Visualizing interfacial collective reaction behaviour of Li-S batteries.

Benefiting from high energy density (2,600 Wh kg-1) and low cost, lithium-sulfur (Li-S) batteries are considered promising candidates for advanced energy-storage systems1-4. Despite tremendous efforts in suppressing the long-standing shuttle effect of lithium polysulfides5-7, understanding of the interfacial reactions of lithium polysulfides at the nanoscale remains elusive. This is mainly because of the limitations of in situ characterization tools in tracing the liquid-solid conversion of unstable lithium polysulfides at high temporal-spatial resolution8-10. There is an urgent need to understand the coupled phenomena inside Li-S batteries, specifically, the dynamic distribution, aggregation, deposition and dissolution of lithium polysulfides. Here, by using in situ liquid-cell electrochemical transmission electron microscopy, we directly visualized the transformation of lithium polysulfides over electrode surfaces at the atomic scale. Notably, an unexpected gathering-induced collective charge transfer of lithium polysulfides was captured on the nanocluster active-centre-immobilized surface. It further induced an instantaneous deposition of nonequilibrium Li2S nanocrystals from the dense liquid phase of lithium polysulfides. Without mediation of active centres, the reactions followed a classical single-molecule pathway, lithium polysulfides transforming into Li2S2 and Li2S step by step. Molecular dynamics simulations indicated that the long-range electrostatic interaction between active centres and lithium polysulfides promoted the formation of a dense phase consisting of Li+ and Sn2- (2 < n ≤ 6), and the collective charge transfer in the dense phase was further verified by ab initio molecular dynamics simulations. The collective interfacial reaction pathway unveils a new transformation mechanism and deepens the fundamental understanding of Li-S batteries.

Unlocking Dynamic Solvation Chemistry and Hydrogen Evolution Mechanism in Aqueous Zinc Batteries.

Understanding the interfacial hydrogen evolution reaction (HER) is crucial to regulate the electrochemical behavior in aqueous zinc batteries. However, the mechanism of HER related to solvation chemistry remains elusive, especially the time-dependent dynamic evolution of the hydrogen bond (H-bond) under an electric field. Herein, we combine in situ spectroscopy with molecular dynamics simulation to unravel the dynamic evolution of the interfacial solvation structure. We find two critical change processes involving Zn-electroplating/stripping, including the initial electric double layer establishment to form an H2O-rich interface (abrupt change) and the subsequent dynamic evolution of an H-bond (gradual change). Moreover, the number of H-bonds increases, and their strength weakens in comparison with the bulk electrolyte under bias potential during Zn2+ desolvation, forming a diluted interface, resulting in massive hydrogen production. On the contrary, a concentrated interface (H-bond number decreases and strength enhances) is formed and produces a small amount of hydrogen during Zn2+ solvation. The insights on the above results contribute to deciphering the H-bond evolution with competition/corrosion HER during Zn-electroplating/stripping and clarifying the essence of electrochemical window widened and HER suppression by high concentration. This work presents a new strategy for aqueous electrolyte regulation by benchmarking the abrupt change of the interfacial state under an electric field as a zinc performance-enhancement criterion.

Gradient Interphase Engineering Enabled by Anionic Redox for High-Voltage and Long-Life Li-Ion Batteries.

Intelligent utilization of the anionic redox reaction (ARR) in Li-rich cathodes is an advanced strategy for the practical implementation of next-generation high-energy-density rechargeable batteries. However, due to the intrinsic complexity of ARR (e.g., nucleophilic attacks), the instability of the cathode-electrolyte interphase (CEI) on a Li-rich cathode presents more challenges than typical high-voltage cathodes. Here, we manipulate CEI interfacial engineering by introducing an all-fluorinated electrolyte and exploiting its interaction with the nucleophilic attack to construct a gradient CEI containing a pair of fluorinated layers on a Li-rich cathode, delivering enhanced interfacial stability. Negative/detrimental nucleophilic electrolyte decomposition has been efficiently evolved to further reinforce CEI fabrication, resulting in the construction of LiF-based indurated outer shield and fluorinated polymer-based flexible inner sheaths. Gradient interphase engineering dramatically improved the capacity retention of the Li-rich cathode from 43 to 71% after 800 cycles and achieved superior cycling stability in anode-free and pouch-type full cells (98.8% capacity retention, 220 cycles), respectively.

