Decomposition Of Ethylene Carbonate Research Articles

Revisit coupling of CO2 to ethylene carbonate with an integrated imidazolium and zinc halides catalyst by a study on its decomposition: Active center and mechanism

Coupling of carbon dioxide (CO2) to cyclic carbonates with an epoxide and further into linear carbonates as the indispensable solvents for the lithium-ion batteries has been being one of the hottest topics in transforming CO2 into high value-added chemicals. Nevertheless, the extremely ultrahigh purity requirement for them causes a low efficiency and a modest yield due to the undesired reactions and byproducts occurring in the separation and purification sections. In this work, a novel imidazolium based ionic liquid catalyst system with different anions and cations was studied by a thorough insight into the decomposition reaction of ethylene carbonate (EC), which reveals the synergistic mechanism of the composite catalysts both in section of cyclic carbonate separation from the catalyst for recirculated utilization and purification from the byproducts, and in section of synthesis with high efficiency. Effects of imidazolium cations and halogen anions in the ionic liquid molecule on EC decomposition were investigated by the elaborately designed experiments, thermodynamic estimations, molecular dynamics simulation and DFT assessment. It was found that combining imidazole salts and zinc halides can greatly boost the activity of EC synthesis and decomposition with the effective structure of catalytic center being [EMIm]ZnX4. Furthermore, it was indicated that Br- has a higher activity for EC synthesis and its reversed pyrolysis, while Cl- will promote a side reaction of ring-opening polymerization of EC. Finally, a most favorable catalyst composition with ZnBr2 and [EMIm]Br for EC synthesis was achieved with a high efficiency in synthesis and a low tendency to initiate the side reactions in separation section.

Catalytic decomposition of ethylene carbonate by pyrrolic-N to stabilize solid electrolyte interphase on graphite anode under extreme conditions

Despite the dominated anode materials of graphite for lithium-ion batteries (LIBs), the poor rate capability and inferior cyclability restain their applications under extreme conditions due to their deficient interfacial chemistry. Herein, an ultrathin but Li2CO3-rich solid electrolyte interphase (SEI) is manipulated by the decomposition of ethylene carbonate (EC) on graphite anode with nitrogenous amorphous carbon coating (denoted as GMIB-2P). Specifically, Li2CO3 can enable a decreased charge transfer resistance and faster Li+ diffusion capability. In addition, we demonstrate the catalytic effect of pyrrolic-N on graphite surface with prominent wettability induces the rapid decomposition of EC with low reaction energy barrier through density function theory (DFT) calculation. Consequently, GMIB-2P delivers an outstanding capacity retention ratio of 90.82% after 300 cycles at 1 C and an improved initial Coulombic efficiency (ICE, 90.77% vs. 85.35%). More importantly, GMIB-2P possesses durable-cycling capability even with lean electrolytes of 5 μL mg-1 and as well as 85.68% capacity retention for over 300 cycles at 60 ℃. Our work highlights the key role of EC in tailoring interfacial chemistry from structure regulations and provides new guidelines to design more reliable anodes for LIBs under extreme conditions.

Role of Oxygen in the Formation of the Solid-Electrolyte Interphase Evaluated by Online Electrochemical Mass Spectrometry and Electrochemical Atomic Force Microscopy

Solid-electrolyte interphase (SEI) is a passive film formed on the anode surface of Li-ion batteries by decomposing organic electrolyte solutions during the initial charging process. It plays an essential role in the stability and cyclability of Li-ion batteries. We found that a small amount of oxygen can significantly influence the SEI formation process and its properties on the carbon anode surface of Li-ion batteries in ethylene carbonate (EC)-based electrolyte solution. Herein, a quantitative evaluation of the effect of oxygen concentration was conducted by detecting volatile product evolution during SEI formation by online mass spectrometry. In addition, electrochemical atomic force microscopy was employed to observe the morphological features of the electrode surface during the SEI formation process in argon and oxygen atmospheres. Furthermore, attenuated total reflection Fourier transform infrared spectroscopy was used to investigate the chemical composition of the SEI. Our results show that the presence of oxygen strongly suppressed the generation of ethylene (C2H4) molecules during SEI formation. We propose a novel mechanism for SEI formation in the oxygen-containing EC electrolyte. In the presence of oxygen, the oxygen reduction reaction (ORR) generates superoxide ions which can induce EC decomposition and form a loose SEI film on the electrode surface. As the potential becomes more negative, EC molecules can be electrochemically reduced. The decomposition products can deposit on the defects and voids of the loose SEI film formed during the ORR stage, making the SEI film thicker and denser.

