CN116154101A - Electrochemical devices and electronic devices - Google Patents

Abstract

The application provides an electrochemical device and an electronic device, wherein the electrochemical device comprises a positive electrode, a negative electrode and electrolyte; the positive electrode includes a positive electrode active material having P6 ₃ mc crystal structure; the mass content of boron element on the surface of the positive electrode is n by utilizing X-ray photoelectron spectroscopy analysis ₁ The mass content of oxygen element on the surface of the positive electrode is n percent ₂ % and n ₁ /n ₂ >0.2, wherein the electrolyte comprises at least one of lithium difluorooxalato borate, lithium tetrafluoroborate or lithium bistrifluoro methanesulfonimide. Electrochemical device and electronic device provided by the applicationThe device can improve the structural stability of the positive electrode active material in the circulation process under the ultrahigh pressure, and improve the circulation stability of the electrochemical device.

Description

Electrochemical devices and electronic devices

This application is a divisional application of the Chinese invention with application number 202110564152.4 and invention name “Electrochemical device and electronic device”.

Technical Field

The present application relates to the field of energy storage technology, in particular, to electrochemical devices and electronic devices.

Background Art

Lithium-ion batteries are widely used in portable electronic products, electric transportation, defense aviation, energy storage and other fields due to their high energy density, good cycle performance, environmental protection, safety and no memory effect. In order to meet the needs of social development, it is urgent to seek lithium-ion batteries with higher energy density and power density, which requires the cathode materials used to have higher specific capacity and higher voltage platform.

In order to obtain higher specific energy, positive electrode active materials are developing towards high voltage. At present, as the voltage of positive electrode active materials increases, a large amount of Li ⁺ will be released, and the crystal structure of the material will undergo a series of irreversible damage, causing the lattice oxygen to be released, and the battery cycle performance will be greatly reduced.

Therefore, there is an urgent need to seek an electrochemical device to improve the positive electrode interface stability and cycle stability under high voltage.

Summary of the invention

In view of this, the present application proposes an electrochemical device, which can improve the structural stability of the positive electrode active material during the cycle under ultra-high pressure and enhance the cycle stability of the electrochemical device.

In a first aspect, the present application provides an electrochemical device, comprising a positive electrode, a negative electrode, a separator and an electrolyte; the positive electrode comprises a positive electrode active material having a P6 ₃ mc crystal structure; using X-ray photoelectron spectroscopy analysis, the mass content of boron element on the surface of the positive electrode is n ₁ %, the mass content of oxygen element on the surface of the positive electrode is n ₂ %, and n ₁ /n ₂ >0.2.

In the above scheme, the positive electrode active material in the electrochemical device has a P6 ₃ mc crystal structure, which is stable. It combines with the boron element in the electrolyte to stabilize the lattice oxygen in the positive electrode active material under high voltage, thereby improving the structural stability of the positive electrode active material during the cycle, reducing the oxidation of the electrolyte by the positive electrode active material due to the loss of active oxygen under high voltage, and improving the cycle stability of the electrochemical device under ultra-high voltage.

In combination with the first aspect, in a feasible implementation manner, the mass content of the boron element on the surface of the positive electrode is n ₁ , 5<n ₁ <15.

In combination with the first aspect, in a feasible implementation manner, the mass content of oxygen element on the surface of the positive electrode is n ₂ %, 8<n ₂ <25.

In combination with the first aspect, in a feasible implementation, when the electrochemical device is in a fully charged state, using X-ray photoelectron spectroscopy analysis, the positive electrode active material has a characteristic peak in the range of 17.5° to 19°, and the half-peak width of the characteristic peak is 0.05° to 0.1°.

In combination with the first aspect, in a feasible embodiment, the positive electrode active material satisfies at least one of the following characteristics (a) to (c): (a) the average particle size of the positive electrode active material is 8μm to 30μm; (b) the tap density of the positive electrode active material is 2.2g/ ^cm3 to 3g/ ^cm3 ; (c) the positive electrode active material comprises a lithium metal composite oxide having an oxygen element and an M element, wherein the M element includes at least one of Al, Mg, Ti, Mn, Fe, Ni, Zn, Cu, Nb, Cr or Zr.

In combination with the first aspect, in a feasible implementation, the positive _{electrode active} material includes _{LixNazCo1 - yMyO2} , wherein 0.6<x<1.02 _, 0≤y<0.15, 0≤z<0.03, and the M element includes at least one of Al, Mg, Ti, Mn, Fe, Ni, Zn, Cu, Nb, Cr or Zr.

In combination with the first aspect, in a feasible implementation, the electrolyte includes at least one of lithium difluorooxalatoborate, lithium tetrafluoroborate or lithium bis(trifluoromethanesulfonyl)imide.

In combination with the first aspect, in a feasible implementation, the electrolyte includes at least one of lithium difluorooxalatoborate, lithium tetrafluoroborate or lithium bis(trifluoromethanesulfonyl)imide.

In combination with the first aspect, in a feasible embodiment, the electrolyte satisfies at least one of the following conditions (d) to (f): (d) the electrolyte includes lithium difluorooxalatoborate, and the mass of the lithium difluorooxalatoborate in the electrolyte is X%, and the value range of X is 8 to 25; (e) the electrolyte includes lithium difluorooxalatoborate, and the mass of the lithium difluorooxalatoborate in the electrolyte is X%. The electrolyte also includes lithium tetrafluoroborate, and the mass of the lithium tetrafluoroborate in the electrolyte is Y%. The value range of Y is 1 to 10, and 0.8≤X/Y≤25; (f) the electrolyte includes lithium difluorooxalatoborate, the mass of lithium difluorooxalatoborate in the electrolyte is X%, the electrolyte also includes lithium tetrafluoroborate, the mass of lithium tetrafluoroborate in the electrolyte is Y%, the electrolyte also includes lithium bis(trifluoromethanesulfonylimide), the mass of lithium bis(trifluoromethanesulfonylimide) in the electrolyte is Z%, the value range of Z is 2 to 30, and 0.3≤(X+Y)/Z≤17.5.

