Electrolyte solution, secondary battery, and electric device

Abstract

The invention relates to an electrolyte, a secondary battery and an electric device, the electrolyte comprises a lithium salt and a non-aqueous solvent, the electrolyte further comprises a fluoroborate compound, and the mass ratio of the fluoroborate compound is 0.8%-5% based on the mass of the electrolyte. When the electrolyte is decomposed, the fluoroborate compound with the preset content can be decomposed preferentially, and a CEI layer containing boron and fluorine is formed on the surface of the positive electrode after decomposition, so that continuous decomposition of the electrolyte on the surface of the positive electrode can be relieved, and the CEI layer can also form a coordination effect with transition metal ions, so that the service life of the electrolyte is prolonged. Therefore, the dissolution of transition metal ions (such as Mn) in the positive electrode material can be reduced. The secondary battery using the electrolyte has relatively good cycling stability, rate capability and high-temperature storage performance.

Description

Technical Field

[0001] The present application relates to the field of electrochemical energy storage, and in particular to an electrolyte, a secondary battery, and an electrical device. Background Art

[0002] Lithium manganese iron phosphate (LMFP)-based cathode materials have high energy density, excellent low-temperature performance, high safety, and low cost, and are considered to be one of the most promising cathode materials for next-generation batteries.

[0003] However, there are side reactions between LMFP and the electrolyte, which lead to the decomposition of the electrolyte, excessive consumption of active lithium, and even the formation of a thick and unstable cathode electrolyte interface (CEI) layer, which is not conducive to the diffusion of lithium ions. As a result, the LMFP-based cathode material has slow kinetics, increased impedance, severe high-temperature gas production, and poor cycle performance and rate performance. Summary of the Invention

[0004] In view of this, the present application provides an electrolyte, a secondary battery, and an electrical device to solve at least one of the above technical problems.

[0005] To achieve the above objectives, in a first aspect, the present application provides an electrolyte, the electrolyte comprising a lithium salt and a non-aqueous solvent, the electrolyte further comprising a fluoroborate-based compound, the general formula of the fluoroborate group being: Wherein, R1~R6 are each independently selected from one or more of hydrogen, halogen, C1~C20 alkyl, C1~C20 haloalkyl, C6~C26 aryl, C6~C26 haloaryl and alkoxy, and at least one of R1~R6 includes one or more fluorine-containing groups; based on the mass of the electrolyte, the mass proportion of the fluoroborate ester compound is 0.8% to 5%.

[0006] When the electrolyte of the present application decomposes, the preset content of the fluoroborate-based compound can decompose preferentially and form a dense boron and fluorine-containing CEI layer on the surface of the positive electrode after decomposition. The CEI layer can timely reduce the direct contact between the positive electrode and the electrolyte, thereby helping to alleviate the continuous decomposition of the electrolyte on the positive electrode surface, thereby alleviating the excessive consumption of lithium salts in the electrolyte; the borate or fluoroborate in the CEI layer can also form a coordination effect with the transition metal ions in the positive electrode material, thereby helping to reduce the dissolution of transition metal ions (such as Mn) in the positive electrode material. In addition, the borate or fluoroborate free in the electrolyte can also form a coordination effect with the transition metal ions in the positive electrode material, thereby reducing the migration and reduction of the dissolved transition metal ions to the negative electrode. Therefore, the preset content of fluoroborate-based compounds in the electrolyte of the present application can reduce the dissolution of transition metal ions in the positive electrode material and reduce the migration and reduction of the dissolved transition metal ions to the negative electrode, and can also reduce the unstable lithium insertion or extraction process of the negative electrode, thereby facilitating the transmission of lithium ions. It can also form a solid electrolyte interface (SEI) layer on the negative electrode. The SEI layer has good mechanical strength, can reduce the direct contact between the negative electrode and the electrolyte, and is beneficial to alleviate the strain caused by the volume change during the charging and discharging process of the silicon negative electrode.

[0007] Based on the first aspect, in some possible implementations, based on the mass of the electrolyte, the mass of the fluoroborate-based compound accounts for 0.8% to 3%.

[0008] Based on the first aspect, in some possible implementations, the non-aqueous solvent includes one or more of ethylene carbonate, propylene carbonate, diethyl carbonate, ethyl methyl carbonate, ethyl acetate, and propyl acetate.

[0009] Based on the first aspect, in some possible implementations, the lithium salt includes one or more of lithium hexafluorophosphate, lithium bis(oxalatoborate), lithium bis(fluorosulfonylimide), and lithium bis(trifluoromethylsulfonylimide).

[0010] Based on the first aspect, in some possible implementations, the fluoroborate compound is selected from

[0011] One or more of .

[0012] In a second aspect, the present application provides a secondary battery comprising a positive electrode sheet, a negative electrode sheet and an isolation membrane arranged between the positive electrode sheet and the negative electrode sheet, the positive electrode sheet comprising a positive electrode collector and a positive electrode material arranged on the surface of the positive electrode collector, the negative electrode sheet comprising a negative electrode collector and a negative electrode material arranged on the surface of the negative electrode collector, the positive electrode material comprising lithium iron manganese phosphate, and the secondary battery also comprising the above-mentioned electrolyte.

