Secondary battery and electronic equipment

Abstract

The invention relates to a secondary battery and electronic equipment, and belongs to the technical field of batteries. The secondary battery comprises a positive electrode, a negative electrode and an electrolyte, wherein the positive electrode comprises a current collector and a positive electrode material layer which is arranged on at least one side surface of the current collector and contains Co; the electrolyte comprises a first solvent, a first additive and a second additive; based on the total mass of the electrolyte, the mass percentage content of the first solvent is a%, the mass percentage content of the first additive is b%, and the mass percentage content of the second additive is d%; based on the total mass of the positive electrode material layer, the mass percentage content of Co is e%; 0.1 < = a / (10 * b) < = 5, and 1 < 10 (b + d) / e < 5. The secondary battery has good high-temperature cycle performance, standing cycle performance and high-temperature storage performance.

Description

Technical Field

[0001] The present invention belongs to the technical field of batteries, and in particular relates to a secondary battery and an electronic device. Background Art

[0002] The electrolyte in lithium-ion batteries not only provides a migration channel for lithium ions between the positive and negative electrodes to ensure a stable ion concentration during battery operation, but also forms a solid electrolyte interface (CEI membrane or SEI membrane) on the positive and negative electrodes to ensure that the battery can continuously and stably supply power. However, in high-voltage systems, problems such as the dissolution of transition metal ions, the continuous oxidation of conventional carbonate solvents, and the instability of the electrode / electrolyte interface can easily lead to a rapid decline in battery cycling performance. In other words, existing conventional electrolytes are difficult to apply to high-voltage lithium cobalt oxide-graphite (LCO-Gr) lithium-ion batteries. Summary of the Invention

[0003] The object of the present invention is to overcome the deficiencies of the prior art and provide a secondary battery and an electronic device.

[0004] To achieve the above object, the technical solution adopted by the present invention is:

[0005] In a first aspect, the present invention provides a secondary battery comprising a positive electrode, a negative electrode and an electrolyte, wherein the positive electrode comprises a current collector and a positive electrode material layer comprising Co provided on at least one side of the current collector;

[0006] The electrolyte includes a first solvent, a first additive and a second additive;

[0007] The chemical structural formula of the first solvent is shown in Formula A:

[0008]

[0009] R1 and R2 in formula A are each independently selected from any one of an unsubstituted alkyl group having 1 to 5 carbon atoms, a H atom, and a F atom;

[0010] The chemical structural formula of the first additive is shown in Formula B:

[0011]

[0012] R1 and R2 in formula B are each independently selected from any one of an unsubstituted alkyl group having 1 to 5 carbon atoms, an amino group, a H atom, a F atom, and a cyano group;

[0013] Based on the total mass of the electrolyte, the mass percentage of the first solvent is a%, the mass percentage of the first additive is b%, and the mass percentage of the second additive is d%;

[0014] Based on the total mass of the positive electrode material layer, the mass percentage of Co is e%;

[0015] 0.1≤a / (10*b)≤5, 1<10(b+d) / e<5.

[0016] As an embodiment of the present invention, at least one of the following is satisfied:

[0017] (1)5≤a≤30;

[0018] (2) 0.5≤b≤10;

[0019] (3)4≤d≤20;

[0020] (4)40≤e≤60.

[0021] As an embodiment of the present invention, 5<b+d<22.

[0022] As an embodiment of the present invention, the first solvent is selected from at least one of Formula A1, Formula A2, and Formula A3:

[0023]

[0024]

[0025] As an embodiment of the present invention, the first additive is selected from at least one of Formula B1, Formula B2, Formula B3, and Formula B4;

[0026]

[0027] As an embodiment of the present invention, the second additive is selected from at least one of fluoroethylene carbonate, succinonitrile, adiponitrile, and 1,3,6-hexanetrinitrile.

[0028] As an embodiment of the present invention, the electrolyte further includes a second solvent, and the second solvent is selected from at least one of ethylene carbonate, propylene carbonate, dimethyl carbonate, 2,2-difluoroethyl acetate, ethyl propionate, and propyl propionate.

[0029] As an embodiment of the present invention, based on the total mass of the electrolyte, the mass percentage of the second solvent is c%, and 30≤c≤70.

[0030] As an embodiment of the present invention, 0.01≤a / c≤1.5.

[0031] In a second aspect, the present invention provides an electronic device comprising the aforementioned secondary battery.

