-phase reactions during sodiation/desodiation [68]. The electrochemical performance of Na2FeP2O7 electrode was greatly improved by different carbon decorations [69, 70]. For example, the Na2FeP2O7 nanoparticles embedded in carbon exhibited a high rate capability and long cycle life (84% capacity retention over 10,000 cycles) [71]. Recently, Na4−αFe2+α/2(P2O7)2(2/3 ≤ α ≤ 7/8) also showed decent performance for SIBs [72, 73, 74]. The Na3.12Fe2.44(P2O7)2 electrode showed a reversible capacity of 107 mAh g−1 with an average potential at around 3.0 V [75]. And high-performance Na3.12Fe2.44(P2O7)2 electrodes with a long cycle life over 5000 cycles were also reported [76].

Another interesting structure is the Na Super Ionic Conductor (NASICON)-type structure, which is well known for its high Na ionic conductivity and stable 3D framework. As electrode materials, the NASICON NaxM2(PO4)3(M = V and Ti, 0 ≤ x ≤ 3) have been widely studied, considering that the NASICON-type lattices ensure long-term cycle life as well as high rate capability. Na3V2(PO4)3 has a 3D framework of VO6 octahedra sharing all of its corners with PO4 tetrahedra, and one Na+ ion occupies the M1 sites and two Na+ ions occupy the M2 sites [7]. And only Na+ ions located at M2 sites are electrochemically active due to the weak bonding to surrounding oxygen atoms. The Na3V2(PO4)3 material exhibits a reversible capacity of 117 mAh g−1 with a flat potential at 3.4 V (Fig. 5b). In situ XRD study indicates a typical two-phase reaction between Na3V2(PO4)3 and NaV2(PO4)3 during the sodium insertion/extraction [77]. Tremendous efforts have been made to improve the electrochemical properties of Na3V2(PO4)3 electrode, including carbon decoration [78, 79, 80, 81], conductive polymer coating [82] and metal ion doping [83, 84, 85]. And high-performance Na3V2(PO4)3 electrodes are reported [86, 87, 88]. For example, Fang et al. reported a facile chemical vapor deposition (CVD) method to construct hierarchical graphitized carbon decorated Na3V2(PO4)3 electrodes, which exhibited a high rate capability (38 mAh g−1 at 500 C) and superior long cycle life (54% capacity retention over 20,000 cycles) (Fig. 5e) [89]. The outstanding electrochemical performance may result from the hierarchical graphitized carbon decoration, with graphene coating and interconnected nanofiber networks. The graphene coating significantly differs from amorphous carbon coatings reported in previous publications. Wei et al. [90] recently reported a 3DHP–NVP@C cathode with a long cycling life over 30,000 cycles at 50 C. The high-performance Na3V2(PO4)3 electrodes indicate the feasible application of Na3V2(PO4)3 material for SIBs.

Recently, a series of NaxVOPO4 (0 ≤ x ≤ 1) materials have been reported to show good sodium storage performance. Goodenough and co-workers have reported a monoclinic NaVOPO4 with a reversible capacity of 90 mAh g−1 [91]. He et al. [92, 93] synthesized tetragonal and orthorhombic VOPO4 by delithiation of the corresponding LiVOPO4 species, and the obtained VOPO4 materials show barely satisfactory sodium storage performance. The above-mentioned NaxVOPO4 materials have tunnel structures in which Na ions are zigzagged to transport through the lattice frameworks, resulting in slow kinetics and small reversible capacity. Zhu et al. [94] reported ultrathin VOPO4 nanosheets, which exhibit a high reversible capacity of 136 mAh g−1 and stable cycle life with 73% capacity retention over 500 cycles. However, in view of technological application, the Na-deficient VOPO4 needs to be pre-sodiated to afford a Na-rich cathode, making these materials quite complex, costly and time-consuming. Recently, Cao’s group reported a novel layered NaVOPO4 material through a facile solvothermal reduction method [95]. The two-dimensional interspace endows the electrodes with a high reversible capacity of 144 mAh g−1 (Fig. 5c) and long cycle life of 67% capacity retention over 1000 cycles. The sodium storage reaction mechanism has been well investigated by in situ synchrotron X-ray diffraction and X-ray near edge adsorption.

2.2.1.2 Amorphous Phases

Amorphous electrodes, lacking of three-dimensional long-range order, are supposed as potential electrodes considering less lattice pressure during electrochemical reaction, which would improve the kinetics, capacity and cycling stability of the electrodes. Nevertheless, it is not easy to explore suitable amorphous structure for sodium storage. Amorphous FePO4 has been widely studied as lithium-ion battery electrodes and was investigated as drop-in electrodes for SIBs. The amorphous FePO4 was reported to deliver a high capacity of 146 mAh g−1 for SIBs with slope discharge/charge curves [96]. Carbon decoration is an effective way to improve the electrochemical performance of amorphous FePO4 material. However, because high-temperature calcination will lead to the crystallization of amorphous FePO4, carbon decoration should be conducted at relatively low temperature. Various carbon matrixes have been introduced to improve the electrochemical properties of amorphous FePO4 [97, 98, 99, 100]. For example, Fang et al. [101] reported a mesoporous amorphous FePO4 embedded in carbon matrix, and the obtained FePO4/C electrode showed a high reversible capacity of 151 mAh g−1 with stable cycle life (Fig. 5d).

The Na-vacant amorphous FePO4 cathodes are not convenient for practical battery applications. Li et al. [102] reported an amorphous NaFePO4 electrode with a large reversible capacity of 152 mAh g−1 and high rate performance. The work demonstrates the feasible synthesis of a Na-rich amorphous NaFePO4 electrode with good performance, and more versatile synthesis routes are needed for the construction of amorphous NaFePO4 material and other Na-rich amorphous materials to enrich the material systems for SIBs.

2.2.2 Sulfates

Some of the metal sulfates also have been introduced for sodium storage, and the iron-based sulfates exhibit decent electrochemical performance and have attracted great attention. Barpanda et al. [103] reported an alluaudite-type Na2Fe2(SO4)3 material (Fig. 6a), which exhibited a reversible capacity of 102 mAh g−1 with a high average potential of 3.8 V (Fig. 6e). The relatively high work potential may result from the high inductive effect of the SO42−. The sloping voltage curve indicates a single-phase (solid solution) mechanism, which was also proved by XRD patterns and Mossbauer Spectra [104]. Yamada and co-workers have carefully investigated the phase purity of off-stoichiometry Na2+2xFe2−x(SO4)3 materials, and a more precise phase with the composition of Na2.4Fe1.8(SO4)3 was isolated [105]. After that, the Na2.4Fe1.8(SO4)3 materials have been widely studied. For example, Deng et al. have reported a free-standing alluaudite Na2+2xFe2−x(SO4)3@ porous carbon-nanofiber hybrid film with a high rate capability of 40 C and long cyclic life over 500 cycles [106]. Some analogous sulfates, such as NaFe(SO4)2 [107], Na2Mg(SO4)2·4H2O [108] and Na2.44Mn1.79(SO4)3 [109], were also reported, but the performances are not satisfying.
Fig. 6

