JP7483222B2 - Positive electrode active material precursor for secondary battery, positive electrode active material for secondary battery, catholyte for secondary battery, positive electrode for secondary battery, and secondary battery - Google Patents

Description

The present invention relates to a positive electrode active material precursor for a secondary battery, a positive electrode active material for a secondary battery, a catholyte for a secondary battery, a positive electrode for a secondary battery, and a secondary battery.

Lithium ion secondary batteries _, which have the highest energy density among secondary batteries _, have been put to practical use. In recent years, various transition metal oxides ( _LiFePO4 , _{LiNi0.8Mn0.1Co0.1O2 ,} etc.) and their derivatives have been widely used as positive electrode active materials for secondary batteries (especially positive electrode active materials for lithium ion secondary batteries).

However, while the demand for next-generation lithium-ion batteries and other secondary batteries is increasing year by year, their charge and discharge capacities are structurally limited due to the ion intercalation storage mechanism. In addition, some metal elements are not only expensive but also toxic and pose safety issues. In contrast, economically abundant organic compounds, including carbonyls and polysulfides, can potentially serve as positive electrode active materials for secondary batteries, and these organic compounds are advantageous for constructing high-capacity batteries due to their unique charge storage mechanism that allows multiple electron transfer per molecule.

For example, Non-Patent Document 1 reports the synthesis of a cyclohexanehexone-based positive electrode active material and its application to lithium-ion batteries. This material is difficult to dissolve in highly polar ionic liquid electrolytes, and the capacity retention rate was 82% even after 100 cycles. Non-Patent Document 2 also confirmed that dimethyl trisulfide is a promising high-capacity positive electrode active material for rechargeable lithium batteries, and a lithium metal battery using this positive electrode not only achieved a high initial capacity, but also retained 82% of its capacity over 50 cycles at a 0.1C rate.

Angew. Chem. Int. Ed. 2019,58,7020-7024 Angew. Chem. Int. Ed. 2016,55,10027-10031

However, there are few reports of positive electrode active materials for secondary batteries using organic compounds, and the range of choices is narrow. In addition, in the methods of Non-Patent Documents 1 and 2, the cost of ionic liquid is high, and the rate characteristics and cycle stability of the battery are poor. In addition, the obtained organic cathode has a severe shuttle effect that can easily cause self-discharge of the battery and consume energy. Therefore, the present invention aims to provide an organic compound that can be used to manufacture a secondary battery with excellent electrochemical reversibility characteristics, rate characteristics, and cycle stability.

As a result of intensive research into achieving the above object, the inventors have found that the above object can be achieved by applying a compound having a pyridine skeleton with at least one sulfur-containing group. The present invention was completed based on this finding and further research. That is, the present invention includes the following configurations.

Item 1. A positive electrode active material precursor for a secondary battery, containing a compound having at least one sulfur-containing group and having one or more pyridine skeletons in the molecule.

Item 2. The compound has the general formula (1):

[In the formula, R ₁₁ , R ₁₂ , R ₁₃ , R ₁₄ and R ₁₅ are the same or different and each represents a monovalent group, provided that at least one of R ₁₁ , R ₁₂ , R ₁₃ , R ₁₄ and R ₁₅ is a sulfur-containing group.]
A pyridine compound represented by the formula:
General formula (2):

[In the formula, R ₂₁ , R ₂₂ , R ₂₃ , R ₂₄ , R ₂₅ , R ₂₆ , R ₂₇ and R ₂₈ are the same or different and represent a monovalent group, provided that at least one of R ₂₁ , R ₂₂ , R ₂₃ , R ₂₄ , R ₂₅ , R ₂₆ , R ₂₇ and R ₂₈ is a sulfur-containing group.]
and a bipyridine compound represented by general formula (3):

[In the formula, R ₃₁ , R ₃₂ , R ₃₃ , R ₃₄ , R ₃₅ , R ₃₆ , R ₃₇ and R ₃₈ are the same or different and represent a monovalent group, provided that at least one of R ₃₁ , R ₃₂ , R ₃₃ , R ₃₄ , R ₃₅ , R ₃₆ , R ₃₇ and R ₃₈ is a sulfur-containing group, and n represents an integer of 2 to 4.]
Item 2. The positive electrode active material precursor for a secondary battery according to item 1, wherein the pyridine ring compound is at least one selected from the group consisting of pyridine ring compounds represented by the formula:

Item 3. The positive electrode active material precursor for a secondary battery according to item 1 or 2, wherein the sulfur-containing group is a thiol group or a derivative group thereof.

Item 4. A positive electrode active material precursor for a secondary battery according to any one of items 1 to 3, which is a positive electrode active material precursor for a lithium ion secondary battery.

Item 5. A positive electrode active material for a secondary battery, containing a lithium compound having at least one sulfur-containing group and one or more pyridine skeletons in the molecule.

Item 6. The compound has the general formula (1):

[In the formula, R ₁₁ , R ₁₂ , R ₁₃ , R ₁₄ and R ₁₅ are the same or different and each represents a monovalent group, provided that at least one of R ₁₁ , R ₁₂ , R ₁₃ , R ₁₄ and R ₁₅ is a sulfur-containing group.]
A pyridine compound represented by the formula:
General formula (2):

[In the formula, R ₂₁ , R ₂₂ , R ₂₃ , R ₂₄ , R ₂₅ , R ₂₆ , R ₂₇ and R ₂₈ are the same or different and represent a monovalent group, provided that at least one of R ₂₁ , R ₂₂ , R ₂₃ , R ₂₄ , R ₂₅ , R ₂₆ , R ₂₇ and R ₂₈ is a sulfur-containing group.]
and a bipyridine compound represented by general formula (3):

[In the formula, R ₃₁ , R ₃₂ , R ₃₃ , R ₃₄ , R ₃₅ , R ₃₆ , R ₃₇ and R ₃₈ are the same or different and represent a monovalent group, provided that at least one of R ₃₁ , R ₃₂ , R ₃₃ , R ₃₄ , R ₃₅ , R ₃₆ , R ₃₇ and R ₃₈ is a sulfur-containing group, and n represents an integer of 2 to 4.]
Item 6. The positive electrode active material for a secondary battery according to item 5, wherein the pyridine ring compound is at least one selected from the group consisting of pyridine ring compounds represented by the formula:

Item 7. The positive electrode active material for a secondary battery according to claim 5 or 6, wherein the sulfur-containing group is a thiol group or a derivative group thereof.

