JPH021353B2 - - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to a lithium secondary battery that is small in size and has a large charge/discharge capacity, and more particularly to a battery that uses lithium as a negative electrode active material.

Many proposals have been made for high energy density batteries that use lithium as a negative electrode active material, for example, batteries that use graphite and fluorine intercalation compounds as positive electrode active materials and lithium metal as negative electrode active materials are known. (See US Pat. No. 3,514,337). In addition, lithium batteries using graphite fluoride as a positive electrode active material (manufactured by Matsushita Electric) and lithium batteries using manganese dioxide as a positive electrode active material (manufactured by Sanyo Electric) are already on the market. However, these batteries have the disadvantage that they are not rechargeable and cannot be used as secondary batteries.

In addition, secondary batteries using sulfides, selenides, and tellurides of titanium, zirconium, hafnium, niobium, tantalum, and vanadium as positive electrode active materials (see U.S. Pat. No. 4,089,052), chromium oxide, niobium selenide, etc. Secondary battery using [J.Electrochem.Soc, vol.124.No.7
p. 968 and p. 325 (1977)], but these batteries could not necessarily be said to be sufficient due to their battery characteristics.

The present invention has been made in view of the current situation, and its purpose is to provide a secondary battery that is small in size and has excellent charge/discharge characteristics.

To summarize the present invention, in the battery of the present invention, the positive electrode active material is MX ₅ (M: Zr, Hf; X: Te),
The negative electrode active material is lithium, the electrolyte is chemically stable with respect to the positive electrode active material and lithium,
The present invention is a lithium- _MX5 secondary battery characterized in that lithium ions are a substance that moves to perform an electrochemical reaction with a positive electrode active material.

According to the present invention, the positive electrode active material has the general formula:
By using the compound represented by MX ₅ , a secondary battery that is small and has good charge and discharge characteristics can be produced.

The present invention will be explained in more detail.

As described above, the positive electrode active material used in the lithium- _MX5 secondary battery according to the present invention is _MX5 (However, M
is a compound represented by Zr and/or Hf, X represents Te).

This compound is shown in Figure 1 (S. Okada et al. J. phys.
Soc.Japan vel.49, August (1980) P839)
As shown in the figure, it has a unique structure in which zirconium or hafnium and tellurium are intertwined with a one-dimensional triangular prism chain structure and a two-dimensional layered structure of tellurium. It is particularly high among low-dimensional materials characterized by a layered anisotropic crystal structure (more than 700Ω _-1 cm ^-1 at room temperature).

That is, in transition metal chalcogenides
The one-dimensional needle-like structure of MX ₂ and the two-dimensional layered structure of MX ₃ are relatively popular, and these structures are common to more than a dozen types of transition metal chalcogenides.

On the other hand, MX ₅ has an acicular and layered hybrid structure in which the acicular "MX ₃ " chains are linked in a layered manner by the remaining "X-X" molecules, and to date, fiber metal chalcogenides with this structure have In, ZrTe ₅
It has a unique structure, of which only two types, HfTe 5 and HfTe ₅ , have been found.

Here, the “MX ₃ ” chain in Figure 1B is
The triangular prism part shown by the chemical formula of MX ₃ (shown as B')
The "X-X" molecule is the part shown in "B".

MX ₅ has a hybrid structure that combines a one-dimensional "MX ₃ " chain and a two-dimensional "XX" layer.

In Figure 1, white circles and black circles indicate x=0 and x=0.5.
(1/2height), and the inserted Li is in the van der Waals gap area shown by the shaded area in the figure.

When forming a positive electrode using a compound represented by MX ₅ as the positive electrode active material in the present invention, the positive electrode is made of powder of the compound represented by MX ₅ or a mixture of these and a binder powder such as polytetrafluoroethylene. It is pressure-molded into a film shape on a support such as stainless steel. Alternatively, a conductor powder such as acetylene black may be mixed with the compound powder represented by MX ₅ to provide more conductivity, and in some cases, a binder powder such as polytetrafluoroethylene may be further added to the mixture. It is formed by placing the mixture into a metal container, or by pressure-molding the mixture onto a support such as nickel or stainless steel.

Lithium, which is the negative electrode active material, is formed in the form of a sheet, as in general lithium batteries, or by pressing the sheet onto a conductor network made of nickel, stainless steel, etc. to form the negative electrode.

As the electrolyte, aprotic organic solvents such as propylene carbonate, ethylene carbonate, γ-butyrolactone, dimethyl sulfoxide, acedotril, formamide, dimethylformamide, nitromethane, and LiClO ₄ , LiAlCl ₄ ,
Known electrolyte materials commonly used in batteries using lithium as negative electrode active material, such as combinations with lithium salts such as TLiBF ₄ , LiCl, LiPF ₆ , LiAsF ₆ or solid electrolytes or molten salts with Li ⁺ as the conductor. can be used.

Furthermore, if necessary due to the battery structure, a diaphragm made of porous polypropylene or the like may be used.

Although the details of the aforementioned reaction between MX ₅ and Li ⁺ are still unclear, it is thought that Li ⁺ probably enters the Van der Waals gap (shaded area in Figure 1) of MX ₅ and reacts topochemically. That is, reversible positive electrode reactions can be roughly divided into two types: deproposition and topochemical reactions. In the former case, the positive electrode crystal structure changes significantly during the reaction, including its symmetry (for example,
(e.g., CuF ₂ ), whereas the latter undergo little or only slight structural changes (e.g., lattice constant elongation) (e.g., transition metal chalcogenides). Like other transition metal chalcogenides, MX ₅ also has a Van der Waals gap between its layers, and is a topochemical type positive electrode in which Li enters and exits during charging and discharging.

