Theoretical Study of the Reductive Decomposition of Ethylene Sulfite: A Film-Forming Electrolyte Additive in Lithium Ion Batteries

An electrolyte additive for the improved high voltage performance of LiNi0.5Mn1.5O4 (LNMO) cathodes in Li-ion batteries

High-voltage cathode materials are important for the implementation of high-energy-density Li-ion batteries. However, with increasing cut-off voltages, interfacial instabilities between electrodes and the electrolyte limit their commercial development. This study addresses this issue by proposing a new electrolyte additive, (3-aminopropyl)triethoxysilane (APTS). APTS stabilises the interface between the LiNi_0.5Mn_1.5O₄ (LNMO) cathode and the electrolyte in LNMO‖Li half-cells due to its multifunctional character. The amino groups in APTS facilitate the formation of a robust protective cathode layer. Its silane groups improve layer stability by neutralising the electrolyte's detrimental hydrogen fluoride and water. Electrochemical measurements reveal that the addition of 0.5 wt% APTS significantly improves the long-term cycling stability of LNMO‖Li half-cells at room temperature and 55 °C. APTS-addition to the electrolyte delivers excellent capacity retention of 92% after 350 cycles at room temperature and 71% after 300 cycles at 55 °C (1C) contrasting with the much lower performances of the additive-free electrolyte. The addition of a 0.5 wt% (3-glycidyloxypropyl)trimethoxysilane (GLYMO) additive, which contains only the siloxane group, but lacks the amine group, displayed a capacity retention of 73% after 350 cycles at room temperature but degraded significantly upon cycling at 55 °C.

The Impact of Polymer Electrolyte Properties on Lithium-Ion Batteries

In recent decades, the enhancement of the properties of electrolytes and electrodes resulted in the development of efficient electrochemical energy storage devices. We herein reported the impact of the different polymer electrolytes in terms of physicochemical, thermal, electrical, and mechanical properties of lithium-ion batteries (LIBs). Since LIBs use many groups of electrolytes, such as liquid electrolytes, quasi-solid electrolytes, and solid electrolytes, the efficiency of the full device relies on the type of electrolyte used. A good electrolyte is the one that, when used in Li-ion batteries, exhibits high Li+ diffusion between electrodes, the lowest resistance during cycling at the interfaces, a high capacity of retention, a very good cycle-life, high thermal stability, high specific capacitance, and high energy density. The impact of various polymer electrolytes and their components has been reported in this work, which helps to understand their effect on battery performance. Although, single-electrolyte material cannot be sufficient to fulfill the requirements of a good LIB. This review is aimed to lead toward an appropriate choice of polymer electrolyte for LIBs.

Dictating the interfacial stability of nickel-rich LiNi0.90Co0.05Mn0.05O2 via a diazacyclo electrolyte additive - 2-Fluoropyrazine

Nickel-rich (Ni-rich) cathode materials, LiNi_xCo_yMn_zO₂ (NCM, x ≥ 0.9, x + y + z = 1) hold great promise for developing high energy density lithium ion batteries especially for vehicle electrification. However, the practical application of Ni-rich cathode materials is still suffered from fast structural and interfacial degradation, and the resulted capacity decay. In this study, a diazacyclo type electrolyte additive, 2-fluoropyrazine (2-FP), was explored for the first time to boost the interfacial stabilization of single crystal LiNi_0.90Co_0.05Mn_0.05O₂ (NCM90) cathode. The capacity retention of the NCM90 is evidently promoted from 72.3% to 82.1% after 200 cycles at 1C (180 mA g^-1) when adding 0.2% 2-FP into the electrolyte. The improvement of the electrochemical performance is ascribed to the generation of a compact and homogeneous cathode electrolyte interphase (CEI) film through ring-opening electrochemical polymerization of 2-FP upon the NCM90 electrode particles. This enhanced CEI layer benefits the suppression of the decomposition of LiPF₆ electrolyte and the dissolution of the transition metals (Co and Mn), thus preventing the detrimental side reactions between the NCM90 electrode and the electrolyte.

