Composite positive electrode material containing long / short-range conductive agent and preparation method and application thereof

Abstract

The application belongs to the technical field of lithium ion batteries, and particularly relates to a composite positive electrode material containing long / short-range conductive agents and a preparation method and application thereof. The material comprises positive electrode active particles, zero-dimensional short-range conductive agents connected to adjacent positive electrode active particles through physical adsorption, and one-dimensional long-range conductive agents connected to the positive electrode active particles through physical adsorption, and the gaps between the positive electrode active particles are filled with a conductive network formed by the zero-dimensional short-range conductive agents and the one-dimensional long-range conductive agents; the zero-dimensional short-range conductive agents have a nanosphere structure; and the one-dimensional long-range conductive agents have a one-dimensional linear structure. The material has a long-range / short-range composite conductive network, can be tightly filled between the active particles, significantly improves the electronic and ionic conductivities and structural stability of the positive electrode active material, and realizes excellent rate performance.

Description

TECHNICAL FIELD

[0001] The application belongs to the technical field of lithium ion batteries, and particularly relates to a composite positive electrode material containing long / short-range conductive agents and a preparation method and application thereof. BACKGROUND

[0002] As a mature commercialized positive electrode material, lithium cobalt oxide (LiCoO2) has a theoretical specific capacity as high as 270 mAh·g-1. -1 It has achieved great success in the field of consumer electronics such as notebook computers, mobile phones, electronic bracelets and mobile power sources. However, LiCoO2 material only plays about half of the theoretical specific capacity in the safety cut-off voltage interval of 3.0-4.2 V, and its poor electronic conductivity and ionic conductivity limit its application in high-power scenarios. For example, at room temperature, the electronic conductivity of LiCoO2 material is only 5×10 -8 S·cm -1 , which is far lower than the electronic conductivity 4.1×10 -3 S·cm -1 of NCM 811 material; and the ionic conductivity of LiCoO2 material is only 2.3×10 -7 S·cm -1 , while the ionic conductivity of NCM 811 material reaches 6.3×10 -3 S·cm -1 .

[0003] Therefore, whether from the electronic conductivity or the ionic conductivity, LiCoO2 material shows poor rate performance among existing positive electrode materials, and in the process of charging and discharging, the surface of LiCoO2 material is prone to side reactions with electrolyte to form impurities such as Li2CO3 and LiOH with low conductivity, which further affects the rate performance of LiCoO2 positive electrode material. SUMMARY

[0004] Therefore, the purpose of the present application is to provide a composite positive electrode material containing long / short-range conductive agents and a preparation method and application thereof, which can significantly improve the electronic and ionic conductivity and structural stability of the positive electrode active material, thereby realizing excellent rate performance.

[0005] In order to achieve the above purpose, the present application provides the following technical solutions:

[0006] The present application provides a composite positive electrode material containing long / short-range conductive agents, comprising: positive electrode active particles, zero-dimensional short-range conductive agents connecting adjacent positive electrode active particles through physical adsorption, and one-dimensional long-range conductive agents connecting the positive electrode active particles through physical adsorption, and the gaps between the positive electrode active particles being filled with the conductive network formed by the zero-dimensional short-range conductive agents and the one-dimensional long-range conductive agents.

[0007] The zero-dimensional short-range conductive agent has a nanospherical structure.

[0008] The one-dimensional long-range conductive agent has a one-dimensional linear structure.

[0009] Preferably, the zero-dimensional short-range conductive agent comprises one or more of carbon black, conductive graphite and ketjen black.

[0010] Preferably, the one-dimensional long-range conductive agent comprises vapor grown carbon fiber and / or carbon nanotube; the aspect ratio L / D of the vapor grown carbon fiber is greater than or equal to 30:1, and the aspect ratio L / D of the carbon nanotube is greater than or equal to 1000:1.

[0011] Preferably, the positive electrode active particle comprises one or more of lithium cobalt ternary material, lithium iron phosphate ternary material, lithium nickel ternary material, nickel cobalt manganese ternary material and nickel cobalt aluminum ternary material.

[0012] Preferably, the mass ratio of the one-dimensional long-range conductive agent and the zero-dimensional short-range conductive agent is (1-4):(1-4).

[0013] The application further provides a preparation method of the composite positive electrode material containing long / short-range conductive agent as described in the above technical solution, comprising the following steps:

[0014] Mixing the one-dimensional long-range conductive agent, the zero-dimensional short-range conductive agent and the organic solvent, and performing water bath ultrasonic treatment to obtain a conductive agent dispersion liquid;

[0015] Mixing the conductive agent dispersion liquid and the positive electrode active particle and performing ball milling to obtain the composite positive electrode material containing long / short-range conductive agent.

[0016] Preferably, the temperature of the water bath ultrasonic treatment is less than or equal to 20℃; the time of the water bath ultrasonic treatment is 5-60min; and the power of the water bath ultrasonic treatment is 450-600W.

[0017] Preferably, the ball-to-material ratio of the ball milling is 1:1-20; the rotation speed of the ball milling is 100-500rpm; and the time of the ball milling is 30-720min.

[0018] The application further provides an application of the composite positive electrode material containing long / short-range conductive agent as described in the above technical solution or the composite positive electrode material containing long / short-range conductive agent prepared by the preparation method as described in the above technical solution in a lithium ion battery.

