Negative electrode, preparation method thereof and battery

Abstract

In order to reduce the volume expansion rate of a silicon-based negative electrode and improve the capacity retention ratio and the first efficiency of a battery, the invention provides a negative electrode, a preparation method thereof and a battery, the negative electrode comprises a current collector, an active material layer and a functional coating, the active material layer is arranged on at least one surface of the current collector, and the functional coating is arranged on the active material layer. The functional coating is arranged on one surface, deviating from the current collector, of the active substance layer; the active substance layer comprises an active substance, and the active substance contains a silicon element; and the functional coating comprises a polystyrene-polyoxyethylene block copolymer.

Description

TECHNICAL FIELD

[0001] The application belongs to the technical field of lithium ion battery negative electrode materials, and particularly relates to a negative electrode and a preparation method thereof and a battery. BACKGROUND

[0002] In recent years, silicon-based negative electrode materials have attracted extensive attention due to their high theoretical specific capacity, low lithium intercalation / deintercalation potential, rich raw material sources and environmental friendliness. However, the silicon-based negative electrode materials have the following technical defects: (1) Volume expansion: the silicon material undergoes Li-Si alloying reaction during lithiation / delithiation, resulting in a volume expansion rate of more than 300%. The volume expansion problem leads to electrode structure damage: cracks and pulverization of silicon particles during the cycle process, resulting in the separation of active materials and the current collector; unstable SEI film: the volume change leads to repeated rupture and regeneration of the SEI film, consuming active lithium (thickness increases by about 200%) and increasing the interfacial impedance (>50%); and destruction of the conductive network: the expansion stress increases the contact resistance between the conductive agent and the silicon particles (typical value >100Ω·cm).

[0003] (2) Conductivity defects: the electrical conductivity of silicon material (1×10 -3 S / cm) is 5 orders of magnitude lower than that of graphite (1×10² S / cm), resulting in an increase in electrode polarization voltage (>0.3V) and a decrease in rate performance (2C capacity retention rate <50%).

[0004] (3) Low initial efficiency: the amount of lithium ions consumed in the formation of SEI film during the first cycle is 20-30%, and the typical initial efficiency is 70-75%, which is significantly lower than that of graphite negative electrode (>90%). SUMMARY

[0005] The purpose of the present application is to provide a negative electrode and a preparation method thereof and a battery, so as to reduce the volume expansion rate of the silicon-based negative electrode and improve the capacity retention rate and the initial efficiency of the battery.

[0006] The technical solution adopted by the present application to solve the above technical problems is as follows: In a first aspect, the present application provides a negative electrode, comprising a current collector, an active material layer and a functional coating, the active material layer being arranged on at least one side of the current collector, and the functional coating being arranged on the side of the active material layer away from the current collector. The active material layer comprises an active material containing silicon element, and the functional coating comprises polystyrene-polyethylene oxide block copolymer.

[0007] Optionally, the mass ratio of polystyrene units to polyethylene oxide units in the polystyrene-polyethylene oxide block copolymer is (1:4) to (4:1).

[0008] Optionally, the mass ratio of polystyrene units to polyethylene oxide units in the polystyrene-polyoxyethylene block copolymer is (1:1) to (1:1.5).

[0009] Optionally, the molecular weight range of the polystyrene units in the polystyrene-polyoxyethylene block copolymer is 5k to 50kDa.

[0010] Optionally, the molecular weight range of the polyethylene oxide units in the polystyrene-polyoxyethylene block copolymer is 5k to 50kDa.

[0011] Optionally, the thickness of the active material layer is 20~100μm; the thickness of the functional coating is 1~5μm.

[0012] Optionally, the volume swelling degree of the functional coating is less than 20%.

[0013] Secondly, the present invention provides a method for preparing a negative electrode, comprising the following steps: A solution containing a functional coating is prepared, the solution is coated onto the active material layer, and the negative electrode is obtained through post-processing.

[0014] Optionally, the solution is coated onto the active material layer and then dried to obtain the negative electrode. The concentration of the solution is 5% to 15%, and the coating speed is 0.5 to 2 m / min. The drying is vacuum drying, with a drying temperature range of 60℃ to 100℃ and a drying time range of 6h to 24h. Alternatively, the solution is spin-coated onto the active material layer and then annealed to obtain the negative electrode. The concentration of the solution is 2% to 8%, the coating speed is 2000 to 5000 rpm, the annealing temperature range is 80℃ to 120℃, and the annealing time range is 5 min to 20 min.

