High-capacity lithium-rich layered sulfide positive electrode material, preparation method thereof and application of high-capacity lithium-rich layered sulfide positive electrode material in lithium ion battery

Abstract

The invention relates to a novel layered sulfide positive electrode material of a lithium ion battery, which has a general formula of Li (1.33-0.33 x) VxTi (0.67-0.67 x) S2, and x is more than or equal to 0 and less than or equal to 0.5. The positive electrode material belongs to a hexagonal system O3 type crystal structure, the corresponding space group is # imgabs0 #, the microstructure of the positive electrode material is granular, the particle size of the particles is about 20-50 microns, and all elements are uniformly distributed on the surfaces and inside the particles. In the positive electrode material, almost all capacities come from the oxidation-reduction reaction of sulfur, and the capacity contribution of vanadium can be almost ignored. An electrochemical test and a solid-state nuclear magnetic characterization result show that the Li1. 27V0. 2Ti0. 53S2 positive electrode material has the reversible capacity as high as 308mAh g <-1 >, and reversible de-intercalation of all Li < + > in a system is realized. Meanwhile, the material shows extremely excellent rate and cycle performance, still has a capacity retention ratio of 81.0% after 4300 cycles, and almost has no voltage attenuation in the cycle process.

Description

Technical Field

[0001] The invention relates to a lithium-rich layered sulfide positive electrode material, a preparation method and application in a lithium-ion battery, and belongs to the technical field of lithium-ion battery positive electrode materials. Background Art

[0002] Lithium-ion batteries are considered to be one of the most promising portable electrochemical energy storage devices due to their high capacity, high specific energy and long life. Lithium-rich layered cathode materials are a new type of cathode materials, among which the most representative are lithium-rich manganese-based cathode materials (xLi 2 MnO 3 (1–x)LiTMO 2 , TM = Ni, Mn, Co, etc.), these materials usually have a capacity of more than 350 mAh g -1 Theoretical capacity and 350mAh g -1 Reversible capacity is considered to be one of the most commercially promising lithium-ion cathode materials. Its higher capacity is not only due to the redox of transition metals, but also due to the unique anion redox mechanism. This redox mechanism involving lattice oxygen provides more capacity for lithium-rich layered cathode materials, which is expected to achieve reversible intercalation and deintercalation of more than one lithium ion. The redox of lattice oxygen at higher potentials is usually not completely reversible, that is, the oxidized oxygen ions will be partially converted into peroxide ions, superoxide ions and even molecular oxygen. This irreversible reaction will lead to severe structural rearrangement and transition metal migration, causing the collapse of the layered structure and irreversible phase change. Summary of the invention

[0003] The technical problem to be solved by the present invention is that when the existing lithium-rich layered oxide positive electrode materials are used as positive electrode materials for lithium-ion batteries, there are problems such as lattice oxygen loss, transition metal migration, and kinetic retardation. The present invention proposes a lithium-rich layered sulfide positive electrode material with higher specific capacity and better electrochemical performance.

[0004] A lithium-rich layered sulfide positive electrode material, the general formula of which is Li 1.33-0.33x V x Ti 0.67-0.67x S 2 , and 0<x≤0.5, it belongs to the hexagonal O3 type crystal structure, and the corresponding space group is

[0005] 0.1<x≤0.4, 0.15<x≤0.25.

[0006] With 430-480Wh kg -1 energy density.

[0007] The preparation method of the above-mentioned lithium-rich layered sulfide cathode material comprises the following steps:

[0008] Step 1: Mix Li 2 S powder, vanadium powder, titanium powder and sulfur powder, grind them, and perform tablet pressing treatment;

[0009] Step 2: After isothermal heat treatment, quench to obtain the cathode material.

[0010] The heat treatment temperature is 600 - 800 °C, and the time is 20 - 40 h.

[0011] During the heat treatment process, the vacuum degree is maintained at 10 -4 -10 -6 mbar or so.

[0012] The application of the above-mentioned layered lithium-rich sulfide cathode material in a lithium-ion battery.

[0013] In the lithium-ion battery described above, a lithium metal negative electrode is used, which has a charge-discharge capacity retention rate of 85 - 90% after 4300 cycles.

