NASICON-Structured NaTi2(PO4)3 for Sustainable Energy Storage
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For the first time, we fully presented the recent progress of the application of NaTi2(PO4)3 on sodium-ion batteries including non-aqueous batteries, aqueous batteries, aqueous batteries with desalination, and sodium-ion hybrid capacitors.
The unique NASICON structure of NaTi2(PO4)3 and the various strategies on improving the performance of NaTi2(PO4)3 electrode have been presented and summarized in detail.
KeywordsNaTi2(PO4)3 Sodium superionic conductor Anode Batteries Hybrid capacitors
In recent years, with the increasing consumption of fossil fuels, numerous studies have investigated the development of various types of renewable and clean energy devices [1, 2, 3, 4, 5, 6, 7, 8, 9, 10]. Among current technologies, lithium-ion batteries (LIBs) have been considerably developed and widely used in portable electronic devices and large-scale grid storage applications because of their high energy density and long lifespan [11, 12]. However, the limited lithium resources and the rising cost of LIBs have stimulated research on the similar sodium-ion batteries (SIBs) on account of sodium’s abundance in nature and environmental benignity [13, 14, 15, 16, 17, 18, 19, 20]. At the same time, some other sodium-based energy devices such as aqueous batteries with desalination have also been developed due to their wide range of applications [21, 22].
Generally, both LIBs and SIBs rely on the reversible intercalation and deintercalation process of lithium or sodium ions between the positive and negative electrodes via the electrolyte during the charging and discharging process to complete energy conversion . In particular, active electrode materials, especially anode materials, play an important role in the performance of batteries. In that regard, the development of suitable anode materials with high capacity, long cycle life, and excellent rate performance is of significant importance [15, 18, 23, 24, 25, 26, 27, 28].
At present, although research on LIBs and SIBs has made considerable progress, and the most advanced batteries usually have high energy density, the slow kinetics of ion intercalation and deintercalation limits the achievement of higher power density and better rate performance [14, 25, 29].
Supercapacitors (SCs), characterized by electric double-layer capacitors (EDLCs), are a new type of energy storage device that stores energy by physical adsorption/desorption of an electric charge to form an electric double layer on the electrode-electrolyte interface. The excellent electrode dynamics of physical absorption and desorption enable features such as high power capability, excellent cycling performance, and long lifespan [30, 31]. However, EDLCs are inferior to batteries in terms of energy density [32, 33]. Therefore, merging the merits of the high-energy-density battery with the Faradaic electrode and high-power-density supercapacitor with the non-Faradaic electrode to develop a hybrid capacitor is a promising strategy to increase the energy density without sacrificing the high power density and long lifetime [30, 34, 35, 36, 37].
A systematic overview of the emerging critical progress is an urgent necessity. In this review, we will cover the recent progress in NTP-based electrodes for both NIBs and NHCs. The underlying synthesis methods, materials modification strategies, and electrochemical properties will be summarized in detail. Further, the major challenges and perspectives regarding the prospects for the use of NTP-based electrodes in energy storage systems will be summarized.
2 Sodium-Ion Batteries
2.1 Non-aqueous Batteries
Delmas et al. first reported the reversible sodiation of NaTi2(PO4)3 in an organic electrolyte and revealed that two Na+ ions could be reversibly intercalated to form Na3Ti2(PO4)3 via a two-phase mechanism  showing a pair of typical well-defined redox peaks at 2.2/2.0 V within the potential window of 1.6–2.6 V (vs. Na/Na+) , and a number of research groups conducted a detailed structural elucidation of the electrochemical transition and structural control related to this compound [51, 52, 53, 54, 55, 56]. To enhance the inherent low electronic conductivity of the phosphate framework especially for high-power SIBs application, numerous efforts have been made to improve its electrochemical performance by nanoarchitecturing the NTP particles and incorporating conductive carbon coating/networks (e.g., amorphous carbon, CNTs, or graphene) [45, 48, 57, 58, 59, 60, 61, 62, 63].
2.1.1 Nanoarchitectures of NTP
Crystalline order or the degree of crystallinity in NTP also plays an important role in electrochemical properties such as capacity and electrode kinetics. Ko et al. studied the correlation of electrochemical performance with crystalline order. Starting with an amorphous NTP powder prepared by the Pechini method, varied NTP nanoparticulates of different degrees of crystallinity were derived via calcination. It was observed that poorly crystalline NTP samples (derived at 500–600 °C) exhibited low specific capacities and broad voltammetric features for Na+ insertion, characteristic of surface-limited processes; and highly crystalline NTP samples (derived at 700–800 °C) with the well-formed NASICON structure exhibited sharp voltammetric peaks and diffusion-limited kinetics in both organic (i.e., non-aqueous) and aqueous electrolytes. Further integration of nanocrystalline NTP with conductive networks can enhance the local electronic conductivity to a theoretical specific capacity in a non-aqueous electrolyte and an adequate capacity in a mildly aqueous electrolyte with significantly improved long-term stability .
