A positive electrode and battery with a spatial conductive network structure

Abstract

This invention provides a positive electrode with a spatial conductive network structure, its matrix system, and a battery. The positive electrode includes a binder, a conductive agent, and a positive electrode active material. The positive electrode active material is a transition metal sulfide. The binder includes polyacrylonitrile and glucose, with the glucose content relative to polyacrylonitrile at a mass ratio of 40%-70%. This invention utilizes the high molecular crosslinking of polyacrylonitrile to form an effective binding network for the positive electrode active material. Small-molecule glucose is dispersed within this network for buffering and bridging, while the conductive agent interpenetrates across regions, effectively suppressing positive electrode volume expansion and improving the electrochemical performance of the lithium-ion battery.

Description

Technical Field

[0001] This invention relates to the field of battery materials technology, and more specifically to a positive electrode with a spatial conductive network structure and a battery using the positive electrode. Background Technology

[0002] Currently, lithium-ion batteries have the highest energy density among commercially available batteries and are widely used in various small electronic products and electric vehicles. In recent years, the rapidly developing electric vehicle and energy storage industries have placed higher demands on the lifespan and rate capability of lithium-ion batteries.

[0003] Binders are an important component of lithium-ion batteries. Although used in small quantities, they play a crucial role. Binders bind active materials and conductive agents together with the current collector, reducing the adverse effects of volume changes during lithium insertion / extraction processes on the electrode sheet and stabilizing its internal structure.

[0004] Currently, the main binder used in lithium-ion batteries is polyvinylidene fluoride (PVDF), an organic solvent-based binder. However, the solvent for PVDF, N-methylpyrrolidone (NMP), is volatile, highly toxic, and pollutes the environment. Compared to organic solvent-based binders, aqueous binders release no solvent, are low-cost, non-flammable, and safe to use, making them an important development direction for lithium-ion battery binders.

[0005] Currently, transition metal sulfides, represented by FeS2, are diverse, possess high energy density and theoretical specific capacity, and are abundant, with many minerals existing in sulfide form, making them inexpensive and ideal materials for high-energy-density lithium-ion batteries. Among them, FeS2, as a lithium-ion battery cathode material, exhibits charging platforms of 1.8 and 2.4V and discharging platforms of 2.1 and 1.5V. Its theoretical specific capacity is as high as 894 mAh / g. However, these transition metal sulfides all exhibit significant volume expansion during charge-discharge cycling (the volume expansion rate of FeS2 to 2Li2S+Fe is approximately 164.2%) and severe polysulfide formation (Li2S... n (n>2) The dissolution shuttle effect results in poor conductivity, which leads to poor cycle stability of this type of material in lithium secondary batteries.

[0006] Therefore, transition metal sulfides have excellent development prospects as cathode materials for lithium secondary batteries due to their high theoretical specific capacity and high energy density, but their poor cycle stability and other shortcomings still need to be addressed. Summary of the Invention

[0007] This invention addresses the shortcomings of existing technologies by providing a positive electrode with a spatial conductive network structure, which is applicable to batteries using transition metal sulfides as positive electrode materials for lithium secondary batteries. It can effectively suppress volume expansion and polysulfide shuttle, thereby improving the electrochemical performance of lithium-ion batteries.

[0008] Specifically, the positive electrode with a spatial conductive network structure provided by the present invention includes a binder, a conductive agent, and a positive electrode active material. The positive electrode active material is a transition metal sulfide, and the binder includes polyacrylonitrile and glucose, wherein the mass ratio of glucose to polyacrylonitrile is 40%-70%.

[0009] This invention utilizes the high molecular crosslinking of polyacrylonitrile to construct an effective binding network for the positive electrode active material. This improves adhesion while effectively limiting the volume expansion of transition metal sulfides. To achieve the binding effect, the polyacrylonitrile possesses a high degree of polymerization and relatively low network flexibility. Small-molecule glucose, as a linear molecule with multiple hydroxyl active groups, can be dispersed within the crosslinked polyacrylonitrile network, acting as a buffer and bridge. Furthermore, the formation of the polyacrylonitrile binding network also reduces the contact between positive electrode active materials to some extent. The addition of conductive agents facilitates electrical conduction between the positive electrode active materials, and the presence of glucose promotes the interpenetration of conductive agents such as carbon nanotubes within different regions of the polyacrylonitrile network, improving the dispersion stability of the conductive agents in the binder system. Therefore, the binder and conductive agent of this invention can jointly serve as the matrix system for the battery positive electrode, jointly constructing a spatial conductive network structure that simultaneously binds and effectively conducts positive electrode active materials (such as transition metal sulfides).

