Executive Summary
LiNixCoyMn1−x−yO2 (NCM) cathode materials have attracted considerable interest due to their favorable electrochemical properties. NCM cathode materials typically form ultra-dense agglomerates with limited porosity (~5%), raising the question of whether these agglomerates can be treated as solid particles in electrochemical evaluations and battery modeling. In this work, we develop and employ a single-particle experimental setup to directly measure the kinetics of high-nickel NCM cathode materials at the single-particle scale. We decouple bulk and interfacial transport processes and explore the relationship between electrochemical kinetics and agglomerate structure for LiNi0.8Co0.1Mn0.1O2 and LiNi0.9Co0.08Mn0.02O2 cathode materials. Our extensive dataset of key kinetic parameters—exchange current density (i0) and diffusion coefficient (DLi)—demonstrates that electrolyte penetration within ultra-dense agglomerates must be considered in physics-based battery models, challenging the assumption that such agglomerates behave as solid particles. These insights, along with the large kinetic dataset, are essential for refining battery models and optimizing battery design.

Abstract
We developed and validated a single-particle experimental setup, achieving measurements of high-nickel NCM cathode materials with the commercial LiPF6 electrolytes—a challenging and seldom reported in the literature. Two sample series with controlled variables were employed to explore the relationship between particle size and electrochemical performance (Figure 1). One sample series consists of six single LiNi0.8Co0.1Mn0.1O2 (NCM80) particles with varying Figure but consistent primary particle sizes, while the other series comprises nine single LiNi0.9Co0.08Mn0.02O2 (NCM90) particles with varying primary but consistent secondary particle sizes. All particles were pre-cycled for activation prior to electrochemical evaluation. Key kinetic parameters, diffusion coefficients (DLi) and the exchange current density (i0), which reflect bulk diffusion and interfacial reaction rates, were determined at various open-circuit potentials (OCPs) using electrochemical impedance spectroscopy (EIS) and the galvanostatic intermittent titration technique (GITT). Agglomerate particle model (APM) and solid particle model (SPM) were employed to fit the experimental data. The SPM does not account for electrolyte-filled pores, assuming that electrochemical reactions occur on the surface of secondary particles and that lithium ions diffuse radially within secondary particles. In contrast, the APM considers electrolyte-filled pores, assuming that electrochemical reactions occur on the surface of primary particles and that lithium ions diffuse radially within primary particles.

i0 and DLi against the secondary particle sizes (diameter) at four different OCPs are shown in Figure 2. The dashed lines represent the linear fitting results for i0 and DLi, with the slopes of the lines and the coefficient of determination (R2) indicated. The average R2 values for i0 are 0.14 and 0.19 for SPM and APM, respectively, indicating no statistically significant correlations between i0 and secondary particle sizes. DLi shows an increasing trend with increasing secondary particle sizes under the SPM assumption, with an average R2 of 0.82. In contrast, the average slope of the fitting lines under the APM assumption decreased by a factor of 10, suggesting that DLi is independent of secondary particle sizes. These results provide direct evidence that for high-nickel NCM materials, both i0 and DLi are independent of the secondary particle size.

The dependence of electrochemical parameters on the primary particle size was further examined. We synthesized NCM90 cathode materials with three distinct primary particle sizes using the co-precipitation method. The primary particle size was tuned by adjusting the NH3·H2O feeding rate, resulting in D50 values of 396 nm, 664 nm, and 949 nm. We measured the electrochemical parameters i0 and DLi for nine NCM90 secondary particles (three particles for each primary particle size sample) using EIS and GITT. Figure 3a-b present the measured i0 and DLi for each particle at different OCPs. Compared to NCM80, the i0 values of NCM90 are one to two orders of magnitude higher, indicating that NCM90 has faster reaction kinetics at the cathode–electrolyte interface than NCM80 used in this work. The i0 values initially increase with increasing OCP, then decrease, reaching a maximum at mid-range OCPs. The difference in i0 for a single particle can be as large as tenfold. The DLi values of NCM90 are slightly higher than those of NCM80, consistent with the diffusion coefficients calculated using ab initio calculations.[1] The DLi values of all NCM90 particles show a similar trend with OCP. DLi initially decreases with increasing OCP, reaching a minimum at approximately 3.7 V. This initial decrease is due to the increased activation energy of Li diffusion with the oxidation of nickel ions, while the subsequent increase is due to a change in the dominant Li hopping mechanism from oxygen dumbbell hopping to tetrahedral site hopping, the latter having a smaller diffusion barrier.[1] We plotted i0 and DLi against the primary particle diameters at five different OCPs, as shown in Figure 3c-d. The dashed lines represent the linear fitting results for i0 and DLi, with the slopes of the lines and the coefficients of determination (R2) indicated. Both i0 and DLi show a decreasing trend with increasing primary particle sizes under the SPM assumption, with average R2 values of 0.95 and 0.76, respectively. Under the APM assumption, the average slope of the fitting lines decreased significantly, suggesting that i0 and DLi are independent of the primary particle size. Although primary particle sizes affect the stress distribution in the polycrystalline particles,[2] thus affecting diffusion coefficients,[3] our results show a limited impact, guiding the establishment of electrochemo-mechanical models.[4, 5]

Our extensive dataset demonstrated that both i0 and DLi are independent of both secondary and primary particle sizes, under the assumption of electrolyte penetration into ultra-dense agglomerates. However, while the diffusion and reaction time constants are unaffected by secondary particle size, they are dependent on the primary particle size. This finding aligns with the expectation that kinetic parameters are generally size-independent and supports the view that electrochemical reactions occur on the surfaces of primary particles, with radial lithium-ion diffusion. These insights provide a more accurate representation of the electrochemical conditions within agglomerate particles, which are vital for accurately describing physical processes in battery systems. Moreover, the large dataset of kinetic parameters gathered in this study will be valuable for improving the accuracy of performance predictions in battery models.