JP2001297751A - High-capacity nonaqueous electrolyte lithium secondary cell - Google Patents

Abstract

PROBLEM TO BE SOLVED: To control dissolving of manganese-ion in manganese-acid lithium when charging in discharging and to improve a cycle characteristic of lithium- ion cell, although manganese-ion tends to be dissolved into electrolyte and a cycle characteristic and a capacity of charging and discharging are lowered when manganese-acid lithium is used for positive electrode of lithium-ion cell. SOLUTION: This manganese-acid lithium of lithium-ion cell positive electrode active material is mixed with ferrate (VI) to generate a positive electrode, and Mn (II)-ion or the like which are generated by a non-uniform reaction or the like in charging and discharging Mn (III)-ion, are oxidized with ferrate (VI), and also coated with insoluble ferrate (VI), whereby, Mn solution is largely prevented.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

The present invention relates to a lithium manganese-based non-aqueous electrolyte secondary battery, in which irreversible capacity loss due to Mn dissolution is suppressed, performance is improved, and high capacity is achieved.
A high temperature stable non-aqueous electrolyte lithium secondary battery is obtained.

[0002]

2. Description of the Related Art Lithium ion batteries have a large discharge capacity, a high voltage and a high energy density. At present, non-aqueous electrolyte lithium-ion batteries using lithium cobaltate as a positive electrode are mainly used in terms of structural stability, high capacity, and ease of commercialization. However, lithium ion batteries using lithium nickelate or lithium manganate as a positive electrode have been actively studied due to problems such as high price of raw material cobalt and instability of production, and some of them have been commercialized.

Lithium manganate, a positive electrode active material such as LiMn ₂ O ₄ , is a very interesting positive electrode active material because it is abundant in resources, low in price, and low in toxicity. Before the battery is completely used up, manganese is dissolved, the cycle characteristics are poor, and the discharge capacity is significantly reduced. To suppress manganese dissolution, a non-stoichiometric composition such as Li [Li _Y Mn ³⁺ _1.0 M
n ⁴⁺ _{1.0] O 4} or the like, at room temperature,
Manganese dissolution can be significantly improved. But 40
The problem of manganese dissolution has not been solved when used at a high temperature of ℃ or more. Manganese dissolution ^{Mn 4+} and ^{Mn 2+} are generated by disproportionation reactions ^{Mn 3+} generated in charging and discharging, ^{Mn 2+} is exchanged 2Li and ions, ^{Mn 2+} ions are dissolved in the electrolyte. This manganese dissolution is a major cause of irreversible capacity loss of the electrode.

[0004] Conventionally, as a method of suppressing manganese dissolution in a LiMn ₂ O ₄ spinel positive electrode, LiMn ₂ O ₄
By dissolving a third component such as cobalt, nickel, and aluminum in a positive electrode active material such as ₂ O ₄ or forming a non-stoichiometric composition or a defect structure as described above, the manganese dissolution is reduced. However, even if the dissolution of manganese can be suppressed at room temperature, the dissolution of manganese cannot be suppressed when exposed to a high temperature of 50 ° C. or more for a long time as in an electric vehicle.

[0005]

However, according to the above method, the non-stoichiometric spinel and the third component are made of LiMn ₂ O ₄
However, even when added to or dissolved in a manganese composite oxide, manganese dissolution could not be suppressed in the charge / discharge step of the positive electrode active material at a high temperature. Accordingly, an object is to suppress irreversible capacity loss of the positive electrode due to dissolution of manganese.

SUMMARY OF THE INVENTION The present invention has been made to solve the above-mentioned problem, and a non-aqueous electrolyte lithium secondary battery having a higher discharge capacity and a higher cycle characteristic even at a high temperature than before, without reducing the productivity of the battery. The purpose is to provide.

