Small polaron transport in cathode materials of rechargeable ion batteries

Small polarons are quasi-particles in materials formed by a charge carrier and its self-induced distortion. In cathode materials containing transition metal elements, small polarons play an important role in the diffusion of charge carriers. In order to grasp the fundamentals of small polarons inside cathodes, we highlight the recent progress from both experiments and simulations focusing on the formation of small polarons and their migration. Because of their strong binding to small polarons, alkali ions accompany polarons during diffusion and thus the polaron – ion couple can be considered a complex quasi-particle. The diffusion mechanism of alkali ions inside cathodic materials can be explored by a simultaneous diffusion simulation approach, which naturally and accurately depicts the diffusion of alkali ions (and alkali vacancies) and their accompanying small polarons as complex diffusion particles during alkali intercalation and de-intercalation. The successful employment of this approach paves a new direction toward elucidating the diffusion mechanism in several promising cathode materials containing transition metals. This model and its application to the investigation of the electrochemical properties of several typical cathodic materials such as NASICON, PO44−–, SO44−–, and SiO44−– anion– based compounds are comprehensively demonstrated.