Joule
Volume 5, Issue 4, 21 April 2021, Pages 975-997
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Article
Cycling mechanism of Li2MnO3: Li–CO2 batteries and commonality on oxygen redox in cathode materials

https://doi.org/10.1016/j.joule.2021.02.004Get rights and content
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Highlights

  • Charge plateau of Li2MnO3 is from oxygen release and surface carbonate reactions

  • Mn(III/IV) redox is solely responsible for the reversible bulk Li2MnO3 cycling

  • Oxygen redox shares common nature in both Li-rich and conventional cathodes

  • Li2MnO3 and alkali-rich materials could be superior catalysts for Li–CO2/air batteries

Context & scale

In the debates on how to achieve high energy density battery cathodes, Li-rich compounds are often considered superior over conventional materials due to their high capacity associated with the oxygen redox reactions. Here, we clarify both the bulk and surface reaction mechanisms of Li2MnO3 during the initial and later cycles. Our results reveal that the initial charge plateau is from two types of surface activities, followed by predominating Mn redox reactions with no sign of reversible lattice oxygen redox. The surface chemistry of Li2MnO3 indicates a highly reactive surface to facilitate carbonate formation and decomposition, inspiring a Li-CO2/air battery with Li2MnO3 as a superior electrocatalyst. The comparison between Li-rich, conventional, and Li2MnO3 suggests that the oxygen redox in Li-rich and conventional materials is of the same nature and origin.

Summary

Li2MnO3 has been considered to be a representative Li-rich compound with active debates on oxygen activities. Here, by evaluating the Mn and O states in the bulk and on the surface of Li2MnO3, we clarify that Mn(III/IV) redox dominates the reversible bulk redox in Li2MnO3, while the initial charge plateau is from surface reactions with oxygen release and carbonate decomposition. No lattice oxygen redox is involved at any electrochemical stage. The carbonate formation and decomposition indicate the catalytic property of the Li2MnO3 surface, which inspires Li-CO2/air batteries with Li2MnO3 acting as a superior electrocatalyst. The absence of lattice oxygen redox in Li2MnO3 questions the origin of the oxygen redox in Li-rich compounds, which is found to be of the same nature as that in conventional materials based on spectroscopic comparisons. These findings provide guidelines on understanding and controlling oxygen activities toward high-energy cathodes and suggest opportunities on using alkali-rich materials for catalytic reactions.

Keywords

Li-ion battery
Li-air battery
Li-CO2 battery
high energy density cathode
electrocatalyst and catalyst
Li-rich material
conventional cathode material
anionic redox reaction
soft X-ray spectroscopy
resonant inelastic X-ray scattering (RIXS)

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11

These authors contributed equally

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