Formation method of silicon-based material doped graphite lithium battery

Abstract

The invention relates to a formation method of a graphite lithium battery doped with a silicon-based material, which comprises the following steps: (1) charging to a cut-off voltage with low current and constant current, the cut-off voltage being not greater than 3.15 V; (2) carrying out constant-voltage charging at a cut-off voltage to 0.05 C, and cutting off; (3) charging to 3.4 V with large current and constant current; (4) charging at a constant voltage of 3.4 V, and stopping at 0.05 C; all the steps are carried out in a high-temperature negative-pressure state. According to the formation method of the graphite lithium battery doped with the silicon-based material, the material is charged and lithium is embedded by using small current, so that the film formation of a negative electrode and the expansion of the silicon-based material are facilitated, and various properties of the lithium battery are further improved.

Description

Technical Field

[0001] The invention belongs to the technical field of lithium ion batteries, and in particular relates to a formation method of a silicon-based graphite lithium battery. Background Art

[0002] The theoretical lithium storage capacity of silicon is as high as 4200mAh / g, which is more than 10 times that of commercial graphite-based negative electrode materials. It is a high-specific capacity negative electrode material with the most development potential. At present, most of the negative electrode materials in the market are doped with silicon-based materials based on artificial graphite as negative electrode materials for lithium-ion batteries. However, during the charging and discharging process, the volume expansion of silicon-based materials is nearly 300% due to Li-Si alloying, which can easily cause silicon particles to pulverize and lose activity. On the other hand, the poor electrical contact between the active particles and the current collector forms an "island effect", and new SEI films are repeatedly formed on the fracture surface, resulting in irreversible capacity loss of the material, low coulombic efficiency, and poor cycle performance.

[0003] At present, the formation method of graphite lithium batteries doped with silicon-based materials generally uses a gradient current from small to large to carry out formation charging in 2-3 steps. Some are terminated by charging time, and some are terminated by charging voltage. Different negative electrode materials have different lithium insertion potentials. For silicon-based materials, the larger the current, the more severe the expansion of the silicon-based materials will be, and even cause the material to fall off in the later stage, affecting the cycle life and various electrical properties. Summary of the invention

[0004] The purpose of the present invention is to provide a formation method for a silicon-based graphite lithium battery, which uses a small current to charge the material to insert lithium, which is beneficial to the film formation of the negative electrode and the expansion problem of the silicon-based material, and further improves the various performances of the lithium battery.

[0005] The technical solution adopted by the present invention to solve the above problems is: a formation method of a silicon-doped graphite lithium battery, comprising the following steps: (1) charging with a small constant current to a cut-off voltage, wherein the cut-off voltage is not greater than 3.05V; (2) Charge at a constant voltage until the cut-off voltage reaches 0.05C; (3) Charge to 3.4V with high constant current; (4) Charge at 3.4V constant voltage and 0.05C cut-off.

[0006] The above steps are all carried out under high temperature and negative pressure conditions.

[0007] Preferably, the small current in step (1) does not exceed 0.02C.

[0008] More preferably, the small current is 0.01C.

[0009] Preferably, the cut-off voltage is 3.05V. During the charging process, the potential of silicon-based materials for lithium insertion is higher than that of graphite materials, which means that lithium ions are more easily embedded in silicon grains. During the discharge process, the potential goes from low to high, and the graphite negative electrode first removes lithium ions, and silicon removes lithium ions later. Due to the characteristics of the material itself, the lithium insertion potential of graphite is about 0.05-0.1V, while the lithium insertion potential of silicon-based materials is about 0.4-0.45V. The potential of lithium iron phosphate is 3.4V, and the positive electrode potential minus the negative electrode potential is equal to the battery voltage, that is: 3.4-0.45=2.95V. Considering the polarization effect of the battery of 0.1V, before the formation charge to 3.05V, the silicon-based material begins to partially insert lithium, and the artificial graphite is basically not embedded with lithium. After 3.05V, a very small part of the artificial graphite is embedded with lithium, and the silicon-based material is partially embedded with lithium. After 3.2V, most of the artificial graphite begins to insert lithium, and the silicon-based material is partially embedded with lithium.

[0010] Preferably, the high current in step (3) does not exceed 0.5C.

[0011] Preferably, the high current is 0.2C-0.5C.

[0012] Preferably, the high temperature negative pressure state is specifically: temperature 45±3°C and pressure -90±5kpa.

