Interfacial Stability of Ru-Based Li-Rich Layered Oxide Cathode by Guaiacol as an Organic Superoxide Dismutase Mimics
Authors: Lee Jeongin , Kim Jonghak , Hwang Chihyun , Song Hyun-Kon 
Published: December 2024

As the battery-based energy storage market develops, the demand for Li-ion batteries (LIBs) with high energy density is increasing, which leads to the development of new electrode materials. Li-rich layered oxides (LLO), Li2MO3 where M represents a transition metal such as Mn, Ru or Ir, are attracting attention as promising cathode materials which deliver high discharge capacities exceed 250 mAh g-1 originated from oxygen redox in addition to typical transition (TM) metal redox. The oxygen redox provides progressive insights for higher energy density cathode, however, the oxygen redox is also the origin of the structural instability. The structural distortion of LLOs evolves along a sequence of lattice TM oxidation, ligand-to-metal charge transfer from lattice oxygen (O2
-) to oxidized TM, peroxo-like species formation and dioxygen de-coordination followed by cationic disordering and stacking faults. The de-coordinated dioxygen is reduced to be a reactive oxygen species (ROS) represented by superoxide radical (O2
•-) by oxidizing carbonate solvent molecules and/or carbon materials in electrodes. Besides, the irreversible superoxo-like species generated during excessive charge aggravate the electrolyte decomposition of its nucleophilicity. The reactive oxygen species deteriorates LLO by a circular chain of superoxide generation – electrolyte decomposition – water-induced HF formation – TM dissolution – structural distortion – superoxide generation.

To scavenge superoxide radicals for guaranteeing the interfacial stability of LLO by breaking the LLO-deteriorating chain mechanism, we have noticed the antiaging mechanisms of biological system suffering from ROS issues such as enzymatic and antioxidant mechanisms. Superoxide dismutase (SOD) catalytically transforms superoxide into the less reactive O2 and O2
2-, serving as a reversible catalyst. Conversely, antioxidants undergo oxidation by reacting with superoxide before it induces damage to other cellular constituents. The numerous papers have reported the application of antioxidants in LIBs, despite their consumption nature in scavenging superoxide radicals, whereas the lack of application of SOD in LIBs has been noted. One is the malonic-acid-decorated fullerene (MA-C60) that our group was presented, as the mobile catalyst to disproportionate lithium superoxide (Li2O) to lithium peroxide (Li2O2) for LLO. This class of catalyst was named superoxide dismutase mimic (SODm) as the enzymes responsible for the analogous reaction in aerobic organisms.

In this work, we introduce the first organic SODm additive, which inherits the SODm characteristics of the inorganic MA-C60 for antiaging a LLO based on 4d-TM (Li2RuO3, LRO) for LIBs. Among phenolic antioxidants, guaiacol was chosen as the organic SODm, a mono-phenolic compound having two functional groups to grab superoxide for disproportionation by an associative manner. The antioxidizing capacities of phenolic compounds against superoxide have been documented. Both monophenols and polyphenols act as Brønsted acids in aprotic solvents, transferring protons to superoxide radicals. This proton transfer initiates a protonation-facilitated disproportionation reaction, leading to the formation of peroxide anions and dioxygen. However, these protonation-facilitated disproportionation was limited in LIB electrolytes with abundant lithium ions. Instead, we demonstrated that the ability of guaiacol to scavenge superoxide arises from the potential for chelation of a superoxide radical by the two oxygens, one from the hydroxyl group and the other from the methoxy group due to their close proximity. This was verified by comparing guaiacol with phenol through cyclic voltammograms and DFT calculations. Moreover, the phenolic components undergo oxidative polymerization to form cathode-electrolyte interphase (CEI) layers. Such phenol-derived CEI layers were previously reported to improve charge transfer when compared to conventional electrolyte-derived CEI layers.

The SODm ability of guaiacol successfully enhance the interfacial stability of cathode. In presence of guaiacol, ruthenium dissolution and structural distortion was significantly reduced. The guaiacol-derived CEI layer exhibited reduced thickness and less pronounced growth over cycling, with the presence of lithium peroxide confirming the SODm activity of guaiacol. Consequently, guaiacol effectively mitigated the collapse of LRO, resulting in a capacity of 150 mAh g-1 after 400 cycles at 1C, twice the capacity in the absence of guaiacol under the same conditions.

To improve the electrochemical stability of LRO, most of endeavors were devoted to stabilizing the lattice oxygen redox chemistry by metal co-doping and tunning of local symmetry around the lattice oxygen without focusing on the ROS issue. On the other hand, we tried to find a solution in electrolyte instead of active material itself. This work highlights the importance of deactivating reactive oxygen species in 4d TM-based LLO proposes a simple solution through the application of an additive with antioxidant properties, guaiacol.

Figure 1