Gel-type polymer electrolytes with different types of ceramic fillers and lithium salts for lithium-ion polymer batteries

Gel-type polymer electrolytes are prepared using PVdF/PEGDA/PMMA, LiPF₆/LiCF₃SO₃ mixed lithium salts and ceramic fillers such as Al₂O₃, BaTiO₃ and TiO₂. The electrochemical properties of these electrolytes, such as electrochemical stability, ionic conductivity and compatibility with electrodes are investigated in addition to the physical properties. The charge–discharge performances of lithium-ion polymer batteries using these get-type polymer electrolytes are investigated. The gel-type polymer electrolytes containing a mixed lithium salt of LiPF₆/LiCF₃SO₃ (10/1, wt.%) exhibit more stable ionic conductivity and lower interfacial resistance than those containing only LiPF₆. In addition, an Al₂O₃ filler improves interfacial stability between the electrode and the polymer electrolyte. Stacking cells (MCMB 1028/LiCoO₂, 8 cm × 13 cm × 7 ea) composed of gel-type polymer electrolytes based on PVdF/PEGDA/PMMA, LiPF₆/LiCF₃SO₃ (10/1, wt.%) and Al₂O₃ filler maintain 95% of initial capacity after 100 cycles at a C/2 rate.     

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.     

Thermal stability of propylene carbonate and ethylene carbonate–propylene carbonate-based electrolytes for use in Li cells

The thermal stability of mixed-solvent electrolytes used in lithium cells was investigated by differential scanning calorimetry (DSC) through the use of airtight containers. The electrolytes used were propylene carbonate (PC) and ethylene carbonate (EC)+PC, in which was dissolved 1 M LiPF₆, 1 M LiBF₄, 1 M LiClO₄, 1 M LiSO₃CF₃, 1 M LiN(SO₂CF₃)₂, or 1.23 M LiN(SO₂CF₃)(SO₂C₄F₉). The influence of lithium metal or the Li_0.5CoO₂ addition on the thermal behavior of these electrolytes was also investigated. The peak temperature of PC-based electrolytes increased following the order of LiPF₆<LiClO₄<LiBF₄<LiN(SO₂CF₃)₂<LiSO₃CF₃<LiN(SO₂CF₃)(SO₂C₄F₉). The order of peak temperature of EC–PC-based electrolytes shows a similar tendency to that of EC–PC-based electrolytes, with the exception of the LiN(SO₂CF₃)₂ electrolyte. The EC–PC-based electrolytes with Li metal show a more stable profile compared with the DSC curves of the PC-based electrolytes with the Li metal. The solid electrolyte interphase (SEI) covers the surface of the Li metal and prevents further reduction of the electrolytes. EC may form a better SEI compared with PC. The PC-based electrolytes of LiSO₃CF₃, LiN(SO₂CF₃)₂ and LiN(SO₂CF₃)(SO₂C₄F₉) with the coexistence of Li_0.49CoO₂ show a broad peak at around 200 °C, which may be caused by the reaction of the Li_0.49CoO₂ surface and electrolytes. The PC-based electrolytes of LiPF₆, LiClO₄ and LiBF₄ with Li_0.49CoO₂ show exothermic peaks at higher temperatures than 230 °C. The peak temperatures of the EC–PC-based electrolytes with the coexistence of Li_0.49CoO₂ are nearly the same temperature as the EC–PC-based electrolytes.     

Interface properties between a lithium metal electrode and a poly(ethylene oxide) based composite polymer electrolyte

The interface resistance between a lithium metal electrode and a polymer electrolyte has been measured for composite polymer electrolytes using various ceramic fillers with poly(ethylene oxide) (PEO) and lithium salts (LiX). The interface resistance depended on the properties of added fillers and lithium salts. The PEO with LiClO₄ electrolyte contacted with lithium metal showed the high interfacial resistance of 1000 Ω cm² at 70°C for 25 days. In contrast, the interface resistance between lithium metal and PEO with Li(CF₃SO₂)₂N was as low as 67 Ω cm² after contacting at 80°C for 30 days. The interface stability and the lithium ion conductivity were improved by addition of a small amount of ferroelectric BaTiO₃ as the filler into the PEO–LiX electrolyte.     

