Theoretical analysis of electrode-dependent interfacial structures on hydrate-melt electrolytes

Unraveling the origin of reductive stability of super-concentrated electrolytes from first principles and unsupervised machine learning

Developing electrolytes with excellent electrochemical stability is critical for next-generation rechargeable batteries. Super-concentrated electrolytes (SCEs) have attracted great interest due to their high electrochemical performances and stability. Previous studies have revealed changes in solvation structures and shifts in lowest unoccupied molecular orbitals from solvents to anions, promoting the formation of an anion-derived solid-electrolyte-interphase (SEI) in SCE. However, a direct connection at the atomic level to electrochemical properties is still missing, hindering the rational optimization of electrolytes. Herein, we combine ab initio molecular dynamics with the free energy calculation method to compute redox potentials of propylene carbonate electrolytes at a range of LiTFSI concentrations, and moreover employ an unsupervised machine learning model with a local structure descriptor to establish the structure-property relations. Our calculation indicates that the network of TFSI- in SCE not only helps stabilize the added electron and renders the anion more prone to reductive decomposition, but also impedes the solvation of F- and favors LiF precipitation, together leading to effective formation of protective SEI layers. Our work provides new insights into the solvation structures and electrochemistry of concentrated electrolytes which are essential to electrolyte design in batteries.

Evaluating the behaviour of deep eutectic solvent electrolytes on 2D Ca2C MXene anode for the Li-ion batteries

Rechargeable Li-ion batteries (LIBs) are one of the green energy storage devices that have been utilized in large-scale devices. Hence, improving the LIBs performance plays a crucial role in many industrial sectors. Herein, we introduce a novel electrode and electrolytes for improving the LIBs efficiency. The deep eutectic solvents (DESs) electrolytes based on lithium bis[(trifluoromethyl)sulfonyl] imide (Li[TFSI]) and two different ratios of 2,2,2-trifluoroacetamide (TFA): (Li[TFSI] : 2TFA and Li[TFSI] : 4TFA), and the calcium carbide monolayer (Ca₂C-ML) MXene were used as an anode in the LIBs. The molecular dynamics (MD) simulation and density functional theory (DFT) calculations are performed to evaluate the interaction and orientation of DESs on Ca₂C-ML. The density profiles, pair correlation functions, mean square displacement (MSD), diffusion coefficient, ionic conductivity, molecular orientation, and charge density profiles analyses are performed to determine the behavior of DESs on Ca₂C-ML. The results indicate that in both DESs, the adsorption of Li⁺ cations and TFA species on the Ca₂C surface is more than that of the [TFSI]^- anions. However, the interaction of Li⁺ cations on the Ca₂C surface in Li[TFSI]:2TFA is stronger than in Li[TFSI]:4TFA. Because the adsorption of Li⁺ on the Ca₂C occurs favorably, the low intercalation potential of Li⁺ on the Ca₂C anode can be predicted. Additionally, the simulations are carried out at higher temperatures (333.15 K, 353.15 K, and 373.15 K), and the enhancement in MSD, diffusion coefficient, and ionic conductivity is observed by increasing the temperature. Meanwhile, the low open-circuit voltage (0.30 V) during the Li-ion intercalation processes further shows the advantages of Ca₂C MXene as a potential candidate for LIB anodes. Overall, it is hoped that these findings will provide guidance for the future design of high efficiency LIBs using the Li-based DESs electrolytes and novel MXene anodes.

Applying Classical, Ab Initio, and Machine-Learning Molecular Dynamics Simulations to the Liquid Electrolyte for Rechargeable Batteries

Rechargeable batteries have become indispensable implements in our daily life and are considered a promising technology to construct sustainable energy systems in the future. The liquid electrolyte is one of the most important parts of a battery and is extremely critical in stabilizing the electrode-electrolyte interfaces and constructing safe and long-life-span batteries. Tremendous efforts have been devoted to developing new electrolyte solvents, salts, additives, and recipes, where molecular dynamics (MD) simulations play an increasingly important role in exploring electrolyte structures, physicochemical properties such as ionic conductivity, and interfacial reaction mechanisms. This review affords an overview of applying MD simulations in the study of liquid electrolytes for rechargeable batteries. First, the fundamentals and recent theoretical progress in three-class MD simulations are summarized, including classical, ab initio, and machine-learning MD simulations (section 2). Next, the application of MD simulations to the exploration of liquid electrolytes, including probing bulk and interfacial structures (section 3), deriving macroscopic properties such as ionic conductivity and dielectric constant of electrolytes (section 4), and revealing the electrode-electrolyte interfacial reaction mechanisms (section 5), are sequentially presented. Finally, a general conclusion and an insightful perspective on current challenges and future directions in applying MD simulations to liquid electrolytes are provided. Machine-learning technologies are highlighted to figure out these challenging issues facing MD simulations and electrolyte research and promote the rational design of advanced electrolytes for next-generation rechargeable batteries.

High-voltage liquid electrolytes for Li batteries: progress and perspectives

Since the advent of the Li ion batteries (LIBs), the energy density has been tripled, mainly attributed to the increase of the electrode capacities. Now, the capacity of transition metal oxide cathodes is approaching the limit due to the stability limitation of the electrolytes. To further promote the energy density of LIBs, the most promising strategies are to enhance the cut-off voltage of the prevailing cathodes or explore novel high-capacity and high-voltage cathode materials, and also replacing the graphite anode with Si/Si-C or Li metal. However, the commercial ethylene carbonate (EC)-based electrolytes with relatively low anodic stability of ∼4.3 V vs. Li⁺/Li cannot sustain high-voltage cathodes. The bottleneck restricting the electrochemical performance in Li batteries has veered towards new electrolyte compositions catering for aggressive next-generation cathodes and Si/Si-C or Li metal anodes, since the oxidation-resistance of the electrolytes and the in situ formed cathode electrolyte interphase (CEI) layers at the high-voltage cathodes and solid electrolyte interphase (SEI) layers on anodes critically control the electrochemical performance of these high-voltage Li batteries. In this review, we present a comprehensive and in-depth overview on the recent advances, fundamental mechanisms, scientific challenges, and design strategies for the novel high-voltage electrolyte systems, especially focused on stability issues of the electrolytes, the compatibility and interactions between the electrolytes and the electrodes, and reaction mechanisms. Finally, novel insights, promising directions and potential solutions for high voltage electrolytes associated with effective SEI/CEI layers are proposed to motivate revolutionary next-generation high-voltage Li battery chemistries.

Optimal water concentration for aqueous Li+ intercalation in vanadyl phosphate

Development of high-performance aqueous batteries is an important goal for energy sustainability owing to their environmental benignity and low fabrication costs. Although a layered vanadyl phosphate is one of the most-studied host materials for intercalation electrodes with organic electrolytes, little attention has been paid to its use in aqueous Li⁺ systems because of its excessive dissolution in water. Herein, by controlling the water concentration, we demonstrate the stable operation of a layered vanadyl phosphate electrode in an aqueous Li⁺ electrolyte. The combination of experimental analyses and density functional theory calculations reveals that reversible (de)lithiation occurs between dehydrated phases, which can only exist in an optimal water concentration.

We demonstrate aqueous Li⁺ intercalation chemistry of vanadyl phosphate in the form of monohydrates, which are only stable in an optimal water-concentration environment.