Electrolyte and lithium ion battery thereof

Abstract

The present invention discloses an electrolyte and a lithium-ion battery thereof. The electrolyte comprises, by mass percentage, 20-70% of an organic solvent, 0.1-4.0% of an additive a, 5-20% of ethyl fluoroacetate, 5-15% of an additive b, and the remainder being an electrolyte salt. The general structural formula of additive a is the following formula I, and the structural formula of ethyl fluoroacetate is the following formula A: #imgabs0# wherein, in formula I, R1 is a C1-C5 alkylene group; R2, R3, R4, and R5 are each independently a C1-C6 alkylene group, a fluoroalkylene group, an alkyleneoxy group, or a fluoroalkyleneoxy group; and the additive b is two or more of vinylene carbonate, 1,3-propane sultone, vinyl sulfate, succinonitrile, adiponitrile, 1,3,6-hexanetrinitrile, propylene sultone, methylene methanedisulfonate, ethylene glycol bis(propionitrile) ether, and a fluorinated ether. The electrolyte of the present invention does not deteriorate the low-temperature performance, is not prone to a cliff-like drop in cycle capacity, and takes into account the high and low temperature performance of the high-voltage system.

Description

Technical Field

[0001] The present invention relates to the technical field of lithium ion batteries, and in particular to an electrolyte and a lithium ion battery thereof. Background Art

[0002] Lithium-ion secondary batteries are widely used due to their high energy density, long lifespan, and environmental friendliness. As electronic products continue to expand and enrich their functionality, the demand for higher energy density is becoming increasingly urgent. Increasing the upper cutoff voltage of batteries is currently one of the main methods for increasing energy density.

[0003] However, the structural stability of the cathode material deteriorates at high voltages, making it more susceptible to oxygen release, which oxidizes the electrolyte and produces gas, deteriorating battery cell performance. Generally, the cathode structure is stabilized by increasing the amount of trinitrile or dinitrile in the electrolyte. However, adding too many nitriles can worsen the battery's low-temperature performance and can also lead to a sharp drop in cycle capacity in the later stages of the battery cycle. Summary of the Invention

[0004] The purpose of the present invention is to provide an electrolyte and a lithium ion battery thereof, which does not deteriorate the low-temperature performance of the lithium battery electrolyte, takes into account the high and low temperature performance of the high voltage system, and improves the cycle performance of the high voltage system.

[0005] The present invention discloses an electrolyte, which comprises, by mass percentage:

[0006]

[0007]

[0008] The general structural formula of additive a is the following formula I, and the structural formula of ethyl fluoroacetate is the following formula A:

[0009]

[0010] Wherein, in formula I, R1 is a C1-C5 alkylene group; R2, R3, R4, and R5 are each independently a C1-C6 alkylene group, a fluoroalkylene group, an alkyleneoxy group, or a fluoroalkyleneoxy group; and the additive b is two or more of vinylene carbonate (VC), 1,3-propane sultone (PS), vinyl sulfate (DTD), succinonitrile (SN), adiponitrile (ADN), 1,3,6-hexanetrionitrile (HTCN), propene sultone (PST), methylene methanedisulfonate (MMDS), ethylene glycol bis(propionitrile) ether (EGBE), and a fluorinated ether (D2).

[0011] Optionally, the organic solvent is two or more of ethylene carbonate (EC), propylene carbonate (PC), diethyl carbonate (DEC), ethyl methyl carbonate (EMC), ethyl propionate (EP), and propyl propionate (PP).

[0012] Optionally, the electrolyte salt is one or more of lithium hexafluorophosphate (LiPF6), lithium difluorooxalatoborate (LiODFB), lithium difluorodioxalatophosphate (LiDFOP), lithium tetrafluoroborate (LiBF4), lithium bis(trifluoromethylsulfonyl)imide (LiTFSI), lithium bis(fluorosulfonyl)imide (LiFSI), and lithium difluorophosphate (LiPOF2).

[0013] Optionally, the additive a has the general structural formula of Formula II, III or IV:

[0014]

[0015] Optionally, the additive a has the general structural formula of Formula V, VI, VII or VIII:

[0016]

[0017] Optionally, the organic solvent is ethylene carbonate (EC), propylene carbonate (PC) and propyl propionate (PP), and the electrolyte salt is lithium hexafluorophosphate (LiPF6).

[0018] Optionally, the concentration of lithium hexafluorophosphate (LiPF6) is 1.2 mol / L.

[0019] Optionally, the mass ratio of ethylene carbonate (EC), propylene carbonate (PC), ethyl fluoroacetate and propyl propionate (PP) is 1:1:1:2.

[0020] The present invention also discloses a lithium ion battery comprising the above electrolyte.