[0007]

That is, according to the present invention, there is provided a non-aqueous solution comprising a positive electrode containing a lithium-manganese composite oxide, a negative electrode capable of doping or dedoping lithium, and a non-aqueous electrolyte in which an electrolyte is dissolved in an organic solvent. In the lithium electrolyte secondary battery, the positive electrode active material constituting the positive electrode is LiMn.
_It is a mixed complex oxide of a lithium manganese complex oxide such as ₂ O ₄ and LiMnO ₂ and a ferrate (VI) such as KFeO ₄ (VI).

Further, in the present invention, the mixing ratio of ferrate (VI) in the mixed composite oxide is 3 to 97% by weight.
It is characterized by being.

[0009]

The effects of the present invention include the following. For convenience, a spinel-based composite oxide LiMn ₂ O ₄ mainly composed of lithium and manganese and an orthorhombic lithium manganese oxide LiMnO ₂ are used as Li _X Mn _Y O 2.
₄ , ferrates such as calcium ferrate and barium ferrate are represented by MFeO ₄ (VI). M represents a divalent metal, but when it is a monovalent metal, it is K ₂ or Li ₂ .

[0010] _Li X _Mn Y _{O 4} according to the present invention and MFe
By mixing O ₄ , an inexpensive high-capacity non-aqueous electrolyte secondary battery with high capacity, good cycle characteristics even at high temperatures, abundant in resources, and environmentally friendly is obtained.

[0011] In _addition, Li X _Mn Y _{O 4} and MFeO ₄ (V
By mixing I), lithium 2 having a high discharge capacity can be obtained.
The following battery is obtained. For example, the theoretical discharge capacity of LiMn ₂ O ₄ is 148 mAh / g, and the actual discharge capacity up to 4.2 to 3.5 V is about 110 to 120 mAh / g. On the other hand, MF
In eO ₄ (VI), K ₂ FeO ₄ (VI), Li
₂ The theoretical discharge capacities of FeO ₄ (VI) and BaFeO ₄ (VI) are 406, 601, 313 mAh / g, respectively.
The actual discharge capacity also has a value of about 300 mAh / g or more. Therefore, a considerably higher energy density can be obtained than in a lithium secondary battery using only Li _X Mn _Y O ₄ as a positive electrode active material.

_Furthermore, by mixing the Li X _Mn Y _{O 4} and MFeO _4, inhibit irreversible capacity loss caused by dissolution of manganese Lithium secondary battery using only Li X _Mn Y _{O 4} as a cathode active material It becomes possible. Manganese dissolution during charge and discharge is caused by divalent manganese generated by the following disproportionation reaction.

The positive electrode composed of 2Mn ³⁺ → Mn ⁴ ++ Mn ²⁺ MFeO ₄ (VI) has a three-electron reduction ability, and has a three-fold reduction ability compared to the one-electron reduction ability of Li _X Mn _Y O _4. I have. Conventional KMnO ₄ , Cr
Oxidants such as O ₃ and K ₂ CrO ₄ have strong oxidizing performance, but these metals are environmentally problematic. Further, in order to be able to be used for the nonaqueous electrolyte lithium secondary battery of the present invention, divalent manganese ions are preferentially oxidized to trivalent manganese or partially tetravalent manganese, and the nonaqueous electrolyte and the electrolyte are not oxidized. It is necessary to have selective oxidation ability. Conventional oxidants do not have these properties. However, the MFeO ₄ (VI) of the present invention has these properties.

[0013]

BEST MODE FOR CARRYING OUT THE INVENTION The present invention relates to a high-capacity non-aqueous electrolyte lithium secondary battery including a positive electrode containing a lithium-manganese composite oxide, a negative electrode, and a non-aqueous electrolyte, and a positive electrode active material constituting the positive electrode Are LiMn ₂ O ₄ and LiMnO
A mixed oxide of a complex oxide of lithium and manganese (A) such as No. ₂ and MFeO4 (as Fe: VI, M: II valent metal) (B) is used. Thus, a high-capacity nonaqueous electrolyte lithium secondary battery having a high energy density and excellent cycle characteristics even at a high temperature can be obtained.

