1 Introduction

Sodium-ion batteries (SIBs) have garnered significant attention in the field of energy storage in recent years and are making rapid strides in large-scale energy storage battery applications due to their similar operating principle to lithium-ion batteries, abundant sodium resources, and low cost [1,2,3,4]. Cathode material substantially determines the energy density of batteries. Among many candidate cathode materials, layered transition metal oxide materials (NaxMO2, M = Fe, Mn, Ni, Co, Cr, Ti, V, and their combinations) have been extensively studied owing to their compact crystal structure, high specific capacity, simple synthesis process, and easy to be mass produced [5]. However, the NaxMO2 cathode material of SIBs still poses challenges, such as irreversible phase changes during cycling, poor air stability and electrode-electrolyte interface instability [6].

Several methods have been employed to improve the irreversible phase change and air stability of NaxMO2, such as elemental doping [7,8,9], surface coating [10, 11], etc. Besides, improving the compatibility of electrodes with electrolytes to stabilize their interface is also a key to optimizing battery performance. In SIBs, the ionic conductivities of single solvent electrolytes are generally lower than those of binary solvent electrolyte systems, especially for cyclic solvent molecules, e.g., ethylene carbonate (EC). On the other hand, various properties of electrolytes, such as Na+ solvation structure, viscosity and electrochemical stability, and electrochemical performance of batteries are heavily influenced by solvents [12]. Notably, the compatibility of the electrolyte with the cathodes at high potential has a decisive influence on interface stability within the batteries and even their safety. Therefore, comprehensively understanding the electrochemistry at cathode/electrolyte interface and its critical influence on battery performance is of paramount importance in further enhancing life and stability of batteries.

Herein, a micron-sized spherical cathode material, namely, NaNi1/3Fe1/3Mn1/3O2 (NFM) with a dominant (003) crystal plane was synthesized using a solid-phase method. The sodium storage performance of the NFM cathode and interface chemistry were investigated in a widely used NaPF6 salt and dual-solvent electrolyte system via electrochemical analysis, differential electrochemical mass spectrometry (DEMS), transmission electron microscopy (TEM), and density functional theory (DFT) calculations. Related results show that the EC/dimethyl carbonate (DMC) electrolyte exhibits a strong Na+ solvation structure on the strongly nucleophilic cathode surface due to lower steric hindrance and high oxidation stability, resulting in an even and robust cathode electrolyte interphase (CEI) (Scheme 1). It effectively stabilizes the electrode-electrolyte interface and ensures the long-term cycling stability of the cells.

Scheme 1
scheme 1

Na+ solvation structure behaviors of different electrolytes and interface chemistry on strongly nucleophilic (003) plane of oxide cathode

Full size image

2 Results and discussion

Fig. 1
figure 1

Characterizations of as-prepared NFM materials: (a) Rietveld refinement XRD profiles of NFM; (b) Schematic crystal structure and (003) atomic arrangement; (c) SEM images; (d) TEM images, inset: low-resolution TEM image; (e) Elemental mapping; (f) EXAFS patterns

Full size image

2.1 Material structure and morphology

NFM is synthesized by a facile solid-state reaction (as described in the experiment section in supporting information). X-ray diffraction (XRD) pattern of as-synthesized NFM material and its Rietveld refinement are shown in Fig. 1a. All diffraction peaks are matched well with the NFM, showing high purity and typical O3 layered structure that belongs to the R-3 m group space. Two thetas located at 16.5 o, 33.4 o, 35.3 o, 36.6 o, 41.7 o, 53.4 o, 58.2 o, and 62.5 o correspond to (003), (006), (101), (012), (104), (107), (018), and (110) crystalline planes, respectively. Among them, the (003) and (104) crystal planes exhibit strong diffraction intensity, indicating the dominant growth plane for NFM crystal. Its lattice parameters are determined to be a = b = 2.965 Å and c = 16.04 Å. More detailed refinement parameters are shown in Table S1. Figure 1b shows the NFM crystal structure with its (003) atomic arrangement on crystal plane, where Na ions occupy the octahedral position of the NaO2 layer, while Fe, Ni and Mn ions are located in the octahedral position of MO2.

