Atomistic Simulation Studies of the Electrochemical Activity in Nanocrystalline Li2MnO3

Layered-layered oxide composite cathodes, typically of composition xLi₂MnO₃·(1−x)LiMO₂, have attracted much attention, for essentially doubling the capacity available in the earlier generation of Li-ion batteries based on LiCoO₂ [1].  However, their use is currently limited by, amongst others, voltage fade [2]. Hence, further studies on one of the end members of the composites, Li₂MnO₃, could shed valuable insights on such problems.

Li₂MnO₃ is known to be electrochemically inactive in the parent bulk form and can be rendered Li-active by leaching Li or Li₂O from the structure; these Li-deficient compounds are reported to have high intercalation capacities and good reversibility [3]. Alternatively, when synthesized in a nanocrystalline form, Li₂MnO₃ has been shown to be electrochemically active, yielding capacities up to 200 mAh/g, and excellent capacity retention over multiple cycles [4].

In the current study simulated amorphisation recrystallisation method [5] is used to nucleate and crystallise ternary nanoparticle of Li₂MnO₃. Simulations of the charging of Li₂MnO₃ reveal that the reason nanocrystalline- Li₂MnO₃ is electrochemically active, in contrast to the parent bulk- Li₂MnO₃, is because in the nanomaterial the alternating Li planes are held apart by Mn ions, which act as a pseudo ‘point defect scaffold’. The Li ions are then able to diffuse, via a vacancy driven mechanism, throughout the nanomaterial in all spatial dimensions while the ‘Mn defect scaffold’ maintains the structural integrity of the layered structure during charging. Our findings reveal that oxides, which comprise cation disorder, can be potential candidates for electrodes in rechargeable Li-ion batteries.

References

[1] Thackeray, M.M., Johnson, C.S., Vaughey, J.T. Li, N. and Hackney, S.A., J. Mater. Chem. 15, 2257, 2005.

[2] Croy J.R., Gallagher K.G., Balasubramanian M., Long B.R. and Thackeray M.M., J. Electrochem. Soc. 161, A318, 2014.

[3] Thackeray M. M. Progr. Solid State Chem. 25, 1, 1997.

[4] Jain, G.R., Yang, J.S., Balasubramanian, M. and Xu, J.J., Chem. Mater. 17, 3850, 2005.      

[5] P.E.Ngoepe, R.R. Maphanga and D.C. Sayle, (2013), “Towards the Nanoscale”, Chapter 9 in Computational Approaches to Energy Materials, pp 261-290, edited by C.R.A. Catlow, A. Sokol and A. Welsch, John Wiley and Sons Ltd.