Ultimately, the goal of in situ neutron diffraction is to track the Li content and location in crystalline electrode materials as a function of charge and discharge. This is a difficult task, that until recently was only demonstrated at limited battery states of charge [191] with long collection times and effectively under equilibrium conditions. Other studies have inferred Li content via electrochemical approxima­tions (amount of charge transferred) and the evolution of reflection intensities [196]. If the Li composition of an electrode can be reliably determined as a function of discharge/charge this would give a direct measure of the capacity of the battery, or more accurately, the capacity of the battery that is stored in the crystalline com­ponent of the electrode.

Recently, the ability to track the Li location and content as a function of time (and charge/discharge) has been demonstrated using commercial Li1+yMn2O4 cathodes [189]. Arguably, this represents the most Li-centric view of a Li-ion battery during operation. The Li evolution is found to differ at a structural level during charge/discharge (Fig. 7.29) accounting for the ease of discharging these types of cathodes, relative to charging. Additionally, the Li evolution is shown to progress from one to two crystallographic sites during the charge/discharge pro­cesses. The lattice parameter follows a linear relationship with Li content during single Li site processes (Vegard’s law) and during processes involving two Li sites the relationship between the lattice parameter and the Li occupancy and site is a linear combination of the individual single site processes (Fig. 7.30). This work provided unparalleled insight into the function of the cathode and is used to understand the origins of how the electrode functions. Further studies on structural permutations may provide insight on how these electrodes can be improved from the perspective of the Li.

LiNi05Mn15O4 is attracting significant attention for cathode applications due to the high-voltage redox couple during battery function, and Li4Ti5O12 is attracting attention for anode applications due its small volume change during Li insertion and extraction. By specifically constructing a neutron-friendly cell made of this elec­trode-combination it was possible to study the structural evolution of these mate­rials using time-resolved in situ neutron diffraction [199]. This highlights another advantage of using custom-made cells for in situ neutron diffraction experiments, where research is not limited to commercially-available materials. In this case, it was possible to determine the evolution of Li occupation in the cathode and indi­rectly infer the Li occupation in the anode (Fig. 7.31) in addition to determining the reaction mechanism evolution for the electrodes. It was found that a solid-solution reaction occurred at the cathode with the Ni2+/Ni3+ redox couple at *3.1 V and a two-phase reaction with the Ni3+/Ni4+ redox couple at * 3.2 V. Thus, the extraction of Li from the cathode and insertion of Li into the anode during charge was directly determined, again in real-time. This opens up a way to evaluate a range of materials used as electrodes in Li-ion batteries, where how Li is extracted and inserted while a battery functions can be determined.

Fig. 7.29 a The evolution of Li at the 8a (black) and 16c sites and in a formula unit of Li1+yMn2O4 (lower plot) during charge/discharge. The discharge process shows an increase in Li content, whilst charge shows a decrease. b A representation of the sites described in (a) for Li1+yMn2O4. Reprinted from (N. Sharma, D. Yu, Y. Zhu, Y. Wu, V. K. Peterson, Chem. Mater. 25, 754 (2013)) [189]
Fig. 7.30 The relationship between Li site occupancies and the lattice parameters of Li1+yMn2O4. Single-site regions are shown in black and red and mixed-site regions are in blue and purple. Reprinted from (N. Sharma, D. Yu, Y. Zhu, Y. Wu, V. K. Peterson, Chem. Mater. 25, 754 (2013)) [189]
8.22
8.20
to 8.14
a 8.12 —
8.10 —
8.08 —
8.06 —
Lithium content in the formula unit