Stereo-composite crystal polymer electrolyte, preparation method thereof and lithium metal battery

Abstract

The invention discloses a stereocomplex crystal polymer electrolyte, a preparation method thereof and a lithium metal battery, and the preparation method comprises the following steps: grafting D-polylactic acid to a flexible polycaprolactone chain segment, and synthesizing to obtain a PDLA-PCL-PDLA triblock copolymer; the preparation method comprises the following steps: dissolving a PDLA-PCL-PDLA triblock copolymer and L-polylactic acid in trichloromethane, and adding a lithium salt to obtain a mixed solution; and preparing a film to obtain the stereocomplex crystal polymer electrolyte. The electrolyte has a stereocomplex crystal structure. According to the lithium metal solid-state polymer electrolyte, a three-dimensional composite crystal is constructed to form a bicontinuous phase structure, excessive phase separation of two phases can be relieved, the problems of negative electrode side lithium dendrite growth and poor cycling stability in the cycling process of the lithium metal solid-state polymer electrolyte are solved, long circulation of 1200 circles or above can be kept under the high rate of 5C in a charge-discharge cycle test, and the capacity retention rate reaches up to 87.7%.

Description

Technical Field

[0001] This invention relates to the field of stereocomposite polymer electrolyte preparation technology, and in particular to a stereocomposite polymer electrolyte and its preparation method, and a lithium metal battery. Background Technology

[0002] Solid polymer electrolytes typically exhibit low ionic conductivity at room temperature due to excessive crystallinity, and are prone to melting at high temperatures, limiting their practical applications. Single polymer systems often struggle to simultaneously meet multiple requirements, including high ionic conductivity, good mechanical strength, and a wide electrochemical stability window. To overcome this limitation, current technologies often employ a strategy of blending two or more polymer matrices. This strategy eliminates the need for complex chemical synthesis, simplifies the process, and reduces costs. By disrupting the regular arrangement of polymer chains, it reduces crystallinity, thereby improving room-temperature ionic conductivity and broadening the operating temperature range.

[0003] In blended polymer electrolytes, phase-separated solid polymer electrolytes achieve synergistic optimization of functions by inducing controllable phase separation structures at the microscale between two or more incompatible polymers. One phase forms a continuous ion transport channel, which helps improve ion conductivity and transport number; the other phase forms a high-modulus framework, providing the necessary mechanical strength and toughness, which not only facilitates processing and molding but also inhibits lithium dendrite growth and enhances battery safety. Although this type of electrolyte has solved the contradiction between ion conduction and mechanical properties in traditional solid polymer electrolytes to some extent, in practical applications, excessive phase separation can lead to discontinuous and isolated ion-conducting regions, resulting in decreased ion conductivity, deterioration of mechanical properties, and reduced electrode / electrolyte interface stability.

[0004] Currently, the mainstream methods for improving phase separation structures include adding plasticizers or using physical electric field induction. While plasticizers can promote chain segment movement, inhibit crystallization, and improve ionic conductivity, they also cause a decrease in the overall polymer matrix modulus, weakening its ability to suppress lithium dendrites. Physical electric field induction technology relies on an external field to orient charged molecular chains or ion clusters, requiring long-term processing under specific temperature and electric field conditions. This results in complex equipment, high energy consumption, and time-consuming processes, making it difficult to meet the demands of large-scale, high-efficiency manufacturing in the battery industry. Summary of the Invention

