Electrochemical devices and electronic devices

Abstract

The present application relates to an electrochemical device and an electronic device. Specifically, the present application provides an electrochemical device comprising a positive electrode and an electrolyte, wherein the positive electrode comprises: a positive electrode current collector; and a positive electrode active material layer, wherein the positive electrode active material layer is located on at least one surface of the positive electrode current collector, wherein the positive electrode active material layer comprises a first positive electrode active material layer and a second positive electrode active material layer, wherein the first positive electrode active material layer comprises a first positive electrode active material and is located between the positive electrode current collector and the second positive electrode active material layer, and wherein the second positive electrode active material layer comprises a second positive electrode active material and a binder, wherein the density of the binder is ag / cm 3 , 0.6≤a≤1.5. The above cathode design can significantly improve the safety performance of electrochemical devices under high temperature and high pressure.

Description

Technical Field

[0001] The present application relates to the field of energy storage, and in particular to an electrochemical device and an electronic device, in particular a lithium-ion battery. Background Art

[0002] In recent years, with the rapid development of electronic products such as smart phones, tablets and smart wearables, consumers have increasingly higher requirements for the energy density of electrochemical devices (such as lithium-ion batteries) considering the different usage time and working environments of electronic products. At present, the energy density of lithium-ion batteries is mainly improved by using high-voltage (4.4V and above) lithium cobalt oxide positive electrode active materials and high-capacity, high-density graphite negative electrode materials. However, with the increase of temperature and voltage, the cycle performance and safety performance of such lithium-ion batteries will deteriorate significantly. At the same time, with the intensification of harsh environments such as global warming (such as in special use areas such as India and Africa), higher requirements are placed on the high-temperature performance of batteries.

[0003] In view of the above, there is a real need to provide electrochemical devices and electronic devices with improved high temperature performance. Summary of the Invention

[0004] The present application provides an electrochemical device with improved high-temperature performance by at least improving the positive electrode of the electrochemical device, so as to solve the problems existing in the prior art to some extent.

[0005] According to one aspect of the present application, the present application provides an electrochemical device comprising a positive electrode and an electrolyte, wherein the positive electrode comprises: a positive electrode current collector; and a positive electrode active material layer, wherein the positive electrode active material layer is located on at least one surface of the positive electrode current collector, wherein the positive electrode active material layer comprises a first positive electrode active material layer and a second positive electrode active material layer, wherein the first positive electrode active material layer comprises a first positive electrode active material and is located between the positive electrode current collector and the second positive electrode active material layer, and the second positive electrode active material layer comprises a second positive electrode active material and a binder, wherein the density of the binder is ag / cm 3 , 0.6≤a≤1.5.

[0006] According to an embodiment of the present application, the average particle size of the second positive electrode active material is b μm, and 5≤b≤20.

[0007] According to an embodiment of the present application, 4≤b / a≤25.

[0008] According to an embodiment of the present application, the thickness of the positive electrode active material layer is H, and the thickness of the second positive electrode active material layer is H2, where h=H2 / H, 0.6≤h≤0.98.

[0009] According to an embodiment of the present application, 0.45≤h / a≤1.2.

[0010] According to an embodiment of the present application, the electrolyte includes propyl propionate, wherein the content of the propyl propionate is x% based on the total weight of the electrolyte and 10≤x≤65.

[0011] According to an embodiment of the present application, 7≤x / a≤90.

[0012] According to an embodiment of the present application, the electrolyte includes a phosphorus-containing compound, which includes at least one of the following: lithium difluorophosphate or difluorophosphite, wherein the content of the phosphorus-containing compound is y% based on the total weight of the electrolyte, and 0.01≤y≤3.

[0013] According to an embodiment of the present application, 10≤x / y≤400.

[0014] According to an embodiment of the present application, the difluorophosphite includes at least one of the compound represented by the following formula 1 or the compound represented by the following formula 2:

[0015] A-OPF2 (Formula 1);

[0016] A-(OPF2)2 (Formula 2),

[0017] wherein A is one of a substituted or unsubstituted C1-C10 alkyl group, a C2-C10 alkenyl group, a C2-C10 oxyalkyl group, or a C2-C10 oxyalkenyl group;

[0018] When substituted, the substituent is at least one of halogen or cyano.

[0019] According to an embodiment of the present application, the difluorophosphite includes at least one of the following compounds:

[0020]

[0021]

[0022] According to an embodiment of the present application, the electrolyte includes at least one of fluoroethylene carbonate, 1,3-propane sultone, vinyl sulfate, vinyl ethylene carbonate, phosphoric acid cyclic anhydride, ethylene glycol bis(2-cyanoethyl) ether, 1,2,3-tris(2-cyanoethoxy)propane, 1,2,4-tris(2-cyanoethoxy)butane, 1,1,1-tris(cyanoethoxymethylene)ethane, 1,1,1-tris(cyanoethoxymethylene)propane, 3-methyl-1,3,5-tris(cyanoethoxy)pentane, 1,2,7-tris(cyanoethoxy)heptane, 1,2,6-tris(cyanoethoxy)hexane or 1,2,5-tris(cyanoethoxy)pentane.

[0023] According to another aspect of the present application, the present application provides an electronic device, which includes the electrochemical device described in the above embodiment.

[0024] The present application improves the stability of the interior and multi-layer interface of the positive electrode active material layer by adopting a multi-layer positive electrode active material layer design in the positive electrode and using a low-density binder at least in the positive electrode active material layer away from the positive electrode current collector, thereby improving the safety performance of the electrochemical device under high temperature and high pressure.

[0025] Additional aspects and advantages of the embodiments of the present application will be described, shown, or explained in part in the following description through implementation of the embodiments of the present application. DETAILED DESCRIPTION

[0026] The embodiments of the present application will be described in detail below. The embodiments of the present application should not be interpreted as limiting the present application.

[0027] Unless expressly stated otherwise, the following terms used herein have the meanings indicated below.

[0028] In the detailed description and claims, a list of items linked by the term "at least one of" can mean any combination of the listed items. For example, if items A and B are listed, the phrase "at least one of A and B" means only A; only B; or A and B. In another example, if items A, B, and C are listed, the phrase "at least one of A, B, and C" means only A; or only B; only C; A and B (excluding C); A and C (excluding B); B and C (excluding A); or all of A, B, and C. Item A can contain a single element or multiple elements. Item B can contain a single element or multiple elements. Item C can contain a single element or multiple elements. The term "at least one of" has the same meaning as the term "at least one of."

[0029] The terms "including," "containing," and "comprising" are used in their open, non-limiting sense.

[0030] The term "alkyl" encompasses both straight and branched chain alkyl groups. For example, the alkyl group can be a C1-C50 alkyl, a C1-C40 alkyl, a C1-C30 alkyl, a C1-C20 alkyl, a C1-C12 alkyl, a C1-C10 alkyl, a C1-C6 alkyl, a C2-C6 alkyl, or a C2-C5 alkyl. In some embodiments, the alkyl group includes a methyl group, an ethyl group, a propyl group, an isopropyl group, a butyl group, an isobutyl group, a tert-butyl group, a pentyl group, a hexyl group, a heptyl group, an octyl group, or the like. In addition, the alkyl group can be optionally substituted.

[0031] The term "alkenyl" encompasses straight-chain and branched alkenyl groups. For example, the alkenyl group can be a C2-C50 alkenyl, a C2-C40 alkenyl, a C2-C30 alkenyl, a C2-C20 alkenyl, a C2-C12 alkenyl, a C2-C10 alkenyl, or a C2-C6 alkenyl. In addition, the alkenyl group can be optionally substituted.

[0032] The term "oxyalkyl" refers to an organic group having -OR, where R is a linear or branched alkyl group. For example, the alkyl group can be a C1-C50 alkyl group, a C1-C40 alkyl group, a C1-C30 alkyl group, a C1-C20 alkyl group, a C1-C12 alkyl group, a C1-C10 alkyl group, a C1-C6 alkyl group, a C2-C6 alkyl group, or a C2-C5 alkyl group. In addition, the alkyl group can be optionally substituted.

