A lithium-ion secondary battery

Abstract

The present invention relates to a lithium-ion secondary battery that uses in-situ generated thin-layer graphene in combination with a silicon negative electrode. The negative electrode utilizes a silicon-based material and multilayer graphene, and propylene carbonate is used in the electrolyte. During the battery's charge-discharge cycle, the propylene carbonate in the electrolyte embeds into the multilayer graphene, causing interlayer exfoliation and producing thinner layers of graphene in situ. The combination of the silicon-based negative electrode, multilayer graphene, and propylene carbonate yields a lithium-ion secondary battery with high capacity, excellent cycle performance, low impedance, good safety, and excellent low-temperature performance.

Description

Technical Field

[0001] The present invention belongs to the field of lithium ion secondary batteries, in particular to a lithium ion battery design scheme and a lithium ion secondary battery using the same. Background Art

[0002] Driven by the country's strategic push to develop new energy, new energy vehicles and lithium-ion battery technology are rapidly developing. However, concerns about electric vehicle range are limiting further development. Improving the energy density of lithium-ion batteries has become a top priority in electric vehicle research. Further increases in battery energy density are facing increasing resistance in the positive electrode, whether by increasing the nickel content of ternary materials or raising the operating voltage. On the negative electrode side, the actual capacity of commercial graphite is approaching its theoretical energy density limit, leaving little room for further improvement. In recent years, increased attention has focused on silicon-based materials, which offer a theoretical energy density nearly ten times that of graphite. However, silicon-based materials are alloyed negative electrodes, and the significant volume expansion and contraction associated with charge and discharge pose significant challenges to their practical application. This can lead to SEI film fragmentation, exacerbating negative electrode-electrolyte side reactions, increasing battery gassing, and losing active lithium, compromising cycle and safety performance. Furthermore, this volume effect can also lead to poor electrical contact between negative electrode particles in the later stages of battery cycling, deteriorating cycle performance. Silicon is a semiconductor material and its conductivity is inferior to that of carbon. Improving the conductivity of silicon-based anodes is also a research issue for their application. Therefore, to meet the commercialization needs of silicon-based materials, improving the cycle life, safety performance, and rate performance of batteries containing silicon-based anodes is the main research direction of silicon-based anode batteries.

[0003] Graphene is a new material that has been hotly researched in recent years and has a very broad range of applications. Some researchers use silicon-based materials and graphene together in the hope of improving the performance of silicon-based negative-electrode lithium-ion batteries. Currently, there are several main strategies: First, graphene is used as a coated carbon and coated on the surface of the silicon-based negative electrode. However, the problem is that the preparation of thin-layer graphene is very difficult, and the technical and cost pressures of large-scale preparation are very high. The production capacity is difficult to support the needs of commercial applications. Second, graphene is used simply as a conductive additive to improve the electrical contact between the particles of the negative electrode material. This solution only uses graphene as a high-efficiency conductive agent, but cannot take into account the volume effect of the silicon material.

[0004] Patent CN103050666A discloses a method for preparing graphene-coated silicon-carbon composite materials, which significantly improves the electronic conductivity of silicon-based materials. By utilizing the toughness of the graphene layer, the volume effect of the silicon-based material is alleviated, and the structural stability of the material during the cycle is improved. Patent CN105762364 discloses a method for preparing lithium-ion battery negative electrode sheets by mixing a small amount of graphene as a conductive agent with silicon-based materials, which effectively improves the conductive network of the negative electrode sheet and enhances the battery's cycle performance.

[0005] After analysis, we believe that thin-layer graphene, with its softer material and more conductive sites, can improve the electrical contact between anode material particles while mitigating the volume effect of silicon materials, making it a highly promising material for improving the performance of silicon-based anode batteries. Given the technical difficulties and high costs of producing thin-layer graphene, we proposed replacing thin-layer graphene with multi-layer graphene in combination with silicon-based materials. Using a PC-containing electrolyte, we then exfoliate the multi-layer graphene during the battery's charge and discharge process to obtain thin-layer graphene in situ. Summary of the Invention

[0006] In order to address the deficiencies in the prior art, the present invention provides a lithium-ion secondary battery comprising a positive electrode sheet, a negative electrode sheet, a separator and an electrolyte, wherein the negative electrode sheet comprises a silicon-based material and multilayer graphene, and the electrolyte comprises propylene carbonate.

