Thermal Stability and Reactivity of Cathode Materials for Li-Ion Batteries

The thermal stability of electrochemically delithiated Li0.1Ni0.8Co0.15Al0.05O2 (NCA), FePO4 (FP), Mn0.8Fe0.2PO4 (MFP), hydrothermally synthesized VOPO4, LiVOPO4 and electrochemically lithiated Li2VOPO4 is investigated by differential scanning calorimetry (DSC) and thermogravimetric analysis, coupled with mass spectrometry (TGA-MS). The thermal stability of the delithiated materials is found to be in the order: NCA\textbackslash textless VOPO4\textbackslash textless MFP\textbackslash textless FP. Unlike the layered oxides and MFP, VOPO4 does not evolve O2 on heating. Thus VOPO4 is less likely to cause a thermal runaway in batteries at elevated temperature, and so is inherently safer. The lithiated materials LiVOPO4, Li2VOPO4 and LiNi0.8Co0.15Al0.05O2 are found to be stable in the presence of electrolyte, but sealed capsule high-pressure experiments show a phase transformation of VOPO4 {$\rightarrow$} HVOPO4 {$\rightarrow$} H2VOPO4 when VOPO4 reacts with electrolyte (1 M LiPF6 in EC: DMC=1:1) between 200 and 300 {$^\circ$}C. Using first principles calculations, we confirm that the charged VOPO4 cathode is indeed predicted to be marginally less stable than FP, but significantly more stable than NCA in the absence of electrolyte. An analysis of the reaction equilibria between VOPO4 and EC using a multi-component phase diagram approach yields products and reaction enthalpies that are highly consistent with the experiment results.