Ionic and electronic conductivity in structural negative electrodes

• Transport of ions and electrons through the thickness of electrode laminas is experimentally verified. • Ionic conductivity in the structural battery electrolyte remains unchanged after the incorporation of carbon fibres. • Through-thickness electronic conductivity in thin electrode laminas is determined by electronically insulating electrolyte regions, inter-fibre contact points. • Enhanced ionic conductivity in fully-delithiated electrodes is observed due to microcracks formed upon cycling. The concept of structural battery presents great potential for achieving substantial weight and volume reduction in electrified transportation. A unidirectional electrode lamina consists of carbon fibres embedded in a heterogeneous structural battery electrolyte. Dielectric spectroscopy measurements reveal the presence of both ionic and electronic conductivity through-the-thickness of electrode laminas. We report that the ionic conductivity, facilitated by the ions diffusing through the structural battery electrolyte, is reminiscent of that found in free-standing structural battery electrolytes and increases with a higher content of the ion-conducting phase. On the other hand, the through thickness electronic conductivity is dictated by the size of the electronically insulating electrolyte regions and the inter-fibre contact points forming the interconnected carbon fibre network. For suppressing the out-of-plane electronic conductivity and potential battery short-circuits in thin electrode laminas (≤ 700 μm) a glass-fibre separator is required. After galvanostatic cycling, we show that the fully-delithiated electrode lamina exhibits enhanced ionic conductivity. It indicates the presence of open microcracks, formed due the extensive expansion/contraction of carbon fibres upon charging/discharging. Our study provides valuable impedance and electrochemistry data in structural battery electrodes and half-cells, paving the way for the design of innovative, lightweight structural battery cells.