[0007] In the present invention, the silicon-based material refers to anode materials containing silicon elements, including but not limited to nano-silicon, silicon dioxide and other materials;

[0008] In the present invention, the number of the multilayer graphene layers is 2-200 layers; preferably, the number of the multilayer graphene layers is 3-100 layers; preferably, the number of the multilayer graphene layers is 5-50 layers.

[0009] In the present invention, the multilayer graphene is graphene with one number of layers, or a mixture of multiple graphenes with different numbers of layers, or a mixture of multilayer graphene with one or more numbers of layers and single-layer graphene.

[0010] In the present invention, the size of the multilayer graphene platelets is 0.5-20 μm.

[0011] In the present invention, the negative electrode plate further comprises other negative electrode materials, including but not limited to hard carbon, graphite, lithium titanate, etc., or a combination of several of them.

[0012] In the present invention, the mass percentage of the multilayer graphene in the negative electrode plate is 2%-90%, preferably 5%-90%, and more preferably 5%-75%.

[0013] In the present invention, the mass percentage of the silicon-based material in the negative electrode plate is 2%-50%; preferably, the mass percentage of the silicon-based material in the negative electrode is 5%-35%; preferably, the mass percentage of the silicon-based material in the negative electrode is 10%-25%.

[0014] In the present invention, the electrolyte is composed of a solvent, a lithium salt and an additive; in addition to propylene carbonate, the electrolyte solvent may also include but is not limited to ethylene carbonate, fluoroethylene carbonate, dimethyl carbonate, diethyl carbonate, ethyl methyl carbonate, propyl acetate, ethyl propionate, and propyl propionate, or a combination of several thereof.

[0015] In the present invention, the mass percentage of the propylene carbonate in the electrolyte solvent is 5%-40%; preferably, the mass percentage of the propylene carbonate in the electrolyte solvent is 10%-30%.

[0016] In the present invention, the lithium salt is selected from one or more combinations of lithium hexafluorophosphate, lithium tetrafluoroborate, lithium bis(fluorosulfonyl)imide, lithium bis(trifluoromethylsulfonyl)imide, lithium bis(oxalatoborate), lithium bis(fluorooxalatoborate), lithium difluorophosphate, and lithium perchlorate. Preferably, the mass proportion of the lithium salt in the electrolyte is 10%-20%.

[0017] In the present invention, the additive is selected from one or more of vinyl sulfate, fluoroethylene carbonate, vinylene carbonate, methylene methanedisulfonate, 1,3-propane sultone, 1-propylene-1,3-sultone, and tris(trimethylsilyl)phosphate. Specifically, the additive accounts for less than 5% by weight in the electrolyte.

[0018] In the present invention, the main material of the positive electrode plate is a combination of one or more materials selected from, but not limited to, NCM ternary materials, NCA ternary materials, lithium cobalt oxide, lithium manganese oxide, lithium iron manganese phosphate, and lithium iron phosphate.

[0019] In the present invention, the preparation method of the positive electrode sheet comprises the following steps: 1) uniformly mixing the main material of the positive electrode sheet and conductive carbon black; 2) adding a binder and a solvent and mixing them uniformly to obtain a slurry; 3) coating the obtained slurry on a conductive current collector, and obtaining the desired positive electrode sheet through continuous baking, rolling, slitting, cutting and other steps.

