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Anode Materials for Rechargeable Batteries
Si-based anodes for Li-ion batteries
With the growing market for electric vehicles, lithium ion batteries (LIBs) have received a great deal of attention as suitable power sources owing to their high energy and power densities. Over the past several decades, the main challenge facing LIBs has been finding new materials which can deliver higher capacities than currently commercialized materials. Silicon (Si) has been regarded as a promising anode material for LIBs because of its highest theoretical capacity (∼3580 mA h g−1, Li15Si4) among alloy-type anode materials and relatively low discharge potential (∼0.4 V vs Li/Li+). Despite its attractive features, the practical use of Si is still limited by its poor cycle performance, which is associated with its severe volume changes during Li+ insertion and extraction. In an attempt to overcome these limitations of Si, much attention has been devoted to the nano engineering of Si.
1. SiOx-based anodes
1.1 Si/SiOx nanospheres for high-capacity lithium storage materials
Si/SiOx composite materials have been explored for their commercial possibility as high-performance anode materials for lithium ion batteries, but suffer from the complexity of and limited synthetic routes for their preparation. We have developed Si/SiOx nanospheres using a nontoxic and precious-metal-free preparation method based on sol−gel reaction. This approach based on a scalable sol−gel reaction enables not only the development of Si/SiOx with various nanostructured forms, but also reduced production cost for mass production of nanostructured Si/SiOx.
1.2 Mesoporous SiOx lithium storage materials
Mesoporous silicon-based materials gained considerable attention as high-capacity lithium-storage materials. However, the practical use is still limited by the complexity and limited number of available synthetic routes. We have developed carbon-coated porous SiOx as high capacity lithium storage material prepared by using a sol–gel reaction and templating. The anode exhibits a high capacity and outstanding cycling performance without significant dimensional changes. Carbon-coated porous SiOx also showed highly stable thermal reliability comparable to that of graphite. These promising properties come from the mesopores in the SiOx matrix, which ensures reliable operation of lithium storage in SiOx. The scalable sol–gel process can open up a new avenue for the versatile preparation of porous SiOx lithium storage materials.
1.3 Dual-size silicon nanocrystal-embedded SiOx nanocomposites as high-capacity lithium storage materials
SiOx-based materials attracted a great deal of attention as high-capacity Li+ storage materials for lithium-ion batteries due to their high reversible capacity and good cycle performance. However, these materials still suffer from low initial Coulombic efficiency as well as high production cost, which are associated with the complicated synthesis process. We propose a dual-size Si nanocrystal-embedded SiOx nanocomposite as a high-capacity Li+ storage material prepared via cost-effective sol–gel reaction with commercially available Si nanoparticles. In the proposed nanocomposite, dual-size Si nanocrystals are incorporated into the amorphous SiOx matrix, providing a high capacity with a notably improved initial efficiency and stable cycle performance. The highly robust electrochemical and mechanical properties of the dual-size Si nanocrystal-embedded SiOx nanocomposite are mainly attributed to its peculiar nanoarchitecture. This approach is one of the most promising routes for advancing SiOx-based Li+ storage materials for practical use.
1.4 High-performance Si/SiOx nanosphere anode materials by multipurpose interfacial engineering with black TiO2−x
The commercial use of these SiOx is still impeded by low initial Coulombic efficiency and high production cost associated with a complicated synthesis process. We have demonstrated that Si/SiOx nanosphere anode materials show much improved performance enabled by electro-conductive black TiO2−x coating in terms of reversible capacity, Coulombic efficiency, and thermal reliability. The introduction of a TiO2−x layer induces further reduction of the Si species in the SiOx matrix phase, thereby increasing the reversible capacity and initial Coulombic efficiency. Besides the improved electrochemical performance, the TiO2−x coating layer plays a key role in improving the thermal reliability of the Si/SiOx nanosphere anode material at the same time. This multipurpose interfacial engineering approach provides another route toward high-performance Si-based anode materials on a commercial scale.
