Neutron imaging (radiography) is becoming increasingly important in the study of Li-ion batteries as the spatial and temporal resolution of the detectors continually improve, and more advanced computational methods allow tomographic and/or three-dimensional rendering [207]. Neutron radiography (NR) is used to show macroscopic information concerning the Li distribution within a Li-ion battery, and in some cases while a process is occurring or at different states-of-charge [208]. Additionally, the H distribution in the electrolyte can be probed [209]. Examples of such studies include the Li distribution at the charged state versus the discharged state, during high-temperature battery operation, during fast charge/discharge cycling, and during overcharging [207, 208, 210, 211].

Neutron imaging has also been used to study alkaline [212,213] and Li-air batteries [214]. The future for neutron radiography relies on new instruments with improved spatial resolution, but also temporal resolution to allow time-resolved in situ exper­iments. Another method under considerable investigation is the combination of dif­fraction and imaging, which requires the definition of a gauge volume which is imaged and from which diffraction data can also be collected. This has been demonstrated for physically-larger batteries such as Na metal halide batteries, which usually have larger electrodes [215]. However, to be pertinent for Li-ion battery research, the gauge volume has to be reduced to become comparable to the thickness of electrode layers.

Early work on imaging Li-ion batteries explored the different types of battery construction, e. g. coin, prismatic, and cylindrical cells, and the distribution of Li at various battery states or during charge/discharge [216]. Figure 7.35 (left) shows images of a coin cell (CR1220 from Panasonic) from the charged to the discharged state, where lighter (white) regions at the charged state correspond to the Li anode (Li metal) and electrolyte (arrow). Over the course of discharge the Li-ions move towards the cathode (MnO2) resulting in an even distribution of white regions. The authors comment that if standard components and standardised cells are constructed then a more quantitative description of the Li distribution can be made. They also explored charging rates and other constructions, some of which had further experimental difficulties due to the internal structure of the batteries and the need to account for absorption by various layers. An example of the same electrode chemistry in the cylindrical case (CR1/3-1H) is shown in Fig. 7.35 (right) before and after discharge [158] showing similar Li distributions at the charged and discharged states.

In commercial batteries overcharging can be a potentially-devastating failure mechanism, and imaging studies on commercial graphite//LiNi0.8Co0 .15Al0.05O2 (NCA) batteries show what is deposited on the graphite anode during overcharge [208]. By performing an in situ measurement the deposition of a material on the graphite anode was studied (Fig. 7.36) and later determined to be Li. In addition, the authors were able to characterize ‘where’ the Li deposits during battery processes. Another work explored ‘fresh’ and ‘fatigued’ batteries, where batteries that had been cycled 200 times. The 18650 cylindrical batteries showed no differences at the macroscopic level in the neutron images of between fatigued and fresh batteries [182], even though neutron diffraction data indicated less Li insertion in fatigued graphite.

Another study illustrated that a 14 pm spatial resolution is attainable for battery samples using neutron imaging [210]. This work used a purpose-built graphite — containing cell to quantify Li content during charge/discharge and the residual Li content after each cycle, showing quantification over several cycles (e. g. capacity loss). Figure 7.37 shows the evolution of Li content and its distribution in graphite during the first discharge. The Li distribution was compared during cycling and between cycles. A slight difference in Li content between the separator and current collector was found. Further work investigated LiFePO4llgraphite pouch-cells and revealed Li concentration gradients across electrodes and in their bent regions [211]. Figure 7.38 shows the distribution of Li in the layers of the pouch cell at various states-of-charge. The authors used the Beer-Lambert law to correlate colour gradients, shown in Fig. 7.38, to the Li concentration. One advantage of using a pouch cell is that one image contains many layers, so an increase in electrode thickness can be seen in multiple layers verifying the result (as can Li concentration gradients). Clearly, this information can direct the development of better per­forming electrodes.

More recent work used cold neutrons rather than thermal neutrons, harnessing the stronger interaction of colder neutrons with matter, to visualize Li-ion distributions in Li-I batteries used in pacemakers [217]. This work also used three-dimensional imaging (tomography) and discussed methods to improve the signal-to-noise ratio in the images. The authors collected 50 images at 0.3 s for each angular step (rotation) of 0.91° which were then used to construct the three-dimemsional image. Figure 7.39 is an example of a cross section of a neutron tomography image of the fresh battery (left) and after a certain period of discharge (right). The battery is made of plates of Li and I. An unexpected change in the Li distribution (white) was observed in this study, where the smooth distribution became highly irregular.

The irregularity of the Li distribution after discharge is extracted in the three­dimensional image shown in Fig. 7.39, where the Li formations are clearly seen.

Further experimentation was undertaken on a Li-ion polymer battery using monochromatic imaging with cold neutrons, specifically targeting the anode and the processes that occur within it [218]. The LiC6 compound, but no evidence of the staging phenomenon often observed in LixC6 anodes was seen. This work was the first real-time in situ imaging of a commercial Li-ion battery, with some results shown in Fig. 7.40.

A subset of radiography research using commercial batteries, and in some cases custom-made batteries, is the study of gas evolution [209]. One in situ study showed how excess electrolyte present in batteries is consumed in the first charge-cycle, resulting in the formation of the SEI layer and some volume expansion. Additionally, gases were found to be evolved during the first charge. Figure 7.41 shows the consumption of excess electrolyte in these cells. The authors were also able to approximate the amount of expansion and contraction of the electrodes indirectly.

In situ neutron radiography has been extensively used to study the interface between graphite and a range of gel-based electrolytes [219]. By using this tech­nique, the generation of gas bubbles in the first charge can be visualized and quantified. This information allows the best electrolyte to be proposed, noting that the generation of gas bubbles, particularly on graphite surfaces, leads to perfor­mance degradation. This measurement also provided information on the spatial distribution and kinetic evolution of gas bubbles, as well as the electrolyte dis­placement and volume expansion in graphite. In order to undertake these mea­surements, specialised cells were developed. A neutron image of the cell is shown in Fig. 7.42. For the in situ experiment the exposure time for each image was 20 s and an image was recorded every 2 min. Figure 7.42 shows how channels of gases are formed seemingly-randomly in the cell and their evolution at different times. It was found that LiC6 is formed only where gas emission is absent, illustrating some heterogeneities in the charge distribution and electrode composition. The gel-based electrolytes tested in this study show less gas evolution (3 %) compared to liquid — based electrolytes (60 %) and this was related to the smaller amount of gas evo­lution during the first cycle.

7.7 Perspectives

This chapter has aimed at demonstrating how neutron-scattering methods allow researchers to elucidate crucial structural and kinetic properties of electrodes, electrolytes, and complete batteries. Neutron-scattering techniques play a key role in the development of new materials by relating structure to functional properties. Future battery research and development will in particular profit from the advances in in situ neutron-scattering techniques, probing complete battery systems. This gives the opportunity to relate battery performance to material and electrode structural, morphological, and dynamic properties under non-equilibrium and ageing conditions, which is vital information for the design of future batteries.