WO2021134800A1

WO2021134800A1 - Electrode assembly, electrochemical device, and electronic device

Info

Publication number: WO2021134800A1
Application number: PCT/CN2020/070348
Authority: WO
Inventors: 章婷; 姜道义; 陈志焕; 崔航
Original assignee: 宁德新能源科技有限公司
Priority date: 2020-01-03
Filing date: 2020-01-03
Publication date: 2021-07-08

Abstract

An electrode assembly, an electrochemical device, and an electronic device. The electrode assembly comprises: a positive electrode piece (10); a negative electrode piece (12); an isolation membrane (11) provided between the positive electrode piece (10) and the negative electrode piece (12); the capacity per unit area of the negative electrode piece (12) is a mAh/cm 2, 0.97 < a < 8.84, the half-charge shrinkage rate of the isolation membrane (11) is s t, s t = (L0-L1)/L0, L0 is the initial length of the isolation membrane (11) of the electrode assembly, L1 is the length of the isolation membrane (11) when the electrode assembly is charged to 3.7V-4.0V, 0.5% < s t < 5%, and a and s t satisfy: 19.4 < a/s t < 1768. The capacity the per unit area a of the negative electrode piece (12) of the electrode assembly and the half-charge shrinkage rate s t of the isolation membrane (11) are controlled to make a and s t satisfy 19.4 < a/s t < 1768, thereby being able to reduce the deformation of the electrode assembly, and improving the cycle stability of the electrochemical device comprising the electrode assembly.

Description

Electrode assembly, electrochemical device and electronic device

Technical field

The present disclosure relates to the field of electronic technology, and in particular to an electrode assembly, an electrochemical device, and an electronic device.

Background technique

Currently, electrode assemblies are widely used in electrochemical devices such as lithium ion batteries. Due to the volume expansion of the positive and negative pole pieces during the charge-discharge cycle, the electrode assembly has the problems of deformation and wrinkles after the formation and capacity test. Especially, for example, the electrode assembly using silicon-based anode material, because the silicon-based anode material has a volume expansion of about 300% during the lithium insertion process, the electrode assembly using the silicon-based anode material is more prone to deformation after formation and capacity testing. The deformation of the electrode assembly will cause the cycle stability of the electrode assembly to decrease.

Summary of the invention

In view of the above-mentioned shortcomings of the prior art, the purpose of the present disclosure is to provide an electrode assembly, an electrochemical device, and an electronic device. The electrode assembly of the present disclosure has a capacity per unit area of the negative pole piece and a separation membrane within a specific range. Half-charge shrinkage rate to relieve the deformation of the electrode assembly.

The present disclosure provides an electrode assembly, including: a positive pole piece; a negative pole piece; a separator, arranged between the positive pole piece and the negative pole piece; wherein the capacity per unit area of the negative pole piece is a mAh/cm ² , 0.97<a<8.84, the half-charge shrinkage rate of the isolation membrane is s _t , s _t = (L0-L1)/L0, and L0 is the initial length of the isolation membrane of the electrode assembly, L1 is the length of the isolation membrane when the electrode assembly is charged to 3.7-4.0V, 0.5%<s _t <5%, and a and _st satisfy: 19.4<a/s _t <1768.

In the above electrode assembly, wherein the negative pole piece has a negative active material layer, and the negative active material layer includes a silicon-based material.

In the above-mentioned electrode assembly, wherein the positive pole piece has a positive active material layer, and the positive active material layer includes one of lithium cobaltate, lithium manganate, lithium iron phosphate, lithium nickelate, and lithium nickel cobaltate Or multiple.

In the above-mentioned electrode assembly, wherein, the silicon-based material includes one or more of nano-silicon-based particles, silicon oxide, silicon-carbon composite particles, and silicon alloys; the silicon-based material contains lithium and magnesium The average particle size of the silicon-based material is 500 nm-30 μm; the surface of the silicon-based material has one or two of carbon materials and polymer materials.