[0032] Compared with the prior art, the present invention has the following beneficial effects:

[0033] The electrolyte in the secondary battery of the present invention uses a fluorinated carbonate compound of a specific structure as the first solvent, and is combined with a nitrile pyrimidine compound of a specific structure as the first additive. The fluorinated carbonate solvent with low HUMO and LUMO energy levels is used to improve the chemical stability of the electrolyte, and the solvation structure is regulated, the desolvation energy barrier of lithium ions is reduced, the reaction kinetics is accelerated, and the side reactions between the electrolyte and the positive electrode are reduced; at the same time, the -CN group in the structure of the nitrile pyrimidine compound is used to effectively inhibit the dissolution of Co in the positive electrode, and the F atomic group in its structure is used to weaken the interaction between PC and Li. + The coordination of the solvent structure is regulated to form a stable SEI film on the negative electrode surface; at the same time, by regulating the first solvent and the second additive in the electrolyte and the Co in the positive electrode material layer to meet a specific relationship, the performance of the LCO-Gr system lithium-ion battery at high voltage is significantly improved. DETAILED DESCRIPTION

[0034] In order to better illustrate the purpose, technical solutions and advantages of the present invention, the present invention will be further described below in conjunction with specific embodiments and comparative examples. Its purpose is to understand the content of the present invention in detail, rather than to limit the present invention. All other embodiments obtained by those of ordinary skill in the art without making creative work premise all fall within the protection scope of the present invention. The experimental reagents and instruments involved in the implementation of the present invention are all conventional common reagents and instruments unless otherwise specified.

[0035] According to a first aspect of the present invention, a secondary battery is provided, comprising a positive electrode, a negative electrode, and an electrolyte, wherein the positive electrode comprises a current collector and a positive electrode material layer comprising Co provided on at least one side of the current collector;

[0036] The electrolyte includes a first solvent, a first additive, and a second additive (for example, the second additive may be selected from at least one of fluoroethylene carbonate and nitrile compounds);

[0037] The chemical structural formula of the first solvent is shown in Formula A:

[0038]

[0039] R1 and R2 in Formula A are each independently selected from an unsubstituted alkyl group having 1 to 5 carbon atoms (for example, the number of carbon atoms may be 1, 2, 3, 4 or 5, and the alkyl group may be a linear or branched alkyl group), an H atom, or an F atom;

[0040] The chemical structural formula of the first additive is shown in Formula B:

[0041]

[0042] R1 and R2 in Formula B are each independently selected from an unsubstituted alkyl group having 1 to 5 carbon atoms (for example, the number of carbon atoms may be 1, 2, 3, 4 or 5, and the alkyl group may be a linear or branched alkyl group), an amino group, a H atom, a F atom, or a cyano group;

[0043] Based on the total mass of the electrolyte, the mass percentage of the first solvent is a%, the mass percentage of the first additive is b%, and the mass percentage of the second additive is d%;

[0044] Based on the total mass of the positive electrode material layer, the mass percentage of Co is e%;

[0045] 0.1≤a / (10*b)≤5 (for example, a / (10*b) can be any one of 0.1, 0.5, 1, 1.3, 1.5, 1.7, 2, 2.2, 2.5, 2.8, 3, 3.3, 3.5, 3.7, 4, 4.3, 4.5, 4.8, 5 or any two of the range values), 1<10(b+d) / e<5 (for example, 10(b+d) / e can be any one of 1.2, 1.4, 1.6, 1.8, 2, 2.2, 2.4, 2.6, 2.8, 3, 3.2, 3.4, 3.6, 3.8, 4, 4.2, 4.4, 4.6, 4.8 or any two of the range values).

[0046] Although adding film-forming additives (such as fluoroethylene carbonate) and nitrile compound additives (such as succinonitrile) to the electrolyte can improve the interfacial stability of the electrode / electrolyte and thus improve the cycle performance of the battery, fluoroethylene carbonate is prone to undergo F removal reaction under the action of Lewis acid in the electrolyte under high temperature conditions, producing HF and various other acids, which in turn causes the battery capacity to decay.

[0047] The present invention uses a fluorinated carbonate compound of a specific structure as the first solvent, combined with a nitrile pyrimidine compound of a specific structure as the first additive, and utilizes a fluorinated carbonate solvent with low HUMO and LUMO energy levels to improve the chemical stability of the electrolyte, and regulates the solvation structure, reduces the desolvation energy barrier of lithium ions, accelerates the reaction kinetics, and reduces the side reactions between the electrolyte and the positive electrode; at the same time, the -CN group in the structure of the nitrile pyrimidine compound is used to effectively inhibit the dissolution of Co in the positive electrode, and the F atomic group in its structure is used to weaken the interaction between PC and Li + The coordination of Gr and LCO regulates the solvation structure, forming a stable SEI film on the negative electrode surface, thereby significantly improving the high-temperature performance of LCO-Gr system lithium-ion batteries under high voltage.

