Interfacial Reactivity of a High Capacity Manganese Rich (HCMRTM) Li-Ion Positive Electrode

High capacity manganese rich (HCMR™) materials are promising candidates for commercial Li-ion battery positive electrodes for applications in electric and plug-in hybrid electric vehicles.¹ These oxides, also denoted as xLiMO₂ - (1-x)LiMnO₃ (M = Co, Mn, Ni), deliver a high discharge capacity (>240 mAh/g) at operating voltages exceeding 3.5 V vs. Li/Li⁺.² However, these materials have significant limitations and suffer from high first cycle irreversible capacity loss, impedance rise and voltage fade during cycling.^2-4 Extensive studies of HCMR™ electrodes indicate a structural modification/ rearrangement as one of the major reasons for the electrode/material degradation. In this work, the surface and interfacial phenomena of HCMR™ electrodes were investigated using in situ and ex situ spectroscopic techniques as a function of electrode potential and charge/discharge cycling.

HCMR™ materials surface structural changes were investigated during electrochemical cycling by confocal Raman microscopy in a custom-designed spectro-electrochemical cell. A carbon- and binder-free cathode was prepared by pressing the active material onto Al foil, which is then loaded into the spectro-electrochemical cell. Figure 1 shows Raman spectra collected during a first charge-discharge cycle. Surface structural changes due to the delithiation of the HCMR™ material are observed by the change in position of the Raman peaks during cycling.

While the oxidation of Ni²⁺ to Ni⁴⁺ appears to be a fully reversible process, other surface structural changes manifest themselves during the prolonged cycling (Figure 2). A shift of the major peak Mn-O band from 595 cm^-1 to higher frequency, as well as an increase in intensity for another Mn-O mode at 650 cm^-1 was observed.

Shifting from the Raman spectra indicates significant structural deviation of the HCMR™ material during cycling. The correlation of the structural changes with the observed interfacial phenomena will be discussed in regards to their influence on the electrode and cell electrochemical performance and lifetime.

Acknowledgements

This work was supported by the Assistant Secretary for Energy Efficiency and Renewable Energy, Office of Vehicle Technologies of the U.S. Department of Energy under Contract No. DE-AC02-05CH11231, under the Applied Battery Research for Transportation (ABR) Program and Award Number DE-EE0006443.

 

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