Study on sodium storage properties of manganese‐doped sodium vanadium phosphate cathode materials

Na+ superionic conductor (NASICON)‐structured Na4VMn(PO4)3 (NVMP) possesses stable cycling performance at 2.5–3.8 V by replacing V with lower cost Mn but suffers rapid capacity decay when further widening the voltage to 2.5–4.2 V, owing to a less stable V4+/V5+ redox couple. Herein, to stabilize the V4+/V5+ couple and improve the reversibility, a series of carbon‐coated NVMP (NVMP@C) with different V/Mn ratios are compared, among which, Na3.25V1.75Mn0.25(PO4)3@C delivers an additional reversible V4+/V5+ capacity of 7 mAh g−1 at the voltage of 3.9–4.2 V. Based on this, to further activate the capacity of V4+/V5+, Al‐doped Na3.25V1.75−yMn0.25Aly(PO4)3 structures are synthesized. When 0.5 Al is doped, 15 mAh g−1 capacity corresponding to V4+/V5+ can be released. In addition, it is found that the activation of V4+/V5+ is not conducive to electrochemical reversibility and C‐rate performance, owing to the shrinkage of the whole framework structure with the oxidization of V4+ to V5+.


| INTRODUCTION
Owing to the unsustainability and pollution of fossil fuels, the development and utilization of clean energy are becoming more and more critical for the green world. [1][2][3] But the most popular clean energies of solar and wind are both less stable and cannot be directly plugged into the power-supplying grid, which needs a large-scale energystorage system to uniform the current and voltage, providing stable, controllable, and continuous electric energy. [4][5][6] Among the current energy-storage devices, sodium-ion batteries (SIBs) have become a competitive candidate because of the abundant distribution of elemental Na in nature and lower cost, especially when considering the high-efficiency recovery of transition metals. 7,8 To facilitate the practical application of SIBs, the energy density needs to be further promoted, which requires the employment of cathode materials with high voltage and large capacity. [9][10][11][12][13][14][15][16] Compared with the layered transition-metal oxide cathode materials, the framework cathode structures generally own superior cycling stability, which is highly desired for the large-scale energy-storage system with a requirement of running over 20 years. [17][18][19] As the most typical framework structure, Na + superionic conductor (NASICON)-based cathodes not only own high stability but also show a fast Na + -intercalation/extraction capability. 14,20,21 The general NASICON-structured Na x M 2 (PO 4 ) 3 consists of a MO 6 octahedron and PO 4 tetrahedron, where M represents transition-metal ions that can be changed flexibly and x is theoretically in the range from 1 to 4. 22 Na 3 V 2 (PO 4 ) 3 is one of the most widely studied NASICON cathode materials, which possesses a discharge capacity of 117 mAh g −1 and an energy density of 370 Wh kg −1 , through the reversible change of the V 3+ /V 4+ redox couple at 3.4 V. 23,24 Despite the excellent cycling stability, the high cost of V and relatively low voltage limit its application in large-scale energy storage. To overcome these challenges, different transition-metal ions were doped into Na 3 V 2 (PO 4 ) 3 . [25][26][27][28][29][30][31][32][33][34][35] Among them, Na 4 MnV(PO 4 ) 3 (NVMP) not only significantly lowers the cost by the 1 mol eq. substitution of V with Mn but also achieves improved energy density through Mn 2+ /Mn 3+ redox reaction at a slightly higher voltage of 3.6 V. 36 Although at most three Na + are theoretically active in NVMP, it is still hard to reversibly extract/intercalate more than two Na + and release more capacity, might owing to the less structure stability arising from the V 4+ /V 5+ couple. 37 To get more capacity, it is necessary to explore the mechanism that how the V 4+ /V 5+ reaction impacts the cell performance and obtain an optimized V/Mn ratio to improve the reversibility of the V 4+ /V 5+ .
