Lithium iron phosphate battery

Abstract

A lithium iron phosphate battery. The lithium iron phosphate battery comprises a positive electrode, a negative electrode, a separator, and an electrolyte. The main material of the positive electrode comprises lithium iron phosphate, and the main material of the negative electrode comprises graphite and a silicon based material. The areal density of the positive electrode is greater than or equal to 23 mg / cm2, and the areal density of the negative electrode is greater than or equal to 7.8 mg / cm2. The electrolyte comprises a lithium salt, a solvent, and an additive. The solvent mainly comprises ethylene carbonate and a small-molecule linear solvent. The additive comprises fluoroethylene carbonate (FEC) and a negative electrode film-forming additive, and the negative electrode film-forming additive comprises vinylene carbonate and a lithium salt additive. The mass fraction of the small-molecule linear solvent is 35%-60%, the mass fraction of the FEC is M, the mass fraction of silicon in a negative electrode layer is W1, 2%≤M≤2×W1, and the mass fraction of the negative electrode film-forming additive is 4%-6%. By means of doping of the silicon based material into the negative electrode, thick electrode design, and electrolyte formulation adjustment, the lithium iron phosphate battery can improve the energy density of battery cells, and can also take into account both the cycle life and fast charging capability of battery cells.

Description

A lithium iron phosphate battery

[0001] Cross-references to related applications

[0002] This application claims priority to Chinese Patent Application No. CN202411513097.6, filed on October 28, 2024, entitled "A Lithium Iron Phosphate Battery", the entire contents of which are incorporated herein by reference. Technical Field

[0003] This application relates to the field of new energy technology, specifically to a lithium iron phosphate battery. Background Technology

[0004] Lithium-ion batteries have become the main power source for power batteries. Based on the different cathode materials used, lithium-ion batteries are mainly divided into two categories: lithium iron phosphate batteries and ternary lithium batteries. Among them, lithium iron phosphate batteries, with their long lifespan, high safety, environmental friendliness, absence of rare elements, and low cost, have become the mainstream product in the market.

[0005] However, compared to ternary lithium batteries, lithium iron phosphate batteries still have a relatively low energy density (180Wh / kg vs 250Wh / kg), thus limiting the driving range of electric vehicles. Furthermore, the poor kinetic performance of the lithium iron phosphate cathode, coupled with the high viscosity and low conductivity of the electrolyte, results in relatively poor fast-charging performance.

[0006] In currently commercialized lithium iron phosphate batteries, the anode mainly uses graphite, with a theoretical capacity of 372 mAh / g, which is relatively low compared to alloy anodes, only 20% to 30% of that of materials such as SiO and SiC. This limits the energy density of current lithium iron phosphate single cells. At the same time, the current electrolyte system has poor wettability and conductivity, which is not conducive to supporting cell designs with thick electrodes and high compaction, further limiting the improvement of cell energy density.

[0007] CN117007337A discloses a high-energy-density fast-charging lithium iron phosphate battery and its preparation method, including a positive electrode sheet, a negative electrode sheet, a separator, and an electrolyte. The positive electrode sheet includes a positive current collector and a positive electrode coating applied to the surface of the positive current collector. The negative electrode sheet includes a negative current collector and a negative electrode coating applied to the surface of the negative current collector. The positive electrode coating material includes a positive active material, a conductive agent, a binder, and a positive solvent. The negative electrode coating material includes a negative active material, a conductive agent, a dispersant, a binder, and deionized water. The positive active material includes a composite lithium iron phosphate material and a lithium replenishment material. The negative active material includes fast-charging graphite and silicon-based materials. This patent utilizes the combination of lithium iron phosphate and lithium replenishment materials to provide a higher specific capacity. The lithium replenishment material provides additional active lithium, and the energy density of the cell can be further improved with the increase of silicon negative electrode addition. The combination of fast-charging graphite and silicon-based materials further improves the electrochemical performance of the materials. However, this patent increases the energy density of the battery cell by adding lithium to the positive electrode and silicon to the negative electrode, which significantly increases the cost of the battery cell. In particular, adding lithium to the positive electrode will generate a large amount of oxygen during the pre-charge formation process of the battery cell, which requires modification of the battery production line to achieve, making its practical application less feasible.

[0008] CN109687014A discloses a high-energy-density fast-charging lithium-ion power battery, which modifies the positive and negative electrode materials; designs the positive and negative electrode sheets as multi-tabs or all-tabs with a void structure; designs the conductive agent as a "point-line-surface" conductive network; and improves the formulation of the separator and electrolyte. The lithium-ion power battery designed in this way has high energy density and good fast-charging performance. However, this patent only targets nickel-cobalt-manganese ternary materials or nickel-cobalt-aluminum ternary material systems, which already have high energy density, and does not design a solution to improve the energy density of the more mainstream lithium iron phosphate battery.

[0009] CN107959050A improves battery cycle performance, safety performance, lifespan, energy density, and discharge specific capacity by optimizing the electrolyte composition. The electrolyte chamber is obtained by increasing the lithium salt concentration in a conventional electrolyte by 1.5 to 2.5 times; the conventional electrolyte consists of a sulfone solvent, a linear carbonate solvent, and a conductive lithium salt; the conductive lithium salt is a fluorine-containing lithium salt. However, improving battery cycle performance by increasing the lithium salt concentration and using high-viscosity sulfone solvents significantly increases the cost of the electrolyte and reduces its conductivity, making it impossible to design and implement fast-charging batteries.