[0019] The application further provides a lithium ion battery comprising a positive electrode, a negative electrode, a separator and an electrolyte, wherein the active material in the positive electrode is the composite positive electrode material containing long / short-range conductive agent as described in the above technical solution or the composite positive electrode material containing long / short-range conductive agent prepared by the preparation method as described in the above technical solution.

[0020] The present application provides a composite cathode material containing long / short-range conductive agents, comprising: cathode active particles, zero-dimensional short-range conductive agents connecting adjacent cathode active particles through physical adsorption, and one-dimensional long-range conductive agents connecting the cathode active particles through physical adsorption, the space between the cathode active particles being filled with the conductive network formed by the zero-dimensional short-range conductive agents and the one-dimensional long-range conductive agents; the zero-dimensional short-range conductive agents have a nanosphere structure; and the one-dimensional long-range conductive agents have a one-dimensional linear structure. In the present application, the long-range / short-range conductive network not only provides "point-point contact" electron conduction between adjacent active material particles, but also provides "point-line contact" electron conduction between multiple active material particles in a local area. This microstructure can not only alleviate the volume strain accompanied by the charging and discharging process, effectively improve the capacity loss caused by the loss of electrical contact of the active material particles, but also introduce a conductive network with stable structure and large-area fast conduction capability in the entire electrode, which can achieve better rate performance on the entire electrode scale. The present application ingeniously designs a long-range / short-range composite conductive path for the disordered packed cathode active particles, and improves the fast electron and ion transport on the surface of the irregularly distributed cathode active particles, so that the cathode active particles have high electronic and ionic conductivity.

[0021] The present application also provides a preparation method of the above-mentioned composite cathode material containing long / short-range conductive agents. The present application obtains a uniformly dispersed electrode slurry through ultrasonic and ball milling, and the obtained electrode material has a long-range / short-range composite conductive network, can be tightly filled between active particles, and significantly improves the electronic and ionic conductivity and structural stability of the cathode active material, and realizes excellent rate performance. BRIEF DESCRIPTION OF DRAWINGS

[0022] Figure 1 The microstructure schematic diagram of the electrode formed by the composite cathode material in Example 1 and Comparative Examples 1-2, wherein (a) is a short-range conductive agent treatment, (b) is a long-range conductive agent treatment, and (c) is a long-range / short-range conductive agent treatment;

[0023] Figure 2 The SEM images of LiCoO2 particles, VGCF, LiCoO2 / VGCF-CNTs-super C electrodes in Example 6, wherein (a) is the LiCoO2 particles, (b) is the VGCF, (c) is the surface morphology of the LiCoO2 / VGCF-CNTs-super C electrode, (d) is the cross-sectional morphology of the LiCoO2 / VGCF-CNTs-super C electrode, (e) is the local enlarged view of the LiCoO2 / VGCF-CNTs-super C electrode, and (f) is Co, O, C, F four elements;

[0024] Figure 3Cyclic voltammogram of the batteries obtained in Application Example 1 and Comparative Application Example 1, wherein (a) is a LiCoO2 electrode containing only a short-range conductive agent, and (b) is a LiCoO2 / VGCF-CNTs-super C composite electrode containing a long-range conductive agent and a short-range conductive agent;

[0025] Figure 4 Impedance comparison of the batteries obtained in Application Example 1 and Comparative Application Example 1, wherein (a) is a LiCoO2 electrode containing only a short-range conductive agent, and (b) is a LiCoO2 / VGCF-CNTs-super C composite electrode containing a long-range conductive agent and a short-range conductive agent;

[0026] Figure 5 CV curve of the batteries obtained in Application Example 1 corresponding to different cut-off voltage ranges;

[0027] Figure 6 Comparison of the cycle performance of LiCoO2 cathodes with different pretreatment temperatures and VGCF ratios;

[0028] Figure 7 Influence of different pretreatment temperatures and VGCF ratios on the rate performance of LiCoO2 cathodes;

[0029] Figure 8 Influence of different conductive agents and binder types on the rate performance of LiCoO2 cathodes;

[0030] Figure 9 Influence of different separator types on the rate performance of LiCoO2 cathodes;

[0031] Figure 10 Influence of different electrode surface modification layer thicknesses on the rate performance of LiCoO2 cathodes;

[0032] Figure 11 Influence of different VGCF:CNT ratios on the rate performance of cathodes;

[0033] Figure 12 Surface morphology of the LiCoO2 / VGCF-CNTs-super C electrode prepared in Example 6 on the surface of an Al foil. DETAILED DESCRIPTION

[0034] The present application provides a composite cathode material containing long / short-range conductive agents, comprising: cathode active particles, zero-dimensional short-range conductive agents connecting adjacent cathode active particles through physical adsorption, and one-dimensional long-range conductive agents connecting the cathode active particles through physical adsorption, the voids between the cathode active particles being filled with the conductive network formed by the zero-dimensional short-range conductive agents and the one-dimensional long-range conductive agents;

[0035] The zero-dimensional short-range conductive agent has a nanospherical structure.

[0036] The one-dimensional long-range conductive agent has a one-dimensional linear structure.

[0037] The source of the raw materials used in the present application is not particularly limited, and commercially available products known to those skilled in the art can be used.

