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

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 [Ni_xCo_(1-x)F₂]. However, the presence of [Ni_xCo_(1-x)F₂] has never been reported. Our recent work presents the synthesis and the electrochemical properties of  nanostructured mixed nickel-cobalt difluoride [Ni_xCo_(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 [Ni_xCo_(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 [Ni_xCo_(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 [Ni_xCo_(1-x)F₂] was also demonstrated in the work.

References

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2.           Teng, Y. T., Pramana, S. S., Ding, J., Wu, T. & Yazami, R. Investigation of the conversion mechanism of nanosized CoF₂. Electrochimica Acta 107,301–312 (2013).

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4.           Shannon, B. Y. R. D. et al.Revised Effective Ionic Radii and Systematic Studies of Interatomie Distances in Halides and Chaleogenides Central Research and Development Department , Experimental Station , E . L Du Pont de Nemours The effective ionic radii of Shannon & Prewitt. Acta . (1976).