Nanocrystalline cellulose-reinforced composite mats for lithium-ion batteries: electrochemical and thermomechanical performance

Nanocrystalline cellulose (NCC)-reinforced poly(vinylidenefluoride-co-hexafluoropropylene) (PVdF-HFP) composite mats have been prepared by electrospinning method. Polymer electrolytes formed by activating the composite mats with 1 M lithium bis(trifluoromethanesulfonyl)imide/1-butyl-3-methypyrrolidinium bis(trifluoromethanesulfonyl)imide electrolyte solution. The addition of 2 wt% NCC in PVdF-HFP improved the electrolyte retention and storage modulus of the separator by 63 and 15 %, respectively. The developed electrolyte demonstrated high value of ionic conductivity viz. 4?×?10^?4?S?cm^?1 at 30 °C. Linear scan voltammetry revealed a wide electrochemical stability of the composite mat separator up to 5 V (vs. Li⁺/Li). Cyclic voltammetry of the polymer electrolyte with a graphite electrode in 2.5 to 0 V (vs. Li⁺/Li) potential range showed a reversible intercalation/de-intercalation of Li⁺ ions in the graphite. No peaks were observed related to the reduction of the electrolyte on the anode.     

A Perovskite Electrolyte That Is Stable in Moist Air for Lithium‐Ion Batteries

Solid‐oxide Li⁺ electrolytes of a rechargeable cell are generally sensitive to moisture in the air as H⁺ exchanges for the mobile Li⁺ of the electrolyte and forms insulating surface phases at the electrolyte interfaces and in the grain boundaries of a polycrystalline membrane. These surface phases dominate the total interfacial resistance of a conventional rechargeable cell with a solid–electrolyte separator. We report a new perovskite Li⁺ solid electrolyte, Li_0.38Sr_0.44Ta_0.7Hf_0.3O_2.95F_0.05, with a lithium‐ion conductivity of σ_Li=4.8×10^?4 S cm^?1 at 25 °C that does not react with water having 3≤pH≤14. The solid electrolyte with a thin Li⁺‐conducting polymer on its surface to prevent reduction of Ta⁵⁺ is wet by metallic lithium and provides low‐impedance dendrite‐free plating/stripping of a lithium anode. It is also stable upon contact with a composite polymer cathode. With this solid electrolyte, we demonstrate excellent cycling performance of an all‐solid‐state Li/LiFePO₄ cell, a Li‐S cell with a polymer‐gel cathode, and a supercapacitor.     

PVDF/PAN/SiO<Subscript>2</Subscript> polymer electrolyte membrane prepared by combination of phase inversion and chemical reaction method for lithium ion batteries

PVDF/PAN/SiO₂ polymer electrolyte membranes based on non-woven fabrics were prepared via introducing a chemical reaction into Loeb-Sourirajan (L-S) phase inversion process. It was found that physical properties (porosity, electrolyte uptake and ionic conductivity) and electrochemical properties were obviously improved. A favorable membrane structure with fully connective porous and uniform pore size distribution was obtained. The effects of PVDF/PAN weight ratio on the morphology, crystallinity, porosity, and electrochemical performances of membranes were studied. The optimized PVDF/PAN (70/30 w/w) (designated as M_pc30) polymer electrolyte membrane delivered excellent electrolyte uptake of 246.8 % and the highest ionic conductivity of 3.32 × 10^?3 S/cm with electrochemical stability up to 5.0 V (vs. Li/Li⁺). In terms of cell performance, the Li/M_pc30 polymer electrolyte/LiFePO₄ battery exhibited satisfactory electrochemical properties including high discharge capacity of 149 mAh/g at 0.2 C rate and good discharge performance at different current densities. The promising results reported here clearly indicated that PVDF/PAN/SiO₂ polymer electrolyte membranes prepared by the combination of phase inversion and chemical reaction method were promising enough to be applied in power lithium ion batteries.     

