At present, the energy supply around the world comes from fossil fuels and is one of the main causes of greenhouse gas emissions. Attempts have been made to generate electricity using renewable sources, such as wind and solar, but it could be realized through the use of reliable electrochemical storage systems. Batteries can be used in combination with renewable energies to store energy when the demand is low and supply to the grid when demand is high. Among the known secondary batteries, Li ion batteries (LIBs) find applications ranging from small mobile electronics to transportation and even to stationary, grid storage systems. LIBs exhibit the highest power and energy densities over other known batteries. The most frequently used cathodes in LIBs include LiCoO₂, LiNi_1/3Mn_2/3Co_1/3O₁₂ and LiFePO₄ and graphite is the most popular choice for anode. Typical electrolyte used in the LIB consists of lithium salt (e.g., LiPF₆, LiBF₄ or LiClO₄) dissolved in an organic carbonate such as ethylene carbonate, diethyl carbonate or propylene carbonate.

The current LIB research is mainly focused on improving the practical gravimetric and volumetric energy densities by developing advanced Li ion electrolytes and electrodes. Attempts have been made to replace unstable polymer members with high temperature stable and highly conducting ceramic Li ion electrolytes, including Li₃N, Li-β-alumina, A-site deficient perovskite-type and garnet-type metal oxides. These solid electrolytes offer the advantages of being safe and compatible with advanced electrode materials, as well as a having a wide temperature range of operation and high Li ion conductivity. In this talk, recent advances in most promising garnet-type Li₅La₃M₂O₁₂ (M = Nb,Ta) oxides will be discussed. Since their discovery as Li ion conductors, there has been an increased interest in the development of garnet-type Li ion electrolytes for all solid-state LIBs. Some of the members of the garnet have been proven to be stable against chemical reaction with Li, as well as electrochemically stable up to 6 V vs. Li/Li⁺. These attributes, together with the good total Li ion conductivity, make them ideal electrolyte candidates for all solid-state LIBs.