Ion-conducting lithium bis(oxalato)borate-based polymer electrolytes

Poly(2-ethoxyethyl methacrylate) polymer gel electrolytes containing immobilised lithium bis(oxalato)borate in aprotic carbonates: propylene carbonate (PC), propylene carbonate–ethylene carbonate (PC–EC 50:50 vol.%) and diethyl carbonate–ethylene carbonate (DEC–EC 50:50 vol.%) were prepared by a direct radical polymerisation. The electrolyte composition was optimised to achieve suitable ionic conductivity 0.5 and 2.4 mS cm⁻¹ at 25 and 70 °C respectively along with good mechanical properties. The electrochemical stability up to 5.1 V vs. Li/Li⁺ was determined on gold electrode by voltammetrical measurements. The polymer electrolytes with high-boiling solvents (PC and PC/EC) showed higher thermal stability (up to 110–120 °C) compared to the liquid electrolytes. The proposed area of application is in the lithium-ion batteries with cathodes operating at elevated temperatures of 70 °C, where higher electrochemical stability of the polymer electrolytes is employed.     

Ionic liquid doped PEO-based solid polymer electrolytes for lithium-ion polymer batteries

The influence of adding the room-temperature ionic liquid 1-ethyl-3-methyllimidazolium bis(trifluoromethylsulfonyl)imide (EMImTFSI) to poly(ethylene oxide) (PEO)–lithium difluoro(oxalato)borate (LiDFOB) solid polymer electrolyte and the use of these electrolytes in solid-state Li/LiFePO₄ batteries has been investigated. Different structural, thermal, electrical and electrochemical studies exhibit promising characteristics of these polymer electrolyte membranes, suitable as electrolytes in rechargeable lithium-ion batteries. The crystallinity decreased significantly due to the incorporation of ionic liquid, investigated by X-ray diffraction (XRD) and differential scanning calorimetry (DSC). The ion–polymer interaction, particularly the interaction of cations in LiDFOB and ionic liquid with ether oxygen atom of PEO chains, has been evidenced by FT-IR studies. The polymer electrolyte with ～40 wt% of ionic liquid offers a maximum ionic conductivity of ～1.85 × 10^?4 S/cm at 30 °C with improved electrochemical stabilities. The Li/PEO-LiDFOB-40 wt% EMImTFSI/LiFePO₄ coin-typed cell cycled at 0.1 C shows the 1st discharge capacity about 155 mAh g^?1, and remains 134.2 mAh g^?1 on the 50th cycle. The addition of the ionic liquid to PEO₂₀-LiDFOB polymer electrolyte has resulted in a very promising improvement in performance of the lithium polymer batteries.     

Lithium bis(fluorosulfonyl)imide (LiFSI) as conducting salt for nonaqueous liquid electrolytes for lithium-ion batteries: Physicochemical and electrochemical properties

Lithium bis(fluorosulfonyl)imide (LiFSI) has been studied as conducting salt for lithium-ion batteries, in terms of the physicochemical and electrochemical properties of the neat LiFSI salt and its nonaqueous liquid electrolytes. Our pure LiFSI salt shows a melting point at 145 °C, and is thermally stable up to 200 °C. It exhibits far superior stability towards hydrolysis than LiPF₆. Among the various lithium salts studied at the concentration of 1.0 M (= mol dm⁻³) in a mixture of ethylene carbonate (EC)/ethyl methyl carbonate (EMC) (3:7, v/v), LiFSI shows the highest conductivity in the order of LiFSI > LiPF₆ > Li[N(SO₂CF₃)₂] (LiTFSI) > LiClO₄ > LiBF₄. The stability of Al in the high potential region (3.0-5.0 V vs. Li⁺/Li) has been confirmed for high purity LiFSI-based electrolytes using cyclic voltammetry, SEM morphology, and chronoamperometry, whereas Al corrosion indeed occurs in the LiFSI-based electrolytes tainted with trace amounts of LiCl (50 ppm). With high purity, LiFSI outperforms LiPF₆ in both Li/LiCoO₂ and graphite/LiCoO₂ cells.     

