Identifying the Conversion Mechanism of NiCo₂O₄ during Sodiation–Desodiation Cycling by In Situ TEM

For alkali metal ion batteries, probing the ion storage mechanism (intercalation- or conversion-type) and concomitant phase evolution during sodiation–desodiation cycling is critical to gain insights into understanding how the electrode functions and thus how it can be improved. Here, by using in situ transmission electron microscopy, the whole sodiation–desodiation process of spinel NiCo₂O₄ nanorods is tracked in real time. Upon the first sodiation, a two-step conversion reaction mechanism has been revealed: NiCo₂O₄ is first converted into intermediate phases of CoO and NiO that are then further reduced to Co and Ni phases. Upon the first desodiation, Co and Ni cannot be recovered to original NiCo₂O₄ phase, and divalent metal oxides of CoO and NiO are identified as desodiated products for the first time. Such asymmetric conversion reactions account for the huge capacity loss during the first charging–discharging cycle of NiCo₂O₄-based sodium-ion batteries (SIBs). Impressively, a reversible and symmetric phase transformation between CoO/Co and NiO/Ni phases is established during subsequent sodiation–desodiation cycles. This work provides valuable insights into mechanistic understanding of phase evolution during sodiation–desodiation of NiCo₂O₄, with the hope of assistance in designing SIBs with improved performance.