Polymer electrolytes

Abstract

The objective is to provide a polymer electrolyte that has sufficient ionic conductivity and improved low-temperature properties. [Solution] A polymer electrolyte that, when differential scanning calorimetry is performed involving heating from -80°C to 150°C and then cooling to -80°C, exhibits an endothermic peak associated with the phase transition from a crystalline phase to a viscous crystalline phase and an endothermic peak associated with melting during the heating process, and does not exhibit an exothermic peak associated with solidification during the cooling process.

Description

[Technical Field]

[0001] This disclosure relates to polymer electrolytes. In particular, this disclosure relates to polymer electrolytes comprising a substance having flexible crystalline properties and a polymer. [Background technology]

[0002] In recent years, non-aqueous electrolyte batteries using polymer electrolytes have attracted attention, and among them, those using lithium are particularly noteworthy due to their high energy density and ability to generate high electromotive force. Polymer electrolytes have advantages in terms of ease of processing, low density resulting in small mass, and the potential for green batteries that can be synthesized using renewable resources.

[0003] Conventional polymer electrolytes include polymer compositions containing thermoplastic copolyesters, metal salts, and organic nitrile components in certain weight percentages (see, for example, Patent Document 1). According to the polymer composition of Patent Document 1, excellent power conductivity levels can be achieved.

[0004] Materials with viscous crystalline (plastic crystal) properties, such as succinonitrile (SN), are being considered for application in environmentally friendly solid-state batteries. "Viscous crystalline properties" refer to properties intermediate between liquid and solid, where the crystalline position is fixed, but the material exhibits rotational motion and other dynamic states. In many cases, succinonitrile (SN) is used as an additive in polymer electrolytes to enhance ionic conductivity, but SN itself can also be applied as a solid electrolyte with ionic conductivity. It has been demonstrated that material properties, such as mechanical stability, can be improved by changing the molecular structure and concentration of lithium salts, or by adding polymers.

[0005] Non-patent document 1 describes the influence of polymers on the thermomechanical and electrochemical properties of SN-based electrolytes, stating that the presence of polymers such as polyethylene oxide (PEO) shifts the initiation of the SN sublimation process to a higher temperature, resulting in increased mechanical strength. [Prior art documents] [Patent Documents]

[0006] [Patent Document 1] Special Publication No. 2021-514095 [Non-patent literature]

[0007] [Non-Patent Document 1] Vanessa van Laack, et al. “Succinonitrile-Polymer Composite Electrolytes for Li-Ion Solid-State Batteries-The Influence of Polymer Additives on Thermomechanical and Electrochemical Properties.” ACS OMEGA Volume 8, Issue 10(2023):9058-9066. [Overview of the Initiative] [Problems that the invention aims to solve]

[0008] Conventional polymer electrolytes sometimes failed to provide polymer electrolytes that possessed sufficient ionic conductivity while also exhibiting sufficient low-temperature properties.

[0009] Against this backdrop, the present disclosure aims to provide a polymer electrolyte that has sufficient ionic conductivity and improved low-temperature properties. [Means for solving the problem]

[0010] The problems described herein can be solved by the following embodiments of the present invention: <Aspect 1> When differential scanning calorimetry was performed involving heating from -80°C to 150°C and then cooling down to -80°C, an endothermic peak associated with the phase transition from the crystalline phase to the viscous crystalline phase and an endothermic peak associated with melting were observed during the heating process, and During the cooling process, it does not show an exothermic peak associated with coagulation. Polymer electrolyte. <Aspect 2> The polymer electrolyte according to embodiment 1, wherein the polymer electrolyte includes a substance exhibiting flexible crystalline properties and a lithium salt polymer having anionic functional groups and forming a salt with lithium. <Aspect 3> The polymer electrolyte according to embodiment 2, wherein the anionic functional group is present in the side chain of the polymer. <Aspect 4> The polymer electrolyte according to embodiment 2 or 3, wherein the substance exhibiting the flexible crystalline properties is present in an amount greater than 70 mol% and less than 90 mol% of the total amount of the substance exhibiting the flexible crystalline properties and the polymer. <Aspect 5> The polymer electrolyte according to any one of embodiments 2 to 4, wherein the substance having the aforementioned flexible crystalline properties is succinonitrile. <Aspect 6> The polymer electrolyte according to any one of embodiments 2 to 5, wherein the lithium salt polymer is poly((trifluoromethane)sulfonimide lithium methacrylate) or poly((trifluoromethane)sulfonimide lithium styrene). [Effects of the Invention]

