CN111785921B

CN111785921B - Lithium ion battery anode and lithium ion battery

Info

Publication number: CN111785921B
Application number: CN202010760983.4A
Authority: CN
Inventors: 郭明奎; 苏树发; 谢涛
Original assignee: Svolt Energy Technology Co Ltd
Current assignee: Svolt Energy Technology Co Ltd
Priority date: 2020-07-31
Filing date: 2020-07-31
Publication date: 2022-05-17
Anticipated expiration: 2040-07-31
Also published as: CN111785921A

Abstract

The present disclosure relates to a lithium ion battery anode, which includes a current collector and an anode active material coating coated on a surface of the current collector, wherein the anode active material coating includes a first coating coated on the surface of the current collector and a second coating coated on a surface of the first coating, and a porosity of the first coating is lower than a porosity of the second coating. The lithium ion battery prepared by the anode can effectively improve the energy density of the battery core, simultaneously ensure the electrical property of the battery core, the long-term reliability of the battery core and the safety performance of the battery core, and meet the requirements of the current power battery core.

Description

Lithium ion battery anode and lithium ion battery

Technical Field

The disclosure relates to the technical field of lithium ion batteries, in particular to a lithium ion battery anode and a lithium ion battery.

Background

Along with the development of the electric vehicle, the requirement on the energy density of the power battery is higher and higher, the energy density of the power battery, as a core component of the electric vehicle, affects the design of the whole vehicle and comprises the performance and cost control of the whole vehicle, the high-energy-density power battery can effectively control the weight of the whole vehicle and the design of other parts, in addition, the cost of the power battery in the electric vehicle is nearly 50%, the energy density of the power battery is improved, the cost of non-energy units such as mechanical parts can be reduced, and the cost of the whole vehicle is effectively controlled.

At present, the normal level of the energy density of the power battery is 180-: (1) in the cathode aspect: the high Ni system improves the capacity exertion of the cathode unit weight by improving the Ni content in the cathode material, and the current battery core level technical state and level are 240-280 wh/kg; the high-voltage system improves the capacity exertion of the cathode unit weight by improving the upper limit voltage in the cathode material charging process, and the current technical state and level of the battery cell level is 230-; (2) in the aspect of an anode: alloy doping, namely improving the capacity exertion of the unit weight of the anode by adding Si/SiO2 alloy, wherein the current cell level technical state and level is 250-350 wh/kg; (3) the process aspect is as follows: the effective capacity exertion of unit weight is improved by increasing the coating weight of the cathode and the anode and reducing the thickness of a base material (aluminum foil and copper foil) and a diaphragm, and the current technical state and level of the battery cell level is 230 and 270 wh/kg.

However, the above three methods may affect the long-term service life of the cell. For example: (1) in the cathode aspect: in high Ni systems, an increase in Ni content lowers the potential for cathodic oxygen evolution, thereby presenting a cell high gassing risk that will deteriorate cell long term reliability, e.g.Cycle life, storage life and cell expansion control, while the higher Ni content lowers the temperature threshold at which thermal runaway occurs in the cathode material, which will deteriorate the safety of the cell under relevant applications, such as high temperature, overcharge, extrusion, etc.; in the high-voltage system, the oxidation of the charging terminal cathode is improved by increasing the upper limit service voltage of the cathode, and the oxidation of the charging terminal cathode to the electrolyte and the diaphragm is accelerated, so that gas generation is deteriorated, and the long-term reliability is adversely affected. (2) In the aspect of an anode: alloy doping, addition of Si/SiO₂The battery core has very large shrinkage and expansion in the charging and discharging process, and the anode has the phenomena of demoulding and powder falling under a high shrinkage and expansion ratio along with the circulation, and meanwhile, the stability and the integrity of an SEI film on the surface of the anode are influenced, so that the long-term service life and the capacity maintenance of the battery core are greatly deteriorated; (3) the process aspect is as follows: an increase in the coating weight of the cathode and anode will deteriorate the power, charge window and long cycle life of the cell, while also presenting significant challenges to the process and equipment; the reduction of the base material (aluminum foil and copper foil) mainly affects the process manufacturing, the thin base material is easy to break in the manufacturing process of the battery cell, and the excellent rate of the battery cell manufacturing process is seriously affected, so that the cost is increased; the diaphragm is used as a part for isolating the direct contact short circuit of the cathode and the anode, the thickness of the diaphragm is important for controlling the safety of the short circuit in the battery cell, and the safety risk of the short circuit in the battery cell caused by the thickness reduction is reduced.