Oxygen-Bridged Long-Range Dual Sites Boost Ethanol Electrooxidation by Facilitating C-C Bond Cleavage.

Optimizing the interatomic distance of dual sites to realize C-C bond breaking of ethanol is critical for the commercialization of direct ethanol fuel cells. Herein, the concept of holding long-range dual sites is proposed to weaken the reaction barrier of C-C cleavage during the ethanol oxidation reaction (EOR). The obtained long-range Rh-O-Pt dual sites achieve a high current density of 7.43 mA/cm2 toward EOR, which is 13.3 times that of Pt/C, as well as remarkable stability. Electrochemical in situ Fourier transform infrared spectroscopy indicates that long-range Rh-O-Pt dual sites can increase the selectivity of C1 products and suppress the generation of a CO intermediate. Theoretical calculations further disclose that redistribution of the surface-localized electron around Rh-O-Pt can promote direct oxidation of -OH, accelerating C-C bond cleavage. This work provides a promising strategy for designing oxygen-bridged long-range dual sites to tune the activity and selectivity of complicated catalytic reactions.

Injecting Excess Na into a P2-Type Layered Oxide Cathode to Achieve Presodiation in a Na-Ion Full Cell.

The initial Na loss limits the theoretical specific capacity of cathodes in Na-ion full cell applications, especially for Na-deficient P2-type cathodes. In this study, we propose a presodiation strategy for cathodes to compensate for the initial Na loss in Na-ion full cells, resulting in a higher specific capacity and a higher energy density. By employing an electrochemical presodiation approach, we inject 0.32 excess active Na into P2-type Na0.67Li0.1Fe0.37Mn0.53O2 (NLFMO), aiming to compensate for the initial Na loss in hard carbon (HC) and the inherent Na deficiency of NLFMO. The structure of the NLFMO cathode converts from P2 to P'2 upon active Na injection, without affecting subsequent cycles. As a result, the HC||NLFMOpreNa full cell exhibits a specific capacity of 125 mAh/g, surpassing the value of 61 mAh/g of the HC||NLFMO full cell without presodiation due to the injected active Na. Moreover, the presodiation effect can be achieved through other engineering approaches (e.g., Na-metal contact), suggesting the scalability of this methodology.

Quantifying the Influence of Li Plating on a Graphite Anode by Mass Spectrometry.

The prominent problem with graphite anodes in practical applications is the detrimental Li plating, resulting in rapid capacity fade and safety hazards. Herein, secondary gas evolution behavior during the Li-plating process was monitored by online electrochemical mass spectrometry (OEMS), and the onset of local microscale Li plating on the graphite anode was precisely/explicitly detected in situ/operando for early safety warnings. The distribution of irreversible capacity loss (e.g., primary and secondary solid electrolyte interface (SEI), dead Li, etc.) under Li-plating conditions was accurately quantified by titration mass spectroscopy (TMS). Based on OEMS/TMS results, the effect of typical VC/FEC additives was recognized at the level of Li plating. The nature of vinylene carbonate (VC)/fluoroethylene carbonate (FEC) additive modification is to enhance the elasticity of primary and secondary SEI by adjusting organic carbonates and/or LiF components, leading to less "dead Li" capacity loss. Though VC-containing electrolyte greatly suppresses the H2/C2H4 (flammable/explosive) evolution during Li plating, more H2 is released from the reductive decomposition of FEC.

Manipulated Fluoro-Ether Derived Nucleophilic Decomposition Products for Mitigating Polarization-Induced Capacity Loss in Li-Rich Layered Cathode.