Item 8. The positive electrode active material for a secondary battery according to any one of items 5 to 7, which is a positive electrode active material for a lithium ion secondary battery.

Item 9. A cathode solution for a secondary battery, containing a lithium compound having at least one sulfur-containing group and one or more pyridine skeletons in the molecule.

Item 10. The compound has the general formula (1):

[In the formula, R ₁₁ , R ₁₂ , R ₁₃ , R ₁₄ and R ₁₅ are the same or different and each represents a monovalent group, provided that at least one of R ₁₁ , R ₁₂ , R ₁₃ , R ₁₄ and R ₁₅ is a sulfur-containing group.]
A pyridine compound represented by the formula:
General formula (2):

[In the formula, R ₂₁ , R ₂₂ , R ₂₃ , R ₂₄ , R ₂₅ , R ₂₆ , R ₂₇ and R ₂₈ are the same or different and represent a monovalent group, provided that at least one of R ₂₁ , R ₂₂ , R ₂₃ , R ₂₄ , R ₂₅ , R ₂₆ , R ₂₇ and R ₂₈ is a sulfur-containing group.]
and a bipyridine compound represented by general formula (3):

[In the formula, R ₃₁ , R ₃₂ , R ₃₃ , R ₃₄ , R ₃₅ , R ₃₆ , R ₃₇ and R ₃₈ are the same or different and represent a monovalent group, provided that at least one of R ₃₁ , R ₃₂ , R ₃₃ , R ₃₄ , R ₃₅ , R ₃₆ , R ₃₇ and R ₃₈ is a sulfur-containing group, and n represents an integer of 2 to 4.]
Item 10. The catholyte for a secondary battery according to item 9, wherein the pyridine ring compound is at least one selected from the group consisting of pyridine ring compounds represented by the formula:

Item 11. The cathode solution for a secondary battery according to item 9 or 10, wherein the sulfur-containing group is a thiol group or a derivative group thereof.

Item 12. The cathode fluid for a secondary battery according to any one of items 9 to 11, which is a cathode fluid for a lithium ion secondary battery.

Item 13. A positive electrode for a secondary battery, comprising the positive electrode active material for a secondary battery according to any one of items 5 to 8 or the catholyte for a secondary battery according to any one of items 9 to 12.

Item 14. The positive electrode for a secondary battery according to Item 13, which is a positive electrode for a lithium ion secondary battery.

Item 15. A secondary battery comprising the positive electrode for a secondary battery according to item 13 or 14.

Item 16. The secondary battery according to item 15, which is a lithium ion secondary battery.

The present invention provides an organic compound that can be used to manufacture a secondary battery having excellent electrochemical reversibility characteristics, rate characteristics, and cycle stability.

The results of XPS analysis of 2-MP-Li obtained by lithiating 2-MP are shown below. 1 shows the results of electrochemical measurements (cycle stability and coulombic efficiency) using the coin cell of Comparative Example 1. 1 shows the charge/discharge curve (a) and cyclic voltammogram (b) of the coin cell of Example 1. 1 shows the results of electrochemical measurements ((a) rate characteristics and (b) cycle stability) using the coin cell of Example 1. 1 shows the charge/discharge curve (a) and cyclic voltammogram (b) of the coin cell of Example 2. 6 shows the results of electrochemical measurement (cycling stability) using the coin cell of Example 2. In Fig. 6(a), for comparison, the cycling stability using the coin cell of Example 1 is shown as 20 μL-catholite-0.3 C, and the cycling stability using the coin cell of Example 2 is shown as 40 μL-catholite-0.3 C.

In this specification, "containing" is a concept that encompasses all of "comprise," "consist essentially of," and "consist only of."

In addition, in this specification, the notation "A-B" means "A or more and B or less."

1. Positive electrode active material precursor for secondary battery The positive electrode active material precursor for secondary battery of the present invention is composed of a compound having at least one sulfur-containing group and having one or more pyridine skeletons in the molecule. By using such a compound having a pyridine skeleton, a secondary battery having excellent electrochemical reversibility characteristics, rate characteristics, and cycle stability can be manufactured.

The type of sulfur-containing group of the compound having a pyridine skeleton is not particularly limited. For example, a thiol group or a derivative group thereof can be mentioned. Examples of derivative groups of a thiol group include groups in which the hydrogen atom of the thiol group is substituted with a halogen atom (fluorine atom, chlorine atom, bromine atom, iodine atom, etc.), an alkyl group (an alkyl group having 1 to 10 carbon atoms such as a methyl group or an ethyl group), an alkenyl group (an alkenyl group having 2 to 10 carbon atoms such as a vinyl group or an allyl group), an alkoxy group (an alkoxy group having 1 to 10 carbon atoms such as a methoxy group or an ethoxy group), an amino group, etc.