[0014] Beneficial effects

[0015] Compared with the lithium-rich layered oxide cathode material, the lithium-rich sulfide cathode material can form a stronger metal-ligand covalent interaction with common 3d transition metal ions and achieve a highly reversible anion redox reaction, so that the lithium-rich sulfide cathode material has a more stable structure and a higher specific capacity, overcoming the adverse problems related to the anion redox reaction. The lithium-rich layered cathode material can usually only reversibly deintercalate and intercalate 1 Li + during charge and discharge, which has not reached its theoretical capacity, so there is still a large room for improvement. The new material Li 1.27 V 0.2 Ti 0.53 S 2 involved in the present invention can exhibit a reversible capacity as high as 308 mAh g -1 , realizing the completely reversible deintercalation and intercalation of all Li + in the system. Li 1.27 V 0.2 Ti 0.53 S 2During the charge-discharge process, sulfur mainly participates in charge compensation, and the capacity contribution of vanadium is almost negligible. In addition, the positive electrode material still has an initial capacity retention rate of 80.3% after 4300 long cycles, and there is almost no voltage decay during the cycling process. These electrochemical properties are superior to most of the reported lithium-rich layered positive electrode materials at present. In-situ and ex-situ XRD tests show that there is no obvious phase change in the material during the charge-discharge process, which is a solid solution behavior. At the same time, the reversible deintercalation and intercalation processes of lithium in the lithium layer and lithium in the transition metal layer during the charge-discharge process are also observed by solid-state nuclear magnetic resonance. Description of the Drawings

[0016] Figure 1 : A series of prepared Li 1.33-0.33x V x Ti 0.67-0.67x S 2 (0 ≤ x ≤ 0.5) XRD patterns of the samples.

[0017] Figure 2 : Li 1.27 V 0.2 Ti 0.53 S 2 Rietveld refined XRD patterns of the samples.

[0018] Figure 3 : Li 1.27 V 0.2 Ti 0.53 S 2 Microscopic morphology characterization of the powder samples. a. SEM image at low magnification. b. SEM image at high magnification.

[0019] Figure 4 : Li 1.27 V 0.2 Ti 0.53 S 2 TEM characterization of the samples. a. HAADF-STEM image along the

[010] direction, and the bright spots are transition metals (V, Ti) and sulfur atoms. The corresponding O3-type crystal model diagram is on the right, where the blue, yellow, and green spheres represent transition metal (V, Ti) atoms, sulfur atoms, and lithium atoms, respectively. b. ABF-STEM image along the

[010] direction, and the corresponding O3-type crystal model diagram is on the right. c-g. HAADF images and the corresponding EDS spectra of Ti, V, and S.

[0020] Figure 5 : Li 1.33-0.33x V x Ti 0.67-0.67x S 2 (0 ≤ x ≤ 0.5) Electrochemical performance characterization. a-f. A series of components with different vanadium doping contents (0 ≤ x ≤ 0.5) at 20 mA g–1 The first three charge-discharge capacity-voltage curves obtained by testing in the voltage range of 1.8 - 3.0 V at a current density.

[0021] Figure 6 : A series of Li 1.33-0.33x V x Ti 0.67-0.67x S 2 (0 ≤ x ≤ 0.5) The theoretical capacity calculated by removing all lithium ions from the material and the true specific capacity obtained by actual testing at a small current density of 20 mA g -1 . When x = 0.2, the actual capacity and the theoretical capacity of the corresponding component are almost equal.

[0022] Figure 7 : The rate performance of a series of Li 1.33-0.33x V x Ti 0.67-0.67x S 2 (0 ≤ x ≤ 0.5) cathode materials. Among them, the component corresponding to x = 0.2 (Li 1.27 V 0.2 Ti 0.53 S 2 ) has the best rate performance and can still maintain a capacity of 217.5 mAh g -1 at a current density of 1000 mA g -1 .

[0023] Figure 8 : In-situ S 2p XPS spectra of the Li 1.27 V 0.2 Ti 0.53 S 2 cathode. The first charge-discharge curve on the left and the different charge-discharge states selected for testing the XPS spectra are shown.