Crystal structures of a Na2Fe2(SO4)3 and b Na2FePO4F, c Na3V2(PO4)2F3, d Na4Fe3(PO4)2(P2O7). e Galvanostatic charging and discharging profiles of Na2−xFe2(SO4)3 cathode cycled between 2.0 and 4.5 V at a rate of C/20 (2 Na in 20 h) at 25 °C. f Galvanostatic charge/discharge curves of the Na/Na2FePO4F cell cycled at a rate of 6.2 mA g−1. Capacity retention of the Na2FePO4F samples synthesized with or without ascorbic acid is shown in the inset (f). g Galvanostatic charging–discharging profiles of Na3V2(PO4)2F3@C nanocomposite at various current rates. h Galvanostatic charge/discharge profiles of Na4Fe3(PO4)2(P2O7) in a Na-ion cell at the C/20 rate. The inset presents the cycle performance at various C-rates (C/20, C/10, and C/5). a, e Reproduced with permission [103]. Copyright 2014, Nature Publishing Group. f Reproduced with permission [111]. Copyright 2011, Elsevier. g Reproduced with permission [116]. Copyright 2015, Royal Society of Chemistry. h Reproduced with permission [129]. Copyright 2013, American Chemical Society

2.2.3 Mixed-Anion Materials

A lot of materials with mixed-anion have been reported with good sodium storage performance. The mixed-anion materials not only provide different host lattice structures; on the other hand, the interactions between different anions can have a great effect on the electrochemical characteristics, such as increasing the working potential.

As discussed above, the olivine NaFePO4 is usually synthesized through chemical/electrochemical displacement from the olivine LiFePO4 analogue. However, a Na2FePO4F material can be directly synthesized by solid-state reaction and ionothermal method [110]. The Na2FePO4F material exhibits a reversible capacity of 110 mAh g−1 with an average potential of 3 V (Fig. 6f) [111]. And the electrochemical reaction mechanisms have been well investigated by solid-state NMR and XRD [112, 113]. Recently, a high-performance Na2FePO4F/C material with outstanding cycling stability of 70% capacity retention over 5000 cycles has been reported by Dun et al. [114].

F-substitution can enrich largely the family of V-based electrode materials. The tetragonal structure Na3V2(PO4)2F3 belongs to a P42/mnm space group, featuring a strongly covalent 3D framework with large interstitial spaces for ion diffusion (Fig. 6c) [115]. The Na3V2(PO4)2F3 material could deliver a high reversible capacity of 130 mAh g−1 with three plateaus at 3.3, 3.7 and 4.2 V, along with a high rate capability (57 mAh g−1 at 30 C) and long cycle life (50% capacity retention over 3000 cycles) (Fig. 6g) [116]. The sodium storage mechanism has been well studied. For example, the phase diagram during Na+ insertion/extraction was revealed, and four intermediate phases existed during charge process and only one of these phases underwent a solid solution reaction [117]. It is worth noting that, a so-called NaVPO4F material has also been reported by some researchers. However, there is no structural data about the NaVPO4F phase. And the reported XRD data of NaVPO4F material match well with the NASICON Na3V2(PO4)3 compound. As the formation of a NASICON-type phase Na3V2(PO4)3 instead of NaVPO4F could be possible, the samples were prepared by the high-temperature solid-state method and sublimation of VF3 could occur. The existence of the NaVPO4F phase was questioned by some researchers [118, 119].

For Na3V2O2x(PO4)2F3−2x (0 < x ≤ 1), the oxygen substitutes for fluorine in the vanadium-centered bi-octahedra V2O8+2yF3−2y advances the vanadium to a higher average oxidation state (V(3+y)+). In all cases, the samples have similar XRD and charge/discharge curves with two voltage plateaus at the same voltage region, suggesting that all the materials belong to the same family with similar structures [118]. The increase in oxygen content creates a slightly lower operating voltage, and a more sloping voltage curve is caused by weaker sodium ordering and a smaller unit cell [120]. The sodium storage mechanisms during Na+ insertion/extraction include the combination of solid solution and two-phase reaction behavior [121]. Park et al. [122] also found that the redox mechanism and phase reactions varied with fluorine content. By optimizing the structure, Peng et al. [123] reported a RuO2-coated Na3V2O2(PO4)2F nanowires with a superior long cycle life over 1000 cycles.

Multivalent anions can also combine together to form new structures. The compounds containing (PO4)(P2O7) are in the spotlight due to the good sodium storage performance. Na4M3(PO4)2(P2O7) (M = Fe, Mn, Co, Ni) materials, with double chains built up from PO4 tetrahedron and MO6 octahedra sharing corners, have been structurally characterized in early years (Fig. 6d) [124, 125, 126]. The tunnels are extended along the three main crystallographic directions [100], [010] and [001], giving a high Na+ ionic conductivity (10−6 S cm−1). Kang’s group has investigated a series of Na4M3(PO4)2(P2O7) (M = Fe, Mn) materials [127, 128, 129], the Na4Fe3(PO4)2(P2O7) exhibited a reversible capacity of 129 mAh g−1 with a average potential of 3 V (Fig. 6h), while they were 109 mAh g−1 and 3.8 V for the Na4Mn3(PO4)2(P2O7) materials. Combining the Fe and Mn together, they got Na4MnxFe3–x(PO4)2(P2O7) (x = 1 or 2) materials with variable charge/discharge curves and capacities [130]. Na4Co3(PO4)2(P2O7) and Na7V4(P2O7)4(PO4) materials were also reported for sodium storage [131, 132, 133]. Both materials exhibit high operating potential with a capacity around 90 mAh g−1. Thus, the synergistic effect between (PO4) and (P2O7) can endow the composites with enhanced redox potentials. More related materials are expected to be reported by researchers.

Other mixed-anion materials were also reported, such as Na2.24FePO4CO3 [134], Na3MnPO4CO3 [135], NaFe2PO4(SO4)2 [136] and Na2MnPO4F [137], but more works should be done to improve their electrochemical performance.

2.3 Ferrocyanide Materials

As a unique type of metal–organic-framework materials, the Prussian blue-type materials exhibit promising potential for sodium storage due to the rigid open framework with large interstitial sites and weak bonding of Na+ ions with (C≡N), which endows great durability and fast kinetics. NaxMFe(CN)6 (0 ≤ x ≤ 2; M = Fe, Mn, Co, Ni, Cu, Zn, et al.), Prussian blue and its analogues are expected to show a large capacity due to the potential two-electron redox reaction of M and Fe in the structure. The electrochemical performance depends entirely on the phase purity, crystallinity, defects, crystalline water content and carbon decoration. Yang’s group has reported single-crystal FeFe(CN)6 nanoparticles with high purity, which ensured a high reversible capacity of 120 mAh g−1 with a long cycle life over 500 cycles [138]. Guo and co-workers reported that high-quality Na0.61Fe [Fe(CN)6]0.94 crystals showed much enhanced electrochemical performance in terms of high reversible capacity and long cycle life [139]. And Goodenough and co-workers reported a high Na concentration Na1.92Fe [Fe(CN)6] sample with a high capacity of 160 mAh g−1 and two defined potentials at 3.3 and 3.0 V, respectively (Fig. 7a) [140].
Fig. 7

a Galvanostatic charge and discharge curves of Na1.92Fe[Fe(CN)6] at a current of 10 mA g−1 at the first cycle (chronoamperograms were embedded); b Charge/discharge profiles of dehydrated Na1.9MnFe(CN)6·0.3H2O at various C-rates, inset is corresponding charge/discharge capacities at various C-rates; c Charge/Discharge profiles of Na2CoFe(CN)6 sample. a Reproduced with permission [140]. Copyright 2015, American Chemical Society. b Reproduced with permission [142]. Copyright 2015, American Chemical Society. c Reproduced with permission [148]. Copyright 2016, American Chemical Society

