Lithium and Electrolyte Distribution in 18650-Type Lithium-Ion Batteries
Authors: Petz Dominik , Mühlbauer Martin , Baran Volodymyr , Hofmann Michael , Müller-Buschbaum Peter , Senyshyn Anatoliy 
Published: July 2021

Extensive cycling of lithium-ion batteries leads to a partial loss of their capacity due to various side effects like formation of the solid-electrolyte-interface (SEI), loss of active lithium, drying out of the cell [1, 2]
etc. Typical profiles of side reactions, instantaneous temperature[3] and current density[4] are non-uniform, which leads to a heterogeneous lithium distribution in the battery. The loss of lithium inventory is typically correlated with the formation of SEI during cycling, whereas the quantitative role of the electrolyte in the cell operation and cell fatigue remains not fully understood yet.
Primarily, this is related to the fact that quantification of the liquid electrolyte in an electrically and environmentally isolated system such as lithium-ion batteries is not a simple task. Opening the system would inhibit changes of its state, e.g. evaporation of electrolyte. Typical X-ray or neutron-based methods utilized in non-destructive cell characterization are not capable to quantify the amount of liquid electrolyte with desired accuracy. Recently it has been shown that liquid electrolyte (DMC-based) exhibit a long range order, when frozen at T<260 K [5]. This long range order can be detected by neutron diffraction and the amount of frozen electrolyte can be quantified. Furthermore, applying spatially-resolved neutron powder diffraction (Ref. [6]) pave the way for the simultaneous quantification of lithium amount in graphite and relative concentration of frozen electrolyte (Figure 1).
The aim of the current study is a non-destructive quantification of lithium and electrolyte, their spatial distribution throughout the cell and concentration changes vs. cell fatigue. Combined experimental studies embedding electrochemistry, X-ray computed tomography, and neutron diffraction is applied for 18650-type cylinder cell based on NCA|C chemistry, where high-resolution neutron diffraction independently reveals a direct correlation between losses of active lithium in the graphite anode and those of liquid electrolyte (both volume-averaged). The 3D lithium distribution is probed by spatially resolved neutron powder diffraction, thereby displaying the non-trivial character of active lithium/electrolyte losses and complex profile of the cell capacity fading.
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Figure 1