Aqueous-Based Post-Treatment of Li- and Mn-Rich Ncm
Authors: Hartmann Louis , Hin Ching Cheuck , Kipfer Tim , Gasteiger Hubert Andreas 
Published: July 2022

To achieve lithium-ion batteries with high energy density at competitive prices for automotive and large-scale applications, cathode active materials (CAMs) based on Li- and Mn-rich NCMs (LMR-NCMs), like Li1.14(Ni0.26Co0.14Mn0.60)0.86O2, are promising candidates.[1] However, LMR-NCMs still suffer from high gassing, particularly during cell formation, and detrimental voltage and capacity fading over their cycle life.[2,3] Different approaches can be utilized to try to mitigate these issues, such as the use of electrolyte additives, novel material designs (compositional gradients, etc.), or post-treatments.[4–7]

In this study, we investigated the effect of a water-based post-treatment of LMR-NCM. It consists of a washing process of the LMR-NCM that results in a partial delithiation of its near-suface region by a lithium/proton ion exchange, while at the same time avoiding transition metal dissolution. A recalcination of this protonated near-surface layer of the LMR-NCM particles results in the formation of a protective spinel-like surface layer. We observed that after this treatment, the gassing during formation is decreased by »10-fold. Furthermore, the cycling performance of graphite/LMR-NCM full-cells is also drastically increased.
By conducting on-line electrochemical mass spectrometry (OEMS) measurements, we analyzed the gas evolution of as-received and post-treated LMR-NCMs during the first activation cycle. It is known from the literature that the activation of LMR-NCMs is accompanied by a strong O2 and CO2 evolution during the first charge.[2] As seen in Figure 1, CO2 is evolved simultaneously with O2 from Li/LMR-NCM half-cells, prepared with untreated, as-received LMR-NCM (as-received, black line). With post-treated LMR-NCM, both CO2 and O2-evolution during the activation cycle are reduced by »10-fold (post-treated, green line). Only a small amount of the first-charge capacity (<10%) is lost due to the post-treatment, as seen in Figure 1a, reflecting the slight extent of delithiation that is part of the post-treatment.
As will be shown, cycling tests of graphite/LMR-NCM full-cells with a post-treated LMR-NCM reveal a greatly increased cycling stability in comparison to cells with an as-received material. Using TGA-MS, XPS and ICP-OES, we further elucidate the beneficial mechanism of the here developed water-based post-treatment.

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Acknowledgements

This work is financially supported by the BASF SE Network on Electrochemistry and Battery Research.
Figure 1: OEMS measurements of the first lithiation half-cycle to 4.8 V of Li/LMR-NCM half-cells with either an as-received (black line) or a post-treated LMR-NCM (green line). a) Cell voltage vs. time at a C/rate of C/10 (referenced to 250 mAh/g delithiation capacity). b) CO2 evolution given in units of μmol/gCAM (determined from the signal at m/z = 44). c) O2 evolution (from m/z = 32). The half-cells were charged at 25°C, using an FEC/DEC (2:8) electrolyte with 1.0 m LiPF6.

Figure 1