High-voltage cycling behavior of thin-film LiCoO₂ cathodes

Thin-film LiCoO₂ cathodes having grains of 0.3 μm diam combined with the Lipon electrolyte exhibit excellent capacity retention when cycled up to 4.4 V. High capacity (170 ± 10 mAh/g) and energy density (690 ± 60 mWh/g) were obtained between 3.0 and 4.4 V at 0.1 mA/cm², with the capacity fade rate <0.02% per cycle. No evidence of grain fracture was detected by direct observation of the cycled thin-film cathodes by transmission electron microscopy. When cycled to >4.4 V, there is a more rapid increase in cell resistance and reduced capacity at high current densities. This is attributed to the onset of a phase transition at >4.4 V accompanied by an abrupt change in the c parameter. When a LiCoO₂ cathode is completely delithiated by charging to 5.0 V, cycling ability is still retained, but Li reinsertion into CoO₂ is not a fully reversible reaction of the Li extraction from LiCoO₂.