Monoclinic-Orthorhombic Na1.1Li2.0V2(PO4)(3)/C Composite Cathode for Na+/Li+ Hybrid-Ion Batteries

Abstract

Monoclinic Li3V2(PO4)(3) (LVP) has been considered a promising cathode material for lithium-ion batteries for the past decade because of its high average potential (>4.0 V) and specific capacity (197 mAh g(-1)). In this paper, we report a new monoclinic-orthorhombic Na1.1Li2.0V2(PO4)(3)/C (NLVP/C) composite cathode synthesized from monoclinic LVP via a soft ion-exchange reaction for use in Na+/Li+ hybrid-ion batteries. High-resolution synchrotron X-ray diffraction (XRD), thermal studies, and electrochemical data confirm room temperature stabilization of the monoclinic-orthorhombic NLVP/C composite phase. Specifically, we report the application of a monoclinic-orthorhombic NLVP/C composite as cathode material in a Na half-cell. The cathode delivered initial discharge capacities of 115 and 145 mAh g(-1) at a current density of 7.14 mA g(-1) in the 2.5-4 and 2.5-4.6 V vs Na/Na+ potential windows, respectively. In the lower potential window (2.5-4 V), the composite electrode demonstrated a two-step voltage plateau during the insertion and extraction of Na+/Li+ ions. Corresponding in situ synchrotron XRD patterns recorded during initial electrochemical cycling clearly indicate a series of two-phase transitions and confirm the structural stability of the NLVP/C composite cathode during insertion and extraction of the hybrid ions. Under extended cycling, excessive storage of Na ions resulted in the gradual transformation to the orthorhombic NLVP/C symmetry due to the occupancy of Na ions in the available orthorhombic sites. Moreover, the estimated average working potential and energy density at the initial cycle for the monoclinic-orthorhombic NLVP/C composite cathode (3.47 V vs Na/Na+ and 102.5 Wh kg(-1), respectively) are higher than those of the pyro-synthesized rhombohedral Na3V2(PO4)(3) (3.36 V vs Na/Na+ and 88.5 Wh kg(-1)) cathode. Further, the cathode performance of the composite material was significantly higher than that observed with pure monoclinic LVP under the same electrochemical measurement conditions. The present study thus showcases the feasibility of using a soft ion-exchange reaction at 150 degrees C to facilitate the formation of composite phases suitable for rechargeable hybrid-ion battery applications.