In the above-mentioned electrode assembly, wherein the average particle diameter of the nano-silicon-based particles is less than 100 nm.

In the above electrode assembly, wherein the carbon material includes one or more of amorphous carbon, carbon nanotubes, carbon nanoparticles, carbon fibers, and graphene; and/or the polymer material includes polyvinylidene fluoride , Polyvinylidene fluoride derivatives, carboxymethyl cellulose, carboxymethyl cellulose derivatives, sodium carboxymethyl cellulose, sodium carboxymethyl cellulose derivatives, polyvinylpyrrolidone, polyvinylpyrrolidone derivatives, One or more of polyacrylic acid, polyacrylic acid derivatives and polystyrene butadiene rubber.

In the above-mentioned electrode assembly, wherein the separation film includes one or more of polyethylene, polypropylene, and polyvinylidene fluoride.

In the above-mentioned electrode assembly, wherein the electrode assembly is a wound electrode assembly.

The present disclosure also provides an electrochemical device, including the above-mentioned electrode assembly.

The present disclosure also provides an electronic device including the above-mentioned electrode assembly or the above-mentioned electrochemical device.

The present disclosure controls the capacity a per unit area of the negative pole piece of the electrode assembly and the half-charge shrinkage rate s _{t of the} separator so that a and _st meet 19.4<a/s _t <1768, which can reduce the deformation of the electrode assembly and improve the The cycle stability of the electrochemical device of the electrode assembly.

Description of the drawings

FIG. 1 is a schematic diagram of the electrode assembly of the present disclosure.

2 is a schematic diagram of the electrode assembly after 100 cycles of the lithium ion battery in Comparative Example 1 of the present disclosure.

3 is a schematic diagram of the electrode assembly after 100 cycles of the lithium ion battery in Example 2 of the present disclosure.

4 is a schematic diagram of the electrode assembly after 100 cycles of the lithium ion battery in Comparative Example 4 of the present disclosure.

FIG. 5 is a diagram of the wrinkle state of the separator after the lithium ion battery is cycled for 100 cycles in Comparative Example 3 of the present disclosure.

Detailed ways

The following examples may enable those skilled in the art to understand the application more comprehensively, but do not limit the application in any way.

Some embodiments of the present disclosure provide an electrode assembly, which includes a positive pole piece 10, a negative pole piece 12, and an isolation film 11 disposed between the positive pole piece 10 and the negative pole piece 12. The positive pole piece 10 may include a positive current collector and a positive active material layer coated on the positive current collector. In some embodiments, the positive pole piece 10 has a positive active material layer, and the positive active material layer may only be coated on a partial area of the positive current collector. The positive active material layer may include a positive active material, a conductive agent, and a binder. Al foil can be used as the positive electrode current collector, and similarly, other positive electrode current collectors commonly used in this field can also be used. The conductive agent may include one or a combination of conductive carbon black, flake graphite, graphene, and carbon nanotubes. The binder may include polyvinylidene fluoride, vinylidene fluoride-hexafluoropropylene copolymer, styrene-acrylate copolymer, styrene-butadiene copolymer, polyamide, polyacrylonitrile, polyacrylate, poly Acrylic acid, polyacrylate, sodium carboxymethyl cellulose, polyvinyl acetate, polyvinylpyrrolidone, polyvinyl ether, polymethyl methacrylate, polytetrafluoroethylene and polyhexafluoropropylene or one of Several combinations. The positive electrode active material includes, but is not limited to, one or more of lithium cobaltate, lithium nickelate, lithium manganate, lithium nickel manganate, lithium nickel cobaltate, lithium iron phosphate, lithium nickel cobalt aluminate, and lithium nickel cobalt manganate. The above positive active material may include a positive active material that has been doped or coated.

In some embodiments, the isolation film 11 includes one or a combination of polyethylene, polypropylene, polyvinylidene fluoride, polyethylene terephthalate, polyimide, and aramid. For example, polyethylene includes one or a combination of several selected from high-density polyethylene, low-density polyethylene, and ultra-high molecular weight polyethylene. Especially polyethylene and polypropylene, they have a good effect on preventing short circuits, and can improve the stability of the battery through the shutdown effect.

