Structure and state of sodium in carbon or phosphorous anode active materials of sodium ion battery studied using multinuclear solid state NMR

Abstract

Non-graphitizable carbon (hard carbon; HC) is expected as one of the most promising negative electrode materials for sodium ion batteries (NIB) due to its high working voltage, large capacity and superior charge-discharge cyclability. Phosphorus is also a fascinating material for NIB because its theoretical capacity is one of the highest (over 2000 mAh g⁻¹). However, it is difficult to clarify the structures and states of sodium in these anode materials by diffraction methods because the materials are amorphous. In this research, multinuclear (²³Na and ³¹P) solid state nuclear magnetic resonance (NMR) is applied to investigate the states. Models of Na and Li storage in HC based on the calculated cluster structures were proposed, which elucidates why the adequate heat treatment temperature of HC for high-capacity sodium storage is higher than the temperature for lithium storage. The fully sodiated sample prepared at higher carbonization temperature shows a higher ²³Na NMR shift associated with Na clusters. A higher dehydration temperature of the precursor is favorable to form Na clusters with the higher shift, which implies that the HC pore structure is influenced by not only carbonization temperature but also the heat treatment process. Sodium-phosphorus compounds synthesized by thermal reaction or electrochemical sodiation were characterized using NMR. The Na and P NMR spectra reveal that sodiation and desodiation processes are reversible after the second cycle. During the sodiation and desodiation, Na₃P compounds and three amorphous compositions are observed.