LiCoO2 -Based Composite Cathode with PO4-O2 Hybrid Framework for Lithium Ion Batteries

In recent decades, lithium-ion batteries have become the most widely used power source for portable electronic devices and hybrid electric vehicles (HEVs) and plug-in HEVs because it can offer high energy and power density. The most representative cathode material for commercial Li-ion batteries is LiCoO₂ due to its high capacity and excellent cycle life. LiCoO₂ has the hexagonal α-NaFeO₂ phase consisting of the layered rock-salt structure with the order of Li⁺ and Co³⁺ on alternating (111) planes in its cubic structure. When a Li/Li_1-xCo_xO₂ cell is typically charged within limited composition range (0 < x < 0.5, 4.2V), it shows reasonably good capacity retention. However, the discharge capacity under the cut-off voltage of 4.2 V is around only 140 mAh/g, which is much lower than the theoretical value (274 mAh/g) of LiCoO₂. Unfortunately, the practical use of LiCoO₂ has been limited because its stability could be rapidly deteriorated at potentials higher than 4.2 V. Some research groups have reported that the poor cycling performance above 4.2 V is caused by structural instability induced by a phase transition from hexagonal phase to a monoclinic phase, which accompanies a volume change of ~2.6 % along the c-axis. To overcome the above problems, many researchers have developed lots of surface modifications that can improve the structural stability of LiCoO₂.

 In this study, we tried to control the interlayer distance variation (lattice parameter c) though the substitution of phosphorus for Li⁺ sites by a phosphidation process. As a result, the unwanted phase transition could be suppressed by the existence of PO₄ framework formed on the surface of LiCoO₂ dramatically improving the cycleability and rate capability of LiCoO₂ even above 4.5 V. Using this approach, we could easily change the surface O₂-framework of LiCoO₂ to PO₄-framework. Consequently, phosphidated LiCoO₂ came to retain very high structural stability and a stable surface film was formed in contact with electrolyte during charging/discharging. Phosphidated LiCoO₂ exhibited greater bulk and surface stability compared to pristine LiCoO₂ even in high voltage range, suggesting that this methodology will be also promising for other high-voltage cathode materials in lithium ion batteries.