[en] This work not only summarizes the previous doping research that focused on the optimization of a bulk doping composition but also introduces a new doping strategy, namely, "electrochemical reaction mechanism control doping." The new electrochemical mechanism control technology enables the study of the precise deterioration mechanism of layered cathode materials for Li-ion batteries (LIBs). Accordingly, tremendous efforts have been devoted to the development of various types of layered cathode materials, such as lithium-rich, nickel-rich, and cobalt-rich materials, by using an electrochemical functioning doping method. This progress report also gives a perspective on potential future directions for this field. In this context, detailed methodological approaches are suggested for advanced doping studies, where the consideration of the doping method takes significance as great as designing doping configurations, e.g., chemical composition, doping depth, and doping site control, for the modification of battery material properties. As an instance of the methodological approaches for doping studies, a new "secondary doping" is shown with exemplary experimental results showing that functioning dopants (primary dopants) are homogeneously dispersed on the layered cathode materials by using supporting dopants (secondary dopants). This study will provide insights into the future direction of doping research of LIBs, as well as the history of the development of atomic substitution in layered cathode materials. (© 2020 Wiley-VCH GmbH)