Fluorine Rich Borate Salt Anion Based Electrolyte for High Voltage Sodium Metal Battery Development.

This study demonstrates the enhanced performance in high-voltage sodium full cells using a novel electrolyte composition featuring a highly fluorinated borate ester anion (1 M Na[B(hfip)4].3DME) in a binary carbonate mixture (EC:EMC), compared to a conventional electrolyte (1 M Na[PF6] EC:EMC). The prolonged cycling performance of sodium metal battery employing high voltage cathodes (NVPF@C@CNT and NFMO) is attributed to uniform and dense sodium deposition along with the formation of fluorine and boron-rich solid electrolyte interphase (SEI) on the sodium metal anode. Simultaneously, a robust cathode electrolyte interphase (CEI) is formed on the cathode side due to the improved electrochemical stability window and superior aluminum passivation of the novel electrolyte. The CEIs on high-voltage cathodes are discovered to be abundant in C-F, B-O, and B-F components, which contributes to long-term cycling stability by effectively suppressing undesirable side reactions and mitigating electrolyte decomposition. The participation of DME in the primary solvation shell coupled with the comparatively weaker interaction between Na+ and [B(hfip)4]- in the secondary solvation shell, provides additional confirmation of labile desolvation. This, in turn, supports the active participation of the anion in the formation of fluorine and boron-rich interphases on both the anode and cathode.

Computational Investigation of LiF Formation at Graphite-Electrolyte Interfaces.

The performance of rechargeable batteries is strongly influenced by the solid-electrolyte interphase (SEI), and a comprehensive understanding of SEI formation from the atomic level is crucial for effective battery design. The dynamics of the electrode-electrolyte interface is important and needs to be considered when evaluating the mechanism of the SEI formation. Here, we employed ab initio molecular dynamics (AIMD) and density functional theory (DFT) calculations to examine interfacial behaviors and LiF formation. Through molecular dynamics and structure sampling, we successfully constructed an electrochemical stability diagram correlating the thermodynamic free energy with the potential, which is determined by the work function of electrode surfaces. DFT calculations revealed that LiF formation at the graphite-electrolyte interfaces occurs easily via the intermediate LiHF complex. Interestingly, LiF tends to be solvated by solvents rather than directly deposited onto electrode surfaces (e.g., the Au electrode), a phenomenon we identify as a critical determinant of the porous and uneven nature of the LiF layer observed on graphite electrodes. Our finding offers new mechanistic insights into LiF formation at graphite-electrolyte interfaces.

Cation-induced changes in the inner- and outer-sphere mechanisms of electrocatalytic CO2 reduction.

The underlying mechanism of cation effects on CO2RR remains debated. Herein, we study cation effects by simulating both outer-sphere electron transfer (OS-ET) and inner-sphere electron transfer (IS-ET) pathways during CO2RR via constrained density functional theory molecular dynamics (cDFT-MD) and slow-growth DFT-MD (SG-DFT-MD), respectively. Our results show without any cations, only OS-ET is feasible with a barrier of 1.21 eV. In the presence of K+ (Li+), OS-ET shows a very high barrier of 2.93 eV (4.15 eV) thus being prohibited. However, cations promote CO2 activation through IS-ET with the barrier of only 0.61 eV (K+) and 0.91 eV (Li+), generating the key intermediate (adsorbed CO[Formula: see text]). Without cations, CO2-to-CO[Formula: see text](ads) conversion cannot proceed. Our findings reveal cation effects arise from short-range Coulomb interactions with reaction intermediates. These results disclose that cations modulate the inner- and outer-sphere pathways of CO2RR, offering substantial insights on the cation specificity in the initial CO2RR steps.

Cation-Coordinated Inner-Sphere CO2 Electroreduction at Au-Water Interfaces.