[0024] Figure 9 : XAS spectra of the Li 1.27 V 0.2 Ti 0.53 S 2 cathode material in the pristine, charged, and discharged states. (a) Normalized Ti K-edge XANES spectra. (b) k 3 -weighted Fourier-transformed Ti K-edge EXAFS spectra.

[0025] Figure 10 : Li 1.27 V 0.2 Ti 0.53 S 2Ex-situ XAS spectroscopic characterization of the cathode material. a. Different charge-discharge states selected during the first charge-discharge process for testing XANES spectra. b. Normalized V K-edge XANES spectra at different charge-discharge states. (c) k 3 Weighted Fourier-transformed V K-edge EXAFS spectra.

[0026] Figure 11 : Li 1.27 V 0.2 Ti 0.53 S 2 7Li MAS ssNMR spectra of the cathode material at the original state, charged to 2.8 V, charged to 3.0 V, discharged to 2.5 V, and discharged to 1.8 V.

[0027] Figure 12 : Li 1.27 V 0.2 Ti 0.53 S 2 Long cycle performance of the electrode. a. Capacity retention and corresponding Coulombic efficiency measured at a current density of 1000 mA g -1 . b. Energy density and corresponding energy efficiency measured at a current density of 1000 mA g -1 . Detailed implementation mode

[0028] Example 1

[0029] Li 1.33-0.33x V x Ti 0.67-0.67x S 2 , (0 ≤ x ≤ 0.5) synthesis of the material. Taking Li 1.27 V 0.2 Ti 0.53 S 2 as an example, samples were prepared by an improved solid-state synthesis method.

[0030] First, stoichiometric amounts of Li 2 S powder, vanadium powder, titanium powder, and sulfur powder were thoroughly ground in a mortar. After mixing evenly, they were pressed into a sheet-shaped precursor with a diameter of 16 mm using a tablet press mold. Then the precursor was sealed in a quartz tube, and the vacuum was maintained at about 10 -5 mbar. Subsequently, the sealed precursor was placed in a tube furnace, slowly heated to 750 °C, and annealed for 35 h. Immediately after the annealing was completed, it was quenched in cold water to rapidly cool it to room temperature. Finally, the cooled sample was transferred to a glove box, thoroughly ground into powder, and stored for later use. Preparation of Li 1.27 V 0.2 Ti 0.53 S 2Contact with air should be avoided throughout the process.

[0031] Structure and morphology characterization

[0032] First, XRD characterization tests were carried out on a series of synthesized Li 1.33-0.33x V x Ti 0.67-0.67x S 2 (0 ≤ x ≤ 0.5) samples. The 2θ range of the test was 10 - 80°, and the test was as Figure 1 shown. The XRD pattern of the undoped Li 2 TiS 3 (x = 0) sample presented typical O3-type layered structure characteristics. Lithium ions occupied the octahedral sites in the lithium layer, and some lithium and titanium ions occupied the octahedral sites in the transition metal layer. It was noted that superlattice peaks appeared around 18 - 20°, which was due to the ordered structure of the LiTi 6 motif in the transition metal layer. With the introduction of vanadium ions (0.1 ≤ x ≤ 0.4), it was observed that the position of the main peak only shifted slightly, and no impurity phases were generated. This indicated that vanadium had been successfully doped into the Li 2 TiS 3 system. At the same time, it was noted that as the vanadium content increased, the signal of the superstructure peak gradually weakened and disappeared. This was because the introduction of vanadium ions disrupted the ordered structure of the transition metal layer. When the vanadium content increased to 0.4 ≤ x ≤ 0.5, it was observed that the (003) main peak split, indicating that further increasing the vanadium content would generate a small amount of impurity phases.

[0033] The Rietveld refinement of the XRD pattern of the Li 1.27 V 0.2 Ti 0.53 S 2 sample was carried out using GSASⅡ software, and the results were as Figure 2 and Table 1 showed. All the peaks in the XRD pattern corresponded to the space group. Vanadium ions replaced the position of titanium ions in the transition metal layer, and the positions of lithium, vanadium, and titanium ions between the transition metal layers were long-range disordered, so no superstructure peaks appeared in the XRD. The lattice parameters obtained by refinement were More detailed parameters are shown in Table 1. In addition, the ratios of various elements were verified by ICP test, and the results are shown in Table 2. The ratios of Li, V, and Ti in the Li 1.27 V 0.2 Ti 0.53 S 2 sample were consistent with the expectations. The normalized chemical formula was Li 1.27 V 0.2 Ti 0.53 S 2 .

