Novel solid state electrochemical cells
The behaviour of cells of the type Au/Solid Electrolyte/Au has been investigated. At high temperature (e.g. >400°C), they may be charged under an applied voltage of N0.5-1.5 V and the charge stored may be subsequently released on discharging. The amounts of charge stored and discharged were in the range 1 to 100 C g-lt this is 6 to 9 orders of magnitude larger than expected, since at lower temperature, such cells behave as blocking electrode systems with double layer capacitances of ml( )-6 F cm 2. The charging/discharging effect has been observed with various solid electrolytes including P-Alumina, both single crystal and polycrystalline, Li4SiO4 and Li2TiO31; the effects were largest in Li2Ti03. The processes involved in the operation of these cells have been studied in some detail for Li2Ti03 and Li4SiO4. It appears that during charging, electrochemical decomposition of the solid electrolyte occurred giving a) Li2C03 and Ti02, and b) Li2C03 and Li2SiO3, respectively. It was found that atmosphere, temperature and charging voltage have a large effect on the charging and discharging characteristics of the cells, but that surface roughness and the dimensions of the solid electrolyte pellet are less important. The open circuit voltage of charged cells was measured and by using a third reference electrode, the behaviour of the anode and cathode voltages was studied during charging, discharging and on open circuit. In order to check that the process and products of decomposition during charging had been correctly identified, primary cells were constructed, i.e. Au, Li2CO3/Li4SiO4/Li2SiO3, Au and Au, Li2CO3/Li2TiO3/Ti02i Au, and their discharge characteristics noted. These new cells have possible applications as i) thermal batteries and ii) gas sensors. The a.c. conductivity of the cells containing Li4SiO4 and Li2Ti03 was measured over a wide frequency range; in order to facilitate analysis of the data, the behaviour of several ideal circuits composed of resistance and capacitance elements, has been simulated. It was found that at high temperatures and low (relatively) frequencies the a.c. response of the cells was influenced greatly by chemical reactions, which occurred at the electrodes. Preliminary studies on the voltage dependence of the a.c. response were made and the presence in the equivalent circuit of the cells of voltage dependent resistances was observed. These results shed new light on the high temperature conductivity of Li4SiO4. They may also have wider implications for a.c. measurements on solid electrolytes, since Au electrodes are commonly used and are assumed to be blocking to the passage or discharge of ions.