[0013] Compared with the prior art, the advantages of the present invention are: The formation method of the silicon-based graphite lithium battery doped with the present invention, especially charging to 3.05V with a small current constant current, can not only form a dense and stable SEI film, which is beneficial to the improvement of electrical performance and cycle life, but also can reduce the expansion of the negative electrode, effectively prevent the silicon-based material from falling off, ensure the use of the silicon-based material's gram capacity, and improve the energy density of the battery. In addition, it can further reduce the impedance of the lithium battery, reduce the heat generated by the battery, and further improve the rate performance. BRIEF DESCRIPTION OF THE DRAWINGS

[0014] Figure 1 It is a cycle curve diagram of the lithium battery of Examples 1-2 of the present invention and the comparative example. DETAILED DESCRIPTION

[0015] The present invention is further described in detail below with reference to the accompanying drawings. Example 1

[0016] Take the 48173170-130Ah shell size as an example 1. Preparation before the test The raw materials of the positive electrode slurry are as follows: Defang Nano DY-1 lithium iron phosphate, binder PVDF900, superconducting graphite SP, N-methylpyrrolidone, and carbon nanotubes CNT.

[0017] Lithium iron phosphate (DY-1): PVDF900: SP: CNT = 95.5%: 2.5%: 1%: 1% (weight percentage) The raw materials of negative electrode slurry are as follows: Shanshan FSN-1 graphite, BYD silicon oxide material (450mah / g) superconducting graphite SP, binder CMC2200, water-based glue LA133, and deionized water.

[0018] Shanshan Graphite (FSN-1): Silicon-based material (BTR450): Water-based adhesive (LA133): SP: CMC2200 = 92.15%: 2.85%: 2.2%: 1.2%: 1%: 0.6% (weight percentage) Diaphragm: Cangzhou Mingzhu diaphragm 9+3*147mm (single-sided ceramic).

[0019] Electrolyte: Contains 1.2M lithium hexafluorophosphate LiPF6, and the solvent is a mixed solvent of ethyl methyl carbonate: dimethyl carbonate: ethylene carbonate = 1:1:1 (volume ratio).

[0020] 2. Make batteries The positive and negative electrode slurries were stirred in turn (viscosity of the positive electrode material was 8962 mpa.s, and viscosity of the negative electrode material was 7652 mpa.s), coated (single-sided surface density of the positive electrode coating was 186 g / m 2 , negative electrode coating surface density 90g / m 2 ), roller pressing (compacted density 2.5g / m 3 ), slitting, die-cutting, winding, baking (temperature 105°C, time 48h), liquid injection (liquid injection amount 620g), and high temperature standing for 48h to obtain the battery to be formed.

[0021] The formation method of the above battery comprises the following steps: (1) Charge to 3.05V at 0.01C constant current; (2) Charging at 3.05V constant voltage and 6.5A (0.05C) cut off; (3) 0.2C high current constant current charging to 3.4V; (4) Charge at 3.4V constant voltage and cut off at 6.5A (0.05C).

[0022] The above formation steps are all carried out under high temperature negative pressure conditions, with a temperature of 45°C and a pressure of -90kpa. Example 2

[0023] Take the 48173170-130Ah shell size as an example 1. Preparation before the test The specific raw materials of the positive electrode slurry are as follows: Defang Nano DY-1 lithium iron phosphate, binder PVDF900, superconducting graphite SP, N-methylpyrrolidone, and carbon nanotubes CNT.

[0024] Lithium iron phosphate (DY-1): PVDF900: SP: CNT = 95.5%: 2.5%: 1%: 1% (weight percentage) The raw materials of negative electrode slurry are as follows: Shanshan FSN-1 graphite, BYD silicon oxide material (450mah / g) superconducting graphite SP, binder CMC2200, water-based glue LA133, and deionized water.

[0025] Shanshan Graphite (FSN-1): Silicon-based material (BTR450): Water-based adhesive (LA133): SP: CMC2200 = 92.15%: 2.85%: 2.2%: 1.2%: 1%: 0.6% (weight percentage) Diaphragm: Cangzhou Mingzhu diaphragm 9+3*147mm (single-sided ceramic).

[0026] Electrolyte: Contains 1.2M lithium hexafluorophosphate LiPF6, and the solvent is a mixed solvent of ethyl methyl carbonate: dimethyl carbonate: ethylene carbonate = 1:1:1 (volume ratio).