[0005] To address the aforementioned technical problems, this invention provides a method for preparing a stereocomposite polymer electrolyte and a lithium metal battery. This invention designs a polylactic acid stereocomposite bicontinuous phase solid polymer electrolyte using a solution casting method, forming a bicontinuous phase structure solid polymer electrolyte. In fact, the stereocomposite crystal can form a bridge at the interface between the two phases, avoiding excessive phase separation that could lead to structural damage, improving ionic conductivity and ion transference number, and suppressing lithium dendrite growth. This successfully solves the problem of the incompatibility between ionic conductivity and mechanical strength, and the poor long-cycle performance of composite solid electrolytes. Specifically: In a first aspect, embodiments of the present invention provide a method for preparing a stereocomposite polymer electrolyte, comprising: PDLA-PCL-PDLA triblock copolymer was synthesized by grafting dextrorotatory polylactic acid onto flexible polycaprolactone segments. The PDLA-PCL-PDLA triblock copolymer and L-polylactic acid were dissolved in chloroform, and lithium salt was added to obtain a mixed solution; The mixed solution is prepared into a film to obtain the stereocomposite polymer electrolyte; the stereocomposite polymer electrolyte has a stereocomposite crystal structure, which is formed by the complexation of the two helical chains of polylactic acid (PLA) and polylactic acid (PLA) through hydrogen bonds and dipole-dipole interactions.

[0006] The following are preferred technical solutions of the present invention, but are not intended to limit the technical solutions provided by the present invention. The purpose and beneficial effects of the present invention can be better achieved and realized through the following preferred technical solutions.

[0007] As a preferred technical solution, in the method for preparing the stereocomposite polymer electrolyte, the molar ratio of the PDLA-PCL-PDLA triblock copolymer to the lithium salt is 1:5-6.

[0008] As a preferred technical solution, the preparation method of the stereocomposite polymer electrolyte, wherein dextrorotatory polylactic acid is grafted onto flexible polycaprolactone segments to synthesize a PDLA-PCL-PDLA triblock copolymer, specifically includes: Hydroxyl-terminated polycaprolactone, D-lactide, and stannous isooctanoate were mixed and heated under vacuum to obtain the PDLA-PCL-PDLA triblock copolymer; wherein the heating temperature was 130-145℃ and the heating time was 18-24h.

[0009] As a preferred technical solution, the method for preparing the stereocomposite polymer electrolyte, wherein the mixed solution is prepared into a film to obtain the stereocomposite polymer electrolyte, specifically includes: The mixed solution is poured into a mold; The mold containing the mixed solution is placed in a vacuum device, sealed and left for 15-24 hours, then dried and transferred to a glove box to stand, thus obtaining the stereocrystalline polymer electrolyte.

[0010] As a preferred technical solution, in the method for preparing the stereocomposite polymer electrolyte, the drying temperature is 70-90℃.

[0011] In a second aspect, a stereocomplex polymer electrolyte is provided, wherein the stereocomplex polymer electrolyte is prepared using any of the preparation methods described above.

[0012] As a preferred technical solution, the stereocomposite polymer electrolyte comprises an ion-conducting phase and a mechanically supported phase, and an interface reinforcement layer formed at the interface between the two phases. The interface reinforcement layer is composed of PDLA-PCL-PDLA triblock copolymer pairs formed by hydrogen bonding, and is used to anchor the ion-conducting phase and the mechanically supported phase to maintain the stability of the dual continuous phase structure.

[0013] Thirdly, a lithium metal battery, wherein the electrolyte comprises any of the stereocrystalline polymer electrolytes described above.

[0014] As a preferred technical solution, the lithium metal battery retains a capacity of more than 87% after 1200 cycles at a 5C rate.

[0015] As a preferred technical solution, the lithium metal battery, when paired with an NCM811 positive electrode at a high voltage of 4.3V, retains a capacity of more than 81% after 300 cycles at 0.5C.

[0016] Beneficial effects: Compared with the prior art, this invention first grafts dextrorotatory polylactic acid (PLA) onto polycaprolactone (PVC), and then blends it with levorotatory polylactic acid (PLA). The two helical chains of PLA are complexed through hydrogen bonds and dipole-dipole interactions to construct a stereocomposite crystal, which effectively improves the connection at the phase interfaces, enhances the ion conduction between phases, and increases the ionic conductivity. It successfully solves the problems of lithium dendrite growth on the negative electrode side and poor cycle stability during the cycling of lithium metal solid polymer electrolytes. The novel electrolyte with stereocomposite crystals has a conductivity of 5.6 × 10⁻⁶. -4 With a high ionic conductivity of S / cm and a lithium-ion transference number of 0.56, it can maintain polarization stability for more than 4000 hours and a low polarization voltage. In charge-discharge cycle tests, it can maintain a long cycle of more than 1200 cycles at a high rate of 5C, with a capacity retention rate of up to 87.7%. Attached Figure Description