[0034] Table 1 Li 127 V 02 Ti 053 S 2 XRD Rietveld refinement results of the samples.

[0035]

[0036] Table 2 Li obtained from ICP-AES results 1.27 V 0.2 Ti 0.53 S 2 Stoichiometric ratios of each element in the samples.

[0037]

[0038] The morphology of Li 1.27 V 0.2 Ti 0.53 S 2 samples was also characterized by SEM, such as Figure 3 shown. Li 1.27 V 0.2 Ti 0.53 S 2 The samples presented a granular morphology, and the particle size was mainly distributed in the range of 20 - 50 μm. Obvious layered structures could be seen in the magnified particles, which are common morphological features of layered cathode materials.

[0039] To further characterize the microcrystalline structure of Li 1.27 V 0.2 Ti 0.53 S 2 samples, advanced aberration-corrected transmission electron microscopy (STEM) combined with high-angle annular dark-field (HAADF) and annular bright-field (ABF) imaging was also used to directly observe the atomic arrangement order of Li 1.27 V 0.2 Ti 0.53 S 2 in the samples ( Figure 4 a - b). Among them, HAADF-STEM is mainly used for imaging samples containing heavy elements, while ABF-STEM can be used for imaging light elements. Figure 4 a of 1.27 V 0.2 Ti 0.53 S 2HAADF-STEM images of the sample, where the bright spots indicate the positions of heavy elements (transition metals and sulfur). The atomic arrangement in the figure conforms to the stacking sequence of the O3 layered structure and can perfectly coincide with the crystal model. The transition metal layer and the lithium layer are alternately arranged, and the layer spacing is about. Figure 4 Figure b is the ABF-STEM image of the corresponding area, and the bright spots in the figure indicate the occupation positions of lithium ions. In addition, energy-dispersive X-ray spectroscopy (EDS) was also used to study the distribution of various elements in the Li 1.27 V 0.2 Ti 0.53 S 2 sample in the sample particles, and the test results are as shown in Figure 4 c - f. The distribution of Ti, V, and S in the particles is uniform, indicating that the preparation has good homogeneity.

[0040] Electrochemical Characterization

[0041] To determine the optimal vanadium doping ratio, the prepared Li 1.33-0.33x V x Ti 0.67-0.67x S 2 (0 ≤ x ≤ 0.5) cathode materials were subjected to electrochemical tests. The cathode active material, conductive agent (acetylene black), and binder (PVDF) were uniformly mixed in a ratio of 7:2:1 to make a cathode electrode sheet, which was assembled with a lithium metal anode into a half-cell. The electrolyte used was 1.0 M LiPF 6 dissolved in a mixed solvent of ethylene carbonate (EC), dimethyl carbonate (DMC), and ethyl methyl carbonate (EMC) with a volume ratio of 1:1:1. The test voltage range was 1.8 - 3.0 V. First, a small current (20 mA g -1 ) was used for charge-discharge tests, as shown in Figure 5 . Consistent with expectations, the anionic redox activity of Li 2 TiS 3 is very low, and the sulfur redox plateau near 2.8 V can only contribute a reversible capacity of about 80 mAh g -1 , although its theoretical capacity is as high as 339 mAh g -1 ( Figure 5 a). After doping a small amount of vanadium (x = 0.1), the reversible capacity reached 140 mAh g -1 , indicating that vanadium doping can significantly activate the redox activity of sulfur ( Figure 5 b). When the doping content of vanadium ions increased to x = 0.2, the reversible capacity reached 308 mAh g -1 , which is almost close to that of Li 1.27 V 0.2 Ti 0.53 S 2The theoretical value of the positive electrode is 313 mAh g -1 (c) of Figure 5 This indicates that in the Li 1.27 V 0.2 Ti 0.53 S 2 For the positive electrode, lithium ions in both the transition metal layer and the lithium layer can be reversibly deintercalated and intercalated during small current tests. It was also noted that the charge-discharge curves basically coincided during subsequent cycles, with almost no attenuation of capacity and voltage, indicating that Li 1.27 V 0.2 Ti 0.53 S 2 has good structural stability. When the vanadium content is further increased (0.3 ≤ x ≤ 0.5), the reversible capacity slightly decreases. This may be because the increase in vanadium content is not conducive to the system maintaining the O3-type layered structure, which is consistent with the observed splitting trend of the main peak in the XRD pattern ( Figure 1 and Figure 5 d-f). At the same time, the relative atomic mass of vanadium is 50.942 and that of titanium is 47.867. Increasing the vanadium content will also lead to a decrease in the theoretical capacity. Nevertheless, these materials still exhibit reversible capacities as high as 287 mAh g -1 (x = 0.3), 278 mAh g -1 (x = 0.4) and 242 mAh g -1 (x = 0.1), and there is no attenuation of capacity and voltage during subsequent cycles.