[0011] This disclosure provides a polymer electrolyte having sufficient ionic conductivity and improved low-temperature properties. In particular, the present invention has advantageous effects in a polymer electrolyte comprising a substance having flexible crystalline properties and a polymer. [Brief explanation of the drawing]

[0012] [Figure 1]The results of DSC measurement for the polymer electrolyte of Example 1 according to the present invention are shown. [Figure 2] The results of ionic conductivity measurement for the polymer electrolyte of Example 1 according to the present invention are shown. [Figure 3] The results of DSC measurement for the polymer electrolyte of Comparative Example 1 are shown. [Figure 4] The results of ionic conductivity measurement for the polymer electrolyte of Comparative Example 1 are shown. [Figure 5] The results of DSC measurement for the polymer electrolyte of Comparative Example 2 are shown. [Figure 6] The results of ionic conductivity measurement for the polymer electrolyte of Comparative Example 2 are shown.

Mode for Carrying Out the Invention

[0013] <<Polymer Electrolyte>> When differential scanning calorimetry (DSC) involving heating from -80°C to 150°C and subsequent cooling to -80°C is performed on the polymer electrolyte according to the present invention, in the heating process, an endothermic peak associated with the phase transition from the crystalline phase to the plastic crystal phase (flexible viscous crystal phase) and an endothermic peak associated with melting are shown, and in the cooling process, it does not show an exothermic peak associated with solidification (which may be referred to as "the DSC characteristics of the present disclosure" below). Thereby, in addition to sufficient ionic conductivity, it has improved low-temperature characteristics.

[0014] Although not limited to theory, the polymer electrolyte according to the present invention shows an endothermic peak associated with the phase transition from the crystalline phase to the flexible viscous crystal phase and an endothermic peak associated with melting in the heating process by DSC, so it has flexible viscous crystal characteristics, and thereby it is considered that the ionic conductivity is improved.

[0015] Furthermore, the polymer electrolyte according to the present invention does not show an exothermic peak associated with solidification in the cooling process by DSC, so it has the characteristic of being difficult to solidify at low temperatures, and thereby it is considered that sufficient ionic conductivity is ensured even at low temperatures without a decrease in mobility.

[0016] "Low temperature" in this specification means, for example, -80°C to 40°C, particularly -30°C to 20°C. Further, in the present invention, the "low temperature characteristics" of the polymer electrolyte means that at a low temperature (for example, -80°C to 40°C, particularly -30°C to 20°C), the functions as a polymer electrolyte, such as ionic conductivity, are sufficiently ensured.

[0017] Hereinafter, each component of the aspect of the invention according to the present disclosure will be described in more detail.

[0018] <DSC Measurement> Differential scanning calorimetry (DSC measurement) can be performed using a differential scanning calorimeter as follows: In a glove box with an inert atmosphere (argon atmosphere), about 5 mg of the polymer electrolyte is weighed into a sealed aluminum pan for DSC (differential scanning calorimetry) to serve as a DSC sample. As the differential scanning calorimeter, for example, DSC 214 polyma (manufactured by Netzsch) can be used. After cooling the polymer electrolyte sample to -80°C, it is heated from -80°C to 150°C at a heating rate of 10°C / min (the first heating process), and then cooled from 150°C to -80°C at a cooling rate of 10°C / min, and the heat change during this process is measured.

[0019] Regarding peak determination, in the heat flow (W / g) - temperature (°C) graph obtained from DSC measurement, a change in slope of +10% or -10% or more from the slope of the baseline may be regarded as a peak. "Not showing an exothermic peak associated with solidification" particularly means that the peak area of the exothermic peak associated with solidification is 1% or less of the peak area of the endothermic peak associated with melting.

