JP5593919B2 - Secondary battery negative electrode and secondary battery using the same - Google Patents

Description

  The present invention relates to a secondary battery negative electrode, and more particularly to a conversion-type secondary battery negative electrode capable of improving charge / discharge efficiency and a capacity retention rate exhibiting durability, and a secondary battery using the negative electrode.

In recent years, lithium secondary batteries have been put into practical use as batteries having high voltage and high energy density. Due to the expansion of the use of lithium secondary batteries in a wide range of fields and the demand for high performance, various studies have been conducted to further improve battery performance.
For example, various materials have been studied for the negative electrode, and carbon materials, tin, and the like have been put into practical use as negative electrode materials for practical batteries. However, although carbon materials are widely used because they can provide high capacity, the ratio of occupied volume inside the battery increases because the specific gravity is small. Further, it is known that the performance of carbon materials has already been improved to a level where further improvement is difficult. For this reason, in order to improve the performance of the battery, it is indispensable to increase the charge / discharge efficiency by using a negative electrode material other than the carbon material, tin, silicon alloy, etc., and studies on negative electrode materials other than the carbon material have been made.

On the other hand, Li _x FeO _y (x = about 1, 1.8 ≦ y ≦ 2.2), for example, LiFeO ₂ has been proposed as an active material. While this active material is mainly used as a positive electrode active material, there is also an attempt to use it as a negative electrode active material.
For example, Patent Document 1 uses an electrode active material mainly composed of a compound represented by Li _x FeO _y (0 ≦ x ≦ 5, y = 1.5 + x / 2) in either the positive electrode or the negative electrode. Non-aqueous electrolyte lithium secondary batteries are described. However, Patent Document 1 describes that α-LiFeO ₂ was obtained as an electrode active material, and specific examples using this material as a negative electrode active material as well as a positive electrode active material were described. The specific value of the capacity maintenance rate which shows charging / discharging efficiency and durability is not shown.

Further, Non-Patent Document 1, the intercalation of Li into α-Fe ₂ O ₃ in the affected by the particle size of α-Fe ₂ O _3, may improve reversibility Smaller particle size Is described. However, specific values of the charge and discharge efficiency and capacity maintenance ratio indicating the durability of the case of using an active material intercalated with Li as a negative electrode active material in the α-Fe ₂ O ₃ is not shown.

JP-A-10-116616

Journal of The Electrochemical Society, 150 (1) A133-139 (2003)

As described above, Fe ₂ O ₃ is known as a negative electrode active material other than carbon, but no specific example having a high capacity retention rate showing high charge / discharge efficiency and durability as a negative electrode active material is known.
This can achieve reversibility according to an insertion reaction using a material other than carbon as a negative electrode active material (or sometimes referred to as an intercalation reaction), but the charge / discharge capacity obtained is low, On the other hand, the negative electrode active material other than the carbon due to the conversion reaction has a large irreversible capacity, and it is difficult to achieve a high capacity retention ratio exhibiting high charge / discharge efficiency and durability.
Accordingly, an object of the present invention is to provide a negative electrode for a secondary battery that can simultaneously improve charge / discharge efficiency and a capacity retention ratio exhibiting durability using a material other than carbon that is charged and discharged by a conversion reaction as a negative electrode active material, and A secondary battery using a negative electrode is provided.

The present invention provides a negative electrode for a lithium secondary battery conversion type, _Fe 2 _{O 3} and _Ni as the _{catalyst, Nb 2 O 5, La m} Sr n CoO 3 ( where, 0 <m <1,0 <n < 1, at least one selected from m + n = 1).
The present invention also relates to a lithium secondary battery that performs conversion-type charging / discharging, the battery using the negative electrode.

ADVANTAGE OF THE INVENTION According to this invention, the secondary battery negative electrode which can improve simultaneously the capacity maintenance rate which shows charging / discharging efficiency and durability by using materials other than carbon which charge / discharge by conversion reaction as a negative electrode active material can be obtained. .
In addition, according to the present invention, a secondary battery capable of simultaneously improving high charge / discharge efficiency and high capacity retention rate exhibiting durability using a material other than carbon that performs charge and discharge by a conversion reaction as a negative electrode active material is obtained. be able to.

FIG. 1 is a graph showing a comparison of charge and discharge curves of a secondary battery using a negative electrode within the scope of the present invention and outside the scope of the present invention. FIG. 2 is a graph showing a comparison of charge and discharge efficiencies of secondary batteries when the amount of catalyst in the negative electrode is changed.

