Non-aqueous secondary battery

Abstract

To provide a non-aqueous secondary battery that has a simple configuration and can suppress a decrease in capacity retention rate.SOLUTION: A non-aqueous secondary battery includes a positive electrode, a lithium metal negative electrode, and an electrolyte solution, and the electrolyte solution contains a pyrrolidinium cation and a bis(fluorosulfonyl)imide anion (FSI), the molar ratio of lithium ions to the pyrrolidinium cation is 10 to 40, and the molar concentration of the bis(fluorosulfonyl)imide anion is 3 to 4 mol / L.SELECTED DRAWING: None

Description

[Technical field]

[0001] The present disclosure relates to non-aqueous secondary batteries. [Background technology]

[0002] 2. Description of the Related Art With the widespread use of small mobile devices such as mobile phones and notebook computers, much development has been carried out on non-aqueous secondary batteries that are repeatedly charged and discharged. Patent Document 1 discloses a rechargeable lithium battery that uses, as the electrolyte for the lithium metal negative electrode, an electrolyte solution with a high salt concentration having a lithium imide salt concentration of at least 2 mol per liter of organic solvent. [Prior art documents] [Patent documents]

[0003] [Patent Document 1] Special Publication No. 2018-505538 Summary of the Invention [Problem to be solved by the invention]

[0004] It is known that lithium metal has a very low reaction potential and that the solvent in the electrolyte is decomposed as it is deposited. This decomposition is thought to result in a decrease in cycle characteristics (capacity retention rate). Usually, the lithium ions in a low-salt electrolyte are coordinated with a solvent, and the coordination ratio of anions increases as the salt concentration increases. It is known that anions become susceptible to reductive decomposition by being coordinated with lithium ions, the ratio of the coating derived from the anions increases, and the deposition and dissolution efficiency of the lithium metal negative electrode improves. On the other hand, since a high-salt electrolyte actively decomposes anions, it is thought that there is a limit to the improvement of cycle characteristics by only increasing the salt concentration, as in Patent Document 1.

[0005] The present disclosure has been made in view of the above problems, and has an object to provide a nonaqueous secondary battery that can suppress a decrease in capacity retention rate with a simple configuration. [Means for solving the problem]

[0006] One aspect for achieving the above object is a nonaqueous secondary battery comprising a positive electrode, a lithium metal negative electrode, and an electrolyte, the electrolyte containing a pyrrolidinium cation and a bis(fluorosulfonyl)imide anion (FSI), the molar ratio of lithium ions to the pyrrolidinium cation being 10 to 40, and the molar concentration of the bis(fluorosulfonyl)imide anion being 3 to 4 mol / L.

[0007] In the non-aqueous secondary battery according to the present disclosure, the electrolyte contains a specific amount of bis(fluorosulfonyl)imide anions and a specific molar ratio of pyrrolidinium cations to lithium ions. By allowing the specific anions and cations to exist together with the lithium ions in this way, the solvation structure can be controlled by the interaction between the anions coordinated to the lithium ions and the cations, and the amount of decomposition of the anions can be suppressed. In addition, even if localized current concentration occurs due to the presence of the specific cations, the concentration of lithium ions can be alleviated by the steric hindrance effect of the cations, improving the lithium precipitation and dissolution efficiency, and as a result, the capacity retention rate can be improved, and the charge and discharge efficiency can be improved. Effect of the Invention

[0008] According to the present disclosure, it is possible to provide a nonaqueous secondary battery that can suppress a decrease in capacity retention rate with a simple configuration. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0009] Hereinafter, specific embodiments to which the present disclosure is applied will be described in detail. However, the present invention is not limited to the following embodiments.

[0010] The following factors are thought to be the causes of the decrease in cycle capacity retention rate (discharge capacity retention rate) of a non-aqueous secondary battery (non-aqueous lithium ion secondary battery), in other words, the decrease in charge / discharge efficiency. These factors include irreversible reactions of lithium metal, i.e., charge consumption due to decomposition of the electrolyte, charge consumption due to micro-short circuiting, and electrical isolation of lithium metal.

