阅读说明：本技术 一种负极片及锂离子电池 (Negative plate and lithium ion battery ) 是由 张柯 马永军 郭姿珠 于 2020-05-27 设计创作，主要内容包括：本发明提供了一种负极片,包括集流体和设置在集流体表面的涂层,所述涂层包括至少两层活性层和设于相邻两层活性层之间的多孔扩散层,所述多孔扩散层的孔隙率大于相邻所述活性层的孔隙率,所述多孔扩散层的厚度小于相邻所述活性层的厚度。具有该结构的负极片,由于在负极片的活性涂层间引入了孔隙率大且厚度薄的多孔扩散层,使得既不会影响负极片整体的能量密度,还有助于锂离子在涂层中的传输,不仅可提高电池的大倍率充放电性能,还可使负极片中活性材料的容量得到充分发挥,进一步提高电池的能量密度。(The invention provides a negative plate which comprises a current collector and a coating arranged on the surface of the current collector, wherein the coating comprises at least two active layers and a porous diffusion layer arranged between the two adjacent active layers, the porosity of the porous diffusion layer is greater than that of the adjacent active layers, and the thickness of the porous diffusion layer is smaller than that of the adjacent active layers. The negative plate with the structure has the advantages that the porous diffusion layer with high porosity and thin thickness is introduced between the active coatings of the negative plate, so that the overall energy density of the negative plate is not influenced, the transmission of lithium ions in the coatings is facilitated, the high-rate charge-discharge performance of the battery can be improved, the capacity of active materials in the negative plate can be fully exerted, and the energy density of the battery is further improved.)

1. The negative plate comprises a current collector and a coating arranged on the surface of the current collector, and is characterized in that the coating comprises at least two active layers and a porous diffusion layer arranged between the two adjacent active layers, the porosity of the porous diffusion layer is greater than that of the adjacent active layer, and the thickness of the porous diffusion layer is smaller than that of the adjacent active layer.

2. The negative electrode sheet according to claim 1, wherein the porosity of the porous diffusion layer is 15 to 75%, and the porosity of the active layer is 5 to 30%.

3. The negative electrode sheet according to claim 1, wherein the thickness of the porous diffusion layer is 5-20 μm, and the thickness of the active layer is 25-200 μm.

4. The negative electrode sheet according to claim 1, wherein a thickness ratio of the active layer to the adjacent porous diffusion layer is not more than 10.

5. Negative electrode sheet according to claim 1, characterized in that the thickness ratio of the active layer to the adjacent porous diffusion layer is not less than 1.5, preferably not less than 3.

6. The negative electrode sheet according to claim 1, wherein the average pore diameter of the porous diffusion layer is not less than 1.2 nm.

7. The negative electrode sheet according to claim 1, wherein the thickness of the active layer away from the current collector is no greater than the thickness of the active layer close to the current collector adjacent to the active layer.

8. The negative electrode sheet according to claim 1, wherein adjacent active layers, the active layer further from the current collector, have a compacted density not greater than the compacted density of the active layer closer to the current collector.

9. The negative electrode sheet according to claim 1, wherein the thickness of the adjacent porous diffusion layer far from the current collector is not greater than the thickness of the porous diffusion layer near the current collector.

10. The negative electrode sheet according to claim 1, wherein the porosity of the porous diffusion layer far from the current collector is not less than the porosity of the porous diffusion layer near the current collector adjacent to the porous diffusion layer.

11. The negative electrode sheet according to claim 1, wherein the active layer contains a negative electrode active material capable of intercalating and deintercalating lithium, and the porous diffusion layer contains a porous dielectric material.

12. The negative electrode sheet according to claim 1, wherein the negative electrode sheet comprises a current collector and a first active layer, a first porous diffusion layer, and a second active layer sequentially disposed on a surface of the current collector.

13. The negative electrode sheet according to claim 1, wherein the negative electrode sheet comprises a current collector and a first active layer, a first porous diffusion layer, a second active layer, a second porous diffusion layer, and a third active layer sequentially disposed on a surface of the current collector.

14. The negative electrode sheet according to claim 1, wherein the coating layer is formed with through-holes penetrating the coating layer in a direction perpendicular to the current collector.

15. The negative plate according to claim 14, wherein the aperture of the through hole is 100nm to 100 μm, preferably 500nm to 50 μm.

16. Negative electrode sheet according to claim 15, characterized in that the average pore spacing between the through holes is 10-100 times the pore diameter, preferably 10-30 times the pore diameter.

17. Negative electrode sheet according to claim 14, characterized in that the hole shape of the through hole is circular.

18. A lithium ion battery comprising the negative electrode sheet according to any one of claims 1 to 17.

19. The lithium ion battery according to claim 18, wherein when the design magnification of the lithium ion battery is greater than 1.5C, the thickness ratio of the active layer to the adjacent porous diffusion layer in the negative electrode sheet is not greater than 5.

20. The lithium ion battery according to claim 18, wherein when the design magnification of the lithium ion battery is not greater than 1.5C, the thickness ratio of the active layer to the adjacent porous diffusion layer in the negative electrode sheet is greater than 5.

Technical Field

The application relates to the technical field of lithium ion batteries, in particular to a negative plate and a lithium ion battery.

