Lithium ion battery composite positive electrode material, preparation method thereof and formation method of lithium ion battery

Abstract

The invention discloses a composite positive electrode material of a lithium ion battery, a preparation method of the composite positive electrode material and a formation method of the lithium ion battery, the composite positive electrode material of the lithium ion battery comprises a positive electrode active material and a lithium-sodium mixed solid electrolyte coated on the surface of the positive electrode active material, sodium in the lithium-sodium mixed solid electrolyte and lithium on the surface of the positive electrode active material are subjected to electrochemically-driven in-situ lithium-sodium ion exchange, that is, sodium in the lithium-sodium mixed solid electrolyte enters a lithium layer of the positive electrode active material, and lithium with the corresponding content on the surface of the positive electrode active material enters a vacancy of sodium in the lithium-sodium mixed solid electrolyte; based on a dual modification strategy of lithium-sodium mixed solid electrolyte coating and ion exchange driving in-situ sodium doping, the capacity exertion, electrochemical stability, thermal stability and the like of the high-nickel ternary material or the lithium-rich manganese-based positive electrode material can be effectively improved, and a support is provided for application of the high-nickel ternary material or the lithium-rich manganese-based positive electrode material in a high-specific-energy and high-safety lithium battery.

Description

Technical Field

[0001] The present invention relates to the field of electrochemistry technology, and particularly relates to a composite cathode material for lithium-ion batteries, a preparation method thereof, and a formation method for lithium-ion batteries. Background Art

[0002] High specific energy lithium batteries are crucial for electric vehicles with long cruising ranges and renewable energy devices for efficient energy storage. Among the components of lithium-ion batteries, the electrode material is the most critical part, and the energy density of the electrode material largely determines the energy density of the battery. High-nickel ternary cathode materials LiNi x Co y Mn z O2 (NCM) and lithium-rich manganese-based cathode materials xLi2MnO3·(1 - x)LiMO2 (0 < x < 1), M = Ni, Co, Mn and other transition metals and their combinations (LMR), have become the core technologies to meet this demand because they can significantly improve the energy density and cruising ability of the battery. However, the poor stability and severe capacity decay of layered materials under high voltage seriously restrict the practical applications of these two cathode materials. Summary of the Invention

[0003] The purpose of the present invention is to overcome the defect that the electrochemical performance of the cathode material of lithium-ion batteries is poor under high voltage, which in turn affects the cycle performance and safety performance of lithium-ion batteries.

[0004] To achieve the above purpose, the present invention provides a composite cathode material for lithium-ion batteries, which comprises: a cathode active material; a lithium-sodium mixed solid electrolyte, and the lithium-sodium mixed solid electrolyte is coated on the surface of the cathode active material.

[0005] Optionally, the cathode active material has a layered structure.

[0006] Optionally, the cathode active material includes any one of high-nickel ternary materials and lithium-rich manganese-based materials.

[0007] Optionally, the average particle size of the cathode active material is 5 μm to 20 μm, and the average particle size of the lithium-sodium mixed solid electrolyte is 50 nm to 300 nm.

[0008] Optionally, the lithium-sodium mixed solid electrolyte is selected from Li 3-x Na x Zr2Si2PO 12 、Li 3-x Na x V2(PO4)3、Li 3- x Na x La(PO4)2、Li 3-xNa x Hf2Si2PO 12 Any one of them, where 0.3 < x < 1.

[0009] Optionally, the lithium-sodium mixed solid electrolyte accounts for 1 wt.% to 5 wt.% of the composite cathode material of the lithium-ion battery.

[0010] Optionally, the preparation method of the lithium-sodium mixed solid electrolyte comprises the following steps:

[0011] S1. Mix the sodium ion conductor, lithium salt LiTFSI, and ionic liquid evenly, put them into a reaction kettle, place the reaction kettle in a homogeneous reactor, rotate and stir at a speed of 50 rpm / min for 24 h to 72 h, and the temperature is 120 °C;

[0012] S2. Take out the reaction product in the reaction kettle, centrifuge and dry it, and calcine it at 400 °C to 500 °C to obtain the lithium-sodium mixed solid electrolyte.

