Electrochemical Growth of Very Long (∼80 μm) Crystalline Li2O2 Nanowires on Single-Layer Graphene Covered Gold and Their Growth Mechanism

To DISP or Not? The Far-Reaching Reaction Mechanisms Underpinning Lithium-Air Batteries

The short history of research on Li-O2 batteries has seen a remarkable number of mechanistic U-turns over the years. From the initial use of carbonate electrolytes, that were then found to be entirely unsuitable, to the belief that (su)peroxide was solely responsible for degradation, before the more reactive singlet oxygen was found to form, to the hypothesis that capacity depends on a competing surface/solution mechanism before a practically exclusive solution mechanism was identified. Herein, we argue for an ever-fresh look at the reported data without bias towards supposedly established explanations. We explain how the latest findings on rate and capacity limits, as well as the origin of side reactions, are connected via the disproportionation (DISP) step in the (dis)charge mechanism. Therefrom, directions emerge for the design of electrolytes and mediators on how to suppress side reactions and to enable high rate and high reversible capacity.

Nature of Li2O2 and its relationship to the mechanisms of discharge/charge reactions of lithium-oxygen batteries

Lithium-air batteries (LABs) are considered one of the most promising energy storage devices because of their large theoretical energy density. However, low cyclability caused by battery degradation prevents its practical use. Thus, to realize practical LABs, it is essential to improve cyclability significantly by understanding how the degradation processes proceed. Here, we used online mass spectrometry for real-time monitoring of gaseous products generated during charging of lithium-oxygen batteries (LOBs), which was operated with pure oxygen not air, with 1 M lithium bis(trifluoromethanesulfonyl)imide (LiTFSI) tetraethylene glycol dimethyl ether (TEGDME) electrolyte solution. Linear voltage sweep (LVS) and voltage step modes were employed for charge instead of constant current charge so that the energetics of the product formation during the charge process can be understood more quantitatively. The presence of two distinctly different types of Li2O2, one being decomposed in a wide range of relatively low cell voltages (2.8-4.16 V) (l-Li2O2) and the other being decomposed at higher cell voltages than ca. 4.16 V (h-Li2O2), was confirmed by both LVS and step experiments. H2O generation started when the O2 generation rate reached a first maximum and CO2 generation took place accompanied by the decomposition of h-Li2O2. Based on the above results and the effects of discharge time and the use of isotope oxygen during discharge on product distribution during charge, the generation mechanism of O2, H2O, and CO2 during charging is discussed in relation to the reactions during discharge.

Aprotic Lithium-Oxygen Batteries Based on Nonsolid Discharge Products

Aprotic lithium-oxygen (Li-O2) batteries are considered to be a promising alternative option to lithium-ion batteries for high gravimetric energy storage devices. However, the sluggish electrochemical kinetics, the passivation, and the structural damage to the cathode caused by the solid discharge products have greatly hindered the practical application of Li-O2 batteries. Herein, the nonsolid-state discharge products of the off-stoichiometric Li1-xO2 in the electrolyte solutions are achieved by iridium (Ir) single-atom-based porous organic polymers (termed as Ir/AP-POP) as a homogeneous, soluble electrocatalyst for Li-O2 batteries. In particular, the numerous atomic active sites act as the main nucleation sites of O2-related discharge reactions, which are favorable to interacting with O2-/LiO2 intermediates in the electrolyte solutions, owing to the highly similar lattice-matching effect between the in situ-formed Ir3Li and LiO2, achieving a nonsolid LiO2 as the final discharge product in the electrolyte solutions for Li-O2 batteries. Consequently, the Li-O2 battery with a soluble Ir/AP-POP electrocatalyst exhibits an ultrahigh discharge capacity of 12.8 mAh, an ultralow overpotential of 0.03 V, and a long cyclic life of 700 h with the carbon cloth cathode. The manipulation of nonsolid discharge products in aprotic Li-O2 batteries breaks the traditional growth mode of Li2O2, bringing Li-O2 batteries closer to being a viable technology.

Cathode Chemistries of Lithium-Oxygen Batteries in Nanoconfined Space

In lithium-oxygen batteries, although the porous carbon cathodes are widely utilized to tailor the properties of discharged Li2O2, the impact of nanopore size on the Li2O2 formation and decomposition reactions remain incompletely understood. Here, we provide the straightforward elucidation on the effect of pore size in a range of 25-200 nm, using a highly ordered porous cathode matrix based on the carbon-coated anodic aluminum oxide membrane formed on an Al substrate (C/AAO_Al). When the nanopore size is 25 nm, film-like Li2O2 with a thickness of 2-5 nm is formed, possibly via a surface-driven mechanism. When the nanochannel becomes larger, the Li2O2 film thickness saturates at ca. 10 nm, along with crystalline Li2O2 particles possibly formed by a solution-mediated mechanism.