[0038] The composite positive electrode material containing long / short-range conductive agents provided by the present application comprises: positive electrode active particles; and the space between the positive electrode active particles is filled with a conductive network formed by the zero-dimensional short-range conductive agent and the one-dimensional long-range conductive agent. In the present application, the positive electrode active particles preferably comprise one or more of lithium cobalt oxide ternary material, lithium iron phosphate ternary material, lithium nickelate ternary material, nickel-cobalt-manganese ternary material and nickel-cobalt-aluminum ternary material, and more preferably lithium cobalt oxide ternary material; the particle size of the positive electrode active particles is preferably 100 nm to 100 μm, and more preferably 100 nm to 50 μm; the positive electrode active particles comprise large-particle-size particles and small-particle-size particles, wherein the particle size of the large-particle-size particles is preferably > 50 μm and ≤ 100 μm, and the particle size of the small-particle-size particles is preferably 100 nm to 50 μm; and the volume ratio of the large-particle-size particles to the small-particle-size particles is preferably 1:1.

[0039] The composite positive electrode material containing long / short-range conductive agents provided by the present application comprises: zero-dimensional short-range conductive agents connecting adjacent positive electrode active particles through physical adsorption; and the zero-dimensional short-range conductive agents have a nanospherical structure. In the present application, the zero-dimensional short-range conductive agents preferably comprise one or more of carbon black, conductive graphite and Ketjen black, and more preferably carbon black or Ketjen black; the carbon black is preferably Super C 45; and the particle size of the zero-dimensional short-range conductive agents is preferably 0.05 to 15 μm, and more preferably 50 to 100 nm.

[0040] The composite positive electrode material containing long / short-range conductive agents provided by the present application comprises: one-dimensional long-range conductive agents connecting positive electrode active particles through physical adsorption; and the one-dimensional long-range conductive agents have a one-dimensional linear structure. In the present application, the one-dimensional long-range conductive agents preferably comprise vapor-grown carbon fibers VGCF and / or carbon nanotubes CNTs, and more preferably vapor-grown carbon fibers; the aspect ratio of the vapor-grown carbon fibers is preferably L / D ≥ 30:1, and the aspect ratio of the carbon nanotubes is preferably L / D ≥ 1000:1.

[0041] The VGCF and CNTs in the application have a one-dimensional linear structure, can generally wrap around the active material particles to form point-line contact, form a uniform point-line connection conductive network inside the electrode, have a large aspect ratio, and even if the active material is irreversibly deformed, the gap formed between the particles can be reconnected by the VGCF and CNTs, so that the electron transmission path is not interrupted, at the same time, the entire electrode benefits from the high strength of the VGCF and CNTs material, increases the adhesion between the active material particles, and will not be seriously shed due to deformation. In addition, the VGCF and CNTs have a special hollow structure, and a large amount of electrolyte can be stored inside, which is beneficial to the rapid transmission of lithium ions around, shortens the ion transmission path, and the electrode containing the long-range conductive agent can exhibit high-rate charge and discharge performance.

[0042] In the application, the mass ratio of the positive active particles and the zero-dimensional short-range conductive agent is preferably (70-95):(1-10), and more preferably (70-85):(4-10); the mass ratio of the one-dimensional long-range conductive agent and the zero-dimensional short-range conductive agent is preferably (1-4):(1-4), and more preferably (2-3):(2-3).

[0043] The application also provides a preparation method of the composite positive electrode material containing long / short-range conductive agents as described in the above technical solution, which comprises the following steps:

[0044] The one-dimensional long-range conductive agent, the zero-dimensional short-range conductive agent and the organic solvent are mixed and subjected to water bath ultrasonic treatment to obtain a conductive agent dispersion liquid.

[0045] The conductive agent dispersion liquid and the positive active particles are mixed and subjected to ball milling to obtain the composite positive electrode material containing long / short-range conductive agents.

[0046] The one-dimensional long-range conductive agent, the zero-dimensional short-range conductive agent and the organic solvent are mixed and subjected to water bath ultrasonic treatment to obtain a conductive agent dispersion liquid.

[0047] In the application, the organic solvent is preferably N-methyl pyrrolidone (NMP); the mass ratio of the sum of the zero-dimensional short-range conductive agent and the one-dimensional long-range conductive agent to the organic solvent is preferably (0.1-4):(1-6), and more preferably 0.2:4.

[0048] In the application, the temperature of the water bath ultrasonic treatment is preferably ≤20℃, and more preferably 10-15℃; the time of the water bath ultrasonic treatment is preferably 5-60 min, and more preferably 20 min; the power of the water bath ultrasonic treatment is preferably 450-600 W, and more preferably 500-550 W.

[0049] After obtaining the conductive agent dispersion liquid, the present application mixes the conductive agent dispersion liquid and the positive active particles to perform ball milling, thereby obtaining the composite positive electrode material containing long / short-range conductive agent.

[0050] When the positive active particles are lithium cobalt oxide ternary material, before mixing the conductive agent dispersion liquid and the positive active particles, the present application preferably further comprises: preheating the positive active particles; the preheating is preferably performed in a mixed atmosphere of argon and oxygen; the volume ratio of the argon and oxygen is preferably (100-95):(0.5-5), more preferably 98:2; the temperature of the preheating is preferably 550-600℃, more preferably 550℃ or 600℃, most preferably 550℃; the holding time of the preheating is preferably 2h; the heating rate to the preheating temperature is preferably 5℃ / min. The present application compensates for the lack of lattice oxygen in commercial LiCoO2 through preheating, and other types of positive active materials generally do not use a preheating step.