[0013] Optionally, the concentration of the lithium salt LiTFSI in the ionic liquid is 0.1 mol / L to 0.5 mol / L, and the molar ratio of the lithium salt LiTFSI to the sodium ion conductor is 3:1.

[0014] The present invention also provides a preparation method of the composite cathode material of the lithium-ion battery as described above, comprising the following steps:

[0015] S1. Weigh the cathode active material and the lithium-sodium mixed solid electrolyte;

[0016] S2. Mix the cathode active material and the lithium-sodium mixed solid electrolyte and place them in a mechanical fusion device, and fuse them at a speed of 1000 rpm / min to 1500 rpm / min for 15 min to 30 min to obtain the composite cathode material of the lithium-ion battery.

[0017] A formation method of a lithium-ion battery prepared from the composite cathode material of the lithium-ion battery as described above, comprising the following steps:

[0018] First formation step:

[0019] Step 1: Constant current charge at a rate of 0.05C to 120% of the designed capacity of the lithium-ion battery, and stand still for 5 - 10 min;

[0020] Step 2: Constant current discharge at a rate of 0.05C to 120% of the designed capacity of the lithium-ion battery, and stand still for 5 - 10 min;

[0021] Step 3: Repeat steps 1 - 2 for a total of 2 times;

[0022] Step 4: After the above steps are completed, perform the first vacuum sealing.

[0023] The second formation step:

[0024] Step 1: Constant current charge at a rate of 0.1C to 100% of the designed capacity of the lithium-ion battery.

[0025] Step 2: Constant voltage charge until the current drops to 0.01C, and then stand still for 5 - 10 minutes.

[0026] Step 3: Constant current discharge at a rate of 0.1C to 100% of the designed capacity of the lithium-ion battery, and then stand still for 5 - 10 minutes.

[0027] Step 4: Repeat Steps 1 - 3 for a total of 2 times.

[0028] Step 5: After the above steps are completed, perform the second vacuum sealing.

[0029] Compared with the prior art, the beneficial effects of the present invention at least include:

[0030] (1) The composite cathode material of the lithium-ion battery of the present invention includes a cathode active material and a lithium-sodium mixed solid electrolyte coated on the surface of the cathode active material. By innovating the formation system (allowing more lithium ions in the cathode to slowly escape at the initial stage of formation), during the initial operation of the battery, lithium and sodium ions in the lattice of the cathode surface and the lithium-sodium mixed solid electrolyte body phase exchange dynamically, realizing the lattice matching and ion transport channel matching between the cathode and the electrolyte, and in-situ constructing a highly stable and low-impedance electrolyte / cathode interface.

[0031] (2) During the charge and discharge process, sodium ions in the lithium-sodium mixed solid electrolyte are in-situ exchanged into the lattice of the layered cathode. By constructing an efficient ion rapid transport channel, the interface resistance between the cathode active material and the lithium-sodium mixed solid electrolyte coating layer is reduced, and the capacity utilization ability of the cathode is improved; at the same time, based on the lattice support effect of sodium ions, the cation mixing and structure collapse on the cathode surface are inhibited, and the structural stability of the cathode active material is improved, thereby realizing the double improvement of the cathode stability and capacity utilization ability.

[0032] (3) The composite cathode material of the lithium-ion battery prepared by the present invention is applied to a lithium-ion battery. The lithium-sodium mixed solid electrolyte and in-situ doping can realize the synergistic regulation of the structure and performance of the cathode active material, making it have excellent capacity performance, cycle performance and safety performance. Moreover, the preparation process is green and efficient, the preparation process is simple, the applicable range is wide, and it can be mass-produced industrially. Description of the Drawings

[0033] Figure 1Schematic diagram of the electrochemical-driven in-situ lithium / sodium ion exchange between the cathode active material of the present invention and the lithium-sodium mixed solid electrolyte.

[0034] Figure 2 SEM-Mapping image of the lithium-ion battery composite cathode material prepared in Example 1 of the present invention.

[0035] Figure 3 XRD images of pure NCM811 (A) and the lithium-ion battery composite cathode material (B) prepared in Example 1 of the present invention after formation.

