JPH0329135B2 - - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to a material used as a positive electrode of a storage battery, particularly a secondary battery. An object of the present invention is to obtain a material that, when used as a positive electrode of a secondary battery, exhibits a high degree of reversibility during repeated charging and discharging of the battery. Another object of the invention is to obtain a material that is easy to manufacture and relatively inexpensive for use in battery positive electrodes.

The inventors have discovered that lithium molybdenum disulfide (LIxMoS ₂ ) compounds exhibit several different stages ("states") of behavior when used as positive electrodes in batteries with lithium negative electrodes.

During discharge of a newly formed battery (referred to by the inventors as "state 1"), lithium cations interrupt the positive electrode and increase the concentration of lithium in the positive electrode. The inventor has discovered that the voltage of a battery decreases during discharge to a special point from which a plateau begins. The plateau represents a region where the voltage remains constant but the lithium concentration in the positive electrode continues to increase. Once a specific lithium concentration in the positive electrode is reached, the battery continues to discharge in what the inventors call "state 2." In the state 2 region, as the lithium concentration in the positive electrode decreases or increases, the potential of the cell increases or decreases reversibly within a certain range. The state 2 region overlaps the plateau where the transition from state 1 to state 2 occurs in that the lithium concentration in the positive electrode in state 2 is equal to the concentration observed during the transition from state 1 to state 2, but the concentration Equality is at a voltage higher than the voltage at which the transition occurs. state 1
The transition between and state 2 does not appear to be reversible along the plateau. State 2 is the preferred state of operation since excellent reversibility is observed in cells made with positive electrodes conditioned to operate in state 2. As detailed below,
The first discharge to state 2 is at room temperature (e.g. about 20°C)
Although lower temperatures are generally preferred to effect the transition relatively quickly (this is necessary when the positive electrode is relatively thick).

When the potential of a battery operating in state 2 drops to a special level, a second plateau is reached, and the lithium concentration at the positive electrode remains constant until the battery potential reaches a third state ("state 3"). increase In state 3, the potential changes reversibly again as the lithium concentration increases or decreases. In state 3 operation, the lithium concentration decreases to overlap the lithium concentration values seen in state 2 and the plateau where the transition from state 2 to state 3 occurs. Batteries operating in state 3 are not as highly reversible as batteries operating in state 2 and will rapidly lose capacity with repeated charge and discharge cycles. However, since the energy density of state 3 is significantly higher than state 2, state 3 operation may be preferable to state 2 operation in some applications. It is to be understood that the term "reversible" as used herein does not imply complete or 100% reversibility.

The figure shows typical characteristics of a battery made with a lithium negative electrode and a positive electrode made of an aluminum foil substrate coated with molybdenum disulfide (MoS ₂ ). The quantity x represents the concentration of lithium in the positive electrode and increases as lithium cations enter the positive electrode during discharge of the cell. As will become clear below, the figures are exemplary, but it should be understood that the characteristics shown will vary somewhat in practice depending on various parameters.

The figure shows the battery discharging along path AB from an initial voltage of 3 volts or more (3.3 volts is generally standard). As the battery voltage decreases along this path, lithium ions enter the positive electrode, and the concentration of lithium in the positive electrode increases as shown. Path AB shows the voltage decreasing from an initial value of about 3.3 volts to a plateau (path BD), and x shows a corresponding increase from 0 to about 0.2. This was found to be typical at room temperature. However, it has been found that at low temperatures (e.g., 0 DEG C.), point B is at a point where x is slightly greater than 0, and path AB becomes much steeper than shown. In some cases, point B has been observed to reach approximately x=0.5 in room temperature discharge. However, this is thought to be due to decomposition of some electrolyte due to discharge or the presence of some impurity. The inventor defined the physical structure of the positive electrode as "state 1" in which the battery changes along path AB where point B is a point where the positive electrode lithium concentration ” he called.

