Performance improvement of lithium ion battery using PC as a solvent component and BS as an SEI forming additive

The electrochemical behavior of propylene carbonate (PC)-based electrolytes with and without butyl sultone (BS) on graphite electrode and the performance of lithium ion batteries with these electrolytes were studied with cyclic voltammetry (CV), energy dispersive spectroscopy (EDS), as well as density functional theory (DFT) calculation. It is found that the co-insertion of PC with lithium ions into graphite electrode can be inhibited to a great extent by adjusting the composition of solvent in electrolytes. With the application of PC in the electrolyte without any additive, the discharge capacity of lithium ion battery is improved under high temperature or low temperature, however it decays under room temperature compared with the battery without PC. This drawback can be overcome by using BS as a solid electrolyte interphase (SEI) forming additive. BS has a lower LUMO energy and can be more easily electro-reduced than other components of solvent in electrolyte on a graphite electrode, forming a stable SEI film. With the application of BS in the electrolyte, the discharge capacity and cyclic stability of lithium ion battery is improved significantly under room temperature.     

A novel flame retardant and film-forming electrolyte additive for lithium ion batteries

Allyl tris(2,2,2-trifluoroethyl) carbonate (ATFEC) was synthesized as a bi-functional additive of flame retardant and film former in electrolytes for lithium ion batteries (LIBs). The flame retardancy of the additive was characterized with differential scanning calorimetry (DSC) and self-extinguishing time (SET). It is shown that adding 1 vol.% ATFEC in 1 M LiPF₆/propylene carbonate (PC) can effectively enhance the thermal stability of the electrolyte and suppress the co-intercalation of PC into the graphitic anode. Further evaluation indicates that the additive hardly affect the conductivity of electrolyte. These support the feasibility of using ATFEC as an additive on formulating an electrolyte with multiple functions such as film-forming enhancement, high thermal stability and high ionic conductivity.     

Reconsideration of SEI stability: reversible lithium intercalation into graphite electrodes in trans-2,3-butylene carbonate

Lithium ion batteries with graphitic carbon anodes and LiCoO₂ cathodes are cycled reversibly in electrolytes based on trans-2,3-butylene carbonate (t-BC), even in the absence of ethylene carbonate. While the poor interfacial film (the solid electrolyte interface (SEI)) on the lithium electrode can be readily explained in terms of previous models of its stability, this highly reversible behavior of graphite is hard to account for. To explain this profound difference in the SEI stability of the two electrodes, we have taken into account the influence that the nature of the electrode (lithium metal versus graphite) and the type of the reaction site (basal plane versus edge sites) exert on the solvent reduction pathways.     

Functional interface of polymer modified graphite anode

Graphite electrodes were modified by polyacrylic acid (PAA), polymethacrylic acid (PMA), and polyvinyl alcohol (PVA). Their electrochemical properties were examined in 1 mol dm⁻³ LiClO₄ ethylene carbonate:dimethyl carbonate (EC:DMC) and propylene carbonate (PC) solutions as an anode of lithium ion batteries. Generally, lithium ions hardly intercalate into graphite in the PC electrolyte due to a decomposition of the PC electrolyte at ca. 0.8 V vs. Li/Li⁺, and it results in the exfoliation of the graphene layers. However, the modified graphite electrodes with PAA, PMA, and PVA demonstrated the stable charge–discharge performance due to the reversible lithium intercalation not only in the EC:DMC but also in the PC electrolytes since the electrolyte decomposition and co-intercalation of solvent were successfully suppressed by the polymer modification. It is thought that these improvements were attributed to the interfacial function of the polymer layer on the graphite which interacted with the solvated lithium ions at the electrode interface.     