In a compound having a pyridine skeleton, the pyridine skeleton means a structure derived from a pyridine ring. In a compound having a pyridine skeleton, the number of pyridine skeletons is not particularly limited as long as it is one or more. From the viewpoint of ease of production of a compound having a pyridine skeleton and ease of imparting excellent electrochemical reversibility characteristics, rate characteristics, and cycle stability to a secondary battery, the number of pyridine skeletons is preferably 1 to 5, more preferably 1 to 3, even more preferably 1 or 2, and particularly preferably 1.

In a compound having a pyridine skeleton, the number of sulfur-containing groups is not particularly limited as long as it is one or more. From the viewpoint of ease of production of a compound having a pyridine skeleton and ease of imparting excellent electrochemical reversibility characteristics, rate characteristics, and cycle stability to a secondary battery, the number of sulfur-containing groups is preferably 1 to 3, more preferably 1 or 2, and even more preferably 1.

Therefore, the compound having a pyridine skeleton is preferably a compound having one pyridine skeleton and one sulfur-containing group, and more preferably a compound having one pyridine skeleton and one thiol group or a derivative thereof.

In a compound having a pyridine skeleton, the sulfur-containing group can be directly bonded to the pyridine skeleton, and the bonding position is not particularly limited.

The type of compound having a pyridine skeleton is not particularly limited as long as it is a compound having one or more pyridine skeletons and one or more sulfur-containing groups in the molecule. For example, a compound having a pyridine skeleton is represented by the general formula (1):

[In the formula, R ₁₁ , R ₁₂ , R ₁₃ , R ₁₄ and R ₁₅ are the same or different and each represents a monovalent group, provided that at least one of R ₁₁ , R ₁₂ , R ₁₃ , R ₁₄ and R ₁₅ is a sulfur-containing group.]
A pyridine compound represented by the formula:
General formula (2):

[In the formula, R ₂₁ , R ₂₂ , R ₂₃ , R ₂₄ , R ₂₅ , R ₂₆ , R ₂₇ and R ₂₈ are the same or different and represent a monovalent group, provided that at least one of R ₂₁ , R ₂₂ , R ₂₃ , R ₂₄ , R ₂₅ , R ₂₆ , R ₂₇ and R ₂₈ is a sulfur-containing group.]
A bipyridine compound represented by the formula:
General formula (3):

[In the formula, R ₃₁ , R ₃₂ , R ₃₃ , R ₃₄ , R ₃₅ , R ₃₆ , R ₃₇ and R ₃₈ are the same or different and represent a monovalent group, provided that at least one of R ₃₁ , R ₃₂ , R ₃₃ , R ₃₄ , R ₃₅ , R ₃₆ , R ₃₇ and R ₃₈ is a sulfur-containing group, and n represents an integer of 2 to 4.]
The compound may be at least one selected from the group consisting of pyridine ring compounds represented by the following formula:

In general formulas (1) to (3), the sulfur-containing groups can be those described above.

In the general formulas (1) to (3), when the monovalent group is a group other than a sulfur-containing group, there are no particular limitations on the group, and various groups can be selected as long as the effects of the present invention are not impaired. For example, examples of monovalent groups include halogen atoms (fluorine atoms, chlorine atoms, bromine atoms, iodine atoms, etc.), alkyl groups (methyl groups, ethyl groups, and other alkyl groups having 1 to 10 carbon atoms), alkenyl groups (vinyl groups, allyl groups, and other alkenyl groups having 2 to 10 carbon atoms), alkoxy groups (methoxy groups, ethoxy groups, and other alkoxy groups having 1 to 10 carbon atoms), and amino groups. These monovalent groups may be the same or different.

In the general formula (1), R ₁₁ , R ₁₂ , R ₁₃ , R ₁₄ and R ₁₅ are preferably the same or different and are a hydrogen atom or a sulfur-containing group. Among them, it is particularly preferable that at least one of R ₁₁ , R ₁₂ , R ₁₃ , R ₁₄ and R ₁₅ is a sulfur-containing group (preferably a thiol group or a derivative group thereof), and the remaining are hydrogen atoms. In this case, it is possible to bring about particularly excellent electrochemical reversibility characteristics, rate characteristics and cycle stability to the secondary battery. For example, in the general formula (1), either one of R ₁₁ and R ₁₅ can be a sulfur-containing group (preferably a thiol group or a derivative group thereof). In this case, a particularly preferred compound is 2-mercaptopyridine.

In the general formula (2), R ₂₁ , R ₂₂ , R ₂₃ , R ₂₄ , R ₂₅ , R ₂₆ , R ₂₇ and R ₂₈ are preferably the same or different and are a hydrogen atom or a sulfur-containing group. Among them, it is particularly preferable that at least one of R ₂₁ , R ₂₂ , R ₂₃ , R ₂₄ , R ₂₅ , R ₂₆ , R ₂₇ and R ₂₈ is a sulfur-containing group (preferably a thiol group or a derivative group thereof), and the remaining is a hydrogen atom. In this case, it is possible to bring about particularly excellent electrochemical reversibility characteristics, rate characteristics and cycle stability to the secondary battery. For example, in the general formula (2), either one of R ₂₁ and R ₂₅ can be a sulfur-containing group (preferably a thiol group or a derivative group thereof).

In the general formula (3), R ₃₁ , R ₃₂ , R ₃₃ , R ₃₄ , R ₃₅ , R ₃₆ , R ₃₇ and R ₃₈ are preferably the same or different and are a hydrogen atom or a sulfur-containing group. Among them, it is particularly preferable that at least one of R ₃₁ , R ₃₂ , R ₃₃ , R ₃₄ , R ₃₅ , R ₃₆ , R ₃₇ and R ₃₈ is a sulfur-containing group (preferably a thiol group or a derivative group thereof), and the remaining are hydrogen atoms. In this case, it is possible to provide the secondary battery with particularly excellent electrochemical reversible characteristics, rate characteristics and cycle stability. For example, in the general formula (3), either one of R ₃₁ and R ₃₈ can be a sulfur-containing group (preferably a thiol group or a derivative group thereof).

