Preparation method of lithium ion battery composite negative electrode and application of lithium ion battery composite negative electrode in lithium battery

Abstract

The application provides a preparation method of a lithium ion battery composite negative electrode and application of the lithium ion battery composite negative electrode in a lithium battery. The composite negative electrode is mainly composed of a negative electrode active material, a low-melting-point polymer, a nano inorganic solid electrolyte capable of being lithiated, a conductive additive and a binder. In the preparation process, a layer of polymer is first coated on the surface of the negative electrode particles, and the nano inorganic solid electrolyte is introduced. The electrolyte exhibits ion and electron mixed conductivity after being lithiated, so as to further improve the ion and electron channels in the negative electrode sheet, effectively reduce the negative electrode polarization and improve the rate performance. Meanwhile, the lower lithium intercalation potential of the solid electrolyte can inhibit the lithium precipitation on the surface of the negative electrode in the fast charging and long cycle process of the lithium secondary battery, and the solid electrolyte after being lithiated can act as a lithium storage device and continuously release free lithium ions in the long cycle process, so as to prolong the service life of the lithium ion battery.

Description

TECHNICAL FIELD

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

[0002] With the rapid development of new energy vehicles and energy storage industries, the demand for high-performance power lithium batteries is growing. The anode has a decisive influence on the energy density, cycle performance, charge-discharge rate and low-temperature discharge performance of lithium ion batteries. The current commercialized negative electrode materials of lithium ion batteries mainly include carbon-based negative electrodes, silicon-based negative electrodes and lithium titanate, each of which has its own characteristics and advantages, but also has its own limitations. In addition to the properties of the active material itself, the structure of the negative electrode sheet is also crucial to the performance of the battery. By adding an appropriate amount of conductive additives and binders, an effective electron path can be constructed inside the negative electrode, and the penetration of electrolyte in the porous electrode can form a good ion path. Under the development requirement of high energy density, high active material loading and high compactness thick electrode is an inevitable trend. However, the ion and electron transport efficiency in thick electrode will be significantly affected. Thick electrode can cause limited electron transport, thereby increasing the charge transfer resistance. This will affect the electrochemical performance of the battery, especially at high rate charge and discharge. At the same time, due to the limitation of electron and ion transport, the lithiation state may be unevenly distributed within the electrode, and this heterogeneity can affect the stability and cycle life of the battery. Therefore, in order to ensure that lithium ions and electrons can quickly pass through the thick electrode, a continuous ion-electron conductive network needs to be constructed.

[0003] In the research and development of lithium ion battery negative electrode materials, coating modification is a very common means. Among them, the solid electrolyte is widely used in negative electrode coating due to its high ion conductivity. Patent CN118507695A proposes a solid electrolyte coated graphite negative electrode material and its preparation method and application, and reports that the solid electrolyte containing Ti has ion and electron conductivity, and coating on the surface of graphite through liquid phase deposition can improve the rate performance and lithium precipitation problem of graphite material. However, wet coating often needs to be combined with high temperature sintering and other post-processing, and it is also easy to cause impurity in the coating layer. At the same time, the cost of wet process is relatively high, and the industrial application prospect is poor. Dry coating has strong practicability, but the uniformity of coating is difficult to guarantee, which affects the electrochemical performance of the material, especially the rate performance. Therefore, it is necessary to develop a simple, efficient and economical solid electrolyte coating method to improve the comprehensive electrochemical performance of the negative electrode. SUMMARY

[0004] In order to improve the electrochemical performance of the lithium ion battery negative electrode, the present application proposes that the nano-state inorganic solid electrolyte which can be lithiated is coated on the surface of the negative electrode active material by the modified solid phase stirring method, first a layer of low melting point polymer is constructed on the surface of the negative electrode material, and the heat generated by mechanical friction in the planetary stirring process is used to make the polymer on the surface of the negative electrode melt, so as to enhance the adsorption of the inorganic nano solid electrolyte particles, so as to obtain the negative electrode material coated uniformly, and the composite negative electrode sheet prepared by this method can form an ion electron conductive network in its interior after lithiation, so as to improve the large rate charge and discharge performance of the lithium ion negative electrode. At the same time, since the lithiated solid electrolyte still has lithium storage space, the risk of lithium precipitation of the negative electrode can be reduced, and lithium ions can be continuously released during the cycle process to supplement the lack of free lithium ions, and the safety performance and cycle life of the lithium ion battery are improved.

