Heejin Kim

Publications (2)19.81 Total impact

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

  Chan Sun Park, Heejin Kim, Rana A Shakoor, Eunjeong Yang, Soo Yeon Lim, Ramazan Kahraman, Yousung Jung, Jang Wook Choi
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  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 a Mn-based pyrophosphate, Na2MnP2O7, as a new SIB cathode material. Unlike most Mn-based cathode materials that 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 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.
  Journal of the American Chemical Society 01/2013; · 9.91 Impact Factor
• Article: Site-specific transition metal occupation in multicomponent pyrophosphate for improved electrochemical and thermal properties in lithium battery cathodes: a combined experimental and theoretical study.

  Rana A Shakoor, Heejin Kim, Woosuk Cho, Soo Yeon Lim, Hannah Song, Jung Woo Lee, Jeung Ku Kang, Yong-Tae Kim, Yousung Jung, Jang Wook Choi
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  ABSTRACT: As an attempt to develop lithium ion batteries with excellent performance, which is desirable for a variety of applications including mobile electronics, electrical vehicles, and utility grids, the battery community has continuously pursued cathode materials that function at higher potentials with efficient kinetics for lithium insertion and extraction. By employing both experimental and theoretical tools, herein we report multicomponent pyrophosphate (Li(2)MP(2)O(7), M = Fe(1/3)Mn(1/3)Co(1/3)) cathode materials with novel and advantageous properties as compared to the single-component analogues and other multicomponent polyanions. Li(2)Fe(1/3)Mn(1/3)Co(1/3)P(2)O(7) is formed on the basis of a solid solution among the three individual transition-metal-based pyrophosphates. The unique crystal structure of pyrophosphate and the first principles calculations show that different transition metals have a tendency to preferentially occupy either octahedral or pyramidal sites, and this site-specific transition metal occupation leads to significant improvements in various battery properties: a single-phase mode for Li insertion/extraction, improved cell potentials for Fe(2+)/Fe(3+) (raised by 0.18 eV) and Co(2+)/Co(3+) (lowered by 0.26 eV), and increased activity for Mn(2+)/Mn(3+) with significantly reduced overpotential. We reveal that the favorable energy of transition metal mixing and the sequential redox reaction for each TM element with a sufficient redox gap is the underlying physical reason for the preferential single-phase mode of Li intercalation/deintercalation reaction in pyrophosphate, a general concept that can be applied to other multicomponent systems. Furthermore, an extremely small volume change of ~0.7% between the fully charged and discharged states and the significantly enhanced thermal stability are observed for the present material, the effects unseen in previous multicomponent battery materials.
  Journal of the American Chemical Society 06/2012; 134(28):11740-8. · 9.91 Impact Factor