Anomalous Manganese Activation of a Pyrophosphate Cathode in Sodium Ion Batteries: A Combined Experimental and Theoretical Study

Abstract

Sodium ion batteries (SIBs) have many advantages such as the low price and abundance of sodium raw materials that are suitable for large-scale energy storage applications. Herein, we report an Mn-based pyrophosphate, Na₂MnP₂O₇, as a new SIB cathode material. Unlike most Mn-based cathode materials, which suffer severely from sluggish kinetics, Na₂MnP₂O₇ exhibits good electrochemical activity at ∼3.8 V vs Na/Na⁺ with a reversible capacity of 90 mAh g^–1 at room temperature. It also shows an excellent cycling and rate performance: 96% capacity retention after 30 cycles and 70% capacity retention at a c-rate increase from 0.05C to 1C. These electrochemical activities of the Mn-containing cathode material even at room temperature with relatively large particle sizes are remarkable considering an almost complete inactivity of the Li counterpart, Li₂MnP₂O₇. Using first-principles calculations, we find that the significantly enhanced kinetics of Na₂MnP₂O₇ is mainly due to the locally flexible accommodation of Jahn–Teller distortions aided by the corner-sharing crystal structure in triclinic Na₂MnP₂O₇. By contrast, in monoclinic Li₂MnP₂O₇, the edge-sharing geometry causes multiple bonds to be broken and formed during charging reaction with a large degree of atomic rearrangements. We expect that the similar computational strategy to analyze the atomic rearrangements can be used to predict the kinetics behavior when exploring new cathode candidates.