[0027] 2. Make batteries The positive and negative electrode slurries were stirred in turn (viscosity of the positive electrode material was 8962 mpa.s, and viscosity of the negative electrode material was 7652 mpa.s), coated (single-sided surface density of the positive electrode coating was 186 g / m 2 , negative electrode coating surface density 90g / m 2 ), roller pressing (compacted density 2.5g / m 3 ), slitting, die-cutting, winding, baking (temperature 105°C, time 48h), liquid injection (liquid injection amount 620g), and high temperature standing for 48h to obtain the battery to be formed.

[0028] The formation method of the above battery comprises the following steps: (1) Charge to 3.05V at 0.02C constant current; (2) Charging at 3.05V constant voltage and 6.5A (0.05C) cut off; (3) 0.3C high current constant current charging to 3.4V; (4) Charge at 3.4V constant voltage and cut off at 6.5A (0.05C).

[0029] The above formation steps are all carried out under high temperature negative pressure conditions, with a temperature of 45°C and a pressure of -90kpa.

[0030] Comparative Example The only difference from Example 1 is that the formation method is different.

[0031] The specific steps of formation are: (1) Charge to 3.2V at 0.2C constant current; (2) Charge to 3.4V at 0.3C constant current; (4) Charge at 3.4V constant voltage and cut off at 6.5A (0.05C).

[0032] The lithium batteries of Examples 1-2 and the comparative example were subjected to high-temperature aging and capacity separation treatment to obtain finished lithium batteries. The internal resistance of the finished lithium batteries was tested, and the negative electrode sheets of the finished lithium batteries were disassembled after being fully charged to test the full-charge thickness of the negative electrode sheets and the rebound percentage of the negative electrode sheets.

[0033] After formation, the lithium battery is aged at high temperature for 48 hours and then processed for capacity division, as follows: (1) Charge at 0.3C constant current to 3.65V, and cut off at 0.05C constant voltage; (2) Discharge at a constant current of 0.3C to 2.5V; (3) Charge at a constant current of 0.5C to 3.65V, and then cut off at a constant voltage of 0.05C; (4) Discharge at 0.5C constant current to 2.5V; (5) Charge at 0.5C constant current to 3.4V.

[0034] Among them, full charge is specifically: charging to 3.65V at a constant current of 0.5C and cut off at a constant voltage of 0.05C.

[0035]

[0036] The finished lithium battery is tested for cycle life, specifically: Step 1: Let stand for 30 minutes; Step 2: 1P constant power charging to 3.65V; Step 3: Let stand for 30 minutes; Step 4: 1P constant power discharge to 2.5V; Step 5: Cycle steps 1-4 5000 times.

[0037] The lithium battery cycle curves of Examples 1-2 and Comparative Example are as follows: Figure 1 shown.

[0038] In addition to the above embodiments, the present invention also includes other implementation modes. Any technical solutions formed by equivalent transformation or equivalent replacement should fall within the protection scope of the claims of the present invention.

Claims

1. A formation method for a silicon-doped graphite lithium battery, characterized in that: The following steps are involved: (1) charging with a small constant current to a cut-off voltage, wherein the cut-off voltage is no greater than 3.15V; (2) Charge at a constant voltage until the cut-off voltage reaches 0.05C; (3) Charge to 3.4V with high constant current; (4) Charging at 3.4V constant voltage, 0.05C cut-off; The above steps are all carried out under high temperature and negative pressure conditions.

2. The formation method of the silicon-based graphite lithium battery according to claim 1, characterized in that: In step (1), the current should not exceed 0.02C.

3. The formation method of the silicon-based graphite lithium battery according to claim 2, characterized in that: The small current is 0.01C.

4. The formation method of the silicon-based graphite lithium battery according to claim 1, characterized in that: The cut-off voltage is 3.05V.

5. The formation method of the silicon-based graphite-doped lithium battery according to claim 1, characterized in that: The high current in step (3) does not exceed 0.5C.

6. The formation method of the silicon-based graphite-doped lithium battery according to claim 5, characterized in that: The high current is 0.2C-0.5C.

7. The formation method of the silicon-based graphite-doped lithium battery according to claim 1, characterized in that: The high temperature negative pressure state is specifically: temperature 45±3°C and pressure -90±5kpa.

Images