Diphenyloctyl phosphate and tris(2,2,2-trifluoroethyl) phosphite as flame-retardant additives for Li-ion cell electrolytes at elevated temperature

The effect of diphenyloctyl phosphate (DPOF) and tris(2,2,2-trifluoroethyl) phosphite (TTFP) as flame-retardant (FR) additives in the liquid electrolyte of Li-ion cells is evaluated at both elevated temperature (40 °C) and room temperature (RT, 25 °C). The tested cells use mesocarbon microbeads (MCMB) and LiCoO₂ as the anode and cathode materials, respectively. Cell characteristics are investigated by means of electrochemical impedance spectroscopy (EIS) and scanning electron microscopy (SEM). The results of the cycle performance tests demonstrate the superior discharge capacity and capacity retention of the DPOF-containing cell compared will TTFP after cycling at both RT and 40 °C. Therefore, these results confirm the promising potential of DPOF as an FR additive for improving the electrochemical performance of Li-ion batteries.     

Effects of functional electrolyte additives for Li-ion batteries

The electrochemical behaviour and thermal stability of functional electrolyte additives for Li-ion batteries is investigated. The Li-ion cell systems is comprised of an anode of mesocarbon microbeads (MCMB) and a cathode (LiCoO₂) in a solution of 1.1 M LiPF₆ dissolved in ethylene carbonate and ethylmethyl carbonate (EC:EMC; 4:6, v/v). Vinyl acetate (VA) and vinylene carbonate (VC) in an ionic electrolyte containing triphenylphosphate (TPP) are tested as functional electrolyte additives. The main analysis tools used in this study are cyclic voltammetry (CV), differential scanning calorimetry (DSC), electrochemical impedance spectroscopy (EIS), and scanning electron microscopy (SEM). Cells containing VA or VC exhibit excellent irreversible capacity, coulombic efficiency, rate capability and cycleability. These features confirming the effectiveness of VC addition for improving both the cell performance and the thermal stability of electrolytes in TPP-containing solutions for Li-ion batteries.     

Sodium tungstate as electrolyte additive to improve high-temperature performance of nickel–metal hydride batteries

Sodium tungstate (Na₂WO₄) used as new electrolyte additive to enhance the high-temperature performance of Nickel–metal hydride (Ni–MH) battery is investigated in this paper. The effects of Na₂WO₄ on nickel hydroxide electrodes are investigated using cyclic voltammetry (CV), electrochemical impedance spectroscopy (EIS), and a charge/discharge test. It is found that the Ni–MH cell with the conventional KOH electrolyte containing 1 wt.% Na₂WO₄ additive exhibits higher discharge retention and better cycling performance than the cell without Na₂WO₄ additive at both 25 °C and 70 °C. These performance improvements are ascribed to the enhancement of oxygen evolution overvoltage and lower electrochemical impedance, as indicated by CV and EIS. The results suggest that the proposed approach be an effective way to improve the high temperature performance of Ni–MH batteries.     

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.     

LiBOB-based gel electrolyte Li-ion battery for high temperature operation

In this work, we evaluated the chemical compatibility of 1.0m (molality) lithium bis(oxalate)borate (LiBOB) 1:1 (w/w) propylene carbonate (PC)/ethylene carbonate (EC) liquid electrolyte with lithium metal and spinel LiMn₂O₄ cathode using storage and cycling tests at high temperatures. Impedance analyses show that LiBOB and lithium are very compatible due to the formation of a stable passivation layer on the surface of lithium. Cycling tests of Li/Cu and Li/LiMn₂O₄ cells, respectively, show that lithium can be plated and stripped in LiBOB-based electrolyte with more than 80% cycling efficiency, and that this electrolyte can support LiMn₂O₄ cycling reversibly up to 60 °C without visible capacity loss. Using LiBOB-based liquid electrolyte and porous Kynar^® membrane, microporous gel electrolyte (MGE) Li-ion cells were assembled and evaluated. Results show that the MGE cell presents an improved cycling performance compared with a liquid cell, especially at elevated temperatures. It is confirmed that the LiBOB-based gel electrolyte Li-ion batteries can be operated at 60 °C with good capacity retention.     

