CN113036298B

CN113036298B - Negative pole piece and secondary battery and device containing same

Info

Publication number: CN113036298B
Application number: CN201911246501.7A
Authority: CN
Inventors: 张小文; 黄亚萍; 金海族; 林永寿
Original assignee: Contemporary Amperex Technology Co Ltd
Current assignee: Contemporary Amperex Technology Co Ltd
Priority date: 2019-12-06
Filing date: 2019-12-06
Publication date: 2022-02-11
Anticipated expiration: 2039-12-06
Also published as: CN113036298A

Abstract

The application relates to a negative pole piece, a secondary battery containing the same and a device. Specifically, the application provides a negative pole piece, which comprises a negative pole current collector and a negative pole film layer arranged on at least one surface of the negative pole current collector, wherein the negative pole film layer comprises a first negative pole film layer and a second negative pole film layer, the first negative pole film layer comprises a first negative pole active material and is arranged on at least one surface of the negative pole current collector, the second negative pole film layer comprises a second negative pole active material and is arranged on the first negative pole film layer, the first negative pole active material comprises natural graphite, and the OI value of the first negative pole film layer is 10-35; the second negative electrode active material comprises artificial graphite, and the OI value of the second negative electrode film layer is 2-15. The negative pole piece can enable a secondary battery containing the negative pole piece to have good rate capability and long cycle life.

Description

Negative pole piece and secondary battery and device containing same

Technical Field

The application belongs to the technical field of electrochemistry, and more specifically relates to a negative pole piece and a secondary battery and a device containing the same.

Background

New energy automobiles represent the direction of development of the automobile industry in the world. The lithium ion secondary battery is used as a novel high-voltage and high-energy-density power battery, has the outstanding characteristics of light weight, high energy density, no pollution, no memory effect, long service life and the like, and is widely applied to new energy automobiles.

As the market for power batteries has been expanding, the demand for energy density of power batteries has also been increasing. However, the technical means adopted in the prior art for improving the energy density of the battery often result in the deterioration of the performance of the battery in other aspects. Therefore, there is an urgent need for a new technology that can improve the energy density of a battery without degrading or even improving other electrical properties of the battery.

Disclosure of Invention

In view of the problems existing in the background art, the first aspect of the present application provides a negative electrode tab, and after the negative electrode tab is used for a secondary battery, the secondary battery can have good cycle performance and rate performance on the premise of having higher energy density.

In order to achieve the above object, the negative electrode sheet of the first aspect of the present application includes a negative electrode current collector and a negative electrode film layer disposed on at least one surface of the negative electrode current collector, the negative electrode film layer includes a first negative electrode film layer and a second negative electrode film layer, the first negative electrode film layer includes a first negative electrode active material and is disposed on at least one surface of the negative electrode current collector, the second negative electrode film layer includes a second negative electrode active material and is disposed on the first negative electrode film layer, the first negative electrode active material includes natural graphite, and the first negative electrode film layer has an OI value of 10 to 35; the second negative electrode active material comprises artificial graphite, and the OI value of the second negative electrode film layer is 2-15.

In a second aspect of the present application, there is provided a secondary battery comprising the negative electrode tab of the first aspect of the present application.

In a third aspect of the present application, there is provided an apparatus comprising the secondary battery described in the second aspect of the present application.

The application at least comprises the following beneficial effects:

the negative pole piece comprises a multilayer structure, each layer comprises a specific negative active material and a specific pole piece OI range, and under the combined action of the negative pole piece and the pole piece OI range, the battery can have good cycle performance and rate capability simultaneously on the premise of high energy density.

Drawings

Fig. 1 is a schematic diagram of an embodiment of a secondary battery of the present application.

Fig. 2 is a schematic diagram of an embodiment of a battery module.

Fig. 3 is a schematic diagram of an embodiment of a battery pack.

Fig. 4 is an exploded view of fig. 3.

Fig. 5 is a schematic diagram of an embodiment of an apparatus in which the secondary battery of the present application is used as a power source.

Wherein the reference numerals are as follows:

1 Battery pack

2 upper box body

3 lower box body

4 cell module

5 Secondary Battery

Detailed Description

The present application is further described below in conjunction with the detailed description. It should be understood that these specific embodiments are merely illustrative of the present application and are not intended to limit the scope of the present application.

