A micro-lithium-rich lithium nickelate positive electrode material with a layered structure, a preparation method, and its application in lithium-ion batteries

Abstract

The present invention discloses a preparation method of a micro-lithium-rich nickel lithium oxide cathode material with a layered structure and its application in a lithium-ion battery. First, a nickel source, a lithium salt, and a molten salt additive are mixed evenly. The molar ratios of the lithium salt, the molten salt additive to the nickel source are 1.1 - 1.7 and 0.1 - 0.5 respectively. Oxygen is introduced, and it is kept at 550 - 650 °C for 10 - 20 hours and then cooled to room temperature. After mechanical crushing, the excessive lithium salt and molten salt additive are removed, and it is dried. Then, pure oxygen is introduced again, and the temperature is raised to 450 - 550 °C. After keeping it at this temperature for 2 - 5 hours, it is cooled to room temperature. The chemical formula of the lithium-rich nickel lithium oxide cathode material prepared by the present invention is Li1+xNi1-xO2, where 0.02 < x < 0.08; the crystal structure is a layered α-NaFeO2 type, and the space group is R-3m type; the surplus Li occupies the octahedral voids in the Ni layer and is randomly distributed. The lithium-rich nickel lithium oxide obtained by the present invention has a thermodynamically stable layered phase, and the space group of the cathode material is R-3m type. The micro-lithium-rich nickel lithium oxide of the present application simultaneously has high cycle stability, high rate performance, and high thermal stability.

Description

Technical Field

[0001] The present invention belongs to the technical field of lithium-ion batteries, and in particular relates to a slightly lithium-rich lithium nickelate positive electrode material with a layered structure and a preparation method thereof. Background Art

[0002] Ternary materials represented by lithium nickel cobalt manganese oxide and lithium nickel cobalt aluminum oxide can provide good energy density and power density, and have been widely used in the manufacture of power batteries. Increasing the nickel content can increase the energy density of the ternary positive electrode material, while reducing the cycle stability and thermal stability of the material. Lithium nickel oxide (LiNiO2) has a high energy density (>900Wh / kg material layer), but it is not easy to obtain a phase that conforms to the theoretical stoichiometric ratio during the synthesis of this type of material, and it is usually lithium-deficient (the ratio of the amount of Li and Ni is less than 1). During the electrochemical cycle, the material is prone to structural transformation from a layered phase to a rock salt phase, and is also prone to losing lattice oxygen, so the cycle stability is poor, and the structural transformation also leads to poor rate performance. In order to improve the energy density, the existing technology has proposed a variety of lithium-rich cathode materials (usually the ratio of the amount of Li and transition metal substances is greater than 1.1:0.9). However, the traditional lithium-rich cathode materials are composed of two structural units, namely R-3m (layered structure LiMO2, M = Ni, Co, Mn and other transition metals) and c / 2m (monoclinic structure Li2MnO3). Materials containing c / 2m units (Li2MnO3 units) need to be charged to a high voltage (>4.5V vs Li due to their thermodynamic properties). + / Li) can carry out lithium ion deintercalation and achieve high energy density. Such a high voltage exceeds the stability window of the electrolyte, resulting in poor interface stability and affecting the cycle life. To solve this problem, the present invention introduces a slightly excessive amount of lithium ions into the nickel layer in the layered structure through the method of molten salt chemistry. Without changing the layered crystal structure, the obtained slightly lithium-rich lithium nickelate material is applied to the positive electrode material of lithium ion batteries and is charged to 4.3V (vs Li + / Li) can achieve an energy density of >900Wh / kg, and the cycle stability and rate performance are relatively excellent. Summary of the Invention

[0003] The present invention aims to overcome the deficiencies of the prior art and to prepare a lithium-rich nickelate cathode material having a layered structure, thereby improving the electrochemical cycle stability and rate performance of the compound.