New electrolytes for lithium ion batteries using LiF salt and boron based anion receptors

The thermal and electrochemical stability, as well as compatibility with various bench mark cathode and anode materials of two new lithium fluoride salt (LiF) based electrolytes have been studied. These two new electrolytes are formed by using boron-based anion receptors, tris(pentafluorophenyl) borane (TPFPB), or tris(2H-hexafluoroisopropyl) borate (THFPB) as additives, which were designed and synthesized at Brookhaven National Laboratory (BNL), to dissolve the LiF salt in carbonate solvents. The transference number of Li⁺ for these electrolytes is as high as 0.7 and the room-temperature conductivity is around 2 × 10⁻³ S cm⁻¹. The electrolytes containing propylene carbonate (PC) show superior low-temperature conductivity properties. The electrochemical window is approaching 5.0 V. It was also found that the new electrolytes work well with LiCoO₂ or LiMn₂O₄ cathodes. However, when PC containing electrolytes were used, PC co-intercalation is still a problem for graphite anodes. The formation of a stable solid electrolyte interface layer on the surface of anode in this type of electrolyte needs to be studied further.     

The cycling behaviour and stability of the lithium electrode in propylene carbonate and acetonitrile electrolytes

The conductivity and chemical stability with lithium of various electrolytes containing propylene carbonate (PC) and acetonitrile (AN) were determined. Addition of AN improved the conductivity of LiClO₄/PC and LiAsF₆/PC electrolytes, and the LiAsF₆/PC-AN electrolyte showed remarkable chemical stability in contact with lithium. The lithium cycling efficiency was determined on nickel and aluminium substrates in the various electrolytes over a range of current density. While the efficiencies observed on nickel substrates were very poor for all AN-containing electrolytes, efficiencies approaching those for electrolytes containing only PC were obtained with the LiAsF₆/PC-AN electrolyte at low current densities (～1 mA cm^?2) on aluminium substrates. It was concluded that the LiAsF₆/PC-AN electrolyte had generally favourable characteristics and may prove suitable for primary battery applications.     

Chelate complexes with boron as lithium salts for lithium battery electrolytes

The electrolytic conductivity and charge–discharge characteristics of lithium electrodes are examined in propylene carbonate (PC)- and ethylene carbonate (EC)-based binary solvent electrolytes containing lithium bis[1,2-benzenediolato(2-)-O,O′]borate (LBBB), lithium bis[2,3-naphthalenediolato(2-)-O,O′]borate (LBNB) and lithium bis[2,2′-biphenyldiolato(2-)-O,O′]borate (LBBPB). The LBBPB exhibits high thermal and electrochemical stability compared with LBBB and LBNB. Conductivities in PC-THF and EC-THF binary solvent electrolytes at XTHF (mole fraction of tetrahydrofuran, THF)=0.5 containing 0.5 M LBBB and LBNB are nearly equal to that in 0.5 M LiCF₃SO₃ electrolyte as a typical lithium battery electrolyte. The conductivity in 0.3 M LBBPB/PC-DME (DME: 1,2-dimethoxyethane) electrolyte is fairly low compared with that in other electrolytes. The energy density with the LBNB electrolyte is higher than that with LBBB or LBBPB electrolyte. In general, lithium cycling efficiencies in THF-based LBBB and LBNB electrolytes become higher than those in DME-based electrolytes. The 0.5 M LBNB/PC-THF electrolyte is a moderately rechargeable lithium battery electrolyte. The 0.3 M LBBPB/PC-DME equimolar solvent electrolyte displays the highest cycling efficiency, viz., >70%, at a high range of cycle number.     

Improving the electrochemical properties of graphite/LiCoO2 cells in ionic liquid-containing electrolytes

The electrochemical performance of graphite/lithium cobalt oxide (LiCoO₂) cells in N-methoxymethyl-N,N-dimethylethylammonium bis(trifluoromethane-sulfonyl) imide (MMDMEA-TFSI)-containing electrolytes is significantly enhanced by the formation of a fluoroethylene carbonate (FEC)-derived protective film on an anode during the first cycle. The electrochemical intercalation of MMDMEA cations into the graphene layer is readily visualized by ex situ transmission electron microscopy (TEM). Moreover, differences in the X-ray diffraction (XRD) patterns of graphite electrodes in cells charged with and without FEC in dimethyl carbonate (DMC)/MMDMEA-TFSI are clearly discernible. Conclusively, the presence of FEC in MMDMEA-TFSI-containing electrolytes leads to a remarkable enhancement of discharge capacity retention for graphite/LiCoO₂ cells as compared with ethylene carbonate (EC) and vinylene carbonate (VC).     

