Improving electrochemical performance of high-voltage spinel LiNi0

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Improving electrochemical performance of high-voltage spinel LiNi0.5Mn1.5O4 cathode by cobalt surface modification Yuan Xue, Lili Zheng, Jian Wang, Jigang Zhou, Fu-Da Yu, Guo-Jiang Zhou, and Zhen-Bo Wang ACS Appl. Energy Mater., Just Accepted Manuscript • DOI: 10.1021/acsaem.9b00564 • Publication Date (Web): 01 Apr 2019 Downloaded from http://pubs.acs.org on April 3, 2019

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ACS Applied Energy Materials

Improving electrochemical performance of high-voltage spinel LiNi0.5Mn1.5O4 cathode by cobalt surface modification Yuan Xue,a,b,1 Li-Li Zheng,b,1 Jian Wang,c Ji-Gang Zhou,c Fu-Da Yu,b Guo-Jiang Zhou,a* Zhen-Bo Wang,b* a. School of Envionmental and Chemical Engineering, Heilongjiang University of Sciences and Technology, No.2468 Puyuan road, songbei district, Harbin, 150022 China. b. MIIT Key Laboratory of Critical Materials Technology for New Energy Conversion and Storage, School of Chemistry and Chemical Engineering, Harbin Institute of Technology, No.92 West-Da Zhi Street, Harbin, 150001 China. c. Canadian Light Source Inc., Saskatoon, SK S7N 2V3, Canada. 1: These two authors contribute equally. * Corresponding author. Tel.: +86-451-86417853; Fax: +86-451-86418616. Email: [email protected] (Z.B. Wang)

Abstract: High-voltage spinel LiNi0.5Mn1.5O4 is considered as a promising cathode material for the next generation of lithium ion battery, but it is plagued by poor long cycling performances especially at high temperature. The as-prepared LiNi0.5Mn1.5O4 material is surface modified by cobalt at 500 °C and 700 °C, respectively in this work. The obtained samples are studied by scanning transmission X-ray microscopy and electrochemical tests in detail. After surface modification at 500 °C, cobalt oxide coating is formed on the surface of LiNi0.5Mn1.5O4 and electrochemical performance is not improved. While after surface modification at 700 °C, the cobalt enters into the LiNi0.5Mn1.5O4 surface layer, leading to cobalt surface doping. Cobalt surface doping decreases the Ni concentration and increases the oxidation state of Mn on the surface, which enhances the long cycling and high temperature cycling performances. After 200 cycles at 55 oC, 1

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the capacity retention increases from 82 % to 93 % because of cobalt surface doping. After 2000 cycles at 5 C, the capacity of cobalt surface doped LiNi0.5Mn1.5O4 remains 93 mAh g-1 with capacity retention of 81 %. Besides, surface doping increases the disordered phase and Mn3+ content in LiNi0.5Mn1.5O4. This helps to improve the rate performance. Keywords: Cathode material; LiNi0.5Mn1.5O4; High voltage; Cycling performance; Surface modification

1. Introduction In order to handle with environmental issues such as global warming and air pollution, it is necessary to develop sustainable green energy technologies. It is necessary to develop rechargeable batteries with high energy and power density to meet the needs of grid energy storage systems, portable electronic devices and electric vehicles. Among current commercial secondary batteries, lithium ion batteries have high energy density, good electrochemical efficiency, long cycle life, and good environmental friendliness, and have become the most widely used secondary batteries.[1-4] Currently, the energy density of lithium ion batteries is limited by the cathode material. Among the cathode materials, high-voltage spinel LiNi0.5Mn1.5O4 is considered to be a promising cathode material for next-generation lithium-ion batteries.[5-9] Due to the high potential platform (4.7 V) and high specific capacity (147 mAh g-1), the theoretical energy density of LiNi0.5Mn1.5O4 is higher than 600 Wh·kg-1. LiNi0.5Mn1.5O4 has spinel structure with Li and O atoms occupying 8a and 32e positions, respectively. LiNi0.5Mn1.5O4 contains disordered and ordered phases.[10, 11] When calcined at high temperatures above 700 °C, loss of oxygen occurs with the appearance of Mn3+, resulting in disordered phase where Ni and Mn atoms occupy the 16d position. The loss of oxygen is reversible and can be recovered by annealing at 700 °C, which can also reduce Mn3+ content, resulting in ordered 2