[0011] Here, in the rechargeable lithium battery described in Patent Document 1, a high-concentration electrolyte solution containing a lithium imide salt, specifically, lithium bis(fluorosulfonyl)imide (LiN(SO2)2; hereinafter, also referred to as LiFSI), is used as the electrolyte for the lithium metal negative electrode. Thus, electrolytes containing a high concentration of LiFSI exhibit a relatively high Li deposition and dissolution efficiency, but there is room for improvement from the standpoint of suppressing electrolyte decomposition.

[0012] As mentioned above, there are various possible reasons for the decrease in cycle capacity retention rate, but the main cause is thought to be charge consumption due to electrolyte decomposition. Lithium metal has a very base reaction potential, and the solvent decomposes as it deposits. Therefore, it is important to reduce the surface area of ​​the deposited lithium metal to suppress the growth of lithium dendrites and to have a solvation structure that suppresses electrolyte decomposition.

[0013] Normally, the lithium ions in low-salt electrolytes are coordinated with the solvent, and as the salt concentration increases, the coordination ratio of anions (for example, FSI, described below) increases. It is known that anions become more susceptible to reductive decomposition when coordinated with lithium ions, increasing the ratio of anion-derived coatings and improving the precipitation and dissolution efficiency of the lithium metal negative electrode. On the other hand, as the salt concentration increases, anions are actively decomposed, so it is thought that there is a limit to the improvement in efficiency that can be achieved by increasing the salt concentration.

[0014] The nonaqueous secondary battery according to the present disclosure (hereinafter also referred to as the present secondary battery) comprises a positive electrode, a lithium metal negative electrode, and a specific electrolyte solution, and may further comprise a separator. The positive electrode, lithium metal negative electrode, and separator in the present secondary battery may be any of those conventionally known in the field of nonaqueous secondary batteries, as long as the effects of the present disclosure can be obtained. Moreover, the secondary battery contains pyrrolidinium cations and bis(fluorosulfonyl)imide anions (also referred to as FSI) in the electrolyte. Furthermore, in the electrolyte, from the viewpoint of improving the discharge capacity retention rate, the molar ratio of lithium ions to pyrrolidinium cations (Li / cation) is 10 to 40. Furthermore, in the electrolyte, from the viewpoint of improving the discharge capacity retention rate, the molar concentration of FSI is 3 to 4 mol / L (M).

[0015] In this way, in the present secondary battery, since a specific cation exists in the electrolyte in addition to the lithium ion, the charge is dispersed by interaction with the bis(fluorosulfonyl)imide anion coordinated to the lithium ion, and the amount of decomposition of the anion can be suppressed, and as a result, the decrease in the discharge capacity retention rate can be suppressed. In addition, the presence of specific cations in addition to lithium ions can alleviate the concentration of lithium ions due to the three-dimensional hindrance effect of the cations even in the event of localized current concentration, thereby suppressing the formation of lithium dendrites, thereby suppressing the decrease in the discharge capacity retention rate. In this manner, in the present secondary battery, by controlling the solvation structure in the electrolyte, the charge / discharge efficiency is improved and the discharge capacity retention rate is improved.

[0016] In the present secondary battery, from the viewpoint of molecular size and intramolecular charge distribution, the pyrrolidinium cation is preferably a 1-methyl-1-propylpyrrolidinium cation (Pyr13) or a 1-butyl-1-methylpyrrolidinium cation (Pyr14).

[0017] In the present secondary battery, the electrolyte (for example, LiFSI high-concentration electrolyte) may contain Pyr13-FSI or Pyr14-FSI as an ionic liquid in addition to LiFSI as an electrolyte. The content ratio of the electrolyte in the electrolyte may be appropriately set within a range that satisfies the above-mentioned conditions of molar concentration and molar ratio and provides the effects of the present disclosure.