Therefore, it is urgently needed to find a suitable solution for improving the energy density of the power battery and ensuring the long-term reliability of the battery.

Disclosure of Invention

The purpose of the disclosure is to effectively improve the energy density of the power battery on the premise of conventional voltage cathodes with conventional Ni content, conventional graphite anode systems, normal cathode and anode coating weight, normal thickness cathode and anode base materials and diaphragms, and simultaneously ensure that the long-term reliability and safety performance of the power battery are not affected.

In order to achieve the above object, a first aspect of the present disclosure provides an anode for a lithium ion battery, including a current collector and an anode active material coating layer coated on a surface of the current collector, wherein the anode active material coating layer includes a first coating layer coated on the surface of the current collector and a second coating layer coated on a surface of the first coating layer, and a porosity of the first coating layer is lower than a porosity of the second coating layer.

Optionally, the porosity of the first coating is 20-40% and the porosity of the second coating is 40-60%;

preferably, the porosity of the first coating layer is 25 to 35% and the porosity of the second coating layer is 45 to 55%.

Optionally, the anode active material coating layer has a total thickness of 50 to 300 μm, and a ratio of the thickness of the first coating layer to the thickness of the second coating layer is 1: 0.2 to 50;

preferably, the anode active material coating layer has a total thickness of 80 to 240 μm, and a ratio of the thickness of the first coating layer to the thickness of the second coating layer is 1: 0.3-30.

Optionally, the coating of anode active material comprises two or more coatings, the porosity of the coatings decreasing from outer layer to inner layer by layer.

Optionally, the anode active material coating includes an anode active material selected from a graphite material selected from at least one of natural graphite, artificial graphite, soft carbon, and hard carbon.

A second aspect of the present disclosure provides a lithium ion battery comprising an anode, a cathode, an electrolyte and a separator, wherein the anode is the above-mentioned lithium ion battery anode.

Optionally, the system capacity of the cathode is higher than the system capacity of the anode per unit area.

Optionally, the cathode includes a cathode active material that is LiNi_xCo_yMn_zFe_aAl_bP_cO₂(wherein x is more than or equal to 0 and less than or equal to 1, y is more than or equal to 0 and less than or equal to 1, z is more than or equal to 0 and less than or equal to 1, a is more than or equal to 0 and less than or equal to 1, b is more than or equal to 0 and less than or equal to 0.8, and c is more than or equal to 0 and less than or equal to 4).

Optionally, the electrolyte comprises a solvent and a lithium salt, and the lithium salt is LiPF₆、LiClO₄、LiBO₂、LiAsF₆And LiBF₄At least one of;

the solvent is at least one of tetrahydrofuran, ethylene glycol dimethyl ether, gamma-butyrolactone, ethylene carbonate, propylene carbonate, ethyl methyl carbonate, methyl propyl carbonate, dimethyl carbonate, diethyl carbonate, dipropyl carbonate, N-methylpyrrolidone, N-methylformamide, N-methylacetamide, acetonitrile, N-dimethylformamide, sulfolane, dimethyl sulfoxide, dimethyl sulfite, dimethylformamide and methyl acetate.

Optionally, the separator is at least one selected from the group consisting of a polyethylene film, a polyolefin microporous film, a polyethylene felt, a glass fiber felt, and a micro glass fiber paper.