[0010] However, even by optimizing the electrolyte composition, the energy density of lithium iron phosphate batteries is still difficult to exceed 200Wh / kg, which is still significantly lower than that of ternary lithium batteries (>240Wh / kg).

[0011] Therefore, improving the energy density and fast-charging performance of batteries through electrode and electrolyte design optimization is a technical problem that urgently needs to be solved.

[0012] Application content

[0013] In view of this, the purpose of this application is to provide a lithium iron phosphate battery.

[0014] The lithium iron phosphate battery of this application has a cell energy density >200Wh / kg at a charge / discharge rate of 0.33C and a cycle life of 2000 cycles at room temperature under a 2.2C fast charging regime (10%–80% SOC). The 2000-cycle capability refers to a capacity retention of ≥80% after 2000 cycles.

[0015] In one implementation, the lithium iron phosphate battery cell capacity at 0.33C charge / discharge is >100Ah.

[0016] In a first aspect, this application provides a lithium iron phosphate battery, including a positive electrode, a negative electrode, a separator and an electrolyte, wherein the main material of the positive electrode is lithium iron phosphate and the main material of the negative electrode is graphite and silicon-based materials.

[0017] The areal density ρ1 of the positive electrode is ≥ 23 mg / cm³. 2 The areal density ρ2 of the negative electrode is greater than 7.8 mg / cm³. 2 ;

[0018] The electrolyte includes lithium salt, solvent and additives. The solvent mainly includes ethylene carbonate (EC) and small molecule linear solvents. The additives include fluoroethylene carbonate (FEC) and negative electrode film-forming additives. The negative electrode film-forming agent includes vinylene carbonate and lithium salt additives.

[0019] The small molecule linear solvent is a carboxylic acid ester solvent, ether solvent, or nitrile solvent with a relative molecular mass ≤90. Based on the total mass of the electrolyte as 100%, the mass fraction of the small molecule linear solvent is 35% to 60%. The mass fraction of the fluoroethylene carbonate is M. The mass fraction of silicon in the negative electrode layer is W1, where 2% ≤ M ≤ 2 × W1. The mass fraction of the negative electrode film-forming agent is 4% to 6%.

[0020] In this application, the areal density ρ1 of the positive electrode is ≥ 23 mg / cm³. 2 For example, it could be 23 mg / cm³ 2 24mg / cm 2 25mg / cm 2 26mg / cm 2 27mg / cm 2 28mg / cm 229mg / cm 2 30mg / cm 2 wait.

[0021] In this application, the areal density ρ2 of the negative electrode is ≥ 7.8 mg / cm³. 2 For example, it could be 7.8 mg / cm³ 2 8.0 mg / cm 2 8.5 mg / cm 2 9mg / cm 2 9.5 mg / cm 2 10mg / cm 2 wait.

[0022] In this application, the mass fraction of the small molecule linear solvent is 35% to 60%, for example, it can be 35%, 37.5%, 40%, 42.5%, 45%, 47.5%, 50%, 52.5%, 55%, 57.5% or 60%, etc.

[0023] In this application, the mass fraction W1 of silicon element in the negative electrode layer satisfies 2% ≤ M ≤ 10%. For example, W1 can be 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%, etc.

[0024] In this application, the mass fraction of the negative electrode film-forming agent is 4.0% to 6.0%, for example, it can be 4.0%, 4.2%, 4.5%, 4.8%, 5.0%, 5.2%, 5.5%, 5.8%, or 6.0%, etc. If the content of the negative electrode film-forming agent is too high, it will lead to a decrease in room temperature cycling performance.

[0025] This application improves the energy density and fast-charging performance of batteries through design optimization of electrodes and electrolytes. The main principle is as follows:

[0026] ① By mixing a certain amount of high-capacity silicon-based material (such as silicon-carbon material) into the negative electrode main material, the average specific capacity of the negative electrode is increased, the mass of the negative electrode material is reduced, and thus the energy density of the battery is improved.

[0027] ② Introducing a large amount of small-molecule linear solvent into the electrolyte solvent can reduce the electrolyte density and injection volume, further reducing the weight of the battery cell and increasing energy density. Simultaneously, the low viscosity and low surface tension of the small-molecule linear solvent can improve the wettability of the electrolyte to the electrodes. This improved wettability allows for an increase in areal density (e.g., an increase of 2 mg / cm³ compared to existing technologies). 2 ~5mg / cm 2A cell design with thick electrodes can increase the proportion of active materials in the cell, thereby further improving the energy density of the battery.

[0028] ③ By rationally designing the electrolyte formulation, the electrolyte achieves high conductivity, low viscosity, and good wettability. This ensures stable cycling of the silicon-containing anode, allowing the battery to be compatible with thick-electrode cell designs, good fast-charging capabilities, and long cycle life. Specifically:

[0029] First, since the expansion of silicon-based anodes is more than 10 times greater than that of graphite anodes, the SEI formed on the surface of silicon-based anodes is prone to cracking, which greatly degrades cycle performance. FEC additives have the dual advantages of forming inorganic LiF and polymer components. In order to ensure that the silicon-doped anode in this application has excellent cycle stability, the content of FEC should be controlled within a suitable range to continuously repair the SEI on the surface of the silicon-doped anode.

[0030] Secondly, the electrolyte of this application contains a large amount of small molecule linear solvents, which are prone to decomposition and gas generation on the surface of the negative electrode. Therefore, by using two functional negative electrode film-forming agents, vinylene carbonate and lithium salt additives, and controlling their total content, the film formation can be guaranteed to have high chemical and mechanical stability, thereby effectively solving the above-mentioned gas generation problem and enabling the silicon-doped negative electrode to cycle stably, ensuring that the battery has excellent room temperature cycle life.