[0033] The term "oxyalkenyl" is an organic group having -OQ, wherein Q is a straight or branched alkenyl. For example, the alkenyl group can be a C2-C50 alkenyl, a C2-C40 alkenyl, a C2-C30 alkenyl, a C2-C20 alkenyl, a C2-C12 alkenyl, a C2-C10 alkenyl, or a C2-C6 alkenyl. In addition, the alkenyl group can be optionally substituted.

[0034] When the above groups are substituted, the substituents may be independently selected from cyano, halogen, silane, siloxane, amino, ether, ester, carboxyl, sulfonic acid, mercapto, or combinations thereof.

[0035] I. Positive electrode

[0036] Common methods for improving the energy density of electrochemical devices (e.g., lithium-ion batteries) include using high-voltage (4.4V and above) lithium cobalt oxide positive electrode active materials and high-capacity, high-density graphite negative electrode materials. However, with the increase of temperature and voltage, the structural stability of lithium cobalt oxide deteriorates, and metal ions are easily dissolved from the positive electrode and reduced and deposited on the negative electrode surface, destroying the structure of the negative electrode solid electrolyte interface (SEI) film, resulting in a continuous increase in negative electrode impedance and battery thickness, thereby leading to capacity loss and cycle performance degradation of the electrochemical device. In addition, under high temperature and high pressure, the electrolyte is easily oxidized and decomposed on the positive electrode surface to produce a large amount of gas, causing the electrochemical device to swell and destroy the electrode interface, thereby deteriorating the electrochemical performance of the electrochemical device. At the same time, under high temperature and high voltage, due to the high oxidation activity of lithium cobalt oxide, the side reactions between it and the electrolyte are intensified, causing the decomposition products of the electrolyte to continuously deposit on the positive electrode surface, which will further increase the internal resistance of the electrochemical device and reduce the high-temperature cycle capacity retention rate and the residual capacity of the high-temperature storage battery cell. The above factors will cause great safety hazards to electrochemical devices under high temperature and high pressure.

[0037] In at least one aspect, the present application overcomes the above-mentioned defects of the prior art by providing multiple positive electrode active material layers in the positive electrode and using a low-density binder at least in the positive electrode active material layer away from the positive electrode current collector.

[0038] Specifically, the positive electrode described in the present application includes a positive electrode current collector and a positive electrode active material layer located on at least one surface of the positive electrode current collector, wherein the positive electrode active material layer contains a positive electrode active material. The main feature of the positive electrode described in the present application is that the positive electrode active material layer includes a first positive electrode active material layer and a second positive electrode active material layer, the first positive electrode active material layer includes a first positive electrode active material and is located between the positive electrode current collector and the second positive electrode active material layer, and the second positive electrode active material layer includes a second positive electrode active material and a binder, and the density of the binder is ag / cm 3 , 0.6≤a≤1.5.

[0039] In some embodiments, the first positive electrode active material and the second positive electrode active material may be the same or different.

[0040] On the one hand, the present invention provides a plurality of positive electrode active material layers in the positive electrode to achieve improved electrical performance. In addition, the binder used in at least the positive electrode active material layer on the side away from the current collector (i.e., the second positive electrode active material layer) is thicker than the conventional binder (density is generally greater than 1.5 g / cm 3 ) has a lower density. When the density of the positive electrode binder is greater than 1.5g / cm 3 When the density of the positive electrode binder is less than 0.6 g / cm 3 When the density of the positive electrode binder is controlled at 0.6 g / cm 3 Up to 1.5g / cm 3Within the range of , not only good adhesion can be achieved, but also the flexibility of the positive electrode can be enhanced, and the risk of breakage during the winding process can be reduced. At the same time, the low-density binder used in the present application is easy to form a cavity structure with the surrounding active materials, in which the electrolyte can be accommodated. This structure improves the wettability of the electrolyte and the positive electrode active material to a certain extent, and also effectively reduces the side reactions caused by the interaction between the electrolyte and the active material. In addition, during the charge and discharge cycle under high temperature and high pressure, the low-density binder is relatively stable and is not easy to react with the components in the electrolyte. The low-density binder can also be coated on the surface of the positive electrode active material particles to improve the stability of the interface of the positive electrode active material particles. The combination of an insulating layer containing aluminum elements and a positive electrode active material layer containing a low-density binder helps to improve the high-temperature performance of the electrochemical device, especially in that it can effectively reduce the thermal abuse thickness expansion rate and high-temperature short-circuit deformation rate of the electrochemical device.

[0041] In some embodiments, 0.8≤a≤1.2. In some embodiments, a may be 0.6, 0.7, 0.8, 0.9, 1.0, 1.1, 1.2, 1.3, 1.4, 1.5, or within a range consisting of any two of the foregoing values.

[0042] In some embodiments, the binder includes polyvinylidene fluoride (PVDF).

[0043] In some embodiments, further applying the low-density binder in the positive electrode active material layer close to the current collector (ie, the first positive electrode active material layer) can further improve the performance of the electrochemical device, especially the high-temperature performance.

[0044] In some embodiments, the average particle size of the second positive electrode active material is b μm, where 5 ≤ b ≤ 20. In some embodiments, 10 ≤ b ≤ 18. In some embodiments, b is 5, 8, 10, 12, 14, 15, 16, 17, 18, 20, or within a range consisting of any two of the foregoing values.

[0045] On the one hand, when the average particle size of the positive electrode active material is within the above range, a positive electrode active material with a high tap density can be obtained, thereby improving the electrochemical performance of the electrochemical device (for example, improving its energy density). On the other hand, during the preparation process of the positive electrode (that is, the positive electrode active material, the conductive material and the binder are uniformly mixed with the solvent to form a slurry and coated on the current collector), it can prevent the generation of streaks. In addition, the use of two or more positive electrode active materials with different average particle sizes in the positive electrode can further optimize the above effects.

[0046] When the positive electrode active material is in the form of primary particles, the average particle size of the positive electrode active material refers to the primary particle size of the positive electrode active material particles. When the primary particles of the positive electrode active material particles aggregate to form secondary particles, the average particle size of the positive electrode active material refers to the secondary particle size of the positive electrode active material particles.

[0047] The average particle size of the positive electrode active material can be measured using a laser diffraction / scattering particle size distribution analyzer. In some embodiments, the LA-920 manufactured by HORIBA can be used as a particle size distribution analyzer for measurement, wherein a 0.1% sodium hexametaphosphate aqueous solution is used as the dispersion medium during the measurement, and after 5 minutes of ultrasonic dispersion, the refractive index is set to 1.24 for measurement. In some embodiments, the average particle size of the positive electrode active material can also be measured by a laser diffraction particle size analyzer (Shimadzu SALD-2300) and a scanning electron microscope (ZEISS EVO18, sampling number not less than 100).

[0048] In some embodiments, 4≤b / a≤25. In some embodiments, 6≤b / a≤20. In some embodiments, 10≤b / a≤15. In some embodiments, b / a is 4, 5, 6, 7, 8, 9, 10, 11, 12, 13, 14, 15, 16, 17, 18, 19, 20, 21, 22, 23, 24, 25, or is within a range consisting of any two of the above values. When b / a satisfies the above relationship, on the one hand, it can promote the dissolution and uniform distribution of the binder in the positive electrode slurry, improve the adhesion of the positive electrode active material layer, and reduce the risk of the positive electrode active material layer falling off the current collector; on the other hand, it can reduce the agglomeration of the positive electrode active material particles, thereby reducing or avoiding scratches during coating. Based on at least the above two factors, when b / a satisfies the above relationship, the safety performance of the electrochemical device under high temperature and high pressure can be further improved.

