Lithium ion secondary battery and liquid injection method

Abstract

The invention relates to a lithium ion secondary battery and a liquid injection method, the lithium ion secondary battery comprises an electrolyte, and the electrolyte comprises a first electrolyte and a second electrolyte; the first electrolyte comprises a sodium salt and a first lithium salt, the second electrolyte comprises a second lithium salt, and the following relational expressions are met: S1Na / S1Li is greater than or equal to 3, and S1Li * W1 + S2Li * W2gt; s1Na * W1, S1Na is the mass ratio of sodium salt in the first electrolyte, S1Li is the mass ratio of first lithium salt in the first electrolyte, S2Li is the mass ratio of second lithium salt in the second electrolyte, W1 is the mass ratio of the first electrolyte in the electrolyte, and W2 is the mass ratio of the second electrolyte in the electrolyte. The irreversible consumption of the negative electrode to lithium ions is improved by mixing a large amount of sodium salt in the first electrolyte, the first efficiency of the battery can be effectively improved, and the SEI membrane structure is optimized by combining secondary liquid injection, so that the SEI membrane is of a hollow structure to buffer the volume change of the negative electrode, and the cycle performance of the secondary battery is effectively improved; the method is especially suitable for a silicon-based lithium ion secondary battery system.

Description

Technical Field

[0001] The present invention relates to the technical field of batteries, and in particular to a lithium-ion secondary battery and a liquid injection method. Background Art

[0002] Lithium-ion batteries, due to their high energy density, long cycle life, and environmental friendliness, have been widely used in electric vehicles, laptops, and energy storage, particularly in power batteries. With the increasing demand for energy density, the specific capacity of conventional graphite systems has reached its upper limit. Silicon-based anodes, due to their extremely high theoretical specific capacity (~4200mAh / g, far exceeding the 372mAh / g of graphite anodes) and suitable operating potential, are considered a key material for the next generation of high-energy-density batteries.

[0003] However, there are still many problems in the practical application of silicon-based negative electrodes (negative electrode silicon content ≥ 5wt%). For example, during the first charge and discharge of silicon-based materials, some lithium will be irreversibly combined with silicon to form some lithium-silicon compounds that cannot be completely deintercalated during the subsequent discharge process. These lithiums are "trapped" inside the material and cannot return to the positive electrode, further exacerbating the loss of active lithium and resulting in a decrease in the initial coulombic efficiency. In addition, when silicon alloys with lithium during the charge and discharge process, its volume changes greatly (up to 300% or more), which will not only cause electrode pulverization and active material shedding, but also aggravate the repeated rupture and reconstruction of SEI, resulting in the consumption of active lithium and affecting subsequent cycle performance.

[0004] Existing improvement strategies often solve the problem of irreversible initial silicon-lithium alloy by pre-lithiation of the silicon-based material itself, and suppress the volume expansion problem during the silicon-based cycling process by coating the silicon-based material. However, this often requires further increasing the cost and process complexity of the silicon-based negative electrode, and it cannot fundamentally solve the dynamic instability of the silicon-electrolyte interface. Summary of the Invention

[0005] To solve the above problems, the present invention provides a lithium-ion secondary battery and an injection method. The lithium-ion secondary battery is obtained by secondary injection of a first electrolyte containing a high concentration of sodium salt and a low concentration of lithium salt and a second electrolyte containing lithium salt. Through the primary injection and formation, an irreversible silicon-sodium alloy and a SEI film substrate mainly composed of sodium-based compounds are formed. This not only avoids the subsequent formation of silicon-lithium alloy, but also helps to improve the initial coulombic efficiency of the battery. In combination with the secondary injection, an SEI film with a lithium-based compound as the surface layer is constructed. As the formed SEI film is cyclically charged and discharged, the sodium-based compound of the SEI film substrate partially dissolves into the electrolyte, so that the SEI film presents a hollow structure to achieve dynamic stability, which can effectively buffer the volume change of the negative electrode and improve the battery cycle performance.

[0006] The present invention provides the following technical solutions:

[0007] A first aspect of the present invention provides a lithium ion secondary battery, the lithium ion secondary battery comprising an electrolyte, the electrolyte comprising a first electrolyte and a second electrolyte;

[0008] The first electrolyte contains a sodium salt and a first lithium salt, and the second electrolyte contains a second lithium salt, and the following relationship is satisfied: S1 Na / S1 Li ≥3, S1 Li ×W1+S2 Li ×W2>S1 Na ×W1; and 6%≤S1 Na ≤15%, 40%≤W1≤80%;

[0009] Among them, S1 Na is the mass ratio of the sodium salt in the first electrolyte;

[0010] S1 Li is the mass ratio of the first lithium salt in the first electrolyte;

[0011] S2 Li is the mass ratio of the second lithium salt in the second electrolyte;

[0012] W1 is the mass proportion of the first electrolyte in the electrolyte;

[0013] W2 is the mass proportion of the second electrolyte in the electrolyte.

[0014] Furthermore, based on the total mass of the electrolyte, the total mass of the first lithium salt and the second lithium salt in the electrolyte accounts for 5%-20% of the total mass of the electrolyte.

[0015] Furthermore, 2%≤S1 Li ≤5%, more preferably 2.5%≤S1 Li ≤3.5%.

[0016] Furthermore, 12% ≤ S2 Li ≤22%, more preferably 15%≤S2 Li ≤19%.

[0017] Furthermore, 8%≤S1 Na ≤10%, 50%≤W1≤70%.

