Dramatic reduction in the densification temperature of garnet-type solid electrolytes

Cubic garnet Li₇La₃Zr₂O₁₂ (LLZO) and similar compositions of fast ion-conducting solid-state electrolytes have shown great potential for the development of high-energy-density solid-state Li-ion batteries. Although these materials have shown unprecedented ionic conductivities and chemical stability, these materials require high processing temperatures for synthesis. For many of the common compositions of LLZO, temperatures above 1000 °C are required to form the cubic garnet phase and to achieve high conductivities. Therefore, lowering the processing temperatures of these materials is of great interest for the purposes of scalability and fabrication. It has been reported that a Bi co-dopant not only stabilizes the cubic garnet phase but also lowers the densification temperature. In this study, Li₆La₃ZrBiO₁₂ (LLZBO) was prepared by a rapid-induction hot-pressing technique and characterized using a variety of techniques, including X-ray diffraction, scanning electron microscopy, and Raman spectroscopy. We demonstrate the ability to synthesize phase-pure LLZBO with higher relative densities (~?94%) than can be achieved by pressure-less sintering methods, at pressing temperatures of only 850 °C. The ionic conductivity was measured to be 0.1 mS cm^?1, which is comparable to the best reported conductivities of high-density LLZO. This demonstrates the ability to fabricate dense, phase-pure, and high-conductivity LLZBO at temperatures significantly lower than other garnet compositions, which will prove useful for scalability and reducing reactivity with cathodes during densification.     

Mechanical behavior of Li-ion-conducting crystalline oxide-based solid electrolytes: a brief review

Li-ion-conducting solid electrolytes are receiving considerable attention for use in advanced batteries. These electrolytes would enable use of a Li metal anode, allowing for batteries with higher energy densities and enhanced safety compared to current Li-ion systems. One important aspect of these electrolytes that has been overlooked is their mechanical properties. Mechanical properties will play a large role in the processing, assembly, and operation of battery cells. Hence, this paper reviews the elastic, plastic, and fracture properties of crystalline oxide-based Li-ion solid electrolytes for three different crystal structures: Li_6.19Al_0.27La₃Zr₂O₁₂ (garnet) [LLZO], Li_0.33La_0.57TiO₃ (perovskite) [LLTO], and Li_1.3Al_0.3Ti_1.7(PO₄)₃ (NaSICON) [LATP]. The experimental Young’s modulus value for (1) LLTO is ~?200 GPa, (2) LLZO is ~?150 GPa, and (3) for LATP ~?115 GPa. The experimental values are in good agreement with density functional theory predictions. The fracture toughness value for all three of LLTO, LLZO, and LATP is approximately 1 MPa m^?2. This low value is expected since, they all exhibit at least some degree of covalent bonding, which limits dislocation mobility leading to brittle behavior.     

PEG crosslinked poly(vinylbenzene boronic acid) polymer electrolytes for Li-ion batteries

Poly(4-vinylbenzeneboronic acid), PVBBA was synthesized via free-radical polymerization of 4-vinylbenzeneboronic acid (4-VBBA) and followed by crosslinking with polyethylene glycol (PEG) with different molecular weights to produce boron containing crosslinked polymers. Prior to crosslinking, the materials were doped with CF₃SO₃Li at several stoichiometric ratios to get PVBBAPEGX-Y where X is the molecular weight of PEG and Y is the EO/Li ratio. The materials were characterized by using Fourier transform infrared spectroscopy (FT-IR), thermogravimetric analysis (TGA) and differential scanning calorimeter (DSC). The ionic conductivity of these novel crosslinked electrolytes was studied by dielectric-impedance spectroscopy. Li-ion conductivity of these polymer electrolytes depends on the length of the side units as well as the doping ratio. PVBBAPEG200-10 illustrated a satisfactory ionic conductivity of 3.1 × 10^?5 S/cm at 20 °C and 1.8 × 10^?3 S/cm at 100 °C.     

