Investigation of the Degradation of LiPF6- in Polar Solvents through Deep Potential Molecular Dynamics.

The nonaqueous electrolyte based on lithium hexafluorophosphate (LiPF6) is the dominant liquid electrolyte in lithium-ion batteries (LIBs). However, trace protic impurities, including H3O+, alcohols, and hydrofluoric acid (HF), can trigger a series of side reactions that lead to rapid capacity fading in high energy density LIBs. It is worth noting that this degradation process is highly dependent on the polarity of the solvents. In this work, a deep potential (DP) model is trained with a certain commercial electrolyte formula through a machine learning method. H3O+ is anchored with polar solvents, making it difficult to approach the PF6-, and suppressing the degradation process quickly at room temperature. Control experiments and simulations at different temperatures or concentrations are also performed to verify it. This work proposes a precise model to describe the solvation structure quantitatively and offers a new perspective on the degradation mechanism of PF6- in polar solvents.

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.

Toward a molecular understanding of the conductivity of lithium-ion conducting polyanion polymer electrolytes by molecular dynamics simulation.

With the improved lithium-ion transference number near unity, the low conductivity of single lithium-ion conducting solid polymer electrolytes (SLIC-SPEs) still hinders their application in high-rate batteries. Though some empirical conclusions on the conducting mechanism of SLIC-SPEs have been obtained, a more comprehensive study on the quantitative relationship between the molecular structure factors and ionic conduction performance is expected. In this study, a model structure that contains adjustable main chain and anion groups in the polyethylene oxide (PEO) matrix was used to clarify the influence of molecular structural factors on ionic conductivity and electrochemical stability of SLIC-SPEs. The anionic group was further disassembled into the intermediate group and end group while the main chain structure was distinguished into different degrees of polymerization and various lengths of the spacers between anions. Therefore, a well-defined molecular structure was employed to describe its relationship with ionic conductivity. In addition, the dissociation degree of salts and mobility of ions changing with the molecular structure were also discussed to explore the fundamental causes of conductivity. It can be concluded that the anion group affects the conductivity mainly via the dissociation degree, while the main chain structure impacts the conductivity by both dissociation degree and mobility.

Bifunctional Localized High-Concentration Electrolyte for the Fast Kinetics of Lithium Batteries at Low Temperatures.

Traditional lithium batteries cannot work well at low temperatures due to the sluggish desolvation process, which limits their applications in low-temperature fields. Among various previously reported approaches, solvation regulation of electrolytes is of great importance to overcome this obstacle. In this work, a tetrahydrofuran (THF)-based localized high-concentration electrolyte is reported, which possesses the advantages of a unique solvation structure and improved mobility, enabling a Li/lithium manganate (LMO) battery to cycle stably at room temperature (retains 85.9% after 300 cycles) and to work at a high rate (retains 69.0% at a 10C rate). Apart from that, this electrolyte demonstrates superior low-temperature performance, delivering over 70% capacity at -70 °C and maintaining 72.5 mAh g-1 (≈77.1%) capacity for 200 cycles at a 1C rate at -40 °C. Also, even when the rate increases to 5C, the battery could still operate well at -40 °C. This work demonstrates that solvation regulation has a significant impact on the kinetics of cells at low temperatures and provides a design method for future electrolyte design.

A Janus Separator based on Cation Exchange Resin and Fe Nanoparticles-decorated Single-wall Carbon Nanotubes with Triply Synergistic Effects for High-areal Capacity Zn-I2 Batteries.

Zn-I2 batteries stand out in the family of aqueous Zn-metal batteries (AZMBs) due to their low-cost and immanent safety. However, Zn dendrite growth, polyiodide shuttle effect and sluggish I2 redox kinetics result in dramatically capacity decay of Zn-I2 batteries. Herein, a Janus separator composed of functional layers on anode/cathode sides is designed to resolve these issues simultaneously. The cathode layer of Fe nanoparticles-decorated single-wall carbon nanotubes can effectively anchor polyiodide and catalyze the redox kinetics of iodine species, while the anode layer of cation exchange resin rich in -SO3 - groups is beneficial to attract Zn2+ ions and repel detrimental SO4 2- /polyiodide, improving the stability of cathode/anode interfaces synergistically. Consequently, the Janus separator endows outstanding cycling stability of symmetrical cells and high-areal-capacity Zn-I2 batteries with a lifespan over 2500 h and a high-areal capacity of 3.6 mAh cm-2 .

