The solvation structure,transport properties and reduction behavior of carbonate-based electrolytes of lithium-ion batteries

Despite the extensive employment of binary/ternary mixed-carbonate electrolytes (MCEs) for Li-ion batteries, the role of each ingredient with regards to the solvation structure, transport properties, and reduction behavior is not fully understood. Herein, we report the atomistic modeling and transport property measurements of the Gen2 (1.2 M LiPF₆ in ethylene carbonate (EC) and ethyl methyl carbonate (EMC)) and EC-base (1.2 M LiPF₆ in EC) electrolytes, as well as their mixtures with 10 mol% fluoroethylene carbonate (FEC). Due to the mixing of cyclic and linear carbonates, the Gen2 electrolyte is found to have a 60% lower ion dissociation rate and a 44% faster Li⁺ self-diffusion rate than the EC-base electrolyte, while the total ionic conductivities are similar. Moreover, we propose for the first time the anion–solvent exchange mechanism in MCEs with identified energetic and electrostatic origins. For electrolytes with additive, up to 25% FEC coordinates with Li⁺, which exhibits a preferential reduction that helps passivate the anode and facilitates an improved solid electrolyte interphase. The work provides a coherent computational framework for evaluating mixed electrolyte systems.

The different roles of the anion, cyclic and linear carbonates, and additive in mixed-carbonate electrolytes are revealed. The anion–solvent exchange mechanism and factors influencing the solid electrolyte interphase (SEI) formation are deciphered.     

Electrode-compatible fluorine-free multifunctional additive regulating solid electrolyte interphase and solvation structure for high-performance lithium-ion batteries

The rapid development and widespread application of lithium-ion batteries(LIBs) have increased demand for high-safety and high-performance LIBs. Accordingly, various additives have been used in commercial liquid electrolytes to severally adjust the solvation structure of lithium ions, control the components of solid electrolyte interphase, or reduce flammability. While it is highly desirable to develop low-cost multifunctional electrolyte additives integrally that address both safety and perform...     

Fabrication of high Li:water molar ratio electrolytes for lithium-ion batteries

Hydrous electrolytes with high electrochemical potentials were obtained by hydrating water molecules into solutes to form high Li:water molar ratio electrolytes(HMRE).Solid polyethylene glycol(PEG) were e mployed to enha nce the molar ratio of Li+to water in the electrolytes while reducing the consumption of Li-salt.The obtained mole ratio of Li~+ to wa ter molecules in the hydrous electrolytes was greater than 1:1;however,the mass fraction of Li-salt was reduced to 61%(approximately 5.5 mol/kg,based on water and PEG).Compared with that of water-in-salt electrolytes,the mass fraction of Li-salt could be remarkably reduced by adding solid PEG.The electrochemical stability of the electrolytes improved considerably because of the strong hydration of Li~+ by the water molecules.A beneficial passivation effect,arising from the decomposition of the electrolyte,at a wide potential window was observed.     

Rational design on separators and liquid electrolytes for safer lithium-ion batteries

As the energy density of lithium-ion batteries (LIBs) continues to increase,their safety has become a great concern for further practical large-scale applications.One of the ultimate solution of the safety issue is to develop intrinsically safe battery components,where the battery separators and liquid electrolytes are critical for the battery thermal runaway process.In this review,we summarize recent progress in the rational materials design on battery separators and liquid electrolyte towards the goal of improving the safety of LIBs.Also,some strategies for further improving safety of LIBs are also briefly outlooked.     

固态聚合物电解质的锂离子传导机理与研究进展

锂离子电池(lithiumionbatteries,LIBs)在储能领域已取得了巨大的成功.然而,商用LIBs含有高挥发性易燃有机电解液,使其存在严重的安全隐患.固态聚合物电解质具有解决相应安全性问题的潜力,有望成为下一代高安全性全固态LIBs的电解质材料.然而,固态聚合物电解质存在离子电导率不高等问题,限制了其在固态LIBs中的实际应用.研究者们为了提高该类电解质的离子电导率、锂离子迁移数等综合电化学性能,已在寻找新锂盐、对聚合物进行改性以及向聚合物电解质中添加填料等方面进行了较多的研究.本文简要概述了固态聚合物电解质的锂离子传导机理以及在提高固态聚合物电解质综合电化学性能方面的研究进展.     

