Amorphous SiO₂ is promising anode material for next generation lithium-ion batteries (LIBs) that could potentially provide both high specific capacities, a theoretical capacity of 1965 mAh·g^-1, and excellent cycling stability [1]. Diatom frustules have been shown to be a sustainable source of naturally nanostructured SiO₂ which can be used in anode fabrication, displaying capacities of 800 mAh·g^-1 after 100 cycles [2], [3]. However, SiO₂ undergoes a complicated array of reactions at multiple potentials during lithiation resulting in the formation of electrochemically active silicon dispersed in a complex matrix of electrochemically inert lithium silicates and lithium oxides [4]. The formation of these irreversible phases is often linked to the low initial columbic efficiencies of SiO₂ anode materials, as large reservoirs of lithium are trapped irreversibly. Electrochemical activation of SiO₂ material is also crucial for electronic and ionic transport properties which improves after initial lithiation [5], [6]. However, driving forces and reaction pathways for SiO₂ lithiation are not well understood due to the amorphous nature of lithiation reaction products, and it is imperative to fully grasp the mechanisms behind these reactions to utilize SiO₂ anodes to their full potential. Improved knowledge on the evolution of the conversion reactions would for example aid the selection of the most appropriate SiO₂ nanostructure as well as the appropriate cycling parameters.

To study the reaction mechanisms behind SiO₂ lithiation, electrodes consisting of 75 wt% active SiO₂ diatom frustule, 15 wt% carbon black conductive additive and 10 wt% of sodium alginate binder were assembled into 2032 half-cells with lithium metal as the counter electrode. The electrodes were cycled to different potentials in the first two lithiation/delithiation cycles and were subsequently held at that potential for 48 hours. This potentiostatic step was introduced to ensure the establishment of an electrochemical equilibrium within the electrode material. Chemical state analysis of both the electrode surface and buried interphases at different states of charge and cycle number were obtained by X-ray photoelectron spectroscopy (XPS) and Hard X-ray Photoelectron Spectroscopy (HAXPES, BM25 ESRF) respectively. High-resolution Transmission Electron Microscopy (TEM) and Selected Area Electron Diffraction (SAED) characterizations were also conducted on these samples for more accurate identification of the lithium silicate phases as well as to provide insight on the spatial distribution of the lithiation products.

Using a specially designed in-operando cell set-up, X-ray total scattering data in conjunction with computed tomography data were collected during initial SiO₂ lithiation using hard X-rays (DESY). This allowed for real time tracking of SiO₂ lithiation reaction and transition to amorphous phases, complementing therefore results from XPS, HAXPESH and TEM.  

Preliminary result analysis clearly indicates the formation of lithium silicates in conjunction with pure silicon species after SiO₂ lithiation. An increase proportion of SiO₂ lithiation products can be found closer to the particle centers, indicating a reaction mechanism with lithiation beginning from the center of the SiO₂ particles which propagates outwards towards the surface of the particles. This can help explain differences in SiO₂ nanostructures with respect to their efficiency toward silicon formation, and also provide guidelines for identification of the most suitable diatom frustules.