[0020] The "exothermic peak associated with solidification" in the cooling process is a peak corresponding to the "endothermic peak associated with melting" in the heating process. The fact that no "exothermic peak associated with solidification" sample corresponding to the "endothermic peak associated with melting" is observed in the above DSC measurement indicates that it supercools and liquefies.

[0021] In DSC measurements, endothermic peaks associated with the phase transition from the crystalline phase to the viscous crystalline phase during the heating process can be observed, for example, in the temperature range of -50 to -10°C (particularly -40 to -15°C or even -35 to -20°C). In DSC measurements, endothermic peaks associated with melting during the heating process can be observed, for example, in the temperature range of 40 to 80°C (particularly 45 to 75°C or even 50 to 70°C). In DSC measurements, exothermic peaks associated with solidification during the cooling process can be observed, for example, at temperatures of 10 to 40°C (particularly 15 to 35°C or even 20 to 35°C).

[0022] The polymer electrolyte of the present invention exhibiting the DSC properties described above may be lithium-ion conductive and may contain a lithium salt. Known lithium salts can be used, for example, Li-TFSI (lithium bis(trifluoromethanesulfonyl)imide). Alternatively, lithium salt polymers described later may be used as lithium salts.

[0023] The polymer electrolyte of the present invention exhibiting the DSC properties described above may contain a substance exhibiting flexible crystalline properties (e.g., succinonitrile). Details of the substance having flexible crystalline properties will be described later.

[0024] The polymer electrolyte of the present invention exhibiting the DSC properties described above may contain a polymer in addition to a substance exhibiting viscous crystalline properties (e.g., succinonitrile). Examples of polymers include PEO (polyethylene oxide), polyacrylate, polymethacrylate, polystyrene, and lithium salt polymers described later. The polymer may have an average molecular weight of 1,000 to 1,000,000, or even 5,000 to 300,000. The average molecular weight of the polymer is the weight-average molecular weight (Mw). The weight-average molecular weight (Mw) of the polymer can be measured by gel filtration chromatography. In one embodiment, the polymer electrolyte may have a substance exhibiting viscous crystalline properties, a polymer, and a lithium salt.

[0025] In a further embodiment of the polymer electrolyte of the present invention exhibiting the DSC properties described above, the polymer electrolyte comprises a substance exhibiting viscous crystalline properties and a lithium salt polymer. The lithium salt polymer is a polymer having anionic functional groups and forming a salt with lithium via these anionic functional groups. Preferably, in the polymer electrolyte according to one embodiment, the lithium salt polymer and the substance exhibiting viscous crystalline properties are contained in an amount of at least 80% by mass, or more preferably 90% by mass or 95% by mass, and more preferably, the polymer electrolyte according to one embodiment of the present invention comprises a lithium salt polymer and a substance exhibiting viscous crystalline properties.

[0026] Preferably, the substance having viscous crystalline properties is present in an amount greater than 70 mol% and less than 90 mol%, particularly 71 mol% or more, 72 mol% or more, 73 mol% or more, 74 mol% or more, 75 mol% or more, 76 mol% or more, 77 mol% or more, 78 mol% or more, or 79 mol% or more, and / or 89 mol% or less, 88 mol% or less, 87 mol% or less, 86 mol% or less, 85 mol% or less, 84 mol% or less, 83 mol% or less, 82 mol% or less, or 81 mol% or less, based on the total number of moles of the substance having viscous crystalline properties and the polymer (particularly lithium salt polymer). A concentration greater than 70 mol% can yield better ion conductivity. A concentration less than 90 mol% can yield particularly good low-temperature properties. In one embodiment, the substance having viscous crystalline properties is present in an amount of 72-88 mol%, 75-85 mol%, or even 78-82 mol%, relative to the total number of moles of the substance having viscous crystalline properties and the polymer (particularly the lithium salt polymer).

[0027] <Substances exhibiting flexible crystalline properties> In this invention, "flexible crystalline properties" refer to properties intermediate between liquid and solid, where the crystalline positions are fixed, but the material is in a mobile state, such as rotational motion. This allows ions to conduct at high speed, even in a solid state.