LiPF6/methyl difluoroacetate electrolyte with vinylene carbonate additive for Li-ion batteries

A methyl difluoroacetate (MFA)-based LiPF₆ solution was applied as an electrolyte to improve the thermal stability of Li-ion batteries. The addition of vinylene carbonate (VC) improved the electrochemical characteristics of the electrolyte significantly, and satisfactory reversible capacity and cycling performance were obtained with a graphite negative electrode. The thermal stability of the electrolytes was investigated with DSC. Regardless of whether or not VC was used, the electrolyte exothermically decomposed at a temperature higher than 450 °C. The thermal behavior of a mixture of lithiated graphite and VC-added electrolyte was also studied in detail. The ratio between the electrolyte and the electrode was a dominant factor in the heat generation of the mixture. A sharp exothermic peak at about 330 °C was observed when the electrode was superabundant, but the heat value was much smaller than that obtained with 1 mol dm⁻³ LiPF₆/EC-DMC electrolyte under the same conditions. When the electrolyte was superabundant, a mild exothermic decomposition of the electrolyte became the dominant reaction in the mixture. X-ray photoelectron spectroscopic analysis was carried out on delithiated graphite electrodes to study the effect of VC additive on solid electrolyte interphase (SEI) modification. VC-added MFA-based electrolyte was considered to be a good candidate for developing safer Li-ion batteries.     

Improved electrolytes for Li-ion batteries: Mixtures of ionic liquid and organic electrolyte with enhanced safety and electrochemical performance

Physical and electrochemical characteristics of Li-ion battery systems based on LiFePO₄ cathodes and graphite anodes with mixture electrolytes were investigated. The mixed electrolytes are based on an ionic liquid (IL), and organic solvents used in commercial batteries. We investigated a range of compositions to determine an optimum conductivity and non-flammability of the mixed electrolyte. This led us to examine mixtures of ILs with the organic electrolyte usually employed in commercial Li-ion batteries, i.e., ethylene carbonate (EC) and diethylene carbonate (DEC). The IL electrolyte consisted of (trifluoromethyl sulfonylimide) (TFSI) as anion and 1-ethyl-3-methyleimidazolium (EMI) as the cation. The physical and electrochemical properties of some of these mixtures showed an improvement characteristics compared to the constituents alone. The safety was improved with electrolyte mixtures; when IL content in the mixture is ≥40%, no flammability is observed. A stable SEI layer was obtained on the MCMB graphite anode in these mixed electrolytes, which is not obtained with IL containing the TFSI-anion. The high-rate capability of LiFePO₄ is similar in the organic electrolyte and the mixture with a composition of 1:1. The interface resistance of the LiFePO₄ cathode is stabilized when the IL is added to the electrolyte. A reversible capacity of 155 mAh g⁻¹ at C/12 is obtained with cells having at least some organic electrolyte compared to only 124 mAh g⁻¹ with pure IL. With increasing discharge rate, the capacity is maintained close to that in the organic solvent up to 2 C rate. At higher rates, the results with mixture electrolytes start to deviate from the pure organic electrolyte cell. The evaluation of the Li-ion cells; LiFePO₄//Li₄Ti₅O₁₂ with organic and, 40% mixture electrolytes showed good 1st CE at 98.7 and 93.0%, respectively. The power performance of both cell configurations is comparable up to 2 C rate. This study indicates that safety and electrochemical performance of the Li-ion battery can be improved by using mixed IL and organic solvents.     

Physicochemical properties of lithium iron phosphate‐carbon as lithium polymer battery cathodes

Lithium iron phosphate‐carbon (LiFePO₄/multiwalled carbon nanotubes (MWCNTs)) composite cathode materials were prepared by a hydrothermal method. In this study, we used MWCNTs as conductive additive. Poly (vinylidene fluoride‐co‐hexafluoropropylene)‐based solid polymer electrolyte (SPE) was applied. The structural and morphological performance of LiFePO₄/MWCNTs cathode materials was investigated by X‐ray diffraction and scanning electron microscopy/mapping. The electrochemical properties of Li/SPE/LiFePO₄‐MWCNTs coin‐type polymer batteries were analyzed by cyclic voltammetry, ac impedance and galvanostatic charge/discharge tests. Li/SPE/LiFePO₄‐MWCNTs polymer battery with 5 wt % MWCNTs demonstrates the highest discharge capacity and stable cyclability at room temperature. It is indicated that LiFePO₄‐MWCNTs can be used as the cathode materials for lithium polymer batteries. Copyright © 2011 John Wiley & Sons, Ltd.     