The low-cost manganese-based Prussian blue analogues also attracted much attention. The active Mn2+/Mn3+ and Fe2+/Fe3+ redox couples endow the NaxMnFe(CN)6 materials with high reversible capacity and appropriate working potential. A Na1.72Mn[Fe(CN)6]0.99 electrode was reported by Goodenough and co-workers with a reversible capacity of 135 mAh g−1 and high rate capability of 40 C [141]. Recently, they reported the dehydrated Na2MnFe(CN)6 phase with distorted crystal structure, which exhibited different charge/discharge curves with a capacity of 150 mAh g−1 and a plateau at 3.5 V (Fig. 7b), and long cycle life over 500 cycles [142]. Metal doping and conductive polymer coating were also reported, and showed enhanced electrochemical characteristics [143, 144].

Other NaxMFe(CN)6 (M = Co, Ni, Cu, Ti) hexacyanoferrates have also been investigated [145, 146, 147]. The Na2CoFe(CN)6 sample delivered a high reversible capacity of 150 mAh g−1 with two distinguishable potential plateaus at 3.8 and 3.2 V (Fig. 7c) [148]. And Na2NiFe(CN)6 exhibited a lower capacity with only one-electron redox reaction [149].

In addition, many of the hexacyanoferrates have demonstrated good electrochemical performances in aqueous sodium-ion batteries due to the unique and stable structure and appropriate redox potential [150, 151]. The safe and low-cost characteristic of aqueous sodium-ion batteries should be favored for large-scale energy storage application.

3 Anode Materials

Exploring suitable anode materials for sodium storage is also very important for the development of SIBs [152]. As the sodium storage mechanism of anode electrodes for SIBs is similar to that of lithium-ion battery anodes, a large variety of phases used in LIBs have been studied as drop-in replacements for sodium storage, including carbon-based materials, metal oxides/sulfides/phosphides and alloys. And the sodium mechanism mainly lies in intercalation reaction, conversion reaction and alloying reaction. Based on the different reaction mechanisms, in this section, we will discuss the anode materials in four parts: carbon-based materials, conversion reaction, alloying reaction and titanium-based materials.

3.1 Carbon-Based Materials

Due to the wide-abundant resources, low sodium embedded platform and good cycling stability, carbon-based materials have been considered as promising candidates for sodium storage [9]. The carbon-based materials mainly include graphite, graphene, soft carbon and hard carbon. The soft carbon and hard carbon are amorphous carbon, which are short of long-range ordered structure in plane and ordered stacked structure like graphite. Generally, they consist of randomly distributed graphitized microdomains, distorted graphene nanosheets and voids among. This unique structural feature with randomly distributed and cross-linking graphitic microdomains would prefer to remain the amorphous structure and restrain the transformation into graphitic structure [9]. Hard carbon cannot be graphitized, even the disordered structure is kept under annealing over 2500 °C, while soft carbon can be transformed into graphite carbon.

As graphite has been widely used in commercial LIBs, its sodium storage has been studied earlier. However, the graphite has been proved to be unable to accommodate Na+ ions in carbonate electrolytes, which may be due to the Na+ ions with bigger radius that cannot be inserted into the layer of graphite [153, 154, 155]. Wang and co-workers have reported the expanded graphite, which exhibited a enlarged interlayer lattice distance of 4.3 Å, with a reversible capacity of 284 mAh g−1 and stable cycling life over 2000 cycles [156]. In addition, Na+-solvent co-intercalation into the graphite layer has been reported by some researchers [157, 158, 159]. For example, Adelhelm and co-workers have reported the co-intercalation phenomena in a diglyme-based electrolyte, which enabled the graphite with high energy efficiency and small irreversible loss with a long cycle life over 1000 cycles though the reversible capacity was limited to 100 mAh g−1 [157]. Although the graphite can be utilized through co-intercalation of Na+ and solvent, the intercalation of solvent molecules would lead to the consumption of electrolyte solvent and large volume expansion which could result in pulverization of graphite particles. The high intercalation voltage and low specific capacity also show less advantages than other anode materials.

Graphene with nanosheet structure exhibits huge specific surface and many defects, which provide efficient active sites for sodium storage. And the graphene structure related to different synthesis methods shows different sodium storage performances. The adsorption characteristic may lead to a low initial Coulombic efficiency and high reaction potential. In addition, the high cost and complex synthesis routes may not be suitable for large-scale application.

Soft carbon demonstrates a generally sloping potential profile during sodiation/desodiation with a relatively high redox potential (> 0.5 V), and the initial Coulombic efficiency (about 60%) is very low. The reversible capacity (about 200 mAh g−1) was also not high. Ji and co-workers reported a PTCDA pyrolytic soft carbon, which could deliver a capacity of 200 mAh g−1. And they concluded that the high sodiation potential and high reversible slope capacity are related with the defective local structure of soft carbon, and the irreversible macroscopic structural expansion caused by Na+ ions intercalation led to the low Coulombic efficiency [160]. They also found that the Na ions did intercalate in between the turbostratic graphene layers by observing the reversible dramatic gallery expansion from ∼ 3.6 to ∼ 4.2 Å during electrochemical reaction (Fig. 8a) [160, 161]. The major trunk of reversible capacity of soft carbon lies in the high potential region, where Na plating may not occur, thereby alleviating the concerns of dendrite formation, polarization and limited rate capability.
Fig. 8

a First-cycle sodiation/desodiation profiles of soft carbon at 20 mA g−1; the insets show the in situ selected area electron diffraction (SAED) patterns at different charge states. b Logarithm diffusion coefficient (log D) versus OCV plots at various potentials for HCNP-1150. Schematic illustrations of the mechanisms for Na-ion storage in hard carbon: c “intercalation–adsorption” mechanism; d “adsorption–intercalation” mechanism. e Cycle performances of 3D PCFs anode for SIBs. a Reproduced with permission [160]. Copyright 2017, American Chemical Society. b Reproduced with permission [183]. Copyright 2015, Elsevier. c, d Reproduced with permission [181]. Copyright 2017, Wiley–VCH. d Reproduced with permission [108]. e Reproduced with permission [165]. Copyright 2015, Wiley–VCH

Hard carbon has been considered to be a more promising carbonaceous material for SIBs due to the high capacity and low potential, and it typically exhibits a sodium storage capacity in a high-potential sloping region and a low-potential plateau region. Dahn and co-workers reported the electrochemical sodium storage behavior of glucose-pyrolyzed carbon in 2000, which achieved a reversible capacity of 300 mAh g−1 [162]. The high reversible capacity and low sodiation potential have triggered a lot of researchers to synthesize various kinds of hard carbon for sodium storage. José L. Tirado and co-workers have reported resorcinol and formaldehyde-pyrolyzed carbon, with a reversible capacity of 285 mAh g−1 and most of the capacity (247 mAh g−1) centered below 0.2 V [163]. Cao et al. [155] have reported polyaniline-pyrolyzed hollow carbon nanowires with a initial reversible capacity of 251 mAh g−1 and long cycle life over 400 cycles. The pre-eminent sodium storage performance benefited from the hollow structure with high specific surface that can provide more active sites and enlarged interlayer with effective Na+ ions insertion. Guo et al. designed a sandwich-like hierarchical porous carbon/graphene composite, and the synergistic effect of enlarged interlayer spacing and conductive graphene sheets endowed the composite with a high reversible capacity of 400 mAh g−1 and long cycle life over 1000 cycles [164]. And high-performance polymerized carbon dots with a long cycle life of 10,000 cycles were also reported (Fig. 8e) [165].

