Four volts class solid lithium polymer batteries with a composite polymer electrolyte

Polyethylene oxide (PEO)-based polymer electrolytes with BaTiO₃ as a filler have been examined as electrolytes in 4 V class lithium polymer secondary batteries. A mixture of 90 wt.% LiN(CF₃SO₂)₂–10 wt.% LiPF₆ was found to be the best candidate as the salt in PEO, and showed high electrical conductivity, good corrosion resistance to the aluminum current collector and low interfacial resistance between the lithium metal anode and the polymer electrolyte. The cyclic performance of the cell, Li/[PEO₁₀–(LiN(CF₃SO₂)₂–10 wt.% LiPF₆)]–10 wt.% BaTiO₃/LiNi_0.8Co_0.2O₂/Al, showed good charge–discharge cycling performance. The observed capacity fading on charging up to 4.2 V at 80 °C in the cell was about 0.28% per cycle in the first 30 cycles, compared to that of 0.5% for the polymer electrolyte without LiPF₆ in the lithium salt.     

Rotating ring–disk electrode measurements on Mn dissolution and capacity losses of spinel electrodes in various organic electrolytes

The Mn dissolution and capacity losses of spinel electrodes in lithium-ion cells with the various electrolyte solutions of LiPF₆, LiClO₄, LiBF₄ and LiCF₃SO₃, in ethylene carbonate–dimethyl carbonate (EC–DMC) were studied using rotating ring–disk collection experiments. The cyclic voltammograms are similar at high scan rates for all electrolytes. Electrodes with LiPF₆ electrolyte show the best cycling performance. Cells with both LiPF₆ and LiBF₄ electrolyte solutions exhibited a capacity loss of 0.45% per cycle over 200 cycles at high scan rates, and these were slightly lower than the 0.5% per cycle in LiClO₄ and LiCF₃SO₃ electrolytes. The in situ monitoring of Mn dissolution from various electrolytes was carried out under different conditions. Ring cathodic currents of similar shaped were obtained for all electrolytes, which reveal that the Mn dissolution from the spinel LiMn₂O₄ electrodes exists and the highest Mn dissolution takes place at the top of charge voltage in all of electrolytes. Under both overcharge and overdischarge conditions, the ring current peak at disk potential of 5.0 V in the LiCF₃SO₃ electrolyte is much larger than that in other electrolytes. Moreover, the increase in ring current with cycle number occurs only in the LiCF₃SO₃ electrolyte. These results can be attributed to the oxidation of LiCF₃SO₃ electrolyte due to its voltage breakdown below 5.0 V.     

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.     

Discharge characteristics of chemically prepared MnO2 and electrolytic MnO2 in non-aqueous electrolytes

The discharge performance of chemically prepared MnO₂ (CMD) and electrolytic MnO₂ (EMD) is investigated in various electrolytes. LiPF₆, LiCF₃SO₃, and LiBF₄ are used as lithium salts in a mixed solvent of ethylene carbonate, propylene carbonate, and 1,2-dimethoxyethane (DME). The size and crystal structure of MnO₂ particles is observed by scanning electron microscope and X-ray diffraction, respectively. The particle size of CMD is smaller than that of EMD, but the crystal structures of the two materials are similar. The concentration of dissolved manganese ions from CMD and EMD particles is 148 and 23 mg l⁻¹ in the same electrolyte, respectively. The interfacial electrochemistry of test cells is analyzed by impedance spectroscopy. The discharge performance is poor in the electrolyte containing LiCF₃SO₃ salt. The specific discharge capacity of CMD is superior to that of EMD at high discharge rate.     

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.     

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.     

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.     