[0018] Furthermore, 20%≤W2≤60%, more preferably 30%≤W2≤50%.

[0019] Furthermore, the sodium salt is selected from one or more of sodium hexafluorophosphate, sodium perchlorate, sodium tetrafluoroborate, sodium hexafluoroarsenate, sodium bisoxalatoborate, sodium difluorooxalatoborate, sodium difluorophosphate, sodium bis(trifluoromethylsulfonyl)imide, and sodium bis(trifluoromethylsulfonyl)imide.

[0020] Furthermore, the first lithium salt and the second lithium salt are respectively selected from one or more of lithium hexafluorophosphate, lithium tetrafluoroborate, lithium perchlorate, lithium difluorophosphate, lithium bisoxalatoborate, lithium difluorooxalatoborate, lithium bis(fluorosulfonyl)imide, lithium bis(trifluoromethylsulfonyl)imide, and lithium difluorobis(oxalatophosphate).

[0021] Furthermore, the second electrolyte further contains an additive, and the mass proportion of the additive in the second electrolyte is 2.5%-10%; preferably, the additive is selected from one or more of vinylene carbonate, vinyl ethylene carbonate, fluoroethylene carbonate, vinyl sulfate, 1-propyl phosphoric anhydride, 1,3-propane sultone, vinyl sulfate, adiponitrile, butane sultone, propene sultone, and hexamethyldisilazane.

[0022] Furthermore, the first electrolyte further comprises a first solvent, and the second electrolyte further comprises a second solvent; preferably, the first solvent and the second solvent are respectively selected from one or more of carbonates, carboxylates, ethers, sulfones, fluorinated carbonates, fluorinated carboxylates, fluorinated ethers, and fluorinated sulfones.

[0023] Furthermore, the lithium-ion secondary battery includes a negative electrode plate, which includes a current collector and a negative electrode active layer arranged on at least one side of the current collector along the thickness direction. The negative electrode active layer includes a negative electrode active material, and the negative electrode active material includes a silicon-based material.

[0024] Furthermore, the silicon content W in the negative electrode active material Si ≥5%, and satisfy the following relationship: 3≤W Si / (S1 Li ×W1)≤10.

[0025] A second aspect of the present invention provides a method for injecting liquid into a lithium-ion secondary battery according to the first aspect, comprising the following steps:

[0026] S1. Injecting the first electrolyte into the battery, allowing it to stand and undergo formation;

[0027] S2. Injecting the second electrolyte into the formed battery to complete the injection.

[0028] Furthermore, the chemical formation is carried out using conventional chemical formation methods in the art.

[0029] Compared with the prior art, the present invention has the following beneficial effects:

[0030] The present invention prepares a lithium-ion secondary battery by using a first electrolyte containing a high concentration of sodium salt and a low concentration of lithium salt and two second electrolytes containing lithium salts. The high concentration of sodium salt mixed in the first electrolyte is used to improve the irreversible consumption of lithium ions by the negative electrode silicon-based material in the initial stage, thereby effectively improving the first coulombic efficiency of the battery; at the same time, combined with the two second electrolytes containing high concentrations of lithium salts, an SEI film with a sodium-based compound as a base and a lithium-based compound as a surface layer is formed. Based on the high solubility of the sodium-based compound in the electrolyte, it will partially dissolve into the electrolyte during the battery's cyclic charge and discharge process, thereby optimizing the SEI film structure during the cycle process, making the SEI film present a hollow structure to achieve dynamic stability, and can effectively improve the damage to the SEI caused by the volume change of the negative electrode in the late cycle, thereby improving the battery cycle life. DETAILED DESCRIPTION

[0031] Unless otherwise defined, all technical and scientific terms used herein have the same meaning as commonly understood by those skilled in the art to which the present invention pertains. The terms used herein in the specification of the present invention are for the purpose of describing specific embodiments only and are not intended to limit the present invention. The term "and / or" used herein includes any and all combinations of one or more of the relevant listed items. The term "includes" or "comprising" described in the present invention means that in addition to the components described, it may also include or contain other components. The term "includes" or "comprising" described in the present invention may also be replaced by the closed form "for" or "consisting of..."

[0032] Silicon-based anodes with high theoretical specific capacity have the following problems in practical applications: (1) During the first charge and discharge, the silicon-based material combines with the active lithium to form a lithium-silicon compound that cannot be completely deintercalated, resulting in a decrease in the initial Coulombic efficiency; (2) The silicon-based anode undergoes a huge volume change during the charge and discharge process, which will aggravate the repeated rupture and reconstruction of the SEI, resulting in the consumption of active lithium and affecting subsequent cycle performance. Currently, improving the anode material itself not only increases cost and process complexity, but also fails to fundamentally solve the problem of dynamic instability of the silicon-electrolyte interface.

[0033] To solve the above problems, the embodiments of the present invention provide a lithium-ion secondary battery, wherein the lithium-ion secondary battery includes an electrolyte, wherein the electrolyte includes a first electrolyte and a second electrolyte; the first electrolyte includes a sodium salt and a first lithium salt, and the second electrolyte includes a second lithium salt, and the following relationship is satisfied: S1 Na / S1 Li ≥3, S1 Li ×W1+S2 Li ×W2>S1 Na ×W1; and 6%≤S1 Na≤15%, 40%≤W1≤80%;

[0034] Among them, S1 Na is the mass ratio of the sodium salt in the first electrolyte;

[0035] S1 Li is the mass ratio of the first lithium salt in the first electrolyte;

[0036] S2 Li is the mass ratio of the second lithium salt in the second electrolyte;

[0037] W1 is the mass proportion of the first electrolyte in the electrolyte;

[0038] W2 is the mass proportion of the second electrolyte in the electrolyte.

