JP5061851B2 - Positive electrode for secondary battery and secondary battery - Google Patents

Description

  The present invention relates to a positive electrode for a secondary battery and a secondary battery that can obtain a secondary battery that has excellent charge / discharge characteristics at a large current and a high energy density.

  In recent years, with the widespread use of portable electronic devices such as notebook computers and digital cameras, there is an increasing demand for small high-capacity secondary batteries having high energy density. From the viewpoint of environmental problems, battery cars and hybrid cars using electric power as a part of power have been put into practical use, and there is a demand for higher performance of secondary batteries as power storage means.

  Development of a lithium ion battery is progressing as a promising candidate for a secondary battery that meets these requirements, and development is being carried out to achieve excellent stability and high energy density.

  However, since the lithium ion battery is accompanied by an insertion / release reaction of lithium ions as an active material during charging / discharging, there is a problem that the battery performance deteriorates when a large current of a certain level is passed. For this reason, in order to exhibit a high battery performance, the restriction | limiting that discharge rate and charge rate did not become a certain amount or more was needed.

  On the other hand, a secondary battery using a radical material typified by poly (2,2,6,6-tetramethylpiperidinoxymethacrylate) (PTMA) as a positive electrode uses an ion adsorption / desorption reaction for an ionization reaction. Therefore, it is possible to pass a larger current than a normal lithium ion battery, the cycle characteristics are excellent, and application to portable electronic devices and electric vehicles is expected.

  By the way, in a lithium ion battery, an oxide material can be contained in a positive electrode material in a state containing lithium. Therefore, when aiming at a higher energy density, it is sufficient to increase the lithium oxide material in the positive electrode material. It is sufficient that the salt concentration in the electrolytic solution is such that the necessary ionic conductivity can be realized.

  However, in order to achieve a high energy density in a secondary battery using a radical compound such as PTMA for the positive electrode, the amount of radicals in the positive electrode is increased, and at the same time, the source of ions that react with the radicals is increased in the electrolyte. It was thought that it was necessary to contain. For example, the supporting electrolyte in the electrolytic solution is set to a high concentration according to the capacity of the secondary battery.

Here, when the concentration of the supporting electrolyte as typified by LiPF ₆ is increased, the viscosity of the electrolytic solution is increased, the ion diffusion rate is decreased, and the conductivity of the electrolytic solution is decreased. As a result, the value of the current that can be taken out from the battery is reduced, causing a reduction in output density.

  Furthermore, it has been found that when a large amount of a supporting electrolyte is added to the electrolytic solution, the wettability of the electrolytic solution with respect to the electrode is deteriorated, the internal resistance is increased, and the output of the battery is reduced.

  That is, in a secondary battery using a radical compound, it is difficult to achieve both high energy density and high output density.

  As a conventional technique related to increasing the energy density of a secondary battery using a radical compound, Patent Document 1 discloses a technique in which a positive electrode contains a stable radical compound and holds an electrolyte salt.

However, since the electrolyte salt held in the positive electrode in this technology is soluble in the electrolyte, the electrolyte salt dissolves and is uniformly dispersed throughout the electrolyte during high-temperature storage and long-term storage, increasing the viscosity of the electrolyte. It is presumed that the conductivity of the electrolytic solution is lowered, and as a result, the output of the battery is lowered.
JP 2004-259618 A

  This invention is made | formed in view of the said situation, and provides a secondary battery using the positive electrode for secondary batteries which can obtain the secondary battery in which high energy density and high output density were compatible, and the positive electrode And

In order to solve the above problems, the present inventors have conducted intensive studies. As a result, X ₂ YF ₆ (X: a kind of Li, Na, K, group 14 element excluding Y: C) and a radical compound are contained in the electrode. It was found that the characteristics of the radical compound can be sufficiently exerted by coexisting with.

That is, the positive electrode for a secondary battery of the present invention has a radical compound as a positive electrode active material and X ₂ YF ₆ (X: a kind of Li, Na, K, 14: a group 14 element excluding Y: C). It is characterized by.

