Liquid electrolyte for lithium ion secondary batteries; methods for producing liquid electrolyte and lithium ion secondary batteries.

Abstract

DEPCT63 This invention describes the preparation of a liquid electrolyte for lithium-ion batteries. This will exhibit an increased potential on the oxidized side within a higher potential range than the previous electrolyte. Liquid electrolyte for lithium-ion secondary batteries, where the liquid electrolyte contains the following components: A mixture of lithium salt, sulfonamide, and water, where the liquid electrolyte contains anions. At least one type was selected from a group consisting of orthophosphoric ions, ions of... Pyrophosphate, an anion resulting from the dissociation of phosphoric acid or its salts. Anions are formed from the dissociation of phosphonic acid or its salts. -----------------------------------------------------------

Description

Electrolyte for lithium-ion secondary batteries, method for manufacturing the same, and lithium-ion secondary battery

[0001] This invention relates to an electrolyte for lithium-ion secondary batteries, a method for producing the same, and a lithium-ion secondary battery.

[0002] It is known that aqueous electrolytes for lithium-ion secondary batteries have limitations in the electrochemically stable potential range (potential window) due to the influence of the contained water. As an example of an attempt to expand the potential window of aqueous electrolytes, Non-Patent Document 1 discloses an experimental example in which an aqueous solution using lithium bis(trifluorosulfonyl)imide (LiTFSI), a lithium salt of sulfonimide, as the electrolyte for a lithium-ion battery.

[0003] Suo, L. et al. , Science, 2015, 350, 938-943

[0004] However, since further performance improvements are required for lithium-ion secondary batteries, even the electrolyte described in Non-Patent Document 1 cannot be said to have sufficient performance. The present invention was made in view of the above circumstances, and the object of the present invention is to provide an electrolyte for lithium-ion secondary batteries in which the oxidation side potential of the potential window is improved in an electrolyte containing a lithium salt of sulfonimide and water.

[0005] The electrolyte for a lithium-ion secondary battery of the present invention is an electrolyte for a lithium-ion secondary battery containing a lithium salt of sulfonimide and water, and contains at least one anion selected from the group consisting of an anion generated by dissociation of orthophosphate ion, pyrophosphate ion, phosphorous acid, or its salt, and an anion generated by dissociation of phosphinic acid, or its salt. In the electrolyte for a lithium-ion secondary battery of the present invention, it is preferable that the lithium salt of sulfonimide is at least one selected from the group consisting of lithium bis(trifluoromethylsulfonyl)imide (LiTFSI) and lithium bis(fluorosulfonyl)imide (LiFSI). In the electrolyte for a lithium-ion secondary battery of the present invention, it is preferable that the amount of LiTFSI per 1 kg of water is 5 to 21 mol, and the anion is pyrophosphate ion.

[0006] The method for producing an electrolyte for a lithium-ion secondary battery of the present invention is a method for producing an electrolyte for a lithium-ion secondary battery containing a lithium salt of sulfonimide and water, and a phosphoric oxygen acid having at least one anion selected from the group consisting of orthophosphate ion, pyrophosphate ion, phosphorous acid ion, and phosphinic acid ion, or its salt is added to the electrolyte as an additive. In the method for producing an electrolyte for a lithium-ion secondary battery of the present invention, it is preferable that the lithium salt of sulfonimide is at least one selected from the group consisting of lithium bis(trifluoromethylsulfonyl)imide (LiTFSI) and lithium bis(fluorosulfonyl)imide (LiFSI). In the method for producing an electrolyte for a lithium-ion secondary battery of the present invention, Li ３ PO ４ , LiH ２ PO ４ , NaH ２ PO ４ , Na ２ H ２ P ２ O ７ , Na ２ HPO ３ , and NaPH ２ O ２It is preferable to add at least one type of phosphate salt selected from the group consisting of the following. In the method for producing an electrolyte for lithium-ion secondary batteries of the present invention, the amount of LiTFSI per 1 kg of water is adjusted to 5 to 21 mol, and the phosphate salt is Na ２ H ２ P ２ O ７ It is preferable to add [this ingredient].

[0007] The lithium-ion secondary battery of the present invention is a lithium-ion secondary battery having a negative electrode, a positive electrode, and an electrolyte, wherein the electrolyte contains a lithium salt of sulfonimide and water, and contains at least one anion selected from the group consisting of orthophosphate ions, pyrophosphate ions, phosphorous acid, or anions derived from salts thereof, and phosphinic acid, or anions derived from salts thereof. In the lithium-ion secondary battery of the present invention, it is preferable that the lithium salt of sulfonimide is at least one selected from the group consisting of lithium bis(trifluorosulfonyl)imide (LiTFSI) and lithium bis(fluorosulfonyl)imide (LiFSI). In the lithium-ion secondary battery of the present invention, it is preferable that the amount of LiTFSI per 1 kg of water is 5 to 21 mol, and the anion is a pyrophosphate ion.