[0015] Thirdly, the present invention provides a battery comprising the aforementioned negative electrode.

[0016] In this invention, a functional coating is constructed on the active material layer on the surface of a silicon-based anode using a polystyrene-ethylene oxide (PS-b-PEO) block copolymer. The polyethylene oxide (PEO) segments provide lithium-ion transport channels, resulting in an ionic conductivity of >2×10⁻⁶ for the functional coating. -5 The S / cm polystyrene (PS) segments form a mechanical support network, resulting in an elastic modulus >1 GPa for the functional coating. By setting this functional coating on the surface of the negative electrode active material layer to cover and bind the negative electrode active material, the volume expansion rate of the negative electrode of the present invention can be reduced to 10%. By setting this functional layer, the capacity retention rate of the battery can reach 86% after 100 cycles, and the first efficiency is improved to over 76%. Detailed Implementation

[0017] In order to make the technical problems, technical solutions and beneficial effects solved by the present application clearer, the present application will be further described in detail below in combination with embodiments. It should be understood that the specific embodiments described herein are only used to explain the present application and not to limit the present application.

[0018] An embodiment of the present application provides a negative electrode, comprising a current collector, an active material layer and a functional coating, the active material layer is arranged on at least one side of the current collector, and the functional coating is arranged on a side of the active material layer away from the current collector; the active material layer comprises an active material, the active material contains silicon elements; and the functional coating comprises a block copolymer.

[0019] Specifically, a polystyrene-polyethylene oxide (PS-b-PEO) block copolymer is used to construct a functional coating on the active material layer of the surface of a silicon-based negative electrode, the chemical structural formula of the copolymer is -[CH2CH(C6H5)] m -[CH2CH2O] n -. Among them, the polyethylene oxide (PEO) segment provides a lithium ion transmission channel so that the ionic conductivity of the functional coating is > 2 x 10 -5 S / cm, and the polystyrene (PS) segment forms a mechanical support network so that the elastic modulus of the functional coating is > 1 GPa. By arranging the functional coating on the surface of the negative electrode active material layer to cover and bind the negative electrode active material, the volume expansion rate of the negative electrode of the present application can be reduced to 10%. By arranging the functional layer, the capacity retention rate after 100 cycles of the battery can reach 86%, and the initial efficiency can be increased to more than 76%.

[0020] Further, the active material includes but is not limited to silicon particles, silicon carbon or silicon oxygen.

[0021] In some embodiments, the mass ratio of the polystyrene unit and the polyethylene oxide unit in the polystyrene-polyethylene oxide block copolymer is (1:4) to (4:1).

[0022] Specifically, the mass ratio of the polystyrene unit and the polyethylene oxide unit will affect the ionic conductivity and the elastic modulus of the functional coating. When the proportion of the polyethylene oxide unit is high, the ionic conductivity of the functional coating is high, but the swelling degree of the functional coating will increase, which affects the cycle life of the battery. On the contrary, if the proportion of the polystyrene unit is high, the ionic conductivity of the functional coating is low, the contact resistance increases, and the rate performance decreases. When the mass ratio of the polystyrene unit and the polyethylene oxide unit is in the range of (1:4) to (4:1), the volume expansion rate of the silicon-based negative electrode is reduced, the capacity retention rate and the initial efficiency of the battery are good, and the overall performance of the battery is improved.

[0023] The mass ratio of polystyrene units to polyethylene oxide units includes, but is not limited to, 1:4, 1:3, 2:3, 2:1, or 4:1.

[0024] To further improve the ionic conductivity and elastic modulus of the negative electrode, in a preferred embodiment, the mass ratio of polystyrene units to polyethylene oxide units in the polystyrene-polyethylene oxide block copolymer is (1:1)~(1:1.5).

[0025] The mass ratio of polystyrene units to polyethylene oxide units includes, but is not limited to, 1:1, 1:1.3, or 1:1.5.

[0026] In some embodiments, the molecular weight of the polystyrene units in the polystyrene-polyethylene oxide block copolymer ranges from 5k to 50kDa.

[0027] Specifically, a lower molecular weight of the polystyrene units affects the mechanical strength, and a higher molecular weight reduces the ionic conductivity.

[0028] The molecular weight of the polystyrene units includes, but is not limited to, 5kDa, 10kDa, 22kDa, 35kDa, 40kDa, or 50kDa, etc.

