Concurrently coating and doping high-valence vanadium in Ni-rich

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Materials and Interfaces

Concurrently coating and doping high-valence vanadium in Nirich lithiated oxides for high-rate and stable Li-ion batteries Haifeng Yu, Yugang Li, Yanjie Hu, Hao Jiang, and Chunzhong Li Ind. Eng. Chem. Res., Just Accepted Manuscript • DOI: 10.1021/acs.iecr.8b06162 • Publication Date (Web): 25 Feb 2019 Downloaded from http://pubs.acs.org on March 3, 2019

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Industrial & Engineering Chemistry Research

110th Anniversary: Concurrently coating and doping high-valence vanadium in Ni-rich lithiated oxides for high-rate and stable Li-ion batteries Haifeng Yu, Yugang Li, Yanjie Hu, Hao Jiang*, Chunzhong Li* Key Laboratory for Ultrafine Materials of Ministry of Education, School of Materials Science and Engineering, East China University of Science and Technology, Shanghai 200237, China Email: [email protected] (Prof. H. Jiang) and [email protected] (Prof. C. Z. Li) *Corresponding author: Tel.: +86-21-64250949, Fax: +86-21-64250624

Abstract Surface engineering and hetero-element doping are recognized as highly effective protocols to enhance Ni-rich lithiated oxides. Herein, we report the kilogram-scale preparation of the concurrent coating and doping of high-valence vanadium in Ni-rich LiNi0.815Co0.15Al0.035O2 cathode materials (denoted as V-NCA). HF acid is greatly restricted by removing the residual lithium salts after the reaction with vanadium source. In the subsequent low-temperature process, the coating of vanadium compounds (V2O5, Li3VO4) and the doping of vanadium ions have been realized, which remarkably decrease the dissolution of active materials and the Li+/Ni2+ disorder. Consequently, the stable reversible specific capacity increases by 17.7% at 0.1 C (202.6 mAh g-1) and by 59.6% at 5 C (147 mAh g-1) compared with the pristine NCA. We also assembled a pouch-cell of 650 mAh (3.33 g of V-NCA) by choosing graphite as the anode material, in which 89.2% capacity retention is achieved after 500 cycles at 1 C. Keywords: Ni-rich cathode; surface modification; element doping; lithium-ion 1

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batteries

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1. Introduction The cathode materials of lithium-ion batteries (LIBs) are widely considered pivotal to improving the endurance of electric vehicles compared to anode materials with high specific capacity1-4. Commercial cathode materials generally have a relatively low specific capacity of < 160 mAh g-1 5-7. Low-cost Ni-rich lithium nickel cobalt aluminum ternary layered oxides matching with solid state electrolyte, LiNi1−x−yCoxAlyO2 (1-x-y ≥ 0.8, NCA), can deliver a specific capacity as high as 180 mAh g-1 in 2.75-4.3 V and favorable security, demonstrating huge potential for next-generation LIBs8-13. However, a high Ni content inevitably increases the possibility of cationic disorder, allowing more divalent Ni to enter the Li layer14-16. Excessive lithium salts are added in the synthesis process to alleviate this issue17. Unfortunately, the residual lithium salts react with H2O and CO2 in air to form an inactive LiOH/Li2CO3 layer on the surface of the active materials18, 19 and therefore decrease the Li+ diffusion rate. The side reaction of these lithium compounds and the electrolyte will also generate more HF and cause corrosion of the active materials20, 21. Therefore, starting from the perspective of practical applications, it is indispensable to reduce the electrode surface activity with electrolytes and boost the structural stability. To address the aforementioned issues, extensive studies mainly concentrate on the thin surface coatings and the heterogeneous element doping of the electrodes. For the former, some corrosion-resistant and chemically stable compounds are usually chosen as a coating layer to suppress the surface side reaction, such as metal oxides 3