[0018] The content of the ionic liquid in the electrolyte is preferably 0.93% by mass or more from the viewpoint of positive charge dispersion, and is preferably 2.78% by mass or less from the viewpoint of suppressing an increase in resistance due to lithium migration inhibition and controlling local reactions.

[0019] In the present secondary battery, as the organic solvent used in the electrolyte, a conventionally known organic solvent in the field of non-aqueous secondary batteries can be appropriately used. Examples of the organic solvent include dimethoxyethane (DME), diethoxyethane, triglyme, tetraglyme, ethylene carbonate (EC), propylene carbonate, tetrahydrofuran, tetrahydropyran, and derivatives thereof, as well as any combinations and mixtures thereof. Among these, it is preferable to use dimethoxyethane as the organic solvent from the viewpoints of exhibiting high LiFSI salt solubility, improving ionic conductivity, and improving capacity retention rate.

[0020] In addition, the electrolyte may contain other additives as long as the effects of the present disclosure can be obtained. EXAMPLES

[0021] The present disclosure will be described in more detail below using several examples, but the present disclosure is not limited to these examples.

[0022] [Example 1] (Electrolyte preparation) A given amount of LiFSI (Kishida Chemical) was prepared as an electrolyte, and Pyr13-FSI (Tokyo Chemical Industry Co., Ltd.) was prepared as an ionic liquid, and DME (dimethoxyethane) was added as a solvent to dissolve the solution. The resulting solution was transferred to a measuring flask and diluted to a given volume to obtain an electrolyte. The molar concentration of FSI (anion concentration mol / L) and the molar ratio of Li ion / pyrrolidinium cation (Li / cation) in the resulting electrolyte are shown in Table 1. The content of the ionic liquid in the electrolyte was 2.78% by mass. The abbreviations in Table 1 stand for the following: FSI: Bis(fluorosulfonyl)imide anion TFSI: Bis(trifluoromethanesulfonyl)imide anion Pyr13: 1-methyl-1-propylpyrrolidinium cation Pyr14: 1-butyl-1-methylpyrrolidinium cation EMIm: 1-ethyl-3-methylimidazolium cation Na+: sodium cation

[0023] [Examples 2 to 5, Comparative Examples 1 to 2, 4 to 5, and 9] The electrolyte solutions were prepared in the same manner as in Example 1, except that the molar concentration of FSI and the molar ratio of Li ions / pyrrolidinium cations in the electrolyte solution were changed as shown in Table 1. The content ratio of the ionic liquid in the electrolyte solution in each example was as follows. Example 2: 1.85% by mass, Example 3: 0.93% by mass, Example 4: 2.01% by mass, Example 5: 1.01% by mass, Comparative Example 1: 3.70% by mass, Comparative Example 2: 0.46% by mass, Comparative Example 4: 0.50% by mass, Comparative Example 5: 0.58% by mass, Comparative Example 9: 0.72% by mass.

[0024] [Example 6] In Example 6, the electrolyte solution was prepared in the same manner as in Example 1, except that the ionic liquid was changed to Pyr14-FSI (manufactured by Tokyo Chemical Industry Co., Ltd.) and the molar ratio of Li ion / pyrrolidinium cation was changed as shown in Table 1. The content of the ionic liquid in the electrolyte solution was 2.05% by mass.

[0025] [Comparative Example 3] In Comparative Example 3, an electrolyte solution was prepared in the same manner as in Example 1, except that no ionic liquid was added.

[0026] [Comparative Example 6] In Comparative Example 6, the electrolyte solution was prepared in the same manner as in Example 1, except that the ionic liquid was changed to EMIm-FSI (manufactured by Tokyo Chemical Industry Co., Ltd.) and the molar ratio of Li ion / EMIm cation was changed as shown in Table 1. The content of the ionic liquid in the electrolyte solution was 1.60 mass%.

