Battery cell, method for producing the same, and pouch lithium ion battery and use thereof

Abstract

The application relates to the field of low-temperature mobile power supplies, and discloses a battery cell and a preparation method thereof, and a soft package lithium ion battery and application thereof. The battery cell comprises a positive electrode sheet, a negative electrode sheet, a diaphragm and an electrolyte; wherein the positive electrode sheet comprises modified lithium nickel cobalt manganese oxide, the modified lithium nickel cobalt manganese oxide comprises core-shell type lithium nickel cobalt manganese oxide and a boron-containing compound coated on the surface of the core-shell type lithium nickel cobalt manganese oxide; wherein the content of nickel elements in the core-shell type lithium nickel cobalt manganese oxide gradually decreases from the inside to the outside, the content of cobalt elements is uniformly distributed from the inside to the outside, and the content of manganese elements gradually increases from the inside to the outside; the negative electrode sheet comprises a nano silicon-based negative electrode material, and the nano silicon-based negative electrode material is a mixture of silicon-oxygen material and graphite. The soft package lithium ion battery prepared from the battery cell has good low-temperature performance and safety performance.

Description

TECHNICAL FIELD

[0001] The present application relates to the field of low-temperature mobile power supply, in particular to an electric core, a preparation method thereof, a soft-pack lithium ion battery and application thereof. BACKGROUND

[0002] In recent years, the global economy has maintained a high-speed development trend, and the direct consequence of the rapid development of manufacturing industry and the rapid increase of population is that the global environment is damaged and the energy crisis is more obvious. As an important part of the new energy field, the battery industry will inevitably become a hot spot of global economic development.

[0003] Lithium ion batteries are widely used due to their high working voltage, high specific energy and large capacity, but along with this, the working temperature range is one of the important performance indicators of lithium ion batteries. The working temperature range of-20℃ to 60℃ of conventional consumer batteries cannot fully meet people's needs. For example, emergency rescue, medical electronics, robots, space equipment and the like need to adapt to the large temperature difference caused by region and season, and even need to have good low-temperature discharge performance after long-time low-temperature storage.

[0004] At present, the conductivity of the lithium ion battery will be greatly reduced when the temperature is too low, the impedance of the SEI film will increase sharply, and the impedance of lithium ions in the electrode will also increase; when the temperature is relatively high, the oxidation activity of the positive electrode material is high, the side reaction between the electrode and the electrolyte is intensified, the transition metal is continuously dissolved to the surface of the electrode and deposited on the surface, which increases the internal resistance, and also thickens the thickness of the battery, thereby further affecting its low-temperature performance. In addition, too large impedance will also cause lithium dendrites to be generated on the surface of the negative electrode, which will cause safety problems. The size of the battery is also an actual application index that cannot be ignored for the adaptability of the equipment. In terms of battery pack assembly, the soft-pack battery has stronger flexibility in design of the outer dimensions than the cylindrical battery. Therefore, it is urgent to develop a low-temperature soft-pack lithium ion battery with good cycle performance, good low-temperature discharge performance after long-time low-temperature storage, and high safety. SUMMARY

[0005] The purpose of the present application is to overcome the problems of poor low-temperature discharge performance after long-time low-temperature storage and low safety in the prior art, and to provide an electric core, a preparation method thereof, and a soft-pack lithium ion battery and application thereof.

[0006] In order to achieve the above-mentioned purpose, the first aspect of the present application provides an electric core, which comprises a positive electrode sheet, a negative electrode sheet, a separator and an electrolyte; wherein,

[0007] The positive plate comprises modified lithium nickel cobalt manganese oxide, the modified lithium nickel cobalt manganese oxide comprises core-shell type lithium nickel cobalt manganese oxide and a boron-containing compound coated on the surface of the core-shell type lithium nickel cobalt manganese oxide; wherein the content of nickel element in the core-shell type lithium nickel cobalt manganese oxide gradually decreases from inside to outside, the content of cobalt element uniformly distributes from inside to outside, and the content of manganese element gradually increases from inside to outside.

[0008] The negative plate comprises a nano-silicon-based negative material, the nano-silicon-based negative material is a mixture of silicon-oxygen material and graphite; wherein the content of the graphite is 82-88 wt% based on the total weight of the nano-silicon-based negative material; the silicon-oxygen material comprises silicon monoxide and a carbon coating layer coated on the surface of the silicon monoxide, and the silicon monoxide comprises a SiO2 matrix and nano-silicon crystal particles uniformly distributed in the SiO2 matrix.

[0009] The second aspect of the present application provides a preparation method of an electric core, the preparation method comprising: winding a positive plate, a negative plate and a separator into a pole group, and injecting an electrolyte to obtain the electric core; wherein,

[0010] The positive plate comprises modified lithium nickel cobalt manganese oxide, the modified lithium nickel cobalt manganese oxide comprises core-shell type lithium nickel cobalt manganese oxide and a boron-containing compound coated on the surface of the core-shell type lithium nickel cobalt manganese oxide, and the content of nickel element in the core-shell type lithium nickel cobalt manganese oxide gradually decreases from inside to outside, the content of cobalt element uniformly distributes from inside to outside, and the content of manganese element gradually increases from inside to outside.

[0011] The negative plate comprises a nano-silicon-based negative material, the nano-silicon-based negative material is a mixture of silicon-oxygen material and graphite; wherein the content of the graphite is 82-88 wt% based on the total weight of the nano-silicon-based negative material; the silicon-oxygen material comprises silicon monoxide and a carbon coating layer coated on the surface of the silicon monoxide, and the silicon monoxide comprises a SiO2 matrix and nano-silicon crystal particles uniformly distributed in the SiO2 matrix.

[0012] The third aspect of the present application provides an electric core prepared by the preparation method provided by the present application.

[0013] The fourth aspect of the present application provides a soft package lithium ion battery, the battery comprising an electric core provided by the present application.

[0014] The fifth aspect of the present application provides an application of the soft package lithium ion battery provided by the present application as a low-temperature mobile power supply.

[0015] Through the above technical solution, the present application has the following advantages:

[0016] The active material of the positive electrode sheet of the present application adopts the shell-core type lithium nickel cobalt manganese oxide positive electrode material coated with boron element on the surface, the core of the shell-core type lithium nickel cobalt manganese oxide has high nickel content, the outer layer has high manganese content, the active material of the negative electrode sheet adopts the nano silicon-based negative electrode material obtained by mixing silicon-oxygen material and graphite, and the two cooperate with each other to improve the cycle stability and safety performance of the soft package lithium ion battery.

[0017] In the preferred embodiment of the present application, the active material of the positive electrode sheet adopts the shell-core type lithium nickel cobalt manganese oxide positive electrode material coated with boron element on the surface, the active material of the negative electrode sheet adopts the nano silicon-based negative electrode material, the separator adopts the PE / PP / PE separator coated with PVDF, and the electrolyte adopts the electrolyte with good low temperature and safety performance. The specific combination of materials greatly improves the low temperature performance and safety performance of the battery.

[0018] Further, in the preparation of the positive and negative electrode sheets, the core-shell type lithium nickel cobalt manganese oxide with suitable type and structure is selected and coated with boron as the active material of the positive electrode sheet, the nano silicon-based negative electrode material with suitable composition and structure is selected as the active material of the negative electrode sheet, the coating amount of boron element, the particle size and specific surface area of the modified lithium nickel cobalt manganese oxide, the graphite content, particle size and specific surface area of the nano silicon-based negative electrode material, the coating amount of the slurry, the composition of the slurry, the thickness and compaction density of the positive and negative electrode sheets are selected; in the preparation of the separator, the thickness of the separator and the covering thickness of the polyvinylidene fluoride are selected, and in the preparation of the electrolyte, the solvents and additives with suitable type and proportion are selected, which further improves the low temperature and safety performance of the battery. BRIEF DESCRIPTION OF DRAWINGS

[0019] Figure 1 is the low temperature discharge curve diagram of the soft package lithium ion battery prepared by the example 1 and the comparative examples 1-3 of the present application;

[0020] Figure 2 is the normal temperature 0.5C cycle curve diagram of the soft package lithium ion battery prepared by the example 1 and the comparative examples 1-3 of the present application. DETAILED DESCRIPTION

[0021] The endpoints of the ranges and any values disclosed herein are not limited to the precise values stated. The endpoints of the ranges and the values are approximate values and should be understood to include values approximately near these ranges and values within these ranges. Each individual range endpoint is a separate and independent number and can be combined with any other number in any of the ranges to form a new range.

