Additive-modified lithium manganate positive electrode sheet and preparation method thereof

Abstract

The application discloses an additive modified lithium manganate positive electrode sheet and a preparation method thereof. The preparation method introduces dipotassium hydrogen phosphate as a functional additive into a polyvinylidene fluoride (PVDF) binder system in the preparation process of the spinel lithium manganate positive electrode sheet, mixes the lithium manganate active material and a conductive agent, and finally obtains a uniformly dispersed positive electrode sheet. The application has the advantages of simple process, good compatibility with the existing positive electrode sheet preparation process, and the like. The dipotassium hydrogen phosphate is introduced into the binder system in the preparation process of the positive electrode sheet, so that the additive is uniformly distributed in the positive electrode and on the surface of the lithium manganate particles. On the one hand, the additive can optimize the electrode structure, form a stable interface layer on the surface of the positive electrode, and effectively inhibit the interface side reactions such as manganese dissolution; on the other hand, the presence of phosphate can help improve the diffusion kinetics of lithium ions. Under the synergistic effect of the interface stability and the ion conduction capacity, the cycle performance and the rate performance of the lithium manganate positive electrode prepared by the application are significantly improved.

Description

Technical Field

[0001] This invention relates to the field of lithium-ion battery technology, and more specifically to a method for preparing an additive-modified lithium manganese oxide composite cathode and a lithium battery. Background Technology

[0002] Lithium-ion batteries, with their advantages of high energy density, long cycle life, and low self-discharge rate, have become indispensable energy storage devices in modern society and are widely used in portable electronic devices, electric vehicles, and large-scale energy storage systems. The cathode material, as the core component of a lithium-ion battery, determines its performance and cost.

[0003] Currently, commonly used cathode materials include lithium cobalt oxide, lithium iron phosphate, and lithium manganese oxide. Among them, spinel-type lithium manganese oxide is considered one of the most promising cathode materials for lithium-ion batteries due to its abundant raw material resources, environmental friendliness, simple preparation method, and high safety. However, lithium manganese oxide cathodes suffer from severe capacity decay during cycling, especially under high-temperature conditions, which seriously restricts its further expansion. The reasons can be mainly attributed to two aspects: first, interfacial side reactions occur between lithium manganese oxide and the electrolyte, leading to manganese disproportionation and dissolution; second, under deep discharge conditions, the crystal structure of lithium manganese oxide undergoes Jahn-Teller distortion, resulting in structural instability.

[0004] Currently, the mainstream methods for modifying lithium manganese oxide include element doping, morphology control, and surface coating. Element doping involves incorporating metallic or non-metallic elements, such as Al, Cr, Ni, F, and S, into the lithium manganese oxide lattice to mitigate volume changes during lithium intercalation / deintercalation and suppress Jahn-Teller distortion. However, this method struggles to address manganese dissolution. Morphology control involves manipulating the particle shape, size, or proportion of exposed crystal faces during synthesis to reduce the contact area between the lithium manganese oxide grains and the electrolyte, thus mitigating interfacial side reactions. However, morphology control often comes at the cost of reduced tap density, leading to decreased energy density, and balancing rate performance and cycle stability is difficult. Surface coating is one of the most direct and effective strategies for suppressing manganese dissolution. Coating the lithium manganese oxide surface with a protective material, such as metal oxides, phosphates, carbon materials, or conductive polymers, isolates the material from direct contact with the electrolyte. However, the thickness and uniformity of the coating layer are difficult to control. While atomic layer deposition (ALD) and other techniques have significantly improved coating precision, they are costly and difficult to scale up.