In the present invention, it is a conversion-type secondary battery negative electrode, and it is necessary to contain Fe ₂ O ₃ and a catalyst, whereby a material other than carbon that is charged and discharged by a conversion reaction is used as the negative electrode active material. A secondary battery negative electrode capable of simultaneously improving the charge / discharge efficiency and the capacity retention rate showing durability, and a secondary battery using the secondary battery negative electrode can be obtained.
In the present specification, the fact that the capacity maintenance rate indicating the charge / discharge efficiency and the durability can be improved at the same time means that both are slightly improved as compared with the case of Fe ₂ O ₃ alone. I mean.

The above conversion reaction means that the insertion / release of Fe ₂ O ₃ and Li is not the intercalation reaction of the formula (2) but the reaction of the formula (1).
Fe ₂ O ₃ + 6Li → 3Li ₂ O + 2Fe (1)
Fe ₂ O ₃ + xLi → Li _X Fe ₂ O ₃ (x: 1 or 2) (2)
According to this conversion reaction, a material with a theoretical capacity of 1008 mAh / g in the negative electrode is expected to be extremely high compared to the current carbon-based and LTO oxide-based materials, but the irreversible capacity is large, that is, Li insertion and desorption. The reversible reaction of the active material is difficult and the durability is inferior due to expansion and contraction of the active material, and thus has not been studied so far.

The _Fe 2 _{O 3} in the present invention, _α-Fe 2 _{O 3,} may be either _β-Fe 2 _{O 3.}
The catalyst in the present invention is not particularly limited as long as it is a substance having a catalytic function, for example _{Ni, Nb 2 O 5, La} m Sr n CoO 3 ( where, 0 <m <1,0 <n <1, m + n = 1), at least one selected from the group consisting of Ti, Pd, La, Ag and Pt.
Moreover, the _La m _Sr n CoO ₃ (where, 0 <m <1,0 <n <1, m + n = 1) As a perovskite _La 0.8 _Sr 0.2 CoO ₃ from the viewpoint of ease of availability Although it is mentioned, it is not limited to this.
Ratio of catalyst to the _Fe 2 _{O 3} (catalyst / _Fe 2 _{O 3)} is 25% by weight greater than 0 wt% or less, it is preferable that in particular 2.5 to 20% by weight.

In the present invention, as a negative electrode for a lithium ion secondary battery, by applying the catalyst-added negative electrode active material containing Fe ₂ O ₃ and a catalyst as a negative electrode active material to a conversion reaction, as shown in FIG. A charge / discharge curve having a voltage range in which the battery voltage is flat with respect to the SOC (%) progress is less than 1V. This charge / discharge curve indicates that a conversion reaction occurs.
As shown in FIG. 2, the capacity retention rate indicating the charge / discharge efficiency and durability of the catalyst-added negative electrode active material of the embodiment within the scope of the present invention is only Fe ₂ O ₃ outside the scope of the present invention. Compared to the case, the capacity retention rate showing charge / discharge efficiency and durability is improved to the same or higher level.

Although the theoretical elucidation that the capacity retention rate showing the charge / discharge efficiency and the durability is improved by using the catalyst-added negative electrode active material obtained by mixing the Fe ₂ O ₃ and the catalyst of the present invention has not been made, the scope of the present invention When only the other Fe ₂ O ₃ is used, it is considered that, for example, Li _X Fe ₂ O ₃ is formed as an intermediate without proceeding to the conversion reaction. That is, it is considered that once the intermediate Li _X Fe ₂ O ₃ is formed, the intermediate Li _X Fe ₂ O ₃ is stable, so that the Li elimination reaction does not proceed easily. On the other hand, it is considered that the irreversible capacity can be reduced by using a catalyst-added negative electrode active material obtained by mixing Fe ₂ O ₃ and a catalyst. Alternatively, in the reaction of 3Li ₂ O + 2Fe → Fe ₂ O ₃ + 6Li, it is conceivable that O is desorbed from Li ₂ O and consumed as O ₂ gas before becoming Fe ₂ O _3, and the catalyst causes O ₂ to be consumed. It is considered that the irreversible capacity can be reduced by suppressing the consumption as, and promoting the reaction from Li ₂ O → Fe ₂ O ₃ .