[0010] As an optimized alternative, the conductive agent has a mass ratio of 80%-120% to the binder, which can provide good conductivity.

[0011] As an optimized alternative, the conductive agent may include one or more of carbon nanotubes (CNTs), graphene, carbon nanowires, carbon black, etc.

[0012] As an optimized alternative, the positive electrode active material includes one or more of FeS2, MoS2, WS2, etc.

[0013] As an optimized alternative, the positive electrode active material accounts for 70%-95% of the total mass of the positive electrode material (including binder, conductive agent and positive electrode active material), preferably 80%-90%, which ensures capacity while being effectively bound by the cross-linked network of polyacrylonitrile.

[0014] The present invention also provides a method for preparing the above-mentioned positive electrode with a spatial conductive network structure, comprising the following steps: fully dissolving polyacrylonitrile in water, then adding a conductive agent and stirring and dispersing it fully, then adding a positive electrode active material and stirring and dispersing it fully, finally adding glucose and stirring and dispersing it fully, coating the obtained slurry onto a positive electrode current collector, drying it conventionally and pressing and weighing it, and then vacuum drying it to obtain the final product.

[0015] As an optimized alternative, the stirring time for dissolving the polyacrylonitrile in water is 1-2 hours, the stirring time after adding the conductive agent is 2-3 hours, the stirring time after adding the positive electrode active material is 8-12 hours, and the stirring time after adding glucose is 2-3 hours.

[0016] As an optimized alternative, after the slurry is coated on the positive electrode current collector, the conventional drying temperature is 80±5℃ and the vacuum drying temperature is 120±5℃. The setting of the two-step drying temperature is conducive to the formation and stability of the spatial conductive network structure, and while fully removing solvent water, it prevents the collapse of the network structure or excessive cross-linking stress of polyacrylonitrile.

[0017] The matrix system with spatial conductive network structure described above, the positive electrode including the matrix system, and the battery including the positive electrode of the present invention can all be provided as separate products.

[0018] The above-mentioned product and its preparation method of the present invention, as products of an aqueous material system, have the advantages of being green and environmentally friendly. Simultaneously, while effectively suppressing the volume expansion of the positive electrode active material and the shuttling effect of polysulfides, the electrical performance can be further optimized, improving the first-cycle efficiency and cycle performance of lithium batteries. Attached Figure Description

[0019] The technical solutions of the embodiments of the present invention will be further described in detail below with reference to the accompanying drawings and examples.

[0020] Figure 1 This is the first-cycle charge-discharge curve of FeS2 corresponding to Embodiment 1 of the present invention;

[0021] Figure 2 This is a cycle specific capacity diagram of FeS2 corresponding to Embodiment 1 of the present invention;

[0022] Figure 3 This is the first-cycle charge-discharge curve of FeS2 corresponding to Embodiment 2 of the present invention;

[0023] Figure 4 This is a cycle specific capacity diagram of FeS2 corresponding to Embodiment 2 of the present invention;

[0024] Figure 5 This is the first-cycle charge-discharge curve of FeS2 corresponding to Comparative Example 1 of the present invention;

[0025] Figure 6 This is a cycle specific capacity diagram of FeS2 corresponding to Comparative Example 1 of the present invention;

[0026] Figure 7 The CV curve of FeS2 corresponding to Comparative Example 1 of this invention at a scan rate of 0.5 mV / s at room temperature;

[0027] Figure 8 The CV curve of FeS2 at room temperature and a scan rate of 0.5 mV / s corresponds to Example 2 of the present invention;

[0028] Figure 9 The CV curves of FeS2 corresponding to Example 2 of the present invention are shown at room temperature with varying scan rates of 0.5 mV / s to 4 mV / s.

[0029] Figure 10 The lithium-ion diffusion coefficient of FeS2 at room temperature corresponds to Comparative Example 1 and Example 2 of this invention;

[0030] Figure 11 The SEM surface morphology of FeS2 corresponding to Comparative Example 1 of the present invention after 20 weeks is shown.

[0031] Figure 12 The SEM surface morphology of FeS2 after 20 weeks corresponding to Example 2 of the present invention;

[0032] Figure 13 The image shows the morphology of the SEM cross-section of FeS2 before recycling, corresponding to Comparative Example 1 of this invention.