[0020] In the present invention, the method for preparing the positive electrode sheet comprises a binder selected from PVDF, a solvent selected from NMP, and an aluminum foil as the conductive current collector. The method for preparing the negative electrode sheet comprises the following steps: 1) uniformly mixing the negative electrode material and multilayer graphene, adding the binder and solvent, and stirring to obtain a slurry; 2) coating the obtained slurry on the conductive current collector, and successively baking, rolling, slitting, and cutting to obtain the desired negative electrode sheet.

[0021] In the present invention, the negative electrode main material includes silicon-based materials, multilayer graphene, hard carbon, graphite, lithium titanate and other materials with capacity.

[0022] In the present invention, in the method for preparing the positive electrode sheet, the negative electrode binder is selected from polyacrylic acid binders and sodium carboxymethyl cellulose, the solvent is deionized water, and the conductive current collector is copper foil.

[0023] In the present invention, the preparation method of the electrolyte comprises the following steps: 1) uniformly mixing various solvents required for the electrolyte; 2) adding electrolyte lithium salt and additives to the solvent and uniformly mixing to obtain the desired electrolyte.

[0024] In the present invention, the secondary battery preparation method comprises the following steps:

[0025] The prepared positive electrode sheet, negative electrode sheet and separator are wound in the order of positive electrode, separator and negative electrode, and the bare battery cell is obtained after hot pressing, shaping, and tab welding. The bare battery cell is then sealed on the top and side with aluminum-plastic film, and then placed in an oven for baking. The moisture content in the electrode sheet is tested, and the electrolyte is injected after confirming that the moisture content is qualified. After vacuum packaging, standing, formation, shaping, and capacity separation, the required lithium-ion secondary battery is obtained.

[0026] The oven temperature is 50-100°C and the baking time is more than 24-56 hours.

[0027] The beneficial effects of the present invention are:

[0028] First, the in-situ generated thin-layer graphene has better flexibility than multi-layer graphene, which can better alleviate the volume expansion of silicon-based materials during lithium extraction, and provides better feasibility for the design of high silicon content in the battery negative electrode; second, the in-situ generated thin-layer graphene has more conductive sites than multi-layer graphene, which can improve the problem of poor electrical contact between negative electrode material particles caused by the expansion and contraction process of silicon material during battery cycling, and improve the cycle performance of the battery; third, when the battery is charged and discharged, the propylene carbonate in the electrolyte will be embedded in the multi-layer graphene, and the multi-layer graphene will generate thin-layer graphene in situ. Compared with directly adding thin-layer graphene, the material cost is significantly reduced, and multi-layer graphene has an exponential cost advantage over thin-layer graphene; fourth, this solution uses multi-layer graphene and silicon material for compounding. Compared with directly adding thin-layer graphene, it can better control material dispersion, achieve product stability, and the battery electrode preparation process is simpler. Furthermore, the introduction of propylene carbonate offers improved electrochemical performance compared to ethylene carbonate, commonly used in current battery systems. Propylene carbonate offers greater electrochemical stability and is less susceptible to side reactions with the positive and negative electrodes, resulting in less gas production and improved storage and cycling performance. Propylene carbonate also has lower viscosity and melting point, resulting in lower battery impedance and improved low-temperature performance.

[0029] Therefore, by using a combination of silicon-based negative electrode, multilayer graphene and propylene carbonate, a lithium-ion secondary battery with high capacity, excellent cycle performance, low impedance, good safety and excellent low-temperature performance can be obtained. DETAILED DESCRIPTION

[0030] In order to more clearly illustrate the technical solutions and advantages of the present invention, the present invention will be further described herein in conjunction with embodiments. The description of the embodiments is merely used to explain the present invention but is not intended to limit the present invention.

[0031] Raw materials: Unless otherwise specified, the following raw materials are commercially available products.

