Tri-(4-methoxythphenyl) phosphate: A new electrolyte additive with both fire-retardancy and overcharge protection for Li-ion batteries

A novel compound, tri-(4-methoxythphenyl) phosphate, was synthesized and investigated as a safety electrolyte additive for lithium-ion batteries. It was found that this additive could lower the flammability of the electrolyte, and thereby enhance the thermal stability of the Li-ion battery. Moreover, this molecule can also be polymerized at 4.35 V (vs. Li/Li⁺) to form a conducting polymer, which can protect the batteries from voltage runaway at overcharge by internal bypassing the overcharging current in the batteries. Thus, it is possible to use this electrolyte additive to provide both overcharge protection and flame retardancy for lithium-ion batteries without much influence on the battery performance.     

Influence of electrolyte additives on safety and cycle life of rechargeable lithium cells

The influence of electrolyte additives on the safety and cycle life of 4V-class lithium cells is examined. The electrolyte solution employed was 1 M LiClO₄-propylene carbonate, the most widely used electrolyte in lithium battery research. The additives studied were ten organic aromatic compounds including biphenyl, cyclohexylbenzene and hydrogenated diphenyleneoxide. For safety, focus was given to the overcharging tolerance of the lithium cells. Biphenyl is well-known as an overcharge protection additive. The purpose of this work was to find additives with a higher oxidation potential and longer charge–discharge cycle life than biphenyl. The oxidation potentials and currents of the additives were measured to determine whether or not these compounds work as overcharge protection additives. Charge–discharge cycling efficiencies were examined for lithium metal anodes. The results showed that cyclohexylbenzene and hydrogenated diphenyleneoxide have a higher oxidation potential and a higher lithium cycling efficiency than biphenyl.     

环己苯过充添加剂在锂离子电池中的应用

本文分析了环己苯作为过充保护添加剂在锂离子电池中的应用。笔者采用对所组装的锂离子电池1C倍率过充电,锂离子电池循环性能测试,交流阻抗测试,电解液电导率以及电池自放电测试研究添加环己苯的量对锂离子电池的过充保护效果以及对电池性能的影响。同时,本文分析了环己苯作为过充保护剂的可能工作原理,发现当环己苯的含量大于5％时,能对锂离子电池起到良好的过充保护作用;高于7％时会对电池循环性能产生不良影响。同时环己苯会降低电解液电导率,导致电池自放电增加。笔者认为5％－7％是环己苯作为锂离子电池添加剂的适宜比例。     

Electrochemical behavior of biphenyl as polymerizable additive for overcharge protection of lithium ion batteries

Electrochemical properties and working mechanism of biphenyl as a polymerizable electrolyte additive for overcharge protection of lithium ion batteries are studied by microelectrode voltammetry, charge-discharge measurements and SEM characterization of the overcharged cell’s components. The experimental results reveal that biphenyl can electrochemically polymerize at the overcharge potential of 4.5-4.75 V (versus Li/Li⁺) to form a layer of conductive film on the cathode surface and the polymer deposits may develop to penetrate the separator to reach the anode surface, resulting an internal short-circuit to prevent from the cell voltage runaway. On the other hand, the electro-oxidative polymerization of biphenyl produces excessive gas and heat, which help to enhance the sensitivities of electric disconnecting devices. In addition, it is also found that the use of biphenyl as an electrolyte additive does not significantly influence the normal performances of the lithium ion batteries.     

A xylene electrolyte modified by trimethylchlorosilane for electrodeposition of aluminium

The addition of 2–5 vol % of trimethylchlorosilane to a 2m solution of AlBr₃ in xylene gives a very convenient aluminium electroplating bath. Tendency to dendritic growth ceases and the quality of aluminium improves. Electrodeposition proceeds with high cathodic efficiency. The additive has extended the use of the electrolyte up to 1 year.     

