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 Li 2 FeP 2 O 7 and Li 2 MnP 2 O 7 in the P 2 1 / c symmetry are studied. Above 540 °C, the singly delithiated structure LiFeP 2 O 7 undergoes an irreversible phase transformation to the ground state polymorph with a symmetry of P 2 1 . Intermediate delithiated compounds Li 2- x FeP 2 O 7 (0 < x < 1) convert to a mixture of LiFeP 2 O 7 in the P 2 1 symmetry and Li 2 FeP 2 O 7 in the P 2 1 / c symmetry. No decomposition is observed for both the singly and partially delithiated compounds until 600 °C showing the high thermal stabilities of the compounds. Analysis of phase stabilities reveals that LiFeP 2 O 7 ( P 2 1 / c ) is intrinsically more stable than FePO 4 (olivine) against reduction (high temperature). Similar high thermal stability is also observed for Li 1.4 MnP 2 O 7 . It decomposes to Li 2 MnP 2 O 7, Mn 2 P 2 O 7, LiPO 3, and O 2 at 450 °C, much higher than the olivine counterpart MnPO 4 . The high stability of these metal pyrophosphates is rationalized by the stability of the P 2 O 7 4- anion.