A polymer electrolyte with low interfacial impedance and a preparation method and application thereof

Abstract

The application discloses a polymer electrolyte with low interface impedance and a preparation method and application thereof. The electrolyte comprises the following components in percentage by mass: olefin polymer 30-45%, disodium salt 10-25% and plasticizer 35-50%; the disodium salt comprises sodium tetrafluoroborate, and the mass of the sodium tetrafluoroborate is 50-60% of the mass of the disodium salt. The electrolyte is prepared by mixing raw materials including the olefin polymer, the disodium salt and the plasticizer uniformly under a protective atmosphere, pouring the mixture on a mold, and drying. The solid-state sodium battery prepared based on the polymer electrolyte provided by the application effectively solves the problems of serious electrolyte and electrode interface side reaction and high interface impedance, and greatly improves the cycle stability and structural safety of the sodium solid-state battery. Test shows that the sodium battery provided by the application has a capacity retention rate of up to 94.4% after being cycled at a rate of 1C for 650 cycles.

Description

TECHNICAL FIELD

[0001] The present application relates to a polymer electrolyte, in particular to a polymer electrolyte with low interfacial impedance and a preparation method and application thereof, and belongs to the technical field of sodium solid-state battery electrolyte. BACKGROUND

[0002] Sodium-ion batteries have the advantages of low cost and abundant resources compared with lithium-ion batteries, and are a strong competitor in future battery technology. Moreover, sodium metal has a high theoretical specific capacity (1166 mAh / g) and a low electrochemical potential (-2.71 V), which makes sodium metal batteries (NMBs) have a high theoretical energy density of 275 Wh / kg. However, the organic electrolyte used in traditional sodium-ion batteries has the problems of flammability, explosion, and dendrite penetration, which seriously hinders the further development of sodium-ion batteries. Solid-state electrolytes have excellent thermal stability, mechanical properties, and high energy density, and have attracted widespread attention. They can be mainly divided into inorganic solid-state electrolytes, solid-state polymer electrolytes, and organic-inorganic composite electrolytes. Although inorganic solid-state electrolytes have high ionic conductivity, their poor interface stability and interface contact rigidity limit their development. In contrast, solid-state polymer electrolytes have the characteristics of light weight, excellent processing performance, and good interface contact, and have gradually become a research hotspot.

[0003] At present, the reported sodium-ion polymer matrixes include polyethylene oxide (PEO), polyvinyl alcohol (PVA), polyvinylidene fluoride-hexafluoropropylene (PVDF-HFP), polyacrylonitrile (PAN), polymethyl methacrylate (PMMA), polytrimethylene carbonate (PTMC), etc. However, the room temperature ionic conductivity of the existing sodium solid-state battery polymer electrolyte is usually between 10 - 5 S·cm -1 and 10 -7 S·cm -1 , which is mainly because the polymer matrix exhibits high crystallinity at room temperature, and the existence of the crystalline region hinders the movement of the molecular chain, resulting in difficult ion transmission. In order to effectively improve the room temperature ionic conductivity of the polymer electrolyte, the commonly used strategy is to select a high dissociation degree salt, add inorganic fillers, plasticizers, and ionic liquids, etc.

[0004] Patent No. 202210823149.4 discloses a method for improving the ionic conductivity of sodium-ion battery solid-state electrolyte and a polymer solid-state electrolyte prepared thereby, which improves the polymer conductive performance by adding a high dissociation degree sodium salt and adjusting the ratio of the polymer matrix and the sodium salt.

[0005] In addition, constructing a stable electrode and electrolyte interface is also a key to improve the electrochemical performance of the battery. Many researchers form a thin and dense solid electrolyte interface layer (SEI) at the electrolyte and electrode interface by adding a film-forming additive. This SEI film can effectively uniform ion deposition and inhibit the occurrence of side reactions, thereby improving the cycle life and safety of the battery. However, the polymer electrolyte in the prior art scheme often has the problems of low ionic conductivity and poor cycle stability. The commonly used film-forming additives for sodium batteries are FEC, VC, DTD, NaDFOB and NaBF4, among which the film-forming sodium salt can increase the salt concentration of the electrolyte and form a stable SEI / CEI film, which is a better choice for developing a low impedance interface. Therefore, in order to improve the overall performance of the battery, it is particularly important to select appropriate salts and additives to improve the room temperature ionic conductivity of the electrolyte and optimize the electrolyte and electrode interface.

