A high-temperature resistant battery separator, its preparation method, and a lithium-ion battery

Abstract

This invention provides a battery separator and its preparation method, which is mainly composed of polyolefin-based pyridine material. The preparation method includes the following steps: S1, dissolving olefin-based pyridine monomer in toluene to prepare a solution, adding a metallocene catalyst, controlling the reaction temperature at 20-25°C, reacting for 12-18 hours, quenching with methanol dropwise, adding n-hexane, stirring and allowing to stand, washing and filtering to obtain a white polyolefin-based pyridine material; dissolving the polyolefin-based pyridine material obtained in step S1 in N,N-dimethylformamide or N,N-dimethylacetamide to prepare a spinning solution, and then forming a polymer membrane by electrospinning to obtain a battery separator suitable for high-temperature working environments. The battery separator provided by this invention is made of polyolefin-based pyridine material, has high mechanical strength, a better thermal shrinkage rate than polyolefin membranes, and a faster electrolyte wetting rate, resulting in higher efficiency in battery production and processing.

Description

Technical Field

[0001] This invention belongs to the field of energy battery technology, specifically relating to a high-temperature resistant battery separator, its preparation method, and the corresponding lithium-ion battery. Background Technology

[0002] In the structure of lithium batteries, the anode material and the cathode material are usually separated by a separator. The performance of the separator affects the battery's interface structure, internal resistance, etc., and is directly related to the battery's capacity, cycle performance and safety performance.

[0003] Currently, the most commonly used separators on the market are based on polyolefin materials, such as patent application number [missing information]. However, polyolefin separators have poor electrolyte wettability and poor thermal stability, exhibiting significant shrinkage and melting at high temperatures. Some power batteries employ composite separators, which combine polyolefin materials with reinforcing materials to form multi-layer composite separators. The reinforcing layer prevents high-temperature separator shrinkage, avoiding any impact on battery operation due to separator shrinkage. Existing technologies often use ceramic materials as reinforcing materials, coating them onto polyolefin materials to form a reinforcing layer. For example, patent application number CN201910509073.6 discloses a lithium battery separator ceramic coating slurry and a ceramic-coated separator. In this patented technology, the composite separator includes a battery separator and a ceramic coating applied to one or both sides of the battery separator. The battery separator is composed of a polyethylene film, a polypropylene film, or a polypropylene / polyethylene / polypropylene composite film. While the ceramic layer can enhance the heat shrinkage resistance of the organic polymer film to some extent, it also reduces the electrolyte wetting rate, affecting the battery production and processing efficiency. Furthermore, the reinforcing effect of the ceramic layer is limited at higher temperatures, and it still cannot prevent the membrane from shrinking. Summary of the Invention

[0004] To address the aforementioned problems, this invention provides a battery separator that is resistant to high-temperature environments. This battery separator can maintain its structural and functional integrity at high temperatures and has good wettability with the electrolyte, thereby improving battery performance and lifespan.

[0005] To achieve the above objectives, the present invention adopts the following technical solution:

[0006] A battery separator resistant to high temperature environments, the separator being mainly composed of a polyolefin-based pyridine material, wherein the polyolefin-based pyridine material is formed by polymerization of the olefin-based pyridine monomer represented by formula (I);

[0007] (Ⅰ), where the R group represents a hydrocarbon group.

[0008] In this invention, "the diaphragm is mainly composed of polyolefin-based pyridine material" means that the majority of the diaphragm is composed of polyolefin-based pyridine material, but it does not exclude the presence of a small amount of other additives.

[0009] Most existing lithium battery separators are prepared using the following methods:

[0010] Dry process diaphragm: Polyolefin resin is melted, extruded to form crystalline polymer, and then the melt is blown into a film. After crystallization heat treatment and annealing, a highly oriented multilayer structure is obtained. The film is stretched at high temperature to peel off the crystallization interface and form a porous diaphragm.

[0011] Wet process membrane: Liquid hydrocarbons or some small molecules are mixed with polyolefin resin (usually polyethylene or polypropylene), heated and melted to form a homogeneous mixture, then cooled to separate the phases, pressed into a membrane, and then heated to near the melting point temperature to perform biaxial stretching to orient the molecular chains. Finally, the membrane is kept at a certain temperature for a certain period of time, and the residual solvent is washed away with volatile substances to prepare a microporous membrane material with interconnected structures.