Lithium insertion in graphite from ternary ionic liquid-lithium salt electrolytes: II. Evaluation of specific capacity and cycling efficiency and stability at room temperature

In this paper we report the results about the use of ternary room temperature ionic liquid-lithium salt mixtures as electrolytes for lithium-ion battery systems. Mixtures of N-methyl-N-propyl pyrrolidinium bis(fluorosulfonyl) imide, PYR₁₃FSI, and N-butyl-N-methylpyrrolidinium bis(trifluoromethansulfonyl) imide, PYR₁₄TFSI, with lithium hexafluorophosphate, LiPF₆ and lithium bis(trifluoromethansulfonyl) imide, LiTFSI, containing 5 wt.% of vinylene carbonate (VC) as additive, have been used in combination with a commercial graphite, KS6 TIMCAL. The performance of the graphite electrodes has been considered in term of specific capacity, cycling efficiency and cycling stability. The results clearly show the advantage of the use of ternary mixtures on the performance of the graphite electrode.     

Effects of cyclic carbonates as additives to γ-butyrolactone electrolytes for rechargeable lithium cells

γ-Butyrolactone (GBL) has a high boiling point, a low freezing point, a high flashing point, a high dielectric constant and a low viscosity. GBL is a very preferable solvent for lithium ion batteries. However, GBL readily undergoes reductive decomposition on the surface of the negative electrodes, and it forms a solid electrolyte interphase (SEI) with a large resistance. It is causing deterioration of battery performances. In this work, effects of cyclic carbonates as additives to GBL electrolytes were investigated. As these carbonates, ethylene carbonate (EC), propylene carbonate (PC), vinylene carbonate (VC), vinylethylene carbonate (VEC), and phenylethylene carbonate (PhEC) were investigated using LiCoO₂/graphite cells. The effects of these additives were evaluated from the viewpoints of improvement of the battery performance and suppression of the reductive decomposition of GBL. VC, VEC and PhEC were effective to suppress the excess reductive decomposition of GBL. Battery performances were improved and the following results were obtained from the electrochemical measurements of LiCoO₂/graphite cells with GBL-based electrolytes. Residual capacity was high in the order of VEC > VC > PhEC. Rate capability was high in the order of PhEC > VC > VEC. These additives have advantages and disadvantages. By optimizing electrolyte formulation, the performances of Li-ion batteries using GBL-based electrolytes will be improved further.     

The reductive mechanism of ethylene sulfite as solid electrolyte interphase film-forming additive for lithium ion battery

The reduction mechanism of ethylene sulfite (ES) in propylene carbonate (PC) based electrolyte is investigated using density functional theory in gas phase. Based on the electron affinity energy and lowest unoccupied molecular orbital (LUMO) energy, it can be known that free ES is reduced most easily compared with ES-Li⁺ and ES-Li⁺-PC, generating SO₂ and propanal. However, the binding energy of ES-Li⁺ and ES-Li⁺-PC is quite negative, indicating that both of them are more possible in electrolyte solution than the free ES. The reductive decomposition products of ES-Li⁺ and ES-Li⁺-PC are OSO₂Li, OSO₂Li-R and ethylene. OSO₂Li and OSO₂Li-R are the main compositions of the solid electrolyte interphase film on the anode of lithium ion battery, which inhibits the reductive decomposition of PC. These calculations provide a detailed explanation on the experimental phenomena.     

Incorporation of a stable radical 2,2,6,6-tetramethyl-1-piperidinyloxy (TEMPO) in an electrochromic device

A stable organic radical, 2,2,6,6-tetramethyl-1-piperidinyloxy (TEMPO), was studied. We employed TEMPO as a cathodic radical provider in propylene carbonate (PC) and poly(3,4-propylenedioxythiophene) derivatives (PProDOT-Et₂) as an anodic electrochromic (EC) thin film, which was obtained through electropolymerization. On assembling them together in a device, the electrochemical and optical performances of this hybrid electrochromic device (ECD) showed reversible cycling stability and high absorbance attenuation in the visible range. By selecting proper electrolytes (LiClO₄/PC) and controlling the deposited charge of the PProDOT-Et₂ thin film, it was possible to obtain a transmittance change (ΔT) of up to 59% at 590 nm with no noticeable degradation after operating between 0 and 0.9 V for 1000 cycles. Furthermore, an electrochemical quartz crystal microbalance (EQCM) was used to investigate ion migrations in the PProDOT-Et₂ thin film, which influenced its long-term stability.     

