Caption: 
SwRI categorized Sodium-ion and Lithium-ion  cell performance over a broad range of operating conditions. From this, the Institute developed an application-aware Electrical Interchangeability Index (EII), which is a numerical measure of how two battery technologies can satisfy the same application-specific electrical duty. This EII framework can help engineers compare whether two battery technologies can deliver and absorb the same application-specific power within their usage constraints. 