Background

With the widespread use of lithium ion batteries, people have increasingly pursued higher energy density and rate capability. Most of the existing lithium ion batteries adopt graphite or silicon carbon as a negative electrode active material, and in order to improve the energy density of the batteries, the amount of the negative electrode active material coated in a negative electrode plate is increased, i.e., increasing the thickness of the coating layer in the negative electrode sheet and increasing the compacted density of the negative electrode sheet, however, the increase in the thickness and compacted density of the coating layer results in difficulty in penetration of the electrolyte into the coating layer, particularly, the closer to the current collector, such that the transport of lithium ions in the coating layer is hindered, the transport of lithium ions is not facilitated, further, the battery has poor high-rate charge and discharge performance, and the capacity of the active material in the coating layer close to the current collector is not fully exerted, therefore, in order to pursue the capability of high-rate charge and discharge and avoid the capacity loss of the active material, the coating thickness and the compaction density of the negative electrode sheet are reduced, and the energy density of the negative electrode sheet is reduced. Therefore, the conventional negative electrode sheet cannot simultaneously satisfy high energy density and high rate charge and discharge performance. It is undoubtedly an effective method for increasing the energy density of the negative electrode sheet by increasing the coating amount of the active material in the negative electrode sheet, and thus it is urgently required for the improvement of the high-rate charge and discharge performance of such a negative electrode sheet and the full exertion of the capacity of the active material.

Disclosure of Invention

In order to solve the technical problems of capacity exertion and high-rate charge and discharge performance of a negative plate in the prior art, particularly poor capacity exertion and high-rate charge and discharge performance of the negative plate with a thick coating, namely high energy density, the application provides the negative plate and the lithium ion battery.

In one aspect, the application provides a negative pole piece, including the mass flow body with set up the coating on the mass flow body surface, the coating includes at least two-layer active layer and locates the porous diffusion layer between the adjacent two-layer active layer, the porosity of porous diffusion layer is greater than adjacent the porosity of active layer, the thickness of porous diffusion layer is less than adjacent the thickness of active layer.

Compared with the prior art, the coating of the negative plate provided by the application is provided with the porous diffusion layer, and the porous diffusion layer has higher porosity, namely lower compaction density compared with the adjacent active layer, so that lithium ions can be rapidly transmitted through the porous diffusion layer, and convenience is provided for the transmission of the lithium ions; in addition, the porous diffusion layer with larger porosity can also soak and store more electrolyte, and in the battery circulation process, lithium ions in the porous diffusion layer can be rapidly transmitted to the adjacent active layer to complete the lithium intercalation or lithium deposition process, so that the transmission path of the lithium ions is greatly shortened, and the high-rate charge and discharge performance of the negative plate is favorably improved; in addition, due to the arrangement of the porous diffusion layer, lithium ions can reach the active layer close to the current collector more easily, so that the capacity of the active material in the active layer close to the current collector can be fully exerted, and the energy density of the battery is further improved. In the negative plate in the prior art, lithium ions need to be transmitted from the electrolyte of the battery body to the active layer, the transmission path is long, and the transmission speed is slow, so that when the battery is charged with a large multiplying power, the lithium ions cannot be transmitted to the active layer (the active layer close to the current collector) at a farther position, and the process of lithium intercalation or lithium deposition cannot be completed, so that lithium dendrites are generated, and potential safety hazards are brought to the battery; in addition, the capacity of the active layer near the current collector cannot be sufficiently utilized, resulting in a decrease in battery capacity. The negative plate provided by the application can greatly improve the high-rate charge-discharge performance and the capacity performance of the negative plate, and is not easy to generate lithium dendrites, so that the safety performance of the battery is improved. Moreover, the porous diffusion layer is thin, so that the energy density of the whole negative electrode sheet is not influenced. Therefore, the negative plate provided by the application has high-rate charge and discharge performance while meeting the required energy density, and the theoretical capacity of the active material in the active layer can be fully exerted.

In another aspect, the present application provides a lithium ion battery, including the above negative electrode sheet.

According to the lithium ion battery provided by the application, the porous diffusion layer with higher porosity is arranged in the active layer of the negative plate, so that more channels are provided for the transmission of lithium ions, and the transmission of the lithium ions to the active layer is facilitated; in addition, the porous diffusion layer can better soak and store electrolyte, so that lithium ions in the layer can be nearby transmitted to the active layer in the battery circulation, the path and time for transmitting the lithium ions from bulk electrolyte to the active layer are greatly shortened, the high-rate quick charge performance of the battery is favorably improved, and the requirement of the current society on the quick charge performance of the lithium ion battery is met; in addition, lithium ions can reach the active layer close to the current collector more easily, so that the capacity of the active layer can be fully exerted, and the capacity loss of the battery is avoided.

Detailed Description

In order to make the technical problems, technical solutions and advantageous effects solved by the present application more clear, the present application is further described in detail below with reference to embodiments. It should be understood that the specific embodiments described herein are merely illustrative of the present application and are not intended to limit the present application.