2.1.2 Nanolayer-Coated NTP
Methods for the incorporation of carbon with NaTi2(PO4)3 vary from carbon layer coating [60, 66, 67, 68, 69, 70], electrospinning [71, 72, 73, 74, 75, 76], solvothermal synthesis [49, 62, 77], pyro-synthesis [64, 78], spray-drying , and assembly  to ball milling  for different morphologies or structures, e.g., porous plates , 1D nanofibers [71, 72], nanocubes [49, 73], mesoporous materials [67, 73], and hierarchical nanocomposites [60, 79]. Some related works about carbon-coated architectures have reported excellent or enhanced performance; e.g., the nanosized porous carbon-coated NTP particles prepared by He et al. through a hydrothermal process combined with various carbon coating steps showed superior rate (capacities of 106 mAh g−1 at 10 C over 1000 cycles, 111 mAh g−1 at 30 C) and low-temperature properties (98 mAh g−1 at 10 C and 61 mAh g−1 at 20 C even at − 20 °C). This indicated that the addition of a small amount of Na3Ti2(PO4)3 (NVP) intermediate powder accounts for the in situ catalytic formation of more sp2-type carbon coating, i.e., highly graphitic carbon (graphene-like layers) coating, for excellent electrochemical performance of high-power SIBs .
Zhang et al. synthesized an open holey-structured framework for an NTP/C nanocomposite with open channels in nanocube morphologies for faster Na-ion transport by using a solvothermal reaction followed by pyrolysis. It demonstrated fast Na-ion transport and preferable battery performance, with a very small capacity decrease from 124 to 120 mAh g−1 in the wide range 0.5–50 C. An excellent discharge capacity of 103 mAh g−1 (88.3% retention of the first cycle) was delivered after an ultralong lifespan of 10,000 cycles at a super-high rate of 50 C . Pang et al. synthesized a mesoporous NaTi2(PO4)3/CMK-3 (NTP/C) nanohybrid with high-crystallinity NTP nanoparticles (size of ~ 5 nm) homogeneously embedded in the highly conductive mesoporous CMK-3 matrix via a solvothermal route followed by calcination. The CMK-3 not only served as a rigid, interconnected conductive support but also suppressed agglomeration or overgrowth of NTP nanoparticles. Even over 1000 cycles, the nanohybrid showed an integral structure with a well-crystallized rhombohedral NaTi2(PO4)3 phase (Fig. 2d). Further, the nanohybrid anode showed some typical characteristics including a pair of well-defined sharp and stable redox peaks (located at 2.09 and 2.16 vs. Na+/Na corresponding to the redox reaction of Ti4+/Ti3+ during the reversible insertion–extraction reaction of Na+ in the NTP lattice), high initial charge–discharge capacities (corresponding to a 75% utilization of its theoretical capacity with a high coulombic efficiency (CE) of 98% in the potential voltage window 1–3 V), and enhanced rate capabilities (with distinct charge–discharge voltage plateaus) compared to pure NTP at the same rate (Fig. 2e–g), which can be attributed to the fast Na+ insertion–extraction kinetics and good electrical conductivity of the mesoporous hybrid architecture .
Embedding the NTP nanoparticles in the nanocarbon networks will considerably enhance the Na-ion/electron transfer for highly reversible and ultrafast sodium storage [79, 85, 86, 87, 88]. For example, by using a simple soft-template method, Yu and coworkers designed an NTP/C composite with nanosized NTP particles coated by a thinner carbon shell and interconnected by a carbon network. With the synergistic effects of a lower charge transfer resistance and a larger surface area for the electrolyte to soak in and sufficient void to buffer the volume variation during the repeated Na+ insertion/extraction, the anode materials demonstrated outstanding rate performance (108 mAh g−1 at 100 C; i.e., a discharge/charge time of 36 s) and long cycle life (83 mAh g−1 at 50 C over 6000 cycles), as well as a lower polarization and higher initial CE (ICE; e.g., ~ 98% at 1 C) . Similar to amorphous carbon networks, carbon nanotubes are also a superior framework for enhancing Na+/e− conductivity. Xu et al. fabricated a hierarchical porous nanocomposite architecture consisting of MWCNT-threaded mesoporous NTP nanocrystals for high-performance sodium electrodes. With a high ICE of 99%, high rate capability of 74 mAh g−1 at 50 C, and long-term cycling stability (74 mAh g−1 after 2000 cycles at 10 C) superior to that of the physically mixed reference composite, it provides a general hetero-assembly approach to various types of nanocomposites for high-performance SIBs . Wang et al.  designed a carbon-nanotube-decorated NTP/C nanocomposite with high rate performance; especially impressive is that the composite could exhibit low-temperature (− 20 °C) performance with a capacity of 65 mAh g−1 at 10 C. Wei et al. fabricated porous NaTi2(PO4)3/C hierarchical nanofibers (Fig. 5d, e) via an electrospinning method followed by annealing. The NTP/CNFs as an anode for SIBs exhibited a high reversible capacity of 120 mAh g−1 at 0.2 C, high rate capability (71 mAh g−1 at 20 C), and long cycling stability similar to those of sodium-ion full cells and hybrid sodium-ion capacitors. When assembled using nickel hexacyanoferrate (NiHCF, Na4Fe(CN)6) as cathode material, it showed a high ICE of ~ 90% (corresponding to initial charge and discharge capacities of approximately 122 and 110 mAh g−1) and a capacity retention of ~ 90% after 500 cycles at 150 mA g−1 with a CE that approached 100% as well as excellent rate capabilities when operated between 0.5 and 2.5 V (Fig. 5f, g) . Yu et al. prepared a similar structure of ultrafine nanoparticles encapsulated in 1D N-doped carbon nanofibers and extended the voltage window to 0–3.0 V. The poor electrical conductivity of NTP was significantly improved, and the composite demonstrated stable and ultrafast Na storage capability with a specific capacity of 121 mAh g−1 at 10 C after 2000 cycles and 105 mAh g−1 after 20,000 cycles, as well as superior rate performance from 0.2 to 20 C with a recovery efficiency of 99.4% .