[0021] Optionally, the lithium-ion battery includes a positive electrode and a negative electrode, the active material of the positive electrode is selected from at least one of lithium cobalt oxide, nickel-cobalt-manganese ternary material, lithium iron phosphate and lithium manganese oxide, and the active material of the negative electrode is graphite.

[0022] The electrolyte of the present invention has a high electronegativity and weak polarity of the fluorine atom of ethyl fluoroacetate, so that the fluorinated solvent has a higher oxidative decomposition voltage and is applicable to a high-voltage system. The cyano structure can be combined with a positive electrode active material to reduce the electrode surface activity, effectively suppress the dissolution of high-voltage metal ions, and effectively suppress the decomposition of the electrolyte. The additive a (Formula I) has a stronger complexing effect than commonly used succinonitrile (SN), adiponitrile (ADN), and 1,3,6-hexanetrinitrile (HTCN), can reduce the total amount of nitrile, does not deteriorate the low-temperature performance, is not prone to a cliff-like decline in cycle capacity in the later stage of the cycle, and in combination with the use of the fluorinated solvent, can achieve the high and low temperature performance of the high-voltage system. DETAILED DESCRIPTION

[0023] It should be understood that the terms used herein, the specific structures and functional details disclosed are only for describing specific embodiments and are representative. However, the present invention can be implemented in many alternative forms and should not be construed as being limited to only the embodiments described herein.

[0024] The present invention will be described in detail below with reference to optional embodiments.

[0025] As one embodiment of the present invention, an electrolyte solution is disclosed, comprising, by mass percentage, 20-70% of an organic solvent, 0.1-4.0% of an additive a, 5-20% of ethyl fluoroacetate, 5-15% of an additive b, and the balance being an electrolyte salt. The general structural formula of additive a is Formula I, and the structural formula of ethyl fluoroacetate is Formula A.

[0026]

[0027] Wherein, in formula I, R1 is a C1-C5 alkylene group; R2, R3, R4, and R5 are each independently a C1-C6 alkylene group, a fluoroalkylene group, an alkyleneoxy group, or a fluoroalkyleneoxy group; and the additive b is two or more of vinylene carbonate (VC), 1,3-propane sultone (PS), vinyl sulfate (DTD), succinonitrile (SN), adiponitrile (ADN), 1,3,6-hexanetrionitrile (HTCN), propene sultone (PST), methylene methanedisulfonate (MMDS), ethylene glycol bis(propionitrile) ether (EGBE), and a fluorinated ether (D2).

[0028] Specifically, the organic solvent can be 20%, 30%, 40%, 50%, 60%, or 70%; the additive a can be 0.1%, 0.5%, 1%, 1.5%, 2%, 2.5%, 3%, 3.5%, or 4.0%; the ethyl fluoroacetate can be 5%, 8%, 10%, 13%, 15%, 17%, or 20%; and the additive b can be 5%, 6%, 8%, 10%, 12%, or 15%.

[0029] Specifically, the organic solvent is two or more of ethylene carbonate (EC), propylene carbonate (PC), diethyl carbonate (DEC), ethyl methyl carbonate (EMC), ethyl propionate (EP), and propyl propionate (PP).

[0030] Specifically, the electrolyte salt is one or more of lithium hexafluorophosphate (LiPF6), lithium difluorooxalatoborate (LiODFB), lithium difluorodioxalatophosphate (LiDFOP), lithium tetrafluoroborate (LiBF4), lithium bis(trifluoromethylsulfonyl)imide (LiTFSI), lithium bis(fluorosulfonyl)imide (LiFSI), and lithium difluorophosphate (LiPOF2).

[0031] Specifically, the general structural formula of the additive a is the following formula II, III, IV, V, VI, VII or VIII:

[0032]

[0033] Specifically, the organic solvent is ethylene carbonate (EC), propylene carbonate (PC) and propyl propionate (PP), and the electrolyte salt is lithium hexafluorophosphate (LiPF6).

[0034] Specifically, the concentration of lithium hexafluorophosphate (LiPF6) is 1.2 mol / L.

[0035] Specifically, the mass ratio of ethylene carbonate (EC), propylene carbonate (PC), ethyl fluoroacetate and propyl propionate (PP) is 1:1:1:2.

[0036] The present invention also discloses a lithium-ion battery comprising the above-described electrolyte. Specifically, the lithium-ion battery comprises a positive electrode and a negative electrode. The positive electrode's active material is selected from at least one of lithium cobalt oxide, a nickel-cobalt-manganese ternary material, lithium iron phosphate, and lithium manganese oxide. Preferably, the positive electrode material is lithium cobalt oxide or a nickel-cobalt-manganese ternary material. The negative electrode's active material is graphite.