As the voltage of a battery operating in state 1 decreases along path AB, it reaches a plateau represented by path BD. Although this plateau is shown as being at about 1.0 volts, in reality it is typically in the range of about 0.9-1.1 volts at room temperature. However, at extremely low temperatures it can be as low as 0.7 volts. This plateau is illustrated as starting at x ≈ 0.2, but on the grounds just discussed it actually starts at x
It is to be understood that x starts at a value slightly greater than 0 to approximately x=0.5. Also, although the plateau BD is illustrated as ending at approximately x=1.0, in reality this ending point is approximately x=1.0.
It was observed to occur up to a height of 1.5. Although the reason for this change is not clear, it is thought to be due to unknown impurities in the positive electrode. The illustrated plateau path BD represents a region where the cell operates at a relatively constant potential of about 1.0 volts, but the concentration of lithium for the positive electrode, denoted x, increases during discharge of the cell.

If a battery operating at plateau BD continues to discharge as shown, the battery will discharge along path DE when a positive electrode lithium concentration of approximately x=1.0 is reached. The discharge is stopped at any point on the path DE,
The battery can be recharged along path EC in a substantially reversible manner. The inventor calls the physical structure of the positive electrode in which the potential of the battery changes along the illustrated path CE depending on the lithium concentration of the positive electrode "state 2", and the reversible charging and discharging of the battery along the path CE is called "state 2". The process was called "state 2 action." Once state 2 operation is reached, the battery will not re-enter the BD plateau directly from state 2.

However, although the figure is representative,
Note again that in reality it changes. As shown, the path DE is 0.55 from an initial value of approximately 1.0 volts.
volts, while the value of x increases from about 1 to about 1.5. Similarly, path CE varies from about 2.7 volts to about
Shows voltage dropping to 0.55 volts. In reality, the observed value of x changed somewhat for a given voltage. For example, at point C (approximately 2.7 volts), x is 0
from a point slightly larger than (at low temperatures) about x
= in the range of 0.5. Point D (approximately 1.0 volts) is approximately x
= 1.0 to approximately x = 1.6. Point E (about 0.55 volts) ranges from about x=1.3 to about x=2.0.
However, in all cases the path CE is generally sloped to the lower right. The reason for such a change is not clear, but it can again be attributed to unknown impurities in the positive electrode. Also note that these are at room temperature. At lower temperatures the measured voltage will be somewhat lower for a given value of x. The voltage of 0.55 volts illustrated for point E (and path EG, discussed below) is typical at room temperature, but generally ranges from about 0.4 volts to about 0.6 volts.

The inventors have observed that the most reliable reversible battery operation occurs in batteries with the positive electrode conditioned to operate in state 2. Furthermore, we observed that the most reliable reversible state 2 behavior occurs along path CD. The battery operating in state 2 is the path
It has been found that when discharging along the CE, reversibility worsens when the cell potential drops below about 1 volt. Since the reversibility of a State 2 battery deteriorates when the battery is discharged below about 1 volt, the inventor monitors the battery to ensure that the voltage does not rise above about 2.7 volts and prevents the battery from discharging below about 1 volt. It is recommended to prevent the state 2 operation from occurring and limit the state 2 operation to path ED.

When the potential of a cell with a positive electrode in state 2 is reduced along path CE to about 0.55 volts (this voltage is typical as discussed above), a second plateau is reached, represented by path EG (typical (at a positive electrode lithium concentration of approximately x = 1.5 in the diagram). The transition from state 2 to state 3 occurs at a relatively constant potential along a second plateau. At this time, the lithium concentration in the positive electrode, represented by x, is approximately x
=2.8. Preferably, the value of x is maintained at about 3 or less in practicing the invention.

As a cell operating on the EG plateau continues to discharge, the cell will discharge along path GH when a positive lithium concentration of approximately x=2.8 is reached. The discharge can be stopped at any point along this path and the battery is then recharged by path HF. The inventor calls the physical structure of the positive electrode in which the lithium concentration of the positive electrode and the battery potential change along the path FH "state 3", and the reversible charging and discharging process along the path FH "state 3 operation". I called it. Once in state 3 operation, the battery will not reenter the EG plateau immediately after state 3.

Point H in the figure does not represent the lower limit of the battery's discharge capacity. However, the inventor observed a significant deterioration in the performance of cells discharged below about 0.3 volts of state 3. This deterioration is thought to be related to the fact that lithium ions diffuse into the aluminum substrate of the positive electrode to form a lithium-aluminum alloy.

