Lipoic acid-based additive for high-voltage positive electrode and preparation method and application thereof

Abstract

The invention belongs to the technical field of solid-state batteries, and particularly relates to a binder additive for a rapid self-healing high-voltage positive electrode, a preparation method of the binder additive and a solid-state battery. Lipoic acid and a derivative thereof, a cross-linking agent, a lithium salt and a chelating agent are mixed to obtain a precursor solution; and after the precursor solution is converted into a solid state, obtaining a lipoic acid-based polymer film or dissolving the precursor solution in N-methyl pyrrolidone under a heating condition to obtain a lipoic acid-based binder additive solution. A dynamic disulfide bond and hydrogen bond network structure enables the additive to have high elasticity and compression performance, and mechanical damage in the high-voltage positive electrode circulation process is repaired. Carboxyl groups and disulfide bonds in the additive are coordinated with transition metal ions in a positive electrode, so that the dissolution of the transition metal ions is inhibited. In the cycle process of the high-voltage solid-state battery, the lipoic acid and the derivative thereof can be subjected to self-limiting oxidation reaction to generate an inorganic phase rich in sulfur element, so that the electrochemical stability of a high-voltage positive electrode is enhanced.

Description

Technical Field

[0001] This invention belongs to the field of solid-state battery technology, specifically relating to a thioctic acid-based additive for high-voltage cathodes, its preparation method, and its application. Background Technology

[0002] Solid-state batteries using polymer-based solid electrolytes are considered one of the most promising candidates for next-generation energy storage technology due to their high energy density and inherent safety. However, the internal strain of the cathode increases with increasing charging cutoff voltage, leading to poorer physical contact at solid-solid interfaces, including the cathode / polymer electrolyte interface and the cathode particle / cathode particle interface, as well as discontinuous lithium-ion transport. Furthermore, interfacial side reactions between the high-voltage nickel-rich layered oxide cathode and the polymer electrolyte result in an unstable cathode interface layer, further hindering lithium-ion diffusion, thereby accelerating performance degradation and shortening battery cycle life.

[0003] To address the aforementioned issues, doping, surface coating, and the use of elastic polymer electrolytes for high-voltage cathodes have been extensively reported. These methods not only effectively suppress the internal lattice strain of the cathode active material to accommodate volume changes but also establish physical barriers to hinder the decomposition of the polymer electrolyte during cycling. However, elemental doping and surface coating modification methods are complex and costly, limiting their large-scale application. On the other hand, elastic electrolytes contain abundant hydrogen bonds, leading to interfacial instability with the lithium metal anode of solid-state batteries, resulting in limited electrochemical improvements. Therefore, currently, polymer-based binders are used on high-voltage cathodes to reduce costs and improve the effectiveness of electrochemical modification. However, the commonly used polymer-based binder is polyvinylidene fluoride (PVDF), which undergoes plastic deformation under high cutoff voltages, failing to alleviate volume strain, cathode interfacial instability, and transition metal ion dissolution.

[0004] Therefore, there is an urgent need to develop innovative methods with practical value to address the mechanical and electrochemical stability issues related to high-voltage cathodes in solid-state lithium metal batteries. Summary of the Invention

[0005] To address the aforementioned problems, this invention provides a thioctic acid-based additive for high-voltage cathodes, its preparation method, and its application. It solves the problems of existing commercial high-voltage cathode binders, such as polyvinylidene fluoride (PVDF), undergoing plastic deformation under high cutoff voltages, failing to alleviate volume strain, cathode interface instability, and transition metal ion dissolution. This invention achieves stable cycling of polymer-based high-voltage solid-state lithium metal batteries at high voltages.

[0006] To achieve the above objectives, the technical solution of the present invention is as follows.

[0007] The first aspect of the present invention provides a thioctic acid-based additive for a high-voltage positive electrode. The thioctic acid-based additive is obtained by stirring and mixing lithium salt, crosslinking agent, polymerizing monomer, chelating agent and free radical initiator to obtain a precursor solution. The precursor solution is then subjected to thermal initiation to cause thioctic acid to undergo ring-opening polymerization. At the same time, the polymerizing monomer undergoes a double bond addition reaction under the action of the free radical initiator, and the two form a three-dimensional crosslinked structure. The polymer monomer is at least one of thioctic acid, thiocamide, lithium thiocate, and thioctic acid impurity 9. The mass ratio of the polymer monomer to the crosslinking agent is 1:5 to 4:5.