Influence of the microstructure on the electrochemical performance of thin film WO3 cathode

In this work, WO₃ thin films deposited at different powers and geometries have been evaluated for use as electrodes in thin film batteries. The potential profiles and cycling capacity in lithium electrolyte were investigated. The discharge capacity for samples strongly depends on the deposition conditions, and there is a clear correlation between microstructure and electrochemical performance.     

Electrochemical performance of lithium-ion batteries with triphenylphosphate as a flame-retardant additive

Safety concerns have been the key problem in the practical application of lithium-ion batteries. In the present study, triphenylphosphate (TPP) is used as an electrolyte additive to improve the thermal safety and electrochemical performance of lithium-ion cells. Cyclic voltammetric measurements and thermal stability measurements by means of differential scanning calorimetry are undertaken. Rate capability and cycling performance are evaluated. The flame-retarding additive TPP is electrochemically stable up to 4.9 V. The TPP-containing electrolytes display improved thermal stability compared with TPP-free electrolytes. The addition of 3% TPP to the ionic electrolyte is an optimum content for improvement of cell performance and suppression of electrolyte flammability.     

Effect of vinylene carbonate (VC) as electrolyte additive on electrochemical performance of Si film anode for lithium ion batteries

The effect of VC as electrolyte additive on the electrochemical performance of Si film anode was studied in this paper. The charge/discharge test, scanning electron microscopy (SEM), electrochemical impedance spectrum (EIS), Fourier transform infrared (FTIR) and X-ray photoelectron spectroscopy (XPS) were used to investigate the cycle performance and SEI layer of Si film anode. It was found that the SEI layer formed in VC-containing electrolyte possessed better properties. It was impermeable to electrolyte and its impedance kept almost invariant upon cycling. The presence of VC in electrolyte brought out the VC-reduced products and decreased the LiF content in SEI layer. The major components of SEI layer were similar in VC-free and VC-containing electrolytes, which contained lithium salt (e.g. ROCO₂Li, Li₂CO₃, LiF), polycarbonate and silicon oxide. It was newly found that silicon oxide could be formed in SEI layer of Si film anode due to the reaction of lithiated silicon with permeated electrolyte in both VC-free and VC-containing electrolytes.     

Preparation and electrochemical properties of indium- and sulfur-doped LiMnO2 with orthorhombic structure as cathode materials

The indium- and sulfur-doped LiMnO₂ samples with orthorhombic structure as cathode materials for Li-ion batteries are synthesized via hydrothermal method. The microstructure and composition of the samples were characterized by X-ray diffraction (XRD), transmission electron microscopy (TEM), inductively coupled plasma atom emission spectroscopy (ICP-AES), and X-ray photoelectron spectroscopy (XPS) analysis. It is shown that these samples with the orthorhombic structure have irregular shapes with a grain size of about 100–200 nm. The electrochemical performance of these samples as cathode materials was studied by galvanostatic method. All doped materials can offer improved cycling stability and high rate discharge ability as compared with the un-doped Li_0.99MnO₂. Moreover, dual In/S doping can slow down the capacity decay to a great extent, although the transformation to spinel occurs undesirably for all the doped samples during electrochemical cycling.     

