High-Voltage, High-Energy\nLayered-Spinel Composite\nCathodes with Superior Cycle Life for Lithium-Ion Batteries

Abstract

The unique structural characteristics and their effect\non the electrochemical\nperformances of the layered-spinel composite cathode system <i>x</i>Li­[Li_0.2Mn_0.6Ni_0.17Co_0.03]­O₂–(1 – <i>x</i>)­Li­[Mn_1.5Ni_0.425Co_0.075]­O₄ (0 ≤ <i>x</i> ≤ 1) have been investigated by a systematic analysis\nof the X-ray diffraction (XRD) data, neutron diffraction data (ND),\nelectrochemical charge–discharge profiles, and electrochemical\ndifferential–capacity measurements. In the 0.5 ≤ <i>x</i> < 1 samples, the capacity and energy density of the\ncomposite cathodes gradually increase during 50 cycles with a change\nin the shape of the charge–discharge profiles. Ex situ X-ray\ndiffraction data reveal two important findings, which account for\nthe superior cycle performance: (i) the layered phase in the composite\ncathodes (<i>x</i> = 0.5 and 0.75) undergoes an irreversible\nphase transformation to a cubic spinel phase during extended electrochemical\ncycling, and the newly formed spinel phase exhibits only a 3 V plateau\nwithout any 4 or 4.7 V plateau as both Mn and Ni are present in the\n4+ state; (ii) the parent 5 V cubic spinel phase undergoes a cubic\nto tetragonal transition during discharge, but the volume change is\nsmall (∼5%) for the <i>x</i> = 0.5 and 0.75 compositions.\nBoth the small volume change associated with the cubic to tetragonal\ntransition and the excellent stability of the newly evolved 3 V spinel-like\nphase lead to remarkable cycle life despite a wide voltage range (2–5\nV) involving phase transitions.