[0012] The polyolefin films prepared by the two methods described above often begin to shrink at 55°C, while the operating temperature of lithium batteries can reach 65°C. At this temperature, the polyolefin film is difficult to maintain its structural integrity and is prone to shrinkage, leading to contact between the positive and negative electrodes and generating a large amount of heat inside the battery. This heat can then cause the separator to close its pores, affecting the battery's performance. The battery separator provided by this invention has higher mechanical strength, a better thermal shrinkage rate than polyolefin films, and a faster electrolyte wetting rate, resulting in higher efficiency in battery manufacturing.

[0013] Furthermore, the method for preparing the olefinic pyridine monomer is as follows: α-substituted product is obtained by reacting 4-pyridinecarboxylic acid with an alkyl magnesium reagent, and then the α-substituted product is heated in an ethanol solution of sodium hydroxide to prepare the olefinic pyridine monomer.

[0014] Furthermore, the alkyl magnesium reagent is RCH2CH(Br)-MgBr.

[0015] Furthermore, the structure of the α-substituted product is shown in formula (II):

[0016] (Ⅱ), where the R group represents a hydrocarbon group.

[0017] The preparation principle of the above-mentioned olefinic pyridine monomer is as follows: The α-H on the pyridine ring of the pyridine monomer has a certain acidity and can be replaced by a highly nucleophilic alkyl anion to generate α-alkylpyridine. Specifically, in this invention, 4-pyridinecarboxylic acid reacts with a highly nucleophilic alkyl anion in the presence of acetic acid to generate the α-product. For example, when the reagent providing the alkyl anion is RCH2CH(Br)-MgBr, the structure of the α-substituted product is as shown in formula (II). This invention uses 4-pyridinecarboxylic acid as the raw material for synthesizing olefinic pyridine monomers. The carboxyl group on the pyridine ring can improve the wettability between the polymer and the dissociation solution.

[0018] Furthermore, in the above structural formulas, the R group is an alkane group with 1 to 5 carbon atoms. When the R group is an alkane group with fewer carbon atoms, the steric hindrance on the polymer side chain is smaller, and the polymer's crystalline structure tends to form an α-helix structure with high orientation, which can greatly improve the mechanical strength of the material. At the same time, the side chain alkane group can play a certain role in softening, which can reduce the closure temperature of the membrane made of this material.

[0019] Another object of the present invention is to provide a method for preparing the above-mentioned high-temperature resistant battery separator, the method comprising the following steps:

[0020] S1. Dissolve 0.9-1.5 parts of olefin-based pyridine monomer in toluene to prepare a solution, add 0.02-0.4 parts of metallocene catalyst, control the reaction temperature at 20-25℃, react for 12-18 hours, quench with methanol dropwise, add n-hexane, stir and let stand, wash and filter to obtain white polyolefin-based pyridine material.

[0021] S2. Dissolve the polyolefin-based pyridine material obtained in step S1 in N,N-dimethylformamide or N,N-dimethylacetamide to prepare a spinning solution, and then use the spinning solution to form a polymer membrane by electrospinning, thus obtaining a battery separator that can be used in high-temperature working environments.

[0022] Preferably, the concentration of polyolefin pyridine in the spinning solution is 15-20%.

[0023] This invention obtains a polymer membrane through electrospinning. Electrospinning can form a nanofiber structure, thereby significantly increasing the porosity of the membrane and improving its ability to absorb electrolyte.

[0024] Another object of the present invention is to provide a lithium-ion battery based on the battery separator described above, the lithium-ion battery comprising an anode material, a cathode material, an electrolyte and the battery separator described above.

[0025] In some embodiments, the lithium-ion battery cell can be a wound core structure, which includes five film layers: separator-cathode-separator-anode-separator. These five film layers are wound to form a core, typically with 10 to 50 turns. After winding, the core is placed in an aluminum shell, sealed with a cover plate, and electrolyte is injected through the injection hole on the cover plate. After electrolyte injection, it is subjected to a high-temperature standing period of 45°C for 16 hours, followed by formation under a constant current. After aging at 45°C for 14 hours, the lithium battery is obtained through capacity testing.