[0004] Preparation method:

[0005] The nickel source is evenly mixed with lithium salt and Li2SO4 (molten salt additive), the amount ratio of lithium salt to nickel source is in the range of 1.1-1.7, and the amount ratio of Li2SO4 to nickel source is in the range of 0.1-0.5. Then the mixed powder is added to a corundum porcelain boat, placed in a tube furnace, and pure oxygen is introduced at a certain flow rate (see below). The temperature is raised to a certain temperature (see below) at a certain heating rate (see below). After keeping the same temperature for a certain time (see below), the temperature is lowered to room temperature (first heating) at a certain cooling rate (see below).

[0006] The cooled powder is then removed and mechanically crushed, then deionized water is added to dissolve the excess lithium salt and Li2SO4. The solid portion is filtered and dried at 60°C with forced air. The dried powder is again added to a corundum boat and placed in a tube furnace. Pure oxygen is introduced at a constant flow rate. The temperature is raised to a certain temperature at a certain rate. After maintaining the same temperature for a certain period of time, the temperature is cooled to room temperature at a certain rate (second heating) to obtain the product.

[0007] The Ni source may be NiO, Ni(OH)2, or NiCO3; the lithium salt used may be LiOH or LiOH·H2O.

[0008] The temperature range of the first heating is 550-650 degrees, the heating time is 10-20 hours, the heating / cooling rate is 2-10 degrees / minute, and the oxygen flow rate is 0.1-0.5L / min.

[0009] The second heating temperature range is 450-550 degrees, the heating time is 2-5 hours, the heating / cooling rate is 2-10 degrees / minute, and the oxygen flow rate is 0.1-0.5L / min.

[0010] The temperature unit in the above steps is Celsius.

[0011] Characteristics of the slightly lithium-rich lithium nickelate:

[0012] 1. The chemical formula is Li 1+x Ni 1-x O2, 0.02 <x<0.08;

[0013] 2. The crystal structure is layered α-NaFeO2 type, and the space group is R-3m type;

[0014] 3. The surplus Li (the above x part) occupies the octahedral voids in the Ni layer and is randomly distributed.

[0015] Compared with the existing technology, the slightly lithium-rich lithium nickelate obtained in the present invention has a thermodynamically stable layered phase, and the positive electrode material space group is R-3m type, which overcomes the defect that traditional materials cannot simultaneously meet the cycle stability and rate performance. The slightly lithium-rich lithium nickelate of the present application has high cycle stability, high rate performance and high thermal stability. BRIEF DESCRIPTION OF THE DRAWINGS

[0016] Figure 1 Neutron diffraction test results of Example 1;

[0017] Figure 2 Scanning transmission electron micrograph of Example 1;

[0018] Figure 3 Electrochemical charge and discharge test results of Example 1 and Comparative Example 1;

[0019] Figure 4 . The magnification test results of Example 1 and Comparative Example 1;

[0020] Figure 5 The first charge and discharge curve of Example 1;

[0021] Figure 6 .Thermal safety test results of Example 1 and Comparative Example 1 in the charged state. DETAILED DESCRIPTION

[0022] Example 1

[0023] Mix Ni(OH)2 with LiOH and Li2SO4 evenly (the ratio of the amount of substances among the three is 1:1.3:0.27), then add the mixed powder into a corundum porcelain boat, put it into a tube furnace, introduce pure oxygen at a flow rate of 0.2 liters / minute, increase the temperature to 600 degrees at a rate of 5 degrees / minute, keep it at 600 degrees for 15 hours, and then cool it to room temperature at a rate of 5 degrees / minute.

