Non-aqueous electrolyte and lithium ion battery

Abstract

The invention relates to the field of battery materials, and discloses a non-aqueous electrolyte and a lithium ion battery. The non-aqueous electrolyte provided by the invention comprises an organic solvent, a lithium salt and an additive, the organic solvent comprises a carboxylic ester solvent, the carboxylic ester solvent comprises ethyl butyrate, and the mass ratio of the ethyl butyrate in the electrolyte is m1%; the additive comprises a bridged ring compound with a structural general formula as shown in a formula 1, and the mass ratio of the bridged ring compound in the electrolyte is m2%; 5 < = m1 < = 60, 0.1 < = m2 < = 10. According to the invention, the bridge ring compound and the ethyl butyrate are matched with each other, and the contents of the bridge ring compound and the ethyl butyrate in the electrolyte are controlled, so that compared with other electrolytes, the lithium ion battery assembled in the guarantee period effectively and simultaneously has high-temperature cycling stability and high-temperature storage performance of the battery under high voltage.

Description

Technical Field

[0001] The present invention relates to the field of battery materials, and particularly to a non-aqueous electrolyte and a lithium-ion battery. Background Art

[0002] The development background of high-voltage lithium-ion battery electrolytes mainly stems from the pursuit of higher energy density and safety performance. Under the condition of a certain volume of the lithium battery, the energy density can be increased by raising the upper limit voltage. For the means of raising the upper limit voltage, as the voltage continues to increase (>4.55V), the positive electrode active material will release oxygen, which continuously intensifies the oxidation of the electrolyte, resulting in solvent decomposition, gas generation, and continuous increase in the decomposition of the lithium salt, generating more HF, which in turn destroys the SEI film and corrodes the electrode material. At the same time, the transition metal ions of the positive electrode continuously dissolve and migrate to the surface of the negative electrode, causing certain damage to its interface film and deteriorating the high-temperature cycle stability and high-temperature storage performance of the battery.

[0003] Currently, adding specific additives to the electrolyte can endow the electrolyte with different functions based on the characteristics of the additives. Therefore, it is very necessary to develop an electrolyte that can improve the high-temperature cycle stability and high-temperature storage performance of the battery simultaneously at high voltages. Summary of the Invention

[0004] The present invention provides a non-aqueous electrolyte and a lithium-ion battery, aiming to ensure that the lithium-ion battery can improve the high-temperature cycle stability and high-temperature storage performance of the battery simultaneously at high voltages.

[0005] A non-aqueous electrolyte provided by the present invention includes an organic solvent, a lithium salt, and an additive.

[0006] The organic solvent includes a carboxylic acid ester solvent, and the carboxylic acid ester solvent includes ethyl butyrate, and the mass percentage of ethyl butyrate in the electrolyte is m1%;

[0007] The additive includes a bridged-ring compound with a structural general formula as shown in Formula 1, and the mass percentage of the bridged-ring compound in the electrolyte is m2%;

[0008] Formula 1: Wherein, A represents a carbon atom or an oxygen atom, and B - is selected from at least one of BF4 - , PF6 - , TFSI - , FSI - , and R1-R4 are each independently selected from a halogen, a halogen-substituted or unsubstituted hydrocarbon group having 1-3 carbon atoms;

[0009] 5≤m1≤60, 0.1≤m2≤10.

[0010] The electrolyte satisfies at least one of the following conditions:

[0011] (1) 8 ≤ m1 / m2 ≤ 200;

[0012] (2) 15 ≤ m1 ≤ 50;

[0013] (3) 0.2 ≤ m2 ≤ 5.

[0014] Further, in the bridged-ring compound, A represents an oxygen atom; and / or, the anion group is BF4 - or PF6 - ; and / or, R1-R4 are each independently selected from unsubstituted hydrocarbon groups having 1-3 carbon atoms, preferably unsubstituted hydrocarbon groups having 1 carbon atom;

[0015] Preferably, the bridged-ring compound includes at least one of the structural formulas shown in Formulas 1-1 to 1-4 below:

[0016] Formula 1-1:

[0017] Formula 1-2:

[0018] Formula 1-3:

[0019] Formula 1-4:

[0020] The organic solvent further includes a fluorinated solvent, and the fluorinated solvent includes at least one of fluorinated carbonates, fluorinated carboxylates, and fluoroethers; preferably, the fluorinated solvent includes ethylene fluorocarbonate, propylene fluorocarbonate, methyl ethyl fluorocarbonate, diethyl fluorocarbonate, dimethyl fluorocarbonate, ethyl 3-fluoropropionate (3FEP), ethyl 2,2-difluoroacetate (DFEA), fluoromethyl ethyl carbonate (FEMC), bis(2,2,2-trifluoroethyl) ether, 2,2,2-trifluoroethyl-1,1,2,2-tetrafluoroethyl ether, 1,1,2,2-tetrafluoroethyl-2,2,3,3-tetrafluoropropyl ether (D2), 1H,1H,5H-octafluoropentyl-1,1,2,2-tetrafluoroethyl ether, methyl nonafluorobutyl ether, tris(trifluoroethoxy)methane, ethyl nonafluorobutyl ether, fluorobenzene (FB);

[0021] Preferably, the mass percentage of the fluorinated solvent in the electrolyte is m5%, and 5 ≤ m5 ≤ 45.

