Electrolyte adaptive to lithium iron manganese phosphate positive electrode material and battery

Abstract

The invention discloses an electrolyte adaptive to a lithium iron manganese phosphate positive electrode material and a battery, and belongs to the technical field of batteries. The electrolyte comprises a non-aqueous solvent, a lithium salt and a negative electrode film-forming additive, the mass ratio a of the total mass of the dimethyl carbonate and the carboxylic ester in the non-aqueous solvent, the molar ratio b of the lithium bis (fluorosulfonyl) imide in the lithium salt, the molar ratio y of manganese in the lithium manganese iron phosphate and the compaction density c of the positive pole piece meet the relational expression: # imgabs0 # 0.01 < = a < = 1; 0.01 < = b < = 1; 1.8 < = c < = 2.7 in g / cm < 3 >; the molecular formula of the lithium manganese iron phosphate is LixMnyFe1-yPO4, x is more than or equal to 0.95 and less than or equal to 1.05, and y is more than or equal to 0.3 and less than or equal to 0.8; and the mass ratio d and y of the negative electrode film-forming additive in the electrolyte meet the relational expression that (100 * d) / y is greater than or equal to 3 and less than or equal to 20, and d is greater than or equal to 0.001 and less than or equal to 0.2. The electrolyte can improve the deposition phenomenon of manganese ions on a negative electrode and improve the conductivity and the dynamic performance.

Description

Technical Field

[0001] The present application relates to the technical field of battery materials, and in particular to an electrolyte and a battery adapted to lithium manganese iron phosphate positive electrode materials. Background Art

[0002] Lithium manganese iron phosphate (LiMnFePO4) has a higher voltage platform than lithium iron phosphate (LiFePO4). Theoretically, its energy density is expected to be about 20% higher than that of lithium iron phosphate, which can overcome the limitations of lithium iron phosphate in improving energy density to a certain extent. Compared with ternary lithium battery materials, the energy density of lithium manganese iron phosphate is similar to that of ternary five-series materials. At the same time, it also has higher safety, lower cost and better environmental compatibility. However, the addition of manganese makes the deintercalation and migration of lithium ions more difficult, which leads to poor conductivity of the material, low transmission efficiency of lithium ions between material particles, and poor battery kinetics. During the reaction, due to the addition of Mn 3+ The induced Jahn-Teller effect will cause manganese to dissolve from the positive electrode material and deposit on the negative electrode surface. This deposition will destroy the solid electrolyte interface film (SEI) of the battery, thereby shortening the cycle life of the battery.

[0003] At present, the research direction of electrolytes to improve the cycle performance of lithium manganese iron phosphate is mainly to add positive electrode complexing agents to complex metal Mn ions to prevent Mn deposition at the negative electrode. Or to protect the positive and negative electrodes at the same time to form SEI and CEI; however, general positive electrode complexing agents will increase the impedance of the battery. If the amount added is too much, it will significantly affect the battery performance; if the amount added is too little, the effect will be minimal. Summary of the invention

[0004] The main purpose of the present application is to provide an electrolyte and a battery adapted to lithium iron manganese phosphate positive electrode materials, so as to solve the problem in the prior art that manganese ions in lithium iron manganese phosphate positive electrode materials are easily dissolved and deposited on the negative electrode surface to destroy the interface film.

[0005] In order to achieve the above-mentioned object, according to one aspect of the present application, an electrolyte adapted to lithium manganese iron phosphate positive electrode material is provided, comprising a non-aqueous solvent, a lithium salt and a negative electrode film-forming additive; the non-aqueous solvent comprises a carbonate ester and a carboxylate ester, the carbonate ester comprises dimethyl carbonate; the lithium salt comprises lithium bis(fluorosulfonyl)imide;

[0006] Among them, the mass ratio a of the total mass of dimethyl carbonate and carboxylic acid ester in the non-aqueous solvent, the molar ratio b of lithium bis(fluorosulfonyl)imide in the lithium salt, the molar ratio y of manganese element in the lithium iron manganese phosphate positive electrode material, and the compaction density c of the positive electrode sheet satisfy the following relationship:

[0007]

[0008] In formula I, 0.01≤a≤1; 0.01≤b≤1; 1.8≤c≤2.7, unit: g / cm 3 ;

[0009] The molecular formula of lithium manganese iron phosphate positive electrode material is Li x Mn y Fe 1-y PO4, where 0.95≤x≤1.05, 0.3≤y≤0.8;

[0010] And the mass proportions d and y of the negative electrode film-forming additive in the electrolyte satisfy the following relationship:

[0011] 3≤ 100×d / y≤20 Formula II;

[0012] In formula II, 0.001≤d≤0.2.

[0013] Further, in Formula I,

[0014] Furthermore, in Formula I, the value range of a is: 0.05≤a≤1; further: 0.1≤a≤1; further: 0.1≤a≤0.8,

[0015] Furthermore, in Formula I, 0.05≤b≤1; further, 0.1≤b≤1; further, 0.1≤b≤0.8.

[0016] Furthermore, in Formula I, 0.1≤a+b≤2; further, 0.2≤a+b≤2; further, 0.2≤a+b≤1.5.

[0017] Furthermore, in Formula I, 1.9≤c≤2.6.

[0018] Furthermore, in Formula I, 0.4≤y≤0.8.

[0019] Furthermore, in Formula II, 0.01≤d≤0.15, further 0.01≤d≤0.10; further 0.02≤d≤0.08.

[0020] Furthermore, in Formula II, 5≤100×d / y≤15.

[0021] Furthermore, the mass proportion of the non-aqueous solvent in the electrolyte is 0.5-0.9; the mass proportion of the lithium salt in the electrolyte is 0.05-0.2; the mass proportion of the negative electrode film-forming additive in the electrolyte is 0.01-0.15, and the total is 0.95-1.

[0022] Furthermore, the electrolyte also includes other additives, and the mass proportion of other additives in the electrolyte is 0.01-0.2.

[0023] Furthermore, the carboxylic acid ester in the non-aqueous solvent is selected from at least one of methyl acetate, ethyl acetate, methyl propionate, ethyl propionate, methyl butyrate, propyl acetate, methyl formate, propyl propionate, methyl methylpropionate, methyl isobutyrate, methyl trimethylacetate and ethyl trimethylacetate.

