Impact of thermal gas treatment of Li-rich Mn-based cathode materials for Li-ion batteries

The development of cathode materials with high energy density for rechargeable Li-ion batteries, crucial for applications like electromobility and large-scale use, remains a significant challenge. Among the promising cathode materials capable of delivering substantial energy density and thus gaining attention for commercial use are the Li-rich Mn-based cathode materials. These possess a layered structure, represented by the formula xLi₂MnO₃·(1 − x)Li(M)O₂ (where M includes transition metals such as Mn, Ni, Co, and x is less than 0.5). Notably, these materials boast a high discharge capacity (> 250 mA h g⁻¹) and exhibit a more environmentally friendly profile due to lower cobalt content. Their key advantage lies in accommodating extra lithium ions that substitute a transition metal ion within the lattice, leading to additional capacity.
However, a major issue arises from the irreversible release of oxygen in these Li-rich layered structure cathode materials when subjected to high voltages (≥4.6 V vs. Li⁺/Li). This phenomenon triggers a voltage hysteresis connected to the migration of transition metal ions between specific sites, leading to a gradual loss in voltage. Moreover, the use of liquid carbonate-based electrolytes at elevated potentials results in undesired chemical reactions involving the cathode surface, causing irreversible capacity reduction.
Recent efforts aimed at enhancing stability involved subjecting HE-NCM cathodes to thermal treatment with double gases SO₂ and NH₃, resulting in improved capacity retention, rate capability, and reduced voltage hysteresis. However, a comprehensive understanding of the mechanisms responsible for this enhanced stability remains elusive.

A thorough investigation into the chemical composition and electronic structure alterations of transition metals near the surface of various HE-NCM cathodes (untreated, treated, devoid of carbon and binder) using advanced electron spectroscopy techniques quasi in situ (i.e. in vacuo sample transfer without contact to air)  has been recently carried out at the BACH beamline, lead by dr. Gennady Cherkashinin from the Technical University of Darmstadt. The findings showcase that the treatment involving double gases prompts a partial reduction of Co³⁺ and Mn⁴⁺. The proposed chemical reactions involve electron transfer from SO₂, acting as a Lewis acid, to the transition metal sites, leading to SO₂ decomposition and a distinct surface modification, forming a protective layer for the HE-NCM cathode.

 

Impact of thermal gas treatment on the surface modification of Li-rich Mn-based cathode materials for Li-ion batteries
Mellin M., Liang Z., Sclar H., Maiti S., Píš I., Nappini S., Magnano E., Bondino F., Napal I., Winkler R., Hausbrand R., Hofmann J.P., Alff L., Markovsky B., Aurbach D., Jaegermann W., Cherkashinin G.
Materials Advances 4, 3746-3758(2023)
doi: 10.1039/d3ma00236e