Stabilizing Solid-state Lithium Metal Batteries through In Situ Generated Janus-heterarchical LiF-rich SEI in Ionic Liquid Confined 3D MOF/Polymer Membranes

Pursuing high power density lithium metal battery with high safety is essential for developing next-generation energy-storage devices, but uncontrollable electrolyte degradation and the consequence formed unstable solid-electrolyte interface (SEI) make the task really challenging. Herein, an ionic liquid (IL) confined MOF/Polymer 3D-porous membrane was constructed for boosting in situ electrochemical transformations of Janus-heterarchical LiF/Li₃N-rich SEI films on the nanofibers. Such a 3D-Janus SEI-incorporated into the separator offers fast Li⁺ transport routes, showing superior room-temperature ionic conductivity of 8.17×10⁻⁴ S cm⁻¹ and Li⁺ transfer number of 0.82. The cryo-TEM was employed to visually monitor the in situ formed LiF and Li₃N nanocrystals in SEI and the deposition of Li dendrites, which is greatly benefit to the theoretical simulation and kinetic analysis of the structural evolution during the battery charge and discharge process. In particular, this membrane with high thermal stability and mechanical strength used in solid-state Li||LiFePO₄ and Li||NCM-811 full cells and even in pouch cells showed enhanced rate-performance and ultra-long life spans.     

Ultraviolet-Cured Semi-Interpenetrating Network Polymer Electrolytes for High-Performance Quasi-Solid-State Lithium Metal Batteries

Solid polymer electrolytes with relatively low ionic conductivity at room temperature and poor mechanical strength greatly restrict their practical applications. Herein, we design semi-interpenetrating network polymer (SNP) electrolyte composed of an ultraviolet-crosslinked polymer network (ethoxylated trimethylolpropane triacrylate), linear polymer chains (polyvinylidene fluoride-co-hexafluoropropylene) and lithium salt solution to satisfy the demand of high ionic conductivity, good mechanical flexibility, and electrochemical stability for lithium metal batteries. The semi-interpenetrating network has a pivotal effect in improving chain relaxation, facilitating the local segmental motion of polymer chains and reducing the polymer crystallinity. Thanks to these advantages, the SNP electrolyte shows a high ionic conductivity (1.12 mS cm⁻¹ at 30 °C), wide electrochemical stability window (4.6 V vs. Li⁺/Li), good bendability and shape versatility. The promoted ion transport combined with suppressed impedance growth during cycling contribute to good cell performance. The assembled quasi-solid-state lithium metal batteries (LiFePO₄/SNP/Li) exhibit good cycling stability and rate capability at room temperature.     

An enhanced poly(vinylidene fluoride) matrix separator with high density polyethylene for good performance lithium ion batteries

A sponge-like poly(vinylidene fluoride)/high density polyethylene (PVDF/HDPE) separator exhibiting high ionic conductivity and transference number of Li⁺ ion for lithium ion battery has been prepared by non-solvent induced phase separation (NIPS) method. HDPE fillers with size smaller than 250 nm are prepared with moderated reverse phase emulsion. The ion conductivity of PVDF/HDPE separator saturated with 1.0 M LiPF₆–ethylene carbonate (EC)/dimethyl carbonate (DMC)/ethyl methyl carbonate (EMC) (1:1:1, v/v/v) can be up to 2.54 mS cm^?1 at 25 °C, which is higher than that of pristine PVDF separator (1.85 mS cm^?1). The transference number of lithium ion with PVDF/HDPE separator is 0.495, better than that with commercial PP separator (0.33) and pristine PVDF separator (0.27). What is more, LiCoO₂/Li cells assembled with PVDF/HDPE separator show good C-rate and cycling performance which indicates great potential in serving as a good candidate of polymer separator for lithium ion batteries application.     

