Solvent Decomposition Reactions Research Articles

An iterative analytical model for aging analysis of Li-ion cells

Physics-based aging models are critical for understanding capacity degradation mechanisms in Li-ion batteries. This paper presents a technique for aging analysis of a Li-ion cell due to growth of the solid electrolyte interphase (SEI) layer driven by a solvent decomposition reaction at the electrode surface. The model employs an iterative technique based on the analytical solutions of the underlying conservation equations. The single-particle model describing Li-ion intercalation and de-intercalation processes is solved analytically using Green's function technique. The SEI formation problem is solved using the integral balance method. An iterative technique that combines these analytical solutions is shown to result in a converged result within a few iterations. The model is shown to agree well with results from past studies, as well as a numerical simulation. The capacity fade of Li-ion batteries is investigated under different operating conditions and different regimes, including both cycling and storage. The present model offers much faster computation time than numerical models for modeling the degradation of Li-ion cells. Further, the iterative technique described here may serve as a framework for semi-analytical solutions for other, more complicated problems. This work contributes towards improving the performance and reliability of electrochemical energy conversion and storage systems. • Developed an iterative technique for aging analysis of Li-ion cells. • Models capacity degradation due to cycling as well as calendar aging. • Results shown to agree well with past work and numerical simulations. • Results highlight key aspects of aging and capacity degradation in Li-ion cells.

Effects of charged interfaces on electrolyte decomposition at the lithium metal anode

Lithium–Sulfur batteries are promising candidates to substitute conventional Li-ion batteries due to their higher energy density and reduced cost. However, several challenges related to the reactivity of the lithium metal anode have prevented this technology from becoming broadly commercialized. Lithium's high reactive nature leads to the continuous decomposition of the electrolyte and the formation of the solid-electrolyte interphase (SEI) layer. Thus, a comprehensive understanding of how the SEI film is formed is crucial to help elucidate improvements for this battery technology. In this work, we use density functional theory (DFT) based computational methods to investigate the effect of charged interfaces on the electrolyte degradation at the lithium anode. Several electrolyte mixtures were considered including 1,2-dimethoxyethane (DME) and 1,3-dioxolane (DOL) as solvents, and lithium bis(fluorosulfonyl)imide (LiFSI) and lithium bis(trifluoromethanesulfonyl)imide (LiTFSI) as salts. It is found that the extent of salt decomposition is higher when the interface is charged. In addition, two types of solvent reduction mechanisms are identified: C–O bond cleavage and radical attacks. Charge transfer and evolution analysis are characterized in detail. Finally, solvent decomposition reactions are found to become more thermodynamically favorable and their activation barriers to be diminished under the effect of constant potentials and electric fields.

Lithium-electrolyte solvation and reaction in the electrolyte of a lithium ion battery: A ReaxFF reactive force field study.

In the electrode/electrolyte interface of a typical lithium-ion battery, a solid electrolyte interphase layer is formed as a result of electrolyte decomposition during the initial charge/discharge cycles. Electron leakage from the anode to the electrolyte reduces the Li+-ion and makes it more reactive, resulting in decomposition of the organic electrolyte. To study the Li-electrolyte solvation, solvent exchange, and subsequent solvent decomposition reactions at the anode/electrolyte interface, we have extended the existing ReaxFF reactive force field parameter sets to organic electrolyte species, such as ethylene carbonate, ethyl methyl carbonate, vinylene carbonate, and LiPF6 salt. Density Functional Theory (DFT) data describing Li-associated initiation reactions for the organic electrolytes and binding energies of Li-electrolyte solvation structures were generated and added to the existing ReaxFF training data, and subsequently, we trained the ReaxFF parameters with the aim of finding the optimal reproduction of the DFT data. In order to discern the characteristics of the Li neutral and cation, we have introduced a second Li parameter set to describe the Li+-ion. ReaxFF is trained for Li-neutral and Li+-cation to have similar solvation energies, but unlike the neutral Li, Li+ will not induce reactivity in the organic electrolyte. Solvent decomposition reactions are presumed to happen once Li+-ions are reduced to Li-atoms, which can be simulated using a Monte Carlo type atom modification within ReaxFF. This newly developed force field is capable of distinguishing between a Li-atom and a Li+-ion properly. Moreover, it is found that the solvent decomposition reaction barrier is a function of the number of ethylene carbonate molecules solvating the Li-atom.