Electrochemical characterization of blend polymer electrolytes based on poly(oligo[oxyethylene]oxyterephthaloyl) for rechargeable lithium metal polymer batteries

Solid polymer electrolytes composed of poly(ethylene oxide)(PEO), poly(oligo[oxyethylene]oxyterephthaloyl) and lithium perchlorate have been prepared and characterized. Addition of poly(oligo[oxyethylene]oxyterephthaloyl) to PEO/LiClO₄ reduced the degree of crystallinity and improved the ambient temperature ionic conductivity. The blend polymer electrolyte containing 40 wt.% of poly(oligo[oxyethylene]oxyterephthaloyl) showed an ionic conductivity of 2.0 × 10⁻⁵ S cm⁻¹ at room temperature and a sufficient electrochemical stability to allow application in the lithium batteries. By using the blend polymer electrolytes, the lithium metal polymer cells composed of lithium anode and LiCoO₂ cathode were assembled and their cycling performances were evaluated at 40 °C.     

Effect of ball milling on structural and electrochemical properties of (PEO)nLiX (LiX = LiCF3SO3 and LiBF4) polymer electrolytes

Polymer electrolytes consisting of poly(ethylene oxide) (PEO) and lithium salts, such as LiCF₃SO₃ and LiBF₄ are prepared by the ball-milling method. This is performed at various times (2, 4, 8, 12 h) with ball:sample ratio of 400:1. The electrochemical and thermal characteristics of the electrolytes are evaluated. The structure and morphology of PEO–LiX polymer electrolyte is changed to amorphous and smaller spherulite texture by ball milling. The ionic conductivity of the PEO–LiX polymer electrolytes increases by about one order of magnitude than that of electrolytes prepared without ball milling. Also, the ball milled electrolytes have remarkably higher ionic conductivity at low temperature. Maximum ionic conductivity is found for the PEO–LiX prepared by ball milling for 12 h, viz. 2.52×10⁻⁴ S cm⁻¹ for LiCF₃SO₃ and 4.99×10⁻⁴ S cm⁻¹ for LiBF₄ at 90 °C. The first discharge capacity of Li/S cells increases with increasing ball milling time. (PEO)₁₀LiCF₃SO₃ polymer electrolyte prepared by ball milling show the typical two plateau discharge curves in a Li/S battery. The upper voltage plateau for the polymer electrolyte containing LiBF₄ differs markedly from the typical shape.     

All solid-state lithium-polymer battery using poly(urethane acrylate)/nano-SiO2 composite electrolytes

New composite polymer electrolytes composed of polyurethane acrylate (PUA), nano-size SiO₂ as a ceramic filler, and LiN(CF₃SO₂)₂ as a lithium salt were examined in an all-solid-state lithium-polymer battery (Li/PUA-SiO₂/Li_0.33MnO₂). The addition of hydrophobic SiO₂ could increase the ionic conductivity of polymer electrolyte about one-fold. The dynamic modulus of polymer electrolyte increased 50 and 150% by adding 9.1% hydrophobic and hydrophilic SiO₂, respectively. The addition of nano-size SiO₂ powders enhanced greatly the interfacial stability between polymer electrolytes and lithium electrode. The capacity fading of the cell could be improved by the addition of nano-size SiO₂ powders. The cycling performance of the cell reached about 75 and 45% of initial capacity (192 mAh g⁻¹) after 100, and 500 cycles, respectively, with an efficiency of charge–discharge of about 100% at 60 °C.     

Improved performance of Li hybrid solid polymer electrolyte cells

The seminal research by Wright et al. on polyethylene oxide (PEO) solid polymer electrolyte (SPE) generated intense interest in all solid-state rechargeable lithium batteries. Following this a number of researchers have studied the physical, electrical and transport properties of thin film PEO electrolyte containing Li salt. These studies have clearly identified the limitations of the PEO electrolyte. Chief among the limitations are a low cation transport number (t₊), high crystallinity and segmental motion of the polymer chain, which carries the cation through the bulk electrolyte. While low t₊ leads to cell polarization and increase in cell resistance high T_g reduces conductivity at and around room temperatures. For example, the conductivity of PEO electrolyte containing lithium salt is <10⁻⁷ S cm⁻¹ at room temperature. Although modified PEO electrolytes with lower T_g exhibited higher conductivity (∼10⁻⁵ S cm⁻¹ at RT) the t₊ is still very low ∼0.25 for lithium ion. Numerous other attempts to improving t₊ have met with limited success. The latest approach involves integrating nano domains of inorganic moieties, such as silcate, alumosilicate, etc. within the polymer component. This approach yields an inorganic–organic component (OIC) based polymer electrolyte with higher conductivity and t₊ for Li⁺_. This paper describes the improved electrical and electrochemical properties of OIC-based polymer electrolyte and cells containing Li anode with either a TiS₂ cathode or Mag-10 carbon electrode. Several solid polymer electrolytes derived from silicate OIC and salt-in-polymer constituent based on Li triflate (LiTf) and PEO are studied. A typical composition of the SPE investigated in this work consists of 600 kDa PEO, lithium triflate (LiTf, LiSO₃CF₃) and 55% of silicate based on (3-glycidoxypropyl)trimethoxysilane and tetramethoxysilane at molar ratio 4:1 and 0.65 mol% of aluminum(tri-sec-butoxide) (GTMOS-Al1-900k-55%). Several pouch cells consisting of Li/OIC-based–SPE/cathode containing OIC-based–SPE–LiTf binder were fabricated and tested, these cells are called modified cells. The charge/discharge and impedance characteristics of the new cells (also called modified cells) are compared with that of the pouch cells containing the conventional PEO–LiTf electrolyte as the cathode binder, these cells are called non-modified cells. The new cells can be charged and discharged at 70 °C at higher currents. However, the old cells can be charged and discharged only at 80 °C or above and at lower currents. The cell impedance for the new cells is much lower than that for the old cells.     