In some embodiments, the surface of the isolation membrane may further include a porous layer, the porous layer is disposed on at least one surface of the isolation membrane, the porous layer includes inorganic particles and a binder, and the inorganic particles are selected from alumina (Al ₂ O ₃ ), Silicon oxide (SiO ₂ ), magnesium oxide (MgO), titanium oxide (TiO ₂ ), hafnium dioxide (HfO ₂ ), tin oxide (SnO ₂ ), ceria (CeO ₂ ), nickel oxide (NiO), oxide Zinc (ZnO), calcium oxide (CaO), zirconium oxide (ZrO ₂ ), yttrium oxide (Y ₂ O ₃ ), silicon carbide (SiC), boehmite, aluminum hydroxide, magnesium hydroxide, calcium hydroxide and sulfuric acid One or a combination of barium. The binder is selected from polyvinylidene fluoride, vinylidene fluoride-hexafluoropropylene copolymer, polyamide, polyacrylonitrile, polyacrylate, polyacrylic acid, polyacrylate, sodium carboxymethyl cellulose, polyethylene pyrrole One or a combination of alkanone, polyvinyl ether, polymethyl methacrylate, polytetrafluoroethylene and polyhexafluoropropylene. The porous layer on the surface of the isolation membrane can improve the heat resistance, oxidation resistance and electrolyte infiltration performance of the isolation membrane, and enhance the adhesion between the isolation membrane and the pole piece.

In some embodiments, the negative pole piece 12 may include a negative current collector, and the negative current collector may be one or a combination of copper foil, aluminum foil, nickel foil, and carbon-based current collector. In some embodiments, the negative pole piece 12 has a negative active material layer. In some embodiments, the negative active material layer may be disposed on the negative current collector. The negative pole piece has a first side and a second side. In the embodiments of the present disclosure, a negative active material layer may be provided on either or both of the first side and the second side. Including negative active material. Among them, the capacity per unit area of the negative pole piece is a mAh/cm ² , and 0.97<a<8.84. In the present disclosure, the capacity per unit area of the negative electrode piece refers to the capacity per unit area of the area on the negative electrode piece with the negative electrode active material layer when one surface of the current collector of the negative electrode piece is provided with the negative electrode active material layer. For example, taking the current collector of copper foil as an example, the negative electrode active material is arranged on the coating area on either side of the copper foil to form a negative electrode piece with a negative electrode active material layer on one side. The unit of the negative electrode piece on the coating area is The area capacity is the unit area capacity of the negative pole piece. The capacity per unit area of the negative pole piece depends on the selected negative active material and the amount of negative active material coated on the unit area. When there are negative active material layers on both sides of the negative pole piece, the capacity per unit area of the negative pole piece can be measured by removing the negative active material layer on one side.

In some embodiments of the present disclosure, the half-charge shrinkage rate of the isolation film is s _t , s _t =(L0-L1)/L0, L0 is the initial length of the isolation film of the electrode assembly, and the initial length refers to the cutting of the isolation film The length before the winding is not carried out. Taking the electrode assembly as a wound electrode assembly as an example, the initial length refers to the length of the wound electrode assembly before being wound, and L1 is the electrode assembly being charged to 3.7-4.0V (for example, 3.85V) When the length of the isolation membrane, the charging process can be constant current charging or constant voltage charging. For example, for the wound electrode assembly, the half-charge shrinkage rate is mainly caused by the different winding tensions applied during the winding process. Therefore, the half-charge shrinkage rate can be used to characterize the winding tension. The shrinkage rate is also greater. Similarly, the half-fill shrinkage rate can be adjusted by controlling the winding tension. It should be understood that although the present disclosure takes the wound electrode assembly as an example, the disclosure is not limited to this, and may also include stacked electrode assemblies. At this time, the half-charge shrinkage rate of the separator is affected by the expansion of the positive and negative pole pieces. Impact. In the embodiment of the present disclosure, 0.5%<s _t <5%, and a and _st satisfy: 19.4<a/st<1768. In the present embodiment of the present disclosure, the experimental verification by the deformation condition when said electrode assembly and a s _t meet, can significantly improve the deformation of the electrode assembly and to improve the cycle stability, in particular, can be improved so that the wound electrode assembly Improve cycle stability, thereby increasing the yield of products.

