Recent safety incidents of mobile telephones underline the need for solid electrolytes in high performance batteries. In principle, the advantages offered by solid electrolytes include i) simpler device design and fabrication (facile shaping, patterning and integration); ii) easier miniaturization and no need for liquid containment; iii) better resistance to changes in conditions (e.g. shock vibration or temperature-pressure variation); iv) stability (non-volatile) and safety (non-flammable).
Among the requirements that must be reached for high performance battery operation, one can mention a high ionic conductivity, negligible electronic conductivity (negligible internal short-circuit) and sufficient mechanical stability (ceramics are generally fragile).
The interface compatibility with electrode materials (e.g. large contact area and similar thermal expansion coefficients to avoid delamination) and a high chemical and electrochemical stability (wide potential window) to avoid reductive or oxidative decomposition are further necessities. Solid Electrolyte Interphase (SEI) passivation layers can protect against decomposition, but they need certain electrolyte compositions to form spontaneously.
Electrochemical deposition is a versatile bottom-up technique for the à la carte synthesis of a wide variety of materials including thin layer functional materials. Conformal deposition allows designing objects with complicated shape, for example nano-architectured or nanostructured electrodes, which can be applied in miniaturized energy storage and conversion devices.
In the LIME Lab, nanosized cation-conducting solid polymer electrolytes were prepared by direct electrochemical synthesis on top of nanotubular TiO2. We explored the electrochemical deposition of single Li ion conducting solid polymer electrolytes based on polystyrene sulfonate (PSS) and p-sulfonated poly(allyl phenyl ether) (SPAPE). The successful deposition of a thin and homogeneous layer allows the realization of microbatteries with high areal insertion capacities and very good capacity retention especially at high cycling rates, studied up to 10 C. The resulting microbatteries showed high performance, including good capacity and excellent cycling at high rates. The areal energy and power densities are among the best reported in the literature.