[0049] In some embodiments, the thickness of the positive electrode active material layer is H, and the thickness of the second positive electrode active material layer is H2, wherein h=H2 / H, 0.6≤h≤0.98. In some embodiments, h is 0.6, 0.65, 0.7, 0.75, 0.8, 0.85, 0.9, 0.98 or is within the range consisting of any two of the above values. The first positive electrode active material layer can play a role in protecting the current collector. When the electrode is cold pressed, the presence of the first positive electrode active material layer can play a buffering role, reducing the probability of breakage of the positive electrode active material particles, thereby reducing the occurrence of side reactions. When h is within the above range, it can not only enhance the compaction density of the electrode and reduce the risk of brittle fracture of the electrode during the winding process, but also give full play to the function of the first positive electrode active material layer, further improving the safety performance of the electrochemical device under high temperature and high pressure.

[0050] In some embodiments, 0.45 ≤ h / a ≤ 1.2. In some embodiments, 0.5 ≤ h / a ≤ 1.2. In some embodiments, h / a is 0.45, 0.5, 0.55, 0.6, 0.65, 0.7, 0.8, 0.9, 1.0, 1.1, 1.2, or within a range consisting of any two of the foregoing values. When h / a satisfies the above relationship, it can further ensure that the first positive electrode active material layer plays an adequate buffering role and further promote the dissolution and uniform distribution of the binder in the positive electrode active material slurry, thereby further improving the safety performance of the electrochemical device under high temperature and high pressure.

[0051] The type of positive electrode active material is not particularly limited, as long as it can electrochemically absorb and release metal ions (e.g., lithium ions). In some embodiments, the positive electrode active material is a material containing lithium and at least one transition metal. Examples of positive electrode active materials may include, but are not limited to, lithium transition metal composite oxides and lithium-containing transition metal phosphate compounds.

[0052] In some embodiments, the transition metal in the lithium transition metal composite oxide includes V, Ti, Cr, Mn, Fe, Co, Ni, Cu, etc. In some embodiments, the lithium transition metal composite oxide includes lithium cobalt composite oxides such as LiCoO2, lithium nickel composite oxides such as LiNiO2, lithium manganese composite oxides such as LiMnO2, LiMn2O4, Li2MnO4, etc., LiNi 1 / 3 Mn 1 / 3 Co 1 / 3 O2、LiNi 0.5 Mn 0.3 Co 0.2 O2 and other lithium nickel manganese cobalt composite oxides, wherein a portion of the transition metal atoms serving as the main body of these lithium transition metal composite oxides are replaced by other elements such as Na, K, B, F, Al, Ti, V, Cr, Mn, Fe, Co, Li, Ni, Cu, Zn, Mg, Ga, Zr, Si, Nb, Mo, Sn, W, etc. Examples of lithium transition metal composite oxides may include, but are not limited to, LiNi 0.5 Mn 0.5 O2、LiNi 0.85 Co 0.10 Al 0.05 O2、LiNi 0.33 Co 0.33 Mn 0.33 O2、LiNi 0.45 Co 0.10 Al 0.45 O2、LiMn 1.8 Al 0.2 O4 and LiMn 1.5 Ni 0.5O4, etc. Examples of combinations of lithium transition metal composite oxides include, but are not limited to, combinations of LiCoO2 and LiMn2O4, wherein a portion of the Mn in LiMn2O4 can be replaced by a transition metal (e.g., LiNi 0.33 Co 0.33 Mn 0.33 O2), part of the Co in LiCoO2 can be replaced by transition metals.

[0053] In some embodiments, the transition metal in the lithium-containing transition metal phosphate compound includes V, Ti, Cr, Mn, Fe, Co, Ni, Cu, etc. In some embodiments, the lithium-containing transition metal phosphate compound includes iron phosphates such as LiFePO4, Li3Fe2(PO4)3, and LiFeP2O7, and cobalt phosphates such as LiCoPO4, wherein a portion of the transition metal atoms serving as the main body of these lithium transition metal phosphate compounds are replaced by other elements such as Al, Ti, V, Cr, Mn, Fe, Co, Li, Ni, Cu, Zn, Mg, Ga, Zr, Nb, and Si.

[0054] The surface of the positive electrode active material may be adhered to a substance having a different composition from the positive electrode active material. Examples of surface-attached substances may include, but are not limited to: oxides such as aluminum oxide, silicon dioxide, titanium dioxide, zirconium oxide, magnesium oxide, calcium oxide, boron oxide, antimony oxide, and bismuth oxide; sulfates such as lithium sulfate, sodium sulfate, potassium sulfate, magnesium sulfate, calcium sulfate, and aluminum sulfate; carbonates such as lithium carbonate, calcium carbonate, and magnesium carbonate; carbon, etc. By attaching substances to the surface of the positive electrode active material, the oxidation reaction of the electrolyte on the surface of the positive electrode active material can be suppressed, and the life of the electrochemical device can be improved. When the amount of surface-attached substances is too small, the effect cannot be fully exerted; when the amount of surface-attached substances is too much, the entry and exit of lithium ions will be hindered, and the resistance may sometimes increase. In this application, the positive electrode active material having a substance having a different composition attached to the surface of the positive electrode active material is also referred to as a "positive electrode active material."

[0055] In some embodiments, the “positive electrode active material” is preferably lithium cobalt oxide or lithium nickel cobalt manganese oxide.

[0056] In some embodiments, the shape of the positive electrode active material particles includes, but is not limited to, block, polyhedron, sphere, ellipsoid, plate, needle, and column. In some embodiments, the positive electrode active material particles include primary particles, secondary particles, or a combination thereof. In some embodiments, the primary particles may aggregate to form secondary particles.

[0057] There are no restrictions on the type of positive electrode conductive material; any known conductive material may be used. Examples of positive electrode conductive materials include, but are not limited to, graphites such as natural graphite and artificial graphite; carbon blacks such as acetylene black; carbon materials such as amorphous carbon such as needle coke; carbon nanotubes; and graphene. These positive electrode conductive materials may be used alone or in any combination.

[0058] There is no limitation on the type of solvent used to form the positive electrode slurry, as long as it is a solvent that can dissolve or disperse the positive electrode active material, conductive material, positive electrode binder, and thickener used as needed. Examples of solvents used to form the positive electrode slurry may include any of aqueous solvents and organic solvents. Examples of aqueous media may include, but are not limited to, water and mixed media of alcohol and water. Examples of organic media may include, but are not limited to, aliphatic hydrocarbons such as hexane; aromatic hydrocarbons such as benzene, toluene, xylene, and methylnaphthalene; heterocyclic compounds such as quinoline and pyridine; ketones such as acetone, methyl ethyl ketone, and cyclohexanone; esters such as methyl acetate and methyl acrylate; amines such as diethylenetriamine and N,N-dimethylaminopropylamine; ethers such as diethyl ether, propylene oxide, and tetrahydrofuran (THF); amides such as N-methylpyrrolidone (NMP), dimethylformamide, and dimethylacetamide; aprotic polar solvents such as hexamethylphosphoramide and dimethyl sulfoxide, etc.

[0059] Thickeners are generally used to adjust the viscosity of the slurry. When using an aqueous medium, the slurry can be formed using a thickener and a styrene-butadiene rubber (SBR) emulsion. The type of thickener is not particularly limited, and examples thereof include, but are not limited to, carboxymethyl cellulose, methyl cellulose, hydroxymethyl cellulose, ethyl cellulose, polyvinyl alcohol, oxidized starch, phosphorylated starch, casein, and salts thereof. These thickeners may be used alone or in any combination.

[0060] The type of positive electrode current collector is not particularly limited and can be any known material suitable for use as a positive electrode current collector. Examples of positive electrode current collectors include, but are not limited to, metal materials such as aluminum, stainless steel, nickel plating, titanium, and tantalum; and carbon materials such as carbon cloth and carbon paper. In some embodiments, the positive electrode current collector is a metal material. In some embodiments, the positive electrode current collector is aluminum.