[0036] Figure 4 Time-of-flight secondary ion mass spectrometry (TOF-SIMS) test result graphs of the lithium-ion battery composite cathode material prepared in Example 1 of the present invention before and after formation.

[0037] Figure 5 First-week charge-discharge curves of the lithium-ion battery prepared using the uncoated cathode active material (A) and the lithium-ion battery composite cathode material (B) prepared in Example 2.

[0038] Figure 6 Microcalorimeter test curves of the fully charged uncoated cathode active material (A) and the lithium-ion battery composite cathode material (B) prepared in Example 3. Detailed implementation manners

[0039] The technical solutions of the present invention will be further described below in conjunction with the accompanying drawings and embodiments.

[0040] In view of the serious structural changes, bulk degradation, and interfacial side reactions of NCM and LMR under high charging cut-off voltages, which lead to the failure of the material for multiple reasons, currently, methods such as element doping, surface coating, and composite strategies have been widely used to improve the electrochemical performance of cathode materials under high voltage.

[0041] Element doping is an effective way to modify materials, which can alleviate cation mixing and increase lithium ion diffusion channels. However, traditional doping strategies mainly focus on transition metal sites, such as Chinese invention patents CN117374259B, CN110504448B, etc. This method has some limitations, such as uneven doping and indirect effects on Li + diffusion kinetics. Among the measures to improve discharge specific capacity and cycling performance, coating can reduce or slow down the influence of the external environment such as the electrolyte on the cathode material, stabilize the surface structure, and thus improve the cycling stability of the battery.

[0042] The positive electrode active material is the most crucial raw material for lithium-ion batteries. The positive electrode active material of lithium-ion batteries is the decisive factor for the electrochemical performance of lithium batteries, playing a leading role in the energy density and safety performance of the batteries. However, there are still some problems to be solved for the positive electrode active material with high capacity, such as particle breakage during the cycling process and the generation of oxygen due to side reactions with the electrolyte, which will seriously damage the cycle life and rate performance of the battery. The most effective modification means for layered positive electrode active materials should have the following characteristics: 1) Provide a physical barrier to prevent side reactions; 2) Alleviate the mixing of Li + / Ni 2+ mixing, while increasing the lithium-ion diffusion channels; 3) High ionic conductivity to improve charge transfer at the interface; 4) High matching with the ion transport channels of the positive electrode active material and low interface resistance; 5) Good mechanical properties to adapt to the volume change of the positive electrode material.

[0043] Solid electrolyte materials are expected to improve charge transfer at the positive electrode / electrolyte interface due to their high ionic conductivity. However, the key is to construct a matching ion transport channel between the coating material and the coated material, so as to effectively reduce the interface impedance between the positive electrode material and the active coating layer, and thus construct an efficient ion rapid transport channel.

[0044] As Figure 1 shown, the present invention provides a composite positive electrode material for lithium-ion batteries. First, a lithium-sodium mixed solid electrolyte is synthesized by the lithium-sodium ion exchange method, and it is mechanically fused with the positive electrode active material, and the composite positive electrode material for lithium-ion batteries is obtained after electrochemical pretreatment. On the one hand, the high stability and high ionic conductivity of the lithium-sodium mixed solid electrolyte coating layer itself can strengthen the stability and rapid ion transport of the positive electrode active material. On the other hand, sodium in the lithium-sodium mixed solid electrolyte is easy to undergo in-situ ion exchange with lithium on the surface of the positive electrode active material, constructing a progressive and unobstructed ion transport channel at the interface, realizing the matching with the ion transport channel of the positive electrode active material, greatly reducing the interface impedance, and at the same time realizing sodium doping on the surface of the positive electrode active material, effectively improving the positive electrode capacity performance and the stability of the interface structure.