Lithium garnet based free-standing solid polymer composite membrane for rechargeable lithium battery

Electrolytes with high lithium-ion conductivity, better mechanical strength and large electrochemical window are essential for the realization of high-energy density lithium batteries. Polymer electrolytes are gaining interest due to their inherent flexibility and nonflammability over conventional liquid electrolytes. In this work, lithium garnet composite polymer electrolyte membrane (GCPEM) consisting of large molecular weight (W_avg ~?5?×?10⁶) polyethylene oxide (PEO) complexed with lithium perchlorate (LiClO₄) and lithium garnet oxide Li_6.28Al_0.24La₃Zr₂O₁₂ (Al-LLZO) is prepared by solution-casting method. Significant improvement in Li⁺ conductivity for Al-LLZO containing GCPEM is observed compared with the Al-LLZO free polymer membrane. Maximized room temperature (30 °C) Li⁺ conductivity of 4.40?×?10^?4 S cm^?1 and wide electrochemical window (4.5 V) is observed for PEO₈/LiClO₄?+?20 wt% Al-LLZO (GCPEM-20) membrane. The fabricated cell with LiCoO₂ as cathode, metallic lithium as anode and GCPEM-20 as electrolyte membrane delivers an initial charge/discharge capacity of 146 mAh g^?1/142 mAh g^?1 at 25 °C with 0.06 C-rate.     

Supramolecular Polymer Ion Conductor with Weakened Li Ion Solvation Enables Room Temperature All-Solid-State Lithium Metal Batteries

Improved durability, enhanced interfacial stability, and room temperature applicability are desirable properties for all-solid-state lithium metal batteries (ASSLMBs), yet these desired properties are rarely achieved simultaneously. Here, in this work, it is noticed that the huge resistance at Li metal/electrolyte interface dominantly impeded the normal cycling of ASSLMBs especially at around room temperature (<30 °C). Accordingly, a supramolecular polymer ion conductor (SPC) with “weak solvation” of Li⁺ was prepared. Benefiting from the halogen-bonding interaction between the electron-deficient iodine atom (on 1,4-diiodotetrafluorobenzene) and electron-rich oxygen atoms (on ethylene oxide), the O-Li⁺ coordination was significantly weakened. Therefore, the SPC achieves rapid Li⁺ transport with high Li⁺ transference number, and importantly, derives a unique Li₂O-rich SEI with low interfacial resistance on lithium metal surface, therefore enabling stable cycling of ASSLMBs even down to 10 °C. This work is a new exploration of halogen-bonding chemistry in solid polymer electrolyte and highlights the importance of “weak solvation” of Li⁺ in the solid-state electrolyte for room temperature ASSLMBs.     

Improving the properties of HDPE based separators for lithium ion batteries by blending block with copolymer PE-b-PEG

To improve the performances of HDPE-based separators, polyether chains were incorporated into HDPE membranes by blending with poly(ethylene-block-ethylene glycol) (PE-b-PEG) via thermally induced phase separation (TIPS) process. By measuring the composition, morphology, crystallinity, ion conductivity, etc, the influence of PE-b-PEG on structures and properties of the blend separator were investigated. It was found that the incorporated PEG chains yielded higher surface energy for HDPE separator and improved affinity to liquid electrolyte. Thus, the stability of liquid electrolyte trapped in separator was increased while the interfacial resistance between separator and electrode was reduced effectively. The ionic conductivity of liquid electrolyte soaked separator could reach 1.28 × 10-3 S.cm-1 at 25℃, and the electrochemical stability window was up to 4.5 V (versus Li + /Li). These results revealed that blending PE-b-PEG into porous HDPE membranes could efficiently improve the performances of PE separators for lithium batteries.     

Enhanced electrochemical properties of a novel polyvinyl formal membrane supporting gel polymer electrolyte by Al2O3 modification