In order to further investigate the microstructure of as-prepared NFM materials, scanning electron microscopy (SEM) images are conducted. The prepared NFM materials are highly regular spherical particles with a particle size of 4–5 μm (Fig. 1c). The material consists of numerous irregular nanoblocks stacked into large spherical particles with well-defined angles. This regular spherical morphology is mainly derived from the used precursors (Fig. S1), whose spherical structure is well maintained during the high-energy mixing process. Compared with flake, granular and irregular shaped powders, electrode material powders composed of spherical particles have a higher packing density and have excellent fluidity, dispersion and process properties, which are favorable for making cathode material slurry and coating [13].

The crystal structure of the material is further investigated by TEM images. The inset of Fig. 1d shows a distinct spherical shape and there are some discrete nanoblocks near their edges, which may be unstable nanoblocks of particles shed from the surface during sonication process. The lattice striations of the sample are calibrated by high magnification transmission electron microscopy (HRTEM), and the lattice spacing is determined to be 5.32 Å, which corresponds to the (003) crystallographic plane, which supports the dominant (003) crystallographic exposure in NFM crystals. Elemental mapping also demonstrates an even distribution of Na, Ni, Fe and Mn elements study the valence state of ions within the crystals, which is critical for the reaction mechanism and interface properties of cathode materials (Fig. 1e). Transition metal elements show various valance states in NFM material, including + 2 for Ni, + 3 for Fe and + 4 for Mn (Fig. 1f), which are consistent with the charge of typical oxides [14]. Fourier transform (FT) of extended X-ray absorption fine structure (EXAFS) indicates two dominant peaks related to metal-metal bonds and Fe-O/Mn-O/Ni-O bonds, respectively. The former corresponds to adjacent metal atoms in crystals, and the latter originates from rich O atoms within NFM crystals. Thus, an oxygen-rich surface within oxides is certain.

Fig. 2
figure 2

Sodium storage performance of NFM cathodes in four electrolytes: (a) LSV curves; (b) CV curves; (c) Initial charge/discharge curves; (d) Cycling performance at 150 mA g− 1, inset: photos of disassembled battery; (e) Na+ diffusion coefficient; (f) EIS plots after initial cycle with fitted equivalent circuit model; (g) Obtained Rct from fitted models; (h) Obtained RCEI from fitted models

Full size image

2.2 Electrochemical performance analysis

Galvanostatic charge-discharge tests were performed on NFM cathodes in four electrolytes to study their electrochemical behaviors. To meet the requirements of practice applications, electrolyte systems consisting of NaPF6 salt and combined solvent (EC/DMC, EC/EMC, EC/DEC, EC/PC) were chosen. To provide good antioxidant stability, 5 wt% fluoroethylene carbonate (FEC) was added to all electrolytes [15]. Figure 2a shows the linear scan voltammetry (LSV) plots of the four electrolytes, with their oxidation voltages reaching 5.02 V (EC/EMC), 4.86 V (EC/PC), 4.90 V (EC/DMC) and 4.94 V (EC/DEC), respectively. It is observed that on NFM cathodes, the electrolytes cannot function stably within 2.0–4.0 V (Fig. S2), possibly due to the high catalysis activity of oxides, which induces severe oxidation decomposition of electrolytes. Therefore, a narrower electrochemical window of 2.0-3.9 V is chosen for subsequent electrochemical characterizations of the NFM cathode.

