Method for preparing lithium-rich manganese-based positive electrode material surface-selenized

Abstract

The application discloses a preparation method of a surface-seleniumized lithium-rich manganese-based positive electrode material, and specifically comprises the following steps: placing lithium-rich manganese-based positive electrode material and selenium powder on both ends of a magnetic boat, and then placing the magnetic boat into a tube furnace, wherein the selenium powder is located at the upstream of the tube furnace, and the lithium-rich manganese-based positive electrode material is located at the downstream; and performing selenization under an inert atmosphere, so as to obtain the surface-seleniumized lithium-rich manganese-based positive electrode material. The application makes full use of the low boiling point of the selenium source, and the selenium source is easy to react with the lithium-rich manganese-based positive electrode material under the inert atmosphere to realize surface selenization. The selenium is distributed in the form of anions on the surface of the lithium-rich manganese-based positive electrode material, and the surface oxygen loss of the selenium can effectively inhibit the surface irreversible oxygen loss, while the bulk phase lattice is not changed, so that the advantages of the bulk phase high capacity are not affected, and the electrochemical problems such as transition metal migration, structure phase transition, voltage and capacity continuous attenuation caused by surface oxygen loss are relieved.

Description

Technical Field

[0001] This invention belongs to the field of lithium-ion battery cathode material preparation technology, specifically relating to a method for preparing surface selenized lithium-rich manganese-based cathode materials. Background Technology

[0002] With the explosive growth of electric vehicles, the performance requirements for electrode materials in power lithium-ion batteries are becoming increasingly stringent. Due to the difference in capacity between the positive and negative electrodes, and the ongoing research and development of solid-state electrolytes and all-solid-state lithium batteries, positive electrode materials dominate energy density and cost, becoming a bottleneck restricting the development of lithium-ion batteries. Therefore, developing high-specific-capacity positive electrode materials is key to realizing high-energy-density lithium-ion power batteries for electric vehicles.

[0003] Attributable to the redox reactions of transition metals and the charge compensation mechanism of oxygen, lithium-rich manganese-based cathode materials exhibit ultra-high specific capacity (>250 mAh g). -1 ) and energy density (up to 900Wh / kg) -1 With a specific capacity higher than other power battery cathode materials such as lithium iron phosphate and ternary materials, it is considered one of the most promising next-generation high-energy-density lithium-ion battery cathode materials. However, for lithium-rich manganese-based cathode materials, oxygen redox is a double-edged sword. Bulk reversible oxygen redox contributes to specific capacity, while irreversible oxygen loss causes transition metal migration, structural phase transition, and transition metal reduction during charge and discharge, resulting in reduced electrochemical performance such as capacity / voltage decay, voltage hysteresis, and low initial coulombic efficiency. This is a problem that urgently needs to be solved for lithium-rich manganese-based cathode materials. Since these irreversible oxygen losses mainly occur on the material surface, maintaining reversible bulk oxygen redox and developing effective methods to suppress irreversible surface oxygen loss are key to solving the main problems of lithium-rich manganese-based cathode materials.

[0004] Currently, effective methods to suppress irreversible oxygen loss on the surface of lithium-rich manganese-based cathode materials mainly include: surface coating and doping to enhance the TM-O bonding force and alter the electronic structure of oxygen to stabilize surface oxygen; changing the surface chemical configuration (substituting surface Li-O-Li) and creating oxygen vacancies on the surface (removing surface oxygen) can also alleviate irreversible oxygen release to varying degrees. These methods essentially address the problem of irreversible oxygen loss from the perspectives of stabilizing surface oxygen, rendering surface oxygen electrochemically inactive, and directly removing surface oxygen. Theoretically, surface anion substitution can also directly suppress irreversible oxygen loss, but this has been rarely studied. Suppressing surface oxygen loss in lithium-rich manganese cathode materials can avoid the resulting degradation in electrochemical performance, which is of great significance for the commercial application of high-capacity lithium-rich manganese cathode materials.

[0005] Chinese Patent (Application No.: 202110242324.6, Publication No.: CN112599783B, Publication Date: 2021-03-05) discloses a selenium-doped lithium-rich manganese-based cathode material, its preparation method, and its application. The method involves uniformly mixing the lithium-rich manganese-based cathode material with selenium powder to obtain a mixture. The mixture is then placed in a calcination apparatus for selenization to obtain the selenized lithium-rich manganese-based cathode material mLi₂MnO. 3-δ ·(1-m)LiTMO 2-δ Se 2δ / 3 The selenization temperature is 500-900℃, and the selenization time is 5-10h. High-temperature selenization and sufficient selenization time can ensure the uniform distribution of selenium in lithium-rich manganese-based cathode materials. However, by setting the selenization time and selenization temperature parameters, what is obtained is a bulk selenium-doped lithium-rich manganese-based cathode material, rather than one that targets surface oxygen.