[0026] Further, the anode material preparation method in the lithium-ion battery is as follows: A first slurry is prepared according to the ratio of carbon-based material: SP:CMC:SBR:H2O = (94~98):(1~2):(1~2):1:(120~150). The first slurry is coated by an extrusion coating machine to obtain the anode material. The carbon-based material is selected from one or more of carbon powder, graphite, graphene, carbon fiber, and carbon nanotubes. Preferably, the baking temperature of the front, middle, and rear sections of the coating machine is 90~140℃ to ensure that the electrode is dry, and the areal density of the coating is in the range of 100~300g / ㎡.

[0027] Further, the cathode material preparation method in the lithium-ion battery is as follows: A second slurry is prepared according to the ratio of lithium iron phosphate: SP: CNT: PVDF: NMP = (94~96.5): 1.2: (0.8~1.8): (2~3.5): (50~70). The second slurry is then coated using an extrusion coating machine to obtain the cathode material. Preferably, the baking temperature of the front, middle, and rear sections of the coating machine is 80~120℃ to ensure the electrode is dry, and the areal density of the coating is in the range of 200~400 g / m².

[0028] Further, the electrolyte in the lithium-ion battery comprises an organic solvent, an additive, and a lithium salt; the organic solvent is selected from one or more of γ-butyl lactone, ethylene carbonate, propylene carbonate, diethyl carbonate, propyl acetate, dimethyl carbonate, and ethyl methyl carbonate; the additive is selected from one or more of maleimide, polymaleimide, bismaleimide, polybismaleimide, copolymers of bismaleimide and maleimide, and vinylene carbonate; the lithium salt is selected from one or more of LiPF6 and LiBF4. Preferably, the content of electrolyte 4 is 95~99wt%, and the content of additive is 0.05~0.5wt%.

[0029] In summary, the following beneficial effects can be achieved by applying the solution of the present invention:

[0030] 1. The battery separator provided by the present invention is made of polyolefin-based pyridine material. The battery separator has high mechanical strength, better thermal shrinkage rate than polyolefin film, and faster electrolyte wetting rate, resulting in higher efficiency in battery production and processing.

[0031] 2. The battery separator provided by the present invention does not require the setting of a ceramic layer or other reinforcing layer, making the production process simpler and avoiding the influence of the ceramic layer on the electrolyte wetting rate.

[0032] 3. The battery separator provided by the present invention is made by electrospinning. Electrospinning can form a nanofiber structure, thereby greatly improving the porosity of the separator and improving its ability to absorb electrolyte. Attached Figure Description

[0033] Figure 1 The viscosity test graphs are for spinning solutions with different ratios in the examples. Detailed Implementation

[0034] The present invention will be further described below with reference to the accompanying drawings and specific embodiments, so that those skilled in the art can better understand and implement the present invention. However, the embodiments described are not intended to limit the present invention.

[0035] Example 1

[0036] This embodiment provides a battery separator and a lithium-ion battery based on the battery separator.

[0037] The battery separator preparation method is as follows:

[0038] S1. Synthesis of polyolefin pyridine: 1.5 parts of olefin pyridine monomer with R group methyl was dissolved in 5 parts of toluene to prepare a solution. 0.02 parts of metallocene catalyst were added, and the reaction temperature was controlled at 20℃. After reacting for 12 h, methanol was added dropwise to quench the reaction. After adding n-hexane and stirring and letting stand, the solution was washed with n-hexane and filtered to obtain white polyolefin pyridine.

[0039] S2. Preparation of the fiber lithium battery separator: The polyolefin-based pyridine prepared above was dissolved in N,N-dimethylformamide to obtain a spinning solution with a concentration of 15 wt%. Spinning parameters were set as follows: electrospinning voltage of 30 kV, receiving distance of 18 cm, and electrospinning of the spinning solution at 20–25 °C to obtain the battery separator. The separator thickness was 24 μm, the pore size was 180 nm, and the porosity was 62%.

[0040] The preparation method of lithium-ion batteries is as follows:

[0041] The anode is prepared by mixing and stirring graphite:SP:CMC:SBR:H2O = 96:1.5:1.5:1:(120~150) to obtain the first slurry; the cathode is prepared by mixing lithium iron phosphate:SP:CNT:PVDF:NMP = 96:1.2:0.8:2:(50~70) to obtain the second slurry; the two slurries are coated to obtain the anode electrode and the cathode electrode respectively; after the electrode and separator are cut, they are processed into lithium batteries through winding, casing, welding, liquid injection, high-temperature standing, formation, high-temperature aging, and capacity testing. The high-temperature standing and high-temperature aging times are 16h and 14h respectively.