[0022] The first aspect of the present application provides an electric core, the electric core comprising a positive electrode sheet, a negative electrode sheet, a separator and an electrolyte; wherein,

[0023] The positive plate comprises modified nickel cobalt manganese acid lithium, the modified nickel cobalt manganese acid lithium comprises core-shell type nickel cobalt manganese acid lithium and boron-containing compound coated on the surface of the core-shell type nickel cobalt manganese acid lithium; wherein the content of nickel element in the core-shell type nickel cobalt manganese acid lithium gradually decreases from inside to outside, the content of cobalt element uniformly distributes from inside to outside, and the content of manganese element gradually increases from inside to outside.

[0024] The negative plate comprises nano silicon-based negative material, the nano silicon-based negative material is a mixture of silicon-oxygen material and graphite; wherein the content of the graphite is 82-88wt% based on the total weight of the nano silicon-based negative material; the silicon-oxygen material comprises silicon monoxide and a carbon coating layer coated on the surface of the silicon monoxide, and the silicon monoxide comprises SiO2 matrix and nano silicon crystal particles uniformly distributed in the SiO2 matrix.

[0025] In the present application, the content of nickel element in the core-shell type nickel cobalt manganese acid lithium gradually decreases from inside to outside, the content of cobalt element uniformly distributes from inside to outside, and the content of manganese element gradually increases from inside to outside, which means that the mass percentage of nickel element in the core-shell type nickel cobalt manganese acid lithium gradually decreases from inside to outside, the mass percentage of cobalt element uniformly distributes from inside to outside, and the mass percentage of manganese element gradually increases from inside to outside, based on the total weight of nickel element, cobalt element and manganese element in the core-shell type nickel cobalt manganese acid lithium.

[0026] The core-shell type nickel cobalt manganese acid lithium in the positive plate has high nickel in the core and high manganese in the outer layer, has high structural stability, is beneficial to the improvement of the overall cycle stability and safety performance of the positive material, makes the nickel content in the core higher and the manganese content in the shell higher, and is more beneficial to the improvement of the battery energy density and surface stability. The boron element is coated, on one hand, the B=O bond energy is strong, can effectively inhibit the precipitation of lattice oxygen in the electrochemical reaction process, and reduce the capacity loss caused by irreversible phase change. On the other hand, the high lithium ion conductor Li2O-B2O3 coating layer can stabilize the electrode-electrolyte interface to some extent, thereby further improving the cycle stability.

[0027] The nano silicon-based negative material in the negative plate adopts a mixture of carbon-coated silicon monoxide and graphite material, wherein the silicon monoxide negative electrode filled with nano silicon crystal particles has small volume expansion, reduces the influence of the silicon volume effect on the plate, and produces non-active substances such as Li2O in the charging and discharging process. The Li2O matrix can act as a fast diffusion channel for lithium ions, making the cycle and rate performance better when lithium is embedded. The content of graphite should not be too much or too little, too much cannot meet the demand of high energy density battery in the real environment, and too little will make the negative expansion problem very prominent.

[0028] According to the application, the nanometer silicon crystal particles in the silicon suboxide are uniformly distributed in the SiO2 matrix, then carbon coating is performed on the surface of the obtained silicon suboxide, and the obtained silicon-oxygen material and graphite are mixed to obtain the nanometer silicon-based negative electrode material. In the application, the silicon-oxygen material can be obtained by self-preparation or purchased from the market, for example, it can be purchased from Shanghai Sun Sun Technology Co., Ltd.

[0029] According to the application, preferably, the change rate of the content of the nickel element is 3-5wt% / μm from the core of the core-shell type lithium nickel cobalt manganese oxide to the core surface, and the change rate of the content of the manganese element is 3-5wt% / μm.

[0030] The change rate of the content of the nickel element is 5-6wt% / μm from the core surface of the core-shell type lithium nickel cobalt manganese oxide to the shell surface, and the change rate of the content of the manganese element is 5-6wt% / μm.

[0031] According to the application, preferably, the distance from the core of the core-shell type lithium nickel cobalt manganese oxide to the core surface is 2.7-3.2μm, and the distance from the core surface to the shell surface is 1.5-2.5μm.

[0032] The nickel element and the manganese element are both in a double-gradient concentration distribution from inside to outside, that is, the content of the nickel element decreases less with a change rate of 3-5wt% / μm from the center of the core-shell type lithium nickel cobalt manganese oxide particle to a position 2.7-3.2μm away from the center, and the content of the nickel element decreases more with a change rate of 5-6wt% / μm from the position 2.7-3.2μm away from the center of the particle to the outer surface of the particle; the content of the manganese element increases less with a change rate of 3-5wt% / μm from the center of the core-shell type lithium nickel cobalt manganese oxide particle to a position 2.7-3.2μm away from the center, and the content of the manganese element increases more with a change rate of 5-6wt% / μm from the position 2.7-3.2μm away from the center of the particle to the outer surface of the particle, so that the concentration of transition metal ions (nickel ions and manganese ions) has two different distribution gradients, and the concentration gradient change occurs at a position 2.7-3.2μm away from the center of the particle, and the material has a higher content of nickel in the inside, which is more conducive to obtaining and maintaining high capacity, and has a higher content of manganese in the outer layer, which is more conducive to improving the cycle stability and thermal stability.

[0033] According to the application, preferably, the content of the nickel element in the core, the core surface and the shell surface of the core-shell type lithium nickel cobalt manganese oxide is 75-85wt%, 67-73wt% and 47-62wt%, respectively, based on the total weight of the nickel element, the cobalt element and the manganese element in the core-shell type lithium nickel cobalt manganese oxide.

[0034] According to the present application, preferably, the content of the manganese element at the core of the core-shell type lithium nickel cobalt manganese oxide, at the surface of the core and at the surface of the shell is 5-18wt%, 17-26wt% and 28-46wt% respectively, based on the total weight of the nickel element, the cobalt element and the manganese element in the core-shell type lithium nickel cobalt manganese oxide.

[0035] According to the present application, preferably, the content of the cobalt element at the core of the core-shell type lithium nickel cobalt manganese oxide, at the surface of the core and at the surface of the shell is 7-10wt%, based on the total weight of the nickel element, the cobalt element and the manganese element in the core-shell type lithium nickel cobalt manganese oxide.

[0036] In the present application, a point can be taken at any surface of the core of the core-shell type lithium nickel cobalt manganese oxide, and the content of the element at the surface of the core of the core-shell type lithium nickel cobalt manganese oxide can be obtained by determining the mass percentage of the element at the point; similarly, a point can be taken at any surface of the shell of the core-shell type lithium nickel cobalt manganese oxide, and the content of the element at the surface of the shell of the core-shell type lithium nickel cobalt manganese oxide can be obtained by determining the mass percentage of the element at the point. By comparing the results obtained by analyzing different points, it is proved that the content distribution of nickel, cobalt and manganese in the core-shell type lithium nickel cobalt manganese oxide provided by the present application meets the requirements that the content of the nickel element gradually decreases from inside to outside, the content of the cobalt element is uniformly distributed from inside to outside, and the content of the manganese element gradually increases from inside to outside.

[0037] According to the present application, preferably, the boron-containing compound is at least one selected from the group consisting of diboron trioxide, boron trichloride, boron trifluoride, boric acid and sodium metaborate.

[0038] According to the present application, preferably, the content of the boron element in the boron-containing compound is 2-12wt%, based on the total weight of the modified lithium nickel cobalt manganese oxide. The content of the boron element meets the requirements of optimizing the electrical performance and safety performance, and it is not suitable to be too high or too low. If it is too high, B2O3 which is not reduced will remain on the surface of the material, which will increase the impedance. If it is too low, the coating will not be complete, and the modification will not be obvious.

[0039] According to the present application, preferably, the D50 of the modified lithium nickel cobalt manganese oxide is 8-12μm. 50 The particle size is 8-12μm, and the specific surface area is 0.5-2m 2 / g. The particle size and the specific surface area in this range can ensure that the battery has a high volumetric energy density and good low-temperature charge and discharge performance. If the particle size is too large, the specific surface area is small, the adsorbability of the particle is relatively poor, the positive active material may be separated from the matrix and free in the electrolyte, and once it contacts with the negative material, it will cause local battery short circuit. If the particle size is too small, the specific surface area is too large, the powder is easy to agglomerate, and it is difficult to disperse in the organic solvent. The active material of the electrode sheet is not uniformly distributed, and the battery performance is not reduced. At the same time, the fine particle size can cause surface defects, induce battery polarization and reduce the electrochemical energy of the positive electrode.

[0040] In the present application, the modified lithium nickel cobalt manganese oxide can be obtained by self-preparation or purchased from GEM New Materials Corporation.

[0041] According to the present application, preferably, the D50 of the nano-silicon-based negative electrode material is 7-13 mu m. 50 The particle size is 7-13 mu m, and the specific surface area is 5-9 m 2 The particle size and the specific surface area in this range can meet the requirements of smaller impedance under higher unit load, improve the utilization rate of active materials, and alleviate the problems of huge volume change and cycle deterioration of silicon during lithium extraction. If the particle size is too large, the specific surface area will be too large, which will lead to the generation of a large-area SEI film during the first lithium intercalation process, thereby consuming lithium ions in the battery, and the silicon particles inside the large particle size are prone to cracking during the cycle, which reduces the electrical contact activity between materials. If the particle size is too small, the oxidation resistance and dispersibility will be poor, and the occurrence of electrode surface side reactions will be aggravated.