Sn–Ni/MWCNT nanocomposite negative electrodes for Li-ion batteries: The effect of Sn:Ni molar ratio

Since Ni is used to behave as a buffer component in the Sn-based anode materials for the Li-ion batteries, it is aimed to reveal the optimum Sn:Ni ratio to reduce the electrode pulverization emanated from volume increase during the charge/discharge process. MWCNTs were also co-deposited from the suspended MWCNT in the electrolyte to increase buffering effect and conductivity. To reduce irreversible capacity and improve cycle performance of tin electrodes, Sn–Ni/MWCNT three components nanocomposite electrodes were prepared with different Sn:Ni ratio by pulse electrodeposition method using copper substrate as current collector. The morphology and the structures of the Sn–Ni/MWCNT nanocomposites were characterized by scanning electron microscopy (SEM), X-ray diffraction (XRD). Raman spectroscopy was used to determine the existence of MWCNT in the Sn–Ni matrix. The electrochemical performance of Sn–Ni/MWNT nanocomposites studied by charge/discharge tests and cyclic voltammetry experiments. Results showed that increasing the amount of co-deposited Ni has a strong effect on the electrochemical performances. The best electrochemical results were obtained in the nanocomposite electrodes with Ni content of 29 wt.%     

Improvement of electrochemical and structural properties of LiMn2O4 spinel based electrode materials for Li-ion batteries

Spinel LiMn₂O₄ and LiM_0.02Mn_1.98O₄ (where M is Zn, Co, Ni and In) were produced via facile sol–gel method and Cu/LiMn₂O₄, Cu/LiM_0.02Mn_1.98O₄, Ag/LiMn₂O₄ and Ag/LiM_0.02Mn_1.98O₄ binary composite electrode materials were produced via electroless coating techniques as a positive electrode material for Li-ion batteries. The phase composition, morphology and electrochemical properties of the synthesized materials were investigated by X-ray diffraction (XRD), field emission scanning electron microscopy (FESEM), cyclic voltammometry (CV), galvanostatic charge–discharge tests and electrochemical impedance spectroscopy (EIS). The synthesized cathode active materials are characterized as single phase spinel LiMn₂O₄ with degree of crystallization and uniform particle size distribution. Best results were obtained with electrodes substituted with In and an initial discharge capacity of 134 mAhg⁻¹ after 50 cycles. The improvement in the cycling performance may be attributed to stabilization of spinel structure by smaller lattice constant when manganese ion was partially substituted with In³⁺ ions. EIS analysis also confirms that the obvious improvement in Ag coating is mainly attributed to the accelerated phase transformation from layered phase to spinel phase and highly stable electrolyte/electrode interface due to the suppression of electrolyte decomposition.     

Graft copolymer-based lithium-ion battery for high-temperature operation

The use of conventional lithium-ion batteries in high temperature applications (>50 °C) is currently inhibited by the high reactivity and volatility of liquid electrolytes. Solvent-free, solid-state polymer electrolytes allow for safe and stable operation of lithium-ion batteries, even at elevated temperatures. Recent advances in polymer synthesis have led to the development of novel materials that exhibit solid-like mechanical behavior while providing the ionic conductivities approaching that of liquid electrolytes. Here we report the successful charge and discharge cycling of a graft copolymer electrolyte (GCE)-based lithium-ion battery at temperatures up to 120 °C. The GCE consists of poly(oxyethylene) methacrylate-g-poly(dimethyl siloxane) (POEM-g-PDMS) doped with lithium triflate. Using electrochemical impedance spectroscopy (EIS), we analyze the temperature stability and cycling behavior of GCE-based lithium-ion batteries comprised of a LiFePO₄ cathode, a metallic lithium anode, and an electrolyte consisting of a 20-μm-thick layer of lithium triflate-doped POEM-g-PDMS. Our results demonstrate the great potential of GCE-based Li-ion batteries for high-temperature applications.     

Comparative study of trimethyl phosphite and trimethyl phosphate as electrolyte additives in lithium ion batteries

Safety concerns of lithium ion batteries have been the key problems in their practical applications. Trimethyl phosphite (TMP(i)) and trimethyl phosphate (TMP(a)) were used as the electrolyte additives to improve the safety and electrochemical performance of lithium cells. Gallvanostatic cell cycling, flammability test and thermal stability measurements by means of accelerated rate calorimeter (ARC) and micro calorimeter were performed. It is found that both TMP(i) and TMP(a) reduce the flammability of the electrolyte. The TMP(i) additive not only enhances the thermal stability of the electrolyte, but also improves its electrochemical performance. The TMP(a) additive can improve the thermal stability of the electrolyte at the expense of some degree of degradation of its electrochemical performance. Therefore, TMP(i) is a better flame retardant additive in the electrolyte compared with TMP(a).     