Through the technical scheme, the lithium ion battery provided by the disclosure can effectively improve the energy density of the battery cell, simultaneously ensure the electrical property of the battery cell, the long-term reliability of the battery cell and the safety performance of the battery cell, and meet the requirements of the current power battery cell.

Additional features and advantages of the disclosure will be set forth in the detailed description which follows.

Drawings

The accompanying drawings, which are included to provide a further understanding of the disclosure and are incorporated in and constitute a part of this specification, illustrate embodiments of the disclosure and together with the description serve to explain the disclosure without limiting the disclosure. In the drawings:

fig. 1 is a schematic diagram of an anode structure of a lithium ion battery of the present disclosure.

Description of the reference numerals

1. Current collector

2. First coating

3. Second coating layer

Detailed Description

The following describes in detail specific embodiments of the present disclosure. It should be understood that the detailed description and specific examples, while indicating the present disclosure, are given by way of illustration and explanation only, not limitation.

A first aspect of the present disclosure provides a lithium ion battery anode including a current collector and an anode active material coating coated on a surface of the current collector, wherein the anode active material coating includes a first coating coated on the surface of the current collector and a second coating coated on a surface of the first coating, and a porosity of the first coating is lower than a porosity of the second coating.

In the charging process, lithium ions reach the lower layer through the high porosity of the upper layer, and due to the low porosity of the lower layer, after the graphite of the lower layer is embedded with proper lithium, a large number of lithium ions are gathered on the interface of the upper layer and the lower layer, deposited and separated to form lithium metal at the interface and distributed along the corresponding pores of the interface, so that the position and the shape of the deposited lithium metal of the anode under the condition that the cathode capacity is higher than the anode capacity can be effectively controlled.

According to a first aspect of the present disclosure, the porosity of the first coating layer may be 20-40%, and the porosity of the second coating layer may be 40-60%;

preferably, the porosity of the first coating layer may be 25 to 35%, and the porosity of the second coating layer may be 45 to 55%.

According to the first aspect of the present disclosure, the total thickness of the anode active material coating layer may be 50 to 300 μm, and the ratio of the thickness of the first coating layer to the thickness of the second coating layer may be 1: 0.2 to 50;

preferably, the total thickness of the anode active material coating layer may be 80 to 240 μm, and the ratio of the thickness of the first coating layer to the thickness of the second coating layer may be 1: 0.3-30.

The anode active material coating of the present disclosure may have the number of layers adjusted according to actual conditions, that is, may include more than two coating layers, and the porosity of each coating layer decreases layer by layer from the outer layer to the inner layer.

According to a first aspect of the present disclosure, the anode active material coating includes an anode active material selected from a graphite material selected from at least one of natural graphite, artificial graphite, soft carbon, and hard carbon.

In a specific embodiment of the present disclosure, the hardness of the graphite material in the different coatings is selected from an outer layer to an inner layer, and the hardness gradually decreases, that is, the hardness of the graphite particles at the outermost layer is the largest, and the hardness of the graphite particles at the innermost layer is the smallest, and the test method may be: taking different graphite material powder particles, filling the graphite particles with the same thickness by using a powder tablet press, then extruding the powder particles with the same pressure, unloading the pressure after a certain time, testing the thickness of the powder particles extruded with the same pressure, wherein the larger the thickness is, the higher the hardness of the powder particles is, the larger the hardness of the powder particles is, the smaller the thickness is, the lower the hardness of the powder particles is, and the powder particles are used in the inner layer.

The specific manufacturing method of the anode active material coating can be that parameters and selections of graphite material particles of different layers of the anode active material coating are determined through a graphite material particle hardness test, then the graphite particles are homogenized to be made into slurry, then multi-cavity coating or multi-die coating is adopted for one-time coating to realize coating of a plurality of coatings, and then rolling is carried out to realize the anode active material coating with gradient distribution of porosity, wherein the porosity distribution is gradually reduced from the outer layer of the pole piece to the inner layer of the pole piece.