In addition, the secondary battery of the present invention is a secondary battery having a radical compound as a positive electrode active material and X ₂ YF ₆ (X: a kind of Li, Na, K, group 14 element excluding Y: C). And a negative electrode and an electrolyte solution having an electrolyte.

Secondary battery positive electrode of the present invention, the X ₂ YF ₆ capable of supplying ions to react to radical compound has with radical compounds. In this positive electrode, the radical part of the radical compound is oxidized at the time of charging to become a cation, and participates in the electrode reaction by releasing electrons. Then, in which the cell reaction can continue by cations from the X ₂ YF ₆ to compensate the charge of the cations generated from radical compounds dissociate. As a result, the secondary battery positive electrode of the present invention can provide a secondary battery capable of achieving both high output density and high energy density.

  And the secondary battery of this invention has the positive electrode for secondary batteries of this invention, and can make high power density and high energy density compatible. In particular, since a high energy density can be realized even when the concentration of the supporting electrolyte in the electrolyte is low, an increase in the viscosity of the electrolyte caused by dissolving the supporting electrolyte at a high concentration can be suppressed, resulting in a high output density. It has become.

The positive electrode for a secondary battery of the present invention has a radical compound as a positive electrode active material and X ₂ YF ₆ (X: a kind of Li, Na, K, group 14 element excluding Y: C).

X ₂ YF ₆ is a compound that realizes ion conduction in the positive electrode active material, and is a compound that can desorb ions corresponding to the radical compound in an environment where the positive electrode for a secondary battery of the present invention is applied to a secondary battery. is there. Here, X ₂ YF ₆ is desirably mixed in a mole number (ratio) of 50% or more and 200% or less of the number of moles of the radical compound.

X ₂ YF ₆ is a cation structure that is a partial structure that becomes a cation upon desorption, and a partial structure that forms the anion after the cation structure is desorbed after the cation structure is desorbed. A material having an anionic structure can be exemplified. Specifically, a compound in which X in X ₂ YF ₆ is Li can be exemplified. In a lithium compound having lithium as a cation, a structure such as LiYF ₆ ^— or YF ₆ ²⁻ remains as an anion structure after lithium is dissociated. Furthermore, examples of X ₂ YF ₆ include Li ₂ SiF ₆ or Li ₂ SnF _{6 in} which Y is Si or Sn.

X ₂ YF ₆ is desirably in a solid state when a secondary battery is formed, and further desirably not dissolved when a liquid is used as a supporting electrolyte. That is, X ₂ YF ₆ is preferably insoluble in the electrolytic solution used when forming the secondary battery. Here, the form of X ₂ YF ₆ outside the secondary battery (before applying the secondary battery positive electrode of the present invention to the secondary battery and before manufacturing the secondary battery positive electrode) is not particularly limited.

  The radical compound is a compound that functions as a positive electrode active material in the positive electrode for a secondary battery of the present invention and has a radical in the molecular structure. The radical compound is a compound in which the oxidation-reduction in the radical contained in the molecular structure is directly related to the battery reaction when the secondary battery positive electrode of the present embodiment is applied to the secondary battery. Accordingly, the radical compound can be appropriately selected depending on the type of secondary battery (the electrode to be combined, the type of supporting electrolyte, and the target battery performance) to which the positive electrode for secondary battery of the present invention is applied. Moreover, you may use what has electroconductivity in the structure as a radical compound. When the conductive material is used, the amount of the conductive material can be reduced, so that the amount of the radical compound can be increased. The radical compound is desirably mixed at a ratio of 5% to 60% based on the weight of the positive electrode mixture.

  Examples of radical compounds that can be specifically used when applied to a secondary battery employing lithium ions include nitroxyl radical compounds, oxy radical compounds, aryloxy radical compounds, and compounds having an aminotriazine structure. .

In the positive electrode for secondary battery of the present invention, it is desirable that X ₂ YF ₆ and the radical compound are as close as possible. That is, it is desirable that the radical compound and the lithium compound are mixed. Here, it is desirable that the radical compound and the lithium compound have a high affinity for each other so as to be mixed well.