In a first aspect, the application provides a negative plate, including the mass flow body and the coating of setting on the mass flow body surface, the coating includes at least two-layer active layer and locates the porous diffusion layer between the adjacent two-layer active layer, the porosity on porous diffusion layer is greater than adjacent the porosity on active layer, the thickness on porous diffusion layer is less than adjacent the thickness on active layer.

According to the negative plate, the porous diffusion layer is arranged between the two adjacent active layers, the porosity of the porous diffusion layer is greater than that of the active layers, the porous diffusion layer with the large porosity is easy to soak and store electrolyte, and similarly, the liquid storage layer is arranged between the active layers, so that lithium ions reaching the active layers are not only from bulk electrolyte but also from electrolyte stored in the porous diffusion layer, and the change of a lithium ion transmission path greatly shortens the path and time for the lithium ions to reach the active layers, and is favorable for improving the high-rate charge and discharge performance of the negative plate; in addition, among the prior art, because the active layer of negative pole piece has certain thickness and compaction density, make lithium ion in the electrolyte of bulk phase difficult transmit to the active layer near the mass flow body department in, make the active material's in the active layer near the mass flow body department capacity can not full play, and through introducing porous diffusion layer in the negative pole piece of this application, a certain amount of electrolyte can be stored to porous diffusion layer, lithium ion in this part of electrolyte is changeed and is transmitted to the active layer near the mass flow body side in, thereby be favorable to being close to the active material's in the active layer near the mass flow body side capacity and can full play, further improve the energy density of negative pole piece.

Moreover, the thickness of the porous diffusion layer is small, so that the thickness of the negative plate cannot be additionally increased, the high-rate charge and discharge performance of the negative plate can be improved on the premise of ensuring the energy density of the negative plate, and the capacity of the negative plate can be fully exerted.

Further, the porosity of the porous diffusion layer is 15-75%, and the porosity of the active layer is 5-30%.

Further, the thickness of the single porous diffusion layer is 5-20 mu m, and the thickness of the active layer is 25-200 mu m.

Further, the thickness ratio of the active layer to the adjacent porous diffusion layer is not more than 10.

The porous diffusion layer is mainly used for solving the problems that the capacity of an active substance in the active layer cannot be fully exerted and high-rate charge and discharge cannot be realized due to the fact that lithium ions in a bulk electrolyte are not easy to transmit to the inside of the active layer, and has high porosity and can be used for storing the electrolyte, the stored electrolyte can be used for transmitting the lithium ions between the electrolyte and an adjacent active layer, and therefore the problems of capacity exertion of the active substance in the negative plate and high-rate charge and discharge of the negative plate can be solved. Therefore, the amount of the electrolyte stored in the porous diffusion layer is related to the thickness of the porous diffusion layer, how much electrolyte can be stored can meet the requirement, and is also related to the amount of the active material in the adjacent active layer, and the amount of the active material is related to the thickness of the active layer.

Further, the thickness ratio of the active layer to the adjacent porous diffusion layer is not less than 1.5, and preferably, the thickness ratio of the active layer to the adjacent porous diffusion layer is not less than 3.

When the thickness ratio of the active layer to the adjacent porous diffusion layer is not less than 1.5, the effect of improving the capacity of the negative electrode sheet is exerted more effectively. When the thickness ratio of the active layer to the adjacent porous diffusion layer is not less than 3, not only can the rate performance of the negative electrode sheet be further improved, but also the porous diffusion layer occupies a smaller volume in the negative electrode sheet, and even if the porous diffusion layer does not generally have a battery capacity, the volumetric energy density of the battery is also improved because the total capacity exertion is improved at high rates.

Further, the average pore diameter of the porous diffusion layer is not less than 1.2 nm.

The porous diffusion layer is introduced into the coating of the negative plate, mainly for improving the transmission performance of lithium ions in the layer, so that the lithium ions are transmitted into the active layer more easily and quickly, and therefore, in order to ensure that the lithium ions can be transmitted smoothly, the average pore diameter of the porous diffusion layer is required to be not less than 1.2 nm.

Preferably, the porous diffusion layer has an average pore diameter of 1.2 to 10 nm.

When the average pore diameter of the porous diffusion layer is large, the porous diffusion layer is excessively fluffy, the mechanical strength and the electrical conductivity are also reduced, and the rate capability of the battery is also affected, so that the average pore diameter of the porous diffusion layer is more preferably controlled below 10 nm.

Preferably, the porous diffusion layer has an average pore size of 2-6 nm.

The porous diffusion layer with the average pore diameter within the range not only can provide stronger mechanical strength and larger conductivity, but also can enhance the rate capability of the negative electrode plate while keeping the energy density of the negative electrode plate from height to volume.

Further, the thickness of the active layer far away from the current collector is not more than that of the active layer close to the current collector, and the compaction density of the active layer far away from the current collector is not more than that of the active layer close to the current collector.

For the active layer, the thickness and the compaction density of the active layer may be constant or gradually decreased in a direction away from the current collector. The thickness and the compaction density of the active layer are not changed along the direction far away from the current collector, namely the thickness and the compaction density of each active layer in the negative plate are equal.

Preferably, the thickness of the active layer gradually decreases in a direction away from the current collector.

Preferably, the compacted density of the active layer gradually decreases in a direction away from the current collector.