Electrochemical CO2 reduction reaction (CO2RR) is a promising technology for the clean energy economy. Numerous efforts have been devoted to enhancing the mechanistic understanding of CO2RR from both experimental and theoretical studies. Electrolyte ions are critical for the CO2RR; however, the role of alkali metal cations is highly controversial, and a complete free energy diagram of CO2RR at Au-water interfaces is still missing. Here, we provide a systematic mechanism study toward CO2RR via ab initio molecular dynamics simulations integrated with the slow-growth sampling (SG-AIMD) method. By using the SG-AIMD approach, we demonstrate that CO2RR is facile at the inner-sphere interface in the presence of K cations, which promote the CO2 activation with the free energy barrier of only 0.66 eV. Furthermore, the competitive hydrogen evolution reaction (HER) is inhibited by the interfacial cations with the induced kinetic blockage effect, where the rate-limiting Volmer step shows a much higher energy barrier (1.27 eV). Eventually, a comprehensive free energy diagram including both kinetics and thermodynamics of the CO2RR to CO and the HER at the electrochemical interface is derived, which illustrates the critical role of cations on the overall performance of CO2 electroreduction by facilitating CO2 adsorption while suppressing the hydrogen evolution at the same time.

CO2 activation at Au(110)-water interfaces: An ab initio molecular dynamics study.

The electrochemical reduction of CO2 into valuable chemicals under mild conditions has become a promising technology for energy storage and conversion in the past few years, receiving much attention from theoretical researchers investigating the reaction mechanisms. However, most of the previous simulations are related to the key intermediates of *COOH and *CO using the computational hydrogen electrode approach under vacuum conditions, and the details of the CO2 activation are usually ignored due to the model simplicity. Here, we study the CO2 activation at the Au-water interfaces by considering the dynamics of an explicit water solvent, where both regular ab initio molecular dynamics and constrained ab initio molecular dynamics simulations are carried out to explore the CO2 adsorption/desorption reactions from the atomic level. By introducing K+ cations into Au(110)-water interfacial models, an electrochemical environment under reducing potentials is constructed, where the reaction free energy (0.26 eV) and activation energy (0.61 eV) are obtained for CO2 adsorption based on the thermodynamic integration. Moreover, the Bader charge analysis demonstrates that CO2 adsorption is activated by the first-electron transfer, forming the adsorbed CO2 - anion initiating the overall catalytic reaction.

Constructing Active Sites from Atomic-Scale Geometrical Engineering in Spinel Oxide Solid Solutions for Efficient and Robust Oxygen Evolution Reaction Electrocatalysts.

Spinel oxides are considered as promising low-cost non-precious metal electrocatalysts for oxygen evolution reaction (OER) due to their desirable catalytic activities and fast kinetics. However, as a result of the structural complexity of spinel oxides, systematic and in-depth studies on enhancing the OER performance of spinel oxides remain inadequate. In particular, the construction of active sites regarding the large number of unoccupied octahedral interstices has not yet been explored. Herein, more octahedral sites with high OER activities are constructed on the surface of spinel oxides via a cationic misalignment, which is induced by the defects in the spinel oxide solutions, i.e., MoFe2 O4 and CoFe2 O4 nanosheets supported on an iron foam (MCFO NS/IF). With increased active sites and modified electronic structure, the state-of-the-art electrocatalyst exhibits the excellent OER catalytic activity with an onset potential of 1.41 V versus RHE and an overpotential of 290 mV to achieve a current density of 500 mA cm-2 . Moreover, such an electrocatalyst also demonstrates fast kinetics with the Tafel slope of 38 mV dec-1 and superior durability by maintaining the OER activity at 250 mA cm-2 for 1000 h.

Highly Dispersive Cerium Atoms on Carbon Nanowires as Oxygen Reduction Reaction Electrocatalysts for Zn-Air Batteries.

Highly efficient noble-metal-free electrocatalysts for oxygen reduction reaction (ORR) are essential to reduce the costs of fuel cells and metal-air batteries. Herein, a single-atom Ce-N-C catalyst, constructed of atomically dispersed Ce anchored on N-doped porous carbon nanowires, is proposed to boost the ORR. This catalyst has a high Ce content of 8.55 wt % and a high activity with ORR half-wave potentials of 0.88 V in alkaline media and 0.75 V in acidic electrolytes, which are comparable to widely studied Fe-N-C catalysts. A Zn-air battery based on this material shows excellent performance and durability. Density functional theory calculations reveal that atomically dispersed Ce with adsorbed hydroxyl species (OH) can significantly reduce the energy barrier of the rate-determining step resulting in an improved ORR activity.