Influences of LiCF3SO3 and TiO2 nanofiller on ionic conductivity and mechanical properties of PVA:PVdF blend polymer electrolyte

In recent years, solid polymer electrolytes have been extensively studied due to its flexibility, electrochemical stability, safety, and long life for its applications in various electrochemical devices. Interaction of LiCF₃SO₃ and TiO₂ nanofiller in the optimized composition of PVA:PVdF (80:20—system-A possessing σ ~ 2.8 × 10⁻⁷ Scm⁻¹ at 303 K) blend polymer electrolyte have been analyzed in the present study. LiCF₃SO₃ has been doped in system-A, and the optimized LiCF₃SO₃ doped sample (80:20:15-system-B possessing σ ~ 2.7 × 10⁻³ Scm⁻¹ at 303 K) has been identified. The effect of different concentration of TiO₂ in system-B has been analyzed and the optimized system is considered as system-C (σ ~ 3.7 × 10⁻³ Scm⁻¹ at 303 K). The cost effective, solution casting technique has been used for the preparation of the above polymer electrolytes. Vibrational, structural, mechanical, conductivity, thermal, and electrochemical properties have been studied using FTIR, XRD, stress-strain, AC impedance spectroscopic technique, DSC and TGA, LSV, and CV respectively to find out the optimized system. System-C possessing the highest ionic conductivity, higher tensile strength, low crystallinity, high thermal stability, and high electrochemical stability (greater than 5 V vs Li/Li⁺) is well suitable for lithium ion battery application.

     

Lithium-doped PEO—a prospective solid electrolyte with high ionic conductivity,developed using <Emphasis Type="Italic">n</Emphasis>-Butyllithium in hexane as dopant

The present work is an effort to study the effects of Li doping on the structural and transport properties of the solid polymer electrolyte, poly-ethelene oxide (PEO) (molecular weight, 200,000). Li-doped PEO was synthesized by treating PEO with n-Butyllithium in hexane for different doping concentrations. It is seen that the crystallinity of the doped PEO decreases on increasing the Li doping concentration and XRD and FTIR studies support this observation. FESEM images give better details of surface morphology of doped PEO samples. The TGA curves of PEO and Li-doped PEO samples reveal the weight loss region, and it is observed that the weight loss process of the solid polymer electrolyte is gradual rather than abrupt, contrary to the case of liquid electrolytes. The purity and the electrochemical stability of the samples were established by cyclic voltammetry studies. Impedance measurements were carried out to estimate the ionic conductivity of Li-doped PEO samples. The present value of ionic conductivity observed at room temperature in Li-doped PEO is about five orders higher than that of pure PEO and is quite close to that of liquid electrolytes. It is inferred that, ionic conductivity of the sample is increasing on increasing the Li doping concentration due to enhanced charge carrier density and flexibility of the doped sample structure. The ionic mobility and ionic transport are significantly improved by the less crystallinity and higher flexibility of the Li-doped PEO samples which in turn are responsible for the enhanced ionic conductivity observed.     

固态电解质锂镧锆氧(Li_7La_3Zr_2O_(12))制备及第一性原理研究

固态电解质(SSE)是锂离子电池(LIB)的关键材料.Li_7La_3Zr_2O_(12)(LLZO)固体电解质是全固态锂离子电池开发中的关键部分.采用高温固相法制备了不同烧结温度后的四方Li_7La_3Zr_2O_(12)(t-LLZO)和立方Li_7La_3Zr_2O_(12)(c-LLZO),分析了两种样品的结构性能.800℃下烧结12小时的t-LLZO呈四方相,晶格尺寸为a=b=13.13064?,c=12.66024?,离子电导率为3.42×10~(-8)S·cm~(-1);1000℃下烧结12小时的c-LLZO呈立方相,晶格尺寸为a=b=c=13.03544?,离子电导率为8.48×10~(-5)S·cm~(-1).另基于密度泛函理论(DFT)的第一性原理计算了四方相和立方相的LLZO固体电解质材料的能带结构、晶格参数、态密度和键布居.通过理论计算解释了四方相LLZO离子电导率低于立方相LLZO的原因.     