[0006] Chinese Patent (Application No.: 201510963549.5, Publication No.: CN105576202A, Publication Date: 2015-12-21) discloses a lithium-rich manganese selenium-based cathode material and its preparation method. The method involves weighing a lithium source compound at 1-2 times the sum of the amounts of a soluble salt containing manganese, a soluble selenium compound, and a soluble salt containing M. The lithium source compound is then mixed uniformly with a composite metal precursor to obtain a mixture. Under an oxygen-containing atmosphere, the mixture is heated to 200-500℃ at a heating rate of 0.5-10℃ / min and held at this temperature for 2-12 hours. Then, it is heated to 600-900℃ at a heating rate of 0.5-10℃ / min and calcined for 2-24 hours. The mixture is then cooled to obtain the lithium-rich manganese selenium-based cathode material xLi2Mn. 1-y Se y O3·(1-x)LiMO2. Soluble selenium-containing compounds are H2SeO3, SeO2, or Na2SeO3. This technology involves doping Se into lithium-rich manganese-based cathode materials in the form of cations, addressing the technical problems of poor rate performance and cycle performance of existing lithium-rich manganese-based cathode materials, rather than directly addressing the problem of surface oxygen loss. Summary of the Invention

[0007] The purpose of this invention is to provide a method for preparing a lithium-rich manganese-based cathode material with a selenized surface, thereby mitigating the degradation of electrochemical performance by suppressing oxygen loss on the surface of the lithium-rich manganese-based cathode material.

[0008] The technical solution adopted in this invention is a method for preparing surface-selenized lithium-rich manganese-based cathode material, specifically: the lithium-rich manganese-based cathode material and selenium powder are spread evenly on both ends of a magnetic boat, and then the magnetic boat is placed in a tube furnace, wherein the selenium powder is upstream of the tube furnace and the lithium-rich manganese-based cathode material is downstream, and selenization is carried out under an inert atmosphere to obtain surface-selenized lithium-rich manganese-based cathode material.

[0009] The features of the present invention also lie in that

[0010] The chemical formula of the lithium-rich manganese-based cathode material is Li 1+x M 1-x O2, where 0 < x < 1, and M is at least one of Ni, Co, or Mn.

[0011] The mass ratio of the lithium-rich manganese-based cathode material to the selenium powder is 1:0.05 - 0.20.

[0012] The specific conditions for selenization are as follows: The inert atmosphere is a mixed gas of argon and hydrogen; the selenization temperature is 300 - 450 °C, the heating rate is 2 - 5 °C / min, and the selenization time is 2 - 3 h.

[0013] The volume ratio of argon to hydrogen is 9 - 12:1.

[0014] The beneficial effects of the present invention are as follows: The method for preparing the surface-selenized lithium-rich manganese-based cathode material of the present invention makes full use of the characteristic of the low boiling point of the selenium source, which is prone to undergo a gas-solid reaction with the lithium-rich manganese-based cathode material under an inert atmosphere to achieve surface selenization. Selenium is distributed on the surface of the lithium-rich manganese-based cathode material in the form of anions. Selenium replacing surface oxygen can effectively inhibit surface irreversible oxygen loss, while not changing the bulk lattice, thus not affecting the advantage of high capacity in the bulk phase, and alleviating electrochemical problems such as transition metal migration, structural phase transformation, and continuous attenuation of voltage and capacity caused by surface oxygen loss. In addition, the electronic conductivity of the selenide is higher than that of the oxide, which is beneficial to the improvement of the rate performance. Selenization is more direct and effective compared to surface cation doping and other modification methods. At the same time, this method is simple to operate, has strong controllability, is suitable for large-scale industrial production, and provides a new direction for inhibiting surface oxygen loss of the lithium-rich manganese-based cathode material. Description of the Drawings

[0015] Figure 1 Cycling performance graph of the specific capacity of the CR2032 coin cells assembled with the cathode materials prepared in Comparative Example 1 and Example 1 at a current density of 50 mAh g -1 current density;

[0016] Figure 2a SEM image of the lithium-rich manganese-based cathode material of Comparative Example 1;

[0017] Figure 2b SEM image of the surface-selenized lithium-rich manganese-based cathode material of Example 2;

[0018] Figure 3 Cycling performance graph of the specific capacity of the CR2032 coin cells assembled with the cathode materials prepared in Comparative Example 1 and Example 2 at a current density of 125 mAh g -1 current density;

[0019] Figure 4The cycling performance graph of the discharge mid-voltage of the CR2032 coin cells assembled with the cathode materials prepared in Comparative Example 1 and Example 2 at a current density of 125 mAh g -1 is shown in the following figure. Detailed Description of the Invention

[0020] The present invention will be described in detail below with reference to the accompanying drawings and specific embodiments.

