Oxygen Redox Versus Oxygen Evolution in Aqueous Electrolytes: Critical Influence of Transition Metals

Abstract

Aqueous lithium‐ion batteries are promising electrochemical energy storage devices owing to their sustainable nature, low cost, high level of safety, and environmental benignity. The recent development of a high‐salt‐concentration strategy for aqueous electrolytes, which significantly expands their electrochemical potential window, has created attractive opportunities to explore high‐performance electrode materials for aqueous lithium‐ion batteries. This study evaluates the compatibility of large‐capacity oxygen‐redox cathodes with hydrate‐melt electrolytes. Using conventional oxygen‐redox cathode materials (Li 2 RuO 3 , Li 1.2 Ni 0.13 Co 0.13 Mn 0.54 O 2 , and Li 1.2 Ni 0.2 Mn 0.6 O 2 ), it is determined that avoiding the use of transition metals with high catalytic activity for the oxygen evolution reaction is the key to ensuring the stable progress of the oxygen redox reaction in concentrated aqueous electrolytes.

Na-Salt Eutectic Dihydrate Melt for High-Voltage Aqueous Batteries

“…Another important factor that should be considered in cell optimization is the upshift of electrode potentials (approximately 0.1 V in the present case; Figure b ,,, ) induced by the shallower liquid Madelung potential in the concentrated regime. ,− Fine-tuning of the electrode potentials paired with optimal electrolyte design will enable the preferred overlap of the electrode potentials and stability windows. Further optimization of these factors for hydrate melts may allow 3 V-class aqueous Na-ion batteries to surpass the present nonaqueous Na-ion batteries. ,,, For instance, unstable SEI formation on carbon additives and catalytic effects of transition metals of active materials would accelerate HER and OER, thus hampering the stable operation of aqueous batteries. , Moreover, since the pitting corrosion of Al occurs above 3.5 V vs Na/Na + , the utilization of cost-effective current collector Al instead of Ti at the cathode remains a highly challenging issue.…”

“…7,8,10,19 For instance, unstable SEI formation on carbon additives and catalytic effects of transition metals of active materials would accelerate HER and OER, thus hampering the stable operation of aqueous batteries. 28,29 Moreover, since the pitting corrosion of Al occurs above 3.5 V vs Na/Na + , the utilization of cost-effective current collector Al instead of Ti at the cathode remains a highly challenging issue.…”

Kinetic square scheme in oxygen-redox battery electrodes

“…after the charging process, O 2 gas release upon charge, and different enthalpies on charge versus discharge. [18][19][20][21][22][23][24][25][26][27][28][29][30] However, experimental verification of the square scheme is limited in part due to complicated structural changes during the oxygen-redox reactions. [31][32][33][34][35][36] For example, O3-Li 1.2 Ni 0.2 Mn 0.6 O 2 (O3: lithium ions occupy octahedral sites between the MO 2 layers, and the packing arrangement of the oxide ions is ABCABC) exhibits irreversible structural degradation such as layered-to-spinel transformation and surface cation densification upon cycling.…”

“…16 However, most Li 1+x M 1Àx O 2 electrodes exhibit polarizing reduction of peroxo-like O 2 2À (e.g., Li 2 Ru 0.5 Sn 0.5 O 3 ) or trapped O 2 molecules (e.g., Li 1.2 Ni 0.13 Co 0.13 Mn 0.54 O 2 ) to deliver the discharge capacity at a lower voltage. 11,15,[17][18][19][20][21][22][23][24][25] Considering immediate electron transfer (O 2À -O À + e À ) against subsequent structural deformation (O-O dimerization), the overall hypothetical mechanism of the oxygen-redox reactions is described as a square scheme (Scheme 1): if an oxidized oxide ion (O À ) is stable, it directly contributes to a non-polarizing discharge capacity (nonpolarizing O redox). Meanwhile, unstable O À dimerize to form stable peroxo-like O 2…”

“…, Li 1.2 Ni 0.13 Co 0.13 Mn 0.54 O 2 ) to deliver the discharge capacity at a lower voltage. 11,15,17–25 Considering immediate electron transfer (O 2− → O − + e − ) against subsequent structural deformation (O–O dimerization), the overall hypothetical mechanism of the oxygen-redox reactions is described as a square scheme (Scheme 1): if an oxidized oxide ion (O − ) is stable, it directly contributes to a non-polarizing discharge capacity (non-polarizing O redox). Meanwhile, unstable O − dimerize to form stable peroxo-like O 2 2− , which may be accelerated by cation migration.…”