Heteroatom doping has been considered as effective routes to enlarge the interlayer distance, enhance the surface wettability and tailor the electronic characteristics of hard carbon. A variety of heteroatoms (e.g., B, N, S and P) have been doped in carbonaceous materials and evaluated as anode materials for sodium storage. Lou’s group has reported flexible N-doped carbon-nanofiber films with 7.5 wt% nitrogen content, exhibiting a long cycle life over 7000 cycles and high rate capability (154 mAh g−1 at 5 A g−1) [166]. S-doped carbon materials were also reported [167, 168]. Additional reaction of Na+ ions with S can also be achieved during the sodium storage reaction, enabling high reversible capacity (500 and 126 mAh g−1 for the S-doped carbon and undoped carbon, respectively) [167]. N, P co-doped carbon spheres were also reported with improved sodium storage performance [169].

Due to the renewablility, abundance and eco-friendly properties, biomass-derived carbon materials are quite attractive and widely reported. As a lot of biomass materials mainly consist of elemental C, H, N and O, generating carbonaceous materials from them is practicable. A large variety of biomass materials have been utilized to synthesize carbonaceous materials, such as, banana peels [170], natural cotton [171], corn cobs [172], apple biowaste [173], oatmeal [174], coconut oil [175], okara [176], pomelo peels [177], oak leaves [178]. Different biomass materials have different microstructures and chemical compositions, which determine the microstructure, morphology, specific area and composition of derived carbon materials, finally influencing the sodium storage characteristics. And the biomass materials often contain some heteroatoms (such as, N, P and S), which can be doped into the derived carbon. Choosing proper biomass materials and optimizing the carbonizing process should be the key routes to obtain carbonaceous materials with high sodium storage performance.

It should be noted that the hard carbon electrodes exhibit sodiation/desodiation curve with both slope and plateau, which trigged hot debates about the sodium storage mechanism. Two main sodium storage mechanisms were proposed: “insertion-adsorption mechanism” and “adsorption-insertion mechanism” (Fig. 8c, d) [9, 179]. The “insertion-adsorption mechanism” was first proposed by Dahn and Stevens [162], and they found that the charge/discharge curves of pyrolytic glucose carbon for lithium and sodium system were quite similar. And they suggested that the sodium storage mechanism of hard carbon for sodium-ion battery was similar to its lithium companion, namely, the capacity in the slope region was mainly derived from the insertion/extraction of Na+ between the carbon layers, and the capacity in the plateau region is from the adsorption/desorption of Na+ in the micropores [162]. And in situ XRD and SAXS were introduced to confirm the conclusions [180]. However, for the same precursor, the hard carbon pyrolyzed at low temperature has lots of micropores, but it exhibits few plateau capacities, and in addition, the micropores’ volume of hard carbon decreases with the increase in pyrolytic temperature, but the plateau capacity gradually increases. These phenomena are in contradiction with the “insertion-adsorption mechanism.” Cao and co-workers proposed the “adsorption-insertion mechanism” that high potential slope region is associated with the adsorption/desorption of Na+ ions, and low-potential plateau corresponds to the insertion/extraction of Na+ between the carbon layers [155]. And very recently, they have used various techniques (such as, in situ XRD, NMR, electron paramagnetic resonance) to sufficiently evidence the sodium storage mechanism [181]. During discharge process, Na ions first adsorb on the surface active sites of hard carbon, which leads to a sloping voltage profile due to a wide distribution of adsorption energies; then, Na ions intercalate into graphene layers with suitable spacing to form NaCx compounds, which exhibits a flat voltage plateau similar to the Li–graphite counterpart [181]. This mechanism suggests that the sodium storage capacity depends on appropriate carbon layer spacing rather than microporous structure. So, the reduction in the micropores greatly decreases the specific surface area, leading to the improvement in the initial Coulombic efficiency for hard carbon anode. Through designing low microporous structure and appropriate spacing of carbon layer, the low porosity hard carbon can achieve the high initial Coulombic efficiency of 86.1% and high reversible capacity of 362 mAh g−1 with an improved plateau capacity of 230 mAh g−1 below 0.1 V [181], which provides new insight into high-performance and practicable hard carbon anodes. Ji and co-workers also suggested that the slope region is related to the sodium storage behavior of edges and surface defects of carbon, rather than micropores, and the low-potential plateau region corresponds to the insertion of Na+ into carbon interlayers and minor Na+ adsorption on pore surfaces [182]. The Na+ cations diffusion variation throughout the insertion/extraction processes at low-potential plateau in hard carbon has been measured by Xiao et al. (Figure 8b), which provides helpful insight into the stepwise Na+ cation insertion/extraction phases and rates in hard carbon materials [183]. Recently, they reported that the defects in the graphite layers were directly related to the reversible capacity and initial Coulombic efficiency. A hard carbon electrode with low defect and porosity has been synthesized and exhibited a high initial Coulombic efficiency of 86.1% and reversible capacity of 361 mAh g−1 [184].

3.2 Conversion Reaction Materials

The conversion reaction results in the reversible formation of metallic nanoparticles dispersed intimately within the NaXn (X = O, S, P, Se, F, et al.) matrix that maintains the electronic conductivity. The conductivity of the electrode materials, conversion energy gap and drastic volume expansion show big effect on the performance of the electrodes. A large variety of metal oxides/sulfides/phosphides have been investigated as anode materials for sodium storage.

A lot of metal oxides have been investigated as drop-in anodes for sodium storage, such as, Fe2O3, Fe3O4, Co3O4, NiO, CuO, ZnO, SnO, SnO2. Conductive carbon decoration was introduced to improve the electronic conductivity and buffer the volume expansion. For example, Chen’s group reported a γ-Fe2O3@C nanocomposite which delivered a high reversible capacity of 1000 mAh g−1 with a long cyclic life of 1400 cycles [185]. Qiu’s group reported hybrid carbon nanosheets decorated Fe3O4 quantum dots with a high reversible capacity (over 400 mAh g−1) and stable cyclic life over 1000 cycles [186].

