On the Thermal Stability of Olivine Cathode Materials for Lithium-Ion Batteries

The thermal stability of pristine and electrochemically delithiated LiMPO4 (Carbon coated-LiMnPO4, Carbon coated-LiMn0.8Fe0.2PO4, and Carbon coated-LiFePO4), LiCoO2 and LiNi0.8Co0.15Al0.05O2 (NCA) composite electrodes with LiPF6 solutions in ethylene carbonate (EC)/dimethyl carbonate (DMC) and EC/propylene carbonate (PC), was investigated by differential scanning calorimetry (DSC) and thermogravimetric analysis, coupled with mass spectrometry. The thermal reactions products were measured by XRD and electron microscopy. The LiFePO4 and LiMnPO4 cathode materials were found to have comparable thermal stability in their pristine and fully delithiated states. The onset temperatures of the thermal reactions are lower in EC/DMC than in EC/PC solutions but the specific heat evolution of all the thermal reactions are higher with EC-PC solutions. No evidence was found that delithiated LiMnPO4 or Li[MnFe]PO4 have lower thermal stability than delithiated LiFePO4. The thermal reactivity of the layered LiCoO2 and LiNi0.8Co0.15Al0.05O2 cathode materials was found to be comparable to that of the LiMPO4 materials. Oxygen release was detected from the layered compounds upon their heating around 200°C.