Full cell batteries with high energy and long life cycle remain a significant challenge, and the formation of an SEI on both cathodes and anodes (especially for the hard-carbon-incorporated composites) has revealed a potential way to realize long-term stability of SIB full cells. A pre-cycling of cathodes and anodes leads to pre-formation of an SEI, which mitigates the additional consumption of Na+ ions in full cells for higher ICE as well [89, 90, 91, 92, 93, 94]. In addition, the adoption of Al (compared to the more common choice, Cu) as the anode current collector in NASICON NTP//NVP full cells will enhance the specific energy, although some improvements are needed to achieve better power capability and energy efficiency . The simultaneous optimization of the structural stability of cathode materials will further enhance the cycle performance of the Na-ion full cells .
2.1.3 2D NTP Composites
Fang et al. designed 3D-graphene-decorated NaTi2(PO4)3 microspheres (NTP@rGO) (Fig. 6d) via a hierarchical graphene-embedded process using a facile spray-drying method with post-calcination for superior high rate and ultracycle-stable sodium storage performance as a promising SIB anode . The as-obtained NTP@rGO composite demonstrated a high reversible capacity of 130 mAh g−1 (close to 96% theoretical capacity) at 0.1 C (with almost equal values from 0.1 to 2 C; 1 C = 133 mA g−1), long stable cyclability (77% capacity retention over 1000 cycles at 20 C), and ultrahigh rate capability (38 mAh g−1 at 200 C) (Fig. 6e, f). When paired with Na3V2(PO4)3 as a cathode, the NTP@rGO//NVP/C full cell can deliver a high discharge capacity of 128 mAh g−1 at 0.1 C based on the anode mass, and an outstanding long-life cycling performance with 80% capacity retention over 1000 cycles and a CE of above 99.5%, as well as a high rate performance of 88 mAh g−1 at 50 C. The SIB full cell exhibited excellent specific energy and power densities that are superior to those of hybrid batteries and supercapacitors [102, 103], showing an energy density of 73 Wh kg−1 at a power density of 7.6 W kg−1 (0.1 C) and even maintaining 38.6 Wh kg−1 at a power density up to 3167 W kg−1 (50 C). The excellent properties can be attributed to the combined advantages of the graphene-coated nanosized NTP particles and the presence of the highly conductive 3D graphene network, which remarkably enhanced the ionic/electronic transport and buffered the volume variation during sodiation and desodiation. The novel method for 3D hierarchical spherical structures shows a promising alternative route for realizing superior SIBs.
2.1.4 Flexible or Binder-Free Electrodes
2.1.5 High-Safety SIBs
Development of high-safety and long-lifespan SIBs is urgently needed for large-scale energy storage applications. All-solid-state SIBs have attracted considerable attention for their safety and long-term durability [120, 121, 122, 123, 124, 125]. Solid-state rechargeable SIBs based on ceramic (e.g., Na-βʺ-Al2O3) electrolyte with high sodium ion conductivity can demonstrate an extremely stable voltage plateau of ~ 2.1 V in the half-cell and an initial discharge capacity of 133 mAh g−1, although the cycling and rate performances may be improved via modification of interfacial incompatibility (or cell resistance and intrinsic polarization) compared to that of typical non-aqueous SIBs . To further suppress the formation of Na dendrite, Goodenough and coworkers designed a NASICON ceramic electrolyte assisted by an in situ-formed thin interfacial interlayer or by the introduction of a dry polymer layer for high-temperature performance and safety advantages. The as-prepared all-state-batteries with NTP as the anode showed high cycling stability and CE (99.8 ± 0.02%) at 65 °C due to the enhanced wetting of sodium on the interfacial interlayer that suppresses dendrite formation and growth.
