Flame retardant polymer compositions comprising certain thermally stable salts of pyrophosphonic acid as flame retardant materials, which flame retardant materials comprise one or more than one metals, are prepared and have excellent flame retardant activity.

Flame retardant polymer compositions comprising certain thermally stable salts of pyrophosphonic acid as flame retardant materials, which flame retardant materials comprise one or more than one metals, are prepared and have excellent flame retardant activity.

A process produces zirconium phosphate by flame spray pyrolysis. A solution of at least one zirconium compound, an organic phosphate and a solvent with less than 10% by weight water is subjected to flame spray pyrolysis. Zirconium phosphate obtainable by this process finds application in batteries to encapsulate lithium mixed oxide particles.