Unlocking the Multi-Redox (Nb5+/Nb4+/Nb3+) Reactions in Nasicon- Nb2(PO4)3 Anode for Li- and Na-Ion Batteries

Abstract

NASICON (Natrium Super-Ionic CONductor) type materials are known for their high structural, and thermal stability and superior sodium ion mobility which make this framework an efficient electrode material for Na-ion batteries.1 In recent times NASICON-type compounds have been widely explored as Li- and Na-ion cathodes and solid-state electrolytes but are largely ignored as anodes due to their lower capacities and higher intercalation voltages, which reduce the overall energy densities of Li- and Na-ion batteries (LIBs and SIBs). NASICON structure has a general molecular formula of NaxMM′(PO4)3, where two MO6 octahedra corners are shared with three PO4 tetrahedra along the c-direction resulting in the “lantern units” and each lantern unit joins six other lantern units, generating a large interstitial space that can accommodate up to four alkali cations per formula unit.2 In the NASICON family usually all members contain Na/Li ions in terms of synthesis and electrochemical/chemical oxidation routes must be employed to remove the Na from the parent compound.Herein, for the first time we will present a comprehensive study on the structural and electrochemical properties of the empty NASICON-type (contains no Li or Na ions in its pristine state) mixed valance Nb2(PO4)3 and its potential application as an anode in LIBs and SIBs. Due to the multi-redox activity of Nb, it is possible to attain high insertion capacity (~ 150 mAh g-1 for SIBs and ~ 170 for LIBs) and relatively lower voltages (1.8 V vs. Li+/Li0 and 1.4 V vs. Na+/Na0) compared to Ti or V-based NASICON anodes.3,4 Our in-situ XRD measurements revealed multiple-phase transformations during Li and Na intercalation with the formation of short-range ordered Li3Nb2(PO4)3 and triclinic (P-1)-Na3Nb2(PO4)3 at the end of discharge. The distinct electrochemical Li and Na intercalation behavior of the Nb2(PO4)3 anode can be ascribed to the relative differences in size, filling of crystallographic sites, and chemical characters of Li and Na ions. Our density functional theory calculations are also in agreement with the in-situ XRD measurements in predicting a stable Na3Nb2(PO4)3 composition in the Na-Nb2(PO4)3 pseudo-binary system. X-ray absorption spectroscopy confirms the participation of multi-redox Nb5+/Nb4+/Nb3+ couples. Although the micron-sized anode showed moderate storage capacities (~106 and 84 mAh g-1 for Li and Na cells, respectively) at 1C rate after 200 cycles, further optimization of electrode and electrolytes is expected to produce better performance. In the future, pairing Nb2(PO4)3 anode with suitable cathodes can enable high-energy density batteries.5