Synthesis of Nanoporous Materials using Block Copolymer/Inorganic Nanomaterials Hybrids and their Applications to Energy Storage

Abstract

While environmental pollution and depletion of fossil resources have been growing serious, energy storage system such as lithium ion batteries (LIBs) and supercapacitors have been especially considered as a power source for next generation of automobile as well as portable electronic devices. Recently, a lot of efforts have been made to satisfy high level performance, providing high energy density, high power density, long cycle lifetime, low cost, safety, light weight and high packing density, etc. Especially, in order to utilize plug-in hybrid electric vehicles (PHEVs) and electric vehicles (EVs), lithium ion battery with spinel Li4Ti5O12 as an alternative anode material have been considered as a promising candidate due to several important advantages in spite of lower theoretical capacity (Li4Ti5O12 + 3Li+ + 3e-  Li7Ti5O12, 175 mA h g-1) than already commercialized graphite (372 mA h g-1). First, Ti is an abundant element allowing it to be a cost-effective material. Furthermore, LTO exhibits a Li+ insertion/extraction potential of ~1.55 V (vs. Li/Li+). This enables LTO electrodes not to suffer from many critical problems caused by undesirable electrolyte decomposition that occurs at reductive potentials under ~1 V (vs. Li/Li+). The side reactions not only lead to the formation of a solid electrolyte interphase (SEI) film and gas evolution but also result in low coulombic efficiency especially in the first formation cycle due to a significant loss of Li+. LTO is also known to show exceptional durability because of negligible volume expansion/contraction, < 1%, while graphite experiences ~13% volume change during full charge-discharge. However, the major drawback of LTO is that it is electrically insulating (<10-13 S cm-1). Several approaches to improve the electronic conductivity include carbon coating and metal doping. Another approach to overcome low electrical conductivity is to employ nanostructures, as the reduced dimension significantly shortens the electronic path. In addition, the nanostructures also enable facile transport of the electrolyte to the surfaces of electrochemically active materials, resulting in rapid charge transfer reactions due to the high electrode-electrolyte interface area, and short Li+ diffusion paths. Thus there have been many research efforts to develop nano-sized materials for high rate capability electrodes. However, the nano-sized materials suffer from low packing density, resulting in low volumetric density. Furthermore, it is also possible that the nanoparticles may be released from the electrode surface, cross the separator and cause an internal short circuit. To overcome these problems associated with the nano-sized materials, it is desirable to create a conductive and porous matrix for micrometer-sized particles that provides good particle-particle contact. Namely, mesostructured materials with large open pores, which were first synthesized through a self-assembly method for nanocatalysis, are ideal candidates. Here, we simply synthesized a mesostructured spinel Li4Ti5O12(LTO)-carbon nanocomposite with large ( > 15 nm) and uniform pores via block copolymer self-assembly. Exceptionally high rate capability is then demonstrated for Li-ion battery (LIB) negative electrodes. Polyisoprene- block - poly(ethylene oxide) (PI- b -PEO) with a sp2 -hybridized carbon-containing hydrophobic block is employed as a structure-directing agent. Then the assembled composite material is crystallized at 700 °C enabling conversion to the spinel LTO structure without loss of structural integrity. Part of the PI is converted to a conductive carbon that coats the pores of the Meso-LTO-C. The in situ pyrolyzed carbon not only maintains the porous mesostructure as the LTO is crystallized, but also improves the electronic conductivity. A Meso-LTO-C/Li cell then cycles stably at 10 C-rate, corresponding to only 6 min for complete charge and discharge, with a reversible capacity of 115 mAhg−1 with 90% capacity retention after 500 cycles. In sharp contrast, a Bulk-LTO/Li cell exhibits only 69 mAhg−1 at 10 C-rate. The carbon-coated mesoporous structure enables highly improved electronic conductivity and significantly reduced charge transfer resistance, and a much smaller overall resistance is observed compared to Bulk-LTO. Additionally, to achieve further improvement in rate capability of LTO, a mesostructured Nb doped LTO-carbon composite was successfully synthesized through a simple route of block copolymer assembly with inorganic precursors. During heat-treatment at 700 ˚C, the amorphous metal oxide was crystallized to form the spinel LTO structure and part of the polystyrene in block copolymer was converted to an electrical conductive carbon matrix that not only maintained the pores structure but also provided electrical conductivity to the insulating LTO framework. Pore size was calculated to be 15 nm, large enough for facile diffusion of electrolyte to the LTO framework. The excellent electrochemical performance of 1wt% Nb-LTO, with a capacity of 130 mA h g-1 at 10 C and a capacity of ~ 172 mA h g-1 at 0.5 C, is attributed to the outstanding electrical conductivity of the mesoporous nanoarchitecture combined with generation of Ti3+/Ti4+ mixture as charge compensation by doping Nb5+. This self-assembly method employing block copolymers with a sp2-hybridized carbon and sol-gel process of inorganic materials can simply provide effective ways to improve the electrical conductivity of spinel Li4Ti5O12, which is relevant to fabrication of nano-architectures of Li4Ti5O12, introduction of electrical conductive phase (carbon) and doping with aliovalent metal ion into lattices of Li4Ti5O12.On the other hands, recently, it has been shown that thin-film microbatteries (TFBs) and thin-film microsupercapacitors (TFSCs) are needed to power microelectromechanical system (MEMS)-based sensors and actuators or implanted medical devises (IMDs). To achieve a high energy and power unit on a small foot print area (typically < 1 cm2) in TFBs and TFSCs, three-dimensional nanostructures on thin-film has been suggested. The high-rate capability can also be achieved in 3-D nanostructured electrodes due to facile ionic motions inside nanostructures. We have demonstrated that ordered mesoporous carbon nanofiber array (OMCNA) was fabricated by confined self-assembly of PS-b-PEO and resol inside anodic aluminum oxide (AAO), followed by carbonization, gold sputtering and removal of AAO. The well-aligned structure of mesoporous carbon nanofibers on current collectors enhanced charge transfer and electron transfer rates and increased the number of usable charge storage sites, leading to development of microsupercapacitor electrode with high energy and power density. The new electrode configuration described in this work can be a good candidate for future microsupercapacitor electrodes powering MEMS based devices.