[0024] Furthermore, the carbonate in the non-aqueous solvent includes at least one of ethylene carbonate, propylene carbonate, ethyl methyl carbonate and diethyl carbonate in addition to dimethyl carbonate.

[0025] Further, the non-aqueous solvent includes ethylene carbonate, dimethyl carbonate, ethyl acetate and ethyl methyl carbonate.

[0026] Furthermore, the mass ratio of ethylene carbonate, dimethyl carbonate, ethyl acetate and ethyl methyl carbonate is (1-3):(1-3):(1-3):(3-5).

[0027] Furthermore, the lithium salt includes at least one of lithium tetrafluoroborate, lithium hexafluorophosphate, lithium perchlorate, lithium bis(trifluoromethylsulfonyl)imide and lithium bis(fluorooxalatoborate) in addition to lithium bis(fluorosulfonyl)imide.

[0028] Further, the lithium salt includes lithium bis(fluorosulfonyl)imide and lithium hexafluorophosphate.

[0029] Furthermore, the molar ratio of lithium bis(fluorosulfonyl)imide to lithium hexafluorophosphate is (3-5):(5-7).

[0030] Furthermore, the negative electrode film-forming additive is selected from at least one of vinylene carbonate, ethylene carbonate, methylene carbonate, fluoroethylene carbonate, trifluoromethylethylene carbonate, bisfluoroethylene carbonate, vinyl sulfate, propylene sulfate, methyl vinyl sulfate, vinyl sulfite, 1,3-propane sultone, 1,4-butane sultone, 1,3-propylene sultone and methylene disulfonate.

[0031] Furthermore, the negative electrode film-forming additive is selected from at least one of vinylene carbonate, vinyl ethylene carbonate, fluoroethylene carbonate, vinyl sulfate, vinyl sulfite and methane disulfide sulphur methylene ester.

[0032] Furthermore, the mass percentage of vinylene carbonate in the negative electrode film-forming additive is 30% to 100%.

[0033] According to the second aspect of the present application, a battery is provided, comprising a positive electrode plate, a negative electrode plate, a separator and an electrolyte; the positive electrode material of the positive electrode plate is lithium iron manganese phosphate; and the electrolyte is an electrolyte adapted to the above-mentioned lithium iron manganese phosphate positive electrode material.

[0034] By applying the technical solution of the present application, an electrolyte adapted to lithium iron manganese phosphate positive electrode materials is provided, and the kinetic properties of lithium iron manganese phosphate are improved by introducing low-viscosity, high-conductivity solvents and lithium salts into the electrolyte. At the same time, a suitable film-forming additive is used to protect the negative electrode to prevent damage caused by manganese deposition. In addition, the electrolyte is optimized in a targeted manner according to the ratio of manganese to iron in lithium iron manganese phosphate and the compaction density; the optimized electrolyte can improve the problem that manganese ions in lithium iron manganese phosphate positive electrode materials are easily dissolved and deposited on the negative electrode surface to destroy the interface film; it can increase the lithium ion transfer rate, improve the kinetic performance, and can increase the conductivity, battery rate performance, and cycle performance. DETAILED DESCRIPTION

[0035] It should be noted that, in the absence of conflict, the embodiments and features in the embodiments of the present application can be combined with each other. The present application will be described in detail below in conjunction with the embodiments.

[0036] In the prior art, manganese ions in lithium iron manganese phosphate positive electrode materials are easily dissolved and deposited on the surface of the negative electrode to destroy the interface film. Generally, positive electrode chelating agents are added to the electrolyte, but this will increase the battery impedance. In particular, adding too much will significantly affect the battery performance, and adding too little will have little effect.

[0037] Therefore, the present application provides an electrolyte adapted to lithium manganese iron phosphate positive electrode material, comprising a non-aqueous solvent, a lithium salt and a negative electrode film-forming additive; the non-aqueous solvent comprises a carbonate ester and a carboxylate ester, the carbonate ester comprises dimethyl carbonate; the lithium salt comprises lithium bis(fluorosulfonyl)imide;

[0038] Among them, the mass ratio a of the total mass of dimethyl carbonate and carboxylic acid ester in the non-aqueous solvent, the molar ratio b of lithium bis(fluorosulfonyl)imide in the lithium salt, the molar ratio y of manganese element in the lithium iron manganese phosphate positive electrode material, and the compaction density c of the positive electrode sheet satisfy the following relationship:

[0039]

[0040] In formula I, 0.01≤a≤1; 0.01≤b≤1; 1.8≤c≤2.7, unit: g / cm 3 ;

[0041] The molecular formula of lithium manganese iron phosphate positive electrode material is Li x Mn y Fe 1-y PO4, where 0.95≤x≤1.05, 0.3≤y≤0.8;

[0042] And the mass proportions d and y of the negative electrode film-forming additive in the electrolyte satisfy the following relationship:

[0043] 3≤ 100×d / y≤20 Formula II;

[0044] In formula II, 0.001≤d≤0.2.

[0045] In Formula I of the present application, the value of (a+b) / (y×c) is 0.1, 0.2, 0.3, 0.4, 0.5, 0.6, 0.7, 0.8, 0.9, 1.0, 1.1, 1.2, 1.3, 1.4, 1.5, 1.6, 1.7, 1.8, 1.9, 2.0, 2.1, 2.2, 2.3, 2.4, 2.5, 2.6, 2.7, 2.8, 2.9, 3.0, or any range between any two thereof;

[0046] The value of 100×d / y is any value among 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13, 14, 15, 16, 17, 18, 19, 20 or a range between any two of them;

[0047] The value of a is any value among 0.01, 0.1, 0.2, 0.3, 0.4, 0.5, 0.6, 0.7, 0.8, 0.9, 1.0 or any range between two values;

[0048] The value of b is any value among 0.01, 0.1, 0.2, 0.3, 0.4, 0.5, 0.6, 0.7, 0.8, 0.9, 1.0 or any range between two values;

[0049] The value of c is any value among 1.8, 1.9, 2.0, 2.1, 2.2, 2.3, 2.4, 2.5, 2.6, 2.7 or any range between two values;

[0050] The value of y is any value among 0.3, 0.4, 0.5, 0.6, 0.7, 0.8, or any range between them;

[0051] The value of d is any value among 0.001, 0.1, 0.2, 0.3, 0.4, 0.5, 0.6, 0.7, 0.8, 0.9, 1.0, 1.1, 1.2, 1.3, 1.4, 1.5, 1.6, 1.7, 1.8, 1.9, 2.0 or any range therebetween.