[0031] In this application, the electrolyte formulation design is crucial. If the electrolyte formulation design is not met, it will be impossible to meet the technical requirements such as stable cycling of high silicon anode, high wetting required for thick electrode cell design, and high ionic conductivity required for fast charging cycle. The cell is prone to risks such as large-scale gas generation during cycling and capacity drop.

[0032] This application mainly improves battery energy density and fast charging performance by increasing the areal density of the positive electrode and mixing silicon materials into the negative electrode, combined with a high-conductivity electrolyte, resulting in lower cost and higher application feasibility.

[0033] In one optional embodiment, the negative electrode includes a negative electrode current collector and a negative electrode layer disposed on the surface of the negative electrode current collector, and the positive electrode includes a positive electrode current collector and a positive electrode layer disposed on the surface of the positive electrode current collector. The areal density ρ1 of the positive electrode, the areal density ρ2 of the negative electrode, the areal density ρ3 of the positive electrode current collector, and the areal density ρ4 of the negative electrode current collector satisfy the relationship: ρ1 / ρ3+ρ2 / ρ4>7.5.

[0034] In this application, the areal density ρ1 of the positive electrode refers to the mass of the positive electrode layer loaded on a unit area of ​​the positive electrode current collector. The positive electrode layer includes the main positive electrode material (e.g., lithium iron phosphate), and may also include a conductive agent and / or a binder.

[0035] In this application, the areal density ρ2 of the negative electrode refers to the mass of the negative electrode layer loaded on a unit area of ​​negative electrode current collector. The negative electrode layer includes a main negative electrode material (e.g., graphite and silicon-based materials), and may also include conductive agents and / or binders.

[0036] In this application, the surface density ρ3 of the positive electrode current collector refers to the mass of the positive electrode current collector per unit area.

[0037] In this application, the surface density ρ4 of the negative electrode current collector refers to the mass of the negative electrode current collector per unit area.

[0038] In this application, based on the better wettability of the electrolyte, by increasing the areal density of the electrode, more active material is coated on the current collector per unit area, thereby increasing the mass ratio of active material (positive and negative electrode materials) to inactive material (positive and negative electrode current collectors, battery structural components, etc.) on the single electrode, forming a thick electrode, thereby reducing the number of cell layers and the amount of inactive materials such as copper foil, aluminum foil, and separator, and thus improving the energy density of the battery.

[0039] This application does not specify the material of the positive electrode current collector and the negative electrode current collector. For example, the positive electrode current collector can be aluminum foil and the negative electrode current collector can be copper foil.

[0040] In one optional embodiment, the compaction density of the positive electrode is 2.58 g / cm³. 3 ~2.7g / cm 3 For example, it could be 2.5g / cm³ 3 2.59g / cm 3 2.60g / cm 3 2.61 g / cm 3 2.62 g / cm 3 2.63 g / cm 3 2.64 g / cm 3 2.65g / cm 3 2.66 g / cm 3 2.67 g / cm 3 2.68g / cm 3 2.69 g / cm 3 Or 2.70 g / cm 3 wait.

[0041] In one optional embodiment, the compaction density of the negative electrode is 1.58 g / cm³. 3 ~1.7g / cm 3 For example, it could be 1.58 g / cm³ 3 1.59g / cm 3 1.60g / cm 3 1.61 g / cm 31.62g / cm 3 1.63g / cm 3 1.64 g / cm 3 1.65 gg / cm 3 1.66 g / cm 3 1.67 g / cm 3 1.68g / cm 3 1.69 g / cm 3 or 1.70g / cm 3 wait.

[0042] Based on the improved wetting ability of the electrolyte in this application, the compaction density of the positive and negative electrodes is increased (e.g., by 0.05 g / cm³ compared to the prior art). 3 ~0.1g / cm 3 This can reduce the porosity of the electrodes and further improve the energy density of the battery.

[0043] As a preferred technical solution for the lithium iron phosphate battery described in this application, the small molecule linear solvent is at least one of ethyl acetate, methyl acetate, methyl propionate, ethylene glycol dimethyl ether, acetonitrile, or propionitrile.

[0044] In one optional embodiment, the mass fraction of ethylene carbonate is 15% to 30% based on the total mass of the electrolyte as 100%, for example, it can be 15%, 17%, 18%, 20%, 21%, 22%, 24%, 25%, 26%, 27%, 28%, or 30%.

[0045] In one alternative embodiment, the solvent may further include at least one of dimethyl carbonate and / or ethyl methyl carbonate.

[0046] In one alternative embodiment, the lithium salt comprises at least one of lithium hexafluorophosphate and lithium bis(fluorosulfonyl)imide.

[0047] In one optional embodiment, the total mass fraction of the lithium salt is 10% to 18% based on the total mass of the electrolyte as 100%, for example, it can be 10%, 11%, 12%, 13%, 14%, 15%, 16%, 17%, or 18%, etc.

[0048] In one optional embodiment, the lithium salt additive is selected from at least one of lithium difluorooxalate borate, lithium difluorophosphate, lithium tetrafluoroborate, or lithium difluorobis(oxalate) phosphate.

[0049] In one optional embodiment, the mass fraction of the lithium salt additive is 0.5% to 1.5% based on the total mass of the electrolyte (100%), for example, it can be 0.5%, 0.6%, 0.7%, 0.8%, 0.9%, 1%, 1.1%, 1.2%, 1.3%, 1.4%, or 1.5%, etc.