[0045] Example 1

[0046] A lithium-sodium mixed solid electrolyte Li 2.3 Na 0.7 Zr2Si2PO 12 is obtained by liquid-phase lithium-sodium ion exchange, and its preparation method includes the following steps:

[0047] (1) The sodium ion conductor Na3Zr2Si2PO 12Mix evenly with lithium salt LiTFSI and ionic liquid 1-ethyl-3-methylimidazolium bis(trifluoromethanesulfonyl)imide, and put them into a reaction kettle. The concentration of the lithium salt in the ionic liquid is 0.5 mol / L, and the molar ratio of LiTFSI to Na3Zr2Si2PO 12 is 3:1; put the reaction kettle into a homogeneous reactor and rotate and stir at a speed of 50 rpm for 48 h at a temperature of 120 °C;

[0048] (2) Take out the reaction product in the reaction kettle, centrifuge and separate it, dry it, and calcine it at 450 °C to obtain the lithium-sodium mixed solid electrolyte Li 2.3 Na 0.7 Zr2Si2PO 12 .

[0049] The preparation method of the composite cathode material for lithium-ion batteries is as follows:

[0050] (1) Accurately weigh the cathode active material lithium nickel cobalt manganate (LiNi 0.8 Co 0.1 Mn 0.1 O2, that is, NCM811) and the lithium-sodium mixed solid electrolyte Li 2.3 Na 0.7 Zr2Si2PO 12 according to the mass ratio of 97:3;

[0051] (2) Put the weighed materials into a mechanical fusion device and fuse them at a speed of 1500 rpm for 15 min to obtain the composite cathode material for lithium-ion batteries.

[0052] When the composite cathode material for lithium-ion batteries is applied to lithium-ion batteries, step-by-step formation is required, and the steps are as follows:

[0053] The first formation step:

[0054] Step 1: Constant current charge at a rate of 0.05C to 120% of the designed capacity of the lithium-ion battery, and let it stand for 5-10 min;

[0055] Step 2: Constant current discharge at a rate of 0.05C to 120% of the designed capacity of the lithium-ion battery, and let it stand for 5-10 min;

[0056] Step 3: Repeat steps 1-2 for a total of 2 times;

[0057] Step 4: After the above steps are completed, perform the first vacuum sealing.

[0058] The second formation step:

[0059] Step 1: Constant current charge at a rate of 0.1C to 100% of the designed capacity of the lithium-ion battery;

[0060] Step 2: Constant voltage charge until the current drops to 0.01C, and stand still for 5 - 10 min;

[0061] Step 3: Constant current discharge at a rate of 0.1C until 100% of the designed capacity of the lithium-ion battery, and stand still for 5 - 10 min;

[0062] Step 4: Repeat Steps 1 - 3 for a total of 2 times;

[0063] Step 5: After the above steps are completed, perform the second vacuum sealing.

[0064] As Figure 2 shown, after mechanical fusion, the lithium-sodium mixed solid electrolyte Li 2.3 Na 0.7 Zr2Si2PO 12 is uniformly coated on the surface of the cathode active material lithium nickel cobalt manganese oxide.

[0065] As Figure 3 shown, after the formation of the composite cathode material of the lithium-ion battery, both the diffraction peaks of NCM811 (JCPDS: 74–0919) and the diffraction peaks of Li 2.3 Na 0.7 Zr2Si2PO 12 (marked with ◆) appear in its XRD pattern, proving the stability of Li 2.3 Na 0.7 Zr2Si2PO 12 on the surface of NCM811. Compared with the original NCM811, the (003) diffraction peak of the composite cathode material of the lithium-ion battery shifts towards a lower 2θ angle direction, indicating an increase in the interlayer spacing of the (003) crystal plane, which proves that sodium ions with a larger radius than lithium ions are doped into the lithium layer.

[0066] As Figure 4 shown, this is the time-of-flight secondary ion mass spectrometry (TOF-SIMS) test result of the composite cathode material of the lithium-ion battery prepared in Example 1 of the present invention before and after formation. The test target is a single composite cathode material particle of the lithium-ion battery, and the test depth is 336 nm (approximately the thickness of the coating layer), and sputtering is carried out at a rate of 0.28 nm / s for 1200 s. From Figure 4 it can be seen that after formation, the content of Li + on the surface of the lithium-ion battery composite material increases, the content of Na + decreases, and it tends to be stable at a test depth of 200 nm - 300 nm. Combining Figure 3 the results, it can be speculated that during the formation process, Na 2.3 in Li 0.7 Zr2Si2PO 12 the Na +penetrates into NCM811, and Li in NCM811 + then migrates towards Li 2.3 Na 0.7 Zr2Si2PO 12 migrates in the bulk phase, proving that an in-situ lithium-sodium ion exchange reaction occurs at the interface between the cathode active material and the solid-state electrolyte.