[0052] The content ratio of a, b, and d in Formula I of the present application is the content ratio after the battery is formed.

[0053] The present application considers that the introduction of low-viscosity, high-conductivity solvents and lithium salts into the electrolyte can improve the conductivity and the kinetic properties of lithium iron manganese phosphate after synergistic effects, and selects dimethyl carbonate and carboxylic acid ester non-aqueous solvents as influencing factors. These two ester solvents have low viscosity and high conductivity, and have very good wetting effects on high-voltage dense electrode materials. One or more carboxylic acid ester solvents can be selected for compounding. Lithium bis(fluorosulfonyl)imide is selected as an influencing factor because the lithium salt has good conductivity and can also promote negative electrode film formation; considering that the negative electrode film-forming agent can protect the negative electrode surface to prevent manganese deposition, the negative electrode film-forming agent is used as an influencing factor, and the effect between the manganese element, iron element and compaction density in the lithium iron manganese phosphate is also considered, and the manganese element and compaction density are used as an influencing factor of the electrolyte performance; the multiple influencing factors considered in the present application have a mutually reinforcing and mutually influencing effect. When the above relationship is satisfied between the factors, the capacity attenuation phenomenon caused by manganese deposition as well as the conductivity and kinetic properties can be significantly improved.

[0054] The present application improves the kinetic performance of lithium iron manganese phosphate by introducing low-viscosity, high-conductivity solvents and lithium salts into the electrolyte; at the same time, a suitable film-forming additive is used to protect the negative electrode to prevent damage caused by manganese deposition; and the electrolyte is optimized in a targeted manner according to the ratio of manganese to iron in the lithium iron manganese phosphate and the compaction density; the optimized electrolyte can improve the problem that manganese ions in the lithium iron manganese phosphate positive electrode material are easily dissolved and deposited on the negative electrode surface to destroy the interface film; it can increase the lithium ion transfer rate, improve the kinetic performance, and can increase the conductivity, battery rate performance and cycle performance.

[0055] In some embodiments, in Formula I, Further, 0.55≤(a+b) / (y×c)≤2.

[0056] The present application further limits the proportion of the total weight of carbonate and carboxylate in the non-aqueous solvent, the proportion of the molar amount of lithium bis(fluorosulfonyl)imide in the total molar amount of lithium salt, and the compaction density of the positive electrode plate, so as to better play the synergistic effect between carbonate, carboxylate, lithium bis(fluorosulfonyl)imide and the positive electrode plate and the compaction density, better optimize the electrolyte, and further improve the conductivity and kinetic performance of lithium iron manganese phosphate.

[0057] In some embodiments, in Formula I, the value range of a is, for example, 0.05≤a≤1; for another example, 0.1≤a≤1; for another example, 0.1≤a≤0.8; the value range of b is, for example, 0.05≤b≤1; for another example, 0.1≤b≤1; for another example, 0.1≤b≤0.8; for example, 0.1≤a+b≤2; for another example, 0.2≤a+b≤2; for another example, 0.2≤a+b≤1.5.

[0058] The present application further increases the proportion of the total weight of carbonate and carboxylate in the non-aqueous solvent and the proportion of lithium bis(fluorosulfonyl)imide in the lithium salt, giving full play to the synergistic effect between the solvent and the electrolytic lithium salt, which helps to improve the conductivity of the electrolyte.

[0059] In some embodiments, in Formula I, the value ranges of c and y are, for example: 1.9≤c≤2.6; 0.4≤y≤0.8.

[0060] The present application defines the compaction density as a high compaction density which has a good match with the above-selected ester solvent and lithium salt, and the above-selected ester solvent can also better infiltrate the high compaction density lithium iron manganese phosphate positive electrode plate.

[0061] In some embodiments, in Formula II, the value range of d is, for example, 0.01≤d≤0.15; for another example, 0.01≤d≤0.10; for another example, 0.02≤d≤0.08; for a further example, 5≤100×d / y≤15.

[0062] The present application further limits the content of the negative electrode film-forming additive and the ratio between the negative electrode film-forming additive and the manganese element, thereby better improving the degree of manganese ion deposition and protecting the negative electrode from manganese damaging the interface.

[0063] In some embodiments, the mass proportion of the non-aqueous solvent in the electrolyte is 0.5-0.9 (or 50%-90%), for example, 0.7-0.9, for example, any value of 0.5, 0.6, 0.7, 0.8, 0.9 or any range between two values; the mass proportion of the lithium salt in the electrolyte is 0.05-0.2 (or 5%-2%), for example, 0.1-0.2, for example, 0.05, 0.08, 0.1, 0.12, 0 .15, 0.18, 0.2 or any range between any two of them; the mass proportion of the negative electrode film-forming additive in the electrolyte is 0.01-0.15 (or 1%-15%), for example, 0.02-0.08, for example, 0.01, 0.03, 0.05, 0.08, 0.10, 0.12, 0.15 or any range between any two of them; the total is 0.95-1 (or 95%-100%). In addition to non-aqueous solvents, lithium salts, and negative electrode film-forming additives, the electrolyte of the present application may also include other auxiliary additives, such as positive electrode film-forming additives, etc. The additives can be selected according to actual needs, with a mass ratio of 0.01 to 0.2 (or 1% to 20%), for example, any value among 0.01, 0.03, 0.05, 0.08, 0.10, 0.12, 0.15, 0.18, 0.20 or any range value between two of them.

[0064] In some embodiments, the carboxylic acid ester is selected from at least one of methyl acetate, ethyl acetate, methyl propionate, ethyl propionate, methyl butyrate, propyl acetate, methyl formate, propyl propionate, methyl methylpropionate, methyl isobutyrate, methyl trimethylacetate, and ethyl trimethylacetate;

[0065] The carbonate ester includes at least one of ethylene carbonate, propylene carbonate, dimethyl carbonate, ethyl methyl carbonate and diethyl carbonate in addition to dimethyl carbonate.