Tailoring the p-Band Center of NS Pair for Accelerating High-Performance Lithium-Oxygen Battery

The local coordination environment of catalytical moieties directly determines the performance of electrochemical energy storage and conversion devices, such as Li-O₂ batteries (LOBs) cathode. However, understanding how the coordinative structure affects the performance, especially for non-metal system, is still insufficient. Herein, a strategy that introduces S-anion to tailor the electronic structure of nitrogen-carbon catalyst (SNC) is proposed to improve the LOBs performance. This study unveils that the introduced S-anion effectively manipulates the p-band center of pyridinic-N moiety, substantially reducing the battery overpotential by accelerating the generation and decomposition of intermediate products Li_1-3 O₄ . The lower adsorption energy of discharging product Li₂ O₂ on NS pair accounts for the long-term cyclic stability by exposing the high active area under operation condition. This work demonstrates an encouraging strategy to enhance LOBs performance by modulating the p-band center on non-metal active sites.

Stepping Up the Kinetics of Li-O2 Batteries by Shrinking Down the Li2O2 Granules through Concertedly Enhanced Catalytic Activity and Photoactivity of Se-Doped LaCoO3

Owing to their structural tunability for furnishing high catalytic activity and photoactivity, perovskite oxides are a class of promising materials for high-performance photocathode catalysts in a photoassisted lithium oxygen battery (LOB), which is still in its infancy. Herein, single-crystalline LaCoO3 (LCO) is successfully synthesized through a microwave-assisted approach and selenylated to simultaneously introduce anionic doping and oxygen vacancies, boosting not only the electrocatalytic activity toward reversible Li2O2 formation/decomposition, but also the photoactivity to further reduce the charge/discharge polarization. As a result, LOBs utilizing Se-doped LCO as the photocathode catalyst demonstrate a superior performance under illumination in all aspects of energy efficiency, specific capacity, and cycling stability, ranking among the best reported in the literature for perovskite oxides. The photoenhanced charge kinetics is found to be correlated with the accelerated Li2O2 nucleation with lowered granule size, which is key to both the improved charge/discharge capacity and reversibility. The results underscore the tailoring of perovskite structure to aggrandize both the catalytic activity and photoactivity for concertedly promoting the kinetics of LOBs.

Spin-State Manipulation of Two-Dimensional Metal-Organic Framework with Enhanced Metal-Oxygen Covalency for Lithium-Oxygen Batteries

Aprotic Li-O 2 battery has attracted extensive attention in the past decade owing to the high theoretical energy density, however it is obstructed by the sluggish reaction kinetics at cathodes and large voltage hysteresis. Herein, we regulate the spin state of partial Ni 2+ metal centers ( t 2g 6 e g 2 ) of conductive nickel catecholate framework (Ni II -NCF) nanowire arrays to high-valence Ni 3+ ( t 2g 6 e g 1 ) for Ni III -NCF. The spin-state modulation enables enhanced nickel-oxygen covalency in Ni III -NCF, which facilitates electron exchange between the Ni sites and oxygen adsorbates and accelerates the oxygen redox kinetics. The high affinity of Ni 3+ sites with the intermediate LiO 2 promotes formation of nanosheet-like Li 2 O 2 in the void space among Ni III -NCF nanowires upon discharging. These merit the Li-O 2 battery based on Ni III -NCF with remarkably reduced discharge/charge voltage gaps, superior rate capability, and long cycling stability of over 200 cycles. This work highlights the domination of electron spin state on the redox kinetics and will shed insights into electronic structure regulation of electrocatalysts for Li-O 2 battery and beyond.

Isotopic Depth Profiling of Discharge Products Identifies Reactive Interfaces in an Aprotic Li-O2 Battery with a Redox Mediator

Prior to the practical application of rechargeable aprotic Li-O₂ batteries, the high charging overpotentials of these devices (which inevitably cause irreversible parasitic reactions) must be addressed. The use of redox mediators (RMs) that oxidatively decompose the discharge product, Li₂O₂, is one promising solution to this problem. However, the mitigating effect of RMs is currently insufficient, and so it would be beneficial to clarify the Li₂O₂ reductive growth and oxidative decomposition mechanisms. In the present work, Nanoscale secondary ion mass spectrometry (Nano-SIMS) isotopic three-dimensional imaging and differential electrochemical mass spectrometry (DEMS) analyses of individual Li₂O₂ particles established that both growth and decomposition proceeded at the Li₂O₂/electrolyte interface in a system containing the Br^-/Br₃^- redox couple as the RM. The results of this study also indicated that the degree of oxidative decomposition of Li₂O₂ was highly dependent on the cell voltage. These data show that increasing the RM reaction rate at the Li₂O₂/electrolyte interface is critical to improve the cycle life of Li-O₂ batteries.