The lithium ion battery provided by the disclosure uses the anode with the anode active material coating, the energy density is increased from 180-class 230wh/kg to 230-class 280wh/kg on the existing basis, the electrical property of the battery core and the long-term reliability of the battery core can be ensured, the power, the cyclic storage, the gas generation expansion and the battery core safety of the prepared lithium ion battery can be kept at the existing average level, and the requirements of the current power battery core can be met.

As a preferred embodiment of the present disclosure, the system capacity of the cathode is higher than the system capacity of the anode per unit area. The inventor of the present disclosure finds through a great deal of experiments that, in the case where the cathode capacity is higher than the anode capacity, the safety problem of puncturing the separator can be avoided by effectively controlling the deposition position and shape of the lithium metal of the anode, and for the anode, lithium is partially stored in the anode in a metal form, and the energy density of the cell can be effectively increased due to the high energy density of the lithium metal. The lithium ion battery anode disclosed by the invention can effectively control the position and shape of the anode lithium metal deposition under the condition that the cathode capacity is higher than the anode capacity, thereby effectively improving the energy density of the battery cell under the condition of maintaining the application and process level of the existing mature chemical system, mature base material and diaphragm, and meeting the requirements of the current power battery cell on the premise of keeping the battery performance, the battery reliability and the safety performance.

According to a second aspect of the present disclosure, the cathode includes a cathode active material, which may be LiNi_xCo_yMn_zFe_aAl_bP_cO₂(wherein x is more than or equal to 0 and less than or equal to 1, y is more than or equal to 0 and less than or equal to 1, z is more than or equal to 0 and less than or equal to 1, a is more than or equal to 0 and less than or equal to 1, b is more than or equal to 0 and less than or equal to 0.8, and c is more than or equal to 0 and less than or equal to 4).

According to a second aspect of the present disclosure, the electrolyte comprises a solvent and a lithium salt, wherein the solvent and the lithium salt are well known to those skilled in the art, and the lithium salt may be LiPF₆、LiClO₄、LiBO₂、LiAsF₆And LiBF₄At least one of; the solvent may be at least one of tetrahydrofuran, ethylene glycol dimethyl ether, gamma-butyrolactone, ethylene carbonate, propylene carbonate, ethyl methyl carbonate, methyl propyl carbonate, dimethyl carbonate, diethyl carbonate, dipropyl carbonate, N-methylpyrrolidone, N-methylformamide, N-methylacetamide, acetonitrile, N-dimethylformamide, sulfolane, dimethyl sulfoxide, dimethyl sulfite, dimethylformamide, and methyl acetate.

According to the second aspect of the present disclosure, the separator is at least one selected from the group consisting of a polyethylene film, a polyolefin microporous film, a polyethylene felt, a glass fiber felt, and an ultrafine glass fiber paper.

The present disclosure is further illustrated by the following examples. The raw materials used in the examples are all available from commercial sources.

The test methods of the graphite material particles 1 and the graphite material particles 2 used in the examples and comparative examples of the present disclosure were: and (2) filling graphite particles with the same thickness by using a powder tablet press, then extruding the powder particles with the same pressure, relieving the pressure after a certain time, and testing the thickness of the powder particles extruded with the same pressure, wherein the larger the thickness is, the higher the hardness of the powder particles is, the powder particles are used for the upper layer, and the smaller the thickness is, the lower the hardness of the powder particles is, the powder particles are used for the lower layer. The hardness of the graphite material particles 2 is higher than that of the graphite material particles 1 through testing.

The cathodes of the examples and comparative examples in this disclosure all used LiNi_0.5Co_0.2Mn_0.3O₂The ternary material comprises a cathode aluminum foil base material which is a normal base material with the thickness of 12 micrometers, an anode copper foil base material which is a normal base material with the thickness of 8 micrometers, diaphragms which are all made of polyethylene diaphragms with the thickness of 16 micrometers, and electrolyte which is made of LiPF with the thickness of 1.12M₆Lithium salt and DEC/EC/EMC 1/1/1 solvent.