[0008] According to the present invention, it is possible to provide an electrolyte for lithium-ion secondary batteries in which the oxidation-side potential of the potential window is improved in an electrolyte containing a lithium salt of sulfonimide and water, a method for producing the electrolyte for lithium-ion secondary batteries, and a lithium-ion secondary battery having the electrolyte for lithium-ion secondary batteries.

[0009] Na ２ H ２ P ２ O ７ This graph shows the relationship between the amount of LiTFSI (mol) per 1 kg of water and the potential window for electrolytes with and without additives. This figure shows the linear sweep voltammograms of the electrolytes of Example 4 and Comparative Example 1. This figure shows the relationship between the number of cycles and the charge / discharge efficiency of lithium-ion secondary batteries having the electrolytes of Example 22 and Comparative Example 10.

[0010] The following will describe in detail the electrolyte for lithium-ion secondary batteries according to this disclosure, a method for manufacturing the electrolyte, and a lithium-ion secondary battery having the electrolyte, in that order.

[0011] 1. Electrolyte for Lithium-Ion Secondary Batteries The electrolyte for lithium-ion secondary batteries of this disclosure (hereinafter sometimes simply referred to as the electrolyte of this disclosure) is an electrolyte for lithium-ion secondary batteries containing a lithium salt of sulfonimide and water, and contains at least one anion selected from the group consisting of orthophosphate ions, pyrophosphate ions, phosphorous acid, or anions produced by the dissociation of a salt thereof, and phosphinic acid, or anions produced by the dissociation of a salt thereof.

[0012] As mentioned above, it has been reported that using LiTFSI, a lithium salt of sulfonimide, as the electrolyte in an aqueous electrolyte expands the potential window. This is thought to be because sulfonimide chemically reacts on the electrode surface to form a film (SEI; Solid Electrolyte Interface), and this SEI inhibits contact between water and the electrode, thereby suppressing the decomposition of water. In this disclosure, by including a specific phosphate ion in addition to such a lithium salt of sulfonimide in the aqueous electrolyte, it is possible to further improve the oxidation-side potential of the potential window. The reason why it is possible to further improve the oxidation-side potential of the potential window is not clear, but it is thought that because the electrochemical reaction product of phosphate ions, in addition to the electrochemical reaction product of sulfonimide mentioned above, is included in the SEI, it is possible to form an SEI with good Li ion conductivity, film thickness, and coverage. Here, improving the oxidation potential of the potential window not only expands the potential window of the electrolyte, enabling the manufacture of lithium-ion secondary batteries with higher voltage output, but also brings the advantage of increasing the selection of usable electrode materials. In other words, since electrode materials are selected based on the electrochemically stable potential range of the electrolyte, even if the improvement in the oxidation potential of the potential window is quantitatively small, electrode materials that could not be used before may become usable. Therefore, it is difficult to judge its superiority based solely on the improvement in the oxidation potential.

[0013] The electrolyte of this disclosure uses an aqueous solution containing a lithium salt of sulfonimide and water. The lithium salt of sulfonimide used as an electrolyte in this disclosure is not particularly limited as long as it is suitable for use in electrolytes for lithium-ion secondary batteries, for example, lithium bis(fluorosulfonyl)imide (LiFSI; CAS No. 171611-11-3), lithium bis(trifluorosulfonyl)imide (LiTFSI; CAS No. 90076-65-6), lithium bis(pentafluoroethanesulfonyl)imide (LiBETI; CAS No. 132843-44-8), lithium bis(nonafluorobutanesulfonyl)imide (CAS No. 119229-99-1), lithium nonafluoro-N-[(trifluoromethane)sulfonyl]butanesulfonylamide (CAS No. 176719-70-3), and lithium N,N-hexafluoro-1,3,-disulfonylimide (CAS Examples include No. 189217-62-7). When the electrolyte of this disclosure is used in the electrolyte layer of a lithium-ion secondary battery, the sulfonimide, together with the phosphate ions described later, forms a good quality film (SEI) on the electrode surface. In the electrolyte of this disclosure, a good quality film is formed by the reaction of the imide structure with specific phosphate anions described later. Therefore, if the electrolyte is a lithium salt of sulfonimide, the oxidation-side potential of the potential window can be improved regardless of the type.

[0014] Among these lithium salts of sulfonimides, LiTFSI, LiFSI, and LiBETI are preferred because they have high Li ion conductivity and a high effect in improving the oxidation side potential of the potential window, and LiTFSI and LiFSI are even more preferred. These lithium salts of sulfonimides may be commercially available or synthesized in advance.