In combination with the first aspect, in a feasible implementation manner, 1.16≤(X+Y)/Z≤13.

In a second aspect, the present application provides an electronic device, which includes the electrochemical device described in the first aspect.

Compared with the prior art, this application has at least the following beneficial effects:

The positive electrode active material in the electrochemical device provided in the present application has a P6 ₃ mc crystal structure with high crystal structure stability, which reduces the probability of particle breakage and crystal structure damage, improves the structural stability of the positive electrode active material during high voltage cycling, and thus improves the cycling performance of the electrochemical device.

The lithium salt in the electrolyte of the electrochemical device of the present application can form a positive electrode material solid electrolyte interface film rich in B-O bonds on the surface of the positive electrode active material. Under high voltage, the B-O bonds on the positive electrode surface can stabilize the lattice oxygen in the positive electrode active material and improve the structural stability of the positive electrode active material during high voltage cycling. By modifying the crystal structure of the positive electrode active material, the electrolyte positive electrode film formation protection is enhanced, and the cycle stability of the electrochemical device under ultra-high voltage (>4.55V) is improved.

DETAILED DESCRIPTION

The following is a preferred implementation of the embodiment of the present invention. It should be pointed out that for ordinary technicians in this technical field, several improvements and modifications can be made without departing from the principles of the embodiment of the present invention. These improvements and modifications are also regarded as the protection scope of the embodiment of the present invention.

For simplicity, only some numerical ranges are explicitly disclosed herein. However, any lower limit can be combined with any upper limit to form an unambiguous range; and any lower limit can be combined with other lower limits to form an unambiguous range, and any upper limit can be combined with any other upper limit to form an unambiguous range. In addition, although not explicitly stated, each point or single value between the range endpoints is included in the range. Thus, each point or single value can be combined as its own lower limit or upper limit with any other point or single value or with other lower limits or upper limits to form an unambiguous range.

In the description of this article, it should be noted that, unless otherwise specified, “above” and “below” are inclusive of the number itself, and “multiple” in “one or more” means more than two.

The above invention summary of the present application is not intended to describe each disclosed embodiment or each implementation in the present application. The following description more specifically illustrates exemplary embodiments. In many places throughout the application, guidance is provided by a series of examples, which can be used in various combinations. In each example, enumeration is only used as a representative group and should not be interpreted as exhaustive.

In a first aspect, the present application provides an electrochemical device, comprising a positive electrode, a negative electrode, a separator and an electrolyte; the positive electrode comprises a positive electrode active material, and the positive electrode active material has a P6 ₃ mc crystal structure; using X-ray photoelectron spectroscopy analysis, the mass content of boron element on the surface of the positive electrode is n ₁ %, the mass content of oxygen element on the surface of the positive electrode is n ₂ %, and n ₁ /n ₂ >0.2.

In the electrochemical device of the present application, the lithium salt in the electrolyte can form a BO-rich positive electrode material solid electrolyte interface film on the surface of the positive electrode active material. Under high voltage, the BO bond on the positive electrode surface can stabilize the lattice oxygen in the positive electrode active material, improving the structural stability of the positive electrode active material during high voltage cycling; the positive electrode active material has a P6 ₃ mc crystal structure, which has high crystal structure stability, reduces the probability of particle crushing and crystal structure damage, and improves the structural stability of the positive electrode active material during high voltage cycling. By modifying the crystal structure and enhancing the electrolyte positive electrode film formation protection, the cycle stability of the electrochemical device under ultra-high voltage (>4.55V) can be improved.

As used herein, the positive electrode surface refers to the interface formed on the positive electrode surface between the electrolyte and the positive electrode material after charge and discharge. As used herein, "fully charged state" refers to the state when the electrochemical device is charged to 4.55V or above. That is, when the electrochemical device is in the fully charged state, the charging potential of the positive electrode is above 4.55V, specifically 4.55V, 4.56V, 4.57V, 4.58V, 4.59V or 4.6V, etc.

As an optional technical solution of the present application, the positive electrode includes a positive electrode current collector and a positive electrode active material layer arranged on at least one surface of the positive electrode current collector.

The positive electrode current collector can be made of metal foil, carbon-coated metal foil or porous metal plate, preferably aluminum foil.

The positive electrode active material layer includes a positive electrode active material, and the positive electrode active material has a P6 ₃ mc crystal structure, specifically a hexagonal close-packed crystal structure, the crystal structure has higher stability, lower probability of particle breakage and crystal structure damage, smaller structural changes caused by lithium ion deintercalation and intercalation, and higher stability in air and water, which is beneficial to improving the cycle performance and thermal stability of lithium ion batteries.

As an optional technical solution of the present application, the positive electrode active material is a lithium metal composite oxide containing oxygen and M elements, wherein the M element includes at least one of Al, Mg, Ti, Mn, Fe, Ni, Zn, Cu, Nb, Cr or Zr.

Preferably, the M element includes at least one of Al, Mg, Ti, Mn or Y.

In a specific embodiment of the present application, the chemical formula of the positive _electrode active material is _{LixNazCo1 -yMyO2 ,} wherein 0.6<x<0.85, 0≤y<0.15, 0≤z<0.03, and the M element includes at least one of Al, Mg, Ti, Mn, Fe _, Ni, Zn, Cu, Nb, Cr or Zr.

