Electrochemical Lithium Alloying Behavior of Guest-Free Type II Silicon Clathrates

The guest-free type II Si clathrate (Si₁₃₆) is an open framework polymorph of Si that displays unique electrochemical reactions with Li. Li ions are first topotactically inserted into the vacant clathrate cages, followed by an alloying reaction that forms an amorphous lithium silicide phase. The alloying reaction voltage is higher than those seen in other Si electrodes, suggesting that there are structural differences in the formed amorphous phases. Synchrotron X-ray total scattering measurements and pair distribution function analysis are employed to characterize the amorphous phases formed after lithiation. The results show that the clathrate becomes completely amorphous at an earlier stage of lithiation when compared to diamond cubic Si, forming a phase with comparatively larger amounts of Si-Si bonding. The initial insertion of Li into the clathrate cages establishes important Li diffusion paths that kinetically enable the formation of an amorphous phase with lower Li content than typically seen in other silicon-based electrodes. After the initial crystalline-to-amorphous conversion reaction, lithiation takes place via solid-solution alloying. These results demonstrate how the topotactic insertion of Li into an alloying host can kinetically enable modified reaction pathways leading to more homogeneous lithiation throughout the electrode, which is beneficial for Li-ion battery applications.