[0050] In one alternative embodiment, the overall conductivity of the electrolyte is ≥14.5 mS / cm.

[0051] In one optional embodiment, the mass fraction of silicon element W1 in the negative electrode layer is ≥2%, preferably 2% to 10%, and can be 1%, 2%, 3%, 4%, 5%, 6%, 7%, 8%, 9%, 10%, etc.

[0052] In one alternative embodiment, the silicon-based material is a silicon-carbon material.

[0053] In this application, silicon-carbon material refers to a material that simultaneously includes silicon-containing components and carbon materials. The silicon-containing component can be elemental silicon, silicon oxide, or both elemental silicon and silicon oxide. Furthermore, this application does not specifically limit the form in which the silicon-containing component and carbon materials exist; they can be a composite formed by a carbon coating layer covering the silicon-containing component, or they can be dispersed together to form a mixture.

[0054] The numerical range described in this application includes not only the point values ​​listed above, but also any point values ​​between the above numerical ranges that are not listed. Due to space limitations and for the sake of brevity, this application will not exhaustively list the specific point values ​​included in the range.

[0055] Compared with existing technologies, this application has the following advantages:

[0056] (1) This application improves the energy density and fast charging performance of the battery through the design optimization of the electrode and electrolyte. The main principle is as follows:

[0057] ① By mixing a certain amount of high-capacity silicon-based material (such as silicon-carbon material) into the negative electrode main material, the average specific capacity of the negative electrode is increased, the mass of the negative electrode material is reduced, and thus the energy density of the battery is improved.

[0058] ② Introducing a large amount of small-molecule linear solvent into the electrolyte solvent can reduce the electrolyte density and injection volume, further reducing the weight of the battery cell and increasing energy density. Simultaneously, due to the improved wettability of the electrolyte, it is possible to increase the areal density (e.g., by 2 mg / cm³ compared to existing technologies). 2 ~5mg / cm 2 A cell design with thick electrodes can increase the proportion of active materials in the cell, thereby further improving the energy density of the battery.

[0059] ③ By rationally designing the electrolyte formulation, the electrolyte has high conductivity, low viscosity and good interfacial stability, which can ensure stable cycling of silicon-containing anodes, enabling the battery to be compatible with thick electrode cell design, good fast charging capability and long cycle life.

[0060] (2) By adopting silicon-based materials for the negative electrode, thick electrode design and electrolyte formulation adjustment, this application can increase the mass energy density of lithium iron phosphate cells from 180Wh / kg to 190Wh / kg to more than 200Wh / kg, further improving the range of electric vehicles and enhancing fast charging capability. Detailed Implementation

[0061] The embodiments of this application are described in detail below. Examples of these embodiments are shown in the accompanying drawings, wherein the same or similar reference numerals denote the same or similar elements or elements having the same or similar functions throughout. The embodiments described below with reference to the accompanying drawings are exemplary and intended to explain this application, and should not be construed as limiting this application.

[0062] The following disclosure provides numerous different embodiments or examples for implementing various structures of this application. To simplify the disclosure, specific examples of components and arrangements are described below. These are merely examples and are not intended to limit the scope of this application. Furthermore, reference numerals and / or letters may be repeated in different examples. Such repetition is for simplification and clarity and does not in itself indicate a relationship between the various embodiments and / or arrangements discussed. In addition, various specific examples of processes and materials are provided in this application; however, those skilled in the art will recognize the applicability of other processes and / or the use of other materials.

[0063] In this embodiment of the application, the positive electrode material in the lithium iron phosphate battery is lithium iron phosphate.

[0064] Example 1

[0065] This embodiment provides a lithium iron phosphate battery, which consists of a positive electrode, a negative electrode, a separator, an electrolyte, and a casing. Its manufacturing process is as follows:

[0066] (1) Positive electrode manufacturing: Lithium iron phosphate, conductive carbon black, carbon nanotubes, polyvinylidene fluoride, and dispersant are mixed in a mass ratio of 96.8:1.0:0.5:1.5:0.2. During the mixing process, N-methylpyrrolidone is used to prepare a positive electrode slurry with a solid content of 55%. The slurry is then coated onto aluminum foil current collectors, dried at 60°C, and then rolled and die-cut to form the desired positive electrode sheet. The positive electrode sheet includes an aluminum foil and a positive electrode layer disposed on the surface of the aluminum foil. The areal density ρ1 of the positive electrode is 23 mg / cm³.2 The areal density ρ3 of the aluminum foil current collector is 3.6 mg / cm³. 2 (The same applies below), the compaction density of the positive electrode is 2.58 g / cm³. 3 .

[0067] (2) Anode Manufacturing: Graphite, silicon carbide, conductive carbon black, sodium carboxymethyl cellulose, and styrene-butadiene rubber are prepared in a mass ratio of 91.0:5.5:1.2:0.8:1.5. Using deionized water as a solvent, a slurry with a solid content of 45% is formed. The slurry is then coated onto a 6-micron thick copper foil current collector. After drying at 80°C, it is roll-cut into the desired anode sheet. The anode sheet comprises a copper foil and a anode layer disposed on the surface of the copper foil. The areal density ρ2 of the anode is 8.67 mg / cm². 2 The areal density ρ4 of the copper foil current collector is 5.37 mg / cm³. 2 (The same applies below), the compaction density of the negative electrode is 1.62 g / cm³. 3 The silicon content in the negative electrode layer is 2.75 wt%.