For the sake of brevity, only some numerical ranges are specifically disclosed herein. However, any lower limit may be combined with any upper limit to form ranges not explicitly recited; and any lower limit may be combined with any other lower limit to form a range not explicitly recited, and similarly any upper limit may be combined with any other upper limit to form a range not explicitly recited. Furthermore, each separately disclosed point or individual value may itself, as a lower or upper limit, be combined with any other point or individual value or with other lower or upper limits to form ranges not explicitly recited.

In the description herein, it is to be noted that, unless otherwise specified, "above" and "below" are inclusive and "one or more" mean "several" two or more.

Unless otherwise indicated, terms used in the present application have well-known meanings that are commonly understood by those skilled in the art. Unless otherwise indicated, the numerical values of the parameters mentioned in the present application can be measured by various measurement methods commonly used in the art (for example, the test can be performed according to the methods given in the examples of the present application).

In a secondary battery, the thickness of a film layer is often increased in order to increase the energy density of the battery, but the increase in thickness causes both the cycle performance and the rate performance of the battery to be affected. This is because the negative active material expands during cycling, resulting in a decrease in the binding force between the active material and the substrate, and even a release, which is more severe when the thickness increases; at the same time, the increased thickness increases the diffusion path of the active ions, and thus the rate performance of the battery is also affected. In order to solve the above problem, the following two aspects can be considered: on the one hand, a negative active material having small expansion can be used, but this limits the selection range of the active material; on the other hand, the content of the binder in the negative electrode film layer can be increased to improve the binding power of the pole piece, but the energy density of the battery is reduced due to the increase of the content of the binder. Therefore, how to combine the good cycle performance and rate capability of the battery under the premise of having higher energy density is a great challenge in the technical aspect.

The inventor finds out through a large number of experiments that the technical goal of the application can be achieved by adjusting the preparation process of the negative pole piece. Specifically, the negative pole piece comprises a negative pole current collector and a negative pole film layer arranged on at least one surface of the negative pole current collector, wherein the negative pole film layer comprises a first film layer and a second film layer, the first film layer comprises a first active material and is arranged on at least one surface of the current collector, the second film layer comprises a second active material and is arranged on the first film layer, the first active material comprises natural graphite, and the OI value of the first film layer is 10-35; the second active material comprises artificial graphite, and the OI value of the second film layer is 2-15.

The inventor researches and discovers that when the negative pole piece meets the design conditions, on one hand, the binding force between the negative pole film layer and the current collector is effectively improved, the demoulding phenomenon of the pole piece is improved, and therefore the cycle performance of the battery is improved; on the other hand, the OI values of the upper and lower film layers are controlled within a given range, the pore channel structures of the upper and lower layers are reasonably matched, the dynamic performance of the battery is effectively improved, the diffusion rate of active ions can be ensured even under the condition that the coating thickness is increased, and the rate capability of the battery is effectively improved.

The application the negative pole piece including the mass flow body with set up in two-layer or multilayer rete on at least one surface of the mass flow body. The negative electrode plate can be coated on both surfaces (i.e., the film layers are disposed on both surfaces of the current collector) or only one surface (i.e., the film layer is disposed on only one surface of the current collector). The negative electrode plate can be prepared by various methods commonly used in the field. Generally, a negative electrode current collector is prepared, a negative electrode active material slurry is prepared, the negative electrode active material slurry is coated on one or two surfaces of the negative electrode current collector, and finally, the required negative electrode plate is obtained through post-processing steps of drying, welding a conductive member (tab), cutting and the like.

The negative electrode film layer can be formed by coating the first active material slurry and the second active material slurry on the negative electrode current collector at the same time, or coating the first active material slurry and the second active material slurry at two times.

The negative electrode film layer typically includes a negative electrode active material, and optionally a binder, an optional conductive agent, and other optional adjuvants. The film layer slurry coating is generally formed by dispersing the negative electrode active material and optionally a conductive agent and a binder, etc. in a solvent, such as N-methylpyrrolidone (NMP) or deionized water, and uniformly stirring. Other optional auxiliaries may be, for example, thickening and dispersing agents (e.g. carboxymethylcellulose, CMC), PTC thermistor materials, etc.