Electrolyte engineering is a fascinating choice to improve the performance of Li-rich layered oxide cathodes (LRLO) for high-energy lithium-ion batteries. However, many existing electrolyte designs and adjustment principles tend to overlook the unique challenges posed by LRLO, particularly the nucleophilic attack. Here, we introduce an electrolyte modification by locally replacing carbonate solvents in traditional electrolytes with a fluoro-ether. By benefit of the decomposition of fluoro-ether under nucleophilic O-related attacks, which delivers an excellent passivation layer with LiF and polymers, possessing rigidity and flexibility on the LRLO surface. More importantly, the fluoro-ether acts as "sutures", ensuring the integrity and stability of both interfacial and bulk structures, which contributed to suppressing severe polarization and enhancing the cycling capacity retention from 39 % to 78 % after 300 cycles for the 4.8 V-class LRLO. This key electrolyte strategy with comprehensive analysis, provides new insights into addressing nucleophilic challenge for high-energy anionic redox related cathode systems.

Visualizing and Regulating Dynamic Evolution of Interfacial Electrolyte Configuration during De-solvation Process on Lithium-Metal Anode.

Acting as a passive protective layer, solid-electrolyte interphase (SEI) plays a crucial role in maintaining the stability of the Li-metal anode. Derived from the reductive decomposition of electrolytes (e.g., anion and solvent), the SEI construction presents as an interfacial process accompanied by the dynamic de-solvation process during Li-metal plating. However, typical electrolyte engineering and related SEI modification strategies always ignore the dynamic evolution of electrolyte configuration at the Li/electrolyte interface, which essentially determines the SEI architecture. Herein, by employing advanced electrochemical in situ FT-IR and MRI technologies, we directly visualize the dynamic variations of solvation environments involving Li+-solvent/anion. Remarkably, a weakened Li+-solvent interaction and anion-lean interfacial electrolyte configuration have been synchronously revealed, which is difficult for the fabrication of anion-derived SEI layer. Moreover, as a simple electrochemical regulation strategy, pulse protocol was introduced to effectively restore the interfacial anion concentration, resulting in an enhanced LiF-rich SEI layer and improved Li-metal plating/stripping reversibility.

Graphitic Armor: A Natural Molecular Sieve for Robust Hydrogen Electroxidation.

Carbon coating layers have been found to improve the catalytic performance of transition metals, which is usually explained as an outcome of electronic synergistic effect. Herein we reveal that the defective graphitic carbon, with a unique interlayer gap of 0.342 nm, can be a highly selective natural molecular sieve. It allows efficient diffusion of hydrogen molecules or radicals both along the in-plane and out-of-plane direction, but sterically hinders the diffusion of molecules with larger kinetic diameter (e.g., CO and O2) along the in-plane direction. As a result, poisonous species lager than 0.342 nm are sieved out, even when their adsorption on the metal is thermodynamically strong; at the same time, the interaction between H2 and the metal is not affected. This natural molecular sieve provides a very chance for constructing robust metal catalysts for hydrogen-relevant processes, which are more tolerant to chemical or electrochemical oxidation or CO-relevant poisoning.

Implanting Transition Metal into Li2 O-Based Cathode Prelithiation Agent for High-Energy-Density and Long-Life Li-Ion Batteries.

Compensating the irreversible loss of limited active lithium (Li) is essentially important for improving the energy-density and cycle-life of practical Li-ion battery full-cell, especially after employing high-capacity but low initial coulombic efficiency anode candidates. Introducing prelithiation agent can provide additional Li source for such compensation. Herein, we precisely implant trace Co (extracted from transition metal oxide) into the Li site of Li2 O, obtaining (Li0.66 Co0.11 □0.23 )2 O (CLO) cathode prelithiation agent. The synergistic formation of Li vacancies and Co-derived catalysis efficiently enhance the inherent conductivity and weaken the Li-O interaction of Li2 O, which facilitates its anionic oxidation to peroxo/superoxo species and gaseous O2 , achieving 1642.7 mAh/g~Li2O prelithiation capacity (≈980 mAh/g for prelithiation agent). Coupled 6.5 wt % CLO-based prelithiation agent with LiCoO2 cathode, substantial additional Li source stored within CLO is efficiently released to compensate the Li consumption on the SiO/C anode, achieving 270 Wh/kg pouch-type full-cell with 92 % capacity retention after 1000 cycles.

