Vapour phase conversion of metal oxalates to metal phosphide nanostructures and their use as anode in rechargeable Li, Na and K-ion batteries

Abstract

We report a simple, vapour-phase conversion of two prominent metal oxalates into metal phosphide nanostructures (CoP and FeP) and their application in three alkali ion batteries (Li, Na and K). This method was a low temperature, scalable, single step conversion process using metal oxalates and sodium hypophosphite as originators. Apart from the vapour phase conversion reaction mechanism, structural, morphological, surface chemical, thermo-gravimetric and surface area analysis of these metal phosphides were also carried out. The metal phosphides was imaged using transmission electron microscopy which revealed a macroporous sheet like morphology for CoP and a mesoporous frame work for FeP particles which was further confirmed by BET. CoP and FeP nanostructures delivered reversible specific capacity of 265 and 360 mAh/g respectively for lithium ion battery at a current density of 250 mA/g for 500 cycles. For sodium ion battery, at a current density of 100 mA/g, CoP and FeP exhibited a specific capacity of 122 and 216 mAh/g respectively. For potassium ion battery, CoP and FeP exhibited a specific capacity of 73 and 113 mAh/g for 100 cycles. The alkali ion storage performances correlated well with their physicochemical and electrochemical properties while the surface and bulk storage contributions were also explored.