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

Monday, 27 July 2015: 11:50
Carron (Scottish Exhibition and Conference Centre)
S. H. Min, M. R. Jo, and Y. M. Kang (Dept. of Energy and Materials Eng., Dongguk University)
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/Li1-xCoxO2 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.