Energy efficiency: a critically important but neglected factor in battery research

In fundamental studies of electrode materials for lithium-ion batteries (LIBs) and similar energy storage systems, the main focus is on the capacity, rate capability, and cyclability. The efficiency is usually judged by the coulombic efficiency indicating the electrochemical reversibility. As practical measures, power and energy densities are calculated for discharging, but less attention (if any) is paid to the energy efficiency. Although in the past the energy efficiency was almost close to unity for all electrode materials of LIBs, this factor is critically important for new high-density materials (e.g., based on conversion mechanism) since the energy density can be way below the requirements for the practical development. In fact, a low energy density is due to high overpotentials and is an essential part of the basic research for the material design because it cannot be improved during commercialisation. In large-scale energy storage devices such as batteries in electric vehicles (EVs) or household energy storage systems, the cost of energy consumed to charge the battery is a significant factor and is directly translated into the cost of the energy supplied by the storage device. The present paper clarifies how neglecting the energy efficiency has caused a fraction of research on LIBs to go astray in developing new electrode materials, which are not of practical interest because of huge overpotentials. Particular attention is given to conversion-based electrode materials, which are the common candidates for anode materials of lithiumion batteries and cathode materials of lithium-sulphur (Li-S) and similar batteries. Furthermore, the impact of nanostructures is considered.