The thickness and the compaction density of the active layer are gradually reduced along the direction far away from the current collector, namely the thickness and the compaction density of the active layer close to the current collector in the negative plate are the largest, and the thickness and the compaction density of the active layer far away from the current collector are the smallest, and the design of the structure accords with the characteristic of lithium ion transmission, namely the concentration difference caused by the uneven distribution of lithium ions in the electrode during charging and discharging and provides power for the transmission and diffusion of the lithium ions; the design of the pole piece structure that the thickness and the compaction density of the active layer are gradually reduced along the direction far away from the current collector optimizes the transmission path of lithium ions, and avoids the problem that the lithium ions are difficult to reach the active layer close to the current collector due to unreasonable pore size and thickness distribution, so that the lithium ion intercalation or deposition reaction is difficult to carry out due to overhigh concentration polarization.

Because the transmission path of lithium ions is mainly from the bulk phase electrolyte to the active layer at the innermost layer, namely the active layer closest to the current collector, through the active layer at the surface layer, namely the active layer farthest from the current collector, the lithium ions are transmitted layer by layer to the active layer at the innermost layer, namely the active layer closest to the current collector, so that the lithium ion flow required by the active layer far from the current collector is larger, the lithium ion flow required by the active layer close to the current collector is smaller, the thickness and the compaction density of the active layer are related to the required lithium ion amount, and therefore, according to the required lithium ion amount of each layer, the waste of lithium ions and the waste of active materials in the active layer are avoided, a structure that the thickness and the compaction density of the active layer gradually decrease along the direction far from the current collector is designed, and not only can the large-rate charge and discharge be realized, the capacity of the active material in the active layer can be exerted to a greater extent, and the energy density of the negative plate is further improved.

Further, the thickness of the adjacent porous diffusion layers far away from the current collector is not more than that of the porous diffusion layers close to the current collector, and the porosity of the porous diffusion layers far away from the current collector is not less than that of the porous diffusion layers close to the current collector.

For the porous diffusion layer, the thickness of the porous diffusion layer can be constant or gradually decreased along the direction far away from the current collector, and the porosity of the porous diffusion layer can be constant or gradually increased along the direction far away from the current collector. The thickness and the porosity of porous diffusion layer do not change along the direction of keeping away from the mass flow body, that is to say, when having the porous diffusion layer of multilayer in the negative pole piece, the thickness and the porosity of every layer of porous diffusion layer are all the same, the design of this kind of structure, can satisfy the requirement that lithium ion can be fast transmitted to the active layer through the porous diffusion layer, in addition, the porous diffusion layer still can save electrolyte in advance and make the requirement that lithium ion can be fast transmitted to the active layer in, moreover, lithium ion in the porous diffusion layer also changes to reach in the active layer near mass flow body department, the capacity loss of negative pole piece has been avoided.

Preferably, the thickness of the porous diffusion layer gradually decreases in a direction away from the current collector.

Preferably, the porosity of the porous diffusion layer gradually increases in a direction away from the current collector.

The thickness of porous diffusion layer is decreased progressively along the direction of keeping away from the mass flow body, the porosity of porous diffusion layer is gradually increased progressively along the direction of keeping away from the mass flow body, that is to say, when there is multilayer porous diffusion layer in the negative pole piece, the thickness of the porous diffusion layer that is close to the mass flow body department in the negative pole piece is the biggest, the porosity is the minimum, the thickness of the porous diffusion layer that is kept away from the mass flow body department is the smallest, the porosity is the biggest, the design of this kind of structure, it is different mainly to rely on the required lithium ion's of each layer active layer quantity, thereby make the transmission path of lithium ion more optimized, and the waste of lithium ion and the loss of active material capacity in the active layer have been avoided.

In the battery circulation process, lithium ions are mainly transmitted from a bulk electrolyte to an active layer at the innermost layer, namely an active layer at the closest current collector, through an active layer at the surface layer, namely an active layer at the farthest current collector, layer by layer, or from the active layer at the closest current collector to the bulk electrolyte through the active layer at the farthest current collector, so that the lithium ion flow required by the active layer at the farthest current collector is far greater than that of the active layer at the closest current collector, the lithium ion flow required by a porous diffusion layer closest to the active layer at the closest current collector is smaller, and the thickness and porosity of the porous diffusion layer directly influence the stock and flow of lithium ions, so that the thickness and porosity of the porous diffusion layer at the position close to the current collector in a negative plate are required to be the largest, the smallest, and the thickness and the smallest, The porosity is the largest, and the lithium ion quantity required by the adjacent active layer is met, so that high-rate charge and discharge can be realized, the capacity of the active material in the active layer can be exerted to a greater extent, and the energy density of the negative plate is improved.

Furthermore, the active layer contains a negative electrode active material capable of lithium intercalation and deintercalation, and the porous diffusion layer contains a porous medium material.

The negative active material is selected from one or more of carbon material, tin alloy, silicon, tin, germanium, lithium metal and lithium alloy, and when the carbon material is selected, the carbon material can be non-graphitized carbon, graphite or carbon obtained by high-temperature oxidation of polyacetylene polymer material, or one or more of pyrolytic carbon, coke, organic polymer sinter and activated carbon. When the negative active material is silicon, the active layer also contains conductive agent.

