Effect of metal ion and ball milling on the electrochemical properties of M0.5TiOPO4 (MÂ =Â Ni, Cu, Mg)

Various metal titanium oxyphosphates, M0.5TiOPO4 (M = Ni, Cu, Mg) were synthesized via modified solution route synthesis. The as synthesized M0.5TiOPO4 (M = Ni, Cu, Mg) were electrochemically tested using galvanostatic cycling, cyclic voltammetry, and rate performance measurements in order to investigate the effect of metal ion (M) on the electrochemical performance of this family of materials. All the studied materials reacted with 3 Li+ during the 1st lithiation showing reaction plateaus at different potentials versus Lithium. Similar studies were performed on M0.5TiOPO4 (M = Ni, Cu, Mg) samples with smaller particle size, obtained via ball milling, in order to understand the effect of particle size on the electrochemistry of the materials. The ball milled samples delivered higher specific charge during the 1st cycle showing reaction plateaus at different potentials, poorer capacity retention, and poorer rate capability as compared to the as synthesized ones. This was attributed to a change in morphology and particle size of the samples upon ball milling. Amongst all the tested materials, the as synthesized Cu0.5TiOPO4 showed the best electrochemistry. The ball milled Mg0.5TiOPO4 reacted with ∼5.5 Li+ during 1st lithiation (as compared to 3 Li+ expected from this family of compounds) and 3.3 Li+ during the 1st delithiation (rather than the expected 2 Li+). This suggests a reaction mechanism where Mg0.5TiOPO4 undergoes a phase transformation forming Mg0, which reversibly alloys with 2.5 extra Li+. Thus the electrochemical cycling of Mg0.5TiOPO4 gives insights into the reaction mechanism in this family of materials.