Sodiation and Desodiation via Helical Phosphorus Intermediates in High-Capacity Anodes for Sodium-Ion Batteries

Lauren E Marbella,Clare P Grey,Joseph Nelson,Matthias F Groh,Andrew J Morris,Matthew L Evans,Kent J Griffith

doi:10.1021/jacs.8b04183

Abstract

Na-ion batteries are promising alternatives to Li-ion systems for electrochemical energy storage because of the higher natural abundance and widespread distribution of Na compared to Li. High capacity anode materials, such as phosphorus, have been explored to realize Na-ion battery technologies that offer comparable performances to their Li-ion counterparts. While P anodes provide unparalleled capacities, the mechanism of sodiation and desodiation is not well-understood, limiting further optimization. Here, we use a combined experimental and theoretical approach to provide molecular-level insight into the (de)sodiation pathways in black P anodes for sodium-ion batteries. A determination of the P binding in these materials was achieved by comparing to structure models created via species swapping, ab initio random structure searching, and a genetic algorithm. During sodiation, analysis of 31P chemical shift anisotropies in NMR data reveals P helices and P at the end of chains as the primary structural components in amorphous Na xP phases. X-ray diffraction data in conjunction with variable field 23Na magic-angle spinning NMR support the formation of a new Na3P crystal structure (predicted using density-functional theory) on sodiation. During desodiation, P helices are re-formed in the amorphous intermediates, albeit with increased disorder, yet emphasizing the pervasive nature of this motif. The pristine material is not re-formed at the end of desodiation and may be linked to the irreversibility observed in the Na-P system.

Highlights

Na-ion batteries (NIBs) are promising alternatives for longterm sustainability in terms of both cost and natural abundance compared to Li-ion systems.[1−5] Na is widely available and evenly distributed worldwide, lessening the political tensions that may arise from continued Li use
These new motifs provided a vast library of possible P binding environments to evaluate as potential local environments and motifs that may arise in the amorphous NaxP structures formed on cycling
Near-theoretical capacity was reached on the first discharge (2510 mA h g−1, based on the mass of P alone) and one of the highest reversible capacities observed to date of 2074 ± 80 mA h g−1 without the use of additives is achieved in the first cycle

Summary

Introduction

Na-ion batteries (NIBs) are promising alternatives for longterm sustainability in terms of both cost and natural abundance compared to Li-ion systems.[1−5] Na is widely available and evenly distributed worldwide, lessening the political tensions that may arise from continued Li use. While numerous viable cathode materials for NIBs have been identified from Li analogues,[6,7] many intercalation and alloying anode materials that work well for Li-ion batteries (LIBs) fail for Na chemistries. With an emphasis on battery safety, monitoring tools, and quality assurance, the likelihood that these risks can be mitigated increases

Results

Discussion

Conclusion