Furthermore, the porous medium material can be selected from materials which do not have battery capacity but have lithium ion reversible intercalation or deposition capacity, such as one or more of conductive carbon-based materials such as porous carbon, graphene, carbon nanotubes, conductive polymer materials (such as polyaniline) and the like, and such materials have conductivity, so that no additional conductive agent is required to be added when the porous diffusion layer is prepared. Meanwhile, the porous medium material can also be selected from materials with battery capacity, namely lithium ions can be reversibly intercalated or deposited, such as one or more of silicon alloy, silicon, tin, germanium, metallic lithium, lithium alloy, non-graphitized carbon, graphite, carbon obtained by high-temperature oxidation, pyrolytic carbon, coke, organic polymer sinter and activated carbon. When the porous medium material is selected from silicon materials, a conductive agent is required to be added during the preparation of the porous diffusion layer, so that better conductive performance is achieved.

The porous diffusion layer further contains a binder, which is a binder for various negative electrodes known to those skilled in the art, and may be selected from one or more of polythiophene, polypyrrole, polytetrafluoroethylene, polyvinylidene fluoride, polyethylene, polypropylene, polystyrene, polyacrylamide, ethylene-propylene-diene copolymer resin, styrene butadiene rubber, polybutadiene, fluororubber, polyethylene oxide, polyvinylpyrrolidone, polyester resin, acrylic resin, phenol resin, epoxy resin, polyvinyl alcohol, carboxypropyl cellulose, ethyl cellulose, polyethylene oxide, sodium carboxymethylcellulose (CMC), and styrene butadiene latex (SBR).

Further, the negative plate comprises a current collector, and a first active layer, a first porous diffusion layer and a second active layer which are sequentially arranged on the surface of the current collector.

Wherein, the first active layer and the second active layer can be the same or different.

Further, the negative plate comprises a current collector, and a first active layer, a first porous diffusion layer, a second active layer, a second porous diffusion layer and a third active layer which are sequentially arranged on the surface of the current collector.

The first active layer, the second active layer and the third active layer may be the same or different, and the first porous diffusion layer and the second porous diffusion layer may be the same or different.

Further, the coating layer is formed with through-holes penetrating the coating layer in a direction perpendicular to the current collector.

That is, the vias run through the entire coating, i.e., the vias begin with the active layer closest to the current collector and end with the active layer furthest from the current collector, or the vias begin with the active layer furthest from the current collector and end with the active layer closest to the current collector.

The through holes in the coating are arranged, a lithium ion transmission channel is additionally increased, namely lithium ions in the bulk electrolyte can be rapidly transmitted to the active layer closest to the current collector through the through holes in the coating, so that the capacity of the active layer closest to the current collector can be exerted, the lithium ion transmission paths are increased, and the rate performance of the battery can be improved.

Further, the aperture of the through hole is 100nm-100 mu m, and preferably, the aperture of the through hole is 500nm-50 mu m.

The aperture of the through hole is in the range, the mechanical strength of the coating and the capacity of the negative plate are not influenced, and the lithium ion transmission in the through hole is facilitated.

Further, the hole shape of the through hole is circular.

The shape of the through hole is not required, and only the through hole penetrates through the whole coating; the through holes are preferably circular, the circular through holes can improve the space utilization rate of the coating, and more lithium ion transmission channels can be provided under the same condition.

Further, the average hole pitch between the through holes is 10 to 100 times the hole diameter, and preferably, the average hole pitch between the through holes is 10 to 30 times the hole diameter.

The average hole spacing reflects the hole density of the through holes in the coating, and is in the range, so that the capacity and the mechanical strength of the negative electrode cannot be influenced by the introduction of the holes, more channels can be provided for lithium ion transmission, and the rate capability and the capacity of the negative electrode sheet can be improved.

The preparation method of the through hole in the coating of the negative plate is not specially limited, and only the hole penetrating through the whole coating is obtained. Laser drilling can be adopted, and rolling and pore-forming can also be carried out by adopting metal needles, wherein the diameter of each metal needle is the diameter of each hole, and the distance between every two metal needles is the hole distance. In a second aspect, the application provides a preparation method of the negative plate, which includes preparing active layer slurry and porous diffusion layer slurry, coating the active layer slurry on the surface of a current collector, drying, coating the porous diffusion layer slurry, drying, coating the active layer slurry according to the number of layers of the active layer and the porous diffusion layer required by the negative plate, only needing to enable the porous diffusion layer to be located between two adjacent active layers, and drying and rolling to obtain the required negative plate after coating.

The preparation method of the negative plate does not have a strong requirement, and the preparation method can also be realized by preparing a porous diffusion layer for later use by porous diffusion layer slurry, then stacking the prepared porous diffusion layer on a current collector coated with an active layer, and then coating the active layer on the surface of the porous diffusion layer.