[0066] For example, the non-aqueous solvent is formed by mixing ethylene carbonate, dimethyl carbonate, ethyl acetate, methyl acetate and ethyl methyl carbonate; more specifically, the mass ratio of ethylene carbonate, dimethyl carbonate, ethyl acetate (methyl acetate) and ethyl methyl carbonate is (1-3): (1-5): (1-4): (1-5); within this mass ratio range, the proportion of ethylene carbonate is any value among 1, 2, 3 or a range value between any two of them, the proportion of dimethyl carbonate is any value among 1, 2, 3, 4, 5 or a range value between any two of them, the proportion of ethyl acetate (methyl acetate) is any value among 1, 2, 3, 4 or a range value between any two of them, and the proportion of ethyl methyl carbonate is any value among 1, 2, 3, 4, 5 or a range value between any two of them; for example, the mass ratio is (1-3): (1-3): (1-3): (3-5); further example is 2:2:2:4.

[0067] The above-mentioned carboxylic acid solvents and the carbonates in a specific ratio selected in the present application have better solubility in lithium manganese iron phosphate materials, and can better infiltrate high-density lithium manganese iron phosphate positive electrode plates, thereby improving electrical conductivity and kinetic performance.

[0068] In some embodiments, the lithium salt includes at least one of lithium tetrafluoroborate, lithium hexafluorophosphate, lithium perchlorate, lithium bisfluorosulfonyl imide, lithium bistrifluoromethylsulfonyl imide and lithium bisfluorooxalate borate in addition to lithium bisfluorosulfonyl imide. For example, the lithium salt includes lithium bisfluorosulfonyl imide and / or lithium hexafluorophosphate; specifically, the molar ratio of lithium bisfluorosulfonyl imide to lithium hexafluorophosphate is (3-5): (5-7); for example, 3:5, 3:6, 3:7, 4:5, 4:6, 4:7, 5:5, 5:6, 5:7. The present application selects the above two lithium salts and the above ratios to match them with the above ester solvents selected in the present application, which can improve the conductivity and kinetic properties of the electrolyte.

[0069] In some embodiments, the negative electrode film-forming additive is selected from at least one of vinylene carbonate, ethylene carbonate, methylene carbonate, fluoroethylene carbonate, trifluoromethylethylene carbonate, bisfluoroethylene carbonate, vinyl sulfate, propylene sulfate, methyl vinyl sulfate, vinyl sulfite, 1,3-propane sultone, 1,4-butane sultone, 1,3-propylene sultone, and methylene disulfonate.

[0070] Further, the negative electrode film-forming additive is selected from at least one of vinylene carbonate, vinyl ethylene carbonate, fluoroethylene carbonate, vinyl sulfate, vinyl sulfite and methane disulfide methylene ester; specifically, the mass percentage of vinylene carbonate (VC) in the negative electrode film-forming agent is 30% to 100%, for example, any value among 30%, 40%, 50%, 60%, 70%, 80%, 90%, 100% or any range value between two of them.

[0071] The negative electrode film-forming additive selected in the present application can better form a film interface at the negative electrode, and cooperate with the above-selected solvent and lithium salt to better improve the deposition of manganese at the negative electrode.

[0072] According to the second aspect of the present application, a battery is provided, comprising a positive electrode plate, a negative electrode plate, a separator and an electrolyte; the positive electrode material of the positive electrode plate is lithium iron manganese phosphate; and the electrolyte is an electrolyte adapted to the above-mentioned lithium iron manganese phosphate positive electrode material.

[0073] The present application is further described in detail below in conjunction with specific embodiments. These embodiments should not be construed as limiting the scope of protection claimed in the present application.

[0074] The raw materials used in the examples of this application are prior art and can be purchased commercially.

[0075] Example 1

[0076] Preparation of positive electrode sheet: lithium iron manganese phosphate, binder polyvinylidene fluoride, and conductive agent acetylene black are mixed in a mass ratio of 95:2:3, and then N-methylpyrrolidone (NMP) is added and stirred until the mixture is uniform, thereby preparing positive electrode slurry; the positive electrode slurry is evenly coated on aluminum foil, followed by drying, and then cold pressing and other subsequent processes to finally prepare positive electrode sheet; wherein, the molecular formula of the positive electrode material lithium iron manganese phosphate is LiMn 0.5 Fe 0.5 PO4, compacted density is 2.2g / cm 3 .

[0077] Preparation of negative electrode sheet: negative electrode active material graphite, conductive agent acetylene black, adhesive styrene butadiene rubber and thickener sodium carboxymethyl cellulose are mixed in a mass ratio of 96:2:1:1, deionized water is added, and the negative electrode slurry is fully stirred to obtain the negative electrode slurry, which is coated on the negative electrode collector, and the negative electrode sheet is obtained through drying, cold pressing and other processes.

[0078] Lithium-ion battery assembly: the vacuum-dried positive and negative electrodes and separators are cut, die-cut, and striped, and then the electrolyte is injected under high temperature and negative pressure to assemble into square aluminum shell batteries;

[0079] Among them, the solvent includes ethylene carbonate (EC), dimethyl carbonate (DMC), ethyl acetate (EA) and ethyl methyl carbonate (EMC), and the mass ratio is 2:2:2:4; the lithium salt includes lithium bis(fluorosulfonyl)imide (LiFSI) and lithium salt lithium hexafluorophosphate (LiPF6), and the molar ratio is 3:7; the lithium salt concentration in the electrolyte is 1M; the negative electrode film-forming additive accounts for 4wt% of the total mass of the electrolyte, among which vinylene carbonate (VC) accounts for 3.5wt% and methane disulfide (MMDS) accounts for 0.5wt%; the solvent, lithium salt and negative electrode film-forming additive are mixed to form an electrolyte.

[0080] Example 2

[0081] Preparation of positive electrode sheet: lithium iron manganese phosphate, binder polyvinylidene fluoride, and conductive agent acetylene black are mixed in a ratio of 95:2:3, and then N-methylpyrrolidone (NMP) is added and stirred until the mixture is uniform, thereby preparing positive electrode slurry; the positive electrode slurry is evenly coated on aluminum foil, followed by drying, and then cold pressing and other subsequent processes to finally prepare positive electrode sheet; wherein, the molecular formula of the positive electrode material lithium iron manganese phosphate is LiMn 0.5 Fe 0.5 PO4, compacted density is 2.2g / cm 3 .

[0082] Preparation of negative electrode sheet: negative electrode active material graphite, conductive agent acetylene black, adhesive styrene butadiene rubber and thickener sodium carboxymethyl cellulose are mixed in a mass ratio of 96:2:1:1, deionized water is added, and the negative electrode slurry is fully stirred to obtain the negative electrode slurry, which is coated on the negative electrode collector, and the negative electrode sheet is obtained through drying, cold pressing and other processes.

