The chemical and structural complexity of lithium-ion battery electrodes and their constituting materials requires comprehensive characterization techniques to reveal degradation phenomena at the mesoscale. For the first time, application of single-particle inductively coupled plasma-optical emission spectroscopy enables the investigation of the chemomechanical interplay on the particle level of lithium transition-metal oxide [e.g., Li(Ni1/3Co1/3Mn1/3)O2] cathode materials. The sample-inherent polydisperse size distribution of particles ranging up to 10 μm was effectively restricted with the use of a custom-made gravitational-counter-flow classifier to facilitate complete particle vaporization and excitation. After classification, the particles were transported directly to the plasma by means of an argon flow to prevent chemical alterations in aqueous media due to potentially occurring Li+-H+ exchange reactions. The size-separated particles were monitored online by flow cell particle analysis (FPA). The influence of different gas flow settings and plasma parameters on the peak emission intensity of Li and Mn was evaluated. A particle size detection limit of ∼0.5 μm was estimated based on the 3σ criterion of the baselines and the measured peak intensities for Li and Mn considering the particle size distribution as obtained by FPA. The corresponding analyte masses at the detection limits were ∼30 and ∼180 fg for Li and Mn, respectively. Furthermore, an approach for a matrix-matched external calibration with electrochemically delithiated lithium transition-metal oxides is presented.