[0022] The additive further includes a nitrile additive;

[0023] The mass percentage of the nitrile additive in the electrolyte is m6%, 2 ≤ m6 ≤ 10, and preferably, 3 ≤ m6 ≤ 6;

[0024] Preferably, the nitrile additive includes at least one of a saturated nitrile additive and an unsaturated nitrile additive;

[0025] Preferably, the saturated nitrile additive includes at least one of succinonitrile (SN), adiponitrile (AND), glutaronitrile, pimelonitrile, suberonitrile, sebaconitrile, 1,2-bis(cyanoethoxy)ethane, 1,3,6-hexanetricarbonitrile (HTCN), glycerol trinitrile, 1,3,5-pentanetricarbonitrile, and tris(2-cyanoethyl)phosphine, tetra-[(2-cyanoethoxy)methyl]ethane, and the unsaturated nitrile additive includes 1,4-dicyano-2-butene (DCB).

[0026] The electrolyte further includes at least one of additive B shown in Formula 2 and additive C shown in Formula 3;

[0027] Formula 2: Formula 3:

[0028] R1-R4 are each independently selected from a halogen, a halogen-substituted or unsubstituted hydrocarbon group having 1-3 carbon atoms, and A represents a carbon atom or an oxygen atom.

[0029] The total mass content of additive B shown in Formula 2 and additive C shown in Formula 3 in the electrolyte is m3%, and 0.1 ≤ m3 ≤ 5;

[0030] And / or, additive C shown in Formula 3 is selected from at least one of the following compounds:

[0031] Formula 3-1: Formula 3-2:

[0032] The carboxylic ester solvent further includes one or more of propyl acetate, propyl propionate, n-butyl acetate, isobutyl acetate, n-pentyl acetate, isopentyl acetate, ethyl propionate, n-propyl propionate, methyl butyrate, and n-butyl acetate;

[0033] Preferably, the carboxylic ester solvent contains propyl propionate, and the mass content of propyl propionate in the electrolyte is m4%, and 0 < m4 ≤ 20.

[0034] A lithium ion battery includes an electrolyte, a positive electrode sheet, and a negative electrode sheet, and the electrolyte is the above non-aqueous electrolyte.

[0035] The positive electrode active material in the positive electrode sheet is Li x Co 1-y Me y O2, where Me is a metal doping element, 0.68 < x < 0.74, and 0 ≤ y ≤ 0.15.

[0036] The metal doping elements include one or more of Al, Mg, Ti, Zr, Ni, Mn, Y, La, Sr, W, Sc, Ce, P, Nb, V, Ta, and Te.

[0037] The metal doping element is Mn, and the proportion of manganese element in the positive electrode active material is y, where 10 ≤ m2 / y ≤ 100.

[0038] The technical solution of the present invention has the following advantages:

[0039] A non-aqueous electrolyte provided by the present invention includes ethyl butyrate as an organic solvent and a bridged-ring compound with a structural general formula as shown in Formula 1. By the mutual cooperation of the bridged-ring compound and ethyl butyrate and controlling the contents of the bridged-ring compound and ethyl butyrate in the electrolyte, the high-temperature cycle stability and high-temperature storage performance of a lithium-ion battery can be improved.

[0040] In a non-aqueous electrolyte of the present invention, a fluorinated solvent is further included, and this optimization can effectively improve the ionic conductivity of the electrolyte and further improve the charge and discharge efficiency of the battery.

[0041] In a non-aqueous electrolyte of the present invention, a nitrile additive is further included, and the nitrile additive cooperates with ethyl butyrate to form a stable SEI film on the surface of the negative electrode, further improving the cycle stability.

[0042] When the metal doping element of the positive electrode material in a lithium-ion battery provided by the present invention is selected as Mn and the above non-aqueous electrolyte is used in cooperation with it, the high-temperature cycle capacity retention rate of the battery can be further optimized.

[0043] The additional aspects and advantages of the embodiments of the present invention will be partially described and shown in the subsequent description, or will be explained through the implementation of the embodiments of the present invention. Detailed Embodiments

[0044] The following embodiments are provided to better further understand the present invention. They are not limited to the best embodiment, do not limit the content and protection scope of the present invention, and any product that is the same as or similar to the present invention obtained by anyone under the inspiration of the present invention or by combining the features of the present invention with other prior art features falls within the protection scope of the present invention.

[0045] For those embodiments in which specific experimental steps or conditions are not indicated, the operations or conditions of the conventional experimental steps described in the literature in this field can be followed. For the reagents or instruments whose manufacturers are not indicated, they are all conventional reagent products that can be obtained through commercial purchase.

[0046] In addition, the technical features involved in different embodiments of the present invention described below can be combined with each other as long as they do not conflict with each other.

[0047] The present invention provides a non-aqueous electrolyte, comprising an organic solvent, a lithium salt, and an additive;

[0048] The organic solvent includes a carboxylic acid ester solvent, and the carboxylic acid ester solvent includes ethyl butyrate. The mass percentage of ethyl butyrate in the electrolyte is m1%;

[0049] The additive includes a bridged-ring compound having a structural general formula as shown in Formula 1. The mass percentage of the bridged-ring compound in the electrolyte is m2%;

[0050] Formula 1: wherein A represents a carbon atom or an oxygen atom, and B - is selected from at least one of BF4 - 、PF6 - 、TFSI - 、FSI - ; R1-R4 are each independently selected from a halogen, a halogen-substituted or unsubstituted hydrocarbon group having 1-3 carbon atoms;

[0051] 5≤m1≤60, 0.1≤m2≤10.