[0067] Example 2

[0068] A lithium-sodium mixed solid-state electrolyte Li 2.5 Na 0.5 La(PO4)2 is obtained by liquid-phase lithium-sodium ion exchange. The preparation method includes the following steps:

[0069] (1) Mix the sodium ion conductor Na3La(PO4)2, the lithium salt LiTFSI, and the ionic liquid 1-ethyl-3-methylimidazolium bis(trifluoromethylsulfonyl)imide evenly, and put them into a reaction kettle. The concentration of the lithium salt LiTFSI in the ionic liquid is 0.5 mol / L, and the molar ratio of LiTFSI to Na3La(PO4)2 is 3:1; place the reaction kettle in a homogeneous reactor and rotate and stir at a speed of 50 rpm / min for 36 h, and the temperature is 120 °C;

[0070] (2) Take out the reaction product in the reaction kettle, centrifuge and separate it, dry it, and calcine it at 400 °C to obtain the lithium-sodium mixed solid-state electrolyte Li 2.5 Na 0.5 La(PO4)2.

[0071] The preparation method of the composite cathode material for lithium-ion batteries is as follows:

[0072] (1) Weigh the cathode active material lithium nickel cobalt manganese oxide (LiNi 0.8 Co 0.1 Mn 0.1 O2, that is, NCM811) and the lithium-sodium mixed solid-state electrolyte Li 2.5 Na 0.5 La(PO4)2 accurately according to the mass ratio of 95:5;

[0073] (2) Mix the weighed materials and put them into a mechanical fusion device, and fuse them at a speed of 1000 rpm / min for 30 min to obtain the composite cathode material for lithium-ion batteries.

[0074] When the composite cathode material for lithium-ion batteries is applied to a lithium-ion battery, step-by-step formation is required. The steps are as follows:

[0075] The first formation step:

[0076] Step 1: Constant current charge at a rate of 0.05C to 120% of the designed capacity of the lithium-ion battery, and let it stand for 5 - 10 min;

[0077] Step 2: Constant current discharge at a rate of 0.05C until 120% of the designed capacity of the lithium-ion battery, and let it stand for 5 - 10 min;

[0078] Step 3: Repeat Steps 1 - 2 for a total of 2 times;

[0079] Step 4: After the above steps are completed, perform the first vacuum sealing.

[0080] The second formation step:

[0081] Step 1: Constant current charge at a rate of 0.1C until 100% of the designed capacity of the lithium-ion battery;

[0082] Step 2: Constant voltage charge until the current drops to 0.01C, and let it stand for 5 - 10 min;

[0083] Step 3: Constant current discharge at a rate of 0.1C until 100% of the designed capacity of the lithium-ion battery, and let it stand for 5 - 10 min;

[0084] Step 4: Repeat Steps 1 - 3 for a total of 2 times;

[0085] Step 5: After the above steps are completed, perform the second vacuum sealing.

[0086] As Figure 5 shown, the first-week charge and discharge curves of the lithium-ion battery (A) prepared with the uncoated positive active material (A) and the lithium-ion battery (B) prepared with the composite positive electrode material of the lithium-ion battery in Example 2 are presented. According to the test data, the first-week charge capacity of the uncoated positive active material is 6.52 Ah, the first-week discharge capacity is 5.73 Ah, and the first efficiency is 87.9%. The first-week charge capacity of the composite positive electrode material of the lithium-ion battery is 6.23 Ah, the first-week discharge capacity is 5.92 Ah, and the first efficiency is 95.0%. It can be seen that the first efficiency and capacity performance of the composite positive electrode material of the lithium-ion battery prepared in the present invention are significantly improved.

