Asst. Prof. Montree Sawangphruk
Laboratory of Energy Storage Technology, Energy Science and Engineering, VISTEC
In 2015 the United Nations estimated that the world total number of humans are 7,349 million and keep increasing with the exponential growth. As the result, the huge amount of energy is needed, and the major source of the energy is from the combustion reaction process of fossil fuels releasing carbon dioxide (CO2) gas to the environment. It is reported this year that the concentration of CO2 in atmosphere is 0.04 % or 400 ppm and keep increasing at the rate of 2 ppm a year leading to the global warming issue. To address the global warming issue, green and renewable energy technologies such as solar cells are needed. To enable these technologies, high-efficiency, stable, non-toxic energy storages are needed. The energy storage technology itself is also classified to be one of the disruptive technologies in 2016 since it must be used many disruptive technologies requiring electrical power e.g., electric vehicles, IoTs, advanced robots. As the result, VISTEC focuses on many energy storage technologies e.g., (i) lithium sulfur batteries, (ii) advanced lithium and sodium ions batteries, (iii) supercapacitors and (IV) Li- and Na-capacitors.
Lithium-sulfur batteries (LSBs)
Lithium-sulfur batteries (LSBs) have been greatly developed to meet high-energy requirements in many applications such as electric vehicles (EVs) and hybrid-electric vehicles (HEVs). LSBs have high theoretical capacity (1672 mAh g-1) and energy density (2500 Wh kg-1 or 2800 Wh L-1). In addition, sulfur used as an active material in the cathode of LSBs is abundant, low cost, and environmentally friendly. However, LSBs are still facing many challenges limiting their practical use such as poor conductivity of sulfur and Li2S, soluble long-chain polysulfides (PSs) creating a “shuttle mechanism”, and volume expansion during cycling up to 80% resulting in a material pulverization.
To address these issues, a proper design of the host material for sulfur, ideally having high electrical conductivity and stable porosity, is highly needed. As a result, carbon-based materials have been widely used since they can improve the electron transfer of the sulfur cathode, trap the soluble Li2Sn intermediates and accommodate the volume expansion during charge/discharge process. Besides, other efforts have also been made to improve the stability of LSBs via a modified separator or an inserted layer so-called “interlayer” between sulfur cathode and polymer separator. It is capable for trapping the soluble PSs in the electrolyte via its functionalized surface property. In addition, it can be used to reduce the sulfur cathode resistance resulting in high utilization of sulfur active material, improving the capacity and providing longer cycle life of LSBs.
Advanced Lithium Ion Batteries
Li-ion batteries (LIBs) have been widely used in many applications e.g., mobile phones, laptops, electric vehicles (EVs), and hybrid-electric vehicles (HEVs) due to their high energy densities (~100-170 Wh kg-1) and long cycle life. The present materials used in the cathode of LIBs consist of three main components: (i) ca. 80 wt.% of active materials such as LiCoO2 (LCO), LiMn2O4 (LMO), LiNixCoyAl1-x-yO2 (NCA), LiNixMnyCo1-x-yO2 (NMC), LiFePO4 (LFP), and composite materials, (ii) ca. 10 wt.% conductive additive (i.e. spherical carbon black, Super P), and (iii) ca. 10 wt.% polymer binder (i.e. PVDF and PTFE). For our technology, we turned spherical carbon to graphene-like nanosheets and used as the conductive additive and further improving the coating process with the core-shell structure reducing the charge transfer resistance between active materials and conductive additives.
Due to the increasing demands in energy and power technology, e.g. electric vehicles (EVs) and plug-in/hybrid electric vehicles (PHEVs or HEVs), effective energy storage systems providing high energy density and power density must be developed to meet such requirements. Recently, the state-of-the-art energy storage devices integrating faradaic-based lithium ion battery (LIB) negative electrodes with non-faradaic-based supercapacitor positive electrodes in lithium salt containing electrolytes have been greatly developed so-called lithium ion capacitors (LICs). LICs operate based on the adsorption/desorption of anions on the surface of positive electrode materials and simultaneously lithiation/delithiation of cations in the structure of negative electrode materials.
Sodium-ion hybrid capacitors (NICs) have attracted a great deal of interest over the well-developed lithium-based technologies. One of the main challenges to make hybrid capacitors become practical energy solution is to reduce the cost of the overall device, especially the electrode materials that taking the major cost contribution. Therefore, carbon-based materials have been widely adopted as both positive and negative electrodes in NICs due to its low intercalation potential, low cost, tunable pore structure, natural abundance, small volume change, and good stability. Several biomass-derived materials have also been used as electrode materials for efficient sodium ion storage due to their green, available natural resource, renewable, and sustainable properties.