Proceedings of Materials for Sustainable Development Conference (MAT-SUS) (NFM22)
DOI: https://doi.org/10.29363/nanoge.nfm.2022.103
Publication date: 11th July 2022
LiNi0.8Mn0.1Co0.1O2 (NMC811) is among the cathode materials most discussed for high-performance Li-ion batteries thanks to its capacity of approximately 200 mAh g-1 and its low Co content. Currently, NMC811 is considered for implementation in next-generation electric vehicles, but the high Ni content (> 60 %) still poses challenges, such as oxygen loss at voltages above 4.3 V, impedance rise, transition metal dissolution, and finally, a loss of active lithium in the electrode. These challenges can be mitigated by utilizing appropriate coating and doping strategies and limiting the voltage to standard cycling conditions (2.8 - 3.0 V to 4.3 - 4.5 V). NMC811 can be overlithiated to form Li2NMC811, in analogy to pure LiNiO2, which can form Li2NiO2 [1]. Such a high lithiation degree increases the initial discharge capacity but leads to a fast voltage loss, especially during the first cycle [2]
In this study, we address two intriguing research questions: (i) Can we control the interface of Ni-rich cathode material with the electrolyte and prevent the formation of detrimental solid electrolyte interphase (ii) Is it possible to take advantage of multi-electron cycling by introducing additional lithium into the host NMC811 structure? To address these questions, Li-rich NMC811 thin-film cathodes were prepared by sputtering from a Li2O-NMC811 target. Li-rich NMC811 cathodes were tested with LiPON as a solid electrolyte in a thin-film architecture, which offers a simplified 2D model with direct access to the cathode-electrolyte interface. The solid-state electrolyte helped to stabilize the interface and prevented capacity fading, voltage decay, and interface resistance growth. In combination with LiPON, cycling at extended voltage ranges of 1.5-4.7 V was possible without increasing interfacial resistance. It was possible to reversible cycle Li2NMC811 by lowering the applied potential to 1.5 V. resulting in a discharge capacity of up to 350 mAh g-1 due to multi-electron cycling. Overall, the all-solid-state cells with a lithium metal anode can cycle in the range of 1.5 V to 4.3 V for 1000 cycles with an average coulombic efficiency of 98.79 %. Our results demonstrate how solid electrolytes that are stable against NMC811 cathodes can unlock the full potential of this Ni-rich cathode class.
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