The cooperative effect of tri(β-chloromethyl) phosphate and cyclohexyl benzene on lithium ion batteries

Tri(β-chloromethyl) phosphate (TCEP) and cyclohexyl benzene (CHB) had been studied synchronously as the additives of the lithium ion batteries to improve the high temperature and overcharge safety. The study group used the cyclic voltammetry and environment scanning electron microscopy (ESEM) to investigate the oxidize potentials of the TCEP and CHB at normal environment and 150 °C and the surface characteristics of the positive electrode of the graphite/LiCoO₂(063465) batteries before and after overcharge. The self-extinguishing time (SET) tests were carried out to measure the effect of additives on the electrolyte combustibility. The oven and overcharge tests and other electrochemical testing methods were performed to test the reliability of the protection provided by the TCEP and CHB of the high temperature and overcharge safety and the effect of the additives on the electrochemical performance. The results show that the oxidize potential of the TCEP is about 4.75 V and the CHB about 4.6 V at normal environment and the oxidize potential of the TCEP drops to 4 V and CHB about 4.1 V at 150 °C; the TCEP has favorable effect on the self-extinguish time of the electrolyte; the surface of the LiCoO₂ positive electrode after overcharge formed a layer of CHB polymer; when the content of both additives are over 5 wt%, the batteries show improved safety through the oven tests of 150 °C and can stand 1.3 A (1 C) and constant 10 V overcharge tests; the adverse effect of TCEP and CHB on cycle performance of battery is relatively small. In sum, the cooperative use of these two additives can improve the safety of lithium ion battery greatly.     

The electrodeposition of aluminium from xylene and ether-hydride electrolytes

A comparative study of aluminium electrodeposition from xylene and ether electrolytes is presented. The kinetic parameters of aluminium deposition from the tetrahydrofuran (THF) and xylene electrolytes are presented. In the case of the THF and diethylether solutions investigations of aluminium nucleation were also made. It was found that the rate of aluminium deposition was highest in the THF electrolyte composed of AlCl₃ and LiAlH₄ in a ratio 1:1. This electrolyte is recommended for deposition of aluminium protective coatings of thickness over 20 µm     

Electrolysis of V(IV) and free meso-tetraphenyl porphyrin in two immiscible liquids

The electrochemical decomposition of free and vanadyl tetraphenyl porphyrin in xylene solutions, and in the presence of water, using K₂SO₄ as electrolyte, was studied. Electrolysis of the two-phase system was carried out at constant current density and potential. It was observed that the electrochemical decomposition of the porphyrins, using a two-phase liquid system, is possible. An inexpensive organic solvent (xylene instead of THF, acetonitrile, methylene chloride etc.) can be very effectively used in such a system. The decomposition occurs through an oxidation process, with the subsequent formation of cationic and dicationic radicals, and a final breakdown of the macrocycle.     

Melting-crystallization properties of the sulphur electrolyte in sodium-sulphur batteries

The present communication reports the results of some melting-crystallization studies, undertaken to characterize the thermal behaviour of the sulphur electrolyte. Heat capacities, enthalpies of transition (solid-state) and of fusion have been measured. The electrolyte is observed to have glass forming tendencies and the composition range for this behaviour is characterized. An inverse-crystallization phenomenon is observed as the sulphur content increases. The saturation solubility of sulphur is the sodium pentasulphide layer in the liquid-liquid immiscibility composition range of the sulphur electrolyte, the influence of moisture as an additive in trace amounts, and the thermal character of the chemical events in the formation of polysulphides from sulphur and Na₂S as reactants are also reported.     

Redox shuttle additives for chemical overcharge protection in lithium ion batteries

We disclosed that a few kinds of aromatic compounds having thianthrene derivatives with acetyl or other functional groups were stable up to about 4.2-4.3 V against lithium. These materials, called redox shuttle, have lately been employed as a chemical overcharge protection agent that consumes the excess current during battery overcharge. They oxidized above 4 V and worked as redox shuttle when introduced into the electrolyte of a Carbon/LiCoO₂ prismatic battery within less than one hour rate (1C). We also studied thermal properties of batteries containing the above-mentioned materials with ARC (Accelerating Rate Calorimeter). We ascertained that the current supplied over the full charge was not stored, but instantly and quite completely consumed in an oxidation-reduction reaction.     

Redox shuttles for safer lithium-ion batteries

Overcharge protection is not only critical for preventing the thermal runaway of lithium-ion batteries during operation, but also important for automatic capacity balancing during battery manufacturing and repair. A redox shuttle is an electrolyte additive that can be used as intrinsic overcharge protection mechanism to enhance the safety characteristics of lithium-ion batteries. The advances on stable redox shuttles are briefly reviewed. Fundamental studies for designing stable redox shuttles are also discussed.     