[0013] The secondary battery including the above-mentioned non-aqueous electrolyte helps to form a more stable CEI layer on the surface of lithium iron manganese phosphate, which is beneficial to alleviate the decomposition of lithium salts in the electrolyte and reduce the dissolution of Mn metal ions in lithium iron manganese phosphate, thereby reducing the adverse side reactions between the lithium iron manganese phosphate positive electrode material and the electrolyte and improving the interface stability between the positive electrode material and the electrolyte. At the same time, the secondary battery including the above-mentioned electrolyte also helps to form a SEI layer with higher mechanical strength on the surface of the negative electrode, which helps to reduce the corrosion of the electrode by the electrolyte and alleviate the strain caused by the volume change of the electrode (such as the negative electrode) during the charge and discharge process. Therefore, the above-mentioned electrolyte is beneficial to enhance the reaction kinetics of lithium iron manganese phosphate, reduce the impedance and storage gas production of the battery, and enhance the cycle stability, rate performance and high temperature storage performance of the positive electrode material based on lithium iron manganese phosphate, thereby helping to improve the safety and stability of the secondary battery.

[0014] Based on the second aspect, in some possible implementations, the negative electrode material includes silicon. The electrolyte helps form a mechanically strong SEI layer on the surface of the silicon negative electrode, helping to reduce electrolyte corrosion of the electrode and alleviate strain caused by volume changes in the silicon material in the negative electrode during charge and discharge.

[0015] Based on the second aspect, in some possible implementations, the mass ratio of the fluoroborate-based compound to the lithium manganese iron phosphate is 0.45% to 2.81%. In this case, the fluoroborate-based compound in the electrolyte and the lithium manganese iron phosphate in the positive electrode material have a specific mass ratio. At this mass ratio, the fluoroborate-based compound preferentially decomposes, and after decomposition, a CEI layer containing boron and fluorine can be formed on the surface of the positive electrode, thereby facilitating the continuous decomposition of the electrolyte on the positive electrode surface and forming a stronger coordination effect with the transition metal ions in the positive electrode material, thereby further facilitating the reduction of the dissolution of transition metal ions (such as Mn) in the positive electrode material.

[0016] Based on the second aspect, in some possible implementations, the concentration of the lithium salt in the non-aqueous solvent is 0.8 mol / L to 1.5 mol / L.

[0017] Based on the second aspect, in some possible implementations, the electrolyte includes at least two non-aqueous solvents.

[0018] In a third aspect, the present application provides an electrical device, which includes the above-mentioned secondary battery. DETAILED DESCRIPTION

[0019] The embodiments of the present application are described in detail below. The embodiments described below are illustrative and are only used to explain the present application, and should not be understood as limiting the present application. It should be noted that, unless otherwise defined, all technical and scientific terms used herein have the same meanings as those commonly understood by those skilled in the art to which the present application belongs. In the absence of conflict, the embodiments of the present application and the features in the embodiments may be combined with each other. In the following description, many specific details are set forth to facilitate a full understanding of the present application, and the embodiments described are only a part of the embodiments of the present application, not all of the embodiments.

[0020] In response to the problem of adverse side reactions between lithium manganese iron phosphate in the positive electrode material and the electrolyte, this study found that the residual water in the lithium manganese iron phosphate material causes the common electrolyte components (such as LiPF6) to hydrolyze, generating HF, which corrodes the positive electrode and causes the Mn in the lithium manganese iron phosphate to dissolve. The dissolved Mn produces Mn 2+ It will migrate to the negative electrode through the liquid phase and be reduced to metal Mn at the negative electrode, which will catalyze the decomposition of SEI and hinder the Li + On the other hand, in the Jan-Taylor effect, the elongated Mn-O bond will react with H + The reaction produces H2O, which continues to hydrolyze the electrolyte components (such as LiPF6), exacerbating the decomposition of the electrolyte, resulting in excessive consumption of active lithium and triggering serious side reactions, causing the positive electrode material based on lithium manganese iron phosphate to have slow kinetics, increased impedance, severe high-temperature gas production, and poor cycle performance and rate performance.

[0021] Based on this, the present application studies how to reduce the dissolution of Mn in lithium manganese iron phosphate to reduce the adverse phenomena caused by the dissolution of Mn, thereby achieving the purpose of improving the kinetic efficiency of lithium manganese iron phosphate-based positive electrode materials.

[0022] Based on this, one embodiment of the present application provides a secondary battery comprising a housing, an electrode assembly, and an electrolyte. The battery may be a lithium-ion battery, a sodium-ion battery, or a potassium-ion battery. The electrode assembly and the electrolyte are both located within the housing.

