Unraveling the Conversion Reaction Pathways and Kinetics of FeWO4 Anode during Lithiation via In Situ Transmission Electron Microscopy

Transition metal tungstates (TMTs) are gaining attraction in lithium-ion batteries (LIBs) anode research, primarily attributed to their superior volumetric capacity, good electronic transport property, and stable electrochemical cycling. However, the fundamental phase evolution pathways during lithiation of TMTs largely remain ambiguous owing to the lack of direct evidence. Herein, ferrous tungstate (FeWO₄) is strategically selected as a characteristic TMT anode material to investigate the time-resolved structural evolution and reaction kinetics during lithiation. Previously unexplored two-step conversion reaction mechanism during the lithiation of FeWO₄ is explicitly revealed by coupling in situ transmission electron microscopy (TEM) with synchronized electron diffraction (ED) observation. Impressively, the single-crystal Li₂WO₄ is identified as the intermediate phase by in situ lattice-resolution imaging and density functional theory (DFT) calculation. Upon further Li⁺ intercalation, the lattice spacing of Li₂WO₄ progressively expands along with distortion and cracking of ordered lattice fringes, ultimately resulting in full conversion of Li₂WO₄ into Fe, W and Li₂O phases. The final delithiation products can be recovered to the FeWO₄ phase and therefore reversible phase conversion reactions can be maintained with cycling. The in situ TEM observations not only decipher the whole lithiation process of the FeWO₄ anode, but more importantly establishes a general mechanistic framework for understanding the Li storage mechanism of other TMT anodes.