Further, the present invention includes a composite oxide _Li X _Mn Y _{O 4} with Lithium and manganese 6 ferric of ferrate MFe
The mixing ratio (B / A + B: wt%) of ferrate (VI) (B) in the mixed composite oxide (A + B) mixed with O ₄ is 3
The content is set to be up to 97%, because it is desired to suppress the dissolution of manganese even a little and to improve the energy density. For example, a spinel composite oxide LiMn ₂ O of lithium and manganese
_In the case of No. 4, Li [Mn ³⁺ _1.0 M
n ⁴⁺ _1.0 ] O ₄ , in which manganese at the octahedral site has trivalent and tetravalent cations in halves, and trivalent manganese ions undergo a disproportionation reaction to form trivalent manganese ions. Half become divalent manganese cations. That is, LiMn ₂
One- _fourth manganese becomes a divalent manganese cation in one mole of O ₄ . In order to convert all the divalent manganese cations into trivalent manganese cations, ferrate (VI) (Fe
(Hexavalent ions are converted into trivalent ions), which is only one-third, so that divalent manganese produced from 1 mol of lithium and manganese spinel complex oxide LiMn ₂ O ₄ is converted into trivalent manganese. Requires 1/12 mole of ferrate. LiMn ₂ O ₄ (A) 1 mol (molecular weight 18
In comparison with 1), potassium ferrate (B) becomes 1/12 mol of K ₂ FeO ₄ (VI) (molecular weight 192), so that the weight% (B) of potassium ferrate (B) in mixed mixed oxide (A + B) / A + B)
Is about 8%. That is, in order to prevent the dissolution of manganese, the dissolution of manganese can be greatly reduced by adding 8% or more, in practice, about 8 to 20% to the mixed double oxide. Actually, divalent manganese is oxidized to trivalent manganese and, at the same time, tetravalent manganese is generated at the same time, so it is necessary to add an excessive amount of ferrate.

The composite oxide of lithium and manganese is 4
In the V system, spinel LiMn ₂ O ₄ is often used, and in the 3V system, orthorhombic LiMnO ₂ is often used.
In order to improve cycle characteristics, etc., as described above, C
Although a third component such as o, Ni, Al, Cr, or B is added or solid-solved, or a non-stoichiometric compound or a defective compound is used, the present invention relates to a lithium manganese composite. Oxides are also included. Regarding ferrate (VI), potassium ferrate K ₂ FeO ₄ (VI) and barium ferrate Ba from the viewpoint of solubility in various solvents and synthesis methods.
FeO ₄ (VI) or lithium ferrate Li ₂ FeO ₄ (V
I) is preferred, but Na, Ca, Mg, Zn, Rb, C
Metal salts such as s and In can also be used. As the non-aqueous electrolyte, dimethyl ether, ethylene carbonate, diethyl carbonate, γ-butyrolactone, 1.4 dioxane, acrylonitrile, acetone, hexane, chloroform and the like can also be used. As the electrolyte, lithium perchlorate, lithium trifluoromethanesulfonate, lithium phosphate, lithium borofluoride, or the like is used.

[0016]

EXAMPLES Hereinafter, the present invention will be described with reference to Examples and Comparative Examples.
This will be specifically described. Here, as a composite oxide of lithium and manganese, LiMn ₂ having a spinel structure is used.
O ₄ , potassium ferrate K ₂ FeO ₄ (VI) as a ferrate,
The description will be made by taking lithium ferrate Li ₂ FeO ₄ (VI) as an example.

(Example 1) Spinel-type lithium manganese composite oxide LiMn ₂ O ₄ (A) and potassium ferrate K ₂ FeO ₄ (VI) (B), which are positive electrode active materials, are synthesized by the following methods.