This van der Waals gap can be made by applying an initial charge or repeating charge/discharge cycles.
It is possible to increase the number of Li ⁺ accepting sites (positions), thereby increasing the discharge capacity and energy density. The Van der Waals gap, which is the insertion site for Li, becomes larger than the initial state after charge/discharge cycles, and as it becomes easier for Li to enter and exit, the discharge capacity becomes larger in the first few cycles than in the first cycle. There is. Of course, once a certain number of cycles is exceeded, the factors that cause deterioration of the positive electrode structure become more prevalent, and a decrease in capacity is unavoidable.

Next, examples of the present invention will be described, but the present invention is not limited to these in any way. In addition, in the examples, battery preparation and measurements were performed in a dry box under an Ar atmosphere.

Example 1 FIG. 2 is a sectional view of a coin-type battery that is a specific example of the battery according to the present invention, in which 1 is a stainless steel sealing plate, 2 is a polypropylene gasket, and 3 is a coin-type battery.
is a stainless steel positive electrode case, 4 is a lithium negative electrode,
5 is a polypropylene microporous separator, and 6 is a positive electrode mixture pellet.

ZrTe ₅ synthesized by the following method was used as _the positive electrode active material. As a starting material, Zr and Te were vacuum-sealed in a quartz tube with a composition of 1:5, and this mixture was heated at 450°C for 7 days to obtain the desired polycrystal of ZrTe ₅ (S. Turuseth et al., Actachem. .27
(1973) P2367). The obtained MX ₅ crystals were mixed with a conductive agent (acetylene black powder) and a binder (polytetrafluoroethylene) at a weight ratio of 78:18:4, and then roll-molded to form positive electrode mixture pellets 6 (thickness: 0.5 mm, diameter 17 mm, 250 mg/cell). first,
A metal lithium negative electrode 4 placed under pressure on a sealing plate 1 is inserted into the recess of the gasket 2, a separator 5 and a positive electrode mixture pellet 6 are placed in this order on the metal lithium negative electrode 4, and the electrolyte is 1 stipulation as
LiClO ₄ /propylene carbonate (PC) +1,2
- After injecting and impregnating an appropriate amount of dimethoxyethane (DME) [1:1 vol] (equal volume mixed solvent), a coin-shaped battery with a thickness of 2 mm and a diameter of 23 mm was fabricated by covering and caulking the positive electrode case 3. did. The discharge capacity is determined by the open circuit voltage (measured after discharging for 7 hours at 0.5 mA/cm ² and resting for 50 hours) and 0.5 mA/cm ² ,
Measurements were made at current densities of 1.5 mA/cm ² and 5 mA/cm ² . FIG. 3 is a diagram showing the results of the discharge test. Its discharge capacity (first discharge plateau) decreases as the discharge current density increases, but the first discharge plateau of the open circuit voltage corresponds to a reaction amount of 7 electrons per mole of ZrTe ₅ (260Ah/Kg). do.

This reaction amount can be estimated using the reduction model of each element of MX ₅ (DWMurphy et al. Journal of Crystal
Growth 39 (1977) P185) agrees with the predicted value of 7 electrons.

^{M 4+ (} _X ^{2- )} ₍ ^X ₂ ^{2- )} _{_} 1.7V) is considered to be the reduction potential of the electrolyte solvent propylene carbonate.

Example 2 The same battery was prepared in the same manner as in Example 1 using HfTe ₅ as the positive electrode active material, and a discharge test was conducted on this battery at a discharge current density of 0.5 mA/cm ² .

FIG. 4 shows the results of the discharge test together with ZrTe ₅ of Example 1. The discharge profiles of both are very similar, indicating that the _MX5 structure common to both plays an important role in this battery reaction.

Example 3 Using ZrTe ₅ as the positive electrode active material, the same battery was produced in the same manner as in Example 1, and a constant capacity cycle test was performed on this battery at a discharge current density of 0.5 mA/cm ² . Figure 5 shows 1 mole of ZrTe ₅
These are the cycle results for charge/discharge depth involving electrons, and similarly, FIGS. 6 and 7 are the cycle results for depths involving 2 electrons and 3 electrons, respectively. As the depth of charging and discharging increases, the number of cycles decreases rapidly, but 1
At the charge/discharge depth involving electrons, 180 cycles were possible.

〔Effect of the invention〕

As explained above, the battery of the present invention is small in size, has a large charge/discharge capacity, and has a long charge/discharge life.
Moreover, it has the advantage that it can be used in various fields as a secondary battery that can obtain a large current.

[Brief explanation of drawings]

Figure 1A is a (010) plane of the crystal structure of the compound represented by MX ₅ , Figure 1B is a projection of the crystal structure onto the (100) plane, and Figure 2 is a coin-type battery that is an embodiment of the present invention. Schematic cross-sectional diagrams of cells for measuring characteristics, Figures 3 and 4
The figure shows the discharge voltage change of the battery in Examples 1 and 2 of the present invention, and Figures 5, 6, and 7 show the number of charging/discharging cycles and the charging/discharging cycle of the battery in Example 3 of the present invention. FIG. The numbers attached to each curve here indicate the number of repetitions of charging and discharging. DESCRIPTION OF SYMBOLS 1... Sealing plate, 2... Gasket, 3... Positive electrode case, 4... Metal lithium negative electrode, 5... Separator, 6... Positive electrode mixture pellet.

Claims (1)

[Claims] 1 As a positive electrode active material, the general formula MX ₅ (where M is
Zr and/or Hf; 1. A lithium- _MX5 secondary battery comprising a material capable of transferring .