A novel PEO-based blends solid polymer electrolytes doping liquid crystalline ionomers

A novel PEO-based blends solid polymer electrolytes doping liquid crystalline ionomers (LCI), PEO/PMMA/LiClO₄/LCI, and PEO/LiClO₄/LCI were prepared by solution casting technology. Scanning electron microscope (SEM) and energy-dispersive spectroscopy (EDS) analysis proved that LCI uniformly dispersed into the solid electrolytes and restrained phase separation of PEO and PMMA. Differential scanning calorimetry (DSC) results showed that LCI decreases the crystallinity of blends solid polymer electrolytes. Thermogravimetric analysis (TGA) proved LCI not only improved thermal stability of PEO/PMMA/LiClO₄ blends but also prevent PEO/PMMA from phase separation. Infrared spectra results illustrated that there exists interaction among Li⁺ and O, and LCI that promotes the synergistic effects between PEO and PMMA. The EIS result revealed that the conductivity of the electrolytes increases with LiClO₄ concentration in PEO/PMMA blends, but it increases at first and reaches maximum value of 2.53?×?10^?4 S/cm at 1.0 % of LCI. The addition of 1.0 % LCI increases the conductivity of the electrolytes due to that LCl promoting compatibility and interaction of PEO and PMMA. Under the combined action of rigidity induced crystal unit, soft segment and the terminal ionic groups in LCI, PEO/PMMA interfacial interaction are improved, the reduction of crystallinity degree of PEO leads Li⁺ migration more freely.     

Development of proton conducting biopolymer membrane based on agar–agar for fuel cell

A proton-conducting polymer electrolyte based on agar and ammonium nitrate (NH₄NO₃) has been prepared through solution casting technique. The prepared polymer electrolytes were characterized by impedance spectroscopy, X-ray diffraction, and Fourier transform infra-red spectroscopy. Impedance analysis shows that sample with 60 wt.% NH₄NO₃ has the highest ionic conductivity of 6.57 × 10⁻⁴ S cm⁻¹ at room temperature. As a function of temperature, the ionic conductivity exhibits an Arrhenius behaviour increasing from 6.57 × 10⁻⁴ S cm⁻¹ at room temperature to 1.09 × 10⁻³ S cm⁻¹ at 70 °C. Transport parameters of the samples were calculated using Wagner’s polarization method and thus shows that the increase in conductivity is due to the increase in the number of mobile ions. Fuel cell has been constructed with the highest proton conductivity polymer 40agar/60NH₄NO₃ and the open circuit voltage is found to be 558 mV.

     

Role of silica nanoparticles in conductivity enhancement of nanocomposite solid polymer electrolytes: (PEGx NaBr): ySiO2

Nanocomposite solid polymer electrolytes (NCSPEs) with conducting species other than Li ions are being investigated for solid-state battery applications. Pristine solid polymer electrolytes (SPEs) do not show ionic conductivity suitable for batteries. Addition of inert fillers to SPEs is known to enhance the ionic conductivity. In this paper, we present the role of silica nanoparticles in enhancing the ionic conductivity in NCSPEs with sodium as conducting species. Sodium bromide is complexed with the host polyethylene glycol polymer by solution cast method and silica nanoparticles (SiO₂, average particle size 7 nm) are incorporated into the complex in small amounts. The composites are characterized by powder XRD and IR spectroscopy. Conductivity measurements are undertaken as a function of concentration of salt and also as a function of temperature using impedance spectroscopy. Addition of silica nanoparticles shows an enhancement in conductivity by 1–2 orders of magnitude. The results are discussed in terms of interaction of nanoparticles with the nonconducting anions.     

Electrochemical performance of plasticized PEO-LiTf complex-based composite gel polymer electrolytes with the addition of barium titanate

Gel electrolytes and solid electrolytes have been reported as a potential element to slow down the polysulfide shuttle by reducing its mobility in the electrolytes. The preparation of sulfur-conductive polymer composites, or sulfur-carbon composites, has been reported as softening the impact of the shuttle effects. Unlike Li-ion batteries so far, no electrolyte is found to be optimal for Li–S batteries at all conditions. Taking into account all these factors, in the present study, an attempt has been made to develop solid polymer electrolytes in conjunction with non-aqueous liquid electrolytes along with inert fillers for Li–S batteries. Poly-ethylene oxide (PEO)-based composite gel polymer electrolytes (CGPE) comprising a combination of plasticizers, namely 1,3-dioxolane (DIOX)/tetraethylene glycol dimethylether (TEGDME) and a lithium salt (LiTf) with the addition of ceramic filler, barium titanate (BaTiO₃) have been prepared using a simple solution casting technique in an argon atmosphere. The as-prepared polymer electrolyte films were subjected to SEM, ionic conductivity, TG/DTA, and FTIR analyses. A symmetric cell composed of Li/CGPE/Li was assembled, and the variation of interfacial resistance as a function of time was also measured. The ionic conductivity was found to be increased as a function of temperature. The lithium transference number (Li_t ⁺) was measured, and the value was calculated as 0.7 which is sufficient for battery applications. The electrochemical stability window of the sample was studied by linear sweep voltammetry, and the polymer electrolyte film was found to be stable up to 5.7 V. The TG/DTA analysis reveals that this CGPE is thermally stable up to 350 °C. The compatibility studies exhibited that CGPE has better interracial properties with lithium metal anode. The interaction between the PEO and salt has been identified by an FTIR analysis.     