[0032] EC is the abbreviation for ethylene carbonate;

[0033] PC is the abbreviation of propylene carbonate;

[0034] EMC is the abbreviation of vinyl methyl carbonate;

[0035] FEC is the abbreviation of fluoroethylene carbonate:

[0036] VC is the abbreviation of vinylene carbonate;

[0037] DTD is the abbreviation for diethylene sulfate;

[0038] PS is the abbreviation for 1,3-propane sultone;

[0039] PAA is the abbreviation of polyacrylic acid adhesive. The PAA adhesive used in this experiment was purchased from Suzhou Ruihong Electronic Chemicals Co., Ltd., brand: BH1100H;

[0040] CMC is the abbreviation of sodium carboxymethyl cellulose. The CMC used in this experiment was purchased from Nippon Paper Industries, Ltd., brand: MAC500.

[0041] SP is the abbreviation of Super-P conductive carbon black. The SP used in this experiment was purchased from France Yiruishi, brand: Super P-Li;

[0042] PVDF is the abbreviation of polyvinylidene fluoride. The PVDF used in this experiment was purchased from Solvay Group in the United States, brand: 5130;

[0043] NMP is the abbreviation for N-methylpyrrolidone;

[0044] LiPF6 is the abbreviation of lithium hexafluorophosphate;

[0045] DCR is the DC resistance;

[0046] SOC: Battery state of charge

[0047] Example 1

[0048] The steps for preparing the lithium-ion secondary battery of this embodiment are as follows:

[0049] 1) Preparation of positive electrode sheet:

[0050] First, the positive electrode active material LiNi 0.85 Co 0.05 Mn 0.1 O2 (lithium nickel cobalt manganese oxide) and SP were added to a stirring tank and mixed evenly. The NMP solution of PVDF was added thereto and stirred continuously. After mixing evenly, a black slurry was obtained, in which the mass ratio of the positive electrode active material, SP and PVDF was 96:2:2. The black slurry was coated on aluminum foil and continuously baked (baking conditions: temperature 110 ° C, wind frequency 35HZ, speed 3m / min), roller pressed (compacted density 3.4g / cm 3 ), stripping, cutting steps to obtain a surface density of 34.0g / cm 2 The positive electrode.

[0051] 2) Preparation of negative electrode sheet:

[0052] First, the silicon oxide negative electrode (source: Tianmu Pioneer, brand SL1700A-SOCQ2) and multilayer graphene were added to a stirring tank and mixed evenly. The binder PAA, CMC and deionized water were added and stirred continuously until the mixture was evenly mixed to obtain a black slurry. The mass ratio of silicon oxide negative electrode, multilayer graphene, PAA and CMC in the slurry was 2:91:5:2. The black slurry was coated on copper foil and continuously baked (baking conditions: temperature 95 ° C, wind frequency 30HZ, speed 3m / min), roller pressed (compacted density 1.3g / cm 3 ), slit, and cut into pieces to obtain a surface density of 12.1g / cm 2 The negative electrode.

[0053] 3) Preparation of electrolyte:

[0054] In an argon atmosphere glove box with a water and oxygen content of less than 1ppm, PC:EMC:FEC were mixed in a mass ratio of 30:60:10 as an electrolyte solvent, cooled to below -10°C, and LiPF6 accounting for 12.5% ​​of the mass percentage of the electrolyte was slowly added thereto while maintaining stirring. The process temperature was controlled not to exceed 0°C. After LiPF6 was completely dissolved, DTD accounting for 2% of the mass fraction of the electrolyte, VC accounting for 1% of the mass fraction of the electrolyte, and PS accounting for 1% of the mass fraction of the electrolyte were added. After mixing evenly, the configured electrolyte was obtained.

[0055] 4) Battery production:

[0056] The prepared positive electrode sheet, negative electrode sheet and separator are wound in the order of positive electrode, separator and negative electrode, and the bare battery cell is obtained after hot pressing, shaping, and tab welding. The bare battery cell is then sealed on the top and side with aluminum-plastic film, and then placed in an oven at 90°C for more than 24 hours. The moisture content in the electrode sheet is tested, and the electrolyte is injected after confirming that the moisture content is qualified (less than 200ppm). After vacuum packaging, standing, formation, shaping, and capacity separation, the required lithium-ion secondary battery is obtained.