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phase.[10-12] Besides, the Mn3+ and disorded phase content in the spinel can be increased by cation doping.[15,

16]

It is generally believed that LiNi0.5Mn1.5O4 containing Mn3+ and

disordered phase has higher lithium ion diffusion rate and higher electron conductivity, and therefore has good rate performance.[15, 17, 18] Currently, the main challenges for LiNi0.5Mn1.5O4 are poor long cycling and high temperature cycling performance due to the high voltage Ni2+/4+ redox and Mn3+ ions. The high voltage Ni2+/4+ redox leads to the decomposition of the electrolyte.[17, 18] The main salt in the carbonate-based electrolyte is LiPF6. LiPF6 decomposes into PF5 and LiF, and PF5 further reacts with residual water to form HF and POF3 species. HF corrodes the electrode material. In addition to lithium salts, carbonate-based solvent can react at the surface of high-voltage electrodes, decomposing to form -ROCO2 species. The decomposition product forms an electrolyte membrane on the surface of electrode material. The Mn3+ ions in LiNi0.5Mn1.5O4 are unstable and prone to disproportionation reaction: 2Mn3+ = Mn2+ + Mn4+. The generated Mn2+ ions then dissolve in the electrolyte. The dissolved transition metal ions migrate toward the anode and deposit on the anode surface. The dissolution of manganese leads to loss of capacity.[5, 21, 22] One effective way to solve the above problem of poor cycling performance is surface coating approach.[23-28] Surface coating can reduce the electrode/electrolyte contact area and reduce the electrolyte corrosion to the electrode material. The commonly used coating material is metal oxides, such as Al2O3, ZnO, ZrO2 and so on, because the metal oxides can inhibit the hydrofluoric acid corrosion. However, some surface coating layer blocks the transport of ions and electrons, thereby impairing the electrochemical performance of LiNi0.5Mn1.5O4. Besides ， the doping of Co in the spinel can improve electronic conductivity and structural stability.[29, 30] The Ti surface3



doping method that minimizes Mn dissolution from LiMn2O4 is reported.[31] This method takes advantage of both bulk doping and surface coating, and enhances electrochemical performance of LiMn2O4. Here, LiNi0.5Mn1.5O4 material is surface modified by cobalt at 500 °C and 700 °C, respectively. The as-prepared samples are studied by scanning transmission X-ray microscopy (STXM), TEM, Raman, XPS and electrochemical tests. Surface modification at 500 °C leads to cobalt oxide coating on the surface of LiNi0.5Mn1.5O4 and electrochemical performance is not improved. Surface modification at 700 °C leads to cobalt surface doping and electrochemical performance is improved obviously.

2. Experimental 2.1 Preparation of LNMO LiNi0.5Mn1.5O4 material is prepared by Na2CO3 co-precipitation in continuous stirred tank reactor (CSTR). The metal salt solution (0.5 mol L-1 NiSO4 and 1.5 mol L-1 MnSO4) and the precipitant solution (2 mol L-1 Na2CO3 and 0.08 mol L-1 ammonia) are simultaneously added in the base solution (0.04 mol L-1 NH4HCO3) at a rate of about 1 mL min-1. The solution temperature is kept 55 °C. The pH value is 7.5±0.2, which is controlled by changing the precipitant entering speed. After the dropwise addition, the solution is stirred at 55°C for 12 h. The carbonate precursor is obtained after filtration. Then the carbonate precursor is pre-calcined at 800 °C for 5 h to obtain nickel manganese oxides. The nickel manganese oxides and LiOH are mixed and calcined at 800 °C for 12 h, then cooled at 0.5 °C min-1 in air to obtain LiNi0.5Mn1.5O4 material, which is denoted as LNMO. 2.2 Surface modification LiNi0.5Mn1.5O4 material is surface modified by cobalt as follows. 2 g of LNMO is dispersed into 20 mL of Co(NO3)2·6H2O (0.12 g) solution. The solution is stirred and 4


