Anomalous manganese activation of a pyrophosphate cathode in sodium ion batteries: A combined experimental and theoretical study
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Date
2013Author
Park, Chan SunKim, Heejin
Shakoor, Rana A.
Yang, Eunjeong
Lim, Soo Yeon
Kahraman, Ramazan
Jung, Yousung
Choi, Jang Wook
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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.
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