Thermal behaviors of electrolytes in lithium-ion batteries determined by differential scanning calorimeter

Lithium-ion batteries have been widely used in daily electric appliance, but abusive accidents occurred from time to time. Lithium-ion batteries composed of various electrolytes (containing organic solvents, inorganic salts), binder, and electrode materials, which may be unstable under some abnormal conditions. The formulation or components of electrolytes played a crucial factor in affecting reactive hazards within the cell. In order to meet safety requirements of the lithium-ion batteries on electronic device, reseachers and scholars are continuing to do further studies on the important issues in relation to the lithium-ion batteries. This study aims to apply the differential scanning calorimeter for measuring the thermal or reactive hazards of the organic solvents related to the formulation of the electrolytes. Besides, thermal instabilities of lithiated graphite or deposited lithium with electrolytes were simulated by the reactions between metallic lithium (Li) and organic carbonates of linear and cyclic structures. Exothermic onset temperatures and enthalpy changes were measured and analyzed. Results showed that the thermal behaviors of these organic carbonates themselves or with lithium hexafluorophosphate liberated less enthalpy changes. However, violent exothermic reactions were detected between the linear or cyclic carbonates and lithium metal with larger enthalpy change larger than 1,000 J g^?1 of lithium. This can be observed by Li reacted with diethyl carbonate at surprisingly lower onset temperature about 46.6 °C.     

The effect of coordinating and non-coordinating additives on the transport properties in ionic liquid electrolytes for lithium batteries

In the present study we expand our analysis of using two contrasting organic solvent additives (toluene and THF) in an ionic liquid (IL)/Li NTf(2) electrolyte. Multinuclear Pulsed-Field Gradient (PFG) NMR, spin-lattice (T(1)) relaxation times and conductivity measurements over a wide temperature range are discussed in terms of transport properties and structuring of the liquid. The conductivity of both additive samples is enhanced the most at low temperatures, with THF slightly more effective than toluene. Both the anion and lithium self-diffusivity are enhanced in the same order by the additives (THF > toluene) while that of the pyrrolidinium cation is marginally enhanced. (1)H spin-lattice relaxation times indicate a reasonable degree of structuring and anisotropic motion within all of the samples and both (19)F and (7)Li highlight the effectiveness of THF at influencing the lithium coordination within these systems.     

The impedance of lithium-ion batteries

Cylindrical and flat one-layer lithium-ion batteries, as well as symmetrical cells with like electrodes, are studied using electrochemical impedance spectroscopy. Equivalent circuit is suggested, its parameters found, and the activation energies estimated for different stages of the electrode process.     

Thermal study of organic electrolytes with fully charged cathodic materials of lithium-ion batteries

We systematically investigated thermal effects of organic electrolytes/organic solvents with fully charged cathodic materials (Li_0.5CoO₂) of Li-ion battery under rupture conditions by using oxygen bomb calorimeter. In the six studied systems, both the amount of combustion heat and heat release rates showed a pronounced increase with the increase in mass ratios of cathodic materials to electrolytes/solvents. More importantly, synergistic effects not simply physical mixtures have firstly been observed between cathodic materials and electrolytes/solvents in the complete combustion reactions. The results have been further analyzed by X-ray diffraction spectra, which revealed that Co₃O₄, CoO, and LiCoO₂ were the main solid products for the combustion reactions of studied systems. And there are more CoO and less LiCoO₂ products for the higher ratio of cathodic materials system and more amount of heat generated. It means that the combustion reaction, which produced CoO, generated more amount of heat than LiCoO₂. Electronic supplementary material The online version of this article (doi:) contains supplementary material, which is available to authorized users.     

