Effects of Cation-Disordering in Si4+- Substituted Li3VO4 As a Negative Electrode for Superredox Capacitors

Abstract

Introduction SuperRedox Capacitors (SRC) that combine ultrafast pseudocapacitive battery materials as positive and negative electrodes are novel energy storages with battery-like energy density and supercapacitor-like power density and durability[1]. β-Li3VO4 (LVO) shows promising characteristics as a negative electrode for SRC with regards to its high energy density, with a specific capacity of 394 mAh g-1 (two electrons reaction). In addition, LVO reacts at potentials between 0.4 and 1.3 V vs. Li+/Li, whose range is suitable for the safe operation while achieving high cell voltage[2]. We have reported that disordering all cations-orientation (Li+ and V5+) of LVO increases Li+ diffusion coefficient (D Li) from the pristine LVO by a factor of one hundred, thus facilitating faster charge/discharge reactions[3]. However, such a cation-disordered structure has durability issues because of its thermodynamic metastability over 400˚C.In this study, we introduce a novel method to enhance Li+ conductivity of LVO while maintaining a stable structure. In order to obtain the same effect of cation-disordering, not all cations but only V5+ orientation was disordered by random substitution of V5+ with Si4+. Higher thermodynamic stability over 800˚C was achieved by maintaining specific arrangement between Li+ and Mn+ (Mn+ = V5+ or Si4+). Detailed influential factors of Si4+-substitution to electrochemical performances were discussed from the crystallographic point of view. Experimental Si4+-substituted LVO was prepared by a simple powder calcination process. V2O5, Li2CO3 and SiO2 powder were sealed in a zirconia pod of a planetary ball mill (PL-7, Fritsch) and mixed them at 300 rpm for 30 min. Mixture was calcined to obtain Li3+xV1-xSixO4 (LVSiO, 0≦x≦0.4). Characterization of the crystal structure was performed using X-ray and neutron diffraction techniques. In order to estimate D Li+ in the crystal structure, we used a Galvanostatic Intermittent Titration Technique (GITT). Electrochemical performances were measured using 2032-type coin cells assembled of LVSiO and Li metal electrodes. The used electrolyte composition was a 1.0 M solution of lithium hexafluorophosphate (LiPF6) dissolved in a mixture of ethylene carbonate and diethyl carbonate (50:50 in volume ratio). Results and Discussion Si4+-substituted LVO (LVSiO) was successfully synthesized, as confirmed by both X-ray and neutron diffraction patterns, confirming crystal phase change from a β-phase (Pnm21) of pristine LVO into a high-temperature γ-phase (Pnma) of LVSiO. Rietveld analysis revealed that Si4+ randomly occupied V5+ sites, whereas, Li+ and Mn+ (M = V5+ or Si4+) arrangements were maintained. Thermal stability was confirmed by in situ powder X-ray diffraction measurements while raising temperature in Ar atmosphere; the LVSiO crystal structure remains stable up to 800ºC.Results obtained from GITT measurements showed that D Li+ was maximized at 20 at.% of Si4+ and the value was maintained up to 40 at.% (Fig. 1). The effect of Si4+ substitution on reaction mechanisms were analyzed by in situ XRD electrochemical cell measurements by comparing the crystallographic change of LVO and LVSiO while Li+ insertion/deinsertion. The obtained results confirm that LVO reacts in two-phase separated mechanism and LVSiO reacts in solid solution mechanism, which is similar changes observed in the cation-disordered LVO[2,3]. It is considered that Li+ can diffuse more smoothly in solid solution mechanism than two-phase separated mechanism, since there is no phase boundary that disturbs diffusion of Li+. The D Li+ enhancement contributed to the improved capacity retention at a high rate of 100C from 25% (LVO) to 65% (LVSiO, Si20%). The obtained results show that the improvements of D Li+ and rate capability by Si4+-substitution can be achieved, as similar effect from a fully cation-disordering, while increasing thermodynamic stability. This study suggests that the main factor of effects of cation-disordering was due to the partially disordered orientation of transition metal (V5+, Si4+) cations without Li+.