[0052] "Non-aqueous" in the non-aqueous electrolyte of the present invention refers to an electrolyte solution in which the solvent is not water. Specifically, the electrolyte usually contains a solvent and an electrolyte salt, and may also contain additives as needed; when the solvent is a non-aqueous solvent such as an organic solvent, the electrolyte is a non-aqueous electrolyte.

[0053] The molecular structure of ethyl butyrate in the present invention helps to form a stable solvation layer, which is conducive to maintaining the stable transport of lithium ions at high temperatures, reducing the interfacial impedance and capacity loss, and maintaining an excellent capacity retention rate during high-temperature charge and discharge processes, thereby improving the high-temperature cycle stability. However, under long-term high-temperature storage conditions, the ethyl butyrate solvent will react with trace amounts of water in the electrolyte, accelerating the decomposition of the electrolyte, generating gases and by-products. These by-products may deposit on the electrode surface, resulting in an increase in the resistance of the interfacial SEI film and capacity attenuation. In the present invention, by adding a bridged-ring compound as shown in Formula 1 to the electrolyte, the polarity of the ethyl butyrate solvent can be adjusted, its solvation structure can be changed, the polarity of the solvent can be reduced, and its interaction with water molecules can be decreased, thereby slowing down the hydrolysis reaction, and then ensuring the battery capacity at high temperatures and improving the high-temperature storage performance. In addition, the bridged-ring compound can also inhibit the formation of by-products at the electrode / electrolyte interface, such as the formation of by-products such as LixPOyFz and LixPFy. By reducing the formation of these by-products, it is further helpful to maintain the number of active lithium ions in the battery and improve its high-temperature storage performance. Therefore, by adding the bridged-ring compound as shown in Formula 1 to the electrolyte containing ethyl butyrate and cooperating with each other, the effect of significantly improving the high-temperature storage performance and high-temperature cycle stability can be achieved.

[0054] As an example, the mass percentage m1% of ethyl butyrate in the electrolyte can be 5%, 15%, 20%, 25%, 30%, 35%, 40%, 45%, 50%, 55%, 60% or within the range composed of any two of the above values; the mass percentage m2% of the bridged-ring compound in the electrolyte can be 0.1%, 0.5%, 1%, 2%, 4%, 6%, 8%, 10% or within the range composed of any two of the above values.

[0055] In an alternative embodiment, the electrolyte satisfies at least one of the following conditions:

[0056] (1) 8 ≤ m1 / m2 ≤ 200;

[0057] (2) 15 ≤ m1 ≤ 50;

[0058] (3) 0.2 ≤ m2 ≤ 5.

[0059] Under this condition, the high-temperature cycle stability and high-temperature storage performance can be further improved.

[0060] As an example, m1 / m2 can be 8, 10, 15, 20, 25, 30, 35, 40, 45, 50, 80, 100, 120, 140, 160, 180, 200 or within the range composed of any two of the above values.

[0061] In an optional embodiment, the A represents an oxygen atom; and / or the anionic group is BF4 - or PF6 - ; and / or, said R1-R4 are independently selected from unsubstituted hydrocarbon groups having 1 to 3 carbon atoms, preferably unsubstituted hydrocarbon groups having 1 carbon atom;

[0062] Preferably, the bridged ring compound includes at least one of the structural formulas shown in Formula 1-1 to Formula 1-4 below:

[0063] Formula 1-1:

[0064] Formula 1-2:

[0065] Formula 1-3:

[0066] Formula 1-4:

[0067] In an optional embodiment, the carboxylate solvent also includes one or more of propyl acetate, propyl propionate, n-butyl acetate, isobutyl acetate, n-pentyl acetate, isopentyl acetate, ethyl propionate, n-propyl propionate, methyl butyrate and ethyl butyrate; preferably, when the carboxylate solvent contains propyl propionate, the mass content of propyl propionate in the electrolyte is m4%, 0<m4≤20.

[0068] In the present invention, the thermal storage performance of propyl propionate is better than that of ethyl butyrate. Therefore, the thermal storage performance can be effectively improved by adding propyl propionate. However, since ethyl butyrate has better thermal cycle stability than propyl propionate, the content of propyl propionate should not be too high. Too high a content of propyl propionate will affect the thermal cycle stability. By limiting the content of propyl propionate within the above range, the high-temperature storage performance and high-temperature cycle stability can be further improved, thereby further achieving the purpose of taking into account both the high-temperature storage performance and the high-temperature cycle stability.

[0069] As an example, the mass proportion m4% of propyl propionate in the electrolyte can be 0.1%, 2%, 4%, 6%, 8%, 10%, 12%, 14%, 16%, 18%, 20% or within the range of any two of the above values.

[0070] And / or, the organic solvent further comprises a carbonate solvent, preferably, the carbonate solvent preferably comprises one or more of ethylene carbonate, propylene carbonate, dimethyl carbonate, diethyl carbonate and ethyl methyl carbonate.

[0071] In an optional embodiment, the organic solvent further includes a fluorinated solvent, and the mass proportion of the fluorinated solvent in the electrolyte is m5%, 5≤m5≤45.