[0023] The outer shell can be a packaging bag obtained by packaging film (such as aluminum-plastic film), for example, a soft pack battery. In other embodiments, it can also be a steel shell battery, an aluminum shell battery, etc.

[0024] The electrode assembly includes electrode sheets and separators. The electrode sheets include positive and negative electrodes. The separator is used to separate the positive and negative electrodes and may be disposed between the positive and negative electrodes. In some embodiments, the electrode assembly may be a laminated structure, for example, formed by alternating layers of positive electrodes, separators, and negative electrodes. In other embodiments, the electrode assembly may be a wound structure, for example, formed by stacking positive electrodes, separators, and negative electrodes in sequence and then winding them.

[0025] positive electrode

[0026] The positive electrode sheet includes a positive electrode current collector and an active layer of positive electrode material disposed on at least one surface of the positive electrode current collector. The positive electrode current collector can be made of aluminum foil or nickel foil, or any composite current collector disclosed in the prior art, such as, but not limited to, a current collector formed by combining the aforementioned conductive foil and a polymer substrate.

[0027] The positive electrode material active layer includes a positive electrode material, which includes a compound that reversibly intercalates and deintercalates lithium ions (i.e., a lithiated intercalation compound). In some embodiments, the positive electrode material may include a lithium transition metal composite oxide. The lithium transition metal composite oxide contains lithium and at least one element selected from cobalt, manganese, and nickel.

[0028] In some embodiments, the positive electrode material includes lithium manganese iron phosphate (LMFP). Positive electrode materials based on lithium manganese iron phosphate have high energy density, excellent low-temperature performance, high safety, and low cost.

[0029] The positive electrode material active layer also includes a binder to bond the positive electrode active material particles to facilitate film formation and improve the bonding strength between the positive electrode material active layer and the positive electrode current collector. In some embodiments, the binder may include, but is not limited to, at least one of polyvinyl alcohol, hydroxypropyl cellulose, diacetyl cellulose, polyvinyl chloride, carboxylated polyvinyl chloride, polyvinyl fluoride, ethylene oxide-containing polymers, polyvinyl pyrrolidone, polyurethane, polytetrafluoroethylene, polyvinylidene fluoride, polyethylene, polypropylene, styrene-butadiene rubber, acrylated styrene-butadiene rubber, epoxy resin, or nylon.

[0030] The positive electrode material active layer may further comprise a conductive material, including but not limited to carbon-based materials, metal-based materials, conductive polymers, or any combination thereof. In some embodiments, the carbon-based material may include, but is not limited to, natural graphite, artificial graphite, carbon black, acetylene black, Ketjen black, carbon fiber, or any combination thereof. In some embodiments, the metal-based material may include, but is not limited to, metal powder or metal fiber, such as copper, nickel, aluminum, or silver. In some embodiments, the conductive polymer may be a polyphenylene derivative.

[0031] negative electrode

[0032] The negative electrode sheet includes a negative electrode current collector and an active layer of negative electrode material disposed on at least one surface of the negative electrode current collector. The negative electrode current collector can be made of at least one of copper foil, nickel foil, stainless steel foil, titanium foil, or a carbon-based current collector. It can also be any composite current collector disclosed in the prior art, such as, but not limited to, a current collector formed by combining the aforementioned conductive foil and a polymer substrate.

[0033] The active layer of the negative electrode material includes a negative electrode material, and the negative electrode material includes carbon material, silicon material, silicon-carbon composite material, SiOx (0.5<x<1.6), Li-Sn alloy, Li-Sn-O alloy, Sn, SnO, SnO2, spinel structure lithium titanate, lithiated TiO2-Li4Ti5O 12 , Li-Al alloy and at least one of metallic lithium.

[0034] The negative electrode active material layer also includes a binder to bond the negative electrode active material particles to facilitate film formation and improve the bonding strength between the negative electrode active material layer and the negative electrode current collector. In some embodiments, the binder may include, but is not limited to, at least one of polyvinyl alcohol, hydroxypropyl cellulose, diacetyl cellulose, polyvinyl chloride, carboxylated polyvinyl chloride, polyvinyl fluoride, ethylene oxide-containing polymers, polyvinyl pyrrolidone, polyurethane, polytetrafluoroethylene, polyvinylidene fluoride, polyethylene, polypropylene, styrene-butadiene rubber, acrylated styrene-butadiene rubber, epoxy resin, and nylon.

[0035] The negative electrode active material layer may further include a conductive material, including but not limited to carbon-based materials, metal-based materials, conductive polymers, or any combination thereof. In some embodiments, the carbon-based material may include, but is not limited to, natural graphite, artificial graphite, carbon black, acetylene black, Ketjen black, carbon fiber, or any combination thereof. In some embodiments, the metal-based material may include, but is not limited to, metal powder or metal fiber, such as copper, nickel, aluminum, or silver. In some embodiments, the conductive polymer may be a polyphenylene derivative.

