High-stability Mn–Co–Ni ternary metal oxide microspheres as conversion-type anodes for sodium-ion batteries

For sodium-ion batteries (SIBs), the electrochemical process for electrodes involves ion transport in the solid electrolyte interphase (SEI) and active materials. Generally, the large ion radius of Na+ is often considered as the key factor in poor electrode kinetics. However, for conversion-type metal oxide anodes with low redox potentials, unstable SEIs as well as the corresponding kinetic barrier also have significant effects on electrochemical behaviors and deserve more attention. Herein, porous and micro-spherical (Mn0.6Co0.3Ni0.1)3O4 is tailored as a SIB anode using co-precipitation. A high Mn percentage is beneficial to the formation of a micro-spherical morphology during co-precipitation. Due to its lack of electrochemical activity, Mn also contributes to the morphological stability of active materials during cycling. This allows for a clear observation regarding morphological changes of SEI products generated at the electrode surface. It is revealed that branch-like products are gradually converted into a dense interphase layer at electrode surfaces during cycling. The unstable and uneven topography of these electrode surfaces generates kinetic barriers that account for low rate capacities of the as-obtained (Mn0.6Co0.3Ni0.1)3O4 materials. The synthesized metal oxide is able to retain 98.1% of its initial sodiation capacity after 2000 cycles at 0.5 A g−1.