Surface Gradient Ti-Doped MnO 2 Nanowires for High-Rate and Long-Life Lithium Battery.

Cryptomelane-type α-MnO2 has been demonstrated as a promising anode material for high-energy Li-ion batteries (LIBs) due to its high capacity and intriguing [2 × 2] tunnel structure. However, applications of MnO2 electrode, especially at high current rates and mass active material loading, are limited by the poor mechanical stability, unstable solid electrolyte interphase (SEI) layer, and low reversibility of conversion reactions. Here, we report a design of homogeneous core-shell MnO2 nanowires (NWs) created by near-surface gradient Ti doping (Ti-MnO2 NWs). Such a structurally-coherent core-shell configuration endowed gradient volume expansion from inner core to outer shell, which could effectively release the stress of the NW lattice during the cycling and avoid the pulverization of the electrode. Moreover, the grediently doped Ti is able to avoid the Mn metal coarsening, reducing the metal particle size and improving the reversibility of conversion reaction. In this way, the Ti-MnO2 NWs achieved a high reversible areal capacity (2.3 mAh cm-2 at 200 mA g-1), a superior round-trip efficiency (coulombic efficiency achieved above 99.5% after only 30 cycles), and a long lifetime (a high capacity of 742 mAh g-1 retained after 3000 cycle at 10 A g-1) at a high mass loading level of 3 mg cm-2. In addition, the detailed conversion reaction mechanism was investigated through in-situ TEM, which further evidenced the unique homogeneous core-shell structure could largely suppress the separation of core and shell upon charging and discharging. This new NW configuration could benefit the design of other large-volume-change lithium battery anode materials.