Comparison of Al2O3- and AlPO4-coated LiCoO2 cathode materials for a Li-ion cell

The electrochemical and thermal properties of AlPO4-coated LiCoO2 were compared with those of the Al2O3-coated cathode. Even though cycling stability of the Al2O3-coated cathode was apparently similar to that of the AlPO4-coated sample at 4.6 V cycling, increasing the charge-cutoff voltage to 4.8 V led to the rapid capacity decay, exhibiting ∼20% larger capacity-fading than the AlPO4-coated cathode. The irreversible capacity of the Al2O3-coated cathode (∼34 mAh g−1) was also larger than that of AlPO4-coated cathode (∼24 mAh g−1) at a charge-cutoff voltage of 4.8 V. This was attributed to the increase in the amount of Co dissolution into the electrolyte at higher voltage. Differential scanning calorimetry results showed that the overall exothermic-heat release of the Al2O3-coated cathode was similar to that of the bare cell, but the onset temperature of oxygen evolution from the cathode was increased to ∼190 °C (up from ∼170 °C in the bare cell). On the other hand, AlPO4-coated LiCoO2 showed a much improved onset temperature of the oxygen evolution at ∼230 °C, and a much lower amount of exothermic-heat release, compared to the Al2O3-coated sample. These results were correlated with the 12 V overcharge experiments: the Li-ion cell containing AlPO4-coated LiCoO2 did not show a thermal runaway behavior in contrast to that containing bare, or Al2O3-coated cathode.