[0006] A kind of polymer sodium ion solid electrolyte and its preparation method and application as the application number of 202310457804.3 disclose, by utilizing the synergistic effect of sodium salt and functional adjuvant, the room temperature ionic conductivity and electrochemical performance of sodium ion solid electrolyte are significantly improved. SUMMARY

[0007] In view of the problems in the prior art, the first object of the present application is to provide a polymer electrolyte with low interface impedance. The polymer electrolyte is based on the synergistic effect between raw materials. By selecting a polymer matrix with high dielectric constant, matching with double sodium salt and small molecule plasticizer, the interface impedance can be effectively reduced while improving the conductivity of the electrolyte film, and the occurrence of interface side reactions can be inhibited. Among them, the high dissociation degree sodium salt in the double sodium salt can improve the interface ion mobility of the system. Combined with sodium tetrafluoroborate, a stable SEI film is formed to prevent interface side reactions of high dissociation degree sodium salt under high pressure. Combined with propylene carbonate, the solvation structure of sodium salt is promoted, and the crystallinity of the polymer is reduced, which significantly improves the Na + The transmission rate inside the electrolyte and the interface wettability.

[0008] The second object of the present application is to provide a preparation method of a polymer electrolyte with low interface impedance. The method uses a solution casting method. The raw materials including olefin polymer, sodium salt, plasticizer and solvent are mixed uniformly and then poured into a mold under a protective atmosphere. After drying, the electrolyte is obtained. The method is simple and easy to mass produce. At the same time, the prepared electrolyte has high room temperature ionic conductivity, and the conductivity is maintained at 10 -4 S·cm -1 Around.

[0009] A third object of the present application is to provide an application of a polymer electrolyte with low interface impedance for preparing a sodium ion solid-state battery.

[0010] To achieve the above technical purposes, the present application provides a polymer electrolyte with low interface impedance, characterized in that it comprises the following components by mass percentage: 30-45% of an olefin polymer, 10-25% of a double sodium salt, and 35-50% of a plasticizer; the double sodium salt contains sodium tetrafluoroborate, the mass of which is 50-60% of the mass of the double sodium salt.

[0011] The present application significantly improves the safety, conductivity and cycle life of the polymer electrolyte by introducing a double sodium salt and an organic small molecule plasticizer into the polymer electrolyte system; the synergistic effect of the double salt significantly improves the room temperature ionic conductivity and electrochemical window of the electrolyte membrane; in addition, the introduction of the film-forming sodium salt forms a more stable SEI film at the interface of the sodium metal anode, inhibits the occurrence of interface side reactions, and improves the interface contact; the organic small molecule plasticizer can promote the solvation structure of the salt and reduce the crystallinity of the polymer, thereby improving the fast charging performance of the battery.

[0012] It should be noted that in the technical solution provided by the present application, each raw material component needs to be strictly executed according to the above requirements. If the addition amount of sodium tetrafluoroborate in the double sodium salt system is too high, the ionic conductivity of the electrolyte will decrease, and the battery performance will decrease; if the addition amount of sodium tetrafluoroborate is too low, the occurrence of interface side reactions cannot be effectively inhibited, which will lead to the increase of interface impedance and the decrease of cycle life of the battery.

[0013] As a preferred scheme, the olefin polymer is at least one of polyvinylidene fluoride, poly(vinylidene fluoride-hexafluoropropylene) copolymer, poly(vinylidene fluoride-trifluoroethylene) copolymer, poly(vinylidene fluoride-tetrafluoroethylene) copolymer, and poly(vinylidene fluoride-trifluoroethylene-trifluorochloroethylene) copolymer.

[0014] As a preferred scheme, the double sodium salt is sodium tetrafluoroborate and the double sodium salt is sodium (trifluoromethylsulfonyl) imide, and the mass ratio of the two is 1:0.9-1.1.

[0015] As a preferred scheme, the plasticizer is propylene carbonate.