[0057] Examples 2 to 30:

[0058] The steps for preparing the lithium-ion batteries of Examples 2 to 30 are the same as those of Example 1. The differences are in the types or proportions of materials used in preparing the negative electrode sheets, or the types and contents of components in the electrolyte. The specific differences are shown in Table 1.

[0059] Table 1

[0060]

[0061] Experimental results:

[0062] Test: The lithium-ion secondary batteries prepared in Examples 1 to 30 were tested for capacity, cycle, high-temperature gas generation, DC impedance, and low-temperature performance. The experimental results are shown in Table 2. The specific test methods are as follows:

[0063] 1) Full battery discharge capacity of negative electrode materials:

[0064] During the manufacturing process of the lithium-ion secondary batteries prepared in Examples 1 to 30, the mass of the added negative electrode main materials was recorded.

[0065] The above batteries were charged at 25°C at a constant current of 1C to 4.3V, and then continued to charge at a constant voltage to a cut-off current of 0.05C. After standing for 10 minutes, they were discharged at a constant current of 1C to 2.5V. The discharge capacity of each battery was recorded as C 初始放电容量 .

[0066] Negative electrode main material discharge capacity = C 初始放电容量 / Negative electrode main material quality notes: Negative electrode main materials include silicon-based materials, multilayer graphene, hard carbon, graphite, lithium titanate and other materials with capacity.

[0067] 2) Normal temperature cycle test:

[0068] Room Temperature Cycling: The lithium-ion secondary batteries prepared in Examples 1-30 were charged at 1C constant current to 4.3V at 25°C, then charged at constant voltage to a cutoff current of 0.05C. After standing for 10 minutes, they were discharged at 1C constant current to 2.5V. The initial discharge capacity of each battery was recorded as C0. The batteries were then cycled at 25°C at a charge / discharge rate of 1C / 1C in the range of 2.5 to 4.3V. The test was stopped when the capacity reached 70% of C0, and the number of cycles was recorded.

[0069] 3) DC impedance test:

[0070] The lithium-ion secondary batteries prepared in Examples 1 to 30 were left at rest for 120 minutes at 25°C, discharged at a constant current of 1C to 2.5V, left at rest for 10 minutes, and then charged at a cross-current current of 1C to 4.3V. The batteries were then continuously charged at a constant voltage to a cutoff current of 0.05C. The batteries were left at rest for 10 minutes, and then charged at a constant current of 1C to 2.5V. The discharge capacity was recorded as C1. The batteries were left at rest for 10 minutes, and then charged at a cross-current current of 1C to 4.3V. The batteries were then continuously charged at a constant voltage of 4.3V to a cutoff current of 0.05C. The batteries were left at rest for 10 minutes, and then discharged at a current of 1C1 for 30 minutes to adjust the battery capacity to 50% SOC.

[0071] The battery was placed at 25°C for 10 min, and discharged at 4C for 30 s. The voltage before and after discharge was recorded as U 前 and U 后 .

[0072] DCR=(U 前 -U 后 ) / Discharge current Note: The discharge current here is 4C

[0073] 4) Gas production test:

[0074] The lithium-ion secondary batteries prepared in Examples 1 to 30 were charged at a constant current of 1C to 4.3V and then at a constant voltage of 0.05C. The initial volume was tested using the water displacement method, and then placed in a 70°C incubator for high-temperature storage. Recharge was performed every 15 days (recharge procedure: the battery was cooled to 25°C, charged at a constant current of 1C to 4.3V, and then at a constant voltage of 0.05C). After recharge, high-temperature storage was continued. After 45 days, the volume of each battery was tested using the water displacement method, and the volume data of each test was recorded.