Effect of organic acids and nano-sized ceramic doping on PEO-based solid polymer electrolytes

Composite solid polymer electrolytes (CSPEs) consisting of polyethyleneoxide (PEO), LiClO₄, organic acids (malonic, maleic, and carboxylic acids), and/or Al₂O₃ were prepared in acetonitrile. CSPEs were characterized by Brewster Angle Microscopy (BAM), thermal analysis, ac impedance, cyclic voltammetry, and tested for charge–discharge capacity with the Li or LiNi_0.5Co_0.5O₂ electrodes coated on stainless steel (SS). The morphologies of the CSPE films were homogeneous and porous. The differential scanning calorimetric (DSC) results suggested that performance of the CSPE film was highly enhanced by the acid and inorganic additives. The composite membrane doped with organic acids and ceramic showed good conductivity and thermal stability. The ac impedance data, processed by non-linear least square (NLLS) fitting, showed good conducting properties of the composite films. The ionic conductivity of the film consisting of (PEO)₈LiClO₄:citric acid (99.95:0.05, w/w%) was 3.25 × 10⁻⁴ S cm⁻¹ and 1.81 × 10⁻⁴ S cm⁻¹ at 30 °C. The conductivity has further improved to 3.81 × 10⁻⁴ S cm⁻¹ at 20 °C by adding 20 w/w% Al₂O₃ filler to the (PEO)₈LiClO₄ + 0.05% carboxylic acid composite. The experimental data for the full cell showed an upper limit voltage window of 4.7 V versus Li/Li⁺ for CSPE at room temperature.     

Phase-separated polymer electrolyte based on poly(vinyl chloride)/poly(ethyl methacrylate) blend

The characteristics of polymer electrolytes based on a poly(vinyl chloride) (PVC)/poly(ethyl methacrylate) (PEMA) blend are reported. The PVC/PEMA based polymer electrolyte consists of an electrolyte-rich phase that acts as a conducting channel and a polymer-rich phase that provides mechanical strength. The dual phase was simply developed by a single-step coating process. The mechanical strength of the PVC/PEMA based polymer electrolyte was found to be much higher than that of a previously reported PVC/PMMA-based polymer electrolyte (poly(methyl methacrylate), PMMA) at the same PVC content, and even comparable with that of the PVC-based polymer electrolyte. The blended polymer electrolytes showed ionic conductivity of higher than 10⁻³ S cm⁻¹ and electrochemical stability up to at least 4.3 V. A prototype battery, which consists of a LiCoO₂ cathode, a MCMB anode, and PVC/PEMA-based polymer electrolyte, gives 92% of the initial capacity at 100 cycles upon repeated charge–discharge at the 1 C rate.     

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.     

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).     

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.     

Thermal behavior and decomposition kinetics of six electrolyte salts by thermal analysis

The thermal behavior of five lithium salts commonly used in the electrolytes of lithium-ion battery and one non-lithium salt used as a dominant salt of electrochemical capacitor was studied using simultaneous thermogravimetry (TG)–derivative thermogravimetry (DTG)–differential scanning calorimetry (DSC) under a dynamic nitrogen atmosphere. The results showed that the amount of free acid remained in the five lithium salts and their initial revolution temperature are different and the stability of all six salts falls in the order LiClO₄ > LiCF₃SO₃ > LiTFSI > TEABF₄ > LiBF₄ > LiPF₆. In addition, the reaction heat values associated with their decomposition processes were measured, which showed the stages for the evolution of free acid are endothermic with lower reaction heat, while for the decomposition of salts either endo- or exothermic, mainly depending on their chemical compositions. Kinetic parameters for these decomposition reaction processes were obtained by jointly using two thermal kinetic analysis methods.     

The effect of nanosized TiO2 addition on poly(methylmethacrylate) based polymer electrolytes

The electrochemical, X-ray diffraction, thermal, rheological and spectroscopic studies have been carried out to examine the effect of nanosized TiO₂ addition in different concentration to polymethylmethacrylate (PMMA) based gel polymer electrolytes (GPE). This work demonstrates that with optimum concentration of TiO₂ loadings in GPE, the ionic conductivity enhances with negligible effect on other electrochemical properties. The obtained ionic conductivity value is >10⁻³ S cm⁻¹. An increase in viscosity by an order of magnitude is obtained which also restricts the flow property of GPE. The addition of TiO₂ retains the amorphicity of the GPE while the T_g increases. Enhanced mechanical stability of these composite polymer electrolytes (CPEs) with solid-like behavior is evident from their appearance. The activation energy has been calculated by fitting the conductivity profile in VTF equation, which decreases on the addition of fillers. FTIR characterization also confirms the interaction of filler with CO of PMMA. The capabilities and properties exhibited by these CPEs will be of immense interest for electrochemists to use them in solid-state devices.     