[0024] The cooled powder was taken out, mechanically crushed, and deionized water was added to dissolve excess LiOH and Li2SO4. After filtering, the solid part was taken and dried at 60 degrees. The dried powder was added to the corundum porcelain boat again, placed in a tube furnace, and pure oxygen was introduced at a flow rate of 0.2 liters / minute. The temperature was raised to 500 degrees at a rate of 5 degrees / minute. After keeping at 500 degrees for 5 hours, the temperature was lowered to room temperature at a rate of 5 degrees / minute to obtain Product 1. The crystal structure of Product 1 was characterized by neutron diffraction ( Figure 1 ), confirming that it is a layered structure with a space group of R-3m and does not contain c / 2m Li2MO3 units because no superstructure peaks were observed. Further crystal refinement methods confirmed that the excess Li ions were randomly distributed in the Ni layer. Scanning transmission electron microscopy imaging also confirmed that the material has a layered structure ( Figure 2 The elemental composition of the material was analyzed by ICP (Table 1), and it was confirmed that the molar ratio of Li to Ni was 1.04:0.96 (ie, x=0.04).

[0025] The obtained product 1 was assembled into a button cell with a lithium sheet, a separator, and an electrolyte (1M lithium hexafluorophosphate dissolved in dimethyl carbonate and fluoroethylene carbonate in a volume ratio of 1:1) in an argon-protected glove box and charged and discharged at a rate of 1C in the voltage range of 4.3-2.8V. The electrochemical cycling stability results are shown in Figure 3 The same button cell was used to evaluate the rate performance of the material. After charging to 4.3V at a rate of 0.2C, it was discharged at a rate of 0.5 to 10C. The test results are shown in Figure 4 The first discharge energy density of the material is obtained by integrating the charge and discharge curve ( Figure 5 ), which is 904Wh / kg.

[0026] Example 2

[0027] Mix Ni(OH)2 with LiOH and Li2SO4 evenly (the ratio of the amount of substances among the three is 1:1.1:0.27), then add the mixed powder into a corundum porcelain boat, put it into a tube furnace, introduce pure oxygen at a flow rate of 0.5 liters / minute, increase the temperature to 650 degrees at a rate of 2 degrees / minute, keep it at 600 degrees for 15 hours, and then cool it to room temperature at a rate of 2 degrees / minute.

[0028] The cooled powder was removed, mechanically crushed, and deionized water was added to dissolve excess LiOH and Li2SO4. The solid fraction was filtered and dried at 60°C with forced air. The dried powder was again added to a corundum boat and placed in a tube furnace. Pure oxygen was introduced at a rate of 0.5 L / min. The temperature was raised to 450°C at a rate of 2°C / min. After maintaining the temperature at 450°C for 2 hours, the temperature was then lowered to room temperature at a rate of 2°C / min to obtain Product 2. The elemental composition of the material was analyzed by ICP.

[0029] Example 3

[0030] Mix Ni(OH)2 with LiOH and Li2SO4 evenly (the ratio of the amount of substances among the three is 1:1.7:0.27), then add the mixed powder into a corundum porcelain boat, put it into a tube furnace, introduce pure oxygen at a flow rate of 0.1 liters / minute, increase the temperature to 550 degrees at a rate of 10 degrees / minute, keep it at 550 degrees for 15 hours, and then cool it to room temperature at a rate of 10 degrees / minute.

[0031] The cooled powder was removed and mechanically crushed, followed by addition of deionized water to dissolve excess LiOH and Li2SO4. The solid fraction was filtered and dried at 60°C with forced air. The dried powder was again added to a corundum boat and placed in a tube furnace. Pure oxygen was introduced at a rate of 0.1 L / min, and the temperature was raised to 550°C at a rate of 10°C / min. After maintaining the temperature at 550°C for 4 hours, the temperature was then lowered to room temperature at a rate of 10°C / min to obtain Product 3. The elemental composition of the material was analyzed by ICP.

[0032] Example 4

[0033] Mix Ni(OH)2 with LiOH and Li2SO4 evenly (the ratio of the amount of substances among the three is 1:1.5:0.1), then add the mixed powder into a corundum porcelain boat, put it into a tube furnace, introduce pure oxygen at a flow rate of 0.2 liters / minute, increase the temperature to 600 degrees at a rate of 5 degrees / minute, keep it at 600 degrees for 15 hours, and then cool it to room temperature at a rate of 5 degrees / minute.