Currently, most batteries for electrical energy storage (EES) use highly flammable and volatile organic carbonate esters as electrolytes, which may cause severe safety problems, and an intrinsic system is in high demand, especially for large-scale EES applications . Apart from aqueous electrolytes  and solid-state electrolytes, etc., [121, 126, 129], proton-type organic phosphonates have demonstrated promise for safer SIBs with a wide electrochemical window and high ionic conductivity [127, 130]. Cao and coworkers designed and constructed an interesting all-phosphate-based battery by using a NTP anode, Na3V2(PO4)3 cathode, and trimethyl phosphate (TMP) electrolyte for zero-strain SIBs with intrinsic safety, high rate performance, and long cycle life . The full cells demonstrated good cycling performance (73.7% capacity retention over 1000 cycles) and promising designing flexibility for practical application. Soon after that, a similar self-standing all-phosphate SIB with high mass loading for fast cycling was realized .
For large-scale and low-cost energy storage, non-aqueous semi-solid flow batteries (SSFBs)—a special class of redox flow batteries (RFBs)—based on the rich chemistry of Na-ion intercalating compounds, e.g., the NTP anode and P2-type cathodes, may serve as an inspiration .
2.1.6 Trace-Element-Doped NTP Materials
As an effective method of improving the electrochemical performance of electrode materials, lattice doping has been proposed and widely used in LIBs and SIBs . As for research on NTP-based materials, it was first demonstrated by Mouahid and coworkers  that the doping of Al is beneficial for improving the ionic conductivity of NTP and improves electrochemical performance. Tirado’s group proposed that the low content of iron doping did not change the lattice structure but could enhance the capacity values and improve capacity retention [135, 136, 137]. Goodenough’s group utilized the NASICON-structured Na3MnTi(PO4)3 as both the anode and cathode to construct an aqueous symmetric SIB with an operating voltage of 1.4 V, stable cycle performance, and excellent rate capability . Dai’s group demonstrated that Sn doping on the Ti site shows no obvious effect on the lattice structure and morphology of NaTi2(PO4)3/C but is very beneficial for improving the electrochemical properties of the NaTi2(PO4)3/C anode for aqueous LIBs . Zhang and coworkers reported the synthesis of porous Na3MgTi(PO4)3 aggregates with a sol–gel method. The good rechargeable capacity of 54 mAh g−1 and better capacity retention performance (94.2% after 100 cycles) of Na3MgTi(PO4)3 compared to those of NTP demonstrate that the incorporation of electrochemically inert Mg2+ ions could improve the structural stability of Na storage materials and enhance cycling performance .
2.2 Aqueous Batteries
Aqueous rechargeable alkali-ion (e.g., Li, Na, Mg-ion) batteries that do not employ costly, highly toxic, and flammable organic solvents were first reported by Dahn’s group in 1994 using VO2/LiMn2O4 electrodes and LiNO3 aqueous solution as the electrolyte. Because of the much higher ionic conductivity of the aqueous electrolyte compared to that of organic electrolytes, aqueous batteries always feature high round-trip efficiency and energy density and have attracted interest as possible substitutes for conventional non-aqueous rechargeable systems. However, the narrower stable voltage window and the poor cycling life of aqueous electrolytes compared with those of organic electrolytes prompted researchers to further develop high-performance aqueous rechargeable alkali-ion batteries [43, 141, 142]. Of equal or greater interest, due to the high natural abundance of sodium, low lost, and safety advantages for large-scale or stationary energy storage, are aqueous sodium-ion batteries (ASIBs) or aqueous rechargeable sodium batteries (ARSB) [19, 43, 128, 143, 144, 145]. Okada and coworkers first presented a working demonstration of the aqueous Na-ion full cell using an NTP anode [43, 47] and revealed that an aqueous electrolyte showed a higher conductivity and lower viscosity compared with those of non-aqueous (i.e., organic) electrolytes, as well as much smaller interfacial activation energy or higher kinetics of Na-ion transfer than those of Li-ion secondary batteries, which are advantages for high rate capabilities [47, 57, 146, 147].