[0042] According to the present application, preferably, the graphite has a gram capacity of 300-360 mA·h / g in the nano-silicon-based negative electrode material. The gram capacity of the graphite in this range can ensure that the proportion of the graphite mixed with the silicon-oxygen negative electrode material is 82-88 wt%, and has better compaction density and cycle stability.

[0043] According to the present application, preferably, the graphite is selected from at least one of artificial graphite, natural graphite, and mesocarbon microbeads.

[0044] According to the present application, preferably, the silicon-oxygen material has a gram capacity of 1250-1750 mA·h / g in the nano-silicon-based negative electrode material.

[0045] According to the present application, preferably, the average particle size of the nano-silicon crystal particles is 2-10 nm, and the average particle size of the SiO2 matrix is 3-8 mu m.

[0046] According to the present application, preferably, the content of the nano-silicon crystal particles is 26-28 wt% and the content of the SiO2 matrix is 69-73 wt% based on the total weight of the silicon-oxygen material. The content of the nano-silicon crystal particles and the SiO2 matrix in this range can ensure better cycle stability.

[0047] According to the present application, preferably, the carbon coating layer is amorphous carbon.

[0048] According to the present application, preferably, the thickness of the carbon coating layer is 8-13 nm. The carbon coating is amorphous carbon, and the particle size of the nano-silicon particles and the thickness of the carbon coating layer in this range can ensure better cycle stability and low-temperature performance.

[0049] According to the present application, preferably, the positive electrode sheet is obtained by coating the positive electrode current collector surface with a positive electrode slurry comprising the modified lithium nickel cobalt manganese oxide, and the negative electrode sheet is obtained by coating the negative electrode current collector surface with a negative electrode slurry comprising the nano-silicon-based negative electrode material.

[0050] According to the present application, preferably, the positive electrode current collector is an aluminum foil, preferably an aluminum foil with a thickness of 12-16 μm; and the negative electrode current collector is a copper foil, preferably a copper foil with a thickness of 6-10 μm.

[0051] According to the present application, preferably, the surface density of the positive electrode slurry coated on the positive electrode current collector surface is 110-130 g / cm 2 .

[0052] According to the present application, preferably, the surface density of the negative electrode slurry coated on the negative electrode current collector surface is 70-80 g / cm 2 .

[0053] The surface density of the positive and negative electrode slurries is within the aforementioned range, which can meet the demand of battery energy density while having good lithium ion transmission capacity.

[0054] According to the present application, preferably, the positive electrode slurry and the negative electrode slurry each independently further comprise a conductive agent, a binder, and a solvent.

[0055] According to the present application, preferably, the conductive agent is selected from at least one of conductive carbon black, carbon nanotubes, and graphene.

[0056] According to the present application, preferably, the binder is selected from a polyvinylidene fluoride (PVDF)-based binder, preferably a PVDF-based binder with a weight average molecular weight of 600,000-1,300,000 g / mol, for example, the positive electrode binder can be a PVDF 5130 binder with a weight average molecular weight of 1,200,000 g / mol, and the negative electrode binder can be a PVDF 6020 binder with a weight average molecular weight of 680,000 g / mol.

[0057] According to the present application, preferably, the solvent is N-methyl pyrrolidone (NMP).

[0058] In the present application, the conductive agent, the binder, and the solvent in the positive electrode slurry and the conductive agent, the binder, and the solvent in the negative electrode slurry can be selected from the same conductive agent, binder, and solvent, or can be different.

[0059] According to the present application, preferably, based on the total weight (total powder weight) of the modified lithium nickel cobalt manganese oxide, the conductive agent, and the binder, in the positive electrode slurry, the content of the modified lithium nickel cobalt manganese oxide is 92-98 wt%, the content of the conductive agent is 1-4 wt%, and the content of the binder is 1-4 wt%.

[0060] According to the present application, preferably, in the negative electrode slurry, the content of the nano-silicon-based negative electrode material is 92-98wt%, the content of the conductive agent is 1-4wt%, and the content of the binder is 1-4wt%, based on the total weight of the nano-silicon-based negative electrode material, the conductive agent and the binder (total powder weight).

[0061] According to the present application, preferably, the viscosity of the positive electrode slurry at 25-35℃ is 3500-5500mPa·s.

[0062] According to the present application, preferably, the viscosity of the negative electrode slurry at 25-35℃ is 3000-5000mPa·s.

[0063] According to the present application, preferably, the total content (solid content) of the modified lithium nickel cobalt manganese oxide, the conductive agent and the binder is 65-77wt%, based on the total weight of the positive electrode slurry.

[0064] According to the present application, preferably, the total content (solid content) of the nano-silicon-based negative electrode material, the conductive agent and the binder is 45-55wt%, based on the total weight of the negative electrode slurry.

[0065] According to the present application, preferably, the average thickness of the positive electrode sheet is 82-95μm, and the compaction density is 3.2-3.6g / cm 3 .

[0066] According to the present application, preferably, the average thickness of the negative electrode sheet is 100-110μm, and the compaction density is 1.35-1.65g / cm 3 .

[0067] According to the present application, preferably, the separator is a PE / PP / PE separator with polyvinylidene fluoride (PVDF) on the surface, preferably a PE / PP / PE separator with PVDF on the upper and lower surfaces. The PE / PP / PE separator is a three-layer composite structure film composed of polyethylene (PE), polypropylene (PP) and polyethylene (PE), which combines the advantages of PP film and PE film, has good mechanical strength, and is safer. In the present application, the weight average molecular weight of PE in the PE / PP / PE separator is 200-350 thousand g / mol, and the weight average molecular weight of PP is 350-800 thousand g / mol.

[0068] Since the PE / PP / PE separator is coated with PVDF on both sides, it is easy to form molecular bonds with PVDF in the positive and negative electrodes, and the jig pressure and temperature can be appropriately reduced, the formation time can be shortened, and the reaction between the electrolyte and the surface of the positive and negative electrodes during formation can be reduced, thereby achieving the purpose of increasing the adhesion between the positive and negative electrodes and the separator while reducing the internal resistance of the battery, preventing the deformation of the battery, improving the first efficiency and the electrical performance of the battery.

[0069] According to the present application, preferably, the PVDF coated on both sides of the PE / PP / PE separator has a weight average molecular weight of 500,000-1,000,000 g / mol.

[0070] According to the present application, preferably, the total thickness of the separator is 12-20 μm.

[0071] According to the present application, preferably, the thickness of the polyvinylidene fluoride coated on the surface of the PE / PP / PE separator is 1-4 μm.

[0072] According to the present application, preferably, the electrolyte comprises an organic solvent, a lithium salt and an additive.

[0073] According to the present application, preferably, the content of the organic solvent in the electrolyte is 75-85 wt% based on the total weight of the electrolyte.

[0074] According to the present application, preferably, the organic solvent is at least one selected from ethylene carbonate (EC), propylene carbonate (PC), ethyl methyl carbonate (EMC), diethyl carbonate (DEC), ethyl propionate and propyl propionate, and preferably is a mixture of EC, PC, EMC and DEC.

[0075] According to the present application, preferably, the weight ratio of EC, PC, DEC and EMC is (15-20):(8-12):(33-40):(12-18). By reducing the content of EC and the like having high melting point and high viscosity and appropriately increasing the content of PC and the like having low melting point, the electrolyte viscosity at low temperature can be reduced by limiting the weight ratio of EC, PC, DEC and EMC within this range.

[0076] Further, the weight ratio of EC, PC, DEC and EMC is (16-18):(9-11):(36-38):(14-16).

[0077] According to the present application, preferably, the content of the lithium salt in the electrolyte is 12-15 wt% based on the total weight of the electrolyte.

[0078] According to the present application, preferably, the lithium salt is at least one selected from LiPF6, LiTFSI and LiFSI, and preferably is LiPF6.

[0079] According to the application, preferably, the additive is at least one of fluoroethylene carbonate (FEC), lithium difluorophosphate (LiPO2F2), lithium bis(oxalato)borate (C4BLiO8), tris(trimethylsilyl)borate (TMSB), vinylene carbonate (VC), vinyl ethylene carbonate (VEC), lithium tetrafluoroborate (LiBF) and lithium difluoro(oxalato)borate (LiODFB).