Effect of carbon nanotube on the electrochemical performance of C-LiFePO4/graphite battery

This paper describes the fabrication and testing of C-LiFePO₄/graphite battery with different conductive carbon additives: carbon nanotube (CNT) or carbon black (CB). The discharge capacity, rate capability and cyclic performance of the battery were investigated. Compared with the batteries with CB additive, those with CNT additive show better electrochemical performances with capacity retention ratio of 99.2% after 50 cycles, and the ratio of discharge capacity at 0.1 C rate to that at 1 C rate is 94.6%. The reason for the difference in electrochemical property was studied with cyclic voltammagrams and AC impedance. It was found that, with CNT additive, the polarization voltage was decreased from 0.3 to 0.2 V, and the impedance was decreased from 423.2 to 36.88 Ω. The structures of active materials after cycling were characterized using XRD. The better crystal retaining of LiFePO₄ was found in the active materials with CNT added.     

Dimethyl methylphosphonate (DMMP) as an efficient flame retardant additive for the lithium-ion battery electrolytes

Dimethyl methylphosphonate (DMMP) was used as a flame retardant additive to 1 M LiPF₆/EC + DEC system. The flammability, electrochemical stability and cycling performance of electrolyte containing DMMP were studied. The addition of DMMP to electrolytes provides a significant suppression in the flammability of the electrolyte concluded from the measurements of self-extinguish time and limited oxygen index. The totally nonflammable electrolytes can be achieved with only 10 wt.% DMMP addition—the highly efficient retardant additive. The addition of DMMP causes little damage on the cell electrochemical performance. DMMP is a promising flame retardant additive to improve the safety of lithium-ion batteries.     

Preparation and characterization of 18650 Li(Ni1/3Co1/3Mn1/3)O2/graphite high power batteries

The commercial 18650 Li(Ni_1/3Co_1/3Mn_1/3)O₂/graphite high power batteries were prepared and their electrochemical performance at temperatures of 25 and 50 °C was extensively investigated. The results showed that the charge-transfer resistance (R_ct) and solid electrolyte interface resistance (R_sei) of the high power batteries at 25 °C decreased as states of charge (SOC) increased from 0 to 60%, whereas R_ct and R_sei increased as SOC increased from 60 to 100%. The discharge plateau voltage of batteries reduced greatly with the increase in discharge rate at both 25 and 50 °C. The high power batteries could be discharged at a very wide current range to deliver most of their capacity and also showed excellent power cycling performance with discharge rate of as high as 10 C at 25 °C. The elevated working temperature did not influence the battery discharge capacity and cycling performance at lower discharge rates (e.g. 0.5, 1, and 5 C), while it resulted in lower discharge capacity at higher discharge rates (e.g. 10 and 15 C) and bad cycling performance at discharge rate of 10 C. The batteries also exhibited excellent cycle performance at charge rate of as high as 8 C and discharge rate of 10 C.     

Tris(pentafluorophenyl) borane as an electrolyte additive for LiFePO4 battery

The effects of tris(pentafluorophenyl) borane (TPFPB) additive in electrolyte at the LiFePO₄ cathode on the high temperature capacity fading were investigated by electrochemical impedance spectroscopy (EIS), cyclic voltammetry (CV), cyclability, SEM and Fourier transform infrared (FTIR). According to the study results, tris(pentafluorophenyl) borane has the ability to improve the cycle performance of LiFePO₄ at high temperature. LiFePO₄ electrodes cycled in the electrolyte without the TPFPB additive show a significant increase in charge transfer resistance by EIS analysis. SEM and FTIR disclose evidence of surface morphology change and solid electrolyte interface (SEI) formation. FTIR investigation shows various functional groups are found on the cathode material surface after high temperature cycling tests. The results showed an obvious improvement of high temperature cycle performance for LiFePO₄ cathode material due to the TPFPB additive. The observed improved cycling performance and improved lithium ion transport are attributed to decreased LiF content in the SEI film.