Generally, the incorporation of 3D-carbon-based porous frameworks with high conductivity, high surface area, high structural stability, and good electrolyte penetration has attracted particular attention and exhibited high-efficiency electro/ion transport and better performance, and is thus considered an effective strategy for fabricating high-performance ASIBs . Graphene, as a new and ideal 2D carbon material, has attracted intensive attention for application in energy conversion and storage due to its superior electrical conductivity, high surface area, structural flexibility, etc. The incorporation of graphene to form NTP composites or hybrids as a highly electronically conductive network improved cycling stability and rate performance, as was recently reported [59, 97]. For example, Zhang and coworkers prepared a 2D hybrid nanoarchitecture of NTP/graphene with highly crystalline NTP nanoparticles homogeneously anchored on the surface of conducting graphene nanosheets via a solvothermal method followed by calcination. The nanocomposite (with only 3.4 wt% of graphene) used as anode materials for ASIBs exhibited excellent electrochemical performance with high rate capabilities (110, 85, 65, and 40 mAh g−1 at 2, 5, 10, and 20 C, respectively) and a good cycling stability with 90% retention of the initial capacity over 100 cycles at 2 C. The remarkable improvement in specific capacity, rate performance, and cycling stability can be ascribed to the unique structures and the merits of both ingredients .
Apart from carbon components, a TiN layer was also applied on the surface of NTP to enhance the electronic conductivity in an aqueous electrolyte system. Zhang and coworkers synthesized a TiN-coated NaTi2(PO4)3 as an anode material for aqueous SIBs via a solvothermal routine and a subsequent nitriding process (i.e., calcination in ammonia gas). The optimized TiN-tailored NTP particles showed an improved rate capability and cycling performance with an initial capacity of 132 mAh g−1 and maintained 92 mAh g−1 after 100 cycles at 2 C, a large improvement over the pristine phase .
2.2.1 Full Cells
When combining with the cathodes, e.g., layered NaMnO2 , Na-birnessite with crystal water , alkali-cation-incorporated δ-MnO2 [161, 162, 163], Prussian-blue-type Na2CuFe(CN)6 , Na2NiFe(CN)6 , tunnel-structured Na0.44MnO2 (Na4Mn9O18) [43, 80, 152, 165, 166], NASICON-structured Na3V2(PO4)3 , and Na2VTi(PO4)3 , the aqueous SIBs exhibited high energy density and good cycle stability, which are particularly attractive for stationary energy storage applications. The use of Na-deficient NTP as an anode and Na-rich cathodes in an aqueous electrolyte system may be visually depicted as a “rocking-chair-type” SIBs . In the full cells of an NTP system, it is generally agreed that the low electronic conductivity of the NTP anode was rate limiting, and by eliminating this limitation via NTP/C composite optimization, for example, ultrafast rate capability and superior high rate cycling stability can be obtained [80, 157, 165]. Chiang and coworkers demonstrated an ultrafast rate (> 100 C) and superior high rate cycling (> 1500 cycles) for aqueous NaTi2(PO4)3/Na0.44MnO2 (NTP/NMO) cells with a specific volumetric energy density of up to 127 Wh L−1 from the materials-only level, and a cell level density of ~ 65 Wh L−1 may be expected, which exceeds the energy density of more fully developed active carbon (AC)/NMO systems. The NTP–C nanocomposite synthesized by ball milling a 2–3% pyrolytic carbon from the glucose precursor accounts for the superior performance . Zhang and coworkers further studied an NTP/MWCNTs–Na0.44MnO2 system to improve the electronic conductivity . Thus, aqueous NTP/NMO may become a candidate for safe, low-cost, and high-power storage systems. However, possible causes for low-rate capacity fade may recur due to some complicated side reactions, e.g., partial diffusion of electrode materials, oxidation of the anode in its sodium-inserted state by dissolved oxygen or oxygen generated via water hydrolysis, or oxidation of the aqueous electrolyte by the charged cathode [43, 47]. For flexible aqueous SIBs, Guo et al. designed and fabricated a family of safe flexible SIBs as potential wearable or even implantable electronic devices based on a nanosized NaTi2(PO4)3@C anode, a Na0.44MnO2 cathode, and various Na+-containing aqueous electrolytes (including Na2SO4 solution, normal saline, or cell-culture medium), compared to that of either toxic flammable organic solutions or strong acid/base as electrolytes. The as-prepared belt- and fiber-shaped ASIBs exhibited excellent electrochemical performance (with high volumetric energy and power density, and long life) as well as high flexibility. This fiber-shaped electrode system also exhibited electrochemical deoxygenation and pH-changing features, which might be further applied in biological and medical fields .