[0068] (3) Electrolyte preparation: The components are mixed and prepared according to the following mass ratios: lithium hexafluorophosphate 14.0 wt%, ethylene carbonate (EC) 25 wt%, methyl ethyl carbonate 10%, small molecule linear solvent (ethyl acetate 43 wt%), vinylene carbonate (VC) 4 wt%, fluoroethylene carbonate (FEC) 3 wt%, ethylene sulfate 0.5 wt%, and lithium difluorobis(oxalato) phosphate 0.5 wt%.

[0069] (4) Dry cell manufacturing: The positive and negative electrode sheets prepared above are stacked with ceramic-coated PP separators of corresponding size and thickness of 12 micrometers using a stacking machine to obtain a core with a capacity of 120Ah; after welding the tabs, they are packaged with aluminum-plastic film, baked at 85℃ for 24h, and then injected with liquid (injection amount is 2.8g / Ah), packaged, and left to stand before formation.

[0070] The formation process was carried out in a 45℃ chamber, and the steps were as follows: 1) Charge the battery with a current of 0.1C (12A) for 2 hours, with a cutoff voltage of 3.0V; 2) Let the battery cell stand in the 45℃ chamber for 24 hours to age, remove it, vent it, and reseal it; 3) Charge the battery cell with a constant current and constant voltage of 0.2C to 3.65V, with a cutoff current of 1A; 4) Discharge the battery cell with a constant current of 0.2C, with a cutoff voltage of 2.0V; 5) Continue to charge and discharge the battery cell with a current of 1.33A in the range of 2.0V to 3.65V for 2 weeks.

[0071] After the formation process is completed, final sealing is performed to obtain lithium iron phosphate batteries.

[0072] Example 2

[0073] This embodiment provides a lithium iron phosphate battery, the manufacturing process of which is basically the same as that of Embodiment 1, the only difference being the negative electrode and electrolyte composition:

[0074] Negative electrode manufacturing: Graphite, silicon carbide, conductive carbon black, sodium carboxymethyl cellulose, and styrene-butadiene rubber are prepared in a mass ratio of 86.5:10:1.2:0.8:1.5, mixed with deionized water as a solvent to form a slurry with a solid content of 45%. The slurry is then coated onto a copper foil current collector, dried at 80°C, and then roll-cut to form the desired negative electrode sheet. The negative electrode sheet comprises a copper foil and a negative electrode layer disposed on the surface of the copper foil. The areal density of the negative electrode is 7.9 mg / cm³. 2 The compaction density of the negative electrode is 1.62 g / cm³. 3 The silicon content in the negative electrode layer is 5 wt%.

[0075] (2) Electrolyte preparation: The components are mixed and prepared according to the following mass ratios: lithium hexafluorophosphate 14.0 wt%, ethylene carbonate (EC) 25 wt%, methyl ethyl carbonate 10%, small molecule linear solvent (ethyl acetate 40 wt%), vinylene carbonate (VC) 4 wt%, fluoroethylene carbonate (FEC) 6 wt%, ethylene sulfate 0.5 wt%, and lithium difluorobis(oxalato) phosphate 0.5 wt%.

[0076] Example 3

[0077] This embodiment provides a lithium iron phosphate battery, the manufacturing process of which is basically the same as that of Embodiment 1, the only difference being the negative electrode and electrolyte composition:

[0078] (1) In the negative electrode manufacturing step, the mass ratio of graphite, silicon carbide material, conductive carbon black, sodium carboxymethyl cellulose, and styrene-butadiene rubber is 81.5:15.0:1.2:0.8:1.5. A slurry with a solid content of 45% is prepared by mixing with deionized water as a solvent. The slurry is then coated onto a copper foil current collector, dried at 80°C, and rolled and die-cut to form the desired negative electrode sheet. The negative electrode sheet includes a copper foil and a negative electrode layer disposed on the surface of the copper foil. The areal density of the negative electrode is 7.2 mg / cm³. 2 The compaction density of the negative electrode is 1.62 g / cm³. 3 The silicon content in the negative electrode layer is 10 wt%.

[0079] (2) Electrolyte preparation: The components are mixed and prepared according to the following mass ratios: lithium hexafluorophosphate 14.0 wt%, ethylene carbonate (EC) 25 wt%, methyl ethyl carbonate 7%, small molecule linear solvent (ethyl acetate 40 wt%), vinylene carbonate (VC) 4 wt%, fluoroethylene carbonate (FEC) 9 wt%, ethylene sulfate 0.5 wt%, and lithium difluorobis(oxalato) phosphate 0.5 wt%.

[0080] Example 4

[0081] This embodiment provides a lithium iron phosphate battery, the manufacturing process of which is basically the same as that of Example 1, the only difference being the small molecule linear solvent component in the electrolyte:

[0082] Electrolyte preparation: The components are mixed and formulated according to the following mass ratios: lithium hexafluorophosphate 14.0 wt%, ethylene carbonate (EC) 25 wt%, methyl ethyl carbonate 10%, small molecule linear solvents (ethyl acetate 23 wt%, methyl acetate 20 wt%), vinylene carbonate (VC) 4 wt%, fluoroethylene carbonate (FEC) 3 wt%, ethylene sulfate 0.5 wt%, and lithium difluorobis(oxalato)phosphate 0.5 wt%.