Vibrational,electrical, and ion transport properties of PVA-LiClO<Subscript>4</Subscript>-sulfolane electrolyte with high cationic conductivity

Novel solid polymer electrolytes, poly(vinylalcohol)-lithium perchlorate (PVA-LiClO₄) and PVA-LiClO₄-sulfolane are prepared by solvent casting method. The experimental results show that sulfolane addition enhances the ionic conductivity of PVA-LiClO₄ complex by three orders. The maximum ionic conductivity of 1.14 ± 0.20 × 10^?2 S cm^?1 is achieved for 10 mol% sulfolane-added electrolyte at ambient temperature. Polymer-salt-plasticizer interactions are analyzed through attenuated total reflectance-Fourier transform infrared (ATR-FTIR) spectroscopy. Lithium ion transference number is found by AC impedance spectroscopy combined with DC potentiostatic measurements. The results confirm that sulfolane improves the Li⁺ transference number of PVA-LiClO₄ complex to 0.77 from 0.40. The electrochemical stability window of electrolytes is determined by cyclic voltammetry (CV). The broad electrochemical stability window of 5.45 V vs. lithium is obtained for maximum conducting electrolyte. All-solid-state cell is fabricated using maximum conducting electrolyte, and electrochemical impedance study is carried out. It reveals that electrolyte interfacial resistance with Li electrode is very low. The use of PVA-LiClO₄-sulfolane as a viable electrolyte material for high-voltage lithium ion batteries is ensured.     

New ternary molten salt electrolyte based on alkali metal triflates

A novel molten salt electrolyte composed of lithium triflate (CF₃SO₃Li, LiTf), sodium triflate (CF₃SO₃Na, NaTf), and potassium triflate (CF₃SO₃K, KTf) has been prepared and characterized by thermogravimetry/differential thermal analysis (TG/DTA), electrochemical impedance spectroscopy (EIS), and cyclic voltammetry. TG/DTA shows that the electrolyte was thermally stable when the temperature was under 400 °C. Its thermal stability gradually decreased with increase of LiTf concentration. The ionic conductivity of molten salt electrolyte has been evaluated by EIS and its value exceeds 10⁻² Scm⁻¹ in the temperature range from 230 to 270 °C. The electrochemical window of the electrolyte at the molar ratio of 0.5/1/1 is about 4.7 V at 250 °C. This electrolyte with low melting point exhibits promising characteristics for high-temperature lithium batteries.     

Effects of MnO<Subscript>2</Subscript> nano-particles on the conductivity of PMMA-PEO-LiClO<Subscript>4</Subscript>-EC polymer electrolytes

Li-ion rechargeable batteries based on polymer electrolytes are of great interest for solid state electrochemical devices nowadays. Many studies have been carried out to improve the ionic conductivity of polymer electrolytes, which include polymer blending, incorporating plasticizers and filler additives in the electrolyte systems. This paper describes the effects of incorporating nano-sized MnO₂ filler on the ionic conductivity enhancement of a plasticized polymer blend PMMA–PEO–LiClO₄–EC electrolyte system. The maximum conductivity achieved is within the range of 10⁻³ S cm⁻¹ by optimizing the composition of the polymers, salts, plasticizer, and filler. The temperature dependence of the polymer conductivity obeys the VTF relationship. DSC and XRD studies are carried out to clarify the complex formation between the polymers, salts, and plasticizer.     