When the electrochemical device is in a fully charged state, the positive electrode active material has a characteristic peak in the range of 17.5° to 19° through X-ray photoelectron spectroscopy analysis, and the half-peak width of the characteristic peak is 0.05° to 0.1°.

Specifically, the characteristic peak in the XRD spectrum of the positive electrode active material may be located at 17.5°, 18°, 18.5° or 19°, etc., and the half-peak width of the characteristic peak may be 0.05° to 0.1°.

In some embodiments of the present application, the positive electrode active material includes but is not limited to

Li _0.63 Co _0.985 Al _0.015 O ₂ , Li _0.6 Na _0.01 Co _0.985 Al _0.015 O ₂ , Li _0.7 Na _0.01 Co _0.985 Al _0.015 O ₂ , DD210838I-DIV

Li _0.8 Na _0.01 Co _0.985 Al _0.015 O ₂ , Li _0.7 Na _0.01 Co _0.98 Al _0.02 O ₂ , Li _0.7 Na _0.01 Co _0.975 Al _0.025 O ₂ , Li _0.7 Na _0.015 Co _0.985 Al _0.015 O ₂ , Li _{0. 7} Na _0.02 Co _0.985 Al _0.015 O ₂ , Li _0.7 Na _0.01 Co _0.983 Al _0.015 Mg _0.002 O ₂ , Li _0.7 Na _0.01 Co _0.984 Al _0.015 Ti _0.001 O ₂ , Li _0.7 Na _0.01 Co _0.994 Al _0.003 Mg _0.002 Ti _0.001 O ₂ .

In some embodiments of the present application, the average particle size Dv50 of the positive electrode active material is 8 μm to 30 μm, and the average particle size can be 8 μm, 9 μm, 10 μm, 12 μm, 15 μm, 18 μm, 20 μm, 22 μm, 25 μm, 28 μm or 30 μm, etc., and of course it can also be other values within the above range, which are not limited here. If the average particle size is too large, the diffusion path of lithium ions in large-size particles is longer, and the greater the resistance that needs to be overcome for diffusion, the crystal deformation and volume expansion of the positive electrode active material caused by the embedding process continue to accumulate, making the embedding process gradually difficult to carry out. Controlling the particle size of the positive electrode active material below 30 μm can improve the electrochemical kinetics and rate performance during the charge and discharge process, and reduce polarization, so that the battery has a higher specific capacity, coulombic efficiency and cycle performance. If the average particle size is too small, the specific surface area of the positive electrode active material is often large, and the surface side reactions will increase. The particle size of the positive electrode active material is kept above 8μm to ensure that the particle size of the positive electrode active material is not too small, reduce the side reactions on the material surface, and effectively inhibit the agglomeration of particles of the positive electrode active material with too small a particle size, thereby ensuring that the battery has high rate performance and cycle performance.

The tap density of the positive electrode active material is 2.2 g/cm ³ to 3 g/cm ^3. The tap density can be 2.2 g/cm ³ , 2.3 g/cm ³ , 2.4 g/cm ³ , 2.5 g/cm ³ , 2.6 g/cm ³ , 2.7 g/cm ³ , 2.8 g/cm ³ , 2.9 g/cm ³ or 3 g/cm ³ , etc., and of course it can be other values within the above range, which is not limited here. Making the tap density of the positive electrode active material within the above range is conducive to improving the specific capacity and energy density of the battery, and improving the rate performance and cycle performance of the battery.

Alternatively, the particles of the positive electrode active material may include primary particles and/or secondary particles.

扫描2θ角范围为10°至90°，扫描速率为4°/min。It should be noted that the crystal structure of the positive electrode active material can be determined by an X-ray powder diffractometer, for example, a Brucker D8A_A25 X-ray diffractometer from Brucker AxS of Germany, with CuKα radiation as the radiation source and a radiation wavelength of

The scanning 2θ angle range was from 10° to 90°, and the scanning rate was 4°/min.

The tap density of the positive electrode active material can be measured using instruments and methods known in the art, for example, it can be conveniently measured using a tap density tester, such as a FZS4-4B tap density tester.

The average particle size Dv50 of the positive electrode active material is well known in the art, and the particle size test method refers to GB/T19077-2016. For example, it can be conveniently measured using a laser particle size analyzer, such as the Mastersizer 3000 laser particle size analyzer produced by Malvern Instruments Ltd., UK.

Furthermore, the positive electrode active material layer also includes a binder and a conductive agent.

The binder may be one or more of styrene-butadiene rubber (SBR), water-based acrylic resin, carboxymethyl cellulose (CMC), polyvinylidene fluoride (PVDF), polytetrafluoroethylene (PTFE), ethylene-vinyl acetate copolymer (EVA) and polyvinyl alcohol (PVA).

The conductive agent may be one or more of superconducting carbon, acetylene black, carbon black, Ketjen black, carbon dots, carbon nanotubes, graphene and carbon nanofibers.

The positive electrode can be prepared according to conventional methods in the art. Usually, the positive electrode active material and optional conductive agent and binder are dispersed in a solvent (such as N-methylpyrrolidone, referred to as NMP) to form a uniform positive electrode slurry, and the positive electrode slurry is coated on the positive electrode current collector, and after drying, cold pressing and other processes, the positive electrode is obtained.

Since the positive electrode active material of the first aspect of the present application is adopted, the positive electrode of the present application has higher comprehensive electrochemical performance and safety performance.

Furthermore, the negative electrode may include a negative electrode current collector and a negative electrode active material layer disposed on the negative electrode current collector. For example, the negative electrode current collector includes two opposite surfaces, and the negative electrode active material layer is stacked on either or both of the two surfaces of the negative electrode current collector.

