Novel SEI formation of maleimide-based additives and its improvement of capability and cyclicability in lithium ion batteries

This investigation elucidates three maleimide (MI)-based aromatic molecules as additives in electrolyte that is used in lithium ion batteries. The 1.1 M LiPF₆ in ethylene carbonate (EC):propylene carbonate (PC):diethylene carbonate (DEC) (3:2:5 in volume) containing MI-based additives can prompt the formation of a solid electrolyte interface (SEI); and inhibit the entering into the irreversible state during lithium intercalation and co-intercalation. The reduction potential is 0.71-0.98 V versus Li/Li⁺ as determined by cyclic voltammetry (CV). The morphology and element analysis of the positive and negative electrode after the 100th charge-discharge cycle are examined by scanning electron microscopy (SEM), energy dispersive spectrometry (EDS) and X-ray photoelectron spectroscopy (XPS). Moreover, the MI was used in lithium ion batteries and provided 4.9% capacity increase and 16.7% capacity retention increase when cycled at 1C/1C. The MI-based additive also ensures respectable cycle-ability of lithium ion batteries. MI is decomposed electrochemically to form a long winding narrow SEI strip on the graphite surface. This novel SEI strip not only prevents exfoliation on the graphite electrode but also stabilizes the electrolyte. The MI-based additive also ensures respectable cycle-ability of lithium ion batteries.     

Impedance spectroscopic study for the initiation of passive film on carbon electrodes in lithium ion batteries

The formation of passive film at the interface between the mesocarbon microbeads (MCMB) electrode and the organic electrolyte in a lithium-ion battery during the initial period of intercalation was investigated by a.c.-impedance spectroscopy. An equivalent-circuit model consisting of five parallel RC-circuits in series combination was adopted for the curve-fitting analysis of the obtained impedance spectra. The results indicated that both the total interfacial resistance and the passive film thickness increased with decreasing intercalation potential in the ethylene carbonate (EC) or dimethyl carbonate (DMC) single-solvent system, whereas an opposite trend was observed in the system containing diethyl carbonate (DEC) only. In addition, the total interfacial resistance was clearly affected by the porous structure of the passive film in a single-solvent system. In binary solvent systems such as EC/DEC and EC/DMC, on the other hand, the effect of the porous structure on the total interfacial resistance was negligible. The total interfacial resistance and the passive film thickness were also smaller in these systems than those in single-solvent systems. Finally, the variation of the total interfacial resistance and of the passive film thickness in the EC/DEC (or EC/DMC) system were also found to be similar to those in the parent DEC (or DMC) system during intercalation.     

Effects of solvents and salts on the thermal stability of LiC6

Accelerating rate calorimetry (ARC) was used to study the thermal stability of Li_0.81C₆ in dimethyl carbonate (DMC), diethyl carbonate (DEC), ethylene carbonate (EC), and an EC/DEC mixture as well as in LiPF₆- and LiBOB-based electrolytes. ARC results show that linear carbonates like DMC or DEC react strongly with Li_0.81C₆ and that robust passivating layers do not form. By contrast, the cyclic carbonate, EC, creates a robust passivating film that limits the rate of reaction between Li_0.81C₆ and EC as the temperature increases. X-ray diffraction shows that the addition of LiPF₆ to EC/DEC changes the surface film that forms on Li_0.81C₆ at elevated temperature to one dominated by LiF instead of lithium-alkyl carbonate or lithium carbonate. This increases the thermal stability of Li_0.81C₆ in LiPF₆ electrolyte compared to pure EC/DEC solvent. By an apparently similar mechanism, the addition of only 0.2 M LiBOB to EC/DEC greatly improves the thermal stability of Li_0.81C₆. ARC results for Li_0.81C₆ in pure and mixed salt LiPF₆ and LiBOB EC/DEC electrolytes of various molarities shed light on the reasons for the beneficial effect of the salts.     

Suppressing propylene carbonate decomposition by coating graphite electrode foil with silver

A method has been developed to suppress the decomposition of propylene carbonate (PC) by coating graphite electrode foil with a layer of silver. Results from electrochemical impedance measurements show that the Ag-coated graphite electrode presents lower charge transfer resistance and faster diffusion of lithium ions in comparison with the virginal one. Cyclic voltammograms and discharge-charge measurements suggest that the decomposition of propylene carbonate and co-intercalation of solvated lithium ions are prevented, and lithium ions can reversibly intercalate into and deintercalate from the Ag-coated graphite electrode. These results indicate that Ag-coating is a good way to improve the electrochemical performance of graphitic carbon in PC-based electrolyte solutions.     