[0034] The cooled powder was removed and mechanically crushed, followed by addition of deionized water to dissolve excess LiOH and Li2SO4. The solid fraction was filtered and dried at 60°C with forced air. The dried powder was again added to a corundum boat and placed in a tube furnace. Pure oxygen was introduced at a rate of 0.2 L / min, and the temperature was raised to 500°C at a rate of 5°C / min. After maintaining the temperature at 500°C for 5 hours, the temperature was then lowered to room temperature at a rate of 5°C / min to obtain Product 4. The elemental composition of the material was analyzed by ICP.

[0035] Example 5

[0036] Mix Ni(OH)2 with LiOH and Li2SO4 evenly (the ratio of the amount of substances among the three is 1:1.5:0.5), then add the mixed powder into a corundum porcelain boat, put it into a tube furnace, introduce pure oxygen at a flow rate of 0.2 liters / minute, increase the temperature to 600 degrees at a rate of 5 degrees / minute, keep it at 600 degrees for 15 hours, and then cool it to room temperature at a rate of 5 degrees / minute.

[0037] The cooled powder was then removed and mechanically crushed, followed by addition of deionized water to dissolve excess LiOH and LiSO. The solid fraction was filtered and dried at 60°C with forced air. The dried powder was again added to a corundum boat and placed in a tube furnace. Pure oxygen was introduced at a rate of 0.2 L / min, and the temperature was raised to 500°C at a rate of 5°C / min. After maintaining the temperature at 500°C for 5 hours, the temperature was then lowered to room temperature at a rate of 5°C / min to obtain Product 5. The elemental composition of the material was analyzed by ICP.

[0038] Example 6

[0039] NiO was mixed evenly with LiOH·H2O and Li2SO4 (the molar ratio of the three was 1:1.3:0.27), and then the mixed powder was added to a corundum porcelain boat, placed in a tube furnace, and pure oxygen was introduced at a flow rate of 0.2 liters / minute. The temperature was raised to 600 degrees at a rate of 5 degrees / minute. After keeping at 600 degrees for 15 hours, the temperature was lowered to room temperature at a rate of 5 degrees / minute.

[0040] The cooled powder was removed and mechanically crushed, followed by addition of deionized water to dissolve the excess LiOH·H₂O and Li₂SO₄. The solid fraction was filtered and dried at 60°C with forced air. The dried powder was again added to a corundum boat and placed in a tube furnace. Pure oxygen was introduced at a rate of 0.2 L / min, and the temperature was raised to 500°C at a rate of 5°C / min. After maintaining the temperature at 500°C for 5 hours, the temperature was then lowered to room temperature at a rate of 5°C / min to obtain Product 6. The elemental composition of the material was analyzed by ICP.

[0041] Example 7

[0042] Mix NiCO3 with LiOH and Li2SO4 evenly (the ratio of the amount of substances among the three is 1:1.3:0.27), then add the mixed powder into a corundum porcelain boat, put it into a tube furnace, introduce pure oxygen at a flow rate of 0.2 liters / minute, increase the temperature to 600 degrees at a rate of 5 degrees / minute, keep it at 600 degrees for 15 hours, and then cool it to room temperature at a rate of 5 degrees / minute.

[0043] The cooled powder was then removed and mechanically crushed, followed by addition of deionized water to dissolve excess LiOH and LiSO. The solid fraction was filtered and dried at 60°C with forced air. The dried powder was again added to a corundum boat and placed in a tube furnace. Pure oxygen was introduced at a rate of 0.2 L / min, and the temperature was raised to 500°C at a rate of 5°C / min. After maintaining the temperature at 500°C for 5 hours, the temperature was then lowered to room temperature at a rate of 5°C / min to obtain Product 7. The elemental composition of the material was analyzed by ICP.