Metal sulfides with weak M–S bonds are kinetically more favorable for conversion reactions, and the discharge product (Na2S) of metal sulfides affords better conductivity than that (Na2O) of metal oxides [11]. 2D-layered MoS2 has attracted significant attention. Conductive species, such as graphene [187, 188, 189, 190], carbon tubes [191], carbon fibers [192] and carbon paper [193], were used to construct nanostructured MoS2 electrodes. Dou and co-workers have designed unique hierarchical sandwich-like MoS2/graphene composites, which displayed a high reversible capacity of 640 mAh g−1 with a stable cycle life over 250 cycles [194]. Chen et al. prepared MoS2/C microspheres via aerosol spray pyrolysis method, and the electrode exhibited a long cycle life over 2500 cycles [195]. An ultralong worm-like MoS2 was also reported by Xu et al. (Figure 9a, e) [196]. Tin-based sulfides (SnS [197, 198, 199, 200]) and SnS2 [201, 202]) and antimony sulfide (Sb2S3 [203, 204, 205, 206, 207]) have attracted considerable attention due to the conversion/alloying reactions, in which Sn and Sb can alloy with Na+ to show electrochemical reactivity. Cao and co-workers have reported SnS@ reduced graphene oxide composites with SnS nanoparticles anchored on the surface of graphene, and the composites exhibited a reversible capacity of 457 mAh g−1 with stable cycle life [208]. Lou and co-workers have synthesized carbon-coated hierarchical SnS nanotubes with enhanced sodium storage properties (Fig. 9b) [209]. Guo et al. proposed an efficient method to alter the electrochemical properties of SnS by structural phase transition, and the prepared SnS@GR exhibited a large capacity of 940 mAh g−1 with impressive rate capability (Fig. 9g) [210]. Yu et al. have reported graphene decorated Sb2S3 with a reversible capacity of 730 mAh g−1 and high rate capability, and a full cell constructed by coupling RGO/Sb2S3 anode and Na2/3Ni1/3Mn2/3O2 cathode showed an energy density of 80 Wh kg−1 [204]. FeSx materials were also introduced for sodium storage. Pyrite FeS2 was reported to show a high discharge capacity of 500 mAh g−1 with an average potential at 1.8 V [211, 212, 213]. The discharge process involved a Na+ insertion reaction above 0.8 V and a subsequent conversion reaction below 0.8 V [214, 215]. When cycled in low-potential region, the FeS2 electrode exhibited a high reversible capacity of 900 mAh g−1 and long stable cyclic life [216]. Lou and co-workers designed unique FeS2@C yolk-shell nanoboxes with a long cycle life over 800 cycles (Fig. 9c, f) [217]. Other sulfides, such as CoSx, NiSx, ZnS, MnS, have also been applied in SIBs. Peng et al. [218] reported CoS@rGO composites with a reversible capacity of 540 mAh g−1 and remarkable cycling stability over 1000 cycles.
Fig. 9

a FESEM image of 3D MoS2 of highly ordered hierarchical structures. b TEM image of the hierarchical SnS@C nanotubes. c TEM image of FeS2@C. d HRTEM image of Sn4P3/C composite. e Charge–discharge curves of the as-prepared MoS2. f Discharge/charge voltage profiles of FeS2@C-45 for the first five cycles at a current density of 100 mA g−1. g Galvanostatic discharge–charge profiles for selected cycles of the SnS@graphene electrode at a current density of 30 mA g−1. h Initial charge/discharge curves at a constant current of 50 mA g−1. a, e Reproduced with permission [196]. Copyright 2015, Royal Society of Chemistry. b Reproduced with permission [209]. Copyright 2017, Wiley–VCH. c, f Reproduced with permission [217]. Copyright 2017, Royal Society of Chemistry. g Reproduced with permission [210]. Copyright 2014, American Chemical Society. h Reproduced with permission [224]. Copyright 2014, American Chemical Society

Some selenides have also shown good sodium storage performance. The selenides exhibit similar sodium storage characteristics with sulfides but show higher conductivity, and the large molecular weight also decreases the capacity to some extent. The selenides mainly undergo conversion reactions. Chen and co-workers have reported FeSe2 microspheres with a reversible capacity of 442 mAh g−1 and long cycle life over 2000 cycles [219]. Recently, Fang et al. reported Cu-doped CoSe2 microboxes synthesized via a sequential ion-exchange method. Benefiting from the unique structural and compositional merits, the Cu-doped CoSe2 microboxes exhibit enhanced sodium storage performances with high reversible capacity and stable cyclability [220].

Phosphides have also attracted some attention, such as MxP (M = Fe, Co, Sn) [221, 222, 223]. Among these phosphides, the Sn-based compounds show promising sodium storage performance. Yang and co-workers synthesized Sn4P3/C composites via high-energy mechanical milling method. The Sn4P3/C composites demonstrated a high reversible capacity of 850 mAh g−1 with stable cycle life (Fig. 9d, h) [224]. And the performances have been improved by rational structural design and carbon decoration [225, 226, 227]. Recently, FePx has also been reported for sodium storage [228]. Komaba’s group reported a FeP4 composite with a large reversible capacity of 1200 mAh g−1 and good cyclic life [229]. And a dual-carbon phase-modified amorphous and mesoporous FeP were reported to demonstrate a long-term cyclic life of 500 cycles [230].

3.3 Alloying Reaction Materials

Alloy anode materials with high capacity and low sodiation potential have attracted extensive attention. The sodium storage process includes the forming of Na-metal binary intermetallic compounds with multiple electron reactions, resulting in a high theoretical capacities of 370–2600 mAh g−1 [3]. Group III A (Si, Ge, Sn and Pb) and IV A (P, Sb and Bi) elements have shown efficient sodium storage performance. However, the multiple Na+ ions engaged reactions lead to huge volume changes in the electrodes during sodiation/desodiation, which may result in pulverization of the active materials and rapid capacity fading. Tremendous efforts have been made to improve the electrochemical performance by rational structural design, mainly including carbon decoration, nanosizing and intermetallics.

Sb can alloy with Na to form Na3Sb phase with a high theoretical capacity of 660 mAh g−1 and drastic volume expansion of 390%. Yang and co-workers reported Sb/C composite with a high reversible capacity of 610 mAh g−1 [231]. In situ XRD analysis indicated that Sb formed amorphous phases during the sodiation process except for the fully sodiated crystalline Na3Sb phase [232]. Xiao et al. proposed the report of high capacity alloy reaction for sodium storage based on SnSb/C composites, which were synthesized through a facile high-energy mechanical milling method, and the synergistic Na storage reactions endowed SnSb/C composites with stable cycle life [233]. Wu et al. [234] have designed 1D Sb-C nanofibers ensuring superior cycling stability with 90% capacity retention over 400 cycles (Fig. 10a, e). Similar approaches have been made by rational structural design and carbon decoration to improve the cycling stability of Sb materials, such as Sb@C coaxial nanotubes [235], Sb/C fibers [236], coral-like porous Sb [237], exfoliated Sb nanosheets [238] and hollow Sb@C spheres [239].
Fig. 10

a TEM image of the Sb–C nanofibers. b TEM image of pipe-wire TiO2–Sn@CNFs. c HRTEM image of the a-P/C composite, the inset is the electron diffraction pattern of a-P/C. d HRTEM image of the cross section of the phosphorene–graphene hybrid. e The initial discharge–charge profiles of the Sb–C nanofiber electrode between 0.01 and 2.0 V versus Na/Na+ at a current rate of C/15 (40 mA g−1). f Galvanostatic charge/discharge profiles of pipe-wire TiO2–Sn@CNFs at a current density of 100 mAg−1. g Voltage profiles of the a-P/C sample charged at a current density of 250 mA g−1 and then discharged at a different current density from 250 to 4000 mA g−1. h Galvanostatic discharge–charge curves of the phosphorene–graphene (48.3 wt% P) anode plotted for the first, second and 50th cycles. a, e Reproduced with permission [234]. Copyright 2014, Royal Society of Chemistry. b, f Reproduced with permission [244]. Copyright 2017, American Chemical Society. c, g Reproduced with permission [247]. Copyright 2013, Wiley–VCH. d, h Reproduced with permission [250]. Copyright 2015, Nature Publishing Group

