lithium iron phosphate

Every lithium ion cell has a positive electrode, the cathode, and what that electrode is made of sets the cell's cost, its safety and how much energy it can store. Lithium iron phosphate builds the cathode from iron and phosphate arranged in an olivine crystal structure, rather than from the layered nickel, cobalt and manganese oxides used in the higher energy chemistries common in electric cars. The iron, phosphorus and oxygen bonds in that structure are stronger than the metal and oxygen bonds in the layered oxides, and peer reviewed review work reports the consequence: the material shows no significant weight loss below 230 degrees Celsius, making it the more thermally stable of the two.

The trade is energy against everything else. Layered nickel cobalt manganese cathodes store roughly 160 to 220 watt hours per kilogram against 130 to 140 for lithium iron phosphate, so a car built on it carries more weight for the same range. In return the cells cost less, they tolerate heat better, and their cycle life reaches more than 2,000 charges. For a battery bolted to a pad beside a substation, weight is irrelevant and repeated cycling is the whole job, which is why this chemistry now accounts for around 90 percent of the world's new grid storage.