Dynamically Interfacial pH-Buffering Effect Enabled by N-Methylimidazole Molecules as Spontaneous Proton Pumps toward Highly Reversible Zinc-Metal Anodes.

Aqueous zinc-metal batteries have attracted extensive attention due to their outstanding merits of high safety and low cost. However, the intrinsic thermodynamic instability of zinc in aqueous electrolyte inevitably results in hydrogen evolution, and the consequent generation of OH- at the interface will dramatically exacerbate the formation of dead zinc and dendrites. Herein, a dynamically interfacial pH-buffering strategy implemented by N-methylimidazole (NMI) additive is proposed to remove the detrimental OH- at zinc/electrolyte interface in real-time, thus eliminating the accumulation of by-products fundamentally. Electrochemical quartz crystal microbalance and molecular dynamics simulation results reveal the existence of an interfacial absorption layer assembled by NMI and protonated NMI (NMIH+ ), which acts as an ion pump for replenishing the interface with protons constantly. Moreover, an in situ interfacial pH detection method with micro-sized spatial resolution based on the ultra-microelectrode technology is developed to probe the pH evolution in diffusion layer, confirming the stabilized interfacial chemical environment in NMI-containing electrolyte. Accordingly, with the existence of NMI, an excellent cumulative plating capacity of 4.2 Ah cm-2 and ultrahigh Coulombic efficiency of 99.74% are realized for zinc electrodes. Meanwhile, the NMI/NMIH+ buffer additive can accelerate the dissolution/deposition process of MnO2 /Mn2+ on the cathode, leading to enhanced cycling capacity.

Anion-Containing Solvation Structure Reconfiguration Enables Wide-Temperature Electrolyte for High-Energy-Density Lithium-Metal Batteries.

The demand for high-energy-density lithium batteries (LBs) that work under a wide temperature range (-40 to 60 °C) has been increasing recently. However, the conventional lithium hexafluorophosphate (LiPF6)-based ester electrolyte with a solvent-based solvation structure has limited the practical application of LBs under extreme temperature conditions. In this work, a novel localized high-concentration electrolyte (LHCE) system is designed to achieve the anion-containing solvation structure with less free solvent molecules using lithium difluorophosphate (LiPO2F2) as a lithium salt, which enables wide-temperature electrolyte for LBs. The optimized solvation structure contributes to the cathode-electrolyte interface (CEI) with abundant LiF and P-O components on the surface of the LiNi0.5Co0.2Mn0.3O2 (NCM523) cathode, effectively inhibiting the decomposition of electrolyte and the dissolution of transition-metal ions (TMIs). Moreover, the weakened Li+-dipole interaction is also beneficial to the desolvation process. Therefore, the 4.3 V Li||NCM523 cell using the modified electrolyte maintains a high capacity retention of 81.0% after 200 cycles under 60 °C. Meanwhile, a considerable capacity of 70.9 mAh g-1 (42.0% of that at room temperature) can be released at an extremely low temperature of -60 °C. This modified electrolyte dramatically enhances the electrochemical stability of NCM523 cells by regulating the solvation structure, providing guidelines for designing a multifunctional electrolyte that works under a wide temperature range.

Facile Fabrication of Functionalized Separators for Lithium-Ion Batteries with Ionic Conduction Path Modifications via the γ-Ray Co-irradiation Grafting Process.

Separators play a vital role in electronic insulation and ionic conduction in lithium-ion batteries. The common improvement strategy of polyolefin separators is mostly based on modifications with a coating layer, which is simple and effective to some extent. However, the improvement is often accompanied by negative effects such as the increase of the thickness and the blockage of the porous structure, resulting in the decrease of energy density and power density. The porous structure of the separators serves as a conduction path for ions to travel back and forth between the anode and cathode, which has an important impact on the performance of lithium-ion batteries. If the porous structure of the separators can be modified, it will essentially affect the ionic transport behavior through the whole conduction path. Herein, we provide a simple and effective method to functionalize the porous polyolefin separator via the γ-ray co-irradiation grafting process, where high-energy γ-ray is used to generate active sites on the polymer chain to initiate the grafting polymerization of chosen monomers with selected functional groups. In this work, 4,4,5,5-tetramethyl-2-vinyl-1,3,2-dioxaborolane, a kind of borane molecule with an electron-deficient group, was chosen as the grafting monomer. After the γ-ray co-irradiation grafting process, both the surface and pores of the polyolefin separators were functionalized by electron-deficient groups in the borane molecule and the whole electrolyte conduction path within the separator was activated. Due to the electron-deficient effect of the B atom, the lithium-ion conduction is promoted and the lithium-ion transference number can be increased to 0.5. As a result, the half-cell assembled with the functionalized separator shows better cycle stability and better capacity retention under high current rate.