[0021] The preparation method of the surface-selenized lithium-rich manganese-based cathode material of the present invention is specifically as follows:

[0022] The lithium-rich manganese-based cathode material and selenium powder are spread flat at both ends of the boat, and then the boat is placed in a tube furnace. Among them, the selenium powder is in the upstream of the tube furnace, and the lithium-rich manganese-based cathode material is in the downstream. Selenization is carried out under an inert atmosphere to obtain the surface-selenized lithium-rich manganese-based cathode material;

[0023] The chemical formula of the lithium-rich manganese-based cathode material is Li 1+x M 1-x O2, where 0 < x < 1, and M is at least one of Ni, Co or Mn;

[0024] The mass ratio of the lithium-rich manganese-based cathode material to the selenium powder is 1:0.05 - 0.20;

[0025] The specific conditions for selenization are as follows: the inert atmosphere is a mixed gas of argon and hydrogen with a volume ratio of 9 - 12:1, the selenization temperature is 300 - 450 °C, the heating rate is 2 - 5 °C / min, and the selenization time is 2 - 3 h.

[0026] The method of the present invention replaces the surface oxygen of the lithium-rich manganese-based cathode material through selenization, thereby effectively inhibiting the irreversible oxygen loss on the surface, alleviating problems such as structural phase change and voltage decay caused thereby, and thus improving the electrochemical performance. By adjusting the mass ratio of the lithium-rich manganese-based cathode material to the selenium powder, the surface selenization thickness is controlled. The inert atmosphere prevents the oxidation of selenium. Through the mild selenization temperature (300 - 450 °C) and appropriate selenization time (2 - 3 h), it is ensured from the thermodynamic and kinetic perspectives that only surface modification is carried out without changing the bulk structure and composition. Therefore, on the basis of maintaining the advantages of high specific capacity and energy density of the lithium-rich manganese-based cathode material, the problem of electrochemical performance decay caused by irreversible oxygen loss on the surface is solved.

[0027] Comparative Example 1

[0028] The non-selenized lithium-rich manganese-based cathode material Li 1.2 Mn 0.54 Ni 0.13 Co[[ID=四十二]] 0.13 O2.

[0029] Example 1

[0030] The preparation method of the selenized lithium-rich manganese-based cathode material of the present invention is as follows:

[0031] Weigh out Li in a mass ratio of 1:0.1. 1.2 Mn 0.54 Ni 0.13 Co 0.13 O2 and selenium powder were evenly spread at both ends of the magnetic boat, with the selenium powder upstream of the tube furnace and the lithium-rich manganese-based cathode material downstream. Before selenization, an Ar atmosphere was introduced into the tube furnace for 20 minutes to purge oxygen and moisture. The selenization conditions were: heating to 350°C at a rate of 2°C / min and holding for 3 hours, with an Ar:H2 protective atmosphere of 12:1, thus obtaining surface-selenized Li. 1.2 Mn 0.54 Ni 0.13 Co 0.13 O2.

[0032] Performance Testing: The positive electrode materials obtained in Example 1 and Comparative Example 1 were mixed with conductive carbon black and PVDF at a mass ratio of 8:1:1, using NMP as a solvent. The mixture was homogenized, coated onto aluminum foil, and dried to obtain two types of positive electrode sheets. These were then assembled into CR2032 coin cells using lithium metal sheets as negative electrodes. Evaluation was conducted on the Xinwei Battery testing cabinet. Charge-discharge experiments were performed as required at 25℃ and 2.0-4.8V. Cyclic performance at a current density of 50mAh / g was as follows: Figure 1 As shown, the product of Embodiment 1 of the present invention retains 91.3% of its specific capacity after 80 charge-discharge cycles, which is higher than the 59.78% retention rate of Comparative Example 1. The polarization voltage is 0.2517V, which is lower than the 0.3091V of Comparative Example 1.