[0008] This invention incorporates thioctic acid and its derivatives as polymerizable monomers. These monomers retain disulfide bonds after ring-opening polymerization, providing excellent elasticity and self-healing properties. The added crosslinking agent further forms a three-dimensional network structure, enhancing the additive's elasticity. Furthermore, the addition of a transition metal salt chelating agent to the precursor solution allows for coordination with the carbonyl groups on thioctic acid and its derivatives, further improving the mechanical strength of the binder additive in the self-healing high-voltage cathode, thereby mitigating volume strain during high-voltage cathode cycling. The added lithium salt provides a certain ionic conductivity and decomposes during cycling, further improving the high-voltage cycling performance of the cathode. The thioctic acid-based additive in this invention, as an additive for polyvinylidene fluoride binders, enables stable cycling of polymer-based high-voltage solid-state lithium metal batteries at high voltages.

[0009] In another preferred embodiment, the lithium salt concentration in the precursor solution is 0.5M to 2M, the chelating agent has a mass fraction of 0.01% to 0.2%, and the free radical initiator has a mass fraction of 0.1%.

[0010] In another preferred embodiment, the crosslinking agent is at least one selected from 1,3-diisopropenylbenzene, 1,3-divinylbenzene, and myrcene; The lithium salt is at least one of lithium bis(trifluoromethanesulfonyl)imide, lithium bis(fluorosulfonyl)imide, lithium difluorooxalateborate, and lithium tetrafluoroborate. The chelating agent is at least one of ferric chloride, ferric nitrate, and nickel dichloride.

[0011] A second aspect of this invention provides a method for preparing the aforementioned self-healing high-voltage positive electrode binder additive, comprising the following steps: The monomer and lithium salt were mixed and heated, and then a crosslinking agent and a chelating agent were added. The mixture was stirred and reacted for 15 min to 45 min to obtain a precursor solution. The precursor solution was heated at 160℃~220℃ for 15min~45min to carry out thermally initiated polymerization, thereby obtaining the self-healing high-voltage positive electrode binder additive.

[0012] In another preferred embodiment, the temperature at which the polymer monomer and the lithium salt are mixed and heated is 160°C to 220°C.

[0013] In another preferred embodiment, the chelating agent is further dispersed in an organic solvent, such as acetonitrile or acetone, before being added.

[0014] The third aspect of the present invention provides the application of the thioctic acid-based additive for high-voltage cathodes in the preparation of solid-state batteries.

[0015] In another preferred embodiment, the specific process of obtaining the solid-state battery is as follows: Thioctanoic acid-based additives are added during the preparation of the positive electrode slurry; Place the positive electrode plate on the battery casing, then add the polymer electrolyte, then place the negative electrode plate, and then place the spring sheet, gasket, negative electrode casing and encapsulate the battery in sequence. To enhance interface contact, the encapsulated battery undergoes heat treatment.

[0016] The specific process is as follows: The preparation method for the unmodified positive electrode involves first weighing high-voltage positive electrode powder, conductive carbon, and polyvinylidene fluoride in a weight ratio of 8:1:1, then adding N-methylpyrrolidone to mix and prepare a positive electrode slurry. The resulting positive electrode slurry is then coated onto a carbon-coated aluminum foil current collector and dried under vacuum at 80°C for 24 hours to obtain the positive electrode sheet. The preparation process for the high-voltage positive electrode with added thioctic acid additives is the same as described above, except that 5%–20% (by weight of polyvinylidene fluoride) of the thioctic acid additive is added to the positive electrode slurry. The method for preparing the polymer solid electrolyte is as follows: 0.54 g of PVDF, 0.24 g of lithium bis(trifluoromethanesulfonyl)imide, and 0.12 g of lithium difluorooxalateborate are dissolved in 10 mL of N-methylpyrrolidone, and 0.1 g of alumina nanoparticles (300 nm) are added at the same time. The mixture is stirred at 50°C for 8 h to form a homogeneous solution. The resulting solution is poured onto a 90 mm circular polytetrafluoroethylene plate and dried in a vacuum oven at 90°C for 24 h. After drying, the plate is placed in a glove box and allowed to stand for 12 h before use to obtain the polymer electrolyte.