Reprint of “Suppressing propylene carbonate decomposition by coating graphite electrode foil with silver”

A method has been developed to suppress the decomposition of propylene carbonate (PC) by coating graphite electrode foil with a layer of silver. Results from electrochemical impedance measurements show that the Ag-coated graphite electrode presents lower charge transfer resistance and faster diffusion of lithium ions in comparison with the virginal one. Cyclic voltammograms and discharge-charge measurements suggest that the decomposition of propylene carbonate and co-intercalation of solvated lithium ions are prevented, and lithium ions can reversibly intercalate into and deintercalate from the Ag-coated graphite electrode. These results indicate that Ag-coating is a good way to improve the electrochemical performance of graphitic carbon in PC-based electrolyte solutions.     

Nano-scale copper-coated graphite as anode material for lithium-ion batteries

Nano-scale copper particles were homogeneously deposited on the surface of natural graphite through electroless plating. The co-intercalation of solvated lithium ion and reduction of the electrolyte were effectively depressed after coating of copper particles. Consequently, the graphite showed a significant improvement in charge–discharge properties such as coulombic efficiency, cycle characteristics, and high rate performance as an anode material for lithium ion batteries.     

In situ FTIR study of the Cu electrode/ethylene carbonate + dimethyl carbonate solution interface

The interfacial phenomena between Cu electrode and solution of lithium perchlorate in ethylene carbonate (EC)-dimethyl carbonate (DMC) have been investigated using in situ reflection absorption Fourier transform infrared (FTIR) spectroscopy and single reflection ATR-FTIR spectroscopy. The ATR spectra confirmed the bands due to free EC and DMC and the molecules solvated to lithium ions in the solution. The bands due to the result of the interaction between ClO₄⁻ and DMC in the mixture solution also appeared in the ATR spectra. In the FTIR spectra, the potential dependence on the concentration of EC and DMC in the vicinity of the Cu electrode was observed. It was understood that the reversible changes in the concentration of free EC and DMC and solvated EC and DMC in the diffuse double layer take place with changing in potential. As the potential decreased, the free EC and DMC concentrations increased, while the concentration of the EC and DMC solvated to lithium ions decreased. Thus, it can be concluded that the equilibrium shifts from Li⁺(EC)₂(DMC)₂ to Li⁺(EC)₂(DMC) + DMC or Li⁺(EC)(DMC)₂ + EC as the potential decreases. The bands due to (CH₂OCO₂Li)₂ and CH₃OCO₂Li were observed for an irreversible reaction.     

Triethyl orthoformate as a new film-forming electrolytes solvent for lithium-ion batteries with graphite anodes

Triethyl orthoformate (TEOF) as a new solvent used in propylene carbonate (PC)-based electrolytes together with graphitic anodes in lithium-ion batteries has been investigated. It can be observed that TEOF was capable of suppressing the co-intercalation of PC solvated lithium-ions into the graphite layer during the first lithiation process and the irreversible discharge capacity of the first cycle is the smallest when using 1.0 M LiPF₆ in PC and TEOF at solvent ratio of 1:1 as the electrolytes. The CV, FTIR, EIS, SEM results show that the PC-based electrolytes containing the solvent TEOF can generate an effective solid electrolytes interphase (SEI) film during the first cycling process, and the film is probably mainly composed of ROCO₂Li, ROLi, Li₂CO₃, etc. The formation of a stable passivating film on the graphite surface is believed to be the reason for the improved cell performance. All these results show that TEOF possesses a promising performance for use as an effective film-forming electrolytes solvent in lithium-ion batteries with graphitic anodes.     

