Understanding LiOH Chemistry in a Ruthenium‐Catalyzed Li–O₂ Battery

Abstract

Non‐aqueous Li–O2 batteries are promising for next‐generation energy storage. New battery chemistries based on LiOH, rather than Li2O2, have been recently reported in systems with added water, one using a soluble additive LiI and the other using solid Ru catalysts. Here, the focus is on the mechanism of Ru‐catalyzed LiOH chemistry. Using nuclear magnetic resonance, operando electrochemical pressure measurements, and mass spectrometry, it is shown that on discharging LiOH forms via a 4 e− oxygen reduction reaction, the H in LiOH coming solely from added H2O and the O from both O2 and H2O. On charging, quantitative LiOH oxidation occurs at 3.1 V, with O being trapped in a form of dimethyl sulfone in the electrolyte. Compared to Li2O2, LiOH formation over Ru incurs few side reactions, a critical advantage for developing a long‐lived battery. An optimized metal‐catalyst–electrolyte couple needs to be sought that aids LiOH oxidation and is stable towards attack by hydroxyl radicals.

Defect Chemistry in Discharge Products of Li–O2 Batteries

“…Seeking alternative energy carriers with a low charge overpotential is one of the most effective ways to minimize the negative effects caused by Li 2 O 2 such as the nucleophilic attack of cell components and the resulting high charge overpotential. Two promising strategies have been developed, including stabilizing intermediates of LiO 2 by suppressing Li 2 O 2 formation and converting Li 2 O 2 into other promising energy carriers such as LiOH, and Li 2 CO 3 . In this section, we will focus on the mechanistic study of Li–O 2 batteries using LiO 2 or LiOH/Li 2 CO 3 as alternative energy carriers.…”

“…It is widely accepted that LiOH is one of the main side products in Li–O 2 batteries, where H 2 O is the dominant proton source in humid oxygen and electrolytes. In contrast to conventional Li–O 2 batteries, which work via Li 2 O 2 formation, a novel Li–O 2 battery employing LiOH as the energy carrier has attracted extensive interest . The Li–O 2 battery that works via LiOH formation, 4Li + 2H 2 O + O 2 ⇌ 4LiOH ( E ° = 3.32 V), delivers a low charge potential and high cycling stability because of the enhanced conductivity of LiOH and the alleviation of side reactions caused by the high nucleophilicity of Li 2 O 2 .…”

“…The Li–O 2 battery that works via LiOH formation, 4Li + 2H 2 O + O 2 ⇌ 4LiOH ( E ° = 3.32 V), delivers a low charge potential and high cycling stability because of the enhanced conductivity of LiOH and the alleviation of side reactions caused by the high nucleophilicity of Li 2 O 2 . Additives such as H 2 O and lithium iodide (LiI) greatly impact the conversion of the aforementioned Li–O 2 batteries, acting as either initiators or catalysts to promote the formation of LiOH . The complex effects of H 2 O and LiI on LiOH chemistry will be discussed in this section, mainly focusing on the reaction mechanism.…”

“…Figure b shows the schematic atomic structures of defective Li 2 O 2 materials, including Li‐deficient Li 2 O 2 , doped Li 2 O 2 , Li 2 O 2 with surface/grain boundaries and amorphous Li 2 O 2 . Several strategies have been applied to control these intrinsic defects of Li 2 O 2 , including cathode catalyst and electrolyte additive engineering by controlling their intermediate species as well as nucleation and growth processes. Owing to the structure modification, induced defects and the possibly altered band structure in Li 2 O 2 can impact the major charge carrier and regulate charge carrier concentrations .…”

“…The presence of these defects can significantly enhance the conductivity of Li 2 O 2 and promote charge transport through Li 2 O 2 , thus increasing the cycling stability of Li–O 2 batteries . Employing other energy carriers such as LiOH and LiO 2 has been explored as an effective way to circumvent charge transport issues. Therefore, a comprehensive understanding of defect structures in Li 2 O 2 and the defect formation mechanisms is crucial to realize the rational control of the properties of discharge products.…”