Tin and Tin Compounds for Sodium Ion Battery Anodes: Phase Transformations and Performance

Sodium ion batteries (NIB, NAB, SIB) are attracting interest as a potentially lower cost alternative to lithium ion batteries (LIB), with readily available and geographically democratic reserves of the metal. Tin is one of most promising SIB anode materials, which alloys with up to 3.75 Na, leading to a charge storage capacity of 847 mAh g(-1). In this Account, we outline the state-of-the-art understanding regarding the sodiation-induced phase transformations and the associated performance in a range of Sn-based systems, treating metallic Sn and its alloys, tin oxide (SnO2), tin sulfide (SnS2/SnS), and tin phosphide (Sn4P3). We first detail what is known about the sodiation sequence in metallic Sn, highlighting the most recent insight into the reactions prior to the terminal equilibrium Na15Sn4 intermetallic. We explain why researchers argue that the equilibrium (phase diagram) series of phase transitions does not occur in this system, and rather why sodiation/desodiation proceeds through a series of metastable crystalline and amorphous structures. We also outline the recent modeling-based insight regarding how this phase transition profoundly influences the mechanical properties of the alloy, progressively changing the bonding and the near neighbor arrangement from "Sn-like" to "Na-like" in the process. We then go on to discuss the sodiation reactions in SnO2. We argue that while a substantial amount of experimental work already exists where the focus is on synthesis and testing of tin oxide-based nanocomposites, the exact sodiation sequence is just beginning to be understood. Unlike in Sn and Sn alloys, where capacities near the theoretical are reached at least early during cycling, SnO2 never quite achieves anything close to the 1398 mAh g(-1) that would be possible with a combination of fully reversible conversion and alloying reactions. We highlight recent work demonstrating that contrary to general expectations, it is the Sn to Na15Sn4 alloying reaction that is incomplete and hence limits the capacity of the electrode. We also describe how the oxide conversion reaction goes through an intermediate SnO phase, and how its reversibility in a half-cell is highly dependent on the terminal anodic voltage. We then present what is known about sodiation of tin sulfide and of tin phosphide phases, including emerging microstructural evidence that may explain why both the sulfides and the phosphides are unable to achieve their highly promising theoretical capacities under conventional electrode testing conditions. Finally, we provide a broad comparison of the capacity (cycling and rate) performance for a range of Sn based anode materials, and show that there may be indeed an optimum microstructural architecture.