Highly Conductive Electrolyte for Use in an Ambient Temperature Rechargeable Lithium Battery and Ambient Temperature Rechargeable Lithium Battery Including Said Electrolyte

This invention relates in general to ambient temperature, rechargeable, lithium batteries and in particular to the use of highly conductive electrolytes for the lithium batteries.

The use of organic electrolytes commonly employed in ambient temperature rechargeable lithium batteries has been hampered by the low conductivities of the electrolytes so that the cells can not be recharged at current densities above about 0.3 mA/cm.sup.2 be discharged at current densities above about 3 to 10 mA/cm.sup.2.

The general object of this invention is to provide a high conductivity electrolyte for use in an ambient temperature rechargeable lithium battery. A further object of the invention is to provide such an electrolyte for use in an ambient temperature rechargeable lithium battery that uses lithium intercalation compounds such as TiS.sub.2, WO.sub.2, MoO.sub.2 and graphite etc as anodes instead of lithium metal. A still further object of the invention is to provide such a battery wherein both the charge and discharge rates are higher than those obtained with conventional electrolytes used in lithium batteries. Another object of the invention is to provide such a battery that exhibits excellent cycle life.

It has now been found that the aforementioned objects can be attained by providing an electrolyte of 1.5 molar lithium hexafluoroarsenate (LiAsF.sub.6) in acetonitrile (AN) for use in a battery that includes lithium intercalation compounds such as TiS.sub.2, WO.sub.2, MoO.sub.2, VSe.sub.2, Li.sub.2 TiS.sub.2, Li.sub.2 Se.sub.2, lithiated graphite, lithiated carbon, lithiated petroleum coke etc as anodes and a more voltage positive lithium intercalating cathode such as LiCoO.sub.2 or LiNiO.sub.2.

In lieu of LiAsF.sub.6 as the electrolyte salt one may use other lithium salts that dissolve in acetonitrile such as LiAlCl.sub.4, LiClO.sub.4, LiBF.sub.4, and LiPF.sub.6.

As the organic solvent of the electrolyte, it is preferred to use acetonitrile alone, but the invention also contemplates the use of acetonitrile together with other solvents that are not compatible with metallic lithium.

FIG. 1 is the plot of the conductivities of an electrolyte of 1.5 molar LiAsF.sub.6 in AN.

FIG. 2 shows the discharge curves at current densities of 2 to 20 mA/cm.sup.2 for the cell Li.sub.x TiS.sub.2 /1.5 M LiAsF.sub.6 in AN/Li.sub.1-x CoO.sub.2 where X has a value between 0 and 1.

The electrolyte of 1.5 molar LiAsF.sub.6 in AN has a conductivity of 0.0501 SCm.sup.-1 at 21.degree. C. and increases to 0.0692 SCm.sup.-1 at 61.degree. C. The conductivities of this electrolyte is plotted in FIG. 1 as a function of temperature. These conductivity values are higher than the conductivity of organic electrolytes presently used in ambient temperature rechargeable lithium batteries. Unfortunately, this highly conducting electrolyte can not be used in conventional lithium batteries because the lithium anodes react spontaneously with the acetonitrile solvent. The highly conducting electrolyte solutions can, however, be used in cells that use lithium intercalation compounds such as TiS.sub.2, WO.sub.2, MoO.sub.2, VSe.sub.2, Li.sub.2 TiS.sub.2, Li.sub.2 Se.sub.2, lithiated graphite, lithiated carbon, or lithiated petroleum coke as the anode instead of lithium metal and a more voltage positive lithium intercalating cathode such as LiCoO.sub.2 or LiNiO.sub.2. The intercalation anodes are chemically stable in acetonitrile solutions unlike lithium metal and allow the fabrication of ambient temperature rechargeable lithium cells that can be charged and discharged at higher rates than the conventional lithium batteries.

A cell is then made using the highly conducting acetonitrile solutions as electrolyte. The cell corresponds to Li.sub.x TiS.sub.2 /1.5 molar LiAsF.sub.6 - AN/Li.sub.1-x CoO.sub.2 where X has a value between 0 and 1.

Thin films of titanium disulfide and lithium cobalt oxide are deposited on an aluminum substrate by chemical vapor deposition and sprayed sol gel techniques, respectively. The cell is fabricated and cycled in a glove box. The cell exhibits an open circuit potential of 2.1 volts and can be discharged at current densities up to 20 mA/cm.sup.2. The discharge curves at current densities of 2 to 20 mA/cm.sup.2 are shown in FIG. 2. The cell can also be charged at a current density of 2 mA/cm.sup.2. Both the charge and discharge rates are higher than those obtained with conventional electrolytes used in lithium batteries. The cell also exhibits excellent cycle life and delivers 300 cycles.

We wish it to be understood that we do not desire to be limited to the exact details of construction shown and described for obvious modifications will occur to a person skilled in the art.