Revealing the Intrinsic Atomic Structure and Chemistry of Amorphous LiO2-Containing Products in Li-O2 Batteries Using Cryogenic Electron Microscopy

Crown Ether Electrolyte Induced Li2O2 Amorphization for Low Polarization and Long Lifespan Li-O2 Batteries

Lithium-oxygen batteries possess an extremely high theoretical energy density, rendering them a prime candidate for next-generation secondary batteries. However, they still face multiple problems such as huge charge polarization and poor life, which lay a significant gap between laboratory research and commercial applications. In this work, we adopt 15-crown-5 ether (C15) as solvent to regulate the generation of discharge products in lithium-oxygen batteries. The coronal structure endows C15 with strong affinity to Li+, firmly stabilizes the intermediate LiO2 and discharge product Li2O2. Thus, the crystalline Li2O2 is amorphized into easily decomposable amorphous products. The lithium-oxygen batteries assembled with 0.5 M C15 electrolyte show an increased discharge capacity from 4.0 mAh cm-2 to 5.7 mAh cm-2 and a low charge overpotential of 0.88 V during the whole lifespan at 0.05 mA cm-2. The batteries with 1 M C15 electrolyte can cycle stably for 140 cycles. Furthermore, the amorphous characteristic of Li2O2 product is preserved when matched with redox mediators such as LiI, with the charge polarization further decreasing to 0.74 V over a cycle life of 190 cycles. This provides new possibilities for electrolyte design to promote Li2O2 amorphization and reduce charge overpotential in lithium-oxygen batteries.

Revealing Lithium Nitrate-Mediated Solid-Electrolyte Interphase of Lithium Metal Anode via Cryogenic Transmission Electron Microscopy

The cycle stability of lithium metal anode (LMA) largely depends on solid-electrolyte interphase (SEI). Electrolyte engineering is a common strategy to adjust SEI properties, yet understanding its impact is challenging due to limited knowledge on ultrafine SEI structures. Herein, using cryogenic transmission electron microscopy, we reveal the atomic-level SEI structure of LMA in ether-based electrolytes, focusing on the role of LiNO3 additives in SEI modulation at different temperature (25 and 50 °C). Poor cycle stability of LMA in the baseline electrolyte without LiNO3 additives stems from the Li2CO3-rich mosaic-type SEI. Increased LiNO3 content and elevated operating temperature enhance cyclic performance by forming bilayer or multilayer SEI structures via preferential LiNO3 decomposition, but may thicken the SEI, leading to reduced initial Coulombic efficiency and increased overpotential. The optimal SEI features a multilayer structure with Li2O-rich inner layer and closely packed grains in the outer layer, minimizing electrolyte decomposition or corrosion.

Atomic-Scale Cryo-TEM Studies of the Electrochemistry of Redox Mediator in Li-O2 Batteries

Rechargeable aprotic lithium (Li)-oxygen battery (LOB) is a potential next-generation energy storage technology because of its high theoretical specific energy. However, the role of redox mediator on the oxide electrochemistry remains unclear. This is partly due to the intrinsic complexity of the battery chemistry and the lack of in-depth studies of oxygen electrodes at the atomic level by reliable techniques. Herein, cryo-transmission electron microscopy (cryo-TEM) is used to study how the redox mediator LiI affects the oxygen electrochemistry in LOBs. It is revealed that with or without LiI in the electrolyte, the discharge products are plate-like LiOH or toroidal Li2 O2 , respectively. The I2 assists the decomposition of LiOH via the formation of LiIO3 in the charge process. In addition, a LiI protective layer is formed on the Li anode surface by the shuttle of I3 - , which inhibits the parasitic Li/electrolyte reaction and improves the cycle performance of the LOBs. The LOBs returned to 2e- oxygen reduction reaction (ORR) to produce Li2 O2 after the LiI in the electrolyte is consumed. This work provides new insight on the role of redox mediator on the complex electrochemistry in LOBs which may aid the design LOBs for practical applications.

Constructing Built-In Electric Field in NiCo2 O4 -CeO2 Heterostructures to Regulate Li2 O2 Formation Routes at High Current Densities

Developing catalysts with suitable adsorption energy for oxygen-containing intermediates and elucidating their internal structure-performance relationships are essential for the commercialization of Li-O2 batteries (LOBs), especially under high current densities. Herein, NiCo2 O4 -CeO2 heterostructure with a spontaneous built-in electric field (BIEF) is designed and utilized as a cathode catalyst for LOBs at high current density. The driving mechanism of electron pumping/accumulation at heterointerface is studied via experiments and density functional theory (DFT) calculations, elucidating the growth mechanism of discharge products. The results show that BIEF induced by work function difference optimizes the affinity for LiO2 and promotes the formation of nano-flocculent Li2 O2 , thus improving LOBs performance at high current density. Specifically, NiCo2 O4 -CeO2 cathode exhibits a large discharge capacity (9546 mAh g-1 at 4000 mA g-1 ) and high stability (>430 cycles at 4000 mA g-1 ), which are better than the majority of previously reported metal-based catalysts. This work provides a new method for tuning the nucleation and decomposition of Li2 O2 and inspires the design of ideal catalysts for LOBs to operate at high current density.