Cathodes with different potential plateaus often influence the output voltage of the ASIB system . For example, the Prussian-blue-type Na2NiFe(CN)6 combined with an NTP anode can deliver an average output voltage of 1.27 V, as well as an energy density of 42.5 Wh kg−1 and a capacity retention of 88% over 250 cycles at a 5 C rate . Furthermore, adjustment of the transition metal cations at the M site in the Prussian blue compounds NaxMyFe(CN)6 enabled the Na2CuFe(CN)6 cathode in the same ASIB system to exhibit a higher output voltage of 1.4 V (with a well-defined discharge plateau and a slight decrease from 1.4 to 1.1 V) as well as an enhanced energy density (48 Wh kg−1), rate, and cycling performance . The NASICON-type Na3V2(PO4)3 assembled aqueous full cell with NTP as the anode demonstrated a flat discharge plateau at 1.2 V and could maintain a high rate performance (58 mAh g−1 at 10 A g−1) well, showing a high energy density of 29 Wh kg−1 at a power density of 5145 W kg−1 . Layered NaMnO2 and its full cells with an NTP anode delivered an inclined curve of the voltage profile (e.g., 1.8–0.5 V) without plateaus, as well as an energy density of 30 Wh kg−1 at a power density of 50 W kg−1 and could retain 75% of the initial capacity over 500 cycles at 5 C . The tunnel-structured Na0.44MnO2 full cell coupled with NTP showed a distinct flat plateau and an average operation voltage of 1.13 V, slightly lower than that of a typical commercial battery such as the Ni–Cd battery (1.2 V), but an initial reversible specific capacity of 85 mAh g−1  or a higher capacity of 114 mAh g−1  could be obtained. Through further modification, the Ti-substituted Na0.44MnO2 full cell coupled with an NTP anode could exhibit an average operating voltage of 1.2 V, higher rate capabilities (54 mAh g−1 at 10 C), and much more stable cycling performance (76 mAh g−1 at 2 C with a very small capacity decay up to 300 cycles) for practical applications .
2.3 Electrolyte Dependence of Performance
Various electrolytes have currently been used and developed for non-aqueous SIBs based on the rationale for specific choices regarding cell setup and usage conditions . In regard to aqueous electrolytes, some studies reveal that the full cell with Na2SO4 as electrolyte may show poor cyclability and NaCH3COONa (NaAc) electrolyte exhibits improved performance, for the NaMnO2 cathode suffered from dissolving excessively in the Na2SO4 electrolyte followed by rapid capacity loss of the full cell . Although NaClO4 electrolyte showed even a little better performance than that of the Na2SO4 electrode, its explosiveness and oxidizing abilities may be concerns in the market . In aqueous NaNO3 electrolyte, the NTP anode exhibits higher intercalation/deintercalation kinetics and reactions that are approximately twice as fast as in LiNO3 solution ; further, the NTP anode in Li+ ion aqueous electrolyte (1 M Li2SO4 solution) will suffer from a higher potential plateau together with a suppressed rate performance due to the thermodynamic limitations of the lithium insertion into the Na-containing structure and/or Na+–Li+ repulsive interactions . Another study showed that the full cell (Na2FeP2O7//NTP) with a higher NaNO3 concentration exhibited a large irreversible capacity due to H2 gas evolution and corrosive side reactions . NaOH aqueous electrolyte, however, showed poor performance in cycling stability, which may be due to the decreased stability of the electrode material at higher pH [47, 97, 141].
In aqueous electrolyte SIBs, the salt concentration of the electrolyte affects the ionic conductivity and the rate performance of batteries as well as the diffusion of the reactive species, which can cause self-discharge, particularly in high-mass-loading electrodes. Studies showed that with higher-molarity solutions of typical electrolytes, rate capability and electrode utilization increased significantly (the redox peaks are sharper and closer, which indicates faster kinetics, consistent with the ionic conductivity difference); e.g., by increasing the salt (NaClO4) concentration from 1 to 5 M, the capacity at 1.5 C increased by 38%, and the oxygen-related self-discharge phenomenon diminished, although measurable irreversible capacity loss still occurred with the lowest oxygen content, suggesting that self-discharge and capacity loss are not necessarily causally related . Furthermore, the increase in electrolyte concentration extended the electrochemical window of ASIBs up to 2.8 V (with concentrated 17 mol kg−1 NaClO4 aqueous electrolyte compared to the value of only 1.9 V with diluted 1 mol kg−1, which widened the theoretical voltage restriction of 1.23 V due to practical overvoltage) and could produce a higher discharge plateau of 1.8 V in a full cell with the Prussian-blue-type cathode. Higher concentrations of electrolyte under a higher rate condition benefits the more stable performance in ASIB systems due to the reduced water content of the NTP anode and elution of the cathode by alkalization of the aqueous electrolyte . For an extremely high concentration, superconcentrated “water-in-salt” electrolytes (WiSEs) with the decreased activity of water resulting from its coordination with concentrated salt ions (or much more intense cation–anion interaction and pronounced ion aggregation in Na-ion electrolytes revealed by Raman spectra together with molecular-scale simulations) can even noticeably suppress the electrochemical decomposition of aqueous electrolytes (yet a dense, stable, and repairable SEI simultaneously formed) and significantly enhance the long-term cycling stability (e.g., > 1200 cycles at 1 C with negligible capacity losses—0.006% per cycle; and showing an extraordinarily high CE > 99.2% at 0.2 C over 350 cycles) at both low and high rates [172, 173]. However, it needs to be mentioned that highly concentrated electrolyte can potentially raise challenging issues such as corrosion, especially at extreme electrochemical potentials [147, 170], and therefore there should be a balance between the electrolyte concentration and high performance. In addition, aqueous electrolytes with more extreme pH (i.e., pH > 13) or those exposed to higher temperatures (e.g., 70 °C) will induce significant structural degradation and precipitation of a secondary phase (i.e., via loss of phosphate to layered sodium titanate) . An aqueous/non-aqueous hybrid electrolyte with an expanded electrochemical window of up to 2.8 V and high conductivity was also explored, which inherited the safety feature of aqueous electrolytes and the electrochemical stability of non-aqueous systems .