Characterisation of the SEI formed on natural graphite in PC-based electrolytes

The origin of the different Li⁺ intercalation behaviour of raw and jet-milled natural graphite has been investigated. Jet-milled graphite is found to cycle reversibly in equal solvent mixture of propylene carbonate (PC) and ethylene carbonate (EC), whereas raw graphite does not. Using both Al Kα and synchrotron radiation (SR) Photoelectron Spectroscopy, new insight is obtained into the formation of the solid electrolyte interphase (SEI) on the two different graphite materials during electrochemical cycling in 1 M LiPF₆ in either PC:EC (1:1) or in PC with 5% vinylene carbonate (VC) as additive. Solvent reduction products are found at the surface of both raw and jet-milled graphite cycled in PC:EC (1:1), but differed in composition. The addition of VC reduces primarily the quantities of salt reaction products (LiF and Li_xPF_y compounds) and produces a mainly organic SEI layer. Electron diffraction from the edges for raw and jet-milled graphite particles shows a more disordered surface structure in the jet-milled particles than in the raw graphite. The more disordered surface structure can serve as a physical barrier hindering PC co-intercalation and facilitating the formation of a stable SEI layer.     

Influence of graphite surface properties on the first electrochemical lithium intercalation

The electrochemical insertion of lithium ions into graphite materials having different surface chemistry and defect concentration was studied during the first cycle in half-cell containing 1 M LiPF₆ in an electrolytic solvent mixture of ethylene carbonate (EC) and dimethyl carbonate (DMC). The graphite surface properties were varied by thermal treatments in either hydrogen, oxygen, or nitrogen oxide or chemical treatment in boiling nitric acid. The influence of the surface modifications on the course of the first electrolyte reduction was investigated. The surface group chemistry was analyzed by temperature-programmed desorption coupled with mass spectrometry. The surface defect concentration was determined in terms of the active surface area (ASA) measured by oxygen chemisorption and a subsequent temperature-programmed desorption. The experimental results showed that the ASA parameter governs the exfoliation tendency of the graphite negative electrode material with the existence of a critical value below which the graphite systematically exfoliates. The specific charge loss during the first electrochemical insertion of lithium and the exfoliation behavior of the graphite negative electrode material are not influenced by the type and amount of oxygen surface groups. But hydrogen present on the graphite surface increased the graphite exfoliation tendency even for graphite materials with an ASA above the critical value.     

Electrochemical characteristics of phase-separated polymer electrolyte based on poly(vinylidene fluoride-co-hexafluoropropane) and ethylene carbonate

A polymer electrolyte based on microporous poly(vinylidene fluoride-co-hexafluoropropane) (PVdF-HFP) film was studied for use in lithium ion batteries. The microporous PVdF-HFP (Kynar 2801) matrix was prepared from a cast of homogeneous mixture of PVdF-HFP and solvents such as ethylene carbonate (EC), dimethyl carbonate (DMC), and ethyl methyl carbonate (EMC). After evaporation of DMC and EMC, a sold film of the PVdF-HFP and the EC mixture was obtained. EC-rich phase started its formation in the PVdF-HFP/EC film at EC content of about 60 wt.% based on the total weight of PVdF-HFP and EC. The formation of the new phase resulted in the abrupt increase of the porosity of the PVdF-HFP matrix from 32 to 62%. The ionic conductivity of the film soaked in 1 M LiPF₆-EC/DMC=1/1 was significantly increased from order of 10⁻⁴ S/cm to order of 10⁻³ S/cm at the EC content of 60 wt.%. Thermal and spectroscopic investigations showed that most of the EC interact with PVdF-HFP with the EC content being below 60 wt.%. MCMB/polymer electrolyte/LiCoO₂ cells employing the microporous PVdF-HFP polymer film showed stable charging/discharging characteristics at 1C rate and good rate capability.     

Effect of vinylene carbonate as additive to electrolyte for lithium metal anode

The cycling efficiencies and cycling performance of a lithium metal anode in a vinylene carbonate (VC)-containing electrolyte were evaluated using Li/Ni and LiCoO₂/Li coin type cells. The cycling efficiencies of deposited lithium on a nickel substrate in an EC + DMC (1:1) electrolyte containing LiPF₆, LiBF₄, LiN(SO₂CF₃)₂ (LiTFSI), or LiN(SO₂C₂F₅) (LiBETI) at 25 and 50 °C were improved by presence of VC. However, the lithium cycling efficiencies at low temperature (0 °C) decreased by adding VC to the EC+DMC (1:1) electrolyte. The deposited lithium at low temperature exhibited a dendritic morphology and a thicker surface film. The lithium ion conductivity of the VC derived surface film was lower than that of the VC-free surface film at low temperature. Therefore, we concluded that the cycling efficiency decreased with decreasing temperature. On the other hand, the cell containing VC additive has excellent performance at elevated temperature. The deposited lithium at 50 °C in the VC-containing electrolyte exhibited a particulate morphology and formed a thinner surface film. The VC derived surface film, which consists of polymeric species, suppressed the deleterious reaction between the deposited lithium and the electrolyte.     