[0039] Based on the problems of low first coulombic efficiency and poor cycle performance in current silicon-based lithium-ion batteries, the present invention adopts a mixed salt + two-injection design, using a first electrolyte containing a high concentration of sodium salt and a low concentration of lithium salt for the first injection, and forming an irreversible silicon-sodium alloy through chemical formation. In addition, in the alloying reaction, compared with lithium atoms, the number of sodium atoms bound to each silicon atom when silicon reacts with sodium is relatively small, which can form a relatively small amount of irreversible silicon-sodium alloy and avoid the subsequent formation of silicon-lithium alloy, which is beneficial to improving the first coulombic efficiency of the battery. In addition, excessive sodium salts will form a SEI membrane substrate mainly composed of sodium-based compounds in the formation stage, and then a secondary injection will be carried out. The second electrolyte used for the secondary injection contains a high concentration of lithium salts to provide sufficient lithium ions to ensure the efficient transmission of lithium ions in the later charging and discharging process of the battery. At the same time, the SEI membrane is secondary constructed to form a SEI membrane with sodium-based compounds as the substrate and lithium-based compounds as the surface layer. Based on the relatively high solubility of sodium-based compounds with low Lewis acidity in the electrolyte, in the subsequent battery cycle charge and discharge process, the sodium-based compounds in the SEI membrane will be partially dissolved in the solvent, thereby forming a SEI membrane with a hollow structure and achieving dynamic stability, thereby effectively improving the damage to the SEI membrane caused by the volume change of the silicon-based negative electrode in the late cycle, and improving the cycle life of the battery.

[0040] In order to ensure that the first electrolyte can be normally formed after injection, and that the sodium salt and lithium salt react more in favor of the sodium salt in the initial silicon alloying reaction, it is necessary to ensure that the injection amount of the first electrolyte is not less than 40% of the total electrolyte, and the mass ratio of the sodium salt to the lithium salt in the first electrolyte is not less than 3, that is, S1 Na / S1 Li ≥3; At the same time, in order to ensure that the silicon-sodium alloy can be fully formed after formation, and to build a uniform sodium-containing SEI at the bottom of the SEI to achieve the subsequent formation of the SEI film with a hollow structure, the mass proportion of sodium salt in the first electrolyte is S1Na It must be no less than 6%; however, the injection amount of the first electrolyte and the mass proportion of the sodium salt in the first electrolyte cannot be too high, otherwise the total amount of sodium salt in the total electrolyte will exceed the lithium salt, the battery cell cannot circulate normally, and the capacity will decay rapidly. In order to ensure that the battery can circulate normally and effectively improve the battery's first coulombic efficiency and cycle performance, it is necessary to meet the following requirements at the same time: S1 Na / S1 Li ≥3, S1 Li ×W1+S2 Li ×W2>S1 Na ×W1, and 6%≤S1 Na ≤15% (e.g. S1 Na 6%, 7%, 8%, 9%, 10%, 11%, 12%, 13%, 14%, 15%, etc., including but not limited to the mass proportions listed above), 40%≤W1≤80% (for example, 40%, 45%, 50%, 55%, 60%, 65%, 70%, 75%, 80%, etc., including but not limited to the mass proportions listed above).

[0041] In addition, the negative electrode of silicon-based lithium-ion batteries often contains graphite. Since sodium ions have a large radius and are difficult to embed in graphite, an appropriate amount of lithium salt is introduced into the first electrolyte. On the one hand, the low concentration of lithium salt does not affect the reaction between silicon and sodium salt, and at the same time, it can be embedded in graphite to avoid the risk of sodium precipitation in the early stage of the reaction. Preferably, the mass proportion of the first lithium salt in the first electrolyte is S1 Li Controlled within the range of 2%-5%, such as 2%, 2.5%, 3%, 3.5%, 4%, 4.5%, 5%, etc., including but not limited to the mass proportions listed above.

[0042] In the present invention, based on the total mass of the electrolyte, the total mass of the first lithium salt and the second lithium salt in the electrolyte accounts for 5%-20% of the total mass of the electrolyte, for example, 5%, 6%, 7%, 8%, 9%, 10%, 11%, 12%, 13%, 14%, 15%, 16%, 17%, 18%, 19%, 20%, etc., including but not limited to the mass proportions listed above. The lithium salt content in the electrolyte is controlled within the above range. On the one hand, sufficient lithium ions are provided to ensure efficient transmission of lithium ions in the later charging and discharging process of the battery. On the other hand, excessive lithium salt in the electrolyte is avoided, which increases the viscosity and affects the ionic conductivity.

[0043] In the present invention, the mass proportion of the second lithium salt in the second electrolyte is S2 Li It will affect the cycle performance of the battery. If S2 Li If the value is too large, the total lithium salt content in the battery electrolyte will be high, the viscosity of the electrolyte will increase, resulting in increased polarization and affecting the battery cycle performance; but S2 LiThe value should not be too small. If the mass of the second lithium salt in the second electrolyte is too small, the total lithium salt content in the electrolyte in the battery cell will be low, affecting the conductivity of the electrolyte and thus the cycle performance of the battery. Li Controlled within the range of 12%-22%, for example, 12%, 13%, 14%, 15%, 16%, 17%, 18%, 19%, 20%, 21%, 22%, etc., to obtain a lithium-ion battery with a better cycle life.