[0048] The fluorinated carbonate solvents selected in the present invention have higher chemical stability than carbonates commonly used in electrolytes (such as EC and PC), and are less prone to decomposition or side effects. Fluorinated carbonate compounds can form a more robust SEI film to enhance the stability of the interface between the negative electrode and the electrolyte. The resulting SEI film structure is more compact without increasing impedance, and it can also prevent the electrolyte from being oxidized and decomposed by cobalt ions dissolved from the cathode during circulation. In addition, due to the electron-withdrawing effect of fluorine atoms, replacing hydrogen atoms in carbonates with fluorine atoms can lower the HOMO and LUMO energy levels, thereby enhancing the stability of the positive electrode.

[0049] The nitrile pyrimidine compound selected by the present invention can not only inhibit the dissolution of Co ions, but also preferentially generate an SEI layer on the surface of the negative electrode to protect the negative electrode surface. The nitrile pyrimidine compound additive has good solubility and can fully dissolve lithium salts to ensure the effective transmission of lithium ions in the electrolyte. Moreover, the structure of the nitrile pyrimidine compound is mainly composed of a six-membered heterocyclic compound containing two nitrogen atoms and a cyano group. The nitrogen-containing heterocycle can enable the lithium-ion battery to compete with FEC for film formation during the formation stage, thereby reducing the FEC formation consumption. The SEI film formed at the positive and negative electrode interface has excellent mechanical properties, which significantly improves the cycle life of the battery. In addition, the generated film can also alleviate the rupture of the positive electrode particles due to volume expansion during the charging process, thereby improving the stability and service life of the lithium battery.

[0050] The nitrile pyrimidine compound selected in the present invention has strong hydrophilicity and can effectively absorb trace water and hydrofluoric acid in the electrolyte. The nitrile pyrimidine compound molecules contain nitrogen atoms, which can form hydrogen bonds with trace water molecules and hydrofluoric acid in the electrolyte, thereby reducing electrolyte loss, reducing gas generation, and improving the stability of the SEI film. Furthermore, the nitrile pyrimidine compound can react with hydrofluoric acid, consuming the hydrofluoric acid continuously generated during the lithium-ion battery cycle, thereby inhibiting hydrofluoric acid corrosion of the lithium-ion battery.

[0051] The study found that the value of a / (10*b) plays a key role in the side reactions of the electrolyte during the charge and discharge process of secondary batteries. If the value of a / (10*b) is too large, a large amount of hydrofluoric acid will be generated in the electrolyte, which will cause serious gassing of the secondary battery. If the value of a / (10*b) is too small, the viscosity of the electrolyte will be too high, resulting in poor kinetic performance of the secondary battery and reduced ionic conductivity of the electrolyte. In addition, it was found that the value of 10(b+d) / e has a significant impact on the performance of secondary batteries. If the value of 10(b+d) / e is too large, it will lead to poor intrinsic viscosity and ionic conductivity of the electrolyte. If the value of 10(b+d) / e is too small, it will deteriorate the electrolyte's complexing effect on Co dissolved from the positive electrode material, resulting in poor intrinsic stability of the electrolyte and weakened antioxidant capacity, which in turn leads to poor performance of the secondary battery.

[0052] As an embodiment of the present invention, the electrolyte further includes a lithium salt. The present invention does not particularly limit the lithium salt, as long as the purpose of the present invention can be achieved. For example, the lithium salt can be at least one of lithium hexafluorophosphate (LiPF6), lithium bis(fluorosulfonyl)imide (LiFSI), lithium bis(trifluoromethanesulfonyl)imide (LiTFSI), lithium difluorophosphate (LiPO2F2), lithium difluorobis(oxalate)phosphate (LiDFOP), lithium dioxalate borate (LiBOB), and lithium difluorooxalate borate (LiODFB). Based on the total mass of the electrolyte, the mass percentage of the lithium salt is 6% to 25%, for example, it can be 6%, 7%, 8%, 9%, 10%, 11%, 12%, 12%, 14%, 15%, 16%, 17%, 18%, 19%, 20%, 21%, 22%, 23%, 24%, and 25%.

[0053] As an embodiment of the present invention, at least one of the following is satisfied:

[0054] (1) 5≤a≤30, for example, a can be any one of 5, 8, 10, 13, 15, 18, 20, 23, 25, 28, 30 or any two of the range; by regulating the mass percentage a of the first solvent in the electrolyte within the above range, the solvation structure of the lithium ion electrolyte can be better regulated, the intrinsic oxidation resistance of the electrolyte can be enhanced, the transition metal dissolution of the positive electrode material under high pressure can be reduced, and the interfacial side reactions can be suppressed.

