Mo.sub.6 Se.sub.6 Solid State Electrode for Secondary Lithium Batteries

The invention will be more fully understood by reference to the following detailed description taken in conjunction with the accompanying drawing wherein:

FIG. 1 is an exploded view of a non-aqueous secondary lithium cell in accordance with the invention;

FIGS. 2(a) through 2(d) are graphical representations on coordinates of lithium atoms (x) in Li.sub.x Mo.sub.6 Se.sub.6 against voltage showing the cycling characteristics of lithiated Mo.sub.6 Se.sub.6 at a current density of 1.0 mA/cm.sup.2 with known amounts of pure Mo.sub.6 Se.sub.6 ;

FIG. 3 is a graphical representation on coordinates of lithium atoms (x) in Li.sub.x Mo.sub.6 Se.sub.6 against voltage in volts showing the reversibility of the Li.sub.x Mo.sub.6 Se.sub.6 system during 10 cycles at varying voltage, charging and discharging being effected at 1.5 mA/cm.sup.2 ; and

FIG. 4 is a graphical representation on coordinates of lithium atoms (x) in Li.sub.x Mo.sub.6 Se.sub.6 /Li.sub.9 Mo.sub.6 Se.sub.6 against voltage in volts showing the reversibility of a cell including a Li.sub.x Mo.sub.6 Se.sub.6 cathode and Li.sub.9 Mo.sub.6 Se.sub.6 cathode during the 9th and 10th cycles of cycling at a current density of 1 mA/cm.sup.2.

With reference now more particularly to FIG. 1, there is shown an exploded view of a typical secondary lithium battery of the invention. Shown is cathode 11, in powder form, disposed upon stainless steel disc 12, anode 13 and filter paper 14 which has been soaked in a suitable electrolyte such as LiAsF.sub.6 in 2 methyl tetrahydrofuran. The structure also includes polypropylene fittings 15 and 16, steel disc 17, fitting 18, spring 19, plunger 20, stainless steel rod 21 and cap screws 22 and 23. The fittings, when compressed, provide an air tight ambient for the described battery. In order to prevent electrochemical contact between plunger 20 and the various fittings in the battery, it is advantageous to coat the plunger with a suitable protective film.

In the fabrication of a lithium battery in accordance with the invention, the initial step involves the preparation of the binary molybdenum chalcogenide, Mo.sub.6 Se.sub.6. Molybdenum selenide is a known composition which may be prepared in accordance with conventional commercial methods. Thus, for example, Mo.sub.6 Se.sub.6 may be synthesized by oxidation of In.sub.2 Mo.sub.2 Se.sub.6 in a hydrogen chloride ambient at 450.degree. C. Prior to use, the Mo.sub.6 Se.sub.6 is baked in a vacuum to ensure the removal of gas molecules which may be trapped between particles.

The Mo.sub.6 Se.sub.6 so obtained may then be used for the cathode 11 in the preparation of a structure of the type shown in FIG. 1, wherein lithium is used as the anode 13. Specifically, electrochemical swagelock test cells are prepared in a helium atmosphere using a lithium metal disk anode separated from a molybdenum selenide cathode by suitable porous glass paper soaked in an electrolyte such as 1 molar LiAsF.sub.2 in 2 methyl tetrahydrofuran. The cells so obtained may then be observed to determine their behavior during discharge to various voltages as a function of the change in lithium atom content per formula unit of Mo.sub.6 Se.sub.6.

With reference now to FIG. 2(a), the discharge curve, designated (1), reveals that when discharge is initiated at 2.5 volts the lithium atom content in the formula Li.sub.x Mo.sub.6 Se.sub.6 is 0.3. As discharge continues down to 1.0 volt, the amount of lithium used increases to 2.3 atoms per formula unit. Surprisingly, the cell upon recharging (Curve 2) is reversible and with a voltage of 2.4 volts the lithium content returns to 0.3 atoms per formula unit. Similar behavior is observed with the cells evaluated in FIGS. 2(b), 2(c) and 2(d) wherein discharge was effected from 2.42, 2.9 and 2.5 volts, respectively, to 0.6, 0.5 and 0.4 volts, respectively. It will be noted that the amount of lithium used, that is, lithium introduced into Mo.sub.6 Se.sub.6 increases to 3.2, 5.0 and 7.0 atoms per formula unit in Li.sub.x Mo.sub.6 Se.sub.6. Studies of multiple cycling of these cells revealed a loss in capacity of approximately 10% after the first cycle. However, recharging to 100% of capacity was effected for 15 cycles down to 1 volt, 10 cycles down to 0.6 volt and 2 cycles down to 0.5 volt.