[0015] The concentration of lithium sulfonimide in the electrolyte of this disclosure can be appropriately set according to the desired battery characteristics, within a range that does not exceed the saturation concentration of lithium sulfonimide in the solvent. This is because if solid lithium sulfonimide remains in the electrolyte, the solid component may inhibit the battery reaction. Generally, as the concentration of lithium sulfonimide in the electrolyte increases, the potential window widens, but the viscosity of the solution increases, which tends to decrease the Li ion conductivity. Therefore, generally, the concentration is set according to the desired battery characteristics, taking into account the Li ion conductivity and the effect of widening the potential window. In the electrolyte of this disclosure, regardless of the concentration of lithium sulfonimide set according to the desired battery characteristics, it is possible to improve the oxidation potential of the potential window by including specific phosphate ions, as described later. In this disclosure, the concentration of lithium sulfonimide in the electrolyte is expressed as molality, i.e., the number of moles of lithium sulfonimide per 1 kg of solvent. Compared to conventional technologies, it is possible to obtain an electrolyte that combines a high potential window oxidation-side potential and Li ion conductivity. Therefore, in an electrolyte containing LiTFSI as a sulfonimide along with specific phosphate ions described later, the amount of LiTFSI per 1 kg of water contained as a solvent may be in the range of 10 to 18 mol.

[0016] The electrolyte for lithium-ion batteries of this disclosure contains water as the main solvent. Other components besides water, such as non-aqueous solvents, may be included as solvents, provided they do not affect the effects of the present invention. Specifically, the water content in the solvent may be 95% by mass or more, or 99% or more. The electrolyte of this disclosure may also contain lithium materials other than lithium sulfonimide salts. The molar ratio of lithium sulfonimide salts to the total number of moles of lithium materials in the electrolyte may be 0.1 or more, or 0.5 or more.

[0017] The electrolyte of this disclosure contains at least one anion selected from the group consisting of orthophosphate ions, pyrophosphate ions, phosphorous acid, or anions produced by the dissociation of a salt thereof, and phosphinic acid, or anions produced by the dissociation of a salt thereof.

[0018] There are no particular restrictions on the concentration of the phosphate ions, but as mentioned above, the electrolyte of this disclosure may be a mixed aqueous solution with a relatively high concentration of lithium sulfonimide depending on the desired battery characteristics. In the electrolyte of this disclosure, if the lithium sulfonimide is present at a relatively high concentration and the phosphate ions are difficult to dissolve, it is preferable to make the concentration of phosphate ions as high as possible, and more preferably in a saturated state.

[0019] In this disclosure, the term orthophosphate ion is used as PO ４ ３－ HPO ４ ２－ , and H ２ PO ４ － The three types of anions represented by chemical formulas are shown, and the term for pyrophosphate ion is P ２ O ７ ４－ HP ２ O ７ ３－ H ２ P ２ O ７ ２－ , and H ３ P ２ O ７ － This refers to anions represented by the following four chemical formulas.

[0020] Furthermore, in this disclosure, the term "phosphite ion" or the anion produced by the dissociation of a salt thereof refers to both the phosphonate ion (divalent) and the phosphite ion (trivalent), which may exist as tautomers in the electrolyte. Here, the term "phosphonate ion" refers to PHO ３ ２－ , and HPHO ３ － The anions are represented by two types of chemical formulas, and the term for phosphite ion is PO ３３－ HPO ３ ２－ , and H ２ PO ３ － This refers to anions represented by the following three types of chemical formulas.

[0021] Furthermore, in this disclosure, the term "phosphinic acid" or the anion produced by the dissociation of a salt thereof refers to both the phosphinate ion (monovalent) and the phosphonite ion (divalent), which may exist as tautomers in the electrolyte. Here, the term "phosphinate ion" refers to the pH ２ O ２ － It shows an anion represented by one type of chemical formula, and the term for phosphonic acid ion is PHO ２ ２－ , and HPHO ２ － This refers to anions represented by the following two chemical formulas.

[0022] The phosphate ions contained in the electrolyte of this disclosure undergo different dissociation and interconversion states depending on the pH of the electrolyte and the lithium salt of sulfonimide, making it difficult to determine which state or ratio of the anion represented by the above chemical formula exists. However, if the condition that the electrolyte contains at least one anion selected from the group consisting of the anions represented by the above chemical formula is met, it is possible to achieve the effect of this disclosure, which is to improve the oxidation-side potential of the potential window of the electrolyte.

[0023] In the electrolyte of this disclosure, the anion is preferably an orthophosphate ion or a pyrophosphate ion because it has a high effect in improving the oxidation-side potential of the potential window, and is even more preferably a pyrophosphate ion. Furthermore, when the anion is a pyrophosphate ion, it is preferable that the lithium salt of the sulfonimide is LiTFSI and the amount of LiTFSI per 1 kg of water is 5 to 21 mol, because not only is the oxidation-side potential improved, but the reduction-side potential is also greatly reduced, and the effect of expanding the potential window is enhanced. In particular, it is even more preferable that the amount of LiTFSI per 1 kg of water is 5 to 18 mol, because it results in an electrolyte with an extremely wide potential window and high Li ion conductivity compared to conventional aqueous electrolytes. The reason why a wide potential window is observed when the phosphate ion is a pyrophosphate ion and the amount of LiTFSI per kg of water is 5 to 21 mol is not clear, but it is thought that this is influenced by the fact that pyrophosphate ions have different polarization states than other phosphate ions, resulting in changes in adsorption to the electrode surface, and that pyrophosphate ions have higher solubility compared to other phosphate ions.