The negative electrode current collector may be made of copper foil, nickel foil, stainless steel foil, titanium foil, foamed nickel, foamed copper, a polymer substrate coated with a conductive metal, and a combination thereof.

The negative electrode active material layer generally includes a negative electrode active material and optionally a conductive agent, a binder and a thickener.

As an optional technical solution of the present application, examples of the negative electrode active material may include, but are not limited to, natural graphite, artificial graphite, mesophase microcarbon beads (MCMB for short), hard carbon, soft carbon, silicon, silicon-carbon composite, Li-Sn alloy, Li-Sn-O alloy, Sn, SnO, SnO ₂ or lithiated TiO ₂ -Li ₄ Ti ₅ O ₁₂ of spinel structure, Li metal, at least one of Li-Al alloy. Wherein, the silicon-carbon composite refers to silicon containing at least about 5 weight % based on the weight of the silicon-carbon negative electrode active material.

The conductive agent may be one or more of superconducting carbon, acetylene black, carbon black, Ketjen black, carbon dots, carbon nanotubes, graphene and carbon nanofibers, the binder may be one or more of styrene-butadiene rubber (SBR), water-based acrylic resin and carboxymethyl cellulose (CMC), and the thickener may be carboxymethyl cellulose (CMC). However, the present application is not limited to these materials, and the present application may also use other materials that can be used as negative electrode active materials, conductive agents, binders and thickeners of lithium ion batteries.

The negative electrode can be prepared according to conventional methods in the art. Usually, the negative electrode active material and optional conductive agent, binder and thickener are dispersed in a solvent, which can be deionized water, to form a uniform negative electrode slurry, which is then coated on the negative electrode current collector, and then dried, cold pressed and other processes are performed to obtain the negative electrode.

There is no particular limitation on the above-mentioned isolation membrane, and any known porous structure isolation membrane with electrochemical stability and chemical stability can be selected, for example, it can be a single-layer or multi-layer film of one or more of glass fiber, non-woven fabric, polyethylene, polypropylene and polyvinylidene fluoride.

On the basis of modifying the positive electrode active material, if the electrolyte system is further improved, the interface of the positive electrode active material can be better stabilized, the side reaction between the positive electrode active material and the electrolyte can be inhibited, thereby reducing the release of lattice oxygen of the positive electrode active material and improving the cycle performance of the electrochemical device.

The electrolyte includes an organic solvent, a lithium salt and additives.

As an optional technical solution of the present application, the lithium salt in the electrolyte may include at least one of lithium difluorooxalatoborate, lithium tetrafluoroborate or lithium bis(trifluoromethanesulfonyl)imide. It can be understood that the above lithium salt contains boron, which is conducive to the formation of B-O bonds on the positive electrode surface when the electrolyte contacts and reacts with the positive electrode surface under high voltage, stabilizes the lattice oxygen in the positive electrode active material, and improves the structural stability of the positive electrode active material during high voltage cycling.

As an optional technical solution of the present application, the electrolyte includes lithium difluorooxalate borate (LiDFOB), the mass of lithium difluorooxalate borate (LiDFOB) in the electrolyte is X%, and the value range of X is 8 to 25. Specifically, the mass of lithium difluorooxalate borate (LiDFOB) in the electrolyte can be 8%, 10%, 12%, 14% or 25%, etc., and of course it can also be other values within the above range.

As an optional technical solution of the present application, the electrolyte further includes lithium tetrafluoroborate (LiBF ₄ ), the mass of the lithium tetrafluoroborate (LiBF ₄ ) in the electrolyte is Y%, the value range of Y is 1 to 10, and 0.8≤X/Y≤25.

Specifically, the mass of lithium tetrafluoroborate (LiBF ₄ ) in the electrolyte can be 1%, 2%, 4%, 5%, 6%, 7%, 8% or 10%, etc., and can also be other values within the above range.

The ratio of X/Y may specifically be 0.8, 1, 2, 4, 6, 8, 10, 12, 14 or 25, etc., and may also be other values within the above range.

As an optional technical solution of the present application, the electrolyte further includes lithium bistrifluoromethanesulfonyl imide (LiTFSI), the mass of lithium bistrifluoromethanesulfonyl imide (LiTFSI) in the electrolyte is Z%, the value range of Z is 2 to 30, and 0.3≤(X+Y)/Z≤17.5.

Specifically, the mass of lithium bistrifluoromethanesulfonyl imide (LiTFSI) in the electrolyte can be 2%, 4%, 6%, 8%, 11%, 15%, 20% or 30%, etc., and can also be other values within the above range.

Among them, the ratio of (X+Y)/Z can specifically be 0.3, 1, 2, 3, 6, 7, 9, 10, 11, 12, 13, 14, 15 or 17.5, and of course it can also be other values within the above range.

The organic solvent may include one or more of cyclic carbonates, linear carbonates, and carboxylates. For example, it may be: ethylene carbonate (EC), propylene carbonate (PC), diethyl carbonate (DEC), ethyl methyl carbonate (DEC), dimethyl carbonate (DMC), sulfolane (SF), γ-butyrolactone (γ-BL), propyl carbonate, methyl formate (MF), ethyl formate (MA), ethyl acetate (EA), ethyl propionate (EP), propyl propionate (PP), methyl propionate, methyl butyrate, ethyl butyrate, ethyl methyl fluorocarbonate, dimethyl fluorocarbonate, diethyl fluorocarbonate, etc. At least one of the above.

The electrolyte may further include a functional additive, wherein the functional additive is selected from at least one of vinylene carbonate (VC), vinyl sulfate (DTD), propane sultone (PS), fluoroethylene carbonate (FEC), tris(trimethylsilyl) phosphate (TMSP), adiponitrile (ADN), succinonitrile (SN), 1,3,6-hexanetrinitrile (HTCN) or 1,2,3-tris(2-cyano)propane (TCEP).

