Batteries must be able to undergo a large number of cycles without failure whatever their use. This, however, isn’t easy since the structural changes associated with charge and discharge are really quite dramatic. The (de)intercalation of ions into common electrode materials leads to a drastic changes in volume of 1-10 %. In normal use this leads to dislodging, cracking, and degradation–all of which reduce storage capacity and eventually lead to failure.

Some success in avoiding the effects of insertion strain has been achieved by careful control of microstructure (e.g. electrodes made as nanowires or thin-films) but only two intrinsic “zero-strain” insertion materials have previously been reported. We were interested as to whether we could exploit the flexibility of a molecular framework to ameliorate the volume change of ion insertion.

In our recent paper we show that reductive intercalation of potassium into silver hexacyanoferrate, Ag3[Fe(CN)6], occurs with surprisingly small (0.21 %) volume strain. Ion-storage media that use Na+ or K+, rather than Li+, are increasingly sought after. These earth-abundant elements could be particularly useful for large static batteries for which energy density is relatively unimportant. One of the challenges in developing electrodes for such batteries is accommodating these much larger ions without drastic volume changes. While the reversibility of this intercalation remains to be demonstrated, we are hopeful that this approach of using the anisotropy and inherent flexibility of a molecular framework may prove useful in the development of other low-strain electrode materials.

Zero-strain reductive intercalation in a molecular framework
J A Hill, A B Cairns, J K Lim, S J Cassidy, S J Clarke, and A L Goodwin
CrystEngComm 17, 2925 (2015)

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