Cobalt-based lithium chemistries have a bad thermal resistance coefficient: as they heat up their resistance goes up which causes even more heating. That makes

Cobalt-based lithium chemistries have a bad thermal resistance coefficient: as they heat up their resistance goes up which causes even more heating. That makes them prone to thermal runaway. As they runaway they also trigger the same effect in their neighbors, both in the elements within a cell and between cells. As the Cobalt-Oxygen bond breaks down the Oxygen helps feed any fire present which is fun. The physical size of the Cobalt complex in the cathode is quite different from the "lithiated" vs "unlithiated" states so in the extremes Lithium batteries with cobalt "puff up": they get physically larger. This can physically crush its neighbor which can also lead to a chain reaction. LiFePO4 doesn't have either of these problems. Its physical size is very similar in the lithiated/unlithiated states so these batteries don't puff up or expand. They don't have thermal coefficient problems. And when severely abused the Iron-Oxygen bond is much stronger so they don't feed any potential fire with fresh oxidizer. The only real downside to ditching Cobalt is the lower density of LiFePO4... but for many applications that doesn't matter or is worth the tradeoff. tl;dr: you can over-volt, over-charge, over-discharge, puncture, smash, and/or heat a Lithium Iron Phosphate battery. It will both tolerate way more abuse without being ruined and in no case will it start a runaway reaction that sets the entire battery bank on fire. Lithium batteries with any Cobalt chemistry are way less likely to survive such abuse and way way way more likely to start a runaway fire chain reaction, resulting in a lithium fire.