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(ZnO, Al2O3, TiO2)22-24, metal fluorides (AlF3, CaF2)25, metal phosphates (AlPO4)26, etc. Their low electrochemical activity and ion diffusion will sacrifice the partial specific capacity with increased surface impedance. In the latter, the introduction of heterogeneous elements (such as Ti, Mg, Zr, Mo, etc.)27-30 in the NCA can increase the Li+ conductivity and reduce the cationic disorder, thereby improving the power density of batteries while suppressing the phase transition of the crystal structure. With this tactic, it is difficult to avoid the undesirable side reactions caused by the direct contact of the active materials with electrolyte. Therefore, it is still a challenge to exploit a simple strategy for achieving surface coating and heterogeneous element doping in co-engineered NCA cathode materials with a high specific capacity and long cycle life. Herein, we demonstrate the concurrent coating and doping of high-valence vanadium in NCA cathode materials by a facile surface treatment and the subsequent low-temperature calcination (denoted as V-NCA). In this process, the residual lithium salts on the NCA surface have been effectively cleaned to reduce HF generation by the solution reaction with vanadium source. More importantly, we realized the coating of vanadium compounds (e.g., V2O5, Li3VO4) with high ionic conductivity and the doping with vanadium ions to replace partial nickel ions in NCA by annealing, which decreases the dissolution of the active materials on the NCA surface and the Li+/Ni2+ disorder. Consequently, the V-NCA remarkably enhances the reversible specific capacity of the pristine NCA by 17.7% at 0.1 C (202.6 mAh g-1) and 59.6% at 5 C (147 mAh g-1) with excellent cycling stability. Furthermore, a pouch-cell of 650 mAh 4


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(3.33 g of V-NCA) has been assembled by choosing graphite as the anode material, in which an 89.2% capacity retention was achieved after 500 cycles at 1 C. 2. Experimental Section 2.1 Synthesis of the V-NCA cathode materials The pristine LiNi0.815Co0.15Al0.035O2 (NCA) powder was purchased from Shenzhen BTR New Energy Materials Co. LTD, China. In a typical process, 0.2 g of vanadium triisopropoxy oxide powders were dissolved in 100 mL of mixed solution of anhydrous alcohol and deionized water with the help of an ultrasound treatment. 10 g of NCA powders were gradually added to the above mixed solution with vigorous stirring until the solvents evaporate completely, and then dried in the vacuum oven under 120 ℃ for 12 hours. Finally, the high-valence vanadium-modified NCA materials (V-NCA) were obtained after annealing at 450 ℃ for 3 hours in a pure oxygen atmosphere. 2.2 Material characterization The crystalline phases of the as-obtained samples were confirmed by an X-ray diffractometer (XRD) with Cu Kα radiation under a scanning speed of 5° min-1. Corresponding lattice parameters were refined and calculated via the Rietveld method. Scanning electron microscopy (SEM, S-4800) with an accelerating voltage of 10 kV was performed to investigate the surface microscopic morphology. The detailed crystal structure and element distribution of samples were characterized via transmission electron microscopy (TEM, JEOL-2100) operated at 200 kV with an X-ray energy dispersive spectrometer (EDS). The surface chemical valence states of 5



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the samples were analyzed by X-ray photoelectron spectroscopy (XPS, ESCA PHI500C) with Al Kα radiation (hν = 1486.6 eV), and all the spectra were calibrated with the C 1s peak at 284.6 eV. 2.3 Electrochemical measurements The cathodes were prepared by mixing the V-NCA hybrids, carbon black (Super-P, Timcal graphite Co. LTD.) and polyvinylidene fluoride (PVDF, Solvay Group) at a mass ratio of 90:5:5 in N-methyl pyrrolidone (NMP) to obtain a high-quality slurry, and then coated on pure aluminum foil with a dry process. The loading mass of active materials was approximately 3.0-3.5 mg cm2. The coin-type 2016 half-cells were assembled in an argon-filled glove box with pure lithium foil as the anode, which were separated by a polypropylene membrane (Celgard-2400). The electrolyte was 1 M LiPF6 dissolved in a mixture of ethylene carbonate, dimethyl carbonate and ethyl methyl carbonate (1:1:1 in volume). The pouch cell was assembled by matching the V-NCA cathode and graphite anode with doubling coating. The loading mass and density of cathode materials were ~19.5 mg cm-2 and ~2.5 mg cm-3, respectively. After rolling and slitting (4.3*4 cm2 for each piece), the cathode and anode were stacked in sequence and separated by the membrane, which was then packed and injected with electrolyte in an argon-filled glove box to obtain a pouch-cell. The galvanostatic charge and discharge measurements were performed with LANDCT2001A test system within a voltage range of 2.75-4.3 V at different current densities. Cyclic voltammetry (CV) experiments were carried out in an Autolab

PGSTAT302N

electrochemical

workstation.

For

the

galvanostatic 6


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intermittent titration technique (GITT) experiments, the cells were discharged repeatedly at 0.1 C for 20 min, which then lasts for 2 hours at the open-circuit up to a voltage of 2.75 V.