[0037] The electrolyte of the present invention has a high electronegativity and weak polarity of the fluorine atom of ethyl fluoroacetate, so that the fluorinated solvent has a higher oxidative decomposition voltage and is applicable to a high-voltage system. The cyano structure can be combined with a positive electrode active material to reduce the electrode surface activity, effectively suppress the dissolution of high-voltage metal ions, and effectively suppress the decomposition of the electrolyte. The additive a (Formula I) has a stronger complexing effect than commonly used succinonitrile (SN), adiponitrile (ADN), and 1,3,6-hexanetrinitrile (HTCN), can reduce the total amount of nitrile, does not deteriorate the low-temperature performance, is not prone to a cliff-like decline in cycle capacity in the later stage of the cycle, and in combination with the use of the fluorinated solvent, can achieve the high and low temperature performance of the high-voltage system.

[0038] The technical solution of the present invention is further described in detail below through specific embodiments.

[0039] Battery production

[0040] Preparation of positive electrode sheets: The positive electrode active material lithium cobalt oxide (LiCoO2), the conductive agent CNT, and the adhesive PVDF are fully stirred and mixed in NMP solvent at a weight ratio of 97:1.5:1.5. The slurry is coated on the positive electrode current collector Al foil. After drying, cold pressing, slitting, sheet making, welding, and tab gluing, the positive electrode sheet that meets the winding requirements is made.

[0041] Preparation of negative electrode sheet: Graphite negative electrode, conductive agent SP, thickener CMC, and binder SBR are mixed in a proper amount of deionized water at a mass ratio of 96.3:1:1.2:1.5 to form a uniform negative electrode slurry. This slurry is coated on the negative electrode current collector Cu foil, and then dried, cold pressed, slit, sheeted, welded, and glued to the tabs to produce a negative electrode sheet that meets the winding requirements.

[0042] Preparation of the electrolyte: The electrolyte is prepared by mixing EC / PC / DEC (or ethyl fluoroacetate) / PP in a mass ratio of 1:1:1:2 as the solvent. After uniform mixing, 1.2 mol / L LiPF6 is slowly added to obtain a mixed solution with a LiPF6 concentration of 1.2 mol / L. After the lithium salt is completely dissolved, the corresponding additives are added to obtain the electrolyte.

[0043] Lithium-ion battery production: The positive electrode sheet, separator, and negative electrode sheet are wound together to form a bare cell. The bare cell is placed in a pre-punched aluminum-plastic film and sealed on the top and sides. After high-temperature baking, liquid injection, static standing, formation, capacity separation, and testing, the battery is complete.

[0044] According to the above-mentioned battery manufacturing method, lithium ion batteries of Examples 1 to 11 and Comparative Examples 1 to 3 were prepared. The types and proportions of raw materials in the electrolytes of Examples 1 to 11 and Comparative Examples 1 to 3 were changed as shown in Table 1 below:

[0045] Table 1

[0046]

[0047]

[0048] Wherein, formula A in Table 1 is the fluoroethyl acetate of formula A used in the present invention, formula B is the fluoroethyl acetate of formula B, and the fluoroethyl acetate of formula B is a comparison of the fluoroethyl acetate of formula A of the present invention.

[0049]

[0050] Lithium-ion battery performance testing

[0051] 25℃ 0.7C / 1.0C cycle test: Charge at 0.7C constant current and constant voltage at 25℃ to 4.53V, cut off current 0.05C, stand for 10 minutes, discharge at 1.0C to 3.0V, record discharge capacity C0 as the initial capacity, repeat 500 cycles, and obtain the capacity C500 after 500 cycles. The capacity retention rate = C500 / C0.

[0052] 45℃ 0.7C / 1.0C cycle test: Charge at 45℃ at 0.7C constant current and constant voltage to 4.53V, cut off current 0.05C, stand for 10 minutes, discharge at 1.0C to 3.0V, record discharge capacity C0 as the initial capacity, repeat 500 cycles, and obtain the capacity C300 after 300 cycles. The capacity retention rate = C300 / C0.

[0053] Storage at 85℃ for 6h: Charge at 0.7C constant current and constant voltage at 25℃ to 4.53V, with a cut-off current of 0.05C. Leave for 10min and measure the thickness of the cell (H0). Then store at 85±2℃ for 6h and measure the thickness (H6) by heat. Thickness expansion = (H6-H0) / H0.