  The positive electrode for a secondary battery of the present invention may further contain a conductive material for the purpose of smoothly transferring and receiving electrons that proceed with the radical compound. The conductive material is intended to improve the electrical conductivity between the radical of the radical compound and the current collector that collects current from the radical compound, so it is disposed close to both up to the molecular level. It is desirable that The conductive material is preferably mixed at a rate of 5% to 50%, more preferably 5% to 40%, based on the weight of the positive electrode mixture.

  Specific conductive materials include ketjen black, acetylene black, carbon black, graphite, carbon nanotubes, amorphous carbon, conductive polymers such as polyaniline, polypyrrole, polythiophene, polyacetylene, polyacene, and metal materials. can do.

  Furthermore, the positive electrode for a secondary battery of the present invention can employ a configuration containing an active material related to a battery reaction (such as a compound from which lithium ions are inserted and released) in addition to a radical compound. The active material is desirably mixed in a proportion of more than 0% and 90% or less based on the weight of the positive electrode mixture.

Examples of the positive electrode active material that can be mixed with the positive electrode of the present invention include a lithium-metal composite oxide having a layered structure or a spinel structure. _{Specifically, Li (1-Z) NiO} 2, Li (1-Z) MnO 2, Li (1-Z) Mn 2 O 4, Li (1-Z) CoO 2, Li (1-Z) FeO ₂ etc. can be raised. Here, z in these lithium-metal composite oxides is 0-1. Further, these lithium-metal composite oxides may each be added or substituted with a transition metal such as Li, Mg, Al, Co, Ti, Nb, or Cr. Moreover, not only these lithium-metal composite oxides are used alone, but also a plurality of them can be mixed and used. Among these, at least one lithium-metal composite oxide of a lithium manganese-containing composite oxide, a lithium nickel-containing composite oxide, and a lithium cobalt-containing composite oxide having a layered structure or a spinel structure is preferable.

  Furthermore, the positive electrode for a secondary battery according to the present invention includes a dispersion material in which a lithium compound, a radical compound, an active material, and the like are dispersed, a binder that binds them, and a current collector that collects electrons generated by the radical compound. (It can be formed from a metal foil or the like).

  The dispersion material and the binder are preferably formed from a polymer material, and are desirably materials that are chemically and physically stable in the atmosphere in the secondary battery.

The positive electrode for secondary battery of this embodiment has a radical compound and X ₂ YF ₆ as essential components, and is selected from the above-described positive electrode active material, binder, conductive material and other materials as necessary. In general, the electrode mixture layer is a layer made of an electrode mixture in which an additive is mixed, and the electrode mixture layer formed on the surface of the current collector is formed on the surface of the current collector formed of a metal foil or the like. It is. Examples of a method for forming the electrode mixture layer on the surface of the current collector include a method in which the electrode mixture is dispersed or dissolved in an appropriate dispersion medium and then applied to the surface of the current collector and dried.

The secondary battery of the present invention, a radical compound as a positive electrode active _{material, X 2 YF 6 (X:} Li, Na, kind of K, Y: 14 group elements excluding C) secondary battery having a A positive electrode for use, a negative electrode, and an electrolyte solution having an electrolyte. That is, the secondary battery of the present invention has the above-described positive electrode for a secondary battery of the present invention.

In the secondary battery of the present invention, the above-described positive electrode for a secondary battery is combined with a negative electrode to form a secondary battery. As an active material of the negative electrode to be combined, a compound that occludes lithium ions during charging and releases them during discharging is used. Can be adopted. The negative electrode active material is not particularly limited in its material configuration, and known materials and configurations can be used. For example, a lithium metal, a carbon material such as graphite or amorphous carbon, an alloy material containing silicon, tin, or the like, or an oxide material such as Li ₄ Ti ₅ O ₁₂ or Nb ₂ O ₅ .