[0075] Volume expansion rate after x days = (volume after x days of storage - initial volume) / initial volume * 100%; 4) Low-temperature discharge retention rate test:

[0076] At 25℃, charge the battery to 4.3V with 1C constant current and then charge it to the cut-off current of 0.05C with constant voltage. Let it stand for 10 minutes and then discharge it to 2.5V with 1C constant current. The discharge capacity is recorded as C常温 Then place the battery in a -10℃ environment and let it stand for 120 minutes to allow the battery to cool down sufficiently. Then discharge the battery to 2.5V with a constant current of 1C and record the discharge capacity, which is C. 低温 .

[0077] Low temperature discharge capacity retention rate = C 低温 / C 常温 *100%

[0078] Table 2

[0079]

[0080] Analysis of experimental results:

[0081] 1. Influence of the proportion of silicon-based materials: From the test results of Examples 1 to 8, it can be seen that as the proportion of silicon-based materials increases, the content of multilayer graphene decreases:

[0082] a) The specific capacity of the negative electrode main material is significantly improved because the higher specific capacity of the silicon-based material brings more capacity.

[0083] b) Battery cycle performance significantly decreases. This is because more silicon-based materials bring about greater volume expansion and contraction effects, which increase electrode-electrolyte side reactions and cause active lithium loss. The greater volume effect also leads to more electrical contact problems, resulting in decreased cycle performance.

[0084] c) Battery gas production increases significantly because higher silicon content causes more electrolyte side reactions, leading to increased gas production.

[0085] d) The DC resistance increases significantly. This is because silicon-based materials are semiconductor materials and their conductivity is far inferior to that of graphene. Therefore, as their content increases, the graphene content decreases, and the battery impedance increases.

[0086] e) The low-temperature discharge capacity decreases, which may be due to the increase in silicon content and the decrease in graphene content, resulting in increased battery polarization at low temperatures and reduced low-temperature discharge capacity retention.

[0087] Considering the overall performance of the battery, when the mass percentage of the silicon-based material in the negative electrode is 2%-50%, the battery performance is good; preferably, the mass percentage of the silicon-based material in the negative electrode is 5%-35%; further preferably, the mass percentage of the silicon-based material in the negative electrode is 10%-25%;

[0088] 2. Influence of the number of graphene layers: According to the test results of Example 5, Examples 9 to 16, it can be seen that:

[0089] a) Battery negative electrode specific capacity utilization: When the number of graphene layers is relatively thin, the specific capacity utilization of the negative electrode material increases as the number of graphene layers increases; when the number of graphene layers reaches a certain thickness, the specific capacity utilization of the battery material tends to be stable as the number of graphene layers increases. The reason is that when graphene is too thin, its ability to insert lithium is limited, and the capacity it contributes is relatively small. As the number of graphene layers increases, the lithium insertion ability is enhanced, and the specific capacity utilization of the negative electrode material increases. When the number of graphene layers reaches a certain level, the effect of the number of graphene layers on the lithium insertion ability weakens, and the specific capacity utilization of the battery negative electrode main material tends to be stable.

[0090] b) Cycle performance: When the number of graphene layers is thin, the cycle performance improves as the number of graphene layers increases; when the number of graphene layers reaches a certain thickness, the cycle performance deteriorates as the number of graphene layers continues to increase. The reason is that when the graphene layers are too thin, the specific surface area is too large, which leads to increased electrode-electrolyte side reactions, increased active lithium loss, and deteriorated cycle performance. As the number of graphene layers increases, the specific surface area of ​​the negative electrode decreases, and the cycle performance improves. When the number of graphene layers increases further, the specific surface area is no longer the main influencing factor. Since the graphene is too thick, PC embedding cannot peel off a thin enough graphene. The thick graphene cannot provide enough toughness to alleviate the volume effect of the silicon negative electrode, resulting in deteriorated cycle performance.

[0091] c) Gas production performance: As the number of graphene layers increases, gas production decreases. This is because thinner graphene layers have a larger specific surface area, which increases electrode-electrolyte side reactions and increases gas production. As the number of graphene layers increases, the specific surface area decreases, and gas production decreases.

