Pyrophosphate Chemistry toward Safe Rechargeable Batteries

We demonstrate that pyrophosphate anion can result in metal pyrophosphate cathode materials with high thermal stabilities. High temperature behaviors for the delithiated states of Li2FeP2O7 and Li2MnP2O7 in the P2(1)/c symmetry are studied. Above 540 degrees C, the singly delithiated structure LiFeP2O7 undergoes an irreversible phase transformation to the ground state polymorph with a symmetry of P2(1). Intermediate delithiated compounds Li2-xFeP2O7 (0 < x < 1) convert to a mixture of LiFeP2O7 in the P2(1) symmetry and Li2FeP2O7 in the P2(1)/c symmetry. No decomposition is observed for both the singly and partially delithiated compounds until 600 degrees C showing the high thermal stabilities of the compounds. Analysis of phase stabilities reveals that LiFeP2O7 (P2(1)/c) is intrinsically more stable than FePO4 (olivine) against reduction (high temperature). Similar high thermal stability is also observed for Li1.4MnP2O7. It decomposes to Li2MnP2O7, Mn2P2O7, LiPO3, and O-2 at 450 degrees C, much higher than the olivine counterpart MnPO4. The high stability of these metal pyrophosphates is rationalized by the stability of the P2O74- anion.