A review of recent research on nonequilibrium solid solution behavior in LiXFePO4

LiXFePO4 (0 < X < 1) is one of the most well-studied cathode battery materials and is notable for its large miscibility gap. Although its phase-separating behaviors under equilibrium conditions have been well documented, recent research has shown that phase separation is suppressed at elevated rates of lithium insertion and removal. Specifically, LiXFePO4 exhibits a nonequilibrium solid solution behavior at elevated cycling rates. This article reviews recent research on nonequilibrium solid solution in LiXFePO4; these insights have been largely enabled by operando characterization techniques. Such studies have not only unambiguously confirmed the existence of this solid solution, but also show how surface reaction and diffusion kinetics ultimately affect phase separation and other spatially nonuniform lithiation and delithiation behavior.