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FAST-CHARGING LITHIUM-ION BATTERIES: SPATIAL QUANTIFICATION OF LITHIATION AND LITHIUM-PLATING DYNAMICS
Fast-charging of electric vehicles is an important capability for their widespread uptake. Using high-speed X-ray diffraction (XRD), depth profiles of a graphite electrode s lithiation stages are quantified during a 10-minute charge. The onset of lithium plating on the graphite is linked to the dynamic state of the underlying graphite.
PRINCIPAL PUBLICATION AND AUTHORS
charge configuration Fe3+-Fe2+-Fe3+. However, the B42 site was found to be unique in having a Fe2+-Fe2+(B42)-Fe3+ configuration where another Fe2+ acts as a terminal charge acceptor. The present discovery that B42 is more easily oxidised than other Fe2+ sites is thus attributed to the anomalous nature of the B42 trimeron, in which electron-electron repulsion between localised electrons of the adjacent Fe2+ lowers the potential for ionisation at the B42 site relative to other Fe2+ ions.
Site selective doping of a charge ordered structure has not previously been demonstrated, but this work shows that one site within the spontaneously created charge ordered array of magnetite is selectively oxidised without destroying the rest of the electronically ordered network. This opens new possibilities to create a charge order within a charge order and such materials could be used to provide surface sites for selective redox reactions or for storing information by doping specific sites.
High energy density and the ability to charge quickly are commonly cited as important properties of Li-ion batteries for the success of electric vehicles. One promising approach to increase the energy density of Li-ion cells while using conventional cost-effective electrode materials such as graphite is to increase the ratio of active to inactive materials within the cell, hence manufacturing thicker electrodes relative to inactive components such as the current collector and separator. However, thick electrodes are counterproductive to improving rate capability as they impose transport limitations that deplete Li ions in the electrolyte nearer the current collector, requiring increased potential to charge the cell, especially at higher charging and discharging rates. When concentration gradients in the electrolyte become increasingly severe during fast-charging, particles near the separator experience higher local current densities than those near the current collector. This can lead to solid-state transport limitations within the graphite near the separator, particularly during
the charging step (i.e., graphite lithiation). High- rate lithiation of graphite is also conducive to Li plating where a Li ion is reduced on the surface of the graphite rather than intercalated, which can lead to rapid degradation through side reactions with the electrolyte, the products of which can further inhibit transport.
The rate and location of Li plating in graphite electrodes is not well understood, and the behaviour of plated Li remains uncertain. Most operando studies have been carried out at low temperature (around 0°C) where kinetics are slow and operando techniques that take tens of seconds to minutes can be used to record the dynamic state of graphite lithiation and plating or stripping of Li. Until now, no operando method has quantified the spatial dynamics of heterogenous lithiation and Li plating under fast- charge conditions at room temperature. Here, high-speed depth-profiling XRD at beamline ID15A is used to spatially describe lithiation and Li plating dynamics as a function of depth and time under extreme fast-charge and discharge