Yu-Sheng Wang

Review of gel-type polymer electrolytes for lithium-ion batteries

This review describes the advantages and characteristics of employing polymer electrolytes in solid-state lithium-ion batteries. Criteria for an ideal polymer electrolyte and the differences between polyelectrolytes are discussed. The emphasis of this article is on plasticized or gelled electrolyte systems. Hence, the review focuses on four plasticized systems which have received particular attention from a practical viewpoint, i.e., poly(ethylene oxide) (PEO)-, poly(acrylonitrile) (PAN)-, poly(methyl methacrylate) (PMMA)-, and poly(vinylidene fluoride) (PVdF)-based electrolytes. Some critical concepts and points associated with this emerging technology that still require attention are discussed in the final part of the review.

Design and tailoring of a hierarchical graphene-carbon nanotube architecture for supercapacitors

Stacking of individual graphene sheets (GS) is effectively inhibited by introducing one-dimensional carbon nanotubes (CNTs) to form a 3-D hierarchical structure which significantly enhances the electrochemical capacitive performances of GS-based composites. From SEM images, inserting proper quantity of CNTs as nanospacers can effectively impede the stacking of GS and enlarge the space between GS sheets, leading to obtain a highly porous nanostructure. The specific capacitance of GS-CNTs-9-1 (∼326.5 F g−1 at 20 mV s−1) is much higher than that of GS material (∼83 F g−1). Furthermore, the energy and power densities of GS-CNTs-9-1 are respectively as high as 21.74 Wh kg−1 and 78.29 kW kg−1, revealing that the hierarchical graphene-CNT architecture provides remarkable effects on enhancing the capacitive performance of GS-based composites. Therefore, the GS-CNT composites are promising carbon materials for supercapacitors.

Two- and three-electrode impedance spectroscopy of lithium-ion batteries

Abstract The interfacial impedance between PVDF/HFP-based electrolytes and lithium metal continues to increase and attains a delicate kinetic equilibrium upon prolonged storage. The graphite anode, on the other hand, is found to remain inert towards the electrolytes. Its interfacial impedance does not vary with increasing storage time or in the presence of different lithium salts. In addition, it is found that the impedance of a Li–C half-cell consists of the impedances of two interfaces and is therefore often mistakenly used in the interpretation of the behaviour of a single carbon electrode. Thus, a three-electrode impedance study is required. It is found that an inductive loop appears in the low-frequency region of the impedance spectrum of a carbon electrode immediately after the first lithium-intercalation step, which probably implies that an adsorption–desorption phenomenon might exist at the interface. Moreover, another inductive effect, which arises from the connecting leads, also appears in the high-frequency region. Finally, the cathode is found to be the major source of cell impedance and increases with increasing cycle number.

Conductivity Study of Porous Plasticized Polymer Electrolytes Based on Poly(vinylidene fluoride) A Comparison with Polypropylene Separators

The novel porous plasticized poly(vinylidene fluoride‐co‐hexafluoropropylene) (PVDF/HFP)‐based electrolytes, compared with conventional polypropylene (PP) separators (Celgard® 2400), were studied via electrochemical impedance spectroscopy, gas adsorption/desorption method, differential scanning calorimetry, and a simple wettability test. The obtained impedance spectra of the electrolytes and separators are extensively discussed, inclusive of the effect of poor wetting upon them. The average pore diameter and Brunauer‐Emmett‐Teller surface area of PVDF/HFP‐based electrolyte membranes are 16.4 nm and , respectively. Ionic conductivity and electrolyte retention characteristic of these electrolyte membranes are superior to conventional PP separators. Moreover, PVDF/HFP‐based electrolyte membranes are free from the problem of wetting whereas the poor wetting of PP separators in some electrolytes may cause its effective conductivity to decrease by at least one order of magnitude. The enhanced wettability may be achieved by virtue of the swelling phenomenon between the polymer and the electrolytes. However, the activation energy for the conduction of PVDF/HFP‐based electrolytes is still larger than that of their parent neat electrolytes , which may imply that the influence of PVDF/HFP upon ionic mobility still exists even if they have been made nanoporous.