[0083] Lithium-ion battery assembly: the vacuum-dried positive and negative electrodes and separators are cut, die-cut, and striped, and then the electrolyte is injected under high temperature and negative pressure to assemble into square aluminum shell batteries;

[0084] Among them, the solvent is ethylene carbonate (EC), dimethyl carbonate (DMC), ethyl acetate (EA) and ethyl methyl carbonate (EMC), with a mass ratio of 2:3:1:4; the lithium salt includes lithium bis(fluorosulfonyl)imide (LiFSI) and lithium hexafluorophosphate (LiPF6), with a molar ratio of 5:5, and the lithium salt concentration is 1M; the negative electrode film-forming additive accounts for 3.5wt% of the total mass of the electrolyte, of which vinylene carbonate (VC) accounts for 2.5wt% and vinyl sulfate (DTD) accounts for 1.0wt%; the solvent, lithium salt and negative electrode film-forming additive are mixed to form an electrolyte.

[0085] Example 3

[0086] The difference between Example 3 and Example 1 is that the positive electrode material lithium manganese iron phosphate has a molecular formula of LiMn 0.6 Fe 0.4 PO4, compacted: 1.9g / cm 3 ;

[0087] Among them, the solvent includes ethylene carbonate (EC), dimethyl carbonate (DMC), ethyl acetate (EA) and ethyl methyl carbonate (EMC), with a mass ratio of 2:5:1:2; the lithium salt includes lithium bis(fluorosulfonyl)imide (LiFSI) and lithium hexafluorophosphate (LiPF6) with a molar ratio of 2:8, and the lithium salt concentration is 1M; the negative electrode film-forming additive accounts for 5wt% of the total mass of the electrolyte, of which vinylene carbonate (VC) accounts for 2.0wt%, vinyl sulfate (DTD) accounts for 1.0wt%, and fluoroethylene carbonate (FEC) accounts for 2.0wt%; the solvent, lithium salt and negative electrode film-forming additive are mixed to form an electrolyte.

[0088] Example 4

[0089] The difference between Example 4 and Example 1 is that the positive electrode material lithium manganese iron phosphate has a molecular formula of LiMn 0.5 Fe 0.5 PO4, compacted: 2.2g / cm 3 ;

[0090] Among them, the solvent includes ethylene carbonate (EC), dimethyl carbonate (DMC), methyl acetate (MA) and ethyl methyl carbonate (EMC), with a volume ratio of 2:4:1:4; the lithium salt includes lithium bis(fluorosulfonyl)imide (LiFSI) and lithium hexafluorophosphate (LiPF6) with a molar ratio of 3:7, and the lithium salt concentration is 1M; the negative electrode film-forming additive accounts for 3.5wt% of the total mass of the electrolyte, among which vinylene carbonate (VC) accounts for 3.0wt% and methane disulfide (MMDS) accounts for 0.5wt%; the solvent, lithium salt and negative electrode film-forming additive are mixed to form an electrolyte.

[0091] Example 5

[0092] The difference between Example 5 and Example 1 is that the positive electrode material lithium manganese iron phosphate has a molecular formula of LiMn 0.5 Fe 0.5 PO4, compacted: 2.2g / cm 3 ;

[0093] The solvent includes ethylene carbonate (EC), dimethyl carbonate (DMC), ethyl acetate (EA) and ethyl methyl carbonate (EMC), with a volume ratio of 4:0.3:0.2:5.5; the lithium salt includes lithium bis(fluorosulfonyl)imide (LiFSI) and lithium hexafluorophosphate (LiPF6) with a molar ratio of 4:6, and the lithium salt concentration is 1M; the negative electrode film-forming additive accounts for 3.5wt% of the total mass of the electrolyte, of which vinylene carbonate (VC) accounts for 3.0wt% and vinyl sulfate (DTD) accounts for 0.5wt%; the solvent, lithium salt and negative electrode film-forming additive are mixed to form an electrolyte.

[0094] Example 6

[0095] The difference between Example 5 and Example 1 is that the positive electrode material lithium manganese iron phosphate has a molecular formula of LiMn 0.5 Fe 0.5 PO4, compacted density is 2.2g / cm 3 ;

[0096] The solvent includes ethylene carbonate (EC), dimethyl carbonate (DMC), ethyl acetate (EA) and ethyl methyl carbonate (EMC) in a volume ratio of 3:1:2:4; the lithium salt includes lithium bis(fluorosulfonyl)imide (LiFSI) and lithium hexafluorophosphate (LiPF6) in a molar ratio of 0.05:0.95, and the lithium salt concentration is 1M; the negative electrode film-forming additive accounts for 3.5wt% of the total mass of the electrolyte, of which vinylene carbonate (VC) accounts for 3.0% and vinyl sulfate (DTD) accounts for 0.5%; the solvent, lithium salt and negative electrode film-forming additive are mixed to form an electrolyte.

[0097] Example 7

[0098] The difference between Example 7 and Example 1 is that the positive electrode material lithium manganese iron phosphate has a molecular formula of LiMn 0.5 Fe 0.5 PO4, compacted density is 2.2g / cm 3 ;

[0099] The solvent includes ethylene carbonate (EC), dimethyl carbonate (DMC), ethyl acetate (EA) and ethyl methyl carbonate (EMC), with a volume ratio of 3:0.5:0.5:6; the lithium salt includes lithium bis(fluorosulfonyl)imide (LiFSI) and lithium hexafluorophosphate (LiPF6), with a molar ratio of 1:9 and a lithium salt concentration of 1M; the negative electrode film-forming additive accounts for 3.5wt% of the total mass of the electrolyte, of which vinylene carbonate (VC) accounts for 3.0% and vinyl sulfate (DTD) accounts for 0.5%; the solvent, lithium salt and negative electrode film-forming additive are mixed to form an electrolyte.