[0042] The measured capacities and theoretical capacities of all components in the Li 1.33-0.33x V x Ti 0.67-0.67x S 2 (0 ≤ x ≤ 0.5) materials were summarized as shown in Figure 6 . Among them, the theoretical capacity corresponds to the maximum capacity calculated by removing all lithium ions in the components (including the lithium layer and the transition metal layer), while the measured capacity was tested in the voltage range of 1.8 - 3.0 V at a current density of 20 mA g –1 . As the vanadium ion doping content increases, the theoretical capacity gradually decreases from 339 mAh g -1 (x = 0) to 273 mAh g -1 (x = 5), which is due to the increase in the relative molecular mass of the components caused by the increase in vanadium content. The measured capacity shows a trend of first increasing and then decreasing as the vanadium ion doping content increases. Among them, the component corresponding to x = 0.2 (Li 1.27 V 0.2 Ti 0.53 S 2) The experimental value of the specific capacity reached the maximum and was almost equal to the theoretical value. This preliminarily indicates that the composition corresponding to x = 0.2 is the optimal doping ratio.

[0043] Li 1.33-0.33x V x Ti 0.67-0.67x S 2 (0 ≤ x ≤ 0.5) The rate performance of the cathode material was tested at current densities of 20, 50, 100, 200, 500, and 1000 mA g -1 , and the test results are as Figure 7 shown. The initial capacity of the Li 2 TiS 3 sample was relatively low, only 78 mAh g -1 , and as the current density increased to 1000 mA g -1 , the capacity gradually decreased to 23 mAh g -1 . As the vanadium component gradually increased to x = 0.2, the rate performance was significantly improved. Among them, the specific capacities obtained under the tests at current densities of 20, 50, 100, 200, 500, and 1000 mA g -1 were 308, 289.3, 276.1, 262.6, 238.8, and 217.5 mAh g -1 respectively, and the corresponding capacity retention rates were 100%, 94%, 90%, 85%, 78%, and 71%. When the current returned to 20 mA g -1 again, the capacity fully recovered to the initial capacity level, showing extremely excellent rate performance. Other components (0.3 ≤ x ≤ 0.5) also showed rate performance, but only with a decrease in capacity.