1.5 Swelling-suppressed Si/SiOx nanosphere lithium storage materials
We have designed graphene-enveloped, Si nanocrystals-embedded SiOx nanospheres, in which abundant free spaces are introduced between the graphene envelope and the Si/SiOx nanospheres. The nanoscale free voids formed between the graphene and Si/SiOx nanospheres should ameliorate the technical issues regarding the volume expansion of Si-based anode materials during cycling. Furthermore, the proposed materials can be easily fabricated by a one-step, capillary force-driven aerosol process using graphene oxide particles and Si/SiOx nanospheres prepared by a scalable sol–gel reaction of triethoxysilane in an aqueous solution.
2. Si composite anodes
2.1 Si nanocrystallites embedded in hard alloy matrix as an anode material for Li-ion batteries
Si is an attractive anode material to improve the energy density of Li-ion batteries (LIBs) by replacing graphite anode materials owing to its ten times higher theoretical capacity than graphite and relatively low working potential. However, huge volume changes of Si during cycling and its poor capacity retention limits the wide usage in industry. To address this technical issue, Si-based active-inactive nanocomposite materials have been regarded as alternative material. Among them, Si-metal alloy materials which have hard inactive matrix and electron pathway by metal alloy is regarded as one of the promising Si-based active-inactive nanocomposites.
2.2 Carbon / nanoporous silicon hybrids as high capacity lithium storage materials
We have designed a melt-spun Si alloy nanocomposite. To successfully fabricate three dimensional nanoporous (3DNP) Si particles, we carefully designed the microstructure of the melt-spun Si/metal alloy nanocomposite, in which interconnected nanoscale Si phase is well dispersed in the metal alloy phase. Interestingly, we found that metal impurities remained in the 3DNP Si particles even after chemical etching, but these impurities can be used as a catalyst for growing carbon nanofibers (CNFs) on the surface of 3DNP Si particles without the additional introduction of metal catalyst. As a result, CNFs with a length of a few micrometers were easily grown on the 3DNP Si particles by simple carbon coating using a pitch as a carbon precursor. Considering that CNFs are highly effective in improving the cycle performance of Si-based anode materials.
2.3 Porous silicon–carbon composite materials for high-capacity lithium storage anodes
We have developed a cost-effective simultaneous chemical etching route for porous Si–C composite in which abundant pores as well as micrometer scale Si particles are incorporated into carbon matrix. This mechanically robust design of porous Si–C composites provided highly reversible Li storage over prolonged cycles and good dimensional stability upon alloying and dealloying reaction of Si phase with Li. The highly enhanced battery performances of the proposed Si–C composite is mainly attributed to the pores in the composite, which act as a dynamic buffer phase to accommodate the volume change of the Si phase without significant degradation. This material concept and scalable simultaneous alkaline etching approach provide a means of improving the electrochemical properties of Si-based anode materials for use in commercial LIBs.
Inorganic electrolyte-based Rechargeable battery
1. Nanotechnology Enabled Rechargeable LiAlCl4·3SO2 Batteries
We revisited the rechargeable LiAlCl4·3SO2 battery system using various recently developed nanostructured carbonaceous materials (KB-600JD, MSP-20, OMC, rGO, and CS). The nanotechnology shows the full potential of LiAlCl4·3SO2 rechargeable batteries based on non-flammable SO2-based inorganic electrolytes, which exhibited a reversible capacity of 1000 mA h g-1 with a working potential of 3 V and maintained the initial capacity up to 150 cycles without significant capacity fading.
The electrochemical performance of the LiAlCl4·3SO2 cell could be greatly improved, which makes it a promising candidate as a post-LIB system. Discharge capacity and cycle performance of LiAlCl4·3SO2 cells are highly dependent on the microstructural properties of the carbon materials, as well as on the electrode structure; hierarchical meso and macroporous structures in the carbon material and electrode are desirable to further enhance the capacity and cycle performance of LiAlCl4·3SO2 batteries.
