Stretchable, Compressible and Conductive Metal-Coated PDMS Sponges

Development of flexible, highly conductive electrodes or interconnects with excellent mechanical stability has been a challenging issue in device fabrication for wearable electronics, flexible displays, etc. Researchers at College of Chemistry and Environmental Engineering, Shenzhen University, P. R. China have developed 3D stretchable, compressible and electrically conductive conductors by surface modification of poly(dimethylsiloxane) (PDMS) sponges with poly[2-(methacryloyloxy)ethyl-trimethylammoniumchloride] (PMETAC) polymers followed by electroless deposition of metals.

The PDMS sponges were fabricated using sugar templating method. The sugar cubes were immersed in a mixture of Sylgard 184 and curing agent, degassed in a vacuum desiccator for 2 h followed by baking at 65 °C for 3 h, removal of sugar template by immersion in water at 60 °C for 24 h and drying at 100 °C for 2 h. The PDMS sponges were activated by air plasma treatment for 5 min, functionalized with vinyltrimethoxysilane (VTMS) via silanization followed by in situ free radical polymerization with METAC monomer and potassium persulfate as initiator, leading to the formation of PMETAC-modified PDMS sponges with 3D-interconnected porous structures. Electroless deposition of metals enables the formation of metal-coated PDMS sponges. The schematic of the fabrication process and images of electroless Cu-, Ag/Cu-, and Au/Cu-coated PDMS sponges are shown in Fig. 1.

Fig. 1 (a) Schematic illustration of the fabrication of metal-coated PDMS sponges; (b, c) Optical images of the PDMS sponge and Cu-, Ag/Cu-, and Au/Cu-coated PDMS sponge

The PDMS sponges consist of highly interconnected 3-D porous structures and the sponge-like structure is retained even after coating them with Cu, Ag/Cu and Au/Cu by electroless deposition. The coated metal particles exhibit a close-packed arrangement on the surface of PDMS sponges. The elastomeric property of PDMS sponges enable them to be stretched and compressed while metal coating makes them conductive, thus making them suitable for the fabrication of electrically conductive, stretchable and compressible electrodes. The metal-coated PDMS sponges exhibit remarkable mechanical stability and electrical conductivity, which is evidenced by the overlap of I–V characteristics of Ag/Cu-PDMS sponges while stretching or compressing them from 0% to 50% as well as by the continuous glowing of LED lamps at different extents of stretching, bending, and twisting (Fig. 2). In addition, the metal-coated PDMS sponges remain conducting even after cutting them to two pieces, which indicates that the metal coating is uniform not only on the outer side but also on the inner side of PDMS sponges.

Fig. 2 Flexible LED circuits made of Ag/Cu-PDMS sponge interconnects: (a, b) I–V characteristics of the LED circuit at different (a) tensile; and (b) compressive strains; (c-e) Optical images of the LED circuits with two LEDs at different extent of (c) stretching; (d) bending; and (e) twisting.

The ability of metal-coated PDMS sponges to offer no change in resistance at 40% tensile strain and only ≈20% increments at 50% strain after 5000 cycles of stretching suggests that they can be used for stretchable, compressible, and bendable interconnects or soft electronic contacts.

T.S.N. Sankara Narayanan

For more information, the reader may kindly refer: S.Q. Liang et al., 3D Stretchable, Compressible, and Highly Conductive Metal-Coated Polydimethylsiloxane Sponges, Adv. Mater. Technol. 2016, 1600117, DOI: 10.1002/admt.201600117

 

Flexible, High-Wetting and Fire-Resistant Separators based on Hydroxyapatite Nanowires for Lithium Ion Batteries

Lithium ion batteries (LIBs) have find widespread use as power sources for electric vehicles, grids, and other large-scale energy storage systems. Polyolefin membranes, the most commonly used separators in LIBs, though capable of offering good electrochemical stability, suitable mechanical strength and pore size, suffer from large shrinkages at high temperatures due to the low glass transformation temperature and melting point of the polymer moiety. The incompatibility between the polar organic solvents and nonpolar polyolefin membranes results in poor electrolyte wettability and thus low ionic conductivity. Researchers at Shanghai Institute of Ceramics, Chinese Academy of Sciences, China and School of Materials Science and Engineering, Huazhong University of Science and Technology, China have designed and fabricated a highly flexible and porous separator by self-assembling  hydroxyapatite nanowires (HAP NW) with cellulose fibers (CFs). The HAP/CF separator with a hierarchically cross-linked structure exhibits a good combination of high flexibility, robust mechanical strength, highly porous structure, superior electrolyte wettability, excellent thermal stability and fire resistance. Batteries fabricated using the HAP/CF separator exhibit enhanced cyclability and rate capability when bench marked against the commonly used polypropylene separator in LIBs.