[0044] Example 8

[0045] The synthesis method of the material was the same as that in Example 1. The material was used to assemble soft-pack batteries for thermal safety evaluation. The assembly method is as follows: (1) Preparation of positive electrode sheet: The active material (material of Example 1), conductive agent (carbon black) and binder (5wt% polyvinylidene fluoride / N-methylpyrrolidone solution, NMP) are mixed in a ratio of 94:3:3, NMP is added to adjust the solid content to 65%, and the sheet is coated on a 13μm aluminum foil. After drying, it is punched into 55mm*35mm (H*W) electrode sheets using a punching machine, and then rolled and vacuum-dried at 120℃ for 12 hours; (2) Preparation of negative electrode sheet: The active material (graphite), conductive agent (carbon black) and binder (5wt% polyvinylidene fluoride / NMP solution) are mixed in a ratio of 94:3:3, NMP is added to adjust the solid content to 45%, and the sheet is coated on a 10μm copper foil. After drying, it is punched into 57mm*37mm (H*W) electrode sheets using a punching machine, and then rolled and vacuum-dried at 80℃ for 12 hours. (3) Production of soft-pack cells: 16μm PE diaphragm and positive and negative electrodes are stacked to form a cell, ensuring that each positive electrode has a corresponding negative electrode. The outermost layer is wrapped with a diaphragm and then glued with polyimide tape. Use an ultrasonic welding machine to weld the aluminum and nickel tabs to the exposed tabs of the positive and negative electrodes respectively. Use polyimide tape to stick the welds. Wrap the outer layer of the cell with aluminum plastic film and leave an opening for injecting electrolyte. The prepared cell is vacuum-dried at 60℃ for 6 hours. The cell capacity is set to 30mAh. (4) Battery production: Use a pipette to inject 0.5g of electrolyte into the cell and use a sealing machine to seal the injection port. Lay it flat and let it stand for 10 hours before testing. (5) Battery formation and testing: After clamping the battery with a fixture, test it on a charge and discharge tester. Use a constant current of 0.1C (22mA / g, based on the mass of the positive electrode active material, the same below) to charge to 4.25V and a constant voltage of 0.05C. Use a 0.1C discharge to 2.75V. Repeat the above steps three times to complete the battery formation.

[0046] The method for thermal safety evaluation is: charge two prepared soft-pack batteries at a constant current of 0.1C to 4.25V, and then charge them at a constant voltage of 0.05C for use. Open the cavity of the accelerated calorimeter, stick the front end of the thermocouple to the center of a battery with aluminum tape, then overlap the other battery, stick the two batteries together with aluminum tape, and fix them on a special test stand in the cavity. Cover the upper cover of the accelerated calorimeter and start the accelerated calorimeter program for testing. The specific test procedure is: start the test at 25°C, first enter the heating mode to heat the cavity, heating 5°C each time (10 minutes) and monitor the battery temperature. Then, let it stand for 30 minutes and enter the search mode. During this period, if the temperature rise rate is lower than 0.02°C / min, continue heating after the standing period. If the temperature rise rate is higher than 0.02°C / min, enter the exothermic mode, no longer heat the cavity, and only record the temperature change. After the temperature exceeds 300°C, enter the cooling mode and the test is completed. See the test results. Figure 6The battery equipped with the material of Example 1 had a relatively high T2 temperature (thermal runaway temperature, the temperature at which the self-heating temperature rise is greater than 1 degree / min), which was 253.2 degrees.

[0047] Comparative Example 1

[0048] Ni(OH)2 and LiOH were uniformly mixed (the molar ratio between the two was 1:1.02). The mixed powder was then added to a corundum porcelain boat and placed in a tube furnace. Pure oxygen was introduced at a flow rate of 0.2 liters / minute. The temperature was raised to 485°C at a rate of 5°C / minute and held for 3 hours. The temperature was then further raised to 700°C and held for 20 hours. The temperature was then lowered to room temperature at a rate of 5°C / minute. Comparative Product 1 was obtained. The ICP elemental analysis results are shown in Table 1.

