First principles investigation and design of fluorophosphate sodium-ion battery cathodes

Abstract

Lithium-ion batteries are currently the most widely used consumer energy storage technology. Recently, lithium-ion batteries have been evaluated for use in mitigating the intermittent power supply of leading renewable energy technologies, thereby enabling their use on the electric grid. In order to facilitate the widespread adoption of electric vehicles and renewable energy technologies, the energy-densities, lifetimes, and cost of batteries must be improved. Due to concerns over long-term lithium availability, sodium-ion batteries are currently being investigated as an alternative to lithium-ion batteries in grid-level applications. In this thesis, we use ab inritio methods to characterize th high-voltage sodium-ion fluorophosphate with formula NaxV2(PO4)2O2yF3-2y as an alternative chemistry to Li-ion batteries. In Chapter 3 we investigate the sodium-extraction limitations in the NaxV2(PO4)2O2yF3-2 fluorophosphate. Specifically, we focus on the potential to reversibly extract sodium beyond the 1 </= x </= 3 range. We find that the capacity limitation arises from a combination of the high voltage of the V 4+/'+ oxidation reaction in the 0 </= x </= 1 region, coupled with a strong sodium-vacancy ordering at x = 1, which prevents the formation of mobile defects in the structure. We deduce that the accessible capacity of Na)V2 (PO4 )2F3 can potentially be expanded to 0 </= x </= 3 by introducing defects into the material and reducing the voltage of the active redox couple in the 0 </= x K 1 range. In Chapter 4, we investigate the stability and voltage characteristics of transition metal substitutions on the fluorophosphate framework. We demonstrate that the inferior performance associated with non-vanadium fluorophosphates is the result of a thermodynamic driving force to release oxygen gas upon charging, in tandem with high voltages. From our calculations, we demonstrate that molybdenum is simultaneously stable in the fluorophosphate framework and capable of reducing the sodium extraction voltage in the 0 K x </= 1 range. We conclude with an analysis of the phase stability and voltage curves of mixed transition metal fluorophosphates along the NaxV 2 (PO4) 202yF 3-2y NaxMo 2 (PO4)202yF3-2y composition line. From the results of this study, we identify NaxV2(PO4)2O2yF3-2 as the most promising candidate system, with the potential to improve the capacity of current fluorophosphate cathodes by 37%.