High-Entropy-Stabilized Polyanionic Cathodes with Multiple Redox Reactions for Sodium-Ion Battery Applications

High-entropy (HE) materials containing multiple elements have created a growing interest in exploring the property limits of electrodes in energy storage and understanding the underlying chemical/physical mechanisms. Here, we show a substantial improvement in performance of HE-based cathodes in sodium-ion batteries (SIBs). Polyanionic structure has a large compositional exibility and can incorporate many active transition-metal (TM) species, which is an ideal platform to design HE cathode materials. As a proof of concept, we show that HE sodium superionic conductor (HE-NASICON) materials can be synthesized via a facile sol-gel method. By comparing a group of HE-NASICON cathodes containing different contents of TM species, we demonstrate that the multi-Na-ions intercalation/deintercalation process is highly reversible, whereas capacity and cycling stability are improved. The HE-NASICON cathode with equal molarity of ve TM species achieves a high capacity of 161 mA h g −1 and capacity retention of 85% when cycling at a high rate of 5 C over 1000 cycles. In-situ XRD and spherical-aberration-corrected transmission electron microscope (ACTEM) also demonstrate a robust trigonal phase with a volume change of merely 4.07% during the multi-Na-ions storage. These results reveal the effectiveness of HE concept in expediting high-performance polyanionic cathodes for real SIBs applications.


Introduction
The quick-emerging paradigm of renewable energy development and use of rechargeable Li-ion batteries on a large scale are being challenged by the scarcity of lithium sources and uneven geographical distribution. 1,2,3,4 In this context, sodium-ion batteries (SIBs) technology is an attractive option due to low cost, abundant and environmentally friendly sodium resources. Most importantly, it can be easily and rapidly replicated from Li-ion battery technologies regarding industrial and commercial processes. 5,6,7,8 Major obstacles to using SIBs technology for real applications include the low capacity and unsatisfactory electrochemical stability, which are directly determined by the properties of cathodes in SIBs. Through comparative studies, sodium superionic conductor (NASICON) materials have received considerable interests due to the 3D open framework, structural diversity, and exceptional Na-ion mobilities. 9,10 Recently, researchers incorporate high-valent redox centers into NASICON materials to ensure multiple electrons transfer and high capacities of cathodes, for example Na 3 VCr(PO 4 ) 3 , 11 Na 4 VMn 0.5 Fe 0.5 (PO 4 ) 3 , 12 and Na 4 MnV(PO 4 ) 3 13, 14 etc. Nevertheless, multi-Na-ions intercalation/deintercalation process generally causes a series of issues, including large volume change (ca. 9.89%), high-capacity irreversibility, and irreversible crystal phase evolution (monoclinic → rhombohedral) under high voltage operation (>3.8 V vs. Na/Na + ), which are plaguing the implementation of NASICON cathodes in practical cell. 15,16 Although all these undesirable performance degradations are named with different terminologies, the general strategy is to improve intrinsic structure stability and manipulate multiple redox reactions to enable highly reversible NASICON cathodes.
resistance, and creep resistance. 17,18,19,20 The evolved HE oxides, 18, 21, 22, 23 dichalcogenides, 24 hexacyanometalates, 25 Fig. 1a. The HE-NASICON was synthesized by a general sol-gel method followed with a heat-treatment at 700°C. Rietveld re nement was conducted to analyze X-ray diffraction (XRD) to identify crystal structure of the asprepared material, and the pro le is shown in Fig. 1b occupy the octahedra site of 12c with equal molarities. While there are two types of Na that located in the 6b site (Na1) and 18e site (Na2), with an occupancy factor of 0.539 and 0.970, respectively. Crystal structure of the HE-NASICON is depicted in Fig. 1c, where the TMO 6 octahedra and PO 4 tetrahedra are corner-shared to construct the 3D framework structure. More information about the re ned structure parameters is listed in Table S2. The ngerprint of [PO 4 ] group in the HE-NASICON structure was detected by Raman spectra (Fig. S1). The peaks located at 1347.8 cm −1 and 1594.7 cm −1 correspond to the Dband and G-band of carbon material, which derives from the pyrolysis of organic species in the raw materials. 26 The amount of carbon is around 6.78%, which is measured by thermogravimetry (TG, Fig.  S2).