As an optional technical solution of the present application, the electrolyte includes a heterocyclic sulfonate compound with a structure as shown in Formula I, and the mass of the heterocyclic sulfonate compound in the electrolyte is 0.1% to 2%;

Wherein, M is Na or K; X is O or S;

R ₁ , R ₂ and R ₃ are each independently selected from at least one of hydrogen, halogen and aldehyde.

Specifically, the mass of the heterocyclic sulfonate compound in the electrolyte can be 0.1%, 0.3%, 0.5%, 0.8%, 1%, 1.2%, 1.5%, 1.8% or 2%, etc., and of course it can also be other values within the above range, which is not limited here.

Specifically, the heterocyclic sulfonate compound is selected from at least one of the following compounds:

Due to the use of a positive electrode active material with a P6 ₃ mc crystal structure and the positive electrode film-forming additives of LiDFOB, LiBF ₄ or LiTFSI in the above-mentioned electrolyte, they cooperate with each other and work together to stabilize the lattice oxygen of the positive electrode active material at high voltage, stabilize the structure of the positive electrode active material, and improve the cycle stability of the electrochemical device at high voltage.

As an optional technical solution of the present application, the electrochemical device of the present application further includes a separator disposed between the positive electrode and the negative electrode to prevent short circuit. The present application does not particularly limit the material and shape of the separator used in the electrochemical device, which may be any material and shape disclosed in the prior art. In some embodiments, the separator includes a polymer or inorganic substance formed of a material that is stable to the electrolyte of the present application.

As an optional technical solution of the present application, the isolation film may include a substrate layer and a coating layer. In some embodiments, the substrate layer is a non-woven fabric, a film or a composite film with a porous structure. In some embodiments, the material of the substrate layer may include or be selected from at least one of polyethylene, polypropylene, polyethylene terephthalate or polyimide. In some embodiments, the material of the substrate layer may include or be selected from polyethylene porous film, polypropylene porous film, polyethylene non-woven fabric, polypropylene non-woven fabric or polypropylene-polyethylene-polypropylene porous composite film, etc.

The coating layer may be, but is not limited to, a polymer layer, an inorganic layer or a mixed layer formed by a polymer and an inorganic substance. The coating layer thickness is 0.5 μm to 10 μm, specifically 0.5 μm, 1 μm, 2 μm, 3 μm, 4 μm, 6 μm, 8 μm or 10 μm, etc.

As an optional technical solution of the present application, the inorganic layer includes inorganic particles, and the inorganic particles may include or be selected from one or more of aluminum oxide, silicon oxide, magnesium oxide, titanium oxide, hafnium dioxide, tin oxide, cerium dioxide, nickel oxide, zinc oxide, calcium oxide, zirconium oxide, yttrium oxide, silicon carbide, boehmite, aluminum hydroxide, magnesium hydroxide, calcium hydroxide and barium sulfate. The average particle size of the inorganic particles is 0.001 μm to 3 μm, and specifically can be 0.001 μm, 0.01 μm, 0.1 μm, 0.5 μm, 1 μm, 1.5 μm, 1.8 μm, 2 μm, 2.5 μm or 3 μm, etc., which is not limited here.

As an optional technical solution of the present application, the polymer layer includes a binder, and the binder is selected from at least one of polyvinylidene fluoride (PVDF), polyvinylidene fluoride-hexafluoroethylene copolymer (PVDF-HFP), polyvinyl pyrrolidone (PVP), polyacrylate, pure acrylic emulsion (anionic acrylic emulsion formed by copolymerization of acrylic ester and special functional monomers), styrene acrylic emulsion ((styrene-acrylate emulsion) is obtained by emulsion copolymerization of styrene and acrylate monomers) and styrene-butadiene emulsion (SBR, obtained by copolymerization of butadiene and styrene emulsion).

Those skilled in the art will understand that the electrochemical device of the present application can be a lithium ion battery or any other suitable electrochemical device. Without departing from the disclosure of the present application, the electrochemical device in the embodiments of the present application includes any device in which an electrochemical reaction occurs, and its specific examples include all kinds of primary batteries, secondary batteries, solar cells or capacitors. In particular, the electrochemical device is a lithium secondary battery, including a lithium metal secondary battery, a lithium ion secondary battery, a lithium polymer secondary battery or a lithium ion polymer secondary battery.

The present application is further described below in conjunction with specific examples. It should be understood that these examples are only used to illustrate the present application and are not used to limit the scope of the present application.

1. Preparation of positive electrode active materials

1) Preparation of Li _0.73 Na _0.02 CoO ₂ with P6 ₃ mc structure

Step (1): Cobalt trioxide and sodium carbonate powder are mixed in a ratio of Na to Co of 0.75:1; the uniformly mixed powder is sintered in an oxygen atmosphere at 800° C. for 46 hours to obtain Na _0.75 CoO ₂ with a P6 ₃ mc structure;

Step (2): Na _0.75 CoO ₂ and lithium nitrate are uniformly mixed in a molar ratio of Na to Li of 0.75:5, and reacted at 300° C. in an air atmosphere for 6 hours. The reactants are washed with deionized water for multiple times. After the molten salt is cleaned, the powder is dried to obtain Li _0.73 Na _0.02 CoO ₂ with a P6 ₃ mc structure.

2) Preparation of P6 ₃ mc structure Li _x Na _z Co _1-y Al _y O ₂

Step (1): adding cobalt chloride and aluminum sulfate in a molar ratio of Co to Al of 1-y:y into deionized water, adding a precipitant of sodium carbonate and a complexing agent of ammonia water to adjust the pH value to 7 to cause precipitation; then sintering the precipitate at 600°C for 7h, and grinding to obtain (Co _1-y Al _y ) ₃ O ₄ powder.

