Mechanical stress and morphology evolution in RuO2 thin film electrodes during lithiation and delithiation

RuO2 is a promising candidate for high-performance electrodes in thin film lithium-ion batteries, due to its large volumetric charge capacity, high rate capability, excellent cyclability and compatibility with silicon integrated circuit fabrication techniques. The evolution of mechanical stress associated with the lithiation and delithiation of RuO2 was investigated using in situ stress measurements, and the behavior was found to be fundamentally different from that of previously studied Si and Ge thin film electrodes. Most significantly, the tensile stress during delithiation of RuO2 was found to be very small, especially after the first cycle. The differences can be ascribed to the reversibility of SEI formation on RuO2, which does not occur in the case of Si and Ge. The decomposition of the SEI layer during delithiation allows freer interfacial sliding compared to Si or Ge, which accommodates the stress induced by significant volume changes during cycling. This might explain the relatively better cyclability of RuO2 films compared to Si or Ge films.