[0072] In the organic solvent of the present invention, by further adding a fluorinated solvent, it can cooperate with ethyl butyrate and the bridged-ring compound, thereby optimizing the solvation structure of the non-aqueous electrolyte to increase the migration rate and diffusion coefficient of lithium ions, and thus enhancing the ionic conductivity of the electrolyte.

[0073] As an example, the mass percentage m5% of the fluorinated solvent in the electrolyte can be 5%, 8%, 10%, 12%, 14%, 16%, 18%, 20%, 25%, 30%, 35%, 40%, 45% or within the range composed of any two of the above values.

[0074] Preferably, the fluorinated solvent includes at least one of fluorinated carbonates, fluorinated carboxylates, and fluorinated ethers; the types of fluorinated solvents selected are 1 - 3; specifically, the fluorinated solvent includes at least one of vinylene carbonate fluoride, propylene carbonate fluoride, ethyl methyl carbonate fluoride, diethyl carbonate fluoride, dimethyl carbonate fluoride, ethyl fluoroacetate, methyl fluoropropionate, ethyl fluoropropionate, propyl fluoropropionate; more preferably, the fluorinated solvent includes vinylene carbonate fluoride, propylene carbonate fluoride, ethyl methyl carbonate fluoride, diethyl carbonate fluoride, dimethyl carbonate fluoride, ethyl 3,3,3-trifluoropropionate (3FEP), ethyl 2,2-difluoroacetate (DFEA), fluoromethyl ethyl carbonate (FEMC), bis(2,2,2-trifluoroethyl) ether, 2,2,2-trifluoroethyl-1,1,2,2-tetrafluoroethyl ether, 1,1,2,2-tetrafluoroethyl-2,2,3,3-tetrafluoropropyl ether (D2), 1H,1H,5H-octafluoropentyl-1,1,2,2-tetrafluoroethyl ether, methyl nonafluorobutyl ether, tris(trifluoroethoxy)methane, ethyl nonafluorobutyl ether, fluorobenzene (FB).

[0075] In an alternative embodiment, the lithium salt includes one or more of lithium hexafluorophosphate (LiPF6), lithium difluorophosphate, lithium difluorooxalate borate, lithium bis(trifluoromethanesulfonyl)imide (LiTFSI), lithium difluorobis(oxalate)phosphate, lithium tetrafluoroborate, lithium bis(oxalate)borate, lithium hexafluoroantimonate, lithium hexafluoroarsenate, lithium bis(trifluoromethylsulfonyl)imide, lithium bis(pentafluoroethylsulfonyl)imide, lithium tris(trifluoromethylsulfonyl)methyl, or lithium bis(trifluoromethylsulfonyl)imide.

[0076] In an alternative embodiment, the additive further includes a nitrile additive, and the mass percentage of the nitrile additive in the electrolyte is m6%, 2 ≤ m6 ≤ 10, preferably, 3 ≤ m6 ≤ 6; by adding the nitrile additive in the present invention, which cooperates with ethyl butyrate, it can effectively improve the cycle stability of the battery.

[0077] Specifically, on the one hand, nitrile additives have poor compatibility with the negative electrode and are prone to damaging the negative electrode structure. However, in the solvent structure composed of ethyl butyrate and nitrile additives, it has good compatibility between negative electrode materials (such as graphite), and can form a stable SEI film on the surface of the negative electrode, thereby protecting the negative electrode from being eroded and decomposed by the electrolyte. On the other hand, nitrile additives include at least one of saturated nitrile additives and unsaturated nitrile additives. Ethyl butyrate can promote the polymerization reaction between unsaturated nitrile additives and accelerate the site complexation on the surface of lithium cobaltate at the positive electrode. While saturated nitrile additives have high stability and can provide a better pressure-resistant environment at the positive electrode, which is beneficial for protection at the positive electrode, thereby improving the cycle stability of the battery, especially the high-temperature cycle stability.

[0078] Preferably, the saturated nitrile additives include at least one of succinonitrile (SN), adiponitrile, glutaronitrile, pimelonitrile, suberonitrile, sebaconitrile, 1,2-bis(cyanoethoxy)ethane, 1,3,6-hexanetricarbonitrile (HTCN), glycerol trinitrile, 1,3,5-pentanetricarbonitrile, and tris(2-cyanoethyl)phosphine, tetra-[(2-cyanoethoxy)methyl]ethane, and the unsaturated nitrile additives include 1,4-dicyano-2-butene (DCB).

[0079] As an example, the mass percentage m6% of nitrile additives in the electrolyte can be 2%, 4%, 6%, 8%, 10% or within the range composed of any two of the above values.

[0080] In an alternative embodiment, the electrolyte further includes at least one of additive B shown in Formula 2 and additive C shown in Formula 3;

[0081] Formula 2: Formula 3:

[0082] R1-R4 are each independently selected from halogen, halogen-substituted or unsubstituted hydrocarbon groups having 1-3 carbon atoms, and A represents a carbon atom or an oxygen atom.

[0083] Compound B in the present invention contains a special cyclic structure, which can form a certain steric hindrance around the ethyl butyrate molecule. This steric hindrance can prevent external factors (such as water molecules, HF molecules) from directly attacking the ethyl butyrate molecule, thereby further reducing the possibility of its degradation reactions such as hydrolysis and oxidation. At the same time, the unsaturated double bond chemical structure of nitrogen in compound C containing a nitrogen double bond in the present invention can interact with functional groups (such as carboxyl groups, ester groups) in the ethyl butyrate solvent molecules to form stable chemical bonds or complexes, thereby slowing down the degradation reactions such as hydrolysis and oxidation of ethyl butyrate, thus protecting the ethyl butyrate solvent molecules from being oxidized.