[0055] (2) 0.5≤b≤10, for example, b can be any one of 0.5, 1, 1.5, 2, 2.5, 3, 3.5, 4, 4.5, 5, 5.5, 6, 6.5, 7, 7.5, 8, 8.5, 9, 9.5, 10, or any two of the range values; by regulating the mass percentage b of the first additive in the electrolyte within the above range, not only can the complexing effect of the electrolyte on Co ions be better enhanced, but the viscosity of the electrolyte can also be effectively improved.

[0056] (3) 4≤d≤20, for example, d can be any one of 4, 5, 6, 7, 8, 9, 10, 11, 12, 13, 14, 15, 16, 17, 18, 19, 20, or any two of them; by regulating the mass percentage d of the second additive in the electrolyte within the above range, the stability of the generated SEI film can be better enhanced and good thermal shock resistance can be maintained.

[0057] (4) 40≤e≤60, for example, e can be any one of 40, 41, 42, 43, 44, 45, 46, 47, 48, 49, 50, 51, 52, 53, 54, 55, 56, 57, 58, 59, and 60, or any two of the range values.

[0058] As an embodiment of the present invention, 5 < b + d < 22. For example, b + d can be any one of 5.5, 6, 7, 8, 9, 10, 11, 12, 13, 14, 15, 16, 17, 18, 19, 20, 21, and 21.5, or any two of them. By regulating the total mass percentage b + d of the first additive and the second additive in the electrolyte within the above range, not only can the electrolyte's inhibitory effect on Co ions dissolved from the positive electrode be better ensured, but the electrolyte can also better infiltrate the electrode.

[0059] As an embodiment of the present invention, the first solvent is selected from at least one of Formula A1, Formula A2, and Formula A3:

[0060]

[0061]

[0062] As an embodiment of the present invention, the first additive is selected from at least one of Formula B1, Formula B2, Formula B3, and Formula B4;

[0063]

[0064] As an embodiment of the present invention, the second additive is selected from at least one of fluoroethylene carbonate (FEC), succinonitrile (SN), adiponitrile (ADN), and 1,3,6-hexanetrionitrile (HTCN).

[0065] As an embodiment of the present invention, the electrolyte further includes a second solvent, which is selected from at least one of ethylene carbonate (EC), propylene carbonate (PC), dimethyl carbonate (DEC), 2,2-difluoroethyl acetate (DFEA), ethyl propionate (EP), and propyl propionate (PP).

[0066] As an embodiment of the present invention, based on the total mass of the electrolyte, the mass percentage of the second solvent is c%, and 30≤c≤70. For example, c can be any one of 30, 32, 35, 40, 42, 45, 48, 50, 52, 55, 56, 58, 60, 61, 63, 65, 68, and 70, or any two of them. The viscosity of the second solvent is lower than that of the first solvent. By regulating the mass percentage c of the second solvent in the electrolyte within the above range, not only can the electrolyte maintain a high ionic conductivity to ensure good kinetic performance, but it can also promote the full desolvation effect of the first solvent, effectively improving the intrinsic stability of the electrolyte.

[0067] As an embodiment of the present invention, 0.01≤a / c≤1.5. For example, a / c can be any one of the following ranges: 0.01, 0.05, 0.1, 0.12, 0.15, 0.16, 0.2, 0.25, 0.28, 0.3, 0.33, 0.35, 0.4, 0.42, 0.45, 0.5, 0.55, 0.6, 0.65, 0.7, 0.75, 0.8, 0.85, 0.9, 0.95, 1, and 1.5, or any two of these ranges. By regulating the ratio a / c of ​​the mass percentage of the first solvent to the mass percentage of the second solvent in the electrolyte within the above range, not only can the electrolyte have a suitable viscosity to better wet the electrode, thereby improving the room temperature performance of the battery, but the intrinsic stability of the electrolyte can also be better improved, thereby improving the high temperature performance of the battery.

[0068] In a second aspect, the present invention provides an electronic device comprising the aforementioned secondary battery.

[0069] In order to clearly understand the technical solution of the present invention, the present invention is further described in detail below in conjunction with specific embodiments. These embodiments should not be construed as limiting the scope of protection claimed by the present invention.

[0070] <Raw Materials and Reagents>

[0071] 1. First solvent:

[0072]

[0073]

[0074] 2. The first additive:

[0075]

[0076] <Test methods and equipment>

[0077] 1. Normal temperature cycle performance test

[0078] The divided battery was charged to 4.55V at 1.2C constant current and constant voltage at 25°C, with a cut-off current of 0.05C. It was then discharged to 3.0V at 0.5C constant current for 400 cycles. The capacity retention rate and thickness expansion rate after 400 cycles were calculated using the following formula. The fully charged state refers to 100% SOC:

[0079] 400-cycle capacity retention rate = 400th cycle discharge capacity / first cycle discharge capacity × 100%.

