Hydrostatic pressure effect on structural and transport properties of co-existing layered and disordered rock-salt phase of LixCoO2

It is widely believed that the origin of a significant cause of voltage and capacity fading observed in lithium (Li)-ion batteries is related to structural modifications occurring in the cathode material during the Li-ion insertion/deinsertion process. The Li-ion insertion/deinsertion mechanism and the resulting structural changes are known to exert a severe strain on the lattice and consequently lead to performance degradation. Here, intending to shed more light on the effect of such strain on the structural properties of the cathode material, we have systematically investigated the pressure dependence of structural and transport properties of a LixCoO2 single crystal grown using 5% excess Li in the precursors. Ambient pressure synchrotron diffraction on these crystals reveals that the excess Li during the growth has facilitated the stabilization of a layered rhombohedral phase (space group R3<overline>m) as well as a disordered rock-salt phase (space group Fm3<overline>m). The volume fraction of the rhombohedral and cubic phase is 60:40, respectively, which remains unchanged up to 10.6 GPa. No structural phase transition has been observed up to 10.6 GPa. Further, pressure-dependent structural studies on delithiated mixed-phase material using synchrotrons revealed that the rhombohedral phase of Li0.5CoO2 remains thermodynamically stable at high pressures and prevails as the major phase. An increase in the measured resistance with decreasing temperature revealed the semi-metallic nature of the sample. Further, the application of hydrostatic pressure up to 2.8 GPa shows the enhancement of semi-metallic nature. The experimental results can be explained qualitatively via density functional theory (DFT) and thermodynamics modeling. We find that the calculated density of states was reduced and the activation energy was increased by applied pressure. Our investigations indicate significant phase stability of the mixed-phase crystals under externally applied high pressure and thus suggest the possible use of such mixed-phase materials as a cathode in lithium-ion batteries.