锰基液流电池电解液的体积性质研究

体积性质是锰基液流电池电解液的重要热力学性质,常用于解释溶液中复杂的离子间相互作用关系。本文在283.15-318.15 K温度范围内,测定了浓度为0.5-3.0 mol/kg的MnSO₄水溶液的密度值,得到了MnSO₄溶液的几个热力学参数和弱分子间的相互作用关系。结合Pitzer电解液表观摩尔体积热力学模型,得到了体积参数β(0)V MX,β(1)V MX和CV MX,而且计算值与实验值的相关系数能够达到0.988。这一研究可以更好地理解锰电解液中离子相互作用机制,为优化电解液成分和提高电池性能提供理论支撑。     

The underlying mechanism for reduction stability of organic electrolytes in lithium secondary batteries

Many organic solvents have very desirable solution properties, such as wide temperature range, high solubility of Li salts and nonflammability, and should be able but fail in reality to serve as electrolyte solvents for Li-ion or -metal batteries due to their reduction instability. The origin of this interfacial instability remains unsolved and disputed so far. Here, we reveal for the first time the origin of the reduction stability of organic carbonate electrolytes by combining ab initio molecular dynamics (AIMD) simulations, density functional theory (DFT) calculations and electrochemical stability experiments. It is found that with the increase of the molar ratio (MR) of salt to solvent, the anion progressively enters into the solvation shell of Li⁺ to form an anion-induced ion–solvent-coordinated (AI-ISC) structure, leading to a “V-shaped” change of the LUMO energy level of coordinated solvent molecules, whose interfacial stability first decreases and then increases with the increased MRs of salt to solvent. This mechanism perfectly explains the long-standing puzzle about the interfacial compatibility of organic electrolytes with Li or similar low potential anodes and provides a basic understanding and new insights into the rational design of the advanced electrolytes for next generation lithium secondary batteries.

By theoretical and experimental evidence, the underlying mechanism for the enhanced reduction stability of the HMRE is revealed, suggesting that the interfacial stability of the electrolyte can be adjusted through the modulation of the anion-induced ISC structure.