[0080] Thickness expansion rate after 400 cycles = (thickness at the fully charged state after the 400th cycle - thickness at the fully charged state after the first cycle) / thickness at the fully charged state after the first cycle × 100%.

[0081] 2. High temperature cycle performance test

[0082] In a 45°C environment, the divided battery is charged to 4.55V at a constant current and constant voltage of 0.7C, with a cut-off current of 0.05C, and then discharged to 3.0V at a constant current of 0.5C. This cycle is repeated for 300 cycles. After the 300 cycles, the capacity retention rate and thickness expansion rate are calculated using the following formula. The fully charged state refers to 100% SOC:

[0083] 300-cycle capacity retention rate = 300th cycle discharge capacity / first cycle discharge capacity × 100%;

[0084] Thickness expansion rate after 300 cycles = (thickness at the 300th cycle in a fully charged state - thickness at the first cycle in a fully charged state) / thickness at the first cycle in a fully charged state × 100%.

[0085] 3. Static cycle performance test

[0086] In a 45°C environment, the divided battery is charged to 4.55V at a constant current and constant voltage of 0.7C, with a cut-off current of 0.05C. It is left to stand at full charge for 24 hours, and then discharged to 3.0V at a constant current of 0.5C. This cycle is repeated for 120 cycles. After the cycle, the capacity retention rate and thickness expansion rate at the 120th cycle are calculated. The calculation formula is as follows. The fully charged state refers to 100% SOC:

[0087] 120-cycle capacity retention rate = 120th cycle discharge capacity / first cycle discharge capacity × 100%;

[0088] Thickness expansion rate after 120 cycles = (thickness at full charge in the 120th cycle - thickness at full charge in the first cycle) / thickness at full charge in the first cycle × 100%.

[0089] 4. 85℃, 24h high temperature storage test

[0090] Place the battery at room temperature (25°C) and charge and discharge it once at 0.5C (3.0V-4.55V). Record the battery's discharge capacity before storage (C0). Then charge the battery at constant current and constant voltage to a full charge state of 4.55V (100% SOC). Store the battery in an 85°C constant temperature box for 24 hours. After the battery cools at room temperature for 24 hours, discharge it again at 0.5C constant current to 3.0V. Record the battery's discharge capacity after storage (C1). Calculate the capacity retention rate of the battery after 24 hours of storage at 85°C. The calculation formula is as follows:

[0091] After storage at 85°C for 24 hours, the capacity retention rate = C1 / C0×100%.

[0092] 5. Thermal shock performance test

[0093] Under 25℃ ambient conditions, discharge the battery to 3.0V at a given current of 0.2C; set aside for 5 minutes; charge the battery to 4.55V at a charging current of 0.2C. When the battery voltage reaches 4.55V, change to 4.55V constant voltage charging, until the charging current ≤ the cut-off current of 0.05C; after setting aside for 1 hour, place the battery in an oven, increase the oven temperature to 135±2℃ at a rate of 5±2℃ / min, and keep it for 60 minutes before stopping. Determine whether the battery catches fire or explodes. The test result is recorded as "A / B", where A is the number of battery cells that do not catch fire or explode, and B is the total number of battery cells tested.

[0094] 6. Mass percentage of Co in the positive electrode material layer

[0095] 1) Sample pretreatment:

[0096] Scrape the positive electrode material layer powder (about 0.1 g) into a polytetrafluoroethylene digestion tank; then, add 8 mL of concentrated HNO3 and 2 mL of 30% H2O2, and set the digestion program (microwave digester): 120°C for 10 minutes → 160°C for 15 minutes → 180°C for 20 minutes. After the digestion is completed, cool to room temperature; finally, transfer the digestion solution to a 50 mL volumetric flask, rinse the digestion tank 3 times with deionized water, and combine the washing solution; dilute to the scale line, shake well and set aside.

[0097] 2) ICP-OES analysis:

[0098] Instrument parameter settings:

[0099] Plasma conditions: RF power: 1150 W, nebulizing gas flow rate: 0.7 L / min, auxiliary gas flow rate: 0.2 L / min, pump speed: 50 rpm;

[0100] Analysis spectrum: 228.616nm or 238.892nm (characteristic spectrum of Co);

[0101] Standard curve drawing:

[0102] Prepare a series of concentrations (e.g., 0 mg / L, 5 mg / L, 10 mg / L, 20 mg / L, 50 mg / L) of a 1000 mg / L cobalt standard solution and add the same acid matrix as the sample (e.g., 5% HNO3 by mass);

[0103] Sample determination: Dilute the sample solution to within the range of the standard curve, measure the blank solution and the sample solution in turn, and record the cobalt emission intensity; calculate the cobalt concentration (C, mg / L) in the sample solution according to the standard curve equation.