Cyclic voltammetry (CV) curves of NFM cathodes upon initial cycle are shown in Fig. 2b, in which the oxidation peak of four electrolytes at ~ 3.64 V during charged process corresponds to the phase conversion of NFM crystals from O3 phase to P3 phase, and the reduction peaks of the four electrolytes at ~ 3.54 V/2.45 V and the three electrolytes except EC/PC at ~ 2.78 V during discharged process correspond to the reversible phase transition [16]. The oxidation peak at 3.6 V in EC/DMC electrolyte is slightly higher the than other three electrolytes, indicating a lower electrochemical polarization of battery in EC/DMC electrolyte. The charge/discharge curves of NFM cathodes in four electrolytes are shown in Fig. 2c, whose similarity indicates the same desodiation/sodiation process of NFM cathodes in these electrolytes with a solid-solution reaction [9]. The cathodes have the highest initial discharged capacity of 105.0 mAh g− 1 in EC/DEC electrolyte, followed by 104.6 mAh g− 1 and 101.7 mAh g− 1 in EC/DMC and EC/PC electrolytes, respectively, and the lowest capacity occurs in EC/EMC electrolyte with 98.4 mAh g− 1. After 300 cycles at 150 mA g− 1, the reversible capacities of NFM cathodes are maintained to be 72.5 mAh g− 1 (EC/DMC), 55.8 mAh g− 1 (EC/EMC), 70.1 mAh g− 1 (EC/DEC) and 54.4 mAh g− 1 (EC/PC), corresponding to capacity retention rates of 83.3%, 67.6%, 80.9%, and 65.6%, respectively (Fig. 2d). Furthermore, the DC impedance of all batteries increase as the cycle process, whose growth rates are 87.1% (EC/DMC), 115.3% (EC/EMC), 95.2% (EC/DEC) and 138.3% (EC/PC), respectively (Fig. S3a). There is a good inverse correspondence between increased DC impedance and decreased capacities of NFM cathodes, implying the increased electrochemical polarization mainly contributes to the capacity fading of batteries. In EC/DMC electrolyte, a lower DC impedance growth may be attributed to a robust CEI on NFM cathode, which effectively isolates the electrode from electrolytes with continuous passivation, thus stabilizing the cycling of batteries. Furthermore, the NFM cathode with high loading (3.92 mg cm− 2) can achieve stable cycling in EC/DMC electrolyte with an average Coulomb efficiency above 95% during 200 cycles, supporting better compatibility between NFM cathode and the EC/DMC electrolyte (Fig. S3b). Galvanostatic intermittent titration technique (GITT) further determines the similar Na+ diffusion coefficients (DNa +) of NFM cathodes in the four electrolytes, indicating the ion diffusion inside bulk should be not the main cause for the difference in electrochemical durability (Fig. 2e) [17]. Thus, it can be inferred that the electrochemistry difference between four electrolytes mainly results from their interface.

Electrochemical impedance spectroscopy (EIS) plots are performed on NFM electrodes before and after the initial cycle for the cells, respectively (Fig. S4 and 2f). The Nyquist curves consist of a segment of arc in the mid-high frequency region and a sloping line in the low frequency region. In this case, the first segment of arc in the medium-high-frequency region (consisting of two overlapping semicircles) represents CEI resistance (RCEI) and charge transfer resistance (Rct) (related to the electrochemical bilayer impedance and surface resistance), respectively [18, 19]. Combined equivalent circuit models, the fitted impedance values before and after the initial cycle are shown in Table S2. After initial cycle, the Rct of all four electrolytes decrease after the initial cycle, indicating that the insulating oxide surface is covered by CEI with better ionic conductivity (Fig. 2g). Among them, the CEI formed on EC/DMC and EC/EMC electrolytes show low RCEI value, implying better ion transfer across the interface (Fig. 2h).