[0044] Research on the charge compensation mechanism

[0045] Based on the previous electrochemical characterization tests, it was preliminarily determined according to the redox platform that the redox reaction of sulfur in the Li 1.33-0.33x V x Ti 0.67- 0.67x S 2 (0 ≤ x ≤ 0.5) cathode material was activated by the doped vanadium ions, and showed a trend of first increasing and then decreasing with the increase in the content of vanadium ions. To explore the charge compensation mechanism in the material, the cathode material with the optimal doping ratio of Li 1.27 V 0.2 Ti 0.53 S 2 was selected as the research object for subsequent research. The Li 1.27 V 0.2 Ti 0.53 S 2The valence state change of sulfur in the cathode material during charge and discharge is as follows Figure 8 shown. XPS technology is an important semi - quantitative elemental analysis technique that can be used to analyze the chemical state of elements. Five points were selected to test the ex - situ S2p XPS spectra of the Li 1.27 V 0.2 Ti 0.53 S 2 cathode material, corresponding to different states on the charge - discharge curve on the left. The right side shows the fitting data of the S2p XPS spectra. For the original sample, the S2p XPS spectra can be fitted into a double - peak of 161.5 eV (2p 3 / 2 ) and 162.5 eV (2p 1 / 2 ), which belongs to the chemical state of S 2- , consistent with the design and expectation. As the charging process proceeds, the XPS spectra show a trend of shifting to higher binding energy, and a second set of double - peaks are fitted at 163.0 eV and 164.0 eV, corresponding to the chemical state of S m- (1 < m < 2). This indicates that sulfur is gradually oxidized during the charging process. At the fully charged state, the proportion of the S m- (1 < m < 2) chemical state reaches the maximum value. During the subsequent discharging process, the proportion of the S m- (1 < m < 2) chemical state gradually decreases again, indicating that sulfur is gradually reduced. When the full discharge ends, the peak of the S m- (1 < m < 2) chemical state completely disappears, and only the peak of the S 2- chemical state remains, consistent with the initial state. The XPS spectra of S2p during the whole charge - discharge process show completely reversible changes, that is, the redox of sulfur has good reversibility. According to the XPS analysis results, it has been verified that sulfur provides charge compensation during charge and discharge. According to the material design and the references consulted, the valence states of V, Ti, and S in the Li 1.27 V 0.2 Ti 0.53 S 2 system are +3, +4, and -2 respectively. Among them, Ti with a +4 valence cannot be further oxidized because it has no d electrons in its outermost layer. While V with a +3 valence has a d 2 configuration, so V may participate in the oxidation reaction and be oxidized to +4 or even +5 valence. In order to explore whether the transition metals Ti and V also participate in the redox reaction, ex - situ XAS tests were carried out using synchrotron radiation light source ([[]] Figure 9 and Figure 10 ). Figure 9 What is shown is the Li 1.27 V 0.2 Ti 0.53 S 2XAS spectra of titanium element obtained from the tests of the cathode material in the pristine state, charged state, and discharged state. It can be seen from the XANES spectra that during the charging and discharging processes, the Ti K-edge absorption edge hardly shifts. This indicates that the valence state of Ti remains unchanged (+4 valence) during charge and discharge, that is, it does not participate in charge compensation ( Figure 9 a). In addition, the k 3 weighted Fourier-transformed Ti K-edge EXAFS spectra also show that the bond length of the Ti-S bond hardly changes, that is, the local chemical environment around Ti does not change significantly ( Figure 9 b). The above results fully prove that Ti does not participate in charge compensation during charge and discharge. For V, 7 points were selected to test XAS during the charge and discharge processes in order to more accurately characterize the valence state changes of V at different stages ( Figure 10 a). It should be noted that no obvious shift of the V K-edge absorption edge in the XANES spectra was observed. Although there is a slight shift, the shifted energy is very small, and it is considered that the induced valence state change can be basically ignored. This abnormal behavior may be related to the electron transfer from the ligand sulfur to the metal vanadium, that is, electrons transfer from sulfur to vanadium and then to the external circuit. During the whole process, vanadium neither gains nor loses electrons, and its valence state remains unchanged. k 3 The weighted Fourier-transformed V K-edge EXAFS spectra also show that the bond length of the V-S bond slightly shortens during charging and completely returns to the initial bond length after discharging. And this slight change in the local environment is very likely due to the gain and loss of electrons by sulfur ( Figure 10 b). At the same time, previous electrochemical tests have shown that there is only a single redox peak during charge and discharge, which is consistent with the test results of XAS, that is, only sulfur participates in charge compensation during the whole process, and the valence state changes of titanium and vanadium can be ignored. Based on the above results, it is considered that almost all of the capacity is contributed by the redox reaction of sulfur, and vanadium and titanium themselves do not participate in charge compensation.