[0005] A preparation method of a lithium ion battery composite negative electrode, comprising the following steps:

[0006] (S1) adding the negative electrode active material into a polymer solution with a melting point lower than 180℃, stirring until the solvent is completely evaporated, to obtain the negative electrode active material coated with the polymer;

[0007] (S2) mixing the negative electrode active material coated with the polymer with the nano inorganic solid electrolyte which can be lithiated, and obtaining the negative electrode material uniformly coated with the solid electrolyte after planetary stirring;

[0008] (S3) preparing the negative electrode material uniformly coated with the solid electrolyte into a slurry with the binder, the conductive additive and the solvent, coating on the current collector, and drying to obtain the negative electrode sheet;

[0009] (S4) lithiating the prepared negative electrode sheet to reduce the solid electrolyte to obtain the product lithium ion battery composite negative electrode.

[0010] Further, in step (S1), the polymer with a melting point lower than 180℃ is selected from one or more of polyacrylic acid, polyvinylidene fluoride, polyethylene oxide and polyacrylonitrile; and the concentration of the polymer solution is 0.1-0.5 mol / L.

[0011] Further, in step (S1), the addition amount of the low melting point polymer is 0.2-1wt% of the mass of the negative electrode active material, preferably 0.5-1wt%; and the negative electrode active material is one or a combination of artificial graphite, natural graphite, silicon monoxide, silicon, etc.

[0012] Further, in step (S2), the inorganic solid electrolyte material is a lithium ion conductor material that can react with metal lithium, and is further preferably one or more of lithium titanium phosphate, lithium germanium aluminum phosphate, and lithium titanium oxide; the particle size of the inorganic solid electrolyte is 5-100 nm, and is preferably 10-50 nm.

[0013] Further, in step (S2), the amount of the solid electrolyte is 1-5 wt% of the surface-coated polymer negative active material, and is preferably 2-4 wt%.

[0014] Further, in step (S2), the rotation speed of the planetary stirring is 2000-3000 rpm; the planetary stirring time is 1-2 hours; and the ball-to-material ratio used in the planetary stirring is 10-20:1. The rotation speed of the planetary stirring cannot be too low, otherwise the generated heat can not reach the melting point of the polymer, and the purpose of uniformly coating the negative active material with the polymer cannot be achieved; the rotation speed of the planetary stirring also cannot be too high, otherwise the surface and bulk structure of the negative active material can be damaged.

[0015] Further, in step (S3), the binder is at least one selected from PVDF, CMC, and PAA; the conductive additive is at least one selected from acetylene black, Super P, and carbon tubes; and the solvent is at least one selected from water, N-methyl pyrrolidone, and N,N-dimethylformamide. The mass ratio of the solid electrolyte uniformly coated negative active material, the binder, and the conductive additive is 80-95:2-10:2-10; and the amount of the solvent is such that the solid content of the slurry is 25-30 wt%.

[0016] Further, in step (S4), the lithiation method is electrochemical prelithiation or chemical prelithiation; the electrochemical prelithiation is completed by loading a lithium supplement additive during the assembly of the battery and during the formation of the battery; the lithium supplement additive can be selected from common positive and negative lithium supplement materials; and the chemical prelithiation is mainly through soaking in a polycyclic aromatic lithium solution.

[0017] Further, in step (S4), in the electrochemical prelithiation, the lithium supplement additive is at least one selected from lithium foil and lithium powder, and the amount of the lithium supplement additive is 2-5% of the designed capacity of the negative electrode; in the chemical prelithiation, the polycyclic aromatic lithium is at least one selected from naphthyl lithium and biphenyl lithium, the concentration of the polycyclic aromatic lithium solution is 1 mol / L, and the solvent is at least one selected from tetrahydrofuran, ethylene glycol dimethyl ether, and tetraethylene glycol dimethyl ether.

