Towards the characterization of batteries by in operando NMR - monitoring electrolyte - electrode interactions by PFG NMR techniques

The majority of NMR studies on battery materials have been performed ex situ, where the battery was cycled to a certain state and NMR measurements were conducted after conditioning the material into an NMR-compatible form. While this is a powerful and proven approach, one key downside is the necessity to disassemble the cell, which often requires its destruction. On the other hand, a holistic understanding and characterization of battery cells and battery materials is only achievable during operation, which enables monitoring of components, interface morphologies and material phases with short lifetimes in real time.
Prior to in situ measurements however, a detailed understanding of the components and their interactions is obligatory where one of the major aspects is the characterization of electrode-electrolyte interfaces. A detailed knowledge of electrolyte mobility in pores and interactions with pore surfaces at the electrode is required to improve pore structure and surface morphology and to provide an optimized surface area for a particular electrolyte [1]. Specifically the mobility, ion transport properties, segregation and degradation processes of the electrolyte and the wettability of the pore boundary layers at the electrodes are of fundamental interest. Furthermore, monitoring electrolyte-electrode interactions requires a detailed description of the pore morphology and permeability, where MR methods allow us to conclude on diffusion processes in solvation layers.
As a preliminary investigation towards in operando NMR a complementary sorption study by means of proton MR and BET as well as water vapor sorption was conducted to assess the feasibility of various MR methods for monitoring the wettability of pore boundary layers on the meso- and micropore scale. The wettability of Vulcan XC 72, a highly conducting carbon black substrate with surface area of 250 m² g^-1 was studied for a range of pH-values. MR relaxation time distributions and diffusion times were compared to BET measurements. In a next step we are going to assess organic electrolytes in terms of their interaction with the surface.

We gratefully acknowledge funding by the "German Federal Ministry of Education and Research" (BMBF, grant number 03EK3543 "SABLE")

1. Grey, C.P. and N. Dupré, Chemical reviews, 2004. 104(10): p. 4493-4512.