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evaporated. The obtained mixture is calcined at 500 °C and 700 °C for 1 h, respectively, and cooled with the furnace in air. The surface modified samples are denoted as Co-500 and Co-700, respectively. 2.3 Characterization The obtained material was characterized by scanning electron microscope (SEM) using a Quanta-200 and transmission electron microscopy (TEM, FEI TecnaiF30FEG). Powder X-ray diffraction (XRD) was performed with a D/max-RB diffractometer using Cu Kα source. And the Raman spectra were obtained with a Renishaw in Via Raman microscope. X-ray photoelectron spectroscopy (XPS) was carried out by using the Physical Electronics PHI model 5700 instrument. The samples were analyzed by inductively coupled plasma (ICP, PerkinElmer, Optima 5300DV) tests. Scanning transmission X-ray microscopy (STXM) was performed at the SM beamline of Canadian Light Source (CLS). The monochromatic X-ray beam is focused by a Fresnel zone plate lens to a ~30 nm spot on the sample. Chemical imaging and XANES spectra are obtained using image sequence (stack) scans over a range of photon energies across the Mn L3-edge and O K-edge. STXM data were analyzed by aXis200 (http://unicorn.mcmaster.ca/aXis2000.html). Electrochemical tests of the asprepared samples were carried out by using coin-type cells (2025). The cathode material of the cell was made from a slurry containing 80 wt.% active material, 10 wt.% conductive acetylene black as conductive agent and 10 wt.% polyvinylidene fluoride (PVDF) as binder dissolved in n-methyl pyrrolidinone. The slurry was evenly coated onto an aluminum foil and then dried in a vacuum oven at 120 oC for overnight. Then the foil was punched into a circular electrode (1.4 cm in diameter). The loading weight of the active material on the electrode was about 3 mg cm-2. Cell with lithium metal as the counter electrode was assembled in an argon-filled glove box. The 5



electrolyte was 1 mol L-1 LiPF6 in a mixture of ethylene carbonate and dimethyl carbonate with volume ratio of 1:1. Charge-discharge tests were carried out on a NEWWARE battery tester. When the current densities were higher than 0.5 C, the cells were charged galvanostatically to 4.95 V first. Then the cell voltage was kept at 4.95 V until the current decreased to 0.1 C. Then the cell was discharged to 3.5 V at different rates. The coin cell was tested by cyclic voltammetry (CV; 3.5-5.1 V, 0.1 mV s-1) and electrochemical impendence spectroscope (EIS) measurements were performed on a CHI660E electrochemical workstation. EIS measurements were conducted with AC amplitude of 5 mV at 4.73 V in the frequency range from 105 Hz to 0.01 Hz.

3. Results and discussion LiNi0.5Mn1.5O4 (LNMO) material is prepared by Na2CO3 co-precipitation method. Figure 1 shows the SEM micrographs of carbonate precursors, nickel manganese oxides and LNMO. The carbonate precursors prepared by Na2CO3 coprecipitation method are spherical particles with particle size of about 10 μm. The nickel manganese oxides obtained after pre-calcination are still spherical particles with porous structure. When LiOH is mixed, lithium can enter the pores, which is favorable for more uniform lithium dispersion. The primary nickel manganese oxides particles are approximately 300 nm in size. LNMO materials obtained by high-temperature calcination after lithium mixing are spherical particles, but some of the particles are broken. LNMO primary particle size significantly increased, reaching about 1 μm. And the shape of LNMO primary particle transforms to the spinel octahedron. The molar ratios of Ni/Mn in LNMO were tested by ICP. The ICP results show that the molar ratio of Ni/Mn in LNMO is 0.49/1.51, close to the designed formulas.

6


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Figure 1. SEM micrographs of (a, b, c) carbonate precursor, (d, e, f) nickel manganese oxides and (g, h, i) LiNi0.5Mn1.5O4 prepared by Na2CO3 co-precipitation method.

The surface modification method is to disperse the LNMO particles in a cobalt nitrate solution, and then the solvent is evaporated and calcined at high-temperature. The molar ratio of Co and LiNi0.5Mn1.5O4 is ~4%. The surface modified samples calcined at 500 °C and 700 °C are denoted as Co-500 and Co-700, respectively. Figure 2 shows the SEM and EDS mapping of samples after cobalt surface modification. It can be seen that the surface modification does not change the morphology of LiNi0.5Mn1.5O4. From Figure 2(c, d), it can be seen that the nickel and manganese elements are distributed uniformly in the LNMO, indicating that the Na2CO3 coprecipitation method can uniformly mix the nickel and manganese elements. From the cobalt EDS mapping, it can be seen that the cobalt element is evenly distributed.

7



XRD patterns of the samples are shown in Figure 2e. Surface modification has little effect on XRD patterns. The XRD patterns of all the samples are representative of the LiNi0.5Mn1.5O4 structures (JCPDS Card No.: 80-2162).