The inventors believe that state 3 operation does not exhibit as reliable reversibility as state 2 operation. Furthermore, the inventors were not able to achieve as high a cell potential in state 3 as in state 1 or state 2 operation. However, as previously pointed out, this does not mean that state 3 operation is necessarily undesirable. The energy density of the battery in state 3 is that of state 2
considerably higher than that of Therefore, it is contemplated that State 3 operation may be selected as more desirable than State 2 operation in some applications where energy density requirements lend themselves to improved reversibility and voltage characteristics.

The inventor has found that when a battery operating in state 3 is slowly recharged, the potential reaches about 2.3 volts, and the battery enters state 3.
We discovered that there is a transition from state 2 to state 2.

The inventor has observed that the reversibility of the cell deteriorates in State 2 and State 3 operation when the cell potential drops below about 1 volt. This deterioration is thought to be due to decomposition of the battery's electrolyte. The inventors believe that if the problem of electrolyte instability can be overcome, superior reversibility can be obtained across the entire state 2 pathway, and improved reversibility can be obtained in state 3 operation. There is.

X-ray diffraction analysis conducted by the present inventor revealed that the state 2 structure has a crystal symmetry different from that of the state 1 structure (i.e., a structure in which Mo is octahedrally coordinated to sulfur). It turned out to be a layered compound.

Example 1 I made the battery as follows.

The positive electrode was coated with 3 mg/cm ² on approximately 6 cm ² of aluminum foil.
It has MoS ₂ attached. The negative electrode is a similarly sized lithium foil. The electrodes were separated by a polypropylene separator containing an electrolyte of 0.7 molar LiBr in propylene carbonate (PC). The cell was discharged from state 1 at 1 Å through a first voltage plateau of approximately 1.1 volts until the positive electrode changed to state 2. The cell was then charged and discharged over 100 times between 2.7 volts and 1.0 volts, corresponding to a fully charged state 2 positive electrode in state 2 at 10 Å. The capacity of the battery corresponded to one electron per _two MoS molecules (Δx=1).

Example 2 A battery was produced in the same manner as in Example 1 except that 0.3 mg/cm ² of MoS ₂ was attached to the positive electrode. The positive electrode takes the battery through the first voltage plateau to state 2, approximately
State 3 was entered through a second voltage plateau of 0.55 volts, and so on. The discharge current is 1
It was Å. The cell was then charged and discharged over 100 times at 1 Å between 2.4 volts (corresponding to a fully charged state 3 positive electrode) and 1.0 volts.
The capacity of the battery corresponded to 1.5±0.2 electrons per ₂ MoS molecules (Δx≒1.5±0.2). The battery was charged and discharged several times between 2.4 and 0.5 volts. The capacity of the battery in this case was equivalent to 2±0.2 electrons per ₂ molecules of MoS (Δx≒2±0.2).

Example 3 The battery of Example 2 (at about 100 microamps)
I slowly recharged it to 2.7 Porto. It was found that after recharging in this manner the positive electrode reconverted to state 2 operation.

Example 4 I made the battery as follows.

A 1.3 cm ² aluminum foil coated with 0.5 mg/cm ² of MoS ₂ was used as the positive electrode. The negative electrode is a similar area of lithium foil pressed onto expanded nickel grid. The electrode was suspended in a 50 ml electrolyte of 0.7 M LiBr and PC in a glass beaker. An argon atmosphere was confined within the beaker using a neoprene stopper. This battery was first
Discharge at 100mA, then in state 2
It was conditioned by cycling 82 times between 2.7 volts and 1 volt at 100 microamps.
Further discharge the battery to state 3 to 2.4 volts and 0.5
The bolt was cycled 10 times.

In the above examples, the first conditioning discharge was performed at room temperature (approximately 20°C) and good results were obtained.
It is generally considered desirable to cool the current for conditioning discharges. Otherwise, electrolyte decomposition problems will occur. Since the positive electrodes of the above examples were relatively thin, it was possible to conduct conditioning discharge relatively quickly at room temperature without creating large temperature gradients within the positive electrode. However, the thicker the positive electrode, the more important cooling becomes. At 10 mg/cm ² of MoS ₂ cooling appears to be essential. We have used a cooling temperature as low as -20℃, but the temperature at 0℃ is about 20℃.
It has been found to be very satisfactory for cathode thicknesses up to mg/cm ² MoS ₂ .

