Electrochemical properties of the Li-ion polymer batteries with P(VdF-co-HFP)-based gel polymer electrolyte

Gel polymer electrolytes consisting of 25 wt.% P(VdF-co-HFP), 65 wt.% ethylene carbonate + propylene carbonate and 10 wt.% LiN(CF₃SO₂)₂ are prepared using by a solvent-casting technique. The electrodes are for use in lithium-ion polymer batteries. The electrochemical characteristics of the gel polymer electrolytes are evaluated by means of ac impedance and cyclic voltammetry. The charge–discharge performance of lithium polymer and lithium-ion polymer batteries is examined. A LiCoO₂ | gel polymer electrolyte (GPE) | mesocarbon microbeads (MCMB) cell delivers a discharge capacity of 146.8 and 144.5 mAh g⁻¹ on the first and the 20th cycle, respectively. The specific discharge capacity is greater than 140 mAh g⁻¹ for up to 20 cycle at all the current densities examined.     

A novel composite gel polymer electrolyte for rechargeable lithium batteries

Composite polymer electrolyte (PE) films comprising of thermoplastic polyurethane (TPU) and polyacrylonitrile (PAN) (denoted as TPU–PAN) have been prepared by two different processes. Scanning electron microscope (SEM) of the films reveal the differences in morphology between them. The electrochemical properties of composite electrolyte films incorporating LiClO₄–propylene carbonate (PC) were studied. TPU–PAN based gel PE shows high ionic conductivity at room temperature. Thermogravimetric analysis informs that the composite electrolyte possesses good thermal stability with a decomposition temperature higher than 300 °C. Electrochemical stability in the working voltage range from 2.5 to 4.5 V was evident from cyclic voltammetry. Cycling performances of Li/PE/LiCoO₂ cells were also performed to test the suitability of the composite electrolyte in batteries.     

A novel composite microporous polymer electrolyte prepared with molecule sieves for Li-ion batteries

Molecular sieves of NaY, MCM-41, and SBA-15 were used as fillers in a poly(vinylidene fluoride-co-hexafluoropropylene) (PVdF-HFP) copolymer matrix to prepare microporous composite polymer electrolyte. The SBA-15-based composite polymer film was found to show rich pores that account for an ionic conductivity of 0.50 mS cm⁻¹. However, the MCM-41 and NaY composite polymer films exhibited compact structure without any pores, and the addition of MCM-41 even resulted in aggregation of fillers in the polymer matrix. These differences were investigated and interpreted by their different compatibility with DMF solvent and PVdF-HFP matrix. Results of linear sweep voltammetry (LSV), electrochemical impedance spectroscopy (EIS), X-ray diffraction (XRD) and differential scanning calorimetry (DSC) have revealed that the addition of SBA-15 has extended the electrochemical stability window of polymer electrolyte, enhanced the interfacial stability of polymer electrolyte with lithium electrode, and inhibited also the crystallization of PVdF-HFP matrix. Half-cell of Li/SBA-15-based polymer electrolyte/MCF was assembled and tested. The results have demonstrated that the coulombic efficiency of the first cycle was around 87.0% and the cell remains 94.0% of the initial capacity after 20 cycles, which showed the potential application of the composite polymer electrolyte in lithium 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.     

Effect of chemically modified silicas on the properties of hybrid gel electrolyte for Li-ion batteries

The aim of the presented work was to perform a preliminary study the physico-chemical properties of hybrid organic–inorganic gel electrolytes for Li-ion batteries based on the PVdF–HFP polymeric matrix and surface modified fumed silicas. Modifications were done by means of the so-called dry method using seven different silanes differing in the nature of the principal functional group: N-2-(aminoethyl)-3-amino propyltrimethoxysilane, 3-glycidoxypropyltrimetoxysilane, 3-mercaptopropyltrimetoxysilane, n-octyltriethoxysilane, 3-(chloropropyl)trimethoxysilane, 3-methacryloxypropyltrimetoxysilane, vinyltrimethoxysilane. The PVdF–HFP gels were prepared according to the so-called Bellcore process (two-step method). Impact of the silicas surface functionality on the degree of crystallinity of the polymeric membranes was studied using the differential scanning calorimetry technique. Applicability of the prepared gel electrolytes for the Li-ion technology was estimated on the basis of specific conductivity measurements. It was shown that modification of the silica surface by most of the silanes causes an increase in the gel specific conductivity by about two orders of magnitude as compared to gel with unmodified silica.     

Practical performances of Li-ion polymer batteries with LiNi0.8Co0.2O2, MCMB,and PAN-based gel electrolyte

The practical performances and thermal stability of Li-ion polymer batteries with LiNi_0.8Co_0.2O₂, mesocarbon microbead-based graphite, and poly(acrylonitrile) (PAN)-based gel electrolytes are reported. The gel electrolyte, which shows a fire-retardance by itself as well as good chemical stability effectively improved thermal stability of the Li-ion polymer battery up to 170 °C. We also found that the mesocarbon microbead-based graphite showed better coulombic efficiency even though the gel electrolyte contained PC and GBL. An evaluation of cell performances showed that the electrodes and the gel electrolyte were promising material for a next-generation Li-ion polymer battery.     