Fig. 3
figure 3

Gas evolution NFM||Na cells in the four electrolytes upon initial cycle: (a) EC/DMC, (b) EC/EMC, (c) EC/DEC, (d) EC/PC electrolytes, (e) reaction mechanism during oxidation process

Full size image

To reveal the electrochemistry reaction at cathode/electrolyte interface, the gas evolution of NFM||Na cells with four electrolytes upon initial cycle is firstly carried out by DEMS [20]. Considering the possible reaction products, various gas including H2, O2 and CO2, are chosen as the study gases and a broad voltage window (2.0-4.2 V) is set. The different colored background areas in Fig. 3a-d represent the time period from the start of gas production generation to the peak intensity of gas production during the charging process. It should be noted that the jittery charge/discharge curve in Fig. 3c is strongly related to the cell configuration of the DEMS (Fig. S5) and the preparation of the electrodes. In the four electrolytes, CO2 gas appears around 3.3–3.4 V and becomes rich significantly around 3.6–3.8 V. When the voltage increases to 3.9 V, the CO2 signals reach the strongest intensity, presumably due to increased electrochemical activity of desodiated oxide surface. It promotes the decomposition of the electrolytes, whose derivative quickly covers the cathode surface at this voltage. As shown in Fig. 3e, the oxidative decomposition reaction of FEC, EC or other solvents on the cathode surface contains the generation of CO2 gas. Moreover, the reaction of residual Na2CO3 on cathode surface or in CEI might to further decompose and generate CO2 at high voltage, as shown in Eq. (5) [21,22,23,24,25]. Linear carbonate molecules may also decompose on the cathode surface to produce CO2, although the exact reaction process remains to be investigated [26,27,28]. According to the gas production intensities, it is deduced that the EC/DMC electrolyte suffers from a more serious oxidation decomposition during charged process within 2.0-3.9 V, accompanying with the formation of CO2 gas and CEI. Among them, the EC/DMC and EC/PC electrolyte show higher CO2 intensity, suggesting more severe decomposition reaction of electrolyte and the formation of thick CEI. The evolution of H2 gas is roughly the same in the four electrolytes, with roughly stable level during charged process and sharp increase during discharged process [29]. Therefore, it can be reasonably deduced that H2 gas is not a direct product of electrolytes’ oxidation instead of side reaction that occurs at anode/electrolyte interface during discharged process. It might be due to the diffusion of protic electrolyte oxidation species (R-H+) from the cathode to the anode and their subsequent reduction on Na anode during discharged process [30]. In addition, the O2 intensity does not change significantly in the four electrolytes within the set voltage window, indicating stable crystal structures of NFM cathodes without the visible release of lattice oxygen [31].

Fig. 4
figure 4

Interface evolution on NFM cathodes in four electrolytes upon initial cycle. TEM images (a) at charged to 3.5 V; (b) at charged to 3.9 V; (c) after initial cycle; (d-g) High-resolution C 1s, O 1s, F 1s and P 2p spectra of NFM electrodes after initial cycle