In the positive electrode active material precursor for secondary batteries of the present invention, the above-mentioned pyridine skeleton compound may be contained in only one type, or may contain two or more different types. In addition, these pyridine skeleton compounds may be publicly known or commercially available products.

The positive electrode active material precursor for secondary batteries of the present invention may contain other impurities as long as they do not impair its performance.

The amount of these impurities is preferably within a range that does not impair the performance of the positive electrode active material precursor for secondary batteries of the present invention described above, and is usually preferably 2% by mass or less (0 to 2% by mass) based on 100% by mass of the total amount of the positive electrode active material precursor for secondary batteries of the present invention.

As described above, the positive electrode active material precursor for secondary batteries of the present invention can be used to produce secondary batteries having excellent electrochemical reversibility characteristics, rate characteristics, and cycle stability, and is therefore useful as a positive electrode active material precursor for secondary batteries (particularly as a positive electrode active material precursor for lithium ion secondary batteries).

2. Cathode active material for secondary battery The cathode active material for secondary battery of the present invention contains a lithium compound having at least one sulfur-containing group and one or more pyridine skeletons in the molecule. For the compound having at least one sulfur-containing group and one or more pyridine skeletons in the molecule, the above description of the compound having a pyridine skeleton can be adopted.

The lithium derivatives of the compounds having the pyridine skeleton described above are compounds in which lithium has been introduced into the sulfur-containing group. Usually, a compound is obtained in which one hydrogen atom of the sulfur-containing group is replaced with a lithium atom. Specifically, when the sulfur-containing group is a thiol group, it is lithium-decomposed into a group represented by -SLi.

By using such compounds, it is possible to manufacture secondary batteries with excellent electrochemical reversibility characteristics, rate characteristics, and cycle stability.

In the positive electrode active material for secondary batteries of the present invention, the lithium oxide of the pyridine skeleton compound described above may be only one type, or may be two or more different types. In addition, the lithium oxide of these pyridine skeleton compounds may be synthesized, for example, by reacting a known or commercially available pyridine skeleton compound with a lithium material (lithium metal, etc.). The reaction conditions, etc. at this time are not particularly limited and may be adjusted as appropriate.

The positive electrode active material for secondary batteries of the present invention may contain other impurities as long as they do not impair its performance.

The amount of these impurities is preferably within a range that does not impair the performance of the positive electrode active material for secondary batteries of the present invention described above, and is usually preferably 2% by mass or less (0 to 2% by mass) based on 100% by mass of the total amount of the positive electrode active material for secondary batteries of the present invention.

As described above, the positive electrode active material for secondary batteries of the present invention can be used to produce secondary batteries having excellent electrochemical reversibility characteristics, rate characteristics, and cycle stability, and is therefore useful as a positive electrode active material for secondary batteries (particularly as a positive electrode active material for lithium ion secondary batteries).

3. Catholyte for secondary battery The catholyte for secondary battery of the present invention contains a lithium compound having at least one sulfur-containing group and having one or more pyridine skeletons in the molecule (i.e., the positive electrode active material for secondary battery of the present invention). For the compound having at least one sulfur-containing group and having one or more pyridine skeletons in the molecule and the lithium compound thereof, the above description of the compound having a pyridine skeleton and the lithium compound thereof can be adopted.

The cathode solution for the secondary battery of the present invention is not particularly limited as long as it contains the above-mentioned cathode active material for the secondary battery of the present invention, but typically may contain a solvent and an alkali metal in addition to the cathode active material for the secondary battery of the present invention.

The solvent is not particularly limited, and examples thereof include ethylene carbonate (EC), propylene carbonate (PC), diethyl carbonate (DEC), dimethyl carbonate (DMC), ethyl methyl carbonate (EMC), 1,3-dioxolane (DOL), 1,2-dimethoxyethane (DME), etc. These solvents can be used alone or in combination of two or more.

There are no particular limitations on the amount of solvent used, so long as it is an excess amount. Specifically, it can be used so that the concentration of the positive electrode active material for secondary batteries of the present invention is 0.1 to 5 mol/L, and particularly 0.5 to 3 mol/L.

The alkali metal is not particularly limited, and examples thereof include lithium metal, sodium metal, potassium metal, and the like. These solvents can be used alone or in combination of two or more. In particular, when lithium metal is used, it is useful in that it is possible to synthesize the positive electrode active material for the secondary battery of the present invention and simultaneously produce the catholyte for the secondary battery of the present invention.

The amount of alkali metal used is not particularly limited, but from the viewpoints of electrochemical reversibility, rate characteristics, cycle stability, etc., it is preferable to contain 100 to 300 parts by mass, and particularly 110 to 200 parts by mass, of alkali metal per 100 parts by mass of the positive electrode active material for secondary batteries of the present invention. Note that using a larger amount of alkali metal can make the reaction more thorough, and it is also possible to further increase the purity of the final positive electrode active material for secondary batteries of the present invention.

The cathode fluid for secondary batteries of the present invention may further contain an electrolyte salt, if necessary.

Examples of the electrolyte salt include one or more of lithium nitrate (LiNO ₃ ), lithium hexafluorophosphate (LiPF ₆ ), lithium perchlorate (LiClO ₄ ), lithium tetrafluoroborate (LiBF ₄ ), lithium bisoxalatoborate (LiBOB), lithium hexafluoroarsenate (LiAsF ₆ ), lithium bisfluorosulfonylimide (LiFSI), lithium trifluoromethanesulfonylimide (LiSO ₃ CF ₃ ), and lithium bistrifluoromethanesulfonylimide (LiTFSI).