[0061] In order to reduce the electronic contact resistance between the positive electrode current collector and the positive electrode active material layer, the surface of the positive electrode current collector may include a conductive additive. Examples of conductive additives include, but are not limited to, carbon and precious metals such as gold, platinum, and silver.

[0062] The positive electrode can be made by forming a positive electrode active material layer containing a positive electrode active material and a binder on a current collector. The production of a positive electrode using a positive electrode active material can be carried out by conventional methods, that is, the positive electrode active material and the binder, as well as the conductive material and thickener as needed, are dry-mixed to form a sheet, and the resulting sheet is pressed onto the positive electrode current collector; or these materials are dissolved or dispersed in a liquid medium to form a slurry, and the slurry is applied to the positive electrode current collector and dried to form a positive electrode active material layer on the current collector, thereby obtaining a positive electrode.

[0063] In some embodiments, the mass fraction of the positive electrode active material is M% based on the total weight of the positive electrode active material layer, where 90 ≤ M ≤ 99.5. In some embodiments, 95 ≤ M ≤ 99. In some embodiments, M can be 90, 92, 94, 95, 96, 97, 98, or 99, or within a range consisting of any two of the foregoing values. When the mass fraction of the positive electrode active material in the positive electrode active material layer satisfies the above relationship, the energy density of the electrochemical device can be significantly improved.

[0064] II. Electrolyte

[0065] The electrochemical device of the present application further includes an electrolyte, wherein the electrolyte includes an electrolyte, a solvent for dissolving the electrolyte, and an additive.

[0066] In some embodiments, the electrolyte includes propyl propionate. Propionate can not only form a more uniform solid electrolyte interface film (SEI film) on the surface of the positive electrode active material particles, but also interact with the low-density binder in the positive electrode active material layer, thereby improving the safety of the electrochemical device under high temperature and high pressure.

[0067] In some embodiments, the content of propyl propionate is x%, based on the total weight of the electrolyte, where 10 ≤ x ≤ 65. In some embodiments, 15 ≤ x ≤ 60. In some embodiments, 20 ≤ x ≤ 50. In some embodiments, x is 10, 12, 15, 18, 20, 22, 25, 30, 35, 40, 45, 50, 55, 60, 65, or within a range consisting of any two of the foregoing values.

[0068] In some embodiments, 7≤x / a≤90. In some embodiments, 8≤x / a≤80. In some embodiments, 10≤x / a≤70. In some embodiments, 20≤x / a≤50. In some embodiments, x / a is 7, 10, 15, 20, 25, 30, 35, 40, 45, 50, 55, 60, 65, 70, 75, 80, 85, 90, or within a range consisting of any two of the above values. When x / a satisfies the above relationship, it can not only promote a more uniform distribution and dissolution of the binder in the positive electrode active material slurry, and ensure that a sufficient amount of propyl propionate forms a uniform SEI film on the positive electrode surface; it can also avoid the film-forming reaction between propyl propionate and excessive undissolved binder, thereby affecting the performance of the battery. Based on at least the above factors, when x / a satisfies the above relationship, the safety performance of the electrochemical device under high temperature and high pressure can be further improved.

[0069] In some embodiments, the electrolyte includes a phosphorus-containing compound, wherein the phosphorus-containing compound includes at least one of lithium difluorophosphate or difluorophosphite. The addition of the phosphorus-containing compound can increase the LiF content in the SEI film, thereby enhancing the stability of the SEI film and further reducing the occurrence of side reactions, thereby improving the structural stability and safety performance of the electrochemical device under high temperature and high pressure.

[0070] In some embodiments, the phosphorus-containing compound is present in an amount of y% based on the total weight of the electrolyte, wherein 0.01 ≤ y ≤ 3. In some embodiments, 0.05 ≤ y ≤ 2. In some embodiments, 0.1 ≤ y ≤ 1. In some embodiments, y is 0.01, 0.02, 0.03, 0.04, 0.05, 0.06, 0.07, 0.08, 0.09, 0.1, 0.12, 0.15, 0.2, 0.25, 0.3, 0.35, 0.4, 0.45, 0.5, 0.55, 0.6, 0.65, 0.7, 0.75, 0.8, 0.85, 0.9, 1.0, 1.2, 1.5, 1.8, 2.0, 2.2, 2.5, 2.8, 3.0, or is within a range consisting of any two of the foregoing values.

[0071] In some embodiments, 10≤x / y≤400. In some embodiments, 20≤x / y≤300. In some embodiments, 50≤x / y≤200. In some embodiments, 100≤x / y≤150. In some embodiments, x / y is 10, 20, 30, 50, 70, 100, 120, 150, 170, 200, 220, 250, 280, 300, 320, 350, 370, 400 or is within the range consisting of any two of the above values. By optimizing the content ratio of propyl propionate and phosphorus-containing compounds in the electrolyte, the performance of the electrochemical device, especially the safety performance under high temperature and high pressure, can be further improved. For example, when x / y is within the above range, the synergistic effect between propyl propionate and phosphorus-containing compounds can be better exerted, promoting the formation of a more effective SEI film on the surface of the positive active material, improving the stability of the surface of the positive active material, and optimizing the safety performance of the battery cell. At the same time, when x / y is within the above range, the viscosity of the electrolyte will be further optimized, promoting the infiltration of the electrolyte into the electrode, thereby promoting the electrochemical performance of the battery cell.

[0072] In some embodiments, the difluorophosphite includes at least one of the compound represented by Formula 1 below or the compound represented by Formula 2 below:

[0073] A-OPF2 (Formula 1);

[0074] A-(OPF2)2 (Formula 2),

[0075] wherein A is one of a substituted or unsubstituted C1-C10 alkyl group, a C2-C10 alkenyl group, a C2-C10 oxyalkyl group, or a C2-C10 oxyalkenyl group;

[0076] When substituted, the substituent is at least one of halogen or cyano.

[0077] In some embodiments, A is one of a substituted or unsubstituted C2-C8 alkyl, C2-C8 alkenyl, C3-C8 oxyalkyl, or C3-C8 oxyalkenyl group, wherein, when substituted, the substituent is at least one of a halogen or a cyano group.

[0078] In some embodiments, the difluorophosphite comprises at least one of the following compounds:

[0079]

[0080]

[0081] In some embodiments, the electrolyte includes at least one of fluoroethylene carbonate, 1,3-propane sultone, vinyl sulfate, vinyl ethylene carbonate, phosphoric acid cyclic anhydride, ethylene glycol bis(2-cyanoethyl) ether, 1,2,3-tris(2-cyanoethoxy)propane, 1,2,4-tris(2-cyanoethoxy)butane, 1,1,1-tris(cyanoethoxymethylene)ethane, 1,1,1-tris(cyanoethoxymethylene)propane, 3-methyl-1,3,5-tris(cyanoethoxy)pentane, 1,2,7-tris(cyanoethoxy)heptane, 1,2,6-tris(cyanoethoxy)hexane, or 1,2,5-tris(cyanoethoxy)pentane. When the electrolyte includes one or more of the above compounds, these compounds can form a structurally stable SEI composite film on the surface of the positive electrode, thereby improving the structural stability and safety performance of the electrochemical device under high temperature and high pressure.

[0082] In some embodiments, the electrolyte solution includes any non-aqueous solvent known in the art as a solvent for an electrolyte solution.

[0083] In some embodiments, the non-aqueous solvent includes, but is not limited to, one or more of the following: cyclic carbonates, chain carbonates, cyclic carboxylates, chain carboxylates, cyclic ethers, chain ethers, phosphorus-containing organic solvents, sulfur-containing organic solvents, and aromatic fluorine-containing solvents.

[0084] In some embodiments, examples of the cyclic carbonate may include, but are not limited to, one or more of the following: ethylene carbonate (EC), propylene carbonate (PC), and butylene carbonate. In some embodiments, the cyclic carbonate has 3-6 carbon atoms.