A study on sulfites for lithium-ion battery electrolytes

Because of the similarity in the structures of organic sulfites with those of organic carbonates, the applications of organic sulfites for lithium-ion battery electrolytes were studied. The main differences in the bond lengths and the bond angles, which are resulted from the difference between carbon atom diameter and sulfur atom diameter, are analyzed. The physical properties of organic carbonates and organic sulfites are compared. The results of cyclic voltammetry (CV) test show that the decomposition potentials of propylene sulfite (PS) and dimethyl sulfite (DMS) are much higher than 4.5 V, it is satisfied with the requirements as the solvents for lithium ion batteries. But the decomposition potentials of ethylene sulfite (ES) and diethyl sulfite (DES) are lower than 3.5 V, they can only be used as additives for lithium ion battery electrolytes. The results of charge–discharge tests show that both ES and PS have excellent film-forming properties; the performance of LiCoO₂/graphite cell was improved evidently even with the ES addition as little as 0.3 wt.% in 1 mol L⁻¹ LiPF₆ EC/DMC/DEC (1:2:2) electrolyte. DMS can improve both the conductivities of electrolytes and the capacities of batteries, therefore it is a good electrolyte co-solvent.     

Studies on the enhancement of solid electrolyte interphase formation on graphitized anodes in LiX-carbonate based electrolytes using Lewis acid additives for lithium-ion batteries

The new electrolyte systems utilizing one type of Lewis acids, the boron based anion receptors (BBARs) with LiF, Li₂O, or Li₂O₂ in carbonate solutions have been developed and reported by us. These systems open up a new approach in developing non-aqueous electrolytes with higher operating voltage and less moisture sensitivity for lithium-ion batteries. However, the formation of a stable solid electrolyte interphase (SEI) layer on the graphitized anodes is a serious problem needs to be solved for these new electrolyte systems, especially when propylene carbonate (PC) is used as a co-solvent. Using lithium bis(oxalato)borate (LiBOB) as an additives, the SEI layer formation on mesophase carbon microbeads (MCMB) anode is significantly enhanced in these new electrolytes containing boron-based anion receptors, such as tris(pentafluorophenyl) borane, and lithium salt such as LiF, or lithium oxides such as Li₂O or Li₂O₂ in PC and dimethyl carbonate (DMC) solvents. The cells using these electrolytes and MCMB anodes cycled very well and the PC co-intercalation was suppressed. Fourier transform infrared spectroscopy (FTIR) studies show that one of the electrochemical decomposition products of LiBOB, lithium carbonate (Li₂CO₃), plays a quite important role in the stablizing SEI layer formation.     

Interfacial reactions between graphite electrodes and propylene carbonate-based solutions: Electrolyte-concentration dependence of electrochemical lithium intercalation reaction

This study examines the electrochemical reactions occurring at graphite negative electrodes of lithium-ion batteries in a propylene carbonate (PC) electrolyte that contains different concentrations of lithium salts such as, LiClO₄, LiPF₆ or LiN(SO₂C₂F₅)₂. The electrode reactions are significantly affected by the electrolyte concentration. In concentrated solutions, lithium ions are reversibly intercalated within the graphite to form stage 1 lithium–graphite intercalation compounds (Li–GICs), regardless of the lithium salt used. On the other hand, electrolyte decomposition and exfoliation of the graphene layers occur continuously in the low-concentration range. In situ analysis with atomic force microscopy reveals that a thin film (thickness of ∼8 nm) forms on the graphite surface in a concentrated solution, e.g., 3.27 mol kg⁻¹ LiN(SO₂C₂F₅)₂/PC, after the first potential cycle between 2.9 and 0 V versus Li⁺/Li. There is no evidence of the co-intercalation of solvent molecules in the concentrated solution.     

