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-LiMn _0.8 Fe_0.2PO₄, and Carbon coated-LiFePO ₄), LiCoO₂ and LiNi_0.8Co_0.15Al _0.05O₂ (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 LiFePO ₄ 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 LiMnPO ₄ or LiMnFePO₄ have lower thermal stability than delithiated LiFePO₄. The thermal reactivity of the layered LiCoO ₂ and LiNi_0.8Co_0.15Al_0.05O ₂ 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 200C.