In some embodiments of the present disclosure, the positive pole piece of the electrode assembly has a positive active material layer, and the positive active material layer includes one of lithium cobaltate, lithium manganate, lithium iron phosphate, lithium nickelate, and lithium nickel cobaltate Or multiple. In some embodiments, aluminum foil may be used for the positive pole piece. The positive electrode active material of the positive electrode active material layer in the embodiment of the present disclosure includes, for example, LiNi _0.55 Co _0.05 M _0.4 O ₂ , LiNi _0.55 Co _0.1 M _0.35 O ₂ , LiNi _0.55 Co _0.12 M _0.33 O _2, etc. It may be the above-mentioned positive electrode active material doped with a transition metal.

In some embodiments of the present disclosure, the negative active material layer includes a silicon-based material. In some embodiments, the average particle size of the silicon-based material is 500 nm-30 μm. In some embodiments, the average particle size of the silicon-based material may be one of 600nm-20μm, 800nm-10μm, and 900nm-5μm. This ensures that the content of fine particles in the negative electrode active material is low, and can reduce the side reaction of the electrolyte on the surface of the positive electrode active material, thereby inhibiting gas production and reducing heat generation, thereby improving the deformation of the electrode assembly and improving cycle stability. In addition, if the average particle size of the silicon-based material is too large, the strength of the negative pole piece will decrease, and if the average particle size of the silicon-based material is too small, it will hinder the movement of lithium ions.

In some embodiments of the present disclosure, the silicon-based material includes one or more of nano-silicon-based particles, silicon oxide, silicon-carbon composite particles, and silicon alloys. In some embodiments, the average particle size of the nano-silicon-based particles is less than 100 nm. The existence of nano-silicon-based particles can fill the tiny gaps in the negative electrode active material layer, which is beneficial to increase the compaction density and thus the specific energy, and can improve the transport performance of lithium ions and electrons. If the average particle size of the nano-silicon-based particles is too large, the nano-silicon-based particles will lose this advantage.

In some embodiments, the silicon-based material contains one or both of lithium and magnesium. By embedding lithium and magnesium in the silicon-based material, the negative active material layer can be pre-strained, thereby reducing the amount of deformation of the negative active material layer during charging and discharging, and preventing the negative active material layer from being excessively deformed. Cracks or peeling from the negative current collector.

In some embodiments of the present disclosure, the surface of the silicon-based material may not have any other materials, or other materials may be present. By providing some other materials, the structure of the silicon-based material can be stabilized, the oxidation activity of the electrolyte on the surface of the silicon-based material can be reduced, the side reaction of the electrolyte on the surface of the silicon-based material can be reduced, the gas production can be suppressed, and the heat production can be reduced. In some embodiments, one or both of carbon materials and polymer materials are present on the surface of the silicon-based material. The specific capacity of silicon-based materials is much higher than that of graphite, but silicon-based materials have serious volume expansion during charging and discharging. Carbon anode materials have small volume changes during charging and discharging, and have good cycle stability. The crystal structure and chemical properties of carbon and silicon are similar, and the two can be closely combined. Therefore, when carbon materials are used, silicon-based materials provide higher specific capacity, and carbon materials can buffer the volume changes of silicon-based materials during charge and discharge. Improve the conductivity of silicon-based materials to avoid agglomeration of silicon-based materials during the charge-discharge cycle. In some embodiments of the present disclosure, the carbon material includes one or more of amorphous carbon, carbon nanotubes, carbon nanoparticles, carbon fibers, and graphene.