[0042] Example 2

[0043] This embodiment provides a battery separator and a lithium-ion battery based on the battery separator.

[0044] The battery separator preparation method is as follows:

[0045] S1. Synthesis of polyolefin pyridine: 0.9 parts of olefin pyridine monomer with R group n-pentyl were dissolved in 5 parts of toluene to prepare a solution. 0.02 parts of metallocene catalyst were added, and the reaction temperature was controlled at 25℃. After reacting for 18 h, methanol was added dropwise to quench the reaction. After adding n-hexane and stirring and letting stand, the solution was washed with n-hexane and filtered to obtain white polyolefin pyridine.

[0046] S2. Preparation of the fiber lithium battery separator: The polyolefin-based pyridine prepared above was dissolved in N,N-dimethylacetamide to obtain a spinning solution with a concentration of 18 wt%. Spinning parameters were set as follows: electrospinning voltage of 30 kV, receiving distance of 18 cm, and electrospinning of the spinning solution at 25 °C to obtain the battery separator. The separator thickness was 28 μm, the pore size was 190 nm, and the porosity was 58%.

[0047] The preparation method of lithium-ion batteries is as follows:

[0048] The anode is prepared by mixing and stirring graphite:SP:CMC:SBR:H2O = 96:1.5:1.5:1:(120~150) to obtain a slurry; the cathode is prepared by mixing lithium iron phosphate:CNT:PVDF:NMP = 96:1.2:0.8:2:(50~70) to obtain a slurry; the two slurries are coated to obtain anode and cathode electrodes; after the electrodes and separator are cut, they are manufactured into lithium batteries through processes such as winding, casing, welding, electrolyte injection, high-temperature settling, formation, high-temperature aging, and capacity testing, wherein the high-temperature settling and high-temperature aging times are 16h and 14h, respectively.

[0049] Example 3

[0050] This embodiment provides a battery separator and a lithium-ion battery based on the battery separator.

[0051] The battery separator preparation method is as follows:

[0052] S1. Synthesis of polyolefin pyridine: 1 part of an olefin pyridine monomer with R group n-propyl was dissolved in 5 parts of toluene to prepare a solution. 0.02 parts of metallocene catalyst were added, and the reaction temperature was controlled at 23℃. After reacting for 12 h, methanol was added dropwise to quench the reaction. After adding n-hexane, stirring and letting stand, the solution was washed with n-hexane and filtered to obtain white polyolefin pyridine.

[0053] S2. Preparation of the fiber lithium battery separator: The polyolefin-based pyridine prepared above was dissolved in N,N-dimethylformamide to obtain a spinning solution with a concentration of 19 wt%. Spinning parameters were set as follows: electrospinning voltage of 30 kV, receiving distance of 18 cm, and electrospinning of the spinning solution at 24 °C to obtain the separator. The separator thickness was 31 μm, the pore size was 190 nm, and the porosity was 68%.

[0054] The preparation method of lithium-ion batteries is as follows:

[0055] The anode is prepared by mixing and stirring graphite:SP:CMC:SBR:H2O=96:1.5:1.5:1:(120~150) to obtain a slurry; the cathode is prepared by mixing lithium iron phosphate:SP:CNT:PVDF:NMP=96:1.2:0.8:2:(50~70) to obtain a slurry; the two slurries are coated to obtain anode and cathode electrodes respectively; after the electrodes and separator are cut, they are manufactured into lithium batteries through processes such as winding, casing, welding, liquid injection, high-temperature standing, formation, high-temperature aging, and capacity testing, wherein the high-temperature standing and high-temperature aging times are 16h and 14h respectively.

[0056] Comparative Example

[0057] This comparative example provides a lithium-ion battery, the preparation method of which is as follows:

[0058] The anode is prepared by mixing and stirring graphite:SP:CMC:SBR:H2O = 96:1.5:1.5:1:(120~150) to obtain a slurry; the cathode is prepared by mixing lithium iron phosphate:SP:CNT:PVDF:NMP = 96:1.2:0.8:2:(50~70) to obtain a slurry; the two slurries are coated to obtain the anode and cathode electrodes; after the electrodes are cut, they are combined with commercially available 20μm dry separators and manufactured into lithium batteries through processes such as winding, casing, welding, electrolyte injection, high-temperature settling, formation, high-temperature aging, and capacity testing, with high-temperature settling and high-temperature aging times of 48h and 36h, respectively.