The amount of electrolyte salt used is not particularly limited, but from the viewpoints of electrochemical reversibility characteristics, rate characteristics, cycle stability, etc., it is preferable to contain 100 to 1000 parts by mass, and particularly 200 to 600 parts by mass, of electrolyte salt per 100 parts by mass of the positive electrode active material for a secondary battery of the present invention. Note that using a larger amount of electrolyte salt can further improve the stability of the negative electrode.

The method for producing the cathode solution for secondary batteries of the present invention as described above is not limited. For example, when the alkali metal is lithium metal, when reacting a precursor of the cathode active material for secondary batteries of the present invention with lithium metal in a solvent to synthesize the cathode active material for secondary batteries of the present invention, it is possible to synthesize the cathode active material for secondary batteries of the present invention and produce the cathode solution for secondary batteries of the present invention at the same time by using an excess amount of lithium metal.

4. Positive electrode for secondary battery and secondary battery The positive electrode for secondary battery of the present invention contains the positive electrode active material for secondary battery of the present invention. The positive electrode for secondary battery of the present invention can be suitably used in various secondary batteries, such as a positive electrode for lithium ion secondary batteries, and can provide excellent electrochemical reversibility, rate characteristics, and cycle stability in secondary batteries, particularly lithium ion secondary batteries.

The positive electrode for a secondary battery of the present invention can have the same structure as a known negative electrode for a secondary battery, except that it contains the positive electrode active material for a secondary battery of the present invention.

For example, by immersing a positive electrode collector in the cathode solution for a secondary battery of the present invention, a positive electrode for a secondary battery in which the cathode solution for the secondary battery of the present invention is impregnated in the positive electrode collector can be formed. In this case, it is preferable to use the cathode solution for a secondary battery of the present invention not only as a material for constituting a positive electrode for a secondary battery, but also as an electrolyte for a secondary battery. Specifically, the cathode solution for a secondary battery of the present invention is placed on a negative electrode, a positive electrode collector is dropped on top of it, and the cathode solution for a secondary battery of the present invention is impregnated into the positive electrode collector to form a positive electrode of the present invention, while the cathode solution for a secondary battery of the present invention in the area where the positive electrode collector is not present can be used as an electrolyte for a secondary battery, which is simple.

On the other hand, the method for producing the secondary battery of the present invention is not limited to the above, and the secondary battery can also be produced by the following method. Specifically, the positive electrode collector is immersed in the cathode liquid for the secondary battery of the present invention, or the cathode liquid for the secondary battery of the present invention is applied to the positive electrode collector, thereby forming a positive electrode for the secondary battery of the present invention in which the cathode liquid for the secondary battery of the present invention is impregnated in the positive electrode collector. The secondary battery of the present invention can also be produced by placing a separator impregnated with a liquid electrolyte on the negative electrode, and placing the positive electrode for the secondary battery of the present invention on the separator.

As the positive electrode current collector, various porous current collectors such as carbon fiber paper (CFP), carbon nanotube paper, graphite paper, graphene foam, etc. can be used. In addition, the positive electrode current collector is not limited to the above-mentioned carbon materials, and can also have a structure in which an active material is supported on a metal foil. Examples of the metal foil include aluminum, titanium, platinum, molybdenum, stainless steel, and copper. Examples of the shape of the metal foil include a porous body, a foil, a plate, and a mesh made of fibers.

The method for impregnating or applying the cathode liquid of the present invention to the positive electrode current collector is not particularly limited and can be carried out according to a conventional method. Various conditions can also be adjusted as appropriate.

As the negative electrode, for example, various metal plates can be used. Examples of the metal plate include lithium, sodium, potassium, magnesium, aluminum, zinc, etc. In addition to the various metal plates, for example, a structure in which an active material is supported on a metal foil can be adopted. Examples of the metal foil include aluminum, titanium, platinum, molybdenum, stainless steel, copper, etc. Examples of the shape of the metal foil include a porous body, a foil, a plate, a mesh made of fibers, etc. Examples of the active material of the anode include metals such as Li, Na, K, Mg, Al, and Zn; graphite and other carbon materials; Si(C)-based, Si(O)-based, or Sn-based alloys or metal oxides; Li ₄ Ti ₅ O ₁₂ , etc.

When the separator is impregnated with a liquid electrolyte, the catholyte of the present invention or a known liquid electrolyte can be used as the liquid electrolyte. When a known liquid electrolyte is used, the liquid electrolyte can be a lithium salt dissolved in a polar solvent. Examples of the polar solvent include one or more of carbonate compounds such as propylene carbonate, ethylene carbonate (EC), fluoroethylene carbonate, dimethyl carbonate (DMC), ethyl methyl carbonate, and diethyl carbonate (DEC); ether compounds such as diethylene glycol dimethyl ether (diglyme), triethylene glycol dimethyl ether (triglyme), 1,3-dioxolane (DOL), and 1,2-dimethoxyethane (DME); and ester compounds such as propyl acetate. Examples of the lithium salt include one or more of lithium hexafluorophosphate (LiPF ₆ ), lithium perchlorate (LiClO ₄ ), lithium tetrafluoroborate (LiBF ₄ ), lithium bisoxalatoborate (LiBOB), lithium hexafluoroarsenate (LiAsF ₆ ), lithium bisfluorosulfonylimide (LiFSI), lithium trifluoromethanesulfonylimide (LiSO ₃ CF ₃ ), and lithium bistrifluoromethanesulfonylimide (LiTFSI).

In particular, when the cathode electrolyte of the present invention is used as the liquid electrolyte to be impregnated into the separator, it is possible to further improve cycle stability.