[0080] Further, the additive is a mixture of TMSB, FEC and VC. The addition of TMSB and FEC in the electrolyte has strong oxidation resistance, and forms stable and dense CEI film and SEI film on the surface of the positive and negative electrodes respectively, which reduces the reaction rate of the electrolyte on the surface of the positive and negative electrodes during overcharge of the battery, thereby improving the safety performance of the battery; the VC additive in the electrolyte can undergo a free radical polymerization reaction on the surface of the carbon negative electrode, thereby effectively inhibiting the co-insertion reaction of solvent molecules, and has no side effects on the positive electrode, has good high and low temperature performance and anti-gas expansion function, and can improve the capacity and cycle life of the battery.

[0081] According to the application, preferably, the content of TMSB in the electrolyte is 0.1-0.5wt%, the content of FEC is 1-5wt%, and the content of VC is 0.1-0.5wt%, based on the total weight of the electrolyte.

[0082] Further, the content of TMSB in the electrolyte is 0.2-0.4wt%, the content of FEC is 2-4wt%, and the content of VC is 0.2-0.4wt%, based on the total weight of the electrolyte.

[0083] According to the application, preferably, the amount of the electrolyte is 30-40wt%, based on the total weight of the modified lithium nickel cobalt manganese oxide.

[0084] According to a preferred embodiment of the application, the battery cell comprises a positive electrode sheet, a negative electrode sheet, a separator and an electrolyte; wherein,

[0085] The positive electrode sheet comprises modified lithium nickel cobalt manganese oxide, and the modified lithium nickel cobalt manganese oxide comprises core-shell type lithium nickel cobalt manganese oxide and a boron-containing compound coated on the surface of the core-shell type lithium nickel cobalt manganese oxide; wherein the content of nickel element in the core-shell type lithium nickel cobalt manganese oxide gradually decreases from the inside to the outside, the content of cobalt element uniformly distributes from the inside to the outside, and the content of manganese element gradually increases from the inside to the outside.

[0086] The negative sheet comprises a nano-silicon-based negative material, which is a mixture of a silicon-oxygen material and graphite; wherein the content of the graphite is 82-88wt% based on the total weight of the nano-silicon-based negative material; the silicon-oxygen material comprises silicon monoxide and a carbon coating layer coated on the surface of the silicon monoxide, and the silicon monoxide comprises a SiO2 matrix and nano-silicon crystal particles uniformly distributed in the SiO2 matrix.

[0087] The separator is a PE / PP / PE separator coated with polyvinylidene fluoride (PVDF) on the surface.

[0088] The electrolyte comprises an organic solvent, a lithium salt and an additive.

[0089] The active material of the positive sheet is the shell-core type lithium nickel cobalt manganese oxide positive material coated with boron on the surface, the active material of the negative sheet is the nano-silicon-based negative material, the separator is the PE / PP / PE separator coated with PVDF, and the electrolyte is the electrolyte with good low-temperature and safety performance, so that the specific material combination greatly improves the low-temperature and safety performance of the battery.

[0090] The second aspect of the present application provides a preparation method of an electric core, which comprises: winding a positive sheet, a negative sheet and a separator to form a pole group, and injecting an electrolyte to obtain the electric core; wherein,

[0091] The positive sheet comprises modified lithium nickel cobalt manganese oxide, which comprises a core-shell type lithium nickel cobalt manganese oxide and a boron-containing compound coated on the surface of the core-shell type lithium nickel cobalt manganese oxide, the content of nickel element in the core-shell type lithium nickel cobalt manganese oxide gradually decreases from inside to outside, the content of cobalt element is uniformly distributed from inside to outside, and the content of manganese element gradually increases from inside to outside.

[0092] The negative sheet comprises a nano-silicon-based negative material, which is a mixture of a silicon-oxygen material and graphite; wherein the content of the graphite is 82-88wt% based on the total weight of the nano-silicon-based negative material; the silicon-oxygen material comprises silicon monoxide and a carbon coating layer coated on the surface of the silicon monoxide, and the silicon monoxide comprises a SiO2 matrix and nano-silicon crystal particles uniformly distributed in the SiO2 matrix.

[0093] In the preparation method of the electric core in the second aspect of the present application, the types and amounts of the positive sheet, the negative sheet, the separator and the electrolyte are completely the same as those in the electric core in the first aspect of the present application, and the present application will not be described again in this second aspect to avoid repetition, and the person skilled in the art should not understand it as a limitation of the present application.

[0094] According to the application, preferably, the preparation method of the positive plate comprises: mixing the modified lithium nickel cobalt manganese oxide, a conductive agent, a binder and a solvent to obtain a positive slurry, then coating the positive slurry on the surface of a positive current collector, and then rolling and slitting to obtain the positive plate.

[0095] According to the application, preferably, the preparation method of the negative plate comprises: mixing the nano-silicon-based negative material, a conductive agent, a binder and a solvent to obtain a negative slurry, then coating the negative slurry on the surface of a negative current collector, and then rolling and slitting to obtain the negative plate.

[0096] According to one specific embodiment of the application, the preparation method of the positive plate comprises: adding a binder powder into a N-methyl pyrrolidone solvent to prepare a glue with a binder concentration of 5-10wt%, then sequentially adding a conductive agent and the modified lithium nickel cobalt manganese oxide into the glue, stirring uniformly, adjusting the viscosity to 3500-5500mPa·s and the solid content to 65-77wt% with N-methyl pyrrolidone to obtain a positive slurry, then coating the positive slurry on a positive current collector, rolling and slitting to obtain the positive plate.

[0097] According to one specific embodiment of the application, the preparation method of the negative plate comprises: adding a binder powder into a N-methyl pyrrolidone solvent to prepare a glue with a binder concentration of 5-10wt%, then sequentially adding a conductive agent and the nano-silicon-based negative material into the glue, stirring uniformly, adjusting the viscosity to 3000-5000mPa·s and the solid content to 45-55wt% with N-methyl pyrrolidone to obtain a negative slurry, then coating the negative slurry on a negative current collector, rolling and slitting to obtain the negative plate.

[0098] The third aspect of the application provides a battery cell prepared by the preparation method.

[0099] The fourth aspect of the application provides a soft package lithium ion battery, which comprises the battery cell.

[0100] According to the application, preferably, the preparation method of the battery comprises: packaging the battery cell with an aluminum plastic shell, and then performing formation, final sealing and capacity test to obtain the soft package lithium ion battery.

[0101] According to the application, preferably, the battery has a 1C discharge capacity greater than or equal to 60% at-40℃, and a capacity retention rate greater than or equal to 90% after 500 cycles at normal temperature.

[0102] The fifth aspect of the application provides an application of the soft package lithium ion battery as a low-temperature mobile power supply.

[0103] The present application will be described in detail below by way of examples and comparative examples. In the following examples and comparative examples, unless otherwise specified, the methods are conventional methods; the reagents and materials used, unless otherwise specified, are commercially available. The determination methods involved in each example and comparative example are as follows:

[0104] Element content determination: the desired cross-section of the particle is obtained by cutting the particle with a focused ion beam (FIB), and the concentration changes of nickel / cobalt / manganese from the center of the particle to the surface can be obtained from the line scanning results of the EPMA instrument;

[0105] Particle size determination: 5 g of the sample is weighed to 0.0001 g, and is determined by a laser particle size analyzer;

[0106] Specific surface area determination: 10 g of the sample is weighed to 0.0001 g, and is analyzed by a NOVA2000e specific surface area tester according to the static volumetric method;

[0107] Normal temperature capacity test: at an indoor ambient temperature of 20-25°C, a battery is charged to 4.2V at 0.2C constant current after being fully discharged, and then is charged at constant voltage, and the charging is stopped when the charging current is less than 0.02C. After standing for 1h, the battery is discharged to 2.5V at 0.2C current, and the 0.2C discharge capacity of the battery is determined;

[0108] Low temperature capacity test: the battery is fully charged and is placed at a low temperature of-40°C for 8h in a low temperature box, and then is discharged to 2.5V at 1C constant current at the temperature, and the percentage of the discharge capacity to the initial capacity is determined;

[0109] Cycle performance test: at an indoor ambient temperature of 20-25°C, a battery is charged at 0.5C constant current and constant voltage with a cut-off voltage of 4.2V and a cut-off current of 0.05C, and is discharged at constant current with a cut-off voltage of 2.5V, and the capacity retention rate of the battery is determined after 500 cycles;

[0110] Safety test: the battery is subjected to overcharge, overdischarge, extrusion, heavy charge, hot box and short circuit treatment, and whether the battery ignites or explodes is tested.

[0111] In the following examples, the modified lithium nickel cobalt manganese oxide is purchased from GREEMATERIALS CO., LTD., and the silicon-oxygen material and artificial graphite are purchased from Shanghai Sunstone Technology Co., Ltd.

[0112] The following examples are used to illustrate the preparation of soft package lithium ion batteries.