Herein, a series of carbon-coated Na 3+x V 2−x Mn x (PO 4 ) 3 (NVMP@C) with various V/Mn ratios were synthesized, among which, Na 3.25 V 1.75 Mn 0.25 (PO 4 ) 3 @C delivered the most reversible V 4+ /V 5+ reaction and best cycling stability in the voltage range of 2.7-4.2 V. Moreover, when the Mn content is maintained constant at 0.25 for the same Jahn-Teller effect of Mn 3+ , the substitution of V 3+ with inactive Al 3+ of 0.5 could activate more capacity from V 4+ /V 5+ up to 15 mAh g −1 . However, the oxidation of V 4+ to V 5+ is observed to deteriorate the electrochemical stability in long-term cycling, which can be attributed to the overlarge shrinkage and shape change of framework structures. In addition, it is not conducive to the diffusion of Na + after the oxidation reaction from V 4+ to V 5+ owing to the shrunk Na + -migration channel (Scheme 1).

| Material characterization
The powder X-ray diffraction (XRD) of the as-prepared samples was performed at room temperature and in the 2θ range of 10-120°, and the lattice parameters were refined by FullProf software. The schematic illustration of the structure was carried out using VESTA software. The ex-situ XRD data in different voltages were tested in a Kapton sealed device filled with argon to avoid air contact. During the X-ray photoelectron spectroscopy (XPS) test, all the samples were transferred into the XPS chamber through a sample holder sealed in a glovebox with argon to ensure the complete isolation of air. The specific proportion of carbon content of the samples was defined by thermogravimetric analysis (TGA) at a heating rate of 10°C min −1 and a temperature window of 40-800°C. The morphology, element distribution, and microstructure of the samples were characterized by scanning electron microscope (SEM), energy dispersion spectrum (EDS), and high-resolution transmission electron microscope (HRTEM), respectively.

| Electrochemical characterization
To assemble the half cells, the active cathode materials, carbon black, and PVDF binder were mixed in the mass ratio of 75:15:10, and then were cast on the aluminum foil uniformly and dried at 60°C under vacuum for 18 h. The mass loading of active materials was around 2.5 mg cm −2 on electrode films. In half-cells, sodium foil was used as the anode, glass fiber was employed as the separator, and 1.0 M NaClO 4 in propylene carbonate and ethylene carbonate (PC:EC = 1:1 vol%) without fluoroethylene carbonate was utilized as electrolyte. The galvanostatic charge/discharge test and galvanostatic intermittent titration technique (GITT) were carried out in the voltage ranges of 2.7-4.2 V. Electrochemical impedance spectra (EIS) test in the frequency range from 0.1 Hz to 1 MHz and cyclic voltammetry (CV) curves of various scan rates from 0.1 to 1 mV s −1 were performed on the electrochemical workstation (ENER-GYLAB XM). XPS analysis was measured with an ESCALAB 250Xi to study the valence state.
The crystal structures of Na 3+x V 2−x Mn x (PO 4 ) 3 @C (x = 0.125, 0.25, 0.375, 0.5, 0.75, 1) were first characterized by XRD and Rietveld refinement (Supporting Information: Figure S1), which show the typical diffraction pattern of the NASICON crystal phase, confirming the successful obtainment of the NASICON structure without the impurity phase. The lattice parameters and detailed information on atomic occupation are shown in Supporting Information: Figure S2 and Table S1-6, respectively. In Na 3+x V 2−x Mn x (PO 4 ) 3 @C, the increment of Mn content leads to a gradual promotion of both a and b lattice parameters, due to the introduction of more Na + and the larger radius of Mn 2+ than V 3+ (0.83 vs. 0.64 Å), which is accompanied by the decrease of the c lattice parameter, owing to the declined repulsion between adjacent VO 6 (MnO 6 ) octahedra. As shown in Supporting Information: Figure S3, the Na 3+x V 2−x Mn x (PO 4 ) 3 @C shows the morphology of irregular particles with a size <1 μm. All these particles are coated with a thin amorphous carbon layer, which is beneficial to electronic transfer across the interface since most phosphate-based materials suffer from poor electronic conductivity. 