Polysulfide cluster formation, surface reaction, and role of fluorinated additive on solid electrolyte interphase formation at sodium-metal anode for sodium-sulfur batteries.

Room-temperature sodium-sulfur batteries are promising next-generation energy storage alternatives for electric vehicles and large-scale applications. However, they still suffer from critical issues such as polysulfide shuttling, which inhibit them from commercialization. In this work, using first-principles methods, we investigated the cluster formation of soluble Na2S8 molecules, the reductive decomposition of ethylene carbonate (EC) and propylene carbonate (PC), and the role of fluoroethylene carbonate (FEC) additive in the solid electrolyte interphase formation on the Na anode. The clustering of Na2S8 in an EC solvent is found to be more favorable than in a PC solvent. In the presence of an electron-rich Na (001) surface, EC decomposition undergoes a two-electron transfer reaction with a barrier of 0.19eV for a ring-opening process, whereas PC decomposition is difficult on the same surface. Although the reaction kinetics of an FEC ring opening in the EC and PC solvents are quite similar, the reaction mechanisms of the open FEC are found to be different in each solvent, although both lead to the production of NaF on the surface. The thick NaF layers reduce the extent of charge transfer to Na2S8 at the anode/electrolyte interface, thus decelerating the Na2S8 decomposition reaction. Our results provide an atomistic insight into the interfacial phenomena between the Na-metal anode surface and electrolyte media.

Influence of ligands on lithium salt properties: A comprehensive approach to the structure-function relationship

Complexes of lithium bis(oxalato)borate (LiBOB), lithium difluoro(oxalato)borate (LiODFB) and lithium difluorobis(oxalato) phosphate (LiDFBOP) are the promising additives for advanced lithium ion batteries because of their high reactivity and easy synthesis, in which the changeable central atom and ligand bring diverse properties for salts. In this study, their structure-function relationship has been surveyed by exploring the reduction pathways. According to the analysis of experiments and quantum chemistry calculations, we have established that the steric-hindrance effect of ligands is the key effect on the bond-broken of these lithium salts. Specifically, more oxalate ligands are beneficial to the breaking of C–O and the bond of the central atom (M) with O atom, resulting in CO and CO2 release as well as the generation of MO2-type soluble products. The as-generated MO2-type soluble products undergo a further self-aggregate process to form macromolecular complexes. In contrast, fewer oxalate ligands just encourage the bond breaking of the M − O bond, leading to CO2 release and the generation of MF2-type soluble products. In addition, the universal promoting effect of soluble products on ethylene carbonate decomposition is verified. These results offer theoretic support for the molecular design of advanced functional chelae lithium salts.

Decomposition Pathways of EC and Effects of Additives: Accounting for Liquid, Gas, and Solid Phase Reactions during the First Charge

The majority of lithium-ion batteries (LIBs) utilize carbonate-based electrolytes, due to the good lithium solvation, electrochemical stability, and electrode passivation.1 Studies regarding decomposition pathways and other reaction mechanisms2-4 of carbonates in LIBs are abundant in the literature but few attempt to quantify total losses of these solvents. Quantitative measurement of solvent loss from the electrolyte would significantly inform the comprehensive picture of electrolyte reaction mechanisms in the cell. Furthermore, this approach allows for the study of any effects additives may have on carbonate solvent decomposition. For example, organosilicon (OS) additives have been shown to reduce gassing and improve high temperature cycling in carbonate-based LIBs,5 but their quantitative impact on EC decomposition is not known.In this poster, we detail our methodology for quantitatively tracking and characterizing different forms of ethylene carbonate (EC) decomposition in a LIB pouch cell as well as the effects of organosilicon additives on these reactions. This study focused on the loss of EC after the first charge when the majority of SEI formation occurs. Liquid phase composition, including both EC and its decomposition products, was quantified by NMR analysis (1H and 19F) of extracted electrolyte using internal standards (LiPF6 and 1,4-bis(trifluoromethyl)benzene). Gas phase decomposition was characterized by a combination of the Archimedes method (to quantify gas volume) and GC-MS analysis (to determine gas composition). Finally, the impact of OS additives on the decomposition pathways of EC was examined. Inclusion of OS molecules reduces EC decomposition during the first charge, predominately by reducing the amount of liquid phase decomposition. 1Xu, K. Chem. Rev. 2004, 104, 4303-4417. 2 Campion, C.; Li, W.; Lucht, B. Thermal Decomposition of LiPF6-Based Electrolytes for Lithium-Ion Batteries. Journal of the Electrochemical Society 2005, 152, A2327. 3 Seo, D.; Chalasani, D.; Parimalam, B.; Kadam, R.; Nie, M.; Lucht, B. Reduction Reactions of Carbonate Solvents for Lithium Ion Batteries. EC Electrochemistry Letters 2014, 3, A91-A93. 4 Xing, L.; Li, W.; Wang, C.; Gu, F.; Xu, M.; Tan, C.; Yi, J. Theoretical Investigation on Oxidative Stability of Solvents and Oxidative Decomposition Mechanism of Ethylene Carbonate for Lithium Ion Battery Use. J. Phys. Chem. 2009, 113, 16596-16602. 5 Guillot, S.L.; Usrey, M.L.; Peña-Hueso, A.; Kerber, B.M.; Zhou, L.; Du, P.; Johnson, T. Reduced Gassing in Lithium-Ion Batteries with Organosilicon Additives. J. Electrochem. Soc. 2021, 168, 030533-030543.