Crystal structure and physical properties of lithium difluoro(oxalato)borate (LiDFOB or LiBF2Ox)

The structural characterization and properties of lithium difluoro(oxalato)borate (LiDFOB) are reported. LiDFOB was synthesized as previously described in the literature via direct reaction of boron trifluoride diethyl etherate with lithium oxalate. The crystal structure of the salt was determined from single crystal X-ray diffraction yielding a highly symmetric orthorhombic structure (Cmcm, a = 6.2623(8) Å, b = 11.4366(14) Å, c = 6.3002(7) Å, V = 451.22(9) ų, Z = 4 at 110 K). Single crystal X-ray diffraction of a dihydrate of LiDFOB yielded a monoclinic structure (P2₁/c, a = 9.5580(3) Å, b = 12.7162(4) Å, c = 5.4387(2) Å, V = 634.63(4) ų, Z = 4 at 110 K). Along with the crystal structures, additional structural information and the properties of LiDFOB (via ¹¹B and ¹⁹F NMR, DSC, TGA and Raman spectroscopy) have been compared with those of LiBF₄ and LiBOB to better understand the differences between these lithium battery electrolyte salts.     

Surface layer formed on silicon thin-film electrode in lithium bis(oxalato) borate-based electrolyte

A Si thin-film electrode of 200 nm is prepared using E-beam evaporation and deposition on copper foil. The use of a lithium bis(oxalato) borate (LiBOB)-based electrolyte markedly improves the discharge capacity retention of a Si thin-film electrode/Li half-cell during cycling. The surface layer formed on Si thin-film electrode in ethylene carbonate/diethyl carbonate (3/7) with 1.3 M LiPF₆ or 0.7 M LiBOB is characterized by means of Fourier transform infrared spectroscopy and X-ray photoelectron spectroscopic analysis. The surface morphology of the electrode after cycling is investigated using scanning electron microscopy. The relationship between the physical morphology and the electrochemical performance of Si thin-film electrode is discussed.     

LBDOB, a new lithium salt with benzenediolato and oxalato complexes of boron for lithium battery electrolytes

A new unsymmetrical lithium salt containing C₆H₄O₂²⁻[dianion of 1,2-benzenediol] and C₂O₄²⁻[dianion of oxalic acid], lithium [1,2-benzenediolato(2-)-O,O′ oxalato]borate (LBDOB), is synthesized and characterized. The thermal characteristics of it, lithium bis[1,2-benzenediolato(2-)-O,O′]borate (LBBB) and lithium bis(oxalate)borate (LiBOB) are examined by thermogravimetric (TG) analysis. The thermal decomposition in Ar begins at 250, 256, and 302 °C for LBBB, LBDOB, and LBOB, respectively. The order of the stability toward oxidation of these organoborates is LBOB > LBDOB > LBBB, which is in the same order of the thermal stability. The cyclic voltammetry study shows that the LBDOB solution in PC is stable up to 3.7 V vs. Li⁺/Li. They are soluble in common organic solvents. Ionic dissociation properties of LBDOB and its derivatives are examined by conductivity measurements in PC, PC + DME, EC + DME, PC + THF, EC + THF (molar ratio 1:1) solutions. The conductivity values of the 0.10 mold m⁻³ LBDOB electrolyte in PC, PC + DME, EC + DME, PC + THF, EC + THF solutions are higher than those of LBBB, but lower than those of LBOB electrolytes.     

Electrochemical stability of lithium bis(oxatlato) borate containing solid polymer electrolyte for lithium ion batteries

The electrochemical stability of lithium bis(oxatlato) borate (LiBOB) containing solid polymer electrolyte has been evaluated both by inert electrode and real cathodes. Enhanced intrinsic anodic stability and decreased interface impedance, are obtained by addition of nano-sized MgO to PEO₂₀-LiBOB. It is also found that the LiBOB-containing SPEs exhibit prominent kinetic stability between 3.0 and 4.5 V. For cells using SPEs as the separators, good cycling performance is obtained for real 4 V class cathodes material LiNi_1/3Co_1/3Mn_1/3O₂ and LiCoO₂. The Li|PEO₂₀-LiBOB|LiNi_1/3Co_1/3Mn_1/3O₂ cell takes an initial capacity of 156.8 mAh g⁻¹, with retention of 142.5 mAh g⁻¹ after 20 cycles at 0.2C-rate. The cell also works well up to 1C-rate. The addition of nano-sized MgO into PEO₂₀-LiBOB readily reduces the irreversible capacity per cycle, both for LiNi_1/3Co_1/3Mn_1/3O₂ and LiCoO₂ cathodes. In addition, the critical role of LiBOB in obtaining kinetic stability and passivating ability towards cathodes are specially discussed.     