As an example, the conductive agent may be one or more of graphite, superconducting carbon, acetylene black, carbon black, ketjen black, carbon dots, carbon nanotubes, graphene, and carbon nanofibers.

As an example, the binder may be one or more of Styrene Butadiene Rubber (SBR), water-based acrylic resin (water-based acrylic resin), polyvinylidene fluoride (PVDF), Polytetrafluoroethylene (PTFE), ethylene-vinyl acetate copolymer (EVA), polyvinyl alcohol (PVA), and polyvinyl butyral (PVB).

In the negative electrode plate described herein, the negative active material may further include one or a mixture of several of soft carbon, hard carbon, a silicon-based material, and a tin-based material.

Those skilled in the art understand that: OI value (V) of film layer_OI) Is used to represent an orientation index of the active material in the anode film layer, i.e., a degree of anisotropy of the arrangement of crystal grains in the anode film layer. In the present application, the OI value (V)_OI) Is defined as the area ratio of the (004) characteristic diffraction peak and the (110) characteristic diffraction peak of the negative electrode film layer in an X-ray diffraction spectrum. I.e. V_OI＝C₀₀₄/C₁₁₀Wherein, C₀₀₄Peak area of 004 characteristic diffraction peak, C₁₁₀The peak area of the characteristic diffraction peak is 110.

The OI value of the negative electrode film layer may be determined by methods known in the art, for example, as described in the examples section herein.

In the preparation process of the pole piece, the OI value of the film layer can be controlled by adjusting the following parameters.

First, the average particle diameter D50 of the negative electrode active material and the OI value G of the active material powder_OIHas certain influence on the OI value of the cathode film layer. In general, the larger the D50 of the negative electrode active material, the larger the negative electrode film layer OI value, and the larger the powder OI value of the negative electrode active material, the larger the negative electrode film layer OI value.

Secondly, in the preparation process of the battery, a magnetic field induction technology can be introduced in the coating or drying process, and the arrangement of the negative active material on the pole piece is artificially induced, so that the size of the OI value of the negative film layer is changed; in the cold pressing process, the arrangement of the cathode active material is changed by adjusting the compaction density of the cathode film layer, and the OI value of the cathode film layer is further controlled.

In the negative pole piece, the OI value of the first film layer is 10-35, preferably 12-30, and the OI value of the second film layer is 2-15, preferably 4-12.

In the negative electrode sheet of the present application, the average particle diameter D50 of the first negative electrode active material is preferably 9 to 22 μm, and more preferably 10 to 15 μm. In the negative electrode sheet of the present application, the average particle diameter D50 of the second negative electrode active material is preferably 5 to 12 μm, and more preferably 6 to 10 μm.

In addition, in the preparation process of the negative pole piece, the surface density of the film layer can be controlled, so that the performance of the pole piece is further optimized. In some preferred embodiments of the present application, the first film layer and the second film layer each have an areal density of 3 to 8mg/cm²。

In addition, in the preparation process of the negative pole piece, the compaction density of the film layer can be controlled, so that the performance of the pole piece is further optimized. In some preferred embodiments of the present application, the first film layer has a densified density that is greater than the densified density of the second film layer; more preferablyThe first film layer has a compacted density of 1.65g/cm³～1.7g/cm³The second film layer having a compacted density of 1.5g/cm³～1.65g/cm³。

In the secondary battery of the present application, the total thickness of the film layers of the negative electrode sheet is preferably not less than 60 μm, and more preferably 65 μm to 80 μm.

In the secondary battery of the present application, the upper and lower layer thickness ratio (the thickness ratio of the first negative electrode film layer to the second negative electrode film layer) is preferably 0.55 to 0.95, and more preferably 0.65 to 0.85.

In the secondary battery of the present application, the negative electrode film layer may be disposed on one surface of the negative electrode current collector, and may also be disposed on two surfaces of the negative electrode current collector at the same time. It should be noted that each of the parameters of the negative electrode film layer given in the present application refers to the range of parameters of the single-sided film layer. When the negative electrode film layer is disposed on both surfaces of the negative electrode current collector, the film layer parameters on either surface satisfy the present application, i.e., are considered to fall within the scope of the present application. The ranges of the film thickness, the compacted density, the surface density and the like in the invention refer to the parameters of the pole piece/film layer which is subjected to cold pressing and compaction and is used for assembling the battery.