[0024] Furthermore, while there are no particular restrictions on the pH of the electrolyte of this disclosure as long as it can be used as an electrolyte, it is preferably in the range of 3 to 11, and more preferably in the range of 5 to 9.

[0025] 2. Method for Manufacturing an Electrolyte for Lithium-Ion Secondary Batteries In the method for manufacturing an electrolyte for lithium-ion secondary batteries according to this disclosure, a phosphoric acid having at least one anion selected from the group consisting of orthophosphate ions, pyrophosphate ions, phosphite ions, and phosphinate ions, or a salt thereof, is added as an additive to an electrolyte for lithium-ion secondary batteries containing a lithium salt of sulfonimide and water. However, the lithium-ion secondary battery electrolyte according to this disclosure described above is not necessarily limited to the lithium-ion secondary battery electrolyte manufactured by this method.

[0026] In the manufacturing method of this disclosure, the electrolyte containing a lithium salt of sulfonimide and water, used as the mother liquor to which a phosphate acid or a salt thereof is added, and a preferred embodiment thereof, is the same as the aqueous solution containing a lithium salt of sulfonimide and water described in 1. Electrolyte for lithium-ion secondary batteries above.

[0027] In the manufacturing method of the present disclosure, a phosphorus acid having at least one anion selected from the group consisting of orthophosphate ions, pyrophosphate ions, phosphinate ions, and phosphinate ions, or a salt thereof, is added as an additive to an electrolyte for a lithium-ion secondary battery containing a lithium sulfonimide salt and water. For example, Li ３ PO ４ (Trilithium phosphate), LiH ２ PO ４ (Lithium dihydrogen phosphate), NaH ２ PO ４ (Sodium dihydrogen phosphate), Na ２ H ２ P ２ O ７ (Disodium dihydrogen pyrophosphate), Na ２ HPO ３ (Disodium hydrogen phosphite), and NaPH ２ O ２ Sodium phosphinate (also known as sodium hypophosphite) can be used. By adding the above-mentioned phosphate acid or its salt, anions from orthophosphate ions, pyrophosphate ions, phosphorous acid or its salts, and anions from phosphinate or its salts are supplied to the electrolyte for lithium-ion secondary batteries containing the lithium salt of sulfonimide and water.

[0028] Any compound capable of supplying the specific phosphate ions to the electrolyte can be added in acidic or salt form, and the choice can be made appropriately depending on the pH of the electrolyte. Typically, the pH of the aqueous solution containing the lithium salt of sulfonimide to which the additive is added and water is near neutral, and since the electrolyte of this disclosure is used at a pH close to neutral, it is preferable to add it in salt form.

[0029] There are no particular restrictions on the amount of phosphate acid or its salt added, however, as mentioned above, the amount of phosphate acid or its salt that dissolves in an aqueous solution containing a relatively high concentration of the lithium sulfonimide is relatively small, so it may be added until saturation is achieved. If there is any undissolved acid or salt remaining in the solid state, it may be used as is or removed before use.

[0030] As the phosphate salt mentioned above, Na ２ H ２ P ２ O ７ When adding a lithium sulfonimide, it is preferable to use LiTFSI as the lithium salt of sulfonimide and adjust the amount of LiTFSI per kg of water to the range of 5 to 21 mol. In the resulting electrolyte, not only is the oxidation potential improved, but the reduction potential is also greatly reduced, and the effect of expanding the potential window is enhanced. In particular, it is even more preferable to adjust the amount of LiTFSI per kg of water to the range of 5 to 18 mol / kg, as this yields an electrolyte with an extremely wide potential window and high Li ion conductivity compared to the conventional technology.

[0031] 3. Lithium-ion secondary battery The lithium-ion secondary battery of this disclosure is a lithium-ion secondary battery having a negative electrode, a positive electrode, and an electrolyte, wherein the electrolyte contains a lithium salt of sulfonimide and water, and contains at least one anion selected from the group consisting of orthophosphate ions, pyrophosphate ions, phosphorous acid, or anions produced by the dissociation of a salt thereof, and phosphinic acid, or anions produced by the dissociation of a salt thereof. In addition, a separator, a conductive additive, and a binder may be used as appropriate in the lithium-ion secondary battery of this disclosure.