[0017] The battery assembly process is as follows: Place the positive electrode plate on the battery casing, then add the polymer electrolyte, followed by the negative electrode plate, and then place the spring contact, gasket, negative electrode casing, and finally encapsulate the battery. To ensure electrode alignment and prevent short circuits, the polymer electrolyte has a diameter of 16mm, the negative electrode is 12mm, and the positive electrode is 8mm. To enhance interface contact, the encapsulated battery undergoes heat treatment at 45°C for 3 hours.

[0018] Compared with the prior art, the present invention has the following beneficial effects: The thioctic acid-based additive in this invention contains a rich network of disulfide and hydrogen bonds, giving the additive excellent elastic and self-healing properties. Its application as an additive in high-voltage cathodes can alleviate volumetric strain in cathode particles, ensuring good contact between particles within the cathode and at the cathode / polymer electrolyte interface, which is beneficial for Li... + The continuous transmission of energy; furthermore, the thioctic acid-based additive in this invention can also repair the mechanical damage of the high-voltage cathode during cycling, enhancing the mechanical stability of the high-voltage cathode. The thioctic acid-based additive coordinates with the transition metal ions of the high-voltage cathode, thereby forming a uniform coating on the surface of the high-voltage cathode; during solid-state battery cycling, the thioctic acid-based additive undergoes a self-limiting decomposition reaction, generating a sulfur-rich cathode electrolyte interface layer, inhibiting decomposition from the polymer electrolyte, thereby enhancing the electrochemical stability of the high-voltage cathode. LiNi modified based on thioctic acid-based additive. 0.83 Co 0.12 Mn 0.05 A 4.3V solid-state pouch cell assembled with an O2 (Ni83) cathode can achieve an energy density of 357Wh / kg and retain 79% of its capacity after 100 cycles. Attached Figure Description

[0019] Figure 1 This is a structural diagram of the fast self-healing high-voltage positive electrode binder additive prepared in specific embodiment 1 of the present invention.

[0020] Figure 2 The infrared spectra of the rapid self-healing high-voltage positive electrode binder additive and 1,3-diisopropenylbenzene prepared in Specific Embodiment 1 of the present invention are shown.

[0021] Figure 3 The tensile stress-strain curves of the rapid self-healing high-voltage positive electrode binder additive and PVDF binder prepared in Specific Embodiment 1 of the present invention are shown.

[0022] Figure 4 This is a compressive stress-strain curve of the fast self-healing high-voltage positive electrode binder additive prepared in Specific Embodiment 1 of the present invention.

[0023] Figure 5 Optical photographs of the rapid self-healing high-voltage positive electrode binder additive film prepared in Specific Embodiment 1 of the present invention before and after 30 minutes of self-healing at room temperature.

[0024] Figure 6 Optical photographs of the rapid self-healing high-voltage positive electrode binder additive film prepared in Specific Embodiment 1 of the present invention before and after self-healing under room temperature.

[0025] Figure 7The Li||LiNi prepared in Specific Example 7 and Comparative Example 1 of this invention 0.83 Co 0.12 Mn 0.05 The graph shows the cycle performance of the O2 battery at a cutoff voltage of 4.3V and a 2C rate.

[0026] Figure 8 Li||LiNi prepared in Specific Example 7 of this invention 0.83 Co 0.12 Mn 0.05 The graph shows the cycle performance of the O2 pouch cell at a cutoff voltage of 4.3V and a rate of 0.2C.

[0027] Figure 9 Li||LiNi prepared in Specific Example 7 of this invention 0.83 Co 0.12 Mn 0.05 S 2p XPS spectrum of the positive electrode surface of O2 pouch cell after cycling.

[0028] Figure 10 The Li||LiNi prepared by Comparative Example 1 of this invention 0.83 Co 0.12 Mn 0.05 S2p XPS spectrum of the positive electrode surface of O2 pouch cell after cycling. Detailed Implementation

[0029] To make the objectives, technical solutions, and advantages of this invention clearer, the invention will be further described in detail below with reference to the accompanying drawings and embodiments. It should be understood that the specific embodiments described herein are merely illustrative and not intended to limit the invention. Furthermore, the technical features involved in the various embodiments of this invention described below can be combined with each other as long as they do not conflict with each other.

[0030] This invention provides a lipoic acid-based additive for high-voltage cathodes, its preparation method, and its application. The lipoic acid-based additive is prepared by uniformly mixing lipoic acid and its derivatives, a crosslinking agent, a lithium salt, and a chelating agent at high temperature on a stirring table to obtain a precursor solution. The precursor solution is then poured into a polytetrafluoroethylene mold and naturally cooled in an inert atmosphere. This yields the lipoic acid-based additive for high-voltage cathodes.

