Isocyanate compounds as electrolyte additives for lithium-ion batteries

Several isocyanate compounds have been investigated with regard to their performance as film forming electrolyte additives in propylene carbonate (PC) and EC/EMC-based electrolytes. In situ and ex situ analytical methods were applied to understand the differences in performance. Particular attention was paid to the differences of aromatic and linear isocyanate compounds.     

PVDF-HFP-based porous polymer electrolyte membranes for lithium-ion batteries

As a potential electrolyte for lithium-ion batteries, a porous polymer electrolyte membrane based on poly(vinylidenefluoride-hexafluoropropylene) (PVDF-HFP) was prepared by a phase inversion method. The casting solution, effects of the solvent and non-solvent and addition of micron scale TiO₂ particles were investigated. The membranes were characterized by SEM, XRD, AC impedance, and charge/discharge tests. By using acetone as the solvent and water as the non-solvent, the prepared membranes showed good ability to absorb and retain the lithium ion containing electrolyte. Addition of micron TiO₂ particles to the polymer electrolyte was found to enhance the tensile strength, electrolyte uptake, ion conductivity and the electrolyte/electrode interfacial stability of the membrane.     

Investigation and application of lithium difluoro(oxalate)borate (LiDFOB) as additive to improve the thermal stability of electrolyte for lithium-ion batteries

Lithium difluoro (oxalate) borate (LiDFOB) is used as thermal stabilizing and solid electrolyte interface (SEI) formation additive for lithium-ion battery. The enhancements of electrolyte thermal stability and the SEIs on graphite anode and LiFePO₄ cathode with LiDFOB addition are investigated via a combination of electrochemical methods, nuclear magnetic resonance (NMR), X-ray photoelectron spectroscopy (XPS), Fourier transform infrared-attenuated total reflectance (FTIR-ATR), as well as density functional theory (DFT). It is found that cells with electrolyte containing 5% LiDFOB have better capacity retention than cells without LiDFOB. This improved performance is ascribed to the assistance of LiDFOB in forming better SEIs on anode and cathode and also the enhancement of the thermal stability of the electrolyte. LiDFOB-decomposition products are identified experimentally on the surface of the anode and cathode and supported by theoretical calculations.     

Macroporous nanocomposite polymer electrolyte for lithium-ion batteries

A novel macroporous nanocomposite polymer membrane (NCPM) based on poly(vinylidene difluoride-co-hexafluoropropylene) [P(VDF-HFP)] copolymer was prepared by in situ hydrolysis of Ti(OC₄H₉)₄ using a non-solvent-induced phase separation technique. SEM micrograph shows that the yielding TiO₂ nanoparticles are dispersed uniformly in the polymer matrix and there are a lot of spherical macropores connecting with each other by some smaller pores. DSC results exhibit that the crystallinity of polymer matrix decreases with the incorporation of TiO₂ nanoparticles. The tensile stress of the NCPM is 9.69 MPa and its fracture strain 74.4%. After immersion in 1.0 mol l⁻¹ LiPF₆/ethyl carbonate (EC)–dimethyl carbonate (DMC), the ionic conductivity of the obtained nanocomposite polymer electrolyte (NCPE) is 0.98 × 10⁻³ S cm⁻¹ at 20 °C. Lithium-ion batteries, which use this kind of NCPE as the separator and electrolyte, display good discharging performance at different current densities, presenting promise for its practical application.     

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.     

Cresyl diphenyl phosphate as flame retardant additive for lithium-ion batteries

To improve the safety of lithium-ion batteries, cresyl diphenyl phosphate (CDP) was used as a flame retardant additive in a LiPF₆ electrolyte solution. The flammability of the electrolytes containing CDP and the electrochemical performances of the cells, LiCoO₂/Li, graphite/Li and the battery LiCoO₂/graphite with these electrolytes, were studied by measuring the self-extinguishing time of the electrolytes, the variation of surface temperature of the battery and the charge/discharge curve of the cells or battery. It is found that the addition of CDP to the electrolyte provides a significant suppression in the flammability of the electrolyte and an improvement in the thermal stability of battery. On the other hand, the electrochemical performances of the cells become slightly worse due to the application of CDP in the electrolyte. This alleviated trade-off between the flammability and thermal stability and cell performances provides a possibility to formulate a nonflammable electrolyte by using CDP.     

Ethyl isocyanate—An electrolyte additive from the large family of isocyanates for PC-based electrolytes in lithium-ion batteries

To avoid solvent co-intercalation into graphite, the presence of a solid electrolyte interphase (SEI) is required. This film is formed via the reductive decomposition of electrolyte species, i.e. a film forming electrolyte additives. In this contribution we focus on an isocyanate compound, ethyl isocyanate (EtNCO) which performs well in a propylene carbonate electrolyte at both graphite anode and LiCoO₂ cathode. EtNCO is investigated by in situ Fourier transform infrared (FTIR) spectroscopy. We conclude that the formation of a radical anion via electrochemical reduction of the electrolyte additive is the initiating step of the SEI formation process. The electro-polymerization of isocyanate monomers in small additive amounts in the PC electrolyte is critically discussed.     