Opportunities and Limitations of Ionic Liquid- and Organic Carbonate Solvent-Based Electrolytes for Mg-Ion-Based Dual-Ion Batteries

Dual-ion batteries (DIBs) offer a great alternative to state-of-the-art lithium-ion batteries, based on their high promises due to the absence of transition metals and the use of low-cost materials, which could make them economically favorable targeting stationary energy storage applications. In addition, they are not limited by certain metal cations, and DIBs with a broad variety of utilized ions could be demonstrated over the last years. Herein, a systematic study of different electrolyte approaches for Mg-ion-based DIBs was conducted. A side-by-side comparison of Li- and Mg-ion-based electrolytes using activated carbon as negative electrode revealed the opportunities but also limitations of Mg-ion-based DIBs. Ethylene sulfite was successfully introduced as electrolyte additive and increased the specific discharge capacity significantly up to 93±2 mAh g-1 with coulombic efficiencies over 99 % and an excellent capacity retention of 88 % after 400 cycles. In addition, and for the first time, highly concentrated carbonate-based electrolytes were employed for Mg-ion-based DIBs, showing adequate discharge capacities and high coulombic efficiencies.

Electrolyte Additives in Lithium Ion EV Batteries and the Relationship of the SEI Composition to Cell Resistance and Lifetime

Sulphur, boron and phosphorous containing electrolyte additives were evaluated in cells containing pristine electrodes from a commercial EV lithium ion cell against a standard baseline electrolyte. Following formation and a full cell ageing step, cycling performance and impedance spectroscopy were used to elucidate the most effective additives. The additive tris trimethyl silyl phosphite (TTSPi) showed the most promise; with improved cell capacities and reduced impedances observed after formation. X-ray photoelectron spectroscopy (XPS) measurements on anode elemental surface profiles were correlated with the electrochemical performance. It was observed that increased lithium fluoride content on the surface of the anodes typically produced cells with lower impedance. Sulphur containing additives also showed improved cell behaviours; and the decomposition and chemical reactions of these compounds at the anode surface is discussed in detail. The main influence of TTSPi was to reduce the amount of oxygen (C=O) and sulphur in the electrolyte interphase (SEI) layer; to be replaced with hydrocarbons.

Electrolyte Therapy for Improving the Performance of LiNi0.5Mn1.5O4 Cathodes Assembled Lithium-Ion Batteries

High voltage spinel manganese oxide LiNi₀.₅Mn_1.5O₄ (LNMO) cathodes are promising for practical applications owing to several strengths including high working voltages, excellent operating safety, low costs, and so on. However, LNMO-based lithium-ion batteries (LIBs) fade rapidly mainly owing to unqualified electrolytes, hence becoming a big obstacle toward practical applications. To tackle this roadblock, substantial progress has been made thus far, and yet challenges still remain, while rare reviews have systematically discussed the status quo and future development of electrolyte optimization coupling with LNMO cathodes. Here, we discuss cycling degradation mechanisms at the cathode/electrolyte interface and ideal requirements of electrolytes for LNMO cathode-equipped LIBs, as well as review the recent advance of electrolyte optimization for LNMO cathode-equipped LIBs in detail. And then, the perspectives regarding the future research opportunities in developing state-of-the-art electrolytes are also presented. The authors hope to shed light on the rational optimization of advanced organic electrolytes in order to boost the large-scale practical applications of high voltage LNMO cathode-based LIBs.

