Hydrazine-hydrothermal synthesis of pure-phase O-LiMnO2 for lithium-ion battery application

Submicron-sized orthorhombic LiMnO₂ (O-LiMnO₂) crystals with clear edges can be successfully prepared by a hydrothermal route using LiOH and porous Mn₂O₃ as precursors at 180°C for 16°h. As a reducing agent, hydrazine hydrate (N₂H₄·H₂O) was introduced into the hydrothermal reaction systems of dark-green O-LiMnO₂ at a N₂H₄·H₂O:Mn₂O₃ molar ratio of 10.6:100, inhibiting the common impurity formation of red Li₂MnO₃ therein. At room temperature, the relatively purified O-LiMnO₂ gives a high reversible capacity and good cyclability, showing a maximum discharge capacity of 216 mAh g^-1 and a retention value of 200 mAh g^-1 over 30 cycles at 0.1 C between 2.0 and 4.3 V, whereas the unpurified O-LiMnO₂ showed a maximum discharge capacity of 200 mAh g^-1, and fast decayed to 165 mAh g^-1 during the 30th cycle. Moreover, cyclic voltammetry measurements and X-ray diffraction patterns of the cycled electrode materials prove the irreversible phase transformation of O-LiMnO₂ to spinel LiMn₂O₄ upon cycling, suggesting that the easily Mn(IV)-doped synthesis of O-LiMnO₂ hinder its potential application as lithium-ion battery cathodes.