A new phosphate-based nonflammable electrolyte solvent for Li-ion batteries

A new fire-retardant—dimethyl(2-methoxyethoxy)methylphosphonate (DMMEMP) has been synthesized and evaluated as a high safe electrolyte solvent for lithium-ion batteries. This report summarizes the physical and electrochemical properties of the new compound. It is found that, this nonflammable phosphonate has a moderate viscosity, a high dielectric constant and a good thermal stability. It can provide a wide electrochemical stability window of 0–5.5 V (vs. Li⁺/Li), a high conductivity of 2.0 mS cm⁻¹ at 20 °C with 1 M LiTFSI. The electrochemical performance investigated with the Li/LiFePO₄ half-cells shows a good capacity of 148.1 mAh g⁻¹ and a coulombic efficiency close to 100% at the 10th cycle.     

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.     

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.     

Organic electrolyte solutions for rechargeable lithium batteries

Several combinations of organic solvents and lithium salts have been examined as electrolytes for ambient-temperature, rechargeable lithium batteries. Ethers (1,2-dimethoxyethane, tetrahydrofuran, 1,3-dioxolane, etc.) have been used as the base solvents, as they are electrochemically stable and have a low reactivity towards lithium metal. In the main, mixed-solvent systems have been adopted to improve the solubility of electrolytes and, hence, the electrolytic conductivity of the solution. The charge/discharge characteristics of the lithium negative electrode have been examined in these electrolytes. The cycling characteristics of Li/TiS₂ cells with the electrolytes containing crown ethers have also been investigated. The electrochemical properties of the electrodes and the charge/discharge characteristics of these cells are markedly influenced by the composition of the electrolyte.

The electrode reaction mechanisms are briefly discussed for these systems.     

A novel high-performance gel polymer electrolyte membrane basing on electrospinning technique for lithium rechargeable batteries

Nonwoven films of composites of thermoplastic polyurethane (TPU) with different proportion of poly(vinylidene fluoride) (PVdF) (80, 50 and 20%, w/w) are prepared by electrospinning 9 wt% polymer solution at room temperature. Then the gel polymer electrolytes (GPEs) are prepared by soaking the electrospun TPU-PVdF blending membranes in 1 M LiClO₄/ethylene carbonate (EC)/propylene carbonate (PC) for 1 h. The gel polymer electrolyte (GPE) shows a maximum ionic conductivity of 3.2 × 10⁻³ S cm⁻¹ at room temperature and electrochemical stability up to 5.0 V versus Li⁺/Li for the 50:50 blend ratio of TPU:PVdF system. At the first cycle, it shows a first charge-discharge capacity of 168.9 mAh g⁻¹ when the gel polymer electrolyte (GPE) is evaluated in a Li/PE/lithium iron phosphate (LiFePO₄) cell at 0.1 C-rate at 25 °C. TPU-PVdF (50:50, w/w) based gel polymer electrolyte is observed much more suitable than the composite films with other ratios for high-performance lithium rechargeable batteries.     

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.     

Novel composite polymer electrolyte for lithium air batteries

Hydrophobic ionic liquid–silica–PVdF-HFP polymer composite electrolyte is synthesized and employed in lithium air batteries for the first time. Discharge performance of lithium air battery using this composite electrolyte membrane in ambient atmosphere shows a higher capacity of 2800 mAh g⁻¹ of carbon in the absence of O₂ catalyst, whereas, the cell with pure ionic liquid as electrolyte delivers much lower discharge capacity of 1500 mAh g⁻¹. When catalyzed by α-MnO₂, the initial discharge capacity of the cell with composite electrolyte can be extended to 4080 mAh g⁻¹ of carbon, which can be calculated as 2040 mAh g⁻¹ associated with the total mass of the cathode. The flat discharge plateau and large discharge capacity indicate that the hydrophobic ionic liquid–silica–PVdF-HFP polymer composite electrolyte membrane can effectively protect lithium from moisture invasion.     

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.     

Room temperature rechargeable polymer electrolyte batteries

Polyacrylonitrile (PAN)- and poly(vinyl chloride) (PVC)-based Li⁺-conductive thin-film electrolytes have been found to be suitable in rechargeable Li and Li-ion cells. Li/Li_xMn₂O_y and carbon/LiNiO₂ cells fabricated with these electrolytes have demonstrated rate capabilities greater than the C-rate and more than 375 full depth cycles. Two-cell carbon/LiNiO₂ bipolar batteries could be discharged at pulse currents as high as 50 mA/cm².     