[0104] 3) The mass percentage of Co in the positive electrode material layer was calculated according to the following formula:

[0105]

[0106] Where C is the cobalt concentration in the sample solution (mg / L), V is the constant volume (L), D is the dilution factor (if undiluted, D = 1), and m is the sample mass (g).

[0107] Example 1

[0108] <Preparation of Electrolyte>

[0109] In a glove box filled with argon, the first solvent and the second solvent were mixed uniformly in a mass ratio of 25:75, and then lithium salt was slowly added to the mixture. Then, the first additive and the second additive were added and mixed uniformly to obtain an electrolyte.

[0110] First solvent: Formula A1;

[0111] The second solvent is composed of dimethyl carbonate (DEC), ethyl propionate (EP) and propyl propionate (PP) in a mass ratio of 1:1:1;

[0112] Lithium salt: lithium hexafluorophosphate (LiPF6);

[0113] First additive: Formula B3;

[0114] The second additive is composed of fluoroethylene carbonate (FEC), adiponitrile (AND) and 1,3,6-hexanetrionitrile (HTCN) in a mass ratio of 10:1:1;

[0115] Based on the total mass of the electrolyte, the mass percentage of the lithium salt is 15%, the mass percentage of the first additive is 1%, and the mass percentage of the second additive is 12%.

[0116] <Preparation of positive electrode sheet>

[0117] The positive electrode active material LiCoO2, the conductive agent Super P and the binder polyvinylidene fluoride (PVDF) are fully stirred and mixed in N-methylpyrrolidone solvent (NMP) in a mass ratio of 97:2:1 to obtain a positive electrode slurry; the positive electrode slurry is coated on the current collector aluminum foil, dried at 85°C and then cold pressed, and then trimmed and striped, and dried at 85°C under vacuum conditions for 6 hours, and the tabs are welded to obtain the positive electrode sheet.

[0118] <Preparation of negative electrode sheet>

[0119] The negative electrode active material artificial graphite, the conductive agent acetylene black, the binder styrene-butadiene rubber (SBR) and the thickener sodium carboxymethyl cellulose (CMC) were thoroughly stirred and mixed in a deionized water solvent system at a mass ratio of 96:1:1.5:1.5, and then coated on the negative electrode current collector Cu foil. After drying, cold pressing and slitting, the negative electrode sheet was obtained.

[0120] <Preparation of Separator>

[0121] A polyethylene (PE) porous polymer film (5.5 μm thick) was used as the separator.

[0122] <Preparation of soft-pack lithium batteries>

[0123] The positive electrode sheet, separator, and negative electrode sheet obtained above are stacked in order, with the separator positioned between the positive and negative electrode sheets, and then wound to obtain a bare cell; the cell has a designed capacity of 5.0Ah and a voltage range of 3.0-4.55V. The bare cell is encapsulated in an aluminum-plastic film outer packaging and then baked in an 85°C vacuum oven for 48 hours. The obtained electrolyte is injected into the dried battery at an injection coefficient of 1.5g / Ah. The battery is then encapsulated, allowed to stand, formed, shaped, and volume-divided, and then re-sealed with a liquid retention coefficient of 1.3g / Ah to obtain a soft-pack lithium battery.

[0124] Example 2 to Example 3

[0125] The process was the same as that of Example 1 except that the first solvent in <Preparation of Electrolyte> was different from that in Example 1.

[0126] Example 4 to Example 6

[0127] The process is the same as that of Example 1 except that the first additive in <Preparation of Electrolyte> is different from that in Example 1.

[0128] Example 7 to Example 10

[0129] Except that the mass percentage a of the first solvent and the mass percentage c of the second solvent in the electrolyte in <Preparation of Electrolyte> are different from those in Example 1, the rest are the same as Example 1.

[0130] Example 11 to Example 12

[0131] Except that the mass percentage b of the first additive in the electrolyte and the mass percentage of the lithium salt in <Preparation of Electrolyte> are different from those in Example 1, the rest are the same as those in Example 1.

[0132] Example 13 to Example 14

[0133] Except that the mass percentage d of the second additive in the electrolyte and the mass percentage of the lithium salt in <Preparation of Electrolyte> are different from those in Example 1, the rest are the same as those in Example 1.