Morphology of the HE-NASICON was disclosed by scanning electron microscopy (SEM). As shown in Fig.  S3, the smooth irregular polyhedral shaped particles are displayed with a wide size distribution ranging from nanometer to micrometer scale. Spherical-aberration-corrected transmission electron microscope (ACTEM) was further employed to characterize the detailed structure of the material (Fig. 2a-d). An interphase between the amorphous carbon and crystalline HE-NASICON can be seen in Fig. 2b. The lattice of crystalline HE-NASICON was visualized by high-resolution TEM (HR-TEM). The atomic lattice with an interspacing of 6.22 Å should be indexed to the (012) planes of HE-NASICON structure (Fig. 2c). Survey spectrum of the X-ray photoelectron spectroscopy (XPS) con rms the existence of Na, P, O, Fe, Mn, V, Ti, and Cr for HE-NASICON (Fig. 2g). The detailed spectra of each TM are also probed to analyze their chemical states ( Fig. 2h- species. The Ti 3+ is around 33.8%. 33,34 For the Cr 2p spectrum, it shows two main peaks at 577.98 eV (Cr 2p 3/2 ) and 587.58 eV (Cr 2p 1/2 ), respectively, which can be ascribed to the Cr 3+ species. 31 The overall charge state of TMs was calculated to be 5.62, which is very close to the ideal value of 5.60 in the . The capacity is much larger than the general NASICON materials with 2-sodium storage (> 120 mA h g −1 ), enabling the HE-NASICON a multi-sodium storage cathode as expected. The charge/discharge pro les exhibit a multi-steps pattern, indicating consecutive redox reactions of multiple active transitionmetals in NASICON structured materials. 35 Four pairs of redox potentials at 1.69/1.53 V, 2.25/2.21 V, 3.41/3.40 V, and 4.08/4.08 V are found in the dQ/dV plot (Fig. 3b). The small gaps between the oxidation and reduction potential of each redox pair indicate highly reversible sodium storage process with good electrode kinetics. Fig. 3c  respectively. When the current returns to 0.5 C from the high-rate of 20 C, the discharge capacity can achieve 109 mA h g −1 , implying good reversibility for exible sodium storage. For multi-sodium storage within a wide voltage window, cyclability is a key issue because the too many sodium-ions insertion/extraction in the lattice can signi cantly affect structure stability and cause capacity decay. 16,35 To check the cyclability of multi-sodium storage reaction, the HE-NASICON cathode was operated at 0.5 C for 100 cycles. As displayed in Fig. 3d, a remarkable capacity retention of 93.0% was achieved. Even at the high-rate of 5 C, capacity of the cathode after 1000 cycles still remines 85.3% of its initial capacity (Fig. 3e). Electrode kinetics was then studied to understand the good rate and cycling performance. The apparent sodium diffusion coe cient (D Na ) of HE-NASICON was primarily investigated based on CV curves ( Fig. 3f and Fig. S5). As displayed in Fig. 3g, the calculated D Na uctuates within the range of 10 −9~1 0 −10 cm 2 s −1 , which is on the same order of magnitude as most NASICON materials. Pseudocapacitance behavior investigation was further implemented and the detailed calculation process can be seen in Supporting Information (Fig. S6). It shows that at the scan rate of 1.0 mV s −1 , contribution of capacitance is 67.8% (Fig. 3d). Therefore, the high D Na values and contribution of pseudocapacitance should account for the satisfactory high-rate capability and cycling stability of the material. To better evaluate the performances, we then compared the electrochemical performance of the HE-NASICON with some other typical NASICON materials that previously reported. For a fair comparison, the data adopted are materials with simple carbon compositing as the HE-NASICON does. As displayed in Fig. 4a, in the aspect of long term cyclability, the HE-NASICON with around 0.015% capacity decay of per cycle is outstanding among the various materials. 14,26,36,37,38,39,40,41 Furthermore, in terms of capacity, energy density, and average voltage, the HE-NASICON is also prominent (Fig. 4b). 13,36,38,40,42,43,44,45 The multi-sodium storage contributes a high energy density approaching 500 Wh kg −1 , making the material promising for application of grid-scale energy storage systems.