[0054] Storage at 60°C for 14 days: At 25°C, charge the battery to 4.53V at 0.5C constant current and constant voltage, with a cutoff current of 0.05C. Let it rest for 10 minutes. Measure the voltage, internal resistance, and thickness of the battery cell. Discharge the battery to 3.0V at 0.2C constant current and record the capacity C0. Then charge the battery to 4.53V at 0.5C constant current and constant voltage, with a cutoff current of 0.05C. Let it rest for 10 minutes. Place the battery in a 60°C oven and store it for 14 days. After storage, measure the battery cell voltage, internal resistance, and thermal thickness. Discharge the battery to 3.0V at 0.2C at 25°C, and record the remaining capacity C1. Capacity retention rate = C1 / C0.

[0055] Discharge at -10℃: fully charged at 0.7C at 25℃, cutoff current 0.05C, let stand for 10 min, discharge at 0.2C to 3.0V at 25℃, record the discharge capacity C0, fully charged at 0.7C at 25℃, cutoff current 0.05C, let stand at -10℃ for 2h, then discharge at 0.2C to 3.4V, record the discharge capacity C1, 3.4V capacity retention rate = C1 / C0.

[0056] The test results of Examples 1 to 11 and Comparative Examples 1 to 3 are shown in Table 2 below:

[0057] Table 2

[0058]

[0059] The test results of Examples 2-9 and Comparative Example 2 show that the addition of Formula III optimizes both 45°C cycling and 25°C cycling, indicating a high binding energy between the tetranitrile and the cobalt ion. Furthermore, the appropriate carbon chain length and symmetrical structure provide enhanced protection, resulting in improved high-temperature storage compared to Comparative Example 2, while deteriorating low-temperature discharge. Example 9 exhibits poor low-temperature discharge due to its ether bond-containing film formation on the positive electrode surface and its relatively long chain, which degrades Li+ transport. For Comparative Examples 1-3, the cycling and high-temperature storage performances of A>DEC>B are consistent. Fluorinated solvents improve solvent stability, while the effects of positional substitution vary. Because the dielectric constant of the fluoroacetyl group is higher than that of the fluoroethoxy group, and the viscosity of A is higher than that of B, B exhibits superior low-temperature discharge. The test results of Examples 1, 10, and 11 show that Example 11 exhibits the best overall cycling, high-temperature storage, and low-temperature discharge performance. The fluorosolvent A and Formula III combined with SN exhibit a synergistic effect, improving the 45°C cycling performance of the high-voltage system while also maintaining low-temperature performance.

[0060] It should be noted that the limitations on the various steps involved in this solution do not limit the order of the steps without affecting the implementation of the specific solution. The steps written in front can be executed first, later, or even simultaneously. As long as this solution can be implemented, it should be regarded as falling within the scope of protection of the present invention.

[0061] The above is a further detailed description of the present invention in conjunction with specific optional embodiments, and the specific implementation of the present invention should not be considered to be limited to these descriptions. For those skilled in the art of the present invention, without departing from the concept of the present invention, several simple deductions or substitutions can be made, which should be considered to fall within the scope of protection of the present invention.

Claims

1. An electrolyte, characterized in that Calculated by mass percentage, including: The general structural formula of additive a is the following formula I, and the structural formula of ethyl fluoroacetate is the following formula A: Wherein, the additive b is succinonitrile (SN).

2. The electrolyte according to claim 1, wherein The organic solvent is two or more of ethylene carbonate (EC), propylene carbonate (PC), diethyl carbonate (DEC), ethyl methyl carbonate (EMC), ethyl propionate (EP), and propyl propionate (PP).

3. The electrolyte according to claim 1, wherein The electrolyte salt is one or more of lithium hexafluorophosphate (LiPF6), lithium difluorooxalatoborate (LiODFB), lithium difluorodioxalatophosphate (LiDFOP), lithium tetrafluoroborate (LiBF4), lithium bis(trifluoromethylsulfonyl)imide (LiTFSI), lithium bis(fluorosulfonyl)imide (LiFSI), and lithium difluorophosphate (LiPOF2).

4. The electrolyte according to claim 1, wherein The organic solvent is ethylene carbonate (EC), propylene carbonate (PC) and propyl propionate (PP), and the electrolyte salt is lithium hexafluorophosphate (LiPF6).

5. The electrolyte according to claim 4, wherein The concentration of the lithium hexafluorophosphate (LiPF6) is 1.2 mol / L.

6. The electrolyte according to claim 5, wherein The mass ratio of the ethylene carbonate (EC), propylene carbonate (PC), ethyl fluoroacetate and propyl propionate (PP) is 1:1:1:

2.

7. A lithium-ion battery, characterized in that: Comprising the electrolyte according to any one of claims 1 to 6.

8. The lithium-ion battery according to claim 7, wherein The lithium-ion battery comprises a positive electrode and a negative electrode, the active material of the positive electrode is selected from at least one of lithium cobalt oxide, nickel-cobalt-manganese ternary material, lithium iron phosphate and lithium manganese oxide, and the active material of the negative electrode is graphite.

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