[0005] In summary, while existing lithium manganese oxide modification strategies have yielded various results, they still have limitations to varying degrees, making it difficult to simultaneously achieve cycle stability, rate performance, and large-scale production costs. Summary of the Invention

[0006] The purpose of this invention is to address the shortcomings of existing technologies by providing an additive-modified lithium manganese oxide positive electrode and its preparation method. This preparation method involves introducing dipotassium hydrogen phosphate as a functional additive into a polyvinylidene fluoride (PVDF) binder system during the preparation of a spinel-type lithium manganese oxide positive electrode, followed by mixing with lithium manganese oxide active material and a conductive agent, ultimately yielding a uniformly dispersed positive electrode. During battery charging and discharging, a stable interface layer forms on the positive electrode surface, effectively suppressing interfacial side reactions such as manganese dissolution, while simultaneously improving lithium-ion diffusion kinetics.

[0007] To achieve the above-mentioned objectives, the technical solution of the present invention is as follows:

[0008] The additive-modified lithium manganese oxide positive electrode sheet and its preparation method specifically include the following steps:

[0009] Step 1: Dissolve polyvinylidene fluoride powder in N-methylpyrrolidone solvent and mix evenly to form polyvinylidene fluoride adhesive;

[0010] Step 2: Add dipotassium hydrogen phosphate powder to the above polyvinylidene fluoride adhesive solution, mix and disperse evenly to obtain an adhesive solution with added dipotassium hydrogen phosphate.

[0011] Step 3: Mix lithium manganese oxide active material, conductive agent (such as Super P, acetylene black, Ketjen black, carbon nanotubes, etc.) with binder solution containing dipotassium hydrogen phosphate, and adjust the viscosity to a suitable coating by adding N-methylpyrrolidone to obtain a uniformly mixed positive electrode slurry.

[0012] Preferably, in step 1 above, the mass fraction of the polyvinylidene fluoride adhesive is 1-10 wt%.

[0013] Preferably, in step 1 above, the mixing method is one of ball milling, magnetic stirring, mechanical stirring, and grinding.

[0014] Preferably, in step 2 above, the amount of dipotassium hydrogen phosphate added is 0% to 20% of the mass of polyvinylidene fluoride, excluding 0%.

[0015] Preferably, in step 2 above, the mixing method is one of ball milling, magnetic stirring, mechanical stirring, and grinding.

[0016] Preferably, in step 3 above, the lithium manganese oxide active material is a spinel-type lithium manganese oxide positive electrode active material that is conventionally available in the field of lithium-ion batteries.

[0017] Preferably, in step 3 above, the mass ratio of the lithium manganese oxide active material, the conductive agent, and the binder dry base is the conventional dosage ratio used in the preparation of positive electrode slurry in the art, wherein the mass of the binder dry base is the total mass of polyvinylidene fluoride and dipotassium hydrogen phosphate.

[0018] Compared with the prior art, the present invention has the following advantages:

[0019] This invention introduces dipotassium hydrogen phosphate as a functional additive into the polyvinylidene fluoride binder system during the preparation of spinel-type lithium manganese oxide cathode sheets. This binder is then mixed with lithium manganese oxide active material and a conductive agent to ultimately obtain a uniformly dispersed cathode sheet. On one hand, the dipotassium hydrogen phosphate additive optimizes the electrode structure, forming a stable interface layer on the cathode surface and effectively suppressing interfacial side reactions such as manganese dissolution. On the other hand, the presence of phosphate ions helps improve the diffusion kinetics of lithium ions. Through the synergistic effect of interface stability and ion conductivity, the cycle performance and rate performance of the lithium manganese oxide cathode prepared by this invention are significantly improved.

[0020] Compared with other modifications carried out during the synthesis of lithium manganese oxide, the method of the present invention directly produces a modified binder solution with uniformly dispersed dipotassium hydrogen phosphate, which is then mixed with lithium manganese oxide active material and conductive agent to prepare positive electrode slurry. It is applicable to various conventional spinel-type lithium manganese oxide materials, has good compatibility with existing positive electrode preparation processes, is simple in process, low in cost, and has significance for practical industrial application. Attached Figure Description

[0021] Figure 1 The circuit performance diagram of the battery assembled with the positive electrode sheet is shown in Example 1.

[0022] Figure 2 The rate performance diagram of the battery assembled with the positive electrode sheet is shown in Example 1.