Anodic behavior of Al current collector in 1-alkyl-3-methylimidazolium bis[(trifluoromethyl)sulfonyl] amide ionic liquid electrolytes

The anodic behaviors of aluminum current collector for lithium ion batteries were investigated in a series of 1-alkyl-3-methylimidazolium bis[(trifluoromethyl)sulfonyl] amide room temperature ionic liquids (RTILs) and EC + DMC electrolytes. It was found that the aluminum corrosion, which occurred in EC + DMC electrolytes containing LiTFSI, was not observed in the RTIL electrolytes. Further research showed that a passive film with amide compounds as main components formed firmly on aluminum surface during the anodic polarization in the RTIL electrolytes, which inhabited the aluminum corrosion. In addition, the additives generally used in the batteries, such as ethylene carbonate, ethylene sulfite and vinyl carbonate, as well as temperature did not obviously affect the aluminum passive film, the oxidation of the RTILs increased at the elevated temperature, which only resulted in the corrosion potential of aluminum in the RTIL electrolytes shifted to more negative potential, a passive film still firmly formed on the aluminum surface to surpassed the further oxidation of the aluminum current collector. Those results lead to a potential for the practical use of LiTFSI salt in the room temperature ionic liquid electrolytes for lithium ion batteries.     

Two distinct lithium diffusive species for polymer gel electrolytes containing LiBF4, propylene carbonate (PC) and PVDF

Polymer gel electrolytes have been prepared using lithium tetrafluoroborate (LiBF₄), propylene carbonate (PC) and polyvinylidene fluoride (PVDF) at 20% and 30% concentration by mass. Self diffusion coefficients have been measured using pulse field gradient nuclear magnetic resonance (PFG-NMR) for the cation and anion using ⁷Li and ¹⁹F resonant frequencies respectively. It was found that lithium ion diffusion was slow compared to the much larger fluorine anion likely resulting from a large solvation shell of the lithium. Lithium ion diffusion measurements exhibited two distinct diffusive species, whereas the fluorine ions exhibited only a single diffusive species.     

Cycling performance and thermal stability of lithium polymer cells assembled with ionic liquid-containing gel polymer electrolytes

Gel polymer electrolytes containing 1-butyl-1-methylpyrrolidinium bis(trifluoromethanesulfonyl)imide and a small amount of additive (vinylene carbonate, fluoroethylene carbonate, and ethylene carbonate) are prepared, and their electrochemical properties are investigated. The cathodic limit of the gel polymer electrolytes can be extended to 0 V vs. Li by the formation of a protective solid electrolyte interphase on the electrode surface. Using these gel polymer electrolytes, lithium metal polymer cells composed of a lithium anode and a LiNi_1/3Co_1/3Mn_1/3O₂ cathode are assembled, and their cycling performances are evaluated at room temperature. The cells show good cycling performance, comparable to that of a cell assembled with gel polymer electrolyte containing standard liquid electrolyte (1.0 M LiPF₆ in ethylene carbonate/diethylene carbonate). Flammability tests and differential scanning calorimetry studies show that the presence of the ionic liquid in the gel polymer electrolyte considerably improves the safety and thermal stability of the cells.     

Dimethyl methyl phosphate: A new nonflammable electrolyte solvent for lithium-ion batteries

A new fire retardant-dimethyl methyl phosphate (DMMP) was tested as a nonflammable electrolyte solvent for Li-ion batteries. It is found that in the addition of chloro-ethylene carbonate (Cl-EC) as an electrolyte additive, the electrochemical reduction of DMMP molecules can be completely suppressed and the graphite anode can be cycled very well with high initial columbic efficiency (∼84%) and excellent cycling stability in the DMMP electrolyte. The prismatic C/LiCoO₂ batteries using 1.0 mol L⁻¹ LiClO₄ + 10% Cl-EC + DMMP electrolyte exhibited almost the same charge and discharge performances as those using conventional carbonate electrolytes, suggesting a feasible use of this new electrolyte for constructing nonflammable Li⁺-ion batteries.