A Comprehensive Study on the Effect of Washing on Nickel-Rich Layered Transition Metal Oxides: Surface Characterization after Li+/H+-Exchange
Authors: Wilhelm Rebecca , Oswald Stefan , Kratky Tim , Reinschlüssel Leonhard J. , Piana Michele , Günther Sebastian , Gasteiger Hubert Andreas 
Published: February 2024

With an increasing demand for high-capacity cathode active materials (CAMs) for lithium-ion batteries, nickel-rich layered lithium nickel cobalt manganese oxides (NCMs, LiNixCoyMnzO2, with x+y+z = 1) are the currently favored CAMs [1]. However, a higher nickel content does not only increase the specific capacity of NCMs, but also the reactivity of the CAM surface, which makes the material more prone to surface contaminants [2]. These contaminants consist of lithium salts from excess synthesis precursors [3], but can also be formed during improper storage [4,5]. During battery cycling, these contaminants decompose during the first charge/discharge cycles, leading to excessive gassing [6], decomposition of electrolyte solvents and salts [7], as well as to impedance build-up and capacity fading [8].
Since these contaminants are generally soluble in water, CAMs are usually washed after synthesis. Exposure to liquid or gaseous water, however, is accompanied by a Li+/H+-exchange, whereby protons intercalate into the CAM [9]. Within the framework of this study, we examine the kinetics of the Li+/H+-exchange for a Ni-rich NCM831205 (LiNi0.83Co0.12Mn0.05O2). The amount of protons inserted into the CAM upon washing was characterized by different methods, namely acid/base titration, thermogravimetric analysis, X-ray powder diffraction, and prompt-gamma activation analysis [10]. The effect of ion-exchanged protons on the electrochemistry was then investigated for NCM831205 samples with 1.0 and 1.5 mol% H+. Here it was found that protonated NCMs can be cycled fully reversibly, yet the capacity fading increases with increasing proton concentration while the rate capability decreases [11].
In this work, we now particularly focus on the surface and the bulk of differently washed NCM samples by using X-ray photoelectron spectroscopy (XPS) and X-ray absorption spectroscopy (XAS), respectively. It will be demonstrated that protons intercalate into the near-surface region of the NCM particles, as had only been hypothesized so far [9]. Moreover, the protonated NCM material shows a slightly different Ni-coordination, while maintaining its layered structure. The surface thickness of the protonated layer is determined by quantitative XPS analysis: for example, washing at room temperature for 60 min at a water/CAM mass ratio of 5/1 leads to an approximately 2.5 nm thick protonated surface layer. Lastly, temperature induced phase changes of the protonated surface region are characterized by in-situ by XPS during heating of the sample in vacuum. The data are then correlated with previous results from powder X-ray diffraction.

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Acknowledgement

This work is financially supported by the German Federal Ministry of Education and Research (BMBF) within the project AQua-POp (grant number 03XP0329B) as well as by the BASF SE Network on Electrochemistry and Battery Research.