Research and development on the technology of solid state batteries has become an important field of enquiry in recent years. The history of the field goes back to the 1980’s when thin layers of positive active material, solid electrolyte and evaporated lithium were developed into the so-called micro-battery. These micro batteries were capable of very high cycle life and reasonable current densities, but the capacity is always low and the cost of production is high. The vacuum methods, however, resulted in thin, dense layers which had excellent interfacial contact with the layer above and below, which resulted in the good performance as well as the high cycle life. The discovery of much high conductivity solid electrolytes, some with good stability against lithium metal, has resulted in the renewal of interest in the field.

Research into these new, high conductivity materials has led to attempts to form thicker layers in more conventional methodology with the hopes of lower cost and obviating the interfacial problems of earlier work with thick layers. Eliminating porosity with such methods as well as eliminating ion blocking interfacial layers has proved to be a problem. Also, the original idea that lithium metal would be blocked from access to the positive electrode because of the non-porous solid electrolyte has also proven to difficult to attain. These aspects of the conventional approach will be explained.

More recent attempts have been made with the use of liquid electrolytes to contact the dense solid electrolyte layer as well as fill the porosity in the inherently porous pressed powder positive electrode and sometimes to serve as an intermediate layer between the solid separator and lithium foil. The results of such tests on “hybrid” electrolyte cells will be discussed and an attempt to predict the likelihood of success of such batteries will be made. The approach will also be compared to the more conventional all solid electrolyte systems that have been studied to date.