A Comprehensive Study on the Effect of Washing on Nickel-Rich Layered Transition Metal Oxides: Surface Characterization after Li+/H+-Exchange

Abstract

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. Reference s [1] U. Kim. D. Jun, K. Park, Q. Zhang, P. Kaghazchi, D. Aurbach, D. Major, G. Goobes, M. Dixit, N. Leifer, C. Wang, P. Yan, K. Kim, C. Yoon and Y. Sun, Energy Environ. Sci. 11 1271-1279 (2018).[2] W. Li, E. M. Erickson and A. Manthiram, Nature Energy 5 26 (2020).[3] H.-J. Noh, S. Youn, C.S. Yoon and Y.-K. Sun, J. Power Sources 233 121 (2013).[4]J. Sicklinger, M. Metzger, H. Beyer, D. Pritzl and H.A. Gasteiger,J. Electrochem. Soc. 166 (12) A2322 (2019).[5] L. Hartmann, D. Pritzl, H. Beyer and H.A. Gasteiger, J. Electrochem. Soc. 168 070507 (2021).[6] S. Renfrew and B.D. McCloskey, J. Am. Chem. Soc. 139 17853 (2017).[7] M. Metzger, B. Strehle, S. Solchenbach and H.A. Gasteiger, J. Electrochem. Soc. 163 (7) A1219 (2016).[8] R. Jung, R. Morasch, P. Karayaylali, K. Phillips, F. Maglia, C. Stinner, Y. Shao-Horn and H.A. Gasteiger, J. Electrochem. Soc. 165 (2) A132 (2018).[9] D. Pritzl, T. Teufl, A.T.S. Freiberg, B. Strehle, J. Sicklinger, H. Sommer, P. Hartmann and H.A. Gasteiger, J. Electrochem. Soc. 166 (16) A4056 (2019).[10] S. Oswald, R. Wilhelm, T. Kratky, G. Kieslich, L. Szentmiklósi, B. Maróti, I. Harsányi, M. Piana, S. Günther and H.A. Gasteiger, manuscript in preparation. [11] R. Wilhelm, S. Oswald and H.A. Gasteiger, manuscript in preparation. 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.