Spectroscopic Ellipsometry for Operando Monitoring of (De)Lithiation-Induced Phenomena on LiMn₂O₄ and LiNi_0.5Mn_1.5O₄ Electrodes

Abstract

High voltage cathodes suffer from degradation phenomena that are challenging to observe and identify during cell operation. Dense and smooth sputtered thin films electrodes with an absence of binders and conductive additives allow a direct study of the active material upon Li insertion and extraction at surface and bulk. Using an operando spectroscopic ellipsometry set-up combined with a customized electrochemical-optical cell (EC-SE), the evolution of the optical absorption and thickness of LiMn2O4 and LiNi0.5Mn1.5O4 thin-film electrodes was monitored upon cycling. Mixed Mn3+/4+ valence in the electrodes and evident layer dissolution associated to transition metal dissolution in the non-aqueous electrolyte at the applied polarization potentials was observed. Our results reaffirm EC-SE as a convenient method to study degradation phenomena in cobalt-free transition metal oxide electrodes.

Revealing the Role of Electrolyte Salt Decomposition in the Structural Breakdown of LiNi0.5Mn1.5O4

“…For an accurate thickness distribution, SE mapping is done on a 2 cm diameter area on the LNMO sample after annealing as shown in Figure 1b and fitted using previously developed optical models. 58 The thickness value of 78.42 ± 1.39 nm aligns with the cross-sectional imaging in Figure 1a. XRD pattern in Figure 1c exhibits the expected diffraction peaks of the pure disordered LNMO phase (spinel Fd3̅ m space group) and the peaks corresponding to the platinum current collector.…”

“…Separate and relatively well-adhered layers are observed in the cross-section inspection. For an accurate thickness distribution, SE mapping is done on a 2 cm diameter area on the LNMO sample after annealing as shown in Figure b and fitted using previously developed optical models . The thickness value of 78.42 ± 1.39 nm aligns with the cross-sectional imaging in Figure a.…”

“…Locally generated Cl – will rapidly react with the transition metal oxide in the presence of protons forming Cl compounds and dissolving the LNMO, as stated in eqs and (process 4 in Figure c. This explains why the volume of the active material is rapidly reduced as a consequence of cycling, noticeable as a thickness decrease in the thin film configuration . With the increased acidification of the electrolyte, disproportionation of Mn­(III) into structural Mn­(IV) and Mn­(II) (in soluble MnO) can occur, leading to further active material loss in the liquid electrolyte (process 5 in Figure c).…”

“…This explains why the volume of the active material is rapidly reduced as a consequence of cycling, noticeable as a thickness decrease in the thin film configuration. 58 With the increased acidification of the electrolyte, disproportionation of Mn(III) into structural Mn(IV) and Mn(II) (in soluble MnO) can occur, leading to further active material loss in the liquid electrolyte (process 5 in Figure 8c). Electrolyte decomposition is prone to occur more aggressively on the freshly exposed active material at the fractures that are created upon cycling, so these will become hotspots for the decomposition reactions (eqs 10 and 12).…”

“…Notably, dense LNMO thin films are a convenient model system that can help understand the causes behind the electrode material failure as the active material can be studied individually without the passive components present in composite electrodes (i.e., carbon additives and binders). This simplifies the interpretation of experiments and electrode design and still brings fundamental knowledge that can elucidate the degradation occurring in the composite electrode configuration. − …”