[0051] In the present application, the mixing of the conductive agent dispersion liquid and the positive active particles is preferably performed under magnetic stirring; the rotation speed of the magnetic stirring is preferably 100-800rpm, more preferably 250-550rpm; the time of the magnetic stirring is preferably 5-240min, more preferably 60min.

[0052] In the present application, the ball-to-material ratio of the ball milling is preferably 1:1-20, more preferably 1:5-10; the rotation speed of the ball milling is preferably 100-500rpm, more preferably 200-300rpm; the time of the ball milling is preferably 30-720min, more preferably 60-120min; the ball milling is preferably performed in a agate tank; the grinding balls used in the ball milling are preferably stainless steel balls and / or agate balls, more preferably agate balls; the grinding balls used in the ball milling preferably include grinding balls with a diameter of 10mm and grinding balls with a diameter of 5mm; the mass ratio of the grinding balls with a diameter of 10mm to the grinding balls with a diameter of 5mm is preferably 1:1.

[0053] The VGCF and CNTs used in the present application have the same shortcomings as traditional conductive agents, i.e. the nanoscale materials are not easy to disperse due to electrostatic interaction. The present application successfully prepares electrode slurry with uniform dispersion of conductive agent by high-speed shearing through ultrasonic-ball milling process, thereby solving the dispersion problem.

[0054] After the ball milling, the present application preferably further comprises: drying the slurry obtained by the ball milling to obtain the composite positive electrode material containing long / short-range conductive agent. In the present application, the temperature of the drying is preferably 80-150℃, more preferably 100-120℃; the time of the drying is preferably 2-24h, more preferably 6-12h.

[0055] The beneficial effects of the present application relative to the existing positive electrode modification technology are: the problem of poor positive electrode material conductivity is solved by directly adding different types of conductive agents in the process of ball milling slurry, compared with the method of using complex modification methods such as doping, coating, sintering and the like to modify the positive electrode material to improve its electronic / ion transmission performance, the preparation method provided by the present application is more conducive to the batch preparation of the positive electrode material, and the rate performance is improved.

[0056] The present application also provides the application of the composite positive electrode material containing long / short-range conductive agent in the above technical solution or the composite positive electrode material containing long / short-range conductive agent prepared by the preparation method in the above technical solution in lithium ion batteries.

[0057] The present application also provides a lithium ion battery, which comprises a positive electrode, a negative electrode, a separator and an electrolyte, wherein the active material in the positive electrode is a composite positive electrode material containing long / short-range conductive agent; the composite positive electrode material containing long / short-range conductive agent is the composite positive electrode material containing long / short-range conductive agent in the above technical solution or the composite positive electrode material containing long / short-range conductive agent prepared by the preparation method in the above technical solution.

[0058] In the present application, the preparation method of the positive electrode preferably comprises the following steps:

[0059] The one-dimensional long-range conductive agent, the zero-dimensional short-range conductive agent and the solution containing the binder are mixed, and water bath ultrasonic is performed to obtain a conductive agent dispersion liquid;

[0060] The conductive agent dispersion liquid and the positive electrode active particles are mixed and ball milled, the obtained ball milling slurry is coated on the surface of an Al foil current collector, first dried, and then pressed and second dried.

[0061] In the present application, the preparation method of the binder-containing solution is preferably: mixing the binder and the organic solvent, sequentially performing water bath ultrasonic or magnetic stirring to obtain the binder-containing solution; the binder is preferably polyvinylidene fluoride (PVDF); the model of the polyvinylidene fluoride (PVDF) is preferably HSV900 or 5310; the organic solvent is preferably N-methyl pyrrolidone (NMP); the mass of the binder is preferably 2-5% of the mass of the organic solvent, more preferably 2.5%; the temperature of the water bath ultrasonic is preferably ≤20℃, more preferably 10-15℃; the power of the water bath ultrasonic is preferably 450-600W, more preferably 500-550W; the time of the water bath ultrasonic is preferably 5-30min, more preferably 10-20min; the temperature of the magnetic stirring is preferably 10-50℃, more preferably 10-20℃; the speed of the magnetic stirring is preferably 200-700rpm, more preferably 300-400rpm; the time of the magnetic stirring is preferably 5-60min, more preferably 20min. In the present application, the binder can swell in the solution to form a polymer chain, which plays a role of linking the composite positive electrode material containing long / short-range conductive agents and the current collector.

[0062] In the present application, the process of mixing the one-dimensional long-range conductive agent, the zero-dimensional short-range conductive agent and the binder-containing solution and performing water bath ultrasonic and the process of mixing the conductive agent dispersion and the positive electrode active particles and performing ball milling are consistent with the processes in the preparation method of the composite positive electrode material containing long / short-range conductive agents, which will not be repeated here.

[0063] In the present application, the temperature of the first drying is preferably 100℃, and the time is preferably 6h; the first drying is preferably air drying; the pressure of the pressing is preferably 16MPa, and the time is preferably 10s; the temperature of the second drying is preferably 100℃, and the time is preferably 6h; the second drying is preferably performed in a vacuum drying box.

[0064] In the present application, the loading amount of the composite positive electrode material containing long / short-range conductive agents in the positive electrode is preferably 2-10mg·cm -2 , more preferably 3-4mg·cm -2 .