[0134] Comparative Example 1

[0135] Except that the solvent in the electrolyte is the second solvent (i.e., does not contain the first solvent), the additive is the second additive (i.e., does not contain the first additive), the mass percentage of the lithium salt in <Preparation of the Electrolyte>, and in <Preparation of the Positive Electrode> the mass percentage e of Co in the positive electrode material layer is different from that in Example 1 by regulating the addition amount of the positive electrode active material LiCoO2, the rest is the same as Example 1.

[0136] Comparative Example 2

[0137] Except that the additive in the electrolyte in <Preparation of Electrolyte> is the second additive (i.e., does not contain the first additive), the mass percentage of the lithium salt, and in <Preparation of Positive Electrode> the mass percentage e of Co in the positive electrode material layer is different from that in Example 1 by regulating the addition amount of the positive electrode active material LiCoO2, the rest is the same as Example 1.

[0138] Comparative Example 3

[0139] Except that the solvent in the electrolyte in <Preparation of Electrolyte> is the second solvent (i.e., does not contain the first solvent), and in <Preparation of Positive Electrode Sheet> the mass percentage e of Co in the positive electrode material layer is different from that in Example 1 by regulating the addition amount of the positive electrode active material LiCoO2, the rest is the same as Example 1.

[0140] Comparative Example 4 to Comparative Example 5

[0141] Except that the mass percentage a of the first solvent, the mass percentage c of the second solvent, the mass percentage b of the first additive, the mass percentage d of the second additive, and the mass percentage of the lithium salt in the electrolyte in <Preparation of Electrolyte> are different from those in Example 1, the rest are the same as Example 1.

[0142] Table 1

[0143] serial number First solvent First additive Example 1 Formula A1 Formula B3 Example 2 Formula A2 Formula B3 Example 3 Formula A3 Formula B3 Example 4 Formula A1 Formula B2 Example 5 Formula A1 Formula B1 Example 6 Formula A1 Formula B4 Example 7 Formula A1 Formula B3 Example 8 Formula A1 Formula B3 Example 9 Formula A1 Formula B3 Example 10 Formula A1 Formula B3 Example 11 Formula A1 Formula B3 Example 12 Formula A1 Formula B3 Example 13 Formula A1 Formula B3 Example 14 Formula A1 Formula B3 Comparative Example 1 / / Comparative Example 2 Formula A1 / Comparative Example 3 / Formula B3 Comparative Example 4 Formula A1 Formula B3 Comparative Example 5 Formula A1 Formula B3

[0144] Table 2

[0145]

[0146]

[0147] The “ / ” in Tables 1 and 2 indicates no relevant parameters. In Table 2, a represents the mass percentage of the first solvent in the electrolyte, c represents the mass percentage of the second solvent in the electrolyte, b represents the mass percentage of the first additive in the electrolyte, d represents the mass percentage of the second additive in the electrolyte, and e represents the mass percentage of Co in the positive electrode material layer.

[0148] Table 3

[0149]

[0150]

[0151] According to the data of the above embodiments and comparative examples, the thickness growth and cycle decay of the high-voltage LCO-Gr system lithium-ion battery using conventional electrolyte during the cycle are relatively fast, and the high-temperature cycle and static cycle performance are relatively poor. However, the high-temperature performance of the lithium battery using the electrolyte of the present invention is significantly improved. This is due to the strategy of introducing fluorocarbonate compound A as the first solvent and introducing nitrile pyrimidine compound B as the first additive, which can regulate the Li + Solvation sheath structure, accelerating Li + The desolvation process reduces the interfacial impedance; in addition, the introduction of fluorine atoms with strong electronegativity promotes the formation of a dense and stable SEI film, significantly enhances the interfacial stability of the positive and negative electrodes, and improves the thermal stability of the electrolyte, reducing the continuous decomposition of the electrolyte under high pressure and high temperature conditions.

[0152] Compared with formula A2, formula A1 has fewer F atoms directly connected to C atoms in its structure, avoiding the impact of polyfluorinated substances on the environment. Although formula A3 has more F atoms to generate more SEI films, it will also cause the SEI film to be too thick, hindering the migration of lithium ions and increasing the impedance during the battery cycle. Secondly, more F atoms will also cause it to have the risk of defluorination and gas production at high temperatures. Therefore, the first solvent is preferably formula A1. Compared with formulas B1 and B4, formula B3 replaces the H atoms on the heterocycle with F atoms, which can be preferentially decomposed under reduction / oxidation conditions, promote the formation of a dense SEI film rich in LiF on the negative electrode surface, inhibit the continuous decomposition of the electrolyte, and improve the cycle life. Compared with formula B2, formula B3 has more F atoms, which is also more conducive to the formation of a more stable CEI film at the positive electrode, inhibiting the dissolution and structural collapse of transition metals. Therefore, the first additive is preferably formula B3.