  The electrolyte solution is not particularly limited, but an electrolyte solution (supporting salt) dissolved in a solvent such as an organic solvent, an ionic liquid that is liquid itself, or a solution in which a supporting salt is further dissolved in the ionic liquid. It can be illustrated. As an organic solvent, the organic solvent normally used for the electrolyte solution of a lithium secondary battery can be illustrated. For example, carbonates, halogenated hydrocarbons, ethers, ketones, nitriles, lactones, oxolane compounds and the like can be used. In particular, propylene carbonate, ethylene carbonate, 1,2-dimethoxyethane, dimethyl carbonate, diethyl carbonate, ethyl methyl carbonate, and the like, and mixed solvents thereof are suitable.

  Among these organic solvents mentioned in the examples, in particular, by using one or more non-aqueous solvents selected from the group consisting of carbonates and ethers, the solubility of the supporting salt, the dielectric constant and the viscosity are excellent, and the battery The charge / discharge efficiency is also preferable.

An ionic liquid will not be specifically limited if it is an ionic liquid normally used for the electrolyte solution of a lithium secondary battery. For example, as the cation component of the ionic liquid, or N- methyl -N- propyl piperidinium, dimethylethyl methoxy ammonium cation and the like, and an anionic ^{_{component, BF 4 -, LiN (SO}} 2 C 2 F 5) ₂ ^-, and the like.

The supporting salt used in the electrolytic solution of the secondary battery of the present invention is not particularly limited. For example, LiPF ₆ , LiBF ₄ , LiAsF ₆ , LiCF ₃ SO ₃ , LiN (CF ₃ SO ₂ ) ₂ , LiC (CF ₃ SO ₂ ) ₃ , LiSbF ₆ , LiSCN, LiClO ₄ , LiAlCl ₄ , NaClO ₄ , BClO ₄ , NaI, and salt compounds such as derivatives thereof. Among these, LiPF ₆ , LiBF ₄ , LiClO ₄ , LiAsF ₆ , LiCF ₃ SO ₃ , LiN (CF ₃ SO ₂ ) ₂ , LiC (CF ₃ SO ₂ ) ₃ , LiCF ₃ SO ₃ derivative, LiN (CF ₃ From the viewpoint of electrical characteristics, it is preferable to use one or more salts selected from the group consisting of a derivative of SO ₂ ) _{2 and} a derivative of LiC (CF ₃ SO ₂ ) ₃ .

  It is desirable to interpose a separator that is a member that achieves both electrical insulation and ion conduction between the positive electrode and the negative electrode. When the electrolytic solution is liquid, the separator also plays a role of holding the liquid supporting electrolyte. Examples of the separator include a porous synthetic resin film, particularly a porous film of a polyolefin polymer (polyethylene or polypropylene). Furthermore, it is preferable that the separator has a larger size than the positive electrode and the negative electrode for the purpose of ensuring the insulation between the positive electrode and the negative electrode.

  In general, the positive electrode, the negative electrode, the supporting electrolyte, the separator, and the like are housed in some case. The case is not particularly limited and can be made of a known material and form.

  The positive electrode for secondary battery and the secondary battery of the present invention will be described in more detail based on the following specific examples. However, the following examples are illustrative of the present invention, and the present invention is not limited by the following examples.

Example 1
(Preparation of positive electrode for secondary battery)
Li ₂ SiF ₆ , PTMA as a radical compound, carbon black as a conductive material, polyvinylidene fluoride (PVDF) as a binder, and N-methylpyrrolidone (NMP) as a dispersion medium 10: 57: 28: 5: It was mixed and dispersed at a mass ratio (parts by mass) of 90. Here, the number of moles of Li ₂ SiF ₆ was 50% of the number of moles of the radical compound.

  The obtained slurry was applied to both surfaces of a positive electrode current collector made of an aluminum metal foil, dried and pressed to obtain a positive electrode plate. Thereafter, the positive electrode plate was cut into a predetermined size, and then the positive electrode mixture in the portion to be a lead tab weld for extracting current was scraped off to produce a sheet-like positive electrode.

(Preparation of electrolyte)
LiPF ₆ was added to an organic solvent in which ethylene carbonate (EC) and diethyl carbonate (DEC) were mixed at a mass ratio of 3: 7 to a concentration of 0.6 mol / L to obtain an electrolytic solution.