[0113] Example 1

[0114] The positive active material is modified lithium nickel cobalt manganese oxide (L-7512B), wherein,

[0115] Modified lithium nickel cobalt manganese oxide is a core-shell type lithium nickel cobalt manganese oxide with a surface coating of boron trioxide, a boron-containing compound. The D of modified lithium nickel cobalt manganese oxide... 50 The particle size is 10.5 μm, and the specific surface area is 1.53 m². 2 / g; Based on the total weight of modified nickel cobalt manganese oxide, the boron content is 4wt%. Among them, the distance from the core to the core surface of the core-shell nickel cobalt manganese oxide is 3μm, and the distance from the core surface to the shell surface is 2μm;

[0116] Based on the total weight of nickel, cobalt, and manganese in a core-shell lithium nickel cobalt cobalt oxide (LCO), the nickel content decreases from 81 wt% to 69 wt% from the core to the core surface, a change rate of 4 wt% / μm; the manganese content increases from 10 wt% to 22 wt%, also a change rate of 4 wt% / μm. From the core surface to the shell surface, the nickel content decreases from 69 wt% to 58 wt%, a change rate of approximately 5.5 wt% / μm; the manganese content increases from 22 wt% to 33 wt%, also a change rate of 5.5 wt% / μm. The cobalt content is uniformly distributed at 9 wt%.

[0117] Anode active materials: nano-silicon-based anode materials, among which...

[0118] The anode active material, nano-silicon-based anode material, is composed of silicon-oxygen material (S1550) and artificial graphite (QCG-X1) through grinding and mixing. Based on the total weight of the nano-silicon-based anode material, the content of artificial graphite is 86 wt%. The specific capacity of artificial graphite is 360 mAh / g. The nano-silicon-based anode material D... 50 The particle size is 11.8 μm, and the specific surface area is 8.4 m². 2 / g. The specific capacity of the silicon-oxygen material is 1450mAh / g. The average particle size of the nano-silicon crystal particles in the silicon-oxygen material is 4nm, and the average particle size of the SiO2 matrix is ​​7μm. Based on the total weight of the silicon-oxygen material, the content of nano-silicon crystal particles is 27.4wt%, and the content of SiO2 matrix is ​​69.6wt%. The thickness of the amorphous carbon coating layer is 8nm.

[0119] (1) Preparation of the positive electrode:

[0120] 5130 binder powder with a weight average molecular weight of 1.2 million g / mol was added into N-methyl pyrrolidone solvent, stirred at high speed for 2.5 h to prepare a glue with a binder concentration of 8 wt%, then conductive agent SP and modified lithium nickel cobalt manganese oxide were sequentially added into the glue, stirred for 4.5 h, and then the viscosity of the slurry was adjusted to 5000 mPa·s and the solid content was adjusted to 70 wt% at 25 ℃ using N-methyl pyrrolidone to obtain a positive electrode slurry, then the positive electrode slurry was passed through a 200-mesh screen and coated on an aluminum foil with a thickness of 16 μm, the coating surface density was 125±2.5 g / cm 2 , and after coating and drying, the coated electrode was rolled to a thickness of 86±2 μm, and the compacted density was 3.45 g / cm 2 ; the rolled electrode was then slitted and tab welded to obtain a positive electrode sheet; wherein,

[0121] Based on the total weight of the modified lithium nickel cobalt manganese oxide, the conductive agent SP and the 5130 binder, the content of the modified lithium nickel cobalt manganese oxide was 96 wt%, the content of the conductive agent SP was 2.5 wt%, and the content of the 5130 binder was 1.5 wt%.

[0122] (2) Preparation of a negative electrode sheet:

[0123] 6020 binder powder with a weight average molecular weight of 680,000 g / mol was added into N-methyl pyrrolidone solvent, stirred at high speed for 2.5 h to prepare a glue with a binder concentration of 8 wt%, then conductive agent SP and a nano-silicon-based negative electrode material were sequentially added into the glue, stirred for 4.5 h, and then the viscosity of the slurry was adjusted to 4000 mPa·s and the solid content was adjusted to 55 wt% at 25 ℃ using N-methyl pyrrolidone to obtain a negative electrode slurry, then the negative electrode slurry was passed through a 200-mesh screen and coated on a copper foil with a thickness of 7 μm, the coating surface density was 74±1.5 g / cm 2 , and after coating and drying, the coated electrode was rolled to a thickness of 106±2 μm, and the compacted density was 1.52 g / cm 2 ; the rolled electrode was then slitted and tab welded to obtain a negative electrode sheet; wherein,

[0124] Based on the total weight of the nano-silicon-based negative electrode material, the conductive agent SP and the 6020 binder, the content of the nano-silicon-based negative electrode material was 98 wt%, the content of the conductive agent SP was 1 wt%, and the content of the 6020 binder was 1 wt%.

[0125] (3) The separator was a PE / PP / PE separator (the weight average molecular weight of PE was 300,000 g / mol, and the weight average molecular weight of PP was 700,000 g / mol) coated with PVDF (the weight average molecular weight was 800,000 g / mol) on both sides, the total thickness was 15 μm, and the single-side coating thickness was 3 μm.

[0126] (4) Preparation of electrolyte:

[0127] The weight ratio of solvents EC, PC, DEC and EMC is 17:10:37:15, the electrolyte is LiPF6, and the additive is a mixture of TMSB, FEC and VC. Among them, the content of the solvent is 82.7wt% based on the total weight of the electrolyte, the content of LiPF6 is 13.7wt%, the content of TMSB is 0.3wt%, the content of FEC is 3wt%, and the content of VC is 0.3wt%.

[0128] (5) Preparation of battery: The positive / negative electrode sheet, the separator are made into a pole group by winding, then the electrolyte is injected, and the packaging, formation, final sealing and capacity are carried out to complete the preparation, and a soft package lithium ion battery is obtained; wherein the amount of electrolyte is 34wt% based on the total weight of modified lithium nickel cobalt manganese oxide.

[0129] Example 2

[0130] The soft package lithium ion battery is prepared according to the method of Example 1, except that the powder composition ratio of the positive and negative electrode sheets is different. Specifically, the content of modified lithium nickel cobalt manganese oxide is 95wt%, the content of conductive agent SP is 2wt%, and the content of 5130 binder is 3wt% based on the total weight of modified lithium nickel cobalt manganese oxide (same as Example 1), conductive agent SP and 5130 binder (same as Example 1); the content of nano silicon-based negative electrode material is 96.5wt%, the content of conductive agent SP is 1.5wt%, and the content of 6020 binder (same as Example 1) is 2wt% based on the total weight of nano silicon-based negative electrode material, conductive agent SP and 6020 binder. The soft package lithium ion battery is prepared.

[0131] Example 3

[0132] The soft package lithium ion battery is prepared according to the method of Example 1, except that modified lithium nickel cobalt manganese oxide with different boron element contents is selected. Specifically, the modified lithium nickel cobalt manganese oxide with the brand L-7013B is selected, and the content of boron element is 13wt% based on the total weight of modified lithium nickel cobalt manganese oxide. The soft package lithium ion battery is prepared.

[0133] Example 4

[0134] The soft package lithium ion battery is prepared according to the method of Example 1, except that the particle size and specific surface area of the nano silicon-based negative electrode material are different. Specifically, D 50 The particle size is 13.7μm, and the specific surface area is 4.7m 2The nano-silicon-based negative electrode material has a specific capacity of 1350 mAh / g, the average particle size of the nano-silicon crystal particles in the silicon-oxygen material is 6 nm, the average particle size of the SiO2 matrix is 8 μm, the content of the nano-silicon crystal particles is 27.4 wt% and the content of the SiO2 matrix is 69.6 wt% based on the total weight of the silicon-oxygen material, the thickness of the amorphous carbon coating layer is 8 nm, and the specific capacity of the artificial graphite is 360 mAh / g. A soft package lithium ion battery is prepared.

[0135] Example 5

[0136] The soft package lithium ion battery is prepared according to the method of Example 1, except that the coating amounts of the positive electrode slurry and the negative electrode slurry are different. Specifically, when the positive electrode sheet is prepared, the areal density of the positive electrode slurry coated on the aluminum foil is 140±2.5 g / cm 2 ; and when the negative electrode sheet is prepared, the areal density of the negative electrode slurry coated on the copper foil is 85±1.5 g / cm 2 . A soft package lithium ion battery is prepared.

[0137] Example 6

[0138] The soft package lithium ion battery is prepared according to the method of Example 1, except that the powder composition ratio of the positive and negative electrode sheets is different. Specifically, when the positive electrode sheet is prepared, the content of the modified lithium nickel cobalt manganese oxide is 90 wt%, the content of the conductive agent SP is 5 wt%, and the content of the 5130 binder is 5 wt% based on the total weight of the modified lithium nickel cobalt manganese oxide, the conductive agent SP and the 5130 binder; and when the negative electrode sheet is prepared, the content of the nano-silicon-based negative electrode material is 90 wt%, the content of the conductive agent SP is 5 wt%, and the content of the 6020 binder is 5 wt% based on the total weight of the nano-silicon-based negative electrode material, the conductive agent SP and the 6020 binder. A soft package lithium ion battery is prepared.