Effects of addition of TiO2 nanoparticles on mechanical properties and ionic conductivity of solvent-free polymer electrolytes based on porous P(VdF-HFP)/P(EO-EC) membranes

To enhance the performance (i.e., mechanical properties and ionic conductivity) of pore-filling polymer electrolytes, titanium dioxide (TiO₂) nanoparticles are added to both a porous membrane and its included viscous electrolyte, poly(ethylene oxide-co-ethylene carbonate) copolymer (P(EO-EC)). A porous membrane with 10 wt.% TiO₂ shows better performance (e.g., homogeneous distribution, high uptake, and good mechanical properties) than the others studied and is therefore chosen as the matrix to prepare polymer electrolytes. A maximum conductivity of 5.1 × 10⁻⁵ S cm⁻¹ at 25 °C is obtained for a polymer electrolyte containing 1.5 wt.% TiO₂ in a viscous electrolyte, compared with 3.2 × 10⁻⁵ S cm⁻¹ for a polymer electrolyte without TiO₂. The glass transition temperature, T_g is lowered by the addition of TiO₂ (up to 1.5 wt.% in a viscous electrolyte) due to interaction between P(EO-EC) and TiO₂, which weakens the interaction between oxide groups of the P(EO-EC) and lithium cations. The overall results indicate that the sample prepared with 10 wt.% TiO₂ for a porous membrane and 1.5 wt.% TiO₂ for a viscous electrolyte is a promising polymer electrolyte for rechargeable lithium batteries.     

Is 3-methyl-2-oxazolidinone a suitable solvent for lithium-ion batteries?

3-Methyl-2-oxazolidinone (MeOx) has been mixed to ethylene carbonate (EC) or dimethyl carbonate (DMC) in presence of lithium tetrafluoroborate (LiBF₄) or lithium hexafluorophosphate (LiPF₆) for use as electrolyte in lithium batteries. The optimized electrolytes in term of conductivity and viscosity are MeOx:EC, x(MeOx) = 0.5 and MeOx:DMC, x(MeOx) = 0.4 in presence of LiBF₄ (1 M) or LiPF₆ (1 M). MeOx:EC electrolytes have a better thermal stability than MeOx:DMC electrolytes but the low wettability of the Celgard separator by MeOx:EC prevents its use in lithium batteries. No lithium insertion–deinsertion occurs when LiPF₆ is used as salt in MeOx-based electrolytes. MeOx:DMC, x(MeOx) = 0.4 + LiBF₄ (1 M) exhibits a good cycling ability at a graphite electrode but all the investigated electrolytes containing MeOx have a low stability in oxidation at a lithium cobalt oxide electrode (Li_xCoO₂).     

Lithium batteries composed of aluminate polymer complexes as single-ion conductive solid electrolytes

Single-ionic conductors, which display lithium ion migration exclusively (without anion migration), have been realized as the polymeric solid electrolytes with lithium orthoaluminate repeating units carrying oligo(oxyethylene) main chain and two side chains of endomethoxy{oligo(oxyethylene)}. The ionic conductivity of the aluminate polymer complexes is about 10⁻⁶–10⁻⁷ S/cm at room temperature. Thin film lithium secondary batteries were fabricated into 5.5 cm×4.5 cm×0.02–0.03 cm (thick) cells from lithium foil (anode), aluminate polymer complex (electrolyte) and TiS₂ (cathode). These batteries show minimal decay of output voltage upon constant current discharging and their capacity of first cycle was about 146 mA h/g of active cathode material. By contrast typical bi-ionic conductor of (aluminate polymer complex+5% LiClO₄) hybrid system showed, on the contrary, rapid decay of output voltage, due to polarization.     

