This data set contains all the data presented in the main text figures and extended data figures (for the manuscript titled ‘Operando visualization of kinetically-induced lithium heterogeneities in single-particle layered Ni-rich cathodes’). The information on how the data was acquired and processed is detailed in the open access manuscript which has been deposited in this repository. The check-list ‘Repository_list_Joule.xlsx’ contains the information on the repository files and folders. Main Figures: Figure1: Monolithic NMC cathode and operando optical microscopy. (A) Schematic drawing of the key components of the electrochemical cell for optical microscopy. The cathode is a self-standing electrode composed of numerous NMC particles, carbon black and polytetrafluoroethylene (PTFE) binder. Aluminium mesh is used as a current collector. (WE = working electrode, CE = counter electrode). (B) Optical image of an active NMC particle in the electrochemical cell. (C) SEM image of the same monolithic NMC particle shown in (B), obtained after the optical measurements. (D) Schematic illustration of the crystal structure of the NMC cathode illustrating the alternating layers of LiO6 and TMO6 octahedra, where TM denotes transition metal. (E) Voltage (black) and current (blue) profile (top panel) and the normalised optical intensity of the active particle shown in (B) (bottom panel) during one charge-discharge cycle (comprising a C/3 constant-current (CC) charge and discharge between 4.3 and 3 V, followed by a two-hour constant-voltage (CV) hold at 3 V). Three C/3 cycles were performed prior to this cycle, finishing with two-hour voltage holds at 3 V. The C-rate was calculated based on a practical capacity of 210 mA h g-1, i.e., the current density for C/3 rate is 70 mA g-1. Scale bars, 1 μm. Figure2: Lithium heterogeneity in single-particle NMC at the beginning of charge. (A) Voltage and current profiles during the first one hour of charge at C/3 (top panel), and the normalised intensity changes, obtained by integrating over the whole active particle shown in (B) (bottom panel). (B) Normalised differential images of the active particle during the initial charging, for the time points indicated by black circles in (A). The total contrast is shown, which represents the fractional intensity change between the current frame and the first frame of the cycle (i.e., with no current applied). The colour scale is centred at zero (white), with positive values indicating an overall intensity increase (red) and negative values indicating a decrease (blue). (C) Voltage and current profiles during the first 20 min of charge at C/3 followed by a rest period (top panel), and the normalised intensity changes of a second active particle (bottom panel). (D) Normalised differential images of the active NMC particle during the charge-rest experiment. Scale bars, 1 μm. Figure3: Comparing modelling and experiments. (A) Sketch of the particle used in the modelling. (B) The effective lithium diffusion coefficient D_Li^eff≡D_Li/S as a function of lithium content (S=3.5). (C) Comparison of simulation and experimental imaging results, both conducted at a delithiation rate of C/3. The predicted degree of delithiation (1-θ) on the basal plane of the particle at various times during the charge. (D, E) Evolution of (D) the degree of delithiation in the simulation and (E) the total contrast in the optical images along the horizontal dotted line marked in (C). In (E) we include the corresponding predictions (shown with dashed curves) of the total contrast. (F) Evolution of measurements and predictions of total contrast at the centre of the particle (shown by the vertical dashed lines in (D)). Figure4: Lithium heterogeneity at the end of discharge. (A) Voltage and current profile (top panel) and normalised intensity changes of the active particle during a 1C rate CC charge and CCCV discharge cycle. CV was performed at 3 V for two hours. The dashed grey line (bottom panel) is a guide for the eye, representing 0 intensity change. The initial lithium content is estimated to be ~97% based on the open circuit voltage (OCV, ~3.5 V vs. Li/Li+). Note this near-full lithiation state was achieved by applying a two-hour voltage hold at 3 V after the end of CC discharge in the previous cycle. (B) Differential images of an NMC active particle during the discharge, at the times indicated by black circles in (A) (where a, b, c, d, e and f are at 90, 104, 140, 180, 224 and 298 min, respectively). Scale bars, 1 μm. The current at the end of the CV period was -7.59 μA (equivalent to ~C/200; negative sign denotes discharging current). Figure5: Summary of the lithium-ion distribution within the single active particle at various lithium contents. The circles show schematic representations of single-particle Ni-rich materials at various SoCs. The voltage profile is illustrative of the single-crystal NMC material used in this work, and was obtained in a half-cell cycled with a CC charge and CC discharge (C/20 rate) and a discharge CV hold at 3 V (for 24 hours).