Synthesis of Nickel-Cobalt Difluoride [NixCo(1-x)F2] for Lithium Ion Batteries Application

Abstract

Introduction Transition metal fluorides have been gaining interest as reversible positive electrodes for rechargeable lithium batteries in the recent years due to their high theoretical e.m.f values and their ability to transfer more than 1 electron per formula unit.1 Among the metal difluorides, CoF2 and NiF2 have the highest calculated theoretical discharge voltage1. However, previous experimental work conducted has shown that nanostructured CoF2 has a discharge voltage of about 1.4V and an initial discharge capacity of about 550 mAhg-1. 2 The discharge capacity of 550 mAhg-1 relates to a 2 electron transfer per formula unit of CoF2. As for NiF2, it has previously been reported to have a discharge voltage of about 1.5 V with an initial discharge capacity of about 700 mAhg-1.3 Comparison of their effective ionic radius shows that Ni2+ has an effective ionic radius of 0.70 angstroms while Co2+ with an octahedral coordination has an effective ionic radius of 0.745 angstroms4. This indicates the possibility of the presence of a mixed nickel-cobalt difluoride [NixCo(1-x)F2]. However, the presence of [NixCo(1-x)F2] has never been reported. Our recent work presents the synthesis and the electrochemical properties of nanostructured mixed nickel-cobalt difluoride [NixCo(1-x)F2] in the sizes from 15 nm to 80 nm. Bond valance sum analysis conducted shows that Ni and Co are still in the oxidation state of +2. EDX mapping demonstrated a homogeneous mixing of nickel, cobalt and fluorine which confirms the successful synthesis of [NixCo(1-x)F2] with no phase seperation. Bond length calculations show a decrease in the metal-fluorine bond length as the nickel content was increased, hence leading to a smaller unit cell with smaller crystallites. Lattice parameters calculations conducted on the [NixCo(1-x)F2] show a slight positive deviation from Vegard’s law. Lastly, the possibility of voltage tuning via having a mixed nickel-cobalt difluoride [NixCo(1-x)F2] was also demonstrated in the work.