The loss in capacity sustained after the first cycle is not unexpected since this behavior is frequently noted in secondary lithium cells. This is attributed to the presence of a small amount of intercalated lithium trapped by defects which remain in the structure after recharging the battery.

In order to further evaluate the reversibility of the Li.sub.x Mo.sub.6 Se.sub.6 system at low potential, a cell after being cycled ten times from 2.7 to 0.6 volts was discharged to 0.25 volts and then recycled several times. FIG. 3 reveals that the cell recharged from 0.3 to 0.8 volt (cycle a) and discharged without losing its capacity. However, over subsequent cycles between 0.25 and 0.80 volt, designated first, second and third, a loss in capacity is noted. This behavior is attributed to electrolyte decomposition and/or reaction with cell containers normally encountered at these low potentials.

The following exemplary embodiment which is set forth solely for purposes of exposition, describes the characteristics of Li.sub.x Mo.sub.6 Se.sub.6 /Mo.sub.6 Se.sub.6 cells.

The electrochemical behavior of a Li.sub.x Mo.sub.6 Se.sub.6 /Mo.sub.6 Se.sub.6 cell was studied using 41 milligrams of Mo.sub.6 Se.sub.6 (cathode) and 20 milligrams of Li.sub.6 Mo.sub.6 Se.sub.6. The cell was cycled with a current density of 1 mA/cm.sup.2 between 1.9 and 0.2 volt. The reversibility of the intercalation reaction was noted as well as a small loss in capacity after several cycles (10% after 10 cycles). However, the capacity is low since the narrow range of potential permits the Mo.sub.6 Se.sub.6 to take up only 1.2 lithium atoms.

A larger discharge potential requires a lithiated phase with a greater lithium content. Thus, Li.sub.9 Mo.sub.6 Se.sub.6 was obtained by discharging a Li/Mo.sub.6 Se.sub.6 cell to 0.25 volt. Then, the cell was opened and the lithium replaced by fresh Mo.sub.6 Se.sub.6. The resulting Li.sub.9 Mo.sub.6 Se.sub.6 /Mo.sub.6 Se.sub.6 cell, containing equal amounts of cathode and anode material after charging and recharging behaves in the manner shown in FIG. 4. 22 Milligrams of Li.sub.9 Mo.sub.6 Se.sub.6 and 22 milligrams of Mo.sub.6 Se.sub.6 were employed, cycling being effected at a current density of 1 mA/cm.sup.2. The ninth cycle (solid curve) and tenth cycle (dashed curve) are shown. The reversibility of the cell without significant loss in capacity on cycling enables Li.sub.x Mo.sub.6 Se.sub.6 to be substituted for lithium as the anode in secondary lithium cells based on Mo.sub.6 Se.sub.6. The loss in capacity is of particular interest since it is less than that measured after ten cycles with identical test cells using TiS.sub.2 cathodes.

Based upon the foregoing, it is apparent that Mo.sub.6 Se.sub.6 can be used as both the cathode and anode in secondary lithium cells, the Mo.sub.6 Se.sub.6 being capable of accepting nine lithium atoms reversibly without adversely affecting its linear chain structure. This results in a theoretical volume density of about 1.1 Wh/cm.sup.3 compared to 1.2 for TiS.sub.2. Accordingly, the practical utilization of non-aqueous secondary lithium cells is now attainable.

While the invention has been described in detail in the foregoing specification, the aforesaid is by way of illustration only and is not restrictive in character. It will be appreciated by those skilled in the art that the processing parameters may be varied without departing from the spirit and sense of the invention. Modifications which will readily suggest themselves to persons skilled in the art are all considered within the scope of the invention, reference being had to the appended claims.