Fundamental Understanding of Water-Induced Mechanisms in Li-O2 Batteries: Recent Developments and Perspectives

Triarylmethyl cation redox mediators enhance Li-O2 battery discharge capacities

A major impediment to Li-O2 battery commercialization is the low discharge capacities resulting from electronically insulating Li2O2 film growth on carbon electrodes. Redox mediation offers an effective strategy to drive oxygen chemistry into solution, avoiding surface-mediated Li2O2 film growth and extending discharge lifetimes. As such, the exploration of diverse redox mediator classes can aid the development of molecular design criteria. Here we report a class of triarylmethyl cations that are effective at enhancing discharge capacities up to 35-fold. Surprisingly, we observe that redox mediators with more positive reduction potentials lead to larger discharge capacities because of their improved ability to suppress the surface-mediated reduction pathway. This result provides important structure-property relationships for future improvements in redox-mediated O2/Li2O2 discharge capacities. Furthermore, we applied a chronopotentiometry model to investigate the zones of redox mediator standard reduction potentials and the concentrations needed to achieve efficient redox mediation at a given current density. We expect this analysis to guide future redox mediator exploration.

Reversible Discharge Products in Li-Air Batteries

Lithium-air (Li-air) batteries stand out among the post-Li-ion batteries due to their high energy density, which has rapidly progressed in the past years. Regarding the fundamental mechanism of Li-air batteries that discharge products produced and decomposed during charging and recharging progress, the reversibility of products closely affects the battery performance. Along with the upsurge of the mainstream discharge products lithium peroxide, with devoted efforts to screening electrolytes, constructing high-efficiency cathodes, and optimizing anodes, much progress is made in the fundamental understanding and performance. However, the limited advancement is insufficient. In this case, the investigations of other discharge products, including lithium hydroxide, lithium superoxide, lithium oxide, and lithium carbonate, emerge and bring breakthroughs for the Li-air battery technologies. To deepen the understanding of the electrochemical reactions and conversions of discharge products in the battery, recent advances in the various discharge products, mainly focusing on the growth and decomposition mechanisms and the determining factors are systematically reviewed. The perspectives for Li-air batteries on the fundamental development of discharge products and future applications are also provided.

Critical Factors Affecting the Catalytic Activity of Redox Mediators on Li-O2 Battery Discharge

Redox mediators (RMs) have a substantial ability to govern oxygen reduction reaction (ORR) in Li-O₂ batteries, which can realize large capacity and high-rate capability. However, studies on understanding RM-assisted ORR mechanisms are still in their infancy. Herein, a quinone-based molecule, vitamin K1 (VK1), is first used as the ORR RM for Li-O₂ batteries, together with 2,5-di-tert-butyl-1,4-benzoquinone (DBBQ), to elucidate key factors on the catalytic activity of RMs. By combining experiments and first-principle computations, we demonstrate that the reduced VK1 has strong oxygen affinity and can effectively retard the deposition of Li₂O₂ films on the electrode surface, thereby guaranteeing enough active sites for electron transfer. Besides, the low reaction free energy of disproportionation of the Li(VK1)O₂ intermediate into Li₂O₂ also significantly accelerates the ORR process. Consequently, the catalytic activity of VK1 is significantly boosted, and the discharge capacity of VK1-assisted batteries is 3.2-4.5 times that of DBBQ-assisted batteries. This study provides new insight for better understanding the working roles of RMs in Li-O₂ batteries.

A Moisture-Assisted Rechargeable Mg-CO2 Battery

New sustainable energy conversion and storage technologies are required to address the energy crisis and CO₂ emission. Among various metal-CO₂ batteries that utilize CO₂ and offer high energy density, rechargeable Mg-CO₂ batteries based on earth-abundant and safe magnesium (Mg) metal have been limited due to the lack of a compatible electrolyte, operation atmosphere, and unambiguous reaction process. Herein, the first rechargeable nonaqueous Mg-CO₂ batteries have been proposed with moisture assistance in a CO₂ atmosphere. These display more than 250 h cycle life and maintain the discharge voltage over 1 V at 200 mA g^-1 . Combining with the experimental observations and theoretical calculations, the reaction in the moisture-assisted Mg-CO₂ battery is revealed to be 2 Mg+3 CO₂ +6 H₂ O↔2 MgCO₃ ⋅3 H₂ O+C. It is anticipated that the moisture-assisted rechargeable Mg-CO₂ batteries would stimulate the development of multivalent metal-CO₂ batteries and extend CO₂ fixation and utilization for carbon neutrality.