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.     

Dimethyl methylphosphonate-based nonflammable electrolyte and high safety lithium-ion batteries

Dimethyl methylphosphonate (DMMP) was used as a cosolvent to reformulate the nonflammable electrolyte of 1 M LiPF₆/EC + DEC + DMMP (1:1:2 wt.) in order to improve the safety characteristics of lithium-ion batteries. The flammability, cell performance, low-temperature performance and thermal stability of the DMMP-based electrolyte were compared with the electrolyte of 1 M LiPF₆/EC + DEC (1:1 wt.). The nonflammable electrolyte exhibits good oxidation stability at the LiCoO₂ cathode and poor reduction stability at the mesocarbon microbead (MCMB) and surface-modified graphite (SMG) anodes. The addition of vinyl ethylene carbonate (VEC) to the DMMP-based electrolyte provided a significant improvement in the reduction stability at the carbonaceous electrodes. Furthermore, it was found that the addition of DMMP resulted in optimized low-temperature performance and varied thermal stability of the electrolytes. All of the results indicated the novel DMMP-based electrolyte is a promising nonflammable electrolyte to resolve the safety concerns of lithium-ion batteries.     

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.     

Diphenyloctyl phosphate as a flame-retardant additive in electrolyte for Li-ion batteries

The use of diphenyloctyl phosphate (DPOF) as a flame-retardant additive in liquid electrolyte for Li-ion batteries is investigated. Mesocarbon microbeads (MCMB) and LiCoO₂ are used as the anode and cathode materials, respectively. Cyclic voltammetry (CV), differential scanning calorimetry (DSC), electrochemical impedance spectroscopy (EIS), and scanning electron microscopy (SEM) are used for the analyses. The cell with DPOF shows better electrochemical cell performance than that without DPOF in initial charge/discharge and rate performance tests. In cycling tests, a cell with DPOF-containing electrolyte exhibited better discharge capacity and capacity retention than that of the DPOF-free electrolyte after cycling. These results confirm the viability of using DPOF as a flame-retardant additive for improving the cell performance and thermal stability of electrolytes for Li-ion 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.     

Characteristics of lithium-ion battery with non-flammable electrolyte

To improve the safety of lithium-ion batteries, we studied non-flammable electrolytes made by adding several types of phosphazene-based flame retardants to conventional electrolytes and evaluated their conductivities, electrochemical characteristics, and the effects of flame retardants in terms of safety. Cell performance tests and abuse tests were also conducted using cylindrical test cells. The conductivity of electrolytes decreased when phosphazene-based flame retardants were added to the conventional electrolytes. The reason for this decrease in conductivity may be the increase in electrolyte viscosity caused by adding flame retardants. The conductivity decrease led to a decrease in cell capacity at high current density and at low temperature. However, the cell capacities at 0.2 CA (CA = 750 mA) and at 25 °C were almost the same as those of cells using conventional electrolytes. Flame tests showed that the electrolytes with flame retardants exhibited flame resistance consistent with UL-94V0. We also carried out several abuse tests to check the safety improvements. Both overcharge tests up to 10 V and heating tests up to 200 °C were completed without any extraordinary heat generation. Heating tests using a burner revealed the self-extinguishing properties of these electrolytes which were gushed out by venting. These results indicate that electrolytes with phosphazene-based flame retardants are effective for making lithium-ion batteries safe.     

Low Li binding affinity: An important characteristic for additives to form solid electrolyte interphases in Li-ion batteries

Calculations are made of the lowest unoccupied molecular orbital (LUMO), chemical hardness (η), dipole moment (μ), and binding energy with a Li⁺ ion for 32 organic molecules that are electrolyte additives for solid electrolyte interphase (SEI) formation in lithium-ion batteries (LIBs). The results confirm that both the LUMO and η values are critical indicators of suitable SEI formation. The μ values of the additives are generally smaller than those of widely used solvents in LIBs. It is found that a low Li-ion binding affinity may be an important characteristic for SEI-forming additives. Li⁺ binding affinity is proposed as a factor in the computational screening process used to obtain promising additives.     

Diphenylamine: A safety electrolyte additive for reversible overcharge protection of 3.6 V-class lithium ion batteries

A polymerizable monomer, diphenylamine (DPAn), is reported to act as a safety electrolyte additive for overcharge protection of 3.6 V-class lithium ion batteries. The experimental results demonstrated that the DPAn monomer could be electro-polymerized to form a conductive polymer bridging between the cathode and anode of the battery, and to produce an internal current bypass to prevent the batteries from voltage runaway during overcharge. The charge–discharge tests of practical LiFePO₄/C batteries indicated that the DPAn additive could clamp the cell's voltage at the safe value less than 3.7 V even at the high rate overcharge of 3 C current, meanwhile, this monomer molecule has no significant impact on the charge–discharge performance of the batteries at normal charge–discharge condition.     