Mechanism of Particle-Mediated Inhibition of Demetalation for Single-Atom Catalytic Sites in Acidic Electrochemical Environments.

Demetalation, caused by the electrochemical dissolution of metal atoms, poses a significant challenge to the practical application of single-atom catalytic sites (SACSs) in proton exchange membrane-based energy technologies. One promising approach to inhibit SACS demetalation is the use of metallic particles to interact with SACSs. However, the mechanism underlying this stabilization remains unclear. In this study, we propose and validate a unified mechanism by which metal particles can inhibit the demetalation of Fe SACSs. Metal particles act as electron donors, decreasing the Fe oxidation state by increasing the electron density at the FeN4 position, thereby strengthening the Fe-N bond, and inhibiting electrochemical Fe dissolution. Different types, forms, and contents of metal particles increase the Fe-N bond strength to varying extents. A linear correlation between the Fe oxidation state, Fe-N bond strength, and electrochemical Fe dissolution amount supports this mechanism. Our screening of a particle-assisted Fe SACS led to a 78% reduction in Fe dissolution, enabling continuous operation for up to 430 h in a fuel cell. These findings contribute to the development of stable SACSs for energy applications.

Ga-Modification Near-Surface Composition of Pt-Ga/C Catalyst Facilitates High-Efficiency Electrochemical Ethanol Oxidation through a C2 Intermediate.

In electrochemical ethanol oxidation reactions (EOR) catalyzed by Pt metal nanoparticles through a C2 route, the dissociation of the C-C bond in the ethanol molecule can be a limiting factor. Complete EOR processes producing CO2 were always exemplified by the oxidative dehydrogenation of C1 intermediates, a reaction route with less energy utilization efficiency. Here, we report a Pt3Ga/C electrocatalyst with a uniform distribution of Ga over the nanoparticle surface for EOR that produces CO2 at medium potentials (>0.3 V vs SCE) efficiently through direct and sustainable oxidation of C2 intermediate species, i.e., acetaldehyde. We demonstrate the excellent performance of the Pt3Ga-200/C catalyst by using electrochemical in situ Fourier transform infrared reflection spectroscopy (FTIR) and an isotopic labeling method. The atomic interval structure between Pt and Ga makes the surface of nanoparticles nonensembled, avoiding the formation of poisonous *CHx and *CO species via bridge-type adsorption of ethanol molecules. Meanwhile, the electron redistribution from Ga to Pt diminishes the *O/*OH adsorption and CO poisoning on Pt atoms, exposing more available sites for interaction with the C2 intermediates. Furthermore, the dissociation of H2O into *OH is facilitated by the high hydrophilicity of Ga, which is supported by DFT calculations, promoting the deep oxidation of C2 intermediates. Our work represents an extremely rare EOR process that produces CO2 without observing kinetic limitations under medium potential conditions.

Role of Substitution Elements in Enhancing the Structural Stability of Li-Rich Layered Cathodes.

Element doping/substitution has been recognized as an effective strategy to enhance the structural stability of layered cathodes. However, abundant substitution studies not only lack a clear identification of the substitution sites in the material lattice, but the rigid interpretation of the transition metal (TM)-O covalent theory is also not sufficiently convincing, resulting in the doping/substitution proposals being dragged into design blindness. In this work, taking Li1.2Ni0.2Mn0.6O2 as a prototype, the intense correlation between the "disordered degree" (Li/Ni mixing) and interface-structure stability (e.g., TM-O environment, slab/lattice, and Li+ reversibility) is revealed. Specifically, the degree of disorder induced by the Mg/Ti substitution extends in the opposite direction, conducive to sharp differences in the stability of TM-O, Li+ diffusion, and anion redox reversibility, delivering fairly distinct electrochemical performance. Based on the established paradigm of systematic characterization/analysis, the "degree of disorder" has been shown to be a powerful indicator of material modification by element substitution/doping.

From Li to Na: Exploratory Analysis of Fe-Based Phosphates Polyanion-Type Cathode Materials by Mn Substitution.