[0029] In some embodiments, the molecular weight of the polyethylene oxide units in the polystyrene-polyethylene oxide block copolymer ranges from 5k to 50kDa.

[0030] The molecular weight of the polyethylene oxide units includes, but is not limited to, 5kDa, 18kDa, 25kDa, 38kDa, or 50kDa, etc.

[0031] Specifically, a lower molecular weight of the polyethylene oxide units reduces the ionic conductivity, and a higher molecular weight leads to a decrease in the mechanical strength.

[0032] In some embodiments, the thickness of the active material layer is 20~100μm; and the thickness of the functional coating layer is 1~5μm.

[0033] Specifically, when the functional coating layer is thin, the lithium ion transmission is less hindered, and the rate performance is better, but the thinness leads to an intensified side reaction, poor interface stability, and a higher expansion rate of the silicon-based negative electrode; on the contrary, when the functional coating layer is thick, the expansion rate of the silicon-based negative electrode is lower, but the high interface impedance leads to a decrease in the rate performance.

[0034] In some embodiments, the volume swelling degree of the functional coating layer is less than 20%.

[0035] Specifically, the polystyrene in the polystyrene-polyethylene oxide block copolymer strengthens the polymer network strength and reduces the volume swelling degree of the functional coating layer.

[0036] An embodiment of the present application provides a preparation method of a negative electrode, comprising the following steps: A solution containing a functional coating layer is configured, the solution is coated on the active material layer, and the negative electrode is obtained through post-processing.

[0037] In some embodiments, the solution is scraped onto the active material layer, and then dried to obtain the negative electrode, the concentration of the solution is 5%-15%, the coating speed is 0.5-2 m / min, the drying is vacuum drying, the temperature range of the drying is 60-100 DEG C, and the time range of the drying is 6-24 h.

[0038] In some embodiments, the solution is spin-coated onto the active material layer, and then annealed to obtain the negative electrode, the concentration of the solution is 2%-8%, the coating speed is 2000-5000 rpm, the temperature range of the annealing is 80-120 DEG C, and the time range of the annealing is 5-20 min.

[0039] An embodiment of the present application provides a battery comprising the above negative electrode.

[0040] Specifically, the volume expansion rate of the negative electrode can be reduced to 10%, the capacity retention rate of the battery after 100 cycles can reach 86%, and the initial efficiency of the battery can be increased to more than 76%.

[0041] The present application is further described below through examples.

[0042] Specifically, the present application discloses a negative electrode, a preparation method thereof and a battery.

[0043] Example 1 The present application discloses a negative electrode, a preparation method thereof and a battery. Preparation of the negative electrode: silicon-carbon, butadiene rubber and sodium carboxymethyl cellulose are prepared into negative electrode slurry at a mass ratio of 98:1.5:0.5, coated on two surfaces of a copper foil, and then subjected to roll pressing after 95 DEG C baking; The polystyrene-polyethylene oxide block copolymer is dissolved in tetrahydrofuran to configure a solution with a concentration of 5%.

[0044] The solution is scraped at a coating speed of 1 m / min, and then vacuum dried at 80 DEG C for 2 h to obtain the negative electrode.

[0045] The mass ratio of the polystyrene unit to the polyethylene oxide unit in the polystyrene-polyethylene oxide block copolymer is 1:1, the molecular weight of the polystyrene unit is 5 kDa, the molecular weight of the polyethylene oxide unit is 5 kDa, the thickness of the functional coating layer is 1 mu m, and the thickness of the active material layer after roll pressing is 20 mu m.

[0046] Example 2 This example is used to illustrate the negative electrode and its preparation method and battery disclosed by the present application, most of the operation steps of Example 1, the difference is that: using spin coating coating.

[0047] Using spin coating coating, the solution concentration is 2%, the coating speed is 2000r / min, then 120℃ annealing for 15min, slitting, to obtain the negative electrode.

[0048] Example 3 This example is used to illustrate the negative electrode and its preparation method and battery disclosed by the present application, most of the operation steps of Example 1, the difference is that: the mass ratio of polystyrene unit and polyethylene oxide unit is 4:1.

[0049] Example 4 This example is used to illustrate the negative electrode and its preparation method and battery disclosed by the present application, most of the operation steps of Example 1, the difference is that: the mass ratio of polystyrene unit and polyethylene oxide unit is 1:4.