Poly-acrylonitrile-based gel-polymer electrolytes for sodium-ion batteries

Research and development activities on sodium-ion batteries are becoming prominent in the past few years. Compared to lithium-based batteries, the sodium-based batteries will be cheaper because of the abundancy of sodium raw materials in the earth’s crust and also in seawater. In the current study, we synthesized and characterized poly-acrylonitrile (PAN)-based gel-polymer electrolytes formed with NaClO₄ and dissolved in ethylene carbonate (EC) and propylene carbonate (PC). By systematically varying the weight ratios of polymer, salt, and the solvents, we obtained an optimum room temperature ionic conductivity of 4.5 mS cm⁻¹ for the composition 11PAN-12NaClO₄-40EC-37PC (wt.%), which is reasonably good for practical applications. This value of conductivity is comparable to a few other Na⁺ ion conducting gel-polymer electrolyte systems studied in the recent past. Variation of ionic conductivity with inverse temperature showed Arrhenius behavior. Activation energies estimated for all the samples showed only a slight variation suggesting that a single activation process which depends on the EC/PC co-solvent governs the ionic mobility in these gel-polymer electrolytes. Thermo-gravimetric analysis (TGA) revealed that there is no noticeable weight loss of these electrolytes up to 100 °C and hence the electrolytes are thermally stable for operating temperatures up to 100 °C.

     

Study on ionic conductivity of polymer electrolyte plasticized with PEG–aluminate ester for rechargeable lithium ion battery

We have synthesized poly(ethylene glycol) (PEG)-aluminate ester as a plasticizer for solid polymer electrolytes. The thermal stability, ionic conductivity and electrochemical stability of the polymer electrolyte which consist of poly(ethylene oxide) (PEO)-based copolymer, PEG–aluminate ester and lithium bis-trifluoromethanesulfonimide (LiTFSI) were investigated. Addition of PEG–aluminate ester increased the ionic conductivity of the polymer electrolyte, showing greater than 10^− 4 S cm^− 1 at 30 °C. The polymer electrolyte containing PEG–aluminate ester retained thermal stability of the non-additive polymer electrolyte and exhibited electrochemical stability up to 4.5 V vs. Li⁺/Li at 30 °C.     

6,7Li NMR study of ion mobility on the molecular scale in lithated imidazole complexes

Two model compounds, lithium imidazolium (LiIm) and lithium 2-undecylimidazolium (und-LiIm), were synthesized. These materials are chosen as models of potential lithium ion conductors for use as electrolytes in lithium batteries. Solid-state NMR was used to provide information on the microscopic interactions including ionic mobility and ring reorientations which govern the efficiency of conductivity. Lithium imidazolium was mixed with lithium methylsulfonate, generating a doped complex in which a doubly lithiated imidazole ring was inferred based on the ⁷Li NMR chemical shifts. Our research includes ^6,7Li variable temperature MAS NMR experiments at intermediate spinning speeds, relaxation studies to determine spin-lattice relaxation times (T₁) of lithium ion hopping, and 2D exchange spectroscopy to determine possible chemical exchange processes. The possibility of 2-site ring reorientation for the doubly lithiated imidazole ring was supported by exchange spectroscopy. Comparisons of spin-lattice relaxation times and corresponding activation energies of the lithium imidazolium and the doped complex point to a higher degree of mobility in the latter.Lithium 2-undecylimidazolium was prepared and exhibited a lower melting point than the parent lithium imidazolium, as expected. This small molecule was chosen as representative of a side-chain functionalized polyethylene-based material. ⁷Li MAS spectra show mainly the presence of the doubly lithiated imidazole ring in pure und-LiIm, and in the LiCH₃SO₃–und-LiIm mixture. The data clearly indicate local mobility of the lithium ions in the materials.     

Transport properties of materials with the scheelite structure and their modification by doping

Materials with the scheelite structure exhibit mixed ionic and electronic conductivity, and are of interest as oxidation catalysts. Scheelite materials, with the general composition ABO₄ are also possible candidates for use as electrolytes or cathode materials in solid oxide fuel cells. Our work on the scheelite system, based on BiVO₄, shows that both ionic and electronic components of the conductivity can be modified by doping this material. Both A and B site doping have been investigated in the range of 5 mol% dopant concentration. The A cation was replaced by Ca²⁺ and Ce⁴⁺, and the B site by Mn⁴⁺ and Mo⁶⁺. The phase purity was verified by XRD methods. The total and partial conductivities of pure and doped BiVO₄ were investigated by use of the emf technique and ac impedance spectroscopy. Measurements were made between 550° C and 700° C and the oxygen gradient in the emf cell was established by oxygen and air gas flows with specified flow rates and oxygen partial pressure. Paper presented at the 1st Euroconference on Solid State Ionics, Zakynthos, Greece, 11 – 18 Sept. 1994.     