2. Electrochemical Properties of Sodium Metal Interphase in NaAlCl4∙2SO2 Inorganic Electrolyte for Promising Room-Temperature Sodium Rechargeable Batteries
The NaAlCl4·2SO2 inorganic electrolyte exhibits various promising performances, including high ionic conductivity (up to 0.1 S cm−1), non-flammability, and excellent electrochemical reversibility. Another distinctive feature of NaAlCl4·2SO2 is the high concentration of Na+ cations in the electrolyte. Highly-concentrated NaAlCl4·2SO2 inorganic ionic liquid enables stable solid electrolyte interphase formation on the Na metal as well as dendrite-free polygonal Na electrodeposition, which leads to far superior electrochemical performance to that in conventional organic electrolytes. Our research direction is development of a route for future application of Na metal anode and creates an opportunity to explore a highly stable electrolyte for alkali metal (Na, Li, and K, etc.) anode-based rechargeable batteries.
3. Exploring new electrode materials that are compatible with the SO2-based inorganic electrolytes
The main activity in this research is exploration of various nanostructured electrode materials with high energy density and power density for new battery system employing SO2-based inorganic electrolyte. SO2-based inorganic electrolyte system which has non-flammability, wide temperature range and high ionic conductivity can rule out the concerns on the safety issues. In our previous research, we found that CuCl2 cathode shows high reversible capacity of ~200 mA h g−1 with the working potential of 3.4 V (vs. Na/Na +), and superior cycle performance over 1,000 cycles without significant capacity fading which could not be found in the conventional organic electrolyte. This rechargeable battery delivering all of high energy/power density, guaranteed safety, long cycle-life. Other transition metal halides and transition metal oxides are also possible candidates for this system. Remarkably, this system will not only use cation storage mechanism, but also can store anion for energy carrier in the SO2-based inorganic electrolyte. Understanding this unusual reaction chemistry would offer a wide choice of next-generation energy storage system.
New cation storage site in the nanostructured materials
For Lithium-ion battery
The main goal of our research is designing the new electrode materials with various nanostructures for next generation rechargeable batteries exhibiting high capacities and stabilities. It is well known that the cations, such as Li, Na and Mg, can be stored through several mechanisms: (1) intercalation into host material, (2) insertion mechanism, (3) conversion reaction, and (4) alloy formation with other metal. We focus not only these mechanisms, but also searching new sites which are not reported ever. Due to the current commercialized Li-ion batteries already reached its technological limit, we need to develop new anode materials such as transition metal oxides to achieve high energy density. It is well known that nanostructured materials provide unusual physical and chemical properties, compared to bulk material, because of their large surface area to volume ratio, short distance for mass transport. Nano engineering will be another approach to discover multiple cation storage materials. We will investigate a new cation-storage mechanism that are different from the currently known mechanisms using a transition metal compounds.
The various transition metal oxide nanoparticles have extra capacity beyond theoretical one based on the unclear Li storage conversion mechanisms. Understanding the origin of the new sites of the Li-storage properties and its electrochemical reaction mechanism would be helpful for next generation rechargeable batteries.
For sodium-ion battery
With increase concerns due to global warming and fossil fuel depletion, the demand for efficient energy storage technologies has continued to increase in order to address the technical issues associated with electric vehicles and renewable energies. Lithium ion batteries (LIBs) have maintained a dominant position in the energy storage device market, however, there is a critical issue, being the limited and highly localized resource supply of lithium in the Earth. Accordingly, a lithium shortage may lead to another social problem like the current oil crisis. With this regard, extensive research efforts have been devoted toward alternative power devices to replace commercial LIBs. Sodium ion batteries (SIBs) are regarded as a potential alternative due to the abundance of sodium on Earth in addition to its similarity with lithium on the atomic scale.
Among the various available cathode materials for SIBs, Prussian Blue (PB) and its analogues (PBAs) possess three-dimensional open framework structures and feature a high rate of mass transfer toward alkali cations.
However, the SIB technology is still too premature to replace current LIBs due to the low energy density and reliability of current SIBs. In order to resolve these technical issues, intensive research efforts have been made toward the development of electrode materials that are capable of storing a large amount of Na+ in their structures.