The state-of-the-art electrolytes in Li-ion batteries (LIBs) are mostly based on 1.0 mol L⁻¹ LiPF₆/ethylene carbonate (EC)-based carbonate due to the surface passivation of the graphite anode by forming a stable solid electrolyte interphase (SEI). However, these electrolytes cannot operate well for new electrode materials and battery systems that are expected to have higher voltage, better safety and wider temperature range than current commercial LIBs.^1–3 For example, EC-based carbonate electrolytes are easily oxidized on a high voltage cathode at or above 4.3 V, resulting in depletion of electrolytes, gas evolution and low coulombic efficiency, which reduce the cycle life and create safety hazards for LIBs.⁴ These problems of the conventional electrolyte significantly hinder the development of new generation lithium batteries and limit these batteries for high voltage and/or high capacity applications and operation in a wide temperature range.To overcome these problems, great efforts have been devoted in recent years to the development of new electrolytes, such as solid state electrolytes,⁵ ionic liquids,^6–8 highly-concentrated electrolytes (HCEs),⁹ electrolyte stabilizing additive,^10–13 and so on. Among them, the HCEs or high-molar-ratio electrolytes (HMREs) of salt to solvent have received particular attention, owing to their unusual electrochemical stability, nonflammability, and good compatibility with a wide range of anode and cathode materials.^14–17 These desirable properties are apparently attributed to the solution structure of HCEs, where there exist almost no free solvent molecules, and the parasitic side reactions of solvents are thereby greatly reduced. Due to the lack of solvent molecules in HCEs, anions have to enter into the solvation shell of Li⁺, in order to meet the Li⁺ coordination number of 4–6, to form an ion–solvent-coordinated (ISC) structure.¹⁸ Several studies have shown that the unique ISC structure of HCEs leads to the shift of the lowest unoccupied molecular orbital (LUMO) from solvent to salt, which makes anions preferentially reduced or decomposed to produce a robust anion-derived SEI.^14,19 In recent years, the anion-derived SEI structure has been regarded as the “holy grail” of electrolyte chemistry for understanding the interfacial stability and compatibility of HCEs. However, recent studies have showed that some HCEs containing non-film-forming salts and solvents can still achieve excellent reversible Li⁺ insertion reactions.²⁰ Therefore, an intrinsic origin for the interfacial stability of HCEs still remains unrevealed. In our previous studies on HCEs or HMREs, their interfacial stability was found to depend predominately on the molar ratio (MR) of salt to solvent rather than the molar concentration.^2,21,22 Thus, the HMREs instead of the HCEs in the following study could more clearly describe the nature of electrolyte stability.In this work, we reveal the correlation between the solvation microstructures and the LUMO energy levels of typical ISC structures in the electrolytes at various MRs with non-film-forming lithium salt (LiClO₄) and organic carbonate solvents (PC, DMC, EMC and DEC) by ab initio molecular dynamics (AIMD) simulations and density functional theory (DFT) calculations. The choice of non-film-forming lithium salt and solvent in this study was aimed to exclude the contribution of the formation of the SEI film to the interfacial stability of the electrolytes. It is found from this study that the LUMO energy level of the ISC structure formed at a low MR is lower than that of pure solvent. With the increase of the MR, anions gradually enter into the first solvation shell of Li⁺ to form the anion-induced ISC (AI-ISC) structure, resulting in the increase of the LUMO energy level that enhances the reduction stability of the electrolyte. Also, it is revealed that the LUMO levels of ISC structures at different MRs are always situated at the coordinated solvent molecules, i.e., the strong reduction stability of HMREs is dominated by the modulation of solvent molecules rather than only the formation of the anion-derived SEI. Such a theoretical insight is further unequivocally evidenced by chemical compatibility experiments in this work. These findings reveal the origin of the greatly improved interfacial stability of HMREs and provide a mechanistic insight into the rational design of stable electrolytes for new generation alkali or alkaline metal based batteries.To investigate the specific ISC microstructures of the electrolytes with different MRs, AIMD simulations were first performed (see computational details in the ESI^†). Taking non-film-forming DEC solvent as an example, three types of electrolytes with MRs of LiClO₄ to DEC = 1 : 10, 1 : 5 and 1 : 2 are considered (Table S1^†). After long-time AIMD simulation, the representative images of the equilibrium structures are shown in Fig. 1a–c. To characterize the solution structures, the radial distribution function g(r) of the electrolyte with different MRs is analyzed (Fig. 1e–g), and the changes in the Li⁺ coordination number with the O atoms of solvents and anions are listed in Fig. 1d. In addition, it should be noted that the total coordination number of Li⁺ always remains around 4, which implies that the stable tetragonal solvation shell structure of Li⁺ does not change in the different MR electrolytes; meanwhile, both the coordination numbers of Li⁺ contributed by the solvent and anion change oppositely. This phenomenon can be corroborated experimentally through infrared spectroscopy (IR) because the C O bond of the carbonate group has a strong IR absorption in the carbonyl region (1650–1850 cm⁻¹) and its IR peak position shifts sensitively with its coordination environment. As shown in Fig. 1h, the IR band of carbonyl groups in pure DEC is located at ∼1741 cm⁻¹, which is shifted to ∼1710 cm⁻¹ in a LiClO₄/DEC (MR = 1 : 10) electrolyte due to the coordination of the O atom in C O with Li⁺. With the increase of the MR of Li⁺/DEC, its IR peak at ∼1741 cm⁻¹ gradually disappears, reflecting a gradual decrease in the number of free DEC molecules. In addition, the IR band of free ClO₄⁻ in a LiClO₄/DEC (MR = 1 : 10) electrolyte is located at ∼931 cm⁻¹, which is shifted to ∼942 cm⁻¹ in the 1 : 2 LiClO₄/DEC electrolyte due to the ionic association of Li⁺ and ClO₄⁻ (Fig. S1^†). Combining AIMD simulations and IR experiments, it can be concluded that with the increase of the MR of the electrolyte, the anions gradually enter into the solvation shell of Li⁺, which modulates the chemical stability of the electrolyte.Open in a separate windowFig. 1Snapshots of typical equilibrium trajectories from DFT-MD simulations: (a) 1 : 10 LiClO₄/DEC solution (2-LiClO₄/20-DEC), (b) 1 : 5 LiClO₄/DEC solution (3-LiClO₄/15-DEC) and (c) 1 : 2 LiClO₄/DEC solution (7-LiClO₄/14-DEC). (d) Typical ISC structure extracted from DFT-MD. (e–g) Radial distribution function of lithium–oxygen interaction (short dashed lines) and relationship between the coordination number and bond distances (full lines). (h) FTIR spectra of the carbonyl group in LiClO₄/DEC solution. Atom color: H, white; Li, purple; C, cyan; O, red; Cl, green.Coordination numbers (n(r)) of atom pairs of Li–O(DEC) and Li–O (ClO₄⁻) (cut-off length of r = 2.5 Å)

