Germanium as negative electrode material for sodium-ion batteries

Germanium thin film electrodes show a reversible Na-ion reaction at potentials around 0.15/0.6 V. The reaction is accompanied with a reversible capacity close to 350 mAh g(-1), which matches the value expected for the formation of NaGe. The electrode capacity retention is stable over 15 cycles but subsequently declines. However, using fluoroethylene carbonate (FEC) electrolyte additive positively improves capacity retention and promotes the formation of a thinner SEI. Mechanical degradation due to repeated expansion/shrinkage coupled with SEI formation are the main sources of capacity decline. Preliminary XRD results do not reveal the formation of crystalline phases at full (dis)charge. The excellent charge rate up to 340 C highlights the high potential of nanosized germanium as Na-ion anode. (C) 2013 Elsevier B.V. All rights reserved.