Step (2): Mix (Co _{1-y Aly} ) ₃ O ₄ powder and sodium carbonate powder in a molar ratio of Na to Co of z:y; sinter the uniformly mixed powder in an oxygen atmosphere at 800°C for 46 hours to obtain Na _z Co _{1-y Aly} O ₂ .

Step (3): Na _z Co _{1-y Aly} O ₂ and lithium nitrate are uniformly mixed in a molar ratio of Na to Li of z:10x, and reacted at 300°C in an air atmosphere for 6 hours. The reactants are washed with deionized water for multiple times, and after the molten salt is cleaned, the powder is dried to obtain Li _x Na _z Co _{1-y Aly} O ₂ with a P6 ₃ mc structure.

3) Preparation of P6 ₃ mc structured Li _x Na _z Co _{1-y My} O ₂

The preparation methods of _{LixNazCo1 -yMyO2 and LixNazCo1 - yAlyO2 are basically} the same, the difference lies in the type and/or content of the doping element M. Specifically, M is selected from Al, Mg, Ti, _Mn , Fe, Ni, _Zn , Cu, Nb, Cr or Zr.

4) Non-P6 ₃ mc structure Li _0.58 Na _0.01 Co _0.985 Al _0.015 O ₂

Step (1): adding cobalt chloride and aluminum sulfate into deionized water at a molar ratio of cobalt to aluminum of 0.985:0.015, adding a precipitant of sodium carbonate and a complexing agent of ammonia water, adjusting the pH value to 7, and causing precipitation; then sintering the precipitate, and grinding to obtain (Co _0.985 Al _0.015 ) ₃ O ₄ powder.

Step (2): (Co _0.985 Al _0.015 ) ₃ O ₄ powder and lithium carbonate are uniformly mixed in a molar ratio of lithium to cobalt of 0.58:0.985, sintered at 1000° C. for 12 h in air, and after cooling, ground and sieved to obtain Li _0.58 Na _0.01 Co _0.985 Al _0.015 O ₂ with an R-3m structure.

2. Preparation of positive electrode

The prepared positive electrode active material Li _x Na _z Co _1-y My _{O 2} , conductive carbon black (Super P) and binder polyvinylidene fluoride (PVDF) are mixed in a weight ratio of 97:1.4:1.6, added into N-methylpyrrolidone (NMP), stirred and mixed thoroughly to form a uniform positive electrode slurry; wherein the solid content of the positive electrode slurry is 72wt%; the positive electrode slurry is uniformly coated on the positive electrode current collector aluminum foil; the coated aluminum foil is dried, and then cold pressed, cut into pieces, slit, and dried under vacuum conditions to obtain a positive electrode.

3. Preparation of negative electrode

The negative electrode active material artificial graphite, conductive carbon black (Super P), thickener sodium carboxymethyl cellulose (CMC) and binder styrene butadiene rubber (SBR) were mixed according to a weight ratio of 96.4:1.5:0.5:1.6, and deionized water was added and stirred to obtain a negative electrode slurry, wherein the solid content of the negative electrode slurry was 54wt%. The negative electrode slurry was coated on the negative electrode current collector copper foil, and then dried, cold pressed, cut into pieces, welded to the pole ears, and dried to obtain the negative electrode.

4. Preparation of isolation membrane

A 9 μm thick polyethylene (PE) isolation membrane was selected, and the final isolation membrane was obtained after being coated with polyvinylidene fluoride (PVDF) slurry and Al ₂ O ₃ slurry and dried.

5. Preparation of electrolyte

In a dry argon environment, ethylene carbonate (EC), propylene carbonate (PC), ethyl methyl carbonate (EMC), and diethyl carbonate (DEC) are mixed uniformly in a mass ratio of EC:PC:EMC:DEC=20:20:40:30, and then fully dried lithium salt _LiPF6 (1 mol/kg) is dissolved in the above non-aqueous solvent. Finally, the additive or lithium salt in the embodiment is added to the basic electrolyte, and mixed uniformly to obtain an electrolyte.

6. Preparation of lithium-ion batteries

The positive electrode, the separator, and the negative electrode are stacked in order, so that the separator is placed between the positive electrode and the negative electrode to play an isolating role, and then they are wound to obtain a bare cell; after welding the pole ears, the bare cell is placed in an outer packaging foil aluminum-plastic film, and the prepared electrolyte is injected into the dried bare cell. After vacuum packaging, standing, formation, shaping, capacity testing and other processes, a soft-pack lithium-ion battery (thickness 3.3 mm, width 39 mm, length 96 mm) is obtained.

Performance Testing:

(1) Cyclic performance test

At an ambient temperature of 25°C, the lithium-ion battery prepared in the embodiment and comparative example is charged at a constant current of 3C (the capacity of the soft-pack battery is 2000mAh) to a voltage of 4.6V, and then charged at a constant voltage of 4.6V to a current of 0.05C. The charging capacity at this time is recorded as the first-cycle charging capacity of the lithium-ion battery. After that, it is allowed to stand for 5 minutes, and then discharged at a constant current of 1C to a voltage of 3.0V, and allowed to stand for 5 minutes. This is a cyclic charge and discharge process. The discharge capacity this time is recorded as the first-cycle discharge capacity of the lithium-ion battery, which is also the initial capacity of the lithium-ion battery. The lithium-ion battery is subjected to a 200-cycle charge and discharge test according to the above method, and the discharge capacity of the 200th cycle is detected. Capacity retention rate (%) after 200 cycles at 25°C = discharge capacity of the 200th cycle/first-cycle discharge capacity × 100%.