[0031] The lipoic acid and its derivatives added during the preparation process of this invention still contain disulfide bonds after ring-opening polymerization, providing excellent elasticity and self-healing properties. The added crosslinking agent further forms a three-dimensional network structure, improving the elasticity of the additive. Furthermore, the addition of a transition metal salt chelating agent to the precursor solution allows for coordination with the carbonyl groups on the lipoic acid and its derivatives, further enhancing the mechanical strength of the binder additive in the self-healing high-voltage cathode, thereby mitigating volume strain during high-voltage cathode cycling. The added lithium salt provides a certain ionic conductivity and decomposes during cycling, further improving the high-voltage cycling performance of the cathode.

[0032] The polymerizable monomer is at least one of lipoic acid, lipoamide, lithium lipoate, and lipoic acid impurities. The polymerizable monomers proposed in this invention are all lipoic acid and its derivatives, which can undergo ring-opening polymerization under thermal induction. Simultaneously, the disulfide bonds are retained after polymerization. Therefore, the binder additive for the prepared high-voltage cathode exhibits excellent elasticity and self-healing properties, thereby alleviating stress during the cycling process of the high-voltage cathode. Furthermore, the disulfide bonds can also undergo a self-limiting decomposition reaction on the surface of the high-voltage cathode, generating a sulfur-rich cathode interface layer, thereby improving the cycle stability of the high-voltage solid-state battery.

[0033] The crosslinking agent is at least one of 1,3-diisopropenylbenzene, 1,3-divinylbenzene, and myrcene. The lithium salt is at least one of lithium bis(trifluoromethanesulfonyl)imide, lithium bis(fluorosulfonyl)imide, lithium difluorooxalateborate, and lithium tetrafluoroborate. The chelating agent is at least one of ferric chloride, ferric nitrate, and nickel dichloride. Preferably, to ensure uniform dispersion of the chelating agent in the precursor, the chelating agent can be first dispersed in an organic solvent.

[0034] Preferably, the organic solvent is acetonitrile or acetone, which have low boiling points.

[0035] Preferably, the mass ratio of the polymerizable monomer to the crosslinking agent is 1:5 to 4:5; in the precursor solution, the concentration of the lithium salt is 0.5M to 2M, and the mass fraction of the chelating agent is 0.01% to 0.2%; in this invention, the amount of chelating agent needs to be controlled. When the mass fraction of the chelating agent is less than 0.01%, the polymerized binder additive has low mechanical strength and poor self-healing performance; when the mass fraction of the chelating agent is greater than 0.2%, the polymerized binder additive will harden and have no self-healing or elastic properties.

[0036] The amount of the free radical initiator is 0.1% by mass.

[0037] In this invention, lipoic acid and its derivatives are used as polymer monomers to prepare a lipoic acid-based additive for high-voltage cathodes. This additive exhibits an elongation of up to 2300% and a compression ratio of up to 20% after stretching, and can rapidly self-heal within 30 minutes at room temperature. Due to the excellent elasticity and rapid self-healing properties of the lipoic acid-based additive for high-voltage cathodes, it can enhance the mechanical stability of the high-voltage cathode and repair internal damage. Furthermore, the lipoic acid-based additive coordinates with transition metal ions in the high-voltage cathode, forming a uniform coating on the cathode surface. During solid-state battery cycling, the lipoic acid-based additive undergoes a self-limiting decomposition reaction, generating a sulfur-rich cathode electrolyte interface layer that inhibits the decomposition of the polymer electrolyte. Therefore, the assembled 4.3V solid-state battery retains more than 80% of its capacity after 900 cycles at 2C.

[0038] Furthermore, the specific process of obtaining the precursor is as follows: First, add the thioctic acid and its derivatives and lithium salt to the sample bottle, stir and mix evenly, then add the crosslinking agent, and continue stirring until evenly mixed to obtain the precursor solution; then add the chelating agent solution, continue stirring for 15 minutes, and allow to cool naturally to obtain the additive.