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.     

A new lithium salt with 3-fluoro-1,2-benzenediolato and oxalato complexes of boron for lithium battery electrolytes

A new unsymmetrical lithium salt, lithium [3-fluoro-1,2-benzenediolato(2-)-o,o′ oxalato]borate (FLBDOB), is synthesized and characterized. The thermal characteristics of FLBDOB and its counterparts lithium bis[3-fluoro-1,2-benzenediolato(2-)-o,o]borate (FLBBB) and lithium bis[oxalate]borate (LBOB) are examined and compared by thermogravimetric analysis (TG). The thermal decomposition of these salts in air is found to begin at 302, 262 and 256 °C for LBOB, FLBDOB and FLBBB, respectively. The order of the stability toward oxidation of these organoborates is LBOB > FLBDOB > FLBBB, which is in the same order of the thermal stability. The cyclic voltammetry study shows that the FLBDOB solution in PC is stable up to 4.0 V versus Li⁺/Li. It is soluble in common organic solvents such as propylene carbonate (PC), ethylene carbonate (EC), 1,2-dimethoxyethane (DME), and tetrahydrofuran (THF). Ionic dissociation properties of FLBDOB and the counterparts in PC, PC + THF, PC + DME, EC + DME, EC + THF (molar ratio 1:1) solutions are also examined by conductivity measurements. The conductivity values of the 0.10 mol dm⁻³ FLBDOB electrolyte in PC, PC + THF, PC + DME, EC + DME, EC + THF solutions are higher than those of FLBBB, but lower than those of LBOB electrolytes.     

Preparation and characterization of electrospun poly(acrylonitrile) fibrous membrane based gel polymer electrolytes for lithium-ion batteries

Electrospun, non-woven membrane of high molecular weight poly(acrylonitrile) (PAN) is demonstrated as an efficient host matrix for the preparation of gel polymer electrolytes for lithium-ion batteries. Electrospinning process parameters are optimized to get a fibrous membrane of PAN consisting of bead-free, uniformly dispersed thin fibers with diameter in the range 880-1260 nm. The membrane with good mechanical strength and porosity exhibits high uptake when activated with the liquid electrolyte of 1 M LiPF₆ in a mixture of organic solvents and the gel polymer electrolyte shows ionic conductivity of 1.7 × 10⁻⁵ S cm⁻¹ at 20 °C. Electrochemical performance of the gel polymer electrolyte at 20 °C is evaluated in lithium-ion cell with lithium cobalt oxide cathode and graphite anode. Good performance with a low capacity fading on charge-discharge cycling is demonstrated.     

Lithium difluoro(oxalato)borate/ethylene carbonate + propylene carbonate + ethyl(methyl) carbonate electrolyte for LiMn2O4 cathode

Lithium difluoro(oxalato)borate (LiODFB) was investigated as a lithium salt for non-aqueous electrolytes for LiMn₂O₄ cathode in lithium-ion batteries. Linear sweep voltammetry (LSV) tests were used to examine the electrochemical stability and the compatibility between the electrolytes and LiMn₂O₄ cathode. Through inductively coupled plasma (ICP) analysis, we compared the amount of Mn²⁺ dissolved from the spinel cathode in 1 mol L⁻¹ LiPF₆/EC + PC + EMC (1:1:3 wt.%) and 1 mol L⁻¹ LiODFB/EC + PC + EMC (1:1:3 wt.%). AC impedance measurements and scanning electron microscopy (SEM) analysis were used to analyze the formation of the surface film on the LiMn₂O₄ cathode. These results demonstrate that ODFB anion can capture the dissolution manganese ions and form a denser and more compact surface film on the cathode surface to prevent the continued Mn²⁺ dissolution, especially at high temperature. It is found that LiODFB, instead of LiPF₆, can improve the capacity retention significantly after 100 cycles at 25 °C and 60 °C, respectively. LiODFB is a very promising lithium salt for LiMn₂O₄ cathode in lithium-ion batteries.     