[0153] The capacity retention rates of the soft-pack lithium batteries in Examples 1 to 14 after 400 cycles at room temperature are all greater than or equal to 80.20%, and the thickness expansion rates are all less than or equal to 10.32%. At the same time, the capacity retention rates after 300 cycles at high temperature are all greater than or equal to 82.10%, and the thickness expansion rates are all less than or equal to 12.71%. In addition, the capacity retention rates after 120 cycles at rest are all greater than or equal to 61.30%, and the thickness expansion rates are all less than or equal to 12.95%. The capacity retention rates at 85°C / 24h are all greater than or equal to 80.30%, indicating that the secondary batteries of the present invention have good high-temperature cycling, rest cycling, and high-temperature storage performance.

[0154] According to Example 10, Example 7, Example 1, Example 8 and Example 9, it can be seen that as the content of the first solvent increases (i.e., a increases), the viscosity of the electrolyte will increase accordingly, resulting in a decrease in kinetics, hindering the reaction rate of lithium ion insertion / extraction on the electrode surface, and at the same time also aggravating the side reactions at the electrode-electrolyte interface, accelerating the capacity decay of the battery during normal temperature cycling.

[0155] According to Example 1, Comparative Example 1, Comparative Example 2 and Comparative Example 3, it can be seen that only when the first solvent and the first additive are added to the electrolyte at the same time can the first solvent and the first additive act synergistically, thereby significantly improving the cycle performance of the battery; according to Comparative Examples 4 and 5, it can be seen that when 0.1≤a / (10*b)≤5 and 1<10(b+d) / e<5 cannot be satisfied at the same time, the performance of the battery will be greatly reduced.

[0156] Finally, it should be noted that the above embodiments are only used to illustrate the technical solutions of the present invention and are not intended to limit the scope of protection of the present invention. Although the present invention has been described in detail with reference to preferred embodiments, those skilled in the art should understand that the technical solutions of the present invention may be modified or replaced by equivalents without departing from the essence and scope of the technical solutions of the present invention.

Claims

1. A secondary battery comprising a positive electrode, a negative electrode and an electrolyte, characterized in that: The positive electrode includes a current collector and a positive electrode material layer containing Co provided on at least one side surface of the current collector; The electrolyte includes a first solvent, a first additive and a second additive; The chemical structural formula of the first solvent is shown in Formula A: R1 and R2 in formula A are each independently selected from any one of an unsubstituted alkyl group having 1 to 5 carbon atoms, a H atom, and a F atom; The chemical structural formula of the first additive is shown in Formula B: R1 and R2 in formula B are each independently selected from any one of an unsubstituted alkyl group having 1 to 5 carbon atoms, an amino group, a H atom, a F atom, and a cyano group; Based on the total mass of the electrolyte, the mass percentage of the first solvent is a%, the mass percentage of the first additive is b%, and the mass percentage of the second additive is d%; Based on the total mass of the positive electrode material layer, the mass percentage of Co is e%; 0.1≤a / (10*b)≤5, 1<10(b+d) / e<5.

2. The secondary battery according to claim 1, wherein Satisfy at least one of the following: (1)5≤a≤30； (2)0.5≤b≤10； (3)4≤d≤20； (4)40≤e≤60。 3. The secondary battery according to claim 1, wherein 5<b+d<22.

4. The secondary battery according to claim 1, wherein The first solvent is selected from at least one of Formula A1, Formula A2, and Formula A3:

5. The secondary battery according to claim 1, wherein The first additive is selected from at least one of Formula B1, Formula B2, Formula B3, and Formula B4; 6. The secondary battery according to claim 1, wherein The second additive is at least one selected from fluoroethylene carbonate, succinonitrile, adiponitrile, and 1,3,6-hexanetrinitrile.

7. The secondary battery according to claim 1, wherein The electrolyte further includes a second solvent, which is selected from at least one of ethylene carbonate, propylene carbonate, dimethyl carbonate, 2,2-difluoroethyl acetate, ethyl propionate, and propyl propionate.

8. The secondary battery according to claim 7, wherein Based on the total mass of the electrolyte, the mass percentage of the second solvent is c%, and 30≤c≤70.

9. The secondary battery according to claim 8, wherein 0.01≤a / c≤1.

5.

10. An electronic device, characterized in that: A secondary battery comprising the secondary battery according to any one of claims 1 to 9.

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