Electrochemical Characterization of Na-Ion Charge-Storage Properties for Nanostructured NaTi 2 (PO 4 ) 3 as a Function of Crystalline Order

Abstract

© The Author(s) 2017. We evaluate a series of nanoparticulate NaTi2(PO4)3 (NTP) powders as Na+-insertion hosts in either nonaqueous or aqueous electrolyte, correlating electrochemical properties such as capacity and electrode kinetics (in the form of powder-composite electrodes) with the degree of crystallinity in NTP. Starting with amorphous NTP powders prepared using the Pechini method, calcination from 500 to 800°C was used to induce varying degrees of crystallinity and to remove carbonaceous species. Poorly crystalline NTP powders derived by heating at 500-600°C exhibit low specific capacities and broad voltammetric features for Na+-insertion, characteristic of surface-limited processes. Heating at higher temperatures (700-800°C) yields the nanocrystalline form with the NASICON structure. Nanocrystalline NTP exhibits sharp voltammetric peaks and diffusion-limited kinetics in both aqueous and nonaqueous electrolytes. The electrochemical performance of nanocrystalline NTP is further enhanced when integrated with reduced graphene oxide (rGO) to increase local electronic conductivity; theoretical specific capacity for NTP (133 mAh g-1) is achieved when NTP-rGO is cycled in a nonaqueous electrolyte, and 100 mAh g-1 in a mild aqueous electrolyte. This nanocomposite also exhibits long-term stability (86% capacity retention after 1000 charge/discharge cycles) in a nonaqueous electrolyte.