Conductivities and transport properties of gelled electrolytes with and without an ionic liquid for Li and Li-ion batteries

Conductivity and transport properties have been determined for gelled polymer electrolytes of three compositions: a base PVdF-polymer gel with organic carbonate solvents as plasticizers and LiN(SO(2)C(2)F(5))(2) electrolyte, a second polymer electrolyte with 5 mass % 1-ethyl-3-methylimidazolium bisperfluoroethylsulfonyl imide (EMI-BETI) added to the base polymer electrolyte, and a third PVdF polymer electrolyte using only EMI-BETI as the plasticizer. Conductivities were studied over the temperature range +25 to -40 degrees C, and for all three gels, the temperature dependence of the conductivities was found to follow the VTF equation, which is consistent with the free volume model for ion transport. For the gel containing 5 mass % EMI-BETI, transport numbers were determined from +50 to -20 degrees C and were found to decrease as the temperature decreased. Although there are no theoretical models to treat and interpret the temperature dependence of transport numbers, we found that a modified VTF equation resulted in an excellent fit to the temperature dependence of the transport number, which is another confirmation of a free volume model for transport in these gelled polymer electrolytes.     

Phase behavior and critical properties of size-asymmetric, primitive-model electrolytes

The theory of J. Jiang et al. [J. Chem. Phys. 116, 7977 (2002)] for size-symmetric electrolytes is extended to size-asymmetric electrolytes. When compared to molecular-simulation results, this extension gives the correct trend of critical properties with size asymmetry.     

Synthesis of nanosized materials for lithium-ion batteries by mechanical activation. Studies of their structure and properties

This short review reports on the synthesis of nanosized electrode materials for lithium-ion batteries by mechanical activation (MA) and studies of their properties. Different structural types of compounds were considered, namely, compounds with a layered (LiNi_{1 − x − y} Co _x Mn _y O₂), spinel (LiMn₂O₄, Li₄Ti₅O₁₂), and framework (LiFePO₄, LiTi₂(PO₄)₃) structures. The compounds also differed in electronegativity, which varied from 10⁻⁴ S cm⁻¹ for LiCoO₂ to 10⁻⁹ S cm⁻¹ for LiFePO₄. The preliminary MA of mixtures of reagents in energy intensive mechanoactivators led to the formation of highly reactive precursors, and annealing of the latter formed nanosized products (the mean particle size is 50–200 nm). The local structure of the synthesized compounds and the composition of their surface were studied by spectral methods. An increase in the dispersity and defect concentration, especially in the region of the surface, improved some electrochemical characteristics. It increased the stability during cycling (LiMn₂O₄, at 3 V) and the regions of the formation of solid solutions during cycling (Li₄Ti₅O₁₂, LiFePO₄), led to growth of surface Li-ion conductivity (LiTi₂(PO₄)₃), etc. The mechanochemical approach was also used for the synthesis of core-shell type composite materials (LiFePO₄/C, LiCoO₂/MeO _x ) and materials based on two active electrode components (LiCoO₂/LiMn₂O₄).     

Electrochemical properties of fluoropropylene carbonate and its application to lithium-ion batteries

We have investigated the monofluorination effect of propylene carbonate (PC) on the electrolytic properties and on the performance of lithium-ion cells. The anodic stability of fluoropropylene carbonate (FPC) is higher than that of PC. The use of FPC as a high-polarity solvent decreases the electrolytic conductivity but improves the charge–discharge characteristics of the lithium-ion cells.     