The active layer slurry comprises a negative electrode active substance, a conductive agent, a binder and a solvent, wherein the binder can be selected from one or more of fluorine-containing resin and polyolefin compound, such as polyvinylidene fluoride (PVDF), Polytetrafluoroethylene (PTFE) and Styrene Butadiene Rubber (SBR), the conductive agent can be selected from one or more of acetylene black, carbon nano tubes, carbon fibers and carbon black, and the solvent is selected from one or more of N-methyl pyrrolidone (NMP), water, ethanol and acetone. The mass content of the binder is 0.01-10%, preferably 0.02-5% based on the weight of the active layer slurry; the mass content of the conductive agent is 0.1-20%, preferably 1-10%; the mass content of the solvent is 50-400%.

The slurry of the porous diffusion layer comprises a porous medium material and a solvent, and optionally further comprises a conductive agent and a binder, wherein the conductive agent can be one or more selected from acetylene black, carbon nanotubes, carbon fibers and carbon black; the binder is selected from one or more of polythiophene, polypyrrole, polytetrafluoroethylene, polyvinylidene fluoride, polyethylene, polypropylene, polystyrene, polyacrylamide, ethylene-propylene-diene copolymer resin, styrene butadiene rubber, polybutadiene, fluororubber, polyethylene oxide, polyvinylpyrrolidone, polyester resin, acrylic resin, phenolic resin, epoxy resin, polyvinyl alcohol, carboxypropyl cellulose, ethyl cellulose, polyethylene oxide, sodium carboxymethyl cellulose (CMC) and styrene butadiene latex (SBR); the solvent can be obtained by mixing various solvents with different boiling points, such as deionized water and acetone, acetone and dimethyl carbonate, N, N-dimethylformamide and dimethyl carbonate, and the like.

In a third aspect, the present application provides a lithium ion battery, including the above negative electrode sheet.

The negative plate of the lithium ion battery contains the porous diffusion layer which has a thinner thickness, so that the thickness of the negative plate cannot be additionally increased, and the integral energy density of the negative plate cannot be influenced; in addition, the porous diffusion layer has higher porosity than that of the adjacent active layer, so that lithium ions can be conveniently and rapidly transmitted to the adjacent active layer through the porous diffusion layer, the high-rate charge and discharge performance of the battery can be improved, the porous diffusion layer with the high porosity can better soak and store electrolyte, and similarly, a lithium ion storage layer is introduced into the negative plate, so that the lithium ions required in the active layer can not only come from bulk electrolyte but also come from electrolyte (lithium ions) stored in the adjacent porous diffusion layer, the transmission path and the transmission time of the lithium ions are greatly shortened, and the high-rate charge and discharge performance of the battery is greatly improved; moreover, due to the introduction of the porous diffusion layer, lithium ions can be more easily and quickly transmitted to the active layer close to the current collector, so that the capacity of the negative electrode active material in the active layer close to the current collector can be fully exerted, the capacity loss of the battery can not be caused, and the energy density of the battery is improved.

Further, when the design rate requirement of the lithium ion battery is greater than 1.5C, the thickness ratio of the active layer to the adjacent porous diffusion layer in the negative plate is not greater than 5; when the design rate requirement of the lithium ion battery is not more than 1.5C, the thickness ratio of the active layer to the adjacent porous diffusion layer in the negative plate is more than 5.

The requirement of multiplying power, design multiplying power or similar words mentioned in the application all mean that the capacity of the negative electrode is not less than 80% of the capacity of the negative electrode charged and discharged at 0.5C multiplying power when the negative electrode is charged and discharged at the specified multiplying power in a full cell or a half cell.

That is, the thickness ratio of the active layer to the porous diffusion layer in the negative electrode sheet is different for lithium ion batteries with different rate performance requirements. For a fast-charging battery which is sought at present, for example, the fast-charging battery which is required to realize fast charging needs to adopt large current for charging, namely, the fast-charging battery with large rate, under the charging condition, lithium ions can be rapidly separated from an anode and are transmitted to a cathode through electrolyte, in order to complete the fast-charging process, the lithium ions also need to be rapidly inserted or deposited into a cathode active material, otherwise, the lithium ions can be aggregated and deposited to generate lithium dendrite, and potential safety hazard is brought to the battery. The coating of the negative plate has certain thickness and compaction density, so that lithium ions cannot be rapidly transmitted to each part in the active layer from bulk electrolyte. For the high-rate charging of a battery, a large amount of lithium ions need to enter an active layer within a short time to complete a charging process, so that enough lithium ions need to be stored in an adjacent porous diffusion layer, the amount of the stored lithium ions in the porous diffusion layer is related to the thickness of the porous diffusion layer, the thicker the thickness is, the more the amount of electrolyte can be stored, namely the larger the amount of the lithium ions can be provided for the active layer, and in order to meet the lithium ions required by the high-rate charging, the inventor of the application finds through a large number of theoretical calculations and experimental studies that when the thickness ratio of the active layer to the adjacent porous diffusion layer is not more than 5, a negative plate can meet the requirement of the high-rate charging and discharging of the battery with the current of more than 1.5C; in addition, enough lithium ions can be obtained from the active layer at the current collector, so that capacity loss caused by the fact that the lithium ions cannot reach the active layer at the current collector is avoided, and capacity exertion performance of the negative plate is further improved.