3 Results and discussion

Figure 1. (a) Schematic illustration of the preparation process, (b) low- and (c) high-magnification SEM images of the V-NCA cathode materials, (d-e) the XRD Rietveld refinement of the V-NCA and the pristine NCA, respectively.

Figure 1a schematically illustrates the facile preparation process of the V-NCA. Firstly, the NCA powders are added to the mixed solution of anhydrous alcohol and 7



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deionized water (v/v: 9/1) with the help of ultrasound treatment, in which the vanadium source reacts with residue lithium salts and the reaction products remain on the surface of the NCA particles. Subsequently, the suspension liquids are stirring continuously at 80 ℃ until the solvents evaporate completely and are dried in the vacuum to acquire the intermediates. Finally, the as-obtained intermediates are calcined in a pure oxygen atmosphere to achieve doping of high-valence vanadium ions and a uniform coating of vanadium compounds. The morphology of the pristine NCA, intermediates, and the V-NCA were investigated by field emission scanning electron microscopy (FE-SEM). As observed in Figure 1b and Figure S1a, c, all of the samples are typical secondary spherical particles consisting of small primary particles, which means that the microsphere structure of the host materials is not destroyed after the wet-chemical treatment and calcination process. In comparison with the pristine NCA (Figure S1b), it is conspicuous that the particle surface of the intermediate (Figure S1d) and the V-NCA (Figure 1c) exhibits obvious attachment, implying the retention of the reaction product and the successful construction of the layer covering the surface. To study the effect of the modification process on the crystal structure, the XRD patterns and corresponding Rietveld refinements of the V-NCA and the pristine NCA are displayed in Figure S2 and Figure 1e, f. No peaks corresponding to vanadium compounds can be observed in the XRD patterns of the V-NCA, which can be attributed to the small amount of vanadium compounds in the final products22, 24, 26. The diffraction peaks from two samples are all indexed to a typical hexagonal 8


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α-NaFeO2 structure with the space group R-3 m, and the clear splitting of the (006)/(012) and (018)/(110) doublets suggests a well-ordered layered structure31-33. Accordingly, it is concluded that the modification process will not destroy the macroscopic crystal structure of the NCA materials. The crystal constants of the pristine NCA and the V-NCA are further analyzed by the XRD Rietveld refinements, and the results are presented in Table S1. The lower values of Rw and x2 confirm that the refinements are valid. Compared with the pristine NCA, the lattice parameters (c) and the unit volume of the V-NCA slightly increase, which indicates that the calcination process may allow some of the vanadium cations to diffuse into the crystal lattice34. As pointed out by R. D. Shannon, vanadium cations tend to be in the V4+ oxidation state in an octahedral environment35. Due to the restraint of electrical neutrality, Ni2+ ions are formed at the expense of Ni3+ ions to compensate for the presence of V4+. Moreover, the radii of V(IV) (0.58 Å) and Ni(II) (0.72 Å) are much larger than those of the original Ni(III) (0.56 Å) and Co(III) (0.545 Å), which leads to the increase of the lattice parameter and unit volume. Consequently, it can be speculated that the vanadium ion successfully diffuses into the host structure rather than just distributing on the surface of the NCA particles. Additionally, it is noted that the value (1.78) of I(003)/I(104) for the V-NCA is higher than that of the pristine NCA (1.53), indicating that the incorporation of vanadium ions into the crystal lattice suppresses the migration of Ni2+ from the TM layer to the Li layer, thus moderately alleviating the cation disorder36. Conceivably, the improved lattice parameters and the lower cation disorder are beneficial to boosting the intrinsic Li+ conductivity of the 9



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host material and the stability of the crystal structure during lithiation/delithiation processes.

Figure 2. (a) High-magnification TEM, (b) high-resolution TEM and (c-d) enlarged TEM images taken from the yellow rectangles in (b), (e-i) TEM-mapping of the V-NCA; (j) high-magnification TEM and (k) high-resolution TEM images of the pristine NCA.