[0083] Example 5

[0084] This embodiment provides a lithium iron phosphate battery, the manufacturing process of which is basically the same as that of Example 1, the only difference being the small molecule linear solvent component in the electrolyte:

[0085] Electrolyte preparation: The components are mixed and formulated according to the following mass ratios: lithium hexafluorophosphate 14.0 wt%, ethylene carbonate (EC) 25 wt%, methyl ethyl carbonate 10%, small molecule linear solvents (ethyl acetate 23 wt%, ethylene glycol dimethyl ether 20 wt%), vinylene carbonate (VC) 4 wt%, fluoroethylene carbonate (FEC) 3 wt%, ethylene sulfate 0.5 wt%, and lithium difluorobis(oxalato)phosphate 0.5 wt%.

[0086] Example 6

[0087] This embodiment provides a lithium iron phosphate battery, the manufacturing process of which is basically the same as that of Example 1, the only difference being the composition of the small molecule linear solvent in the electrolyte:

[0088] Electrolyte preparation: The components are mixed and formulated according to the following mass ratios: lithium hexafluorophosphate 14.0 wt%, ethylene carbonate (EC) 25 wt%, methyl ethyl carbonate 10%, small molecule linear solvent (ethyl acetate) 23 wt%, acetonitrile 20 wt%, vinylene carbonate (VC) 4 wt%, fluoroethylene carbonate (FEC) 3 wt%, ethylene sulfate 0.5 wt%, and lithium difluorobis(oxalato)phosphate 0.5 wt%.

[0089] Example 7

[0090] This embodiment provides a lithium iron phosphate battery, whose manufacturing process is basically the same as that of Embodiment 1, except that the total content of VC and lithium salt additives in the electrolyte is adjusted from 4.5% (4% VC + 0.5% lithium salt additives) to 6% (5% VC + 1% lithium salt additives).

[0091] The specific electrolyte composition is as follows: 14.0 wt% lithium hexafluorophosphate, 25 wt% ethylene carbonate (EC), 8.5% methyl ethyl carbonate, 43 wt% small molecule linear solvent (ethyl acetate), 5 wt% vinylene carbonate (VC), 3 wt% fluoroethylene carbonate (FEC), 0.5 wt% ethylene sulfate, and 1 wt% lithium difluorobis(oxalato)phosphate.

[0092] Example 8

[0093] This embodiment provides a lithium iron phosphate battery, the manufacturing process of which is basically the same as that of Embodiment 1, the only difference being the change in the type of lithium salt additive:

[0094] Electrolyte preparation: The components are mixed and formulated according to the following mass ratios: lithium hexafluorophosphate 14.0 wt%, ethylene carbonate (EC) 25 wt%, methyl ethyl carbonate 10%, small molecule linear solvent (ethyl acetate) 43 wt%, vinylene carbonate (VC) 4 wt%, fluoroethylene carbonate (FEC) 3 wt%, ethylene sulfate 0.5 wt%, and lithium difluorooxalate borate 0.5 wt%.

[0095] Comparative Example 1

[0096] It is basically the same as Example 1, except that a carbonate electrolyte is used.

[0097] Carbonate electrolytes have low conductivity and poor wetting, and the areal density and compaction density of the positive and negative electrodes need to be reduced, which will result in a decrease in the energy density of the battery cell.

[0098] The manufacturing process is as follows:

[0099] (1) Positive electrode manufacturing: Lithium iron phosphate, conductive carbon black, carbon nanotubes, polyvinylidene fluoride, and dispersant are mixed in a mass ratio of 96.8:1.0:0.5:1.5:0.2. During the mixing process, N-methylpyrrolidone is used to prepare a positive electrode slurry with a solid content of 55%. The slurry is then coated onto aluminum foil current collectors, dried at 60°C, and then rolled and die-cut to form the desired positive electrode sheet. The areal density of the positive electrode is controlled at 20 mg / cm³. 2 The compaction density of the positive electrode is 2.56 g / cm³. 3 .

[0100] (2) Anode Manufacturing: Graphite, silicon carbide, conductive carbon black, sodium carboxymethyl cellulose, and styrene-butadiene rubber were prepared in a mass ratio of 91.0:5.5:1.2:0.8:1.5. Using deionized water as a solvent, they were mixed to form a slurry with a solid content of 45%. The slurry was then coated onto a copper foil current collector, dried at 80°C, and finally rolled and die-cut to form the desired anode sheet. The areal density of the anode was 7.5 mg / cm³. 2 The compaction density of the negative electrode is 1.55 g / cm³.3 .

[0101] (3) Electrolyte preparation: Prepare one batch of electrolyte by mixing the following mass ratios: lithium hexafluorophosphate 14.5wt%, ethylene carbonate 25wt%, methyl ethyl carbonate 34%, dimethyl carbonate 20%, vinylene carbonate 2.5wt%, fluoroethylene carbonate 3wt%, ethylene sulfate 0.5wt%, and lithium difluorooxalate borate 0.5wt%.

[0102] Comparative Example 2

[0103] It is basically the same as Example 1, except that the amount of FEC used in the electrolyte is 1.5 wt%.

[0104] Electrolyte preparation: The components are mixed and formulated according to the following mass ratios: lithium hexafluorophosphate 14.0 wt%, ethylene carbonate (EC) 25 wt%, methyl ethyl carbonate 10%, small molecule linear solvent (ethyl acetate) 44.5 wt%, vinylene carbonate (VC) 4 wt%, fluoroethylene carbonate (FEC) 1.5 wt%, ethylene sulfate 0.5 wt%, and lithium difluorobis(oxalato)phosphate 0.5 wt%.