[0033] Figure 14 The SEM cross-sectional morphology of FeS2 after 20 cycles, corresponding to Comparative Example 1 of this invention;

[0034] Figure 15 The SEM cross-sectional morphology of FeS2 after 20 cycles, corresponding to Example 2 of the present invention;

[0035] Figure 16 This is a schematic diagram illustrating the principle of the present invention. Detailed Implementation

[0036] To facilitate understanding of the present invention, a more comprehensive description will be provided below, along with preferred embodiments. However, it should be understood that these embodiments are merely for more detailed explanation and should not be construed as limiting the invention in any way, i.e., not intended to limit the scope of protection of the invention.

[0037] Example 1

[0038] 0.7g of polyacrylonitrile (molecular weight approximately 1,000,000) was placed in 10g of deionized water. The mixture was stirred with a magnetic stirrer for about 1 hour until completely dissolved. 1g of CNTs was added, and stirring continued for 2 hours. Finally, 8g of FeS2 with a specific capacity of 893.7mAh / g was added, and the mixture was stirred for 10 hours. 0.3g of glucose was added, and the mixture was slowly stirred at low speed for 2 hours. The resulting slurry was coated onto aluminum foil to a thickness of 1000μm. It was then dried in an 80℃ forced-air oven, processed, and weighed. The foil was then vacuum-stored in a 120℃ vacuum oven for 24 hours, and finally transferred to a glove box to assemble into button batteries, which were then allowed to stand for 12 hours.

[0039] Example 2

[0040] 0.6g of polyacrylonitrile (molecular weight approximately 1,000,000) was placed in 10g of deionized water. The mixture was stirred with a magnetic stirrer for approximately 1 hour until completely dissolved. 1g of CNTs was added, and stirring continued for 2 hours. Finally, 8g of FeS2 with a specific capacity of 893.7mAh / g was added, and the mixture was stirred for 8 hours. 0.4g of glucose was added, and the mixture was slowly stirred at low speed for 2 hours. The resulting slurry was coated onto aluminum foil to a thickness of 1000μm. It was then dried in an 80℃ forced-air oven, processed, weighed, and vacuum-stored in a 120℃ vacuum oven for 24 hours. Finally, it was transferred to a glove box and assembled into button batteries, which were then allowed to stand for 12 hours.

[0041] Comparative Example 1

[0042] Take 1g of polyvinylidene fluoride (PVDF) (molecular weight 200,000) and place it in 10g of N-methylpyrrolidone (NMP). Stir with a magnetic stirrer for about 1 hour until completely dissolved. Add 1g of conductive agent (Super-P) and continue stirring for 2 hours. Finally, add 8g of FeS2 with a specific capacity of 893.7mAh / g and stir for 8 hours. Coat the resulting slurry onto aluminum foil to a thickness of 1000μm. Then dry it in an 80℃ forced-air oven, punch and weigh it, and then vacuum store it in a 120℃ vacuum oven for 24 hours. Finally, transfer it to a glove box to assemble it into a button cell and let it stand for 12 hours.

[0043] The above embodiments and comparative examples underwent relevant performance tests.

[0044] The electrochemical performance of the obtained button cells was tested using a CT 2001A battery testing system manufactured by Wuhan Landian Company. The test conditions were: discharge cut-off voltage of 1.0V, charging cut-off voltage of 3.0V, and charge / discharge rate of 0.5C.

[0045] Test results are as follows Figure 1-6 As shown, compared to Comparative Example 1, Example 1 demonstrates a higher efficiency in the first week ( Figure 1 , 5The specific capacity increased from 70.7% to 95.1% at 600 weeks. Figure 2 , 6 ) Remaining stable, similarly, in Example 2 compared to Comparative Example 1, it can be seen that the first-week efficiency ( Figure 3 , 5 The specific capacity increased from 70.7% to 94.2% at 650 weeks. Figure 4 , 6 The material system of this invention remains stable. It has been confirmed that, compared to conventional PVDF binder systems, the material system significantly improves the first-cycle efficiency and cycle performance of the battery, with noticeable improvements.

[0046] Figure 7 The CV curve of FeS2 corresponding to Comparative Example 1 of this invention at a scan rate of 0.5 mV / s at room temperature; Figure 8 The CV curve of FeS2 at room temperature and a scan rate of 0.5 mV / s corresponds to Example 2 of the present invention; Figure 9 The CV curves of FeS2 corresponding to Embodiment 2 of the present invention are shown at room temperature with varying scan rates of 0.5mV / s-4mV / s. It can be clearly seen that the CV curves of the present invention have a higher degree of overlap after multiple cycles, better conductivity due to the spatial conductive network, and can adapt to variable speed scanning in a higher voltage range.