[0084] In an alternative embodiment, the total mass content of additive B represented by Formula 2 and additive C represented by Formula 3 in the electrolyte is m3%, where 0.1 ≤ m3 ≤ 5.

[0085] As an example, the total mass content m3% of additive B represented by Formula 2 and additive C represented by Formula 3 in the electrolyte can be 0.1%, 0.5%, 1%, 2%, 4%, 5% or within the range composed of any two of the above values.

[0086] In an alternative embodiment, additive C represented by Formula 3 is selected from at least one of the following compounds:

[0087] Formula 2-1: Formula 2-2:

[0088] The present invention also provides a lithium-ion battery, comprising an electrolyte, a positive electrode sheet, and a negative electrode sheet, wherein the electrolyte is the above-mentioned non-aqueous electrolyte.

[0089] In an alternative embodiment, the positive electrode active material in the positive electrode sheet is Li x Co 1-y Me y O2, where Me is a metal doping element, 0.68 ≤ x ≤ 0.74, and 0 ≤ y ≤ 0.15.

[0090] As an example, x in the positive electrode active material can be 0.68, 0.69, 0.70, 0.71, 0.72, 0.73, 0.74 or within the range composed of any two of the above values; y in the positive electrode active material can be 0, 0.01, 0.02, 0.03, 0.04, 0.05, 0.06, 0.07, 0.08, 0.09, 0.10, 0.11, 0.12, 0.13, 0.14, 0.15 or within the range composed of any two of the above values.

[0091] In an alternative embodiment, the metal doping element is one or more of Al, Mg, Ti, Zr, Ni, Mn, Y, La, Sr, W, Sc, Ce, P, Nb, V, Ta, Te.

[0092] In an alternative embodiment, the metal doping element is Mn, and the proportion of manganese element in the positive electrode active material is y, where 10 ≤ m2 / y ≤ 100.

[0093] In the present invention, the doped manganese element in the positive electrode active material can reduce the loss of the battery during charge and discharge, enabling the battery to withstand more charge and discharge cycles without significant attenuation; meanwhile, it can also reduce the risk of thermal runaway of the battery under high-temperature or high-voltage conditions. However, when the content of Mn element is relatively high, under high-temperature and high-voltage conditions, the manganese element in the positive electrode material may dissolve out from the crystal lattice, leading to side reactions inside the battery or instability of the battery structure; while in the present invention, the anion or cation in the bridged-ring compound shown in Formula 1 can coordinate with manganese ions to form a coordination compound, and this coordination can change the electronic structure and geometric configuration of manganese ions, thereby reducing their catalytic reaction activity; by controlling the relationship between the content of the bridged-ring compound and the manganese element in the positive electrode active material, that is, controlling 10≤m2 / y≤100, the high-temperature cycle stability of the lithium-ion battery can be effectively guaranteed.

[0094] As an example, m2 / y can be 10, 15, 20, 25, 30, 35, 40, 45, 50, 55, 60, 65, 70, 75, 80, 85, 90, 95, 100 or within the range composed of any two of the above values.

[0095] The positive electrode sheet in the present invention includes a current collector and a positive electrode active material layer provided on the surface of the current collector, and the positive electrode active material layer includes a positive electrode active material, a conductive agent, and a binder.

[0096] The present invention does not particularly limit the conductive agent in the positive electrode sheet, and it can be selected from the conductive agents commonly used in the art, including but not limited to one or more of acetylene black, conductive carbon black, Ketjen black, conductive graphite, carbon nanotubes, conductive carbon fibers, and graphene;

[0097] The present invention does not particularly limit the binder in the positive electrode sheet, and it can be selected from the binders commonly used in the art, including but not limited to one or more of polyvinylidene fluoride, polytetrafluoroethylene, sodium carboxymethyl cellulose, styrene-butadiene rubber, and polyethylene oxide.

[0098] The negative electrode sheet of the present invention includes a current collector and a negative electrode active material layer provided on the surface of the current collector. The active material of the negative electrode active material layer can be selected from lithium metal, and the current collector of the negative electrode sheet can be selected from copper foils in the art. The negative electrode active material layer also includes a conductive agent and a binder at the same time. The present invention does not particularly limit the types of the conductive agent and the binder in the negative electrode sheet, and the selection range can refer to the types of the conductive agent and the binder in the positive electrode sheet, which will not be elaborated here.

[0099] The negative electrode sheet of the present invention can also be prepared by the following method: Select a lithium alloy and a copper foil with a certain thickness, roll them under a certain pressure and then cut into pieces to obtain a lithium metal negative electrode sheet.

[0100] The present invention does not specifically limit the preparation method of the lithium-ion battery, and the lithium-ion battery can be prepared by using conventional preparation methods in the art. For example, the positive electrode sheet, the separator, and the negative electrode sheet are stacked in sequence, with the separator located between the positive electrode sheet and the negative electrode sheet. The battery cell is obtained through the stacking process, and then through processes such as baking, liquid injection, formation, and encapsulation, the lithium-ion battery of the present invention can be obtained.