Recent Advances in Catalyst Structure and Composition Engineering Strategies for Regulating CO2 Electrochemical Reduction.

Electrochemical CO2 reduction has been recognized as a promising solution in tackling energy- and environment-related challenges of human society. In the past few years, the rapid development of advanced electrocatalysts has significantly improved the efficiency of this reaction and accelerated the practical applications of this technology. Herein, representative catalyst structures and composition engineering strategies in regulating the CO2 reduction selectivity and activity toward various products including carbon monoxide, formate, methane, methanol, ethylene, and ethanol are summarized. An overview of in situ/operando characterizations and advanced computational modeling in deepening the understanding of the reaction mechanisms and accelerating catalyst design are also provided. To conclude, future challenges and opportunities in this research field are discussed.

Superfine MnO2 Nanowires with Rich Defects Toward Boosted Zinc Ion Storage Performance.

The core challenge of MnO2 as the cathode material of zinc-ion batteries remains to be their poor electrochemical kinetics and stability. Herein, MnO2 superfine nanowires (∼10 nm) with rich crystal defects (oxygen vacancies and cavities) are demonstrated to possess high efficient zinc-ion storage capability. Experimental and theoretical studies demonstrate that the defects facilitate the adsorption and diffusion of hydrogen/zinc for fast ion transportation and the build of a local electric field for improved electron migration. In addition, the superfine nanostructure could provide sufficient active sites and short diffusion pathways for further promotion of capacity and reaction kinetics of MnO2. Remarkably, the defect-enriched MnO2 nanowires manifest an energy density as high as 406 W h kg-1 and an excellent durability over 1000 cycles without noticeable capacity degradation. Mechanistic analysis substantiates a reversible coinsertion/extraction process of H+ and Zn2+ with a simultaneous deposition/dissolution of zinc sulfate hydroxide hydrate nanoflakes. This work could enrich the fundamental understanding of defect engineering and nanostructuring on the development of advanced MnO2 materials toward high-performance zinc-ion batteries.

Effects of Solid Electrolyte Interphase Components on the Reduction of LiFSI over Lithium Metal.

The use of a lithium metal anode still presents a challenging chemistry and engineering problem that holds back next generation lithium battery technology. One of the issues facing lithium metal is the presence of the solid electrolyte interphase (SEI) layer that forms on the electrode creating a variety of chemical species that change the properties of the electrode and is closely related to the formation and growth of lithium dendrites. In order to advance the scientific progress of lithium metal more must be understood about the fundamentals of the SEI. One property of the SEI that is particularly critical is the passivating behavior of the different SEI components. This property is critical to the continued formation of SEI and stability of the electrolyte and electrode. Here we report the investigation of the passivation behavior of Li2 O, Li2 CO3, LiF and LiOH with the lithium salt LiFSI. We used large computational chemistry models that are able to capture the lithium/SEI interface as well as the SEI/electrolyte interface. We determined that LiF and Li2 CO3 are the most passivating of the SEI layers, followed by LiOH and Li2 O. These results match previous studies of other Li salts and provide further examination of LiFSI reduction.

pH-Dependent Hydrogen and Water Binding Energies on Platinum Surfaces as Directly Probed through Surface-Enhanced Infrared Absorption Spectroscopy.

The origins of the pH-dependent kinetics of hydrogen evolution and oxidation reactions on Pt surfaces are unsolved dilemmas that have lasted for over half a century. In this study, surface-enhanced infrared absorption spectroscopy is applied to directly monitor the vibrational behaviors of adsorbed hydrogen atoms and interfacial water molecules on Pt surfaces in a wide pH window from 1.1 to 12.9. For the first time, we successfully measure the pH-dependent changes of hydrogen and water binding strength according to their vibrational wavenumbers, which are both monotonously weakened as the solution pH increases. Their changes are the net results of altered electrochemical interface environments and are important contributions to the pH-dependent hydrogen reaction kinetics. Our results add significant new insights into the role of interfacial environments on electrocatalysis.

Role of Inorganic Surface Layer on Solid Electrolyte Interphase Evolution at Li-Metal Anodes.

