Understanding the Interfacial Evolution of Lithium-Ion Batteries Using Second-Harmonic Nonlinear Electrochemical Impedance Spectroscopy
Authors: Ji Yuefan , Schwartz Daniel T. 
Published: December 2024

In nickel-rich NMC, a cathode electrolyte interphase (CEI) forms in aged cells, which contributes to impedance growth and a loss of rate capability. [1,2] Electrochemical impedance spectroscopy (EIS) has been widely used to characterize this impedance growth, but gaining deeper quantitative mechanistic insights generally relies on destructive tests. [3] Second harmonic nonlinear electrochemical impedance spectroscopy (2nd-NLEIS) introduces additional mechanistic discriminating power by driving the battery into the weakly nonlinear regime (ΔV < 20 mV) with current modulation amplitudes moderately larger than used in EIS. [4] We have shown that combined experiments and analysis of conventional EIS and 2nd-NLEIS can non-destructively enable clearer assignment of physicochemical processes to each electrode in a cell, yielding internally self-consistent parameters estimation. [5,6] With the combined EIS and 2nd-NLEIS approach, we quantify the evolution of charge transfer rate and symmetry upon aging and formation of CEI in commercial NMC|C cell using a set of complementary analytical and numerical linear and second harmonic nonlinear models. Our approach provides CEI-specific thermodynamic parameters (slope and curvature of the open circuit potential) which differ from the bulk material insertion thermodynamics. We explore the state of charge (SOC) and state of health (SOH) dependency of the interfacial charge transfer processes and thermodynamics in the CEI and bulk cathode. Our method confirms that the formation of a CEI is responsible for impedance growth and a sluggish charge transfer, and unique to this method is an ability to show a strong SOC- and SOH-dependent breaking of charge transfer symmetry at the aged cathode. Asymmetric charge transfer at an aged cathode favors lower overpotential oxidation (delithiation) and higher overpotential reduction (lithiation), especially at low cell SOC.

References

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