Effect of P<INF>2</INF>O<INF>5</INF> and AlPO<INF>4</INF> Coating on LiCoO<INF>2</INF> Cathode Material

The thermal stability and electrochemical properties of P<INF>2</INF>O<INF>5</INF>-coated and AlPO<INF>4</INF>-coated LiCoO<INF>2</INF> powders were compared with those of bare LiCoO<INF>2</INF>. 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<INF>4</INF>-coated, bare, and P<INF>2</INF>O<INF>5</INF>-coated LiCoO<INF>2</INF>. Differential scanning calorimetry (DSC) results of both the P<INF>2</INF>O<INF>5</INF>-coated and AlPO<INF>4</INF>-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<INF>2</INF>), and the coating greatly reduced the amount of exothermic heat generation by approximately 1 order of magnitude, compared to that of the bare LiCoO<INF>2</INF>. This is consistent with the result of the 12-V overcharge tests in the Li-ion cells with the bare and coated LiCoO<INF>2</INF>. 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<INF>2</INF> under the same conditions caught fire and exploded, with the cell-surface temperature reaching 450 °C.