[0028] Conventional polymer electrolytes containing materials with flexible crystalline properties sometimes exhibited insufficient low-temperature characteristics. In conventional polymer electrolytes, it is believed that materials with flexible crystalline properties crystallize below their crystallization temperature, reducing their mobility and consequently significantly decreasing their ionic conductivity.

[0029] In contrast, in one aspect of the present invention, the polymer electrolyte has the property of being a material with flexible crystalline properties and not exhibiting an exothermic peak associated with solidification during the cooling process. The polymer electrolyte of the present invention is supercooled at low temperatures, and as a result maintains a liquid state even at low temperatures, so it is thought that the ionic conductivity does not decrease significantly.

[0030] Examples of substances (particularly compounds) exhibiting flexible crystalline properties include organic nitriles. "Organic nitrile" is understood to be an organic compound or mixture thereof containing a nitrile functional group, also known as a cyano functional group, such as acrylonitrile and propanenitrile. Organic nitriles can be compounds containing multiple nitrile groups, and / or mixtures of multiple compounds containing nitrile groups. The use of nitriles reduces the interaction with lithium ions.

[0031] In preferred embodiments of the present invention, the substance (particularly the compound) having flexible crystalline properties is preferably aliphatic dinitrile, such as adiponitrile (AN) and / or succinonitrile (SN), because it has the advantage of exhibiting high conductivity. More preferably, the substance (compound) having flexible crystalline properties is succinonitrile (SN), which is particularly advantageous because it has the advantage of showing increased conductivity of the composition over a wide temperature range.

[0032] Compounds having viscous crystalline properties may have a molecular weight of less than 2000 g / mol, more preferably less than 1000 g / mol, even more preferably less than 500 g / mol, and most preferably less than 250 g / mol. The lower limit of molecular weight is not limited, but may be, for example, 60 g / mol or more. The molecular weight of a substance having viscous crystalline properties can be determined by mass spectrometry known in the art.

[0033] <Lithium salt polymer> A "lithium salt polymer" is a polymer that has anionic functional groups and forms salts with lithium. When a polymer electrolyte contains a lithium salt polymer in addition to a substance with flexible crystalline properties, a particularly good lithium ion transport rate can be ensured. Although not limited to theory, it is thought that the transport rate of lithium cations improves because anions become less likely to diffuse when immobilized on the polymer. Furthermore, by including a substance with flexible crystalline properties and a polymer, a solid electrolyte that balances conductivity and moldability can be obtained. The addition of polymers also has the effect of providing a polymer electrolyte with appropriate flexibility, moldability, and ease of processing.

[0034] The lithium salt polymer according to the present invention may have an average molecular weight of 1,500 to 1,000,000, or even more, 5,000 to 300,000. The average molecular weight of the polymer is the weight-average molecular weight (Mw). The weight-average molecular weight (Mw) of the polymer can be measured by gel filtration chromatography.

[0035] Lithium salt polymers can be obtained, for example, by polymerizing a monomer mixture containing monomers having anionic functional groups. In this case, the resulting polymer has anionic structural units (or repeating units). Examples of anionic structural units include those shown in the chemical formulas (II) to (V) described later.

[0036] The lithium salt polymer may have a structural unit (unit or structural unit) that does not contain an anionic functional group. In this case, the proportion of anionic structural units having an anionic functional group among all the structural units constituting the polymer is preferably 70 to 100%, more preferably 80 to 100%, and still more preferably 90 to 100%. In particular, the lithium salt polymer may consist of anionic structural units having an anionic functional group.

[0037] The polymer according to the present invention may have a plurality of anionic functional groups in one molecule.

[0038] Preferably, the lithium salt polymer may be present in an amount of more than 10 mol% and less than 30 mol%, 12 to 28 mol%, 15 to 25 mol%, or 18 to 22 mol% with respect to the total number of moles of the lithium salt polymer and the substance having a soft viscous crystal property.

[0039] (Anionic functional group) The lithium salt polymer according to the present invention has an anionic functional group, thereby forming a salt with lithium. It is considered that the anion is immobilized on the polymer, making it difficult to diffuse, and improving the transport rate of lithium ions.