Atomic thermodynamics and microkinetics of the reduction mechanism of electrolyte additives to facilitate the formation of solid electrolyte interphases in lithium-ion batteries

The formation of a solid electrolyte interphase (SEI) between the anode surface and the electrolyte of lithium-ion batteries (LIBs) has been considered to be the most important yet the least understood issue of LIBs. To further our understanding in this regard, the density functional theory (DFT) B3PW91/6-311++G(3df,3pd) together with the implicit solvent model and the transition state theory were used for the first time to comprehensively explore the electroreduction mechanism of a novel additive, 4-chloromethyl-1,3,2-dioxathiolane-2-oxide (CMDO), and a few other solvents and additives, such as ethylene carbonate (EC), propylene carbonate (PC), dimethyl carbonate (DMC), fluoroethylene carbonate (FEC), and even ethylene sulfite (ES), for comparison. The one-electron reduction potential of Li⁺-coordinated compounds Li⁺(X) for forming decomposition precursors [c-Li⁺(X˙⁻)] decreases in the following sequence: CMDO (1.9–2.2 V vs. Li⁺/Li) ∼ ES(1.9 V) > FEC (0.7 V) > EC (0.47 V) > PC (0.45 V) > DMC (0.38 V); this implies that CMDO is reduced prior to other solvents or additives in the mixture. Although the ring opening of [c-Li⁺(CMDO˙⁻)] is the least kinetically favorable, as reflected by the highest energy barrier (E_a), i.e., CMDO (18.8–22.9 kcal mol⁻¹) ∼ ES (23.4) > FEC (16.2) > PC (12.5) > EC (11.2) > DMC (8.0), CMDO still shows the highest overall reaction rate constant (∼10⁵³ s⁻¹) for forming an open ring radical [o-Li⁺(CMDO˙)⁻]. In addition, the termination reaction of [o-Li⁺(CMDO˙)⁻] for forming LiCl is thermodynamically more favorable than that of Li₂SO₃ or organic disulfite (LiSO₃)₂-R, which supports the experimental observation that the halogen-containing LiF or LiCl additives are predominant over all other halogen-containing species in the SEI layer. Moreover, the hybrid model by including the second solvation shell of Li⁺via a supercluster [(CMDO)Li⁺(PC)₂](PC)₉ and the implicit solvent model (SMD) can result in a reduction potential (∼1.7 V) that is in excellent agreement with the experimental reduction peak.

The formation of a solid electrolyte interphase (SEI) between the anode surface and the electrolyte of lithium-ion batteries (LIBs) has been considered to be the most important yet the least understood issue of LIBs.

Development of Divide-and-Conquer Density-Functional Tight-Binding Method for Theoretical Research on Li-Ion Battery

The density-functional tight-binding (DFTB) method is one of the useful quantum chemical methods, which provides a good balance between accuracy and computational efficiency. In this account, we reviewed the basis of the DFTB method, the linear-scaling divide-and-conquer (DC) technique, as well as the parameterization process. We also provide some refinement, modifications, and extension of the existing parameters that can be applicable for lithium-ion battery systems. The diffusion constants of common electrolyte molecules and LiTFSA salt in solution have been estimated using DC-DFTB molecular dynamics simulation with our new parameters. The resulting diffusion constants have good agreement to the experimental diffusion constants.

Fluorinated Electrolytes for Li-Ion Batteries: The Lithium Difluoro(oxalato)borate Additive for Stabilizing the Solid Electrolyte Interphase