Polyvinyl formal (PVFM)‐based dense polymer membranes with nano‐Al₂O₃ doping are prepared via phase inversion method. The membranes and also their performances as gel polymer electrolytes (GPEs) for lithium ion battery are studied by field emission scanning electron microscope, X‐ray diffraction, differential scanning calorimetry, mechanical strength test, electrolyte uptake test, electrochemical impedance spectroscopy, cyclic voltammetry, and charge–discharge test. The polymer membrane with 3 wt % nano‐Al₂O₃ doping shows the improved mechanical strength of 12.16 MPa and electrolyte uptake of 431.25% compared with 10.47 MPa and 310.59% of the undoped sample, respectively. The membrane absorbs and swells liquid electrolyte to form stable GPE with ionic conductivity of 4.92 × 10^?4 S cm^?1 at room temperature, which is higher than 1.77 × 10^?4 S cm^?1 of GPE from the undoped membrane. Moreover, the Al₂O₃‐modified membrane supporting GPE exhibits wide electrochemical stability window of 1.2–4.8 V (vs. Li/Li⁺) and good compatibility with LiFePO₄ electrode, which implies Al₂O₃‐modified PVFM‐based GPE to be a promising candidate for lithium ion batteries. © 2014 Wiley Periodicals, Inc. J. Polym. Sci. Part B: Polym. Phys. 2014 , 52, 572–577     

Excellent Stability of a Lithium‐Ion‐Conducting Solid Electrolyte upon Reversible Li+/H+ Exchange in Aqueous Solutions

Batteries with an aqueous catholyte and a Li metal anode have attracted interest owing to their exceptional energy density and high charge/discharge rate. The long‐term operation of such batteries requires that the solid electrolyte separator between the anode and aqueous solutions must be compatible with Li and stable over a wide pH range. Unfortunately, no such compound has yet been reported. In this study, an excellent stability in neutral and strongly basic solutions was observed when using the cubic Li₇La₃Zr₂O₁₂ garnet as a Li‐stable solid electrolyte. The material underwent a Li⁺/H⁺ exchange in aqueous solutions. Nevertheless, its structure remained unchanged even under a high exchange rate of 63.6 %. When treated with a 2 M LiOH solution, the Li⁺/H⁺ exchange was reversed without any structural change. These observations suggest that cubic Li₇La₃Zr₂O₁₂ is a promising candidate for the separator in aqueous lithium batteries.     

Kinetic and galvanostatic studies of a polymer electrolyte for lithium-ion batteries

Quaternary polymer electrolyte (PE) based on poly(acrylonitrile) (PAN), 1-ethyl-3-methylimidazolium tetrafluoroborate ionic liquid (EMImBF₄), sulfolane (TMS) and lithium hexafluorophosphate salt (LiPF₆) (PAN-EMImBF₄-sulfolane-LIPF₆) was prepared by the casting technique. Obtained PE films of ca. 0.2–0.3 mm in thickness showed good mechanical properties. They were examined using scanning electron microscopy (SEM), thermogravimetry (TGA, DSC), the flammability test, electrochemical impedance spectroscopy (EIS) and galvanostatic charging/discharging. SEM images revealed a structure consisting of a polymer network (PAN) and space probably occupied by the liquid phase (LiPF₆ + EMImBF₄ + sulfolane). The polymer electrolyte in contact with an outer flame source did not ignite; it rather underwent decomposition without the formation of flammable products. Room temperature specific conductivity was ca. 2.5 mS cm^?1. The activation energy of the conding process was ca. 9.0 kJ mol^?1. Compatibility of the polymer electrolyte with metallic lithium and graphite anodes was tested applying the galvanostatic method. Charge transfer resistance for the C₆Li → Li⁺ + e^? anode processes, estimated from EIS curve, was ca. 48 Ω. The graphite anode capacity stabilizes at ca. 350 mAh g^?1 after the 30th cycle (20 mA g^?1).     

A novel process to prepare porous membranes comprising SnO2 nanoparticles and P(MMA-AN) as polymer electrolyte

We have successfully developed a new process to prepare porous poly(methyl methacrylate-co-acrylonitrile) (P(MMA-AN)) copolymer based gel electrolyte. The porous structure in the polymer matrix is achieved by adding SnO₂ nanoparticles which are mostly used as gas sensor materials. The quasi-aromatic solvent, NMP, has an electron-repulsion effect with the space charge layer on the surface of SnO₂ nanoparticles and forms a special gas–liquid phase interface. Once the cast polymer solution is stored at an elevated temperature to evaporate the solvent, gas–liquid phase separation happens and spherical pores are obtained. The ionic conductivity at room temperature of the prepared gel polymer electrolyte based on the porous membrane is as high as 1.54 × 10⁻³ S cm⁻¹ with the electrochemical stability up to 5.10 V (vs. Li/Li⁺). This method presents another promising way to prepare porous polymer electrolyte for practical use.     