Light-Induced Variation of Lithium Coordination Environment in g-C3N4 Nanosheet for Highly Efficient Oxygen Reduction Reactions

The structure and electronic state of the active center in a single-atom catalyst undergo noticeable changes during a dynamic catalytic process. The metal atom active center is not well demonstrated in a dynamic manner. This study demonstrated that Li metal atoms, serving as active centers, can migrate on a C3N4 monolayer or between C3N4 monolayers when exposed to light irradiation. This migration alters the local coordination environment of Li in the C3N4 nanosheets, leading to a significant enhancement in photocatalytic activity. The photocatalytic H2O2 process could be maintained for 35 h with a 920 mmol/g record-high yield, corresponding to a 0.4% H2O2 concentration, which is far greater than the value (0.1%) of practical application for wastewater treatment. Density functional theory calculations indicated that dynamic Li-coordinated structures contributed to the superhigh photocatalytic activity.

Surface modification of garnet fillers with a polymeric sacrificial agent enables compatible interfaces of composite solid-state electrolytes

The poly (vinylidene fluoride) (PVDF)-based composite solid-state electrolyte (CSE) has garnered attention due to its excellent comprehensive performance. However, challenges persist in the structural design and preparation process of the ceramic-filled CSE, as the PVDF-based matrix is susceptible to alkaline conditions and dehydrofluorination, leading to its incompatibility with ceramic fillers and hindering the preparation of solid-state electrolytes. In this study, the mechanism of dehydrofluorination failure of a PVDF-based polymer in the presence of Li2CO3 on the surface of Li6.4La3Zr1.4Ta0.6O12 (LLZTO) is analyzed, and an effective strategy is proposed to inhibit the dehydrofluorination failure on the basis of density functional theory (DFT). We introduce a molecule with a small LUMO-HOMO gap as a sacrificial agent, which is able to remove the Li2CO3 impurities. Therefore, the approach of polyacrylic acid (PAA) as a sacrificial agent reduces the degree of dehydrofluorination in the PVDF-based polymer and ensures slurry fluidity, promoting the homogeneous distribution of ceramic fillers in the electrolyte membrane and enhancing compatibility with the polymer. Consequently, the prepared electrolyte membranes exhibit good electrochemical and mechanical properties. The assembled Li-symmetric cell can cycle at 0.1 mA cm-2 for 3500 h. The LiFePO4‖Li cell maintains 91.45% of its initial capacity after 650 cycles at 1C, and the LiCoO2‖Li cell maintains 84.9% of its initial capacity after 160 cycles, demonstrating promising high-voltage performance. This facile modification strategy can effectively improve compatibility issues between the polymer and fillers, which paves the way for the mass production of solid-state electrolytes.

Morphology-Dictated Mechanism of Efficient Reaction Sites for Li2O2 Decomposition

In the pursuit of a highly reversible lithium-oxygen (Li-O₂) battery, control of reaction sites to maintain stable conversion between O₂ and Li₂O₂ at the cathode side is imperatively desirable. However, the mechanism involving the reaction site during charging remains elusive, which, in turn, imposes challenges in recognition of the origin of overpotential. Herein, via combined investigations by in situ atomic force microscopy (AFM) and electrochemical impedance spectroscopy (EIS), we propose a universal morphology-dictated mechanism of efficient reaction sites for Li₂O₂ decomposition. It is found that Li₂O₂ deposits with different morphologies share similar localized conductivities, much higher than that reported for bulk Li₂O₂, enabling the reaction site not only at the electrode/Li₂O₂/electrolyte interface but also at the Li₂O₂/electrolyte interface. However, while the mass transport process is more enhanced at the former, the charge-transfer resistance at the latter is sensitively related to the surface structure and thus the reactivity of the Li₂O₂ deposit. Consequently, for compact disk-like deposits, the electrode/Li₂O₂/electrolyte interface serves as the dominant decomposition site, which causes premature departure of Li₂O₂ and loss of reversibility; on the contrary, for porous flower-like and film-like Li₂O₂ deposits bearing a larger surface area and richer surface-active structures, both the interfaces are efficient for decomposition without premature departure of the deposit so that the overpotential arises primarily from the sluggish oxidation kinetics and the decomposition is more reversible. The present work provides instructive insights into the understanding of the mechanism of reaction sites during the charge process, which offers guidance for the design of reversible Li-O₂ batteries.