[0065] The composite positive electrode material containing long / short-range conductive agents provided by the present application can ensure good electrical contact between different active particles inside the positive electrode, and the assembled button cell has low charge transfer impedance and excellent rate performance.

[0066] The present application preferably further comprises: depositing gold (Au) on the positive electrode by magnetron sputtering; the time of the deposition is preferably 5-15s, more preferably 5s, 10s or 15s, and most preferably 15s. The present application does not have special limitations on the process of the magnetron sputtering, and a magnetron sputtering process well known in the art can be used.

[0067] In the present application, the negative electrode is preferably a lithium metal sheet; the separator is preferably a polypropylene (PP) separator or a cellulose separator, and more preferably a polypropylene (PP) separator; the thickness of the separator is preferably 3μm, 5.5μm or 12μm, and more preferably 5.5μm; the electrolyte is preferably a LiPF6 solution; the solvent used in the LiPF6 solution is a mixture of ethylene carbonate EC, dimethyl carbonate DMC and diethyl carbonate DEC; the volume ratio of the ethylene carbonate EC, dimethyl carbonate DMC and diethyl carbonate DEC is preferably 1:1:1; and the concentration of the LiPF6 solution is preferably 1.2mol / L.

[0068] The technical solutions in the present application will be described clearly and completely below in combination with the embodiments in the present application, but they should not be understood as limitations on the protection scope of the present application.

[0069] Example 1

[0070] 0.1g of PVDF was dissolved in 4g of NMP solvent, and ultrasonic treatment was carried out at 10℃ water bath with a power of 500W for 10min to obtain a uniformly dispersed binder-containing solution;

[0071] 0.1g of short-range conductive agent carbon black (Super C 45, particle size of 50-100nm) and 0.1g of long-range conductive agent VGCF (L / D≥1000:1) were added to the binder-containing solution, and ultrasonic treatment was continued at 10℃ water bath with a power of 500W for 20min to obtain a conductive agent dispersion;

[0072] 0.7g of commercial LiCoO2 positive electrode particles were directly added to the above conductive agent dispersion, and magnetic stirring was carried out at 500rpm for 60min, and then transferred to a maroon ball mill tank, and ball milling was carried out for 120min with a ball-to-material ratio of 1:10; the rotation speed of the ball milling was 300rpm, the grinding balls used were maroon balls, the diameters of the grinding balls included two sizes of 10mm and 5mm, and the mass ratio of the grinding balls was 1:1, to obtain a uniform slurry;

[0073] The above uniformly dispersed slurry was coated on an aluminum foil, and dried at 100℃ for 6h to obtain an electrode sheet of a composite positive electrode material loaded with long / short-range conductive agents.

[0074] Example 2

[0075] The difference from Example 1 is that the LiCoO2 positive electrode particles are preheated, an Ar / O2 mixed atmosphere with a volume ratio of 98:2 is selected, and the temperature is raised to 550℃ and 600℃ respectively at a step of 5℃ / min, and each is kept for 2h to make up for the lack of lattice oxygen in commercial LiCoO2, and the rest is the same as Example 1.

[0076] Example 3

[0077] The difference from Example 2 is that the mass ratio of long-range conductive agent VGCF and short-range conductive agent carbon black (Super C 45) is adjusted to VGCF:Super C=0.15g:0.05g, 0.05g:0.15g, 0.2g:0g respectively, and the three ratio composite positive electrode material samples are marked as VGCF-1, VGCF-2, VGCF-3 respectively, and the rest is the same as Example 2.

[0078] Example 4

[0079] The difference from Example 2 is that the short-range conductive agent carbon black (Super C 45) is replaced by Ketjen black (KB), and the rest is the same as Example 2.

[0080] Example 5

[0081] The difference from Example 2 is that the binder is changed from the traditional PVDF (model HSV900) to model 5310, and the rest is the same as Example 2.

[0082] Example 6

[0083] The difference from Example 2 is that the long-range conductive agent VGCF is replaced by part of CNTs, and the mass ratio of long-range conductive agent to short-range conductive agent is VGCF:CNTs:KB=0.12g:0.03g:0.05g, 0.075g:0.075g:0.05g, 0.03g:0.12g:0.05g, and the rest is the same as Example 2.

[0084] Comparative Example 1

[0085] The difference from Example 1 is that no long-range conductive agent is used, only short-range conductive agent is used, and the rest is the same as Example 1, to obtain a composite positive electrode material containing a short-range conductive agent.

[0086] Comparative Example 2

[0087] The difference from Example 1 is that no short-range conductive agent is used, only long-range conductive agent is used, and the rest is the same as Example 1, to obtain a composite positive electrode material containing a long-range conductive agent.

[0088] Application Example 1

[0089] The slurry obtained in Example 1 was uniformly coated on an aluminum foil current collector, and after air drying at 100°C for 6h to completely remove the NMP solvent, the positive electrode was cut into a circular sheet with a diameter of 11mm, and then treated using a hydraulic device at a pressure of 16MPa for 10s, weighed, and placed in a vacuum drying oven at 100°C for 6h to obtain an electrode sheet, which was then directly transferred into a glove box for use;

[0090] The above electrode sheet was used as a positive electrode, a lithium metal sheet was used as a counter electrode and a reference electrode, a conventional polypropylene separator PP (thickness of 12μm) was used as a separator, and a 1.2mol / L LiPF6 solution (solvent: a mixture of ethylene carbonate EC, dimethyl carbonate DMC and diethyl carbonate DEC, volume ratio of 1:1:1) was used as an electrolyte, to assemble a CR 2025 button cell in a glove box.