Transmission electron microscopy (TEM) analysis is carried out to explicitly characterize the detailed surface structural features of the V-NCA. As shown in Figure 2a, a homogeneous cover layer with a thickness of 25 nm is distinctly observed on the surface of the NCA particles. A representative high-resolution TEM (HRTEM) image derived from the edge of the V-NCA is provided in Figure 2b. The 10


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compound present in the coating layer is well-crystallized, where two separated zones marked by the labels of c and d can accurately determine their lattice fringes. Interplanar spacings of 0.44 and 0.21 nm are plainly visible, which can be indexed to the (001) planes of V2O5 (JCPDS 41-1426) and the (112) planes of Li3VO4 (JCPDS 39-0378). Moreover, element mapping images of a V-NCA particle (Figure 2e-i) further signify the uniform distribution of the host elements and the homogeneous vanadium compound coating layer. In contrast, Figure 2j, k show the TEM images of the pristine NCA samples. A solid structure with a smooth and clean surface is presented, while a plane spacing of 0.47 nm is observed, corresponding to the (003) plane of the pristine NCA37. Moreover, an amorphous layer with the thickness of ~2 nm exist on the surface of pristine NCA, which is inactive LiOH/ Li2CO3 formed by the reaction of residual lithium salts with H2O/CO2 in air (Figure S3). According to the results of XRD and TEM, we can conclude that a portion of vanadium ions diffuse into the bulk structure, and the others react with residual lithium salts to form the vanadium compound coating layer on the surface of the NCA particles.

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Figure 3. (a) C 1s XPS patterns of the V-NCA and the pristine NCA; (b) V 2p3/2 and (c) Ni 2p3/2 XPS patterns of the V-NCA; (d) the Ni 2p3/2 XPS pattern of the pristine NCA.

XPS measurements were performed to investigate the composition and valence state of elements in the pristine NCA and the V-NCA. The C 1s peaks (Figure 3a) located at 289.5 eV belong to the carbonate compound (Li2CO3)20. The O 1s peaks (Figure S4a, b) located at 531.2 eV are ascribed to surface oxygen derived from adsorbed species, such as LiOH or Li2CO3, while the peaks at 528.9 and 529.3 eV are related to the lattice oxygen in the crystal framework38, 39. Obviously, the intensities of the C 1s peak at 289.8 eV and the O 1s peak at 531.2 eV dramatically decrease and the intensity of the lattice oxygen peak is significantly enhanced after the modification process, demonstrating that the lower amount of residual lithium salts on the surface due to the formation of the vanadium compounds40, 41. As is well known, the removal 12


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of residual lithium salts is beneficial for suppressing the formation of HF, thereby diminishing the side reactions between active materials and the electrolyte during charging/discharging processes. Figure 3b displays the XPS spectrum of the V 2p3/2 for the V-NCA. The resolved peaks at 517.1 and 516.6 eV are assigned to V5+ and V4+, respectively, directly confirming the presence of vanadium ions on the surface of the samples [33]. As shown in Figure 3c, d, the Ni 2p3/2 spectra clearly reveal the fact that Ni3+ and Ni2+ coexist in the samples, and the resolved peaks located at 855.9 and 854.6 eV correspond to Ni3+ and Ni2+, respectively34,

42.

It is noteworthy that the

Ni3+/Ni2+ intensity ratio decreases after modification, which is because Ni2+ ions are formed to replace Ni3+ ions after the incorporation of V4+ ions to maintain the charge balance. This consequence is confirmed well by the XRD results.

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Figure 4. The first three CV curves of (a) the V-NCA and (b) the pristine NCA at 0.2 mV s-1 in 2.75-4.3 V; (c-d) the linear relationship between the anodic/cathodic peaks current (ip) and the square root of the scan rate (v1/2) for the V-NCA and the pristine NCA; (e) GITT curves of discharging curves and (f) the Li+ diffusion coefficient calculated by GITT of the V-NCA and the pristine NCA.

To explore the superiority of the coating and doping of high-valence vanadium for Ni-rich lithiated oxides, the lithium storage performances were tested in half cells. The first three cyclic voltammetry (CV) curves for the pristine NCA and the V-NCA were measured at 0.2 mV s-1 within 2.75-4.3 V. As depicted in Figure 4a, b, three 14


