Atomic Force Microscopy Applications for Energy Storage: Detecting the Structure and Dynamics of Fluid/Electrode Interfaces

The structure of ions at the electrode-electrolyte interface and their transport properties within electrode materials control the properties of energy storage devices, such as the energy density and rate capability. Tremendous efforts have been undergone to achieve fundamental understanding of interfacial structures, dynamics and reactivity in order to improve the energy efficiency of electric devices. However characterization tools able to probe interaction lengths as small as few nanometers are needed. Atomic Force Microscopy (AFM) techniques are well suited to investigate the nanoscale properties of energy storage materials and to characterize the electrode-electrolyte interface due to the high lateral and vertical resolution and sensitivity to weak forces. In this presentation we will discuss several AFM techniques used to investigate ionic transport across and near solid-liquid interfaces.

The sensitivity of the AFM probe to very weak forces allows determining the structure of an ionic liquid within the electrical double layer with molecular-level resolution [1]. The influence of applied potential on the ions structure can be detected, and the excellent agreement with molecular dynamics simulations provides insight into the ion layering reactivity. Moreover, the unmatched lateral resolution of scanning probe techniques also opens the pathway to mapping the structure of the electrical double layer in a 3D manner.

We will also show that AFM is capable of characterizing the changes induced during the electrochemical storage at the nanoscale for Li-ion electrode materials, where local phenomena at the tip-sample junction can be measured, analyzed and interpreted in terms of local ionic transport [2]. Furthermore, Electrochemical Strain Microscopy (ESM) technique has been used to study the ionic transport from the electrolyte into a variety of materials in-operando. For example, an ESM study of activated carbons of different pore sizes revealed faster cation adsorption kinetics vs. anion in an ionic liquid electrolyte [3]. More recently, AFM was used to evidence the anomalously large volume contraction of a 2D titanium carbide (MXene) electrode upon cation intercalation [4].

As the development of AFM techniques in electrolyte environment improve, achieving time-resolved tracking of ion storage in energy materials becomes more accessible and can lead to tuning surface interactions to improve the energy density of supercapacitors.

This work was supported as part of the Fluid Interface Reactions, Structures and Transport (FIRST) Center, and Energy Frontier Research Center funded by the U.S. Department of Energy, Office of Science, Office of Basic Energy Sciences. Measurements were performed at the Center for Nanophase Materials Sciences, which is sponsored at Oak Ridge National Laboratory by the Scientific User Facilities Division, Office of Basic Energy Sciences.

[1] J. M. Black, D. Walters, A. Labuda, G. Feng, P. C. Hillesheim, S. Dai, P. T. Cummings, S. V. Kalinin, R. Proksch, N. Balke, Nano Lett. 13 (2013) 5954-5960.

[2] N. Balke, S. Jesse, A. N. Morozovska, E. Eliseev, D. W. Chung, Y. Kim, L. Adamczyk, R. E. García, N. Dudney, S.V. Kalinin, Nat. Nanotechnol. 5 (2010) 749-754.

[3] J. M. Black, G. Feng, P. F. Fulvio, P. C. Hillesheim, S. Dai, Y. Gogotsi, P. T. Cummings, S. V. Kalinin, N. Balke, Adv. Energy Mater. 4 (2014).

[4] J. Come, J. M. Black, M. Naguib, M. R. Lukatskaya, M. Beidaghi, A. J. Rondinone, D. J. Wesolowski, S. V. Kalinin, Y. Gogotsi, N. Balke, Nano Energy, accepted.