The N/P ratio of examples and comparative examples in this disclosure is the ratio of the anode capacity to the cathode capacity per unit area, i.e.

N/P is the anode unit area capacity/cathode unit area capacity (anode areal density × anode gram capacity × anode active material content)/(cathode areal density × cathode gram capacity × cathode active material content).

Different N/P ratios can be obtained by fixing the parameters of the cathode unchanged and adjusting the surface density of the anode.

Example 1

In the embodiment, the N/P ratio is 1.03, the graphite material particles 1, SBR (styrene butadiene rubber), CMC (sodium carboxymethylcellulose) and SP (conductive agent) are homogenized according to the weight ratio of 95:2.5:1.5:1 to obtain anode slurry 1, wherein water is added to control the solid content to be 45-55%, and the viscosity is 2000-4000 mpa. After the stirring is completed, the anode slurry 1 is introduced into the lower cavity. And (3) homogenizing the graphite material particles 2, SBR (styrene butadiene rubber), CMC (sodium carboxymethylcellulose) and SP (conductive agent) according to a weight ratio of 95:2.5:1.5:1 to obtain anode slurry 2, wherein water is added to control the solid content to be 45-55%, and the viscosity is 2000-4000mpa · s. After the stirring was completed, the anode slurry 2 was introduced into the upper chamber. Then the upper cavity slurry and the lower cavity slurry are evenly coated on the surface of a copper foil base material with the thickness of 8 mu m, and the coating weight of the two surfaces is 172g/m²The coating weights of the upper layer and the lower layer are controlled to be 86g/m by controlling the upper cavity gasket, the lower cavity gasket and the clamping degree²Namely, the weight distribution ratio of the upper layer to the lower layer is 5: 5, obtaining a first coating layer positioned at the lower layer and a second coating layer positioned at the upper layer, andand then drying, rolling, die cutting and punching to obtain the anode plate.

Example 2

The manufacturing method of the anode piece of this embodiment is the same as that of embodiment 1, except that the ratio of N/P in this embodiment is 0.98.

Example 3

The manufacturing method of the anode plate of this embodiment is the same as that of embodiment 1, except that the ratio of N/P in this embodiment is 0.93.

Example 4

In the embodiment, the N/P ratio is 0.93, the graphite material particles 1, SBR (styrene butadiene rubber), CMC (sodium carboxymethyl cellulose) and SP (conductive agent) are homogenized according to the weight ratio of 95:2.5:1.5:1 to obtain anode slurry 1, wherein water is added to control the solid content to be 45-55%, and the viscosity is 2000-4000 mpa. After the stirring is completed, the anode slurry 1 is introduced into the lower cavity. And (3) homogenizing the graphite material particles 2, SBR (styrene butadiene rubber), CMC (sodium carboxymethylcellulose) and SP (conductive agent) according to a weight ratio of 95:2.5:1.5:1 to obtain anode slurry 2, wherein water is added to control the solid content to be 45-55%, and the viscosity is 2000-4000mpa · s. After the stirring was completed, the anode slurry 2 was introduced into the upper chamber. Then the slurry of the upper cavity and the lower cavity is evenly coated on the surface of a copper foil base material with the thickness of 8 mu m, and the coating weight of the two surfaces is 156g/m²The upper layer coating weight is controlled to be 47g/m by controlling the upper cavity gasket and the lower cavity gasket and the clamping degree²The lower layer coating weight was 109g/m²Namely, the weight distribution ratio of the upper layer to the lower layer is 3: and 7, obtaining a first coating positioned on the lower layer and a second coating positioned on the upper layer, and then carrying out drying, rolling, die cutting and die cutting to obtain the anode piece.