[0017] To more clearly illustrate the technical solutions in the embodiments of the present invention or the prior art, the drawings used in the description of the embodiments or the prior art will be briefly introduced below. Obviously, the drawings described below are only some embodiments recorded in the present invention. For those skilled in the art, other drawings can be obtained based on these drawings without creative effort.

[0018] Figure 1 This is a schematic diagram of the preparation method of the stereocomposite polymer electrolyte provided by the present invention.

[0019] Figure 2 The image shows the XRD pattern of the stereocomposite polymer electrolyte membrane.

[0020] Figure 3 AFM diagram of stereocomplex polymer electrolyte.

[0021] Figure 4 This is a diagram showing the ionic conductivity of stereocomposite polymer electrolytes.

[0022] Figure 5 The lithium-ion transference number is the value of the stereocomplex polymer electrolyte.

[0023] Figure 6 The polarization diagram of a lithium symmetric cell is designed to match the stereocomposite polymer electrolyte.

[0024] Figure 7 The graph shows the long-cycle performance of the LiFePO4 / / Li battery with polymer electrolyte. After 1200 cycles at 60℃ and 5 C rate, the capacity retention is 87.7%.

[0025] Figure 8 The figure shows the long-cycle performance of the NCM811 / / Li battery with polymer electrolyte. After 300 cycles at 60°C and 0.5 C rate, the capacity retention is 81.3%. Detailed Implementation

[0026] To enable those skilled in the art to better understand the present invention, the technical solutions of the present invention will be clearly and completely described below with reference to the accompanying drawings of the embodiments of the present invention. Obviously, the described embodiments are only some embodiments of the present invention, and not all embodiments. Based on the embodiments of the present invention, all other embodiments obtained by those skilled in the art without creative effort are within the scope of protection of the present invention.

[0027] like Figure 1 As shown in the embodiment of the present invention, a method for preparing a stereocomposite polymer electrolyte is provided, which includes the following steps: S10. Grafting dextrorotatory polylactic acid onto flexible polycaprolactone segments to synthesize PDLA-PCL-PDLA triblock copolymer.

[0028] Specifically, hydroxyl-terminated polycaprolactone (OH-PCL-OH) and D-lactide were first dried under vacuum. Then, OH-PCL-OH and D-lactide were mixed in a thick-walled, pressure-resistant bottle, and the catalyst stannous isooctanoate Sn(Oct)2 was added. The reaction was carried out under vacuum with stirring. After cooling to room temperature, the resulting white solid product was fully dissolved in dichloromethane. The dissolved solution was then added dropwise to methanol, a poor solvent, resulting in the precipitation of a white flocculent solid. The solid-solution mixture was centrifuged, the supernatant was discarded, and the precipitate was washed several times with methanol. The precipitate was then placed in a vacuum oven and dried under vacuum to obtain the trimer copolymer PDLA-PCL-PDLA.

[0029] S20. Dissolve the PDLA-PCL-PDLA triblock copolymer and L-polylactic acid in chloroform, and add lithium salt to obtain a mixed solution.

[0030] Specifically, volatile chloroform was chosen as the solvent, and the trimer PDLA-PCL-PDLA and polylactic acid (PLLA) were dissolved in chloroform. Lithium salt LiTFSI was added at a molar ratio of 6:1 to the lithium salt and polymer and stirred to obtain a mixed solution.

[0031] S30. The mixed solution is prepared into a film to obtain the stereocomposite polymer electrolyte.

