This paper proposed a quantitative evaluation approach to respectively define the particle dispersion degree (PDD) of LiCoO₂ particles and the PDD of carbon black (CB) particles based on the scanning electron microscopy (SEM) images of lithium-ion battery (LIB) slurries. A mixed attention module (MAM) is introduced into the instance segmentation algorithm of Mask R-CNN, which successfully identifies LiCoO₂ particles and their pixel coordinates in the SEM images of LIB slurry. Subsequently, the image subtraction method is used to remove LiCoO₂ particles from the SEM images with the aim of obtaining the pixel coordinates of CB particles. Additionally, the PDD of LiCoO₂ particles is comprehensively evaluated by using the proportion of discrete particles, the average value of the minimum distance and the uniform deviation value; while the PDD of CB particles is evaluated by using the proportion of discrete particles and the average value of the minimum distance. Moreover, four different rotational speeds which are 300, 900, 1050 and 1200 rpm are used to differentiate particle distribution status of LIB slurry. Furthermore, in order to verify the correctness of the proposed approach, the electrochemical characteristics of LIB slurries are also analyzed by using electrochemical impedance spectroscopy (EIS) method. After performing the investigations, the conclusions illustrated that both the PDDs of LiCoO₂ and CB particles have the capability of evaluating particle dispersion status within LIB slurry. Specifically, larger proportion of discrete particles and average value of the minimum distance together with smaller uniform deviation value result in higher PDDs of LiCoO₂ and CB particles in the case that rotational speed is 1050 rpm when compared with other cases including 300, 900 and 1200 rpm. Those results are also consistent with the electrochemical characterizations of LIB slurries, which verified the correctness of the proposed approach. The proposed quantitative evaluation approach is of great importance for the quantitative evaluation of the dispersion degree of microscale and nanoscale particles in dense liquid-solid two-phase flow.