[0091] Application Example 2

[0092] The difference from Application Example 1 is that the composite positive electrode material containing long / short-range conductive agents prepared in Example 1 is replaced by the composite positive electrode material prepared in Example 2, and the rest is the same as Application Example 1.

[0093] Application Example 3

[0094] The difference from Application Example 1 is that the composite positive electrode material containing long / short-range conductive agents prepared in Example 1 is replaced by the composite positive electrode material prepared in Example 3, and the rest is the same as Application Example 1.

[0095] Application Example 4

[0096] The difference from Application Example 2 is that the separator is changed to a cellulose separator with thicknesses of 5.5μm and 3μm respectively, and the rest is the same as Application Example 2.

[0097] Application Example 5

[0098] The difference from Application Example 2 is that an Au modification layer is deposited on the surface of the electrode sheet, and the deposition time is designed to be 5s, 10s and 15s respectively, and the rest is the same as Application Example 2.

[0099] Comparative Application Example 1

[0100] The difference from Application Example 1 is that the composite positive electrode material containing long / short-range conductive agents prepared in Example 1 is replaced by the composite positive electrode material containing short-range conductive agents prepared in Comparative Example 1, and the rest is the same as Application Example 1.

[0101] Performance Test

[0102] (1) The electrodes formed by the composite positive electrode materials in Example 1 and Comparative Examples 1-2 were scanned by electron microscopy, and the results are as follows: Figure 1(a) short-range conductive agent treatment, (b) long-range conductive agent treatment, (c) long-range / short-range conductive agent treatment.

[0103] Figure 1 The structural schematic diagrams of three positive electrode materials are given, Figure 1 (a) zero-dimensional spherical particles are dispersed around LiCoO2 particles, Figure 1 (b) one-dimensional linear conductive agents are shown to be wound around LiCoO2 particles, Figure 1 (c) both zero-dimensional and one-dimensional conductive agents are shown to be uniformly distributed in the pores of LiCoO2 particles, forming a uniform conductive network. Figure 1 The electrode structure shown in (c) is an ideal electrode structure, which is conducive to the rapid transmission of electrons between different LiCoO2 particles.

[0104] (2) The LiCoO2 particles in Example 6, VGDF one-dimensional material, LiCoO2 / VGCF-CNTs-super C electrode were analyzed and tested by scanning electron microscope, and the results are shown in Figure 2 (a) LiCoO2 particles, (b) VGCF, (c) surface morphology of LiCoO2 / VGCF-CNTs-super C electrode, (d) cross-sectional morphology of LiCoO2 / VGCF-CNTs-super C electrode, (e) local enlarged view of LiCoO2 / VGCF-CNTs-super C electrode, (f) uniform distribution of Co, O, C, F four elements in the composite electrode.

[0105] Figure 2 It can be seen that after mixing conductive agent materials of different dimensions, a three-dimensional conductive system can be formed with LiCoO2 particles, and "point-point" contact and "point-line" contact are formed between adjacent two or more LiCoO2 particles. Compared with traditional conductive agents, which can only provide local area electron conduction, this three-dimensional conductive system can quickly transmit electrons.

[0106] (3) The batteries obtained in Application Example 1 and Comparative Application Example 1 were transferred to a new Wei charge-discharge tester for constant current charge-discharge test, the cutoff voltage was set to 3.0-4.4V, the test temperature was room temperature 25℃, and the rate performance test was carried out at different current densities, and the current density was set to 0.1-10A·g -1 The results are shown in Figures 3-5 LiCoO2 and LiCoO2(L) in the figure are Comparative Application Example 1 and Application Example 1, respectively.

[0107] Figure 3 ​​As shown in the CV curve, the addition of long-range conductive agent VGCF does not affect the chemical reaction accompanied by the process of deintercalation of lithium ions in LiCoO2, and the positions of the oxidation and reduction peaks remain unchanged. After overcoming the large polarization of the first charge and discharge cycle, the polarization degree of the second and third charge and discharge cycles decreases and remains unchanged. In addition, Figure 4 The electrochemical impedance spectroscopy (EIS) shown more significantly shows the advantage of adding long-range conductive agent. Compared with the LiCoO2 electrode with only short-range conductive agent, LiCoO2(L) has smaller contact impedance, with a value of 3.8Ω, which is smaller than the contact impedance of the LiCoO2 electrode of 6.1Ω. The effect of long-range conductive agent on reducing the contact impedance of the electrode can be directly observed by comparing the EIS spectrum. In order to fully utilize the theoretical specific capacity of the LiCoO2 positive electrode, the method of increasing the cutoff voltage is adopted, and the highest working voltage without causing side reactions is determined by measuring the CV curve corresponding to different cutoff voltage intervals. The results are as shown in Figure 5 As shown, the high cutoff voltage is selected as 4.1V, 4.2V, 4.3V, 4.4V, and 4.5V in turn. It is observed that the CV curve corresponding to 4.3V is incomplete, indicating that the capacity of LiCoO2 is not fully utilized, while a new small peak appears at 4.5V, which may be due to the decomposition of part of the electrolyte and the beginning of side reactions. Therefore, the safe working voltage interval of 3.0-4.4V is finally selected for the LiCoO2 electrode, which can maximize the lithium storage capacity of the active material while ensuring the normal operation of the battery.