[0046] Reversible insertion and extraction mechanism of lithium ions

[0047] Based on the analysis of the previous electrochemical and XRD characterization tests, the reversibility of Li 1.27 V 0.2 Ti 0.53 S 2 in the cathode material, including all Li in the lithium layer and the transition metal layer, can be reversibly inserted and extracted. In order to more accurately characterize the insertion and extraction of Li + during charge and discharge, next, + Li solid-state nuclear magnetic resonance (SSNMR) spectroscopy was used to characterize the chemical shifts of lithium in different charge and discharge states ( 7 ). Figure 11)。Based on the capacity-voltage curve of the first cycle, 5 points were selected for 7 Li solid-state nuclear magnetic resonance tests, namely the original sample, the sample charged to 2.8 V, the sample charged to 3.0 V, the sample discharged to 2.5 V, and the sample discharged to 1.8 V. The peaks appearing around 70 ppm and 0.5 ppm in the original sample generally can be attributed to the lithium in the lithium layer of the transition metal layer. After charging to 2.8 V, it was found that the peak around 70 ppm still existed, indicating that the de-lithiation process preferentially removed the Li in the lithium layer + , which is consistent with the description in the literature. Subsequently, after charging to 3.0 V, it was found that the peak around 70 ppm had basically disappeared, which means that the lithium Li + in the transition metal layer also detached during the end stage of charging. During the subsequent discharging process, it was observed that the peak around 70 ppm gradually recovered to the original height, that is, Li + re-inserted into the transition metal layer and the structure returned to the initial state. Based on the above results, it was proved that Li 1.27 V 0.2 Ti 0.53 S 2 the high capacity of the cathode material comes from the fact that all Li + in the lithium layer and the transition metal layer can undergo reversible de-lithiation and intercalation.

[0048] In addition to capacity and rate performance, long-cycle testing is one of the important indicators for evaluating cathode materials. Based on the above characterizations, the optimal doping ratio (Li 1.27 V 0.2 Ti 0.53 S 2 ) has been determined. Next, the long-cycle performance of this material will be characterized at high current. The long-cycle test was carried out in a constant-current charge-discharge mode, with the set current of 1000 mA g -1 , and the voltage range of 1.8 - 3.0 V. As Figure 12 shown in a of 1.27 V 0.2 Ti 0.53 S 2 the initial capacity tested at a current density of 1000 mA g -1 was 205.0 mAh g -1 , and the capacity after 1000 cycles was 181.5 mA g -1 , with a capacity retention rate of 88.5%, which is better than most of the currently reported sulfide cathode materials. After cycling to 4300 cycles, the discharge capacity was still 166.1 mAh g -1 , and the capacity retention rate was 81.0%. The Coulomb efficiency remained near 100% throughout the cycling process, indicating that Li +The deintercalation has very good reversibility. In addition, the energy density and energy efficiency during the long cycle process were calculated based on the test results, as Figure 12 shown in b of Li 1.27 V 0.2 Ti 0.53 S 2 The initial energy density of is 470.2 Wh kg -1 and still has 365.0 Wh kg after 4300 cycles -1 , and the energy efficiency remains above 86.3%.

Claims

1. A lithium-rich layered sulfide positive electrode material, characterized in that: Its general formula is Li 1.33-0.33x V x Ti 0.67-0.67x S2, and 0<x≤0.5, belongs to the hexagonal O3 type crystal structure, and the corresponding space group is 2. The lithium-rich layered sulfide positive electrode material according to claim 1, characterized in that 0.1<x≤0.4, 0.15<x≤0.

25.

3. The lithium-rich layered sulfide positive electrode material according to claim 1, characterized in that: The positive electrode material has 430-480Whkg -1 energy density.

4. The method for preparing the lithium-rich layered sulfide positive electrode material according to claim 1, characterized in that: The method comprises the following steps: step 1, mixing Li2S powder, vanadium powder, titanium powder and sulfur powder, grinding and tableting; step 2, performing constant temperature heat treatment and quenching to obtain a positive electrode material.

5. The preparation method according to claim 4, characterized in that: Heat treatment temperature is 600-800℃, time is 20-40h.

6. The preparation method according to claim 4, characterized in that: During the heat treatment, the vacuum degree was maintained at 10 -4 -10 - 6 mbar or so.

7. Use of the lithium-rich layered sulfide positive electrode material according to claim 1 in lithium-ion batteries.

8. The use according to claim 7, characterized in that: The lithium-ion battery adopts a lithium metal negative electrode, which has a power retention rate of 85-90% when charging and discharging for 4300 cycles.

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