Understanding Mechanisms of Ethylene Carbonate Gassing and Gas-Reducing Additives Using Isotopically Labeled Solvents

Gas generation during Li-ion battery operation remains a problem that can affect both safety and operational lifetime of the battery. Electrolyte solvents are a key source of gas generation1 and while gas-reducing additives have been identified, the underlying mechanisms require more study to fully understand the role between solvent structure and gas generation. Understanding of these gassing mechanisms can lead to optimized electrolyte systems with reduced gassing for the next generation of Li-ion batteries. Previous studies in the literature2 have detailed the use of 13C-labeled EC and DEC to identify the proportions of CO2 and CO that are derived from solvent sources, which in turn can help inform mechanistic understanding on gas generation in Li-Ion Batteries.As previously studied by Silatronix®, electrolytes containing organosilicon (OS) materials show significantly reduced gas generation during high temperature storage and cycling in pouch cells primarily through CO2 reduction.3 Studies utilizing simple carbonate blends indicated that EC was the primary source of CO2; therefore, it was hypothesized that OS materials reduce gassing by preventing EC decomposition during high temperature testing. In this work, 13C-labeling was used to identify the sources of gas species in ternary carbonate electrolytes after both first charge (SEI formation4) and high temperature storage (extended aging) to allow the effects of electrolyte additives gas reduction to be elucidated.Gas component analysis was performed using a calibrated GC-MS, providing identification and quantification of each gas species (labeled and unlabeled). These data give insight into how the labeled carbonate solvent may decompose into the observed gas phase products during the first charge (i.e., during SEI layer formation) and high temperature storage. Ethylene carbonate (EC) is found to be the primary source of ethylene generation, as well as a significant source of CO2 and CO. With the addition of OS, the most notable effect is that the previously seen reduction in all gas components is due to not only reduction of EC-related sources, but reduction in non-EC gas sources as well. 1 Rowden, B.; Garcia-Araez, N., A Review of Gas Evolution in Lithium-Ion Batteries. Energy Rep. 2020, 6, 10-18. 2Onuki, M.; Kinoshita, S.; Sakata, Y.; Yanagidate, M.; Otake, Y.; Ue, M.; Deguchi, M., Identification of the Source of Evolved Gas in Li-Ion Batteries using 13C-labeled Solvents. Journal of the Electrochemical Society 2008, 155, A794. 3 Guillot, S.L.; Usrey, M.; Peña-Hueso, A.; Kerber, B.; Zhou, L.; Du, P.; Johnson, T., Reduced Gassing In Lithium Ion Batteries With Organosilicon Additives. Journal of the Electrochemical Society 2021, 168, 030533. 4 Ktristiina Heiskanen, S.; Kim, J.; Lucht, B., Generation and Evolution of the Solid Electroyte Interphase of Li-Ion Batteries. Joule 2019, Volume 3 Issue 10, 2322.

Experimental and modeling study on pyrolysis of ethylene carbonate/dimethyl carbonate mixture

