Contributed Talk  - Thursday, 16 September I 14:20 PM (CEST)

Rapidly growing market of electric vehicles (EV) demands secondary batteries with energy and power densities higher than those present on the market. Currently this great demand is met mainly by gradual improvement of conventional batteries, development of new electrode materials, and tweaking the existing ones. For more than a decade particular attention of the research community has been attracted to Li-rich layered metal oxides (LMO) with formula Li(LixMn1-x-y-zNiyCoz)O2 (further referred as Li-rich NMC), which were obtained by adding excessive amount of Li into layered oxides of nickel, manganese and cobalt (NMC). It possess reversible capacity up to 280 mAh/g and average discharge potential exceeding 3.5V vs Li+/Li which is a substantial advance over the commercially available NMC and NCA cathodes with reversible capacity not exceeding 200 mAh/g. Despite of such attractive properties, Li-rich NMC is still at the laboratory stage of development due to rapid voltage decay and poor rate capability. Apart from these major challenges, it possess another puzzling phenomena observed after a certain degree of Li excess, which manifests itself as a gradual capacity increase upon cycling, which is referred as activation. It may take up to several tenth of cycles to reach the maximum discharge capacity. Several hypothesis were proposed in order to explain this phenomena: (1) LiCO₃ remaining after synthesis and hindering Li diffusion 1; (2) activation of electrochemically inactive Li₂MnO₃ phase ²; (3) activation of the Mn redox process. To date none of these hypotheses are sufficiently confirmed by experimental data. In this work we present a complementary microscopic approach for investigation of the activation phenomena in overlithiated Li-rich NMC. By combining functional atomic force microscopy, scanning transmission electron microscopy, and Raman spectroscopy on cross-sections of secondary Li-rich NMC particles, we collected a set of data that favors the Mn redox activation hypothesis. The Mn redox activation proceeds through formation of the core-shell structure within secondary Li-rich NMC particles upon cycling, which was complementary detected by Kelvin Probe Force Microscopy, conductive atomic force microscopy, nanoindentation, and confocal Raman spectroscopy. These results, firmly supported by electrochemical and electron microscopy data, extend our understanding of processes, underlying capacity changes in Li-rich NMC materials. Equally important is demonstration of AFM capabilities for battery research – an area where AFM is underrepresented.

This work was supported by Russian Science Foundation (RSF), grant agreement № 19-73-00095.

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References:

(1)        Pimenta, V.; Sathiya, M.; Batuk, D.; Abakumov, A. M.; Giaume, D.; Cassaignon, S.; Larcher, D.; Tarascon, J. M. Synthesis of Li-Rich NMC: A Comprehensive Study. Chem. Mater. 2017, 29, 9923–9936.

(2)        Xiao, L.; Xiao, J.; Yu, X.; Yan, P.; Zheng, J.; Engelhard, M.; Bhattacharya, P.; Wang, C.; Yang, X. Q.; Zhang, J. G. Effects of Structural Defects on the Electrochemical Activation of Li2MnO3. Nano Energy 2015, 16, 143–151.