Compatibility of quaternary ammonium-based ionic liquid electrolytes with electrodes in lithium ion batteries

Electrochemical intercalation/deintercalation behavior of lithium into/from electrodes of lithium ion batteries was comparatively investigated in 1 mol/L LiClO₄ ethylene carbonate-diethyl carbonate (EC-DEC) electrolyte and a quaternary ammonium-based ionic liquid electrolyte. The natural graphite anode exhibited satisfactory electrochemical performance in the ionic liquid electrolyte containing 20 vol.% chloroethylenene carbonate (Cl-EC). This is attributed to the mild reduction of solvated Cl-EC molecules at the graphite/ionic electrolyte interface resulting in the formation of a thin and homogenous SEI on the graphite surface. However, rate capability of the graphite anode is poor due to the higher interfacial resistance than that obtained in 1 mol/L LiClO₄/EC-DEC organic electrolyte. Spinel LiMn₂O₄ cathode was also electrochemically cycled in the ionic electrolyte showing satisfactory capacity and reversibility. The ionic electrolyte system is thus promising for 4 V lithium ion batteries based on the concept of “greenness and safety”.     

Conductivity and viscosity studies of ethylene carbonate based solutions containing lithium perchlorate

Specific conductivities and viscosities of lithium perchlorate at four different concentrations (0.5, 1.0, 1.5 and 2.0 M) in ethylene carbonate (EC) based binary mixed solvent systems at 25°C are reported. The co-solvents chosen were tetrahydrofuran (THF), 1,2-dimethoxyethane (DME) and 1,3-dioxolane (DOL). Viscosity variations in all the three mixed solvent systems without electrolyte showed negative deviation from ideal behaviour thereby indicating the occurrence of a structure breaking effect in these three different binary systems. The increase in viscosity with increase in concentration of LiClO₄ is attributed to the structural enhancement through the formation of a solvated complex which occupies interstitials in the solvent mixtures. 1 M LiClO₄ solution shows maximum specific conductivity at 30 vol % EC for EC + DME and EC + DOL mixtures and at 50 vol % EC for EC + THF mixtures. Conductivity variations are explained on the basis of preferential solvation of lithium perchlorate by co-solvents (THF, DME and DOL) in their respective mixtures with ethylene carbonate.     

Methyl phenyl bis-methoxydiethoxysilane as bi-functional additive to propylene carbonate-based electrolyte for lithium ion batteries

Methyl phenyl bis-methoxydiethoxysilane (MPBMDS) was prepared and its effects were investigated as an additive in 1.0 mol dm⁻³ LiPF₆-propylene carbonate (PC)/dimethyl carbonate (DMC) (1:1, v/v) electrolyte for lithium ion batteries. The electrochemical properties of the electrolyte with MPBMDS were characterized by discharge/charge tests, cyclic voltammetry, electrochemical impedance spectroscopy, scanning electron microscopy (SEM) and X-ray photoelectron spectroscopy (XPS). The addition of MPBMDS can effectively prevent the decomposition and the co-intercalation of PC. In addition, burning tests showed that the addition of 4–13 wt.% MPBMDS to the bare PC-based electrolyte effectively reduces the flammability. This eco-friendly compound provides a new promising direction for the development of bi- or multi-functional additives for lithium ion batteries.     

High flash point electrolyte for use in lithium-ion batteries

The high flash point solvent adiponitrile (ADN) was investigated as co-solvent with ethylene carbonate (EC) for use as lithium-ion battery electrolyte. The flash point of this solvent mixture was more than 110 °C higher than that of conventional electrolyte solutions involving volatile linear carbonate components, such as diethyl carbonate (DEC) or dimethyl carbonate (DMC). The electrolyte based on EC:ADN (1:1 wt) with lithium tetrafluoroborate (LiBF₄) displayed a conductivity of 2.6 mS cm⁻¹ and no aluminum corrosion. In addition, it showed higher anodic stability on a Pt electrode than the standard electrolyte 1 M lithium hexafluorophosphate (LiPF₆) in EC:DEC (3:7 wt). Graphite/Li half cells using this electrolyte showed excellent rate capability up to 5C and good cycling stability (more than 98% capacity retention after 50 cycles at 1C). Additionally, the electrolyte was investigated in NCM/Li half cells. The cells were able to reach a capacity of 104 mAh g⁻¹ at 5C and capacity retention of more than 97% after 50 cycles. These results show that an electrolyte with a considerably increased flash point with respect to common electrolyte systems comprising linear carbonates, could be realized without any negative effects on the electrochemical performance in Li-half cells.     