[0023] Figure 3 The SEM image and corresponding mapping image of the positive electrode after cycling were obtained for Example 1.

[0024] Figure 4 The cycle performance diagram of the battery assembled with the positive electrode sheet is shown in Example 2.

[0025] Figure 5 The rate performance diagram of the battery assembled with the positive electrode sheet is shown in Example 2.

[0026] Figure 6 The cycling performance diagram of the battery assembled with the positive electrode sheet is shown in Example 3.

[0027] Figure 7 The cycle performance diagram of the battery assembled with the positive electrode is obtained for Comparative Example 1.

[0028] Figure 8 The rate performance diagram of the battery assembled with the positive electrode is obtained for Comparative Example 1.

[0029] Figure 9 The cycle performance diagram of the battery assembled with the positive electrode is obtained for Comparative Example 2.

[0030] Figure 10 The rate performance diagram of the battery assembled with the positive electrode is obtained for Comparative Example 2. Detailed Implementation

[0031] To better understand the technical content of the present invention, specific embodiments are described below in conjunction with the accompanying drawings.

[0032] Various aspects of the invention are described in this disclosure with reference to the accompanying drawings, which illustrate numerous illustrative embodiments. The embodiments of this disclosure are not necessarily intended to encompass all aspects of the invention. It should be understood that the various concepts and embodiments described above, as well as those described in more detail below, can be implemented in any of many ways, because the concepts and embodiments disclosed herein are not limited to any particular implementation. Furthermore, some aspects of the invention disclosed may be used alone or in any suitable combination with other aspects of the invention disclosed.

[0033] Example 1

[0034] Polyvinylidene fluoride powder was dissolved in N-methylpyrrolidone solvent and ball-milled at room temperature for 1 hour to form a uniform transparent colloid with a mass fraction of 5 wt%.

[0035] The measured amount of dipotassium hydrogen phosphate powder was slowly added to the above polyvinylidene fluoride adhesive solution, and the mixture was stirred for 1 hour to ensure complete dispersion, thus obtaining an adhesive solution with added dipotassium hydrogen phosphate. The mass ratio of dipotassium hydrogen phosphate to polyvinylidene fluoride was 1:25.

[0036] The active material of lithium manganese oxide, conductive carbon black and the above-mentioned polyvinylidene fluoride adhesive with added dipotassium hydrogen phosphate are mixed at a dry weight ratio of 8:1:1. An appropriate amount of N-methylpyrrolidone is added to adjust the viscosity of the slurry. The mixture is thoroughly ground and mixed evenly in a mortar. Finally, it is coated and dried to obtain the final electrode sheet.

[0037] Figure 1 The positive electrode prepared in this embodiment is assembled into a coin cell and subjected to charge-discharge cycles at 3.3-4.5V and 1C current density. After 800 cycles, the capacity retention rate is 80.5%. Figure 2 The rate performance diagram shows that at a 5C current density, the capacity retention is 78.7%. Figure 3 The images show the SEM image and corresponding mapping of the positive electrode after sample cycling. The presence of P and O elements proves that dipotassium hydrogen phosphate is indeed uniformly distributed in the positive electrode.

[0038] Example 2

[0039] This embodiment is basically the same as Embodiment 1, except that the mass ratio of dipotassium hydrogen phosphate to polyvinylidene fluoride is 2:25.

[0040] Figure 4 The positive electrode prepared in this embodiment is assembled into a coin cell and subjected to charge-discharge cycles at 3.3-4.5V and 1C current density. After 800 cycles, the capacity retention rate is 79.7%. Figure 5 The rate performance diagram shows that at a 5C current density, the capacity retention is 62.5%.

[0041] Example 3

[0042] This embodiment is basically the same as Embodiment 1, except that the mass ratio of dipotassium hydrogen phosphate to polyvinylidene fluoride is 4:25.

[0043] Figure 6 The positive electrode prepared in this embodiment is assembled into a coin cell and charged and discharged at 3.3-4.5V and 1C current density. The capacity retention rate after 800 cycles is 89.23%, which shows that it has a higher capacity retention rate under long cycle time compared with Example 1 and Example 2.

