Polymer electrolytes

Abstract

The objective is to provide polymer electrolytes with improved electrical performance. [Solution] A polymer electrolyte comprising a lithium salt polymer having anionic functional groups and forming salts with lithium, and a substance having flexible crystals.

Description

[Technical Field]

[0001] This disclosure relates to a polymer electrolyte comprising a polymer and a substance exhibiting flexible crystalline properties. [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 flexible crystalline (plastic crystal) properties, such as succinonitrile (SN), are being considered for application in environmentally friendly solid-state batteries. While often used as additives in polymer electrolytes to enhance ionic conductivity, SN itself also exhibits properties of 10°C at room temperature. -4 It can be applied as a solid electrolyte with ionic conductivity up to S / cm. Electrolytes based on SN may have insufficient dimensional stability and may suffer from a high tendency for SN sublimation. It has been demonstrated that material properties, such as mechanical stability, can be improved by changing the molecular structure and concentration of the lithium salt 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 project] [Problems that the invention aims to solve]

[0008] Conventional polymer electrolytes containing substances with flexible crystalline properties, such as succinonitrile, and polymers, sometimes fail to provide sufficient electrical performance, particularly when the lithium ion transport rate is insufficient.

[0009] Against this backdrop, the objective of this disclosure is to provide a polymer electrolyte with improved electrical performance (particularly lithium ion transport rate). [Means for solving the problem]

[0010] The problems described herein can be solved by the following embodiments of the present invention: <Aspect 1> A polymer electrolyte comprising a lithium salt polymer having anionic functional groups and forming salts with lithium, and a substance having flexible crystalline properties. <Aspect 2> The polymer electrolyte according to Aspect 1, wherein the anionic functional group of the lithium salt polymer is present in the side chain of the lithium salt polymer. <Aspect 3> The polymer electrolyte according to Aspect 1 or 2, wherein the substance having the soft viscous crystal property is succinonitrile. <Aspect 4> The polymer electrolyte according to any one of Aspects 1 to 3, wherein the anionic functional group of the lithium salt polymer has a sulfonimide skeleton or an imide skeleton. <Aspect 5> The polymer electrolyte according to any one of Aspects 1 to 4, wherein the lithium salt polymer is poly((trifluoromethane)sulfonimide lithium methacrylate) or poly((trifluoromethane)sulfonimide lithium styrene).

Advantages of the Invention

[0011] According to the present disclosure, a polymer electrolyte with improved electrical performance is provided, and in particular, a polymer electrolyte with improved lithium ion transference number is provided.

Modes for Carrying Out the Invention

[0012] <<Polymer Electrolyte>> The polymer electrolyte according to the present invention includes a lithium salt polymer having an anionic functional group and forming a salt with lithium, and a substance having a soft viscous crystal property.

[0013] In a conventional lithium ion conductive polymer electrolyte, in a polymer electrolyte containing a substance having a soft viscous crystal property such as succinonitrile and a polymer, a low molecular weight lithium salt, for example, LiTFSI (lithium bis(trifluoromethanesulfonyl)imide) is used, and sufficient electrical performance may not be obtained, and in particular, there is a problem that the lithium ion transference number is not sufficient.

[0014] In contrast, the polymer electrolyte according to the present invention uses a polyanionic lithium salt in which an anionic functional group is fixed to a polymer, and has an improved lithium cation transference number. Without being limited to theory, in a conventional polymer electrolyte, since the anion of the lithium salt is a low molecule and thus easily diffuses, for example, it is considered that both the lithium salt and the anion in the polymer electrolyte move during the operation of the battery, thereby lowering the transference number. In contrast, according to the present invention, since the anion is immobilized on the polymer and thus becomes difficult to diffuse, it is considered that the transference number of the lithium cation is improved.

[0015] Also, according to the polymer electrolyte of the present invention, by including a polymer in addition to a substance having a viscoelastic crystal property, a solid electrolyte having both conductivity and moldability can be obtained.

[0016] Hereinafter, each component of the aspect of the invention according to the present disclosure will be described in more detail.

[0017] <Lithium salt polymer> The polymer electrolyte according to the present invention has a "lithium salt polymer". The "lithium salt polymer" has an anionic functional group and forms a salt with lithium through the anionic functional group. The present invention has such a lithium salt polymer in addition to a substance having a viscoelastic crystal property, whereby the above-described improved transference number of the lithium cation can be obtained, and a polymer electrolyte having appropriate flexibility, moldability, and ease of processing can be obtained. The lithium salt polymer may have an anionic structural unit (repeating unit).

[0018] The molar ratio of the lithium salt polymer and the substance having a viscoelastic crystal property in the polymer electrolyte may be, for example, 1:2 to​​​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.

[0020] The lithium salt polymer according to the present invention can be obtained, for example, by polymerizing a monomer mixture containing monomers having anionic functional groups, in which case the resulting polymer has anionic structural units (repeating units). Examples of anionic structural units include the anionic structural units contained in the chemical formulas (II) to (V) described later.