Both LiFePO4 (LFP) and NaFePO4 (NFP) are phosphate polyanion-type cathode materials, which have received much attention due to their low cost and high theoretical capacity. Substitution of manganese (Mn) elements for LFP/NFP materials can improve the electrochemical properties, but the connection between local structural changes and electrochemical behaviors after Mn substitution is still not clear. This study not only achieves improvements in energy density of LFP and cyclic stability of NFP through Mn substitution, but also provides an in-depth analysis of the structural evolutions induced by the substitution. Among them, the substitution of Mn enables LiFe0.5 Mn0.5 PO4 to achieve a high energy density of 535.3 Wh kg-1 , while NaFe0.7 Mn0.3 PO4 exhibits outstanding cyclability with 89.6% capacity retention after 250 cycles. Specifically, Mn substitution broadens the ion-transport channels, improving the ion diffusion coefficient. Moreover, LiFe0.5 Mn0.5 PO4 maintains a more stable single-phase transition during the charge/discharge process. The transition of NaFe0.7 Mn0.3 PO4 to the amorphous phase is avoided, which can maintain structural stability and achieve better electrochemical performance. With systematic analysis, this research provides valuable guidance for the subsequent design of high-performance polyanion-type cathodes.

Reliable impedance analysis of Li-ion battery half-cell by standardization on electrochemical impedance spectroscopy (EIS).

Electrochemical impedance spectroscopy (EIS) is a powerful characterization technique for the in-depth investigation of kinetic/transport parameters detection, reaction mechanism understanding, and degradation effects exploration in lithium-ion battery (LIB) systems. However, due to the lack of standardized criterion/paradigm, severe misinterpretations occur frequently during an EIS measurement. In this paper, the significance of instrumental accuracy is described and the character/principle of selection on the simulation model is illuminated/proposed, showing that an adequate precision device and an appropriate fitting model are a prerequisite for a correct EIS analysis. Moreover, the drawbacks of conventional two-electrode EIS experiments for typical coin-type cells are rigorously pointed out by comparison with the ideal three-electrode configuration, where the real impedance information of the cathode would be masked by the sum of both the anode film resistance response and the unavoidable inductive loop signal. The three-electrode case enables efficient accurate observations on individual electrodes, thus facilitating abundant and useful information acquisition. Consequently, devices with a sufficient accuracy, rational simulation models, and advanced three-electrode cells are distinctly illustrated as standardized criterion/paradigm for EIS characterizations, which are essentially important for electrode and interface modifications in LIBs.

Electric double layer contribution to sum frequency generation signal from Au electrode.

Understanding the electric double layer (EDL) of the metal electrode-electrolyte interface is essential to electrochemistry and relevant disciplines. In this study, potential-dependent electrode Sum Frequency Generation (SFG) intensities of polycrystalline gold electrodes in HClO4 and H2SO4 electrolytes were thoroughly analyzed. The potential of zero charges (PZC) of the electrodes was -0.06 and 0.38 V in HClO4 and H2SO4, respectively, determined from differential capacity curves. Without specific adsorption, the total SFG intensity was dominated by the contribution from the Au surface and increased similar to that of the visible (VIS) wavelength scanning, which pushed the SFG process closer to the double resonant condition in HClO4. However, the EDL contributed about 30% SFG signal with specific adsorption in H2SO4. Below PZC, the total SFG intensity was dominated by the Au surface contribution and increased with potential at a similar slope in these two electrolytes. Around PZC, as the EDL structure became less ordered and the electric field changed direction, there would be no EDL SFG contribution. Above PZC, the total SFG intensity increased much more rapidly with potential in H2SO4 than in HClO4, which suggested that the EDL SFG contribution kept increasing with more specific adsorbed surface ions from H2SO4.

Nonvolatile and Nonflammable Sulfolane-Based Electrolyte Achieving Effective and Safe Operation of the Li-O2 Battery in Open O2 Environment.