Functional Localized High-Concentration Ether-Based Electrolyte for Stabilizing High-Voltage Lithium-Metal Battery.

Localized high-concentration electrolytes have attracted much attention of researchers due to their low viscosity, low cost, and relatively higher electrochemical performance than their low-concentration counterparts. In our work, 1.5 M (mol L-1) locally concentrated ether-based electrolyte has been obtained by adding 1,1,2,2-tetrafluoroethyl-2,2,3,3-tetrafluoropropyl ether (HFE) into a 4 M LiFSI concentrated dimethoxyethane (DME)-based electrolyte. The optimal ratio is determined by density functional theory (DFT) calculation and experimental combination, and finally, DH(3/5)-1.5M-LiFSI (DME/HFE = 3:5 by volume) is obtained. The electrolyte not only has relatively good physical properties such as low viscosity and high conductivity but also shows decent electrochemical performance. Li∥Cu half-cells can maintain a coulombic efficiency of no less than 99% after circulating for 250 cycles under the condition of 1 mA cm-2 current density and 1 mAh cm-2 lithium deposition for each cycle, and the stable battery polarization voltage was about 50 mV. Furthermore, 0.15 M lithium trifluoromethyl acetate (LiCO2CF3) has been added as an additive to enhance the oxidation stability. The new electrolyte DH(3/5)-1.65M-LiFC (LiFC/LiFSI + LiCO2CF3) makes Li||NCM523 batteries maintain about 83% capacity after cycling for 250 times with a 0.5 C charge current density and a 1 C discharge current density of 160 mAh g-1 when charged to 4.3 V. Furthermore, this new additive has a little negative effect on the Li||Cu half-cell performance under the same condition as before, indicating this new type of localized high-concentration DME-based electrolyte benefits both high-voltage cathode and lithium-metal anode.

Investigation into the reaction mechanism underlying the atmospheric low-temperature plasma-induced oxidation of cellulose.

Atmospheric low-temperature plasma has been widely applied in surface modification of lignocellulose for manufacturing lightweight, strong composites. This study is aimed at elaborating the structural changes of cellulose after plasma treatment and further understanding the mechanism underlying plasma-induced oxidation of cellulose. Experiments suggested that atmospheric low-temperature plasma exhibits strong capacity to cleave covalent bonds, leading to oxidation and degradation of cellulose. Theoretical analysis revealed that cleavage of C4O covalent bond is the first-step reaction during plasma-induced oxidation due to its low bond dissociation energy (229.2 kJ mol-1). Subsequent pyranose ring-breaking reaction dominates dynamically and thermodynamically. Obtained outcomes are vital for fundamentally understanding the plasma-lignocellulose interaction. On that basis, plasma treatment for activation and oxidation of lignocellulose can be optimized and designed for improved efficiency. Wettability of lignocellulose can be thus improved in a short time, providing an opportunity to manufacture lignocellulose-based composites with enhanced efficiency and mechanical properties in future.

Atmospheric Low-Temperature Plasma-Induced Changes in the Structure of the Lignin Macromolecule: An Experimental and Theoretical Investigation.

Atmospheric low-temperature plasma has emerged as a promising pretreatment for lignocellulose to improve bio-refining. Herein, we investigated plasma-induced changes in the chemical structure of lignin to obtain a fundamental understanding of the plasma-lignocellulose interaction. Based on the results, plasma possesses a strong capacity to cleave C-C covalent bonds in the aliphatic region of lignin, accompanied by oxidation. Plasma treatment leads to the degradation and fragmentation of lignin. Pronounced deconstruction of ß-O-4 aryl ether is observed in plasma. The relative content of ß-O-4 aryl ether was reduced from the initial value of 65.1/100Ar to 58.7/100Ar for lignin from corncob and from the initial value of 72.5/100Ar to 63.8/100Ar for lignin from poplar after plasma treatment, respectively. According to the density functional theory analysis, the oxygen atom of ß-O-4 aryl ether is the most likely potential reaction site and the Cß-O covalent bond exhibits the lowest decomposition free energy (50.5 kcal mol-1), which will easily be cleaved in plasma. The dominant reaction pathway of lignin degradation is the cleavage of the Cß-O covalent bond followed by the cleavage of the Cß-Cα bond. We propose that this investigation is beneficial to optimize and expand the applications of plasma treatment in pretreatment of lignocellulose.