As the separator, a known separator that is applied to secondary batteries such as lithium ion secondary batteries can be used, and it is possible to use materials such as polyolefin resins such as polyethylene (PE) and polypropylene (PP); polyimide; polyvinyl alcohol; fluororesins such as aminated polyethylene oxide polytetrafluoroethylene; acrylic resins; nylon; aromatic aramid; inorganic glass; and ceramics, and materials in the form of porous membranes, nonwoven fabrics, woven fabrics, etc. can be used.

There are no particular limitations on the method for assembling the secondary battery, and the secondary battery can be obtained by a method similar to that used for assembling publicly known secondary batteries.

The present invention will be described in more detail below with reference to examples, but the present invention is not limited to these examples.

2-mercaptopyridine (99%), which is a precursor of the positive electrode active material for secondary batteries of the present invention, was purchased from Sigma-Aldrich Co. A 1M solution (containing 1% by mass of LiNO ₃ ) of lithium trifluoromethanesulfonylimide (LiTFSI) in a mixed solvent (volume ratio 1:1) of 1,3-dioxolane (DOL) and 1,2-dimethoxyethane (DME), which is a liquid electrolyte, was purchased from Xiamen Tob New Energy Technology Co., Ltd. China. Hereinafter, this liquid electrolyte may be referred to as "1M LiTFSI in DOL:DME (1:1 vol.% containing 1% LiNO ₃ )".

In the construction of the battery, a glove box (manufactured by Miwa Manufacturing Co., Ltd.) was used for the production of the battery in an inert atmosphere, and the production was carried out in an Ar gas atmosphere in which the _H2O and _O2 concentrations were both 0.1 ppm or less. The separator used in the production of the battery was polypropylene (PP) with a diameter of 16 mm and a thickness of 0.025 mm, the carbon fiber paper (CFP) as the negative electrode current collector was 12 mm in diameter and 0.1 mm in thickness, and the lithium foil as the positive electrode was 12 mm in diameter and 0.1 mm in thickness.

Comparative Example 1: No 2-MP-Li catholyte: Coin cell prototype and performance measurement Carbon fiber paper (CFP), which is a positive electrode current collector, was dried at 110 ° C for 24 hours before use. A CR2025 type coin cell was fabricated in a glove box filled with Ar. First, 20 μL of a blank electrolyte consisting of 1 M LiTFSI in DOL:DME (1:1 vol.% containing 1% LiNO ₃ ) was added to the CFP current collector. Specifically, at room temperature, the CFP current collector was placed in the positive electrode case of the coin cell, and the blank electrolyte was dripped from above with a pipette for 1 to 5 seconds until the CFP current collector was completely wet. Next, a PP separator was placed on top of the obtained CFP electrode (positive electrode), and 20 μL of the above blank electrolyte was added to the top of the PP separator. Specifically, the blank electrolyte was dripped from above with a pipette for 1 to 5 seconds at room temperature until the PP separator was completely wetted, as described above. Lithium foil was then placed on the electrolyte-impregnated PP separator, which was then covered with a stainless steel spacer and placed on a coin-type battery cell for compression bonding. The resulting battery was removed from the glove box and electrochemical measurements were performed at 30°C using a Wuhan LAND electronics charge/discharge device (LAND batteries testing system CT2001A) with a cutoff voltage range of 1.8 to 3.0 V.

Example 1: Preparation of 2-MP-Li cathode solution and 20 μL cathode solution: Coin cell prototype and performance measurement 0.5 mmol of 2-mercaptopyridine (2-MP) was dissolved in 1 mL of a blank electrolyte consisting of 1 M LiTFSI in DOL:DME (1:1 vol.% containing 1% LiNO ₃ ) to form a 0.5 M 2-MP solution. Next, 6.042 mg of lithium metal was added to the above solution to lithiate 2-MP and form a 0.5 M 2-MP-Li cathode solution. As a result, the obtained 2-MP-Li is a compound having the following structure. In addition, the fact that 2-MP-Li was actually obtained was analyzed by X-ray photoelectron spectroscopy (XPS). The results are shown in FIG. 1.

The carbon fiber paper (CFP), which is the positive electrode current collector, was dried at 110°C for 24 hours before use. The CR2025 type coin cell was fabricated in a glove box filled with Ar. First, 20 μL of the prepared catholyte was added to the CFP current collector. Specifically, at room temperature, the CFP current collector was placed in the positive electrode case of the coin cell, and the prepared catholyte was dripped from above with a pipette for 1 to 5 seconds until the CFP current collector was completely wet. Next, a PP separator was placed on top of the obtained CFP electrode (positive electrode), and 20 μL of a blank electrolyte consisting of 1M LiTFSI in DOL:DME (1:1 vol.% containing 1% LiNO ₃ ) was added to the top of the PP separator. Specifically, at room temperature, the blank electrolyte was dripped from above with a pipette for 1 to 5 seconds until the PP separator was completely wet. Then, a lithium foil was placed on the electrolyte-impregnated PP separator, and the lithium foil was covered with a stainless steel spacer, and then placed on a coin-type battery cell and pressed. The resulting battery was removed from the glove box and electrochemical measurements were performed at 30°C using a Wuhan LAND electronics charge/discharge device (LAND batteries testing system CT2001A) with a cutoff voltage range of 1.8 to 3.0 V. The cyclic voltammetry behavior of the above cell was performed at 30°C with a scan rate of 1 mVs ^-1 in the potential range of 1.8 to 3.0 V.

Example 2: Preparation of 2-MP-Li cathode solution and 40 μL cathode solution: Coin cell prototype and performance measurement 0.5 mmol of 2-mercaptopyridine (2-MP) was dissolved in 1 mL of a blank electrolyte consisting of 1 M LiTFSI in DOL:DME (1:1 vol.% containing 1% LiNO ₃ ) to form a 0.5 M 2-MP solution. Next, 6.042 mg of lithium metal was added to the above solution to lithiate 2-MP and form a 0.5 M 2-MP-Li cathode solution. As a result, the obtained 2-MP-Li is a compound having the above structure. In addition, the fact that 2-MP-Li was actually obtained was analyzed by X-ray photoelectron spectroscopy (XPS). The results are shown in FIG. 1.