[0021] Lithium salt polymers may have constituent units (units or repeating units) that do not contain anionic functional groups. In this case, the proportion of anionic constituent units having anionic functional groups among all constituent units of the polymer is preferably 70-100%, more preferably 80-100%, and even more preferably 90-100%. In particular, lithium salt polymers may consist of anionic constituent units having anionic functional groups.

[0022] The polymer according to the present invention may have multiple anionic functional groups in a single molecule.

[0023] The lithium salt polymer may be present in an amount of 1 to 50 mol%, 5 to 40 mol%, or even 10 to 30 mol%, relative to the total number of moles of the lithium salt polymer and the substance having flexible crystalline properties, and preferably in an amount of more than 10 mol% but less than 30 mol%, 12 to 28 mol%, 15 to 25 mol%, or 20 mol%.

[0024] Preferably, in the polymer electrolyte according to the present invention, the lithium salt polymer and the substance having flexible crystalline properties are contained in a total amount of at least 80% by mass, or more preferably 90% or 95% by mass, and more preferably the polymer electrolyte according to the present invention comprises a lithium salt polymer and a substance having flexible crystalline properties.

[0025] (Anionic functional group) The lithium salt polymer according to the present invention has an anionic functional group, which forms a salt with lithium. It is considered that when the anion is immobilized on the polymer, it becomes difficult to diffuse, and the transport rate of lithium ions is improved.

[0026] 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 - , bis(oxalato)borate also known as BOB, -SO2-N - ―SO2-CF3, and -SO2-C - ―SO2-CF3. Particularly preferably, the anionic functional group is -SO2-N - ―SO2-CF3.

[0027] (Side chain) In one aspect, the lithium salt polymer has an anionic functional group in the side chain. Examples of the anionic functional group present in the side chain include those listed above.

[0028] When the polymer has an anionic functional group in the side chain, examples of the main chain of the polymer include polymethacrylate, polyacrylate, polystyrene, polystyrene sulfonic acid, polyethylene oxide (PEO), polyvinyl pyrrolidone (PVP), polyacrylonitrile (PAN), and poly(vinylidene fluoride-co-hexafluoropropylene) (PVDF-HFP).

[0029] 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.

[0030] The anionic functional group preferably has a sulfonimide skeleton or an imide skeleton. For example, a specific chemical structure of a sulfonimide located in a side chain is shown in structural formula (I). In structural formula (I), * indicates a bond with the polymer backbone or linker group. [ka]

[0031] (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.

[0032] 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.

[0033] [ka]

[0034] 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.

[0035] [ka]

[0036] The structural formula (IV) of lithium polyacrylic acid (lithium polyacrylate) is shown below. In the formula, n may be between 5 and 500.

[0037] [ka]

[0038] The structural formula (V) of lithium polystyrene sulfonic acid is shown below. In the formula, n may be between 5 and 500.

[0039] [ka]

[0040] <Substances with flexible crystalline properties> The polymer electrolyte according to the present invention includes a substance (particularly a compound) having flexible crystalline (plastic crystal) properties. "Flexible crystalline properties" refer to properties intermediate between liquid and solid, where the positions of the crystals are fixed, but they are in a mobile state, such as rotational motion. As a result, even in a solid state, ions can be conducted at high speed.

[0041] Examples of substances (particularly compounds) with flexible crystalline properties include organic nitriles. "Organic nitrile" is understood to be an organic compound or mixture thereof containing a nitrile functional group, also called 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.

[0042] In preferred embodiments of the present invention, the substance (compound) having flexible crystalline properties is preferably an aliphatic dinitrile, such as adiponitrile (AN) and / or succinonitrile (SN), because it has the advantage of exhibiting high conductivity. More preferably, the substance 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.

[0043] Substances having flexible 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 the molecular weight is not limited, but may be, for example, 60 g / mol or more. The molecular weight of a substance having flexible crystalline properties can be determined by mass spectrometry known in the art.

[0044] The substance having viscous crystalline properties may be present in an amount of, for example, 50 to 95 mol%, or even more specifically, 60 to 99 mol%, relative to the total number of moles of the substance having viscous crystalline properties and the lithium salt polymer. Preferably, the substance having viscous crystalline properties is present in an amount of more 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, relative to the total number of moles of the substance having viscous crystalline properties and the lithium salt polymer. Better ion conductivity can be obtained by having more than 70 mol%. Particularly good low-temperature properties can be obtained by having less than 90 mol%.

[0045] The viscous crystal properties can be identified, for example, using differential scanning calorimetry (DSC). Specifically, they can be identified by observing endothermic peaks associated with the phase transition from the crystalline phase to the plastic crystal phase and endothermic peaks of melting during a heating process (e.g., -80°C to 150°C) at a heating rate of 10°C / min using a differential scanning calorimetry device.

[0046] <Method for producing polymer electrolytes> The method for producing the polymer electrolyte according to the present invention is not particularly limited, but can be produced by a method including the following steps, for example: (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.