[0016] As a preferred scheme, the volume mass percentage of the propylene carbonate and the olefin polymer is 0.8-1.2 ml / g. Further preferably, the volume mass ratio of the propylene carbonate and the olefin polymer is 1 ml / g. The propylene carbonate can effectively promote the sodium salt solvation structure and reduce the polymer crystallization area. If the addition amount of PC is too much, serious interface side reactions will occur, causing the battery to fail rapidly; if the addition amount of the small molecule plasticizer is too small, the processing performance of the electrolyte will be poor, the curling deformation will occur, and the mechanical properties and mechanical properties required by the sodium ion solid-state battery cannot be met.

[0017] The application further provides a preparation method of the polymer electrolyte with low interface impedance.

[0018] As a preferred scheme, the protective atmosphere is nitrogen and / or argon.

[0019] As a preferred scheme, the raw materials further comprise a solvent, and the volume mass ratio of the solvent and the olefin polymer is 7-12 ml / g.

[0020] As a preferred scheme, the solvent is acetone and / or N-methyl pyrrolidone.

[0021] As a preferred scheme, the raw materials are mixed by stirring, and the stirring conditions are as follows: the temperature is 40-60 DEG C, the stirring rate is 600-1000 r / min, and the time is 16-24 h. The stirring process involves the mixing of the raw materials, and if the temperature is too low, the polymer is not completely dissolved, and the electrolyte membrane of the vegetation is not uniform; if the temperature is too high, the polymer may be depolymerized, broken, and side groups are removed, and the like.

[0022] As a preferred scheme, the drying method is at least one of protective gas drying, vacuum drying and freeze drying.

[0023] As a preferred scheme, the protective gas drying conditions are as follows: the raw materials are dried under the protective atmosphere at room temperature for 5-7 h.

[0024] As a preferred scheme, the protective atmosphere is nitrogen and / or argon.

[0025] As a preferred scheme, the drying process is complete evaporation of the solvent in the raw materials.

[0026] As a preferred scheme, the thickness of the polymer electrolyte is 500-650 mu m.

[0027] The application further provides an application of the polymer electrolyte with low interface impedance, which is used for preparing a sodium ion solid-state battery.

[0028] As a preferred scheme, the specific preparation process of the sodium solid-state battery is as follows: the positive electrode material, the conductive agent and the binder with a mass ratio of 8:1:1 are uniformly mixed and coated on an aluminum current collector, placed in a vacuum oven for drying at a temperature of 80 DEG C for 12 hours, then taken out, and cut by a cutter with a diameter of 12 mm to obtain a positive electrode sheet; similarly, the negative electrode material, the conductive agent and the binder with a mass ratio of 8:1:1 are uniformly mixed and coated on a copper current collector, placed in a vacuum oven for drying at a temperature of 80 DEG C for 12 hours, then taken out, and cut by a cutter with a diameter of 12 mm to obtain a negative electrode sheet; the obtained positive electrode sheet and the negative electrode are packaged, and the sodium solid-state battery is obtained.

[0029] Compared with the prior art, the technical scheme of the present application has the beneficial technical effects that:

[0030] 1) The polymer electrolyte provided by the present application is based on the synergistic effect between raw materials, by selecting a polymer matrix with high dielectric constant, matching with double sodium salt and small molecule plasticizer, the interface impedance can be effectively reduced while the conductivity of the electrolyte film is improved, and the occurrence of interface side reactions is inhibited; among them, the high dissociation degree sodium salt in the double sodium salt can improve the interface ion migration rate of the system, combined with sodium tetrafluoroborate, a stable SEI film is formed to prevent the interface side reaction of high dissociation degree sodium salt under high pressure, combined with propylene carbonate to promote the solvation structure of sodium salt and reduce the crystallinity of the polymer, which significantly improves the Na + The transmission rate in the electrolyte and the interface wettability.

[0031] 2) In the preparation method provided by the present application, a solution casting method is used, and the raw materials including olefin polymer, sodium salt, plasticizer and solvent are mixed uniformly and poured on a mold under a protective atmosphere, and then dried to obtain the electrolyte. The method is simple, easy to mass produce, and the prepared electrolyte has high room temperature ionic conductivity, and the conductivity is maintained at about 10 -4 S·cm -1 .