[0092] d) DC impedance & low-temperature discharge: As the number of graphene layers increases, the impedance increases. This is because thinner graphene has more conductive sites per unit mass, resulting in better electrical contact between negative electrode particles, stronger conductivity, and lower polarization at low temperatures. As the number of graphene layers increases, the number of conductive sites per unit mass decreases, leading to increased battery impedance and reduced low-temperature discharge performance.

[0093] Considering the overall performance of the battery, the battery performance is good when the number of multilayer graphene layers is between 2 and 200. Preferably, the battery performance is good when the number of multilayer graphene layers is between 3 and 100, and more preferably, the battery performance is even better when the number of multilayer graphene layers is between 5 and 50.

[0094] 3. Influence of solvent system:

[0095] According to the test results of Example 13 and Examples 17 to 21, it can be seen that

[0096] a) From the perspective of negative electrode material specific capacity, impedance, and low-temperature performance, the gradual replacement of EC with PC has resulted in a slight increase in battery negative electrode material specific capacity, reduced impedance, and improved low-temperature performance. This is because PC has a lower viscosity than EC at both room and low temperatures, resulting in better lithium ion transport in the electrolyte and reduced battery polarization.

[0097] b) In terms of cycling performance, increasing PC content significantly improves cycling performance. This is due to the following reasons: first, PC-based electrolytes result in smaller polarization in the battery; second, EC cannot exfoliate graphite, preventing the production of thinner graphene layers to mitigate the volume effect of silicon-based materials; and third, PC offers greater electrochemical stability, significantly reducing electrode-electrolyte side reactions.

[0098] c) From the perspective of gas production performance, PC-based electrolyte produces lower gas. The reason is that PC has better electrochemical stability than EC, the electrode-electrolyte side reaction is reduced, and the gas production is improved.

[0099] According to the test results of Example 13, Examples 22 and 23, it can be seen that:

[0100] As the PC content increases further, the battery capacity, cycle, impedance and low-temperature performance deteriorate. The reason is that the higher PC content further increases the viscosity of the electrolyte, significantly reduces the electrolyte's ability to migrate lithium ions, increases battery polarization, and deteriorates performance.

[0101] Therefore, considering the overall performance of the battery, when the mass percentage of propylene carbonate in the electrolyte solvent is 5%-40%, the battery performance is better; preferably, when the mass percentage of propylene carbonate in the electrolyte solvent is 10%-30%, the battery performance is better.

[0102] 3. Influence of introduction of other negative electrode main materials: According to the test results of Example 5 and Examples 24 to 29, it can be seen that hard carbon partially replaces graphene:

[0103] a) The specific capacity of the negative electrode main material is improved to a certain extent because hard carbon has a slightly higher specific capacity than graphene;

[0104] b) From the perspective of cycling performance, as the amount of hard carbon substitution increases, the cycling performance deteriorates, and the degradation is more pronounced at low graphene content. This is because insufficient graphene content cannot fully mitigate the volume effect of the silicon anode, causing cycling performance to deteriorate. When the graphene content reaches a certain level, the ability to mitigate the volume effect of the silicon material is sufficient, and further increasing the content does not improve the performance significantly.

[0105] c) From the perspective of gas production, replacing graphene with hard carbon can improve battery gas production to a certain extent. The reason is that hard carbon has a larger specific surface area than graphene, and the electrode-electrolyte side reaction is reduced.

[0106] d) Impedance and low-temperature performance deteriorate because hard carbon is less conductive than graphene, resulting in increased battery polarization.

[0107] Considering overall battery performance, a certain degree of hard carbon substitution can have an acceptable impact on battery performance. Battery designs that combine silicon-based anodes, multilayer graphite, and PC can also incorporate other anode materials to a certain extent. Battery performance is best at a graphene content of 2%, and optimally, at a graphene content of 5% or more, battery performance is even better.