Sn exhibits a large theoretical capacity of 847 mAh g−1 with the maximally sodiated phase of Na15Sn4. But the huge volume expansion of 523% also results in poor cycle life. The sodium storage mechanisms were carefully studied by DFT calculation, XRD and TEM technologies [240, 241, 242]. The sodium storage performances have been optimized by some groups through rational structural design. Chen and co-workers have designed Sn nanodots encapsulated in N-doped carbon nanofibers through the electrospinning method, and the materials demonstrated a high rate capability (450 mAh g−1 at 10 A g−1) and superior cycle life over 1300 cycles [243]. Wang and co-workers reported pipe-wire TiO2-Sn@ carbon nanofibers with a stable cycle life over 400 cycles (Fig. 10b, f) [244]. Sn nanofibers with high aspect ratios were synthesized via electrodeposition process in aqueous solution. The Sn nanofibers exhibited high sodium storage performance with a reversible capacity of 808 mAh g−1 and stable cycling, which may be attributed to the high mechanical stability of the nanofibers [245].

P is capable to accommodate three Na+ ions with the high theoretical capacity of 2596 mAh g−1 [10]. However, several issues restricted the sodium storage performance of the P materials. First, the red P exhibits a low electron conductivity (~ 10−14 S cm−1), which needs further conductive materials decoration [246]. In addition, the Na+ ion diffusion coefficient in P materials is relatively low. What’s more, the enormous volume expansion (491%) during the sodium uptake process is also an intractable problem. The amorphous P/C composite was reported with good sodium storage (Fig. 10c, g) [247, 248]. Qian et al. [247] reported amorphous P/C nanocomposite synthesized by high-energy ball-milling method, and the P/C electrodes demonstrated a high reversible capacity of 1800 mAh g−1 with a low-potential plateau of 0.2 V. Recently, Zhang et al. [249] demonstrated an amorphous P/nitrogen-doped graphene material with a stable cycle life over 350 cycles. And the black P with layered structure and a high electron conductivity was proposed as a good sodium storage host. Cui and co-workers have demonstrated graphene-sandwiched phosphorene with a remarkably high reversible capacity of 2440 mAh g−1 and stable cycle life (Fig. 10d, h) [250]. And the sodium storage mechanism includes first the intercalation of Na+ ions along the x axis and followed by the alloying reaction to form Na3P. Detailed sodiation mechanism of black P was studied by in situ XRD and first-principles calculations [251, 252]. Other materials, such as Si [253], Bi [254], Ge [255], have also been studied for SIBs, but the performance needs to be further improved.

3.4 Ti-Based Materials

Ti-based compounds have been extensively studied as anode materials for SIBs, due to the appropriate operating voltages, low cost and stability [256]. The Ti-based materials mainly demonstrate an insertion-type reaction with the Ti3+/Ti4+ redox couple, which normally ranges from 0.5 to 1.0 V, avoiding hazardous sodium plating and thus ensuring high safety.

Polymorph titanium dioxides such as amorphous and anatase TiO2-B compounds have been studied for reversible sodium insertion/extraction. Xiong et al. [257] first investigated the amorphous TiO2 nanotube anode for SIBs. After that, a large variety of works have been done to improve the sodium storage properties of TiO2 materials. Cao and co-workers reported graphene-modified TiO2 microspheres with the superior long cycle life over 1000 cycles [258]. Yang and co-workers synthesized TiO2/C nanofibers with anatase TiO2 nanocrystals embedded in conductive carbon fibers. The TiO2/C anode exhibits a high capacity of ~ 302.4 mAh g−1 and about 100% capacity retention over 1000 cycles (Fig. 11a) [259]. And they further designed graphene-supported TiO2 nanospheres with a reversible capacity of 300 mAh g−1 and high rate capability (123.1 mAh g−1 at a current rate of 4.0 A g−1) [260]. And anatase TiO2 nanorods with the ultrahigh rate capability of 100 C were also reported [261]. Huang and co-workers demonstrated the Na+ intercalation pseudo-capacitance in TiO2/graphene composites enabling high rate capability and long cycle life, and sodium storage characteristics were analyzed by electrochemical method and first-principle calculations [262]. Tremendous efforts have been made to improve the sodium storage performance of TiO2 materials through rational structural design and carbon decoration [263, 264, 265, 266, 267, 268, 269].
Fig. 11

a Discharge/charge profiles of the TiO2/C nanofiber anode at a constant current of 20 mA g−1; b Comparison of the initial discharge/charge curves of Li4Ti5O12 electrodes with binder of PVdF, NaAlg and Na-CMC, respectively. c Voltage versus composition profile for the electrochemical reduction in a blank electrode containing only carbon black (blue curve) and a composite electrode containing Na2Ti3O7 and 30% carbon black (red curve). d Typical discharge–charge profiles of the NTP and NTP@rGO electrodes at a current rate of 0.2 C. e Long-term cycling performance of the NTP@rGO electrode at a high current rate of 20 C over 1000 cycles, voltage window is 1.4–3 V. a Reproduced with permission [259]. Copyright 2016, American Chemical Society. b Reproduced with permission [270]. Copyright 2013, Nature Publishing Group. c Reproduced with permission [276]. Copyright 2011, American Chemical Society. d, e Reproduced with permission [287]. Copyright 2016, Wiley–VCH

Spinel Li4Ti5O12 with a “zero-strain” characteristic has also been reported with good sodium storage performance. The material exhibited a reversible capacity of 145 mAh g−1 with a flat potential of ~ 1.0 V (Fig. 11b). Hu and co-workers proposed a three-phase sodium storage mechanism based on in situ XRD and STEM analysis [270], with LiNa6Ti5O12 and Li7Ti5O12 intermediate phases formed during Na+ ions intercalation. Chu and co-workers reported a free-standing electrodecomposition of carbon-coated Li4Ti5O12 nanosheets and reduced graphene oxide, showing long cycle life over 600 cycles [271]. The sodium storage properties were further improved by structural optimizing [272, 273, 274, 275].

Na2Ti3O7 was found to reversibly uptake 2 Na+ ions with a theoretical capacity of 200 mAh g−1 and an average potential of 0.3 V (Fig. 11c) [276, 277]. The exceptionally low sodium storage potential was due to the structural instability of the Na intercalated compound from the calculation of the electrostatic interaction in the structure [278]. And the charge/discharge profile was notably changed depending on the discharge cutoff voltage of the electrode, which was associated with the appearance of an intermediate phase [279]. Casas-Cabanas and co-workers discovered that the SEI formed upon discharge was unstable during electrochemical cycling, resulting in poor cycling stability [280]. Ni et al. [281] deposited a S-doped TiO2 layer on the surface of Na2Ti3O7 nanotube, and the composites exhibited greatly enhanced cyclic life over 10,000 cycles. And Na2Ti3O7 hollow spheres were also reported with a high rate capability of 50 C [282].