Operando Spectroscopy of Alkyl Carbonate-Based Electrolyte Decomposition in LiCoO2 Batteries

Electrolytes composed of alkyl carbonate solvents with lithium hexafluorophosphate (LiPF6) salt have enjoyed near universal adoption in lithium ion batteries. Despite the wide adoption of these electrolytes, significant safety issues can occur under mechanical, electrochemical, or thermal abuse. Safety concerns will only increase with an expanded number of battery applications (including emerging markets for used batteries), higher energy density, and a need for extreme fast charging in electric vehicle applications. The stability of ethylene carbonate (EC) against oxidation at the cathode while forming a stable solid electrolyte interface (SEI) upon reduction at the anode has made this solvent a key ingredient in almost all lithium ion batteries. Linear carbonates (e.g. diethyl carbonate (DEC) or dimethyl carbonate (DMC)) are mixed with EC to reduce viscosity and increase ion mobility, while maintaining a wide electrochemical stability window. While stable against reduction at the anode, the higher volatility and lower boiling points of linear alkyl carbonates increase risk of vapor buildup within the battery. At elevated temperature, these carbonate-based electrolytes become unstable as SEI decomposition, solvent-salt reactions, anode reduction, cathode oxidation, and thermal decomposition lead to thermal runaway, battery venting, fire, or explosion. In an effort to improve safety and explore alternative electrolytes, numerous studies have sought to identify the underlying mechanisms of electrolyte decomposition. Differential scanning calorimetry and accelerated rate calorimetry experiments are commonly used to identify onset temperatures of key reactions, global kinetic parameters, and heat release rates. Ex-situ electrolyte and gas analyses are also used to identify decomposition mechanisms and reaction products. However, much remains unknown about the detailed decomposition chemistry in lithium ion batteries. Furthermore, decomposition chemistry is highly dependent on the composition of the electrolyte and electrode, making it difficult to predict thermal stability in new or modified battery chemistries. Here we present operando Fourier transform infrared (FTIR) spectroscopy measurements of electrolyte decomposition in lithium ion batteries. Using attenuated total reflection (ATR) spectroscopy, we simultaneously monitored electrolyte composition and electrochemical performance of heated LiCoO2 coin cells with EC/DEC/LiPF6 electrolyte and Celgard separators. Using the ATR method, an infrared evanescent wave propagates several microns into the electrolyte-saturated pores of the anode and cathode layers, respectively. The resulting absorption spectra are analyzed to identify electrolyte species and their molecular bonds. The coin cell geometry allows for careful control and monitoring of electrolyte temperature and composition at charge/discharge rates > 1C. Electrolyte decomposition was monitored during battery cycling as a function of electrolyte composition, battery temperature, C-rate, and state of charge. The role of LiPF6 in solvent decomposition reactions was also explored. These measurements captured the dynamics of SEI formation on the graphite anode, including identification of key molecular bonds involved in EC decomposition during formative cycles from density functional theory modeling. At temperatures above 70°C, SEI decomposition was observed, followed by an increase in EC decomposition. Overcharging resulted in irreversible damage of the LiCoO2 cathode without any observable change in electrolyte composition. Spectral analysis further revealed distinct reaction mechanisms for temperature-induced EC decomposition and EC decomposition associated with SEI formation.

Investigation of the Passivation Properties of the Solid Electrolyte Interphase Using a Soluble Redox Couple

The solid electrolyte interphase (SEI) that forms at carbonaceous anodes makes Li-ion battery a viable technology because it inhibits solvent-decomposition reactions. However, passivation is never complete and SEI “leakage” appears as the main contributor to Li-ion battery aging. There has been a great deal of experiments focusing on the chemical analysis of SEIs over the past decades. Still, a direct evaluation of their passive character has not been much regarded. In this work, SEIs formed cathodically on glassy carbon electrodes are characterized using the rotating disk electrode method and ferrocene/ferrocenium as a redox shuttle, as originally proposed in Tang and Newman [M. Tang and J. Newman, This journal, 158(5), A530-A536 (2011)]. A comprehensive model is developed that accounts for transport of soluble redox species across the diffusion layer and the porous SEI as well as charge-transfer kinetics at the modified electrode surface. From a model analysis of electrodes with SEIs formed in various conditions, values of SEI porosity and effective rate constant of ferrocenium reduction are derived and discussed. First attempts are conducted to extend the method to SEIs formed at graphite composite electrodes. Preliminary results suggest SEIs are less passivating than those on glassy carbon.