[0044] In the present invention, W2 is preferably controlled within the range of 20%-60%, such as 20%, 25%, 30%, 35%, 40%, 45%, 50%, 55%, 60%, etc., including but not limited to the mass proportions listed above.

[0045] In some preferred embodiments of the present invention, 8%≤S1 Na ≤10% and 50%≤W1≤70%; 15%≤S2 Li ≤19% and 30%≤W2≤50%.

[0046] In the present invention, the sodium salt is selected from one or more of sodium hexafluorophosphate, sodium perchlorate, sodium tetrafluoroborate, sodium hexafluoroarsenate, sodium bisoxalatoborate, sodium difluorooxalatoborate, sodium difluorophosphate, sodium bis(trifluoromethylsulfonyl)imide, and sodium bis(trifluoromethylsulfonyl)imide. Other electrolyte sodium salts commonly used in the art may also be selected.

[0047] In the present invention, the first lithium salt and the second lithium salt are respectively selected from one or more of lithium hexafluorophosphate, lithium tetrafluoroborate, lithium perchlorate, lithium difluorophosphate, lithium bisoxalatoborate, lithium difluorooxalatoborate, lithium bisfluorosulfonyl imide, lithium bis(trifluoromethylsulfonyl)imide, and lithium difluorobisoxalatophosphate; the present invention does not limit the type of lithium salt, and other electrolyte lithium salts commonly used in the art may also be selected.

[0048] In the present invention, the second electrolyte further contains additives. The content of the additives in the second electrolyte has a great influence on the cycle performance of the battery. If the content of the additives is too much, the SEI outer layer structure will be too thick, the interfacial impedance will increase, the polarization will increase, and the battery cycle performance will deteriorate; if the content of the additives is too little, the SEI outer layer structure will be too thin, and a stable hollow SEI structure cannot be constructed, thereby affecting the cycle life of the battery. Preferably, the mass proportion of the additives in the second electrolyte is 2.5%-10%, for example, 2.5%, 3%, 4%, 5%, 6%, 7%, 8%, 9%, 10%, etc., so that the prepared lithium ion battery exhibits better cycle performance; more preferably, the additives are selected from one or more of vinylene carbonate, vinyl ethylene carbonate, fluoroethylene carbonate, vinyl sulfate, 1-propyl phosphoric anhydride, 1,3-propane sultone, vinyl sulfate, adiponitrile, butane sultone, propylene sultone, and hexamethyldisilazane.

[0049] In the present invention, the first electrolyte further comprises a first solvent, and the second electrolyte further comprises a second solvent; preferably, the first solvent and the second solvent are respectively selected from one or more of carbonates, carboxylates, ethers, sulfones, fluorocarbons, fluorocarboxylates, fluoroethers, and fluorosulfones; wherein carbonate solvents include but are not limited to dimethyl carbonate, diethyl carbonate, ethyl methyl carbonate, ethylene carbonate, propylene carbonate, etc.; carboxylate solvents include but are not limited to ethyl acetate, methyl acetate, propyl propionate, ethyl propionate, etc.; ether solvents include but are not limited to ethylene glycol dimethyl ether, propylene glycol dimethyl ether, etc.; sulfone solvents include but are not limited to Sulfolane, dimethyl sulfite and methyl ethyl sulfone, etc.; fluorocarbonate solvents include but are not limited to fluoroethylene carbonate, fluoroethylene carbonate, etc.; fluorocarboxylate solvents include but are not limited to fluoroethyl acetate, fluoropropionate, ethyl trifluoroacetate, etc.; fluoroether solvents include but are not limited to 1,1,2,2-tetrafluoroethyl-2,2,3,3-tetrafluoropropyl ether, bis(2,2,2-trifluoroethyl) ether, etc.; fluorosulfone solvents include but are not limited to bis(2,2,2-trifluoroethyl) sulfone, 1,1,1-trifluoro-2-(trifluoromethyl)-3-oxa-2-sulfopentane, tetrafluoro-1,3-dioxolane-2-sulfone, etc.

[0050] In the present invention, the lithium-ion secondary battery comprises a negative electrode plate, which comprises a current collector and a negative electrode active layer arranged on at least one side of the current collector along the thickness direction. The negative electrode active layer comprises a negative electrode active material, and the negative electrode active material comprises a silicon-based material.

[0051] In some preferred embodiments of the present invention, the silicon content in the negative electrode active material is W Si ≥5%, and satisfy the following relationship: 3≤W Si / (S1 Li ×W1)≤10, in order to meet the demand for irreversible silicon-sodium alloy without affecting the capacity of subsequent silicon-based materials.

[0052] The embodiments of the present invention further provide a method for injecting liquid into the lithium-ion secondary battery, comprising the following steps:

[0053] S1. Injecting the first electrolyte into the battery, allowing it to stand and undergo formation;

[0054] S2. Injecting the second electrolyte into the formed battery to complete the injection.

[0055] The present invention employs a conventional secondary injection method for injection. A first electrolyte containing a high-concentration sodium salt and a low-concentration lithium salt is first injected. After injection, the electrolyte is allowed to stand and undergoes chemical formation using a conventional chemical formation method. During this stage, an irreversible silicon-sodium alloy and an SEI membrane substrate composed primarily of sodium-based compounds are formed. A second electrolyte containing a high-concentration lithium salt is then injected a second time to complete the injection. After the injection, the battery undergoes conventional aging, shaping, and volume separation to form an SEI membrane with sodium-based compounds as the substrate and lithium-based compounds as the surface layer. The lithium-ion secondary battery prepared using this method can not only effectively improve the initial coulombic efficiency but also increase the cycle life of silicon-based lithium-ion secondary batteries.