In the secondary battery of the present invention, a negative electrode obtained by combining the above-described catalyst-added negative electrode active material, which is a mixture of Fe ₂ O ₃ and a catalyst, with a commonly used binder, conductive agent and solvent can be used.
Examples of the binder include styrene butadiene rubber (SBR), polyacrylate, polyvinylidene fluoride (PVdF), and the like.
Examples of the conductive agent include carbon materials, metals that are difficult to alloy with lithium, conductive polymer materials, and the like, and carbon materials are preferred. As the carbon material, graphite, carbon black, carbon nanotube, carbon nanofiber, fullerene and the like can be used alone or in combination of two or more.

  Examples of the solvent include alcohol, glycol, cellosolve, amino alcohol, amine, ketone, carboxylic acid amide, phosphoric acid amide, sulfoxide, carboxylic acid ester, phosphoric acid ester, ether, and nitrile. Specific examples include methyl alcohol, ethyl alcohol, 2-propanol, 1-butanol, 1,2-ethanediol, 1,2-propanediol, 1,3-propanediol, 1,4-butanediol, diethylene glycol, 2 -Methoxyethanol, 2-ethoxyethanol, 2-aminoethanol, acetone, methyl ethyl ketone, formamide, N-methylformamide, N, N-dimethylformamide, N-methylacetamide, N, N-dimethylformamide, N-methylacetamide, N , N-dimethylacetamide, N-methylpropionamide, N-methyl-2-pyrrolidone, hexamethylphosphoric triamide, dimethyl sulfoxide, sulfolane, acetonitrile, propionitrile.

The proportion of each component in the total solid content in the negative electrode is such that the catalyst-added negative electrode active material obtained by adding a catalyst to Fe ₂ O ₃ is 60% by mass or more and 98.5% by mass or less, and the binder is 1% by mass or more and 20% by mass. % Or less and the conductive agent is preferably 0.5 mass% or more and 30 mass% or less.

When the ratio of the catalyst-added negative electrode active material is less than 60% by mass, it may be difficult to obtain sufficient conductivity and discharge capacity. When the ratio exceeds 98.5% by mass, the ratio of the binder decreases. In some cases, the adhesion to the current collector is reduced, and the negative electrode active material is easily detached.
When the ratio of the binder is less than 1% by mass, the binding property is reduced, and thus the negative electrode active material and the carbon material as the conductive agent may be easily detached from the current collector. Since the ratio of the catalyst-added negative electrode active material and the carbon material as the conductive agent is lowered, there is a possibility that the battery performance is lowered.
In addition, if the proportion of the conductive agent is less than 0.5% by mass, it may be difficult to obtain sufficient conductivity. If the proportion exceeds 30% by mass, the proportion of the catalyst-added negative electrode active material that greatly contributes to battery performance. Therefore, problems such as a decrease in discharge capacity may occur.

  As a method of obtaining a negative electrode using the catalyst-added negative electrode active material of the present invention, a paste containing the catalyst-added negative electrode mixture or a paste is added to the paste and applied onto the negative electrode current collector, followed by drying and pressing. Examples of the coating method include forming a negative electrode material layer on the current collector.

A secondary battery is configured using a negative electrode obtained by using the above-described catalyst-added negative electrode active material, and other constituent materials such as a positive electrode, a separator, and an electrolyte.
The positive electrode has a positive electrode current collector and a positive electrode active material layer provided on at least one surface thereof.
The positive electrode current collector is made of, for example, a metal material such as aluminum, nickel, or stainless steel.

  The positive electrode active material layer includes a positive electrode material such as lithium oxide, particularly a composite oxide containing lithium and a transition metal, lithium sulfide, an intercalation compound containing lithium, and a lithium phosphate compound. The positive electrode active material layer may be a polymer material such as polyaniline, polythiophene, a conductive agent such as graphite, carbon black, acetylene black or ketjen black, carbon nanotube, carbon nanofiber, fullerene, etc. The carbon material which combined these may be contained.

  Examples of the separator include a porous film made of polyolefin such as polypropylene (PP) and polyethylene (PE), and a porous film made of ceramic. Particularly, a porous film made of polyolefin having a multilayer structure, for example, a three-layer structure of PE / PP / PE is preferably used.

  Examples of the electrolyte include an electrolytic solution or a gel electrolyte. The electrolytic solution contains a solvent and an electrolyte salt, and preferred examples of the solvent include ethylene carbonate, propylene carbonate, dimethyl carbonate, diethyl carbonate, and ethyl methyl carbonate. Among them, a mixed solvent obtained by mixing a high-viscosity solvent such as ethylene carbonate or propylene carbonate and at least one or two or more low-viscosity solvents such as dimethyl carbonate, ethyl methyl carbonate, diethyl carbonate, or ethyl methyl carbonate is preferable. . This solvent may contain a cyclic carbonate having an unsaturated bond such as vinylene carbonate or vinylethylene carbonate, or a cyclic carbonate having a halogen such as bis (fluoromethyl) carbonate.