[0036] Isolation film

[0037] The separator includes a porous membrane layer, and its material includes, but is not limited to, at least one of polyethylene, polypropylene, polyvinylidene fluoride, polyethylene terephthalate, polyimide, or aramid. For example, the separator is a polypropylene porous membrane, a polyethylene porous membrane, a polypropylene non-woven fabric, a polyethylene non-woven fabric, or a polypropylene-polyethylene-polypropylene porous composite membrane.

[0038] electrolyte

[0039] The electrolyte includes a lithium salt and a non-aqueous solvent. The electrolyte also includes a fluoroborate compound. The general formula of the fluoroborate compound is: Wherein, R1~R6 are each independently selected from one or more of hydrogen, halogen, C1~C20 alkyl, C1~C20 haloalkyl, C6~C26 aryl, C6~C26 haloaryl and alkoxy, and at least one of R1~R6 includes one or more fluorine-containing groups; based on the mass of the electrolyte, the mass proportion of the fluoroborate ester compound is 0.8% to 5%.

[0040] For example, based on the mass of the electrolyte, the mass proportion of the fluoroborate compound can be 0.8%, 1%, 1.5%, 2%, 2.5%, 3%, 3.5%, 4%, 4.5%, 5% or any value within the range formed by any two of the above values.

[0041] For example, the lithium salt can be selected from, but is not limited to, one or more of lithium hexafluorophosphate (LiPF6), lithium tetrafluoroborate (LiBF4), lithium hexafluoroarsenate (LiAsF6), lithium perchlorate (LiClO4), lithium tetraphenylborate (LiB(C6H5)4), lithium methanesulfonate (LiCH3SO3), lithium bisfluorosulfonyl imide (LiFSI), lithium bistrifluoromethanesulfonyl imide (LiTFSI), lithium trifluoromethanesulfonate (LiCF3SO3), lithium bis(trifluoromethylsulfonyl)imide (LiN(SO2CF3)2, tris(trifluoromethylsulfonyl)methyl lithium (LiC(SO2CF3)3), lithium bisoxalatoborate (LiBOB) and lithium difluorophosphate (LiPO2F2).

[0042] For example, the non-aqueous solvent may be a carbonate compound, a carboxylate compound, an ether compound, a nitrile compound, other organic solvents, or combinations thereof. Examples of carbonate compounds include, but are not limited to, diethyl carbonate (DEC), dimethyl carbonate (DMC), dipropyl carbonate (DPC), methylpropyl carbonate (MPC), ethylpropyl carbonate (EPC), ethyl methyl carbonate (MEC), ethylene carbonate (EC), propylene carbonate (PC), butylene carbonate (BC), vinyl ethylene carbonate (VEC), fluoroethylene carbonate (FEC), 1,2-difluoroethylene carbonate, 1,1-difluoroethylene carbonate, 1,1,2-trifluoroethylene carbonate, 1,1,2,2-tetrafluoroethylene carbonate, 1-fluoro-2-methylethylene carbonate, 1-fluoro-1-methylethylene carbonate, 1,2-difluoro-1-methylethylene carbonate, 1,1,2-trifluoro-2-methylethylene carbonate, trifluoromethylethylene carbonate, or combinations thereof.

[0043] When the electrolyte of the present application decomposes, the preset content of the fluoroborate-based compound can decompose preferentially and form a dense boron and fluorine-containing CEI layer on the surface of the positive electrode after decomposition. The CEI layer can timely reduce the direct contact between the positive electrode and the electrolyte, thereby helping to alleviate the continuous decomposition of the electrolyte on the positive electrode surface, thereby alleviating the excessive consumption of lithium salts in the electrolyte; the borate or fluoroborate in the CEI layer can also form a coordination effect with the transition metal ions in the positive electrode material, thereby helping to reduce the dissolution of transition metal ions (such as Mn) in the positive electrode material. In addition, the borate or fluoroborate free in the electrolyte can also form a coordination effect with the transition metal ions in the positive electrode material, thereby reducing the migration and reduction of the dissolved transition metal ions to the negative electrode. Therefore, the preset content of fluoroborate-based compounds in the electrolyte of the present application can reduce the dissolution of transition metal ions in the positive electrode material and reduce the migration and reduction of the dissolved transition metal ions to the negative electrode, and can also reduce the unstable lithium insertion or extraction process of the negative electrode, thereby facilitating the transmission of lithium ions. It can also form a solid electrolyte interface (SEI) layer on the negative electrode. The SEI layer has good mechanical strength, can reduce the direct contact between the negative electrode and the electrolyte, and is beneficial to alleviate the strain caused by the volume change during the charging and discharging process of the silicon negative electrode.

[0044] It is understandable that when the mass proportion of fluoroborate-based compounds in the electrolyte is too low (less than 0.8%), the content of fluoroborate-based compounds that are preferentially decomposed is too low, and the CEI layer that can be formed on the positive electrode surface after decomposition is too thin. The electrolyte may continue to decompose, resulting in continued consumption of lithium salts, and the effect of reducing the dissolution of transition metal ions is not good. When the mass proportion of fluoroborate-based compounds in the electrolyte is too high (greater than 5%), the CEI layer or SEI layer formed on the electrode surface is too thick, affecting the transport of lithium ions and being unfavorable for improving the rate performance. It is understandable that the electrolyte may also include other functional additives disclosed in the prior art to achieve specific functions.

