New Electrode Materials for Sodium Ion Batteries

Abstract

Sodium-ion batteries (SIBs) are evolving as a low-cost alternative to the state-of-the-art lithium-ion batteries (LIBs). Elementary properties of sodium, high abundance and low cost associated with sodium precursors made it a realistic alternative to lithium-ion chemistry [1, 2]. Research activities on SIBs are growing worldwide and still require a great deal of basic research. Currently, the research on SIBs is focused on the development of new cathode and anode materials or combinations with improved properties such as high energy density, sustainability, and safety. Sodium insertion in hard carbon is strongly debated, and a full and consistent picture of the underlying mechanism is still missing. By combining in-situ Raman spectra obtained during sodium insertion in hard carbon with detailed ab initio studies, for the first time we provide a complete description of the Na insertion process in hard carbon [3]. On the cathode side, we propose weberite-type sodium metal fluorides (SMF), a new class of high voltage and high energy density materials which are so far unexplored as cathode materials for SIBs [4]. Weberite-type is highly favorable for sodium-containing transition metal fluorides, with a large variety of transition metal combinations (M, M’) adopting the corresponding Na2MM’F7 structure. A series of known and hypothetical compounds with weberite-type structure were computationally investigated to evaluate their potential as cathode materials for SIBs. Weberite-type SMFs show quasi-three-dimensional pathways for Na+ diffusion with surprisingly low activation barriers. The high energy density combined with low diffusion barriers for Na+ makes this type of compounds promising candidates for cathode materials in SIBs. We also present the synthesis and electrochemical properties of new sodium vanadium oxy phosphate (NaVOP), Na2+xV3P2O13 (0 ≤ x ≤ 2). NaVOP shows a reversible capacity of 150 mAh g-1 at an average voltage of 2.5 V vs. Na/Na+, with high cycling stability [5]. The post-structural analysis shows Na2+xV3P2O13 (0 ≤ x ≤ 2) is structurally stable over a wide range of sodium extraction and reinsertion demonstrating its potential as cathode material for SIBs.