[0050] Example 5 This example is used to illustrate the negative electrode and its preparation method and battery disclosed by the present application, most of the operation steps of Example 1, the difference is that: the mass ratio of polystyrene unit and polyethylene oxide unit is 1:2.

[0051] Example 6 This example is used to illustrate the negative electrode and its preparation method and battery disclosed by the present application, most of the operation steps of Example 1, the difference is that: the mass ratio of polystyrene unit and polyethylene oxide unit is 1:1.5.

[0052] Example 7 This example is used to illustrate the negative electrode and its preparation method and battery disclosed by the present application, most of the operation steps of Example 1, the difference is that: the mass ratio of polystyrene unit and polyethylene oxide unit is 5:1.

[0053] Example 8 This example is used to illustrate the negative electrode and its preparation method and battery disclosed by the present application, most of the operation steps of Example 1, the difference is that: the mass ratio of polystyrene unit and polyethylene oxide unit is 1:5.

[0054] Example 9 This example is used to illustrate the negative electrode and its preparation method and battery disclosed by the present application, most of the operation steps of Example 1, the difference is that: the thickness of the functional coating is 5μm.

[0055] Example 10 This example is used to illustrate the negative electrode and the preparation method and battery disclosed in the present application, most of the operation steps of Example 1, the difference is that the thickness of the functional coating is 3 μm.

[0056] Example 11 This example is used to illustrate the negative electrode and the preparation method and battery disclosed in the present application, most of the operation steps of Example 1, the difference is that the thickness of the functional coating is 6 μm.

[0057] Example 12 This example is used to illustrate the negative electrode and the preparation method and battery disclosed in the present application, most of the operation steps of Example 1, the difference is that the thickness of the functional coating is 0.5 μm.

[0058] Example 13 This example is used to illustrate the negative electrode and the preparation method and battery disclosed in the present application, most of the operation steps of Example 1, the difference is that the molecular weight of the polystyrene unit is 30 kDa, and the molecular weight of the polyethylene oxide unit is 30 kDa.

[0059] Example 14 This example is used to illustrate the negative electrode and the preparation method and battery disclosed in the present application, most of the operation steps of Example 1, the difference is that the molecular weight of the polystyrene unit is 50 kDa, and the molecular weight of the polyethylene oxide unit is 50 kDa.

[0060] Comparative Example 1 This comparative example is used to illustrate the negative electrode and the preparation method and battery disclosed in the present application, most of the operation steps of Example 1, the difference is that the functional coating is not coated.

[0061] Comparative Example 2 This comparative example is used to illustrate the negative electrode and the preparation method and battery disclosed in the present application, most of the operation steps of Example 1, the difference is that the functional coating is coated between the current collector and the active material layer.

[0062] Comparative Example 3 This comparative example is used to illustrate the negative electrode and the preparation method and battery disclosed in the present application, most of the operation steps of Example 1, the difference is that the functional coating is a polystyrene coating.

[0063] Comparative Example 4 This comparative example is used to illustrate the negative electrode and the preparation method and battery disclosed in the present application, most of the operation steps of Example 1, the difference is that the functional coating is a polyethylene oxide coating.

[0064] Preparation of the battery: the above negative electrode, existing positive electrode, base electrolyte (EC:DEC=3:7) were assembled to prepare lithium ion battery.

[0065] Performance test method: (1) Swelling rate test: the thickness of the silicon-based negative electrode sheet to be tested was accurately tested, and a lithium metal electrode was assembled into a button cell. The silicon-based negative electrode sheet was discharged by 2 mAh / cm2to remove lithium using a battery charge-discharge instrument. 2 Cycles. The cycled battery was disassembled to obtain the electrode sheet, which was cleaned with DMC, then naturally dried in a glove box, and the thickness of the cleaned electrode sheet was measured. The swelling rate is the ratio of the increased thickness to the original thickness.

[0066] (2) Cycle performance test: ① 1C constant current charging to 50% SOC, then 0.65C constant current charging to 4.30V, and the cutoff current is 0.05C; ② 0.5C constant current discharging to 3.0V; steps ① and ② were cycled for 50 and 100 times, and the initial efficiency and capacity retention rate of the cycled battery were recorded; (3) Rate performance (2C) test: at 25°C, the battery prepared in the above examples and comparative examples was tested for discharge capacity at 2C rate.