[0056] The present invention will be further described below with reference to specific examples so that those skilled in the art can better understand the present invention and implement it, but the examples are not intended to limit the present invention.

[0057] Example 1

[0058] This embodiment relates to the preparation of a lithium ion battery, and the specific preparation process is as follows:

[0059] Preparation of positive electrode sheets: positive electrode active material NCM (811), binder polyvinylidene fluoride (PVDF), conductive carbon black and single-walled carbon nanotubes are mixed in a weight ratio of 97.2:1.5:1.2:0.1, N-methylpyrrolidone (NMP) is added, and the mixture is stirred under the action of a vacuum mixer until the mixed system becomes a positive electrode slurry with uniform fluidity; the positive electrode slurry is evenly coated on the current collector aluminum foil, and the desired positive electrode sheets are obtained by drying, rolling and cutting.

[0060] Preparation of negative electrode sheet: negative electrode active material (graphite + silicon oxide, where the silicon content in the negative electrode active material is W si =30%), a thickener, sodium carboxymethyl cellulose (CMC-Na), a binder, styrene butadiene rubber, and a conductive agent, acetylene black, were mixed in a weight ratio of 97:1:1:1, deionized water was added, and a negative electrode slurry was obtained under the action of a vacuum mixer. The negative electrode slurry was evenly coated on a high-strength carbon-coated aluminum foil to obtain a pole piece, and the obtained pole piece was dried, rolled, and slit to obtain a negative electrode sheet.

[0061] Preparation of electrolyte:

[0062] Preparation of the first electrolyte: In a glove box filled with inert gas (H2O <10ppm, O2 <5ppm), fluoroethylene carbonate, ethyl methyl carbonate, and diethyl carbonate were mixed in a mass ratio of FEC:EMC:DEC = 3:5:2, and then 9wt% NaFSI and 3wt% LiPF6 based on the total weight of the electrolyte were slowly added to the mixed solution;

[0063] Preparation of the second electrolyte: In a glove box filled with inert gas (H2O <10ppm, O2 <5ppm), fluoroethylene carbonate, ethyl methyl carbonate, and diethyl carbonate were mixed in a mass ratio of FEC:EMC:DEC = 3:5:2, and then 17wt% of LiPF6 and 5wt% of vinylene carbonate (VC) based on the total weight of the electrolyte were slowly added to the mixed solution;

[0064] Diaphragm: 9 micron thick PP film;

[0065] Preparation of the battery: The positive electrode, diaphragm, and negative electrode prepared above are stacked in order to ensure that the diaphragm is in isolation between the positive and negative electrode sheets. The bare cell is placed in an aluminum-plastic film outer package to obtain a 5Ah dry cell, and the first electrolyte prepared above is injected into the dried battery, packaged, allowed to stand, and formed. The formation process is: 25°C, 0.05C constant current charging to 3.2V, recording the charging capacity V1, after standing for 10min, 0.1C constant current charging to 3.75V, recording the charging capacity V2, and completing the formation step; after formation, the second electrolyte is injected for aging (aging temperature 60°C, 12h), shaping and capacity division, and the capacity division process is: 25°C, 0.33C constant current charging to 4.25V, then constant voltage to 0.05C, recording the charging capacity V3, after standing for 10min, 0.33C constant current discharge to 2.5V, and record the discharge capacity as the first discharge capacity V 放 ;Total charging capacity V 充 =V1+V2+V3, the total charging capacity of the formation and fractionation is V 充 and discharge capacity V 放 , the first coulomb efficiency C=V 放 / V 充 .

[0066] In this embodiment, the mass proportion W1 of the first electrolyte in the total electrolyte is 60%, and the mass proportion W2 of the second electrolyte in the total electrolyte is 40%.

[0067] Example 2

[0068] This embodiment relates to the preparation of a lithium-ion battery. The only difference from Example 1 is that the mass proportion W1 of the first electrolyte in the total electrolyte is 40%, and the mass proportion W2 of the second electrolyte in the total electrolyte is 60%. The remaining operations are the same to prepare the corresponding lithium-ion battery.

[0069] Example 3

[0070] This embodiment relates to the preparation of a lithium ion battery, which differs from Example 1 in that:

[0071] (1) Mass proportion of NaFSI in the first electrolyte S1 NaThe mass ratio of LiPF6 in the first electrolyte is S1 Li is 2.2%; the mass proportion W1 of the first electrolyte in the total electrolyte is 80%;

[0072] (2) The mass proportion of LiPF6 in the second electrolyte is S2 Li The mass proportion W2 of the second electrolyte in the total electrolyte is 20%.

[0073] The remaining operations are the same, and the corresponding lithium-ion battery is prepared.

[0074] Example 4

[0075] This embodiment relates to the preparation of a lithium ion battery, and the only difference from the embodiment 1 is that the mass proportion of NaFSI in the first electrolyte is S1 Na The mass ratio of LiPF6 in the first electrolyte is S1 Li The other operations were the same to prepare the corresponding lithium-ion battery.

[0076] Example 5

[0077] This embodiment relates to the preparation of a lithium ion battery, and the only difference from the embodiment 1 is that the mass proportion of NaFSI in the first electrolyte is S1 Na The mass ratio of LiPF6 in the first electrolyte is S1 Li The other operations were the same to prepare the corresponding lithium-ion battery.