[0032] Specifically, the solution is poured into a petri dish (the mass of the polymer and the size of the petri dish are controlled to control the thickness of the polymer electrolyte membrane). To control the formation of the polymer electrolyte membrane and avoid the formation of pores in the electrolyte membrane due to rapid solvent evaporation, the petri dish containing the precursor solution is first placed in a vacuum oven for evaporation at room temperature under sealed conditions. After the solvent has evaporated, it is dried under vacuum conditions until the solvent is completely dried. Then, the polymer electrolyte membrane is transferred to a glove box to stand and remove water and oxygen.

[0033] In this invention, dextrorotatory polylactic acid (PDLA) is grafted onto polycaprolactone (PLLA) through polymer design, and then blended with levorotatory polylactic acid (PLLA) to form a stereochemical composite crystal of polylactic acid. The unique three-dimensional network structure formed by hydrogen bonding between the CH3···O=C and CH···OC molecular chains, from a crystal formation perspective, results in a stable helical structure formed by the complexation of PDLA and PLLA, improving thermal stability and increasing the mechanical strength of the polymer electrolyte. PLLA and polylactic acid form a bicontinuous phase-separated structure, with one phase being the soft phase of PLLA and the other composed of the hard phase of polylactic acid. Simultaneously, the formation of the stereochemical composite crystal creates a "bridge" between the phases, stabilizing the phase interface and preventing the destruction of the polymer electrolyte structure due to excessive phase separation. Furthermore, under high voltage and high rate long-cycle conditions, it can prevent oxidative decomposition at the positive electrode and inhibit lithium dendrite growth in the negative electrode layer. In addition to the conventional lithium-ion transport via the complexation of carbonyl groups on PCL with Li, the stereocomposite electrolyte membrane also forms an additional lithium-ion transport channel through a three-dimensional network of hydrogen bonds formed by the polylactic acid stereocomposite crystals. This improves ionic conductivity and lithium-ion transference number, alleviates lithium-ion concentration polarization at the negative electrode interface, and solves the problems of lithium dendrite growth and poor cycle stability on the negative electrode side during lithium metal solid polymer electrolyte cycling. The polylactic acid stereocomposite bicontinuous phase solid electrolyte proposed in this invention exhibits excellent performance, such as... Figure 3 and Figure 4 The results show that it has high ionic conductivity and lithium-ion transference number.

[0034] Based on the same inventive concept, this invention also provides a stereocomposite crystalline polymer electrolyte, which is prepared using the preparation method described above. The specific preparation process has been explained in detail above and will not be repeated here.

[0035] Based on the same inventive concept, the present invention also provides a lithium metal battery comprising the above-described stereocomposite polymer electrolyte.

[0036] like Figure 6 As shown, the assembled lithium-ion symmetric battery can cycle stably for over 4000 hours at 60°C, exhibiting a low polarization voltage. Furthermore, we assembled this polylactic acid stereocrystalline dual-phase solid electrolyte into a 2032-type coin cell to test its long-cycle capability at 60°C, and also tested its long-cycle capability with a matched high-voltage NCM811 cathode cell. Figure 7 and Figure 8 As shown, under high-rate conditions of 5C, the capacity retention rate after 1200 cycles is 87.7%, and the NCM811 battery under conditions of 4.3V 0.5C retains 81.3% of its capacity after 300 cycles. This invention successfully solves the problems of low ionic conductivity, low ion transference number, poor battery stability, and poor electrolyte cycle performance of current solid electrolytes.

[0037] The technical solutions provided by the present invention will be further explained and illustrated below through specific embodiments.