[0032] The electrolyte used in the lithium-ion secondary battery of this disclosure is the same as the electrolyte for lithium-ion secondary batteries described in 1. Electrolyte for Lithium-ion Secondary Batteries, so its description is omitted. In the lithium-ion secondary battery of this disclosure, the composition of the electrolyte changes due to charge-discharge reactions (oxidation-reduction reactions), and these reactions accumulate with repeated charging and discharging, making it difficult to precisely specify its composition. However, regardless of the composition, since it contains a lithium salt of sulfonimide and water, and at least one anion selected from the group consisting of orthophosphate ions, pyrophosphate ions, phosphorous acid, or anions derived from their salts, and phosphinic acid or anions derived from its salts, it is possible to make a lithium-ion secondary battery with a high input / output potential on the oxidation side. As described above, if the conditions of containing a lithium salt of sulfonimide and the specific phosphate ions are met, a good SEI is formed on the electrode, and the oxidation side potential of the electrolyte's potential window is improved.

[0033] As the positive electrode active material used for the positive electrode, for example, a material that exhibits redox reactivity with lithium ions and contains at least one transition metal element can be used. Suitable positive electrode active materials include oxide materials, polyanionic materials, and organic materials. As the negative electrode active material used for the negative electrode, for example, a material mainly containing sulfur, Mo ６ S ８ The Chevre, Li ４ Ti ５ O １２ Materials such as lithium titanium oxide, sulfur, carbon, Si and Sn which can form alloys with Li, and metal-organic frameworks (MOFs) can be used. Since good SEI is easily formed, Li is used as the negative electrode active material. ４ Ti ５ O １２ It is preferable to use [this].

[0034] A separator can typically be used in the lithium-ion secondary battery of this disclosure. The separator is placed between the positive electrode and the negative electrode and has the function of preventing contact between the positive and negative electrodes, holding the electrolyte, and forming an electrolyte layer. In the lithium-ion secondary battery of this disclosure, a separator that is normally used in batteries using an aqueous electrolyte can be used as the separator. Examples of materials for the separator include resins such as polyethylene (PE), polypropylene (PP), polyester, cellulose, and polyamide. In addition, in this disclosure, the separator may be a nonwoven fabric such as a resin nonwoven fabric or a cellulose-based nonwoven fabric, and a cellulose-based nonwoven fabric is preferred. Furthermore, the film thickness of the separator is not particularly limited and is similar to the film thickness of a separator used in a general lithium-ion secondary battery.

[0035] For conductive additives and binders used in electrodes and the like, those commonly used in lithium-ion batteries can be used. Conductive additives are not particularly limited as long as they improve conductivity, but examples include acetylene black, carbon black such as furnace black, carbon nanotubes (CNTs), and carbon nanofibers (CNF). Examples of binders include polyvinylidene fluoride (PVdF), polytetrafluoroethylene (PTFE), butylene rubber (BR), and styrene-butadiene rubber (SBR).

[0036] The lithium-ion secondary battery of this disclosure may include a battery case that houses a positive electrode, an electrolyte layer, a negative electrode, etc. Specific examples of battery case shapes include coin-type, flat-plate-type, cylindrical-type, laminate-type, and the like.

[0037] [Example 1] 1. Preparation of electrolyte for lithium-ion secondary battery A mother liquor was prepared by mixing LiTFSI (manufactured by Tokyo Chemical Industry Co., Ltd.) and water so that the amount of LiTFSI per 1 kg of water was 21 mol. Li ３ PO ４Lithium phosphate (manufactured by Aldrich) was added so as to be 1% by mass, and after diffusion in an ultrasonic generator for 15 minutes, the temperature of the mixture was maintained at 40 °C using a thermostat, whereby the dissolution of LiTFSI and Li ３ PO ４ in water was promoted. The electrolyte solution of Example 1 was prepared by leaving the mixture as it was at 40 °C overnight. In the electrolyte solution of Example 1, all of the added Li ３ PO ４ did not dissolve, and it was in a state of a saturated solution in which a part precipitated. Therefore, the electrolyte solution was allowed to stand, and only the supernatant was extracted so as not to be contaminated with the precipitated Li ３ PO ４ and used in the following evaluations.

[0038] 2. Preparation of a cell for potential window evaluation For the evaluation of the potential window, a batch cell for battery evaluation (manufactured by SBI A, Easy Frontier Co., Ltd.) using SUS316 foil (manufactured by Nilaco) for the working electrode and the counter electrode and Ag / AgCl (manufactured by Interchem) for the reference electrode was used. By injecting the above electrolyte solution into the batch cell for battery evaluation, a cell for potential window evaluation was fabricated.

[0039] 3. Preparation of a cell for ion resistance measurement A cell for ion resistance measurement using only the positive electrode and the negative electrode without using the reference electrode in the cell for potential window evaluation was prepared.

[0040] 4. Evaluation conditions As an electrochemical measurement device, a multi-channel potentiostat / galvanostat (manufactured by Bio Logic, model number: VMP3) was used. Also, the cell temperature during evaluation was adjusted by a thermostat (manufactured by Espec, model number: LU-124).

