Modeling solid-electrolyte interfacial phenomena in silicon anodes

Correlating Solid Electrolyte Interphase Composition with Dendrite-Free and Long Life-Span Lithium Metal Batteries via Advanced Characterizations and Simulations

Lithium metal anode attracts great attention because of its high specific capacity and low redox potential. However, the uncontrolled dendrite growth and its infinite volume expansion during cycling are extremely detrimental to the practical application. The formation of a solid electrolyte interphase (SEI) plays a decisive role in the behavior of lithium deposition/dissolution during electrochemical processing. Clarifying the essential relationship between SEI and battery performance is a priority. Research in SEI is accelerated in recent years by the use of advanced simulation tools and characterization techniques. The chemical composition and micromorphology of SEIs with various electrolytes are analyzed to clarify the effects of SEI on the Coulombic efficiency and cycle life. In this review, the recent research progress focused on the composition and structure of SEI is summarized, and various advanced characterization techniques applied to the investigation of SEI are discussed. The comparisons of the representative experimental results and theoretical models of SEI in lithium metal batteries (LMBs) are exhibited, and the underneath mechanisms of interaction between SEI and the electrochemical properties of the cell are highlighted. This work offers new insights into the development of safe LMBs with higher energy density.

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.

Frontiers in Theoretical Analysis of Solid Electrolyte Interphase Formation Mechanism

Solid electrolyte interphase (SEI) is an ion conductive yet electron-insulating layer on battery electrodes, which is formed by the reductive decomposition of electrolytes during the initial charge. The nature of the SEI significantly impacts the safety, power, and lifetime of the batteries. Hence, elucidating the formation mechanism of the SEI layer has become a top priority. Conventional theoretical calculations reveal initial elementary steps of electrolyte reductive decomposition, whereas experimental approaches mainly focus on the characterization of the formed SEI in the final form. Moreover, both theoretical and experimental methodologies could not approach intermediate or transient steps of SEI growth. A major breakthrough has recently been achieved through a novel multiscale simulation method, which has enriched the understanding of how the reduction products are aggregated near the electrode and influence the SEI morphologies. This review highlights recent theoretical achievements to reveal the growth mechanism and provides a clear guideline for designing a stable SEI layer for advanced batteries.

Solid-Electrolyte Interphase During Battery Cycling: Theory of Growth Regimes

The capacity fade of modern lithium ion batteries is mainly caused by the formation and growth of the solid-electrolyte interphase (SEI). Numerous continuum models support its understanding and mitigation by studying SEI growth during battery storage. However, only a few electrochemical models discuss SEI growth during battery operation. In this article, a continuum model is developed that consistently captures the influence of open-circuit potential, current direction, current magnitude, and cycle number on the growth of the SEI. The model is based on the formation and diffusion of neutral lithium atoms, which carry electrons through the SEI. Recent short- and long-term experiments provide validation for our model. SEI growth is limited by either reaction, diffusion, or migration. For the first time, the transition between these mechanisms is modelled. Thereby, an explanation is provided for the fading of capacity with time t of the form tβ with the scaling coefficent β, 0≤β≤1. Based on the model, critical operation conditions accelerating SEI growth are identified.

Dendrite formation in Li-metal anodes: an atomistic molecular dynamics study

Lithium-metal is a desired material for anodes of Li-ion and beyond Li-ion batteries because of its large theoretical specific capacity of 3860 mA h g⁻¹ (the highest known so far), low density, and extremely low potential. Unfortunately, there are several problems that restrict the practical application of lithium-metal anodes, such as the formation of dendrites and reactivity with electrolytes. We present here a study of lithium dendrite formation on a Li-metal anode covered by a cracked solid electrolyte interface (SEI) of LiF in contact with a typical liquid electrolyte composed of 1 M LiPF₆ salt solvated in ethylene carbonate. The study uses classical molecular dynamics on a model nanobattery. We tested three ways to charge the nanobattery: (1) constant current at a rate of one Li⁺ per 0.4 ps, (2) pulse train 10 Li⁺ per 4 ps, and (3) constant number ions in the electrolyte: one Li⁺ enters the electrolyte from the cathode as one Li⁺ exits the electrolyte to the anode. We found that although the SEI does not interfere with the lithiation, the mere presence of a crack in the SEI boosts and guides dendrite formation at temperatures between 325 K and 410.7 K at any C-rate, being more favorable at 325 K than at 410.7 K. On the other hand, we find that a higher C-rate (2.2C) favors the lithium dendrite formation compared to a lower C-rate (1.6C). Thus the battery could store more energy in a safe way at a lower C-rate.

Lithium dendrites (blue) growing through the cracks of a SEI (orange) covering a Li-metal anode (blue) while lithiation by Li-ions (purple). The electrolyte is not shown.

DFT study of nano zinc/copper voltaic cells

To facilitate the development of new materials for use in batteries, it is necessary to develop ab initio full-electron computational techniques for modeling potential new battery materials. Here, we tested density functional theory procedures that are accurate enough to obtain the energetics of a zinc/copper voltaic cell. We found the magnitude of the zero-point energy correction to be 0.01-0.2 kcal/mol per atom or molecule and the magnitude of the dispersion correction to be 0.1-0.6 kcal/mol per atom or molecule for Zn _n , (H₂O) _n , [Formula: see text], [Formula: see text], and Cu _n . Counterpoise correction significantly affected the values of ∆[Formula: see text], ∆[Formula: see text], and ∆E^solv by 1.0-3.1 kcal/mol per atom or molecule at the B3PW91/6-31G(d) level of theory, but by only 0.04-0.4 kcal/mol per atom or molecule at the B3PW91/cc-pVTZ level of theory. The application of B3PW91/6-31G(d) yielded results that differed from macroscopic experimental values by 0.1-7.1 kcal/mol per atom or molecule, whereas applying B3PW91/cc-pVTZ produced results that differed from macroscopic experimental values by 0.1-4.8 kcal/mol per atom or molecule, with the smallest differences occurring for reactions with a small macroscopic experimental ∆E and the largest differences occurring for reactions with a large macroscopic experimental ∆E, implying size consistency.

Simulation Protocol for Prediction of a Solid-Electrolyte Interphase on the Silicon-based Anodes of a Lithium-Ion Battery: ReaxFF Reactive Force Field

We propose the ReaxFF reactive force field as a simulation protocol for predicting the evolution of solid-electrolyte interphase (SEI) components such as gases (C₂H₄, CO, CO₂, CH₄, and C₂H₆), and inorganic (Li₂CO₃, Li₂O, and LiF) and organic (ROLi and ROCO₂Li: R = -CH₃ or -C₂H₅) products that are generated by the chemical reactions between the anodes and liquid electrolytes. ReaxFF was developed from ab initio results, and a molecular dynamics simulation with ReaxFF realized the prediction of SEI formation under real experimental conditions and with a reasonable computational cost. We report the effects on SEI formation of different kinds of Si anodes (pristine Si and SiO_x), of the different types and compositions of various carbonate electrolytes, and of the additives. From the results, we expect that ReaxFF will be very useful for the development of novel electrolytes or additives and for further advances in Li-ion battery technology.