In this manuscript, the authors summarized the recent progress of characterization techniques for investigating the charge storage mechanisms of 2D and layered material-based electrochemical energy storage (EES) devices, such as optical spectroscopy, imaging techniques, X-ray and neutron-based methods, mechanical probing and nuclear magnetic resonance spectroscopy. The working principles and the result analysis methods of the characterization techniques are systematically presented and discussed. Finally, the limitations, challenges, pitfalls and future directions of each characterization method are summarized and projected. This review will serve as a good reference for the characterizations of 2D energy storage materials. Therefore, I would like to recommend it for publication in Nanoscale after some major revisions.
1. The advantages of each testing technique in characterizing 2D energy storage materials are suggested to be further summarized and discussed.
2. The working principles of 2D energy storage materials in performance enhancement are suggested to be well introduced.
3. Typical 2D materials and their classification should be summarized and discussed in the introduction section, such as graphene, transition metal dichalcogenides, MXenes, layered double hydroxides, black phosphorus and other emerging 2D materials.
4. The differences in characterizations between 2D materials and bulk materials are suggested to be described.
5. The advantages of the combined application of different characterization methods are suggested be projected.
6. About the characterizations and advantages of two-dimensional energy storage and conversion materials (e.g., graphene, layered hydroxides), the following papers are suggested for reference: Advanced Energy Materials, 2019, 9, 1901093, Carbon Energy, 2024, e532, Carbon Energy, 2023, 5, e333 and Chemical Engineering Journal, 2024, 487, 150559.

The manuscript has been well revised. I would like to recommend it for publication in Nanoscale.