Correlating Catalyst Design and Discharged Product to Reduce Overpotential in Li-CO2 Batteries

Li-CO₂ batteries with dual efficacy for greenhouse gas CO₂ sequestration and high energy output have been regarded as a promising electrochemical energy storage technology. However, battery feasibility has been hampered by inferior electrochemical performance due to large overpotentials and low cyclability primarily caused by the difficult decomposition of ultra-stable Li₂ CO₃ during charge. The use of cathode catalysts has been highlighted as a promising solution and catalyst properties, as well as the nature of discharge products, are closely correlated with electrochemical performance. Here, the catalyst design strategies that include active site enrichment, electrical transport enhancement, and mass transfer improvement are summarized. Catalyst effects on product decomposition are then subsequently introduced, while product geometry and chemical composition will be explored, with an emphasis on the formation/decomposition of Li₂ C₂ O₄ instead of Li₂ CO₃ . Building on previous research, future directions that facilitate improvements in catalyst design are put forward to reinforce the fundamental development of Li-CO₂ batteries.

α-MnO2/MWCNTs as an electrocatalyst for rechargeable relatively closed system Li-O2 batteries

Herein, a new, relatively closed system Li-O₂ (RCLO) battery, without extra oxygen being involved in the reaction during the charge and discharge process, is reported. This relatively closed system effectively solves the key issue of poor circulation caused by oxygen generation in conventional Li-O₂ batteries.

Theoretical evidence of water serving as a promoter for lithium superoxide disproportionation in Li-O2 batteries

Experimental evidence has demonstrated that the presence of water in non-aqueous electrolytes significantly affects Li-O2 electrochemistry. Understanding the reaction mechanism for Li2O2 formation in the presence of water impurities is important to understand Li-O2 battery performance. A recent experiment has found that very small amounts of water (as low as 40 ppm) can significantly affect the product formation in Li-O2 batteries as opposed to essentially no water (1 ppm). Although experimental as well as theoretical work has proposed mechanisms of Li2O2 formation in the presence of much larger amounts of water, none of the mechanisms provide an explanation for the observations for very small amounts of water. In this work, density functional theory (DFT) was utilized to obtain a mechanistic understanding of the Li-O2 discharge chemistry in a dimethoxyethane (DME) electrolyte containing an isolated water and no water. The reaction pathways for Li2O2 formation from LiO2 on a model system were carefully evaluated with different level of theories, i.e. PBE (PW), B3LYP/6-31G(2df,p), B3LYP/6-311++G(2df,p) and G4MP2. The results indicate that the LiO2 disproportionation reaction to Li2O2 can be promoted by the water in DME electrolyte, which explains why there is a significant difference compared to when no water is present in the experimentally observed discharge product distributions. Ab initio molecular dynamics calculations were also used to investigate the disproportionation of LiO2 dimer in explicit DME. This work adds to the fundamental understanding of the discharge chemistry of a Li-O2 battery.