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pairs of cathodic/anodic peaks located at 3.75/3.66, 4.01/3.95, 4.20/4.15 V are observed during the charge-discharge process, which correspond to the three-phase transformation processes: hexagonal phase (H1) → monoclinic phase (M), monoclinic phase (M) → hexagonal structure (H2), and hexagonal structure (H2) → hexagonal phase (H3)34, 43. Impressively, the first three CV curves of the V-NCA have better consistency than that of the pristine NCA, implying a higher reversibility in the lithiation/delithiation process. The CV at various sweep rates and with the galvanostatic intermittent titration technique (GITT) are employed to analyze the diffusion kinetics of migrant lithium ions. Figure S5 shows the CV curves of the two samples in various scan rates of 0.2, 0.4, 0.6 and 0.8 mV s-1, respectively. The intensity of the current peaks gradually enhances with the increasing sweep rate, and the linear relationship between the anodic/cathodic peak current (ip) and the square root of the scanning rate (v1/2) imply that the reaction of Li+ ion intercalation/deintercalation is diffusion-controlled (Figure 4c, d)10, 44. The obtained data are used to calculate the diffusion coefficient via the Randles-Sevcik equation (see the Supporting Information for more details). According to these calculations, the Li+ ion diffusion coefficients of the pristine NCA and the V-NCA are 5.73×10-10 and 3.65×10-10 cm2 s-1, respectively, indicating that the diffusion coefficient improve after the coating and doping of high-valence vanadium. To more clearly reflect the diffusion dynamics, the GITT analysis on cycled cells was carried out to obtain the specific Li+ diffusion coefficients at each stage of cell operation45, 46. Figure 4e shows the GITT curves of the pristine NCA and the V-NCA 15



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in discharging process, while a typical τ versus V profile for a single titration is revealed in Figure S6a. The testing process and calculation equations are presented in the Supporting Information. The as-calculated different Li+ diffusion coefficients (DLi+) as a function of the Li intercalation/deintercalation content are shown in Figure 4f, and the values of two samples are all approximately 10-10∼10-9 cm2 s-1, which is consistent with the earlier reports47,

48.

Furthermore, the V-NCA have higher Li+

diffusion coefficients than that of the pristine NCA throughout the discharge process. The vanadium doping expands the lattice parameters and decreases the degree of cation disorder. Meanwhile, the vanadium compound coating layer on the surface of the active materials possesses excellent ionic conductivity. Therefore, the modification of high-valence vanadium boosts the rapid transfer of lithium ions at the interface and the interior of the active materials, thereby significantly improving the diffusion kinetics.

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Figure 5. (a) The first charge-discharge curves at 0.1 C, (b) rate performance and (c) cycling stability of the V-NCA and the pristine NCA, (d) the schematic diagram of advantages of the V-NCA compared with the pristine NCA.

The influence of the modification on the initial charge-discharge curves (Figure 5a) was investigated. The first discharge capacities of the pristine NCA and the V-NCA are 166.7 and 199.4 mAh g-1, respectively. The higher capacity of the V-NCA is due to the increased coulombic efficiency compared with the pristine NCA (89.4% instead of 73.1%), which possibly originates from the removal of residual lithium salts and the decreased side reactions with the electrolyte. The coulombic efficiency of the subsequent two curves are all improved to over 95%, but the charge-discharge curves of the V-NCA have preferable overlap compared to that of the pristine NCA (Figure S7), which is consistent with the CV tests. Figure 5b shows the charge-discharge capacities of the two samples with various current densities from 0.1 C to 5 C at 25 ℃. Compared to the capacity at 0.1 C, the capacity retention of the V-NCA at 5 C is 72.4% (147.7 mAh g-1), while that of the pristine NCA just remains at 53.8% (92.1 mAh g-1). Meanwhile, the discharge capacity of the V-NCA can immediately go back to 168.8 mAh g-1 when the current density decreases to 1 C, which directly indicates the superior reversibility and structure stability [6]. Since the cycle stability is also a critical factor for the practical application of LIBs, the cycling performances of coin-type cells are all measured at the current density of 1 C. As shown in Figure 5c, the specific capacity retention of the V-NCA after 100 cycles is 17



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88.2%, corresponding to 147.8 mAh g-1, whereas the pristine NCA only displays a capacity retention of 74.5%. Undoubtedly, these ameliorative electrochemical performances are mainly associated with the change of the structure in the cathode materials. As depicted with a schematic illustration (Figure 5d), the vanadium compounds are formed as a coating layer on the surface of the NCA particles with the consumption of residual lithium salts, which can effectively decrease the formation of HF and the dissolution of the active materials. Unlike the lithium salts, the vanadium compounds with high Li+ conductivity can also boost the Li+ transfer at the interface. In addition, the vanadium ions can also diffuse into the host materials by calcination. The introduction of vanadium ions in the crystal lattice can reduce the cation disorder and expand the lattice parameters, which can stabilize the crystal structure during repeating lithiation/delithiation processes and adequately guarantee the unhindered shuttling back and forth of Li+ in the host materials.