[0032] 3) In the technical scheme provided by the present application, the solid-state sodium battery prepared based on the polymer electrolyte provided by the present application effectively solves the problems of serious interface side reaction of the electrolyte and electrode and high interface impedance, and greatly improves the cycle stability and structural safety of the sodium solid-state battery. Tests show that the sodium battery provided by the present application has a capacity retention rate of up to 94.4% after 650 cycles at a rate of 1C. BRIEF DESCRIPTION OF DRAWINGS

[0033] Figure 1 The physical pictures of the electrolyte film and the sodium negative electrode interface after the sodium solid-state batteries in Comparative Examples 1-2 and Example 1 are cycled at a rate of 1C for 30 times are shown in the following figures:

[0034] Figure 2 The thickness of the polymer electrolyte prepared in Example 1 was 594 μm;

[0035] Figure 3 SEM image of the polymer electrolyte prepared in Example 1;

[0036] Figure 4 Linear voltammetry test curve and electrochemical window data of the polymer electrolytes prepared in Comparative Examples 1-2 and Example 1;

[0037] Figure 5 Electrochemical impedance spectrum and room temperature ionic conductivity data of the polymer electrolytes prepared in Comparative Examples 1-2 and Example 1;

[0038] Figure 6 Physical image of the polymer electrolyte prepared in Comparative Example 3;

[0039] Figure 7 Cycle number vs. discharge specific capacity and charge-discharge efficiency curve of the sodium solid-state battery in Comparative Examples 1-2 and Example 1 at a rate of 1C. DETAILED DESCRIPTION

[0040] In order to have a clearer understanding of the technical features, objects and effects of the present application, the specific embodiments of the present application will be described in detail with reference to the accompanying drawings. It should be noted that the present application is not limited to the following examples, and the following examples are only illustrative and not limiting. Those skilled in the art can make many forms without departing from the spirit and scope of the present application under the inspiration of the present application, and these all belong to the protection of the present application.

[0041] Example 1

[0042] The present embodiment provides a polymer electrolyte with low interfacial impedance and a sodium solid-state battery thereof, and the steps are as follows:

[0043] Step 1, according to the following formula, the raw materials are weighed, poly(vinylidene fluoride-hexafluoropropylene) (PVDF-HFP) 1 g; sodium bis(trifluoromethylsulfonyl)imide (NaTFSI) 0.2 g; sodium tetrafluoroborate (NaBF4) 0.2 g; propylene carbonate (PC) 1 ml; acetone (AC) 10 ml. In an atmosphere protection environment, the above materials are placed in a serum bottle, and stirred at a temperature of 50°C and a speed of 600 r / min for 16 hours; the obtained electrolyte slurry is scraped on a glass plate, and dried in a glove box at room temperature for 5h to remove organic solvents, and a self-supporting polymer electrolyte is obtained;

[0044] Step 2, based on the obtained polymer electrolyte, a stainless steel / polymer electrolyte / stainless steel symmetric battery was assembled, and AC impedance spectrum test and linear sweep voltammetry test were carried out by using an electrochemical workstation; the room temperature ionic conductivity and electrochemical window data of the polymer electrolyte were calculated, and the results are shown in Table 1.

[0045] Step 3, based on the obtained polymer electrolyte, NVP@C was used as the positive electrode, and sodium metal was used as the negative electrode to assemble an NVP@C|Na full battery, and the cycle performance test and interface impedance test were carried out. The specific operation was as follows: in the electrochemical window range of 2.0-3.8 V, the charge and discharge cycle test was carried out at room temperature by using 1C rate, the cycle performance of the above-mentioned battery was tested, the cycle number of the capacity retention rate of 80% was counted; in addition, the battery cycled for 30 cycles at the rate was subjected to AC impedance spectrum test by using an electrochemical workstation, and the interface impedance data were counted, and the data results of the present embodiment are recorded in Table 1.

[0046] Example 2

[0047] Compared with Example 1, the negative electrode in Example 2 used hard carbon material; the rest of the conditions and test types were the same as those in Example 1.

[0048] Example 3

[0049] Compared with Example 1, the negative electrode in Example 3 used anode-free (aluminum current collector); the rest of the conditions and test types were the same as those in Example 1.