The carbon fiber paper (CFP), which is the positive electrode current collector, was dried at 110°C for 24 hours before use. The CR2025 type coin cell was fabricated in a glove box filled with Ar. First, 20 μL of the prepared catholyte was added to the CFP current collector. Specifically, at room temperature, the CFP current collector was placed in the positive electrode case of the coin cell, and the prepared catholyte was dripped from above with a pipette for 1 to 5 seconds until the CFP current collector was completely wet. Next, a PP separator was placed on top of the obtained CFP electrode (positive electrode), and 20 μL of the prepared catholyte was added to the top of the PP separator. Specifically, as above, at room temperature, the prepared catholyte was dripped from above with a pipette for 1 to 5 seconds until the PP separator was completely wet. Then, a lithium foil was placed on the PP separator impregnated with the catholyte, and the PP separator was covered with a stainless steel spacer, and then placed on a coin battery cell and pressed. The resulting battery was removed from the glove box and electrochemical measurements were performed at 30° C. using a Wuhan LAND electronics charge/discharge device (LAND batteries testing system CT2001A) with a cutoff voltage range of 1.8 to 3.0 V. The cyclic voltammetry behavior of the above cell was performed at 30° C. in the potential range of 1.8 to 3.0 V with a scan rate of 1 mV s ⁻¹ .

2 shows the results of electrochemical measurements using the coin cell of Comparative Example 1. When the 2-MP-Li cathode solution was not used, the discharge capacity was almost zero in the coin cell having the CFP positive electrode, blank electrolyte, and lithium metal negative electrode. In other words, it can be understood that the CFP positive electrode current collector does not improve the discharge capacity.

FIG. 3 shows the charge/discharge curve (a) and cyclic voltammogram (b) of the coin cell of Example 1. The coin cell, which had a CFP positive electrode (containing 2-MP-Li), blank electrolyte, and lithium metal negative electrode, impregnated with 20 μL of 2-MP-Li catholyte in the CFP positive electrode current collector, showed excellent electrochemical properties. In addition, in FIG. 3(a), a clear potential plateau was shown in the potential range of 1.8 to 3.0 V under conditions of 0.3 C and 30° C. As shown in FIG. 3(b), the coin cell also showed good reversible properties under conditions of 1 mVs ⁻¹ , which was in good agreement with the voltage profile in FIG. 3(a).

FIG. 4 shows the results of electrochemical measurements (rate characteristics (a) and cycle stability (b)) using the coin cell of Example 1. From FIG. 4(a), in a coin cell composed of a CFP positive electrode (containing 2-MP-Li), blank electrolyte, and lithium metal negative electrode, in which 20 μL of 2-MP-Li cathode solution was impregnated into the CFP positive electrode current collector, discharge capacities of 132.5, 112, 107.2, 102.6, and 101.1 mAhg ⁻¹ were shown at rates of 0.1, 0.3, 0.5, 1.0, and 2.0 C, respectively. Furthermore, even when the rate returned to 0.1 C after cycling at a high rate of 2 C was completed, the discharge capacity was 131.7 mAhg ⁻¹ , reaching 99.4% of the initial discharge capacity. On the other hand, as shown in FIG. 4(b), this coin cell had a Coulombic efficiency of 99.11% even after 150 cycles, and a high cycle stability of 87.45%. These results indicate that the coin cells using 2-MP-Li catholyte have excellent reversible characteristics and cycle stability.

FIG. 5 shows the charge/discharge curve (a) and cyclic voltammogram (b) of the coin cell of Example 2. The coin cell, which was impregnated with a total of 40 μL of 2-MP-Li cathode solution in both the CFP cathode current collector and the PP separator, also showed excellent electrochemical properties. In addition, in FIG. 5(a), a clear potential plateau was shown in the potential range of 1.8 to 3.0 V under conditions of 0.3 C and 30° C. As shown in FIG. 5(b), the cell also showed good reversible properties under conditions of 1 mVs ⁻¹ , which was in good agreement with the voltage profile in FIG. 5(a). These phenomena again indicate that 2-MP-Li has excellent electrochemical stability with lithium metal, and has good lithium ion transport capacity and high electrical insulation.

Figure 6 shows the results of electrochemical measurements (cycle stability) using the coin cell of Example 2. In Figure 6(a), for comparison, the cycle stability using the coin cell of Example 1 is shown as 20 μL-catholite-0.3C, and the cycle stability using the coin cell of Example 2 is shown as 40 μL-catholite-0.3C. From Figure 6(a), in a coin cell composed of a CFP positive electrode (containing 2-MP-Li), an electrolyte (containing 2-MP-Li), and a lithium metal negative electrode, in which a total of 40 μL of 2-MP-Li cathode solution was impregnated into both the CFP positive electrode collector and the PP separator, the discharge capacity at 0.3 C was about twice that of Example 1. Furthermore, this coin cell showed a high cycle stability of 94.72% and a Coulombic efficiency of 97.49% even after 200 cycles of charge/discharge experiment at a high rate of 0.5 C. Compared to cathode sheets produced by a conventional solvent casting method, it is suggested that 2-MP-Li avoids the limitations of ion diffusion in the coin cell of Example 2, improving battery capacity and cycle stability. The above results suggest that the catholyte and battery operation mode of the present invention can also be used in liquid flow batteries. For this reason, it is expected that they will also be applied to large-scale grid energy storage.