[0040] Examples of the anionic functional group include carboxylate (COO , - , - , - , - , - ), sulfonate (SO3 - ), ClO4 - , SCN - , BF4, AsF6 - , CF3SO3 - , Br, I - , PF6 - , CF3CO2 - , (FO2S)2N also known as FSI - ​​​​​​​​

[0041] (side chain) In one embodiment, the lithium salt polymer has anionic functional groups in its side chains. Examples of anionic functional groups present in the side chains include those listed above.

[0042] When lithium salt polymers have anionic functional groups in their side chains, examples of polymer main chains include polymethacrylate, polyacrylate, polystyrene, polystyrene sulfonic acid, polyethylene oxide (PEO), polyvinylpyrrolidone (PVP), polyacrylonitrile (PAN), and poly(vinylidene fluoride-co-hexafluoropropylene) (PVDF-HFP).

[0043] The anionic functional groups present in the side chains may be directly bonded to the polymer's main chain or linked via linker groups. Examples of linker groups include alkylene groups (particularly those with 1 to 6 or even 1 to 4 carbon atoms), phenylene groups, and acrylate groups.

[0044] The anionic functional group preferably has a sulfonimide skeleton or an imide skeleton. An example of a specific chemical structure of a sulfonimide located in the side chain is shown in structural formula (I). In structural formula (I), * indicates a bond with the polymer backbone or linker group. [ka]

[0045] (Specific embodiments of lithium salt polymers) Examples of lithium salt polymers according to the present invention include poly((trifluoromethane)sulfonimide lithium methacrylate) (PMTFSI-Li), poly((trifluoromethane)sulfonimide lithium styrene) (PSTFSI-Li), lithium polyacrylic acid, lithium polystyrene sulfonic acid, and mixtures thereof. Preferred lithium salt polymers are PMTFSI-Li or PSTFSI-Li.

[0046] The structural formula (II) of poly((trifluoromethane)sulfonimide lithium methacrylate) (PMTFSI-Li) is shown below. In the formula, n may be between 5 and 500.

[0047] [ka]

[0048] The structural formula (III) of poly((trifluoromethane)sulfonimide lithium styrene) (PSTFSI-Li) is shown below. In the formula, n may be between 5 and 500.

[0049] [ka]

[0050] The structural formula (IV) of lithium polyacrylic acid (lithium polyacrylate) is shown below. In the formula, n may be between 5 and 500.

[0051] [ka]

[0052] The structural formula (V) of lithium polystyrene sulfonic acid is shown below. In the formula, n may be between 5 and 500.

[0053] [ka]

[0054] <Method for producing polymer electrolytes> The method for producing the polymer electrolyte according to the present invention is not particularly limited. For example, a polymer electrolyte according to one embodiment of the present invention can be produced by a method comprising the following steps: (a) To provide a lithium salt polymer, (b) To provide a material having flexible crystalline properties, (c) Mixing a lithium salt polymer with a substance having flexible crystalline properties to form a polymer electrolyte. Here, the substance having flexible crystalline properties is present in an amount greater than 70 mol% but less than 90 mol% relative to the total amount of the substance having flexible crystalline properties and the lithium salt polymer.

[0055] (Step a) Step a provides a lithium salt polymer. For details regarding the lithium salt polymer, please refer to the above description concerning polymer electrolytes.

[0056] For example, a lithium salt polymer can be obtained by a process that involves polymerizing a monomer mixture containing monomers having anionic functional groups. In this case, the resulting polymer has anionic constituent units.

[0057] The lithium salt polymer may have constituent units that do not contain anionic functional groups. In this case, preferably, the proportion of anionic constituent units having anionic functional groups among all constituent units of the polymer is 70 to 100 mol%, more preferably 80 to 100 mol%, and even more preferably 90 to 100 mol%. In particular, the lithium salt polymer may consist of anionic constituent units having anionic functional groups.

[0058] (Step b) Step b provides a substance (particularly a compound) having flexible crystalline properties. For details on substances having flexible crystalline properties, refer to the description of the polymer electrolyte according to the present invention above.