For the negative electrode sheet described herein, the negative current collector may be a conventional metal foil or a composite current collector. The material of the metal foil may be one or more of copper, copper alloy, nickel alloy, titanium alloy, silver, and silver alloy. For example, a copper foil having a thickness of 5 to 30 μm may be used. The composite current collector is generally formed by forming a metal material (copper, copper alloy, nickel alloy, titanium alloy, silver alloy, or the like) on a polymer base material.

In addition, the negative electrode sheet described herein does not exclude additional functional layers other than the film layer. For example, in certain preferred embodiments, the negative electrode sheet described herein further comprises a conductive primer layer (e.g., consisting of a conductive agent and a binder) disposed on the surface of the current collector, sandwiched between the current collector and the first membrane layer. In some other embodiments, the negative electrode sheet further includes a protective cover layer covering the surface of the second film layer.

The construction and production method of the secondary battery of the present application are known per se, except that the negative electrode sheet of the present application is used. For example, a secondary battery generally mainly comprises a negative electrode plate (i.e., the negative electrode plate of the present application), a positive electrode plate, a separator, and an electrolyte, and active ions move between the positive electrode and the negative electrode with the electrolyte as a medium to realize charging and discharging of the battery. In order to avoid short circuit of the anode and the cathode through the electrolyte, the anode and the cathode need to be separated by a separation film.

The secondary battery of the present application may employ a winding or lamination manufacturing process.

The outer package of the secondary battery of the present application may be a hard case (e.g., an aluminum case, a steel case, etc.), or may be a soft case (e.g., a pouch type, which may be made of a plastic material, such as one or more of polypropylene PP, polybutylene terephthalate PBT, polybutylene succinate PBS, etc.).

In the secondary battery of this application, positive pole piece includes the anodal mass flow body and sets up and collect the body surface and just include anodal active material's anodal active material layer at the anodal mass flow. The positive electrode active material may be a transition metal composite oxide including lithium iron phosphide, lithium iron manganese phosphide, lithium cobalt oxide, lithium nickel oxide, lithium manganese oxide, lithium nickel cobalt aluminum oxide, compounds obtained by adding other transition metals or non-transition metals to these lithium transition metal oxides, or a mixture of two or more of the foregoing; however, the present application is not limited to these materials, and other conventionally known materials that can be used as a positive electrode active material of a secondary battery may be used. These positive electrode active materials may be used alone or in combination of two or more.

In the secondary battery of the present application, the specific types and compositions of the separator and the electrolyte are not particularly limited, and may be selected according to actual requirements.

Specifically, the separator may be selected from, for example, a polyethylene film, a polypropylene film, a polyvinylidene fluoride film, a nonwoven fabric, and a multilayer composite film thereof.

Specifically, the electrolytic solution generally includes an organic solvent and an electrolyte salt. The organic solvent may be selected from a chain carbonate (e.g., dimethyl carbonate DMC, diethyl carbonate DEC, methylethyl carbonate EMC, methylpropyl carbonate MPC, dipropyl carbonate DPC, etc.), a cyclic carbonate (e.g., ethylene carbonate EC, propylene carbonate PC, vinylene carbonate VC, etc.), other chain carboxylic acid (e.g., methyl propionate, etc.), other cyclic ester (e.g., γ -butyrolactone, etc.), a chain ether (e.g., dimethoxyethane, diethyl ether, diglyme, triglyme, etc.), a cyclic ether (e.g., tetrahydrofuran, 2-methyltetrahydrofuran, etc.), a nitrile (acetonitrile, propionitrile, etc.), or a mixed solvent thereof. The electrolyte salt is, for example, LiClO₄、LiPF₆、LiBF₄、LiAsF₆、LiSbF₆Etc. inorganic lithium salt, or LiCF₃SO₃、LiCF₃CO₂、Li₂C₂F₄(SO₃)₂、LiN(CF₃SO₂)₂、LiC(CF₃SO₂)₃、LiC_nF_2n+1SO₃(n is more than or equal to 2) and the like.