NASICON-type NaTi2(PO4)3 also attracted considerable attention as Na+ ions host. The NaTi2(PO4)3 can uptake 2 Na+ ions with a theoretical capacity of 133 mAh g−1, and it displays a flat discharge plateau at 2.1 V associated with a two-phase reaction (Fig. 11d). The moderate voltage range ensures that the NaTi2(PO4)3 can be used as anode or cathode depending on the counter electrodes [7]. Tremendous efforts have been made through carbon decoration of NaTi2(PO4)3 to improve the electronic conductivity of the NaTi2(PO4)3 electrodes. Wu and Wang et al. reported the graphene and carbon nanotube decorated NaTi2(PO4)3 nanoparticles, respectively; both of the electrodes exhibited a high rate capability of 50 C and long cycle life over 1000 cycles. And the sodium storage performances have been further improved with a rate capability over 100 C and long cycle life over 10,000 cycles [283, 284, 285, 286]. Fang et al. [287] designed a hierarchical graphene-supported NaTi2(PO4)3, where graphene-coated nanosized NaTi2(PO4)3 and 3D graphene network could be achieved simultaneously. The NaTi2(PO4)3/graphene electrode exhibited a high specific capacity (130 mAh g−1), an excellent rate capability (38 mAh g−1 at 200 C) and a stable cycle life (77% capacity retention over 1000 cycles) (Fig. 11e). And an all NASICON-type NaTi2(PO4)3//Na3V2(PO4)3 full cell was also assembled with superior capacity and high-power performance.

4 Conclusions and Outlook

SIBs have been considered as promising candidates for grid-scale energy storage applications due to the consideration of low-cost Na resources and its similar chemical/electrochemical characteristic with established lithium-ion batteries. And the applicative utilization of inexpensive Al as anode current collector further lowers the cost. From the application aspects, the most important directions are lowering the cost per energy density and per lifetime cycle, and extending long cycle life. During the past decade, great efforts have been made to explore suitable electrode materials for sodium storage, and the sodium storage properties of typical electrode materials are summarized in Tables 1 and 2. Though the large atomic weight of Na+ ion sacrifices the energy density and big ionic radius requires suitable structure for sodium accommodation, a lot of materials have been reported with high rate capability and stable cycle life even over ten thousand cycles. And some cathode materials have shown high energy densities (vs. metal Na anode) around 500 Wh kg−1 (such as, NaVOPO4 504 Wh kg−1, Na2MnFe(CN)6 525 Wh kg−1, Na2CoFe(CN)6 525 Wh kg−1, Na3V2O2x(PO4)2F3−2x 480 Wh kg−1), which exceed that of Li/LiMn2O4 (429 Wh kg−1) and are very close to those of Li/LiCoO2 (530 Wh kg−1) and Li/LiFePO4 (510 Wh kg−1). For the anodes, the hard carbon exhibits high energy density around 290 Wh kg−1 (vs. Na3V2(PO4)3 as positive electrode, 117 mAh g−1, E = 3.4 V vs. Na/Na+) (Table 2), representing a promising candidate for sodium storage. The materials with alloying reaction (such as, Sn, Sb, P) and conversion reaction (such as, SnSb, SnSx, FePx) also show high energy densities over 250 Wh kg−1. Besides, some full sodium-ion batteries have been constructed [288], such as, hard carbon//Na3V2(PO4)3 (258 Wh kg−1) [289], hard carbon//NaFe0.5Mn0.5O2 (240 Wh kg−1) [290], Sb//Na3V2(PO4)3 (242 Wh kg−1) [78], Fe2O3//Na2FeP2O7 (210 Wh kg−1) [291], hard carbon//Na0.9(Cu0.22Fe0.3Mn0.48)O2 (203 Wh kg−1) [38], Sn//NaFePO4 (150 Wh kg−1) [292], Sb/C//Na3Ni2SbO6 (210 Wh kg−1) [53], NaTi2(PO4)3//Na3V2(PO4)3 (73 Wh kg−1) [287], FeOx//NaxFeFe(CN)6 (136 Wh kg−1) [293]. Some of the full cells exhibit high energy density over 240 Wh kg−1, showing potential application for large-scale energy storage.
Table 1

Overview of the electrochemical properties of typical cathode materials for SIBs

Materials

Structure

Average potential (V. vs Na/Na+)

Reversible capacity (mAh g−1)

Energy density (vs. Na/Na+, Wh kg−1)

References

Na0.44MnO2

Tunnel structure

2.75

128

352

[20]

Na0.54Mn0.50Ti0.51O2

Tunnel structure

2.8

114

319

[27]

Na0.61[Mn0.27Fe0.34Ti0.39]O2

Tunnel structure

3.6

90

324

[29]

NaFeO2

O3-layered structure

3.3

85

281

[32]

NaFe1/2Co1/2O2

O3-layered structure

3.2

160

512

[37]

Na2/3[Fe1/2Mn1/2]O2

P2-layered structure

2.5

190

475

[34]

NaNi1/3Fe1/3Mn1/3O2

O3-layered structure

2.75

100

275

[294]

NaFe0.3Ni0.7O2

O3-layered structure

2.9

135

392

[295]

NaFex(Ni0.5Mn0.5)1−xO2

O3-layered structure

2.9

131

380

[51]

NaFe0.5Co0.5O2

O3-layered structure

3.1

160

496

[37]

Na2/3MnO2

P2-layered structure

2.1

165

347

[43]

Na5/6[Mn0.75Li0.25]O2

O2/P2-layered structure

2.6

190

494

[44]

Na0.67[Mn0.65Ni0.15Co0.15Al0.05]O2

P2-layered structure

3.3

129

428

[49]

Na0.67Mn0.65Fe0.2Ni0.15O2

P2-layered structure

2.5

210

525

[45]

NaNiO2

O3-layered structure

3

120

360

[50]

Na2/3Ni1/3Mn2/3O2

P2-layered structure

3.2

151

483

[296]

NaNi0.5Mn0.5O2

O3-layered structure

2.75

180

495

[51]

NaNi0.5Mn0.5-xTixO2

O3-layered structure

2.9

130

377

[52]

Na0.68Cu0.34Mn0.66O2

P2-layered structure

3.7

67

248

[297]

Na7/9Cu2/9Fe1/9Mn2/3O2

P2-layered structure

3.6

90

324

[39]

Na0.7CoO2

P2-layered structure

2.9

125

363

[58]

Na0.66Co0.5Mn0.5O2

P3/P2-layered structure

2.3

156

359

[298]

NaCrO2

O3-layered structure

3

110

330

[55]

Na3Ni2SbO6

O3-layered structure

3.3

117

386

[53]

NaFePO4

Olivine

2.75

125

345

[63]

Na0.67FePO4

Alluaudite

2.5

143

358

[299]

FePO4

Amorphous

2.4

170

408

[101]

NaFePO4

Amorphous

2.4

152

365

[102]

Na3V2(PO4)3

NASICON

3.4

115

391

[89]

NaVOPO4

Triclinic P1

3.5

144

504

[95]

Na3V2(PO4)2F3

Tetragonal structure

3.8

130

494

[116]

Na3V2O2x(PO4)2F3−2X

Tetragonal structure

3.7

130

481

[118]

Na2.24FePO4CO3

Monoclinic, P21/m

2.7

120

324

[134]

Na2FeP2O7

Triclinic P1

2.9

85

246

[68]