[0059] (Step c) In step c, a lithium salt polymer and a substance having flexible crystalline properties are mixed to form a polymer electrolyte. The mixing method is not particularly limited, and known methods can be used. An organic solvent (e.g., acetonitrile) may be used for mixing.

[0060] A substance having viscous crystalline properties and a lithium salt polymer can be mixed in such a proportion that the substance having viscous crystalline properties is greater than 70 mol% but less than 90 mol%, particularly 71 mol% or more, 72 mol% or more, 73 mol% or more, 74 mol% or more, 75 mol% or more, 76 mol% or more, 77 mol% or more, 78 mol% or more, or 79 mol% or more, and / or 89 mol% or less, 88 mol% or less, 87 mol% or less, 86 mol% or less, 85 mol% or less, 84 mol% or less, 83 mol% or less, 82 mol% or less, or 81 mol% or less, relative to the total number of moles of the substance having viscous crystalline properties and the lithium salt polymer.

[0061] The mixing may be carried out, for example, at a temperature of 40-80°C, preferably 50-70°C, for 20-30 hours, preferably 23-27 hours.

[0062] In one embodiment, a lithium salt polymer having anionic functional groups and a substance having flexible crystalline properties may be mixed by stirring and dissolving them in an organic solvent (e.g., acetonitrile) for, for example, 23 to 27 hours. In this case, the solution obtained by mixing can be allowed to stand at a temperature of, for example, 40 to 60°C, preferably 45 to 55°C to evaporate the organic solvent, and then the polymer electrolyte can be obtained by drying, for example, vacuum drying, at a temperature of, for example, 40 to 60°C, preferably 45 to 55°C.

[0063] Another aspect of the present invention provides a method for producing a substance having flexible crystalline properties, a polymer that is not a lithium salt polymer, and a polymer electrolyte containing a lithium salt. This method can be carried out in the same manner as described above, except that in step (a) of the above method, a polymer that is not a lithium salt polymer and a lithium salt are provided instead of a lithium salt polymer. An example of a polymer that is not a lithium salt polymer is PEO (polyethylene oxide). An example of a lithium salt is Li-TFSI (lithium bis(trifluoromethanesulfonyl)imide).

[0064] <Applications of polymer electrolytes> The polymer electrolyte according to the present invention can be used in solid-state batteries, particularly all-solid-state batteries. In this disclosure, “solid-state battery” means a battery that uses at least a solid electrolyte as its electrolyte, and therefore, for solid-state batteries, a combination of a solid electrolyte and a liquid electrolyte may be used as the electrolyte. Solid electrolytes having a polymer electrolyte, particularly all-solid-state batteries, can be manufactured according to known methods.

[0065] The polymer electrolyte according to the present invention is used, for example, as an additive in electrodes, particularly electrodes for rechargeable batteries. In this disclosure, “additive” means a composition that can constitute a positive electrode active material layer, either as is or by further containing other components. In this disclosure, “additive slurry” means a slurry that includes a dispersion medium in addition to the “additive,” and can be applied and dried to form a positive electrode active material layer, etc.

[0066] When the polymer electrolyte according to the present invention is used, for example, in an electrode stack in a thin-film all-solid-state battery, a method can be used in its manufacture in which a positive electrode active material layer is formed on a substrate, a solid electrolyte layer and a negative electrode active material layer are formed in sequence, and then stacked; however, other variations of the process are also conceivable. The polymer electrolyte of the present invention can be used in both primary and secondary batteries. An all-solid-state battery manufactured using the polymer electrolyte of the present invention may, in addition to the components described above, typically have a separator, a positive electrode current collector, and positive and negative electrode terminals connected to the negative electrode current collector. The materials and shapes of the positive electrode, negative electrode, and other components can be appropriately selected by those skilled in the art depending on the application of the battery.

[0067] The polymer electrolyte according to the present invention is particularly advantageous when used in low-temperature environments (e.g., -80°C to 40°C, especially -30°C to 20°C). [Examples]

[0068] Polymer electrolytes according to Example 1 and Comparative Examples 1-2 were prepared, and DSC measurements and ionic conductivity measurements were performed.

