Cryoporometry for short T ₂ samples: A T ₁ filter method applied to battery electrode characterization

The pore size distribution is often an important parameter for transport processes in porous media. Cryoporometry experiments can provide such data in the meso and macropore size up to 1 μm providing the sample temperature is finely controlled. We use a Peltier based system inserted directly into the NMR probe to control the temperature within 0.05 °C and impose temperature ramps down to 0.002 °C/min, necessary to characterize the largest pore sizes. The pore size information in the macropore range cannot be obtained from gas adsorption techniques while mercury injection is questionable for the material considered here. For porous materials made of paramagnetic minerals such as lithium-iron phosphate (LiFePO₄, LFP) or nickel-manganese-cobalt oxides (NMC) and saturated with octamethylcyclotetrasiloxane (OMCTS), the T ₂ relaxation times are very short such as protons from the liquid and frozen phases cannot be separated. Hence the usual cryoporometry experiment cannot be performed. Instead, we propose to use the T ₁ contrast to separate these phases. The method is studied in detail along with some temperature effects linked with the T ₁ variation of the bulk frozen OMCTS. We show an example on two cathode materials part of industrial battery product.