[0105] Comparative Example 3

[0106] It is basically the same as Example 1, except that lithium salt additive lithium difluorobis(oxalato) phosphate was not used in the electrolyte, and the content of vinyl sulfate was adjusted from 0.5 wt% to 1.0 wt%.

[0107] Electrolyte preparation: The components are mixed and formulated according to the following mass ratios: lithium hexafluorophosphate 14.0 wt%, ethylene carbonate (EC) 25 wt%, methyl ethyl carbonate 10%, small molecule linear solvent (ethyl acetate) 43 wt%, vinylene carbonate (VC) 4 wt%, fluoroethylene carbonate (FEC) 3 wt%, and ethylene sulfate 1.0 wt%.

[0108] Comparative Example 4

[0109] The method is basically the same as in Example 2, except that less vinylene carbonate and lithium salt additives are used in the electrolyte, and the content of ethyl methyl carbonate is adjusted from 10 wt% to 11.8 wt%.

[0110] Electrolyte preparation: The components are mixed and formulated according to the following mass ratios: lithium hexafluorophosphate 14.0 wt%, ethylene carbonate (EC) 25 wt%, methyl ethyl carbonate 11.8 wt%, small molecule linear solvent (ethyl acetate) 43 wt%, vinylene carbonate (VC) 2.5 wt%, fluoroethylene carbonate (FEC) 3 wt%, ethylene sulfate 0.5 wt%, and lithium difluorobis(oxalato)phosphate 0.2 wt%.

[0111] Comparative Example 5

[0112] It is basically the same as Example 2, except that an excessive amount of small molecule linear solvent is used in the electrolyte and the contents of EC and ethyl methyl carbonate are reduced.

[0113] Electrolyte preparation: The components are mixed and formulated according to the following mass ratios: lithium hexafluorophosphate 14.0 wt%, ethylene carbonate (EC) 12 wt%, methyl ethyl carbonate 5%, small molecule linear solvent (ethyl acetate) 61 wt%, vinylene carbonate (VC) 4 wt%, fluoroethylene carbonate (FEC) 3 wt%, ethylene sulfate 0.5 wt%, and lithium difluorobis(oxalato)phosphate 0.5 wt%.

[0114] Table 1 below provides the characteristic relationships between each embodiment and the comparative example in this application. All contents are mass fractions, based on the total mass of the electrolyte. W1 is the mass fraction of silicon in the negative electrode layer, M is the mass fraction of fluoroethylene carbonate, and VC is vinylene carbonate.

[0115] Table 1

[0116] To facilitate the testing of the performance differences between the embodiments and the comparative examples in the following three aspects, the test results of the embodiments and the comparative examples are provided in Table 2.

[0117] (I) Energy density calculation of battery cells:

[0118] At 25℃, the cells from the above schemes are charged to 3.65V at a constant current of 0.33C, and then discharged to 1.5V at a constant current of 0.33C. The rated discharge capacity and rated discharge voltage of the cells can then be obtained. By weighing the cells, the mass energy density can be calculated: Cell energy density = Cell capacity × Rated discharge voltage / Total cell mass.

[0119] (II) Room temperature cycling test:

[0120] The battery cells manufactured in the above embodiments and comparative examples were charged at 0.2C current in the 0-10% SOC range at 25℃ and 45℃ respectively, followed by fast charging at an average current of 2.2C in the 10%-80% SOC range, and then charged at a small current of 0.2C to 3.65V. After the battery cell was fully charged, it was allowed to rest for 10 minutes, and then discharged at a constant current of 1C to 2.0V. This cycle was repeated until the capacity retention rate of the battery cell was less than 80%, and the number of cycles was counted.

[0121] (III) High-Temperature Storage Test:

[0122] First, the battery is charged and discharged three times at 0.5C / 0.5C (3.65V to 2.5V) at room temperature. Then, the fully charged (DOD = 100SOC%) cells are stored in a 55℃ oven for 30 days. After being removed, the cells are left to stand at room temperature for 4 hours, and then discharged at 0.5C at room temperature. This process is repeated three times. The capacity of the last discharge is recorded as the recoverable capacity. The ratio of the recoverable capacity to the initial discharge capacity is the capacity recovery rate after 30 days of storage at 55℃.

[0123] Table 2

[0124] Based on the analysis in Table 2, this application achieves a cell energy density exceeding 200Wh / kg by using thick electrodes and high-pressure compaction design for both the positive and negative electrodes of the cell, resulting in low porosity, low electrolyte density, and small electrolyte injection volume. Furthermore, under a 2.2C fast charging regime, the capacity retention rate at room temperature for 2000 cycles is greater than or equal to 80%, and the capacity recovery rate at 55℃ for 30 days is greater than or equal to 95%.

[0125] Compared with Example 1, Example 2 has more high-capacity silicon-based materials mixed in the negative electrode, which can support the energy density of the battery cell to reach 205Wh / kg.

[0126] Compared to Example 1, Comparative Example 1 uses a carbonate electrolyte, which has relatively poor wetting ability. Therefore, it is necessary to reduce the areal density and compaction density of the positive and negative electrodes to ensure that the cell has good cycling capability and meets the cycling index of 2000 cycles at room temperature. However, the energy density of the cell can only meet 194Wh / kg.

[0127] Compared to Example 2, the FEC content in the electrolyte of Comparative Example 2 was only 1.5 wt%, which does not meet the requirement of ≥2 wt%, making it impossible to stably cycle the silicon-doped negative electrode and meet the project's requirement of 2000 cycles at room temperature. This indicates that using an appropriate amount of FEC is very important in high-silicon content systems.