[0039] To strictly control the degree of polymerization of additives and the interference of water and oxygen during the preparation process, it is preferable that the atmosphere for heating to initiate polymerization is an inert gas atmosphere, while the content of water and oxygen is less than 1 ppm. To ensure uniform mixing of the precursors, the heating temperature of the stirring table is controlled at 160℃~220℃, and the heating time is 30 minutes. The precursors should be placed in sealed sample bottles during the heating and polymerization process.

[0040] The present invention also provides a solid-state battery comprising the binder additives of the aforementioned fast-healing high-voltage positive electrode. The solid-state battery further comprises a positive electrode, a polymer electrolyte, and a negative electrode.

[0041] The present invention also provides a method for preparing a solid-state battery, the method comprising: The preparation method for the unmodified positive electrode involves first weighing high-voltage positive electrode powder, conductive carbon, and PVDF in a weight ratio of 8:1:1, then adding N-methylpyrrolidone to form a slurry. The resulting slurry is then coated onto a carbon-coated aluminum foil current collector and dried under vacuum at 80°C for 24 hours to obtain the positive electrode sheet. The preparation process for the high-voltage positive electrode with added thioctic acid additives is similar to the above, except that a portion of the PVDF binder is replaced with a portion of the thioctic acid additive.

[0042] The method for preparing polymer solid electrolyte is as follows: 0.54 g of PVDF, 0.24 g of lithium bis(trifluoromethanesulfonyl)imide, and 0.12 g of lithium difluorooxalateborate are dissolved in 10 mL of N-methylpyrrolidone. At the same time, 0.1 g of alumina nanoparticles with a particle size of 300 nm are added. The mixture is stirred at 50°C for 8 h to form a homogeneous solution. The resulting solution is poured onto a 90 mm circular polytetrafluoroethylene plate and dried in a vacuum oven at 90°C for 24 h. After drying, the plate is placed in a glove box and allowed to stand for 12 h before use to obtain the polymer electrolyte.

[0043] The battery installation process is as follows: Place the positive electrode plate on the battery casing, then add the polymer electrolyte, then place the negative electrode plate, and then place the spring sheet, gasket, negative electrode casing and encapsulate the battery in sequence. To ensure electrode alignment and prevent short circuits, the polymer electrolyte has a diameter of 16 mm, with the negative electrode being 12 mm and the positive electrode being 8 mm.

[0044] To enhance interface contact, the encapsulated battery is subjected to heat treatment at 45°C for 3 hours.

[0045] The following is a detailed description of an additive for a self-healing high-voltage positive electrode binder, its preparation method, and its application.

[0046] Example 1: A method for preparing a thioctic acid-based additive for a high-voltage positive electrode, comprising the following steps: Under conditions where both water and oxygen concentrations are less than 1 ppm, 25 g of lipoic acid and 2.17 g of lithium bis(trifluoromethanesulfonyl)imide were added to a sample vial. The vial was placed on a 180°C stirring table and heated for 5 minutes until it was completely liquefied. Then, 5 g of 1,3-diisopropenylbenzene was added, and the mixture was stirred for another 3 minutes. Next, 40 μL of acetone solution containing 19.6 mg of ferric chloride was added, and the mixture was stirred for another 15 minutes to obtain the precursor solution. The precursor solution was then introduced into a polytetrafluoroethylene plate, and after natural cooling, a lipoic acid-based additive for high-voltage cathodes was obtained.

[0047] The structure of the lipoic acid-based additive for high-voltage cathodes prepared above was characterized, and the results are shown in Figure 1. Figure 2 It can be seen that this additive contains disulfide bonds but no carbon-carbon double bonds, indicating that lipoic acid and 1,3-diisopropenylbenzene have undergone cross-linking. Figure 3 As shown, the tensile strain of the thioctic acid-based additive is greater than 2300%, which is much higher than that of the PVDF binder. For example... Figure 4 As shown, the film prepared with thioctic acid-based additives can also be reversibly tested 5 times under 20% compressive strain, with a resilience greater than 75%. Figure 5As shown, to verify the self-healing ability of the lipoic acid-based additive, its surface was lightly scratched with a knife. After 30 minutes at room temperature, an optical microscope revealed that the scratches on the supramolecular membrane of the lipoic acid-based additive could essentially recover to their original state. To more directly demonstrate this, the entire lipoic acid-based additive supramolecular membrane was cut into two segments. The cut sections were then reconnected, as shown... Figure 6 As shown, the two cut segments can be completely cured into a single piece and can be re-stretched, achieving an elongation of 200% without breaking. Therefore, the dynamic chemical bonds of the thioctic acid-based additive greatly enhance its elasticity, mechanical strength, and self-healing properties, making it suitable for application in high-voltage cathodes as an additive in PVDF binders.