The electrolytic solution generally contains an electrolyte salt as a supporting salt. Examples of the electrolyte salt include lithium hexafluorophosphate (LiPF ₆ ), lithium tetrafluoroborate (LiBF ₄ ), lithium perchlorate (LiClO ₄ ), lithium hexafluoroarsenate (LiAsF ₆ ), Bis (pentafluoroethanesulfonyl) imidolithium (LiN (C ₂ F ₅ SO ₂ ) ₂ ), lithium trifluoromethanesulfonate (LiCF ₃ SO ₃ ), bis (trifluoromethanesulfonyl) imide lithium (LiN (CF ₃ SO ₂ ) ₂ ), lithium tris (trifluoromethanesulfonyl) methide (LiC (CF ₃ SO ₂ ) ₃ ), lithium chloride (LiCl) or lithium bromide (LiBr), preferably lithium hexafluorophosphate (LiPF ₆ ) Can be mentioned.

  The gel electrolyte can be formed, for example, by preparing a positive electrode and a negative electrode, applying an electrolytic solution containing a solvent and an electrolyte salt thereto, and then volatilizing the solvent.

Examples of a method for obtaining a positive electrode using the positive electrode active material include a method known per se, for example, a method of forming a positive electrode active material layer on a positive electrode current collector, for example, Al by vapor deposition, sputtering, or CVD.
Alternatively, as a method of obtaining a positive electrode using the positive electrode active material, a paste containing the positive electrode active material is applied on a positive electrode current collector and then dried to form a positive electrode active material layer on the positive electrode current collector Law. It can be obtained by adding a solvent, for example, the above-mentioned solvent for producing a negative electrode to the paste containing the positive electrode active material or the paste, and applying it on the positive electrode current collector, followed by drying and pressing.
In addition, by using a negative electrode, a positive electrode, a separator, and an electrolyte obtained by using the catalyst-added negative electrode active material according to the present invention, a secondary battery that can simultaneously improve charge / discharge efficiency and a capacity retention rate showing durability. Can be obtained.
Examples of the secondary battery include those having an arbitrary shape.

Examples of the present invention will be described below.
The following examples are for illustrative purposes only and are not intended to limit the invention.

In each of the following examples, in the battery evaluation, a charge / discharge curve was created with the vertical axis representing voltage and the horizontal axis representing SOC (%). The SOC (State Of Charge) means a state of charge, indicates a remaining capacity, and is a well-known index in the industry.
Example 1
[Preparation of test anode]
Fe ₂ O ₃ powder and Ni powder were mixed at a mass ratio of 80:20 and mixed uniformly in a mortar. Thereafter, the obtained catalyst-added active material, acetylene black as a conductive agent and PVdF powder as a binder are added at a mass ratio of 65:20:15, and kneaded using N-methyl-2-pyrrolidone as a solvent. Thus, a paste was prepared, and a negative electrode current collector was coated, dried and pressed on a copper foil with a doctor blade to prepare a negative electrode having a thickness of 10 μm.

[Preparation of electrolyte]
In a mixed solvent in which EC (ethylene carbonate), DMC (dimethyl carbonate), and EMC (ethyl methyl carbonate) are mixed at a volume ratio of 3: 3: 4, LiPF ₆ as a supporting salt is dissolved at a concentration of 1 mol / L to obtain an electrolytic solution. Was made.
[Battery fabrication]
A battery having the following configuration was produced.
-Battery type: CR2032-type coin cell-Working electrode: negative electrode for testing-Counter electrode: Li metal-Separator: PE / PP / PE polyolefin porous membrane-Electrolyte: EC / DMC / EMC = 3: 3: 4 1M-LiPF ₆

[Battery evaluation]
Charge / discharge (upper and lower limit voltage: 3.0 V to 0.01 V) is performed at a battery evaluation environment temperature of 25 ° C. and a current rate of C / 10, and a charge / discharge curve is obtained to determine a ratio between the discharge capacity and the charge capacity, that is, charge / discharge efficiency. evaluated.
Further, the ratio of the charge capacity (Li desorption capacity) between the first cycle and the tenth cycle, that is, the capacity maintenance rate was evaluated.
The obtained results are shown together with other results in Table 1 and FIG.