Sodium-storage mechanism. To unveil underlying effect of high-entropy on the advanced sodium storage properties, the HE-NASICON was subjected to in-situ XRD to probe crystal structure evolution of the multisodium storage reaction in the voltage range from 1.5 V to 4.5 V vs. Na + /Na. Contour map and the corresponded charge/discharge curves of the cell are presented (Fig. 5a and Fig. 5b). According to the number of peaks re ection and position, the whole sodium storage reaction can be divided into 3 regions.
To portray the reaction process, ex-situ XRD of a HE-NASICON electrode discharged to 1.5 V was primarily analyzed to disclose crystal structure of the material (Fig. 5c), and the diffraction peaks were identi ed based on Rietveld re nement results (Table S4). The peak at around 13.9°, 19.8°, 20.2°, 23.6°, and 24.2° is assigned to (10-2), (104), (110), (113), and (006) re ection, respectively. In region I, the (104) and (110) re ections are very close, leading to a merged broad peak, while the (112) and (006) re ections can be identi ed clearly. As sodium-ions continue to be extracted, in region II, the (104) and (110) separate out from the broad peak while the (112) and (006) merge together. The peaks greatly shift to higher angle when it goes to region III, indicating relatively large crystal structure variation in the high voltage region. For cathode materials, unfavorable crystal structure variation may appear and damage electrochemical performance. Therefore, crystal phase of the HE-NASICON electrode at 4.5 V was further studied (Fig. 5d) and details of the Rietveld re nement results are listed in Table S5. The fully charged electrode material can also be indexed to a trigonal R-3c phase, which is the same as the electrode at 1.5 V, indicating that the crystal framework has no signi cant changes during the charge process. The diffraction peaks variation of HE-NASICON is believed to associate with the changes of content of Na + , valence of transition metals, and their local environments in the host structure. [29,43] The lattice variation in high voltage does not affect reversibility of the material. When discharging to 1.5 V, the XRD recovers to the original state, demonstrating an iso-symmetric reaction pattern as shown in the contour map (Fig. 5a). The peaks shifting and intensity changes should be ascribed to lattice expanding/shrinking and slight local environmental rearrangement during sodium-ion extraction/insertion. 40 To con rm it, crystal structure of the electrode before charging (Fig. 5e), fully charged electrode (Fig. 5f), and fully discharged electrode (Fig. S7) were characterized by ACTEM. FFT patterns of the electrodes are both identi ed to be the trigonal phase along zone-axis of , suggesting that the samples possess the same crystal phase, and show the same exposing facets. The lattice spacing is measured to be 0.647 nm and 0.613 nm for the sample of 1.5 V and 4.5 V, respectively. The decreased spacing con rms lattice shrinkage of the material when sodium-ions are extracted. When the electrode was fully discharged from 4.5 V to 1.5 V, the lattice spacing returned to 0.644 nm (Fig. S7), con rming reversible lattice variation in the stable highentropy NASIOCN crystal structure. As shown in Fig. 5g-i, lattice parameter changes of the HE-NASICON during the reaction are reversible.