Electrodeposition of Cu–In alloys from a choline chloride based deep eutectic solvent for photovoltaic applications

Electrodeposition and post-annealing is a promising low-cost approach to the growth of Cu(In,Ga)Se2 for photovoltaic applications. However, In and Ga electrodeposition is not facile from aqueous electrolytes. Efficient electrodeposition can be achieved by using deep eutectic solvent (DES) based electrolytes since solvent decomposition reactions (such as hydrogen evolution reaction in water) do not occur.In this work, the electrochemical behavior of InCl3 and CuCl2–InCl3 on the DES 1:2 choline chloride:urea (ChCl:U) on a Mo rotating disk electrode is studied via cyclic voltammetry. The deposited Cu–In thin films are characterized by scanning electron microscopy (SEM) and energy dispersive X-ray spectroscopy (EDX) in terms of morphology and composition and X-ray diffraction (XRD) regarding crystal structure. The cyclic voltammetric studies on the ChCl:U–CuCl2–InCl3 system showed multiple cathodic and anodic responses, which were assigned to various crystalline phases. The crystalline phases, composition and morphology of the deposits were potential dependent. Cu–In intermetallics formation is studied as a function of deposition potential and electrolyte composition. Cu–In metal precursors were thermally annealed under selenium pressure to form CuInSe2.

Multimodal Physics-Based Aging Model for Life Prediction of Li-Ion Batteries

An isothermal, multimodal, physics-based aging model for life prediction of Li-ion batteries is developed, for which a solvent-decomposition reaction leading to the growth of a solid electrolyte interphase (SEI) at the carbonaceous anode material is considered as the source of capacity fade. The rate of SEI film growth depends on both solvent diffusion through the SEI film and solvent-reduction kinetics at the carbon surface. The model is able to simulate a wide variety of battery aging profiles, e.g., open-circuit and constant-voltage storage, charge/discharge cycling, etc. An analysis of capacity-fade data from the literature reveals that the same set of aging parameters may be used for predicting cycling and constant-voltage storage. The use of this set of parameters for predicting storage under open-circuit voltage points out that part of the self-discharge is reversible.

Surface films formed on SnO2 anode in lithium secondary batteries by FTIR spectroscopy

The chemical composition of the passivating layer formed on nano SnO2 anodes in 1 M LiClO4 + (ethylene carbonate)EC + (dimethyl carbonate)DMC at different charge/discharge states in lithium secondary batteries was studied using extra reflectance FTIR spectra. Results show that solvent decomposition reaction that generally occurs on the surface of carbon and alkali metal electrodes also takes place on nano-SnO2 anode, and the major constituent of the passivating layer is Li2CO3 and ROCO2Li. Formation of the passivating layer would certainly lead to the irreversible capacity loss.

Electrochemical performance of nano-Pt-supported carbon anode for lithium ion batteries

A nano-Pt-supported carbon anode was prepared by supporting Pt nanoparticles onto carbon powder. Ultrafine Pt nanoparticles could be well distributed on the surface of carbon particles. Transmission electron microscopy (TEM), scanning electron microscopy (SEM), energy dispersive X-ray spectroscopy (EDX), cyclic voltammetry (CV), FTIR, and impedance spectroscopy were used to study on the electrode structure and electrochemical performance. Nano-Pt-supported carbon anode enhanced the Li discharge reaction and suppressed the solvent decomposition reaction, which are favorable for lithium batteries.

Deposition of calcium phosphate coatings with defined chemical properties using the electrostatic spray deposition technique

The electrostatic spray deposition (ESD) technique was used for biomedical purposes in order to deposit calcium phosphate (CaP) coatings onto titanium substrates. The relationship between various deposition parameters and the chemical properties of deposited coatings was investigated in order to be able to deposit CaP coatings with tailored chemical characteristics. The results showed that the chemical properties of the coatings were determined by both physical, apparatus-related factors and chemical, solution-related parameters. By varying the processing parameters of the technique, several crystal phases and phase mixtures were obtained, ranging from carbonate-free phases such as meta- and pyrophosphates, monetite and various tricalcium phosphates to carbonate-containing phases such as various carbonate apatites and calcite. On the basis of these results, a chemical mechanism of coating formation was proposed. Essentially, the deposition of the various crystal phases was the result of an acid–base reaction between basic CO 3 2− groups (originating from solvent decomposition reactions) and acidic HPO 4 2− groups from an intermediate monetite (CaHPO 4) phase of the CaP precipitate. The amount of carbonate incorporation (ranging from 0 to 15 wt%) determined the crystal and molecular structure of the deposited coatings.