Full size image

CEI as the passivation layer on cathode surface, greatly determines the ionic transfer and compatibility of cathode/electrolyte interface [32]. The CEI morphology at different voltage states upon initial cycle is characterized by TEM images. Upon charging to 3.5 V, the CEI begins to form on cathode surface in all four electrolytes (Fig. 4a). This is consistent with the decreased oxidation stability on NFM cathode compared to that on inert steel sheet electrode in LSV curves, indicating the induction of early decomposition of electrolyte by oxide surface. In EC/DMC and EC/PC electrolytes, the CEI has covered the cathode surfaces integrally. While in EC/EMC and EC/DEC electrolytes, the CEI does not show an integral coverage. We further supplemented the TEM images at other regions of electrode samples for better credibility of the date and discussion in Fig. S6. As the charge proceed to 3.9 V, the thickness of CEI formed in all electrolytes increases, along with the increased CEI coverage in EC/EMC and EC/DEC electrolyte (Fig. 4b). At this state, all CEI show good integrity, and their thicknesses decrease in various electrolytes with an order of EC/PC, EC/DMC, EC/EMC and EC/DEC electrolytes. Therefore, it can be reasonably deduced that the formation of CEI on the positive electrode surface is the combined result of the continuous reaction at the electrode/electrolyte interface [33]. During the battery charging process, the cathode surface exhibits a persistently high electric potential. When the Fermi energy of the cathode is lower than the highest occupied molecular orbital (HOMO) of the electrolyte, making the electrolyte lose electrons and be oxidized. The associated decomposition products are deposited as insoluble matter on the electrode surface and constitute passivated CEI. When discharged to 2.0 V, all CEI show a decrease in thickness, which might be due to the reduction of semi-oxides within CEI during discharged process (Fig. 4c). Although the coverage integrity was well maintained, there were large differences in the uniformity and thickness. The CEI in EC/DMC and EC/PC electrolytes have an even coverage but larger CEI thickness, while the CEI in EC/EMC and EC/DEC electrolytes are thinner but have uneven coverage. A more intuitive evolution of CEI’s thickness and evenness in four electrolytes are shown in Fig. S7, demonstrating a thickness order of EC/DMC, EC/EMC, EC/DEC and EC/PC. Above results suggest that an even and moderate thickness CEI is formed in EC/DMC electrolyte after the initial cycle.

The compositions and electrochemical properties of CEI on NFM cathodes in the four electrolytes after initial cycle are further investigated using X-ray photoelectron spectroscopy (XPS) spectra (Fig. 4d-g). In the C 1s spectrum from EC/DMC electrolyte, the C-C/C = C (284.5 eV), C = O (287.4 eV), O = C-O (289.7 eV) and C-F (291.0 eV) peaks were attributed to common SEI compositions like sodium carbonate, alkyl carbonate and polyester [34]. While the O 1s spectrum showed low C = O/CO3 2− (532.3 eV) and C-O (533.4 eV) peaks, implying that the formed CEI has less polyester content. And the Na auger was usually considered to originate from NFM and sodium salts in the electrolyte. For the F 1s spectrum, the C-F and NaF peaks come from the decomposition of FEC and PF6 − anions. Combined with the content ratio of peaks in four electrolytes (Table S2), the CEI in EC/DMC electrolyte contains more fluorinated organic components. Due to its good electrical insulation and flexibility, the CEI can effectively passivate the surface of highly oxidative oxides, and prevent the continuous electrolytes’ decomposition, thus significantly stabilizing the cycling of batteries. Meanwhile, enough inorganics components like NaF and Na2CO3 ensure good ionic conductivity of the CEI with high mechanical properties and robustness [35,36,37]. In other three electrolytes, higher peaks including C-O, C = O, and C = O/CO3 2− were visible, which were associated with large unevenness of their CEI.

Fig. 5
figure 5

Interface chemistry on NFM cathode. (a) Na+ solvation behavior in four electrolytes on (003) planes of NFM materials; (b) The average distance between the Na+ and solvents on the (003) plane and bulk electrolyte; (c) Electron density distributions of (003) plane of NFM crystal; (d) Scheme of electrochemical behavior in different electrolytes