[0069] <Example 1> In Example 1, succinonitrile (SN), a substance having flexible crystalline properties, was used at a concentration of 80 mol% relative to the total amount of SN and PMTFSI-Li (poly((trifluoromethane)sulfonimide lithium methacrylate)) as a lithium salt polymer to prepare a polymer electrolyte.

[0070] (Preparation of polymer electrolytes) In an Ar-atmosphere glove box, succinonitrile and PMTFSI-Li (poly(trifluoromethane)sulfonimide lithium methacrylate, manufactured by Polykey, Spain) were weighed in a molar ratio of SN:PMTFSI-Li = 4:1 and dissolved by stirring in acetonitrile (manufactured by Sigma-Aldrich) for 24 hours. The resulting solution was allowed to stand on a hot plate at 50°C to evaporate the acetonitrile, and then vacuum-dried at 50°C to obtain the polymer electrolyte.

[0071] (Differential scanning calorimetry measurement) Approximately 5 mg of the prepared polymer electrolyte was weighed into a sealed aluminum pan for differential scanning calorimetry (DSC) in an Ar-atmosphere glove box to serve as the DSC sample. DSC measurements were performed using a DSC 214 polyma (Netzsch). After cooling the sample to -80°C, it was heated to 150°C at a heating rate of 10°C / min (first heating process), and then cooled from 150°C to -80°C at a cooling rate of 10°C / min. The change in heat during this process was measured.

[0072] (Measurement of ionic conductivity) The ionic conductivity (S / cm) was measured as follows: A cell with two platinum electrodes of known cell constants was used. The electrolyte to be measured was filled between the electrodes of the cell in an Ar-atmosphere glove box and sealed. The cell was placed in a constant temperature bath and left standing for 30 minutes after the bath reached the measurement temperature. After standing, AC impedance was measured using a VMP-3 (Biologic), and the ionic conductivity was calculated from the obtained resistance and cell constants.

[0073] The results of the DSC measurement and ionic conductivity measurement for Example 1 are shown in Figures 1 and 2, respectively.

[0074] Figure 1 shows that the polymer electrolyte according to Example 1 exhibits an endothermic peak associated with the phase transition from the crystalline phase to the viscous crystalline phase and an endothermic peak associated with melting during the heating process in DSC, and does not exhibit an exothermic peak associated with solidification during the cooling process in DSC.

[0075] Figure 2 shows that the polymer electrolyte according to Example 1 does not experience a significant decrease in ionic conductivity at low temperatures during the cooling process after the heating process.

[0076] <Comparative Example 1> In Comparative Example 1, succinonitrile (SN), a substance with flexible crystalline properties, was used at a concentration of 90 mol% relative to the total of SN and PMTFSI-Li (poly((trifluoromethane)sulfonimide lithium methacrylate)) as a lithium salt polymer to prepare a polymer electrolyte.

[0077] (Preparation of polymer electrolytes) In an Ar-atmosphere glove box, succinonitrile and PMTFSI-Li (poly(trifluoromethane)sulfonimide lithium methacrylate, manufactured by Polykey, Spain) were weighed in a molar ratio of SN:PMTFSI-Li = 9:1 and dissolved by stirring in acetonitrile (manufactured by Sigma-Aldrich) for 24 hours. The resulting solution was allowed to stand on a hot plate at 50°C to evaporate the acetonitrile, and then vacuum-dried at 50°C to obtain the polymer electrolyte.

[0078] The polymer electrolyte obtained in Comparative Example 2 was subjected to DSC and ionic conductivity measurements in the same manner as in Example 1. The results are shown in Figures 3 and 4.

[0079] Figure 3 shows that the polymer electrolyte according to Comparative Example 1 exhibits an endothermic peak associated with the phase transition from the crystalline phase to the viscous crystalline phase and an endothermic peak associated with melting during the heating process in DSC, and an exothermic peak associated with solidification during the cooling process in DSC. Note that a measurement error occurred at approximately -35°C, causing the DSC sample to fall out of the measuring device (this phenomenon was reproducible), making measurements below this temperature impossible.