[0101] The following further describes the present invention in detail with specific embodiments, and these embodiments should not be construed as limiting the scope claimed by the present invention. In all the embodiments and comparative examples of the present invention, the unit wt% represents mass percentage content.

[0102] Example 1

[0103] A lithium-ion battery is prepared as follows:

[0104] (1) Preparation of the positive electrode sheet

[0105] The positive electrode active material Li x Co 1-y Me y O2 (Li 0.7 Co 0.95 Mn 0.05 O2), polyvinylidene fluoride, conductive carbon black, and carbon nanotubes are mixed in a mass ratio of 96:2:1.5:0.5, N-methylpyrrolidone (NMP) is added, and the mixture is stirred under a vacuum mixer until the mixture system becomes a homogeneous and flowable positive electrode active paste; the positive electrode active paste is evenly coated on both surfaces of the aluminum foil; the coated aluminum foil is dried, and then rolled and slit to obtain the required positive electrode sheet.

[0106] (2) Preparation of the negative electrode sheet

[0107] The negative electrode active material (composed of 88 wt% graphite + 12 wt% silicon carbide), styrene-butadiene rubber (SBR), lithium polyacrylate, conductive carbon black (SP), and carbon nanotubes (CNTs) are mixed evenly in a mass ratio of 96.5:1.5:0.5:1.0:0.5, and then an appropriate amount of deionized water is added step by step, and a negative electrode active paste is obtained under the action of a vacuum mixer; the negative electrode active paste is evenly coated on both surfaces of the copper foil through a coater; the coated copper foil is dried, and then rolled and slit to obtain the required negative electrode sheet.

[0108] (3) Preparation of the non-aqueous electrolyte

[0109] In a glove box filled with argon (H2O < 0.1 ppm, O2 < 0.1 ppm), an organic solvent, a lithium salt, and an additive were added according to the composition and content requirements of the non-aqueous electrolyte and mixed evenly to obtain the non-aqueous electrolyte. Among them, the lithium salt was lithium hexafluorophosphate (LiPF6); the additive included a bridged-ring compound of Formula 1-1 and a nitrile additive, and the nitrile additives were succinonitrile (SN) and 1,3,6-hexanetricarbonitrile (HTCN); the organic solvents included ethyl butyrate (EB), ethylene carbonate (EC), 2,2-difluoroethyl acetate (DFEA), and ethyl propionate (EA). In the non-aqueous electrolyte, the content of the lithium salt was 15%, the content of the bridged-ring compound of Formula 1-1 was 1%, the content of the nitrile additive was 4%, the content of ethyl butyrate (EB) was 40%, the content of ethylene carbonate (EC) was 10%, the content of 2,2-difluoroethyl acetate (DFEA) was 10%, and the balance was ethyl propionate (EA); the nitrile additive included succinonitrile (SN) and 1,3,6-hexanetricarbonitrile (HTCN) in a mass ratio of 1:1.

[0110] (4) Obtaining the separator

[0111] The separator was a polyethylene separator with a thickness of 8 μm.

[0112] (5) Preparing the battery

[0113] The positive electrode sheet prepared in step (1), the negative electrode sheet prepared in step (2), and the separator prepared in step (4) were stacked in the order of positive electrode sheet, separator, negative electrode sheet, and separator to obtain an electrode core; the electrode core was placed in an outer packaging aluminum foil, and the non-aqueous electrolyte prepared in step (3) was injected into the outer packaging aluminum foil. After processes such as vacuum packaging, standing, formation, shaping, and sorting, a lithium-ion battery was obtained.

[0114] For the battery of the present invention, the charge-discharge range was 3.0 - 4.55 V.

[0115] Examples 2-23 and Comparative Examples 1-4

[0116] A lithium-ion battery, which is different from that of Example 1 in that in step (3) of preparing the non-aqueous electrolyte, the carboxylic acid ester solvent may further contain propyl propionate (PP), and the additive may further contain additive B shown in Formula 2 and additive C shown in Formula 3; m1 represents the mass percentage of ethyl butyrate (EB) in the electrolyte, m4 represents the mass percentage of propyl propionate (PP) in the electrolyte, m5 represents the mass percentage of 2,2-difluoroethyl acetate (DFEA) in the electrolyte, m2 represents the mass percentage of the bridged-ring compound of Formula 1-1 in the electrolyte, m6 represents the mass percentage of the nitrile additive in the electrolyte, and m3 represents the total mass percentage of additive B shown in Formula 2 and additive C shown in Formula 3 added to the electrolyte; the mass percentages of m1, m2, m3, m4, m5, and m6 are different in different examples, as shown in Table 1 below.

[0117] Table 1

[0118]

[0119] The other parameter conditions of Examples 2-11 and Comparative Examples 1-4 are the same as those of Example 1, and the molar ratios of additive B shown in Formula 2 and additive C shown in Formula 3 added to the electrolyte are the same.

[0120] Examples 24-32

[0121] A lithium-ion battery, which is different from that of Example 1 in that in step (3) of preparing the non-aqueous electrolyte, the type of fluorinated solvent, the type of bridged-ring compound, the type of nitrile additive, or the type of lithium salt is different, as shown in Table 2 below.