Figure 6. (a) Digital photograph of the kilogram level V-NCA powders and the assembled pouch 18


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full cell of 600 mAh in 2.75-4.3 V, (b) the first three charge-discharge curves, (c) rate performance and (d) cycling stability of the pouch cell.

To further detect practicability and feasibility of V-NCA in commercial application, the V-NCA are prepared in kilogram level and a pouch full cell is successfully assembled with the commercial graphite anode (Figure 6a). The initial three charge/discharge profiles were investigated at 0.1 C in the range of 2.75-4.3 V (Figure 6b). The discharge capacity of pouch cell is 622.1 mAh with an initial coulombic efficiency of 88.9%, and the coulombic efficiencies increase to 99% on the second and third cycles. Meanwhile, the discharge curves of the initial three cycles have favorable coincidence, indicating the outstanding reversibility of the pouch cells. We also investigate the rate and cycling performances of the pouch cells. As shown in Figure 6c, the cells reveal high discharge capacities of 662.1, 633, 608.3, 582.4, 535.3, 510.4 mAh at current densities of 0.1 C, 0.2 C, 0.5 C, 1 C, 2 C and 3 C, respectively, demonstrating the excellent rate capability. Moreover, a discharge capacity of 650.6 mAh, occupying 98.2% of the original value (0.1 C), is retained after returning to 0.1 C, further implying the singularly stability of the pouch cells. Impressively, even up to 500 cycles at 1 C, a reversible capacity of 494.3 mAh with high coulombic efficiency of ~100% is still maintained (capacity retention ratio of 89.2%), revealing unprecedented cycle stability (Figure 6d). Undoubtedly, these modified cathode materials (V-NCA) can still exert a remarkable electrochemical performance even in large-scale and high-capacity pouch cells. A comparison 19



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regarding the gravimetric energy density with previously reported lithium ion full-cells is displayed in Table S2, and the values are calculated based on the mass of all active materials (V-NCA and graphite). Our V-NCA/graphite full-cells provides an energy density of 443 Wh kg-1, which is higher than that of most full-cells in previous studies. Combined with its prominent cycle and rate capabilities, the batteries with this configuration are promising in electric vehicle applications with an extended driving range. 4 Conclusion In summary, we develop a scalable preparation of modified Ni-rich LiNi0.815Co0.15Al0.035O2 cathode materials with concurrent coating and doping of high-valence vanadium at the kilogram level via a simple wet-chemical method. The removal of residual lithium salts can effectively reduce the formation of HF in the electrolyte. Meanwhile, the coating of vanadium compounds (V2O5, Li3VO4) with high lithium conductivity and the doping of vanadium ions can decrease the dissolution of active materials and stabilize the crystal structure in the lithiation/delithiation process. Consequently, the V-NCA remarkably exhibits a high reversible specific capacity of 202.6 mAh g-1, 147 mAh g-1 even at a current density of 5 C, which is much higher than that of pristine NCA (92.1 mAh g-1 at 5 C). The V-NCA also demonstrates an excellent cycle stability for LIBs. A pouch-cell of 650 mAh (3.33 g of V-NCA) has been assembled by choosing graphite as the anode material, in which 89.2% capacity retention is achieved after 500 cycles at 1 C. Such a full-cell configuration provides an energy density of 443 Wh kg-1, which is promising 20


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in electric vehicle applications with an extended driving range. Conflict of interests There are no conflicts of interest.

Acknowledgments This work was supported by the National Natural Science Foundation of China (21522602, 91534202), the Innovation Program of Shanghai Municipal Education Commission, the Program for Shanghai Youth Top-notch Talent, the Shanghai Scientific and Technological Innovation Project (18JC1410500), the National Program for Support of Top-Notch Young Professionals, and the Fundamental Research Funds for the Central Universities (222201718002).

Appendix A. Supplementary data The Supporting Information is available free of charge on the ACS Publications website at DOI: xxxxxx References (1) Xiong D.; Li X.; Bai Z.; Lu S. Recent Advances in Layered Ti3C2Tx MXene for Electrochemical Energy Storage. Small 2018, 14, 1703419. (2) Jiang B.; Han C.; Li B.; He Y.; Lin Z. In-Situ crafting of ZnFe2O4 nanoparticles impregnated within continuous carbon network as advanced anode materials. ACS Nano 2016, 10, 2728-2735. (3) Zhao S.; Wang Z.; He Y; Jiang B.; Harn Y.; Liu X.; Yu F.; Feng F.; Shen Q.; Lin 21



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Concurrently coating and doping high-valence vanadium in Ni-rich

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