When the multiplying power performance of the lithium ion battery is not required to be higher, a large amount of lithium ions can not be extracted or inserted in a short time in the charging process, and a large amount of lithium ions are not required in a natural negative electrode active layer in a short time, namely, more electrolyte is not required to be stored in the porous diffusion layer, so that the thickness of the porous diffusion layer can be as thin as possible in order to ensure the integral energy density of the negative electrode plate. Through a large number of theoretical calculations and experimental researches, the inventor of the application finds that when the thickness ratio of the active layer to the adjacent porous diffusion layer is more than 5 and not more than 10, the negative plate not only has high energy density, but also can meet the requirement of the battery on the charge-discharge rate of the battery current of not more than 1.5C; moreover, the porous diffusion layer with the thickness can still provide lithium ions for the active layer close to the current collector, so that the capacity loss caused by the fact that the lithium ions cannot reach the active layer at the current collector is avoided, and the capacity exertion performance of the negative plate is further improved.

The present application is further illustrated by the following specific examples, which are provided only for illustrating and explaining the present application and are not intended to limit the present application.

Example 1

Preparing a porous diffusion layer: mixing 1g of graphene oxide and 0.2g of unsymmetrical dimethylhydrazine, dissolving the mixture in 800 ml of distilled water under a stirring state, adding 0.2g of 35 mass percent aqueous ammonia into the mixed solution, stirring, and carrying out water bath reaction at 100 ℃ for 30 min; then 1g of negative pole artificial graphite is added, and ultrasonic dispersion is carried out for 20 min. And naturally cooling to room temperature, filtering to obtain a graphene oxide and artificial graphite mixed hydrogel film, soaking the mixed hydrogel film in a 10 mass percent lithium nitrate aqueous solution for 12 hours, drying at 100 ℃ for 24 hours, washing with water (washing off residual lithium nitrate solid after drying in pore channels), and drying to obtain the porous diffusion layer supported by graphene oxide. The porous diffusion layer had an average pore diameter of 1.2nm and a porosity of 30%. The graphene oxide is reduced by unsymmetrical dimethylhydrazine and then has good conductive capacity, so that the obtained porous diffusion layer contains an artificial graphite cathode active material and has conductivity.

Preparing a negative plate: adding 9.3 g of negative active material artificial graphite (93%), 0.35 g of binder CMC (3.5%) and 0.35 g of binder SBR (3.5%) into 12 g of dimethylbenzene, stirring in a vacuum stirrer to form stable and uniform negative active layer slurry, uniformly coating the negative active layer slurry on the surface of a current collector copper foil, drying and rolling to obtain a current collector with an active layer, placing the porous diffusion layer on the surface of the active layer, coating the active layer slurry on the surface of the porous diffusion layer, drying and rolling to obtain the required negative plate.

In the prepared negative plate, the surface of a current collector is sequentially provided with an active layer, a porous diffusion layer and an active layer, wherein the thickness of the active layer is 25 mu m, the thickness of the porous diffusion layer is 5 mu m, the porosity of the active layer is 10%, and the porosity of the porous diffusion layer is 30%.

Assembling the prepared negative plate, a diaphragm, a positive plate and electrolyte to obtain the lithium ion battery, wherein the active material in the positive plate is LiCoO₂The electrolyte is LiPF₆1M EC/DMC solution.

Example 2

Unlike example 1, in the preparation of the porous diffusion layer, artificial graphite was replaced with 1g of carbon-coated silica powder having a diameter of 50 to 300 nm, and the obtained porous diffusion layer contained a silica negative electrode active material, and the porosity of the porous layer was 23%.

Example 3

Unlike example 1, in the preparation of the porous diffusion layer, a 10% by mass aqueous solution of lithium nitrate was replaced with a 10% by volume EMIMBF₄(1-ethyl-3-methylimidazolium tetrafluoroborate) ionic liquid, and a porous diffusion layer with an average pore diameter of 6nm and a porosity of 60% is obtained.

Example 4

Different from example 1, in the prepared negative electrode sheet, the thickness of the porous diffusion layer is 8 μm and the average pore size is 1.2nm by additionally adding 0.32g of the used amount of graphene oxide and graphite.

Example 5

The difference from the embodiment 1 is that the negative plate has three active layers and two porous diffusion layers, that is, the surface of the current collector is sequentially provided with a first active layer, a first diffusion layer, a second active layer, a second diffusion layer and a third active layer, the three active layers are completely the same, and the two diffusion layers are completely the same.

Example 6

Different from example 1, when the porous diffusion layer is prepared, the use amount of 0.8g of graphene oxide and graphite is reduced to make the thickness of the porous diffusion layer 2.5 μm, and in the obtained negative electrode sheet, the thickness ratio of the porous diffusion layer to the active layer is 1: 10.

example 7

The difference from the embodiment 1 is that the negative plate comprises three active layers and two porous diffusion layers, namely, the surface of the current collector is sequentially provided with a first active layer, a first diffusion layer, a second active layer, a second diffusion layer and a third active layer, wherein the two diffusion layers are completely the same; by adjusting the active material loading amount and the rolling pressure, the thickness of the first active layer is 30 mu m, the thickness of the second active layer is 25 mu m, the thickness of the third active layer is 20 mu m, and the compaction density of the first active layer is 1.73g/cm³The second active layer had a compacted density of 1.57g/cm³And the third active layer has a compacted density of 1.43g/cm³。

Example 8

The difference from the embodiment 1 is that the negative plate comprises three active layers and two porous diffusion layers, namely, the surface of the current collector is sequentially provided with a first active layer, a first diffusion layer, a second active layer, a second diffusion layer and a third active layer, wherein the three active layers are completely the same; when the porous diffusion layer is prepared, the porosity of the first porous diffusion layer is controlled to be 25%, the porosity of the second porous diffusion layer is controlled to be 30%, and meanwhile the thickness of the first porous diffusion layer is 7 mu m, and the thickness of the second porous diffusion layer is 5 mu m by adjusting the rolling pressure and the input amount of graphene oxide.