33,38,39 The electrochemical properties of Na 3+x V 2−x Mn x (PO 4 ) 3 @C were evaluated in Na-metal half cells. The charge/discharge profiles tested in the voltage window of 2.7-4.2 V are shown in Figure 1A-F. All these six samples deliver a similar discharge capacity of around 100 mAh g −1 in the first cycle, whereas their cycling stability varies. Na 3.25 V 1.75 Mn 0.25 (PO 4 ) 3 @C presents the best cycling stability with a capacity retention rate of 92.3% after 200 cycles ( Figure 1G,H). With the increase of Mn content from 0.25 to 1.0, the cycling stability gradually declines. When the Mn content reaches 1, Na 4 VMn(PO 4 ) 3 @C only shows capacity retention of 37.7% after 200 cycles in the voltage range of 2.7-4.2 V, which is dramatically inferior to that in 2.7-3.8 V, 36 indicating that the involvement of V 4+ /V 5+ heavily deteriorates the structure stability. It is worth noting that there are two voltage plateaus in Na 4 VMn(PO 4 ) 3 between 2.5 and 3.8 V, while only one when Mn content <1 because the voltage plateaus of V 3+ /V 4+ and Mn 2+ /Mn 3+ are very close to each other. When Mn content is lower, less Mn participates in the reaction, only showing one voltage plateau. When the Mn content rises to 1, the voltage plateau from Mn 2+ /Mn 3+ becomes clear since more Mn participate in the reaction, and the platform is split into two distinct steps. It is obvious in the dQ/dV curves that the peak width near 3.4 V is gradually widening with the increase of Mn content, indicating that two redox reactions of V 3+ /V 4+ and Mn 2+ /Mn 3+ are gradually separated with the increase of Mn content (Supporting Information: Figure S4). To further study the impact of the V 4+ /V 5+ reaction on the electrochemical performance, the charge/discharge capacity provided by the V 4+ /V 5+ redox couple during the first cycle is compared in Figure 1I. When the Mn content is 0, the voltage plateau related to V 4+ /V 5+ cannot be observed visibly. [40][41][42] With the doping of Mn, the capacity originating from the V 4+ /V 5+ reaction is activated. When the Mn content is lower than 0.5, all these structures have similar charge capacity (18-20 mAh g −1 ) and discharge capacity (~7 mAh g −1 ) contributed by V 4+ /V 5+ at 3.9-4.2 V. Nevertheless, when the Mn content exceeds 0.75, in spite of dramatically raised charge capacity, the discharge capacity from V 4+ /V 5+ reaction decreases to only~2.5 mAh g −1 , leading to extremely low Coulombic efficiency, suggesting that the over-much V 4+ /V 5+ reaction would lead to both poor reversibility and capacity decrease.
The impact of Mn content on the rate performance exhibits a similar trend to that on the cycling performance (Supporting Information: Figure S5). At the same rate, the capacity of Na 3.25 V 1.75 Mn 0.25 (PO 4 ) 3 @C is higher than that of Na 3.125 V 1.875 Mn 0.125 (PO 4 ) 3 @C, particularly at the high rate of 10 C (86.6 vs. 75.6 mAh g −1 ), and could recover to 99.3 mAh g −1 (higher than 97.9 mAh g −1 for Na 3.125 V 1.875 Mn 0.125 (PO 4 ) 3 @C) when the rate returns to 1 C. However, as the Mn content continuously rises, the rate performance of the composites severely decreases, and even the recovered capacity drops when the rate is back to 1 C; this implies that the excessive V 4+ /V 5+ reaction may be detrimental to structural reversibility. Overall, Na 3.25 V 1.75 Mn 0.25 (PO 4 ) 3 @C possesses the best cycling stability and C-rate performance among these composites, which hence is chosen for further modifications to explore the impacts of the V 4+ /V 5+ reaction.