[0047] Figure 10 The lithium-ion diffusion coefficient of FeS2 at room temperature corresponds to Comparative Example 1 and Example 2 of the present invention. The lithium-ion diffusion coefficient with the spatial conductive network of the present invention is increased by an order of magnitude compared with the lithium-ion diffusion coefficient without the spatial conductive network of the present invention, further demonstrating the excellent conductivity of the spatial conductive network.

[0048] like Figure 11 and 12 As shown, Figure 11 The SEM surface morphology of FeS2 corresponding to Comparative Example 1 of the present invention after 20 weeks is shown. Figure 12 The SEM surface morphology of FeS2 after 20 weeks corresponding to Example 2 of the present invention; Figure 12 It exhibits a finely textured spatial conductive network structure, which suppresses volume expansion.

[0049] Figure 13 , Figure 14 and Figure 15This is a comparison of the surface morphology of the SEM cross-sections of FeS2 before and after 20 cycles for Comparative Example 1 and Example 2 of this invention. According to the size change, the cross-sectional size of Comparative Example 1 and Example 2 increased from the original 35 μm to 80 μm and 50 μm respectively after cycling, and the volume expansion ratio decreased from 130% to 50%. This confirms that the spatial network structure not only has good conductivity and electrical conductivity, but can also significantly suppress the volume expansion of the positive electrode active material.

[0050] Figure 16 This is a schematic diagram illustrating the principle of the spatial conductive network structure of the present invention. It can be seen that the positive electrode active material is bound within a bound network formed by cross-linked polyacrylonitrile (Binder), while glucose is dispersed within the polyacrylonitrile network, promoting cross-linking contact between the bound meshes and forming lithium-ion transport channels. Simultaneously, conductive agents (CNTs) are interspersed between the polyacrylonitrile bound networks, improving conductivity efficiency. Together, they constitute a stable spatial conductive network structure with excellent conductivity.

[0051] The above description is only a preferred embodiment of the present invention and is not intended to limit the present invention. Any modifications, equivalent substitutions, improvements, etc., made within the spirit and principles of the present invention should be included within the protection scope of the present invention.

Claims

1. A positive electrode having a spatial conductive network structure, comprising a binder, a conductive agent, and a positive electrode active material, wherein the binder comprises polyacrylonitrile and glucose, and the mass ratio of glucose to polyacrylonitrile is 40%-70%; The mass ratio of the conductive agent to the binder is 80%-120%; The positive electrode active material is a transition metal sulfide.

2. The positive electrode with a spatial conductive network structure according to claim 1, characterized in that, The conductive agent includes one or more of carbon nanotubes, graphene, carbon nanowires, and carbon black. The transition metal sulfides specifically include one or more of FeS2, MoS2, and WS2.

3. The positive electrode with a spatial conductive network structure according to claim 1, characterized in that, The positive electrode active material accounts for 70%-95% of the total mass of the positive electrode material.

4. The positive electrode with a spatial conductive network structure according to claim 1, characterized in that, The positive electrode active material accounts for 80%-90% of the total mass of the positive electrode material.

5. A method for preparing a positive electrode with a spatial conductive network structure according to any one of claims 1-4, comprising the following steps: fully dissolving polyacrylonitrile in water, then adding a conductive agent and stirring and dispersing it fully, then adding a positive electrode active material and stirring and dispersing it fully, finally adding glucose and stirring and dispersing it fully, coating the obtained slurry onto a positive electrode current collector, drying it conventionally and pressing and weighing it, and then vacuum drying it to obtain the final product.

6. The method for preparing a positive electrode with a spatial conductive network structure according to claim 5, characterized in that, The stirring time for dissolving the polyacrylonitrile in water is 1-2 hours, the stirring time after adding the conductive agent is 2-3 hours, the stirring time after adding the positive electrode active material is 8-12 hours, and the stirring time after adding glucose is 2-3 hours. After the slurry is coated on the positive electrode current collector, the conventional drying temperature is 80±5℃, and the vacuum drying temperature is 120±5℃.

7. A battery having a spatial conductive network structure, comprising a positive electrode having a spatial conductive network structure as described in any one of claims 1-4.

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