[0080] Figure 4 shows that the polymer electrolyte of Comparative Example 1 exhibits a significant decrease in ionic conductivity in the low-temperature region during the cooling process in DSC. Comparing the ionic conductivity in Figures 2 and 4, it can be seen that the polymer electrolyte of Example 1 has superior low-temperature characteristics compared to the polymer electrolyte of Comparative Example 1.

[0081] <Comparative Example 2> In Comparative Example 2, succinonitrile (SN), a substance with flexible crystalline properties, was used at a concentration of 70 mol% relative to the total of SN and PMTFSI-Li (poly((trifluoromethane)sulfonimide lithium methacrylate)) as a lithium salt polymer to prepare a polymer electrolyte.

[0082] (Preparation of polymer electrolytes) In an Ar-atmosphere glove box, succinonitrile and PMTFSI-Li (poly(trifluoromethane)sulfonimide lithium methacrylate, manufactured by Polykey, Spain) were weighed in a molar ratio of SN:PMTFSI-Li = 7:3 and dissolved by stirring in acetonitrile (manufactured by Sigma-Aldrich) for 24 hours. The resulting solution was allowed to stand on a hot plate at 50°C to evaporate the acetonitrile, and then vacuum-dried at 50°C to obtain the polymer electrolyte.

[0083] The polymer electrolyte obtained in Comparative Example 1 was subjected to DSC and ionic conductivity measurements in the same manner as in Example 1. The results are shown in Figures 5 and 6.

[0084] Figure 5 shows that the polymer electrolyte according to Comparative Example 2 does not show an endothermic peak associated with the phase transition from the crystalline phase to the viscous crystalline phase or an endothermic peak associated with melting during the heating process, and does not show an exothermic peak associated with solidification during the cooling process in DSC. Although not limited by theory, it is thought that because the SN is in an amorphous state, it does not show peaks for the crystalline phase transition and melting, i.e., it does not show viscous crystalline properties.

[0085] In Figure 6, no significant decrease in the ionic conductivity of the polymer electrolyte according to Comparative Example 2 is observed at low temperatures. However, when comparing the ionic conductivity in Figures 2 and 6 at the same temperature, it can be seen that the ionic conductivity of the polymer electrolyte according to Example 1, which has flexible crystalline properties, during the cooling process is generally superior to that of the polymer electrolyte according to Comparative Example 1 during both the heating and cooling processes.

[0086] From the above, it can be seen that the embodiment of the present invention, which exhibits an endothermic peak associated with the phase transition from the crystalline phase to the viscous crystalline phase and an endothermic peak associated with melting during the heating process, and does not exhibit an exothermic peak associated with solidification during the cooling process in DSC, exhibits improved low-temperature characteristics along with excellent ionic conductivity, unlike Comparative Examples 1 and 2 which do not satisfy this characteristic.

Claims

1. When differential scanning calorimetry was performed involving heating from -80°C to 150°C and subsequent cooling to -80°C, the heating process showed endothermic peaks associated with the phase transition from the crystalline phase to the viscous crystalline phase and endothermic peaks associated with melting, and During the cooling process, it does not show an exothermic peak associated with coagulation. Polymer electrolyte.

2. The polymer electrolyte according to claim 1, wherein the polymer electrolyte comprises a substance having flexible crystalline properties and a lithium salt polymer having anionic functional groups that form salts with lithium.

3. The polymer electrolyte according to claim 2, wherein the anionic functional group is present in the side chain of the lithium salt polymer.

4. The polymer electrolyte according to claim 2 or 3, wherein the substance having the flexible crystalline properties is present in an amount greater than 70 mol% and less than 90 mol% relative to the sum of the substance having the flexible crystalline properties and the polymer.

5. The polymer electrolyte according to claim 2 or 3, wherein the substance having the flexible crystalline properties is succinonitrile.

6. The polymer electrolyte according to claim 2 or 3, wherein the lithium salt polymer is poly((trifluoromethane)sulfonimide lithium methacrylate) or poly((trifluoromethane)sulfonimide lithium styrene).

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