Photocured PEO-based solid polymer electrolyte and its application to lithium–polymer batteries

A solid polymer electrolyte (SPE) based on polyethylene oxide (PEO) is prepared by photocuring of polyethylene glycol acrylates. The conductivity is greatly enhanced by adding low molecular weight poly(ethylene glycol) dimethylether (PEGDME). The maximum conducticity is 5.1×10⁻⁴ S cm⁻¹ at 30°C. These electrolytes display oxidation stability up to 4.5 V against a lithium reference electrode. Reversible electrochemical plating/stripping of lithium is observed on a stainless steel electrode. Li/SPE/LiMn₂O₄ as well as C(Li)/SPE/LiCoO₂ cells have been fabricated and tested to demonstrate the applicability of the resulting polymer electrolytes in lithium–polymer batteries.     

Lithium ion conductivity of gel polymer electrolytes containing insoluble lithium tetrakis(pentafluorobenzenethiolato) borate

Lithium ion conducting gel polymer electrolytes composed of insoluble lithium tetrakis(pentafluorobenzenethiolato) borate (LiTPSB), poly(vinylidene fluoride-co-hexafluoropropylene) (PVDF-HFP) and ethylene carbonate–propylene carbonate mixed solvent (EC–PC) were prepared and their ionic conductivities and electrochemical stabilities were investigated. Ionic conductivity was largely dependent on the contents of EC–PC and LiTPSB. Gel polymer electrolyte containing optimized content of 50 (LiTPSB)–50 (PVDF-HFP/EC–PC (13:87 wt.%)) exhibited ionic conductivity of 4 × 10⁻⁴ S cm⁻¹ at 30 °C, lithium ion transference number of 0.33 and anodic oxidation potential of 4.2 V.     

Development of PVA based micro-porous polymer electrolyte by a novel preferential polymer dissolution process

A micro-porous polymer electrolyte based on PVA was obtained from PVA–PVC based polymer blend film by a novel preferential polymer dissolution technique. The ionic conductivity of micro-porous polymer electrolyte increases with increase in the removal of PVC content. Finally, the effect of variation of lithium salt concentration is studied for micro-porous polymer electrolyte of high ionic conductivity composition. The ionic conductivity of the micro-porous polymer electrolyte is measured in the temperature range of 301–351 K. It is observed that a 2 M LiClO₄ solution of micro-porous polymer electrolyte has high ionic conductivity of 1.5055 × 10⁻³ S cm⁻¹ at ambient temperature. Complexation and surface morphology of the micro-porous polymer electrolytes are studied by X-ray diffraction and SEM analysis. TG/DTA analysis informs that the micro-porous polymer electrolyte is thermally stable upto 277.9 °C. Chronoamperommetry and linear sweep voltammetry studies were made to find out lithium transference number and stability of micro-porous polymer electrolyte membrane, respectively. Cyclic voltammetry study was performed for carbon/micro-porous polymer electrolyte/LiMn₂O₄ cell to reveal the compatibility and electrochemical stability between electrode materials.     

Oligo(ethylene glycol)-functionalized disiloxanes as electrolytes for lithium-ion batteries

Functionalized disiloxane compounds were synthesized by attaching oligo(ethylene glycol) chains, –(CH₂CH₂O)–_n, n = 2–7, via hydrosilation, dehydrocoupling, and nucleophilic substitution reactions and were examined as non-aqueous electrolyte solvents in lithium-ion cells. The compounds were fully characterized by ¹H, ¹³C, and ²⁹Si nuclear magnetic resonance (NMR) spectroscopy. Upon doping with lithium bis(oxalato)borate (LiBOB) or LiPF₆, the disiloxane electrolytes showed conductivities up to 6.2 × 10^?4 S cm^?1 at room temperature. The thermal behavior of the electrolytes was studied by differential scanning calorimetry, which revealed very low glass transition temperatures before and after LiBOB doping and much higher thermal stability compared to organic carbonate electrolytes. Cyclic voltammetry measurements showed that disiloxane-based electrolytes with 0.8 M LiBOB salt concentration are stable to 4.7 V. The LiBOB/disiloxane combinations were found to be good electrolytes for lithium-ion cells; unlike LiPF₆, LiBOB can provide a good passivation film on the graphite anode. The LiPF₆/disiloxane electrolyte was enabled in lithium-ion cells by adding 1 wt% vinyl ethylene carbonate (VEC). Full cell performance tests with LiNi_0.80Co_0.15Al_0.05O₂ as the cathode and mesocarbon microbead (MCMB) graphite as the anode show stable cyclability. The results demonstrate that disiloxane-based electrolytes have considerable potential as electrolytes for use in lithium-ion batteries.     