Electrolyte additive trimethyl phosphite for improving electrochemical performance and thermal stability of LiCoO2 cathode

The purpose of this paper is to investigate compositions on the interface between LiCoO₂ and electrolyte when trimethyl phosphite TMP(i) is used as an additive in 1 M LiPF₆/EC + DEC electrolyte system and the thermal stability of the electrolyte as well as Li_0.5CoO₂ mixed with the electrolyte. The electrochemical performance of LiCoO₂ electrode in the two electrolyte systems was also studied. It is found that the electrochemical performance, including capacity, cycle performance and 3.6 V plateau efficiency, has been improved in the electrolyte with TMP(i) additive. FTIR analysis indicates that Li_xPO_y is an important surface film composition on the cathode in TMP(i) containing system. A thicker and more passivating surface layer is formed when using TMP(i) additive as an additive. The thermal stability of the cathode is substantially improved in the electrolyte containing TMP(i) additive in the system, especially the exothermic peak around 190 °C, which is associated with the reaction between active surface of cathode and solvents, is obviously restrained.     

Electrochemical impedance and conductivity measurements in a heterogeneous Fe powder particle—electrolyte system with or without electrochemical reaction

The participation of solid Fe powder particles in the transfer of charge through a heterogeneous system consisting of an electrolyte and conducting powder particles was studied by means of electrochemical impedance spectroscopy and conductivity measurements. Different behaviour was encountered in electroinactive Na₂SO₄ electrolyte without metal electrodeposition on the Fe particles and in NiSO₄ electrolyte where Ni electrodeposition occurs on the Fe particle surfaces. From impedance diagrams and the proposed model the formation of aggregates and chains of Fe particles is deduced. The important role of electrochemical reaction proceeding at the particle surface in the charge transfer behaviour in stirred heterogeneous systems was also demonstrated.     

Gas Recombination in Sealed Ni–MHx Cells

This paper focuses on investigation of gas recombination in a positive-limited-sealed Ni–MH_x cell. The positive electrodes were prepared by electrochemical impregnation of fibrous nickel plaques. The metal hydride negative electrodes were made by pasting the mixture of rare-earth hydrogen storage alloy powders, conducting and binding agents on foamed nickel substrates. The measurement of the positive capacity at different charge times was used to estimate the partial current for oxygen evolution at the same time. The effects of charge rate, electrolyte saturation level and initial state of charge of the positive electrodes on the recombination were investigated in sealed Ni–MH_x cells. By determining the differential capacity of nickel hydroxide electrodes, an improved mathematical model was used to evaluate the gas recombination parameters during charge, overcharge, rest and discharge of the positive-limited-sealed Ni–MH_x cell. The gas recombination during rest, discharge and overdischarge was also examined. The oxygen recombination on the nickel hydroxide electrodes can be neglected due to the consumption of water when the nickel hydroxide electrodes were discharged. The longer overdischarge produced an increase in cell pressure for the sealed Ni–MH_x cell at an electrolyte unsaturated level and the evolving gas can be recombined by a following recharge operation. © 1997 SCI.     

Long-term electrolytic hydrogen permeation in nickel and the effect of vanadium species addition

Nickel cathodes have been found to become deactivated under long-term polarization in the H₂ evolution region during alkaline water electrolysis. The cause of deactivation was examined using steady state polarization and measurement of hydrogen permeation through nickel foil in 8 mol/l KOH at 70 °C and 100 mA/cm². The long-term (over 50 h) permeation behaviour was explained by formation and growth of a nickel hydride phase. The rise in hydrogen overpotential was ascribed to an increase of the hydrogen surface coverage on the newly formed hydride. The effect of an electrolyte additive (a vanadium salt) on the hydrogen overpotential and permeation rate was also investigated. Upon addition of dissolved V₂O₅, the permeation rate was found to increase quickly and then slowly decrease to a steady value close to that measured for hydride-free nickel. Meanwhile, the hydrogen overpotential was observed to recover back to nearly its initial value for fresh nickel. The exhibited behaviour was attributed to decomposition of the hydride phase, after deposition of a vanadium-bearing compound. The prolonged contact between Ni and V was proposed as the main reason for hydride decomposition. The addition of more vanadium had no further result on the hydrogen overpotential.     

