The Solid Electrolyte Interface in Emerging Battery Chemistries

Understanding the properties of the solid electrolyte interface (SEI) in alkaline ion batteries is important for improving SEI related phenomena including irreversible capacity, interfacial resistance, cycle life and safety. Analysis of the SEI, however, is complicated by its thinness, sensitivity and dependence on cell history, resulting in differing or conflicting results regarding its composition. Although there has been considerable progress regarding the families of compounds occurring and their formation mechanism,1 the relation between composition, structure and properties of the SEI is still not entirely understood. We will present new insights in the SEI in emerging battery chemistries, such as sodium ion batteries, high capacity anodes and high voltage cathodes.

New battery chemistries are often assessed in half-cells that contain alkali metal counter and reference electrodes. Recently, however, the SEI on sodium metal electrodes was found to be rather instable, raising questions about the reliability of both Na-ion half cells and other low voltage anodes.2 A highly concentrated ethereal electrolyte is introduced that forms a stable and resilient SEI on sodium metal and thereby enables more reliable sodium ion half-cells.

Alkyl carbonates are lead compounds in the SEI on alkali metals and carbonaceous anode materials like graphite or hard carbon. Despite their vital role, there has been limited work on key parameters such as ionic conductivity and mechanical properties.3,4 These parameters have been analyzed for a homologous series of lithium and sodium alkyl carbonates.

High-voltage cathodes suffer from continuous decomposition of carbonate based electrolytes, leading to capacity fade and self-discharge. Several promising additives have been introduced to mitigate this issue, including anhydrides, sultones, phosphates and borates.5-8 The SEI formed by such additives was investigated using combined in-situ and ex-situ analysis.