The search for a solid electrolyte,as a polysulfide barrier,for lithium/sulfur batteries

Composite Li₁₀SnP₂S₁₂ (LSPS)/polyethylene oxide (PEO) films, containing 25 to 50 % polymer, were electrophoretically deposited from acetone-based suspension and tested as possible candidates for polysulfide barriers in Li/S batteries. It was found by XRD and XPS tests that saturation of composite films by LiI salt, followed by prolonged annealing at 90 °C, diminishes the crystallinity of neat LSPS and results in the formation of a novel composite Li_10+xI_xSnP₂S₁₂ (LISPS)/P(EO)₃/LiI solid electrolyte (x < 1). The high room-temperature ion conductivity of amorphous sulfide Li_10+xI_xSnP₂S₁₂ (0.1–0.3 mS cm^?1) is restricted by slow ion transport via the polymer electrolyte (PE) imbedded in ceramics and grain boundaries between the PE and sulfide. Increase in polymer content and temperature improves total ion transport in the LISPS/PEO system. Conformal EPD coating of sulfur and lithium sulfide cathodes by the developed composite electrolyte increased the reversible capacity and Faradaic efficiency of the Li/S and Li/Li₂S cells and enabled their operation at 60 °C.     

A novel sandwiched membrane as polymer electrolyte for lithium ion battery

A novel kind of sandwiched polymer membrane was prepared by coating three layers of poly(vinyl difluoride) (PVDF), poly(methyl methacrylate) (PMMA) and PVDF, separately. Its characteristics were investigated by scanning electron microscopy, FT-IR, X-ray diffraction, and differential thermal analysis. It consists of two phases. The outer PVDF layers are porous, and the inner PMMA layer is solid. Since the PMMA has a good compatibility with the carbonate-based liquid electrolyte, the membrane can easily absorb the electrolyte to form a gelled polymer electrolyte (GPE). As a result, the evaporation peak of the liquid electrolyte is increased to 160 °C. Due to very low evaporation of the liquid electrolyte, LiCoO₂ shows good cycling behavior in the range of 4.4–3.0 V when this GPE is used as the separator and polymer electrolyte, and lithium as the counter and reference electrode. This unique sandwiched membrane is promising for application in scale-up lithium ion batteries with high safety and high energy density.     

Ordered and Fast Ion Transport of Quasi-solid-state Electrolyte with Regulated Coordination Strength for Lithium Metal Batteries

Polymer based quasi-solid-state electrolyte (QSE) has attracted great attention due to its assurance for high safety of rechargeable batteries including lithium metal batteries (LMB). However, it faces the issue of low ionic conductivity of electrolyte and solid-electrolyte-interface (SEI) layer between QSE and lithium anode. Herein, we firstly demonstrate that the ordered and fast transport of lithium ion (Li⁺) can be realized in QSE. Due to the higher coordination strength of Li⁺ on tertiary amine (−NR₃) group of polymer network than that on carbonyl (−C=O) group of ester solvent, Li⁺ can diffuse orderly and quickly on −NR₃ of polymer, significantly increasing the ionic conductivity of QSE to 3.69 mS cm⁻¹. Moreover, −NR₃ of polymer can induce in situ and uniform generation of Li₃N and LiN_xO_y in SEI. As a result, the Li||NCM811 batteries (50 μm Li foil) with this QSE show an excellent stability of 220 cycles at ≈1.5 mA cm⁻², 5 times to those with conventional QSE. LMBs with LiFePO₄ can stably run for ≈8300 h. This work demonstrates an attractive concept for improving ionic conductivity of QSE, and also provides an important step for developing advanced LMB with high cycle stability and safety.     

