Electrochemical and multinuclear solid-state NMR studies of tin composite oxide glasses as anodes for Li ion batteries

Electrochemical and multi-nuclear solid-state NMR studies of various tin oxide and two tin composite oxide (TCO Sn1.0Al0.42B0.56P0.40O3.6 and Sn-rich TCO Sn1.5Al0.42B0.56P0.40O4.2) samples are described, which give a coherent picture of the different processes occurring within these systems. 6,7Li NMR results demonstrate that the agglomeration of Li–Sn domains is inhibited in TCO; in contrast, in SnO, the aggregation of particles is observed. This difference results in part from the facile back-reaction between Sn and O. The interfacial energy of the most highly divided particles (TCO) allows the "back-reaction" of lithium with oxygen to be reversible at a lower potential than predicted from simple thermodynamic considerations that exclude surface energy contributions. Thus, the proximity and availability of oxygen in the host matrix may indirectly enhance the reversibility and cyclability of the cell in these materials, by "trapping the Sn particles". Aggregation may also be limited in TCO owing to the participation of the matrix observed by 27Al, 31P, and 11B NMR, where reversible changes in the coordination environment are observed during lithium uptake and removal. The size-limiting role of the matrix ions is key to the enhanced electrochemical properties of the TCO glass. The initial rearrangement of the glass network is kinetically limited, as demonstrated by galvanostatic intermittent titration technique (GITT) experiments. The combined results of this study demonstrate the unique nature of the reaction between lithium and TCO.