Ethylene carbonate (EC) and dimethyl carbonate (DMC) are important chemical substances of the lithium-ion battery (LIB) electrolytes. Although the carbonate esters are regarded as a cause of the LIB fires, there are few studies on the pyrolysis/combustion characteristics of EC. This study aims to investigate the pyrolysis characteristics of EC by performing theoretical calculations and species measurements, and to propose the first EC pyrolysis sub-mechanism. The theoretical calculations performed for a gas-phase EC decomposition at the G4 level of theory indicate that a dissociation channel producing acetaldehyde and carbon dioxide, EC = CH3CHO + CO2 (R1), is an energetically and entropically preferred decomposition reaction channel. The rate constant of R1 calculated in the present study is 7.4 times higher than that of the CO2 elimination reaction of DMC, which produces dimethyl ether, DMC = DME + CO2, at 1000 K. Species measurements were performed for the pyrolysis of EC/DMC/N2 mixtures at atmospheric pressure using a time-of-flight mass spectrometer and a gas chromatograph connected to a micro flow reactor with a controlled temperature profile. Oxygenates (EC, DMC, formaldehyde, acetaldehyde and DME), inorganic species (H2, CO and CO2) and hydrocarbons (CH4, C2H2, C2H4 and C2H6) were measured. Experimental results indicate that the consumption of EC occurs at a lower temperature range (Tw = 1000–1050 K) than that of DMC (Tw = 1050–1100 K), as was expected from the theoretical calculations. The measured acetaldehyde showed a rapid increase at Tw = 1000–1050 K, implying that acetaldehyde can be a good marker for the EC decomposition. One-dimensional computations using the present model involving the EC pyrolysis sub-mechanism reproduced the measured results well. Based on the reaction path analysis, R1 accounts for approximately 95% of the contribution to EC consumption at 1080 K, where 30% of EC is consumed.

Investigating the dominant decomposition mechanisms in lithium-ion battery cells responsible for capacity loss in different stages of electrochemical aging

Cell aging is a major issue in battery cells, as it affects the application capabilities. The mechanisms contributing to aging and capacity loss are not yet fully understood, so that performance enhancements are difficult to achieve. Here, the decomposition mechanisms responsible for capacity loss in LiNi0.6Co0.2Mn0.2O2 (NCM622)/graphite lithium-ion pouch cells containing 1 M LiPF6 in ethylene carbonate (EC)/dimethyl carbonate (DMC) 1:1 by weight with 3 wt% vinylene carbonate (VC) are analyzed. For this purpose, absolute amounts of the electrolyte components are determined in cells at six stages of electrochemical aging using High Performance Liquid Chromatography. The resulting, absolute consumptions of the electrolyte components reveal the dominant degradations. Furthermore, complementary analysis methods, namely X-ray photoelectron spectroscopy, gas chromatography, scanning electron microscopy, energy-dispersive X-ray spectroscopy, and inductively coupled plasma optical emission spectrometry are applied. Two phases of electrochemical aging are identified: During formation and short-term cycling, preferential decomposition of EC and VC is observed accompanied by solid electrolyte interphase (SEI) buildup at the graphite particle edges. During long-term cycling, non-preferential decomposition of each electrolyte component is found associated with SEI growth at edges and basal planes of the graphite particles induced by manganese contamination and/or crack formation in the graphite during de-/lithiation.

Quantifying Absolute Amounts of Electrolyte Components in Lithium-Ion Cells Using HPLC