Example 5

In the embodiment, the N/P ratio is 0.93, the graphite material particles 1, SBR (styrene butadiene rubber), CMC (sodium carboxymethylcellulose) and SP (conductive agent) are homogenized according to the weight ratio of 95:2.5:1.5:1 to obtain anode slurry 1, wherein water is added to control the solid content to be 45-55%, and the viscosity is 2000-4000 mpa. After the stirring is completed, the anode slurry 1 is introduced into the lower cavity. Homogenizing graphite material particles 2, SBR (styrene butadiene rubber), CMC (sodium carboxymethylcellulose) and SP (conductive agent) according to a weight ratio of 95:2.5:1.5:1 to obtain anode slurry 2, wherein water is added to control solid content to be45-55%, viscosity 2000-. After the stirring was completed, the anode slurry 2 was introduced into the upper chamber. Then the upper cavity slurry and the lower cavity slurry are evenly coated on the surface of a copper foil base material with the thickness of 8 mu m, and the coating weight of the two surfaces is 156g/m²The upper coating weight is controlled to be 109g/m by controlling the upper cavity gasket and the lower cavity gasket and the clamping degree²The lower layer coating weight was 47g/m²Namely, the weight distribution ratio of the upper layer to the lower layer is 7: and 3, obtaining a first coating positioned on the lower layer and a second coating positioned on the upper layer, and then carrying out drying, rolling, die cutting and die cutting to obtain the anode piece.

Comparative example 1

In the comparative example, the N/P ratio was 1.08, and the graphite particles 1 were homogenized with SBR (styrene butadiene rubber), CMC (sodium carboxymethylcellulose) and SP (conductive agent) in a weight ratio of 95:2.5:1.5:1, wherein water was added to control the solid content to 45-55%, and the viscosity to 2000-4000mpa s. After stirring, the anode slurry was uniformly coated on the surface of a copper foil substrate of 8 μm with a double-side coating weight of 180g/m²And then drying, rolling, die cutting and punching to obtain the anode plate.

Comparative example 2

In the comparative example, the N/P ratio was 1.08, and the graphite particles 2 were homogenized with SBR (styrene butadiene rubber), CMC (sodium carboxymethylcellulose) and SP (conductive agent) in a weight ratio of 95:2.5:1.5:1, wherein water was added to control the solid content to 45-55%, and the viscosity to 2000-4000mpa s. After stirring, the anode slurry was uniformly coated on the surface of a copper foil substrate of 8 μm with a double-side coating weight of 180g/m²And then drying, rolling, die cutting and punching to obtain the anode plate.

Test example 1

The anode sheets prepared in examples 1 to 5 and comparative examples 1 to 2 were tested for coating porosity and coating thickness for each coating by the following test methods:

coating thickness test: and testing the thickness of the first coating, namely taking the pole piece only coated with the first coating, obtaining the thickness of the first coating through a micrometer test, then taking the double-layer pole piece coated with the first coating and the second coating, obtaining the thickness of the double-layer pole piece through the micrometer, wherein the thickness of the second coating is equal to the thickness of the double-layer electrode minus the thickness of the first coating.

And (3) porosity testing: and testing the porosity of the first coating, namely taking the pole piece only coated with the first coating, calculating the porosity of the first coating through a true density tester, and then taking the double-layer pole piece coated with the first coating and the second coating, and calculating the porosity of the double-layer electrode through the true density tester. Porosity of the second coating layer ═ (bilayer electrode porosity · bilayer electrode thickness-first coating layer porosity · first coating layer thickness)/second coating layer thickness

The specific structure is shown in Table 1.

TABLE 1

Test example 2

Taking cathode pole pieces and the anode pole pieces in the embodiments 1-5 and the comparative examples 1-2, stacking the cathode pole pieces and the anode pole pieces layer by layer according to the sequence of anode-diaphragm-cathode-diaphragm-anode to manufacture a bare cell, controlling the thickness of the comparative examples to be consistent with that of the bare cell in the embodiments by controlling the number of cathode-anode laminations, and then putting the bare cell into a shell, baking, injecting liquid, forming and sealing to manufacture each group of cells.