[0050] Example 4

[0051] Compared with Example 1, the positive electrode in Example 4 used layered oxide; the rest of the conditions and test types were the same as those in Example 1.

[0052] Example 5

[0053] Compared with Example 1, the positive electrode in Example 5 used Prussian blue; the rest of the conditions and test types were the same as those in Example 1.

[0054] Comparative Example 1

[0055] Compared with Example 1, in Comparative Example 1, the double salt was changed to contain only a single salt sodium bis(trifluoromethylsulfonyl)imide (NaTFSI) 0.4 g; the rest of the conditions and test types were the same as those in Example 1.

[0056] Comparative Example 2

[0057] Compared with Example 1, in Comparative Example 2, the double salt was changed to contain only a single salt sodium tetrafluoroborate (NaBF4) 0.4 g; the rest of the conditions and test types were the same as those in Example 1.

[0058] Comparative Example 3

[0059] Comparative Example 3 Sodium bis(trifluoromethylsulfonyl)imide (NaTFSI) 0.3 g; Sodium tetrafluoroborate (NaBF4) 0.1 g; the rest of the conditions and test types are the same as Example 1.

[0060] Comparative Example 4

[0061] Comparative Example 4 Sodium bis(trifluoromethylsulfonyl)imide (NaTFSI) 0.1 g; Sodium tetrafluoroborate (NaBF4) 0.3 g; the rest of the conditions and test types are the same as Example 1.

[0062] Comparative Example 5

[0063] Comparative Example 5 No PC; the rest of the conditions and test types are the same as Example 1. But in Comparative Example 5, the polymer electrolyte slowly shrinks and gradually hardens during the solvent evaporation process, and the battery cannot be assembled.

[0064] Comparative Example 6

[0065] Comparative Example 6 PC content is 0.5 ml; the rest of the conditions and test types are the same as Example 1. But in Comparative Example 6, the polymer electrolyte slowly shrinks and gradually hardens during the solvent evaporation process, and the battery cannot be assembled.

[0066] Comparative Example 7

[0067] Comparative Example 7 PC content is 0.8 ml; the rest of the conditions and test types are the same as Example 1.

[0068] Comparative Example 8

[0069] Comparative Example 8 PC content is 1.5 ml; the rest of the conditions and test types are the same as Example 1.

[0070] Comparative Example 9

[0071] Comparative Example 9 PC content is 2 ml; the rest of the conditions and test types are the same as Example 1.

[0072] Comparative Example 10

[0073] Comparative Example 10 The negative electrode uses hard carbon material; the rest of the conditions and test types are the same as Example 1.

[0074] Comparative Example 11

[0075] Comparative Example 11 The negative electrode uses no negative electrode (aluminum current collector); the rest of the conditions and test types are the same as Example 1.

[0076] Comparative Example 12

[0077] Compared with Comparative Example 1, in Comparative Example 12, the positive electrode adopts a layered oxide; the rest of the conditions and test types are the same as those in Comparative Example 1.

[0078] Comparative Example 13

[0079] Compared with Comparative Example 1, in Comparative Example 13, the positive electrode adopts Prussian blue; the rest of the conditions and test types are the same as those in Comparative Example 1.

[0080] Table 1

[0081]

[0082]

[0083] As can be seen from the data of Example 1 and Comparative Examples 3 and 4 in Table 1, as the proportion of sodium tetrafluoroborate increases, the cycle life and interface impedance both increase first and then decrease, and the electrolyte obtained when the proportion of sodium tetrafluoroborate is 50% of the mass of the disodium salt has the best performance; further, as can be seen from Comparative Examples 6 to 9, under the premise that the proportion of sodium tetrafluoroborate is 50% of the mass of the disodium salt, as the proportion of PC increases, the ionic conductivity is significantly improved, the interface impedance gradually decreases, and the cycle stability is also improved, and when the mass / volume ratio of PC to the polymer matrix is 1 ml / g, the solid-state battery has the best performance, but further increasing the content of PC will lead to a downward trend in the improvement effect of the cycle performance of the battery. Therefore, the proportion of sodium tetrafluoroborate in the disodium salt and the proportion of propylene carbonate must be within the range required by the present application in order to achieve the corresponding technical purpose.