[0078] Example 6

[0079] This embodiment relates to the preparation of a lithium ion battery, and the only difference from the embodiment 1 is that the mass proportion of the additive VC in the second electrolyte is S 添 The other operations were the same to prepare the corresponding lithium-ion battery.

[0080] Example 7

[0081] This embodiment relates to the preparation of a lithium ion battery, and the only difference from the embodiment 1 is that the mass proportion of the additive VC in the second electrolyte is S 添 The remaining operations were the same to prepare the corresponding lithium-ion battery.

[0082] Example 8

[0083] This embodiment relates to the preparation of a lithium ion battery, which differs from the embodiment 1 only in that the mass proportion of LiPF6 in the second electrolyte is S2 Li The other operations were the same to prepare the corresponding lithium-ion battery.

[0084] Example 9

[0085] This embodiment relates to the preparation of a lithium ion battery, which differs from the embodiment 1 only in that the mass proportion of LiPF6 in the second electrolyte is S2 Li The remaining operations were the same to prepare the corresponding lithium-ion battery.

[0086] Example 10

[0087] This embodiment relates to the preparation of a lithium-ion battery. The only difference from Example 1 is that an equal amount of LiFSI is used to replace LiPF6 in the first electrolyte and the second electrolyte; the other operations are the same, and the corresponding lithium-ion battery is prepared.

[0088] Example 11

[0089] This embodiment relates to the preparation of a lithium-ion battery. The only difference from Example 1 is that an equal amount of NaPF6 is used to replace NaFSI in the first electrolyte; the other operations are the same, and the corresponding lithium-ion battery is prepared.

[0090] Example 12

[0091] This embodiment relates to the preparation of a lithium ion battery. The only difference from Example 1 is that an equal amount of propylene sulfite (PS) is used to replace VC in the second electrolyte. The remaining operations are the same to prepare the corresponding lithium ion battery.

[0092] Example 13

[0093] This embodiment relates to the preparation of a lithium-ion battery. The only difference from the embodiment 1 is that the silicon content W in the negative electrode active material is si =50%; the rest of the operations are the same, and the corresponding lithium-ion battery is prepared.

[0094] Example 14

[0095] This embodiment relates to the preparation of a lithium ion battery, and the only difference from the embodiment 1 is that the mass proportion of the additive VC in the second electrolyte is S 添 The other operations were the same to prepare the corresponding lithium-ion battery.

[0096] Example 15

[0097] This embodiment relates to the preparation of a lithium ion battery, and the only difference from the embodiment 1 is that the mass proportion of the additive VC in the second electrolyte is S 添 The other operations were the same to prepare the corresponding lithium-ion battery.

[0098] Comparative Example 1

[0099] This comparative example relates to the preparation of a lithium-ion battery, which differs from Example 1 only in that no NaFSI is added to the first electrolyte; the remaining operations are the same, and the corresponding lithium-ion battery is prepared.

[0100] Comparative Example 2

[0101] This comparative example relates to the preparation of a lithium-ion battery, which differs from Example 1 only in that no NaFSI is added to the first electrolyte, and the silicon content in the negative electrode active material is W si =50%; the rest of the operations are the same, and the corresponding lithium-ion battery is prepared.

[0102] Comparative Example 3

[0103] This comparative example relates to the preparation of a lithium-ion battery, which differs from Example 1 only in that the mass proportion W1 of the first electrolyte in the total electrolyte is 90%, and the mass proportion W2 of the second electrolyte in the total electrolyte is 10%; the remaining operations are the same, and the corresponding lithium-ion battery is prepared.

[0104] Comparative Example 4

[0105] This comparative example relates to the preparation of a lithium-ion battery, which differs from Example 1 only in that the mass proportion W1 of the first electrolyte in the total electrolyte is 20%, and the mass proportion W2 of the second electrolyte in the total electrolyte is 80%; the remaining operations are the same, and the corresponding lithium-ion battery is prepared.

[0106] Comparative Example 5

[0107] This comparative example relates to the preparation of a lithium ion battery, which differs from Example 1 only in that the mass proportion of NaFSI in the first electrolyte is S1. Na The mass ratio of LiPF6 in the first electrolyte is S1 Li The other operations were the same to prepare the corresponding lithium-ion battery.

[0108] Comparative Example 6

[0109] This comparative example relates to the preparation of a lithium ion battery, which differs from Example 1 only in that the mass proportion of NaFSI in the first electrolyte is S1. Na 3%, the mass proportion of LiPF6 in the first electrolyte is S1 Li The other operations were the same to prepare the corresponding lithium-ion battery.

[0110] Comparative Example 7

[0111] This comparative example relates to the preparation of a lithium ion battery, which differs from Example 1 only in that the mass proportion of NaFSI in the first electrolyte is S1. NaThe mass ratio of LiPF6 in the first electrolyte is S1 Li The other operations were the same to prepare the corresponding lithium-ion battery.

[0112] Comparative Example 8

[0113] This comparative example relates to the preparation of a lithium-ion battery, which differs from Example 1 only in that the mass proportion W1 of the first electrolyte in the total electrolyte is 100%, and the mass proportion W2 of the second electrolyte in the total electrolyte is 0; the remaining operations are the same, and the corresponding lithium-ion battery is prepared.