(Production of coin-type battery)
A coin-type battery was manufactured using the manufactured positive electrode 1 of this example. A coin-type battery 1 of this example is shown in FIG. The coin-type battery of this example includes a positive electrode 2, a negative electrode 3, an electrolytic solution 4, and a separator 7. The negative electrode 3 was made of metallic lithium, the electrolytic solution 4 was the prepared electrolytic solution, and the separator 7 was a 25 μm thick polyethylene porous membrane. The positive electrode 2 has a positive electrode current collector 2a, and the negative electrode 3 has a negative electrode current collector 3a.

  These power generation elements were housed in a stainless steel case (consisting of a positive electrode case 50 and a negative electrode case 51). The positive electrode case 50 and the negative electrode case 51 serve as a positive electrode terminal and a negative electrode terminal. A gasket 6 made of polypropylene is interposed between the positive electrode case 50 and the negative electrode case 51 to ensure sealing and insulation between the positive electrode case 50 and the negative electrode case 51.

(Example 2)
Li ₂ SiF ₆ and PTMA as a radical compound, carbon black as a conductive material, PVDF as a binder, and NMP as a dispersion medium are mixed and dispersed in a mass ratio (parts by mass) of 17: 52: 26: 5: 90. I let you. Here, the number of moles of Li ₂ SiF ₆ is the same number (100%) as the number of moles of the radical compound.

  The positive electrode and coin-type battery of this example were produced from the prepared slurry in the same manner as in Example 1.

(Example 3)
Li ₂ SiF ₆ mixed with PTMA as a radical compound, carbon black as a conductive material, PVDF as a binder, and NMP as a dispersion medium in a mass ratio (parts by mass) of 29: 44: 22: 5: 90 Dispersed. Here, the number of moles of Li ₂ SiF ₆ is 200% of the number of moles of the radical compound.

  The positive electrode and coin-type battery of this example were produced from the prepared slurry in the same manner as in Example 1.

Example 4
Li ₂ SiF ₆ and PTMA as a radical compound, carbon black as a conductive material, PVDF as a binder, and NMP as a dispersion medium are mixed and dispersed in a mass ratio (parts by mass) of 17: 52: 26: 5: 90. I let you. Here, the number of moles of Li ₂ SiF ₆ is the same number (100%) as the number of moles of the radical compound.

  The positive electrode of this example was produced from the prepared slurry in the same manner as in Example 1.

This example was the same as Example 1 except that LiPF ₆ was added at a concentration of 1.0 mol / L to an organic solvent in which EC and DEC were mixed at a mass ratio of 3: 7 to obtain an electrolytic solution. Manufactured a coin-type battery.

(Comparative Example 1)
A positive electrode of this comparative example was manufactured in substantially the same manner as in Example 1 except that Li ₂ SiF ₆ was not added.

  As a positive electrode, PTMA as a radical compound, carbon black as a conductive material, PVDF as a binder, and NMP as a dispersion medium are mixed and dispersed at a mass ratio (parts by mass) of 63: 31: 6: 90. Otherwise, a positive electrode of this comparative example was produced in the same manner as in Example 1.

This comparative example was the same as in Example 1 except that LiPF ₆ was added to an organic solvent in which EC and DEC were mixed at a mass ratio of 3: 7 at a concentration of 1.0 mol / L to obtain an electrolytic solution. Manufactured a coin-type battery.

(Comparative Example 2)
The positive electrode of this comparative example is a positive electrode manufactured in the same manner as in comparative example 1.

  A coin-type battery of this comparative example was manufactured in the same manner as in Comparative Example 1 except that an organic solvent obtained by mixing EC and DEC at a mass ratio of 3: 7 was used as it was as an electrolytic solution.

(Comparative Example 3)
The positive electrode of this comparative example is a positive electrode manufactured in the same manner as in comparative example 1.

In the same manner as in Comparative Example 1, except that Li ₂ SiF ₆ was added at a concentration of 1.0 mol / L to an organic solvent in which EC and DEC were mixed at a mass ratio of 3: 7, this electrolyte was used. A coin-type battery of a comparative example was manufactured. Here, the number of moles of Li ₂ SiF ₆ is the same number (100%) as the number of moles of the radical compound.

