Unveiling the In Situ Evolution of Li2O‐Rich Solid Electrolyte Interface on CoOx Embedded Carbon Fibers as Li Anode Host

Abstract Modification of three‐dimensional (3D) carbon hosts with metal oxides has been considered as advantageous for the formation of Li 2 O‐rich solid electrolyte interface (SEI), which can show fast Li + diffusion, and meanwhile alleviate dendrite problems caused by fragility and nonuniformity of native SEIs. However, the lack of convincing experimental evidence has made it difficult to unveil the true origin of oxygen in Li 2 O‐rich SEIs until now. Herein, CoO x embedded carbon nanofibers (CNF‐CoO x ) are successfully prepared as high‐performance Li anode hosts. By employing 18 O isotope labeling, the role of CoO x during SEI evolution is elucidated, revealing that CoO x contributes significantly to Li 2 O formation by delivering oxygen. Benefiting from the rich Li 2 O content, the as‐formed SEIs greatly improve the Li + migration kinetics, and therefore, the CNF‐CoO x @Li anode can exhibit excellent cycling stability in half, symmetrical, and full cells.