All-solid-state SIBs have attracted considerable attention for their safety and long-term durability [122, 123, 124, 125]. Apart from the typical SIBs based on liquid electrolytes, solid-state rechargeable SIBs based on ceramic (e.g., Na-βʺ-Al2O3) electrolyte with high sodium-ion conductivity can demonstrate an extremely stable voltage plateau of ~ 2.1 V in the half-cell and an initial discharge capacity of 133 mAh g−1, although the cycling and rate performances may be improved via modification of interfacial incompatibility (or cell resistance and intrinsic polarization) compared to that of typical non-aqueous SIBs .
2.4 Aqueous Batteries with Desalination
Further study showed that the ion transport from the bulk electrolyte to the electrode surface limited the rate of ion removal and the round-trip CE, and the presence of non-inserting ions in water would reduce the ionic flux and ion removal capacity due to the containment of interfacial transport following the accumulation effect . Generally, optimizing the operating current density and cutoff voltage window (to reduce the parasitic water splitting reaction) as well as advective ion transport to the electrode surface will improve the ion removal performance in dilute aqueous systems . The excellent performance of the EDI system based on NTP composites has made it a promising desalination technology and provides significant potential for direct seawater desalination in the future. By combining commercial photovoltaics, the goal of “renewables to usable electric energy and desalted water” can be achieved .
3 Hybrid Capacitors
In the past few years, many articles on NHCs or hybrid sodium-ion capacitors (NICs, or SICs) have been published [37, 212, 213, 214, 215]. Apart from the high-electrical-conductivity carbon-based materials and high-power-performance metal oxides such as TiO2, Nb2O5 and so on, NaTi2(PO4)3 with a NASICON structure features high ionic conductivity and structural stability with the excellent kinetics of sodium, and therefore will be a suitable material for sodium-ion hybrid capacitor applications [32, 212, 216, 217]. However, it has poor electron conductivity, and therefore many methods such as reducing the particle size, coating conductive materials, adopting suitable counter electrode materials, etc., have been used to rationally design NTP materials to solve these problems [218, 219].
Zhang and his coworkers first reported that a NaTi2(PO4)3/C composite synthesized by the ball milling method used as the anode of an aqueous sodium-ion hybrid supercapacitor with AC as the cathode could deliver a high energy density of 31.6 Wh kg−1. When tested at a current density of 200 mA g−1, the cell delivered an excellent cycle performance of less than 11.7% capacitance loss after 2000 cycles. This electrochemical performance is derived from the unique structure of the as-synthesized NaTi2(PO4)3/C composite. First, the NASICON structure of NaTi2(PO4)3 could promote rapid and easy migration of Na+ ions in the 3D open framework structure. On the other hand, the intimate mixing of acetylene black and sucrose with the precursor material through ball milling results in the formation of a uniform amorphous carbon layer of approximately 7 nm on the surface of the NTP particles so that the electron conductivity of the NaTi2(PO4)3/C composite could be significantly improved . However, the as-synthesized NaTi2(PO4)3/C composite is in the range of 0.5–2 mm, which might be detrimental to the diffusion of ions and electrons in the electrode material. Roh et al. reported a NaTi2(PO4)3/rGO microsphere composite synthesized by a facile spray-drying method used as a high-rate insertion anode for sodium-ion capacitors. The spray-drying method produced a structure of NTP nanoparticles with sizes < 80 nm, which considerably reduced the diffusion length of the Na+ ion inside the material. Moreover, during the synthesis process, components of titanium were ionic species, which caused the chemical bonding of high-conductivity reduced graphene oxide (rGO) with NTP and finally significantly improved the electrical conductivity of the composite. When fabricated with an AC counter electrode to construct an NHC, a maximum energy density of 53 Wh kg−1 at a power density of 334 W kg−1 with good cycling stability was obtained .
Similar work has also been reported by Lee and his coworkers. They synthesized NTP nanoparticles grown on graphene nanosheets as an anode with the graphene nanosheets as a cathode in an organic electrolyte NHC. This new system features a high specific surface area and a high-conductivity nanosheet-like graphene cathode. Unlike the activated carbon electrode, which is porous with pores that are not conducive to electron transport, the surface of the 2D nanosheet can be easily contacted by ions in the electrolyte, thereby reducing the ion transport distance. This new system delivers a high energy density of ≈ 80 Wh kg−1 and a high specific power of 8 kW kg−1. An ultralow performance fading of ≈ 0.13% per 1000 cycle (90%–75,000 cycles) outperforms previously reported sodium-ion capacitors .
