Hard X-ray spectromicroscopy of Ni-rich cathodes under in situ liquid heating conditions

In situ microscopy involves imaging of samples under real reaction conditions. For electron microscopy, micro-electromechanical systems (MEMS) chips have previously been developed that can hold a liquid or gas inside the vacuum of the electron microscope, with electrical contacts that allow for heating or biasing of the sample. These chips have paved the way for high-resolution imaging of dynamic chemical reactions. Here, we report the use of such MEMS chips in an in-house developed setup for a hard X-ray nanoprobe, applied to Ni-rich cathode materials. We investigate the chemical and structural changes in nickel-rich cathodes upon exposure to electrolyte and under heating conditions using hard X-ray spectromicroscopy. As such, we find marked differences in the behaviour of pure LiNiO₂ compared to Co and Mn substituted material, NMC811. The use of hard X-ray spectromicroscopy allows for imaging and observation of: (i) the oxidation state of nickel, changing from Ni³⁺ to Ni²⁺, (ii) the effect of a preexisting fracture in the sample and (iii) the structural degradation of the sample during accelerated aging.