[0108] (4) The effects of different preheating temperatures in Example 2 and different ratios of long-range conductive agent and short-range conductive agent in Example 3 on the cycle performance of LiCoO2 electrode, as shown in Figure 6 The LiCoO2 without preheating has a higher discharge specific capacity in the first 200 cycles, but the discharge specific capacity rapidly decreases from the 200th cycle, with an average decrease of 1.2mA·h·g -1 -1 per cycle. The LiCoO2 electrode preheated at 550°C or 600°C has a more stable structure due to the supplement of lattice oxygen missing in LiCoO2, and the discharge specific capacity slowly decreases during the entire cycle process, with an average decrease of 0.2mA·h·g -1Meanwhile, LiCoO2 preheated at 550℃ was selected. During the preparation of the electrode slurry, the mass ratio of LiCoO2:PVDF:VGCF:Super C was designed as 0.7g:0.1g:0.2g:0g, 0.7g:0.1g:0.15g:0.05g, and 0.7g:0.1g:0.05g:0.15g, respectively. These three samples were labeled VGCF-1, VGCF-2, and VGCF-3. After comparing their charge-discharge cycle performance, the high proportion of long-range conductive agent showed excellent cycle performance. The tentatively determined mass ratio of LiCoO2:PVDF:VGCF:Super C is 0.7g:0.1g:0.15g:0.05g. Simultaneously, the rate performance of all LiCoO2 electrodes in Examples 2 and 3 was compared, and the results are as follows: Figure 7 As shown. Within 0.1–7.0 A·g -1 Under increasing current density, the sample preheated to 550℃ and the mass ratio of LiCoO2:PVDF:VGCF:Super C of 0.7g:0.1g:0.15g:0.05g exhibit excellent rate performance at a maximum current density of 7A·g. -1 The corresponding reversible capacity under these conditions is 76 mA·h·g -1 This is higher than the reversible capacity of 49 mA·h·g for an untreated LiCoO2 electrode at this current density. -1 .

[0109] (5) To further improve the rate performance of the LiCoO2 electrode, a new comparative experiment was designed based on the type of conductive agent and binder, corresponding to the samples in Examples 4 and 5. In Example 4, the short-range conductive agent carbon black was replaced with Ketjen black (KB), while the corresponding mass ratio remained unchanged. The corresponding rate performance was as follows: Figure 8 As shown, the type of short-range conductive agent had no significant effect on the rate performance of the LiCoO2 electrode in the range of 0.1–2.0 A·g. -1 At current densities of 0.1 A·g, no significant difference in discharge specific capacity was observed, for example, at 0.1 A·g. -1 and 2.0A·g -1 At the given current density, both the sample using Super C45 and the sample using KB exhibit a reversible specific capacity of 174 mA·h·g. -1 and 104 mA·h·g -1 .

[0110] (6) The effects of different diaphragm types on the rate performance of the LiCoO2 electrode were compared, such as... Figure 9As shown, for common PP separator, three types of separators with thickness of 3 μm, 5.5 μm and 12 μm were selected, and a cellulose separator was also added. Under the same electrode material and test conditions, it was found that the ultra-thin 3 μm PP separator corresponded to a higher reversible specific capacity at a low current density, but the separator was not easy to spread due to electrostatic effect and was easy to break during the lamination operation. With the increase of current density, it was found that the PP separator with a thickness of 5.5 μm showed more excellent performance. The cellulose separator and the 12 μm PP separator had a large thickness and ion shuttle path side length, and had no advantage in rate performance. After comprehensive consideration, the PP separator with a thickness of 5.5 μm was selected.

[0111] (7) In order to optimize the interface between LiCoO2 electrode and electrolyte, metal gold (Au) was deposited on the surface of LiCoO2 electrode by magnetron sputtering technology, and the influence of different deposition times (0 s, 5 s, 10 s, 15 s) on the rate performance was compared, and the test results are shown in Figure 10 . The results show that the surface modification of Au has no obvious effect on the rate performance of LiCoO2 electrode, and the reversible specific capacity of the sample deposited for 15 s increases slightly. Considering the complexity of the magnetron sputtering process and the continuity of the entire electrode preparation, the interface between LiCoO2 electrode and electrolyte is not optimized.

[0112] (8) Starting from optimizing the proportion of conductive agent, on the basis of the above optimization selection, the mass ratio of LiCoO2: PVDF: VGCF: SuperC is 0.7 g: 0.1 g: 0.15 g: 0.05 g, and 0.15 g of VGCF is replaced by VGCF: CNTs = 0.12 g: 0.03 g, 0.075 g: 0.075 g, 0.03 g: 0.12 g, i.e. the mass ratio of VGCF: CNTs is marked as 4:1, 2.5:2.5, 1:4 in turn, and the influence of the above on the rate performance of LiCoO2 electrode is compared, and the results are shown in Figure 11 . The 1:4 sample corresponds to the optimal rate performance, for example, at the same current density of 0.5 A·g -1 , the reversible specific capacity of the 1:4 sample is 163 mA·h·g -1 , which is higher than the specific capacity of 150 mA·h·g -1 of the electrode with only VGCF.