Enhanced ionic conductivity in novel nanocomposite gel polymer electrolyte based on intercalation of PMMA into layered LiV3O8

In the present work, a novel polymer electrolyte based on poly(methyl methacrylate) (PMMA)/layered lithium trivanadate (LiV₃O₈) nanocomposite has been investigated. X-ray diffraction (XRD) study shows that d-spacing is increased from 6.3?±?0.1 Å to 12.8?±?0.1 Å upon intercalation of the polymer into the layered LiV₃O₈. Room temperature ionic conductivity of the obtained nanocomposite gel polymer electrolyte is found to be superior to that of conventional PMMA-based gel polymer electrolyte. Enhancement in ionic conductivity of the nanocomposite gel electrolyte is attributed to the formation of a two-dimensional channel as a result of decreased interaction between Li⁺ and V₃O ₈ ^? layers as confirmed by FTIR. SEM results show aggregation of nanocomposite particles resulting from extension of some of the polymer chains from interlayer to the edge providing paths for Li⁺ ion transport. Interfacial stability of nanocomposite gel electrolyte is also found to be better than that of the conventional PMMA-based gel polymer electrolyte.     

Single Lithium‐Ion Conducting Polymer Electrolytes Based on a Super‐Delocalized Polyanion

A novel single lithium‐ion (Li‐ion) conducting polymer electrolyte is presented that is composed of the lithium salt of a polyanion, poly[(4‐styrenesulfonyl)(trifluoromethyl(S‐trifluoromethylsulfonylimino)sulfonyl)imide] (PSsTFSI^?), and high‐molecular‐weight poly(ethylene oxide) (PEO). The neat LiPSsTFSI ionomer displays a low glass‐transition temperature (44.3 °C; that is, strongly plasticizing effect). The complex of LiPSsTFSI/PEO exhibits a high Li‐ion transference number (t_Li⁺=0.91) and is thermally stable up to 300 °C. Meanwhile, it exhibits a Li‐ion conductivity as high as 1.35×10^?4 S cm^?1 at 90 °C, which is comparable to that for the classic ambipolar LiTFSI/PEO SPEs at the same temperature. These outstanding properties of the LiPSsTFSI/PEO blended polymer electrolyte would make it promising as solid polymer electrolytes for Li batteries.     

The network gel polymer electrolyte based on poly(acrylate-co-imide) and its transport properties in lithium ion batteries

Poly (acrylate-co-imide)-based gel polymer electrolytes are synthesized by in situ free radical polymerization. Infrared spectroscopy confirms the complete polymerization of gel polymer electrolytes. The ionic conductivity of gel polymer electrolytes are measured as a function of different repeating EO units of polyacrylates. An optimal ionic conductivity of the poly (PEGMEMA₁₁₀₀-BMI) gel polymer electrolyte is determined to be 4.8 × 10^–3 S/cm at 25 °C. The lithium transference number is found to be 0.29. The cyclic voltammogram shows that the wide electrochemical stability window of the gel polymer electrolyte varies from −0.5 to 4.20 V (vs. Li/Li⁺). Furthermore, we found the transport properties of novel gel polymer electrolytes are dependent on the EO design and are also related to the rate capability and the cycling ability of lithium polymer batteries. The relationship between polymer electrolyte design, lithium transport properties and battery performance are investigated in this research.     

Morphology of lithium droplets electrolytically deposited in LiCl–KCl–Li2O melt

We report the growth of electrochemically deposited liquid lithium droplet in LiCl–KCl–Li₂O melt at 673–723 K. To understand the transient behavior of liquid lithium in the electrolyte, the interface between the electrodeposited molten metal phase and the molten salt system was observed in situ using a high-speed digital microscope. We found that the droplets on the electrode are slightly flattened, when the colloidal Li content decreases due to an increasing Li₂O content. This mechanism indicates that the heterogeneous distribution of the colloid Li may be due to the local Li solubility in the electrolyte.