Direct measurement of lithium transport in graphite electrodes using neutrons

Lithium intercalation into graphite electrodes is widely studied, but few direct in situ diagnostic methods exist. Such diagnostic methods are desired to probe the influence of factors such as charge rate, electrode structure and solid electrolyte interphase layer transport resistance as they relate to lithium-ion battery performance and durability. In this work, we present a continuous measurement of through-plane lithium distributions in a composite graphite/lithium metal electrochemical cell. Capacity change in a thick graphite electrode was measured during several charge/discharge cycles with high resolution (14 μm) neutron imaging. A custom test fixture and a method for quantifying lithium are described. The measured lithium distribution within the graphite electrode is given as a function of state of charge. Bulk transport resistance is considered by comparing intercalation rates through the thickness of the electrode near the separator and current collector. The residual lithium content associated with irreversible capacity loss that results from cycling is also measured.

► Spatiotemporal measurements of lithium through the electrode thickness were quantified with high resolution neutron imaging.
► A nonuniform lithium distribution was observed early in the first intercalation cycle but relaxed as the electrode filled with lithium.
► Through-plane transport resistance in the bulk of the graphite composite electrode was measured.
► The distribution of lost capacity associated with trapped lithium was quantified and linked to regions with low intercalation rates.