In some embodiments of the present disclosure, the polymer material includes polyvinylidene fluoride, polyvinylidene fluoride derivatives, carboxymethyl cellulose, carboxymethyl cellulose derivatives, sodium carboxymethyl cellulose, carboxymethyl cellulose One or more of sodium derivatives, polyvinylpyrrolidone, polyvinylpyrrolidone derivatives, polyacrylic acid, polyacrylic acid derivatives, and polystyrene butadiene rubber.

In some embodiments of the present disclosure, the electrode assembly is a wound electrode assembly.

The embodiments of the present disclosure also provide an electrochemical device including the above-mentioned electrode assembly. In some embodiments, the electrochemical device includes a lithium ion battery, but the present disclosure is not limited thereto. For example, the electrochemical device may include the above-mentioned electrode assembly and electrolyte. In some embodiments, the electrolyte includes dimethyl carbonate (DMC), ethyl methyl carbonate (EMC), diethyl carbonate (DEC), ethylene carbonate (EC), propylene carbonate (PC), propylene propionate At least two of esters (PP). In addition, the electrolyte may additionally include at least one of vinylene carbonate (VC), fluoroethylene carbonate (FEC), and dinitrile compounds as electrolyte additives.

In some embodiments of the present disclosure, taking a lithium ion battery as an example, the positive pole piece, the separator film, and the negative pole piece are sequentially wound or stacked to form an electrode piece, and then packed in, for example, an aluminum plastic film for packaging, and injection electrolysis Lithium-ion battery is made by liquid, formed and packaged. Then, perform performance test and cycle test on the prepared lithium-ion battery.

Those skilled in the art will understand that the method for preparing an electrochemical device (for example, a lithium ion battery) described above is only an example. Without departing from the content disclosed in this application, other methods commonly used in the art can be used.

The embodiments of the present disclosure also provide an electronic device including the above-mentioned electrode assembly or an electronic device including the above-mentioned electrochemical device. In some embodiments, the electronic device may include any electronic device that uses a rechargeable battery, such as a mobile phone, a tablet computer, and a charging device.

Some specific examples and comparative examples are listed below to better illustrate the present disclosure.

In the following Examples 1-12 and Comparative Examples 1-8 of the present disclosure, a lithium ion battery with a wound electrode assembly is used as an example, and the raw materials, the ratio of raw materials, and the preparation method used to prepare the lithium ion battery are only used as Examples, the present disclosure is not limited to the raw materials, raw material ratios, and production methods used in the following examples.

Preparation of positive pole piece: After taking positive active material LiCoO ₂ , conductive carbon black, and binder polyvinylidene fluoride (PVDF) at a weight ratio of 96.7:1.7:1.6 in an N-methylpyrrolidone solvent system, stir and mix well. Coating on the aluminum foil, and then drying and cold pressing to obtain a positive pole piece. In Examples 1-12 and Comparative Examples 1-8, the prepared positive pole pieces are the same.

Negative pole piece preparation: mix graphite and silicon-based materials in a certain ratio to design a mixed powder with a capacity of 500mAh/g as the negative active material, and mix the mixed powder, conductive agent acetylene black, and polyacrylic acid in a weight ratio of 95:1.2:3.8 After fully stirring and mixing uniformly in a deionized water solvent system, coating on the copper foil to form a negative active material layer, and drying and cold pressing the coated copper foil to obtain a negative pole piece. In Examples 1-12 and Comparative Examples 1-8, by controlling the amount of the negative electrode active material coated on the copper foil, negative electrode pieces with different unit area capacities were obtained.

