JPH0732010B2 - Non-aqueous electrolyte secondary battery - Google Patents

Description

TECHNICAL FIELD The present invention relates to improvement of a non-aqueous electrolyte secondary battery, particularly a positive electrode thereof.

Conventional technology Until now, alkali metals such as Li and Na are used as a negative electrode active material, and γ-butyrolactone, tetrahydrofuran,
A secondary battery using a so-called non-aqueous electrolyte in which LiClO ₄ , LiBF ₄ , LiCl and the like are dissolved as a solute in a solvent such as propylene carbonate and dimethoxyethane is under development.

However, this type of secondary battery has not yet been put to practical use.

The reason is that the life of charge and discharge is short and the charge and discharge efficiency at the time of charge and discharge is low, and the cause of this performance deterioration is mainly chemical or physical reversible charge and discharge of the positive electrode and the negative electrode active material. It is a decrease in sex.

As a positive electrode active material for a non-aqueous electrolyte secondary battery, oxides and chalcogen compounds such as Ti, V, Cr and Mo having a layered structure or a tunnel structure have been known so far.

In the compounds having these structures, alkali metal ions such as Li ⁺ go in and out of the compound layer or tunnel when the battery is charged and discharged. Therefore, the crystal structure of the compound itself merely expands and contracts, and the structure is not significantly destroyed. Therefore, the compound is suitable for a positive electrode active material for a secondary battery.

By the way, MnO ₂ is applied to a non-aqueous electrolyte primary battery as a positive electrode active material having a high voltage and a large discharge capacity, that is, a high energy density, and is widely used as a power source for small electronic devices. MnO ₂ has a rutile type crystal structure and has the above-mentioned tunnel structure. As the battery discharges, alkali metal ions such as Li ⁺ enter and move into this tunnel, causing so-called insertion reaction.

During this discharge process, the crystal structure of MnO ₂ expands, but the crystal structure itself is not destroyed. Therefore, in the discharge process, Mn
Alkali metal ions that have penetrated into O ₂ can easily move in the MnO ₂ tunnel. Nevertheless, it is difficult to extract alkali metal ions from MnO ₂ by charging. Therefore, when MnO ₂ is used in a secondary battery, the charge / discharge capacity deteriorates significantly due to charge / discharge cycles,
Due to its short life, MnO ₂ is said to be unsuitable as a positive electrode active material for secondary batteries.

Conventionally, in a primary battery using MnO ₂ as a positive electrode active material, MnO ₂ has been used as a means for improving the performance of this type of battery.
A method using a mixture or compound of O ₂ and other various oxides as a positive electrode active material has been proposed. For example,
There are a mixture of MnO ₂ and chromium oxide (JP-A-59-66062) and a complex oxide of Mn and Cr (59-132568).

Problems to be Solved by the Invention However, with such a configuration, the charge / discharge life cannot be extended even when a charge / discharge cycle is tried as a secondary battery. This is for the following reason.

In other words, as a result of considering the cause of deterioration of the charge / discharge cycle,
This is a decrease in electron conductivity of MnO ₂ particles themselves due to the discharge, it found to be a current collector failure due to separation of the conductive material particles ₂ such as particles of carbon black MnO due to shrinkage of the MnO ₂ particles during charging It was In particular, the separation of MnO ₂ particles and conductive agent particles is because Li _x MnO ₂ generated by discharge is close to an insulator.
Charging is not performed effectively and the cycle characteristics are significantly affected.

In the above-mentioned first example (Japanese Patent Laid-Open No. 59-66062), the positive electrode active material is a simple mixture of MnO ₂ and chromium oxide, and is not a modification of MnO ₂ itself. Therefore, when the charge / discharge cycle is performed, the capacity deterioration due to the MnO ₂ portion is remarkable. In another example (Japanese Patent Laid-Open No. 59-132568), it is a composite oxide of Mn and Cr and has a tunnel or layered structure peculiar to MnO ₂ which is a condition under which a charge / discharge cycle can be favorably performed. Absent. Therefore, this active material is useful only as a primary battery, and is not suitable as a positive electrode active material for a secondary battery.

The present invention eliminates such conventional drawbacks, with a simple configuration, even if repeated charging and discharging cycle,
It is an object of the present invention to provide a highly reliable non-aqueous electrolyte secondary battery having excellent charge / discharge behavior by producing a positive electrode active material having a small separation from conductive agent particles.

Means for Solving the Problems The non-aqueous electrolyte secondary battery of the present invention has a positive electrode, an alkali metal ion conductive non-aqueous electrolyte, and a negative electrode having an alkali metal as an active material as constituent elements. Active material becomes MnO ₂
CrO ₂ is a solid solution oxide whose crystal form is MnO ₂
Is the same shape, Cr / Mn atomic ratio in the solid solution is 0.02 ~ 0.2
And the peak of the X-ray diffraction (Kα ray of Cu) of the solid solution is shifted to the high angle side by 0.1 to 0.4 degrees from the diffraction peak of MnO ₂ .

When the positive electrode active material of the present invention is used, the degree of expansion and contraction of the active material particles due to the penetration and release of alkali metal ions such as Li ^{+ during} charge and discharge is small, and therefore, with the conductive agent particles such as carbon black. There is little separation. Therefore, even if the charge / discharge cycle is repeated, the current collection state is good and cycle deterioration is suppressed.