[0114] Comparative Example 9

[0115] This comparative example relates to the preparation of a lithium ion battery, which differs from Example 1 in that:

[0116] (1) Mass proportion of NaFSI in the first electrolyte S1 Na 3%, the mass proportion of LiPF6 in the first electrolyte is S1 Li is 9%; the mass proportion W1 of the first electrolyte in the total electrolyte is 100%;

[0117] (2) The mass proportion W2 of the second electrolyte in the total electrolyte is 0.

[0118] The remaining operations are the same, and the corresponding lithium-ion battery is prepared.

[0119] The electrolyte parameters and negative electrode silicon content of the lithium ion batteries prepared in the above examples and comparative examples are shown in Table 1 below:

[0120] Table 1

[0121]

[0122]

[0123] Performance Testing

[0124] The lithium-ion batteries prepared in the above examples and comparative examples were subjected to the following electrochemical performance tests. The specific test methods are as follows:

[0125] First coulombic efficiency test: as described in Example 1.

[0126] Initial discharge capacity and cycle performance test: Set the temperature to 25°C, charge at a constant current of 1C to 4.25V, then discharge at a constant current of 1C to 2.5V, and record the discharge capacity as the initial discharge capacity; repeat the above steps until the discharge capacity drops to 80% of the initial capacity, and record the number of cycles.

[0127] The test results are shown in Table 2 below

[0128] Table 2

[0129]

[0130]

[0131] As can be seen from Table 2, compared with Comparative Example 1, the lithium-ion batteries prepared in Examples 1-15 by using one injection of a first electrolyte containing a high concentration of sodium salt and a low concentration of lithium salt and two injections of a second electrolyte containing lithium salt have higher initial discharge capacity and first coulombic efficiency.

[0132] It can be seen from Examples 1-3 and Comparative Examples 3 and 4 that the ratio of the injection amount of the first electrolyte and the second electrolyte will affect the electrochemical performance of the prepared lithium-ion battery. This is because the injection amount of the first electrolyte will affect whether the formation can proceed normally, the formation of silicon-sodium alloy and the construction of SEI structure. If the mass ratio W1 value of the first electrolyte in the total electrolyte is relatively low (for example, Example 2), it can be normally formed to form a silicon-sodium alloy, but due to the low sodium salt content, a better SEI structure cannot be constructed, affecting the battery cycle performance, and if W1 is too small (for example, Comparative Example 3), the injection amount of the first electrolyte is too low, it cannot be normally formed, and there is no performance; but the W1 value cannot be too large (Example 3, Comparative Example 4), the injection amount of the first electrolyte is too high, and the corresponding injection amount of the second electrolyte is too small, then the overall lithium salt content of the battery cell is less, which will cause the conductivity of the electrolyte itself to decrease. If the sodium salt content in the electrolyte exceeds the lithium salt, the battery cell cannot circulate normally, resulting in rapid decay. Therefore, the injection amount of the first electrolyte needs to be controlled within a suitable range, such as 40%-80%, to obtain a lithium-ion battery with higher first coulombic efficiency and better cycle performance.

[0133] It can be seen from Examples 1, 4, 5 and Comparative Examples 5 and 6 that the mass ratio of the sodium salt and the first lithium salt in the first electrolyte will affect the first effect and cycle performance of the battery. When the sodium salt content in the first electrolyte is too low, the silicon-sodium alloy cannot be fully formed, and the lithium salt content is too low to be normally formed, which will deteriorate the first effect. In addition, too low a content of sodium salt cannot effectively construct the SEI substrate, affecting the formation of the hollow structure, further affecting the cycle life of the battery. At the same time, the content of sodium salt and the first lithium salt in the first electrolyte should not be too high. Excessive sodium salt will hinder the normal reaction of lithium ions, reducing efficiency, and too high a salt content will increase the viscosity of the electrolyte, resulting in increased polarization, affecting the long-term cycle performance of the battery.

[0134] From Example 1 and Comparative Example 7, it can be seen that in the silicon alloy reaction at the formation stage, in order to ensure that the sodium salt can form a silicon-sodium alloy with the silicon material better than the lithium salt, the mass ratio of the sodium salt to the first lithium salt in the first electrolyte needs to be controlled to be greater than 3, that is, S1 Na / S1 Li ≥3; if S1 is not met Na / S1 Li ≥3, on the one hand, the lithium salt will compete with the sodium salt to form a silicon-sodium alloy to improve the initial efficiency, and it will not be able to build a uniform sodium-containing SEI at the bottom of the SEI, resulting in the subsequent "hollow" SEI being unable to form, affecting the battery's cycle performance. Therefore, compared with Example 1, the lithium-ion battery prepared in Comparative Example 7 not only has a relatively low initial efficiency, but also a significantly reduced cycle life.

[0135] As shown in Examples 1, 6, 7, 14, and 15, the additive content in the second electrolyte significantly affects the battery's cycling performance. Excessive additive content can lead to an excessively thick SEI outer layer, increased interfacial impedance, and increased polarization, worsening the battery's cycling performance. Excessive additive content can result in an excessively thin SEI outer layer, preventing the formation of a stable hollow SEI structure, which in turn affects the battery's cycle life. Preferably, when the additive content in the second electrolyte is controlled within the range of 2.5% to 10%, the resulting lithium-ion battery exhibits superior cycling performance.