[0085] In some embodiments, examples of the chain carbonates may include, but are not limited to, one or more of the following: dimethyl carbonate, ethyl methyl carbonate, diethyl carbonate (DEC), methyl n-propyl carbonate, ethyl n-propyl carbonate, di-n-propyl carbonate, etc. Examples of fluorine-substituted chain carbonates may include, but are not limited to, one or more of the following: bis(fluoromethyl) carbonate, bis(difluoromethyl) carbonate, bis(trifluoromethyl) carbonate, bis(2-fluoroethyl) carbonate, bis(2,2-difluoroethyl) carbonate, bis(2,2,2-trifluoroethyl) carbonate, 2-fluoroethyl methyl carbonate, 2,2-difluoroethyl methyl carbonate, and 2,2,2-trifluoroethyl methyl carbonate.

[0086] In some embodiments, examples of the cyclic carboxylic acid ester may include, but are not limited to, one or more of the following: one or more of γ-butyrolactone and γ-valerolactone. In some embodiments, some hydrogen atoms of the cyclic carboxylic acid ester may be substituted with fluorine.

[0087] In some embodiments, examples of the chain carboxylic acid esters may include, but are not limited to, one or more of the following: methyl acetate, ethyl acetate, propyl acetate, isopropyl acetate, butyl acetate, sec-butyl acetate, isobutyl acetate, tert-butyl acetate, methyl propionate, ethyl propionate, propyl propionate, isopropyl propionate, methyl butyrate, ethyl butyrate, propyl butyrate, methyl isobutyrate, ethyl isobutyrate, methyl valerate, ethyl valerate, methyl pivalate, and ethyl pivalate. In some embodiments, some of the hydrogen atoms of the chain carboxylic acid esters may be substituted with fluorine. In some embodiments, examples of fluorine-substituted chain carboxylic acid esters may include, but are not limited to, methyl trifluoroacetate, ethyl trifluoroacetate, propyl trifluoroacetate, butyl trifluoroacetate, and 2,2,2-trifluoroethyl trifluoroacetate.

[0088] In some embodiments, examples of the cyclic ether may include, but are not limited to, one or more of the following: tetrahydrofuran, 2-methyltetrahydrofuran, 1,3-dioxolane, 2-methyl1,3-dioxolane, 4-methyl1,3-dioxolane, 1,3-dioxane, 1,4-dioxane, and dimethoxypropane.

[0089] In some embodiments, examples of the chain ether may include, but are not limited to, one or more of the following: dimethoxymethane, 1,1-dimethoxyethane, 1,2-dimethoxyethane, diethoxymethane, 1,1-diethoxyethane, 1,2-diethoxyethane, ethoxymethoxymethane, 1,1-ethoxymethoxyethane and 1,2-ethoxymethoxyethane, etc.

[0090] In some embodiments, examples of the phosphorus-containing organic solvent may include, but are not limited to, one or more of the following: trimethyl phosphate, triethyl phosphate, dimethyl ethyl phosphate, diethyl methyl phosphate, ethylidene methyl phosphate, ethylidene ethyl phosphate, triphenyl phosphate, trimethyl phosphite, triethyl phosphite, triphenyl phosphite, tris(2,2,2-trifluoroethyl) phosphate, and tris(2,2,3,3,3-pentafluoropropyl) phosphate, etc.

[0091] In some embodiments, examples of the sulfur-containing organic solvent may include, but are not limited to, one or more of the following: sulfolane, 2-methylsulfolane, 3-methylsulfolane, dimethyl sulfone, diethyl sulfone, ethyl methyl sulfone, methylpropyl sulfone, dimethyl sulfoxide, methyl methanesulfonate, ethyl methanesulfonate, methyl ethanesulfonate, ethyl ethanesulfonate, dimethyl sulfate, diethyl sulfate, and dibutyl sulfate. In some embodiments, some of the hydrogen atoms of the sulfur-containing organic solvent may be substituted with fluorine.

[0092] In some embodiments, the aromatic fluorine-containing solvent includes, but is not limited to, one or more of the following: fluorobenzene, difluorobenzene, trifluorobenzene, tetrafluorobenzene, pentafluorobenzene, hexafluorobenzene, and trifluoromethylbenzene.

[0093] In some embodiments, the solvent used in the electrolyte of the present application includes cyclic carbonates, chain carbonates, cyclic carboxylates, chain carboxylates, and combinations thereof. In some embodiments, the solvent used in the electrolyte of the present application includes an organic solvent selected from the group consisting of ethylene carbonate, propylene carbonate, diethyl carbonate, ethyl propionate, propyl propionate, n-propyl acetate, ethyl acetate, and combinations thereof. In some embodiments, the solvent used in the electrolyte of the present application includes ethylene carbonate, propylene carbonate, diethyl carbonate, ethyl propionate, propyl propionate, γ-butyrolactone, and combinations thereof.

[0094] In some embodiments, the electrolyte is not particularly limited, and any substance known as an electrolyte can be used arbitrarily. In the case of a lithium secondary battery, a lithium salt is generally used. Examples of the electrolyte may include, but are not limited to, inorganic lithium salts such as LiPF6, LiBF4, LiClO4, LiAlF4, LiSbF6, LiWF7; lithium tungstates such as LiWOF5; lithium carboxylates such as HCO2Li, CH3CO2Li, CH2FCO2Li, CHF2CO2Li, CF3CO2Li, CF3CH2CO2Li, CF3CF2CO2Li, CF3CF2CF2CO2Li, CF3CF2CF2CF2CO2Li; lithium salts such as FSO3Li, CH3SO3Li, , CH2FSO3Li, CHF2SO3Li, CF3SO3Li, CF3CF2SO3Li, CF3CF2CF2SO3Li, CF3CF2CF2CF2SO3Li and other lithium sulfonates; LiN(FCO)2, LiN(FCO)(FSO2), LiN(FSO2)2, LiN(FSO2)(CF3SO2), LiN(CF3SO2)2, LiN(C2F5SO2)2, cyclic 1,2-perfluoroethane bissulfonimide lithium, cyclic 1,3-perfluoropropane bissulfonimide Lithium, LiN (CF3SO2) (C4F9SO2) and other imide lithium salts; LiC (FSO2) 3, LiC (CF3SO2) 3, LiC (C2F5SO2) 3 and other methylated lithium salts; bis (malonate) borate lithium salts, difluoro (malonate) borate lithium salts and other (malonate) borate lithium salts; tri (malonate) phosphate lithium, difluorobis (malonate) phosphate lithium, tetrafluoro (malonate) phosphate lithium salts and other (malonate) phosphate lithium salts; and LiPF4 (CF3) 2, LiPF4 (C2F5) 2, Fluorine-containing organic lithium salts such as LiPF4(CF3SO2)2, LiPF4(C2F5SO2)2, LiBF3CF3, LiBF3C2F5, LiBF3C3F7, LiBF2(CF3)2, LiBF2(C2F5)2, LiBF2(CF3SO2)2, LiBF2(C2F5SO2)2; lithium oxalatoborate salts such as lithium difluorooxalatoborate and lithium bis(oxalato)borate; lithium oxalato phosphate salts such as lithium tetrafluorooxalato phosphate, lithium difluorobis(oxalato)phosphate and lithium tris(oxalato)phosphate, etc.

[0095] In some embodiments, the electrolyte is selected from LiPF6, LiSbF6, FSO3Li, CF3SO3Li, LiN(FSO2)2, LiN(FSO2)(CF3SO2), LiN(CF3SO2)2, LiN(C2F5SO2)2, cyclic 1,2-perfluoroethane bissulfonyl imide lithium, cyclic 1,3-perfluoropropane bissulfonyl imide lithium, LiC(FSO2)3, LiC(CF3SO2)3, LiC(C2F5SO2)3, LiBF3CF3, LiBF3C2F5, LiPF3(CF3)3, LiPF3(C2F5)3, lithium difluorooxalatoborate, lithium bis(oxalato)borate or lithium difluorobis(oxalato)phosphate, which helps to improve the output power characteristics, high-rate charge and discharge characteristics, high-temperature storage characteristics and cycle characteristics of the electrochemical device.