To explore the relationship between structural changes and charge/discharge capacity of Na 3.25 V 1.75 Mn 0.25 (PO 4 ) 3 @C during V 3+ /V 4+ and V 4+ /V 5+ reactions, the exsitu XRD is tested in different voltage points in the range of 2.7-4.2 V (Figure 2). The weak peaks of Al are resulted from the current collector of the cathode. When the cell is charged to 3.9 V, the diffraction peaks (104) and (116) of Na 3.25 V 1.75 Mn 0.25 (PO 4 ) 3 exhibit a more obvious shift to a higher angle compared with that is charged to 3.4 V (Figure 2B), indicating a further volume shrinkage when V 4+ is oxidized to V 5+ . This is reasonable since V 5+ is only 0.54 Å, which is smaller than V 4+ (0.58 Å) and far smaller than V 3+ (0.64 Å). This heavy volume alternation during Na + -extraction should bring challenges to structural stability and cycling life. Figure 2C  octahedrons and PO 4 tetrahedrons are interconnected by corner-shared oxygen to establish the three-dimensional framework, allowing the transportation of Na + . There are two different coordination environments of Na, Na1 (6b) and Na2 (18e), in this NASICON structure, and the corresponding occupation ratios are obtained by Rietveld refinement, as listed in Supporting Information: Table S1-6. During the charging/discharging process, only Na2 can be reversibly extracted/intercalated in the Na-O polyhedron with zigzag diffusion routes Na2 → Na1 → Na2. 43 The area of bottleneck gradually shrinks with the release of Na + from 3.25 to 1 (Supporting Information: Table S7), indicating a severer volume shrinkage when V 4+ is further oxidized to V 5+ , making it more difficult for Na + to diffuse. When the Na content is 1.25, the bottleneck area is minimized due to the Jahn-Teller effect leading to abnormal lattice distortion.
To directly prove the existence of V 3+ /V 4+ and V 4+ / V 5+ reactions at different voltages, XPS is conducted to monitor the valence change of V at an open circuit voltage (OCV), 3.7 and 4.2 V. At OCV ( Figure 2D), two characteristic peaks at 524.0 and 517.0 eV are observed, which correspond to the V 2p 1/2 and V 2p 3/2 of V 3+ , respectively. When charged to 3.7 V, two peaks of V 3+ shift to 524.7 and 517.3 eV, matching well with the oxidation of V 3+ to V 4+ . 43 When further charged to 4.2 V, the above peaks further shift to 525.0 and 517.7 eV, matching well with the further oxidation of V 4+ to V 5+ .
3.2 | Na 3.25 V 1.75−y Mn 0.25 Al y (PO 4 ) 3 @C From the results above, it is obvious that the large volume shrinkage from V 4+ to V 5+ deteriorates the cycling stability, especially when considering there already are volumetric shrinkage of Mn 2+ → Mn 3+ and V 3+ → V 4+ . To stabilize the framework structure, it is necessary to introduce an electrochemically inactive cation without changing size and so help to mitigate the volumetric change. Based on this consideration, the electrochemically inactive Al 3+ was employed to partially replace V 3+ , and the Mn content was still kept at 0.25 to ensure the Jahn-Teller effect of Mn 3+ at a constant level, for separately analyzing the structural impacts on electrochemical performance.
The XRD Rietveld refinement of Na 3.25 V 1.75−y Mn 0.25 Al y (PO 4 ) 3 @C (y = 0, 0.25, 0.5) (Figure 3) confirms the successful preparation of Na 3.25 V 1.75−y Mn 0.25 Al y (PO 4 ) 3 @C composites with pure NASICON crystal structures. All the diffraction peaks are matched well with the typical NASICON structure with the R_3c space group, and the strong intensity of peaks indicates a high degree of crystallinity for all samples. No obvious diffraction signal of carbon is detected, indicating the amorphous state of coated carbon. The lattice parameters decrease with increasing Al content because the larger V 3+ (0.640 Å) is replaced by the slightly smaller Al 3+ (0.535 Å), as shown in Supporting Information: Figure S6 and Table S8-9, respectively.
