Electrochemical Performance of NASICON-structured Na3-x V2-xTix(PO4)3 (0.0 < x < 1.0) as aqueous Na-ion battery positive electrodes

Abstract

Phosphate frameworks with NASICON structure are among the most studied and applied Li- and Na-ion battery electrode and electrolyte materials. In this work, the NASICON-structured Na3-xV2-xTix(PO4)3 with x = 0.0, 0.25, 0.5, 0.75 and 1.0 are successfully prepared by conventional solid-state synthesis and characterized in detail as potential aqueous Na-ion battery positive electrodes with improved charge capacity and cycling stability. Structural analysis using powder X-ray diffractometry indicates that titanium substitutes vanadium at arbitrary concentration without significant distortion of the NASICON structure. The results show that titanium content in this system directly correlates with its aqueous stability when cycled in simple 1 M Na2SO4 aqueous electrolyte within the vanadium redox potential range. Electrochemical kinetics and charge capacity measurements show Na2VTi(PO4)3 as well as Na2.25V1.25Ti0.75(PO4)3 to be stable positive electrodes in simple aqueous electrolyte solutions. Hybrid density functional theory analysis of V-O chemical bonding suggests that it is stabilized by the presence of titanium in the NASICON structure. In this work, we show that the observed capacity loss in full symmetric cells is caused by the capacity imbalance between positive and negative electrodes which progresses during cycling but not the aqueous materials stability per se. This imbalance is caused by several parasitic reactions, the most important being the oxygen reduction reaction catalyzed by Ti(III) species. Careful mitigation and management of this reaction could, in principle, allow for the preparation of truly capacity balanced cells (i.e. without a need of any electrode overcapacity), and superior cycling stability.