The Li-O2 battery should operate effectively/safely in an open O2 environment for practical applications, but not trapped in sealed/closed atmosphere. However, the typical use of volatile and flammable electrolyte restricts Li-O2 battery to be able to be running in open O2 environment. We report herein, for the first time, a highly electrochemical reversible Li-O2 battery operated in an open O2 environment, i.e., under the condition of keeping O2 flowing continuously based on a nonvolatile and nonflammable sulfolane (TMS) solvent. The electrochemical irreversibility of Li2O2/O2 conversion and incompatibility between Li metal anodes and electrolyte have been addressed via dissolving LiNO3 in concentrated TMS electrolyte. The tuned electrolyte not only enables a stable solid electrolyte interphase (SEI) with conformal inorganic components (including LiF, LiNxOy, and Li2O) that promotes a uniform Li electro-plating/stripping process but also results in a low charge overpotential, a stable discharge terminal plateau, and reversible O2 generation of the Li-O2 battery conducted in an open O2 environment.

Long-Life Aqueous Zn-I2 Battery Enabled by a Low-Cost Multifunctional Zeolite Membrane Separator.

Aqueous zinc iodide (Zn-I2) batteries are promising large-scale energy-storage devices. However, the uncontrollable diffuse away/shuttle of soluble I3- leads to energy loss (low Coulombic efficiency, CE), and poor reversibility (self-discharge). Herein, we employ an ordered framework window within a zeolite molecular sieve to restrain I3- crossover and prepare zeolite molecular sieve particles into compact, large-scale, and flexible membranes at the engineering level. The as-prepared membrane can confine I3- within the catholyte region and restrain its irreversible escape, which is proved via space-resolution and electrochemical in situ time-resolution Raman technologies. As a result, overcharge/self-discharge and Zn corrosion are effectively controlled by zeolite separator. After replacing the typically used glass fiber separator to a zeolite membrane, the CE of Zn-I2 battery improves from 78.9 to 98.6% at 0.2 A/g. Besides, after aging at the fully charged state for 5.0 h, self-discharge is restrained and CE is enhanced from 44.0 to 85.65%. Moreover, the Zn-I2 cell maintains 91.0% capacity over 30,000 cycles at 4.0 A/g.

Stabilizing Li-O2 Batteries with Multifunctional Fluorinated Graphene.

As a full cell system with attractive theoretical energy density, challenges faced by Li-O2 batteries (LOBs) are not only the deficient actual capacity and superoxide-derived parasitic reactions on the cathode side but also the stability of Li-metal anode. To solve simultaneously intrinsic issues, multifunctional fluorinated graphene (CFx, x = 1, F-Gr) was introduced into the ether-based electrolyte of LOBs. F-Gr can accelerate O2- transformation and O2--participated oxygen reduction reaction (ORR) process, resulting in enhanced discharge capacity and restrained O2--derived side reactions of LOBs, respectively. Moreover, F-Gr induced the F-rich and O-depleted solid electrolyte interphase (SEI) film formation, which have improved Li-metal stability. Therefore, energy storage capacity, efficiency, and cyclability of LOBs have been markedly enhanced. More importantly, the method developed in this work to disperse F-Gr into an ether-based electrolyte for improving LOBs' performances is convenient and significant from both scientific and engineering aspects.

Titration Mass Spectroscopy (TMS): A Quantitative Analytical Technology for Rechargeable Batteries.

Development of high-energy-density rechargeable battery systems not only needs advanced qualitative characterizations for mechanism exploration but also requires accurate quantification technology to quantitatively elucidate products and fairly assess numerous modification strategies. Herein, as a reliable quantification technology, titration mass spectroscopy (TMS) is developed to accurately quantify O-related anionic redox reactions (Li-O2 battery and nickel-cobalt-manganese (NCM)/Li-rich cathodes), parasitic carbonate deposition and decomposition (derived from air-exposure degradation and electrolyte oxidation), and dead Li0 formation (Li-metal battery and over-discharged graphite anode). TMS technology can harvest key information on products (e.g., quantification of oxidized lattice oxygen and solid electrolyte interphase (SEI)/cathode electrolyte interphase (CEI) components) and guide corresponding design strategy by enhancing understanding of the mechanism (e.g., clearly distinguish the catalytic target of highly oxidative Ni4+ on the NCM cathode). Not limited as a rigid quantification tool for widely known products/mechanisms, TMS technology has been demonstrated as a powerful and versatile tool for the investigations of advanced batteries.