[0041] 4-1. Evaluation method of potential window The evaluation of the potential window was performed by cyclic voltammetry (CV) using a cell for potential window evaluation. The cell temperature was set at 25 °C. First, starting from the open circuit potential (OCP), the sweep was started at a sweep rate of 1 mV / s in the noble potential direction, and the sweep was repeated with 5.2 V (vs Li / Li ＋ as the upper limit. From the OCP to 5.2 V (vs. Li / Li ＋The potential at which the current, thought to correspond to the oxidative decomposition of water, begins to rise during the sweep up to 1.4V (vs. Li / Li) was defined as the oxidation-side potential of the electrolyte. In addition, a similar cell was prepared separately, and a sweep was started from OCP in the direction of a lesser potential at a sweep rate of 1 mV / s, and 1.4V (vs. Li / Li) was obtained. ＋ The sweep was repeated with the lower limit being ). From OCP, 1.4V (vs. Li / Li ＋ The potential at which the current, thought to correspond to the reductive decomposition of water, begins to rise during the sweep up to ) was defined as the reducing potential of the electrolyte. Furthermore, the potential region between the oxidation potential and the reduction potential was defined as the potential window of the electrolyte, and the wider this potential window (ΔV), the wider the electrochemically stable potential region can be considered to be.

[0042] 4-2. Method for Measuring Ion Resistance Ion resistance was measured using an ion resistance measurement cell and the AC impedance method. The detailed measurement conditions are as follows: Current: 10 mV AC frequency: 1 to 100 kHz Cell temperature: 25°C

[0043] [Examples 2 to 21, Comparative Examples 1 to 9] Except for changing the concentration of LiTFSI and the additives used in "1. Preparation of Electrolyte for Lithium-ion Batteries" in Example 1 as shown in Table 1 below, the electrolytes for Examples 2 to 21 and Comparative Examples 1 to 9 were prepared in the same manner as in Example 1. Note that each additive is LiH ２ PO ４ (Lithium dihydrogen phosphate, manufactured by Aldrich), NaH ２ PO ４ (Sodium dihydrogen phosphate, manufactured by Aldrich), Na ２ H ２ P ２ O ７ (Disodium dihydrogen pyrophosphate, manufactured by Aldrich), Na ２ HPO ３ (Disodium hydrogen phosphite, manufactured by Aldrich), and NaPH ２ O ２ (Sodium phosphinate, manufactured by Aldrich), and Na ２ S ２ O ３(Sodium thiosulfate, manufactured by Aldrich), NaSCN (sodium thiocyanate, manufactured by Aldrich), and K ２ S (potassium sulfide, manufactured by Aldrich) was used. Using these electrolytes, a cell for potential window evaluation and a cell for ion resistance measurement were prepared in the same manner as in Example 1, and these cells were subjected to evaluation.

[0044]

[0045] [Example 22, Comparative Example 10] 1. Preparation of electrolyte for lithium-ion secondary battery The electrolyte for Example 22 was prepared in the same manner as in Example 4, except that the LiTFSI (manufactured by Tokyo Chemical Industry Co., Ltd.) used as the electrolyte in Example 4 was changed to LiFSI (manufactured by Kishida Chemical Co., Ltd.). The electrolyte for Comparative Example 10 was prepared in the same manner as in Comparative Example 1, except that the LiTFSI used as the electrolyte in Comparative Example 1 was changed to LiFSI. 2. Evaluation of potential window Using the electrolytes of Example 22 and Comparative Example 10, cells for evaluating the potential window were prepared in the same manner as in Example 1, and these cells were subjected to evaluation. 3. Evaluation of charge / discharge efficiency Using the electrolytes of Example 22 and Comparative Example 10, a lithium-ion secondary battery for evaluating charge / discharge efficiency was prepared, equipped with electrodes fabricated as follows: 1.62 to 2.32 V (vs. Li / Li ＋ The charge-discharge reaction was repeated 100 times within the range of ), and the charge-discharge efficiency was determined for each cycle from the amount of charge during the charge-discharge reaction. [Electrode fabrication conditions] The active material was Li for the working electrode (negative electrode). ４ Ti ５ 0 １２ (LTO) is connected to LiMn as the opposite electrode (positive electrode). ２ 0 ４(LMO) was used. Acetylene blank was used as the conductive additive, and PVdF was used as the binder. SUS316L foil (manufactured by Niraco) was used for both the positive and negative electrodes as current collectors. First, the active material and conductive additive were mixed in a mortar, and then PVdF was added. The mass ratio of the active material, conductive additive, and PVdF was set to active material:conductive additive:PVdF = 85:10:5. NMP was added while checking the viscosity, and mixing in the mortar continued until uniform. After that, it was transferred to an ointment container and mixed at 3000 rpm for 10 minutes in a mixer (product name: Awatori Rentaro, manufactured by Shinky Co., Ltd.). The slurry obtained in this way was placed on metal foil and coated with a doctor blade. After that, it was left to stand overnight in a 60°C dryer to dry the solvent and prepare the electrodes. Each of the obtained electrodes was punched out to a diameter of φ16 mm and subjected to a roll press so that the porosity was 40%. The capacity is 0.3 mAh / cm² LTO. ２ LMO is 0.6 mAh / cm³ ２ I made it so that it would be like that.

