Steady-State and "Activated" State in Li Ion Insertion Systems
Authors: Moskon Joze , Krizan Gregor , Dominko Robert , Gaberscek Miran 
Published: February 2020

In parallel with the fast development of new active materials and introducing novel synthetic routes, materials characterization and optimization, there has been also a tremendous research effort aiming at understanding of the key mechanisms governing the operation of battery insertion materials. Accordingly during the last two decades several different models have been proposed as regards the insertion materials [1-3]. In this work we present some general, carefully proved experimental facts observed in Li ion insertion systems that have not yet been satisfactorily addressed.
As observed experimentally already almost a decade ago a LiFePO4 based cell exhibited an effect of reduced total electrode resistance when subjected to an increase of a current driven through the cell [4]. Later on this “activation” phenomenon was confirmed in the work of other groups [5]. Recently in an extensive experimental observation of LiFePO4 based electrodes using synchrotron Scanning Transmission X-Ray Microscopy (STXM) Chue et al. [6] have demonstrated that the fraction of the phase-transforming particles depends on C-rate. In the light of the results of this study we here propose that observed decrease of total electrode resistance with increasing current originates from the increase of population of LiFePO4 particles that simultaneously undergo phase-transition.
In this work we will display how impedance spectroscopy (EIS) can be utilized for the observation of the major switch between the stady-state and dynamical state in a Li ion electrode. We show that obtained results of the “dynamic” impedance are directly correlated to the voltage hysteresis observed in the ordinary galvanostatic charge/discharge cycles. We propose a generalized phenomenological model that effectively captures the transition by introducing a switch-resistor element in the electrochemical equivalent circuit of a cell. Further we show results of simple galvanostatic step-wise current increase measurements where we show that “activated” state in Li ion insertion electrode is meta-stable (on the order of at least 100 h). This gives us first experimental evidence that the “activation” observed in Li ion insertion systems is indeed differing in its nature from the phenomena observed in conventional electrochemical systems where the kinetic properties of the electrolyte-metal interface are often successfully modeled by Butler-Volmer type relations.

Figure 1. a) Comparison of mass-normalized kinetic resistance, R
kinetic×m, of three typical Li ion insertion materials as a function of mass-normalized current, I
m, obtained from the ordinary galvanostatic charge/discharge cycles. Strong decrease of electrode resistance with increasing I
m can be observed in the case of LiMnPO4 and LiFePO4 based electrodes, while in the case of LiCoO2 electrode the effect is much less pronounced. b) Results of a simple galvanostatic step-wise current increase experiment where a normal galvanostatic discharge measurement (base C-rate, brown curve) is at some point interrupted by applying a significantly larger current for certain amount of charge being passed through a cell and thereafter followed by a completion of a discharge using base C-rate. In the case shown on Fig. 1b the base –C/100 measurement (black curve) was interrupted by -1C "pulse". In the period after "pulse" completion a distinctive decrease of overvoltage, Δη, is regularly observed - indicating a meta-stable nature of "activated" state.
[1] V. Srinivasan and J. Newman, J. Electrochem. Soc., 151, A1517 (2004).
[2] L. Laffont et al., Chem. Mater., 18, 5520 (2006).
[3] C. Delmas, M. Maccario, L. Croguennec, F. Le Cras, and F. Weill, Nat. Mater., 7,
665 (2008).
[4] M. Gaberscek, M. Küzma, and J. Jamnik, Phys. Chem. Chem. Phys., 9, 1815–1820 (2007).
[5] C. Fongy, S. Jouanneau, D. Guyomard, J. C. Badot, and B. Lestriez, J. Electrochem. Soc., 157, A1347 (2010).
[6] Y. Li et al., Nat. Mater., 13, 1149–1156 (2014).

Figure 1