A new polymer electrolyte system (PEO)n:NaPO3

A new Na⁺ ion conducting polymer electrolyte, based on poly(ethylene oxide) (PEO) and sodium meta phosphate (NaPO₃) is investigated. (PEO)_n:NaPO₃ polymer metal salt complexes with different [ethylene oxide]/Na ratios (n = 3, 4, 6, 8 and 10) are prepared by the solution casting method. Dissolution of the salt into the polymer host is confirmed by X-ray diffraction, differential scanning calorimetry (DSC) and scanning electron microscopy. Further, interaction of the polymer chains with the metal salt is substantiated by Fourier transform infrared spectroscopy. The electrical conductivity of the samples is measured over the temperature range 322–351 K. The temperature dependent conductivity exhibits two different activation energies, below and above the softening point of the polymer. The composition (PEO)₆:NaPO₃ is found to exhibit the least crystallinity but the highest conductivity 2.8 × 10⁻⁸ S cm⁻¹ at 351 K. The electronic transport number, measured by the dc polarization technique, shows that the conducting species are ionic in nature. The effect of ethylene carbonate on the best conducting composition is investigated by DSC and impedance spectroscopy. The addition of 20 wt.% ethylene carbonate, increases the amorphous phase and enhances the conductivity by two orders of magnitude.     

Ion pair formation and its effect in PEO:Mg solid polymer electrolyte system

In poly(ethylene oxide) (PEO) based solid polymer electrolytes, the interaction between cations and the ether oxygen plays a major role in ion conductivity. Measurements with differential scanning calorimetry (DSC) illustrated clearly the modification of the PEO crystalline structure with increasing content of magnesium salt. FTIR spectral studies suggest interaction of Mg²⁺ cations with the ether oxygen of PEO, where a 1100 cm⁻¹ broad band corresponds to COC stretching and severe deformation occurs. A spectral band at ∼623 cm⁻¹ corresponds to the ClO₄⁻ anion and shows the growth of a shoulder at a higher wave number with increasing salt content. The apparent new envelope at ∼634.5 cm⁻¹ clearly indicates ClO₄⁻–Mg²⁺ ion pairing. Ionic conductivity increases with salt content, and is optimized at 15 wt.% Mg salt (O:Mg ratio 28:1). The decrease in ion conductivity at higher salt contents is due to ion–ion association, which leads to ion pair formation (i.e. aggregation of ionic salt) and retards the motion of ions.     

A study on sulfites for lithium-ion battery electrolytes

Because of the similarity in the structures of organic sulfites with those of organic carbonates, the applications of organic sulfites for lithium-ion battery electrolytes were studied. The main differences in the bond lengths and the bond angles, which are resulted from the difference between carbon atom diameter and sulfur atom diameter, are analyzed. The physical properties of organic carbonates and organic sulfites are compared. The results of cyclic voltammetry (CV) test show that the decomposition potentials of propylene sulfite (PS) and dimethyl sulfite (DMS) are much higher than 4.5 V, it is satisfied with the requirements as the solvents for lithium ion batteries. But the decomposition potentials of ethylene sulfite (ES) and diethyl sulfite (DES) are lower than 3.5 V, they can only be used as additives for lithium ion battery electrolytes. The results of charge–discharge tests show that both ES and PS have excellent film-forming properties; the performance of LiCoO₂/graphite cell was improved evidently even with the ES addition as little as 0.3 wt.% in 1 mol L⁻¹ LiPF₆ EC/DMC/DEC (1:2:2) electrolyte. DMS can improve both the conductivities of electrolytes and the capacities of batteries, therefore it is a good electrolyte co-solvent.     

Nanofiller incorporated poly(vinylidene fluoride–hexafluoropropylene) (PVdF–HFP) composite electrolytes for lithium batteries

Composite polymer electrolyte (CPE) membranes, comprising poly(vinylidene fluoride–hexafluoropropylene) (PVdF–HFP), aluminum oxyhydroxide (AlO[OH]_n) of two different sizes 7 μm/14 nm and LiN(C₂F₅SO₂)₂ as the lithium salt were prepared using a solution casting technique. The prepared membranes were subjected to XRD, impedance spectroscopy, compatibility and transport number studies. Also Li Cr_0.01Mn_1.99O₄/CPE/Li cells were assembled and their charge–discharge profiles made at 70 °C. The incorporation of nanofiller greatly enhanced the ionic conductivity and the compatibility of the composite polymer electrolyte. The film which possesses a nanosized filler offered better electrochemical properties than a film with micron sized fillers. The results are discussed based on Lewis acid–base theory.