What is the rarest metal used in batteries?

The rarest metal used in batteries is often considered to be lithium, primarily due to its limited availability and the specific geological conditions required for its extraction. However, other rare metals like cobalt and nickel, which are also critical for battery production, can be quite scarce depending on market demand and geopolitical factors. While lithium is more commonly associated with lithium-ion batteries, the overall rarity can vary based on the specific battery chemistry used.