First-Principles Study of the Calcium Insertion in Layered and Non-Layered Phases of Vanadia
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MRS Advances © 2018 Materials Research Society DOI: 10.1557/adv.2018.468
First-Principles Study of the Calcium Insertion in Layered and Non-Layered Phases of Vanadia Daniel Koch1 and Sergei Manzhos2 Department of Mechanical Engineering, National University of Singapore, 9 Engineering Drive 1, Singapore 117576, Singapore; 1email: [email protected]; 2email: [email protected]
ABSTRACT
We investigate the insertion energetics of Ca at low concentrations in four promising vanadium oxide phases (α and δ vanadium pentoxide (V 2O5) polymorphs as well as rutile- (R) and bronze-type (B) vanadium dioxide (VO2)) using density functional theory (DFT). We find α-V2O5 to be the most suitable material for an application as cathode, driven by a stable coordinative environment, while VO2(R) does not exhibit a stable low-concentration CaxVO2 phase due to severe distortions of the host lattice due to the large calcium ion. The results provide insight into the possibility of employing these phases as active cathode materials of Ca-ion batteries.
INTRODUCTION In the search for viable alternatives to lithium (Li) for future high-performance secondary batteries, earth-abundant metals such as sodium (Na), magnesium (Mg), zinc (Zn), or aluminum (Al), are frequently investigated in combination with intercalationtype anode and cathode materials. Especially multivalent metals have attracted attention due to their potentially high gravimetric capacity and the possibly advantageous use of bulk metal as anode material without dendrite formation during cycling. A drawback of such elements as Mg, Zn, or Al, on the other hand, is that their low-magnitude standard potentials limit the achievable full battery voltages and therefore lead to lower energy densities which, as of today, cannot compete with those achieved with Li-ion batteries [1]. Exceptions to this trend are the higher alkaline earth metals calcium (Ca) and strontium (Sr), of which only Ca can be considered as a serious candidate element due to its abundance and safety. Several studies have investigated the use of vanadium pentoxide (α-V2O5) as cathode material for Ca-based batteries and reported appreciable capacities and voltages (2.4 – 3.1 V [2]) for this material. The layered α-V2O5 (space group: Pmmn) has been reported to undergo a series of phase transformations upon insertion of several metals. For our investigations we decided to include α-V2O5 as well as the metastable δ-V2O5, the latter being found to form upon Li intercalation into the α phase [3]. The δ phase (space group: Cmcm) of vanadium pentoxide can be imagined as shear-distorted α-V2O5 with increased interlayer distance [4]. The inclusion of the Cmcm phase further allows studying the effect of larger insertion sites and the possible influence of steric effects on the intercalation of Ca2+, which has an ionic radius
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