Figure 2. (a)SEM images and (b,c,d) EDS mapping of Co-700; (e) XRD patterns of samples LNMO, Co-500 and Co-700.

TEM images of LNMO, Co-500, and Co-700 samples are shown in Figure 3. Comparing with LNMO and Co-500, it can be seen that there is a coating on the surface of LiNi0.5Mn1.5O4 after surface modification at 500 °C. The coating thickness is about 10 nm. The coating is the cobalt oxide transformed from cobalt nitrate. However, the coating on the surface disappeared after surface modification at 700 °C, as shown in Figure 3c. This indicates that cobalt enters into the LiNi0.5Mn1.5O4 surface layer, leading to surface doping, at higher calcining temperature. The select area electron diffraction of sample Co-700 is corresponding to [001] zone of spinel.

8


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Figure 3. TEM images of (a) LNMO, (b,d) Co-500 and (c,e) Co-700; (f)select area electron diffraction of Co-700; (g) Raman spectra of samples.

Samples are tested by Raman spectroscopy to evaluate the cation ordering in spinel. The results are shown in Figure 3d. The peaks at 405 and 496 cm-1 are related to the Ni2+-O mode. The peaks at 640 cm-1 are related to the symmetric Mn-O mode of 9



MnO6 group. All samples show splitting of peaks at the 588-623 cm-1 region, which are characteristic of the cation ordered phase.[32,33] The peak split of the sample Co-700 is relatively insignificant, indicating that the cobalt surface doping reduces the cation ordering. This is because the surface doping introduces disordered phase in LiNi0.5Mn1.5O4. Besides, sample Co-500 shows a peak at 691 cm-1, which is assigned to Co3O4. This shows that there is a Co3O4 coating on the surface of LiNi0.5Mn1.5O4 in sample Co-500. However, the peak at 691 cm-1 does not exist in sample Co-700, indicating that the Co3O4 coating disappeared. It is because that cobalt enters into the LiNi0.5Mn1.5O4 surface layer during 700 °C calcination. TEM and Raman results show that surface modification at 500 °C leads to cobalt oxide coating on the surface of LiNi0.5Mn1.5O4 and surface modification at 700 °C leads to cobalt surface doping. In order to study the surface chemistry of the samples, the LNMO, Co-500 and Co-700 samples are tested by XPS. The results are shown in Figure 4(a~c) and Table 1. It can be seen that the content of Ni and Mn are significantly reduced after surface modification, and cobalt peaks appear. The decrease of the surface Ni and Mn concentration can reduce the decomposition of the electrolyte and Mn dissolution. It is beneficial to improve the cycle performance. When the surface modification temperature is increased from 500 °C to 700 °C, the cobalt content decreases (Figure 4c). This is because cobalt enters into the LiNi0.5Mn1.5O4 surface layer during 700 °C calcination, which is also revealed by TEM and Raman results.

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Figure 4. XPS spectra of (a) Mn, (b) Ni and (c) Co of samples; Mn 2p3/2 XPS curve fitting of (d) LNMO, (e) Co-500 and (f) Co-700. Table 1 The relative content (%) of Mn, Ni and Co obtained by XPS test Sample

LNMO

Co-500

Co-700

Mn

5.09

3.7

4.42

Ni

1.29

1.1

1.22

Co

0

1.57

1.08

In addition, the surface Mn3+ and Mn4+ contents are compared by fitting the Mn 2p3/2 XPS curve. The results are shown in Figure 4(d~f). The binding energies of Mn4+ and Mn3+ are 642.9 and 641.9 eV, respectively. The ratios of the peak areas of Mn4+ and Mn3+ of LNMO, Co-500 and Co-700 are 1.24, 1.16 and 1.66, respectively, indicating that surface doping at 700 °C decrease the surface Mn3+ content. The XANES spectra at Mn L3-edge and O K-edge was collected from the surface and subsurface toward the center of the LNMO particles as the internal reference spectra as shown in Figure 5(a,b) and Figure 5(d,e) for pristine and modified LNMO, respectively . These reference spectra were used to fit STXM image stacks to 11



