Revealing the unconventional lithium storage mechanism of ordered mesoporous NiO for lithium-ion batteries

Transition metal oxides have been intensively developed for use as anode materials to overcome the capacity limitations of commercial graphite. In this study, a highly ordered mesoporous NiO electrode material is fabricated using a hard templating method, and exhibits a reversible capacity of approximately 940 mAh g⁻¹ that is much higher than the theoretical value based on the conversion reaction (717 mAh g⁻¹). Combined analyses that include synchrotron-based X-ray techniques and controlled X-ray photoelectron spectroscopies attribute the lithium storage behaviors to both the conversion reaction of NiO framework and the reversible electrolyte-derived surface layer. Interestingly, the contribution of the reversible electrolyte-derived surface layer (∼440 mAh g⁻¹) to the capacity is comparable to that of the conversion reaction with NiO (∼500 mAh g⁻¹). The results also demonstrate that incomplete conversion occurs due to the high bonding energy of Ni–O in the framework during the electrochemical reaction, and prove that the distinctive nano-structural characteristics of the mesoporous NiO surface cause the reversible behavior of the electrolyte-derived surface layer.