And (3) at room temperature, taking each pair of the electric cores in each proportion and each three electric cores in each embodiment, adopting a charging and discharging test cabinet to charge the electric cores to 4.2V at a constant current and a constant voltage according to the charging 0.33C, standing for 10min, discharging the electric cores to 2.8V according to the discharging 0.33C, and recording the discharging capacity. The cell impedances of the comparative examples and examples were measured with a resistance tester and the values recorded. The cell weights of the comparative examples and examples were measured using an electronic scale, and the cell weight energy density is discharge capacity discharge plateau voltage/cell weight. The test results are shown in Table 2.

As can be seen from table 2, the cell capacity and energy density in the examples are significantly increased relative to the comparative examples, which is probably because the low N/P reduces the anode coating weight and more active material can be filled in the same space, thereby improving the effective energy exertion. It can be seen that the resistance did not deteriorate and was slightly better than the comparative example, and as the N/P ratio decreased, there was an advantage in that the resistance could be decreased, probably because the low N/P ratio resulted in a slightly lower anode coating weight, and the number of lamination layers increased, corresponding to an increase in the number of parallel connections in the cell. While the ratio of the first coating to the second coating did not differ significantly in resistance and capacity performance, indicating that it had no effect on capacity performance within a range of suitable first coating to second coating ratios.

Test example 3

Cells were prepared at room temperature using the anode sheets prepared in examples 1-5 and comparative examples 1-2, by the same method as in test example 2, charging to 4.2V with a constant current and a constant voltage of 0.33C, then discharging for 30min to 50% SOC with 1C, discharging for 10S with a current of 4C, recording the voltage values before and after discharge, and measuring two cells per group, the results of which are shown in table 3.

Dc impedance (voltage before discharge-voltage after discharge)/discharge current;

power (pre-discharge voltage-lower limit voltage) lower limit voltage/dc impedance

TABLE 3

As can be seen from table 3, the dc impedance and power in the examples did not deteriorate and were slightly better than those in the comparative examples, and as the N/P ratio decreased, there was an advantage in that the phenomenon was intensified, which is probably because the low N/P ratio resulted in a slightly lower anode coating weight, and the number of lamination layers increased, which corresponds to an increase in the number of parallel connections in the cell, so that the impedance could be decreased. The proportions of the first and second coatings were not significantly different, indicating that the proportions of the first and second coatings had no effect on short-term electrical properties.

Test example 4

At room temperature, cells were prepared by using the anode sheets prepared in examples 1 to 5 and comparative examples 1 to 2, which were prepared in the same manner as in test example 2, by charging to 4.2V at a constant current and a constant voltage at 0.33C, allowing the cells to stand for 5min, then discharging to 2.8V at 0.33C, and recording the discharge capacity, the capacity retention rate being the corresponding cycle discharge capacity/initial discharge capacity. The process is repeated until the capacity retention rate is less than or equal to 80%, the number of cycles is recorded, two battery cells are measured in each group, and the measurement results are shown in table 4. Wherein, a gas production tester and an expansion inner test device are adopted to test the gas production and the expansion force change condition in the circulation process.

The prepared battery cell is charged to 4.2V by adopting a 0.33C constant current and constant voltage, then the battery cell is placed in a high-temperature 45 ℃ thermostat and stored for 500 days, the capacity retention rate is tested after the battery cell is taken out every 30 days, and the measurement result is shown in table 4.

TABLE 4

As can be seen from Table 4, the examples showed no deterioration in the cycle, gassing, swelling and storage, and were slightly superior to the comparative examples, and the advantage of the enhanced phenomenon was that the low N/P ratio resulted in a lower cathode potential at 100% SOC, which reduced the oxidation of the electrolyte. The ratios of the first and second coatings were not significantly different, indicating that the ratios of the first and second coatings had no effect on long term electrical properties.

