Simon Engelkeabc (firstname.lastname@example.org)
a Department of Chemistry, University of Cambridge, Lensfield Road, Cambridge CB2 1EW, United Kingdom
b Institute for Manufacturing, Department of Engineering, 17 Charles Babbage Road, University of Cambridge, Cambridge, CB3 0FS, United Kingdom.
c Cambridge Graphene Centre, University of Cambridge, 9 JJ Thomson Avenue, Cambridge, CB3 0FA, United Kingdom
There is an ever-increasing demand for batteries with greater capacity and higher rate performance. One approach to improve those characteristics for batteries is to modify their electrode structure to overcome lithium transport limitations. A variety of methods exists to engineer anisotropic structures to enhance diffusion across the electrode including extrusion 1, freeze-drying 2, chemical vapour deposition (CVD) 3, magnetic alignment 4, and templating methods 5. However, direct measurement of 3D diffusion in these non-transparent nanoscale pores is extremely challenging with classic optical techniques. To address this challenge, we developed a technique to measure anisotropic diffusion in a model porous silicon substrate with pulse field gradient (PFG) NMR. We show that NMR provides resolution for solvent (here, H2O, DMSO, and the battery electrolyte LIPF6:DC:EMC) inside and outside of the pores in the Si substrate. When the diffusivity of in-pore NMR peak is analyzed, the root mean squared displacement correlates well with the pore dimensions measured with electron microscopy. These results suggest that PFG NMR can serve as a non-destructive characterisation method for both in- and ex-situ analysis of materials in complex battery electrodes.
Other approaches are to incorporate air into the electrode (lithium-air batteries) and to use liquids as a storage medium (flow batteries). Setting suitable targets and the integration of such batteries into devices and electricity grids will be of great importance.
1 C.-J. Bae, C. K. Erdonmez, J. W. Halloran and Y.-M. Chiang, Design of Battery Electrodes with Dual-Scale Porosity to Minimize Tortuosity and Maximize Performance, Adv. Mater., 2013, 25, 1254–1258.
2 K. H. Lee, Y.-W. Lee, S. W. Lee, J. S. Ha, S.-S. Lee and J. G. Son, Ice-templated Self-assembly of VOPO4–Graphene Nanocomposites for Vertically Porous 3D Supercapacitor Electrodes, Sci. Rep., 2015, 5, 13696.
3 S. Ahmad, D. Copic, C. George and M. De Volder, Hierarchical Assemblies of Carbon Nanotubes for Ultraflexible Li-Ion Batteries, Adv. Mater., 2016, 1–6.
4 J. S. Sander, R. M. Erb, L. Li, A. Gurijala and Y.-M. Chiang, High-performance battery electrodes via magnetic templating, Nat. Energy, 2016, 1, 16099.
5 P. L. Taberna, S. Mitra, P. Poizot, P. Simon and J.-M. Tarascon, High rate capabilities Fe3O4-based Cu nano-architectured electrodes for lithium-ion battery applications, Nat. Mater., 2006, 5, 567–573.