[0045] The secondary battery including the above-mentioned non-aqueous electrolyte helps to form a more stable CEI layer on the surface of lithium iron manganese phosphate, which is beneficial to alleviate the decomposition of lithium salts in the electrolyte and reduce the dissolution of Mn metal ions in lithium iron manganese phosphate, thereby reducing the adverse side reactions between the lithium iron manganese phosphate positive electrode material and the electrolyte and improving the interface stability between the positive electrode material and the electrolyte. At the same time, the secondary battery including the above-mentioned electrolyte also helps to form a SEI layer with higher mechanical strength on the surface of the negative electrode, which helps to reduce the corrosion of the electrode by the electrolyte and alleviate the strain caused by the volume change of the electrode (such as the negative electrode) during the charge and discharge process. Therefore, the above-mentioned electrolyte is beneficial to enhance the reaction kinetics of lithium iron manganese phosphate, reduce the impedance and storage gas production of the battery, and enhance the cycle stability, rate performance and high temperature storage performance of the positive electrode material based on lithium iron manganese phosphate, thereby helping to improve the safety and stability of the secondary battery.

[0046] In some embodiments, the mass percentage of the fluoroborate-based compound is 0.8% to 3% based on the mass of the electrolyte. For example, the mass percentage of the fluoroborate-based compound can be 0.8%, 1%, 1.5%, 2%, 2.5%, 3%, or any value within a range consisting of any two of the above values, based on the mass of the electrolyte. When the mass percentage of the fluoroborate-based compound meets the above range, it is beneficial to further improve the capacity retention rate of the resulting secondary battery while exerting a good effect of inhibiting the dissolution of transition metals.

[0047] In some embodiments, the mass ratio of the fluoroborate-based compound to the lithium iron manganese phosphate is 0.45% to 2.81%. For example, the mass ratio of the fluoroborate-based compound to the lithium iron manganese phosphate can be 0.45%, 0.55%, 0.70%, 0.85%, 1%, 1.25%, 1.5%, 1.75%, 2%, 2.25%, 2.5%, 2.75%, 2.81%, or any value within the range formed by any two of the above values. The fluoroborate-based compound in the electrolyte and the lithium iron manganese phosphate in the positive electrode material are controlled to have a specific mass ratio. Under this mass ratio, the fluoroborate-based compound preferentially decomposes and, after decomposition, can form a CEI layer containing boron and fluorine on the surface of the positive electrode, thereby facilitating the continuous decomposition of the electrolyte on the positive electrode surface. The CEI layer can also form a stronger coordination effect with the transition metal ions in the positive electrode material, thereby further facilitating the reduction of the dissolution of the transition metal ions (such as Mn) in the positive electrode material.

[0048] It can be understood that constructing the mass ratio of the fluoroborate-based compound to the lithium iron manganese phosphate can make the fluoroborate-based compound more compatible with the content of the lithium iron manganese phosphate, thereby facilitating the improvement of the effect of the preset content of the fluoroborate-based compound in reducing the dissolution of Mn in the lithium iron manganese phosphate, thereby further improving the cycle performance of the resulting secondary battery.

[0049] In some embodiments, the fluoroborate compound is selected from:

[0050]

[0051] One or more of .

[0052] In some embodiments, the lithium salt includes one or more of lithium hexafluorophosphate, lithium bis(oxalatoborate), lithium bis(fluorosulfonyl imide), and lithium bis(trifluoromethylsulfonyl imide). For example, LiPF6 is selected as the lithium salt because it can provide high ionic conductivity and improve cycle characteristics.

[0053] In some embodiments, the concentration of the lithium salt in the non-aqueous solvent is 0.8 mol / L to 1.5 mol / L. For example, the concentration of the lithium salt in the non-aqueous solvent can be 0.8 mol / L, 0.9 mol / L, 1 mol / L, 1.1 mol / L, 1.2 mol / L, 1.3 mol / L, 1.4 mol / L, 1.5 mol / L, or any value within a range formed by any two of the foregoing values. The lithium salt concentration directly affects the ionic conductivity of the electrolyte, and controlling the lithium salt concentration within the foregoing range is beneficial for maintaining the migration of active ions.

[0054] In some embodiments, the non-aqueous solvent includes one or more of ethylene carbonate, propylene carbonate, diethyl carbonate, ethyl methyl carbonate, ethyl acetate, and propyl acetate.

[0055] In some embodiments, the electrolyte includes at least two non-aqueous solvents. Different non-aqueous solvents have different dielectric constants and viscosities. Mixing these solvents can balance these parameters, increase the dissociation degree of the lithium salt and the ion mobility, and thus improve the conductivity.