Vinylene carbonate and vinylene trithiocarbonate as electrolyte additives for lithium ion battery

Vinylene carbonate (VC) and vinylene trithiocarbonate (VTC) are studied as electrolyte additives in two kinds of electrolytes: (1) propylene carbonate (PC) and diethyl carbonate (DEC) (1:2 by weight) 1 mol dm⁻³ LiPF₆; (2) ethylene carbonate (EC) and DEC (1:2 by weight) 1 mol dm⁻³ LiPF₆. Characterization is performed by cyclic voltammetry, impedance spectroscopy, scanning electron microscopy (SEM), electrochemical impedance spectroscopy (EIS), X-ray photoelectron spectroscopy (XPS) and half cell tests. Cyclic life is better in either electrolyte with VC than either electrolyte with/without VTC. SEM shows VC and VTC both form well developed passivation films on the graphite anode, but the films with VTC are thicker than with VC. EIS shows the VTC films have significantly higher charge transfer resistance. The VTC film in PC fails to protect against exfoliation. XPS indicates VTC has different reaction pathways in PC relative to EC. In EC/DEC, VTC forms polymeric C-O-C-like components and sulfide species (C-S-S-C, S and C-S-C). In PC/DEC, VTC does not form polymeric species, instead forming a film mainly containing LiF and Li₂S. It appears that a thinner polymeric film is preferential. The specific data herein are of interest, and the general conclusions may help development of improved additives for enhanced Li-ion battery performance.     

Effect of propane sultone on elevated temperature performance of anode and cathode materials in lithium-ion batteries

A detailed investigation of the effect of the thermal stabilizing additive, propane sultone (PS), on the reactions of the electrolyte with the surface of the electrodes in lithium-ion cells has been conducted. Cells were constructed with meso-carbon micro-bead (MCMB) anode, LiNi_0.8Co_0.2O₂ cathode and 1.0 M LiPF₆ in 1:1:1 EC/DEC/DMC electrolyte with and without PS. After formation cycling, cells were stored at 75 °C for 15 days. Cells containing 2% PS had better capacity retention than cells without added PS after storage at 75 °C. The surfaces of the electrodes from cycled cells were analyzed via a combination of TGA, XPS and SEM. The addition of 2% PS results in the initial formation of S containing species on the anode consistent with the selective reduction of PS. However, modifications of the cathode surface in cells with added PS appear to be the source of capacity resilience after storage at 75 °C.     

Functional electrolytes: Synergetic effect of electrolyte additives for lithium-ion battery

We have found that certain combinations of specific additives show a very interesting behavior in Li-ion batteries. During the course of investigating further improvements in the performance of the triple-bonded compounds, which we very recently reported, a novel and unique effect of an additive combination was observed. The combination of the triple-bonded compounds and the double-bonded compounds has proven to show a much improved battery performance, especially in cycleability and gas evolution than the case when they are singly used. Especially, the synergetic effect of propargyl methanesulfonate and vinylene carbonate is remarkable. To clarify the synergetic effect, the electrochemical properties of the additives and the electrode analyses were investigated. It is assumed that the higher battery performance of the combination effect resulted not only from the thin and dense SEI on the negative electrode but also from the positive electrode surface co-polymerized film produced by the synergetic decomposition of the additives. We suggest that the keys for producing the synergetic functions are (1) a structural difference in the unsaturated moiety, and (2) a greater difference in the reduction potential.     

SnCo nanowire array as negative electrode for lithium-ion batteries

Amorphous SnCo alloy nanowires (NWs) grown inside the channels of polycarbonate membranes by potentiostatic codeposition of the two metals (SnCo-PM) were tested vs. Li by repeated galvanostatic cycles in ethylene carbonate-dimethylcarbonate - LiPF₆ for use as negative electrode in lithium ion batteries. These SnCo electrodes delivered an almost constant capacity value, near to the theoretical for an atomic ratio Li/Sn of 4.4 over more than 35 lithiation-delithiation cycles at 1 C. SEM images of fresh and cycled electrodes showed that nanowires remain partially intact after repeated lithiation-delithiation cycles; indeed, several wires expanded and became porous. Results of amorphous SnCo nanowires grown inside anodic alumina membranes (SnCo-AM) are also reported. The comparison of the two types of NW electrodes demonstrates that the morphology of the SnCo-PM is more suitable than that of the SnCo-AM for electrode stability over cycling. Optimization of NW technology should thus be a promising route to enhancing the mechanical strength and durability of tin-based electrodes.