[0033] Example 2

[0034] Weigh out Li in a mass ratio of 1:0.05. 1.2 Mn 0.54 Ni 0.13 Co 0.13 O2 and selenium powder are evenly spread at both ends of the magnetic boat, with the selenium powder upstream of the tube furnace and the lithium-rich manganese-based cathode material downstream. Before selenization, an Ar atmosphere is introduced into the tube furnace for 20 minutes to purge oxygen and moisture. During selenization, the temperature is increased to 400°C at a rate of 3°C / min and held for 2 hours. The protective atmosphere is Ar:H2 = 9:1, thus obtaining surface-selenized Li. 1.2 Mn 0.54 Ni 0.13 Co 0.13 O2.

[0035] Figure 2a , Figure 2bThe SEM images of Comparative Example 1 and Example 2 show that the selenization treatment did not change the basic morphology of the lithium-rich manganese-based cathode material.

[0036] Performance Testing: The positive electrode materials obtained in Example 2 and Comparative Example 1 were weighed together with acetylene black and PVDF at a mass ratio of 8:1:1. Using NMP as a solvent, the mixtures were homogenized, coated onto aluminum foil, and dried to obtain two types of positive electrode sheets. These were then assembled into CR2032 coin cells using lithium metal sheets as negative electrodes. Evaluation was conducted on the Xinwei Battery testing cabinet. Charge-discharge experiments were performed as required at 25℃ and 2.0-4.8V. The cycle performance at a current density of 125mAh / g was as follows: Figure 3 , Figure 4 As shown, after 100 charge-discharge cycles, the specific capacity and discharge voltage retention rates of the product of Embodiment 2 of the present invention are still 85.78% and 92.30%, respectively, which are higher than the retention rates of 66.4% and 89.88% of Comparative Example 1.

[0037] The mechanism of this invention is as follows: A gas-phase anion exchange method is used for surface anion substitution. First, hydrogen reacts with selenium to generate hydrogen selenide, a low-boiling-point substance. The gaseous hydrogen selenide further reacts with the lithium-rich manganese-based cathode material to achieve surface selenization. This gas-solid contact method results in extremely high surface modification uniformity. A suitable selenization temperature (300–450°C) and a short selenization time (2–3 hours) ensure that only oxygen of varying thicknesses on the surface is replaced by selenium, while the bulk (internal) structure and composition remain unchanged. This effectively suppresses irreversible oxygen loss and alleviates problems such as structural phase transitions and voltage decay caused by it, while retaining the advantages of high specific capacity and high energy density of the lithium-rich manganese-based cathode material. Furthermore, the electronic conductivity of selenides is higher than that of oxides, which is beneficial for improving rate performance.

[0038] Example 3

[0039] The surface-selenized lithium-rich manganese-based cathode material of this invention exhibits reversible specific capacity in the bulk phase. Surface oxygen is replaced, directly suppressing irreversible oxygen loss and thus avoiding the resulting structural and electrochemical performance degradation, resulting in improved electrochemical performance. When Li... 1.2 Mn 0.54 Ni 0.13 Co 0.13 When the mass ratio of O2 to selenium powder is 1:0.1 (Example 1), at 25°C, 2.0-4.8V, and 50mAh g... -1 After 100 cycles at the current density, the specific capacity retention increased from 59.78% to 91.3%. This provides a feasible solution for improving the electrochemical performance of surface-modified lithium-rich manganese-based cathode materials.

Claims

1. A method for preparing a surface-selenized lithium-rich manganese-based cathode material, characterized in that, Specifically, lithium-rich manganese-based cathode material and selenium powder are spread evenly on both ends of a magnetic boat, and then the magnetic boat is placed in a tube furnace, with the selenium powder upstream and the lithium-rich manganese-based cathode material downstream. Selenization is carried out in a mixed atmosphere of argon and hydrogen, where hydrogen is used to react with the selenium powder in situ to generate highly active hydrogen selenide gas, so as to achieve chemical selenization of the surface of the lithium-rich manganese-based cathode material, that is, to obtain a surface-selenized lithium-rich manganese-based cathode material. The chemical formula of the lithium-rich manganese-based cathode material is Li 1+x M 1-x O2, where M is a combination of Ni, Co, and Mn, and the chemical formula is Li. 1.2 Ni 0.13 Co 0.13 Mn 0.54 O2; The mass ratio of the lithium-rich manganese-based cathode material to selenium powder is 1:0.05; The specific conditions for selenization are as follows: selenization temperature is 400℃, heating rate is 3℃ / min, selenization time is 2h; and the volume ratio of argon to hydrogen is 9:1.

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