[0048] Example 2: A method for preparing a thioctic acid-based additive for a high-voltage positive electrode, comprising the following steps: S1. Under conditions where the water and oxygen concentrations are both less than 1 ppm, add 25 g of thioctic acid and 2.17 g of lithium bis(trifluoromethanesulfonyl)imide to a sample vial. Place the sample vial on a 180°C stirring table and heat for 5 minutes until it is completely converted into a liquid state. Then add 5 g of 1,3-diisopropenylbenzene and continue stirring for 3 minutes. After that, add 20 μL of acetone solution containing 9.8 mg of ferric chloride and continue stirring for 15 minutes. Introduce the precursor into a polytetrafluoroethylene plate and allow it to cool naturally to obtain a thioctic acid-based additive for high-voltage cathodes.

[0049] Example 3: A method for preparing a lipoic acid-based additive for a high-voltage positive electrode, comprising the following steps: Under conditions where the water and oxygen concentrations are both less than 1 ppm, 25 g of lipoic acid and 2.17 g of lithium bis(trifluoromethanesulfonyl)imide were added to a sample vial. The sample vial was placed on a 180°C stirring table and heated for 5 minutes until it was completely converted into a liquid state. Then, 5 g of 1,3-diisopropenylbenzene was added and the mixture was stirred for another 3 minutes. After that, 4 μL of acetone solution containing 1.96 mg of ferric chloride was added and the mixture was stirred for another 15 minutes. The precursor was then introduced into a polytetrafluoroethylene plate and allowed to cool naturally to obtain a lipoic acid-based additive for high-voltage cathodes.

[0050] Example 4: A method for preparing a thioctic acid-based additive for a high-voltage positive electrode, comprising the following steps: Under conditions where the water and oxygen concentrations are both less than 1 ppm, 25 g of lipoic acid and 2.17 g of lithium bis(trifluoromethanesulfonyl)imide were added to a sample vial. The sample vial was placed on a 180 °C stirring table and heated for 5 minutes until it was completely converted into a liquid state. Then, 25 g of 1,3-diisopropenylbenzene was added and the mixture was stirred for another 3 minutes. After that, 40 μL of acetone solution containing 19.6 mg of ferric chloride was added and the mixture was stirred for another 15 minutes. The precursor was then introduced into a polytetrafluoroethylene plate and allowed to cool naturally to obtain a lipoic acid-based additive for high-voltage cathodes.

[0051] Example 5: A method for preparing a thioctic acid-based additive for a high-voltage positive electrode, comprising the following steps: Under conditions where the water and oxygen concentrations are both less than 1 ppm, 25 g of lipoic acid and 2.17 g of lithium bis(trifluoromethanesulfonyl)imide were added to a sample vial. The sample vial was placed on a 180 °C stirring table and heated for 5 minutes until it was completely converted into a liquid state. Then, 75 g of 1,3-diisopropenylbenzene was added and the mixture was stirred for another 3 minutes. After that, 20 μL of acetone solution containing 9.8 mg of ferric chloride was added and the mixture was stirred for another 15 minutes. The precursor was then introduced into a polytetrafluoroethylene plate and allowed to cool naturally to obtain a lipoic acid-based additive for high-voltage cathodes.

[0052] Example 6: A method for preparing a lipoic acid-based additive for a high-voltage positive electrode, comprising the following steps: Under conditions where the water and oxygen concentrations are both less than 1 ppm, 25 g of thioctic acid and 1.08 g of lithium difluorooxalate borate were added to a sample vial. The sample vial was placed on a stirring table at 180 °C and heated for 5 minutes until it was completely converted into a liquid state. Then, 5 g of 1,3-diisopropenylbenzene was added and the mixture was stirred for another 3 minutes. After that, 40 μL of acetone solution containing 19.6 mg of ferric chloride was added and the mixture was stirred for another 15 minutes. The precursor was then introduced into a polytetrafluoroethylene plate and allowed to cool naturally to obtain a thioctic acid-based additive for high-voltage cathodes.