[0047] (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.

[0048] 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 (or repeating units).

[0049] 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.

[0050] (Step b) Step b provides a substance having flexible crystalline properties. For details on the substance having flexible crystalline properties, refer to the description of the polymer electrolyte according to the present invention above.

[0051] (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. A solvent, such as an organic solvent (e.g., acetonitrile), may be used for mixing.

[0052] The mixing ratio of the lithium salt polymer and the substance having flexible crystalline properties can be appropriately determined, for example, to yield a suitable proportion of these substances with respect to the polymer electrolyte of the present invention.

[0053] 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.

[0054] In one embodiment, a lithium salt polymer 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.

[0055] <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.

[0056] 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.

[0057] 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. [Examples]

[0058] Polymer electrolytes according to Examples 1 and 2 and Comparative Example 1 were manufactured, and evaluation cells were then manufactured using the obtained polymer electrolytes to evaluate the lithium ion transport fraction.

[0059] <Example 1: PMTFSI-Li+SN> In Example 1, a polymer electrolyte was prepared using PMTFSI-Li, a lithium salt polymer in which a TFSI ((trifluoromethane)sulfonimidolithium) group is bonded to a methacrylate main chain as a side-chain anionic functional group, and succinonitrile (SN) as a material having flexible crystalline properties.

[0060] (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.

[0061] (Measurement of lithium ion transport fraction) The obtained polymer electrolyte was heated to 60°C and impregnated into a polypropylene separator. Using the separator impregnated with the polymer electrolyte, a coin cell was fabricated with the configuration of Li metal / separator / Li metal. The fabricated coin cell was left to stand in a constant temperature bath at 50°C for 12 hours. After standing, AC impedance measurements from 1 Hz to 1 MHz were performed at 50°C using a VMP3 (manufactured by Biologic). The measured resistance at this time was denoted as R0. Next, DC polarization measurements were performed at 10 mV for 3600 seconds. The initial current value at this time was denoted as I0, and the steady-state current value (after 3600 seconds) was denoted as Is. Next, AC impedance measurements were performed from 1 Hz to 1 MHz. The measured resistance at this time was denoted as Rs. Substituting the obtained values ​​into equation (1), the lithium ion transport fraction t was obtained. Li+ I obtained it. t Li+ ={Current after polarization Is(A) × (Applied voltage V(V) - Current before polarization I0(A) × Impedance before polarization R0(Ω))} / {Current before polarization I0(A) × (Applied voltage V(V) - Current after polarization Is(A) × Impedance after polarization Rs(Ω))} ··· (1)

[0062] <Example 2: PSTFSI-Li+SN> The same method as in Example 1 was used for production, except that PMTFSI-Li was replaced with PSTFSI-Li (poly((trifluoromethane)sulfonimide lithium styrene)), and the lithium ion transport fraction was evaluated.

[0063] <Comparison example 1: PEO+LiTFSI+SN> In Comparative Example 1, a polymer electrolyte was prepared containing PEO (polyethylene oxide), a polymer without anionic functional groups; LiTFSI (lithium bis(trifluoromethanesulfonyl)imide), a low molecular weight lithium salt; and succinonitrile (SN), a substance with flexible crystalline properties.

[0064] In an Ar-atmosphere glove box, succinonitrile (Sigma-Aldrich) and LiTFSI (Sigma-Aldrich) were weighed in a molar ratio of SN:LiTFSI = 4:1 and stirred at 70°C for 24 hours. 10 wt% Mn=6000 PEO (polyethylene oxide, Sigma-Aldrich) was added to the resulting solution and stirred further at 70°C for 24 hours to obtain a polymer electrolyte. The polymer electrolyte of Comparative Example 1 was evaluated in the same manner as in Example 1.

[0065] <Result>

[0066] Table 1 shows the results of lithium ion transport fraction measurements for Examples 1 and 2 and Comparative Example 1.

[0067] [Table 1]

[0068] The results in Table 1 show that the polymer electrolytes of Examples 1 and 2, which have lithium salt polymers with anionic functional groups, exhibit superior lithium ion transportability compared to the polymer without anionic functional groups and the polymer electrolyte of Comparative Example 1, which has a low molecular weight lithium salt.

Claims

1. A polymer electrolyte comprising a lithium salt polymer having anionic functional groups and forming salts with lithium, and a substance having flexible crystalline properties.

2. The polymer electrolyte according to claim 1, wherein the anionic functional group of the lithium salt polymer is present in the side chain of the lithium salt polymer.

3. The polymer electrolyte according to claim 1 or 2, wherein the substance having the flexible crystalline properties is succinonitrile.

4. The polymer electrolyte according to claim 1 or 2, wherein the anionic functional group of the lithium salt polymer has a sulfonimide skeleton or an imide skeleton.

5. The polymer electrolyte according to claim 1 or 2, wherein the lithium salt polymer is poly((trifluoromethane)sulfonimide lithium methacrylate) or poly((trifluoromethane)sulfonimide lithium styrene).

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