As shown in Figure 4A,B, most Na 3.25 V 1.5 Mn 0.25 Al 0.25 (PO 4 ) 3 @C and Na 3.25 V 1.25 Mn 0.25 Al 0.5 (PO 4 ) 3 @C particles are less than 1 μm and aggregate heavily with each other, owing to the addition of a carbon precursor and its carbonization. The HRTEM image shows that the thickness of the carbon layer on the Na 3.25 V 1.25 Mn 0.25 Al 0.5 (PO 4 ) 3 surface is about 8 nm ( Figure 4C) and the weight content is around 6 wt% according to the TGA (Supporting Information: Figure S7A-C). Additionally, the lattice fringes with an interplanar distance of 0.38 nm can be well assigned to the (113) crystal plane of the NASICON crystalline structure. According to the energy-dispersive X-ray spectroscopy (EDS) mapping images shown in Figure 4D, Na, V, Mn, P, O, and Al are uniformly distributed on the particles, further verifying the successful introduction of Al. The electrochemical performance of Na 3.25 V 1.75−y Mn 0.25 Al y (PO 4 ) 3 @C is evaluated with the aim of investigating the influence of Al-doping on the V 4+ /V 5+ redox couple. Figure 5A-C shows the CV curves of Na 3.25 V 1.75−x Mn 0.25 Al x (PO 4 ) 3 @C in the initial three cycles at 0.2 mV s −1 . It can be observed that the intensity of the V 4+ /V 5+ redox current increases obviously in the first cycle with the increase of Al content from 0 to 0.5, but the current undergoes a gradual drop in the second and third cycles when Al content is 0.5 ( Figure 5C), demonstrating that the increase of the V 4+ /V 5+ reaction would, in turn, deteriorate its reversibility and stability. As shown in Figure 5D-F, the initial discharge capacities of Na 3.25 V 1.75 Mn 0.25 (PO 4 ) 3 @C, Na 3.25 V 1.5 Mn 0.25 Al 0.25 (PO 4 ) 3 @C, and Na 3.25 V 1.25 Mn 0.25 Al 0.5 (PO 4 ) 3 @C are 101.4, 97, and 98.9 mAh g −1 , respectively. The introduction of inactive Al 3+ is helpful in stabilizing the framework structure and benefits the initiation of the V 4+ /V 5+ reaction; the discharge capacity increases to 15 mAh g −1 when the Al content is 0.5. 36 However, the discharge capacity undergoes faster decay when more V 4+ /V 5+ reactions are involved, owing to the failure of reversible Na + reinsertion, resulting in a significantly declined capacity retention. As shown in Supporting Information: Figure S8, the capacity of Na 3.25 V 1.75 Mn 0.25 (PO 4 ) 3 @C in the high-voltage region (>3.6 V) showed a better capacity retention ratio than that of Na 3.25 V 1.25 Mn 0.25 Al 0.5 (PO 4 ) 3 @C in long-term cycling. Although their stabilized capacities over 3.8 V are almost the same, the overall capacity will be hurted by the reduction of high-voltage capacity ( Figure 5). These phenomena indicate that the excessive activation of the V 4+ /V 5+ redox couple leads to poor structural stability, which further leads to poor cycling performance and rate capability. At the same time, Cr 3+ doping was carried out to obtain Na 3.25 V 1.5 Mn 0.25 Cr 0.25 (PO 4 ) 3 @C with the NASICON structure (Supporting Information: Figure S9A). Electrochemical tests show that Cr 3+ doping can also activate more V 4+ /V 5+ reactions but is accompanied by the degradation of cycling performance and rate capability compared with Na 3.25 V 1.75 Mn 0.25 (PO 4 ) 3 @C, further confirming that the excessive activation of V 4+ /V 5+ reactions is not conducive to the cycling stability (Supporting Information: Figure S9B-E). Therefore, it is necessary to optimize the degree of V 4+ /V 5+ involved in the Na + -storage to balance the capacity, voltage, and cycling stability.
