Li-ion conversion materials have a higher theoretical capacity than commercialized intercalation compounds.[1] During the conversion reaction, more than one electron are used per transition metal ion, which is promising energy storage compared to one-electron reaction of intercalation compounds. Among conversion materials, transition metal difluorides, MF₂ (M=Fe, Ni and Cu), have a relatively higher theoretical reaction voltage (2.66, 2.96 and 3.55 V vs. Li/Li⁺).[2,3] The voltage discrepancy, however, is a well known issue on transition metal difluorides. The experimental voltage is lower than thermodynamically derived theoretical voltage.[4] In this study, we develop the formalism of conversion reaction voltage as a function of size of metal nanoparticle formed during the lithiation. Density functional theory (DFT) calculation shows that the conversion reaction voltage depends on the size of a metal nanoparticle generated. We take annual dark field-scanning transmission electron microscopy (ADF-STEM) on the lithiated CuF₂ and NiF₂ cell, which demonstrates the formation of Cu and Ni nanoparticles with average sizes of ~ 2.5 and 1.5 nm, respectively. Near-equilibrium potentiostatic intermittent titration technique (PITT) is conducted to display the lower voltage with respect to a thermodynamic voltage. We find that the surface energy of metal nanoparticles leads to lower voltage relative to the thermodynamic voltage.

In addition, we develop a reversible CuF₂ for the first time by coating NiO. Electron energy loss spectroscopy (EELS) elemental maps prove that the lithiation process mostly occurs in high NiO domain. We propose NiO as an artificial solid electrolyte interphase, which alleviates Cu dissolution during the conversion reaction.