[0108] The above is a specific embodiment of the present invention, and the description is relatively detailed. However, it should not be understood as limiting the scope of protection of the present invention. In actual application, appropriate changes in form or details should also be within the scope of protection of the present invention. For the sake of brevity, this article does not describe all possible combinations of the technical features of the embodiments. However, as long as these combinations do not conflict, they should be considered to be within the scope of the present invention.

Claims

1. A lithium-ion secondary battery, characterized in that: The invention comprises a positive electrode sheet, a negative electrode sheet, a separator and an electrolyte, wherein the negative electrode sheet comprises a silicon-based material and multilayer graphene, and the electrolyte comprises propylene carbonate; the number of the multilayer graphene layers is 2-200; the mass percentage of the multilayer graphene in the negative electrode sheet is 2%-90%, the mass percentage of the silicon-based material in the negative electrode is 2%-50%; and the mass percentage of the propylene carbonate in the electrolyte solvent is 5%-40%.

2. The lithium-ion secondary battery according to claim 1, wherein The silicon-based material refers to a negative electrode material containing silicon element.

3. The lithium-ion secondary battery according to claim 1, wherein The silicon-based materials refer to nano-silicon and silicon monoxide.

4. The lithium-ion secondary battery according to claim 1, wherein The multilayer graphene has 3 to 100 layers; and / or the multilayer graphene is graphene with one number of layers, or a mixture of multiple graphenes with different numbers of layers, or a mixture of multilayer graphene with one or more numbers of layers and single-layer graphene; and / or the size of the multilayer graphene platelets is 0.5 to 20 μm.

5. The lithium-ion secondary battery according to claim 1, wherein The number of layers of the multi-layer graphene is 5-50.

6. The lithium-ion secondary battery according to any one of claims 1 to 5, wherein: The negative electrode plate also contains hard carbon, graphite, lithium titanate or a combination of these.

7. The lithium-ion secondary battery according to any one of claims 1 to 5, wherein: The mass percentage of the multilayer graphene in the negative electrode plate is 5%-90%; in the negative electrode plate, the mass percentage of the silicon-based material in the negative electrode is 5%-35%.

8. The lithium-ion secondary battery according to any one of claims 1 to 5, wherein: The mass percentage of the multilayer graphene in the negative electrode plate is 5%-75%; in the negative electrode plate, the mass percentage of the silicon-based material in the negative electrode is 10%-25%.

9. The lithium-ion secondary battery according to any one of claims 1 to 5, wherein: The electrolyte is composed of a solvent, a lithium salt and an additive; in addition to propylene carbonate, the electrolyte solvent includes one or a combination of ethylene carbonate, fluoroethylene carbonate, dimethyl carbonate, diethyl carbonate, ethyl methyl carbonate, propyl acetate, ethyl propionate and propyl propionate.

10. The lithium-ion secondary battery according to claim 9, wherein The mass percentage of the propylene carbonate in the electrolyte solvent is 10%-30%.

11. The lithium-ion secondary battery according to claim 9, wherein The lithium salt is selected from a combination of one or more of lithium hexafluorophosphate, lithium tetrafluoroborate, lithium bis(fluorosulfonyl)imide, lithium bis(trifluoromethylsulfonyl)imide, lithium bis(oxalatoborate), lithium bis(fluorooxalatoborate), lithium difluorophosphate, and lithium perchlorate, and the mass proportion of the lithium salt in the electrolyte is 10%-20%.

12. The lithium-ion secondary battery according to any one of claims 9 to 11, wherein: The additive is selected from a combination of one or more of vinyl sulfate, fluoroethylene carbonate, vinylene carbonate, methylene disulfonate, 1,3-propane sultone, 1-propylene-1,3-sultone, and tris(trimethylsilyl)phosphate; the mass proportion of the additive in the electrolyte is less than 5%.

13. The lithium ion secondary battery according to any one of claims 1 to 5, characterized in that: The main material of the positive electrode plate is a combination of one or more selected from NCM ternary material, NCA ternary material, lithium cobalt oxide, lithium manganese oxide, lithium iron manganese phosphate, and lithium iron phosphate.

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