Effect of 1-butyl-1-methylpyrrolidinium hexafluorophosphate as a flame-retarding additive on the cycling performance and thermal properties of lithium-ion batteries

1-Butyl-1-methylpyrrolidinium hexafluorophosphate (BMP-PF₆) was used as a flame-retarding additive in the liquid electrolyte, and the influence of BMP-PF₆ content on cycling performance and thermal properties of lithium-ion batteries was investigated. Self-extinguishing time and DSC studies demonstrated that the addition of BMP-PF₆ to the electrolyte provided a significant suppression in the flammability of the electrolyte and an improvement in the thermal stability of the cell. The optimum BMP-PF₆ content in the electrolyte was found to be 10 wt.% for improving safety without degrading cycling performance of the cell.     

N-(triphenylphosphoranylidene) aniline as a novel electrolyte additive for high voltage LiCoO2 operations in lithium ion batteries

We report N-(triphenylphosphoranylidene) aniline (TPPA) as a new electrolyte additive for high performance lithium cobalt oxide (LiCoO₂) electrodes during high voltage operations. When cycled in the voltage range of 3.0–4.4 V, graphite-LiCoO₂ full cells with 0.2 wt% TPPA exhibited 10% increased discharge capacities after 200 cycles compared to those of control cells with no such additive. The enhanced cycling performance is attributed to the additive effect toward the modified surface films on LiCoO₂ electrodes that suppress the decomposition of both solvent and salt in the electrolyte. This additive effect was characterized by electrochemical impedance spectroscopy (EIS) and X-ray photoelectron spectroscopy (XPS).     

Studies of dielectric relaxation and a.c. conductivity in [(100−<Emphasis Type="Italic">x</Emphasis>)PEO + <Emphasis Type="Italic">x</Emphasis>NH<Subscript>4</Subscript>SCN]: Al-Zn ferrite nano composite polymer electrolyte

Present work deals with findings on dielectric behaviour and a.c. conduction in a ferrite doped polymer nano composite electrolyte system, namely [(100−x) PEO + xNH₄SCN]: ferrite. The formation of nano composite and structural behavior of electrolyte was studied by XRD and SEM images. The effect of salt and ferrite on conductivity behaviour of PEO based nano composite polymer electrolyte has been investigated by the impedance spectroscopy at room temperature. The variation of dielectric permittivity and dielectric loss with frequency was carried out at ambient temperature. The a.c. conductivity seems to follow the universal power law.     

Correlation between AlPO4 nanoparticle coating thickness on LiCoO2 cathode and thermal stability

The thickness of an AlPO₄ coating significantly affects the thermal stability of a LiCoO₂ cathode. Increasing the coating thickness leads to not only a decrease in the exothermic reaction between the cathode and the electrolyte but also to an improvement in the cycling performance. A 1 C rate overcharge experiment up to 12 V is a good example of the thermal stability of the cathode in the Li-ion cell. Furthermore, increasing the AlPO₄ coating thickness results in the lowest cell surface temperature, which is indicative of the degree of heat generation.     

Influence of lithium difluorophosphate additive on the high voltage LiNi0.8Co0.1Mn0.1O2/graphite battery

Lithium-ion batteries (LIBs) possessing high energy densities are driven by the growing demands of electric vehicles (EVs) and hybrid electric vehicles (HEVs). One of the most effective strategies to improve the energy density of LIBs is to enlarge the charge cut-off voltage via a lithium salt additive for the conventional electrolyte system. Herein, lithium difluorophosphate (LIDFP) is employed to optimize and reconstruct the composition of the structure and interface for both cathode and anode, which can effectively restrain the oxidation decomposition of electrolyte as well as refrain the dissolve out of transition metals. The LiNi_0.8Co_0.1Mn_0.1O₂ (LNCM811)/graphite pouch cell with 1 wt% LIDFP in electrolyte delivers a discharge capacity retention of 91.3% at a high voltage of 4.4 V over 100 cycles, which is higher than the 82.0% of that without LIDFP additive. Additionally, the remaining capacity of LNCM811/C battery with 1 wt% LIDFP additive which is left at 60 °C for 14 days is 85.2%, and the recovery capacity is 93.3%. The LIDFP-containing electrolyte demonstrates a great application future for the LiBs operating under the high-voltage condition and high-temperature storage performance.