[0046]

[0047] 5. Results and Discussion Table 1 shows the electrolyte composition, potential window evaluation results, and ion resistance measurement results for Examples 1 to 21 and Comparative Examples 1 to 9, which used LiTFSI as the electrolyte. Table 2 shows the electrolyte composition, potential window evaluation results, and potential window evaluation results for Example 22 and Comparative Example 10, which also used LiTFSI as the electrolyte. Note that Figure 1 shows Na ２ H ２ P ２ O ７Figure 2 shows a graph illustrating the relationship between the amount of LiTFSI per 1 kg of water and the potential window (ΔV) for electrolytes with and without the additive. Figure 2 also shows, for reference, linear sweep voltammograms of the electrolytes for Example 5, Comparative Example 1, and Comparative Example 2. Furthermore, Figure 3 shows a graph illustrating the relationship between the number of cycles and charge / discharge efficiency of lithium-ion secondary batteries prepared from the electrolytes of Example 22 and Comparative Example 10, using LiTFSI as the electrolyte. From the results of Comparative Examples 1 to 6, it was observed that as the amount of LiTFSI per 1 kg of water (concentration of LiTFSI in the electrolyte) increased, the oxidation potential tended to increase and the reduction potential to decrease. In Comparative Example 1, where the amount of LiTFSI per 1 kg of water was 21 mol, the oxidation potential was 4.79 V vs. Li / Li ＋ And the reduction side potential is 2.01V vs. Li / Li ＋ This showed that the potential window (ΔV) was at its maximum of 2.78V. In the mother liquor with the same amount of LiTFSI per 1 kg of water as in Comparative Example 1 (21 mol), Na ２ S ２ O ３ NaSCN, K ２ In the electrolytes of Comparative Examples 7-9, where sulfur was added, the oxidation potential was lower than in Comparative Example 1, which did not contain any additives, and the overall potential window was also reduced. It became clear that the addition of sulfide-based compounds actually had an adverse effect, as the anions generated from these additives adhered to the electrode surface, accelerating the decomposition of water.

[0048] In contrast, when comparing these using the same amount of LiTFSI per 1 kg of water, the mother liquor containing LiTFSI and water contains Li ３ PO ４ LiH ２ PO ４ NaH ２ PO ４ Na ２ H ２ P ２ O ７ Na ２ HPO ３ , and NaPH ２ O ２The electrolytes of Examples 1 to 21, to which each additive was added, showed a higher oxidation potential than the electrolytes of Comparative Examples 1 to 6, which did not contain the additives.

[0049] In particular, in the electrolytes of Examples 2, 5-7, 11-13, 15, 17, and 20, in which the aforementioned phosphate salts were added within the range of 10-18 mol of LiTFSI per 1 kg of water, the amount of LiTFSI per 1 kg of water was 21 mol, exhibiting a higher oxidation potential than the electrolyte of Comparative Example 1, which did not contain any additives, and demonstrating an extremely excellent effect of improving ionic conductivity as well. As shown in Comparative Examples 1-6, in conventional electrolytes using only LiTFSI, increasing the amount of LiTFSI per 1 kg of water (LiTFSI concentration in the electrolyte) to improve the oxidation potential increases viscosity, and therefore ionic resistance also increases. In the electrolytes of this disclosure, by adding specific phosphate salts, the oxidation potential can be improved without increasing the viscosity of the electrolyte, and it is considered that an electrolyte with a higher oxidation potential and Li ion conductivity than conventional electrolytes using only LiTFSI can be obtained.

[0050] Also, Na ２ H ２ P ２ O ７ In the electrolytes of Examples 4 to 9 to which Na was added, as shown in Table 1 and Figures 1 and 2, it was also revealed that, in the range of 5 to 21 mol of LiTFSI per 1 kg of water, not only the oxidation potential but also the reduction potential was expanded compared to the comparative example with the same amount of LiTFSI per 1 kg of water, resulting in an extremely wide potential window (ΔV) of 2.88 to 3.33 V. ２ H ２ P ２ O ７ The reason why the addition of pyrophosphate ions results in an extremely wide potential window is not clear, but it is thought that this is due to the fact that the polarization state of pyrophosphate ions differs from that of other phosphate ions, resulting in a change in adsorption to the electrode surface, and that pyrophosphate ions have higher solubility compared to other phosphate ions.