[0059] Spinning solution viscosity test: The polyolefin pyridine prepared in Example 1 was dissolved in N,N-dimethylacetamide to prepare spinning solutions with mass fractions of 1%, 3%, 5%, 10%, 15%, 16%, 17%, 18%, 19%, 20%, 25%, 30%, and 35%, respectively. The solution viscosity was tested using an NDJ-5S rotational viscometer. The test results are as follows: Figure 1 As shown.

[0060] Depend on Figure 1 It can be seen that when the polymer is dissolved in N,N-dimethylacetamide, the viscosity decreases with increasing viscosity in the range of 15-20% by mass, proving that the polymer has a certain orientation and has the characteristics of a liquid crystal polymer. When the critical viscosity is reached, a nematic liquid crystal is formed. In this state, electrospinning can obtain a high degree of orientation at a lower power, avoiding damage to the fiber caused by internal stress generated when the fiber is electrostatically stretched, thereby improving the mechanical strength of the membrane.

[0061] The following are the performance tests of the diaphragms in the examples and comparative examples. The test results are shown in Table 1.

[0062] Diaphragm performance testing:

[0063] The tensile strength test shall be conducted in accordance with GB / T 1040.3-2006, using a diaphragm sample with a specification of 100mm×20mm.

[0064] The puncture strength test shall be conducted in accordance with GB / T 21302-2007. A diaphragm sample with a specification of 100mm×100mm shall be taken and a flat-headed needle with a diameter of 1mm shall be used at a speed of 100mm / s.

[0065] The closed-cell temperature was measured using a German Netzsch thermomechanical analyzer with a heating rate of 1℃ / min and a load of 20mN. The closed-cell temperature was the temperature at the inflection point of deformation near the melting point.

[0066] Immersion rate test: Take an appropriate amount of diaphragm (10±1g in this experiment), record the mass m0, pour the electrolyte into a beaker, keep the diaphragm immersed in the electrolyte, and set the standing time t (3h in this experiment). After the time is up, remove the diaphragm, wipe off the electrolyte on the surface, weigh it, and record the mass as m. t The wetting rate is (m t -m0) / t; The wetting rate measured by this method has a certain deviation from the actual wetting rate of the diaphragm, and is only used for performance comparison here.

[0067] Liquid absorption test: Take an appropriate amount of diaphragm (10±1g in this experiment) and record the mass as m1. Pour the electrolyte into a beaker and keep the diaphragm immersed in the electrolyte. The standing time is t1 (24h in this experiment). After the time is up, take out the diaphragm, wipe off the electrolyte on the surface, weigh it and record the mass as m2. The liquid absorption is m2-m1. The liquid absorption measured by this method has a certain deviation from the actual liquid absorption of the diaphragm. It is only used for performance comparison here.

[0068] Table 1

[0069]

[0070] As can be seen from Table 1, the mechanical properties of Examples 1 and 2 are superior to those of the comparative example. This also proves that the polymer solution can easily form nematic liquid crystals. Furthermore, the closed-cell temperature of the examples is lower than that of the comparative example, and the thermal shrinkage rate is slightly lower than that of the comparative example. When the external environment heats up, the membrane shrinks less, ensuring that the positive and negative electrodes can be properly separated. This improves the safety performance of the lithium battery and enhances the working environment of the battery, thus ensuring its safety performance at high temperatures.

[0071] Meanwhile, Table 1 shows that the electrolyte wetting rate and liquid absorption of the embodiment are much greater than those of the comparative example, indicating that the wettability of the separator in the embodiment is better than that in the comparative example, which can reduce the high-temperature standing and high-temperature aging time in the battery manufacturing process, and thus improve the production efficiency of the battery.

[0072] The following are the performance tests of the lithium-ion batteries in the examples and comparative examples. The test results are shown in Table 2.

[0073] Battery performance test:

[0074] For room temperature rate discharge, the test method is in accordance with the national standard GB / T 31486-2015, and the 5C discharge capacity / 1C discharge capacity is calculated.