[0096] There is no particular limitation on the content of the electrolyte, as long as the effect of the present application is not impaired. In some embodiments, the total molar concentration of lithium in the electrolyte is greater than 0.3 mol / L, greater than 0.4 mol / L or greater than 0.5 mol / L. In some embodiments, the total molar concentration of lithium in the electrolyte is less than 3 mol / L, less than 2.5 mol / L or less than 2.0 mol / L. In some embodiments, the total molar concentration of lithium in the electrolyte is within the range consisting of any two of the above values. When the electrolyte concentration is within the above range, there will not be too little lithium as a charged particle, and the viscosity can be kept within an appropriate range, thereby easily ensuring good conductivity.

[0097] When two or more electrolytes are used, the electrolyte includes at least one salt selected from the group consisting of monofluorophosphate, borate, oxalate and fluorosulfonate. In some embodiments, the electrolyte includes a salt selected from the group consisting of monofluorophosphate, oxalate and fluorosulfonate. In some embodiments, the electrolyte includes a lithium salt. In some embodiments, based on the weight of the electrolyte, the content of the salt selected from the group consisting of monofluorophosphate, borate, oxalate and fluorosulfonate is greater than 0.01% or greater than 0.1%. In some embodiments, based on the weight of the electrolyte, the content of the salt selected from the group consisting of monofluorophosphate, borate, oxalate and fluorosulfonate is less than 20% or less than 10%. In some embodiments, the content of the salt selected from the group consisting of monofluorophosphate, borate, oxalate and fluorosulfonate is within the range composed of any two of the above values.

[0098] In some embodiments, the electrolyte comprises one or more substances selected from the group consisting of monofluorophosphates, borates, oxalates, and fluorosulfonates, and one or more salts other than these. Salts other than these include the lithium salts exemplified above, and in some embodiments, include LiPF6, LiN(FSO2)(CF3SO2), LiN(CF3SO2)2, LiN(C2F5SO2)2, cyclic 1,2-perfluoroethane bissulfonyl imide lithium, cyclic 1,3-perfluoropropane bissulfonyl imide lithium, LiC(FSO2)3, LiC(CF3SO2)3, LiC(C2F5SO2)3, LiBF3CF3, LiBF3C2F5, LiPF3(CF3)3, and LiPF3(C2F5)3. In some embodiments, the salt other than these is LiPF6.

[0099] In some embodiments, the amount of the other salts is greater than 0.01% or greater than 0.1% based on the weight of the electrolyte. In some embodiments, the amount of the other salts is less than 20%, less than 15%, or less than 10% based on the weight of the electrolyte. In some embodiments, the amount of the other salts is within a range consisting of any two of the above values. The above amounts of the other salts help balance the conductivity and viscosity of the electrolyte.

[0100] III. Negative electrode

[0101] The negative electrode includes a negative electrode current collector and a negative electrode active material layer disposed on at least one surface of the negative electrode current collector, wherein the negative electrode active material layer contains a negative electrode active material. The negative electrode active material layer can be one or more layers, and each layer of the multi-layer negative electrode active material can contain the same or different negative electrode active materials. The negative electrode active material is any substance that can reversibly insert and extract metal ions such as lithium ions. In some embodiments, the rechargeable capacity of the negative electrode active material is greater than the discharge capacity of the positive electrode active material to prevent the unintentional precipitation of lithium metal on the negative electrode during charging.

[0102] As a current collector for retaining the negative electrode active material, any known current collector can be used. Examples of negative electrode current collectors include, but are not limited to, metal materials such as aluminum, copper, nickel, stainless steel, and nickel-plated steel. In some embodiments, the negative electrode current collector is copper.

[0103] When the negative electrode current collector is a metal material, the negative electrode current collector may be in the form of, but is not limited to, metal foil, metal cylinder, metal coil, metal plate, metal film, expanded metal, stamped metal, foamed metal, etc. In some embodiments, the negative electrode current collector is a metal film. In some embodiments, the negative electrode current collector is copper foil. In some embodiments, the negative electrode current collector is a rolled copper foil using a rolling process or an electrolytic copper foil using an electrolytic process.

[0104] In some embodiments, the thickness of the negative electrode current collector is greater than 1 μm or greater than 5 μm. In some embodiments, the thickness of the negative electrode current collector is less than 100 μm or less than 50 μm. In some embodiments, the thickness of the negative electrode current collector is within the range formed by any two of the above values.

[0105] There are no particular limitations on the negative electrode active material, as long as it can reversibly occlude and release lithium ions. Examples include, but are not limited to, carbon materials such as natural graphite and artificial graphite; metals such as silicon (Si) and tin (Sn); or oxides of metal elements such as Si and Sn. Negative electrode active materials can be used alone or in combination.

[0106] The negative electrode active material layer may further include a negative electrode binder. The negative electrode binder can improve the bonding between the negative electrode active material particles and the bonding between the negative electrode active material and the current collector. The type of negative electrode binder is not particularly limited, as long as it is a material that is stable to the electrolyte or the solvent used in electrode manufacturing. In some embodiments, the negative electrode binder includes a resin binder. Examples of resin binders include, but are not limited to, fluororesins, polyacrylonitrile (PAN), polyimide resins, acrylic resins, polyolefin resins, and the like. When an aqueous solvent is used to prepare the negative electrode mixture slurry, the negative electrode binder includes, but is not limited to, carboxymethyl cellulose (CMC) or its salt, styrene-butadiene rubber (SBR), polyacrylic acid (PAA) or its salt, polyvinyl alcohol, and the like.

[0107] The negative electrode can be prepared by applying a negative electrode mixture slurry containing a negative electrode active material, a resin binder, etc. on a negative electrode current collector, drying it, and then rolling it to form a negative electrode active material layer on both sides of the negative electrode current collector, thereby obtaining a negative electrode.

[0108] IV. Isolation membrane

[0109] In order to prevent short circuit, a separator is usually provided between the positive electrode and the negative electrode. In this case, the electrolyte of the present application is usually used by permeating the separator.

[0110] There is no particular restriction on the material and shape of the isolation membrane, as long as the effect of the present application is not significantly impaired. The isolation membrane may be a resin, glass fiber, inorganic substance, etc. formed of a material that is stable to the electrolyte of the present application. In some embodiments, the isolation membrane includes a porous sheet or a non-woven fabric-like material with excellent liquid retention. Examples of materials for resin or glass fiber isolation membranes may include, but are not limited to, polyolefins, aromatic polyamides, polytetrafluoroethylene, polyethersulfone, etc. In some embodiments, the polyolefin is polyethylene or polypropylene. In some embodiments, the polyolefin is polypropylene. The materials of the above-mentioned isolation membranes can be used alone or in any combination.

[0111] The isolation film may also be a material formed by laminating the above materials, and examples thereof include, but are not limited to, a three-layer isolation film formed by laminating polypropylene, polyethylene, and polypropylene in this order.

[0112] Examples of inorganic materials include, but are not limited to, oxides such as aluminum oxide and silicon dioxide, nitrides such as aluminum nitride and silicon nitride, and sulfates (e.g., barium sulfate, calcium sulfate, etc.) The inorganic material may be in the form of, but is not limited to, particles or fibers.

[0113] The separator may be in the form of a thin film, examples of which include, but are not limited to, nonwoven fabrics, woven fabrics, and microporous films. In the thin film form, the separator has a pore size of 0.01 μm to 1 μm and a thickness of 5 μm to 50 μm. In addition to the above-mentioned independent thin film separators, separators may also be used: separators formed by forming a composite porous layer containing the above-mentioned inorganic particles on the surface of the positive electrode and / or negative electrode using a resin-based binder. For example, a separator formed by forming a porous layer on both sides of the positive electrode using fluororesin as a binder with aluminum oxide particles having a particle size of 90% less than 1 μm.

