Impact of crystal density on the electrochemical behavior of lithium-ion anode materials: Exemplary investigation of (Fe-doped) GeO2

For insertion-type lithium battery active materials, the crystal structure is of utmost importance, as it determines the possibility, reversibility, and kinetics of the Li+ de-/insertion. For alloying-type and conversion-type materials, however, the impact of the initial crystal structure has so far been considered less important, as it is commonly not recovered after the first lithiation. Herein, we had a closer look at the impact of the crystal structure by comparatively studying the electrochemical behavior of low-density hexagonal (α-quartz) and high-density tetragonal (rutile) GeO2 as model compounds. Based on the results obtained via a comprehensive set of complementary ex/in situ and operando techniques, it was found that the initial lithiation and the reversibility of the de-/lithiation reaction in general are greatly enhanced when starting from the low-density crystal structure. The introduction of iron stabilizes this low-density phase even at elevated calcination temperatures and, in addition, increases the contribution of the reconversion reaction to the reversible capacity as a result of the formation of ultrasmall metallic iron grains during the first lithiation.