[0075] For room temperature rate charging, the test method is in accordance with the national standard GB / T 31486-2015, and the 5C charging capacity / 1C charging capacity is calculated.

[0076] High-temperature discharge was tested according to the national standard GB / T 31486-2015, and the capacity retention rate at 45℃, 55℃, and 65℃ was calculated.

[0077] Table 2

[0078]

[0079] As can be seen from Table 2, the first-efficiency, internal resistance rate discharge, rate charging, and high-temperature discharge performance of the embodiment are all greater than those of the comparative example. In particular, the internal resistance and 65°C discharge performance of the embodiment are much higher than those of the comparative example, which proves the excellent performance of the separator in the embodiment at high temperatures. It can maintain a low thermal shrinkage rate and a small abnormal pore size, ensuring the normal operation of the battery at high temperatures.

[0080] The above-described embodiments are merely preferred embodiments provided to fully illustrate the present invention, and the scope of protection of the present invention is not limited thereto. Equivalent substitutions or modifications made by those skilled in the art based on the present invention are all within the scope of protection of the present invention. The scope of protection of the present invention is defined by the claims.

Claims

1. A battery separator resistant to high temperature environments, characterized by, The diaphragm is mainly composed of a polyolefin-based pyridine material, which is formed by polymerization of an olefin-based pyridine monomer represented by formula (I); (I), wherein the R groups represent hydrocarbon groups, R is an alkane group having a number of carbon atoms of from 1 to 5; The diaphragm is formed by the following method: S1, 0.9-1.5 parts of the olefin-based pyridine monomer is dissolved in toluene to prepare a solution, 0.02-0.4 parts of a metallocene catalyst is added, the reaction temperature is controlled at 20-25℃, after 12-18h of reaction, methanol is added dropwise for quenching, n-hexane is added for stirring and standing, and then the white polyolefin-based pyridine material is obtained by washing and filtration; S2, the polyolefin-based pyridine material prepared in step S1 is dissolved in N,N-dimethylformamide or N,N-dimethylacetamide to prepare a spinning solution, and then the spinning solution is prepared into a polymer film by an electrostatic spinning method, thereby obtaining a battery diaphragm that can be used in a high-temperature working environment.

2. The high temperature resistant battery separator of claim 1, wherein, The preparation method of the olefin-based pyridine monomer is as follows: an alpha substitution product is obtained by reacting 4-pyridine carboxylic acid with an alkyl magnesium reagent, and the olefin-based pyridine monomer is prepared by heating the alpha substitution product in a sodium hydroxide ethanol solution.

3. The high temperature resistant battery separator of claim 2, wherein, The alkyl magnesium reagent is RCH2CH(Br)-MgBr.

4. The high temperature resistant battery separator of claim 3, wherein, The alpha substitution product structure is shown as formula (II): (II), wherein the R group represents a hydrocarbon group.

5. A lithium-ion battery, characterized by, The battery comprises an anode material, a cathode material, an electrolyte and the battery diaphragm according to any one of claims 1-4.

6. The lithium-ion battery of claim 5, wherein, The preparation method of the anode material is as follows: a first slurry is prepared according to the ratio of carbon-based material: SP: CMC: SBR: H2O = (94-98): (1-2): (1-2): 1: (120-150), and the first slurry is coated by an extrusion coater to obtain the anode material; the carbon-based material is selected from one or more of carbon powder, graphite, graphene, carbon fiber and carbon nanotube.

7. The lithium-ion battery of claim 5, wherein, The preparation method of the cathode material is as follows: a second slurry is prepared according to the ratio of lithium iron phosphate: SP: CNT: PVDF: NMP = (94-96.5): 1.2: (0.8-1.8): (2-3.5): (50-70), and the second slurry is coated by an extrusion coater to obtain the cathode material.

8. The lithium-ion battery of claim 5, wherein, The electrolyte comprises an organic solvent, an additive and a lithium salt; the organic solvent is selected from one or more of gamma-butyrolactone, ethylene carbonate, propylene carbonate, diethyl carbonate, propyl acetate, dimethyl carbonate and methyl ethyl carbonate; the additive is selected from one or more of maleimide, polymaleimide, bismaleimide, polybismaleimide, a copolymer of bismaleimide and maleimide and vinylene carbonate; and the lithium salt is selected from one or more of LiPF6 and LiBF4.

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