[0114] The thickness of the separator is arbitrary. In some embodiments, the separator has a thickness greater than 1 μm, greater than 5 μm, or greater than 8 μm. In some embodiments, the separator has a thickness less than 50 μm, less than 40 μm, or less than 30 μm. In some embodiments, the separator has a thickness within a range defined by any two of the above values. When the separator has a thickness within the above range, insulation and mechanical strength can be ensured, and the rate characteristics and energy density of the electrochemical device can be ensured.

[0115] When a porous material such as a porous sheet or non-woven fabric is used as the separator, the porosity of the separator is arbitrary. In some embodiments, the porosity of the separator is greater than 10%, greater than 15%, or greater than 20%. In some embodiments, the porosity of the separator is less than 60%, less than 50%, or less than 45%. In some embodiments, the porosity of the separator is within a range consisting of any two of the above values. When the porosity of the separator is within the above range, insulation and mechanical strength can be ensured, membrane resistance can be suppressed, and the electrochemical device has good safety characteristics.

[0116] The average pore size of the isolation membrane is also arbitrary. In some embodiments, the average pore size of the isolation membrane is less than 0.5 μm or less than 0.2 μm. In some embodiments, the average pore size of the isolation membrane is greater than 0.05 μm. In some embodiments, the average pore size of the isolation membrane is within a range consisting of any two of the above values. If the average pore size of the isolation membrane exceeds the above range, short circuits are likely to occur. When the average pore size of the isolation membrane is within the above range, the electrochemical device has good safety characteristics.

[0117] V. Electrochemical Devices

[0118] The electrochemical device of the present application includes any device that generates an electrochemical reaction, and specific examples thereof include lithium metal secondary batteries or lithium ion secondary batteries.

[0119] VI. Electronic devices

[0120] The present application further provides an electronic device, which includes the electrochemical device according to the present application.

[0121] The use of the electrochemical device of the present application is not particularly limited, and it can be used in any electronic device known in the prior art. In some embodiments, the electrochemical device of the present application can be used in, but is not limited to, laptop computers, pen-type computers, mobile computers, e-book players, portable phones, portable fax machines, portable copiers, portable printers, head-mounted stereo headphones, video recorders, LCD televisions, portable cleaners, portable CD players, mini-discs, transceivers, electronic notepads, calculators, memory cards, portable recorders, radios, backup power supplies, motors, cars, motorcycles, power-assisted bicycles, bicycles, lighting fixtures, toys, game consoles, clocks, power tools, flashlights, cameras, large household batteries, and lithium-ion capacitors.

[0122] The preparation of lithium-ion batteries is described below using lithium-ion batteries as an example and in combination with specific embodiments. Those skilled in the art will understand that the preparation method described in this application is only an example, and any other suitable preparation method is within the scope of this application.

[0123] 1. Preparation of lithium-ion batteries

[0124] 1. Preparation of negative electrode

[0125] Artificial graphite, styrene-butadiene rubber, and sodium carboxymethyl cellulose were mixed with deionized water in a mass ratio of 96%:2%:2% and stirred evenly to form a slurry. The slurry was then coated onto 9μm copper foil. The resulting material was dried, cold-pressed, cut into pieces, and the tabs welded to form the negative electrode.

[0126] 2. Preparation of positive electrode

[0127] By controlling the step heating speed and pressure as well as the time of each step, polyvinylidene fluoride (PVDF) with different densities was synthesized. 3 The binder polyvinylidene fluoride (PVDF) is mixed with deionized water in a mass ratio of 96.5%: 1%: 2.5%, stirred evenly, and a lower slurry is prepared. Lithium cobalt oxide, Super-P and PVDF of different densities are mixed with N-methylpyrrolidone (NMP) in a mass ratio of 97.5%: 1%: 1.5%, stirred evenly, and an upper slurry is prepared. The lower slurry is coated on both sides of the positive electrode collector made of aluminum foil until the thickness on each side is H1μm as the first positive electrode active material layer, and then the upper slurry is coated thereon until the thickness of the upper slurry on each side is H2μm as the second positive electrode active material layer, and the total thickness of the single-sided coating is Hμm. Then, it is dried, cold pressed, cut into pieces, and welded to the pole ears to obtain the positive electrode.

[0128] 3. Preparation of electrolyte

[0129] Under a dry argon atmosphere, EC, PC, and DEC (weight ratio of 1:1:1) were mixed, and LiPF6 was added and mixed thoroughly to form a base electrolyte solution, wherein the LiPF6 concentration was 12.5%. As needed, different amounts of additives were added to the base electrolyte solution to obtain electrolyte solutions for different examples and comparative examples.

[0130] The abbreviations and names of the components in the electrolyte are shown in the following table:

[0131] Material name abbreviation Material name abbreviation Ethylene carbonate EC Ethylene carbonate PC Diethyl carbonate DEC Ethylene glycol bis(2-cyanoethyl) ether EDN 1,2,3-Tris(2-cyanoethoxy)propane TCEP 1,2,4-Tris(2-cyanoethoxy)butane MJ-2 1,1,1-Tris(cyanoethoxymethylene)ethane MJ-3 1,1,1-Tris(cyanoethoxymethylene)propane MJ-4 3-Methyl-1,3,5-tris(cyanoethoxy)pentane MJ-5 1,2,7-Tris(cyanoethoxy)heptane MJ-6 1,2,6-Tris(cyanoethoxy)hexane MJ-7 1,2,5-Tris(cyanoethoxy)pentane MJ-8 lithium difluorophosphate LDP Propyl propionate PP

[0132] 4. Preparation of isolation membrane

[0133] A polyethylene porous polymer film is used as the separator.

[0134] 5. Preparation of lithium-ion batteries

[0135] The resulting positive electrode, separator, and negative electrode are wound in order and placed in an outer packaging foil, leaving a liquid injection port. The electrolyte is poured through the injection port, packaged, and then undergoes formation and capacity testing to produce a lithium-ion battery.

[0136] 2. Test Method

[0137] 1. Thermal abuse thickness expansion test of lithium-ion batteries

[0138] At 25°C, let the lithium-ion battery stand for 30 minutes, and measure its thickness as T1. Then, start heating it to 130°C at a heating rate of 5°C / min, keep it for 30 minutes, and measure its thickness as T2. The thermal abuse thickness expansion rate of the lithium-ion battery is calculated by the following formula:

[0139] Thickness expansion ratio = [(T2-T1) / T1] x 100%.

[0140] 2. High temperature short circuit deformation rate test of lithium-ion batteries

[0141] At 25°C, the lithium-ion battery was allowed to rest for 30 minutes, then charged to 4.7V at a constant current rate of 0.5C. It was then charged to 0.05C at a constant voltage rate at 4.7V and allowed to rest for 60 minutes. The thickness T3 of the lithium-ion battery was measured. The battery was then short-circuited for 10 seconds at 100mΩ, and the thickness T4 of the lithium-ion battery was measured. The high-temperature short-circuit deformation rate of the lithium-ion battery was calculated using the following formula:

[0142] Short-circuit deformation rate = [(T4-T3) / T3]×100%.

[0143] 3. Test Results

[0144] Table 1 shows the effects of the density of the binder used in the second positive electrode active material layer and the average particle size of the second positive electrode active material on the safety performance of lithium-ion batteries under high temperature and high pressure, wherein in the embodiment described in Table 1, H1=5, H2=50, and H=55.

[0145] Table 1

[0146]

[0147]

[0148] As can be seen from the electrochemical test results in Table 1, compared with Comparative Examples 1-1 and 1-2, the density of the binder used in the second positive electrode active material layer of Examples 1-1 to 1-10 of the present application is 0.6 g / cm 3 Up to 1.5g / cm 3 The obtained electrochemical device has a lower thickness expansion rate during thermal abuse and a lower deformation rate during short circuit.