Fluorinated electrolytes based on fluoroethylene carbonate (FEC) have been considered as promising alternative electrolytes for high-voltage and high-energy capacity lithium-ion batteries (LIBs). However, the compatibility of the fluorinated electrolytes with graphite negative electrodes is unclear. In this paper, we have systematically investigated, for the first time, the stability of fluorinated electrolytes with graphite negative electrodes, and the result shows that unlike the ethylene carbonate (EC)-based electrolyte, the FEC-based electrolyte (EC was totally replaced by FEC) is incapable of forming a protective and effective solid electrolyte interphase (SEI) that protects the electrolyte from runaway reduction on the graphite surface. The reason is that the lowest unoccupied molecular orbital energy levels are also lowered by the introduction of fluorine into the solvent, and the FEC solvent has poorer resistance against reduction, leading to instability on the graphite negative electrode. To tackle this problem, two lithium salts of lithium bis(oxalato)borate and lithium difluoro(oxalato)borate (LiDFOB) have been investigated as negative-electrode film-forming additives. Incorporation of only 0.5 wt % LiDFOB to a FEC-based electrolyte [1.0 M LiPF6 in 3:7 (FEC-ethyl methyl carbonate)] results in excellent cycling performance of the graphite negative electrode. This improved property originates from the generation of a thinner and better quality SEI film with little LiF by the sacrificial reduction of the LiDFOB additive on the graphite negative electrode surface. On the other hand, this additive can stabilize the electrolyte by scavenging HF. Meanwhile, the incorporated LiDFOB additive has positive influence on the interphase layer on the positive electrode surface and significantly decreases the amount of HF formation, finally leading to improved cycling stability and rate capability of LiNi0.5Mn1.5O4 electrodes at a high cutoff voltage of 5 V. The data demonstrate that the LiDFOB additive not only exhibits a superior compatibility with graphite but also improves the electrochemical properties of high-voltage spinel LiNi0.5Mn1.5O4 positive electrodes considerably, confirming its potential as a prospective, multifunctional additive for 5 V fluorinated electrolytes in high-energy capacity lithium-ion batteries (LIBs).

New insights into the electroreduction of ethylene sulfite as an electrolyte additive for facilitating solid electrolyte interphase formation in lithium ion batteries

To help understand the solid electrolyte interphase (SEI) formation facilitated by electrolyte additives of lithium-ion batteries (LIBs) the supermolecular clusters [(ES)Li+(PC)m](PC)n (m = 1-2; n = 0, 6 and 9) were used to investigate the electroreductive decompositions of the electrolyte additive ethylene sulfite (ES) as well as the solvent propylene carbonate (PC) with density functional theory. The results show that ES can be reduced prior to PC, resulting in a reduction precursor that will then undergo a ring opening decomposition to yield a radical anion. A new concerted pathway (path B) was located for the ring opening of the reduced ES, which has a much lower energy barrier than the previously reported stepwise pathway (path A). The transition state for the ring opening of PC induced by the reduced ES (path C, indirect path) is closer to that of path A than path B in energy. The direct ring opening of the reduced PC (path D) has a lower energy barrier than paths A, B and C, yet it is less favorable than the latter paths in terms of thermodynamics (vertical electron affinity or reduction potential and dissociation energy). The overall rate constant including the initial reduction and the subsequent ring opening for path B is the largest among the four paths, followed by paths A > C > D, which further signifies the importance of the concerted new path in facilitating the SEI formation. The hybrid models, the supermolecular clusters augmented by a polarized continuum model, PCM-[(ES)Li+(PC)2](PC)n (n = 0, 6 and 9), were used to further estimate the reduction potential by taking into account both explicit and implicit solvent effects. The second solvation shell of Li+ in [(ES)Li+(PC)2](PC)n (n = 6 and 9) partially compensates the overestimation of solvent effects arising from the PCM for the naked (ES)Li+(PC)2, and the theoretical reduction potential of PCM-[(ES)Li+(PC)2](PC)6 (1.90-1.93 V) agrees very well with the experimental one (1.8-2.0 V).

Improved Interfacial Properties of MCMB Electrode by 1-(Trimethylsilyl)imidazole as New Electrolyte Additive To Suppress LiPF6 Decomposition