The construction and the production method of the secondary battery of the present application are briefly described below.

Firstly, preparing a battery positive pole piece according to a conventional method in the field. The positive active material used for the positive pole piece is not limited in the application. In general, it is necessary to add a conductive agent (for example, a carbon material such as carbon black), a binder (for example, PVDF), and the like to the positive electrode active material. Other additives such as PTC thermistor materials and the like may also be added as necessary. The materials are usually mixed together and dispersed in a solvent (such as NMP), uniformly coated on a positive current collector after being uniformly stirred, and dried to obtain the positive pole piece. As the positive electrode current collector, a metal foil such as an aluminum foil or a porous metal plate can be used. Aluminum foil with a thickness of 8-30 μm is commonly used. In general, when a positive electrode sheet is manufactured, a positive electrode coating is not formed on a portion of a current collector, leaving a portion of the current collector as a positive electrode lead portion. Of course, the lead portion may be added later.

Then, the negative electrode sheet of the present application (as a negative electrode sheet) was prepared as described above.

Finally, stacking the positive pole piece, the isolating membrane and the negative pole piece in sequence to enable the isolating membrane to be positioned between the positive pole piece and the negative pole piece to play an isolating role, and then winding (or laminating) to obtain a bare cell; and placing the bare cell in an outer package, drying, injecting electrolyte, and performing vacuum packaging, standing, formation, shaping and other processes to obtain the secondary battery.

The shape of the secondary battery is not particularly limited, and may be a cylindrical shape, a square shape, or any other shape. A secondary battery 5 of a square structure is shown as an example in fig. 1.

In some embodiments, the secondary batteries may be assembled into a battery module, and the number of the secondary batteries contained in the battery module may be plural, and the specific number may be adjusted according to the application and capacity of the battery module.

Fig. 2 is a battery module 4 as an example. Referring to fig. 2, in the battery module 4, a plurality of secondary batteries 5 may be arranged in series along the longitudinal direction of the battery module 4. Of course, the arrangement may be in any other manner. The plurality of secondary batteries 5 may be further fixed by a fastener.

Alternatively, the battery module 4 may further include a case having an accommodating space in which the plurality of secondary batteries 5 are accommodated.

In some embodiments, the battery modules may be assembled into a battery pack, and the number of the battery modules contained in the battery pack may be adjusted according to the application and the capacity of the battery pack.

Fig. 3 and 4 are a battery pack 1 as an example. Referring to fig. 3 and 4, a battery pack 1 may include a battery case and a plurality of battery modules 4 disposed in the battery case. The battery box comprises an upper box body 2 and a lower box body 3, wherein the upper box body 2 can be covered on the lower box body 3 and forms a closed space for accommodating the battery module 4. A plurality of battery modules 4 may be arranged in any manner in the battery box.

A third aspect of the present application provides a device comprising the secondary battery according to the second aspect of the present application. The secondary battery may be used as a power source of the device. The device may be, but is not limited to, a mobile device (e.g., a mobile phone, a laptop computer, etc.), an electric vehicle (e.g., a pure electric vehicle, a hybrid electric vehicle, a plug-in hybrid electric vehicle, an electric bicycle, an electric scooter, an electric golf cart, an electric truck, etc.), an electric train, a ship, a satellite, an energy storage system, etc.

The device may select a secondary battery, a battery module, or a battery pack according to its use requirements.

Fig. 5 is an apparatus as an example. The device is a pure electric vehicle, a hybrid electric vehicle, or a plug-in hybrid electric vehicle, etc. In order to meet the demand of the device for high power and high energy density of the secondary battery, a battery pack or a battery module may be employed.

As another example, the device may be a cell phone, a tablet, a laptop, etc. The device is generally required to be thin and light, and a secondary battery may be used as a power source.

The following examples are provided to further illustrate the benefits of the present application.

Examples

In order to make the objects, technical solutions and advantageous technical effects of the present invention more clear, the present invention is further described in detail with reference to the following embodiments. However, it should be understood that the embodiments of the present application are only for explaining the present application and are not intended to limit the present application, and the embodiments of the present application are not limited to the embodiments given in the specification. The examples were prepared under conventional conditions or conditions recommended by the material suppliers without specifying specific experimental conditions or operating conditions.