(Evaluation)
As an evaluation of the examples and comparative examples, the capacity ratio of the coin-type battery was determined.

(Coin-type battery evaluation test method)
The coin type batteries of Examples and Comparative Examples are placed in a constant temperature bath at 25 ° C., and are charged at a constant current up to 4.1 V at a current value equivalent to 1 C (1 C is a current value that can discharge the battery capacity in 1 hour). A constant current discharge was performed up to 3.0 V at a considerable current value. After this test was performed five times, the discharge capacity value of the fifth time was used as the capacity value of each coin battery, and the capacity ratio was calculated based on the capacity of Comparative Example 1. The obtained results are shown in Table 1.

As shown in Table 1, it can be confirmed that the coin batteries of Examples 1 to 3 have a higher capacity ratio than Comparative Example 1. That is, by mixing Li ₂ SiF ₆ in the positive electrode having a radical compound, even when the salt concentration of the supporting salt in the electrolytic solution is low, a capacity equal to or higher than that of Comparative Example 1 can be realized.

Furthermore, from the coin batteries of Examples 1 to 3, the capacity ratio was improved by increasing the amount of Li ₂ SiF ₆ added, and after Li of Li ₂ SiF ₆ was dissociated, the anion portion was generated from the radical compound. It was confirmed that the battery reaction was performed by compensating the charge of the cation.

  Moreover, in the coin type battery of Example 4, it can be confirmed that when the salt concentration of the supporting salt in the electrolytic solution is increased, a higher capacity ratio is obtained.

Here, in the coin type battery of Comparative Example 3, Li ₂ SiF ₆ was added to the electrolytic solution, but was not dissolved in the electrolytic solution. And the capacity | capacitance is not expressed in the coin-type battery of the comparative example 3. That is, as in each example, it was confirmed that the effect was exhibited only when it was present in the vicinity of the radical compound (PTMA) in the positive electrode.

As is clear from Table 1, a high capacity can be realized by mixing a radical compound and Li ₂ SiF ₆ in the positive electrode even when the concentration of the supporting salt in the electrolytic solution is low. It has been clarified that a high output density and a high energy density can both be achieved by lowering the concentration of the supporting salt in the electrolyte and increasing the conductivity of the electrolyte.

It is a longitudinal cross-sectional view which shows roughly the structure of the coin-type battery manufactured in the present Example.

Explanation of symbols

1: coin-type battery 2: positive electrode 2a: positive electrode current collector 3: negative electrode 3a: negative electrode current collector 4: electrolyte 50: positive electrode case 51: negative electrode case 6: gasket 7: separator

Claims (10)

A positive electrode for a secondary battery, comprising: a radical compound as a positive electrode active material; and X ₂ YF ₆ (X: a kind of Li, Na, K, a group 14 element excluding Y: C). The positive electrode for a secondary battery according to claim 1, wherein the radical compound and the X ₂ YF ₆ are mixed. The positive electrode for a secondary battery according to claim 1, wherein in the compound X ₂ YF ₆ , X is Li. The _Li 2 XF _{6 is} a positive electrode for a secondary battery according to claim 1 is Li 2 SiF ₆ or _Li 2 SnF _6. The positive electrode for a secondary battery according to claim 1, wherein the X ₂ YF ₆ is insoluble in an electrolytic solution. A positive electrode for a secondary battery having a radical compound as a positive electrode active material and X ₂ YF ₆ (X: a kind of Li, Na, K, group 14 element excluding Y: C), a negative electrode, and an electrolyte A secondary battery comprising an electrolyte solution. The secondary battery according to claim 6, wherein the radical compound and the X ₂ YF ₆ are mixed. The secondary battery according to claim 6, wherein X of the compound X ₂ YF ₆ is Li. The _Li 2 XF _6, the secondary battery according to any one of claims 6 to 8 is a Li 2 SiF ₆ or _Li 2 SnF _6. The secondary battery according to claim 6, wherein the compound X ₂ YF ₆ is insoluble in an electrolytic solution.