The study of surface films formed on SnO anode in lithium rechargeable batteries by FTIR spectroscopy

Fourier-transform infrared (FTIR) spectra of nanometer SnO anodes at different charge/discharge states in lithium rechargeable batteries were recorded. Solvent decomposition reaction that generally presents on the surface of carbon or alkali metal is also presented on SnO anode. The surface film is composed of Li 2CO 3 and ROCO 2Li. In addition, FTIR data indicates that absorption bands of ROCO 2Li are changed below 0.7 V, which is believed to be related to the change of the surface layer of SnO anode around 0.7 V versus Li/Li + during the first discharge process.

An X-ray photoelectron spectroscopy study on the surface film on carbon black anode in lithium secondary cells

The property of the surface film formed on the carbon black anode of a lithium cell at the initial reduction stage was investigated. About 90% of the carbon black surface was covered with a smooth film about 10–15 Å in thickness after five cyclic voltammetry cycles. The X-ray photoelectron spectroscopy analysis revealed that this film contained oxygen atom bonding directly to the carbon atoms and was formed by decomposition of the solvent. The inhibiting effect of the surface film against further solvent decomposition reaction disappeared after washing with ethanol or heat treatment above 200 °C. The surface film was confirmed to form also by chemical reduction using lithium naphthalide, where lithium insertion in the carbon black took place. The inhibiting effect of a chemically formed surface film against the solvent decomposition was less marked than that of an electrochemically formed film.

The anodic dissolution of chalcopyrite in water + acetonitrile solutions

The characteristics of chalcopyrite anodic electrodissolution in aqueous acetonitrile solutions have been studied. Experiments with a high-grade sample, of massive chalcopyrite have been carried out at 25°C in solutions containing 50 g l −1 NaCl and different concentrations of sulphuric acid and acetonitrile. Galvanostatic and potentiostatic conditions were employed and the results obtained in solutions with and without acetonitrile compared. The characteristics of the mechanism involved in the chalcopyrite electrodissolution precluded acetonitrile having an influence at low potentials. However its influence is more noticeable at higher potentials where solvent decomposition reactions are involved. Chronopotentiometric experiments in acetonitrile aqueous solutions indicate the existence of diffusional (in the solid) and coulombic phenomena in originating the potential jump.

The anodic dissolution of chalcopyrite in water + acetonitrile solutions

The characteristics of chalcopyrite anodic electrodissolution in aqueous acetonitrile solutions have been studied. Experiments with a high-grade sample, of massive chalcopyrite have been carried out at 25°C in solutions containing 50 g l −1 NaCl and different concentrations of sulphuric acid and acetonitrile. Galvanostatic and potentiostatic conditions were employed and the results obtained in solutions with and without acetonitrile compared. The characteristics of the mechanism involved in the chalcopyrite electrodissolution precluded acetonitrile having an influence at low potentials. However its influence is more noticeable at higher potentials where solvent decomposition reactions are involved. Chronopotentiometric experiments in acetonitrile aqueous solutions indicate the existence of diffusional (in the solid) and coulombic phenomena in originating the potential jump.

Cathodic decomposition reactions of propylene carbonate

The cathodic decomposition of propylene carbonate was investigated. Whereas at graphite the reaction occurs with reasonable high current densities predominantly via the formation of alkali metal intercalation compounds (EC mechanism) at active electrode surfaces such as e.g. sintered nickel, copper- or steel-felt, the reaction occurs at rather small current densities by direct electrochemical reduction. In both cases the reaction products are propene and carbonate. An interpretation of the observed effects due to reactions of impurities such as water can be excluded. Solvent decomposition reactions have to be taken into account for the use of solvents as e.g. PC in lithium batteries.

Cathodic decomposition reactions of propylene carbonate

The cathodic decomposition of propylene carbonate was investigated. Whereas at graphite the reaction occurs with reasonable high current densities predominantly via the formation of alkali metal intercalation compounds (EC mechanism) at active electrode surfaces such as e.g. sintered nickel, copper- or steel-felt, the reaction occurs at rather small current densities by direct electrochemical reduction. In both cases the reaction products are propene and carbonate. An interpretation of the observed effects due to reactions of impurities such as water can be excluded. Solvent decomposition reactions have to be taken into account for the use of solvents as e.g. PC in lithium batteries.