[0122] Table 2

[0123] Types of fluorinated solvents Types of bridged ring compounds Types of nitrile additives Types of lithium salts Example 24 FEMC Formula 1-1 SN / HTCN = 1 / 1 LiPF6 Example 25 D2 Formula 1-1 SN / HTCN = 1 / 1 LiPF6 Example 26 FB Formula 1-1 SN / HTCN = 1 / 1 LiPF6 Example 27 3FEP Formula 1-1 SN / HTCN = 1 / 1 LiPF6 Example 28 DFEA Formula 1-2 SN / HTCN = 1 / 1 LiPF6 Example 29 DFEA Formula 1-1 AND / HTCN = 1 / 1 LiPF6 Example 30 DFEA Formula 1-1 SN / AND / HTCN = 1 / 1 / 2 LiPF6 Example 31 DFEA Formula 1-1 SN / HTCN = 1 / 1 LiTFSI Example 32 DFEA Formula 1-1 SN / HTCN = 1 / 1 LiTFSI / LiPF6 = 1 / 1

[0124] The other parameter conditions of Examples 24-32 are the same as those of Example 1.

[0125] Examples 33-38

[0126] A lithium-ion battery, which is different from that of Example 1 in that in step (1) of preparing the positive electrode sheet, the positive electrode active material Li x Co 1-y Me y O2 is different, as shown in Table 3 below.

[0127] Table 3

[0128] x y Me m2 / y Example 33 0.68 0.15 Mn 6.7 Example 34 0.7 0.12 Mn 8.3 Example 35 0.72 0.1 Mn 10 Example 36 0.74 0.05 Mn 20 Example 37 0.74 0 Mn - Example 38 0.7 0.05 Zn 20

[0129] The other parameter conditions of Examples 33-38 are the same as those of Example 1.

[0130] Experimental Example

[0131] The lithium-ion batteries obtained from the examples and comparative examples were respectively tested for capacity, charge-discharge efficiency, high-temperature cycle stability, and high-temperature storage performance, and the test results are shown in Tables 4 - 6 below.

[0132] 1. Capacity test

[0133] First, the battery was charged to the upper limit voltage of 4.55V under the condition of constant current and constant voltage at 0.2C, and then discharged to the lower limit voltage of 3.0V at a current of 0.2C. During the discharge process, the amount of electricity generated by the battery was recorded, and the capacity (mAh) of the battery was calculated accordingly.

[0134] 2. High-temperature cycle stability

[0135] The batteries prepared from the examples and comparative examples were subjected to a cycle test. The specific test method is as follows: at 45°C, the battery was subjected to constant current charge-discharge cycling at a rate of 1C within the charge-discharge cut-off voltage range (3.0V - 4.55V) for 600 cycles; the discharge capacity Q1 in the first cycle and the discharge capacity Q600 in the 600th cycle were recorded, and the capacity retention rate (%) after 600 cycles at 45°C was calculated by Q600 / Q1×100%, and the results were recorded in the table.

[0136] 3. High-temperature storage performance

[0137] The batteries prepared from the examples and comparative examples were subjected to a high-temperature storage test. The specific test method is as follows: at 25°C ± 1°C, the voltage, internal resistance, and thickness d0 of the sample were tested;

[0138] Step1: Stand still at 25°C ± 5°C for 10 min;

[0139] Step2: Discharge at 0.2C to the lower limit voltage (3.0V); stand still for 10 min;

[0140] Step3: Charge at 0.7C to the upper limit voltage (4.55V), cut off at 0.05C, and stand still for 10 min;

[0141] Step4: Discharge at 0.2C to the lower limit voltage (for initial capacity test);

[0142] Step5: Stand still for 10 min;

[0143] Step6: Charge at 0.7C to the upper limit voltage, cut off at 0.05C;

[0144] Step7: Stand still for 10 min;

[0145] Step8: Store at 85°C ± 2°C for 6 hours;

[0146] Take it out after storage is completed and leave it at room temperature for 2 h. Immediately test the thermal thickness d1 of the sample taken out after storage is completed, and calculate the thickness change rate: (d1 - d0) / d0. The high-temperature storage performance is represented by the thickness change rate (%), and the results are recorded in the table.

[0147] Table 4

[0148]

[0149]

[0150] Table 5

[0151] Capacity (mAh) Capacity retention rate (%) Thickness change rate (%) Example 24 3493 89.88 4.89 Example 25 3464 87.39 5.09 Example 26 3480 89.25 5.01 Example 27 3478 88.86 8.61 Example 28 3481 88.41 5.34 Example 29 3483 88.84 6.23 Example 30 3485 87.83 7.11 Example 31 3454 86.34 6.86 Example 32 3474 86.96 7.31

[0152] Table 6

[0153] Capacity (mAh) Capacity retention rate (%) Thickness change rate (%) Example 33 3486 85.77 13.19 Example 34 3497 86.70 13.64 Example 35 3502 92.27 4.08 Example 36 3510 91.70 4.05 Example 37 3481 86.55 13.43 Example 38 3508 92.11 4.64

[0154] From the results of Table 4 - Table 6 above, it can be seen that in the present invention, by cooperating the bridged-ring compound with ethyl butyrate and controlling the contents of the bridged-ring compound and ethyl butyrate in the electrolyte, it is possible to ensure that the assembled lithium-ion battery has both high-temperature cycle stability and high-temperature storage performance at high voltages compared with other electrolyte compositions.