[0139] Example 7

[0140] The soft package lithium ion battery is prepared according to the method of Example 1, except that the average thickness and the compaction density of the positive and negative electrode sheets are different. Specifically, when the positive electrode sheet is prepared, the coating and drying are followed by rolling to a thickness of 100±2 μm and a compaction density of 3.65 g / cm 2 ; and when the negative electrode sheet is prepared, the coating and drying are followed by rolling to a thickness of 115±2 μm and a compaction density of 1.65 g / cm 2 . A soft package lithium ion battery is prepared.

[0141] Example 8

[0142] The soft package lithium ion battery was prepared according to the method of Example 1, except that the selected separator was different. Specifically, a PP / PE / PP composite base film with a thickness of 16 μm was selected as the separator. The soft package lithium ion battery was prepared.

[0143] Example 9

[0144] The soft package lithium ion battery was prepared according to the method of Example 1, except that the thickness of the polyvinylidene fluoride covered on the surface of the PE / PP / PE separator was different, and the single-side coating thickness was 8 μm, and the total thickness of the separator was 20 μm. The soft package lithium ion battery was prepared.

[0145] Example 10

[0146] The soft package lithium ion battery was prepared according to the method of Example 1, except that the composition of the solvent in the electrolyte was different. Specifically, the EMC in the solvent was replaced with an equal weight of DMC. The soft package lithium ion battery was prepared.

[0147] Example 11

[0148] The soft package lithium ion battery was prepared according to the method of Example 1, except that the amount of EC in the electrolyte was increased, and the amount of PC was reduced. Specifically, the solvent in the electrolyte was a mixed solvent with a weight ratio of EC, PC, DEC and EMC of 23:4:37:15. The soft package lithium ion battery was prepared.

[0149] Example 12

[0150] The soft package lithium ion battery was prepared according to the method of Example 1, except that the proportions of TMSB, FEC and VC in the additive were different. Specifically, based on the total weight of the electrolyte, the content of TMSB was 1 wt%, the content of FEC was 2 wt%, and the content of VC was 0.6 wt%. The soft package lithium ion battery was prepared.

[0151] Example 13

[0152] The soft package lithium ion battery was prepared according to the method of Example 1, except that the element distribution in the core-shell type nickel-cobalt-manganese acid lithium in the modified nickel-cobalt-manganese acid lithium was different. Specifically, the modified nickel-cobalt-manganese acid lithium with a trade name of L-7012B was selected. The D 50 particle size of the modified nickel-cobalt-manganese acid lithium was 11.2 μm, and the specific surface area was 0.71 m 2 / g. Among them, the distance from the core to the core surface of the core-shell type nickel-cobalt-manganese acid lithium was 3.1 μm, and the distance from the core surface to the shell surface was 2.2 μm;

[0153] The content of the nickel element decreases from 72wt% to 54wt% from the core of the core-shell type lithium nickel cobalt manganese oxide to the core surface, and the content variation rate is 5.8wt% / μm; the content of the manganese element increases from 19wt% to 37wt%, and the content variation rate is 5.8wt% / μm. From the core surface of the core-shell type lithium nickel cobalt manganese oxide to the shell surface, the content of the nickel element decreases from 54wt% to 43wt%, and the content variation rate is about 5wt% / μm; the content of the manganese element increases from 37wt% to 48wt%, and the content variation rate is 5wt% / μm. The content of the cobalt element is uniformly distributed, which is 9wt%. The soft package lithium ion battery is prepared.

[0154] Example 14

[0155] The soft package lithium ion battery is prepared according to the method of Example 1, except that the type of lithium salt in the electrolyte is different. Specifically, LiPF6 is replaced by an equal weight of LiTFSI. The soft package lithium ion battery is prepared.

[0156] Comparative Example 1

[0157] The soft package lithium ion battery is prepared according to the method of Example 1, except that the active material of the negative electrode sheet is different. Specifically, when preparing the negative electrode sheet, graphite is used instead of an equal weight of nano-silicon-based negative electrode material. The soft package lithium ion battery is prepared.

[0158] Comparative Example 2

[0159] The soft package lithium ion battery is prepared according to the method of Example 1, except that the active material of the positive electrode sheet is different. Specifically, when preparing the positive electrode sheet, NCA represented by the chemical formula LiNi 0.8 Co 0.15 Al 0.05 O2 is used instead of an equal weight of modified lithium nickel cobalt manganese oxide. The soft package lithium ion battery is prepared.

[0160] Comparative Example 3

[0161] The soft package lithium ion battery is prepared according to the method of Example 1, except that the active materials of the positive electrode sheet and the negative electrode sheet are different. Specifically, when preparing the positive electrode sheet, NCA represented by the chemical formula LiNi 0.8 Co 0.15 Al 0.05 O2 is used instead of an equal weight of modified lithium nickel cobalt manganese oxide, and when preparing the negative electrode sheet, graphite is used instead of an equal weight of nano-silicon-based negative electrode material. The soft package lithium ion battery is prepared.

[0162] Comparative Example 4

[0163] The soft package lithium ion battery was prepared according to the method of Example 1, except that the content of graphite in the nanometer silicon-based negative electrode material was different. Specifically, the content of graphite in the nanometer silicon-based negative electrode material was 60% by weight based on the total weight of the nanometer silicon-based negative electrode material. The soft package lithium ion battery was prepared.

[0164] Test Example

[0165] The soft package lithium ion batteries prepared in each of the examples and comparative examples were subjected to normal temperature capacity test, low temperature capacity test, cycle performance test and safety test, respectively. The results of the discharge capacity of the battery in the normal temperature capacity test, the percentage of the discharge capacity in the low temperature capacity test to the initial capacity, the capacity retention rate of the battery in the cycle performance test and whether fire or explosion occurred in the safety test are shown in Table 1, the low temperature discharge curve of the soft package lithium ion battery prepared in Example 1 and Comparative Examples 1-3 is shown in Figure 1 , and the normal temperature cycle curve is shown in Figure 2 .

[0166] Table 1

[0167]

[0168]

[0169] Comparative Example 1 changed the type of the negative electrode active material, Comparative Example 2 changed the type of the positive electrode active material, and Comparative Example 3 changed the types of the positive electrode active material and the negative electrode active material at the same time. As can be seen from Figure 1 , compared with Comparative Examples 1-3, the percentage of the low temperature discharge capacity to the initial capacity of the battery obtained in Example 1 is obviously higher, which indicates that the battery prepared by using the positive electrode active material and the negative electrode active material of the present application still has excellent low temperature discharge performance after being stored at low temperature; as can be seen from Figure 2 , compared with Comparative Examples 1-3, the capacity retention rate of the battery obtained in Example 1 after being cycled at normal temperature for 500 cycles is obviously higher than that of Comparative Examples 1-3, which indicates that the battery prepared by using the positive electrode active material and the negative electrode active material of the present application has good cycle performance. In addition, Comparative Example 4 reduced the content of graphite in the nanometer silicon-based negative electrode material, and compared with Example 1, the capacity retention rate of the battery obtained in Comparative Example 4 after being cycled at normal temperature for 500 cycles is obviously reduced, which indicates that the content of graphite in the nanometer silicon-based negative electrode material has great influence on the cycle performance of the battery. The battery obtained in Example 1 of the present application has good low temperature discharge performance and normal temperature cycle performance, and has excellent safety performance.

[0170] In addition, the content of boron in the modified lithium nickel cobalt manganese oxide is changed in Example 3, the particle size and specific surface area of the nano-silicon-based negative electrode material are changed in Example 4, the coating amount of the positive electrode slurry and the negative electrode slurry is changed in Example 5 when the positive and negative electrode sheets are prepared, the powder composition ratio of the positive and negative electrode sheets is changed in Example 6, the average thickness and the compaction density of the positive and negative electrode sheets are changed in Example 7, the separator selected in Example 8 is different, the thickness of the polyvinylidene fluoride covered on the surface of the separator in Example 9 is different, the solvent composition in the electrolyte is changed in Example 10, the component content in the electrolyte is changed in Example 11, the component content in the additive is changed in Example 12, different core-shell type lithium nickel cobalt manganese oxides are selected in Example 13, and the type of lithium salt in the electrolyte is changed in Example 14. Compared with Example 1, the percentage of the low-temperature discharge capacity of the battery obtained in Examples 3-11 and 13 to the initial capacity and the capacity retention rate after 500 cycles at room temperature are both reduced to a certain extent. The battery obtained in Example 12 not only has a reduced percentage of the low-temperature discharge capacity to the initial capacity and a reduced capacity retention rate after 500 cycles at room temperature, but also catches fire and explodes during overcharge. Although the battery obtained in Example 14 has a high percentage of the low-temperature discharge capacity to the initial capacity, the capacity retention rate after 500 cycles at room temperature is reduced, and the battery catches fire and explodes during overcharge. Therefore, the types and properties of the positive and negative active materials, the coating amount of the positive and negative electrode slurries in the positive and negative electrode sheets, the powder composition, thickness and compaction density of the positive and negative electrode sheets, and the composition of the separator, electrolyte and additive jointly affect the low-temperature discharge performance, room-temperature cycle performance and safety performance of the battery.

