Observation of the highest Mn3+/Mn2+ redox potential of 4.45 V in a Li2MnP2O7 pyrophosphate cathode

By exploring the pyrophosphate chemistry for rechargeable Li-ion batteries, we report the synthesis and electrochemical characterization of a Li 2 MnP 2 O 7 cathode. Easily prepared by conventional solid-state synthesis (at 600 °C), the Li 2 MnP 2 O 7 displays an Mn 3+ /Mn 2+ redox potential centered at 4.45 V versus lithium. It has registered the highest voltage ever obtained for the Mn 3+ /Mn 2+ redox couple in any Mn-based cathode material. Following the pristine and partially (Mn) substituted Li 2 FeP 2 O 7 (3.5-3.9 V vs. Li) and Li 2 CoP 2 O 7 (4.9 V vs. Li), which show the highest Fe 3+ /Fe 2+ and Co 3+ /Co 2+ redox potentials among the known cathode compounds, the highest Mn 3+ /Mn 2+ redox voltage ( ca. 4.45 V) in Li 2 MnP 2 O 7 establishes 'pyrophosphates' as a novel family displaying the highest M 3+ /M 2+ redox potentials among all polyanionic compounds. Fundamental study of these pyrophosphates can provide useful insights into design of high-voltage cathode materials. Exploring the pyrophosphate chemistry, we have observed an Mn 3+ /Mn 2+ redox potential at 4.45 V in Li 2 MnP 2 O 7 cathode, benchmarking the highest ever Mn-redox potential among all known oxide and polyanionic Mn-based cathode materials.