Firstly, preparation of battery

Example 1

1) Preparation of positive pole piece

LiNi-Co-Mn ternary active material LiNi_0.5Co_0.2Mn_0.3O₂(NCM523) with conductive carbon black Super-P and binder polyvinylidene fluoride (PVDF) in a weight ratio of 94: 3 in N-methylpyrrolidoneAnd (3) after fully stirring and uniformly mixing the solvent, coating the slurry on an aluminum foil substrate, and drying, cold pressing, slitting and cutting to obtain the positive pole piece.

2) Preparation of negative pole piece

Fully stirring and uniformly mixing first negative active material natural graphite, conductive carbon black Super-P, binder Styrene Butadiene Rubber (SBR) and thickener sodium carboxymethylcellulose (CMC) in deionized water according to a weight ratio of 96: 1: 2: 1 to obtain first negative active material slurry, coating the first negative active material slurry on a negative current collector copper foil, cold pressing and drying to obtain a first negative film layer, wherein the average particle size D50 of the natural graphite is 11.2 mu m, the powder OI is 4.2, and the surface density of the film layer is 5.5mg/cm²The compacted density of the film layer is 1.65g/cm³。

Fully stirring and uniformly mixing a second negative electrode active material artificial graphite, conductive carbon black Super-P, a binder Styrene Butadiene Rubber (SBR) and a thickening agent sodium carboxymethyl cellulose (CMC) in deionized water according to a weight ratio of 96: 1: 2: 1 to obtain a second negative electrode active material slurry, coating the second negative electrode active material slurry on a first negative electrode film layer to obtain a second negative electrode film layer, wherein the average particle size D50 of the artificial graphite is 9.4 mu m, the powder OI is 3.1, and the surface density of the film layer is 5.5mg/cm²The compacted density of the film layer is 1.65g/cm³(ii) a And (3) obtaining the negative pole piece by cold pressing, drying, slitting and cutting, wherein the total thickness of the negative pole film layer (the sum of the first negative pole film layer and the second negative pole film layer) is 74 m.

In the preparation process of the negative pole piece, the particle size D50 of the negative pole active material and the powder OI value of the negative pole active material have certain influence on the OI value of the negative pole film layer, and the required negative pole film layer OI value can be obtained by controlling the particle size of the negative pole active material and the powder OI value; a magnetic field induction technology can also be introduced in the slurry coating process, the arrangement of the negative active material on the negative pole piece is artificially induced, and the OI value of the negative pole film layer is changed; in the cold pressing process, the arrangement of the negative active materials on the negative electrode plate can be changed by adjusting the compaction density, so that the OI value of the negative electrode film layer is changed.

3) Isolation film

A PE film is selected as an isolating film.

4) Preparation of the electrolyte

Ethylene Carbonate (EC), ethylmethyl carbonate (EMC), diethyl carbonate (DEC) were mixed in a volume ratio of 1: 1, and then a well-dried lithium salt LiPF₆Dissolving the electrolyte into a mixed organic solvent according to the proportion of 1mol/L to prepare the electrolyte.

5) Preparation of the Battery

And (3) stacking the positive pole piece, the isolating membrane and the negative pole piece in sequence, winding to obtain a battery core, putting the battery core into an outer package, adding the electrolyte, and carrying out processes of packaging, standing, formation, aging and the like to obtain the secondary battery.

Examples 2-13 and comparative examples 1-4 were prepared similarly to example 1, with the different product parameters detailed in Table 1.

