Diffusion coefficients are important parameters for the characterization of new electrode materials, but they are also essential for the study of cell aging and as input parameters in battery modeling. In this report, the applicability of the galvanostatic intermittent titration technique (GITT) on commercial cells is studied. A GITT protocol is applied on a set of commercial cells with graphite anodes and various cathode materials. The cell response is then compared with the ones of the individual electrodes, obtained in three-electrode and half-cell configurations. In particular, mostly due to the particular potential profile of graphite, the full cell GITT response corresponds to the anode and cathode response at low and high state of charge, respectively. Therefore, it is possible to estimate the diffusion coefficients of the individual electrodes by a simple experiment on commercial cells, although only in limited ranges of SOC. If the experiments are performed at different temperatures, it is also possible to determine the activation energies of the diffusion coefficients. In conclusion, GITT allows an estimation of the diffusivity data in commercial cells, and can be therefore used as fast analytical tool for the study of aging and for the modeling of lithium-ion batteries.
An operando dual-edge X-ray absorption spectroscopy
on both transition-metal ordered and disordered LiNi0.5Mn1.5O4 during electrochemical delithiation
and lithiation was carried out. The large data set was analyzed via
a chemometric approach to gain reliable insights into the redox activity
and the local structural changes of Ni and Mn throughout the electrochemical
charge and discharge reaction. Our findings confirm that redox activity
relies predominantly on the Ni2+/4+ redox couple involving
a transient Ni3+ phase. Interestingly, a reversible minority
contribution of Mn3+/4+ is also evinced in both LNMO materials.
While the reaction steps and involved reactants of both ordered and
disordered LNMO materials generally coincide, we highlight differences
in terms of reaction dynamics as well as in local structural evolution
induced by the TM ordering.
The electrodes and especially the separator of a lithium-ion-cell are exposed to mechanical stress due to expansion of electrode materials during operation if the electrodes are integrated in a stiff cell casing e.g. in a 18650 cell. As a result, the porosity and therefore the lithiumdiffusion characteristics are altered. In this paper a simulative study of spatially resolved effects from inhomogeneous separator porosity reduction induced by mechanical strain is presented. The findings show significantly changed spatial current densities and local anode voltage potentials that match findings of post-mortem investigations.
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