[0038] Example 1 First, hydroxyl-terminated polycaprolactone (OH-PCL-OH) and D-lactide were vacuum dried. Then, 3 g of OH-PCL-OH and 6 g of D-lactide were mixed in a thick-walled, pressure-resistant bottle, and 28 μL of stannous isooctanoate Sn(Oct)2 catalyst was added. The mixture was reacted under vacuum at 137 °C and a stirring speed of 400 r / min for 24 h. After cooling to room temperature, the white solid product was completely dissolved in dichloromethane. The dissolved solution was added dropwise to methanol, a poor solvent, resulting in the precipitation of a white flocculent solid. The solid-solution mixture was centrifuged at 9000 r / min for 10 min, the supernatant was discarded, and the precipitate was washed several times with methanol. The precipitate was placed in a vacuum oven and dried under vacuum at 40 °C for 24 h to obtain the trimer copolymer PDLA-PCL-PDLA.

[0039] 0.5 g of trimer PDLA-PCL-PDLA and 0.5 g of polylactic acid (PLLA) were dissolved in 30 ml of chloroform. 0.488 g of lithium salt LiTFSI was added at a lithium salt to polymer molar ratio of 6:1, and the mixture was stirred for 12 h. This solution was poured into 9 cm diameter culture dishes. The culture dishes containing the precursor solution were first placed in a vacuum oven and allowed to evaporate at room temperature for 24 h under sealed conditions. After the solvent had evaporated, the mixture was dried at 80 °C under vacuum for 8 h to completely dry the solvent. The polymer electrolyte membrane was then transferred to a glove box and allowed to stand for 4 h to remove water and oxygen.

[0040] Example 2 First, hydroxyl-terminated polycaprolactone (OH-PCL-OH) and D-lactide were vacuum dried. Then, 3 g of OH-PCL-OH and 6 g of D-lactide were mixed in a thick-walled, pressure-resistant bottle, and 28 μL of stannous isooctanoate Sn(Oct)2 catalyst was added. The mixture was reacted under vacuum at 145 °C and a stirring speed of 400 r / min for 20 h. After cooling to room temperature, the white solid product was completely dissolved in dichloromethane. The dissolved solution was added dropwise to methanol, a poor solvent, resulting in the precipitation of a white flocculent solid. The solid-solution mixture was centrifuged at 9000 r / min for 10 min, the supernatant was discarded, and the precipitate was washed several times with methanol. The precipitate was placed in a vacuum oven and dried under vacuum at 40 °C for 24 h to obtain the trimer copolymer PDLA-PCL-PDLA.

[0041] 0.5 g of trimer PDLA-PCL-PDLA and 0.5 g of polylactic acid (PLLA) were dissolved in 30 ml of chloroform. Lithium salt LiTFSI was added at a lithium salt to polymer molar ratio of 5:1 and stirred for 10 h. This solution was poured into a 9 cm diameter culture dish. The culture dish containing the precursor solution was first placed in a vacuum oven and allowed to evaporate at room temperature for 24 h under sealed conditions. After the solvent had evaporated, it was dried at 90 °C under vacuum for 8 h to completely dry the solvent. The polymer electrolyte membrane was then transferred to a glove box and allowed to stand for 4 h to remove water and oxygen.

[0042] In summary, this invention provides a stereocomposite polymer electrolyte and its preparation method, as well as a lithium metal battery. The preparation method includes: grafting dextrorotatory polylactic acid (DLA) onto flexible polycaprolactone (PVC) segments to synthesize a PDLA-PCL-PDLA triblock copolymer; dissolving the PDLA-PCL-PDLA triblock copolymer and levorotatory polylactic acid (LLA) in chloroform and adding lithium salt to obtain a mixed solution; and preparing the mixed solution into a film to obtain the stereocomposite polymer electrolyte. In this invention, DLA is grafted onto PVC and then blended with LLA. PVC and PVC form a bicontinuous phase-separated structure. The enantiomeric polymers of LLA and DLA form a stereocomposite polylactic acid structure. The hydrogen-bonded three-dimensional network structure of the molecular chains improves mechanical strength and thermal stability. Simultaneously, the hydrogen-bonded network at the biphase interface prevents excessive separation of the two phases, improving the stability between the phases. Compared to traditional polymer electrolytes, this more stable structure can suppress lithium dendrite growth at the interface and oxidative decomposition of solid polymer electrolytes under high rate and high voltage conditions, achieving long-term cycling at high rate and high voltage. The continuous ion transport pathways and additional ion transport channels of the hydrogen bond network improve ionic conductivity and lithium-ion transference number, alleviate ion concentration polarization at the interface, suppress interfacial side reactions, and simultaneously enhance the battery's long-term cycling stability. The unique stereocomposite crystalline phase-separated structure, high ionic conductivity, and high ion transference number electrolyte enable the battery to achieve polarization stability exceeding 4000 hours and a long cycling life of 1200 cycles with a capacity retention of up to 87.7%. Furthermore, when matched with a high-voltage NCM811 cathode, it can cycle for 300 cycles at 4.3V.