Na3.12Fe2.44(P2O7)2

Triclinic P2

3

107

321

[73]

Na2.4Fe1.8(SO4)3

Alluaudite

3.8

102

388

[103]

Fe2(MoO4)3

NASICON

2.6

90

234

[300]

Na2FePO4F

Orthorhombic

3

110

330

[111]

Na4Fe3(PO4)2P2O7

Orthorhombic Pn21a

3

105

315

[127]

Na4Mn3(PO4)2P2O7

Orthorhombic Pn21a

3.84

109

419

[128]

Na4Co3(PO4)2P2O7

Orthorhombic Pn21a

4

95

380

[131]

Na4Fe(CN)6

3.3

89

294

[301]

FeFe(CN)6

Cubic Fm3 m

3

135

405

[138]

Na1.92Fe[Fe(CN)6]

Rhombohedral R-3

3.2

160

512

[140]

Na2MnFe(CN)6

Monoclinic P21/n

3.5

150

525

[142]

Na2CoFe(CN)6

Rhombohedral R-3

3.5

150

525

[148]

NaFeF3

Orthorhombic Pnma

2.8

170

476

[302]

Table 2

Overview of the electrochemical properties of typical anode materials for SIBs

Materials

Reaction mechanism

Average potential (V. vs Na/Na+)

Reversible capacity (mAh g−1)

Energy density (Wh kg−1) (vs. Na3V2(PO4)3 as positive electrode, 117 mAh g−1, E = 3.4 V vs. Na/Na+)

References

Hard carbon

Insertion-adsorption/adsorption-insertion

0.1

361

292

[181]

Soft carbon

Insertion-adsorption

0.5

200

214

[160]

S-doped carbon

Insertion-adsorption and conversion

1.7

500

161

[167]

TiO2

Insertion

1

300

202

[259]

Li4Ti5O12

Insertion

1

145

155

[270]

Na2Ti3O7

Insertion

1.2

230

171

[281]

NaTi2(PO4)3

Insertion

2

130

86

[287]

Sn

Alloying

0.25

808

322

[245]

Sb

Alloying

0.7

610

265

[231]

P

Alloying

0.6

1764

307

[247]

SnSb

Alloying

0.5

544

279

[233]

SnO2

Conversion and alloying

0.3

500

294

[303]

SnS

Conversion and alloying

1

940

250

[210]

SnS2

Conversion and alloying

1

680

240

[201]

Sb2S3

Conversion and alloying

1.2

835

226

[203]

Sn4P3

Conversion and alloying

0.6

850

288

[224]

MoS2

Conversion

1.5

504

180

[191]

γ-Fe2O3

Conversion

1.3

450

195

[304]

Fe3O4

Conversion

1.5

450

176

[186]

FeS2

Conversion

1.2

800

225

[216]

FeP4

Conversion

0.6

1200

298

[229]

For the cathode materials, the layered oxides materials demonstrate that unique sodium storage characteristics differ from the oxide materials in lithium-ion batteries. Depending on transition metal constituents, the theoretical capacity ranges may be in the 230–250 mAh g−1. The multiphases transition reactions and effective metal doping/substitution make the layered oxides more nontrivial. And there are lots of work can be done to tailor the chemical composition to optimize the sodium storage performance, and the complicated reaction mechanism should be further studied. In addition, most of the layered oxides are sensitive to moisture, thus increasing the cell cost as the moisture-free processes are needed during materials synthesis and cell preparation. Thus, the polyanionic materials with robust structure and small volume change should be a good choice. The family of polyanionic compounds exhibits high average potential due to the inductive effect of the polyanionic group. And some of the polyanion materials have been reported to demonstrate high rate capability over 100 C rate and long cycle life over thousands of cycles. However, due to the large polyanionic groups, the polyanion materials often exhibit low electronic conductivity and moderate capacity lower than 130 mAh g−1. A lot of work about rational structure design, such as, carbon decoration [7, 89, 287], nanosizing [78, 101], should be done to obtain electrodes with high reversible capacity and high rate capability. The Prussian blue and analogues have shown low-cost characteristics and high capacity benefiting from their two-electron redox reactions. The synthesis routes should be paid special attention since the phase purity, crystallinity, defects, and crystal water have great effect on the electrochemical performance. And their poor thermal and electrochemical stability may also lead to some problems for wide application. In all, considering the cost of scaling up and intrinsic electrochemical performance of the electrodes, O3-Na0.9Cu0.22Fe0.30Mn0.48O2 and P2-Na2/3Fe1/2Mn1/2O2 are promising electrodes among the layered oxides. High-capacity Na3V2(PO4)3 and NaVOPO4 and low-cost Na4Fe3(PO4)2(P2O7) materials stand out among various polyanionic compounds. And about the hexacyanoferrates, NaxFeFe(CN)6 and NaxMnFe(CN)6 exhibit as the most promising candidates for commercial application.

For the anode materials, hard carbon can deliver a high capacity of ca. 350 mAh g−1, close to that of Li+ ions intercalation in graphite, making it the most promising anode material for SIBs. Heteroatom doping and structural optimizing could effectively enhance the sodium storage properties, but the doping type and amount should be considered. For large-scale application, the biomass-derived carbon materials should be preferred. Reducing the processing cost, improving the sodium storage capacity and increasing the initial Coulombic efficiency and rate capability are crucial issues needing further consideration. Through a deeper understanding of the mechanism about sodium insertion into the hard carbon, it will indicate a shortcut for the development of high-performance and practicable hard carbon anode. Conversion reaction materials show high reversible capacities, but the cycling stability and rate capability should be further improved. The huge volume change and relatively poor electronic conductivity suppress the achievement of satisfactory performance. Much effort needs to be made through nanostructuring and carbon decoration to overcome these issues. And the large potential hysteresis should be further understood and reduced. Alloying reaction materials hold great promise for high capacity anodes, especially the Sn, Sb and P. The dramatic volume change during sodium uptake leads to rapid capacity degradation. Through carbon coating, nanosizing and intermetallics, the cyclic stability of the materials can be improved. Exploring suitable nanostructure and intermetallic compound remains great challenges. Ti-based anodes have also attracted some attention, the moderate sodium storage performance should be further improved, and the catalytic reaction between electrode and electrolyte should be better understood and suppressed.

Though considerable achievements have been made with SIBs, many enormous challenges may still lie ahead. With the rapid advances in material innovations and nanotechnology developments, and the sustained interest from industrial and academic communities, we believe that sodium-ion batteries with low cost and long-life span will be commercialized for large-scale energy storage application in the future.

Notes

Acknowledgements

We thank financial support by the National Key Research Program of China (No. 2016YFB0901500), National Natural Science Foundation of China (Nos. 21673165 and 21373155) and the Fundamental Research Funds for the Central Universities (2042018kf0007).

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Copyright information

© Shanghai University and Periodicals Agency of Shanghai University 2018

Authors and Affiliations

  1. 1.College of Chemistry and Molecular Sciences, Hubei Key Laboratory of Electrochemical Power SourcesWuhan UniversityWuhanChina
  2. 2.State Key Laboratory of Advanced Technology for Materials Synthesis and ProcessingWuhan University of TechnologyWuhanChina
  3. 3.School of Power and Mechanical EngineeringWuhan UniversityWuhanChina

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