[0171] The above describes the preferred embodiments of the present application in detail, but the present application is not limited thereto. Within the technical concept of the present application, various simple modifications can be made to the technical solutions of the present application, including the combination of various technical features in any other suitable manner, and these simple modifications and combinations should also be considered as disclosed by the present application and fall within the protection scope of the present application.

Claims

1. A battery cell, characterized in that, The battery cell includes a positive electrode, a negative electrode, a separator, and an electrolyte; wherein, The positive electrode comprises modified lithium nickel cobalt manganese oxide, which includes core-shell lithium nickel cobalt manganese oxide and a boron-containing compound coating the surface of the core-shell lithium nickel cobalt manganese oxide. In the core-shell lithium nickel cobalt manganese oxide, the nickel content gradually decreases from the inside out, the cobalt content is uniformly distributed from the inside out, and the manganese content gradually increases from the inside out. Based on the total weight of nickel, cobalt, and manganese in the core-shell lithium nickel cobalt manganese oxide, the change rate of nickel content from the core to the core surface is 3-5 wt% / μm, and the change rate of manganese content is 3-5 wt% / μm. From the core surface to the shell surface, the change rate of nickel content is 5-6 wt% / μm, and the change rate of manganese content is 5-6 wt% / μm. The negative electrode sheet comprises a nano-silicon-based negative electrode material, which is a mixture of silicon-oxygen material and graphite; wherein, based on the total weight of the nano-silicon-based negative electrode material, the graphite content is 82-88 wt%; the silicon-oxygen material comprises silicon suboxide and a carbon coating layer on the surface of the silicon suboxide, and the silicon suboxide comprises a SiO2 matrix and nano-silicon crystal particles uniformly distributed in the SiO2 matrix; The electrolyte includes LiPF6 as the lithium salt and a mixture of tris(trimethylsilane)borate, fluoroethylene carbonate, and vinylene carbonate as additives. Based on the total weight of the electrolyte, the content of tris(trimethylsilane)borate in the electrolyte is 0.1-0.5 wt%, the content of fluoroethylene carbonate is 1-5 wt%, and the content of vinylene carbonate is 0.1-0.5 wt%.

2. The battery cell according to claim 1, characterized in that, The distance from the core to the core surface of the core-shell lithium nickel cobalt manganese oxide is 2.7-3.2 μm, and the distance from the core surface to the shell surface is 1.5-2.5 μm. And / or, the boron-containing compound is selected from at least one of boron trioxide, boron trichloride, boron trifluoride, boric acid, and sodium metaborate; And / or, based on the total weight of the modified lithium nickel cobalt manganese oxide, the boron content in the boron-containing compound is 2-12 wt%.

3. The battery cell according to claim 1 or 2, characterized in that, The modified lithium nickel cobalt manganese oxide D 50 The particle size is 8-12 μm, and the specific surface area is 0.5-2 m². 2 / g.

4. The battery cell according to claim 1 or 2, characterized in that, The D of the nano-silicon-based anode material 50 The particle size is 7-13 μm, and the specific surface area is 5-9 m². 2 / g; And / or, in the nano-silicon-based anode material, the specific capacity of the graphite is 300-360 mA·h / g; And / or, the graphite is selected from at least one of artificial graphite, natural graphite and mesophase carbon microspheres; And / or, in the nano-silicon-based anode material, the specific capacity of the silicon-oxygen material is 1250-1750 mA·h / g; And / or, the average particle size of the nano-silicon crystal particles is 2-10 nm, and the average particle size of the SiO2 matrix is ​​3-8 μm; And / or, based on the total weight of the silicon-oxygen material, the content of the nano-silicon crystal particles is 26-28 wt%, and the content of the SiO2 matrix is ​​69-73 wt%; And / or, the carbon coating layer is amorphous carbon; And / or, the thickness of the carbon coating layer is 8-13 nm.

5. The battery cell according to claim 3, characterized in that, The D of the nano-silicon-based anode material 50 The particle size is 7-13 μm, and the specific surface area is 5-9 m². 2 / g; And / or, in the nano-silicon-based anode material, the specific capacity of the graphite is 300-360 mA·h / g; And / or, the graphite is selected from at least one of artificial graphite, natural graphite and mesophase carbon microspheres; And / or, in the nano-silicon-based anode material, the specific capacity of the silicon-oxygen material is 1250-1750 mA·h / g; And / or, the average particle size of the nano-silicon crystal particles is 2-10 nm, and the average particle size of the SiO2 matrix is ​​3-8 μm; And / or, based on the total weight of the silicon-oxygen material, the content of the nano-silicon crystal particles is 26-28 wt%, and the content of the SiO2 matrix is ​​69-73 wt%; And / or, the carbon coating layer is amorphous carbon; And / or, the thickness of the carbon coating layer is 8-13 nm.

6. The battery cell according to any one of claims 1, 2, and 5, characterized in that, The positive electrode sheet is obtained by coating the positive electrode slurry including the modified lithium nickel cobalt manganese oxide onto the surface of the positive electrode current collector, and the negative electrode sheet is obtained by coating the negative electrode slurry including the nano-silicon-based negative electrode material onto the surface of the negative electrode current collector.

7. The battery cell according to claim 6, characterized in that, The positive electrode slurry and the negative electrode slurry each independently include a conductive agent, a binder, and a solvent; And / or, based on the total weight of the modified nickel cobalt manganese oxide, the conductive agent, and the binder, the content of the modified nickel cobalt manganese oxide in the positive electrode slurry is 92-98 wt%, the content of the conductive agent is 1-4 wt%, and the content of the binder is 1-4 wt%. And / or, based on the total weight of the nano-silicon-based anode material, the conductive agent, and the binder, the content of the nano-silicon-based anode material in the anode slurry is 92-98 wt%, the content of the conductive agent is 1-4 wt%, and the content of the binder is 1-4 wt%. And / or, the viscosity of the positive electrode slurry at 25-35℃ is 3500-5500 mPa·s; the viscosity of the negative electrode slurry at 25-35℃ is 3000-5000 mPa·s; And / or, the average thickness of the positive electrode sheet is 82-95 μm, and the compaction density is 3.2-3.6 g / cm³. 3 The negative electrode sheet has an average thickness of 100-110 μm and a compaction density of 1.35-1.65 g / cm³. 3 .

8. The battery cell according to claim 3, characterized in that, The positive electrode sheet is obtained by coating the positive electrode slurry including the modified lithium nickel cobalt manganese oxide onto the surface of the positive electrode current collector, and the negative electrode sheet is obtained by coating the negative electrode slurry including the nano-silicon-based negative electrode material onto the surface of the negative electrode current collector.

9. The battery cell according to claim 8, characterized in that, The positive electrode slurry and the negative electrode slurry each independently include a conductive agent, a binder, and a solvent; And / or, based on the total weight of the modified nickel cobalt manganese oxide, the conductive agent, and the binder, the content of the modified nickel cobalt manganese oxide in the positive electrode slurry is 92-98 wt%, the content of the conductive agent is 1-4 wt%, and the content of the binder is 1-4 wt%. And / or, based on the total weight of the nano-silicon-based anode material, the conductive agent, and the binder, the content of the nano-silicon-based anode material in the anode slurry is 92-98 wt%, the content of the conductive agent is 1-4 wt%, and the content of the binder is 1-4 wt%. And / or, the viscosity of the positive electrode slurry at 25-35℃ is 3500-5500 mPa·s; the viscosity of the negative electrode slurry at 25-35℃ is 3000-5000 mPa·s; And / or, the average thickness of the positive electrode sheet is 82-95 μm, and the compaction density is 3.2-3.6 g / cm³. 3 The negative electrode sheet has an average thickness of 100-110 μm and a compaction density of 1.35-1.65 g / cm³. 3 .

10. The battery cell according to claim 4, characterized in that, The positive electrode sheet is obtained by coating the positive electrode slurry including the modified lithium nickel cobalt manganese oxide onto the surface of the positive electrode current collector, and the negative electrode sheet is obtained by coating the negative electrode slurry including the nano-silicon-based negative electrode material onto the surface of the negative electrode current collector.