[0100] Example 8

[0101] The difference between Example 8 and Example 1 is that the positive electrode material lithium manganese iron phosphate has a molecular formula of LiMn 0.5 Fe 0.5 PO4, compacted density is 2.2g / cm 3 ;

[0102] The solvent includes ethylene carbonate (EC), dimethyl carbonate (DMC), ethyl acetate (EA) and ethyl methyl carbonate (EMC) in a volume ratio of 2:2:2:4; the lithium salt includes lithium bis(fluorosulfonyl)imide (LiFSI) and lithium hexafluorophosphate (LiPF6) in a molar ratio of 3:7, and the lithium salt concentration is 1M; the negative electrode film-forming additive accounts for 2wt% of the total mass of the electrolyte, of which vinylene carbonate (VC) accounts for 1.5% and vinyl sulfate (DTD) accounts for 0.5%; the solvent, lithium salt and negative electrode film-forming additive are mixed to form an electrolyte.

[0103] Example 9

[0104] The difference between Example 9 and Example 1 is that the positive electrode material lithium manganese iron phosphate has a molecular formula of LiMn 0.5 Fe 0.5 PO4, compacted density is 2.2g / cm 3 ;

[0105] The solvent includes ethylene carbonate (EC), dimethyl carbonate (DMC), ethyl acetate (EA) and ethyl methyl carbonate (EMC) in a volume ratio of 2:2:2:4; the lithium salt includes lithium bis(fluorosulfonyl)imide (LiFSI) and lithium hexafluorophosphate (LiPF6) in a molar ratio of 3:7, and the lithium salt concentration is 1M; the negative electrode film-forming additive accounts for 6wt% of the total mass of the electrolyte, of which vinylene carbonate (VC) accounts for 3%, methane disulfide (MMDS) accounts for 2%, and vinyl sulfate (DTD) accounts for 1%; the solvent, lithium salt and negative electrode film-forming additive are mixed to form an electrolyte.

[0106] Example 10

[0107] The difference between Example 10 and Example 1 is that the positive electrode material lithium manganese iron phosphate has a molecular formula of LiMn 0.5 Fe 0.5 PO4, compacted density is 2.2g / cm 3 ;

[0108] The solvent includes ethylene carbonate (EC), dimethyl carbonate (DMC) and ethyl methyl carbonate (EMC), with a volume ratio of 3:0.01:6.99; the lithium salt includes lithium bis(fluorosulfonyl)imide (LiFSI) and lithium hexafluorophosphate (LiPF6), with a molar ratio of 0.1:9.9, and the lithium salt concentration is 1M; the negative electrode film-forming additive accounts for 3.5wt% of the total mass of the electrolyte, of which vinylene carbonate (VC) accounts for 3.0% and vinyl sulfate (DTD) accounts for 0.5%; the solvent, lithium salt and negative electrode film-forming additive are mixed to form an electrolyte.

[0109] Embodiment 11

[0110] The difference between Example 11 and Example 1 is that the positive electrode material lithium manganese iron phosphate has a molecular formula of LiMn 0.5 Fe 0.5 PO4, compacted density is 2.2g / cm 3 ;

[0111] The solvent includes ethylene carbonate (EC), dimethyl carbonate (DMC), ethyl acetate (EA) and ethyl methyl carbonate (EMC), with a volume ratio of 2:2:2:4; the molar ratio of lithium salt lithium bis(fluorosulfonyl)imide (LiFSI) and lithium salt lithium hexafluorophosphate (LiPF6) is 3:7, and the lithium salt concentration is 1M; the negative electrode film-forming additive accounts for 10wt% of the total mass of the electrolyte, of which vinylene carbonate (VC) accounts for 7%, methane disulfide (MMDS) accounts for 2%, and vinyl sulfate (DTD) accounts for 1%; the solvent, lithium salt, and negative electrode film-forming additive are mixed to form an electrolyte.

[0112] Example 12

[0113] The difference between Example 12 and Example 1 is that the positive electrode material lithium manganese iron phosphate has a molecular formula of LiMn 0.5 Fe 0.5 PO4, compacted density is 2.2g / cm 3 ;

[0114] The solvent includes ethylene carbonate (EC), dimethyl carbonate (DMC), ethyl acetate (EA) and ethyl methyl carbonate (EMC), with a volume ratio of 2:2:2:4; the molar ratio of lithium salt lithium bis(fluorosulfonyl)imide (LiFSI) and lithium salt lithium hexafluorophosphate (LiPF6) is 3:7, and the lithium salt concentration is 1M; the negative electrode film-forming additive accounts for 1.5wt% of the total mass of the electrolyte, of which vinylene carbonate (VC) accounts for 1.0% and vinyl sulfate (DTD) accounts for 0.5%; the solvent, lithium salt and negative electrode film-forming additive are mixed to form an electrolyte.

[0115] Example 13

[0116] The difference between Example 13 and Example 1 is that the positive electrode material lithium manganese iron phosphate has a molecular formula of LiMn 0.5 Fe 0.5 PO4, compacted density is 2.2g / cm 3 ;

[0117] The solvent includes ethylene carbonate (EC), dimethyl carbonate (DMC) and ethyl acetate (EA) in a volume ratio of 2:6:2; the lithium salt is lithium bis(fluorosulfonyl)imide (LiFSI) and the lithium salt concentration is 1M; the negative electrode film-forming additive accounts for 3.5wt% of the total mass of the electrolyte, of which vinylene carbonate (VC) accounts for 3.0% and vinyl sulfate (DTD) accounts for 0.5%; the solvent, lithium salt and negative electrode film-forming additive are mixed to form an electrolyte.

[0118] Comparative Example 1

[0119] The difference between Comparative Example 1 and Example 1 is that the positive electrode material lithium manganese iron phosphate has a molecular formula of LiMn 0.5 Fe 0.5 PO4, compacted density is 2.2g / cm 3 ;

[0120] The solvent includes ethylene carbonate (EC), dimethyl carbonate (DMC) and ethyl methyl carbonate (EMC), with a volume ratio of 3:0.05:6.95; the lithium salt is lithium hexafluorophosphate (LiPF6), and the lithium salt concentration is 1M; the negative electrode film-forming additive accounts for 3.5wt% of the total mass of the electrolyte, of which vinylene carbonate (VC) accounts for 3.0% and methane disulfide methyl ester (MMDS) accounts for 0.5%; the solvent, lithium salt and negative electrode film-forming additive are mixed to form an electrolyte.

