Interfaces Promoted Conversion-Type Reactions for Energy Storage
Authors: Cheng Yingwen , Pan Huilin , Chen Junzheng , Shao Yuyan , Liu Jun 
Published: February 2020

Conversion-type reactions based battery chemistry presents a great promise for next-generation energy storage. It is based on using the chemical reaction (also called chemical transformation) of the working ion to store energy, different than intercalation-type reactions as used in today’s Li-ion batteries. Good examples include Li-S, Li-O2, and broadly speaking metal electrodes (Li, Mg, Zn, etc.). Interfaces generally play a more crucial role in these battery chemistry systems because of the complexity of this type of reaction even though each of the systems has its unique characteristics. This not only includes electrode/electrolyte interfaces, but also the interfaces within the electrode materials. Here, we use two examples of battery chemistry to demonstrate how the manipulation of interfaces can promote the conversion-type reactions for energy storage: Mg chemistry and Li-S chemistry.
In Mg battery chemistry, due to the divalent nature of Mg2+, the ion transport kinetics in solid host materials is limited. Therefore, the development of Mg battery has been limited by poor performance of electrode materials. We have developed two conversion-type electrode materials with improved Mg2+ storage performance: SnSb alloys for anode1
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2 and V2O5 nanoclusters for cathode.3 More importantly, our experimental and theoretical modeling reveal that the interfaces between the multicomponent phases generated during repeated magnesiation–demagnesiation is responsible for the improved performance of SnSb; a surface-controlled reaction of Mg2+ and V2O5, denoted as a molecular energy storage mechanism, improves the reaction kinetics which increases the rates and capability.
In Li-S battery chemistry, control of solution chemistry and nucleation of polysulfides (on cathode host surface) is crucial for improved capacity and cycling stability. We have demonstrated that, through tuning the solubility of polysulfides in electrolytes, an improved cycling stability is achieved. The controlled nucleation of polysulfides on carbon nanofiber surface leads to the formation of a unique porous microsphere structure of Li2S. This unique porous Li2S microsphere enables a close to 100% utilization of S (specific capacity >1600mAh/gS); it also decreases the passivation of cathode. This opens up a new avenue for Li-S R&D of using low surface area carbon host materials, thus a great potential for high pack density and low electrolyte/sulfur ratio in Li-S cells.
In this talk, we will present our understanding on these interface-related conversion-type reactions and recent progress of material innovation that improves device performance. New electrode architecture engineering will also be discussed.
1. Y. W. Cheng, Y. Y. Shao, L. R. Parent, M. L. Sushko, G. S. Li, P. V. Sushko, N. D. Browning, C. M. Wang and J. Liu, Adv. Mater., 2015, 27, 6598-6505.
2. L. R. Parent, Y. W. Cheng, P. V. Sushko, Y. Y. Shao, J. Liu, C. M. Wang and N. D. Browning, Nano Lett., 2015, 15, 1177-1182.
3. Y. W. Cheng, Y. Y. Shao, V. Raju, X. L. Ji, B. L. Mehdi, K. S. Han, M. H. Engelhard, G. S. Li, N. D. Browning, K. T. Mueller and J. Liu, Advanced Functional Materials, 10.1002/adfm.201505501, 2016.