Electrospinning is a classic method of preparing a 3D network carbon nanofiber with uniform morphology and good electrical conductivity. Recently, Wei et al. reported that porous NTP/C nanofibers (NTP/CNFs) obtained via an electrospinning method’s anode of NHCs could deliver a maximum specific energy density of 56 Wh kg−1 at a specific power density of 174 W kg−1. At a current density of 1 A g−1, the specific capacitance remained at 91.4% after 500 cycles with nearly 100% CE. The electrospinning method uniformly dispersed the NTP nanoparticles with an average crystal size of ~ 15 nm in the carbon matrix. Characterizations suggested that NTP/CNFs have a typical porous structure with a high specific surface area that will facilitate electrolyte infiltration and finally produce high electrochemical performance .
4 Conclusion and Perspectives
Despite the fact that NTP has considerable potential for the development of high-performance SIBs, due to its relatively high voltage plateau as anodes, when used for a full cell, it will require cathodes with a high discharge potential to match and realize high power output. Thus, a higher technological requirement for the cathode materials should be proposed [45, 85, 107, 222]. From the viewpoint of practical battery applications, a high tap density is desirable for higher energy density in addition to high-power performance. In addition, the self-discharge rate has not been systematically evaluated, compared to the typical commercial LIBs with a value of 2% per month; the NTP-based SIBs and NHCs should be further investigated for practical application.
Although the ASIBs are more cost-effective and safer for large-scale energy storage, they often have a lower capacity and cycling life compared to that of organic SIBs. Because capacity fade in aqueous electrolytes remains poorly understood, further studies are needed, including a possible multi-step mechanism followed by a local pH change and alkaline oxidation of the carbon conductive additives .
The decay mechanism and stability of electrode materials in an aqueous electrolyte should be further studied and improved, although some efforts have shown promise by tailoring the electrolytes (including adjusting the pH values, locally generated destructive OH− ions), nanocoating the electrode materials, or eliminating oxygen in the electrolytes to suppress capacity fading upon cycling. An appropriate potential window (cutoff voltage) and corresponding anode and cathode materials (with adjusted mass ratio) should be selected to avoid or suppress the highly irreversible capacity loss due to H2 and/or O2 evolution in the aqueous electrolytes.
For alternative electrolytes, the intrinsically safe organic phosphates for all-phosphate SIBs efficiently avoid firing as usually encountered in the carbonate electrolytes and severe side reactions such as hydrogen and oxygen evolution in aqueous electrolytes. The all-solid-state SIBs with safety, long-term operation capacity, and high-temperature performance advantages show considerable commercial potential. However, more related research on the interface contact is necessary, and the cycling performance of these batteries needs to be further improved for wide practical application.
The realization of full cell SIBs or hybrid Na-ion capacitors with high energy and long cycle life remains challenging. The controlled formation of an SEI on both anodes (especially for the hard-carbon-incorporated composites) and cathodes will be an effective way to achieve long-term stability for full cells. Pre-cycling (or pre-sodiation) of anodes and cathodes will lead to pre-formation of SEI, and hence mitigate the additional consumption of Na ions in full cells for higher ICE as well. With the improvement in aqueous electrolytes, including highly concentrated and even superconcentrated WiSEs or hybrid aqueous/non-aqueous electrolytes, the prospects are promising for large-scale and high-energy-density electrochemical energy storage with the advantages of low cost, eco-friendliness, and long lifespan.
Compared to conventional SIBs relayed on Cu/Al current collectors to support active materials and to serve as conductive pathways, free-standing or flexible electrodes (including graphene papers, graphene foams, and electrospun CNFs) without these metallic current collectors significantly reduce the weight and cost of batteries and have been an emerging demand for today’s battery development. However, more efforts are needed to develop better-performing free-standing electrode materials with a facile preparation route, low cost, and robust mechanical advantages for next-generation batteries.
Compared with traditional static capacitive deionization (CDI) using carbon electrodes, Faradaic capacitive (intercalation) electrodes including NTP can remove ion species with high efficiency (typically, an ultrahigh salt removal capacity of more than 100 mg g−1). These electrodes are promising for not only being a commercially viable alternative for treating water but also for saving energy. However, the stability of these electrodes (particularly associated with electrochemical leakage of metal ions) may be currently a major concern. Furthermore, it is necessary to develop robust and cost-effective intercalation cathodes and anodes to meet the critical requirement of capturing multiple cations and anions from real saline water or seawater.
This work was supported by the National Natural Science Foundation of China (No. 51302079), the Natural Science Foundation of Hunan Province (No. 2017JJ1008), and theKey Research and Development Program of Hunan Province of China under Grant 2018GK2031.
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