[0067] (4) Swelling degree test: the sample to be tested was weighed and immersed in an EC:DEC=3:7 mixed solution, and was placed in a closed container at room temperature for 24h to allow it to swell fully. The swelled sample was taken out, the surface residual solvent was wiped clean and weighed again. The ratio of the increased weight to the original weight is the swelling degree.

[0068] Test results: The specific test results of the swelling rate test, cycle performance test and swelling degree test are shown in Table 1: Table 1 Specific test results of swelling rate test, cycle performance test and swelling degree test The specific test results of the battery initial efficiency test and rate performance test are shown in Table 2: Table 2 Specific test results of SEM analysis, initial efficiency test and rate performance test As can be seen from the data in Tables 1 and 2, the test results of Examples 1-8 show that when the mass ratio of polystyrene units to polyethylene oxide units ranges from (1:4) to (4:1), the volume expansion rate of the silicon-based negative electrode is reduced, the capacity retention rate and the first efficiency of the battery are good, and the overall performance of the battery is improved; when the mass ratio ranges from (1:1) to (1:1.5), the overall performance of the battery is optimal. Compared with Example 4, Example 3 has a higher proportion of polystyrene units, and the volume expansion rate of the silicon-based negative electrode is reduced, but the ionic conductivity of the functional coating is low, the contact resistance is increased, and the rate performance of the battery is reduced.

[0069] The test results of Examples 9-12 show that when the thickness of the functional coating is 1-5 μm, the volume expansion rate of the silicon-based negative electrode is low, and the capacity retention rate and the first efficiency of the battery are maintained at a high level; when the thickness is less than 1 μm, the thin thickness leads to an increase in side reactions, poor interface stability, and a high expansion rate of the silicon-based negative electrode; when the thickness is greater than 5 μm, high interface impedance leads to a reduction in rate performance.

[0070] The test results of Examples 13-14 show that when the molecular weight of polystyrene units and polyethylene oxide units ranges from 5k to 50kDa, the mechanical strength and ionic conductivity of the functional coating are good, which can reduce the volume expansion and structural damage of the silicon-based negative electrode, reduce the interface impedance, and improve the rate performance of the battery.

[0071] The above only describes the preferred embodiments of the present application and is not intended to limit the present application. Any modification, equivalent replacement, and improvement made within the spirit and principle of the present application shall be included in the protection scope of the present application.

Claims

1. A negative electrode, characterized by comprising: The negative electrode comprises a current collector, an active material layer arranged on at least one side of the current collector, and a functional coating arranged on a side of the active material layer away from the current collector. The active material layer comprises an active material containing silicon elements; and the functional coating comprises a polystyrene-polyethylene oxide block copolymer.

2. The negative electrode according to claim 1, characterized by The mass ratio of polystyrene units to polyethylene oxide units in the polystyrene-polyethylene oxide block copolymer is (1:4) to (4:1).

3. The negative electrode according to claim 2, characterized by The mass ratio of polystyrene units to polyethylene oxide units in the polystyrene-polyethylene oxide block copolymer is (1:1) to (1:1.5).

4. The negative electrode according to claim 1, wherein The molecular weight of polystyrene units in the polystyrene-polyethylene oxide block copolymer ranges from 5k to 50kDa.

5. The negative electrode according to claim 1, wherein The molecular weight of polyethylene oxide units in the polystyrene-polyethylene oxide block copolymer ranges from 5k to 50kDa.

6. The negative electrode according to claim 1, wherein The thickness of the active material layer is 20 to 100μm; and the thickness of the functional coating is 1 to 5μm.

7. The negative electrode according to claim 1, wherein The volume swelling degree of the functional coating is less than 20%.

8. The method of producing a negative electrode according to any one of claims 1 to 7, wherein The method comprises the following steps: A solution containing a functional coating is prepared, the solution is coated on the active material layer, and the negative electrode is obtained after post-processing.

9. The production method according to claim 8, characterized by, The solution is blade-coated on the active material layer and then dried to obtain the negative electrode, the concentration of the solution is 5% to 15%, and the coating speed is 0.5 to 2m / min. Alternatively, the solution is spin-coated on the active material layer and then annealed to obtain the negative electrode, the concentration of the solution is 2% to 8%, and the coating speed is 2000 to 5000rpm.

10. A battery, characterized by The negative electrode as claimed in any one of claims 1 to 9.

Images