Advanced neutron imaging of energy devices in 2D and 3D

Abstract

Lithium-ion batteries are widely used to store electrical energy in the form of chemical energy, either due to the need of mobility or due to the need to address intermittent aspects of renew able sources. In the past years, the growing market of electric vehicles and mobile devices, as well as the transition to intermit tent renewable resources, has established a continuously rising demand for Li-ion batteries. This constant increasing demand leads researchers to develop batteries with higher power densities, higher energy densities (capacity), lower cost, environmentally friendly, safer and with a longer life cycle. In the present study is described the neutron tomography investigation of two batteries with LiFeP O4- C chemistry at different stages in their cycling life. The investigation aimed to correlate the Li distribution to the cycling age and performance. The properties of a Li-ion battery are strongly influenced by the ability of the active materials to insert and extract Li ions in and from their structure. The change in stoichiometry of Li is associated with crystallographic phase changes. Neutron imaging is a powerful non-destructive technique, which can be tailored to investigate the spatial distribution of lithium and gain insight on the evolution of crystallographic phases while cycling the lithium through charge-discharge cycles. A Li-ion half-cell battery was developed to study the crystallographic phase evolution throughout the thickness of a graphitic electrode and correlating it with the current flow distribution. The design of the cell was optimised for neutron imaging in-plane investigations, a configuration where the electrodes are imaged from the side. Pushing attenuation and diffraction contrast imaging methods to their current limits, it was demonstrated that it is possible, to determine the spatial distribution of crystallographic phases and their evolution while operando investigation batteries with such a cell design. The method was advanced by improving the cell design, the data analysis, and by combining it with neutron powder diffraction. Neutron powder diffraction, a complementary technique, was used simultaneously with the previously mentioned imaging techniques, to investigate the diffraction pat terns arising from the graphite elect rode. The multimodal analysis proved to be a more robust operando method of studying batteries with the perspective of very detailed combined morphology-crystallography-ion distribution-electrochemical performance studies. Finally, the study of electrical current distribution through a graphitic electrode material using polarised neutron imaging technique revealed that the current paths are accommodated to the geometry of the electrode material.