Layered lithium transition metal oxides, typically containing nickel, cobalt and manganese (NCM, Li_1+δ[Ni_xCo_yMn_z]_1-δO₂ with x+y+z=1), are the most widely used cathode active material (CAM) in Li-ion batteries for portable devices and battery electric vehicles.¹ The class of NCM materials has several advantageous properties such as high energy and power densities and good cycling stability. However, depending on the Ni:Co:Mn ratio, the electrochemical performance characteristics of the materials can differ significantly. Increasing the Ni content is a current research focus, as it typically allows for a higher reversible lithium usage at a given cell potential, thereby improving the specific capacity.² At the same time, highly Ni-rich NCM materials undergo severe structural and mechanical changes, which adversely affect their cycling stability. In this respect, NCM-811 (Li_1+δ[Ni_0.8Co_0.1Mn_0.1]_1-δO₂) is currently one of the most Ni-rich NCM materials with proven cycling stability.³ In this work, we study the structural processes in NCM-811 over the course of 1000 cycles, cycled at a C-rate of C/2 until 4.5 V vs. Li⁺/Li in a pouch-cell setup at the Diamond Light Source (see Figure 1a). By means of in-situ synchrotron X-ray powder diffraction, the structural evolution of NCM-811 was monitored in regular intervals both in the completely charged and discharged state (under open circuit voltage conditions). As the lattice parameter ratio, c/a, directly correlates with the lithium amount, x_Li, in the bulk material,^4,5 the application of a calibration curve, x_Li = f(c/a), allows for monitoring the effective capacity window of the NCM-811 CAM during cycling, and thus for clarifying the observed electrochemical capacity loss (ΔC_EC, see Figure 2b). Due to an increasing cathode overpotential, the accessible capacity determined by XRD analysis becomes smaller within the fixed potential cut-offs both in the charged (ΔC_charge) and discharged state (ΔC_discharge). As the sum of these two loss terms, ΔC_XRD, does not reach the actual capacity loss, ΔC_EC, some fraction of the NCM-811 material has to be entirely lost during cycling (see Figure 1c). By using additional techniques such as electrochemical impedance spectroscopy and X-ray photoelectron spectroscopy, this material loss can be assigned to a nm-thick, resistive surface layer around the primary particles. This resistive layer is in turn responsible for the observed overpotential losses. In contrast to the surface layer, the bulk material turned out to be stable, with a constant Ni disorder of ≈3% on the Li layer over 1000 cycles. For this reason, we think that future research on Ni-rich materials has to make substantial progress in controlling and stabilizing their surface properties. Figure 1: NCM-811/pre-lithiated graphite pouch-cells cycled at the Diamond Light Source over 1000 cycles at C/2 and ≈22°C in the potential window of 3.0-4.5 V vs. Li⁺/Li (cathode potential controlled vs. Li reference-electrode). (a) Discharge capacity of two cells. (b) Comparison of the observed electrochemical capacity loss and the losses determined by XRD analysis relative to the 18^th cycle, illustrated for cell 1. (c) Absolute material loss under these cycling conditions relative to the pristine state for both cells (accounting to ≈2 mAh/g already in the 18^th cycle). Acknowledgements: We want to acknowledge BASF SE for the support within the frame of its scientific network on electrochemistry and batteries. We also thank the Diamond Light Source for access to beamline I11 (Beamtime Award EE16866). References: G. E. Blomgren, J. Electrochem. Soc., 164, A5019–A5025 (2017). D. Andre, S.-J. Kim, P. Lamp, S. F. Lux, F. Maglia, O. Paschos, B. Stiaszny, J. Mater. Chem. A, 3, 6709–6732 (2015). H.-H. Ryu, K.-J. Park, C. S. Yoon, and Y.-K. Sun, Chem. Mater., 30, 1155–1163 (2018). I. Buchberger, S. Seidlmayer, A. Pokharel, M. Piana, J. Hattendorff, P. Kudejova, R. Gilles, H. A. Gasteiger, J. Electrochem. Soc., 162, A2737–A2746 (2015). L. de Biasi, A. O. Kondrakov, H. Geßwein, T. Brezesinski, P. Hartmann, J. Janek, J. Phys. Chem. C, 121, 26163–26171 (2017). Figure 1