[0018] The application also provides a lithium secondary battery comprising the lithium ion battery composite negative electrode prepared by the preparation method.

[0019] This invention achieves uniform coating of solid electrolyte on the surface of negative electrode material in a simple and efficient manner through a modified solid-phase method, further expanding the new application of inorganic solid electrolyte in lithium battery negative electrodes. It effectively reduces the charge transport resistance of the negative electrode by utilizing the ionic-electronic mixed conductivity of its lithiation products, and significantly improves the overall electrochemical performance of lithium-ion battery negative electrodes. Attached Figure Description

[0020] Figure 1 SEM image of LATP coating on graphite surface in Example 1;

[0021] Figure 2 SEM image of LATP coating on the graphite surface in Comparative Example 1;

[0022] Figure 3 SEM image of LATP coating on the graphite surface in Comparative Example 2;

[0023] Figure 4 Comparison chart of 4C charging and -0.1C discharging curves of lithium-ion batteries in Example 1 and Comparative Example 2;

[0024] Figure 5 Comparison of the cycle performance of lithium-ion batteries in Example 1 and Comparative Example 2. Detailed Implementation

[0025] The present invention will be further described below with reference to specific embodiments.

[0026] Unless otherwise specified, the experimental methods described in the following examples are conventional methods; the reagents and materials are all commercially available.

[0027] Example 1

[0028] (I) Preparation of composite graphite anode sheets

[0029] (S1) Dissolve 0.5 wt% (based on graphite mass) of polyethylene oxide (PEO, melting point about 65°C) in deionized water to obtain a polymer solution with a mass concentration of 0.1 mol / L. Add graphite powder to the polymer solution and heat and stir until the solvent is completely evaporated to obtain a negative electrode active material with polymer coating on the surface.

[0030] (S2) The surface-coated polymer negative electrode active material is combined with Li particles with a particle size of approximately 10 nm. 1.4 Al 0.4 Ti 1.6 (PO4)3(LATP) was mixed at a mass ratio of 98:2 and stirred at 2000 rpm for 2 hours to obtain LATP-coated graphite powder.

[0031] (S3) The LATP-coated graphite powder, Super P, and PAA are mixed in a deionized water solvent at a mass ratio of 93:2:5 and slurryed to prepare a slurry with a solid content of 25wt%. The obtained slurry is coated onto copper foil and dried at 60°C for 12 hours. Then it is cut into 10mm round sheets to obtain the graphite negative electrode sheet.

[0032] (S4) In a glove box, the graphite negative electrode sheet is brought into contact with the lithium metal foil, and a small amount of electrolyte is added to promote its lithiation. After reacting for 10 hours, the excess lithium metal is removed, and the composite graphite negative electrode sheet is obtained after cleaning and drying with dimethyl carbonate solvent.

[0033] (II) Lithium-ion secondary battery assembly

[0034] In an argon-atmosphere glove box, the composite graphite negative electrode obtained in (I) and the lithium iron phosphate positive electrode were assembled into 2032 button cells with an N / P ratio of 1.05 for testing. The electrolyte used was 1 mol / L lithium hexafluorophosphate (LiPF6) dissolved in EC / DEC / DMC (volume ratio 1:1:1), with an addition amount of 50 μL. The separator used was Celgard PP / PE / PP separator.

[0035] (III) Performance Testing

[0036] (3.1) Rate performance test: The obtained batteries were tested for charge and discharge at different rates on the Blue Battery Tester. The voltage range was set to 2.5 to 4.2V, the charging rate was 0.1C, 0.2C, 0.5C, 1C, 2C, 4C (1C = 170mAh / g), and the discharge rate was 0.1C. The batteries were cycled 5 times in the order of charging first and then discharging at each rate.