State operation can also be performed when MoS ₂ is partially oxidized to MoO ₂ . Partial oxidation to MoO ₂ can improve conductivity without significant loss in capacity.

Example 5 MoS ₂ partially oxidized to MoO ₂ as follows
They created a battery with a positive electrode containing .

(a) A film of a slurry obtained by mixing MoS ₂ powder with an average particle size of about 20 microns with propylene glycol in a 1:1 volume ratio was applied to an aluminum foil substrate.

(b) The film-coated substrate is baked at 580°C for about 10 minutes in an atmosphere containing about 0.4 mole percent oxygen in nitrogen to form about 20 mole percent MoO ₂ and about 80 mole percent MoS. We created a positive electrode containing ₂ .

The battery was constructed using two stainless steel flanges separated by a neoprene O-ring sealer. The negative electrode is a 6 cm ² sheet of lithium. (about
6 cm ² of the resulting positive electrode (on which 43 milligrams of partially oxidized MoS ₂ was deposited) was used as the positive electrode of the cell.
A separator sheet of polyporous polypropylene soaked in a 1 molar solution of lithium perchlorate in propylene carbonate was placed between the negative and positive electrodes.

The resulting cell was conditioned by initial discharge to a low cut-off voltage of approximately 0.85 volts at 4 Å. During this initial discharge, the battery voltage drops to a plateau of about 1 volt in about 20 minutes and then an additional 2 volts.
It dropped almost linearly to about 0.85 volts in time. This battery has a minimum voltage of about 0.85 volts at about 4 Å and about
It was cycled 66 times between charge and discharge to a maximum voltage of 2.7 volts.

[Brief explanation of the drawing]

The figure is a graph showing typical characteristics of a battery with a positive electrode coated with molybdenum disulfide (MoS ₂ ) on an aluminum foil substrate, a negative electrode of lithium foil, and an electrolyte of 1 molar LiCIO ₄ in propylene carbonate. The vertical axis is the battery voltage (expressed in volts);
The horizontal axis is the lithium concentration x in the positive electrode, which has the general formula _LIxMoS2 .

Claims (1)

[Claims] 1. A first molybdenum disulfide battery having a lithium negative electrode, a non-aqueous electrolyte, and a positive electrode containing MoS ₂
By discharging to a voltage plateau, further discharging the cell along the first voltage plateau, and further discharging the cell to a voltage below the first voltage plateau but greater than or equal to 0.3 volts, the chemical formula: LixMoS ₂ [ In the above formula, 0<x≦3, where x changes reversibly within the above range when the battery is charged and discharged within the range of battery voltage from 2.7 volts to 0.5 volts, and when the voltage is 0.8 volts. Lithium disulfide has a structure in which lithium ions are intercalated in MoS ₂ in a state selected from state 2 and state 3, where x exceeds 0.5 and becomes 3 or less (0.5<x≦3). A method for manufacturing a lithium molybdenum disulfide battery comprising a lithium negative electrode, a non-aqueous electrolyte, and a positive electrode comprising the lithium molybdenum disulfide material, characterized in that a molybdenum material is formed in the positive electrode. 2. By discharging the battery along the first voltage plateau and then discharging it to a voltage lower than the first voltage plateau, but not less than 0.6 volts, the chemical formula: LixMoS ₂ (where 0<x≦2, x When the above battery is charged and discharged between 2.7 volts and 0.8 volts, the positive electrode changes reversibly within the above _range . The manufacturing method according to claim 1, characterized in that the manufacturing method is produced by: 3 discharging the battery along the first voltage plateau, then discharging the battery to a second voltage plateau lower than the first voltage plateau, further discharging along the second voltage plateau, and then discharging the battery along the second voltage plateau; By discharging to a voltage lower than the plateau but above 0.3 volts, the chemical formula: LixMoS ₂ (where 0<x
≦3, x changes reversibly within the _above range when the above battery is charged and discharged between a voltage of 2.4 volts and 0.5 volts). The manufacturing method according to claim 1, characterized in that the material having the above-mentioned properties is produced as a positive electrode.