Claims (10)

The present invention relates to a compound having at least one sulfur-containing group and having one or more pyridine skeletons in the molecule, the sulfur-containing group being a thiol group or a derivative thereof,
The compound has the general formula (1):
[In the formula, R ₁₁ , R ₁₂ , R ₁₃ , R ₁₄ and R ₁₅ are the same or different and each represents a monovalent group, provided that at least one of R ₁₁ , R ₁₂ , R ₁₃ , R ₁₄ and R ₁₅ is a thiol group or a derivative group thereof.]
A pyridine compound represented by the formula:
General formula (2):
[In the formula, R ₂₁ , R ₂₂ , R ₂₃ , R ₂₄ , R ₂₅ , R ₂₆ , R ₂₇ and R ₂₈ are the same or different and represent a monovalent group. However, at least one of R ₂₁ , R ₂₂ , R ₂₃ , R ₂₄ , R ₂₅ , R ₂₆ , R ₂₇ and R ₂₈ is a thiol group or a derivative group thereof.]
A bipyridine compound represented by the formula:
General formula (3):
[In the formula, R ₃₁ , R ₃₂ , R ₃₃ , R ₃₄ , R ₃₅ , R ₃₆ , R ₃₇ and R ₃₈ are the same or different and represent a monovalent group, provided that at least one of R ₃₁ , R ₃₂ , R ₃₃ , R ₃₄ , R ₃₅ , R ₃₆ , R ₃₇ and R ₃₈ is a thiol group or a derivative group thereof, and n represents an integer of 2 to 4.]
The present invention relates to a positive electrode active material precursor for a secondary battery , which is at least one selected from the group consisting of pyridine ring compounds represented by the following formula : The positive electrode active material precursor for a secondary battery according to claim 1 , which is a positive electrode active material precursor for a lithium ion secondary battery. The present invention includes a lithium compound having at least one sulfur-containing group and having one or more pyridine skeletons in the molecule, the sulfur-containing group being a thiol group or a derivative thereof,
The compound has the general formula (1):
[In the formula, R ₁₁ , R ₁₂ , R ₁₃ , R ₁₄ and R ₁₅ are the same or different and each represents a monovalent group, provided that at least one of R ₁₁ , R ₁₂ , R ₁₃ , R ₁₄ and R ₁₅ is a thiol group or a derivative group thereof.]
A pyridine compound represented by the formula:
General formula (2):
[In the formula, R ₂₁ , R ₂₂ , R ₂₃ , R ₂₄ , R ₂₅ , R ₂₆ , R ₂₇ and R ₂₈ are the same or different and represent a monovalent group. However, at least one of R ₂₁ , R ₂₂ , R ₂₃ , R ₂₄ , R ₂₅ , R ₂₆ , R ₂₇ and R ₂₈ is a thiol group or a derivative group thereof.]
A bipyridine compound represented by the formula:
General formula (3):
[In the formula, R ₃₁ , R ₃₂ , R ₃₃ , R ₃₄ , R ₃₅ , R ₃₆ , R ₃₇ and R ₃₈ are the same or different and represent a monovalent group, provided that at least one of R ₃₁ , R ₃₂ , R ₃₃ , R ₃₄ , R ₃₅ , R ₃₆ , R ₃₇ and R ₃₈ is a thiol group or a derivative group thereof, and n represents an integer of 2 to 4.]
The positive electrode active material for a secondary battery is at least one selected from the group consisting of pyridine ring compounds represented by the following formula : The positive electrode active material for a secondary battery according to claim 3 , which is a positive electrode active material for a lithium ion secondary battery. The present invention includes a lithium compound having at least one sulfur-containing group and having one or more pyridine skeletons in the molecule, the sulfur-containing group being a thiol group or a derivative thereof,
The compound has the general formula (1):
[In the formula, R ₁₁ , R ₁₂ , R ₁₃ , R ₁₄ and R ₁₅ are the same or different and each represents a monovalent group, provided that at least one of R ₁₁ , R ₁₂ , R ₁₃ , R ₁₄ and R ₁₅ is a thiol group or a derivative group thereof.]
A pyridine compound represented by the formula:
General formula (2):
[In the formula, R ₂₁ , R ₂₂ , R ₂₃ , R ₂₄ , R ₂₅ , R ₂₆ , R ₂₇ and R ₂₈ are the same or different and represent a monovalent group. However, at least one of R ₂₁ , R ₂₂ , R ₂₃ , R ₂₄ , R ₂₅ , R ₂₆ , R ₂₇ and R ₂₈ is a thiol group or a derivative group thereof.]
A bipyridine compound represented by the formula:
General formula (3):
[In the formula, R ₃₁ , R ₃₂ , R ₃₃ , R ₃₄ , R ₃₅ , R ₃₆ , R ₃₇ and R ₃₈ are the same or different and represent a monovalent group, provided that at least one of R ₃₁ , R ₃₂ , R ₃₃ , R ₃₄ , R ₃₅ , R ₃₆ , R ₃₇ and R ₃₈ is a thiol group or a derivative group thereof, and n represents an integer of 2 to 4.]
The compound is at least one selected from the group consisting of pyridine ring compounds represented by the formula:
Positive electrolyte for secondary batteries. The positive electrode liquid for a secondary battery according to claim 5 , which is a positive electrode liquid for a lithium ion secondary battery. A positive electrode for a secondary battery, comprising the positive electrode active material for a secondary battery according to claim 3 or 4 or the positive electrode liquid for a secondary battery according to claim 5 or 6 . The positive electrode for a secondary battery according to claim 7 , which is a positive electrode for a lithium ion secondary battery. A secondary battery comprising the positive electrode for secondary batteries according to claim 7 or 8 . The secondary battery according to claim 9 , which is a lithium ion secondary battery.

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