A comprehensive electrochemical impedance study is conducted on LiNi 0.80 Co 0.15 Al 0.05 O 2 electrode material as a function of state-of-charge and aging. Electrodes are harvested from four commercial batteries with different state-of-health conditions. Odd random phase electrochemical impedance spectroscopy and the symmetric cell approach are applied in this work in order to obtain reliable impedance results. An equivalent electrical circuit model is constructed. The parameters related to the generalized finite-space Warburg element, the CPE element and the charge transfer resistance are further interpreted. Valuable information is obtained and closely linked to the physical phenomena. The charge transfer resistance has been proved to be the most reliable parameter for the estimation of state-of-health.

The class of nonlinear time-varying (NLTV) systems includes all possible systems and, hence, is difficult to identify. Still, when the nonlinearities are not too strong then, depending on the application, a linear model might be sufficient for approximating the true response. To quantify the approximation error of the linear model, detecting and quantifying the nonlinear behavior is of key importance. In this paper we propose a fully automated procedure for detecting, classifying and quantifying the nonlinear distortions in the response, possibly subject to a trend, of a specific class of NLTV systems to odd random phase multisine excitations. The result is a measurement of the timevarying frequency response function together with uncertainty bounds due to noise and nonlinear distortions. The user only has to specify four integer numbers: an upper bound on (i) the degree on the time-domain polynomial modelling of the trend, (ii) the degree of the frequency-domain polynomial basis function and (iii) the number of frequency-domain hyperboliclike basis functions, all used for modeling the output spectrum; and (iv) a quality measure -called degrees-of-freedom -of the noise variance estimate. Guidelines are provided for obtaining reasonable values for these upper bounds.

Kelvin probe force microscope (KPFM) is an AFM-based technique, which can be used for studying lithium ion batteries (LIBs). The capabilities of this technique for practical applications have been explored further in this work. A direct correlation between the local surface potential measured by KPFM and the electrochemical potential of LiNi 0.80 Co 0.15 Al 0.05 O 2 cathode material is revealed. Based on this correlation, a quantitative approach to investigate the local behavior of the cathode material has been developed. We then demonstrate the potentials of this correlation by applying this methodology in three dierent situations. The surface inhomogeneity of the electrode is visualized and estimated from the micro-scale down to the nanoscale. In addition, the surface over-delithiation state induced by insucient Li + solid-state diusivity, is 1 also observed on the electrodes charged at elevated C-rates. Accordingly, it indicates the presence of an uneven delithiation from the surface towards the bulk of the active particle. This work demonstrates that KPFM is a robust technique to investigate LIB systems due to its prominent experimental compatibility, surface sensitivity and high spatial resolution (< 100 nm).

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