Analysis of All-Solid Li-Ion Batteries By Photoelectron Spectroscopy: Experimental Procedures, Interpretation and Results

Electrochemical devices such as lithium-ion secondary batteries consist of complex layer systems with interfaces and interphases that play a crucial role in cell performance and degradation. The understanding of the energy level alignment and double layer formation especially at electrode-electrolyte interfaces is therefore mandatory. Photoelectron Spectroscopy (PES) is a reliant tool and widely accepted method in surface science to probe the electronic structure of thin, electronically conducting layer systems on a nanometer scale, e.g. of semiconductor devices.  For such systems, an aligned Fermi level (i.e. electrochemical potential of electrons) can usually be presumed resulting in well-defined experimental conditions for PES. For ionic systems involving (solid) electrolytes, the experimental conditions are less clear due to the lack of sufficient electronic conductivity. Nevertheless, also in such cases PES proofs to be a useful tool, i.e. for analysis of solid electrolytes or characterization of full cells and interfaces [1].

In this contribution, we focus on the different electrolyte- and ionic-interface related effects, such as charging and Fermi level misalignment, and discuss experimental strategies and interpretation of PES measurements. By example of a LiCoO2|LiPON|Li thin film battery, we demonstrate how interface experiments and analysis of the top electrode of a layer stack can be performed and interpreted. Key factors taken into consideration are the electrochemical equilibrium of the lithium ions at the interfaces as well as the ability of the electrolyte to sustain an electric (ionic) current, given corresponding (de)intercalation reactions are triggered.  Next to information on electron energy level alignment and double layer formation, information on ion energy levels, ionic and electronic contribution to cell voltage, formation of reaction layers as well as electronic structure of intercalation electrodes can be obtained from suitable experiments (see e.g.[1-3]).

1.            Hausbrand, R., D. Becker, and W. Jaegermann, A surface science approach to cathode/electrolyte interfaces in Li-ion batteries: Contact properties, charge transfer and reactions. Progress in Solid State Chemistry, 2014. 42(4): p. 175-183.

2.            Tonti, D., C. Pettenkofer, and W. Jaegermann, In-Situ Photoelectron Spectroscopy Study of a TiS2 Thin Film Cathode in an Operating Na Intercalation Electrochemical Cell. Ionics, 2000. 6(3-4): p. 196-202.

3.            Schwöbel, A., R. Hausbrand, and W. Jaegermann, Interface reactions between LiPON and lithium studied by in-situ X-ray photoemission. Solid State Ionics, in press.