Semi-Solid Flow Batteries: New Electrochemical Challenges

Redox flow batteries (RFB) are promising technologies for energy storage due to the long life, low cost, high round-trip efficiency and independent scalability of energy and power capabilities. Semi-solid flow batteries (SSFBs) are a special class of RFB, in which anolyte and catholyte consist of flowable suspen­sions of solid active materials rather than dissolved redox species. Thus, the concentration of active redox centers in the anolyte and catholyte of the SSFB can be significantly increased.¹ Using intercalation type active materials such as those typically used in Li-ion batteries (LIBs), e.g. Li₄Ti₅O₁₂, LiCoO₂ or LiNi_0.5Mn_1.5O₄, the energy densities can reach up to 300–500 W h L^-1, which is more than 10 times higher than that of all-vanadium RFBs (40 W h L^-1). Compared to LIBs, SSFBs present several advantages: (i) power and energy can be scaled independently, (ii) the amount of inactive materials such as current col­lectors or housing is decreased, and (iii) the manufacturing processes become simpler and more cost-effective.

Since SSFBs deploy Li-ion or Na-ion host materials, SSFBs and classic solid electrode batteries share the same chemistry. However, the replacement of solid electrodes of classic ion batteries by the fluid elec­trodes employed in semi-solid flow batteries brings new electrochemical challenges. After revising the fundaments of the operating principles of SSFBs, two particular new challenges are presented and dis­cussed: i) the critical influence of the electrical conductivity of the active materials,² and ii) the new role of the solid electrolyte interphase (SEI).^3,4

In the first case, the weaker electrical conductivity of fluid electrodes allows ionic and electric pheno­mena to be easily differentiated. Electron-transfer phenomena in the charge/discharge mechanism of bat­tery materials can be solely studied at slow C-rates, at which ionic factors do not interfere. In particular, the case of titania materials (Li₄Ti₅O₁₂ and TiO₂), which changes from electrically insulating to conduc­ting upon charge/discharge, is discussed and compared to LiCoO₂, which is a good conductor at all states of charge. The proposed electron bottleneck mechanism can now explain behaviors which were not com­pletely understood up to now, such as the improved performance of oxygen deficient Li₄Ti₅O_12-x and TiO_2-xas active material in classic LIBs

In the second case, the SEI is shown to change from an ionic to electron barrier, for classic solid electrode batteries and SSFBs, respectively. This fact brings new limitations since SEI ceases to be a beneficial film, which enables the use of active materials operating at very cathodic potentials, e.g. graphite. In SSFBs, the new implications of the SEI prevent the use of those materials (graphite, Si, Sn, etc), which suggests redirecting the search for novel negative electrode materials in this technology.

References.

1. M. Duduta, B. Y. Ho, V. C. Wood, P. Limthongkul, V. E. Brunini, W. C. Carter and Y.-M. Chiang, Adv. Energy Mater., 2011, 1, 511

2. E. Ventosa, M. Skoumal, F. J. Vazquez, C. Flox, J. Arbiol, J. R. Morante, ChemSusChem, 2015, DOI: 10.1002/cssc.201500349

3. E. Ventosa, D. Buchholz, S. Klink, C. Flox, L. Gomes-Chagas, C. Vaalma, W. Schuhmann, S. Passerini and J. R. Morante, Chem. Commun., 2015, 51, 7298

4. E. Ventosa, G. Zampardi, C. Flox, F. La Mantia, W. Schuhmann, J. R. Morante, in preparation.

Acknowledgement The research leading to these results has received funding from the European Union Seventh Framework Programme (FP7/2007- 2013) under grant agreement n8 608621