Abstract
As a promising charge storage method, hybrid charge storage has a high energy density, high power density, and long cycle life due to its combination of the mechanisms of secondary batteries and electrochemical capacitors. However, the difference in the charge storage mechanisms of the cathode and anode and thus the strong coupling makes it impossible to match cathode and anode in all situations. Research has investigated cell configuration, material design, electrolyte composition, etc., for matching the cathode and anode of hybrid charge storage devices, but there is no complete understanding and analysis from an electrochemical perspective. To better guide and promote the development of hybrid charge storage, this study discusses the matching and coupling of the anode and cathode from the following aspects, using hybrid capacitors as a typical example and combining the analysis of mainstream electrochemical systems, strategies, and materials. First, the charge storage mechanism and the major problems involved in matching the cathode and anode, as well as the “self-matching” of potential and zero-voltage potential, are considered as the basis of coupling. Second, from the perspective of electrochemical behavior and potential range of electrodes, we analyze the conflicts and correlations in coupling between each match and discuss the problems and solutions faced in specific matching processes. Third, the problem of matching a practical but complex electrochemical system is analyzed from the perspective of the coupling relationship. Fourth, the design and development of hybrid charge storage, ideas for future research, and the use of machine learning for electrode matching and coupling are proposed.
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In this review, we propose a transformative discussion idea to analyze to maximize the performance of hybrid charge storage devices from an electrochemical point of view. Hybrid charge storage requires simultaneous optimal performance of the anode and cathode. However, various properties of electrodes will affect and restrict each other (which was defined as “coupling”), making it difficult for them to achieve optimum performance simultaneously. For in-depth discussion, zero-voltage potential and “self-matching” were defined as the basis of coupling to analyze the interrelationship and influence among the matchings of capacity, kinetics, and cycle life in detail. What is more, the influence of non-ideal electrochemical behavior and other possible factors on matching was also thoroughly discussed.
The idea of matching and coupling does not apply only to hybrid charge storage. Considering the difference in the charge storage process of electrodes, matching and coupling are unavoidable to any charge storage system. What is more, the theoretical research on matching and coupling means that the electrochemical behavior can be modeled and analyzed through machine learning so as to predict the overall performance of the device and optimize the design based on the data collection in future. Our analysis, discussion, and insight could provide a deeper understanding of the design concepts for hybrid charge storage and other charge storage systems from an electrochemical perspective.
Despite being proposed as an ideal charge storage method, the performance of hybrid charge storage devices is constrained by the matching problem between cathode and anode. To this end, the research ideas of coupling and matching are proposed. Combined with well-defined self-matching and zero-voltage potential, the problems faced in the construction of various electrochemical systems are discussed in depth, and the design, development, and future research of hybrid charge storage are comprehensively prospected.