[0043] It should be understood that the present invention is not limited to the precise structure described above and shown in the accompanying drawings, and various modifications and changes can be made without departing from its scope. The scope of the invention is limited only by the appended claims.

[0044] 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 method for preparing a stereocomposite polymer electrolyte, characterized in that, include: PDLA-PCL-PDLA triblock copolymer was synthesized by grafting dextrorotatory polylactic acid onto flexible polycaprolactone segments. The PDLA-PCL-PDLA triblock copolymer and L-polylactic acid were dissolved in chloroform, and lithium salt was added to obtain a mixed solution; The mixed solution is prepared into a film to obtain the stereocomposite polymer electrolyte; the stereocomposite polymer electrolyte has a stereocomposite crystal structure, which is formed by the complexation of the two helical chains of polylactic acid (PLA) and polylactic acid (PLA) through hydrogen bonds and dipole-dipole interactions.

2. The method for preparing the stereocomposite polymer electrolyte according to claim 1, characterized in that, The molar ratio of the PDLA-PCL-PDLA triblock copolymer to the lithium salt is 1:5-6.

3. The preparation method of the stereocomposite polymer electrolyte according to claim 1, characterized in that, By grafting dextrorotatory polylactic acid (DLA) onto flexible polycaprolactone (PVC) segments, a PDLA-PCL-PDLA triblock copolymer was synthesized, specifically comprising: Hydroxyl-terminated polycaprolactone, D-lactide, and stannous isooctanoate were mixed and heated under vacuum to obtain the PDLA-PCL-PDLA triblock copolymer; wherein the heating temperature was 130-145℃ and the heating time was 18-24h.

4. The method for preparing the stereocomposite polymer electrolyte according to claim 1, characterized in that, The mixed solution is prepared into a film to obtain the stereocomposite polymer electrolyte, specifically comprising: The mixed solution is poured into a mold; The mold containing the mixed solution is placed in a vacuum device, sealed and left for 15-24 hours, then dried and transferred to a glove box to stand, thus obtaining the stereocrystalline polymer electrolyte.

5. The method for preparing the stereocomposite polymer electrolyte according to claim 4, characterized in that, The drying temperature is 70-90℃.

6. A stereocomposite polymer electrolyte, characterized in that, The stereocomplex polymer electrolyte is prepared using any one of the preparation methods described in claims 1-5.

7. The stereocomposite polymer electrolyte according to claim 6, characterized in that, The stereocomposite polymer electrolyte comprises an ion-conducting phase and a mechanically supported phase, as well as an interface reinforcement layer formed at the interface between the two phases. The interface reinforcement layer is composed of PDLA-PCL-PDLA triblock copolymer pairs formed by hydrogen bonding, and is used to anchor the ion-conducting phase and the mechanically supported phase to maintain the stability of the dual continuous phase structure.

8. A lithium metal battery, characterized in that, The electrolyte comprises the stereocomposite polymer electrolyte according to any one of claims 6-7.

9. The lithium metal battery according to claim 8, characterized in that, After 1200 cycles at a 5C rate, the capacity retention is greater than 87%.

10. The lithium metal battery according to claim 8, characterized in that, When paired with the NCM811 positive electrode at 4.3V high voltage, the capacity retention rate is greater than 81% after 300 cycles at 0.5C.

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