Trace water content in the electrolyte causes the degradation of LiPF₆, and the decomposed products further react with water to produce HF, which alters the surface of anode and cathode. As a result, the reaction of HF and the deposition of decomposed products on electrode surface cause significant capacity fading of cells. Avoiding these phenomena is crucial for lithium ion batteries. Considering the Lewis-base feature of the N-Si bond, 1-(trimethylsilyl)imidazole (1-TMSI) is proposed as a novel water scavenging electrolyte additive to suppress LiPF₆ decomposition. The scavenging ability of 1-TMSI and beneficiary interfacial chemistry between the MCMB electrode and electrolyte are studied through a combination of experiments and density functional theory (DFT) calculations. NMR analysis indicated that LiPF₆ decomposition by water was effectively suppressed in the presence of 0.2 vol % 1-TMSI. XPS surface analysis of MCMB electrode showed that the presence of 1-TMSI reduced deposition of ionic insulating products caused by LiPF₆ decomposition. The results showed that the cells with 1-TMSI additive have better performance than the cell without 1-TMSI by facilitating the formation of solid-electrolyte interphase (SEI) layer with better ionic conductivity. It is hoped that the work can contribute to the understanding of SEI and the development of electrolyte additives for prolonged cycle life with improved performance.

SEI-forming electrolyte additives for lithium-ion batteries: development and benchmarking of computational approaches

SEI-forming additives play an important role in lithium-ion batteries, and the key to improving battery functionality is to determine if, how, and when these additives are reduced. Here, we tested a number of computational approaches and methods to determine the best way to predict and describe the properties of the additives. A wide selection of factors were evaluated, including the influences of the solvent and lithium cation as well as the DFT functional and basis set used. An optimized computational methodology was employed to assess the usefulness of different descriptors.

Additive effect of fluoroethylene and difluoroethylene carbonates for the solid electrolyte interphase film formation in sodium-ion batteries: a quantum chemical study

To reveal the FEC–DFEC mystery, the reduction decomposition mechanism study of Na⁺–PC, Na⁺–FEC and Na⁺–DFEC complexes is carried out by using DFT. This study concludes that the higher activation barrier and absence of NaF complexes make DFEC futile.

Ring-chain synergy in ionic liquid electrolytes for lithium batteries

Lithium-ion batteries have been attracting much attention which enables the revolution of wireless global communication. Ionic liquids are regarded as promising candidates for lithium-ion battery electrolytes because they can overcome the limitations of high operating temperatures and flammability concerns of traditional electrolytes. However, at low temperatures they suffer from low ionic conductivity and phase transition. In this paper mixed electrolyte systems are described based on N-methoxyethyl-N-methylpyrrolidinium bis(trifluoromethanesulfonyl)-imide (Pyr_1,2O1TFSI) and lithium difluoro(oxalate)borate (LiODFB) lithium salt, with ethylene sulphite (ES) or dimethyl sulphite (DMS) as a cosolvent. The mixed electrolyte system exhibits good ion transport properties (a conductivity of 8.163 mS cm^-1), a wide electrochemical window (5.2 V), non-flammability, the ability to form films to protect the anode and a large operating temperature range (-40 °C to 60 °C). We compare the performance and function of the new mixed electrolyte system with a variety of ionic liquid/cosolvent electrolyte systems developed in previous studies. The ring-chain synergy takes advantage of the availability of both high permittivities based on the ring-like components and low viscosities based on the chain-like components in the mixed electrolyte system and causes the electrolyte to exhibit a good overall performance in safety, ion transport and compatibility with electrodes.

Oxidative stability and reduction decomposition mechanism studies on a novel salt: lithium difluoro(sulfato)borate

The density functional theory calculation of the oxidative stability and reduction decomposition mechanism is quite an important factor for practical application.

Mechanism of action of ethylene sulfite and vinylene carbonate electrolyte additives in LiNi1/3Mn1/3Co1/3O2/graphite pouch cells: electrochemical, GC-MS and XPS analysis

The SEI films formation/composition were dominated by VC resulting in better electrochemical performance of LiNi_1/3Mn_1/3Co_1/3O₂ (NMC)/graphite pouch cells.

Novel benzimidazole salts for lithium ion battery electrolytes: effects of substituents

Promising highly dissociating and oxidatively stable anions that can offer better performance than the experimentally reported salts are reported.