Advances in understanding mechanisms underpinning lithium-air batteries

The rechargeable lithium-air battery has the highest theoretical specific energy of any rechargeable battery and could transform energy storage if a practical device could be realised. At the fundamental level, little was known about the reactions and processes that take place in the battery, representing a significant barrier to progress. Here we review recent advances in understanding the chemistry and electrochemistry that govern the operation of the lithium-air battery, especially the reactions at the cathode. The mechanisms of O2 reduction to Li2O2 on discharge and the reverse process on charge are discussed in detail, as are their consequences for the rate and capacity of the battery. The various parasitic reactions involving the cathode and electrolyte during discharge and charge are also considered. We also provide views on understanding the stability of the cathode and electrolyte and examine design principles for better lithium-air batteries

Advances in understanding mechanisms underpinning lithium-air batteries

The rechargeable lithium-air battery has the highest theoretical specific energy of any rechargeable battery and could transform energy storage if a practical device could be realised. At the fundamental level, little was known about the reactions and processes that take place in the battery, representing a significant barrier to progress. Here we review recent advances in understanding the chemistry and electrochemistry that govern the operation of the lithium-air battery, especially the reactions at the cathode. The mechanisms of O2 reduction to Li2O2 on discharge and the reverse process on charge are discussed in detail, as are their consequences for the rate and capacity of the battery. The various parasitic reactions involving the cathode and electrolyte during discharge and charge are also considered. We also provide views on understanding the stability of the cathode and electrolyte and examine design principles for better lithium-air batteries

Impact of intermediate sites on bulk diffusion barriers: Mg intercalation in Mg2Mo3O8

The ongoing search for high voltage positive electrode materials for Mg batteries has been primarily hampered by poor Mg mobility in bulk oxide frameworks. Motivated by the presence of Mo3 clusters that can facilitate charge redistribution and the presence of Mg in a non-preferred (tetrahedral) coordination environment, we have investigated the Mg (de)intercalation behavior in layered-Mg2Mo3O8, a potential positive electrode. While no electrochemical activity is observed, chemical demagnesiation of Mg2Mo3O8 is successful but leads to amorphization. Subsequent first-principles calculations predict a strong thermodynamic driving force for structure decomposition at low Mg concentrations and high activation barriers for bulk Mg diffusion, in agreement with experimental observations. Further analysis of the Mg diffusion pathway reveals an O-Mg-O dumbbell intermediate site that creates a high Mg2+ migration barrier, indicating the influence of transition states on setting the magnitude of migration barriers

Impact of intermediate sites on bulk diffusion barriers: Mg intercalation in Mg2Mo3O8

The ongoing search for high voltage positive electrode materials for Mg batteries has been primarily hampered by poor Mg mobility in bulk oxide frameworks. Motivated by the presence of Mo3 clusters that can facilitate charge redistribution and the presence of Mg in a non-preferred (tetrahedral) coordination environment, we have investigated the Mg (de)intercalation behavior in layered-Mg2Mo3O8, a potential positive electrode. While no electrochemical activity is observed, chemical demagnesiation of Mg2Mo3O8 is successful but leads to amorphization. Subsequent first-principles calculations predict a strong thermodynamic driving force for structure decomposition at low Mg concentrations and high activation barriers for bulk Mg diffusion, in agreement with experimental observations. Further analysis of the Mg diffusion pathway reveals an O-Mg-O dumbbell intermediate site that creates a high Mg2+ migration barrier, indicating the influence of transition states on setting the magnitude of migration barriers

Toward the Development of a High-Voltage Mg Cathode Using a Chromium Sulfide Host

Development of Mg-ion batteries as advanced electrochemical energy storage systems relies on the design and discovery of high-voltage positive electrode (cathode) materials. To date, a variety of sulfide cathodes have been reported (e.g., MgxMo6S8, MgxTi2S4, etc.), but the voltages of these materials are too low to prepare a high energy density Mg cell. Theoretical computations predicted that MgxCr2S4 operating with the high-voltage Cr3+/4+ redox couple would serve as a suitable cathode candidate, but experimental attempts to extract Mg2+ from the lattice have been largely unsuccessful. We show that reversible electrochemical activity within a thiospinel framework (AB2S4) relies on a redox-active transition metal present in the B site; otherwise, anionic redox activity triggers decomposition of the spinel structure. Since Cr and S states are highly coupled in MgCr2S4, the Cr3+/4+ redox couple is inaccessible so that reversible (de)intercalation of Mg2+ cannot occur and charging leads to dissolution of the active material. These findings point to an insufficiency in the screening criteria previously used to identify MgCr2S4 as a promising Mg cathode. Thus, a computable descriptor based on the electronic structure of the discharged material is proposed to predict the prevalence of cation vs anion redox and improve future surveys of potential candidates. It is unlikely that the high-voltage Cr redox couple will be accessible to oxidation in the presence of sulfur within the restrictions of a spinel framework; however, it is possible that a suitable layered MgxCrS2 structure could serve as a reversible high-voltage Mg cathode

Lithium-oxygen batteries and related systems: potential, status, and future

The goal of limiting global warming to 1.5 C requires a drastic reduction in CO2 emissions across many sectors of the world economy. Batteries are vital to this endeavor, whether used in electric vehicles, to store renewable electricity, or in aviation. Present lithium-ion technologies are preparing the public for this inevitable change, but their maximum theoretical specific capacity presents a limitation. Their high cost is another concern for commercial viability. Metal-air batteries have the highest theoretical energy density of all possible secondary battery technologies and could yield step changes in energy storage, if their practical difficulties could be overcome. The scope of this review is to provide an objective, comprehensive, and authoritative assessment of the intensive work invested in nonaqueous rechargeable metal-air batteries over the past few years, which identified the key problems and guides directions to solve them. We focus primarily on the challenges and outlook for Li-O2 cells but include Na-O2, K-O2, and Mg-O2 cells for comparison. Our review highlights the interdisciplinary nature of this field that involves a combination of materials chemistry, electrochemistry, computation, microscopy, spectroscopy, and surface science. The mechanisms of O2 reduction and evolution are considered in the light of recent findings, along with developments in positive and negative electrodes, electrolytes, electrocatalysis on surfaces and in solution, and the degradative effect of singlet oxygen, which is typically formed in Li-O2 cells