At an ambient temperature of 45°C, the lithium-ion battery prepared in the embodiment and comparative example is charged at a constant current of 3C (the capacity of the soft-pack battery is 2000mAh) to a voltage of 4.6V, and then charged at a constant voltage of 4.6V to a current of 0.05C. The charging capacity at this time is recorded as the first-cycle charging capacity of the lithium-ion battery. After that, it is allowed to stand for 5 minutes, and then discharged at a constant current of 1C to a voltage of 3.0V, and allowed to stand for 5 minutes. This is a cyclic charge and discharge process. The discharge capacity this time is recorded as the first-cycle discharge capacity of the lithium-ion battery, which is also the initial capacity of the lithium-ion battery. The lithium-ion battery is subjected to a 200-cycle charge and discharge test according to the above method, and the discharge capacity of the 200th cycle is detected. Capacity retention rate (%) after 200 cycles at 45°C = discharge capacity of the 200th cycle/first-cycle discharge capacity × 100%.

(2) After the formation, the positive electrode is tested by X-ray photoelectron spectroscopy:

The formed battery cell was fully discharged to 3V, and the battery cell was disassembled in the glove box. Part of the positive electrode was washed with dimethylaminoethyl methacrylate (DM), and then dried in the glove box for 48 hours. The positive electrode was then sealed with a sample bag for X-ray photoelectron spectroscopy analysis (i.e., the positive electrode surface). O and B elements were generally selected in the test elements, and then the positive electrode surface related X-ray photoelectron spectroscopy analysis spectrum and related element mass percentage were obtained. The test data are shown in Table 1.

According to the above preparation process, the batteries of Examples 1 to 13 and Comparative Examples 1 and 2 were obtained, wherein:

In Examples 1 to 13, the electrolyte does not contain LiPF ₆ , and the positive electrode active material is Li _0.63 Na _0.01 Co _0.985 Al _0.015 O ₂ of P6 ₃ mc structure; the type and mass of lithium salt added to the electrolyte are shown in Table 1, and the other conditions are the same as those in Comparative Example 2.

In Comparative Example 1, the lithium salt is 1 mol/kg of LiPF ₆ , and the positive electrode active material is Li _0.58 Na _0.01 Co _0.985 Al _0.015 O ₂ of non-P6 ₃ mc structure.

In Comparative Example 2, the lithium salt is 1 mol/kg of LiPF ₆ , and the positive electrode active material is Li _0.63 Na _0.01 Co _0.985 Al _0.015 O ₂ of P6 ₃ mc structure.

After testing, the mass content of each element and the mass content ratio of boron element and oxygen element in the positive electrode XPS test after formation of Comparative Example 1 and Example 7 are shown in Table 1.

Table 1

The test results of Examples 1 to 13 and Comparative Examples 1 and 2 are shown in Table 2:

Table 2

It should be noted that “/” indicates that the component is not added or does not exist.

According to the test data of Examples 1 to 6 in the above table, it can be seen that with the increase of the LiDFOB content in the electrolyte, the ratio of the mass content of the boron element to the mass content of the oxygen element on the positive electrode surface after formation gradually increases. Since the increase in the ratio of the mass content of the boron element to the mass content of the oxygen element means that more B-O bonds can be formed in the interface, it is beneficial to stabilize the lattice oxygen in the positive electrode active material. Therefore, the cycle capacity of the battery at room temperature (25°C) or high temperature (45°C) first increases, and then the basic change is not obvious. The appropriate concentration and the boron/oxygen element mass content ratio on the positive electrode surface synergistically act with the positive electrode active material to improve the cycle stability of the battery under voltage.

According to the test data of Examples 2, 7 to 10 in the above table, it can be seen that with the increase of the _LiBF4 content in the electrolyte, the ratio of the mass content of the boron element to the mass content of the oxygen element on the surface of the positive electrode after formation also shows an increasing trend, which is beneficial to stabilize the lattice oxygen in the positive electrode active material, increase the cycle capacity retention rate, and improve the cycle stability of the battery under voltage.

According to the test data of Examples 8, 11 to 13 in the above table, it can be seen that with the increase of the content of LiTFSI in the electrolyte, the ratio of the mass content of the boron element to the mass content of the oxygen element on the surface of the positive electrode after formation does not increase significantly, which shows that LiTFSI can slightly improve the high voltage cycle because LiTFSI is more stable than LiDFOB and _LiBF4 and can provide stable lithium ion transmission.

Since embodiments 1 to 13 use positive electrode active materials with a P6 ₃ mc structure, the lattice oxygen in the crystal structure has good structural stability during high voltage cycling. In addition, the electrolytes of embodiments 1 to 13 contain boron. The boron element and the positive electrode active materials with a P6 ₃ mc structure work synergistically to stabilize the lattice oxygen on the surface of the positive electrode material, thereby improving the high voltage cycling.

It can be seen from the test data of Comparative Example 1 and Examples 1 to 10 that since the positive electrode active material in Comparative Example 1 is Li _0.58 Na _0.01 Co _0.985 Al _0.015 O ₂ of non-P6 ₃ mc structure, its lattice structure is unstable. During the high-voltage cycle process, the lattice oxygen in the lattice of the positive electrode active material is released to oxidize the electrolyte, causing the cycle capacity retention rate of the battery at room temperature (25°C) or high temperature (45°C) to decrease significantly.