Electrical properties of heavily doped fluorite-structured BaF2:RF3 (R=La, Pr, Nd, Gd, Tb, Y, Sc) single crystals

The superionic conductivity and dielectric response of heavily doped fluorite-structured Ba_1−xR_xF_2+x (R=La, Pr, Nd, Gd, Tb, Y, Sc; x=0.005–0.45) crystals are reported. The highest ionic conductivity is found for R=Sc and x=0.1. Upon ScF₃ doping, small Sc³⁺ ions rearrange their surroundings, create excessive fluoride interstitial ions and bring about a high ionic conductivity. For each dopant, the concentration dependence of the ionic conductivity is non-linear. A monotonous concentration dependence of the ionic conductivity is found only for La³⁺ doping. Upon doping with Nd³⁺, Gd³⁺, Tb³⁺, Y³⁺ and Sc³⁺ ions, a conductivity maximum is observed at x=0.1–0.2. Upon Pr³⁺ doping, this maximum is split. The influence of defect clustering on the concentration dependence of the conductivity is discussed. Paper presented at the 6th Euroconference on Solid State Ionics, Cetraro, Calabria, Italy, Sept. 12–19, 1999.     

Structural,optical and electrical properties of CuSCN nano-powders doped with Li for optoelectronic applications

Pure and Li-doped CuSCN nano-powders were prepared using an in situ method. Structural, optical and electrical properties of the prepared samples were investigated using X-ray diffraction (XRD), UV–Visible spectrophotometer and simple electrical circuit. XRD measurements showed that all pure and doped samples with 1%–7% Li have the hexagonal structures. The crystallite size of CuSCN decreased from 39.46 nm to 36.42 nm with increasing Li concentration from 0 to 7%. The values of direct and indirect optical band gap energies of pure and Li-doped CuSCN nano-powders were calculated. Direct optical band gap energy increased from 3.60 eV to 4.20 eV and indirect optical band gap energy increased from 2.36 eV to 3.20 eV by doping CuSCN with Li. The dc electrical conductivity was calculated at room temperature for all prepared CuSCN samples. Electrical conductivity decreased from 6.04 × 10⁻⁸ (Ω.cm)⁻¹ to 2.82 × 10⁻⁸ (Ω.cm)⁻¹ with increasing Li concentration from 0 to 7%. The optoelectronic performance of CuSCN was improved by doping with Li. As a result, Li-doped CuSCN could be a good candidate material as a window layer and as a hole transport layer (HTL) for producing more efficient solar cells.     

A preliminary study on a new LiBOB/acetamide solid phase transition electrolyte

New solid electrolytes containing acetamide and lithium bioxalato borate (LiBOB) with different molar ratios have been investigated. Their melting points (T_m) are around 42 °C. The ionic conductivities and activation energies vary drastically below and above T_m, indicating a typical feature of phase transition electrolyte. The ionic conductivity of the LiBOB/acetamide electrolyte with a molar ratio of 1:8 is 5 × 10^? 8 S cm^? 1 at 25 °C but increases to 4 × 10^? 3 S cm^? 1 at 60 °C. It was found that anode materials, such as graphite and Li₄Ti₅O₁₂, could not discharge and charge properly in this electrolyte at 60 °C due to the difficulty in forming a stable passivating layer on the anodes. However, a Li/LiFePO₄ cell with this electrolyte can be charged properly after heating to 60 °C, but cannot be charged at room temperature. Although the LiBOB/acetamide electrolytes are not suitable for Li-ion batteries due to poor electrode compatibility, the current results indicate that a solid electrolyte with a slightly higher phase transition temperature than room temperature may find potential application in stationary battery for energy storage where the electrolyte is at high conductive liquid state at elevated temperature and low conductive solid state at low temperature. The interaction between acetamide and LiBOB in the electrolyte is also studied by Raman and FTIR spectroscopy.