Effect of P₂O₅ and AlPO₄ Coating on LiCoO₂ Cathode Material

The thermal stability and electrochemical properties of P 2 O 5 -coated and AlPO 4 -coated LiCoO 2 powders were compared with those of bare LiCoO 2 . Even though all samples had a similar initial capacity at a low current rate, the capacity retention after 20 cycles at 1 C (= 140 mA/g) was in the order of AlPO 4 -coated, bare, and P 2 O 5 -coated LiCoO 2 . Differential scanning calorimetry (DSC) results of both the P 2 O 5 -coated and AlPO 4 -coated samples (charged up to 4.3 V) showed that the initial exothermic-reaction temperatures with the flammable electrolytes increased to ∼230 °C (from ∼180 °C in bare LiCoO 2 ), and the coating greatly reduced the amount of exothermic heat generation by approximately 1 order of magnitude, compared to that of the bare LiCoO 2 . This is consistent with the result of the 12-V overcharge tests in the Li-ion cells with the bare and coated LiCoO 2 . Upon charging up to 12 V at the rate of 2 C, the coated samples did not exhibit thermal runaway, and the cell-surface temperature remained below 120 °C. On the other hand, bare LiCoO 2 under the same conditions caught fire and exploded, with the cell-surface temperature reaching 450 °C.