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.     

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.     

Charge–discharge characteristics of LiCoO2/mesocarbon microbeads battery with poly(vinyl chloride)-based composite polymer electrolyte

Polyvinyl chloride (PVC)-based composite polymer electrolyte films consisting of PVC–LiCF₃SO₃–SiO₂ are prepared by the solution-casting method. The electrical properties of the electrolyte are investigated for ionic conductivity and its dependence on temperature. The electrolyte with the highest ionic conductivity is used to fabricate a LiCoO₂/PVC–LiCF₃SO₃–SiO₂/mesocarbon microbeads (MCMB) battery. The charge–discharge characteristics and performance of the battery at room temperature are evaluated to ascertain the effective viability, of these solid electrolytes in lithium-polymer batteries. Battery performances is also investigated at 313, 323 and 333 K.     

Studies of cycling behavior,ageing, and interfacial reactions of LiNi0.5Mn1.5O4 and carbon electrodes for lithium-ion 5-V cells

The cycling and storage behavior of LiNi_0.5Mn_1.5O₄ and MCMB electrodes for 5-V Li-ion batteries was investigated at elevated temperatures using a variety of electrochemical (CV, EIS) and spectroscopic (XPS, micro-Raman) tools. It was established that LiNi_0.5Mn_1.5O₄ electrodes could be cycled highly reversibly, demonstrating sufficient capacity retention at 60 °C by a constant current/constant voltage mode in DMC–EC/1.5 M LiPF₆ solutions. By studying the influence of temperature on the impedance of LiNi_0.5Mn_1.5O₄ electrodes, we conclude that when the initial electrode's surface chemistry is developed at a high temperature (60 °C) it becomes nearly invariant, and hence, their impedance remains steady upon cycling and storage. Prolonged storage of these electrodes at 60 °C may result in local Mn and Ni dissolution and transformation of the active material to λ-MnO₂. We have found that the surface chemistry of aged LiNi_0.5Mn_1.5O₄ electrodes (free of carbon black and PVdF) involves the formation of LiF, C–F and P–F_x species. Storage of MCMB electrodes in LiPF₆ containing solutions at open circuit conditions (before their first lithiation) leads to significant morphological changes and the formation of lithium fluoride on the electrode surface, as determined by the XRD studies. LiF is probably a product of a catalytic thermal decomposition of LiPF₆. These initial changes further influence the impedance and kinetics of the lithiated electrodes.     

Ionic liquid–polymer gel electrolytes based on morpholinium salt and PVdF(HFP) copolymer

New ionic liquid–polymer gel electrolytes (IPGEs) are prepared from N-ethyl-N-methylmorpholinium bis(trifluoromethanesulfonyl)imide (Mor_1,2TFSI) and poly(vinylidene fluoride)-hexafluoropropylene copolymer (PVdF(HFP)). To investigate the effect of propylene carbonate (PC) on the ionic conductivity of the IPGEs, the preparation methods are roughly divided into two groups according to the presence or absence of PC. The ionic conductivity for each IPGE is measured with increasing temperature and changing weight ratio of Mor_1,2TFSI. The results show that the ionic conductivity increases as the temperature and weight ratio of the Mor_1,2TFSI increase, and that the added PC improves the ionic conductivity of the IPGEs. In addition, thermogravimetric analysis and the data from infrared spectroscopy demonstrate the thermal stability of each IPGE and the presence of PC in the polymer network. Although the IPGEs that contain PC display high conductivity (∼1.1 × 10⁻² S cm⁻¹) at 60 °C, they are thermally unstable.