Electrochemical energy storage systems include all types of secondary batteries. The basic principle of battery functioning entails the conversion of chemical energy contained in the batteries’ active materials into electric energy by an electrochemical oxidation-reduction reverse reaction. Normally, the corresponding kinetics involved during this conversion is usually transport controlled and then determined by chemical diffusion. Research has shown that the chemical diffusion coefficient does not only decide upon the rate of diffusion-controlled solid-state reactions but also upon the rate performance of transport-controlled storage/removal in battery electrodes. From the thermodynamics of space charge zones, it is obvious that even without any structural anomalies at the contacts, composites of two phases can show not only anomalous conductivity but also anomalous storage properties.
Most striking is the ability of job-sharing composites to store components that cannot be stored by the individual constituents at all. In addition, the fact that composites of ion and electron conducting phases allow for interfacial storage of a neutral component via space charge effects has necessitated more research.
Recently, a team of researchers led by Professor Joachim Maier from the Max Planck Institute for Solid State Research in Germany evaluated the mass transport kinetics in job-sharing composites. Additionally, they looked in depth on the kinetics of solid-state supercapacitors and dual-phase permeation membranes. To achieve their goals, they chose to concentrate on materials with extremely small screening lengths such that the space charge zones collapse to single layers. Their work is currently published in the research journal, Advanced Functional Materials.
In brief, the research method applied commenced by considering the rate of transport-controlled incorporation and ex-corporation in artificial mixed conductors. Next, the researchers determined the upper limit of the relaxation time by interfacial job-sharing diffusion which was then analyzed in greater detail. The job-sharing diffusion coefficient was then addressed in great detail. A generalization of homogeneous chemical diffusion, job-sharing chemical diffusion, and dual phase transport was then provided. Lastly, they performed finite element calculations on different morphologies so as to handle the complexity of mass transport in composites.
The authors observed that the Stranski–Krastanov structure showed similar charging behavior as the monolayer structure, indicating the transport was dominated by job-sharing chemical diffusion. Moreover, a significant enhancement of charging rate was observed by an extreme case in which a coalesced island was present. For the two comparisons involving monolayer: solid circles and hollow circles, the latter was seen to be less sensitive to the number of islands.
In summary, the Joachim Maier and his research team successfully presented a detailed investigation of the kinetics of space charge storage in composites of ion and electron conductors. They noted that the rate of mass transport was generally described by the dual-phase transport, and thus the relaxation time was significantly inﬂuenced by the morphologies. Altogether, the treatment presented in this study is vital for understanding the kinetics of artificial mixed conductors and even of great interest to applications such as solid-state batteries and solid-state supercapacitors.
Chia-Chin Chen, Edvinas Navickas, Jürgen Fleig, Joachim Maier. Kinetics of Space Charge Storage in Composites. Advanced Functional Materials 2018, volume 28, 1705999Go To Advanced Functional Materials