[0113] (9) The rate performance of LiCoO2 electrode corresponding to different embodiments was tested, and the button cell was assembled according to the same method as above, and the test results are shown in Table 1.

[0114] Table 1 Rate performance of LiCoO2 electrode corresponding to different embodiments

[0115]

[0116]

[0117] As shown in Table 1, the preparation method selected in this invention and the types and proportions of long-range and short-range conductive agents screened can effectively improve the rate performance of the LiCoO2 electrode. In the preferred embodiment of this invention, the final selected electrode composition has a mass ratio of LiCoO2:PVDF:VGCF:CNTs:Super C45 of 0.7::0.1:0.03:0.12:0.05, which corresponds to the highest reversible specific capacity at different current densities. This is because the long-range / short-range conductive network constructed inside the LiCoO2 electrode can provide good electrical contact at high current densities, reducing the loss of electrical contact by active material particles and exhibiting high reversible specific capacity. In contrast, the unmodified LiCoO2 electrode not only has poor cycle life, but its discharge specific capacity also decays rapidly at high current densities.

[0118] (10) Figure 12 Surface morphology of the LiCoO2 / VGCF-CNTs-super C electrode prepared for Al foil surface in Example 6.

[0119] Depend on Figure 12 It can be seen that the three-dimensional conductive network used in this invention has a good inhibitory effect on the aggregation of LiCoO2 particles. The components are evenly distributed throughout the entire electrode scale, the separated LiCoO2 particles are evenly distributed on the entire electrode surface, and the particles are surrounded by conductive material.

[0120] Although the above embodiments have provided a detailed description of the present invention, they are only some embodiments of the present invention and not all embodiments. People can obtain other embodiments based on these embodiments without creative effort, and these embodiments all fall within the protection scope of the present invention.

Claims

1. A composite cathode material containing a long / short range conductive agent, characterized in that, The composite positive electrode material containing long / short-range conductive agents comprises positive active particles, zero-dimensional short-range conductive agents connecting adjacent positive active particles through physical adsorption, and one-dimensional long-range conductive agents connecting the positive active particles through physical adsorption, and the space between the positive active particles is filled with the conductive network formed by the zero-dimensional short-range conductive agents and the one-dimensional long-range conductive agents. The zero-dimensional short-range conductive agents have a nanospherical structure. The one-dimensional long-range conductive agents have a one-dimensional linear structure. The preparation method of the composite positive electrode material containing long / short-range conductive agents comprises the following steps: The one-dimensional long-range conductive agents, the zero-dimensional short-range conductive agents and an organic solvent are mixed and subjected to water bath ultrasonic treatment to obtain a conductive agent dispersion liquid. The conductive agent dispersion liquid and the positive active particles are mixed and subjected to ball milling to obtain the composite positive electrode material containing long / short-range conductive agents. The zero-dimensional short-range conductive agents comprise one or more of carbon black, conductive graphite and Ketjen black.

2. The composite cathode material containing long / short-range conductive agent according to claim 1, characterized in that, The one-dimensional long-range conductive agents comprise vapor phase grown carbon fibers and / or carbon nanotubes; the vapor phase grown carbon fibers have an aspect ratio L / D≥30:1, and the carbon nanotubes have an aspect ratio L / D≥1000:

1.

3. The composite cathode material containing long / short range conductive agent according to claim 1, characterized in that, The positive active particles comprise one or more of lithium cobalt ternary material, lithium iron phosphate ternary material, lithium nickel ternary material, nickel cobalt manganese ternary material and nickel cobalt aluminum ternary material.

4. The composite cathode material containing long / short range conductive agent according to claim 1, characterized in that, The mass ratio of the one-dimensional long-range conductive agents to the zero-dimensional short-range conductive agents is (1-4):(1-4).

5. The composite cathode material containing long / short range conductive agent according to claim 1 or 2 or 3, characterized in that, The preparation method of the composite positive electrode material containing long / short-range conductive agents comprises the following steps:

6. The method of producing the composite cathode material containing the long / short-range conductive agent according to any one of claims 1 to 5, characterized by, The one-dimensional long-range conductive agents, the zero-dimensional short-range conductive agents and an organic solvent are mixed and subjected to water bath ultrasonic treatment to obtain a conductive agent dispersion liquid. The conductive agent dispersion liquid and the positive active particles are mixed and subjected to ball milling to obtain the composite positive electrode material containing long / short-range conductive agents. The temperature of the water bath ultrasonic treatment is ≤20℃; the time of the water bath ultrasonic treatment is 5-60 min; and the power of the water bath ultrasonic treatment is 450-600 W.

7. The preparation method according to claim 6, characterized in that, The ball-to-material ratio of the ball milling is 1:1-20; the rotation speed of the ball milling is 100-500 rpm; and the time of the ball milling is 30-720 min.

8. The preparation method according to claim 6, characterized in that, 9. The composite positive electrode material containing long / short-range conductive agents according to any one of claims 1-5 or prepared by the preparation method according to any one of claims 6-8 is applied to a lithium ion battery. The active material in the positive electrode is the composite positive electrode material containing long / short-range conductive agents according to any one of claims 1-5 or prepared by the preparation method according to any one of claims 6-8.

10. A lithium ion battery comprising a positive electrode, a negative electrode, a separator and an electrolyte, characterized in that, ​

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