Cu-Substituted NiF₂ as a Cathode Material for Li-Ion Batteries

Metal fluorides usually have a large electronegativity and are promising electrode materials for high-power lithium-ion batteries. However, like other conversion-reaction-based materials, large volumetric expansions and large capacity losses in cycling are the major issues for metal fluorides. Here, we explore substitution of Ni with Cu for binary NiF₂ and its effects on the electrochemical properties. By in situ transmission electron microscopy, the structural evolutions of several ternary metal fluorides with different Cu/Ni ratios are observed and correlated with their electrochemical properties. With increased Cu substitution from 0 to 25 wt %, the areal expansion during the first lithiation is reduced. Meanwhile, the fluorine loss (due to reaction irreversibility) in the delithiation cycle is also reduced. This provides an explanation for the advantage of Cu substitution for improved cycling stability and capacity. We believe that our observations provide insight into the development of better ternary metal fluorides as cathodes for high power density lithium-ion batteries.