Quantitative Delineation of the Low Energy Decomposition Pathway for Lithium Peroxide in Lithium-Oxygen Battery

Identification of a low-potential decomposition pathway for lithium peroxide (Li2O2) in nonaqueous lithium-oxygen (Li-O2) battery is urgently needed to ameliorate its poor energy efficiency. In this study, experimental data and theoretical calculations demonstrate that the recharge overpotential (η RC) of Li-O2 battery is fundamentally dependent on the Li2O2 crystallization pathway which is intrinsically related to the microscopic structural properties of the growing crystals during discharge. The Li2O2 grown by concurrent surface reduction and chemical disproportionation seems to form two discrete phases that have been deconvoluted and the amount of Li2O2 deposited by these two routes is quantitatively estimated. Systematic analyses have demonstrated that, regardless of the bulk morphology, solution-grown Li2O2 shows higher η RC (>1 V) which can be attributed to higher structural order in the crystal compared to the surface-grown Li2O2. Presumably due to a cohesive interaction between the electrode surface and growing crystals, the surface-grown Li2O2 seems to possess microscopic structural disorder that facilitates a delithiation induced partial solution-phase oxidation at lower η RC (<0.5 V). This difference in η RC for differently grown Li2O2 provides crucial insights into necessary control over Li2O2 crystallization pathways to improve the energy efficiency of a Li-O2 battery.

Li-air Battery with a Superhydrophobic Li-Protective Layer

Li-air  batteries operated in ambient air are imperative toward real practical applications. However, the passivation of lithium metal anodes induced by attacking air hinders their long-term running, accelerating the degradation of Li-air batteries. Herein, a hydrogel-derived hierarchical porous carbon (HDHPC) layer with superhydrophobicity is proved as an effective Li-protective layer for a Li-air battery that suppresses the H₂O attack and lithium dendrite formation during cycling. Accordingly, the HDHPC protective layer-based Li-air cell exhibits eminent cycling stability in ambient air [relative humidity (RH) of ∼40%], which is far better than that of the Li-air cell without the HDHPC protective layer. It is also demonstrated that the conversion of O₂/Li₂O₂ in Li-air batteries adversely affects the decomposition of the byproduct and electrolyte. The usage of the HDHPC protective layer pioneers a new avenue of developing high-performance Li-air batteries in ambient air.

3D printing of cellular materials for advanced electrochemical energy storage and conversion

3D printing, an advanced layer-by-layer assembly technology, is an ideal platform for building architectures with customized geometries and controllable microstructures. Bio-inspired cellular material is one of most representative 3D-printed architectures, and attracting growing attention compared to block counterparts. The integration of 3D printing and cellular materials offer massive advantages and opens up great opportunities in diverse application fields, particularly in electrochemical energy storage and conversion (EESC). This article gives a comprehensive overview of 3D-printed cellular materials for advanced EESC. It begins with an introduction of advanced 3D printing techniques for cellular material fabrication, followed by the corresponding material design principles. Recent advances in 3D-printed cellular materials for EESC applications, including rechargeable batteries, supercapacitors and electrocatalysts are then summarized and discussed. Finally, current trends and challenges along with in-depth future perspectives are provided.

Stable Lithium Anode of Li-O2 Batteries in a Wet Electrolyte Enabled by a High-Current Treatment

Rechargeable Li-air (O₂) batteries have attracted a great deal of attention because of their high theoretical energy density and been regarded as a promising next-generation energy storage technology. Among numerous obstacles to Li-air (O₂) batteries preventing their use in practical applications, water is a representative impurity for Li-air (O₂), which could hasten the deterioration of the anode and accelarate the premature death of cells. Here, we report an effective in situ high-current pretreatment process to enhance the cycling performance of Li-O₂ batteries in a wet tetraethylene glycol dimethyl ether-based electrolyte. With the help of certain levels of H₂O (from 100 to 2000 ppm) in the electrolyte, adequate Li₂O formed on the lithium anode surface after high-current pretreatment, which is necessary for a robust and uniform solid electrolyte interphase layer to protect Li metal during the long-term discharge-charge cycling process. This in situ high-current pretreatment method in a wet electrolyte is shown to be an effective approach for enhancing the cycling performance of Li-O₂ batteries with a stable Li metal anode and promoting the realization of practical Li-air batteries.

Guidelines and trends for next-generation rechargeable lithium and lithium-ion batteries

This review article summarizes the current trends and provides guidelines towards next-generation rechargeable lithium and lithium-ion battery chemistries.