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 realized. 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. Lithium–air batteries offer great promise for high-energy storage capability but also pose tremendous challenges for their realization. This Review surveys recent advances in understanding the fundamental science that governs lithium–air battery operation, focusing on the reactions at the oxygen electrode.

/pdf/advances-in-understanding-mechanisms-underpinning-lithium-4541jd7tbe.pdf

Advances in understanding mechanisms underpinning lithium–air batteries

Li−O2 Batteries Jun Lu,† Li Li,‡ Jin-Bum Park, Yang-Kook Sun,* Feng Wu,*,‡ and Khalil Amine*,†,∥ †Chemical Sciences and Engineering Division, Argonne National Laboratory, 9700 South Cass Avenue, Lemont, Illinois 60439, United States ‡Beijing Key Laboratory of Environmental Science and Engineering, School of Chemical Engineering and the Environment, Beijing Institute of Technology, Beijing 100081, China Department of Energy Engineering, Hanyang University, Seoul 133-791, South Korea Chemistry Department, Faculty of Science, King Abdulaziz University, 80203 Jeddah, Saudi Arabia

Aprotic and Aqueous Li–O2 Batteries

Alan C. Luntz*,† and Bryan D. McCloskey‡,§ †SUNCAT, SLAC National Accelerator Laboratory, 2575 Sand Hill Road, Menlo Park, California 94025, United States ‡Department of Chemical and Biomolecular Engineering, University of California, Berkeley, California 94720, United States Environmental Energy Technologies Division, Lawrence Berkeley National Laboratory, Berkeley, California 94720, United States

Nonaqueous Li-air batteries: a status report.

Rechargeable energy storage systems with high energy density and round-trip efficiency are urgently needed to capture and deliver renewable energy for applications such as electric transportation Lithium–air/lithium–oxygen (Li–O2) batteries have received extraordinary research attention recently owing to their potential to provide positive electrode gravimetric energies considerably higher (∼3 to 5×) than Li-ion positive electrodes, although the packaged device energy density advantage will be lower (∼2×) In light of the major technological challenges of Li–O2 batteries, we discuss current understanding developed in non-carbonate electrolytes of Li–O2 redox chemistry upon discharge and charge, oxygen reduction reaction product characteristics upon discharge, and the chemical instability of electrolytes and carbon commonly used in the oxygen electrode We show that the kinetics of oxygen reduction reaction are influenced by catalysts at small discharge capacities (Li2O2 thickness less than ∼1 nm), but not at large Li2O2 thicknesses, yielding insights into the governing processes during discharge In addition, we discuss the characteristics of discharge products (mainly Li2O2) including morphological, electronic and surface features and parasitic reactivity with carbon On charge, we examine the reaction mechanism of the oxygen evolution reaction from Li2O2 and the influence of catalysts on bulk Li2O2 decomposition These analyses provide insights into major discrepancies regarding Li–O2 charge kinetics and the role of catalyst In light of these findings, we highlight open questions and challenges in the Li–O2 field relevant to developing practical, reversible batteries that achieve the anticipated energy density advantage with a long cycle life

Lithium–oxygen batteries: bridging mechanistic understanding and battery performance

Lithium metal-based battery is considered one of the best energy storage systems due to its high theoretical capacity and lowest anode potential of all. However, dendritic growth and virtually relative infinity volume change during long-term cycling often lead to severe safety hazards and catastrophic failure. Here, a stable lithium–scaffold composite electrode is developed by lithium melt infusion into a 3D porous carbon matrix with “lithiophilic” coating. Lithium is uniformly entrapped on the matrix surface and in the 3D structure. The resulting composite electrode possesses a high conductive surface area and excellent structural stability upon galvanostatic cycling. We showed stable cycling of this composite electrode with small Li plating/stripping overpotential (

/pdf/composite-lithium-metal-anode-by-melt-infusion-of-lithium-1rk1yg1eub.pdf

Composite lithium metal anode by melt infusion of lithium into a 3D conducting scaffold with lithiophilic coating.

The thermodynamic stability and electronic structure of 40 surfaces of lithium peroxide (Li2O2) and lithium oxide (Li2O) were characterized using first-principles calculations. As these compounds constitute potential discharge products in Li–oxygen batteries, their surface properties are expected to play a key role in understanding electrochemical behavior in these systems. Stable surfaces were identified by comparing 23 distinct Li2O2 surfaces and 17 unique Li2O surfaces; crystallite areal fractions were determined through application of the Wulff construction. Accounting for the oxygen overbinding error in density functional theory results in the identification of several new Li2O2 oxygen-rich {0001} and {1100} terminations that are more stable than those previously reported. Although oxygen-rich facets predominate in Li2O2, in Li2O stoichiometric surfaces are preferred, consistent with prior studies. Surprisingly, surface-state analyses reveal that the stable surfaces of Li2O2 are half-metallic, despi...