Battery preparation: Polyethylene porous polymer film is used as the separator, and the positive pole piece, separator film, and negative pole piece are stacked in sequence, so that the separator is in the middle of the positive and negative pole pieces for isolation, and rolled Wind the electrode assembly. The electrode assembly is placed in the outer packaging aluminum plastic film, and the electrolyte containing ethylene carbonate (EC) and propylene carbonate (PC) is injected and packaged. After forming, degassing, trimming and other processes, a lithium ion battery is obtained. In Examples 1-12 and Comparative Examples 1-8, by controlling the tension during the winding process, a lithium ion battery _{with a corresponding isolation film st was obtained.}

Capacity test per unit area of negative pole piece

The negative active material, conductive carbon black, and binder polyacrylic acid used in the examples were mixed with deionized water in a mass ratio of 95:1.2:3.8 to form a slurry, and then coated with a 100um thick coating. After being dried in a vacuum drying oven for an hour, use a punch in a dry environment to cut into a disc with a diameter of 1.4cm (area 1.5386cm ² ), use a metal lithium sheet as the counter electrode in the glove box, and select a plug for the isolation membrane. Germany (ceglard) composite membrane, adding electrolyte to assemble a button cell. Use the LAND series battery tester to charge and discharge the battery, test its charge and discharge performance, and obtain the gram capacity C1 (mAh/g) of the ^{prepared 1.5386cm 2 wafer, then the examples 1-12 and the comparison} The capacity per unit area of the negative pole piece in ratios 1-8 a=C1(mAh/g)*coating weight(g)/1.5386(cm ² ), where the coating weight refers to Examples 1-12 and Comparative Example 1. ^{The weight of the negative active material coated on 1.5386 cm 2} of the negative pole piece in -8 is different in different embodiments, so the capacity per unit area of the negative pole piece in different embodiments is different.

Deformation test of electrode assembly

Define PPG as the thickness of the plate surface of the electrode assembly. Use an electrode assembly plate thickness gauge to measure the thickness. Place the electrode assembly under a pressure of 700g, measure the surface thickness of the electrode assembly, measure three times in sequence, and take the average of the three data; Define MMC as the point thickness of the electrode assembly. By taking three points spaced along the vertical direction of the positive electrode tab, measure the thickness of these three points and take the average value; define Ripple=PPG/MMC-1, and take the battery cycle The thickness data of the electrode assembly after 100 cycles of the performance test is the judgment point. When Ripple>2% is deformed, Ripple>4% is severely deformed.

Battery cycle performance test

The test temperature is 45°C, and it is charged to 4.4V at a constant current of 0.7C, then charged to 0.025C at a constant voltage, and discharged to 3.0V at 0.5C after standing for 5 minutes. The capacity obtained in this step is the initial capacity, and the 0.7C charge/0.5C discharge is carried out for a cycle test. The capacity at each step is used as the ratio of the initial capacity to obtain the capacity attenuation curve, and the number of cycles at which the capacity attenuates to 80% of the initial capacity is counted. .

Data statistics are performed on Examples 1-12 and Comparative Examples 1-8, and the statistical results are shown in Table 1.

Table 1

Comparing the results of Example 1, Example 2 and Comparative Example 1 in conjunction with Figure 2 and Figure 3, it can be seen that when the capacity per unit area of the negative pole piece is the same, by controlling the different half-charge shrinkage rates of the separator, the cycle The degree of deformation of the rear electrode assembly is different. It can be seen from Comparative Example 1 that when the half-charge shrinkage rate of the separator reaches 5.6% (greater than 5%), the electrode assembly deforms greatly after the cycle, and the Ripple reaches 3.5%. The cycle stability of Comparative Example 1 deteriorated significantly, and the number of cycles at which the capacity decayed to 80% was significantly less than that of Example 1 and Example 2.

Comparing the results of Example 3, Example 4 and Comparative Example 2 in combination with Figures 4 and 5, or comparing the results of Example 5, Example 6 and Comparative Example 3, the same conclusion can be obtained. When the capacity per unit area of the sheet is the same, after the half-charge shrinkage rate of the separator exceeds 5%, the electrode assembly will undergo greater deformation, and the number of cycles at which the capacity decays to 80% will be significantly reduced, and the cycle stability will be poor. This can also be seen from the comparison results between Example 7 and Comparative Example 4 and the comparison results between Example 8 and Comparative Example 5. The half-charge shrinkage rate of the separators in Comparative Example 4 and Comparative Example 5 is greater than 5%. The electrode assembly is severely deformed, so the _{st of the} electrode assembly is defined to be less than 5% in the embodiment of the present disclosure.