It can be seen from the test data of Comparative Example 2 and Examples 1 to 10 that although the positive electrode active material is Li _0.63 Na _0.01 Co _0.985 Al _0.015 O ₂ of P6 ₃ mc structure, its electrolyte does not contain LiTFSI, LiDFOB or LiBF ₄ , and there is no boron element that can form BO bonds with oxygen elements, which is not conducive to stabilizing the lattice oxygen in the positive electrode material. In addition, the positive electrode film-forming ability of the electrolyte LiPF ₆ is lower than that of the electrolyte LiDFOB, which is not conducive to the formation of a stable positive electrode interface film, and the battery cycle capacity retention rate is also significantly reduced.

Further, Examples 14 to 23 were prepared according to the above preparation method. The positive electrode active materials in Examples 14 to 23 are shown in Table 2, and the other conditions are the same as those in Example 5. The test results of Examples 5, 14 to 23 and Comparative Example 1 are shown in Table 3:

Table 3

From the comparative analysis of the data in Table 2, it can be seen that the _{LixNazCo1 -yMyO2 with} the _P63mc structure in Examples 5, ₁₄ to ₂₃ of the present application has a characteristic peak containing the (002) crystal plane located between 17.5° and 19°, and a half-peak width of the characteristic peak between 0.05° and 0.1°. The boron-containing element contained in the electrolyte and the positive electrode active material act synergistically to improve the stability of the positive electrode active material to air, water and carbon dioxide, and to improve the cycle stability of the battery at room temperature or high temperature and high voltage.

Further, Examples 24 to 29 were prepared according to the above preparation method. The amount of the compound represented by Formula I-1 added to the electrolyte of Examples 24 to 29 was different, and the other conditions were the same as those of Example 5. The test results of Example 5 and Examples 24 to 29 are shown in Table 4:

Table 4

According to the test data of Example 5 and Examples 24 to 29 in Table 3, as the amount of the compound of Formula I-1 added increases, the cycle capacity retention rate of the battery changes as shown in Table 4. This is because the compound of Formula I-1 is conducive to forming a stable solid electrolyte film at the positive electrode interface, and synergistically acts with the positive electrode active material containing boron lithium salt and P6 ₃ mc structure to reduce the side reaction of the electrolyte, thereby improving the battery cycle performance.

Although the present application is disclosed as above with preferred embodiments, it is not intended to limit the claims. Any technical personnel in this field may make several possible changes and modifications without departing from the concept of the present application. Therefore, the scope of protection of the present application shall be based on the scope defined by the claims of the present application.

Claims (9)

1. An electrochemical device comprising a positive electrode, a negative electrode and an electrolyte; characterized in that: The positive electrode includes a positive electrode active material having a P6 ₃ mc crystal structure; using X-ray photoelectron spectroscopy analysis, the mass content of boron on the surface of the positive electrode is n ₁ %, the mass content of oxygen on the surface of the positive electrode is n ₂ %, and n ₁ /n ₂ >0.2; The electrolyte includes at least one of lithium difluorooxalatoborate, lithium tetrafluoroborate or lithium bis(trifluoromethanesulfonyl)imide. 2 . The electrochemical device according to claim 1 , wherein 5<n ₁ <15. 3. The electrochemical device according to claim 1 is characterized in that, when the electrochemical device is in a fully charged state, using X-ray photoelectron spectroscopy analysis, the positive electrode active material has a characteristic peak in the range of 17.5° to 19°, and the half-peak width of the characteristic peak is 0.05° to 0.1°. 4. The electrochemical device according to claim 1, wherein the positive electrode active material satisfies at least one of the following characteristics (a) to (c): (a) the average particle size of the positive electrode active material is 8 μm to 30 μm; (b) the tap density of the positive electrode active material is 2.2 g/cm ³ to 3 g/cm ³ ; (c) The positive electrode active material comprises a lithium metal composite oxide having an oxygen element and an M element, wherein the M element comprises at least one of Al, Mg, Ti, Mn, Fe, Ni, Zn, Cu, Nb, Cr or Zr. 5. The electrochemical device according to claim 1, characterized in that the positive _electrode active material comprises _{LixNazCo1 - yMyO2} , wherein 0.6<x< _1.02 , 0≤y<0.15, 0≤z<0.03, and the M element comprises at least one of Al, Mg, Ti, Mn, Fe _, Ni, Zn, Cu, Nb, Cr or Zr. 6. The electrochemical device according to claim 1, wherein the electrolyte satisfies at least one of the following conditions (d) to (f): (d) the electrolyte comprises lithium difluorooxalatoborate, the mass of the lithium difluorooxalatoborate in the electrolyte is X%, and the value of X is in the range of 8 to 25; (e) the electrolyte includes lithium difluorooxalatoborate, the mass of the lithium difluorooxalatoborate in the electrolyte is X%, the electrolyte also includes lithium tetrafluoroborate, the mass of the lithium tetrafluoroborate in the electrolyte is Y%, the value range of Y is 1 to 10, and 0.8≤X/Y≤25; (f) The electrolyte includes lithium difluorooxalatoborate, the mass of which in the electrolyte is X%, the electrolyte also includes lithium tetrafluoroborate, the mass of which in the electrolyte is Y%, the electrolyte also includes lithium bis(trifluoromethanesulfonylimide), the mass of which in the electrolyte is Z%, the value range of Z is 2 to 30, and 0.3≤(X+Y)/Z≤17.5. 7. The electrochemical device according to claim 1, characterized in that the electrolyte comprises a heterocyclic sulfonate compound of formula I, and the mass of the heterocyclic sulfonate compound in the electrolyte is 0.1% to 2%;

Wherein, M is Na or K; X is O or S; R ₁ , R ₂ and R ₃ are each independently selected from at least one of hydrogen, halogen and aldehyde. 8. The electrochemical device according to claim 7, characterized in that the heterocyclic sulfonate compound is selected from at least one of the following compounds:

9. An electronic device, characterized in that the electronic device comprises the electrochemical device according to any one of claims 1 to 8.