Second, performance parameter testing method

1. Testing of parameters of negative active material and negative film layer

(1) OI value testing

The powder OI value of the negative electrode active material and the OI value of the negative electrode film layer can be obtained by using an X-ray powder diffractometer (X' pertPRO), and an X-ray diffraction pattern is obtained according to the general rule of X-ray diffraction analysis and the measurement method of lattice parameters of graphite JIS K0131-1996, JB/T4220-2011, wherein the OI value is C004/C110, wherein C004 is the peak area of an 004 characteristic diffraction peak, and C110 is the peak area of a 110 characteristic diffraction peak. Specifically, the method for testing the powder OI value of the negative active material comprises the following steps: putting a certain mass of negative electrode active material powder into an X-ray powder diffractometer, and obtaining the peak area of a 004 crystal face diffraction peak and the peak area of a 110 crystal face diffraction peak by an X-ray diffraction analysis method, thereby obtaining the powder OI value of the negative electrode active material particles. Specifically, the method for testing the OI value of the negative electrode film layer comprises the following steps: and directly placing the prepared negative pole piece in an X-ray powder diffractometer, and obtaining the peak area of the 004 crystal face diffraction peak and the peak area of the 110 crystal face diffraction peak by an X-ray diffraction analysis method so as to obtain the OI value of the negative pole film layer.

(2) Particle diameter of negative electrode active material

The particle diameter D50 of the negative electrode active material was measured by a particle size distribution laser diffraction method (specifically, GB/T19077-2016) using a laser diffraction particle size distribution measuring instrument (Mastersizer 3000), and the average particle diameter was represented by a median value D50 of the volume distribution.

(3) Areal density of the negative electrode film layer

The area density of the negative electrode film layer is m/s, m represents the weight of the film layer, s represents the area of the film layer, m can be measured by an electronic balance with an accuracy of 0.01g or more, and s can be a small disc with a size of 1540.25mm 2.

(4) Compacted density of negative film layer

The compacted density of the negative electrode film layer is m/V, m represents the weight of the film layer, V represents the volume of the film layer, m can be obtained by weighing with an electronic balance with the precision of more than 0.01g, and the product of the surface area of the film layer and the thickness of the film layer is the volume V of the film layer, wherein the thickness of the film layer can be obtained by measuring with a spiral micrometer with the precision of 0.5 mu m.

2. Battery performance testing

(1) Rate capability test

At normal temperature, the manufactured lithium ion battery carries out first charging and discharging with the current of 1C (namely the current value of which the theoretical capacity is completely discharged within 2 h), the charging is constant-current constant-voltage charging, the termination voltage is 4.2V, the cutoff current is 0.05C, the discharge termination voltage is 2.8V, and the theoretical capacity is recorded as C0; then the battery is charged to 4.2V by a constant current and a constant voltage of 1C, and the battery is charged to 0.05C by a constant voltage of 4.2V; the 3C was discharged to 2.8V, and the discharge capacity of the 3C was recorded as C1, and C1/C0 was recorded as the discharge capacity retention rate of the cell 3C.

(2) Cycle performance test

At normal temperature, the manufactured lithium ion battery cell carries out first charging and discharging by using a current of 1C (namely a current value which completely discharges theoretical capacity within 1 h), the charging is constant-current constant-voltage charging, the termination voltage is 4.2V, the cut-off current is 0.05C, the discharging termination voltage is 2.8V, and the discharging capacity Cb of the cell during first circulation is recorded. And then carrying out cycle life detection, wherein the test condition is a normal temperature condition, 1C/1C cycle is carried out, the discharge capacity Ce during cell recording is recorded at any time, the ratio of Ce to Cb is the capacity retention rate in the cycle process, the test is stopped when the capacity retention rate is lower than or equal to 80%, and the number of cycle turns is recorded.

Test results of three, each example and comparative example

The batteries of examples and comparative examples were prepared according to the above-described methods, respectively, and various performance parameters were measured, and the results are shown in the following table.

In addition, compared with a comparative example, the rate performance and the cycle performance of the battery cell in the embodiment are obviously improved; the reason is that when the OI value of the second film layer is small, the graphite is oriented perpendicular to the current collector, and lithium ions are easy to diffuse, so that the second film layer has smaller ion diffusion impedance and high lithium ion diffusion rate, and the multiplying power and the cycle performance of the battery cell are improved. In addition, when the OI value of the first film layer is controlled to be larger, the expansion of the graphite in the width direction in the repeated charging and discharging process is smaller, and higher adhesive force is still kept between the graphite and a current collector in the long cycle process, so that the occurrence of the negative pole piece demoulding phenomenon is avoided (the long cycle performance of the battery cell can be improved, and the service life is prolonged).