Lithium metal is an ideal anode for rechargeable lithium-battery technology. However, the extreme reactivity of Li metal with electrolytes leads to solid electrolyte interphase (SEI) layers that often impede Li+ transport across interfaces. The challenge is to predict the chemical, structural, and topographical heterogeneities of SEI layers arising from a multitude of interfacial constituents. Traditionally, the pathways and products of electrolyte decomposition processes were analyzed with the basic and simplifying presumption of an initial pristine Li-metal surface. However, ubiquitous inorganic passivation layers on Li metal can reduce electronic charge transfer to the electrolyte and significantly alter the SEI layer evolution. In this study, we analyzed the effect of nanometric Li2O, LiOH, and Li2CO3 as surface passivation layers on the interfacial reactivity of Li metal, using ab initio molecular dynamics (AIMD) calculations and X-ray photoelectron spectroscopy (XPS) measurements. These nanometric layers impede the electronic charge transfer to the electrolyte and thereby provide some degree of passivation (compared to pristine lithium metal) by altering the redox-based decomposition process. The Li2O, LiOH, and Li2CO3 layers admit varying levels of electron transfer from a Li-metal slab and subsequent storage of the electronic charges within their structures. As a result, their ability to transfer electrons to the electrolyte molecules, as well as the extent of decomposition of bis(trifluoromethanesulfonyl)imide anions, is significantly reduced compared to similar processes on pristine Li metal. The XPS experiments revealed that when Li2O is the major component on the altered surface, LiF phases formed to a greater extent. The presence of a dominant LiOH layer, however, results in enhanced sulfur decomposition processes. From AIMD studies, these observations can be explained based on the calculated quantities of electronic charge transfer found for each of the passivating films.

Mn3O4 Quantum Dots Supported on Nitrogen-Doped Partially Exfoliated Multiwall Carbon Nanotubes as Oxygen Reduction Electrocatalysts for High-Performance Zn-Air Batteries.

Highly efficient and low-cost nonprecious metal electrocatalysts that favor a four-electron pathway for the oxygen reduction reaction (ORR) are essential for high-performance metal-air batteries. Herein, we show an ultrasonication-assisted synthesis method to prepare Mn3O4 quantum dots (QDs, ca. 2 nm) anchored on nitrogen-doped partially exfoliated multiwall carbon nanotubes (Mn3O4 QDs/N-p-MCNTs) as a high-performance ORR catalyst. The Mn3O4 QDs/N-p-MCNTs facilitated the four-electron pathway for the ORR and exhibited sufficient catalytic activity with an onset potential of 0.850 V (vs reversible hydrogen electrode), which is only 38 mV less positive than that of Pt/C (0.888 V). In addition, the Mn3O4 QDs/N-p-MCNTs demonstrated superior stability than Pt/C in alkaline solutions. Furthermore, a Zn-air battery using the Mn3O4 QDs/N-p-MCNTs cathode catalyst successfully generated a specific capacity of 745 mA h g-1 at 10 mA cm-2 without the loss of voltage after continuous discharging for 105 h. The superior ORR activity of Mn3O4 QDs/N-p-MCNTs can be ascribed to the homogeneous Mn3O4 QDs loaded onto the N-doped carbon skeleton and the synergistic effects of Mn3O4 QDs, nitrogen, and carbon nanotubes. The interface binding energy of -3.35 eV calculated by the first-principles density functional theory method illustrated the high stability of the QD-anchored catalyst. The most stable adsorption structure of O2, at the interface between Mn3O4 QDs and the graphene layer, had the binding energy of -1.17 eV, greatly enhancing the ORR activity. In addition to the high ORR activity and stability, the cost of production of Mn3O4 QDs/N-p-MCNTs is low, which will broadly facilitate the real application of metal-air batteries.

Hierarchical NiCo2O4 Micro- and Nanostructures with Tunable Morphologies as Anode Materials for Lithium- and Sodium-Ion Batteries.

NiCo2O4 microrods with open structures are successfully synthesized using a solvothermal method. Compared with those of dense microspheres, the one-dimensional (1D) porous microrods show much higher capacities and stability for both Li- and Na-ion batteries due to the 1D open structure facilitating fast ion transport and buffering volumetric change during charge/discharge. This work demonstrates that the electrochemical performance of NiCo2O4 is highly dependent on morphologies of the active material.