An application of lithium cobalt nickel manganese oxide to high-power and high-energy density lithium-ion batteries

Evolved gas analysis (EGA) by mass spectroscopy (MS) was carried out for the pyrolysis of Li_1−xCo_1/3Ni_1/3Mn_1/3O₂ (185 mAh g⁻¹ of charge capacity) and the results were compared with that of Li_1−xCoO₂ (140 mAh g⁻¹). Electrochemically prepared Li_1−xCo_1/3Ni_1/3Mn_1/3O₂ clearly shows that O₂ evolution begins at much higher temperature than Li_1−xCoO₂, suggesting that Li_1−xCo_1/3Ni_1/3Mn_1/3O₂ is superior to LiCoO₂ with respect to thermal stability. High-temperature XRD measurements of charged LiCo_1/3Ni_1/3Mn_1/3O₂-electrodes at 4.45 V were also carried out and shown that the decomposition product by heating was identified as a cubic spinel consisting of cobalt, nickel, and manganese. This indicates that phase change from a layered to spinel-framework structure plays a crucial role in the suppression of oxygen evolution from the solid matrix. In order to show practicability of the new material, lithium-ion batteries with graphite-negative electrodes are fabricated and examined in the R18650-hardware. The new lithium-ion batteries show high rate discharge performances, excellent cycle life, and safety together with high-energy density.     

New lithium salts with croconato-complexes of boron for lithium battery electrolytes

A new lithium salt containing C₅O₅²⁻, lithium bis[croconato]borate (LBCB), and its novel derivative, lithium [croconato salicylato]borate (LCSB) were synthesized and characterized. The thermal characteristics of them and lithium bis[salicylato(2-)]-borate (LBSB) were examined by thermogravimetric analysis (TG). The thermal decomposition in Ar begins at 250, 328, and 350 °C for LBCB, LCSB, and LBSB, respectively. The order of the stability toward oxidation of these organoborates is LBCB > LCSB > LBSB, which differs from the thermal stability. The cyclic voltammetry study shows that the LiBCB and LCSB solutions in PC are stable up to 5.5 and 4.8 V versus Li⁺/Li, respectively. They are moderately soluble in common organic solvents, being 0.14, 0.16, and 1.4 mol dm⁻³ at 20 °C in EC + DME (molar ratio 1:1) for LBCB, LCSB, and LBSB, respectively. Ionic dissociation properties of LBCB and its derivatives were examined by conductivity measurements in PC, PC + DME, EC + DME, PC + THF, EC + THF (molar ratio 1:1) solutions. The conductivity values of the 0.10 mol dm⁻³ LBCB electrolyte in PC, PC + DME, EC + DME, PC + THF, EC + THF solutions are higher than those of LCSB and LBSB electrolytes. It means that LBCB has the higher dissociation ability in those solutions.     

Polymer electrolytes based on an electrospun poly(vinylidene fluoride-co-hexafluoropropylene) membrane for lithium batteries

Electrospinning parameters are optimized for the preparation of fibrous membranes of poly(vinylidene fluoride-co-hexafluoropropylene) {P(VdF-HFP)} that consist of layers of uniform fibres of average diameter 1 μm. Electrospinning of a 16 wt.% solution of the polymer in acetone/N,N-dimethylacetamide (DMAc) (7/3, w/w) at an applied voltage of 18 kV results in obtaining membranes with uniform morphology. Polymer electrolytes (PEs) are prepared by activating the membrane with liquid electrolytes. The fully interconnected porous structure of the host polymer membrane enables high electrolyte uptake and ionic conductivities of 10⁻³ S cm⁻¹ order at 20 °C. The PEs have electrochemical stability at potentials higher than 4.5 V versus Li/Li⁺. A PE based on a membrane with 1 M LiPF₆ in ethylene carbonate (EC)/dimethyl carbonate (DMC), which exhibits a low and stable interfacial resistance on lithium metal, is evaluated for discharge capacity and cycle properties in Li/LiFePO₄ cells at room temperature and different current densities. A remarkably good performance with a high initial discharge capacity and low capacity fading on cycling is obtained.     

