Engineering single crystalline Mn₃O₄ nano-octahedra with exposed highly active {011} facets for high performance lithium ion batteries

Well shaped single crystalline Mn₃O₄ nano-octahedra with exposed highly active {011} facets at different particle sizes have been synthesized and used as anode materials for lithium ion batteries. The electrochemical results show that the smallest sized Mn₃O₄ nano-octahedra show the best cycling performance with a high initial charge capacity of 907 mA h g^-1 and a 50th charge capacity of 500 mA h g^-1 at a current density of 50 mA g^-1 and the best rate capability with a charge capacity of 350 mA h g^-1 when cycled at 500 mA g^-1. In particular, the nano-octahedra samples demonstrate a much better electrochemical performance in comparison with irregular shaped Mn ₃O₄ nanoparticles. The best electrochemical properties of the smallest Mn₃O₄ nano-octahedra are ascribed to the lower charge transfer resistance due to the exposed highly active {011} facets, which can facilitate the conversion reaction of Mn₃O₄ and Li owing to the alternating Mn and O atom layers, resulting in easy formation and decomposition of the amorphous Li₂O and the multi-electron reaction. On the other hand, the best electrochemical properties of the smallest Mn₃O₄ nano-octahedra can also be attributed to the smallest size resulting in the highest specific surface area, which provides maximum contact with the electrolyte and facilitates the rapid Li-ion diffusion at the electrode/electrolyte interface and fast lithium-ion transportation within the particles. The synergy of the exposed {011} facets and the smallest size (and/or the highest surface area) led to the best performance for the Mn₃O₄ nano-octahedra. Furthermore, HRTEM observations verify the oxidation of MnO to Mn₃O₄ during the charging process and confirm that the Mn₃O₄ octahedral structure can still be partly maintained after 50 discharge-charge cycles. The high Li-ion storage capacity and excellent cycling performance suggest that Mn ₃O₄ nano-octahedra with exposed highly active {011} facets could be excellent anode materials for high-performance lithium-ion batteries.