Full size image

2.3 Theoretical calculations

To reveal the differences in the electrochemical behavior of NFM electrodes in the four electrolytes, the Na+ solvation structures of four electrolytes on NFM surface were further calculated [38, 39]. Based on the results of XRD diffraction spectra and TEM images, the dominant (003) crystal plane of NFM material was selected as the base plane in models. Figure 5a shows the four calculated models containing four electrolytes on (003) plane, and as a contrast, bulk electrolyte models of four electrolytes are studied (Fig. S8). The average distance between four solvents and Na+ in these models are presented in Fig. 5b. The addition of the (003) crystal plane led to a significant decrease of the average distance between Na+ and solvents in all four electrolyte solvation structures, indicating enhanced electrolyte solvation at cathode/electrolyte interface. Further electron density diffraction spectra of (003) crystal plane showed multiple electron-rich oxygen atoms, giving it a high electron density and strong nucleophilicity (Fig. 5c) [40, 41]. Given that electrolytes preferentially undergo nucleophilic reactions during oxidative decomposition, it is reasonable to infer that this electron-rich electrode surface provides a strong electron supply capacity, which would promote further agglomeration of Na+-solvents coordination structures in electrolytes on cathode surface, exhibiting stronger Na+ solvation structures and interfacial reactivity. This might be an important reason for the reduced oxidative stability on oxide cathodes, which do not cycle stably within the voltage window obtained from Na/steel sheet cells in LSV curves. Among the three linear carbonate DMC, EMC and DEC molecules on (003) plane, the Na+ solvation structure gradually becomes weaker with the increase of linear molecular chains, which may be due to the increase of spatial resistance. For EC/DMC electrolytes, owing to the shortest linear molecular chain of DMC, the smallest average distance between Na+ and solvents (EC and DMC) and the strongest Na+ solvation structure occur, leading to sufficient electrolyte’s decomposition, as well as robust CEI formation (Fig. 5d). The FEC in EC/DMC is closer to the (003) crystal surface and is subjected to the strong positive potential on the electrode surface during charging, and the electrolyte molecules near the electrode surface are more likely to obtain electrons for oxidative decomposition, indicating that the FEC is fully involved in the formation of CEI on the electrode surface. For electrolytes containing EMC and DEC molecules, the Na+ solvation structures suffer from greater spatial site resistance due to their larger chain-like molecular structures, resulting in a sequential decrease in the compactness of solvation structures. As a result, the CEI formed in these electrolytes are thinner but incomplete and uneven, failing to effectively passivate the cathode surface and ensure reversible ion transport across the interface. For the electrolyte containing PC molecules, which also have a weaker solvation structure, the poor oxidative stability of the PC molecules themselves (high HOMO energy) also promotes a strong decomposition of the electrolyte at the interface [42], resulting in the thickest CEI formation and high interfacial impedance, which greatly hinders the rapid transport of Na+ across the interface. Therefore, it is necessary to consider the influence of cathode material surface properties when studying the reaction and evolution of the electrode/electrolyte interface. Simply generalizing the interface rules on one electrode material to another may not be applicable (Fig. S9). For cathode materials with special surface properties, it is necessary to develop suitable electrolytes.

3 Conclusion

In this work, a ternary layered oxide NaNi1/3Fe1/3Mn1/3O2 material with a micron-scale solid spherical structure was prepared using a simple one-step solid-phase method as a cathode for SIBs, which features an advantageous (003) plane and high crystal crystallinity. When matched with widely used ester-based electrolytes (EC/DMC, EC/EMC, EC/DEC, EC/PC), this cathode exhibits significant differences in electrochemical stability. Specifically, the EC/DMC electrolyte system provides the highest cycling stability with 83.3% capacity retention after 300 cycles, which surpasses those observed in the other three electrolytes (80.9%, 67.6%, and 65.6%). TEM images and XPS spectra demonstrate a robust and even CEI formed in EC/DMC electrolyte, ensuring good interfacial stability and reversible Na+ transfer. DFT calculation reveals that the oxide cathode surface has high electron density and strong nucleophilicity, which greatly strengthens the Na+ solvation structures at cathode/electrolyte interface and decreases the oxidation stability. Specifically, the Na+ solvation structures weaken with increasing linear molecular chain length (DMC, EMC, DEC) due to spatial resistances, restricting the formation of effective CEI in weakly solvated electrolytes (EC/EMC, EC/DEC). The electrolyte containing PC exhibits severe decomposition and thick CEI’s formation accompanied by an overlarge interfacial impedance due to the intrinsically poor oxidation stability of PC. This work provides new insights into the surface characteristics of the electrode in interface chemistry, which provides strong guidance for designing advanced electrode materials and electrolytes.