The long-term cycling performance of Na 3.25 V 1.75−y Mn 0.25 Al y (PO 4 ) 3 @C is tested at 1 C ( Figure 6A), giving the capacity retentions of 82.54%, 77.01%, and 75.73% after 500 cycles at the Al contents of 0, 0.25, and 0.5, respectively. It is observed that the more discharge capacity provided by the V 4+ /V 5+ reaction, the worse cycling stability is. The rate performance of the three composites was also evaluated under different current densities from 0.1 to 10 C ( Figure 6B), and the results display a worse C-rate performance when more V 4+ /V 5+ redox couples are involved. Among these structures, Na 3.25 V 1.75 Mn 0.25 (PO 4 ) 3 @C owns the optimal degree of V 4+ /V 5+ reaction and delivers the highest discharge capacities of 107.0, 105.1, 101.6, 98.5, 95.3, 90.8, and 86.4 mAh g −1 at 0.1, 0.2, 0.5, 1, 2, 5, and 10 C, respectively.
As shown in Figures 5 and Supporting Information: S10, it can be seen that the capacity provided by the V 4+ /V 5+ redox couple increases with the increment of Al content in the charge/discharge processes. To study the underlying mechanism of the results above, the reaction kinetic properties were further investigated. The GITT test was employed to measure the Na + diffusion kinetics (D Na + ) of Na 3.25 V 1.75−y Mn 0.25 Al y (PO 4 ) 3 @C composites, as shown in Supporting Information: Figure S10D-F (the detailed calculation process and the corresponding formula are summarized in Supporting Information: Figure S11). In the voltage window from 3.9 to 4.2 V, D Na + of all samples decreases remarkably, demonstrating the harder Na + diffusion after the oxidation of V 4+ to V 5+ . The resistance at different charge/discharge states of Na 3.25 V 1.75 Mn 0.25 (PO 4 ) 3 @C and Na 3.25 V 1.25 Mn 0.25 Al 0.5 (PO 4 ) 3 @C in the first cycle was monitored by electrochemical impedance spectroscopy (EIS), as shown in Figure 7A-C and Supporting Information: Figures S12 and S13. The charge-transfer resistance values of Na 3.25 V 1.25 Mn 0.25 Al 0.5 (PO 4 ) 3 @C are significantly higher than Na 3.25 V 1.75 Mn 0.25 (PO 4 ) 3 @C during the oxidizing process from V 4+ to V 5+ and then decreased quickly with the reduction of V 5+ to V 4+ , demonstrating that a larger charge-transfer resistance needs to be overcome when more V 4+ /V 5+ is utilized in the charge/ discharge process, which reasonably explains why more F I G U R E 6 (A) Long-term cycling performance and (B) rate performance of Na 3.25 V 1.75−y Mn 0.25 Al y (PO 4 ) 3 @C (1 C represents 0.17 mA cm −2 ). capacity from the V 4+ /V 5+ redox couple is not conducive to the fast charge/discharge capability. The ex-situ XRD ( Figure 7E) shows that the peaks of (104) and (116) shift to the higher angles when charged to 4.0 and 4.2 V, indicating a gradually heavier volume shrinkage induced by the V 4+ /V 5+ reaction, further verifying the that larger charge-transfer resistance is originated from the structural shrinkage, inconsistent with the calculated results in Supporting Information: Table S7.

| CONCLUSION
The Na 3+x V 2−x Mn x (PO 4 ) 3 @C composites with various V/Mn ratios were synthesized to explore their impact on the cathode electrochemical performance and corresponding mechanism. When the Mn content varies within 0.25, the V 4+ /V 5+ reaction at 4.0 V with a capacity of 7 mAh g −1 is beneficial to increasing the voltage and energy density of Na 3+x V 2−x Mn x (PO 4 ) 3 @C. Based on the Mn content of 0.25, with the further introduction of Al from 0 to 0.5, increased V 4+ /V 5+ redox couples up to 15 mAh g −1 would be initiated. However, with the involvement of the V 4+ /V 5+ redox-couple, the Na + diffusion rate decreases and the charge-transfer impedance increases, which are caused by the shrinkage of the framework due to the smaller ionic radius of V 5+ . Among these prepared composites, Na 3.25 V 1.75 Mn 0.25 (PO 4 ) 3 @C, with an optimized Mn content of 0.25, is demonstrated to have the best ability to balance the capacity and cycling stability, which exhibits a capacity retention rate of 82.54% after 500 cycles at 1C and a discharge capacity of 86.4 mAh g −1 at 10 C.