[0056] One embodiment of the present application further provides a method for preparing the above-mentioned electrolyte, comprising: mixing a lithium salt, a non-aqueous solvent, and a fluoroborate-based compound.

[0057] In some embodiments, the preparation method comprises:

[0058] Step 1: Purify the non-aqueous solvent, including reducing impurities or water. For example, the purification process may be performed using one or more of molecular sieves, activated carbon, calcium hydride, lithium hydride, anhydrous calcium oxide, calcium chloride, phosphorus pentoxide, alkali metals, and alkaline earth metals.

[0059] Step 2: Mixing lithium salt with purified non-aqueous solvent to obtain a basic electrolyte.

[0060] Step 3: Mixing the fluoroborate compound with the basic electrolyte to obtain an electrolyte.

[0061] Another embodiment of the present application provides an electrical device comprising the aforementioned secondary battery. In some embodiments, the battery of the present application can be used in, but is not limited to, the following electrical devices: 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, automobiles, motorcycles, power-assisted bicycles, bicycles, lighting fixtures, toys, game consoles, clocks, power tools, flashlights, cameras, large household batteries or lithium-ion capacitors, etc.

[0062] The present invention will be explained below in conjunction with the embodiments. It will be understood by those skilled in the art that the following examples are only used to explain the present invention and are not to be construed as limiting the present invention. Unless otherwise indicated, the reagents, software, and instruments not specifically described in the following examples are all conventional commercially available products or open source.

[0063] Example 1:

[0064] A secondary battery, the preparation method of which comprises:

[0065] The first step is to mix cyclic carbonate solvent ethylene carbonate (EC) and linear carbonate solvent ethyl methyl carbonate (EMC) and diethyl carbonate (DEC) in a mass ratio of EC: (EMC+DEC) = 3:7, and purify with molecular sieve to remove impurities and water.

[0066] Step 2: Under room temperature, dissolve 12.5% by mass of lithium hexafluorophosphate in the solvent obtained in the first step, then add vinylene carbonate (VC) and fluoroethylene carbonate (FEC), stir evenly, and obtain a basic electrolyte.

[0067] Step 3: Add a fluoroborate compound to the basic electrolyte obtained in step 2. The molecular formula of the fluoroborate compound used is The amount used is 1.5% of the total mass of the electrolyte to obtain an electrolyte.

[0068] Step 4: Select LiMn 0.6 Fe 0.4 PO4 is used as the positive electrode material to make a positive electrode sheet, graphite silicon is selected as the negative electrode material to make a negative electrode sheet, the obtained positive electrode sheet, negative electrode sheet, isolation membrane and the electrolyte obtained in the third step are assembled into a secondary battery, and the fluoroborate-based compound is reacted with LiMn 0.6 Fe 0.4 The mass ratio of PO4 is 0.84%.

[0069] Example 2: The difference from Example 1 is that in the third step, the molecular formula of the fluoroborate compound used is

[0070] Example 3: The difference from Example 1 is that in the third step, the molecular formula of the fluoroborate compound used is

[0071] Example 4: The difference from Example 1 is that in the third step, the molecular formula of the fluoroborate compound used is

[0072] Example 5: The difference from Example 1 is that in the third step, the molecular formula of the fluoroborate compound used is

[0073] Example 6: The difference from Example 1 is that in the third step, the molecular formula of the fluoroborate compound used is

[0074] Example 7: The difference from Example 1 is that in the third step, the amount of the fluoroborate compound used is 0.8% of the total mass of the electrolyte.

[0075] Example 8: The difference from Example 1 is that in the third step, the amount of the fluoroborate compound used is 3% of the total mass of the electrolyte.

[0076] Example 9: The difference from Example 1 is that in the third step, the amount of the fluoroborate compound used is 5% of the total mass of the electrolyte.

[0077] Comparative Example 1: The difference from Example 1 is that in the fourth step, the basic electrolyte obtained in the second step is used to assemble a secondary battery.

[0078] Comparative Example 2: The difference from Example 1 is that in the third step, the amount of the fluoroborate compound used is 0.2% of the total mass of the electrolyte.

[0079] Comparative Example 3: The difference from Example 1 is that in the third step, the amount of the fluoroborate compound used is 10% of the total mass of the electrolyte.

[0080] The soft-pack batteries of Examples 1-9 and Comparative Examples 1-3 of the present application were subjected to performance tests, including:

[0081] Negative electrode manganese content test: The Mn content of the battery cells stored at 60°C for 30 days was tested by ICP-OES. The specific steps are as follows: (1) Sample stripping and weighing: Use a blade or scraper to carefully scrape the negative electrode active material (including graphite, binder, etc.) from the copper foil current collector to avoid copper foil residue. Accurately weigh about 0.1 to 0.5 g of sample (accurate to 0.1 mg) and record the mass m; (2) Acid digestion and deacidification: Place the sample in a polytetrafluoroethylene (PTFE) digestion tank, add 10 mL of concentrated HNO3 and 2 mL of HCl for microwave digestion (heat to 180°C, hold for 30 min) to ensure that Mn is completely dissolved; heat on a hot plate until the solution is almost dry (about 1 mL remains) to remove excess acid; (3) Volume adjustment and filtration: Use 2% The HNO3 solution was diluted to a 50 mL volumetric flask; filtered through a membrane filter to remove insoluble particles and prevent clogging of the ICP nebulizer; (4) Preparation of Mn standard solution: Use a 1000 mg / L Mn standard solution as the mother solution; prepare a series of standard solutions at 0 (blank), 1, 5, and 10 mg / L (diluted with 2% HNO3); (5) Sample testing and data processing: Test the blank and standard solutions sequentially to establish a standard curve; test the sample solutions, with each sample measured three times in parallel, and record the average value. Manganese concentration C1 = C2 × V / m; where C2 is the Mn concentration measured by the instrument (mg / L), V is the volume of the solution (L), and m is the sample mass (kg).

[0082] Battery High-Temperature Storage Performance Test: The secondary battery was subjected to five charge-discharge cycles at room temperature at a charge-discharge rate of 1C, and finally charged to full charge at a rate of 1C. The 1C capacity Q, battery internal resistance T0, and battery volume V0 were recorded. The fully charged battery was stored at 60°C for 30 days, and the 1C discharge capacity Q1, battery internal resistance T1, and volume V1 were recorded. The battery was then charged and discharged at room temperature at a rate of 1C for 5 weeks. The discharge capacity with the highest discharge capacity was recorded as the discharge capacity Q2. The capacity retention rate (%) of the secondary battery under high-temperature storage conditions was calculated according to the following formulas: Q1 / Q × 100%, capacity recovery rate (%) = Q2 / Q0 × 100%, internal resistance change rate (%) = (T1-T0) / T0 × 100%, and volume change rate (%) = (V1-V0) / V0 × 100%. Please refer to Tables 1 and 2 for the test results.

[0083] Battery rate charging performance test: The secondary battery was subjected to three charge and discharge cycle tests at room temperature at a charge and discharge rate of 0.33C, and finally charged to a fully charged state at a rate of 0.33C. The 0.33C constant current constant voltage charging capacity Q0 was recorded respectively. After the fully charged battery was discharged to 2.0V at the same rate, it was then charged to 4.3V at constant current rates of 0.33C, 0.5C, 1C, 2C, 3C, and 4C, and the constant current charging capacity Q at different rates of 0.33C, 0.5C, 1C, 2C, 3C, and 4C was recorded respectively.x The constant current charging ratio (%) of the battery at different rates is calculated according to the following formula = Q x / Q0×100%, test results please refer to Table 3.

[0084] Table 1. Capacity performance and negative electrode manganese content test results of secondary batteries of Examples 1-9 and Comparative Examples 1-3 of the present application

[0085]

[0086] Table 2. Test results of volume change rate and internal resistance change rate of secondary batteries of Examples 1-9 and Comparative Examples 1-3 of the present application

[0087]

[0088] Table 3. Rate performance test results of secondary batteries of Examples 1-9 and Comparative Examples 1-3 of the present application

[0089]

[0090] The secondary batteries of Examples 1-9 of the present application use a lithium salt non-aqueous electrolyte having a preset content of fluoroborate-based compounds. When the electrolyte decomposes, the preset content of fluoroborate-based compounds can preferentially decompose and form a dense boron and fluorine-containing CEI layer on the surface of the positive electrode after decomposition. The CEI layer can timely reduce the direct contact between the positive electrode and the electrolyte, thereby helping to alleviate the continuous decomposition of the electrolyte on the positive electrode surface, thereby alleviating the excessive consumption of lithium salt in the electrolyte; the borate or fluoroborate in the CEI layer can also form a coordination effect with the transition metal ions in the positive electrode material, thereby helping to reduce the dissolution of transition metal ions (such as Mn) in the positive electrode material. In addition, the borate or fluoroborate free in the electrolyte can also form a coordination effect with the transition metal ions in the positive electrode material, thereby reducing the migration and reduction of the dissolved transition metal ions to the negative electrode. Therefore, the fluoroborate compound with a preset content can reduce the dissolution of transition metal ions in the positive electrode material and reduce the migration and reduction of the dissolved transition metal ions to the negative electrode, and can also reduce the unstable lithium insertion or extraction process on the negative electrode, thereby facilitating the transmission of lithium ions, and can also form a solid electrolyte interface (SEI) layer on the negative electrode. The SEI layer has good mechanical strength, can reduce the direct contact between the negative electrode and the electrolyte, and is conducive to alleviating the strain caused by the volume change during the charge and discharge process of the silicon negative electrode. When applied to secondary batteries, it helps to form a more stable CEI layer on the surface of lithium manganese iron phosphate, alleviate the decomposition of lithium salts in the electrolyte, and reduce the dissolution of Mn metal ions in lithium manganese iron phosphate, thereby reducing the adverse side reactions between the lithium manganese iron phosphate positive electrode material and the electrolyte, and improving the interface stability between the positive electrode material and the electrolyte. At the same time, it also helps to form a SEI layer with higher mechanical strength on the surface of the negative electrode, reduce the erosion of the electrode by the electrolyte, and alleviate the strain caused by the volume change of the electrode (such as the negative electrode) during the charge and discharge process.