[0087] Example 3

[0088] Obtain the lithium-sodium mixed solid electrolyte Li 2.1 Na 0.9 Zr2Si2PO 12 through liquid-phase lithium-sodium ion exchange, and its preparation method includes the following steps:

[0089] (1) Mix the sodium ion conductor Na3Zr2Si2PO 12Mix uniformly with lithium salt LiTFSI and ionic liquid 1-ethyl-3-methylimidazolium bis(trifluoromethylsulfonyl)imide, and put them into a reaction kettle. The concentration of the lithium salt LiTFSI in the ionic liquid is 0.5 mol / L, and the molar ratio of LiTFSI to Na3Zr2Si2PO 12 is 3:1; put the reaction kettle into a homogeneous reactor and rotate and stir at a speed of 50 rpm / min for 24 h, and the temperature is 120 °C;

[0090] (2) Take out the reaction product in the reaction kettle, centrifuge and separate it, dry it, and calcine it at 450 °C to obtain the lithium-sodium mixed solid electrolyte Li 2.1 Na 0.9 Zr2Si2PO 12 .

[0091] The preparation method of the composite cathode material for lithium-ion batteries is as follows:

[0092] (1) Weigh accurately the cathode active material lithium-rich manganese-based Li 1.2 Mn 0.54 Ni 0.13 Co 0.13 O2 and the lithium-sodium mixed solid electrolyte Li 2.1 Na 0.9 Zr2Si2PO 12 at a mass ratio of 95:5;

[0093] (2) Put the weighed materials into a mechanical fusion device and fuse them at a speed of 1500 rpm / min for 15 min to obtain the composite cathode material for lithium-ion batteries.

[0094] When the composite cathode material for lithium-ion batteries is applied to lithium-ion batteries, step-by-step formation is required, and the steps are as follows:

[0095] The first formation step:

[0096] Step 1: Constant current charge at a rate of 0.05C to 120% of the designed capacity of the lithium-ion battery, and let it stand for 5-10 min;

[0097] Step 2: Constant current discharge at a rate of 0.05C to 120% of the designed capacity of the lithium-ion battery, and let it stand for 5-10 min;

[0098] Step 3: Repeat steps 1-2 for a total of 2 times;

[0099] Step 4: After the above steps are completed, perform the first vacuum sealing.

[0100] The second formation step:

[0101] Step 1: Constant current charge at a rate of 0.1C to 100% of the designed capacity of the lithium-ion battery;

[0102] Step 2: Constant voltage charge until the current drops to 0.01C, and stand still for 5 - 10 min;

[0103] Step 3: Constant current discharge at a rate of 0.1C until 100% of the designed capacity of the lithium-ion battery, and stand still for 5 - 10 min;

[0104] Step 4: Repeat Steps 1 - 3 for a total of 2 times;

[0105] Step 5: After the above steps are completed, perform the second vacuum pumping and sealing.

[0106] As Figure 6 shown, the microcalorimeter test curves of the uncoated positive active material (A) in the fully charged state and the lithium-ion battery composite positive material (B) prepared in Example 3 are presented. It can be seen that the thermal decomposition temperature of the uncoated positive active material is 187°C, and that of the lithium-ion battery composite positive material is 210°C. That is to say, the thermal decomposition temperature of the coated material has increased by 23°C, and the thermal stability has been greatly improved.

[0107] In summary, the lithium-ion battery composite positive material of the present invention comprises a positive active material and a lithium-sodium mixed solid electrolyte coated on the surface of the positive active material. During the formation stage of the lithium-ion battery, sodium in the lithium-sodium mixed solid electrolyte and lithium on the surface of the positive active material undergo an in-situ lithium-sodium ion exchange driven electrochemically, that is, sodium in the lithium-sodium mixed solid electrolyte enters the lithium layer of the positive active material, and a corresponding amount of lithium on the surface of the positive active material enters the sodium vacancies in the lithium-sodium mixed solid electrolyte. Based on the dual modification and modification strategy of lithium-sodium mixed solid electrolyte coating and ion-exchange-driven in-situ sodium doping, the present invention can effectively improve the capacity performance, electrochemical stability, and thermal stability of high-nickel ternary materials or lithium-rich manganese-based positive materials, providing support for their application in high specific energy and high safety lithium batteries.