[0051] Also, instead of using LiTFSI as an electrolyte, use LiFSI, Na２ H ２ P ２ O ７ In the electrolyte of Example 22 to which Na was added, as shown in Table 2, ２ H ２ P ２ O ７ Compared to Comparative Example 10, which was LiFSI without additives, it was found that not only the oxidation potential but also the reduction potential was expanded, resulting in an extremely wide potential window (ΔV) of 3.05 V. Therefore, if the electrolyte is a lithium salt of sulfonimide, it is thought that the oxidation potential of the potential window can be improved by combining it with at least one anion selected from the group consisting of orthophosphate ions, pyrophosphate ions, phosphorous acid, or anions produced by the dissociation of their salts, and phosphinic acid, or anions produced by the dissociation of its salts. As shown in Figure 3, Na using LiFSI ２ H ２ P ２ O ７ In the lithium-ion secondary battery prepared from the electrolyte of Comparative Example 10, which did not contain added sodium, the charge-discharge efficiency decreased sharply after the second cycle. In contrast, when using LiFSI, Na ２ H ２ P ２ O ７ In the lithium-ion secondary battery prepared from the electrolyte of Example 22 with added LiFSI and Na, the charge-discharge efficiency increased to approximately 50% by the third cycle, and then maintained an efficiency of approximately 50% up to 100 cycles. This is due to the addition of LiFSI and Na. ２ H ２ P ２ O ７ It is believed that the SEI formed from the lithium titanate inhibits contact between water in the electrolyte and the electrodes, thereby suppressing the decomposition of water as a side reaction, and as a result, the current during the reduction reaction was used to charge more lithium titanate (LTO).

[0052] From the above results, it has been demonstrated that the electrolyte for lithium-ion secondary batteries of the present disclosure, which contains a lithium salt of sulfonimide and water, and contains at least one anion selected from the group consisting of orthophosphate ions, pyrophosphate ions, phosphorous acid, or salts thereof, and phosphinic acid or salts thereof, improves the oxidation-side potential of the potential window.

Claims

DEPCT631. Liquid electrolyte for lithium ion secondary batteries where such liquid electrolyte is composed of lithium sulfonimide salts and water, and the liquid electrolyte contains at least one anion of a type chosen from the group of orthophosphoric ions, pyrophosphate ions, anions resulting from the dissociation of phosphoric acid or its salts, and anions resulting from the dissociation of phosphonic acid or its salts.

2. Liquid electrolyte for lithium ion secondary batteries according to claim 1, where at least one lithium sulfonimide salt is a substance chosen from the group of lithium bis(trifluorosulfonyl)imide (LiTFSI) and lithium bis(fluorosulfonyl)imide (LiFSI).

3. Liquid electrolyte for lithium ion secondary batteries under claim 2, where the amount of LiTFSI per kg of water is between 5 mol and 21 mol and the anion is pyrophosphate ions 4.Method for the production of liquid electrolyte for lithium-ion secondary batteries under patent 4, whereby the liquid electrolyte is composed of lithium sulfonimide salts and water, and whereby phosphorus oxyacid containing at least one anion of a choice from the group of orthophosphoric ions, pyrophosphate ions, phosphoric acid ions, and phosphonic acid ions, or salts of such, is added to the liquid electrolyte as an additive.

5. Method for the production of liquid electrolyte for lithium-ion secondary batteries under patent 4, whereby at least one lithium sulfonimide salt is a choice from the group of lithium bis(trifluorosulfonyl)imide (LiTFSI) and lithium bis(fluorosulfonyl)imide (LiFSI).

6. A method for producing liquid electrolytes for lithium-ion secondary batteries under claim 5 by doping with at least one selected phosphorus oxoate from the group consisting of Li3PO4, LiH2PO4, NaH2PO4, Na2H2P2O7, Na2HPO3, and NaPH2O27.A method for producing liquid electrolyte for lithium ion secondary batteries under claims 5 or 6, where the amount of LiTFSI per kg of water is adjusted to 5 mol to 21 mol and Na2H2P2O7 is added as phosphorus oxoate.

8. A lithium ion secondary battery consisting of an anode, cathode and liquid electrolyte, where the liquid electrolyte is composed of lithium salts of sulfonimide and water, and where the liquid electrolyte contains at least one type of anion selected from the group of orthophosphoric ions, pyrophosphate ions, anions resulting from the dissociation of phosphoric acid or its salts, and anions resulting from the dissociation of phosphonic acid or its salts.

9. Secondary lithium-ion batteries under claim 8, where at least one lithium salt of sulfonimide is selected from the group of lithium bis(trifluorosulfonyl)imide (LiTFSI) and lithium bis(fluorosulfonyl)imide (LiFSI).10.A lithium-ion secondary battery under claim 9 where the amount of LiTFSI per kg of water is between 5 mol and 21 mol and the anion is pyrophosphate ions.