Electrical and electrochemical studies of poly(vinylidene fluoride)-clay nanocomposite gel polymer electrolytes for Li-ion batteries

A study is conducted on the electrical and electrochemical properties of nanocomposite polymer electrolytes based on intercalation of poly(vinylidene fluoride) (PVdF) polymer into the galleries of organically modified montmorillonite (MMT) clay. A solution intercalation technique is employed for nanocomposite formation with varying clay loading from 0 to 4 wt.%. X-ray diffraction results show the β phase formation of PVdF on intercalation. Transmission electron microscopy reveals the formation of partially exfoliated nanocomposites. The nanocomposites are soaked with 1 M LiClO₄ in a 1:1 (v/v) solution of propylene carbonate (PC) and diethyl carbonate (DEC) to obtain the required gel electrolytes. The structural conformation of the nanocomposite electrolytes is examined by Fourier transform infrared spectroscopy analysis. Examination with a.c. impedance spectroscopy reveals that the ionic conductivity of the nanocomposite gel polymer electrolytes increases with increase in clay loading and attains a maximum value of 2.3 × 10⁻³ S cm⁻¹ for a 4 wt.% clay loading at room temperature. The same composition exhibits enhancement in the electrochemical and interfacial properties as compared with that of a clay-free electrolyte system.     

Development of a biodegradable polymer electrolyte for rechargeable batteries

The possibility of producing a biodegradable polymer electrolyte based on poly-ɛ-caprolactone (PCL) with different concentrations of LiClO₄ has been investigated. The maximum ionic conductivity obtained at room temperature was 1.2 × 10⁻⁶ S cm⁻¹ for PCL complexed with 10 wt.% LiClO₄. In this mixture, complete biodegradation occurred after 110 days and was attributed to the presence of ester groups in the polymer matrix. The large electrochemical stability window of approximately 5 V showed that the PCL/LiClO₄ electrolyte had important electrochemical properties that would make it useful in the production of rechargeable batteries with a lower environmental impact.     

Preparation and electrochemical characterization of gel polymer electrolyte based on electrospun polyacrylonitrile nonwoven membranes for lithium batteries

Electrospun membranes of polyacrylonitrile are prepared, and the electrospinning parameters are optimized to get fibrous membranes with uniform bead-free morphology. The polymer solution of 16 wt.% in N,N-dimethylformamide at an applied voltage of 20 kV results in the nanofibrous membrane with average fiber diameter of 350 nm and narrow fiber diameter distribution. Gel polymer electrolytes are prepared by activating the nonwoven membranes with different liquid electrolytes. The nanometer level fiber diameter and fully interconnected pore structure of the host polymer membranes facilitate easy penetration of the liquid electrolyte. The gel polymer electrolytes show high electrolyte uptake (>390%) and high ionic conductivity (>2 × 10⁻³ S cm⁻¹). The cell fabricated with the gel polymer electrolytes shows good interfacial stability and oxidation stability >4.7 V. Prototype coin cells with gel polymer electrolytes based on a membrane activated with 1 M LiPF₆ in ethylene carbonate/dimethyl carbonate or propylene carbonate are evaluated for discharge capacity and cycle property in Li/LiFePO₄ cells at room temperature. The cells show remarkably good cycle performance with high initial discharge properties and low capacity fade under continuous cycling.     

A review on the separators of liquid electrolyte Li-ion batteries

This paper reviews the separators used in liquid electrolyte Li-ion batteries. According to the structure and composition of the membranes, the battery separators can be broadly divided as three groups: (1) microporous polymer membranes, (2) non-woven fabric mats and (3) inorganic composite membranes. The microporous polymer membranes are characterised by their thinness and thermal shutdown properties. The non-woven mats have high porosity and a low cost, while the composite membranes have excellent wettability and exceptional thermal stability. The manufacture, characteristics, performance and modifications of these separators are introduced and discussed. Among numerous battery separators, the thermal shutdown and ceramic separators are of special importance in enhancing the safety of Li-ion batteries. The former consists of either a polyethylene (PE)–polypropylene (PP) multilayer structure or a PE–PP blend which increases safety by allowing meltdown of the PE to close the ionic conduction pathways at a temperature below that at which thermal runway occurs. Whereas the latter comprises nano-size ceramic materials coated on two sides of a flexible and highly porous non-woven matrix which enhances the safety by retaining extremely stable dimensions even at very high temperatures to prevent the direct contact of the electrodes.     

Nonflammable gel electrolyte containing alkyl phosphate for rechargeable lithium batteries

A nonflammable polymeric gel electrolyte has been developed for rechargeable lithium battery systems. The gel film consists of poly(vinylidenefluoride-co-hexafluoropropylene) (PVdF-HFP) swollen with lithium hexafluorophosphate (LiPF₆) solution in ternary solvent containing trimethyl phosphate (TMP). High ionic conductivity of 6.2 mS cm⁻¹ at 20 °C was obtained for the gel electrolyte consisting of 0.8 M LiPF₆/EC + DEC + TMP (55:25:20) with PVdF-HFP, which is comparable to that of the liquid electrolyte containing the same electrolytic salt. Addition of a small amount of vinylene carbonate (VC) in the gel electrolyte improved the rechargeability of a graphite electrode. The rechargeable capacity of the graphite in the gel containing VC was ca. 300 mAh g⁻¹, which is almost the same as that in a conventional liquid electrolyte system.