[0155] Obviously, the above embodiments are merely examples given for clear illustration and are not limitations on the implementation manners. For those of ordinary skill in the art, other different forms of changes or modifications can be made based on the above description. It is not necessary and impossible to enumerate all the implementation manners here. And the obvious changes or modifications derived therefrom are still within the protection scope of the present invention.

Claims

1. A non-aqueous electrolyte comprising an organic solvent, a lithium salt and an additive; characterized in that: The organic solvent includes a carboxylate solvent, the carboxylate solvent includes ethyl butyrate, and the mass proportion of ethyl butyrate in the electrolyte is m1%; The additive includes a bridged ring compound with a general structural formula as shown in Formula 1, and the mass proportion of the bridged ring compound in the electrolyte is m2%; Formula 1: Among them, A represents a carbon atom or an oxygen atom, B - Selected from BF4 - PF6 - TFSI - 、FSI - At least one of the following, R1-R4 are independently selected from halogen, halogen-substituted or unsubstituted hydrocarbon group having 1 to 3 carbon atoms; 5≤m1≤60, 0.1≤m2≤10.

2. The non-aqueous electrolyte according to claim 1, characterized in that The electrolyte satisfies at least one of the following conditions: (1)8≤m1 / m2≤200; (2)15≤m1≤50； (3)0.2≤m2≤5。 3. The non-aqueous electrolyte according to claim 1 or 2, characterized in that The bridged ring compound includes at least one of the structural formulas shown in Formula 1-1 to Formula 1-4: Formula 1-1: Formula 1-2: Formula 1-3: Formula 1-4:

4. The non-aqueous electrolyte according to claim 1 or 2, characterized in that: The organic solvent also includes a fluorinated solvent, and the fluorinated solvent includes one or more of fluoroethylene carbonate, fluoropropylene carbonate, fluoroethyl methyl carbonate, fluorodiethyl carbonate, fluorodimethyl carbonate, ethyl trifluoropropionate, ethyl 2,2-difluoroacetate, trifluoroethyl methyl carbonate, bis(2,2,2-trifluoroethyl) ether, 2,2,2-trifluoroethyl-1,1,2,2-tetrafluoroethyl ether, 1,1,2,2-tetrafluoroethyl-2,2,3,3-tetrafluoropropyl ether, 1H,1H,5H-octafluoropentyl-1,1,2,2-tetrafluoroethyl ether, methyl nonafluorobutyl ether, tris(trifluoroethoxy)methane, ethyl nonafluorobutyl ether, and fluorobenzene; Preferably, the mass proportion of the fluorinated solvent in the electrolyte is m5%, 5≤m5≤45.

5. The non-aqueous electrolyte according to any one of claims 1 to 4, characterized in that: The additives also include nitrile additives, and the mass proportion of the nitrile additives in the electrolyte is m6%, 2≤m6≤10; Preferably, 3≤m6≤6; Preferably, the nitrile additive includes at least one of a saturated nitrile additive and an unsaturated nitrile additive, the saturated nitrile additive includes at least one of succinonitrile, adiponitrile, glutaronitrile, pimelonitrile, suberonitrile, sunflower dinitrile, 1,2-bis(cyanoethoxy)ethane, 1,3,6-hexanetrinitrile, glycerol trinitrile, 1,3,5-pentanetrinitrile, tri(2-cyanoethyl)phosphine, tetrakis-[(2-cyanoethoxy)methyl]ethane, and the unsaturated nitrile additive includes 1,4-dicyano-2-butene.

6. The non-aqueous electrolyte according to claim 1 or 2, characterized in that: The electrolyte further includes at least one of an additive B shown in Formula 2 and an additive C shown in Formula 3; Formula 2: Formula 3: R1-R4 are independently selected from halogen, halogen-substituted or unsubstituted hydrocarbon group having 1 to 3 carbon atoms, and A represents a carbon atom or an oxygen atom.

7. The non-aqueous electrolyte according to claim 6, characterized in that The total mass content of the additive B shown in Formula 2 and the additive C shown in Formula 3 in the electrolyte is m3%, 0.1≤m3≤5; And / or, the additive C shown in Formula 3 is selected from at least one of the following compounds: Formula 3-1: Formula 3-2:

8. The non-aqueous electrolyte according to claim 1 or 2, characterized in that: The carboxylate solvent further comprises one or more of propyl acetate, propyl propionate, n-butyl acetate, isobutyl acetate, n-amyl acetate, isoamyl acetate, ethyl propionate, methyl butyrate and ethyl butyrate; Preferably, the carboxylate solvent contains propyl propionate, and the mass content of propyl propionate in the electrolyte is m4%, 0<m4≤20.

9. A lithium-ion battery comprising an electrolyte, a positive electrode sheet, and a negative electrode sheet, characterized in that: The electrolyte is the non-aqueous electrolyte according to any one of claims 1 to 8.

10. The lithium ion battery according to claim 9, characterized in that: The positive electrode active material in the positive electrode sheet is Li x Co 1-y Me y O2, wherein Me is a metal doping element, 0.68<x<0.74, 0≤y≤0.15; the metal doping element includes one or more of Al, Mg, Ti, Zr, Ni, Mn, Y, La, Sr, W, Sc, Ce, P, Nb, V, Ta, and Te; Preferably, the metal doping element includes Mn, and the mass ratio of manganese element in the positive electrode active material is y, 10≤m2 / y≤100.

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