Test example 5

At room temperature, cells were prepared by using the anode sheets prepared in examples 1 to 5 and comparative examples 1 to 2, in the same manner as in test example 2, charging to 4.2V with a constant current and a constant voltage of 0.33C, then testing the hot box and extrusion separately, starting at a temperature of 25 ℃ in the hot box, at a rate of 5 ℃/min, heating to 130 ℃ and then maintaining for 30min, and observing the cell conditions. And (3) extrusion testing, wherein the extrusion speed is 2mm/s, the voltage reaches 0V or the deformation reaches 15% or the extrusion force reaches 100KN, then the extrusion is stopped, the standing is carried out for 1h, the cell condition is observed, two cells are measured in each group, and the measurement result is shown in table 5.

TABLE 5

As can be seen from table 5, the lithium ion battery of the present disclosure has no difference in safety performance, and can meet the requirements of the current power battery.

The preferred embodiments of the present disclosure have been described in detail above, however, the present disclosure is not limited to the specific details of the above embodiments, and various simple modifications may be made to the technical solution of the present disclosure within the technical idea of the present disclosure, and these simple modifications all fall within the protection scope of the present disclosure.

It should be noted that the various features described in the above embodiments may be combined in any suitable manner without departing from the scope of the invention. In order to avoid unnecessary repetition, various possible combinations will not be separately described in this disclosure.

In addition, any combination of various embodiments of the present disclosure may be made, and the same should be considered as the disclosure of the present disclosure, as long as it does not depart from the spirit of the present disclosure.

Claims

1. The anode of the lithium ion battery is characterized by comprising a current collector and an anode active material coating coated on the surface of the current collector, wherein the anode active material coating comprises a first coating coated on the surface of the current collector and a second coating coated on the surface of the first coating, and the porosity of the first coating is lower than that of the second coating;

the anode active material coating comprises more than two layers of coatings, and the porosity distribution of the coatings is gradually reduced from the outer layer of the pole piece to the inner layer of the pole piece;

the total thickness of the anode active material coating is 80-240 mu m, and the ratio of the thickness of the first coating to the thickness of the second coating is 1: 0.3 to 30;

the anode active material coating layer includes an anode active material selected from a graphite material selected from at least one of natural graphite, artificial graphite, soft carbon, and hard carbon; the hardness of the graphite materials in different coatings is gradually reduced from outside to inside.

2. The lithium ion battery anode of claim 1, wherein the porosity of the first coating is 20-40% and the porosity of the second coating is 40-60%.

3. The lithium ion battery anode of claim 1, the porosity of the first coating layer being 25-35% and the porosity of the second coating layer being 45-55%.

4. A lithium ion battery comprising an anode, a cathode, an electrolyte and a separator, wherein the anode is the lithium ion battery anode of any one of claims 1 to 3;

the system capacity of the cathode is higher than that of the anode;

the energy density of the lithium ion battery is 230-280 wh/kg.

5. The lithium ion battery of claim 4, wherein the cathode comprises a cathode active material that is LiNi_xCo_yMn_zFe_aAl_bP_cO₂Wherein x is more than or equal to 0 and less than or equal to 1, y is more than or equal to 0 and less than or equal to 1, z is more than or equal to 0 and less than or equal to 1, a is more than or equal to 0 and less than or equal to 1, b is more than or equal to 0 and less than or equal to 0.8, and c is more than or equal to 0 and less than or equal to 4.

6. The lithium ion battery of claim 4, wherein the electrolyte comprises a solvent and a lithium salt, the lithium salt being LiPF₆、LiClO₄、LiBO₂At least one of LiAsF6 and LiBF 4;

7. The lithium ion battery according to claim 4, wherein the separator is at least one selected from the group consisting of a polyethylene film, a polyolefin microporous film, a polyethylene felt, a glass fiber felt, and a fine glass fiber paper.