[0037] (3.2) Cyclic performance test: The obtained battery was subjected to constant current charge-discharge cycle test on the Blue Battery Tester. The voltage range was set to 2.5 to 4.2V. The battery was first cycled 3 times at 0.1C, and then 100 times at 1C.

[0038] Example 2

[0039] All other conditions are the same as in Example 1, except that:

[0040] In step (S1), the amount of PEO added is changed to 0.2 wt% of graphite powder.

[0041] Example 3

[0042] All other conditions are the same as in Example 1, except that:

[0043] In step (S1), the amount of PEO added is changed to 1 wt% of graphite powder.

[0044] Example 4

[0045] All other conditions are the same as in Example 1, except that:

[0046] In step (S1), PEO is replaced with an equal mass of polyacrylic acid (PAA).

[0047] Example 5

[0048] All other conditions are the same as in Example 1, except that:

[0049] In step (S2), the mass ratio of graphite to LATP is changed to 99:1.

[0050] Example 6

[0051] All other conditions are the same as in Example 1, except that:

[0052] In step (S2), the mass ratio of graphite to LATP is changed to 95:5.

[0053] Example 7

[0054] All other conditions are the same as in Example 1, except that:

[0055] In step (S2), LATP is replaced with an equal mass of lithium titanate Li4Ti5O 12 (LTO).

[0056] Example 8

[0057] All other conditions are the same as in Example 1, except that:

[0058] In step (S2), the LATP particle size is changed to 50 nm.

[0059] Example 9

[0060] All other conditions are the same as in Example 1, except that:

[0061] In step (S2), the planetary mixer speed is changed to 3000 rpm.

[0062] Example 10

[0063] All other conditions are the same as in Example 1, except that:

[0064] In step (S2), the planetary mixer speed is changed to 1500 rpm.

[0065] Example 11

[0066] All other conditions are the same as in Example 1, except that:

[0067] In step (S2), the planetary mixer speed is changed to 5000 rpm.

[0068] Comparative Example 1

[0069] All other conditions are the same as in Example 1, except that:

[0070] Step (S1) is canceled, meaning that the graphite is not treated with the polymer solution, and the graphite powder and LATP powder are directly mixed in a planetary manner.

[0071] Comparative Example 2

[0072] The assembly and performance testing steps for the lithium secondary battery are the same as in Example 1, except that step (i) is changed as follows:

[0073] Lithium hydroxide, aluminum nitrate nonahydrate, tetrabutyl titanate, and ammonium dihydrogen phosphate were added to ethanol in a molar ratio of 1.4:0.4:1.6:3 and stirred until homogeneous. Graphite powder was then added according to the set ratio of Li... 1.4 Al 0.4 Ti 1.6 (PO4)3 (LATP) was added to the solution at a mass ratio of 98:2. After stirring at 80°C for 2 hours, the solvent was evaporated to obtain graphite powder coated with LATP precursor. The graphite anode material coated with 2wt% LATP was obtained by heating in argon at 650°C for 4 hours.

[0074] The electrochemical performance of the lithium-ion batteries assembled with the negative electrodes of the above embodiments and comparative examples was tested, and the results are shown in Table 1 below.

[0075] Table 1 Experimental conditions and corresponding electrochemical performance

[0076]

[0077]

[0078] As can be seen from the above embodiments, this invention improves the electrochemical performance of the negative electrode by coating an inorganic solid electrolyte onto the surface of the lithium-ion battery negative electrode material and then lithiating it. By adjusting the solid electrolyte material and particle size, coating method, coating amount, activation method, and other conditions, the capacity improvement and cycle stability of the lithium secondary battery at high rates are achieved. The following patterns can be observed from the data in Table 1:

[0079] 1) After pre-coating the graphite surface with polymer, uniform coating of solid electrolyte can be obtained by mechanical stirring. Figure 1 Compared to Comparative Example 1, direct solid-phase stirring of untreated graphite with LATP easily leads to uneven dispersion of LATP. Figure 2This indicates that by pre-constructing a low-melting-point polymer layer, the LATP nanoparticles can be more uniformly adsorbed during the molten state of the polymer in planetary stirring, thereby forming a more uniform solid electrolyte coating layer and further improving the electrochemical performance of the negative electrode. Compared to Comparative Example 2, the LATP distribution on the graphite surface coated by wet precipitation is uneven (…). Figure 3 This is because the precipitation process of metal ions is not completely controllable and is greatly affected by the surface properties of graphite. This indicates that the modified dry coating can effectively regulate the surface properties of graphite and achieve more uniform LATP coating.