[0053] Example 7: A method for preparing a thioctic acid-based additive for a high-voltage positive electrode, comprising the following steps: Under conditions where the water and oxygen concentrations are both less than 1 ppm, 25 g of lipoic acid, 1.08 g of lithium bis(trifluoromethanesulfonyl)imide, and 0.54 g of lithium difluorooxalateborate were added to a sample vial. The sample vial was placed on a stirring table at 180 °C and heated for 5 minutes until it was completely converted into a liquid state. Then, 5 g of 1,3-diisopropenylbenzene was added and the mixture was stirred for another 3 minutes. After that, 40 μL of acetone solution containing 19.6 mg of ferric chloride was added and the mixture was stirred for another 15 minutes. The precursor was then introduced into a polytetrafluoroethylene plate and allowed to cool naturally to obtain a lipoic acid-based additive for high-voltage cathodes.

[0054] The high-voltage positive electrode powder, conductive carbon, PVDF, and thioctic acid-based binder additive were weighed in a ratio of 8:1:0.9:0.1. N-methylpyrrolidone was then added to prepare a slurry. This slurry was then coated onto a carbon-coated aluminum foil current collector and dried under vacuum at 80°C for 24 hours to obtain the positive electrode sheet. The method for preparing the polymer solid electrolyte involves dissolving 0.54 g of PVDF, 0.24 g of lithium bis(trifluoromethanesulfonyl)imide, and 0.12 g of lithium difluorooxalateborate in 10 mL of N-methylpyrrolidone. Simultaneously, 0.1 g of alumina nanoparticles with a particle size of 300 nm were added. The mixture was stirred at 50°C for 8 hours to form a homogeneous solution. This solution was poured onto a 90 mm circular polytetrafluoroethylene plate and dried in a vacuum oven at 90°C for 24 hours. After drying, it was placed in a glove box and allowed to stand for 12 hours before use to obtain the polymer electrolyte.

[0055] The positive electrode plate, polymer electrolyte plate, lithium plate, gasket, spring plate, and negative electrode shell are placed sequentially on the positive electrode shell. The battery is then sealed and placed in a 45°C forced-air oven for 3 hours before being taken out and cooled.

[0056] When the positive electrode is LiNi 0.83 Co 0.12 Mn 0.05 When O2 (Ni83) is present, the cycling performance of the assembled Li||Ni83 battery at room temperature and 2C rate, within a voltage range of 3V~4.3V, is as follows: Figure 7 As shown, the solid-state battery retains 80% of its capacity after 900 cycles, and its average coulombic efficiency is greater than 99%. Further testing of the Li||Ni83 pouch cell's cycle performance was conducted with a positive electrode loading of 20 m³ / cm². Figure 8 As shown, the capacity retention of the solid-state pouch cell was 80% after 100 cycles. XPS characterization was performed on the cycled cathode, as shown... Figure 9 As shown, the intensity of S element is high in the S 2p spectrum, and there are LiTFSI and Li2S and Li2SO3 produced by the decomposition of additives. Therefore, a good positive electrode / polymer electrolyte interface layer can be formed, thereby suppressing interfacial side reactions.

[0057] Comparative Example 1: Assembly of a solid-state battery using PVDF as the sole binder High-voltage positive electrode powder, conductive carbon, and PVDF binder were weighed in a ratio of 8:1:1, and then N-methylpyrrolidone was added to mix and form a slurry. The resulting slurry was then coated onto a carbon-coated aluminum foil current collector and dried in a vacuum environment at 80°C for 24 hours to obtain the positive electrode sheet. The method for preparing the polymer solid electrolyte is as follows: 0.54 g of PVDF, 0.24 g of lithium bis(trifluoromethanesulfonyl)imide, and 0.12 g of lithium difluorooxalateborate were dissolved in 10 mL of N-methylpyrrolidone, and 0.1 g of alumina nanoparticles with a particle size of 300 nm were added simultaneously. The mixture was stirred at 50°C for 8 hours to form a homogeneous solution. The resulting solution was poured onto a 90 mm circular polytetrafluoroethylene plate and dried in a vacuum oven at 90°C for 24 hours. After drying, it was placed in a glove box and allowed to stand for 12 hours before use to obtain the polymer electrolyte.

[0058] The positive electrode plate, polymer electrolyte plate, lithium plate, gasket, spring plate, and negative electrode shell are placed sequentially on the positive electrode shell. The battery is then sealed and placed in a 45°C forced-air oven for 3 hours before being taken out and cooled.