Composition analysis of the passive film on the carbon electrode of a lithium-ion battery with an EC-based electrolyte

Abstract This work examines the formation of a passive film on the carbon electrode of lithium-ion batteries. With a single solvent of EC (ethylene carbonate), the structure of the passive film is found to be (CH2OCOOLi)2. In a DEC (diethyl carbonate) or DMC (dimethyl carbonate) system, C2H5OCOOLi and Li2CO3 are formed on the surface of the carbon electrode. According to results from mass spectra, CO2 gas is the main product when EC is decomposed. By contrast, DEC is decomposed into CO and C2H6, and DMC into CO and CH4. These findings suggest that the composition of the passive film depends on the chosen solvent. In a binary solvent system which contains EC, the passive film contains chiefly (CH2OCOOLi)2, which is identical to a single EC solvent system.

Lightweight and flexible reduced graphene oxide/water-borne polyurethane composites with high electrical conductivity and excellent electromagnetic interference shielding performance.

In this study, we developed a simple and powerful method to fabricate flexible and lightweight graphene-based composites that provide high electromagnetic interference (EMI) shielding performance. Electrospun waterborne polyurethane (WPU) that featured sulfonate functional groups was used as the polymer matrix, which was light and flexible. First, graphene oxide (GO)/WPU composites were prepared through layer-by-layer (L-b-L) assembly of two oppositely charged suspensions of GO, the cationic surfactant (didodecyldimethylammonium bromide, DDAB)-adsorbed GO and intrinsic negatively charged GO, depositing on the negatively charged WPU fibers. After the L-b-L assembly cycles, the GO bilayers wrapped the WPU fiber matrix completely and revealed fine connections guided by the electrospun WPU fibers. Then, we used hydroiodic acid (HI) to obtain highly reduced GO (r-GO)/WPU composites, which exhibited substantially enhanced electrical conductivity (approximately 16.8 S/m) and, moreover, showed a high EMI-shielding effectiveness (approximately 34 dB) over the frequency range from 8.2 to 12.4 GHz.

Effect of Molecular Chain Length on the Mechanical and Thermal Properties of Amine-Functionalized Graphene Oxide/Polyimide Composite Films Prepared by In Situ Polymerization

This study fabricates amine (NH(2))-functionalized graphene oxide (GO)/polyimide(PI) composite films with high performance using in situ polymerization. Linear poly(oxyalkylene)amines with two different molecular weights 400 and 2000 (D400 and D2000) have been grafted onto the GO surfaces, forming two types of NH(2)-functionalized GO (D400-GO/D2000-GO). NH(2)-functionalized GO, especially D400-GO, demonstrated better reinforcing efficiency in mechanical and thermal properties. The observed property enhancement are due to large aspect ratio of GO sheets, the uniform dispersion of the GO within the PI matrix, and strong interfacial adhesion due to the chemical bonding between GO and the polymeric matrix. The Youngs modulus of the composite films with 0.3 wt % D400-GO loading is 7.4 times greater than that of neat PI, and tensile strength is 240% higher than that of neat PI. Compared to neat PI, 0.3 wt % D400-GO/PI film exhibits approximately 23.96 °C increase in glass transition temperature (T(g)). The coefficient of thermal expansion below T(g) is significantly decreased from 102.6 μm/°C (neat PI) to 53.81 μm/°C (decreasing 48%) for the D400-GO/PI composites with low D400-GO content (0.1 wt %). This work not only provides a method to develop the GO-based polyimide composites with superior performances but also conceptually provides a chance to modulate the interfacial interaction between GO and the polymer through designing the chain length of grafting molecules on NH(2)-functionalized GO.