Capacity loss and solid electrolyte interphase (SEI) buildup are accompanied by continuous electrolyte consumption due to electrochemical decomposition reactions[1],[2],[3]. The quantitative analysis of electrolytes in battery cells usually consists in the determination of concentrations or quantity ratios. As the total electrolyte amount in the cell decreases during cell aging due to electrochemical decompositions, concentrations do not reveal reliable information about the dominant decomposition reactions. Hence, the total electrolyte mass and the absolute consumption of the electrolyte components need to be determined to get reliable insights into cell chemistry. For this purpose, a method for electrolyte extraction from pouch cells and subsequent analysis by high-performance liquid chromatography (HPLC) coupled to an electrospray ionization mass spectrometer (ESI/MS) and an ultraviolet/visible light (UV/Vis) detector is presented that enable a quantification of the absolute amounts of the electrolyte components in battery cells[4].The investigated pouch cells are initially filled with 1 M LiPF6 in ethylene carbonate (EC)/dimethyl carbonate (DMC) 1:1 by weight with 3 wt% vinylene carbonate (VC). For the electrolyte extraction, diethyl carbonate (DEC) is injected into the cell, followed by a sealing of the puncture and a storage of the cell for 8 days to enable an intermixing between DEC and the original electrolyte. Afterwards, the electrolyte is extracted through a puncture in the cell. The UV/Vis detector is used for the determination of the concentrations of EC, DMC, VC, and DEC in the extracts, while the quantification of Li+ and PF6 − is performed by the ESI/MS. Based on these concentrations and the weighed mass of the injected DEC, the total mass of the original electrolyte and the absolute amounts of EC, DMC, VC, Li+, and PF6 − in the cell is determined. The investigated electrolytes are extracted from fresh cells, cells after formation, 25 cycles, and about 2000 cycles (with a remaining capacity of 80%). Based on the absolute amounts of EC, DMC, VC, Li+, and PF6 − in the cells at different aging stages, the absolute consumption of the electrolyte components by electrochemical decomposition is eventually calculated.From the concentrations shown in Figure (a), only a VC decomposition during formation and cycling can be concluded. The consideration of the absolute, consumed amounts of substance during formation and long-term cycling in Figure (b), however, suggests that the EC decomposition is dominant and thus much more pronounced than the VC degradation. Besides, a slight salt and DMC decomposition during long-term cycling is observed. Simultaneously, the quantified electrolyte mass is diminished during formation and cycling, as can be seen in Figure (c).The pronounced VC consumption during formation and cycling is expected, as VC is an additive for the SEI buildup. As a high fraction of the initially present VC is decomposed, its decomposition can be identified by the concentrations. The decomposition of the other components cannot be detected based on the concentrations, as lower fractions of the initially present amounts are consumed and the total electrolyte amount in the cell decreases during formation and cycling. The strong EC degradation might be related to the tendency of EC to contribute to the Li+ solvation shell, which is associated with a preferential reduction[5]. These results demonstrate that only the determination of the absolute consumed amounts of substance allows for identification of the decompositions in battery electrolytes. Figure: (a) Overview of the average concentrations of EC, DMC, and VC (quantified by HPLC-UV/Vis), as well as the average concentrations of Li+ and PF6 − (quantified by HPLC-ESI/MS) in cells at different stages of electrochemical aging[4]. (b) The amounts of substance consumed during formation and prolonged cycling altogether determined by HPLC analyses[4]. (c) The electrolyte masses in the fresh cells, the cells after formation, and the cells after prolonged cycling determined by HPLC analyses related to the corresponding initial electrolyte masses[4].

Ethylene Carbonate Adsorption and Decomposition on Pristine and Defective ZnO(101̅0) Surfaces: A First-Principles Study

Fundamental understanding of the reactivity between coating material of Li-ion battery cathode and electrolyte is important in order to obtain suitable coating candidates. Herein, we study ethylene carbonate (EC) adsorption and decomposition reactions on pristine, O vacancy- and Zn vacancy-defected ZnO (10-10) by means of first-principles density functional theory (DFT) calculations. Possible decomposition pathways via H-abstraction and EC ring-opening reaction that leads to the generation of CO2 and C2H4 gases are studied from the thermodynamic and kinetic aspects. Firstly, we find that molecular EC preferably adsorbs on both pristine and defective ZnO (1010) via the bonding between its carbonyl oxygen (OC) and surface Zn. Secondly, subsequent decomposition reactions show large tendency of EC to decompose on both pristine and defective ZnO (10-10). This tendency is indicated by the large thermodynamic driving forces to decompose EC that range from -1.5 eV to -2.5 eV on both pristine and defective ZnO (10-10) (calculated with respect to EC gas phase). The large tendency of EC to decompose, however, is hindered by the high activation barriers of the EC decomposition, shown by the lowest activation barrier of 0.96 eV on Zn vacancy defected ZnO (10-10). Our results thus indicate that EC decomposition on ZnO (10-10) is mainly hindered due to its slow rate of decomposition instead of the thermodynamic factors.

Quantifying Absolute Amounts of Electrolyte Components in Lithium-Ion Cells Using HPLC

To quantify absolute amounts of electrolyte components in lithium-ion cells, we developed a method for electrolyte extraction from pouch cells using a diluent and subsequent analysis by high-performance liquid chromatography (HPLC) coupled to an electrospray ionization mass spectrometer and an ultraviolet/visible light detector. From LiNi1/3Co1/3Mn1/3O2 (NCM111)/graphite lithium-ion pouch cells containing 1 M LiPF6 in ethylene carbonate (EC)/dimethyl carbonate (DMC) 1:1 by weight with 3 wt% vinylene carbonate (VC), electrolytes were extracted using diethyl carbonate as diluent and HPLC analyses were subsequently performed to investigate electrolyte decomposition upon electrochemical aging. Complementary analysis methods, namely X-ray photoelectron spectroscopy, gas chromatography, scanning electron microscopy, energy-dispersive X-ray spectroscopy, and inductively coupled plasma optical emission spectrometry were applied. During formation, only VC decomposition and associated deposition of polymerized VC on the anode was identified. However, after long-term cycling, EC decomposition was dominant. Furthermore, LiPF6 decomposition and deposition of LiF on the anode were detected. Electrochemically untreated cells showed no change in electrolyte composition during storage. Unlike analyzing relative values (concentrations/quantity ratios), where only VC decomposition is detected, our approach determining absolute amounts reveals more details and allows to identify decomposition mechanisms in electrolytes upon electrochemical aging.

