Uniform second Li ion intercalation in solid state ∈-LiVOPO_4

摘要

Full, reversible intercalation of two Li~+ has not yet been achieved in promising VOPO_4 electrodes. A pronounced Li~+ gradient has been reported in the low voltage window (i.e., second lithium reaction) that is thought to originate from disrupted kinetics in the high voltage regime (i.e., first lithium reaction). Here, we employ a combination of hard and soft x-ray photoelectron and absorption spectroscopy techniques to depth profile solid state synthesized LiVOPO_4 cycled within the low voltage window only. Analysis of the vanadium environment revealed no evidence of a Li~+ gradient, which combined with almost full theoretical capacity confirms that disrupted kinetics in the high voltage window are responsible for hindering full two lithium insertion. Furthermore, we argue that the uniform Li~+ intercalation is a prerequisite for the formation of intermediate phases Li150VOPO_4 and Li_(1.75)VOPO_4. The evolution from LiVOPO_4 to Li_2VOPO_4 via the intermediate phases is confirmed by direct comparison between O K-edge absorption spectroscopy and density functional theory.