Effect of Covalent Modification of Graphene Nanosheets on the Electrical Property and Electromagnetic Interference Shielding Performance of a Water-Borne Polyurethane Composite

Flexible and lightweight graphene nanosheet (GN)/waterborne polyurethane (WPU) composites which exhibit high electrical conductivity and electromagnetic shielding performance were prepared. Covalently modifying GNs with aminoethyl methacrylate (AEMA; AEMA-GNs) through free radical polymerization effectively inhibited the restacking and aggregation of the GNs because of the -NH3(+) functional groups grafted on the AEMA-GNs. Moreover, the AEMA-GNs exhibited high compatibility with a WPU matrix with grafted sulfonated functional groups because of the electrostatic attraction, which caused the AEMA-GNs to homogeneously disperse in the WPU matrix. This homogeneous distribution enabled the GNs to form electrically conductive networks. Furthermore, AEMA-GNs with different amounts of AEMA segments were introduced into the WPU matrix, and the effects of the surface chemistry of the GNs on the electrical conductivity and EMI shielding performance of composites were investigated. AEMA-GN/WPU composites with a GN loading of 5 vol % exhibited remarkable electrical conductivity (approximately 43.64 S/m) and EMI shielding effectiveness (38 dB) over the frequency of 8.2 to 12.4 GHz.

Effects of Multiwalled Carbon Nanotubes Functionalization on the Morphology and Mechanical and Thermal Properties of Carbon Fiber/Vinyl Ester Composites

Multiwalled carbon nanotube (MWCNT)/carbon fiber (CF)/vinyl ester (VE) laminate composites have been fabricated in this study. Pristine MWCNTs were treated with acid solution, which formed numerous oxygen-containing functional groups onto their surface, resulting in COOH-MWCNTs. Thereafter, acrylic functional groups were grafted onto the COOH-MWCNTs to generate acryl-MWCNTs. Three types of MWCNTs (pristine MWCNTs, COOH-MWCNTs, and acryl-MWCNTs) were used to reinforce the CF/VE-based composites. The dispersion of MWCNTs in the VE matrix and the interfacial interaction between MWCNTs and the VE matrix were investigated. Thereafter, the individual reinforcement efficiencies of these MWCNTs are compared. The flexural strength of the MWCNT/CF/VE composite with 1.0 phr acryl-MWCNTs content is 29.8% greater than that of neat CF/VE composites, and the flexural modulus of the MWCNT/CF/VE composite is 9.9% higher than that of neat CF/VE composites. Compared with neat CF/VE composites, 1.0 phr acryl-MWCNT/CF/VE composites exhibit an approximately 19.9 °C increase in glass transition temperature (Tg). The coefficients of thermal expansion significantly decreased from 47.2 ppm/°C of the neat CF/VE composites to 35.6 ppm/°C of the acryl-MWCNTs/CF/VE composites with 1 phr acryl-MWCNT content. This study provides a method for developing acryl-MWCNT/CF/VE composites with good dispersion of MWCNTs in VE matrix and strong interfacial interaction between the MWCNTs and VE matrix for enhancing the stress transfer from VE matrix to CF reinforcement.

Effect of Octa(aminophenyl) Polyhedral Oligomeric Silsesquioxane Functionalized Graphene Oxide on the Mechanical and Dielectric Properties of Polyimide Composites

An effective method is proposed to prepare octa(aminophenyl) silsesquioxane (OAPS) functionalized graphene oxide (GO) reinforced polyimide (PI) composites with a low dielectric constant and ultrastrong mechanical properties. The amine-functionalized surface of OAPS-GO is a versatile starting platform for in situ polymerization, which promotes the uniform dispersion of OAPS-GO in the PI matrix. Compared with GO/PI composites, the strong interfacial interaction between OAPS-GO and the PI matrix through covalent bonds facilitates a load transfer from the PI matrix to the OAPS-GO. The OAPS-GO/PI composite film with 3.0 wt % OAPS-GO exhibited an 11.2-fold increase in tensile strength, and a 10.4-fold enhancement in tensile modulus compared with neat PI. The dielectric constant (D(k)) decreased with the increasing content of 2D porous OAPS-GO, and a D(k) value of 1.9 was achieved.