Based on analysis results of the crystal phase evolution, reaction mechanism of the multi-sodium storage process in HE-NASICON was plotted in Fig. 6a. The pristine HE-NASICON, which contains 3.4 sodium ions, was initially discharged to obtain a sodium-rich phase  (Table S4). In the pristine HE-NASICON structure, Na-ions locate at Na1 and Na2 sites. The extra inserted sodium-ions in HE-Na 4.143 partially enters into the Na1 stie, increasing the occupancy factor of Na1 from 0.539 to 0.705. While other inserted sodium-ions settle in a new Na3 site (36f). It has been reported that the coulombic repulsion, which is caused by simultaneous occupation of neighboring Na1 and Na2 sites, would lead sodium-ions to the Na3 site. 38 The fully charged sodium-de cient phase is determined to be  (Table   S5). The number of sodium ions participate in energy storage is then calculated to 2.892, verifying a multi-sodium storage reaction as anticipated. Note that, the overall volume change of the multi-sodium storage in HE-NASICON material is only 4.07%. Compared to some typical low entropy NASICON structured material (with only one or two TMs), such as Na 4 MnV(PO 4 ) 3 , Na 2 TiV(PO 4 ) 3 , and Na 4 MnCr(PO 4 ) 3 , etc. volume change of the HE-NASICON for multi-sodium storage is greatly reduced (Fig. 6b). 11,16,26,38,39,43,45,46,47,48 That is critically important for the cyclability of the material. The electrode after 1000 cycles at 5 C was further characterized to verify stability of the material. Crystal phase of the electrode after the long cycling was rstly checked by XRD, and it shows that crystal phase of the material maintains well (Fig. S8). The robust microstructure of carbon coated trigonal NASICON lattice after long cycling was determined by HR-TEM (Fig. S9). EDS shows that the atomic ratio of the 3d-transition-metals is almost equal (Fig. S10), and all the elemental components still uniformly distributed in the material (Fig. 6c), indicating that no phase segregation or metal-dissolution was found. Therefore, the HE-NASICON material is highly stable to afford long time running. The high-entropy strategy is suggested to present multi-functions to improve the electrochemical properties of NASICON material, as illustrated in Fig. 6d. The high-entropy effect can help to increase the solid solubility of different elements in one phase, and strain effect in high-entropy materials, which is caused by the rich accommodated atoms with different sizes, leading to an intense lattice strain eld that suppress large phase transition and lattice variation. 19 As a result, unfavorable crystal phase transition is suppressed in the high-entropy material even when it is charged to the high voltage of 4.5 V. Besides, volume changes of multi-sodium storage can be greatly reduced.
Interaction between the diverse transition metals also has fundamental impact on the properties of HE-NASICON, such as redox activity. To understand it, valent states of the TMs in the pristine electrode, fully discharged electrode and fully charged electrode were analyzed by ex-situ XPS to determine the electrochemically active species during the sodium-storage reaction (Fig. 7a) peaks, respectively. 50,51 The Ti 3+ was found recover back to Ti 4+ (460.38 eV) in the fully charged sample, while the V 2+ was oxidated to V 5+ (518.11 eV) and V 4+ (516.99 eV), implying that the V undergoes continuous redox reactions. 40,52 Cr 2p 3/2 is the only TM species that almost has no changes during the whole reaction. The XPS results verify the multi active TM centers and multi-redox reactions in the HE-NASICON material, which is in accordance with the multi voltage platforms of the charge/discharge pro les. To further revel the roles of the transition metals to electrochemical performance, ve highentropy NASICON materials with doubled concentration of Fe, Mn, V, Ti, and Cr (noted as HEN-Fe, HEN-Mn, HEN-V, HEN-Ti, HEN-Cr, respectively) were synthesized. All of the samples show the same crystal phase as the HE-NASICON (Fig. S11). EDS mapping demonstrates that the atomic ratios of the elements are close to their theoretical values and the elements are evenly distributed, indicating successful synthesis of the diverse high-entropy materials (Fig. S12-16). Carbon content of the materials were determined by TG (Fig. S17) and were ruled out when counting their sodium storage capacities. Galvanostatic charge/discharge pro les of the samples at 0.1 C are presented in Fig. S18.  Fig. 7b. Larger capacity contributions in the high voltage regions of 3.5 ~ 4.0 V and 4.0 4.5 V are found in HEN-Mn and HEN-Cr, indicating that Mn and Cr are bene cial to achieve high voltage properties of the material (Fig. 7c). Rate performance and cycling capability of the materials are exhibited in Fig. 7d and Fig. S19. For HEN-Fe, although it has relatively large capacity at 0.1 C, its rate performance is poor, with only 49.9 mA h g −1 at 20 C. The HEN-Mn shows the worst rate capability, which is probably because of the relatively sluggish electrode kinetics as many Mn-based materials reported. 6, 53 In the contrast, the HEN-Ti possess the highest capacity of 62.81 mA h g −1 at 20 C, regardless the relatively low capacity at 0.1 C. Besides, HEN-Ti also demonstrates the best cycling performance, with a remarkable capacity retention of 90.92% after 1000 cycles at 5 C (Fig. 7e). While the capacity of HEN-Cr decays very fast. Therefore, the TMs show different features affecting the materials in terms of capacity, voltage, rate capability, and cycling stability. It is the synergistic effect of TMs with suitable molarity and high-entropy nally leads to the HE-NASICON material with high performances. In-situ XRD analysis shows that crystal phase of the HE-NASICON during the multi-sodium storage reaction is stable, and the volume change is only 4.07%. It is suggested that the high-entropy effect can help to stabilize the host framework of NASICON structure, thus leading to the enhanced cyclability. By comparing a group of HE-NASICON cathodes containing different contents of TM species, we further demonstrate the advanced electrochemical performance of HE-NASICON also derives from the synergistic effect of the various transition metals with equal molarity. The high-entropy strategy on NASICON opens a new opportunity to design advanced polyanion compounds for SIBs.