get components distribution map as shown in Figure 5(c) and Figure 5(f) for the pristine and modified LNMO, respectively. The O K-edge spectra show peaks A and B at 530 and 532 eV, corresponding to the hybridization of the O 2p and metal 3d orbitals. And the broad peaks C and D arise from the hybrid states of O 2p with metal 4sp character.As shown in Figure 5(a,b), the spectra at the “surface” and the “bulk” have curves of the same shapes (note that the pixel size in STXM is about 40 nm so the term of surface is just relatively correct). Mn spectra indicate a mixture of Mn4+ and Mn3+ oxidation state in this sample. The difference between surface and bulk at Mn site is mainly due to a thickness relevant distortion rather than structural difference. This indicates that unmodified LNMO has the same electronic and chemical structures both at the surface and in the bulk. However, doping with Co causes the difference between surface and bulk as shown in Figure 5(f). The surface O K-edge XANES spectrum of Co-700 has a different shape, especially in the decreased peak intensity at peak B than that in the bulk. This must be caused by the surface Co doping. Furthermore, in Figure 5d, a much pronounced peak at 643.5 eV corresponding to Mn4+ shows in the surface but not in the bulk, indicating that the manganese in the surface of the modified Co-700 is mainly Mn4+.[34-38]

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Figure 5. (a) Mn L3-edge XANES, (b) O K-edge XANES, (c)STXM image of LNMO and (d) Mn L3-edge XANES, (e) O K-edge XANES, (f)STXM image of Co-700.

The XPS and XANES results indicate the oxidation state of Mn in surface is increased by cobalt surface doping. The decrease of Mn3+ content favors the cycle performance

of

the

LiNi0.5Mn1.5O4

material.

To

study

the

electrochemical

performances, coin cells with lithium metal as the anodes are tested. Figure 6a shows charge and discharge curves at 0.2 C for different samples. The samples have two charging plateaus at around 4.7 V, corresponding to Ni2+/Ni3+ and Ni3+/Ni4+ redox couples, respectively. The Ni2+/Ni3+ plateau of sample Co-700 is lower than 4.7 V, which indicates the introduction of disordered phase.[14,

38]

There are small plateau

around 4 V, corresponding to the Mn3+/Mn4+ redox couple. The 4V plateau can be used to compare the Mn3+ content. Sample Co-700 with longer 4V plateau contains more Mn3+ ions.

13



Figure 6. (a) Charge and discharge profiles at 0.2 C of samples; Cyclic voltammetry curves of samples (b) LNMO, (c) Co-500, (d) Co-700 before and after 200 cycles at 1 C rate.

Cyclic voltammetry tests are performed on different samples before and after 200 cycles of 1 C. The results are shown in Figure 6(b~d). The samples have redox peaks at around 4.7 V, corresponding to the Ni2+/Ni4+ redox. Co-700 has two small peaks at around 4.7 V, because the surface doping introduces disordered phase in LiNi0.5Mn1.5O4.[14,38] The CV curves of the sample Co-700 shows small change before and after the cycle, indicating that it has better cycle stability. A small peak of about 4 V corresponds to the Mn3+/Mn4+ pair. The 4 V peak area of Co-700 is larger, indicating a higher content of Mn3+. Cobalt doping induces part of Mn4+ ions to be reduced to Mn3+ ions due to charge neutrality and the Mn3+ formation increases cation disorder phase content.[15] 14


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The results of Raman, charge-discharge curve and CV show that the cobalt surface doping increases the disordered phase and Mn3+ content in LiNi0.5Mn1.5O4. This helps to improve the rate performance. Figure 7a shows the specific capacities at different current densities. With the increase of current density, the specific capacities of unmodified LNMO decrease rapidly. And the discharge specific capacity is only 32 mAh g-1 at a large rate of 20 C, which keeps 25 % of the 0.2 C discharge capacity. After the surface doping at 700 °C, the rate performance is significantly improved. The discharge specific capacity of sample Co-700 at large rate of 20 C is 69 mAh g-1, which keeps 54 % of the 0.2 C discharge capacity. It is also useful to compare rate performance by the charge and discharge curves at large current density. Charge and discharge capacity curves at the rate of 5 C are shown in Figure 7b. Sample Co-700 shows the smallest polarization, that is, the best rate performance. The improvement is attributed to the presence of disordered phase and Mn3+ in Co-700, which increase the electronic conductivity and lithium ion diffusion of the material.[15, 17, 18]

15



Figure 7. (a) Rate capabilities and (b) Charge and discharge capacity curves at 5 C rate of samples; Cycling performances at (c) 1 C, (d) 55 oC, 1 C and (e) 5 C of samples LNMO, Co500 and Co-700.