[0136] In addition, it can be seen from Examples 1, 8, and 9 that the mass proportion of the second lithium salt in the second electrolyte is S2 Li It has a great influence on the battery cycle performance. If S2 Li If the value is too large, the total lithium salt content in the battery electrolyte will be high, the viscosity of the electrolyte will increase, resulting in increased polarization and affecting the battery cycle performance; but S2 Li The value should not be too small. If the mass of the second lithium salt in the second electrolyte is too small, the total lithium salt content in the electrolyte in the battery cell will be low, affecting the conductivity of the electrolyte and thus the cycle performance of the battery. Li Control it within the range of 12%-22% to obtain a lithium-ion battery with a better cycle life.

[0137] It can be seen from Example 1 and Comparative Example 8 that only one injection is performed with the first electrolyte of high concentration of sodium salt and low concentration of lithium salt. During the initial charge and discharge process, sodium ions are embedded in the silicon material, resulting in a decrease in the negative electrode capacity. Lithium ions cannot be embedded, thereby lithium precipitation, resulting in the battery cell being unable to operate normally. In addition, it can be seen from Example 1 and Comparative Example 9 that if only one injection is performed with the first electrolyte of high concentration of lithium salt and low concentration of sodium salt, lithium salt will dominate the formation of silicon-lithium alloy, and the addition of sodium salt will not significantly improve the initial effect, and a uniform sodium-containing SEI cannot be constructed at the bottom of SEI, resulting in the inability to form a subsequent "hollow" SEI, and the battery cycle performance cannot be effectively improved.

[0138] The above-described embodiments are merely preferred examples for the purpose of fully illustrating the present invention, and the scope of protection of the present invention is not limited thereto. Equivalent substitutions or modifications made by those skilled in the art based on the present invention are within the scope of protection of the present invention. The scope of protection of the present invention shall be subject to the claims.

Claims

1. A lithium-ion secondary battery, characterized in that: The lithium-ion secondary battery comprises an electrolyte, wherein the electrolyte comprises a first electrolyte and a second electrolyte; The first electrolyte comprises a sodium salt and a first lithium salt, and the second electrolyte comprises a second lithium salt, and the following relationship is satisfied: S1 Na / S1 Li ≥3,S1 Li ×W1+S2 Li ×W2>S1 Na ×W1; And 6% ≤ S1 Na ≤15%, 40%≤W1≤80%; Among them, S1 Na is the mass ratio of the sodium salt in the first electrolyte; S1 Li is the mass ratio of the first lithium salt in the first electrolyte; S2 Li is the mass ratio of the second lithium salt in the second electrolyte; W1 is the mass proportion of the first electrolyte in the electrolyte; W2 is the mass proportion of the second electrolyte in the electrolyte.

2. The lithium-ion secondary battery according to claim 1, wherein Based on the total mass of the electrolyte, the total mass of the first lithium salt and the second lithium salt in the electrolyte accounts for 5%-20% of the total mass of the electrolyte.

3. The lithium-ion secondary battery according to claim 1, wherein 2%≤S1 Li ≤5%,12%≤S2 Li ≤22%。 4. The lithium-ion secondary battery according to claim 1, wherein 8%≤S1 Na ≤10%, and 50%≤W1≤70%.

5. The lithium-ion secondary battery according to claim 4, characterized in that 15％≤S2 Li ≤19％。 6. The lithium-ion secondary battery according to claim 1, wherein The sodium salt is selected from one or more of sodium hexafluorophosphate, sodium perchlorate, sodium tetrafluoroborate, sodium hexafluoroarsenate, sodium bisoxalatoborate, sodium difluorooxalatoborate, sodium difluorophosphate, sodium bis(trifluoromethylsulfonyl)imide, and sodium bis(trifluoromethylsulfonyl)imide; The first lithium salt and the second lithium salt are respectively selected from one or more of lithium hexafluorophosphate, lithium tetrafluoroborate, lithium perchlorate, lithium difluorophosphate, lithium bisoxalatoborate, lithium difluorooxalatoborate, lithium bis(fluorosulfonyl)imide, lithium bis(trifluoromethylsulfonyl)imide, and lithium difluorobis(oxalatophosphate).

7. The lithium-ion secondary battery according to claim 1, wherein The second electrolyte further comprises an additive, wherein the additive accounts for 2.5% to 10% by weight in the second electrolyte; The additive is selected from one or more of vinylene carbonate, vinyl ethylene carbonate, fluoroethylene carbonate, vinyl sulfate, 1-propyl phosphoric anhydride, 1,3-propane sultone, vinyl sulfate, adiponitrile, butane sultone, propene sultone, and hexamethyldisilazane.

8. The lithium-ion secondary battery according to claim 1, wherein The first electrolyte further comprises a first solvent, and the second electrolyte further comprises a second solvent; The first solvent and the second solvent are respectively selected from one or more of carbonates, carboxylates, ethers, sulfones, fluorinated carbonates, fluorinated carboxylates, fluorinated ethers, and fluorinated sulfones.

9. The lithium-ion secondary battery according to claim 1, wherein The lithium-ion secondary battery comprises a negative electrode plate, the negative electrode plate comprises a current collector and a negative electrode active layer provided on at least one side of the current collector along the thickness direction, the negative electrode active layer comprises a negative electrode active material, and the negative electrode active material comprises a silicon-based material; The silicon content W in the negative electrode active material Si ≥5%, and satisfy the following relationship: 3≤W Si / (S1 Li ×W1)≤10.

10. A method for injecting liquid into a lithium-ion secondary battery according to any one of claims 1 to 9, characterized in that: The following steps are involved: S1. Injecting the first electrolyte into the battery, allowing it to stand and undergo formation; S2. Injecting the second electrolyte into the formed battery to complete the injection.

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