[0059] When the positive electrode is Ni83, the cycle performance of the assembled Li||Ni83 battery at room temperature and 2C rate in the voltage range of 3~4.3V is as follows: Figure 7 As shown, the solid-state capacity decays rapidly, with only 80% capacity retention after 50 cycles. XPS characterization of the cycled cathode is performed, as shown... Figure 10 As shown, the intensity of S element decreased in the S 2p spectrum, and the content of Li2S was low, which could not effectively protect the high voltage cathode.

[0060] As can be seen from the above embodiments, the thioctic acid-based high-voltage cathode binder additive, possessing high elasticity, high tensile strength, and rapid self-healing properties, can alleviate volume strain in the cathode during charging and discharging, ensuring continuous lithium-ion transport within the cathode. Simultaneously, a self-limiting oxidation reaction occurs, generating a sulfur-rich cathode interface layer, improving the cycle stability of the high-voltage cathode. Furthermore, the carboxyl groups of the thioctic acid-based additive can coordinate with transition metal ions, inhibiting their dissolution. It will be readily understood by those skilled in the art that the above descriptions are merely preferred embodiments of the present invention and are not intended to limit the invention. Any modifications, equivalent substitutions, and improvements made within the spirit and principles of the present invention should be included within the scope of protection of the present invention.

Claims

1. A lipoic acid-based additive for high-voltage cathodes, characterized in that, The lipoic acid-based additive is obtained by stirring and uniformly mixing a lithium salt, a crosslinking agent, a polymerization monomer, a chelating agent and a free radical initiator to obtain a precursor solution, and by heat initiation to make the polymerization monomer undergo ring-opening polymerization to reserve a disulfide bond, and at the same time, the polymerization monomer undergoes a double bond addition reaction under the action of the free radical initiator to form a three-dimensional crosslinked structure. The polymerization monomer is at least one of lipoic acid, lipoamide, lithium lipoate and lipoic acid impurity 9. The mass ratio of the polymerization monomer to the crosslinking agent is 1:5-4:

5.

2. The lipoic acid-based additive for high-voltage cathodes according to claim 1, characterized by the fact that, In the precursor solution, the concentration of the lithium salt is 0.5M-2M, the mass fraction of the chelating agent is 0.01%-0.2%, and the mass fraction of the free radical initiator is 0.1%.

3. The lipoic acid-based additive for high-voltage cathodes according to claim 2, characterized by the fact that, The crosslinking agent is at least one of 1,3-diisopropenyl benzene, 1,3-divinyl benzene and laurylene. The lithium salt is at least one of lithium bistrifluoromethylsulfonylimide, lithium bisfluorosulfonylimide, lithium difluoro(oxalato)borate and lithium tetrafluoroborate. The free radical initiator is azobis isobutyronitrile. The chelating agent is at least one of ferric chloride, ferric nitrate and nickel dichloride.

4. A process for the preparation of a lipoic acid based additive for high voltage cathodes as claimed in claim 3, characterized in that, The method comprises the following steps: The polymerization monomer and the lithium salt are mixed and heated, the crosslinking agent and the chelating agent are added and the stirring reaction is continued for 15-45 minutes to obtain a precursor solution; The precursor solution is heated at 160-220℃ for 15-45 minutes for heat initiation polymerization to obtain the self-healing high-voltage positive electrode adhesive additive.

5. The method of claim 4, wherein the preparation of the lipoic acid-based additive for high-voltage cathodes is characterized by, The temperature for mixing and heating the polymerization monomer and the lithium salt is 160-220℃.

6. The process for the preparation of a lipoic acid-based additive for high-voltage cathodes according to claim 4, characterized by the fact that, Before the chelating agent is added, it is dispersed in an organic solvent; the organic solvent is acetonitrile or acetone.

7. Use of the lipoic acid-based additive for high-voltage positive electrodes according to claim 2 in the preparation of a solid-state battery.

8. Use of a lipoic acid-based additive for high-voltage cathodes according to claim 7 for the preparation of solid-state batteries, characterized by the fact that, The specific obtaining process of the solid-state battery is as follows: The lipoic acid-based additive is added in the preparation of a positive electrode slurry; A positive electrode sheet is placed on a battery shell, followed by a polymer electrolyte, a negative electrode sheet, a spring, a gasket, a negative electrode shell and a packaged battery, and the packaged battery is subjected to heat treatment to obtain the solid-state battery.

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