[0080] 2) After coating the graphite surface with LATP, the kinetic performance of the negative electrode was improved. In Example 1, the discharge specific capacity of the lithium iron phosphate / graphite full cell at 0.1C under 4C charging increased to 131 mAh / g. Figure 4 This is because the lithium-ionized nano-inorganic solid electrolyte constructs a better ion-electron conductivity network in the negative electrode, reducing charge transfer resistance, thereby suppressing lithium plating and achieving higher capacity.

[0081] 3) The cycling stability of full cells assembled using activated LATP-coated graphite anodes was also improved, such as... Figure 5 As shown, the full cell of Example 1 retains 95.9% of its capacity after 100 cycles at 1C. This is because the lithiated inorganic solid electrolyte can act as a lithium storage medium. During full cell cycling, as polarization increases, the negative electrode potential rises, and lithium in the solid electrolyte is slowly released to replenish the lost free lithium ions, thereby improving cycle stability.

[0082] 4) Comparing the results of Examples 1, 2 and 3, it can be seen that the rate performance and cycle stability of the lithium secondary battery first increase and then decrease with the increase of PEO addition. This is because too little PEO cannot completely coat the graphite surface, resulting in uneven coating of LATP in the later stage, while too much PEO will reduce the proportion of negative electrode active material and also affect the charge transport on the graphite surface, resulting in a reduction in the actual capacity of the negative electrode.

[0083] 5) Comparing the results of Examples 1 and 4, it can be seen that, compared with PAA, the lithium secondary battery assembled after coating with PEO-treated graphite has better rate performance and cycle stability. This is because PEO has a lower melting point and a better mechanical melting effect, which can promote the formation of a more uniform LATP coating effect.

[0084] 6) Comparing the results of Examples 1, 5 and 6, it can be seen that the rate performance and cycle stability of lithium secondary batteries first increase and then decrease with the increase of LATP coating amount. This is because if the LATP particle size is too small, it cannot form an effective three-dimensional ion / electron conductive network, while if there is too much LATP, it will reduce the proportion of negative electrode active material, resulting in a decrease in the actual capacity of the negative electrode.

[0085] 6) Comparing the results of Examples 1, 9, 10, and 11, it can be seen that the planetary stirring speed should be moderate. Too high or too low a speed will not allow the electrochemical performance of the composite anode material to be fully utilized. If the speed is too low, the polymer melting point may not be reached, or the coating effect may be poor; if the speed is too high, the structure of the anode active material will be damaged to a certain extent, resulting in a decrease in capacity and cycle performance.

[0086] 8) Comparing with Examples 1 and 7, coating the graphite surface with LATP and LTO can improve the rate performance and cycle stability of lithium secondary batteries. Compared with LTO, LATP coating has the best improvement effect. This is because LATP has a higher ionic conductivity and the lithiation product has a higher electronic conductivity. At the same time, it can induce the formation of a more stable negative electrode interface layer.

[0087] The present invention has been described in detail above with reference to specific embodiments and exemplary examples; however, these descriptions should not be construed as limiting the present invention. Those skilled in the art will understand that various equivalent substitutions, modifications, or improvements can be made to the technical solutions and embodiments of the present invention without departing from the spirit and scope of the invention, and all such modifications and improvements fall within the scope of the present invention. The scope of protection of the present invention is defined by the appended claims.