In order to compare the long cycle performance and high temperature performance, the samples are tested at 1 C, 1C at a high temperature of 55 °C and 5 C. The results are shown in Figure 7(c~e). It can be seen that the cycling performance of LNMO and Co-500 is poor. The cycling performance of sample Co-700 is obviously 16


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improved. After 200 cycles at 55 °C, the capacity retention rate increases from 82 % to 93 %. After 1000 cycles at 5 C, the capacity of sample LNMO is 55 mAh g-1 and the capacity retention is 61 %. The capacity of sample Co-700 after 2000 cycles at 5 C is 93 mAh g-1, and the capacity retention is 81 %. Sample Co-700 shows excellent long cycling performance, better than most of reported results.[7,19,39] The long cycle performance and high temperature performance are improved because the surface modification reduces the content of Ni and Mn3+ on the surface, which reduce the decomposition of the electrolyte and the dissolution of manganese. Electrochemical impedance tests are performed on the cells before and after 200 cycles of 1 C. The results are shown in Figure 8. The depressed semicircles in the high frequency region reflect the interface impedance of the interface layer and the charge transfer reaction. The slope in the low frequency region is related to the diffusion of lithium ions in the electrode. Impedance fitting is performed using the equivalent circuit shown in the inset of Figure 8a. Rs is the electrolyte resistance. RSEI and CSEI represent the resistance and geometric capacitance of the electrode/electrolyte interface, respectively. Rct represents the charge transfer resistance. C is the double layer capacitance. ZW is the Warbugr diffusional impedance. The fitting results are listed in Table 2. It can be seen that the Rct of Co-500 is higher than that of unmodified LNMO. This is because the sample Co-500 has coating layer that impedes surface charge transfer. The Rct and RSEI of LNMO and Co-500 increase significantly after the cycle. The Rct and RSEI of the sample Co-700 is the smallest before and after the cycle, and the increase is far less after the cycle. This is because Co-700 contains less Ni in the surface. It inhibits electrolyte decomposition, thus reduces the surface film formation. And cobalt surface-doped increases the content of Mn3+ and disordered phases in Co700, so that the electronic conductivity and ionic conductivity are improved.[15, 17, 18] 17



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Figure 8. EIS of samples (a) LNMO, (b) Co-500 and (c) Co-700 before and after 200 cycles at 1 C rate. Table 2 Fitting results for the impedance spectra in Figure 8 Sample

LNMO

Co-500 Co-700

RSEI before cycle

9.0

5.1

3.6

RSEI after cycle

17.6

15.8

4.5

Rct before cycle

46.1

52.3

28.4

Rct after cycle

68.7

78.4

34.5

4. Conclusions The LiNi0.5Mn1.5O4 material is surface modified by cobalt to improve its electrochemical performance. The surface modification method is to disperse the LNMO particles in cobalt nitrate solution, and then the solvent is evaporated and calcined at 500 °C and 700 °C, respectively. After surface modification at 500 °C, there is cobalt oxide coating on the surface of LiNi0.5Mn1.5O4. While after surface modification at 700 °C, the coating enters into the LiNi0.5Mn1.5O4 surface layer, leading to surface doping, as revealed by TEM, Raman and XPS results. The cobalt surface doping decreases the Ni concentration and increases the oxidation state of Mn on the surface, as revealed by the XPS and X-ray absorption near-edge structure spectroscopy. This gives LiNi0.5Mn1.5O4 excellent long cycle performance and high temperature cycling performance. After 2000 cycles at 5 C, the capacity of Co-700 is 93 mAh g-1 with 18


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capacity retention of 81 %. After 200 cycles at 1C under 55 oC, the capacity retention rate increases from 82 % to 93%. Besides, according to the results of Raman, chargedischarge curve and CV tests, the surface doping increases the disordered phase and Mn3+ content in LiNi0.5Mn1.5O4, which helps to improve the rate performance.

Acknowledgements We acknowledge the National Natural Science Foundation of China (Grant No. 21273058 ， 21673064 and 51802059), China postdoctoral science foundation (Grant No.

2017M621285

and

2018T110292),

Harbin

technological

achievements

transformation projects (2016DB4AG023) and Heilongjiang provincial university scientific and technological achievements industrialization support project (2018) for their financial support.

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Improving electrochemical performance of high-voltage spinel LiNi0

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