First-Principles Simulations for the Surface Evolution and Mn Dissolution in the Fully Delithiated Spinel LiMn2O4.

The interfacial stability between the cathode and electrolyte is an essential issue in the development of high-energy-density and long-life lithium-ion batteries. The deterioration of capacity dominated by Mn dissolution makes LiMn2O4 a representative case for studying the evolution of interfaces. Here, we use the ab initio molecular dynamics (AIMD) method to simulate the interface reaction between the ethylene carbonate (EC) molecules and the (110) surface of completely delithiated LiMn2O4 where most severe Mn dissolution is observed in the experiment. It is found that the intrinsic oxygen loss on the surface will drive the initial migration of surface Mn atoms to the electrolyte while reducing them. The EC molecules will decompose after transferring electrons to the surface Mn atoms, and its decomposition products further promote the Mn dissolution. In addition, oxygen loss and EC decomposition are in a competitive relationship when transferring electrons to the surface Mn atoms. This work provides a complete picture of the step-by-step dissolution of Mn atoms along with the interfacial evolution in the spinel LiMn2O4 system and also provides a scope for the study of transition-metal dissolution in other cathode materials and electrolytes.

Effect of Fluoroethylene Carbonate Additives on the Initial Formation of the Solid Electrolyte Interphase on an Oxygen-Functionalized Graphitic Anode in Lithium-Ion Batteries.

The formation of a solid electrolyte interphase (SEI) at the electrode/electrolyte interface substantially affects the stability and lifetime of lithium-ion batteries (LIBs). One of the methods to improve the lifetime of LIBs is by the inclusion of additive molecules to stabilize the SEI. To understand the effect of additive molecules on the initial stage of SEI formation, we compare the decomposition and oligomerization reactions of a fluoroethylene carbonate (FEC) additive on a range of oxygen-functionalized graphitic anodes to those of an ethylene carbonate (EC) organic electrolyte. A series of density functional theory (DFT) calculations augmented by ab initio molecular dynamics (AIMD) simulations reveal that EC decomposition on an oxygen-functionalized graphitic (112̅0) edge facet through a nucleophilic attack on an ethylene carbon site (CE) of an EC molecule (S2 mechanism) is spontaneous during the initial charging process of LIBs. However, decomposition of EC through a nucleophilic attack on a carbonyl carbon (CC) site (S1 mechanism) results in alkoxide species regeneration that is responsible for continual oligomerization along the graphitic surface. In contrast, FEC prefers to decompose through an S1 pathway, which does not promote alkoxide regeneration. Including FEC as an additive is thus able to suppress alkoxide regeneration and results in a smaller and thinner SEI layer that is more flexible toward lithium intercalation during the charging/discharging process. In addition, we find that the presence of different oxygen functional groups at the surface of graphite dictates the oligomerization products and the LiF formation mechanism in the SEI.

A Computational Comparison of Ether and Ester Electrolyte Stability on a Ca Metal Anode

Ca-ion batteries (CIBs) have the potential to provide inexpensive energy storage, but their realization is impeded by the lack of suitable electrolytes. Motivated by recent experimental progress, we perform ab initio molecular dynamics simulations to investigate early decomposition reactions at the anode-electrolyte interface. By examining different combinations of solvent—tetrahydrofuran (THF) or ethylene carbonate (EC)—and salt—Ca(BH 4 ) 2 , Ca(BF 4 ) 2 , Ca(BCl 4 ) 2 , and Ca(ClO 4 ) 2 —we identify a variety of behavioral trends between electrolyte solutions. Next, we perform a separate trajectory with pure THF and gradually increased negative charge; despite an addition of -32 e , no THF decomposition is detected. Charge analysis reveals that in a reductive environment, THF distributes excess charge evenly across its hydrocarbon backbone, while EC concentrates charge on its ester oxygens and carbonyl carbon, resulting in decomposition. Graphs of charge vs. time for both solvents reveal that EC decomposition products can be reduced by up to five electrons, while those of THF are limited to a single electron. Ultimately, we find Ca(BH 4 ) 2 and THF to be the most stable solution investigated herein, corroborating experimental evidence of its suitability as a CIB electrolyte.