11. The battery cell according to claim 10, characterized in that, The positive electrode slurry and the negative electrode slurry each independently include a conductive agent, a binder, and a solvent; And / or, based on the total weight of the modified nickel cobalt manganese oxide, the conductive agent, and the binder, the content of the modified nickel cobalt manganese oxide in the positive electrode slurry is 92-98 wt%, the content of the conductive agent is 1-4 wt%, and the content of the binder is 1-4 wt%. And / or, based on the total weight of the nano-silicon-based anode material, the conductive agent, and the binder, the content of the nano-silicon-based anode material in the anode slurry is 92-98 wt%, the content of the conductive agent is 1-4 wt%, and the content of the binder is 1-4 wt%. And / or, the viscosity of the positive electrode slurry at 25-35℃ is 3500-5500 mPa·s; the viscosity of the negative electrode slurry at 25-35℃ is 3000-5000 mPa·s; And / or, the average thickness of the positive electrode sheet is 82-95 μm, and the compaction density is 3.2-3.6 g / cm³. 3 The negative electrode sheet has an average thickness of 100-110 μm and a compaction density of 1.35-1.65 g / cm³. 3 .

12. The battery cell according to any one of claims 1, 2, 5, 7-11, characterized in that, The diaphragm is a PE / PP / PE diaphragm with polyvinylidene fluoride covering its surface; And / or, the total thickness of the diaphragm is 12-20 μm.

13. The battery cell according to claim 12, characterized in that, The thickness of the polyvinylidene fluoride coating on the surface of the PE / PP / PE diaphragm is 1-4 μm; And / or, the weight-average molecular weight of the polyvinylidene fluoride is 500,000 to 1,000,000 g / mol.

14. The battery cell according to claim 3, characterized in that, The diaphragm is a PE / PP / PE diaphragm with polyvinylidene fluoride covering its surface; And / or, the total thickness of the diaphragm is 12-20 μm.

15. The battery cell according to claim 14, characterized in that, The thickness of the polyvinylidene fluoride coating on the surface of the PE / PP / PE diaphragm is 1-4 μm; And / or, the weight-average molecular weight of the polyvinylidene fluoride is 500,000 to 1,000,000 g / mol.

16. The battery cell according to claim 4, characterized in that, The diaphragm is a PE / PP / PE diaphragm with polyvinylidene fluoride covering its surface; And / or, the total thickness of the diaphragm is 12-20 μm.

17. The battery cell according to claim 16, characterized in that, The thickness of the polyvinylidene fluoride coating on the surface of the PE / PP / PE diaphragm is 1-4 μm; And / or, the weight-average molecular weight of the polyvinylidene fluoride is 500,000 to 1,000,000 g / mol.

18. The battery cell according to claim 6, characterized in that, The diaphragm is a PE / PP / PE diaphragm with polyvinylidene fluoride covering its surface; And / or, the total thickness of the diaphragm is 12-20 μm.

19. The battery cell according to claim 18, characterized in that, The thickness of the polyvinylidene fluoride coating on the surface of the PE / PP / PE diaphragm is 1-4 μm; And / or, the weight-average molecular weight of the polyvinylidene fluoride is 500,000 to 1,000,000 g / mol.

20. The battery cell according to any one of claims 1, 2, 5, 7-11, 13-19, characterized in that, The electrolyte also includes an organic solvent.

21. The battery cell according to claim 20, characterized in that, Based on the total weight of the electrolyte, the content of the organic solvent in the electrolyte is 75-85 wt%. And / or, the organic solvent is selected from at least one of ethylene carbonate, propylene carbonate, methyl ethyl carbonate, diethyl carbonate, ethyl propionate, and propyl propionate; And / or, based on the total weight of the electrolyte, the content of the lithium salt in the electrolyte is 12-15 wt%.

22. The battery cell according to claim 21, characterized in that, The organic solvent is a mixture of ethylene carbonate, propylene carbonate, methyl ethyl carbonate and diethyl carbonate.

23. The battery cell according to claim 3, characterized in that, The electrolyte also includes an organic solvent.

24. The battery cell according to claim 23, characterized in that, Based on the total weight of the electrolyte, the content of the organic solvent in the electrolyte is 75-85 wt%. And / or, the organic solvent is selected from at least one of ethylene carbonate, propylene carbonate, methyl ethyl carbonate, diethyl carbonate, ethyl propionate, and propyl propionate; And / or, based on the total weight of the electrolyte, the content of the lithium salt in the electrolyte is 12-15 wt%.

25. The battery cell according to claim 24, characterized in that, The organic solvent is a mixture of ethylene carbonate, propylene carbonate, methyl ethyl carbonate and diethyl carbonate.

26. The battery cell according to claim 4, characterized in that, The electrolyte also includes an organic solvent.

27. The battery cell according to claim 26, characterized in that, Based on the total weight of the electrolyte, the content of the organic solvent in the electrolyte is 75-85 wt%. And / or, the organic solvent is selected from at least one of ethylene carbonate, propylene carbonate, methyl ethyl carbonate, diethyl carbonate, ethyl propionate, and propyl propionate; And / or, based on the total weight of the electrolyte, the content of the lithium salt in the electrolyte is 12-15 wt%.

28. The battery cell according to claim 27, characterized in that, The organic solvent is a mixture of ethylene carbonate, propylene carbonate, methyl ethyl carbonate and diethyl carbonate.

29. The battery cell according to claim 6, characterized in that, The electrolyte also includes an organic solvent.

30. The battery cell according to claim 29, characterized in that, Based on the total weight of the electrolyte, the content of the organic solvent in the electrolyte is 75-85 wt%. And / or, the organic solvent is selected from at least one of ethylene carbonate, propylene carbonate, methyl ethyl carbonate, diethyl carbonate, ethyl propionate, and propyl propionate; And / or, based on the total weight of the electrolyte, the content of the lithium salt in the electrolyte is 12-15 wt%.

31. The battery cell according to claim 30, characterized in that, The organic solvent is a mixture of ethylene carbonate, propylene carbonate, methyl ethyl carbonate and diethyl carbonate.

32. The battery cell according to claim 12, characterized in that, The electrolyte also includes an organic solvent.

33. The battery cell according to claim 32, characterized in that, Based on the total weight of the electrolyte, the content of the organic solvent in the electrolyte is 75-85 wt%. And / or, the organic solvent is selected from at least one of ethylene carbonate, propylene carbonate, methyl ethyl carbonate, diethyl carbonate, ethyl propionate, and propyl propionate; And / or, based on the total weight of the electrolyte, the content of the lithium salt in the electrolyte is 12-15 wt%.

34. The battery cell according to claim 33, characterized in that, The organic solvent is a mixture of ethylene carbonate, propylene carbonate, methyl ethyl carbonate and diethyl carbonate.

35. A method for preparing a battery cell according to any one of claims 1-34, characterized in that, The preparation method includes: winding a positive electrode sheet, a negative electrode sheet, and a separator into an electrode assembly, and injecting an electrolyte to obtain the battery cell; wherein, The positive electrode comprises modified lithium nickel cobalt manganese oxide, which includes core-shell lithium nickel cobalt manganese oxide and a boron-containing compound coated on the surface of the core-shell lithium nickel cobalt manganese oxide; wherein, the content of nickel in the core-shell lithium nickel cobalt manganese oxide gradually decreases from the inside to the outside, the content of cobalt is uniformly distributed from the inside to the outside, and the content of manganese gradually increases from the inside to the outside. The negative electrode sheet comprises a nano-silicon-based negative electrode material, which is a mixture of silicon-oxygen material and graphite; wherein, based on the total weight of the nano-silicon-based negative electrode material, the graphite content is 82-88 wt%; the silicon-oxygen material comprises silicon suboxide and a carbon coating layer on the surface of the silicon suboxide, and the silicon suboxide comprises a SiO2 matrix and nano-silicon crystal particles uniformly distributed in the SiO2 matrix.

36. The preparation method according to claim 35, characterized in that, The method for preparing the positive electrode sheet includes: mixing the modified lithium nickel cobalt manganese oxide, conductive agent, binder and solvent to obtain a positive electrode slurry, then coating the positive electrode slurry onto the surface of the positive electrode current collector, and then rolling and slitting it to obtain the positive electrode sheet; And / or, the method for preparing the negative electrode sheet includes: mixing the nano-silicon-based negative electrode material, conductive agent, binder and solvent to obtain a negative electrode slurry, then coating the negative electrode slurry onto the surface of the negative electrode current collector, and then rolling and slitting it to obtain the negative electrode sheet.

37. A battery cell prepared by the method of claim 35 or 36.

38. A soft-pack lithium-ion battery, characterized in that, The battery comprises the battery cell according to any one of claims 1-34, 37; And / or, the battery has a 1C discharge capacity of ≥60% at -40°C and a capacity retention rate of ≥90% after 500 cycles at room temperature.

39. An application of the pouch lithium-ion battery of claim 38 as a low-temperature mobile power source.

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