Methods
Synthesis of HE-NASICON and other high-entropy NASICON materials with different atomic ratio. The HE-NASICON was synthesized by a typical sol-gel method. All chemical reagents were used without further puri cation. Iron nitrate nonahydrate, manganese acetate, ammonium metavanadate, titanium (IV) isopropoxide, and chromium(III) nitrate nonahydrate were used as the Fe, Mn, V, Ti, Cr sources, respectively. Firstly, stoichiometric amount of anhydrous citric acid and ammonium metavanadate are dissolved in 20 mL deionized water, forming solution A. Then, sodium carbonate and the Fe, Mn, Cr sources are added in another 40 mL deionized water, forming solution B. A was then mixed with B at 80 ℃ and kept constant stirring for 30 min. After that, ammonium biphosphate were added into the mixed solution. When the ammonium biphosphate was fully dissolved, appropriate amount of titanium (IV) isopropoxide that dissolved in 20 mL absolute ethanol was nally added into the mixed solution drop by drop. The nal solution kept constant stirring at 80 ℃ until a gel was formed. The gel was dried at 120 ℃ in an oven. Finally, the porous precursor was calninated at 700 ℃ for 12 h under Ar to obtain the HE-NASICON material. Other high-entropy NASICON materials with different atomic ratio were also synthesized through the same sol-gel route. The amounts of reagents used to synthesize the materials are listed in Table S1.
Materials characterization. X-ray diffraction (XRD) data and were collected from 10° to 80° in PANalytical Empyrean 2. The XRD re nements were conducted by GSAS software. Morphology and composition of the materials was disclosed by eld-emission scanning electron microscopy (SEM, JSM-IT300LA) equipped with energy-dispersive X-ray spectroscopy (EDS). Micro-structure of materials was further studied by spherical-aberration-corrected transmission electron microscopy (ACTEM, Titan G2 60-300). Electron diffraction images were got by Fast Fourier Transform (FFT). Valence states of Fe, Mn, V, Ti and Cr were determined by X-ray photoelectron spectroscopy (XPS, ESCALAB 250Xi). Raman measurements were performed on LabRAM HR Evolution. Carbon content of the materials were determined by thermogravimetric (TG) analysis from room temperature to 650°C with a heating rate of 10°C min −1 under air atmosphere.
Electrochemical measurements. For the fabrication of electrodes for electrochemical performance tests, the active materials, carbon black and polyvinylidene uoride (PVDF) with a weight ratio of 7: 2: 1 were mixed in appropriate amount of 1-Methyl-2-pyrrolidinone (NMP) to make a slurry. The slurry was costed on Al foil and dried at 80°C. Loading mass of active materials was in the range of 1.2~1.4 mg cm −2 . Na metal and glass ber lters (Whatman) were used as counter electrode and separator, respectively. 1M NaClO 4 dissolving in PC solution with 5 vol% addition of uoroethylene carbonate (FEC) was used as the electrolyte. Electrochemical performance of the cells was carried out on a Land battery testing system (CT2001A) within the voltage range of 4.5-1.5 V (vs. Na/Na + ).