The tradeoffs that rule out most chemistries
A robot's battery has to satisfy several competing demands at once: it needs to store a lot of energy in a small, light package, it needs to release that energy quickly enough to spin motors under load, and it needs to survive hundreds of charge cycles without its performance collapsing. Older chemistries fail at least one of these. Lead-acid batteries are cheap and rugged but so heavy per watt-hour that they eat into a mobile robot's payload before it even starts working. NiMH and NiCd batteries are lighter than lead-acid and tolerant of abuse, but their energy density is still well behind lithium chemistries, and NiCd carries the added baggage of toxic cadmium and a pronounced 'memory effect' that degrades capacity if it's habitually recharged before fully depleted.
Lithium Polymer (LiPo) and Lithium-ion (Li-ion) cells win on the metrics that matter most for robotics: energy density (watt-hours per kilogram) and power density (how fast they can safely discharge). LiPo cells use a soft pouch format and a gel-like polymer electrolyte, which lets manufacturers shape them into flat, lightweight packs and gives them very high discharge rates, making them the default choice for combat robots, drones, and anything with power-hungry motors. Li-ion cells use a rigid cylindrical or prismatic can and a liquid electrolyte, trading some peak discharge rate for higher energy density per cell and better long-term stability, which is why Li-ion dominates in laptops, e-bikes, and robots optimized for long runtime over peak power, like delivery or inspection robots.