LiMn ₂ O ₄ is used as a starting material for electrolysis 2
Manganese oxide and lithium hydroxide LiOH.H ₂ O
i / Mn = 1.00 / 2.00 to 1.05 / 2.00, mixed in air, at a temperature of 750 to 800 ° C,
The synthesis was performed in a firing time of 20 hours. This product was pulverized by a pulverizer and then classified to obtain LiMn ₂ O ₄ .

Potassium ferrate K ₂ FeO ₄ (VI) is used as a starting material with respect to 3 moles of potassium hypochlorite KClO produced by blowing chlorine gas into a cold caustic solution, and nitric acid
iron. Nonahydrate _{Fe (NO) 3 · 9H 2} O 2 molar temperatures 0
C. for 1 hour with good agitation.
10 to 15 mol (about PH11) slowly added to the solution,
This produces potassium ferrate K ₂ FeO ₄ (VI). The resulting potassium ferrate K ₂ FeO ₄ (VI) is filtered, washed twice with 1 molar aqueous potassium hydroxide solution, then washed with n-pentane, and the water is replaced with n-pentane to remove water. Further, the resultant is washed with methanol to remove impurities such as KOH, and then washed with diethyl ether, vacuum-suctioned and dried. The yield of potassium ferrate K ₂ FeO ₄ (VI) is about 75
%, The purity was 98%.

The positive electrode active material obtained as described above was prepared by adjusting the weight ratio of LiMn ₂ O ₄ having a spinel structure, graphite as a conductive agent, and polyvinylidene fluoride binder to 85/1.
LiMn ₂ O ₄ cathode mixture (a) mixed at a ratio of 2/3
And potassium ferrate K ₂ FeO ₄ (VI) as a ferrate and Li
The weight ratio of OH, graphite and the binder is 60/25/12.
A K ₂ FeO ₄ (VI) -based positive electrode mixture (b) mixed well at a ratio of / 3 is prepared. The two positive electrode mixtures a and b are combined with K ₂
The mixture is mixed at a ratio of FeO ₄ (VI) / LiMn ₂ O ₄ = 15/85 (weight ratio) to prepare a positive electrode mixture. This positive electrode mixture was dispersed in n-methyl-2-pyrrolidone to form a slurry of a positive electrode active material mixture, uniformly applied to both surfaces of an aluminum foil having a thickness of 20 μm, dried, and pressed to form a positive electrode. Created.

The negative electrode comprises 9 parts of a graphite material as an active material and a binder.
The mixture was mixed at a ratio of 0/10 (weight ratio) to prepare a negative electrode mixture. A negative electrode was prepared in the same manner as the positive electrode except that the applied metal foil was a copper foil.

The prepared strip-shaped positive electrode and negative electrode were wound many times through a separator made of a microporous polyolefin film to prepare a multi-turn electrode body. The multi-turn electrode was housed in a nickel-plated iron cylindrical battery can, and insulators were placed above and below the multi-turn electrode. The aluminum current collector lead was led out of the positive electrode and welded to the protrusion of the safety valve installed on the battery lid. On the other hand, a current collecting lead made of aluminum was led out of the negative electrode and was welded to the bottom of the battery can.

After injecting the electrolytic solution into this, the battery can was caulked through a sealing gasket to fix the battery lid, thereby producing a cylindrical battery having an outer diameter of 18 mm and a height of 65 mm. As the electrolytic solution, an electrolytic solution obtained by dissolving lithium perchlorate LiClO ₄ in a mixed solvent of ethylene carbonate and dimethyl carbonate (volume ratio of 1 to 1) at a concentration of 1 mol / l was used. This battery is referred to as a lithium secondary battery of Example 1.

Example 2 LiMn ₂ O ₄ having a spinel structure of a mixed cathode active material and potassium ferrate K ₂ Fe as a ferrate
O ₄ (VI) and K ₂ FeO ₄ (VI) / LiMn ₂ O
₄ was prepared in the same manner as in Example 1 except that the ratio was 50/50 (weight ratio). This battery was used as the lithium secondary battery of Example 2.