From the results of Example 10 and Comparative Example 6, it can be seen that when the half-charge shrinkage rate of the separator is less than 0.5%, the electrode assembly is also severely deformed. When the half-charge shrinkage rate of the separator is less than 0.5%, the electrode assembly deforms seriously. The reason may be that the tension of the isolation film during the winding process is too small, causing the isolation film itself to appear wrinkles, causing deformation after winding, which affects the subsequent cycle test. It can be seen that, in the embodiments of the present disclosure, limiting the electrode assembly to satisfy 0.5%< _st <5% can reduce deformation and improve cycle test performance.

From the statistical results of Examples 7-12, it can be found that with the increase of the capacity per unit area of the negative pole piece, the shrinkage rate of the separator is reduced in the interval of 0.5%<st<5%, and the deformation of the electrode assembly after cycling is There is no big difference, but the number of cycles at which the capacity decays to 80% gradually decreases, that is, the cycle stability tends to gradually decrease. At the same time, comparing Example 12 with Comparative Example 7 and Comparative Example 8, it can be found that when the half-charge shrinkage rate of the control separator is the same, as the capacity per unit area of the negative electrode piece increases, the deformation of the electrode assembly also deteriorates significantly. At the same time, the cycle stability is also significantly reduced. This is mainly caused by the different silicon content on the negative pole piece. The increase in the unit area capacity a of the negative pole piece means an increase in the negative electrode active material per unit area, that is, the increase in silicon content. Different silicon content will cause the pole piece In addition to the different degree of volume expansion, the structural instability of the silicon-based material itself during the cycle makes the cycle stability worse. Therefore, in the embodiment of the present disclosure, the capacity a per unit area of the negative pole piece of the electrode assembly is less than 8.84 mAh/cm ² .

Therefore, the application of different winding tensions of the separator film will cause the difference in the degree of deformation of the electrode assembly. At the same time, as the capacity per unit area of the negative pole piece increases, reducing the corresponding shrinkage of the separator film can further suppress the deformation of the electrode assembly. When the capacity per unit area of the negative pole piece is in the ^{interval of 0.97<a<8.84mAh/cm 2} , the half-charge shrinkage rate of the separator is adjusted to be in the interval of 0.5%<st<5%, so that the capacity per unit area of the negative pole piece is in the interval a and The quotient of the winding tension of the separator is in the range of 19.4<a/s _t <1768, and the deformation degree of the electrode assembly is significantly reduced. The application of reducing the winding tension of the isolation film can further reduce the internal stress of the isolation film (internal stress of the isolation film: when the isolation film is subjected to an external force, the internal parts interact to resist this deformation force is called stress. When the external force is removed, the remaining force inside the isolation film is called the internal stress of the isolation film; the internal stress of the isolation film = the tension of the isolation film substrate itself + the coating tension + the slitting tension + the winding tension-the tension released during the preparation process ), so as to suppress the deformation of the electrode assembly after cycling, improve the cycle stability, and increase the yield of the product.