[0091] Therefore, referring to Tables 1 to 3, the reaction kinetics of lithium manganese iron phosphate in the secondary batteries of Examples 1-9 of the present application are significantly improved, the impedance and storage gas production of the secondary batteries are significantly reduced, and they have good cycle stability, rate performance and high-temperature storage performance.

[0092] Compared with Examples 1-9, the secondary battery of Comparative Example 1 uses a basic electrolyte and does not add a fluoroborate-based compound. There is a serious side reaction between the lithium manganese iron phosphate in the positive electrode material and the electrolyte, which leads to decomposition of the electrolyte, excessive consumption of active lithium, or further formation of a thick and unstable positive electrode electrolyte interface (CEI), which is not conducive to the diffusion of lithium ions. As a result, the positive electrode material based on lithium manganese iron phosphate has slow kinetics, large impedance changes, severe high-temperature gas production, and poor cycle performance and rate performance.

[0093] Compared with Examples 1-9, the electrolytes used in the secondary batteries of Comparative Examples 2 and 3 have lower and higher contents of fluoroborate compounds, respectively. In Comparative Example 2, the content of the preferentially decomposed fluoroborate compound is too low, and the CEI layer formed on the positive electrode surface after decomposition is too thin. The electrolyte may continue to decompose, resulting in continued consumption of lithium salts and poor effect in reducing the dissolution of transition metal ions. In Comparative Example 3, the CEI layer or SEI layer formed on the electrode surface by the decomposed fluoroborate compound is too thick. Although it can inhibit the dissolution of transition metal ions, the excessive amount of fluoroborate compound affects the solvation structure, which in turn leads to poor electrochemical performance and affects the transport of lithium ions. Therefore, the cycle performance and rate performance of the secondary batteries of Comparative Examples 2 and 3 are not as good as those of the embodiments.

[0094] The above embodiments are only used to illustrate the technical solutions of the present application and are not intended to limit the present application. Although the present application has been described in detail with reference to the above preferred embodiments, those skilled in the art should understand that modifications or equivalent replacements of the technical solutions of the present application should not depart from the spirit and scope of the technical solutions of the present application.

Claims

1. An electrolyte comprising a lithium salt and a non-aqueous solvent, characterized in that: The electrolyte further comprises a fluoroborate compound, the general formula of which is: wherein R1 to R6 are each independently selected from one or more of hydrogen, halogen, C1 to C20 alkyl, C1 to C20 haloalkyl, C6 to C26 aryl, C6 to C26 haloaryl, and alkoxy, and at least one of R1 to R6 includes one or more fluorine-containing groups; Based on the mass of the electrolyte, the mass proportion of the fluoroborate-based compound is 0.8% to 5%.

2. The electrolyte according to claim 1, wherein Based on the mass of the electrolyte, the mass proportion of the fluoroborate-based compound is 0.8% to 3%.

3. The electrolyte according to claim 1, wherein The electrolyte satisfies at least one of the following conditions: (1) The non-aqueous solvent comprises one or more of ethylene carbonate, propylene carbonate, diethyl carbonate, ethyl methyl carbonate, ethyl acetate and propyl acetate; (2) The lithium salt includes one or more of lithium hexafluorophosphate, lithium bis(oxalatoborate), lithium bis(fluorosulfonylimide) and lithium bis(trifluoromethylsulfonylimide).

4. The electrolyte according to claim 1, wherein The fluoroborate compound is selected from One or more of .

5. A secondary battery comprising a positive electrode sheet, a negative electrode sheet, and a separator disposed between the positive electrode sheet and the negative electrode sheet, wherein the positive electrode sheet comprises a positive electrode current collector and a positive electrode material disposed on the surface of the positive electrode current collector, characterized in that: The positive electrode material includes lithium manganese iron phosphate, and the secondary battery further includes the electrolyte according to any one of claims 1 to 4.

6. The secondary battery according to claim 5, wherein The negative electrode sheet includes a negative electrode current collector and a negative electrode material disposed on a surface of the negative electrode current collector, wherein the negative electrode material includes a silicon material.

7. The secondary battery according to claim 5, wherein The mass ratio of the fluoroborate-based compound to the lithium manganese iron phosphate is 0.45% to 2.81%.

8. The secondary battery according to claim 5, wherein The concentration of the lithium salt in the non-aqueous solvent is 0.8 mol / L to 1.5 mol / L.

9. The secondary battery according to claim 5, wherein The electrolyte includes at least two non-aqueous solvents.

10. An electrical device, characterized in that: The electrical device comprises the secondary battery according to any one of claims 5 to 9.

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