[0149] Comparing Examples 1-1 to 1-4 with Example 1-6 shows that, when the binder density is the same, an average particle size bμm of the second positive electrode active material within the range of 5 ≤ b ≤ 20 μm can further reduce the thermal abuse thickness expansion rate and short-circuit deformation rate of the electrochemical device. Furthermore, when b / a is within the range of 4 ≤ b / a ≤ 25 μm, the resulting electrochemical device exhibits even better safety performance under high temperature and high pressure conditions.

[0150] Table 2 shows the effects of the ratio of the thickness of the second positive electrode active material layer to the positive electrode active material layer and the density of the positive electrode binder on the safety performance of lithium-ion batteries under high temperature and high pressure. Examples 2-1 to 2-11 in Table 2 differ from Example 1-1 only in the parameters shown in Table 2, where H1 remains unchanged and the value of H2 is adjusted based on h.

[0151] Table 2

[0152]

[0153] The electrochemical test results in Table 2 show that, when the binder density used in the second positive electrode active material layer is the same, compared to Examples 1-1 and 2-6, the h ratios of Examples 2-1 to 2-5 are within the range of 0.6-0.98, resulting in electrochemical devices with lower thickness expansion rates during thermal abuse and lower deformation rates during short circuits. When h / a is within the range of 0.45-1.2, the resulting electrochemical devices exhibit superior high-temperature performance.

[0154] Table 3 shows the effects of adding PP to the electrolyte and the density of the binder in the second cathode active material layer on the safety performance of lithium-ion batteries under high temperature and high pressure. The only difference between Examples 3-1 to 3-14 in Table 3 and Example 1-1 is the parameters shown in Table 3.

[0155] Table 3

[0156]

[0157] Referring to the electrochemical test results in Table 3, it can be seen that compared with Example 1-1, after adding propyl propionate to the electrolyte, the electrochemical devices obtained in Examples 3-1 to 3-12 have lower thickness expansion rate during thermal abuse and lower deformation rate during short circuit.

[0158] When the density of the binder used in the second positive electrode active material layer is the same, compared with Examples 3-6 and 3-7, the PP content in Examples 3-1 to 3-5 and 3-8 is in the range of 10%-65%, and the resulting electrochemical device exhibits better safety performance under high temperature and high pressure.

[0159] When x / a is in the range of 7-90, the obtained electrochemical device exhibits more excellent high-temperature performance.

[0160] Table 4 shows the effects of adding PP and a phosphorus-containing compound (lithium difluorophosphate or difluorophosphite) to the electrolyte on the safety performance of lithium-ion batteries under high temperature and high pressure. Examples 4-1 to 4-18 in Table 4 differ from Example 3-8 only in the parameters shown in Table 4.

[0161] Table 4

[0162]

[0163]

[0164] Referring to the electrochemical test results in Table 4, it can be seen that compared with Examples 3-8, Examples 4-1 to 4-18 further added lithium difluorophosphate or difluorophosphite to the electrolyte, and the corresponding electrochemical devices had a lower thickness expansion rate during thermal abuse and a lower deformation rate during short circuit.

[0165] In addition, when PP and a phosphorus-containing compound are added to the electrolyte at the same time and the contents of the two satisfy 10≤x / y≤400, the resulting electrochemical device exhibits better safety performance under high temperature and high pressure.

[0166] Table 5 shows the effects of adding other additives to the electrolyte on the high-temperature performance of lithium-ion batteries. The only difference between Examples 5-1 to 5-15 in Table 5 and Example 3-8 is the parameters shown in Table 5.

[0167] Table 5

[0168]

[0169]

[0170] Referring to the electrochemical test results in Table 5, it can be seen that when fluoroethylene carbonate, 1,3-propane sultone, vinyl sulfate, vinyl ethylene carbonate, phosphoric acid cyclic anhydride, ethylene glycol di(2-cyanoethyl) ether or 1,2,3-tris(2-cyanoethoxy)propane is added to the electrolyte, the safety performance of the electrochemical device under high temperature and high pressure can be further improved.

[0171] References throughout this specification to "an embodiment," "part of an embodiment," "one embodiment," "another example," "an example," "a specific example," or "a portion of an example" mean that at least one embodiment or example in this application includes the specific features, structures, materials, or characteristics described in that embodiment or example. Therefore, descriptions appearing throughout this specification, such as, "in some embodiments," "in an embodiment," "in one embodiment," "in another example," "in an example," "in a specific example," or "an example," are not necessarily references to the same embodiment or example in this application. In addition, the specific features, structures, materials, or characteristics described herein may be combined in any suitable manner in one or more embodiments or examples.

[0172] Although illustrative embodiments have been shown and described, those skilled in the art should understand that the above embodiments should not be construed as limitations on the present application, and that changes, substitutions, and modifications may be made to the embodiments without departing from the spirit, principles, and scope of the present application.

Claims

1. An electrochemical device comprising a positive electrode and an electrolyte, wherein the positive electrode comprises: positive electrode current collector; and A positive electrode active material layer, the positive electrode active material layer being located on at least one surface of the positive electrode current collector, wherein the positive electrode active material layer comprises a first positive electrode active material layer and a second positive electrode active material layer, the first positive electrode active material layer comprising a first positive electrode active material and being located between the positive electrode current collector and the second positive electrode active material layer, and the second positive electrode active material layer comprising a second positive electrode active material and a binder, wherein the density of the binder is ag / cm 3 , 0.6≤a≤1.5; The electrolyte includes propyl propionate, wherein the content of the propyl propionate is x% based on the total weight of the electrolyte and 10≤x≤65; the electrolyte also includes a phosphorus-containing compound, wherein the phosphorus-containing compound includes at least one of the following: lithium difluorophosphate or difluorophosphite, wherein the content of the phosphorus-containing compound is y% based on the total weight of the electrolyte, and 0.01≤y≤3, 10≤x / y≤400. 2 . The electrochemical device according to claim 1 , wherein the average particle size of the second positive electrode active material is b μm, and 5≤b≤20. The electrochemical device according to claim 2 , wherein 4≤b / a≤25. 4 . The electrochemical device according to claim 1 , wherein the thickness of the positive electrode active material layer is H, and the thickness of the second positive electrode active material layer is H2, wherein h=H2 / H, and 0.6≤h≤0.

98. The electrochemical device according to claim 4 , wherein 0.45≤h / a≤1.

2. The electrochemical device according to claim 1 , wherein 7≤x / a≤90.

7. The electrochemical device according to claim 1, wherein the difluorophosphite comprises at least one of the compound represented by Formula 1 below or the compound represented by Formula 2 below: A-OPF2 (Formula 1); A-(OPF2)2 (Formula 2), wherein A is one of a substituted or unsubstituted C1-C10 alkyl group, a C2-C10 alkenyl group, a C2-C10 oxyalkyl group, or a C2-C10 oxyalkenyl group; in, When substituted, the substituent is at least one of halogen or cyano.

8. The electrochemical device according to claim 7, wherein the difluorophosphite comprises at least one of the following compounds:

9. The electrochemical device of claim 1 , wherein the electrolyte comprises at least one of fluoroethylene carbonate, 1,3-propane sultone, vinyl sulfate, vinyl ethylene carbonate, phosphoric acid cyclic anhydride, ethylene glycol bis(2-cyanoethyl) ether, 1,2,3-tris(2-cyanoethoxy)propane, 1,2,4-tris(2-cyanoethoxy)butane, 1,1,1-tris(cyanoethoxymethylene)ethane, 1,1,1-tris(cyanoethoxymethylene)propane, 3-methyl-1,3,5-tris(cyanoethoxy)pentane, 1,2,7-tris(cyanoethoxy)heptane, 1,2,6-tris(cyanoethoxy)hexane, or 1,2,5-tris(cyanoethoxy)pentane.

10. An electronic device comprising the electrochemical device according to any one of claims 1 to 9.

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