Claims

1. A method for preparing a lithium-ion battery composite anode, characterized in that, Comprising the following steps: (S1) adding a negative electrode active material to a polymer solution with a melting point below 180℃, stirring until the solvent evaporates completely to obtain a polymer-coated negative electrode active material on the surface; (S2) mixing the polymer-coated negative electrode active material with a nano inorganic solid electrolyte that can be lithiated, and after planetary stirring, obtaining a solid electrolyte uniformly coated negative electrode material; (S3) preparing a slurry of the solid electrolyte uniformly coated negative electrode material, a binder, a conductive additive, and a solvent, coating on a current collector, and drying to obtain a negative electrode sheet; (S4) subjecting the prepared negative electrode sheet to lithiation to reduce the solid electrolyte to obtain a product lithium ion battery composite negative electrode.

2. The production method according to claim 1, characterized by, In step (S1), the polymer with a melting point below 180℃ is selected from one or more of polyacrylic acid, polyvinylidene fluoride, polyethylene oxide, and polyacrylonitrile; the concentration of the polymer solution is 0.1-0.5 mol / L.

3. The preparation method according to claim 1, characterized in that, In step (S1), the amount of low-melting-point polymer added is 0.2-1 wt% of the mass of the negative electrode active material; the negative electrode active material is one or a combination of artificial graphite, natural graphite, silicon monoxide, and silicon.

4. The method of claim 1, wherein, The amount of low-melting-point polymer added is 0.5-1 wt% of the mass of the negative electrode active material.

5. The preparation method according to claim 1, characterized in that, In step (S2), the inorganic solid electrolyte material is a lithium ion conductor material that can react with metallic lithium; the particle size of the inorganic solid electrolyte is 5-100 nm.

6. The method of claim 1, wherein, In step (S2), the inorganic solid electrolyte material is one or more of lithium titanium aluminum phosphate, lithium germanium aluminum phosphate, and lithium titanium oxide; the particle size of the inorganic solid electrolyte is 10-50 nm.

7. The preparation method according to claim 1, characterized in that, In step (S2), the amount of solid electrolyte is 1-5 wt% of the polymer-coated negative electrode active material.

8. The preparation method according to claim 7, characterized in that, The amount of solid electrolyte is 2-4 wt% of the polymer-coated negative electrode active material.

9. The method of claim 1, wherein, In step (S2), the rotational speed of the planetary stirring is 2000-3000 rpm; the planetary stirring time is 1-2 hours; the ball-to-material ratio used for planetary stirring is 10-20:

1.

10. The method of claim 1, wherein, In step (S3), the binder is selected from at least one of PVDF, CMC, and PAA; the conductive additive is selected from at least one of acetylene black, Super P, and carbon nanotubes; the solvent is selected from at least one of water, N-methyl pyrrolidone, and N,N-dimethylformamide.

11. The method of claim 10, wherein, The mass ratio of the solid electrolyte uniformly coated negative electrode material, the binder, and the conductive additive is 80-95:2-10:2-10; the amount of solvent is such that the solid content of the slurry is 25-30 wt%.

12. The method of claim 1, wherein, In step (S4), the lithiation method is electrochemical prelithiation or chemical prelithiation; electrochemical prelithiation is achieved by loading a lithium supplement additive during battery assembly and activating during battery formation; Chemical prelithiation is achieved by soaking in a polycyclic aromatic hydrocarbon lithium solution.

13. The method of claim 12, wherein, In the electrochemical prelithiation, the lithium supplement additive is at least one of lithium foil and lithium powder, and the amount of the lithium supplement additive is 2-5% of the designed capacity of the negative electrode; in the chemical prelithiation, the polycyclic aromatic lithium is at least one of naphthyl lithium and biphenyl lithium, the concentration of the polycyclic aromatic lithium solution is 1 mol / L, and the solvent is at least one of tetrahydrofuran, ethylene glycol dimethyl ether and tetraethylene glycol dimethyl ether. 14.A lithium secondary battery comprising the lithium ion battery composite negative electrode prepared by the preparation method of any one of claims 1-13.

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