[0027] [Comparative Example 7] In Comparative Example 7, an electrolyte solution was prepared in the same manner as in Example 1, except that no ionic liquid was added, instead NaFSI (manufactured by Tokyo Chemical Industry Co., Ltd.) was added, and the molar ratio of Li ions / Na cations was set as shown in Table 1.

[0028] [Comparative Example 8] In Comparative Example 8, the ionic liquid was changed to lithium bis(trifluoromethanesulfonyl)imide (LiTFSI) (manufactured by Tokyo Chemical Industry Co., Ltd.), and the molar concentration of TFSI (anion concentration mol / L) and the molar ratio of Li ion / pyrrolidinium cation were changed as shown in Table 1. Except for the above, the electrolyte solution was prepared in the same manner as in Example 1. The content of the ionic liquid in the electrolyte solution was 2.32 mass%.

[0029] (Cell preparation) LiNi x Co y Mn (1-x-y) A positive electrode was produced by mixing O2 (x=0.5, y=0.2), a conductive additive (product name: AB, manufactured by Denka), and a binder (product name: PVdF, manufactured by Kureha), applying the mixture to an Al foil, and pressing it. Next, electrolytic copper foil was used as a negative electrode, which was placed opposite the positive electrode via a separator, and the electrolyte prepared in each example was poured into the negative electrode to prepare a coin cell.

[0030] (Evaluation of Discharge Capacity Retention Rate) 0.4mA / cm 2The cell was activated at a current density of 4 mA / cm and a voltage range of 3.0 to 4.3 V. 2 A cycle test was performed for 20 cycles at a current density of 1000 and a voltage range of 3.0 to 4.3 V. The discharge capacity retention rate was calculated by dividing the discharge capacity at the 20th cycle of the cycle test by the discharge capacity at the 1st cycle of the cycle test. The discharge capacity retention rate of each example was then divided by the discharge capacity retention rate of Comparative Example 3, which did not use the ionic liquid, to calculate the discharge capacity retention rate ratio. The evaluation results for each example are shown in Table 1 below.

[0031] [Table 1]

[0032] As shown in Table 1, this secondary battery containing a specific concentration of bis(fluorosulfonyl)imide anions and a specific molar ratio of pyrrolidinium cations to lithium ions in the electrolyte has a superior capacity retention rate ratio compared to secondary batteries that do not contain ionic liquid.

[0033] According to the present invention related to the present embodiment described above, it is possible to provide a nonaqueous secondary battery capable of suppressing a decrease in the capacity retention rate with a simple configuration.

[0034] The present disclosure is not limited to the above-described embodiment, and can be modified as appropriate without departing from the spirit and scope of the present disclosure.

Claims

1. A positive electrode and a lithium metal anode; An electrolytic solution, The electrolyte solution contains a pyrrolidinium cation and a bis(fluorosulfonyl)imide anion (FSI), the molar ratio of lithium ions to pyrrolidinium cations is 10 to 40; The nonaqueous secondary battery is characterized in that the molar concentration of the bis(fluorosulfonyl)imide anion is 3 to 4 mol / L.

2. 2. The nonaqueous secondary battery according to claim 1, wherein the pyrrolidinium cation is a 1-methyl-1-propylpyrrolidinium cation (Pyr13) or a 1-butyl-1-methylpyrrolidinium cation (Pyr14).

3. 2. The nonaqueous secondary battery of claim 1, wherein the electrolyte solution comprises dimethoxyethane.

4. 4. The nonaqueous secondary battery according to claim 2, wherein the electrolytic solution contains LiFSI as an electrolyte and Pyr13-FSI or Pyr14-FSI as an ionic liquid.

5. The nonaqueous secondary battery according to claim 4, wherein the content of the ionic liquid in the electrolyte is 0.93 to 2.78 mass%.

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