HAP NWs were prepared by solvo-thermal method using calcium oleate as the precursor (Fig. 1(a)). The HAP NWs and CFs (Fig. 1(b)) were subjected to a self-assembly (hybridization) process (Fig. 1 (c)) in aqueous solution wherein the branched CFs were wrapped with network-structured HAP NWs through hydrogen bonding and van der Waals force. The self-assembled hierarchically cross-linked hybrid fibers were filtrated under vacuum suction (Fig. 1(d)) and dried. The HAP-CF separator was used to assemble the LiFePO₄/separator/Li half cells (Fig. 1(e)).

Fig. 1 Various stages involved in the fabrication of HAP/CF separator

The morphological features of HAP NW, CF and HAP/CF separator (Fig. 2) indicate that the CFs are uniformly embedded in the porous HAP NW networks, leading to the formation of HAP/CF separator with open, continuous, and interconnected nanopores. The porosity of the HAP/CF separator is ~81% with an average pore size of 120.9 nm and a narrow pore size distribution ranging from about 110 to 130 nm, suggesting its suitability as separators for LIBs. The HAP/CF separator can be rolled, twisted, folded, scrunched and unscrunched with no visible damages (Fig. 3), which indicate its good strength and high flexibility. The synergistic combination of the van der Waals force and hydrogen bonding enables the HAP/CF separator to achieve a higher tensile strength of 13.21 MPa. The HAP/CF separator exhibits good mechanical strength even at 200 °C, demonstrating its excellent thermal stability.

Fig. 2 Morphological features of (a) HAP NW networks; (b) CFs; and (c, d) HAP/CF separator

Fig. 3 Flexibility of the HAP/CF separator under different bending conditions: (a) rolled; (b) twisted; (c) folded; and (d) scrunched.

The rapid penetration of the electrolyte droplet within 5 s indicates the high wettability, which enables the HAP/CF separator with an electrolyte uptake of 253%. The ability of HAP/CF separator to maintain 77% of the initial weight even at a high temperature of 900 °C, suggests its excellent thermal stability. When ignited, the HAP/CF separator wetted with electrolyte gets self-extinguished due to the non-flammable nature and strong affinity of HAP NW for the electrolytes. In contrast, the commonly used PP separator wetted with electrolyte gets ignited and continuously combusted (Fig. 4).

Fig. 4 Fire-resistant characteristics of (a, b) PP separator; and (c, d) HAP/CF separator; (a, c) before burning; and (b, d) after burning.

The cycling performance and rate capability of the LiFePO₄/separator/Li half cells assembled using the HAP/CF separator and commercial PP separator (Fig. 5) indicate that the initial discharge capacity of the cell with HAP/CF separator (138 mAh/g) is higher than those obtained using the PP separator (130.1 mAh/g) at 0.5 C. Cells with the HAP/CF separator exhibit a higher discharge capacity of 135.4 mAh/g when compared to those obtained with the PP separator (129.5 mAh/g) after 145 cycles at 1 C. Cells with the HAP/CF separator exhibit a capacity retention of 85.7%, which is much higher than those exhibited by the cells with PP separators (65.7%).

Fig. 5 (a) Cycling performance; and (b) rate capability of theLiFePO₄/separator/ Li half cells using the HAP/CF and PP separators

The electrochemical performance of the batteries prepared using HAP/CF and commercial PP separators, for the initial 5 cycles at room temperature and subsequent 20 cycles at 150 °C (Fig. 6 (a)) indicates that the battery constructed with the HAP/CF separator exhibits a good cycling performance with a higher discharge capacity of 157.8 mAh/g and an average Coulombic efficiency of >98%. In contrast, the battery constructed with PP separator fails to offer a good performance at 150 °C (Fig. 6(a)). The large shrinkage of the PP separator at 150 °C causes internal short circuit, which is evidenced by the sudden drop in open-circuit voltage (OCV) while the battery with the HAP/CF separator could maintain its initial voltage throughout the whole testing process (Fig. 6(b)). The battery equipped with HAP/CF separator can safely light up two 3.0 V LED lamps at a temperature as high as 150 °C, suggesting the extraordinary thermal stability of the HAP/CF separator and the great potential for its application in high-temperature-related batteries (Figs. 6 (c) and 6(d)).

Fig. 6 (a) Cycling performance of the batteries constructed using HAP/CF and PP separators at 2 C for the initial 5 cycles at room temperature and subsequent 20 cycles at 150 °C; (b) OCV curves of the LiFePO4/separator/Li batteries with the HAP/CF separator and the PP separator at 150 °C; (c, d) battery prepared using the HAP/CF separator working at 150 °C.

The superior electrolyte wettability, mechanical robustness, high thermal stability, and fire resistance of HAP/CF separator appears to be promising for LIBs with enhanced performance and safety.

T.S.N. Sankara Narayanan

For more information, the reader may kindly refer: Heng Li et al., Adv. Mater. 2017, 1703548, DOI: 10.1002/adma.201703548