In summary, it can be seen from the above examples and comparative examples that when the half-charge shrinkage rate s _t satisfies 0.5%<s _t <5%, and _{the quotient of a and st} satisfies 19.4<a/s _t <1768 , The degree of deformation of the electrode assembly is significantly reduced. It can be seen from Comparative Example 1 that when the half-charge shrinkage rate is greater than 5%, although a/s _{t is} within the above-mentioned limited range, the electrode assembly still undergoes severe deformation, and the cycle stability is also significantly deteriorated. This is because the winding tension applied in the process of preparing the electrode assembly exceeds the range that the separator can withstand without irreversible shrinkage and deformation. On the other hand, the large winding tension (the main factor for the shrinkage of the separator) The application, coupled with the force generated by the volume expansion of the silicon-based negative electrode material during the cycle, increases the internal stress of the isolation membrane, which together lead to severe deformation of the electrode assembly. Among the influencing factors of the deformation of the electrode assembly, the internal stress of the separator is a major factor, which is mainly composed of the tension of the separator substrate itself, coating tension, slitting tension, winding tension, and tension released during the preparation process. The winding tension of the separator is the last production process that produces the internal stress of the separator. Therefore, the control of the winding tension during the preparation process (that is, the control of the half-charge shrinkage rate of the separator) affects the internal stress of the separator in the electrode assembly. The stress has a great influence, thereby affecting the degree of deformation of the electrode assembly. At the same time, the severe deformation of the electrode assembly resulted in the formation of voids between the pole piece and the isolation membrane, and the formation of micro-cracks on the pole piece. The solid electrolyte interphase (SEI) membrane ruptured and reorganized, which consumed the electrolyte and changed the cycle performance of the electrode assembly. difference.

The above description is only a preferred embodiment of the present disclosure and an explanation of the applied technical principles. Those skilled in the art should understand that the scope of disclosure involved in this disclosure is not limited to the technical solutions formed by the specific combination of the above technical features, and should also cover the above technical features or technical solutions without departing from the above disclosed concept. Other technical solutions formed by arbitrarily combining the equivalent features. For example, the above-mentioned features and the technical features with similar functions disclosed in the present disclosure are replaced with each other to form a technical solution.

Claims

An electrode assembly, characterized in that it comprises:

Positive pole piece

Negative pole piece

Separating membrane, arranged between the positive pole piece and the negative pole piece;

Wherein, the capacity per unit area of the negative pole piece is a mAh/cm 2 , 0.97<a<8.84, and the half-charge shrinkage rate of the separator is st , st =(L0-L1)/L0, L0 is The initial length of the isolation membrane of the electrode assembly, L1 is the length of the isolation membrane when the electrode assembly is charged to 3.7-4.0V, 0.5%<s t <5%, and a and st satisfy: 19.4<a/s t <1768.
The electrode assembly according to claim 1, wherein the negative pole piece has a negative active material layer, and the negative active material layer comprises a silicon-based material.
The electrode assembly according to claim 1, wherein the positive pole piece has a positive active material layer, and the positive active material layer includes lithium cobaltate, lithium manganate, lithium iron phosphate, lithium nickelate, and nickel cobalt. One or more of lithium acid.
The electrode assembly according to claim 2, wherein the silicon-based material comprises one or more of nano-silicon-based particles, silicon oxide, silicon-carbon composite particles, and silicon alloys;

The silicon-based material contains at least one of lithium and magnesium;

The average particle size of the silicon-based material is 500nm-30μm;

One or two of carbon materials and polymer materials are present on the surface of the silicon-based material.
4. The electrode assembly of claim 4, wherein the average particle size of the nano-silicon-based particles is less than 100 nm.
The electrode assembly according to claim 4, wherein the carbon material comprises one or more of amorphous carbon, carbon nanotubes, carbon nanoparticles, carbon fibers, and graphene;

The polymer materials include polyvinylidene fluoride, polyvinylidene fluoride derivatives, carboxymethyl cellulose, carboxymethyl cellulose derivatives, sodium carboxymethyl cellulose, sodium carboxymethyl cellulose derivatives, polyethylene One or more of pyrrolidone, polyvinylpyrrolidone derivatives, polyacrylic acid, polyacrylic acid derivatives, and polystyrene butadiene rubber.
The electrode assembly according to claim 1, wherein the isolation film comprises one or more of polyethylene, polypropylene, and polyvinylidene fluoride.
The electrode assembly according to claim 1, wherein the electrode assembly is a wound electrode assembly.
An electrochemical device, characterized by comprising the electrode assembly according to any one of claims 1 to 8.
An electronic device characterized by comprising the electrode assembly according to any one of claims 1 to 8 or the electrochemical device according to claim 9.