Gas Evolution in Li-Ion Batteries: Modeling Ethylene Carbonate Decomposition on LiCoO2 in the Presence of Surface Magnetism

Decomposition of liquid electrolytes used in Li-ion batteries can result in the formation of gaseous species and the release of flammable solvent vapor. These undesirable “gassing” reactions can be mediated by interactions between the electrode surfaces and the electrolyte. At present, the detailed mechanisms responsible for electrolyte gassing are not well understood. Here, first-principles calculations are used to characterize the thermodynamics and kinetics of potential gas-forming reactions that involve surface-mediated decomposition of ethylene carbonate (EC)—a commonly used electrolyte solvent—at the (101̅4) surface of a LiCoO2 (LCO) cathode. The magnetic ordering of the (101̅4) surface was explicitly taken into account. Comparisons of EC adsorption on nonmagnetic, ferromagnetic, and antiferromagnetic surface orderings reveal differences in the adsorption geometry and reaction energetics. The antiferromagnetic surface exhibited the lowest surface energy overall; EC adsorption on this surface preferentially occurs on Li sites. In contrast, the nonmagnetic surface exhibits a stronger attraction for EC—adsorption is 0.5 eV more exothermic per molecule than for the antiferromagnetic surface—and adsorption occurs on Co sites. The thermodynamic driving force for EC decomposition on the antiferromagnetic surface was predicted for several potential reaction products resulting from more than 30 bond-breaking scenarios. The most exothermic of these reactions results in the formation of CO2 and acetaldehyde (C2H4O), gaseous products that have been observed in prior experiments. Nevertheless, an evaluation of the minimum energy path for CO2/C2H4O formation on fully lithiated LCO reveals a large reaction barrier for this process, implying a kinetically limited reaction. These data suggest that the rate of solvent decomposition at the cathode may be maximized in the charged state, where LCO is partially delithiated.

The Effect of Epoxy Group on Ethylene Carbonate Decomposition at Carbon Anode of Sodium Ion Batteries: Theoretical Study

Sodium-ion batteries (SIBs) have received much attention as promising alternatives to Li-ion batteries as large scale and low-cost energy storage systems owing to close electrochemical similarity between lithium and sodium, and the natural abundance of sodium resources. However, many challenges must be overcome to make SIBs well-positioned in commercialization such as low cyclability, and low stability of the solid-electrolyte interphase (SEI) formation, results from the decomposition of organic solvents in the electrolyte on the anode of SIBs. The SEI has a profound effect on cycle life and performance of the batteries. Therefore, understanding the SEI compositions and its formation mechanisms is crucial for SIBs development. Carbon-based anode materials are commonly used as the anode for SIBs because of their appropriate electrochemical properties, an abundance of carbon and safety. The oxygen-containing groups often present in the carbon-based anode and they usually actively involved in chemical reactions. In this work, we performed density functional theory (DFT) calculations to elucidate the effect of oxygen containing group of epoxy on decomposition mechanisms at the initial stages of sodiation of ethylene carbonate (EC) molecule which is one of common electrolytes applied in ion-battery. The calculation results indicated that EC decompositions on pristine graphene were initiated by CE-OE bond cleavage which is rate-limiting step followed by Cc-OE bond cleavage in the second step producing CO2 and acetaldehyde as products (Figure 1). The presence of an epoxy group on graphene does not directly change the mechanisms, however, it significantly increased the activation barriers on all decomposition pathways compared to those on pristine graphene. The strong electrostatic interaction between negatively charged epoxy group and positively charged Na ion weakens interaction between EC and carbon surface. Also, the presence of an epoxy group facilitates carbon surface to be more positively charged and electron transfer to EC is less favorable than that on pristine graphene. Furthermore, we investigated the solvation effect on the mechanisms by increase the number of EC molecules to model a solvation shell. The results showed that the inclusion of the electrolyte environment reveals other possible decomposition mechanisms including proton transfer from EC molecule to epoxy group producing hydroxyl group on carbon surface prior to the EC ring-opening reaction step. This work suggests that the presence of oxygenated functional group on anode carbon surface and solvent environment can have significant effects on EC decomposition mechanisms both thermodynamic and kinetic aspects. Including the electrolyte solvation shell is crucial for electrolyte decomposition investigation using molecular modeling. Figure 1

Operando Fourier Transform Infrared Investigation of Cathode Electrolyte Interphase Dynamic Reversible Evolution on Li1.2Ni0.2Mn0.6O2.

