Invited Presentation: NMR and Diffraction Studies of Electrode Reactions: The Effect of Structure, Rate and Electrode Formulations on Reaction Mechanisms

A full understanding of the operation of a battery requires that we utilize methods that allow devices or materials to be probed while they are operating (i.e., in-situ). This allows, for example, the transformations of the various cell components to be followed under realistic conditions without having to disassemble and take apart the cell.  To this end, the application of new in and ex-situ Nuclear Magnetic Resonance (NMR) and X-ray diffraction (XRD) approaches to correlate structure and dynamics with function in lithium-ion batteries will be described. The in-situ approach allows processes to be captured, which are very difficult to detect directly by ex-situ methods.  For example, we can detect side reactions involving the electrolyte and electrode materials and non-equilibrium processes that can occur during extremely fast charging and discharging.  Ex-situ NMR investigations allow more detailed structural studies to be performed to correlate local and long-range structure with performance in battery materials.

In this talk, I will describe the use of NMR spectroscopy and X-ray diffraction to probe local structure changes in lithium ion batteries focusing on our work on olivines, spinels and lithium air cathodes. In the olivine work, we show how the use of fast charging allows us to capture metastable/non-equilibrium states.  Mapping of the electrodes allows the distribution of the reaction front across the electrode to be followed.  Second, the development of new NMR approaches to investigate paramagnetic battery materials, both in and ex situ, will be discussed, the approach making use of both theory and experiment.  Although it is difficult to achieve high-resolution spectra from these paramagnetic materials (see for example the Li_1.08-xMn_1.92O₄ resonance in Figure 1) in the in situ experiments, measurements of the relaxation time allow access to the dynamics of the lithium ions in real time as a function of state of charge.

Figure 1: ⁷Li in situ NMR of Li_1.08Mn_1.92O₄ during galvanostatic cycling (electrochemistry shown on the left).  The red arrows indicate the evolution of the resonance from the tetrahedral sites in Li_1.08-xMn_1.92O₄.  Short recycle delays are used so as to suppress the resonance from the electrolyte.  Note the apparent increase in intensity at the end of the 4.0 V process/beginning of 4.2 V process on charge and the end of the 4.2 V process on discharge.  The behaviour originates from a change in spin-spin relaxation behaviour and indicates a slowing down of the cation/electron mobility. This phenomena has been explored as a function of temperature and spinel stoichiometry.