[0084] The Example 1 provided by the present application improves the cycle stability to 650 cycles and the capacity retention rate is as high as 94.4% through the synergistic effect of the two sodium salts, and the cycle life is three times that of the battery without the addition of sodium tetrafluoroborate. In addition, through the addition of sodium tetrafluoroborate, the ionic conductivity of the polymer electrolyte film is improved by 20% to 30%, and the interface impedance is reduced by 20% to 30%. Figure 5 It can also be seen that the introduction of sodium tetrafluoroborate can significantly improve the electrochemical window of the polymer electrolyte film, and through the addition of sodium tetrafluoroborate, the ionic conductivity of the polymer electrolyte film is improved by 20% to 30%, and the interface impedance is reduced by 20% to 30%. Figure 4 It can be seen that the ionic conductivity of the disodium salt still remains at 10 -4 S·cm -1 The level can meet the market demand. In order to prove the interface stability, the batteries after cycling in Comparative Example 1, Comparative Example 2 and Example 1 are disassembled, and the results are as follows. Figure 1As shown, the electrolyte membrane in Comparative Example 1 is blackened, indicating that a serious side reaction occurs at the interface. In comparison, the electrolyte membranes in Example 1 and Comparative Example 2 have no serious discoloration, indicating that no serious side reaction occurs at the interface. However, in Comparative Example 2, NaBF4 is used alone as the sodium salt, and the cycle stability is only 3 weeks, and stable charging and discharging cannot be performed. In summary, only by using a double sodium salt system containing sodium tetrafluoroborate can the occurrence of interface side reactions be significantly inhibited under the premise of improving the cycle stability of the electrolyte, and the battery interface is stabilized.

Claims

1. A polymer electrolyte with low interfacial impedance, characterized in that, It comprises the following components by mass percentage: 30-45% olefin polymer, 10-25% disodium salt, and 35-50% plasticizer; wherein the disodium salt contains sodium tetrafluoroborate, which accounts for 50-60% of the mass of the disodium salt; wherein the disodium salt is sodium tetrafluoroborate and the disodium salt is sodium (trifluoromethanesulfonyl)imide, and the mass ratio of the two is 1:0.9-1.

1.

2. The polymer electrolyte with low interfacial impedance according to claim 1, characterized in that: The olefin polymer is at least one of polyvinylidene fluoride, poly(vinylidene fluoride-hexafluoropropylene) copolymer, poly(vinylidene fluoride-trifluoroethylene) copolymer, poly(vinylidene fluoride-tetrafluoroethylene) copolymer, and poly(vinylidene fluoride-trifluoroethylene-trifluorochloroethylene) copolymer.

3. The polymer electrolyte with low interfacial impedance according to claim 1, characterized in that: The plasticizer is propylene carbonate.

4. A method for preparing a low interfacial impedance polymer electrolyte according to any one of claims 1 to 3, characterized in that: Under a protective atmosphere, raw materials including olefin polymers, disodium salts and plasticizers are mixed evenly and then poured into a mold. After drying, the product is obtained.

5. The method for preparing a polymer electrolyte with low interfacial impedance according to claim 4, characterized in that: The protective atmosphere is nitrogen and / or argon; the raw materials also include a solvent, the volume-to-mass ratio of the solvent to the olefin polymer is 7~12 ml / g; the solvent is acetone and / or N-methylpyrrolidone.

6. The method for preparing a polymer electrolyte with low interfacial impedance according to claim 4, characterized in that: The raw materials are mixed by stirring, under the following conditions: temperature 40~60℃, stirring rate 600~1000r / min, and time 16~24h.

7. The method for preparing a polymer electrolyte with low interfacial impedance according to claim 4, characterized in that: The drying method is at least one of protective gas drying, vacuum drying and freeze drying; the conditions for protective gas drying are: drying at room temperature under a protective atmosphere for 5 to 7 hours; the protective atmosphere is nitrogen and / or argon.

8. The method for preparing a polymer electrolyte with low interfacial impedance according to claim 5, characterized in that: The drying process involves completely evaporating the solvent in the raw material; the thickness of the polymer electrolyte is 500~650μm.

9. The application of the low interfacial impedance polymer electrolyte according to any one of claims 1 to 3, characterized in that: Used to prepare sodium-ion solid-state batteries.

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