[0108] Although the content of the present invention has been described in detail through the above preferred embodiments, it should be recognized that the above description should not be considered as a limitation of the present invention. After those skilled in the art have read the above content, various modifications and substitutions to the present invention will be obvious. Therefore, the protection scope of the present invention should be defined by the appended claims.

Claims

1. A composite cathode material for lithium-ion batteries, characterized in that, The composite cathode material of the lithium-ion battery comprises: a cathode active material; a lithium-sodium mixed solid electrolyte, and the lithium-sodium mixed solid electrolyte is coated on the surface of the cathode active material.

2. The composite cathode material for lithium ion battery according to claim 1, characterized in that, The cathode active material has a layered structure.

3. The composite cathode material for lithium ion battery according to claim 2, wherein The cathode active material includes any one of a high-nickel ternary material and a lithium-rich manganese-based material.

4. The composite cathode material for lithium-ion battery according to claim 1, characterized in that The average particle size of the cathode active material is 5 μm to 20 μm, and the average particle size of the lithium-sodium mixed solid electrolyte is 50 nm to 300 nm.

5. The composite cathode material for lithium ion battery according to claim 1, characterized in that, The lithium-sodium mixed solid electrolyte is selected from Li 3-x Na x Zr2Si2PO 12 、Li 3-x Na x V2(PO4)3, Li 3-x Na x La(PO4)2, Li 3-x Na x Hf2Si2PO 12 wherein any one of them, and 0.3 < x < 1.

6. The composite cathode material for lithium-ion battery according to claim 1, wherein The lithium-sodium mixed solid electrolyte accounts for 1 wt.% to 5 wt.% of the composite cathode material of the lithium-ion battery.

7. The composite cathode material for lithium-ion battery according to claim 1, wherein The preparation method of the lithium-sodium mixed solid electrolyte comprises the following steps: S1. Mix a sodium ion conductor, lithium salt LiTFSI, and an ionic liquid evenly, put them into a reaction kettle, place the reaction kettle in a homogeneous reactor, and rotate and stir at a speed of 50 rpm / min for 24 h to 72 h, with the temperature being 120°C; S2. Take out the reaction product in the reaction kettle, centrifuge and dry it, and calcine it at 400°C to 500°C to obtain the lithium-sodium mixed solid electrolyte.

8. The composite cathode material for lithium ion battery according to claim 7, characterized in that, The concentration of the lithium salt LiTFSI in the ionic liquid is 0.1 mol / L to 0.5 mol / L, and the molar ratio of the lithium salt LiTFSI to the sodium ion conductor is 3:

1.

9. A method for preparing a composite cathode material for a lithium-ion battery according to any one of claims 1 to 8, characterized in that, Comprises the following steps: S1. Weigh the cathode active material and the lithium-sodium mixed solid electrolyte; S2. Mix the cathode active material and the lithium-sodium mixed solid electrolyte and place them in a mechanical fusion device, and fuse them at a speed of 1000 rpm / min to 1500 rpm / min for 15 min to 30 min to obtain the composite cathode material of the lithium-ion battery.

10. A formation method for a lithium-ion battery prepared from the lithium-ion battery composite cathode material according to any one of claims 1 to 8, characterized in that, Comprises the following steps: The first formation step: Step 1: Constant current charge at a rate of 0.05C to 120% of the designed capacity of the lithium-ion battery, and let it stand for 5 - 10 min; Step 2: Constant current discharge at a rate of 0.05C to 120% of the designed capacity of the lithium-ion battery, and let it stand for 5 - 10 min; Step 3: Repeat Steps 1 - 2 for a total of 2 times; Step 4: After the above steps are completed, perform the first vacuum sealing; The second formation step: Step 1: Constant current charge at a rate of 0.1C to 100% of the designed capacity of the lithium-ion battery; Step 2: Constant voltage charge until the current drops to 0.01C, and let it stand for 5 - 10 min; Step 3: Constant current discharge at a rate of 0.1C to 100% of the designed capacity of the lithium-ion battery, and let it stand for 5 - 10 min; Step 4: Repeat Steps 1 - 3 for a total of 2 times; Step 5: After the above steps are completed, perform the second vacuum sealing.

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