High-Voltage, High-Energy Layered-Spinel Composite Cathodes with Superior Cycle Life for Lithium-Ion Batteries

The unique structural characteristics and their effect on the electrochemical performances 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 of the X-ray diffraction (XRD) data, neutron diffraction data (ND), electrochemical charge–discharge profiles, and electrochemical differential–capacity measurements. In the 0.5 ≤ <i>x</i> < 1 samples, the capacity and energy density of the composite cathodes gradually increase during 50 cycles with a change in the shape of the charge–discharge profiles. Ex situ X-ray diffraction data reveal two important findings, which account for the superior cycle performance: (i) the layered phase in the composite cathodes (<i>x</i> = 0.5 and 0.75) undergoes an irreversible phase transformation to a cubic spinel phase during extended electrochemical cycling, and the newly formed spinel phase exhibits only a 3 V plateau without any 4 or 4.7 V plateau as both Mn and Ni are present in the 4+ state; (ii) the parent 5 V cubic spinel phase undergoes a cubic to tetragonal transition during discharge, but the volume change is small (∼5%) for the <i>x</i> = 0.5 and 0.75 compositions. Both the small volume change associated with the cubic to tetragonal transition and the excellent stability of the newly evolved 3 V spinel-like phase lead to remarkable cycle life despite a wide voltage range (2–5 V) involving phase transitions