[0049] The comparative product 1 was assembled into a button cell in an argon-protected glove box with lithium sheets, a separator, and an electrolyte (1M lithium hexafluorophosphate dissolved in dimethyl carbonate and fluoroethylene carbonate in a volume ratio of 1:1). The cells were charged and discharged at a rate of 1C in the voltage range of 4.3-2.8V. Figure 3 The same button cell was used to evaluate the rate performance of the material. After charging to 4.3V at a rate of 0.2C, it was discharged at a rate of 0.5 to 10C. The test results are shown in Figure 4 .

[0050] Comparative Example 2

[0051] The material synthesis method is the same as that of Comparative Example 1. The assembly method and thermal safety evaluation method of the soft pack battery are the same as those of Example 8. The test results are shown in Figure 6 The T2 temperature (thermal runaway temperature, the temperature at which the self-heating temperature rise is greater than 1 degree / min) is 179.0 degrees. Therefore, Comparative Example 2 is more likely to experience thermal runaway than Example 8.

[0052] Table 1. Elemental analysis results of Examples and Comparative Examples

[0053] The ratio of the amount of Li and Ni <![CDATA[x in Li 1+x Ni 1-x the value of O2]]> Example 1 1.04：0.96 0.04 Example 2 1.02：0.98 0.02 Example 3 1.08：0.92 0.08 Example 4 1.06：0.94 0.06 Example 5 1.05：0.95 0.05 Example 6 1.03：0.97 0.03 Example 7 1.04：0.97 0.04 Comparative Example 1 0.98：1.02 -0.02

Claims

1. A layered structured slightly lithium-rich lithium nickelate positive electrode material, characterized in that: The chemical formula of the micro lithium-rich lithium nickelate cathode material is Li 1+x Ni 1-x O2, where 0.02 < x < 0.08; the crystal structure is a layered α-NaFeO2 type, and the space group is R-3m type; the surplus Li occupies the octahedral voids in the Ni layer and is randomly distributed.

2. Application of the layered slightly lithium-rich lithium nickelate positive electrode material according to claim 1 in lithium-ion batteries.

3. A method for preparing the layered slightly lithium-rich lithium nickelate positive electrode material according to claim 1, characterized in that: The preparation method comprises the following steps: (1) First heating The nickel source, lithium salt and molten salt additive are mixed evenly, the ratio of the amount of lithium salt to the nickel source is in the range of 1.1-1.7, and the ratio of the amount of molten salt additive to the nickel source is in the range of 0.1-0.5, and then the mixed powder is placed in a tube furnace, pure oxygen is introduced at a flow rate of 0.1-0.5 L / min, and the temperature is raised to 550-650 degrees at a rate of 2-10 degrees / minute. After keeping the temperature for 10-20 hours, the temperature is lowered to room temperature at a rate of 2-10 degrees / minute; (2) Second heating The cooled powder is taken out, mechanically crushed, and deionized water is added to dissolve excess lithium salt and molten salt additives. After filtering, the solid part is taken and blown dry at 60 degrees. The dried powder is placed in a tube furnace again, and pure oxygen is introduced at a flow rate of 0.1-0.5 L / min. The temperature is raised to 450-550 degrees at a rate of 2-10 degrees / minute. After keeping warm for 2-5 hours, the temperature is cooled to room temperature at a rate of 2-10 degrees / minute to obtain the product.

4. The method for preparing a layered slightly lithium-rich lithium nickelate positive electrode material according to claim 3, characterized in that: The nickel source is one or more of NiO, Ni(OH)2 or NiCO3.

5. The method for preparing a layered slightly lithium-rich lithium nickelate positive electrode material according to claim 3, characterized in that: The lithium salt is LiOH or LiOH·H2O.

6. The method for preparing a layered slightly lithium-rich lithium nickelate positive electrode material according to claim 3, characterized in that: The molten salt additive is Li2SO4.

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