Revisiting the Role of Discharge Products in Li-CO2 Batteries

Rechargeable lithium-carbon dioxide (Li-CO2 ) batteries are promising devices for CO2 recycling and energy storage. However, thermodynamically stable and electrically insulating discharge products (DPs) (e.g., Li2 CO3 ) deposited at cathodes require rigorous conditions for completed decomposition, resulting in large recharge polarization and poor battery reversibility. Although progress has been achieved in cathode design and electrolyte optimization, the significance of DPs is generally underestimated. Therefore, it is necessary to revisit the role of DPs in Li-CO2 batteries to boost overall battery performance. Here, a critical and systematic review of DPs in Li-CO2 batteries is reported for the first time. Fundamentals of reactions for formation and decomposition of DPs are appraised; impacts on battery performance including overpotential, capacity, and stability are demonstrated; and the necessity of discharge product management is highlighted. Practical in situ/operando technologies are assessed to characterize reaction intermediates and the corresponding DPs for mechanism investigation. Additionally, achievable control measures to boost the decomposition of DPs are evidenced to provide battery design principles and improve the battery performance. Findings from this work will deepen the understanding of electrochemistry of Li-CO2 batteries and promote practical applications.

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.

In Situ Detecting Thermal Stability of Solid Electrolyte Interphase (SEI)

Solid electrolyte interphase (SEI) plays an important role in regulating the interfacial ion transfer and safety of Lithium-ion batteries (LIBs). It is unstable and readily decomposed releasing much heat and gases and thus triggering thermal runaway. Herein, in situ heating X-ray photoelectron spectroscopy is applied to uncover the inherent thermal decomposition process of the SEI. The evolution of the composition, nanostructure, and the released gases are further probed by cryogenic transmission electron microscopy, and gas chromatography. The results show that the organic components of SEI are readily decomposed even at room temperature, releasing some flammable gases (e.g., H₂ , CO, C₂ H₄ , etc.). The residual SEI after heat treatment is rich in inorganic components (e.g., Li₂ O, LiF, and Li₂ CO₃ ), provides a nanostructure model for a beneficial SEI with enhanced stability. This work deepens the understanding of SEI intrinsic thermal stability, reveals its underlying relationship with the thermal runaway of LIBs, and enlightens to enhance the safety of LIBs by achieving inorganics-rich SEI.

Engineering the Electronic Interaction between Atomically Dispersed Fe and RuO2 Attaining High Catalytic Activity and Durability Catalyst for Li-O2 Battery

It is significant to develop catalysts with high catalytic activity and durability to improve the electrochemical performances of lithium-oxygen batteries (LOBs). While electronic metal-support interaction (EMSI) between metal atoms and support has shown great potential in catalytic field. Hence, to effectively improve the electrochemical performance of LOBs, atomically dispersed Fe modified RuO₂ nanoparticles are designed to be loaded on hierarchical porous carbon shells (Fe_SA -RuO₂ /HPCS) based on EMSI criterion. It is revealed that the Ru-O-Fe₁ structure is formed between the atomically dispersed Fe atoms and the surrounding Ru sites through electron interaction, and this structure could act as the ultra-high activity driving force center of oxygen reduction/evolution reaction (ORR/OER). Specifically, the Ru-O-Fe₁ structure enhances the reaction kinetics of ORR to a certain extent, and optimizes the morphology of discharge products by reducing the adsorption energy of catalyst for O₂ and LiO₂ ; while during the OER process, the Ru-O-Fe₁ structure not only greatly enhances the reaction kinetics of OER, but also catalyzes the efficient decomposition of the discharge products Li₂ O₂ by the favorable electron transfer between the active sites and the discharge products. Hence, LOBs based on FeSA-RuO₂ /HPCS cathodes show an ultra-low over-potential, high discharge capacity and superior durability.

Kinetically Accelerated Lithium Storage in High-Entropy (LiMgCoNiCuZn)O Enabled By Oxygen Vacancies

High-entropy oxides (HEOs) are gradually becoming a new focus for lithium-ion battery (LIB) anodes due to their vast element space/adjustable electrochemical properties and unique single-phase retention ability. However, the sluggish kinetics upon long cycling limits their further generalization. Here, oxygen vacancies with targeted functionality are introduced into rock salt-type (MgCoNiCuZn)O through a wet-chemical molten salt strategy to accelerate the ion/electron transmission. Both experimental results and theoretical calculations reveal the potential improvement of lithium storage, charge transfer, and diffusion kinetics from HEO surface defects, which ultimately leads to enhanced electrochemical properties. The currently raised strategy offers a modular approach and enlightening insights for defect-induced HEO-based anodes.