[0128] Compared to Example 2, the electrolyte in Comparative Example 3 did not use lithium salt additives, and therefore could not meet the SOR (Self-Rating Orientation) requirement for 2000 cycles at room temperature. This indicates that inorganic lithium salt additives are crucial for achieving good cycling performance.

[0129] Compared to Example 2, the electrolyte in Comparative Example 4 contained insufficient amounts of VC and lithium salt additives, failing to meet the SOR (Self-Rating Orientation) requirement for 2000 cycles at room temperature. This indicates that the appropriate amount of negative electrode film-forming additive is crucial for achieving good cycle performance.

[0130] Compared to Example 2, Comparative Example 5 used an excessive amount of small molecule linear solvent (>60%) and had a low EC content, which led to a decrease in the conductivity of the electrolyte, deteriorating the battery's cycle performance and high-temperature calendar life.

[0131] Although embodiments of this application have been shown and described, those skilled in the art will understand that various changes, modifications, substitutions and alterations can be made to these embodiments without departing from the principles and spirit of this application, the scope of which is defined by the claims and their equivalents. Industrial applicability

[0132] This application improves the energy density of the battery cell by modifying the silicon-doped negative electrode material, using a thick electrode design, and adjusting the electrolyte formulation, while also considering the cell's cycle life and fast-charging capability. The lithium iron phosphate battery of this application exhibits a cell gravimetric energy density >200Wh / kg at a 0.33C charge / discharge rate and a 2000-cycle capability at room temperature under a 2.2C fast-charging regime (10%–80% SOC). The 2000-cycle capability refers to a capacity retention of ≥80% after 2000 cycles.

Claims

1. A lithium iron phosphate battery, comprising a positive electrode, a negative electrode, a separator, and an electrolyte, wherein the main material of the positive electrode comprises lithium iron phosphate, characterized in that, The negative electrode material includes graphite and silicon-based materials; The areal density ρ1 of the positive electrode is ≥ 23 mg / cm³. 2 The areal density ρ2 of the negative electrode is ≥ 7.8 mg / cm³. 2 ; The electrolyte comprises lithium salt, solvent, and additives. The solvent mainly comprises ethylene carbonate and small-molecule linear solvents. The additives include fluoroethylene carbonate and negative electrode film-forming additives. The negative electrode film-forming agent comprises vinylene carbonate and lithium salt additives. The small-molecule linear solvent is a carboxylic acid ester solvent, ether solvent, or nitrile solvent with a relative molecular mass ≤90. Based on the total mass of the electrolyte as 100%, the mass fraction of the small-molecule linear solvent is 35% to 60%, the mass fraction of the fluoroethylene carbonate is M, the mass fraction of silicon in the negative electrode layer is W1, 2% ≤ M ≤ 2 × W1, and the mass fraction of the negative electrode film-forming agent is 4% to 6%.

2. The lithium iron phosphate battery according to claim 1, characterized in that, The negative electrode includes a negative electrode current collector and a negative electrode layer disposed on the surface of the negative electrode current collector. The positive electrode includes a positive electrode current collector and a positive electrode layer disposed on the surface of the positive electrode current collector. The areal density ρ1 of the positive electrode, the areal density ρ2 of the negative electrode, the areal density ρ3 of the positive electrode current collector, and the areal density ρ4 of the negative electrode current collector satisfy the relationship: ρ1 / ρ3+ρ2 / ρ4>7.

5.

3. The lithium iron phosphate battery according to claim 1 or 2, characterized in that, The compaction density of the positive electrode is 2.58 g / cm³. 3 ~2.7g / cm 3 ; Preferably, the compaction density of the negative electrode is 1.55 g / cm³. 3 ~1.7g / cm 3 .

4. The lithium iron phosphate battery according to any one of claims 1 to 3, characterized in that, The small molecule linear solvent is at least one of ethyl acetate, methyl acetate, methyl propionate, ethylene glycol dimethyl ether, acetonitrile, or propionitrile.

5. The lithium iron phosphate battery according to any one of claims 1 to 4, characterized in that, Based on the total mass of the electrolyte (100%), the mass fraction of ethylene carbonate is 15% to 30%.

6. The lithium iron phosphate battery according to any one of claims 1 to 5, characterized in that, The solvent also includes at least one of dimethyl carbonate and / or ethyl methyl carbonate.

7. The lithium iron phosphate battery according to any one of claims 1 to 6, characterized in that, In the electrolyte, the lithium salt includes at least one of lithium hexafluorophosphate and lithium bis(fluorosulfonyl)imide; Preferably, the total mass fraction of the lithium salt is 10% to 18% based on the total mass of the electrolyte (100%).

8. The lithium iron phosphate battery according to any one of claims 1 to 7, characterized in that, The lithium salt additive is selected from at least one of lithium difluorooxalate borate, lithium difluorophosphate, lithium tetrafluoroborate or lithium difluorobis(oxalate) phosphate. Preferably, the mass fraction of the lithium salt additive is 0.5% to 1.5% based on the total mass of the electrolyte (100%).

9. The lithium iron phosphate battery according to any one of claims 1 to 8, characterized in that, The overall conductivity of the electrolyte is ≥14.5 mS / cm.

10. The lithium iron phosphate battery according to any one of claims 1 to 9, characterized in that, The mass fraction of silicon in the negative electrode layer, W1, is ≥2%, preferably 2% to 10%. Preferably, the silicon-based material is a silicon-carbon material.

Images