Example 2
A negative electrode and a battery were produced in the same manner as in Example 1 except that Fe ₂ O ₃ powder and Nb ₂ O ₅ powder were mixed at a mass ratio of 80:20.
Using this battery, the battery was evaluated in the same manner as in Example 1.
The obtained results are shown together with other results in Table 1 and FIG.

Example 3
A negative electrode and a battery were produced in the same manner as in Example 1 except that Fe ₂ O ₃ powder and perovskite-type La _0.8 Sr _0.2 CoO ₃ powder were mixed at a mass ratio of 80:20.
Using this battery, the battery was evaluated in the same manner as in Example 1.
The obtained results are shown together with other results in Table 1 and FIG.

Comparative Example 1
A negative electrode and a battery were produced in the same manner as in Example 1 except that no catalyst was added to the Fe ₂ O ₃ powder.
Using this battery, the battery was evaluated in the same manner as in Example 1.
The obtained results are shown together with other results in Table 1 and FIG.

Comparative Example 2
A negative electrode and a battery were produced in the same manner as in Example 1 except that MnO ₂ powder was added instead of Ni powder.
Using this battery, the battery was evaluated in the same manner as in Example 1.
The obtained results are shown together with other results in Table 1.

Comparative Example 3
A negative electrode and a battery were produced in the same manner as in Example 1 except that Co phthalicyanine powder was added instead of Ni powder.
Using this battery, the battery was evaluated in the same manner as in Example 1.
The obtained results are shown together with other results in Table 1.

From FIG. 1 and Table 1, compared with the comparative example 1 which did not add a catalyst, in Example 1, by adding Ni as a catalyst, charging / discharging efficiency and capacity maintenance rate, that is, reversibility and durability are slightly In Example 2, the reversibility is slightly improved and durability is improved by adding Nb ₂ O ₅ as a catalyst. In Example 3, perovskite type La _0.8 Sr _0.2 Co ₃ is used as a catalyst. By adding, reversibility and durability are improved and improved.
Further, from FIG. 1, _Fe 2 _{O 3} powder to Ni, the case of adding _Nb 2 _{O 5} or perovskite _La 0.8 _Sr 0.2 Co _3, the plateau in the case of _Fe 2 _{O 3} powder alone It is understood that there is no change in the voltage position of the charge / discharge curve that is included, and that these additive substances do not function as active materials but function as catalysts.
Furthermore, it is understood from Table 1 that the substance added to the Fe ₂ O ₃ powder needs to function as a catalyst.

Examples 4-5
Fe ₂ O ₃ powder and Nb ₂ O ₅ powder were added at a mass ratio of 95: 5 (Example 4) or added at a mass ratio of 92.5: 7.5 (Example 5). In the same manner as in Example 2, a negative electrode and a battery were produced.
Using this battery, the battery was evaluated in the same manner as in Example 1.
The obtained results are shown together with the results of Example 2 in FIG.

From Figure 2, the ratio of catalyst to _Fe 2 _{O 3} (catalyst / _Fe 2 _{O 3)} may be 0 mass% 25 mass% with greater than or less, it is preferable to be particularly 2.5 to 20 wt% Understood.

  ADVANTAGE OF THE INVENTION According to this invention, the lithium secondary battery which the capacity maintenance rate which shows charging / discharging efficiency and durability by conversion reaction using negative electrode active materials other than a carbon material improved can be obtained.

Claims (5)

A negative electrode for a lithium secondary battery-conversion, _Ni as _Fe 2 _{O 3} and the _{catalyst, Nb 2 O 5, La m} Sr n CoO 3 ( where, 0 <m <1,0 <n <1, m + n = The negative electrode comprising at least one selected from 1). 2. The secondary according to claim 1, wherein the La _m Sr _n CoO ₃ (where 0 <m <1, 0 <n <1, m + n = 1) is a perovskite type La _0.8 Sr _0.2 CoO _3. Battery negative electrode. The _Fe 2 ratio of the catalyst to _{O 3} (catalyst / _Fe 2 _{O 3)} is a secondary battery negative electrode according to claim 1 or 2 or less 25% by weight greater than 0 wt%. The Fe ₂ O ₃ is the secondary battery negative electrode according to any one of claims 1 to 3 comprising used premixed with the catalyst. It is a lithium secondary battery which performs charge / discharge of conversion type, Comprising: The said battery which uses the negative electrode of Claim 1.

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