Comparative Example 1 K ₂ Fe was used as a positive electrode active material.
O ₄ (VI) / LiMn ₂ O ₄ = 0/100 (weight ratio)
Except that only LiMn ₂ O ₄ was used. This battery was used as a lithium secondary battery of Comparative Example 1.

Comparative Example 2 K ₂ Fe was used as a positive electrode active material.
O ₄ (VI) / LiMn ₂ O ₄ = 100/0 (weight ratio)
, Ie, except that only K ₂ FeO ₄ (VI) was used. This battery was used as a lithium secondary battery of Comparative Example 2.

Using the lithium secondary batteries prepared as described above in Examples 1 and 2 and Comparative Examples 1 and 2, the battery was charged to 4.2 V at a constant temperature of 50 ° C. and 0.2 C, The battery was charged for another 2 hours and then discharged from 4.2 to 3.5V. The ratio between the discharge capacity at the first cycle and the discharge capacity at the 100th cycle was determined and defined as the capacity retention.

After 100 cycles, the manganese ions dissolved in the battery electrolyte were measured, and the positive electrode active material LiMn ₂
The solubility of O ₄ in the manganese was measured. The manganese solubility is shown in Table 1 "Discharge capacity, cycle test result and manganese solubility".

As can be seen from Table 1, the spinel lithium manganate LiMn ₂ O ₄ is L
When iMn ₂ O ₄ alone was used as the positive electrode active material,
The initial discharge capacity is 125 mAh / g,
When the 0-cycle charge / discharge was repeated, the capacity retention was 64% at the 100th cycle, which means that manganese was dissolved by 57% of the manganese amount in LiMn ₂ O ₄ . But
As in Example 1, the mixed positive electrode active material composition LiMn ₂ O ₄ /
In the case of K ₂ FeO ₄ (VI), that is, when A / B = 85/15, the initial discharge capacity is 152 mAh / g, the capacity retention at the 100th cycle is 85%, and the manganese dissolution amount is 20%. Further, the LiMn ₂ O ₄ / K ₂ FeO _{4 of} Example 2 was used.
When (VI) = 50/50, the initial discharge capacity is 210 mA.
h / g, the capacity retention at the 100th cycle was 90%, and the manganese dissolution rate was 10%. As described above, in the mixed composite oxide positive electrode in which potassium ferrate was mixed with the lithium manganate composite oxide, the capacity retention was large, and the manganese solubility was significantly reduced.

[0030]

[Table 1]

The lithium secondary battery of the present invention is not limited to the starting materials, the production method, the positive electrode, the negative electrode, the electrolyte, the separator, the battery shape, and the like of the positive electrode active material described in the above embodiments. As the electrolyte, a solid electrolyte or the like can be used.

[0032]

As described above, according to the present invention, the positive electrode active material of a lithium secondary battery such as spinel lithium manganate has an irreversible capacity loss due to dissolution of manganese during use, and particularly at a high temperature of 50 ° C. or more. Is very problematic, but by mixing a ferrate (VI) such as potassium ferrate, irreversible loss of discharge capacity due to manganese dissolution can be greatly reduced. A high-capacity non-aqueous electrolyte lithium secondary battery with a higher discharge capacity and a lower price than lithium alone can be provided.

Claims (2)

[Claims] 1. A high-capacity nonaqueous electrolyte lithium secondary battery comprising a positive electrode containing a lithium manganese composite oxide, a negative electrode and a nonaqueous electrolyte, wherein the positive electrode constituting the positive electrode is LiMn ₂ O ₄ , A composite oxide of lithium and manganese (A) such as LiMnO ₂ and MFeO 4 (Fe:
VI, M: a high-capacity nonaqueous electrolyte lithium secondary battery characterized by being a mixed composite oxide with (B) (as a II-valent metal). 2. A mixing ratio of ferrate (VI) (B) in the mixed composite oxide (A + B) (B / A + B: wt%)
2. The high-capacity nonaqueous electrolyte lithium secondary battery according to claim 1, wherein