[0121] Comparative Example 2

[0122] The difference between Comparative Example 2 and Example 1 is that the positive electrode material lithium manganese iron phosphate has a molecular formula of LiMn 0.5 Fe 0.5 PO4, compacted: 2.2g / cm 3 ;

[0123] The solvent includes ethylene carbonate (EC) and ethyl methyl carbonate (EMC) in a volume ratio of 3:7; the lithium salt includes lithium bis(fluorosulfonyl)imide (LiFSI) and lithium hexafluorophosphate (LiPF6) in a molar ratio of 0.1:9.9, and the lithium salt concentration is 1M; the negative electrode film-forming additive accounts for 3.5wt% of the total mass of the electrolyte, of which vinylene carbonate (VC) accounts for 3.0% and methane disulfide (MMDS) accounts for 0.5%; the solvent, lithium salt and negative electrode film-forming additive are mixed to form an electrolyte.

[0124] Comparative Example 3

[0125] The difference between Comparative Example 3 and Example 1 is that the positive electrode material lithium manganese iron phosphate has a molecular formula of LiMn 0.5 Fe 0.5 PO4, compacted density is 2.2g / cm 3 ;

[0126] The solvent includes ethylene carbonate (EC), dimethyl carbonate (DMC), ethyl acetate (EA) and ethyl methyl carbonate (EMC) in a volume ratio of 2:2:2:4; the lithium salt includes lithium bis(fluorosulfonyl)imide (LiFSI) and lithium hexafluorophosphate (LiPF6) in a molar ratio of 3:7, and the lithium salt concentration is 1M; the negative electrode film-forming additive accounts for 0.5wt% of the total mass of the electrolyte, which is vinyl sulfate (DTD); the solvent, lithium salt and negative electrode film-forming additive are mixed to form an electrolyte.

[0127] Comparative Example 4

[0128] The difference between Comparative Example 4 and Example 1 is that the positive electrode material lithium manganese iron phosphate has a molecular formula of LiMn 0.5 Fe 0.5 PO4, compacted density is 2.2g / cm 3 ;

[0129] The solvent includes ethylene carbonate (EC), dimethyl carbonate (DMC), ethyl acetate (EA) and ethyl methyl carbonate (EMC) in a volume ratio of 2:2:2:4; the lithium salt includes lithium bis(fluorosulfonyl)imide (LiFSI) and lithium hexafluorophosphate (LiPF6) in a molar ratio of 3:7, and the lithium salt concentration is 1M; the negative electrode film-forming additive accounts for 16wt% of the total mass of the electrolyte, of which vinyl sulfate (DTD) accounts for 6% and fluoroethylene carbonate (FEC) accounts for 10%; the solvent, lithium salt and negative electrode film-forming additive are mixed to form an electrolyte.

[0130] The contents of the electrolyte ester solvent, lithium salt and negative electrode film-forming agent in Examples 1 to 13 and Comparative Examples 1 to 4 are shown in Table 1.

[0131] Table 1

[0132]

[0133]

[0134] The performance of the batteries prepared in Examples 1 to 13 and Comparative Examples 1 to 4 were tested.

[0135] 1. Rate performance test:

[0136] For each embodiment and comparative example at 25°C, the battery is charged with a constant current and constant voltage of 0.5C, the cut-off voltage is 4.2V, and the cut-off current is 0.05C. The battery is discharged with a constant current of 0.5C to 2.5V, and the cycle is repeated 3 times. The discharge capacity of the last time is taken as the initial capacity and recorded as Q0. The battery is then charged with a constant current and constant voltage of 0.5C, the cut-off voltage is 4.2V, and the cut-off current is 0.05C. After being fully charged, it is discharged at a rate of 2C, and the discharge capacity Q1 is recorded. The discharge capacity retention rate is tested to be Q1 / Q0. The test results are shown in Table 2.

[0137] 2. Cycle performance test:

[0138] After each embodiment and comparative example was left at 25° C. for 1 hour, the battery was fully charged and discharged, and the capacity retention rate at 200, 500 and 1000 cycles was recorded. The test results are shown in Table 2.

[0139] Full charge: The battery is charged at a constant current and constant voltage of 0.5C, with a cut-off voltage of 4.2V and a cut-off current of 0.05C;

[0140] Full discharge: The battery is discharged at a constant current of 0.5C to 2.5V

[0141] Table 2

[0142]

[0143] It can be seen from the test data in Table 2 that the battery discharge capacity retention rate of Examples 1 to 13 of the present application is between 86.78% and 92.25%; after 200 charge and discharge cycles, the normal temperature capacity retention rate is between 95.92% and 98.56%, and the high temperature capacity retention rate is between 94.25% and 97.61%; after 500 charge and discharge cycles, the normal temperature capacity retention rate is between 93.45% and 96.64%, and the high temperature capacity retention rate is between 90.91% and 94.93%; after 1000 charge and discharge cycles, the normal temperature capacity retention rate is between 90.26% and 93.98%; the high temperature capacity retention rate is between 86.82% and 91.05%, and the improvement effect is significant; it shows that the optimized electrolyte of the present application can improve the problem that manganese ions in the lithium iron manganese phosphate positive electrode material are easily dissolved and deposited on the negative electrode surface to destroy the interface film; it can improve the lithium ion transfer rate, improve the kinetic performance, improve the conductivity, and improve the battery rate performance and cycle performance.

[0144] The overall performance of Comparative Examples 1 to 4 is slightly worse than that of Examples 1 to 13; among them, the lithium salt of Comparative Example 1 does not use LiFSI, but all uses LiPF6, the DMC+carboxylate content is low, and the calculation result of Formula I is less than 0.1. Due to the low DMC+carboxylate content, the kinetic performance is poor, the quality of the formed SEI film deteriorates, and the battery rate performance and cycle performance are poor.

[0145] In Comparative Example 2, dimethyl carbonate (DMC) and ethyl acetate (EA) were not used in the non-aqueous solvent, and other carbonate solvents were selected. The LiFSI content was low, and the calculation result of Formula I was less than 0.1. Due to the low LiFSI content, DMC+carboxylate was not used, the kinetic performance was poor, and the battery rate performance and cycle performance were poor.

[0146] In Comparative Example 3, the content of the negative electrode film-forming additive is too low, and the calculation result of Formula II is less than 3. When the content of the negative electrode film-forming additive is small, a dense SEI film cannot be formed or the damaged SEI film cannot be repaired in the later cycles, resulting in serious attenuation of the battery cycle life.