In addition, for a thick coating weight of the cell, the rate and cycle performance will generally deteriorate due to the increased diffusion path; but the power cycle performance of the thick coating weight battery cell can be improved to a great extent by adopting the negative pole piece in the application.

From the above test results it can be seen that: the conventional single-layer film layer of the negative pole piece is changed into two or more film layers with specific IO value distribution, so that the cycle performance of the battery can be greatly improved, and the rate performance of the battery cannot be deteriorated or even improved.

It should be further noted that, based on the disclosure and guidance of the above description, those skilled in the art to which the present application pertains may make appropriate changes and modifications to the above-described embodiments. Therefore, the present application is not limited to the specific embodiments disclosed and described above, and some modifications and variations of the present application are also within the scope of the claims of the present application. In addition, although specific terms are used herein, they are used in a descriptive sense only and not for purposes of limitation.

Claims

1. A negative pole piece is characterized in that: the negative electrode current collector comprises a negative electrode current collector and a negative electrode film layer arranged on at least one surface of the negative electrode current collector, wherein the negative electrode film layer comprises a first negative electrode film layer and a second negative electrode film layer, the first negative electrode film layer comprises a first negative electrode active material and is arranged on at least one surface of the negative electrode current collector, the second negative electrode film layer comprises a second negative electrode active material and is arranged on the first negative electrode film layer, the first negative electrode active material comprises natural graphite, the OI value of the first negative electrode film layer is 10-35, and the powder OI value of the first negative electrode active material is 4-10; the second negative active material comprises artificial graphite, and the OI value of the second negative film layer is 2-15, wherein the OI value of the negative film layer is the area ratio of a 004 characteristic diffraction peak to a 110 characteristic diffraction peak of the negative film layer in an X-ray diffraction spectrogram, and the powder OI value is the ratio of the peak area of the 004 crystal plane diffraction peak to the peak area of the 110 crystal plane diffraction peak of the material obtained by an X-ray diffraction analysis method.

2. The negative pole piece of claim 1, wherein the first negative pole film layer has an OI value of 12 to 30;

and/or the OI value of the second negative electrode film layer is 4-11.

3. The negative electrode tab of claim 1, wherein the natural graphite is spherical or spheroidal;

and/or the artificial graphite is in a sheet shape or a block shape.

4. The negative electrode sheet according to claim 1, wherein the first negative electrode active material has an OI value of 4.5 to 8;

and/or the OI value of the second negative electrode active material is 1-5.

5. The negative electrode tab of claim 1, wherein the second negative electrode active material has an OI value of 2 to 4.

6. The negative electrode sheet according to claim 1, wherein the average particle diameter D50 of the first negative electrode active material is 9 to 22 μm;

and/or the average particle size D50 of the second negative electrode active material is 5-12 μm.

7. The negative electrode sheet according to claim 1, wherein the average particle diameter D50 of the first negative electrode active material is 10 to 15 μm;

and/or the average particle size D50 of the second negative electrode active material is 6-10 μm.

8. The negative electrode plate as claimed in claim 1, wherein the thickness of the negative electrode film layer is greater than or equal to 60 μm.

9. The negative pole piece of claim 8, wherein the thickness of the negative pole film layer is 65 μm to 80 μm.

10. The negative electrode plate as claimed in claim 1, wherein the ratio of the thickness of the first negative electrode film layer to the thickness of the second negative electrode film layer is 0.55-0.95.

11. The negative electrode plate as claimed in claim 10, wherein the ratio of the thickness of the first negative electrode film layer to the thickness of the second negative electrode film layer is 0.65-0.85.

12. The negative electrode tab of claim 1, wherein the first negative electrode film layer has a compacted density that is greater than the compacted density of the second negative electrode film layer.

13. The negative electrode sheet of claim 12, wherein the first negative electrode film layer has a compacted density of 1.65g/cm³~1.7g/cm³The compacted density of the second negative electrode film layer is 1.5g/cm³~1.65g/cm³。

14. The negative electrode plate of claim 1, wherein the first negative active material and/or the second negative active material further comprises one or more of soft carbon, hard carbon, and silicon-based material.

15. A secondary battery comprising the negative electrode tab of any one of claims 1 to 14.

16. An apparatus characterized by comprising the secondary battery according to claim 15.