Nanophase ZnCo<SUB>2</SUB>O<SUB>4</SUB> as a high performance anode material for Li-ion batteries

ZnCo<SUB>2</SUB>O<SUB>4</SUB> has been synthesized by the low-temperature and cost-effective urea combustion method. X-ray diffraction (XRD), HR-TEM and selected area electron diffraction (SAED) studies confirmed its formation in pure and nano-phase form with particle size ~15-20 nm. Galvanostatic cycling of nano-ZnCo<SUB>2</SUB>O<SUB>4</SUB> in the voltage range 0.005-3.0 V versus Li at 60 mA g<SUP>&#8722;1</SUP> gave reversible capacities of 900 and 960 mA h g<SUP>&#8722;1</SUP>, when cycled at 25&#176;C and 55&#176;C, respectively. These values correspond to ~8.3 and ~8.8 mol of recyclable Li per mole of ZnCo<SUB>2</SUB>O<SUB>4</SUB>. Almost stable cycling performance was exhibited in the range 5-60 cycles at 60 mA g<SUP>&#8722;1</SUP> and at 25&#176;C with ~98 % coulombic efficiency. A similar cycling stability at 55&#176;C, and good rate-capability both at 25 and 55&#176;C were found. The average discharge- and charge-potentials were ~1.2 V and ~1.9 V, respectively. The ex-situ-XRD, -HRTEM, -SAED and galvanostatic cycling data are consistent with a reaction mechanism for Li-recyclability involving both de-alloying-alloying of Zn and displacement reactions, viz., LiZn&#8596;Zn&#8596;ZnO and Co&#8596;CoO&#8596;Co<SUB>3</SUB>O<SUB>4</SUB>. For the first time we have shown that both Zn- and Co-ions act as mutual beneficial matrices and reversible capacity contribution of Zn through both alloy formation and displacement reaction takes place to yield stable and high capacities. Thus, nano-ZnCo<SUB>2</SUB>O<SUB>4</SUB> ranks among the best oxide materials with regard to Li-recyclability