[0147] In Comparative Example 4, the content of the negative electrode film-forming additive is too high, and the calculation result of Formula II is greater than 20. When the content of the negative electrode film-forming additive is large, the impedance is large, which deteriorates the kinetic performance and cycle performance, causing serious attenuation of the battery cycle life.

[0148] It should be noted that the terms "first", "second", etc. in the specification and claims of the present application are used to distinguish similar objects, and are not necessarily used to describe a specific order or sequence. It should be understood that the terms used in this way can be interchangeable where appropriate, so that the embodiments of the present application described herein can be implemented in an order other than those described herein, for example.

Claims

1. An electrolyte suitable for lithium manganese iron phosphate positive electrode material, characterized in that: The electrolyte includes a non-aqueous solvent, a lithium salt and a negative electrode film-forming additive; the non-aqueous solvent includes a carbonate ester and a carboxylate ester, the carbonate ester includes dimethyl carbonate; the lithium salt includes lithium bis(fluorosulfonyl)imide; The following relationship is satisfied between the total mass ratio a of the dimethyl carbonate and the carboxylate in the non-aqueous solvent, the molar ratio b of the lithium bis(fluorosulfonyl)imide in the lithium salt, the molar ratio y of the manganese element in the lithium iron manganese phosphate positive electrode material, and the compaction density c of the positive electrode sheet: In formula I, 0.01≤a≤1; 0.01≤b≤1; 1.8≤c≤2.7, unit: g / cm 3 ; The molecular formula of the lithium manganese iron phosphate positive electrode material is Li x Mn y Fe 1-y PO4, where 0.95≤x≤1.05, 0.3≤y≤0.8; And the mass proportion d of the negative electrode film-forming additive in the electrolyte and y satisfy the following relationship: 3≤ 100×d / y≤20 Formula II; In formula II, 0.001≤d≤0.

2.

2. The electrolyte adapted to lithium manganese iron phosphate positive electrode material according to claim 1, characterized in that: In the formula I, 3. The electrolyte adapted to lithium manganese iron phosphate positive electrode material according to claim 1 or 2, characterized in that: In the formula I, 0.05≤a≤1; preferably 0.1≤a≤1; more preferably 0.1≤a≤0.8; And / or, in the formula I, 0.05≤b≤1; preferably 0.1≤b≤1; more preferably 0.1≤b≤0.8; And / or, in the formula I, 0.1≤a+b≤2; preferably 0.2≤a+b≤2; more preferably 0.2≤a+b≤1.

5.

4. The electrolyte adapted to lithium manganese iron phosphate positive electrode material according to any one of claims 1 to 3, characterized in that: In the formula I, 1.9≤c≤2.6; and / or, 0.4≤y≤0.

8.

5. The electrolyte adapted to lithium iron manganese phosphate positive electrode material according to any one of claims 1 to 4, characterized in that: In the formula II, 0.01≤d≤0.15; preferably 0.01≤d≤0.10; more preferably 0.02≤d≤0.08; And / or, in the formula II, 5≤100×d / y≤15.

6. The electrolyte adapted to lithium iron manganese phosphate positive electrode material according to any one of claims 1 to 5, characterized in that: The mass proportion of the non-aqueous solvent in the electrolyte is 0.5-0.9; the mass proportion of the lithium salt in the electrolyte is 0.05-0.2; the mass proportion of the negative electrode film-forming additive in the electrolyte is 0.01-0.15, and the total is 0.95-1; Preferably, the electrolyte further comprises other additives, and the mass proportion of the other additives in the electrolyte is 0.01-0.

2.

7. The electrolyte adapted to lithium manganese iron phosphate positive electrode material according to any one of claims 1 to 6, characterized in that: The carboxylic acid ester in the non-aqueous solvent is selected from at least one of methyl acetate, ethyl acetate, methyl propionate, ethyl propionate, methyl butyrate, propyl acetate, methyl formate, propyl propionate, methyl methylpropionate, methyl isobutyrate, methyl trimethylacetate and ethyl trimethylacetate; And / or, the carbonate in the non-aqueous solvent includes at least one of ethylene carbonate, propylene carbonate, ethyl methyl carbonate and diethyl carbonate in addition to dimethyl carbonate; Preferably, the non-aqueous solvent includes the ethylene carbonate, the dimethyl carbonate, the ethyl acetate, the methyl acetate, and the ethyl methyl carbonate; Preferably, the mass ratio of the ethylene carbonate, the dimethyl carbonate, the ethyl acetate and the ethyl methyl carbonate is (1-3):(1-3):(1-3):(3-5).

8. The electrolyte adapted to lithium iron manganese phosphate positive electrode material according to any one of claims 1 to 7, characterized in that: The lithium salt includes at least one of lithium tetrafluoroborate, lithium hexafluorophosphate, lithium perchlorate, lithium bis(trifluoromethylsulfonyl)imide and lithium bis(fluorooxalatoborate) in addition to the lithium bis(fluorosulfonyl)imide; Preferably, the lithium salt includes the lithium bis(fluorosulfonyl)imide and the lithium hexafluorophosphate; Preferably, the molar ratio of the lithium bis(fluorosulfonyl)imide to the lithium hexafluorophosphate is (3-5):(5-7).

9. The electrolyte adapted to lithium iron manganese phosphate positive electrode material according to any one of claims 1 to 8, characterized in that: The negative electrode film-forming additive is selected from at least one of vinylene carbonate, vinyl ethylene carbonate, methylene carbonate, fluoroethylene carbonate, trifluoromethylethylene carbonate, bisfluoroethylene carbonate, vinyl sulfate, propylene sulfate, methyl vinyl sulfate, vinyl sulfite, 1,3-propane sultone, 1,4-butane sultone, 1,3-propylene sultone and methylene disulfonate; Preferably, the negative electrode film-forming additive is selected from at least one of vinylene carbonate, vinyl ethylene carbonate, fluoroethylene carbonate, vinyl sulfate, vinyl sulfite and methane disulfide sulfonate; Preferably, the mass percentage of the vinylene carbonate in the negative electrode film-forming additive is 30% to 100%.

10. A battery comprising a positive electrode sheet, a negative electrode sheet, a separator and an electrolyte; characterized in that: The positive electrode material of the positive electrode plate is lithium iron manganese phosphate; and the electrolyte is the electrolyte adapted to the lithium iron manganese phosphate positive electrode material as described in any one of claims 1 to 9.

Images