[0044] Comparative Example 1

[0045] Polyvinylidene fluoride powder was dissolved in N-methylpyrrolidone solvent and ball-milled at room temperature for 1 hour to form a uniform transparent adhesive solution with a mass fraction of 5 wt%.

[0046] Lithium manganese oxide active material, conductive carbon black, and polyvinylidene fluoride adhesive were mixed at a mass ratio of active material: conductive agent: binder of 8:1:1. An appropriate amount of N-methylpyrrolidone was added to adjust the viscosity of the slurry. The mixture was then thoroughly ground and mixed in a mortar. Finally, the mixture was coated and dried to obtain the final electrode sheet.

[0047] Figure 7 The positive electrode prepared in this embodiment was assembled into a coin cell and subjected to charge-discharge cycles at 3.3-4.5V and 1C current density. After 800 cycles, the capacity retention rate was 51.2%, which is much lower than that of Examples 1-3 modified with dipotassium hydrogen phosphate. Figure 8 The rate performance diagram shows that at a 5C current density, the capacity retention is only 54.9%.

[0048] Comparative Example 2

[0049] This embodiment is basically the same as Embodiment 1, except that the mass ratio of dipotassium hydrogen phosphate to polyvinylidene fluoride is 6:25.

[0050] Figure 9 The positive electrode prepared in this embodiment is assembled into a coin cell and subjected to charge-discharge cycles at 3.3-4.5V and 1C current density. After 800 cycles, the capacity retention rate is 56.4%. Figure 10The rate performance graph shows that at a 5C current density, the capacity retention is 58.6%. This indicates that adding too much dipotassium hydrogen phosphate can actually decrease performance.

Claims

1. An additive-modified lithium manganese oxide positive electrode and its preparation method, characterized in that, Includes the following steps: Step 1: Dissolve polyvinylidene fluoride powder in N-methylpyrrolidone solvent and mix evenly to form polyvinylidene fluoride adhesive; Step 2: Add dipotassium hydrogen phosphate powder to the above polyvinylidene fluoride adhesive solution, mix and disperse evenly to obtain an adhesive solution with added dipotassium hydrogen phosphate. Step 3: Mix the lithium manganese oxide active material, conductive agent and binder solution with added dipotassium hydrogen phosphate, and adjust the viscosity to a suitable coating by adding N-methylpyrrolidone to obtain a uniformly mixed positive electrode slurry. Finally, coat and dry to obtain the final electrode sheet.

2. The preparation method according to claim 1, characterized in that, In step 1, the mass fraction of the polyvinylidene fluoride adhesive is 1-10 wt%.

3. The preparation method according to claim 1, characterized in that, In step 1, the mixing method is one of ball milling, magnetic stirring, mechanical stirring, and grinding.

4. The preparation method according to claim 1, characterized in that, In step 2, the amount of dipotassium hydrogen phosphate added is 0% to 20% of the mass of polyvinylidene fluoride, excluding 0%.

5. The preparation method according to claim 1, characterized in that, In step 2, the mixing method is one of ball milling, magnetic stirring, mechanical stirring, and grinding.

6. The preparation method according to claim 1, characterized in that, In step 3, the lithium manganese oxide active material is a spinel-type lithium manganese oxide positive electrode active material that is conventionally available in the field of lithium-ion batteries.

7. The preparation method according to claim 1, characterized in that, In step 3, the mass ratio of the lithium manganese oxide active material, conductive agent and binder dry base is the conventional dosage ratio for preparing positive electrode slurry in the art, wherein the mass of the binder dry base is the total mass of polyvinylidene fluoride and dipotassium hydrogen phosphate.

8. An additive-modified lithium manganese oxide positive electrode sheet prepared by the method according to any one of claims 1-7.

9. A lithium-ion battery, characterized in that, A lithium manganese oxide positive electrode sheet modified with the additives described in claim 8.

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