Solvents effects on electrochemical characteristics of graphite fluoride—lithium batteries

A study was made of the electrochemical characteristics of graphite fluoride—lithium batteries in various non-aqueous solvents. Two types of graphite fluorides, (C₂F)_n and (CF)_n were used as cathode materials. The discharge characteristics of graphite fluorides were better in dimethylsulfoxide, γ-butyrolactone, propylene carbonate and sulfolane in that order. The relation between electrode potential of graphite fluoride and solvation energy of lithium ion with each solvent indicates that solvated lithium ion is intercalated into traphite fluoride layers by the electrode reaction. Both the difference in the overpotentials and in the rates of OCV recovery among these solvents further supports the proposed reaction mechanism.     

Pyrolytic polyurea encapsulated natural graphite as anode material for lithium ion batteries

Carbon encapsulated graphite was prepared by coating polyurea on the surface of natural graphite particles via interfacial polymerization followed by a pre-oxidation at 250 °C in air and a heat treatment at 850 °C in nitrogen. FT-IR spectroscopy, X-ray diffraction (XRD) and scanning electron microscopy (SEM) were employed to investigate the structure of the graphite before and after the surface modification. Galvanostatic cycling, dc impedance spectroscopy, and cyclic voltammetry were used to investigate the electrochemical properties of the modified graphite as the anode material of lithium cells. The modified graphite shows a large improvement in electrochemical performance such as higher reversible capacity and better cycleability compared with the natural graphite. It can work stably in a PC-based electrolyte with the PC content up to 25 vol.% because the encapsulated carbon can depress the co-intercalation of solvated lithium ion. The initial coulombic efficiency of C-NG and NG in non-PC electrolyte is 74.9 and 88.5%, respectively.     

A study of electrochemical kinetics of lithium ion in organic electrolytes

Limiting current densities equivalent to the transport-controlling step of lithium ions in organic electrolytes were measured by using a rotating disk electrode (RDE). The diffusion coefficients of lithium ion in the electrolyte of PC/LiClO₄, EC : DEC/LiPF₆ and EC : DMC/LiPF₆ were determined by the limiting current density data according to the Levich equation. The diffusion coefficients increased in the order of PC/LiClO₄<EC : DEC/LiPF₆<EC : DMC/ LiPF₆ with respect to molar concentration of lithium salt. The maximum value of diffusivity was 1.39x10-5cm²/s for 1M LiPF₆ in EC : DMC=1 : 1. Exchange current densities and transfer coefficients of each electrolyte were determined according to the Butler-Volmer equation.     

Calorimetric investigation of the reactivity of the passivation film on lithiated graphite at elevated temperatures

Thermal storage of lithiated graphite electrodes has been performed between 40 and 90 °C for 8 h to 3 weeks. The results were compared for two separators: Celgard 2402 and a microporous PVdF membrane. The effects of storage on the capacity losses have been discussed with respect to the passivation film on the graphite electrodes in contact with the electrolyte solution EC:DMC:DEC (2:2:1)-1 M LiPF₆. The capacity loss shows a thermally activated character, which has been related to transformations of the passivation film at moderate temperatures. At higher temperatures, reaction of the intercalated lithium takes place, controlled by Li⁺-ion diffusion. DSC measurements were performed on passivated and lithiated graphite electrodes. Two peaks could be distinguished. An effect of the elevated temperature storage on the intensity and onset temperature of the first peak in DSC is evidenced. This peak could be attributed to the transformation of the passivation film. The second peak is due to the diffusion of lithium ions and the subsequent reaction with the liquid electrolyte.The effect of washing the electrode with DMC was thoroughly investigated. Our results allowed to attribute the transformation of the passivation film upon DSC analysis to a reaction taking place in the presence of LiPF₆.