Mn Dissolution and Migration in Mn-Based Lithium Ion Batteries: Fundamental Understanding and Protective Interphase Modifications
Authors: Lu Jun , Zhan Chun , Qiu Xinping , Amine Khalil 
Published: February 2020

Dissolution and migration of manganese from cathode to anode lead to severe capacity fading of Li2MnO4-carbon cells. Overcoming this major problem requires better understanding of the mechanism of manganese dissolution, migration, and deposition. Here, we apply a variety of advanced analytical methods to the study of LiMn2O4 cathodes cycled with different anodes. We provide solid evidence, for the first time, that oxidation state of manganese deposited on the anodes is +2, which differs from the results reported earlier. The results also suggest a metathesis reaction between Mn2+ and some species on the SEI during the deposition of Mn on the anodes, rather than a reduction reaction that leads to the formation of metallic Mn otherwise, as speculated in earlier studies. The capacity fading is caused by changing of the SEI on the graphite anode due to the continuous reaction with the dissolved Mn2+from the cathode.
Suppressing the dissolution of Mn from the cathode is critical to reducing capacity fade of LiMn2O4–based cells. Therefore, we apply a nanoscale surface-doping approach to minimize Mn dissolution from LiMn2O4. This approach exploits advantages of both bulk doping and surface coating methods by stabilizing the surface crystal structure of LiMn2O4 through cationic doping while maintaining the bulk spinel LiMn2O4 structure, and protecting the bulk LiMn2O4 from electrolyte corrosion while maintaining ion and charge transport channels on the surface through the electrochemically active doping layer. As a consequence, the surface-doped LiMn2O4demonstrates significantly enhanced electrochemical performance in terms of cycleability and capacity at elevated temperature. This study provides encouraging evidence that surface doping could be a promising alternative to improve the cycling performance of lithium-ion batteries.
On the anode side, we address a new perspective on solving Mn deposition issue: tuning the Mn deposition reaction instead of simply decreasing the concentration of Mn deposited on anode. We find that two commonly used electrolyte additives, fluorinated ethylene carbonate (FEC) and vinylene carbonate (VC), improve the cycle performance of the battery but surprisingly increase the concentration of Mn deposited on the anode. According to the ion-exchange model mentioned above, the increased amount of Mn2+ deposited on anode can be attributed to the increased amount of the Li+ ions in the additive modified SEI layers, as the Mn deposition can occur via the ion-exchange reaction between the Li+ in the SEI and the Mn2+ in the electrolyte. The improved cycle performance can be attributed to enhanced robustness of the SEI layers against the attack of Mn2+ ion from the electrolyte because of the sufficient amount of mobile Li+ in the SEI layer for the ion transport, which was confirmed by in-situ EIS measurements. This work sheds some new light on solving the capacity fade issue of the Mn-based Li ion batteries by manipulating the Mn-Li ion-exchange process rather than simply suppressing the deposition of Mn on anodes.

Figure 1