Synthesis and electrochemical properties of graphene-SnS2 nanocomposites for lithium-ion batteries

Graphene-SnS₂ nanocomposites were prepared via a solvothermal method with different loading of SnS₂. The nanostructure and morphology of the samples were characterized by X-ray diffraction (XRD), scanning electron microscopy (SEM), and transmission electron microscopy (TEM). The XRD patterns revealed that hexagonal SnS₂ was obtained. SEM and TEM results indicated that SnS₂ particles distributed homogeneously on graphene sheets. The electrochemical properties of the samples as active anode materials for lithium-ion batteries were examined by constant current charge–discharge cycling. The composite with weight ratio between graphene and SnS₂ of 1:4 had the highest rate capability among all the samples and its reversible capacity after 50 cycles was 351 mAh/g, which was much higher than that of the pure SnS₂ (23 mAh/g). With graphene as conductive matrix, homogeneous distribution of SnS₂ nanoparticles can be ensured and volume changes of the nanoparticles during the charge and discharge processes can be accomodated effectively, which results in good electrochemical performance of the composites.     

The effects of the size and content of BaTiO3 nanoparticles on solid polymer electrolytes for all-solid-state lithium-ion batteries

Journal of Solid State Electrochemistry - For all-solid-state lithium-ion batteries, several disadvantages such as low ionic conductivity and poor interfacial stability have been concerned....     

On the gassing behavior of lithium-ion batteries with NCM523 cathodes

Gas evolution has a profound effect on the functioning of state-of-the-art lithium-ion batteries. On one hand, it is the natural concomitant of solid electrolyte interphase (SEI) formation on the anode (reduction of electrolyte components). On the other hand, because of the demand for high terminal voltages, it is also the consequence of electrolyte and/or cathode material oxidation. Overall, gassing happens on the expense of Coulombic efficiency and additionally raises safety issues. Herein, the gassing behavior of one of the most important commercialized cathode materials, namely Ni-rich Li_1 + x Ni_0.5Co_0.2Mn_0.3O₂ (NCM523 with 0.01 < x < 0.05), is reported for the first time. We analyze the generation pattern of the most typical gases H₂, C₂H₄, CO₂, and CO during 30 cycles by means of differential electrochemical mass spectrometry combined with Fourier transform infrared spectroscopy. In a long-term test of an NCM523/graphite cell, we monitor its potential-resolved gas evolution and evaluate the total amount of gas from cycle to cycle. An explanation on the characteristic features of pressure versus time curves during cycling is given by combining the spectrometric and total gas pressure data. With additional information from graphite/lithium cells, the identity of gases formed during SEI formation is revealed.     

Effect of ion solvation on the diffuse layer structure in mixed electrolytes

A “solvionic” model of a multicomponent electrochemical system (mixed electrolyte) is considered. An ion in the solution is considered as a point charge rigidly fixed inside its solvation shell. The corresponding equations for the diffuse layer on an ideally polarizable electrode are derived, and an effective method of their numerical solution is formulated. The calculations are performed in order to follow the changes in the diffuse layer structure with variations in the electrode charge and electrolyte composition. Far from the zerocharge potential of solution, the dependences of distributions of solution components over the diffuse layer on the electrode charge radically differ from those within the classic Gouy-Chapman theory. Analytical equations (asymptotics at large electrode charges) for concentrations of solvated ions in the plane of their maximum approach and for their “surface excesses” (diffuse adsorption) are determined. Results of numerical calculations for a 0.2 M LiCl + 0.05 M BaCl₂ solution are plotted.     

Li2S-P2S5体系电解质及其在全固态锂离子电池中的应用研究进展

全固态锂离子电池具有安全性能好、能量密度高、工作温区广等优点,被广泛应用于便携式电子设备。固态电解质是全固态锂离子电池的关键材料之一,其中的硫化物电解质具有离子电导率高、电化学窗口宽、晶界电阻低和易成膜等特点,被认为最有希望应用于全固态锂离子电池。本文综述了Li_2S-P_2S_5体系电解质的发展状况,包括固态电解质的制备、改性、表征以及电极/固态电解质之间的固-固界面的稳定兼容问题。本文还涉及了以Li_2S-P_2S_5为电解质的全固态锂离子电池性能的研究进展。