Example 9

Different from the embodiment 1, the negative plate comprises three active layers and two porous diffusion layers, namely, the surface of the current collector is provided with a first active layer, a first diffusion layer, a second active layer, a second diffusion layer and a third active layer in sequence; when the active layers are prepared, the loading amount and the rolling pressure of the active materials are adjusted, so that the thickness of the first active layer is 30 mu m, the thickness of the second active layer is 25 mu m, the thickness of the third active layer is 20 mu m, and the compaction density of the first active layer is 1.73g/cm³The second active layer had a compacted density of 1.57g/cm³The third active layer had a compacted density of 1.43g/cm³(ii) a When the porous diffusion layer is prepared, the rolling pressure and the input amount of the graphene oxide are adjusted to control the first porous layerThe porosity of the diffusion layer is 25%, the porosity of the second porous diffusion layer is 30%, and meanwhile the thickness of the first porous diffusion layer is 7 mu m and the thickness of the second porous diffusion layer is 5 mu m.

Example 10

Different from the example 1, in the process of preparing the porous diffusion layer, the thickness of the obtained porous layer is 20 μm by increasing the input amount of 5 g of graphene oxide and graphite.

Example 11

The difference from the example 1 is that the prepared negative electrode sheet uses metal needles with the diameter of 25 mu m and the average distance of 1000 mu m, and the whole negative electrode sheet is array-rolled, so that the metal needles penetrate through the coating of the negative electrode sheet, and the negative electrode sheet with a vertical pore structure is obtained.

Example 12

The difference from example 11 is that the diameter of the metal needles is 50 μm and the average pitch of the needles is 1000 μm.

Comparative example 1

Unlike example 1, the negative electrode sheet did not have a porous diffusion layer, and the surface of the current collector copper foil was coated with three active layers.

Comparative example 2

Different from example 1, in the preparation of the porous diffusion layer, the thickness of the obtained porous diffusion layer was 50 μm by increasing the input amount of 10g of graphene oxide and graphite.

Comparative example 3

Unlike example 1, a porous diffusion layer having an average pore diameter of 0.88 nm and a porosity of 5% was obtained without performing a soaking operation in a 10% by mass aqueous solution of lithium nitrate when preparing the porous diffusion layer.

Battery performance testing

The lithium ion batteries obtained in examples 1 to 12 and comparative examples 1 to 3 were subjected to a battery cycle performance test by the following method:

the batteries prepared in the examples and the comparative examples are respectively 28, and the batteries of the examples and the comparative examples are respectively subjected to charge-discharge cycle tests at 0.1C, 0.5C, 1C, 2C and 3C on a LAND CT 2001C secondary battery performance detection device under the condition of 298 +/-1K. The method comprises the following steps:

standing for 10 min; constant current charging is carried out until 4.2V is cut off; standing for 10 min; constant current discharge to 1.5V is 1 cycle, the step is repeated for 10 times, and the average value is the capacity under the multiplying power, and the result is shown in Table 1, wherein the discharge specific capacity is calculated by the whole mass of the anode material. Furthermore, when the battery capacity was less than 80% of the first discharge capacity during the cycling, the cycling was terminated, the number of cycles was the cycle life of the battery, and the results are averaged for each group and shown in table 2.

TABLE 1

	0.1C specific discharge capacity/mAh/g	0.5C specific discharge capacity/mAh/g	1C specific discharge capacity/mAh/g	2C specific discharge capacity/mAh/g	3C specific discharge capacity/mAh/g
						Example 1	153	143	125	113	107
Example 2	157	139	121	105	95
						Example 3	143	137	134	127	115
Example 4	146	140	131	125	111
						Example 5	153	145	139	131	123
Example 6	141	132	112	97	89
						Example 7	151	143	139	132	125
Example 8	147	144	141	131	123
						Example 9	155	149	143	135	129
Example 10	139	135	127	119	103
						Example 11	155	151	147	142	137
Example 12	154	151	147	145	143
						Comparative example 1	137	121	86	56	43
Comparative example 2	103	96	89	71	65
						Comparative example 3	132	105	73	49	31

TABLE 2

	Number of cycles of 1C	Number of cycles of 3C
			Example 1	715	632
Example 2	673	597
			Example 3	820	752
Example 4	745	676
			Example 5	703	651
Example 6	617	552
			Example 7	790	721
Example 8	810	733
			Example 9	875	771
Example 10	710	632
			Example 11	841	790
Example 12	883	821
			Comparative example 1	535	426
Comparative example 2	735	660
			Comparative example 3	493	415