Effect of P2O5 and AlPO4 coating on LiCoO2 cathode material

Abstract

The thermal stability and electrochemical properties of P2O5-coated and AlPO4-coated LiCoO2 powders were compared with those of bare LiCoO2. 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 AlPO4-coated, bare, and P2O5-coated LiCoO2. Differential scanning calorimetry (DSC) results of both the P2O5-coated and AlPO4-coated samples (charged up to 4.3 V) showed that the initial exothermic-reaction temperatures with the flammable electrolytes increased to similar to230 degreesC (from similar to180 degreesC in bare LiCoO2), and the coating greatly reduced the amount of exothermic heat generation by approximately 1 order of magnitude, compared to that of the bare LiCoO2. This is consistent with the result of the 12-V overcharge tests in the Li-ion cells with the bare and coated LiCoO2. 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 degreesC. On the other hand, bare LiCoO2 under the same conditions caught fire and exploded, with the cell-surface temperature reaching 450 degreesC.