One of the keys to the cycling stability of electrode materials is the formation of a stable interface film on cathode materials, which is called a cathode electrolyte interphase (CEI). For a Li/Mn-rich cathode, especially, the high working voltage will cause an extremely unstable electrolyte environment, becoming a challenge for the stable interface film formation. In this work, an operando-attenuated total reflection-Fourier transform infrared (ATR-FTIR) technique is developed to monitor in real time the dynamic mechanism of CEI formation in a carbonate-based electrolyte with or without the moderate additive tris(trimethylsilyl)borate (TMSB), which could promote the formation and stability of high-quality CEI films when it charges to 4.8 V. It is interesting that the components of CEI are basically generated in the first cycle owing to ethylene carbonate (EC) priority decomposition. Besides, the presence of TMSB can suppress the decomposition of EC in part and modify the stability of the CEI film. This is because TMSB containing an electron-deficient boron atom can easily combine with an electron-rich F- and PF6- forming a polyanion group initially, which will weaken the electrostatic force between the anionic groups and EC to reduce the concentration of EC on the cathode surface and prevent the continuous decomposition of EC at a high voltage. X-ray photoelectron spectroscopy also verifies the presence of polyanion groups and their further participation in CEI formation. This work highlights the dynamical stability of CEI modified by moderate TMSB and the formation mechanism of this dynamical change during cycling characterized by the operando ATR-FTIR technique, which paves the way for a better understanding of the complex and hard-characterized cathode interface reactions.

Rational Design of Bian Based Anode Binder/Additive for High Performance Li Ion Secondary Batteries

Bisiminoacenaphthene (BIAN) has been an important ligand structure for a variety of metal complex and widely employed as a ligand for olefin polymerization catalysts. However, in spite of their unique electrochemistry, it has not been widely utilized for energy devices. In the present work, firstly BIAN based novel conjugated polymer was prepared by Sonogashira-coupling polymerization and the obtained polymer (P-BIAN) was employed as anode binder in Li/EC:DEC/C type anodic half cells. Second, novel additive material bearing BIAN structure (BIANODA) was synthesized, which was used as additive to improve the characteristics of MNC cathode material (LiMn1/3Ni1/3Co1/3O2). Electrochemistry of these materials along with characteristics of fabricated Li ion battery cells under these design protocol will be presented in detail. The BIAN based polymer, P-BIAN shows lower LUMO level in comparison with ethylene carbonate (EC). This means reductive doping of P-BIAN takes place prior to reductive decomposition of EC at anode surface, which will restrict thick SEI formation at anode. When cyclic voltammetry measurements were carried out for coin cell with P-BIAN, peak due to reductive decomposition of EC was not observed, unlike the case of PVDF. Further, after charge-discharge cycles, coin cell with P-BIAN exhibited much smaller internal resistance when compared with PVDF. This indicates that reductive doping of P-BIAN and restriction of SEI decomposition had synergistically decreased internal resistance of battery cells. As a result, coin cell with P-BIAN showed 1.5 times higher discharging capacity in comparison with coin cell with PVDF. On the other hand, BIANODA has higher HOMO level than that of EC, which enables oxidative polymerization of BIANODA prior to oxidative decomposition of EC. This will restrict thick SEI formation at MNC cathode surface. Use of Schiff base can trap HF generated during the cycling, and strong binding property of BIAN can stabilize MNC cathode for long term use. Consequently, BIANODA additive had enhanced the discharging performance of MNC/EC:DC/Li cathodic half cells. Both P-BIAN as anodic binder material and BIANODA as MNC cathode additive were found to be remarkably effective to improve the performance of Li ion battery cells.

Quantification and simulation of thermal decomposition reactions of Li-ion battery materials by simultaneous thermal analysis coupled with gas analysis

Differential Scanning Calorimetry (DSC) and Thermal Gravimetry (TGA) combined with gas analysis are used to identify the main decomposition processes and to develop reaction kinetic models for thermal runaway modelling of Graphite – Lithium Nickel–Manganese–Cobalt-Oxide (NMC 111) cells. Heating rates of 5, 10 and 15 °C min−1 with multiple replications are performed to determine the frequency factor, activation energy and the heat of reaction of the different sub-processes. It is found that both the anode and cathode decompose in multiple parallel and consecutive reactions between 5 °C and 600 °C. A double breakdown mechanism of the protecting Solid Electrolyte Interface (SEI) is suggested to describe the anode decomposition reactions. For the cathode, decomposition and evaporation of ethylene carbonate (EC), decomposition of NMC with the liberation of oxygen, combustion of EC with the liberated oxygen, decomposition of binder, decomposition of EC and combustion of carbon additive reactions are identified and modelled. The proposed model can be used to simulate thermal runaway initiation methods in a realistic way.