Overhauser Dynamic Nuclear Polarization of Lithiated Graphite Anodes: Probing Bulk and Surface Structures.

Abstract

Graphite is used, almost ubiquitously, as an anode material in today's high energy density Li-ion batteries. Both artificial and natural graphites are widely used, and there are large differences in the production methods, cost, particle morphologies, sizes, and percentage of defects in their structures, all these parameters affecting use and performance. The success of graphite as an anode depends on the formation of a Li-ion-conducting passivation layer (the solid electrolyte interphase (SEI)) on the first cycle, with the nature of this layer still being under investigation with a range of approaches. During lithiation (charge in a full cell), graphite is lithiated in stages and becomes electronically conductive. The conduction electrons of lithiated graphite anodes are exploited in this work to enhance the nuclear magnetic resonance (NMR) signal of bulk and surface nuclei via Overhauser dynamic nuclear polarization (DNP). The parameters directly affecting the enhancement factor (leakage factor, saturation factor, and coupling factor) are examined in detail for an artificial graphite at different lithiation stages. Four additional (natural and artificial) graphites are then studied to explore the effects of particle size and morphology, electron relaxation times, and conductivity on the observed DNP enhancements. Finally, the polarization transfer between bulk and surface (SEI) species is explored through ^6,7Li, ¹H, and ¹³C DNP NMR experiments.