The above action is obtained when the Cr / Mn atomic ratio is 0.02 to 0.2, there is no action when it is less than 0.02, and when it exceeds 0.2, Mn and Cr
Therefore, it is useful only for primary batteries and is not preferable for secondary batteries from the viewpoint of cycle characteristics.

EXAMPLES Hereinafter, the present invention will be described in detail with reference to Examples.

The positive electrode oxide serving as a test electrode was obtained by mixing a predetermined amount of MnO ₂ and CrO ₃ , adding water and stirring until a paste was formed, and heating this in air at 250 ° C. for 10 hours. When firing at a temperature or time higher than this, a mixed oxide of Mn and Cr is formed, and a mixed crystal cannot be obtained. The starting material MnO ₂ has a γ
The type used was. X-ray diffraction after heat treatment with CrO ₃ (Cu
Kα line), all peaks were shifted to the high angle side by about 0.1 to 0.4 degrees as compared with that of MnO ₂ which was heat-treated under the same conditions without adding CrO ₃ . And CrO ₃ , Cr ₃ O ₈ , Cr ₂
No oxides of Cr alone such as O ₅ were found.
It is considered that the single oxide was incorporated into the crystal lattice of MnO ₂ to form a solid solution. This is also called a mixed crystal,
Chromium oxide is a solid solution in MnO ₂ crystal,
It is distinguished from the mixture of MnO ₂ and chromium oxide, or eutectic. The crystalline form of the chromium oxide solid solution into the MnO _2, the peak of X-ray diffraction is only shifted MnO ₂
Be inferred.

In addition, β-type MnO ₂ and γ / β that is an intermediate between γ-type and β-type
It is almost the same when β-type MnO ₂ is used as the starting material, but γ
The type using MnO ₂ of the type as a starting material had the best battery characteristics.

In addition, all the tests were performed on the flat type battery.

The above products, carbon black, and tetrafluoroethylene resin were mixed in a weight ratio of 100: 5: 10. Mixture 200
mg was molded and crimped into a battery case in which an expanded metal current collector of titanium was spot-welded. The diameter of the electrode plate is 17.5
mm. A 0.38 mm-thick metallic lithium was used as the negative electrode, and a nickel expanded metal current collector was pressure-welded to a spot-welded sealing plate.

For the electrolyte, a mixture of propylene carbonate and dimethoxyethane in an equal volume ratio was added at a ratio of 1 mol / l.
It was prepared by dissolving the LiClO _4. Further, polypropylene non-woven fabric was used for the separator in order to prevent an internal short circuit due to dendrite generated in the metal lithium electrode.

The battery used as a comparative example also had a γ-type positive electrode active material.
The configuration was similar except that MnO ₂ was used. The battery thus configured was charged / discharged at a constant current of 2 mA and in a voltage range of 2.0 to 3.8 V.

FIG. 1 shows a mixed crystal of MnO ₂ and CrO ₂ (Cr
When A / Mn atomic ratio = 0.1) is used as the positive electrode active material, A
6 is a plot of the discharge capacity in each cycle of B when γ-type MnO ₂ which is a comparative example is used. From this, it can be seen that the battery of the present invention has a small discharge capacity at the beginning of the number of cycles, but the discharge capacity in each cycle is substantially constant, and is excellent in reliability.

FIG. 2 is a diagram in which the cycle deterioration amount is plotted when the Cr / Mn atomic ratio is changed. The cycle deterioration amount here was calculated by the following equation.

Cycle deterioration amount = (Fig. 10 cycle discharge capacity-second cycle discharge capacity) / 9 From this, it can be seen that the cycle deterioration amount decreases when the Cr / Mn atomic ratio is 0.02 or more, and when it exceeds 0.2, cycle deterioration The amount increases again. This is because, as described above, when the Cr / Mn atomic ratio exceeds 0.2, a composite oxide of Mn and Cr is formed.

Effects of the Invention As described above, according to the present invention, the charge / discharge behavior of MnO ₂ can be remarkably improved, and a highly reliable non-aqueous electrolyte secondary battery with stable charge / discharge capacity can be obtained.

[Brief description of drawings]

FIG. 1 is a diagram in which the discharge capacities of the nonaqueous electrolyte secondary batteries of Examples of the present invention and Comparative Examples are plotted, and FIG. 2 shows the cycle deterioration amount with respect to the Cr / Mn atomic ratio. It is the plotted figure.

Claims (1)

[Claims] 1. A positive electrode, an alkali metal ion-conducting non-aqueous electrolyte, and a negative electrode having an alkali metal as an active material as constituent elements, and the active material of the positive electrode is an oxidation of CrO ₂ dissolved in MnO ₂ as a solid solution. The crystal form of which is the same as that of MnO ₂ , the Cr / Mn atomic ratio in the solid solution is 0.02 to 0.2, and the peak of X-ray diffraction (Cu Kα line) of the solid solution is MnO _2. The non-aqueous electrolyte secondary battery is characterized in that it is shifted to the high angle side by 0.1 to 0.4 degrees from the diffraction peak of.