Lithium Peroxide Surfaces Are Metallic, While Lithium Oxide Surfaces Are Not

The properties of the Li2O2 discharge phase are expected to impact strongly the performance of Li–air batteries. Although both crystalline Li2O2 (c-Li2O2) and amorphous Li2O2 (a-Li2O2) have been reported to form in Li–air cells, little is known regarding possible differences in charge and mass transport within these phases. To reveal these differences, here we predict the properties of a-Li2O2 using first-principles “melt-and-quench” molecular dynamics and percolation theory. We find that the band gaps and equilibrium electrochemical potentials of c-Li2O2 and a-Li2O2 are similar; nevertheless, their transport properties are quite different. Importantly, the ionic conductivity of a-Li2O2 is predicted to be 2 × 10–7 S/cm, which is 12 orders of magnitude larger than that in the crystalline phase. This enhancement arises from increases in both the concentration and mobility of negative lithium vacancies. The electronic conductivity of a-Li2O2 is also enhanced, but to a much smaller extent (4 orders of magnitu...

Enhanced Charge Transport in Amorphous Li2O2

The surface properties of the Li2O2 discharge phase are expected to impact strongly the capacity, rate capability, and rechargeability of Li-oxygen batteries. Prior calculations have suggested that the presence of half-metallic surface states in Li2O2 may mitigate electrical passivation resulting from the growth of Li2O2, which is a bulk insulator. Here we revisit the electronic structure of bulk Li2O2 and the dominant Li2O2 {0001} surface by comparing results obtained with the PBE GGA functional, the HSE06 hybrid functional, and quasiparticle GW methods. Our results suggest that the bulk band gap lies between the value predicted by the G0W0 method, 5.15 eV, and the value predicted by the self-consistent quasiparticle GW (scGW) approximation, 6.37 eV. The PBE, HSE06, and scGW methods agree that the most stable surface, an oxygen-rich {0001} termination, is indeed half-metallic. This result supports the notion that the electronic structure of surfaces may play an important role in understanding performance limitations in Li-oxygen batteries.

/pdf/electronic-structure-of-li2o2-0001-surfaces-vnx4wvtk5b.pdf

Electronic structure of Li2O2 {0001} surfaces

During the chemical vapor deposition (CVD) of MoSe2, controlling its selenization reaction and understanding its reaction mechanisms are of great significance to obtain high‐quality 2D transition metal selenide semiconductors. Herein, a variable‐pressure CVD (VPCVD) method is reported to achieve the controllable transformation from MoO2 to MoSe2 monolayer based on gas pressure‐mediated selenization. At the gas pressure lower than 20 KPa, high‐temperature decomposition of MoO3 in the Ar/H2 mixture only produces MoO2 nanosheets without any selenization. When the gas pressure is between 20 and 60 KPa, quadrilateral MoO2 nanosheets are partially selenized. By further increasing the gas pressure from 60 to 100 KPa, they are completely selenized. At the downstream margins of as‐selenized films, 2D MoSe2 monolayers demonstrate a morphological evolution from triangle to hexagon and then to a continuous film. In addition, their Se vacancy concentrations and nucleation sizes depend directly on gas pressure. Therefore, the gas pressure‐mediated selenization provides a feasible way for the in situ synthetic control of chemical composition and vacancy doping of 2D transition metal selenides/oxides.

Controllable Selenization Transformation from MoO2 to MoSe2 by Gas Pressure‐Mediated Chemical Vapor Deposition

The two-dimensional MAX phases with compositional diversity are promising functional materials for electrochemical energy storage. Herein, we report the facile preparation of the Cr2GeC MAX phase from oxides/C precursors by the molten salt electrolysis method at a moderate temperature of 700°C. The electrosynthesis mechanism has been systematically investigated, and the results show that the synthesis of the Cr2GeC MAX phase involves electro-separation and in situ alloying processes. The as-prepared Cr2GeC MAX phase with a typical layered structure shows the uniform morphology of nanoparticles. As a proof of concept, Cr2GeC nanoparticles are investigated as anode materials for lithium-ion batteries, which deliver a good capacity of 177.4 mAh g−1 at 0.2 C and excellent cycling performance. The lithium-storage mechanism of the Cr2GeC MAX phase has been discussed based on density functional theory (DFT) calculations. This study may provide important support and complement to the tailored electrosynthesis of MAX phases toward high-performance energy storage applications.

/pdf/molten-salt-electrosynthesis-of-cr2gec-nanoparticles-as-2pg764rp.pdf

Feng Tian

Papers

Lithium Peroxide Surfaces Are Metallic, While Lithium Oxide Surfaces Are Not

Enhanced Charge Transport in Amorphous Li2O2

Electronic structure of Li2O2 {0001} surfaces

Controllable Selenization Transformation from MoO2 to MoSe2 by Gas Pressure‐Mediated Chemical Vapor Deposition

Molten salt electrosynthesis of Cr2GeC nanoparticles as anode materials for lithium-ion batteries