Preparation and electrochemical characterization of electrospun,microporous membrane-based composite polymer electrolytes for lithium batteries

Poly(vinylidene fluoride-co-hexafluoropropylene) {P(VdF-HFP)} membranes incorporating 0, 6 and 10 wt.% of nano-meter sized particles of SiO₂ were prepared by electrospinning. These membranes served as host matrix for the preparation of polymer electrolytes (PEs) by activating with the non-volatile and safe room temperature ionic liquid (RTIL), 1-butyl-3-methylimidazolium bis(trifluoromethanesulfonylimide) (BMITFSI). The membranes consisted of layers of fibers with average fiber diameter of 2–5 μm and had a porosity of ∼87%. PEs with SiO₂ exhibited higher ionic conductivity with a maximum of 4.3 × 10⁻³ S cm⁻¹ at 25 °C obtained with 6% SiO₂. The optimum PE based on the membrane with 6% SiO₂ exhibited better compatibility with lithium metal electrode on storage and resulted in enhanced charge–discharge performance in Li/LiFePO₄ cells at room temperature, delivering the theoretical specific capacity of 170 mAh g⁻¹ at 0.1 C-rate. The PEs exhibited a very stable cycle property as well, demonstrating their suitability for lithium battery applications.     

Solid polymer electrolytes with sulfur based ionic liquid for lithium batteries

The electrochemical properties of a solid hybrid polymer electrolyte for lithium batteries based upon tri-ethyl sulfonium bis(trifluorosulfonyl) imide (S₂TFSI), lithium TFSI, and poly(ethylene oxide) (PEO) is presented. We have synthesized homogenous freestanding films that possess low temperature ionic conductivity and wide electrochemical stability. The hybrid electrolyte has demonstrated ionic conductivity of 0.117 mS cm⁻¹ at 0 °C, and 1.20 mS cm⁻¹ at 25 °C. At slightly elevated temperature ionic conductivity is on the order of 10 mS cm⁻¹. The hybrid electrolyte has demonstrated reversible stability against metallic lithium at the anodic interface and >4.5 V vs. Li/Li⁺ at the cathodic interface.     

On the electrochemical and thermal behavior of lithium bis(oxalato)borate (LiBOB) solutions

In recent years, lithium bis(oxalato)borate, LiB(C₂O₄)₂ (LiBOB) has been proposed as an alternative salt to the commonly used electrolyte, LiPF₆. There is evidence of the enhanced stability of Li-ion battery electrodes in solutions of this salt, due to a unique surface chemistry developed in LiBOB solutions. The present study is aimed at further exploring the electrochemical and thermal properties of LiBOB solutions in mixtures of alkyl carbonates with non-active metal, graphite and lithium electrodes. FTIR spectroscopy, XPS, EQCM, in situ AFM imaging, and DSC were used in conjunction with standard electrochemical techniques. The study also included a comparison between LiBOB and LiPF₆ solutions. The development of a favorable surface chemistry in LiBOB solutions that provides better passivation to Li and Li-graphite electrodes was clearly evident.     

Mixture of LiBF4 and lithium difluoro(oxalato)borate for application as a new electrolyte for lithium-ion batteries

An equimolar mixture of fluoroborate salts: LiBF₄ and lithium difluoro(oxalate)borate LiBF₂(C₂O₄) (MIX-LiFBs) was obtained from a simple one-step reaction of lithium oxalate and boron fluoride. Voltamperommetry shows that the salt obtained is stable in the potential range of 4.9 V. Impedance measurements of liquid electrolytes involving imidazolium ionic liquid and aliphatic carbonates have been carried out, which show the highest ionic conductivity of the order of 10^?3 S cm^?1 (and low activation energy of 0.14 eV) when using carbonates as the solvent. The mixture of fluoroborate salts MIX-LiFBs used as a component of solid polymer electrolytes provides much higher ionic conductivity values at high salt concentrations in “polymer-in-salt systems”. The conductivity of solid polymer electrolytes was considerably increased by adding a low-molecular-weight organic plasticizer.     

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.