Magnetron Sputtering Formation of Germanium Nanoparticles for Electrochemical Lithium Intercalation.

In the drive towards increased lithium based battery capacity, germanium is an attractive material due to its very high lithium storage capacity, second only to silicon. The persistent down-side is the considerable embrittlement accompanying its remarkable volume expansion of close to 300 %. A proven method to accommodate for this lattice expansion is the reduction of the size towards the nanoscale at which the fracturing is prevented by "breathing". In this work we employed a novel magnetron sputtering gas aggregation nanoparticle generator to create unprecedented layers of well-defined germanium nanoparticles with sizes below 20 nm. The electrochemical lithium intercalation was monitored by a suite of techniques under which Raman spectroscopy, which provided clear evidence of the presence of lithium inside the germanium nanoparticles. Moreover, the degree of lattice order was measured and correlated to the initial phases of the lithium-germanium alloy. This was corroborated by electron diffraction and optical absorption spectroscopy, of which the latter provided a strong dielectric change upon lithium intercalation. This study of low lithium concentrations inside layers of well-defined and very small germanium nanoparticles, forms a new avenue towards significantly increasing the lithium battery capacity.