All-solid-state lithium ion batteries
Lithium ion batteries are flammable and so safety is a big issue. Research into their electrolyte, complete solidification, and other matters is actively ongoing. When all-solid-state lithium ion batteries are used, it is predicted that the battery capacity will expand and fast charge and discharge will be possible. With this in mind, we made a quasi-all-solid-state lithium ion battery by using as electrolyte a combination of an oxide garnet type solid electrolyte, Li7La3Zr2O12; an ionic liquid system electrolyte that is a non-flammable electrolyte; spinel high-voltage-type LiNi0.5Mn1.5O4 positive electrode material for the positive-electrode material; and Li5Ti4O12 for the negative-electrode material. All of these electrode and electrolyte materials were synthesized within our company. The present battery capacity is low, at about 40 mAh/g for the positive-electrode material (several 10 s of stable cycle characteristics confirmed), but it has benefits such as non-flammability and usability at high temperatures, which is a merit from the point of view of safety. In the future, we will be aiming for an all-solid-state lithium ion battery that does not use ionic liquid and at increasing the battery capacity further.
Electrode derived from a metal–organic framework (MOF) using a lithium ion battery
At Green Science Alliance, we are carrying out research and development, as well as manufacturing and sales, of a super-porous material called an MOF. We are also doing research and development of electrode material using a battery having this MOF as its material. MOF is a material composed of a metal cation and organic ligand bridging it, and its interesting properties are the shape of the pore space, its size, and its ability to change freely depending on the chemical environment. The structure can be controlled rigorously to nanometer units. Combinations of metal ion and organic ligand are numerous; more than tens of thousands of types of MOF have been reported.
We have already examined the electrochemical properties of lithium ion batteries based on electrode material made using MOF as its material. We confirmed that with a capacity of 500–600 mAh/g and after 50 cycles of charge and discharge, up to 85% or more of the battery capacity is maintained. Since MOF is synthesized within our company, we can try to make lithium ion battery with higher cell capacity and better cell cycle stability, based on different type of MOF which we can synthesize ourselves.
Silicon-based electrode material used by lithium ion batteries
Silicon monoxide is also being considered for use in negative electrode activator material for increasing the capacity of lithium ion batteries. In silicon-based material, when taking in and releasing lithium ions during charging or discharging, a large change in volume occurs and the battery capacity deteriorates, which is problematic. However, the theoretical capacity of silicon monoxide is about 2000 mAh/g and so it is a very attractive material. In our company, initially about 1800 mAh/g battery capacity has been achieved by making improvements to the silicon monoxide coating ink, and to the charging and discharging at conditions of 0.1 C,. Even after 10 cycles, the lowered battery capacity is kept within 10%. At present, the capacity under a larger current and the degradation characteristics are being assessed, and improvements are being continued.
Lithium–sulfur batteries are storage batteries having sulfur as the positive electrode active material and lithium metal for the negative electrode, with charge and discharge occurring by the redox reaction between sulfur and lithium. When electricity is discharged, the lithium oxidizes and dissolves at the negative electrode; at the positive electrode, sulfur is reduced stepwise, and through a reaction intermediate comprising multiple types of lithium polysulfide, it is reduced to lithium sulfide. At the time of charging, lithium ion is reduced to lithium metal and deposited at the negative electrode, and lithium sulfide is oxidized to sulfur at the positive electrode. For the electrolytes, as with lithium ion batteries, an electrolyte solution using organic solvent together with Ionic liquids is first being considered. High energy density is an advantage, with the theoretical capacity of sulfur positive electrodes at 1672 mAh/g, greatly exceeding the lithium ion battery oxide-based positive electrode at 100–250 mAh/g. Voltage is lower than that of the lithium ion battery, but the theoretical energy density reaches 2500 Wh/kg. At present, the capacity of a well-functioning lithium ion battery is generally in the range of 200 Wh/kg. In addition, the positive-electrode sulfur is in good supply and is cheaply obtainable; hence, there are advantages from the point of view of cost and supply. However, there is an issue in that during charging and discharging, the sulfur positive electrode decomposes, and so there is a large change in volume and a rapid decrease in cell capacity, which is problematic. In our company, the sulfur positive electrode initially gives a battery capacity of about 1200 mAh/g, but because of poor cycle characteristics, we will continue work on improvements, including improving battery capacity and cycle characteristics.
Lithium-excess-type positive electrode
As seen from the viewpoint of the battery’s function, positive-electrode active material for a lithium ion battery requires high potential, high capacity, excellent reversibility, as well as excellent charge and discharge speed, chemical and thermal stability, and high density. Low cost, reproducibility, as well as a synthetic method with good yield, and good handling, are required for practical mass production. Various oxide compounds have been considered to improve the battery function; in recent years, consideration has been given mainly to the layered rock salt structure, spinel-type structure, transition-metal oxides in olivine-type structures, and related compounds. These compounds have an average potential of 3–4 V and a theoretical capacity of 170–280 mAh/g, with measured values of 100–200 mAh/g. Moreover, the energy density of lithium ion batteries combined with carbon-based negative electrodes presently on the market is limited to a best figure of about 250 Wh/kg. Thus, in order to increase the energy storage amount, a material with higher potential and higher capacity is needed. In consideration of these issues, lithium-excess-type positive electrode material is recently being considered. For lithium-excess-type material, the molar ratio of lithium to the transition metal in the material is greater than 1. With a high theoretical capacity of 280–320 mAh/g, it is a promising positive electrode material for the future. In Green Science Alliance, we have been recently starting to succeed in synthesizing lithium-excess-type positive electrode material with good property. As we have shown it to initially have about 250 mAh/g capacity, we have expectations for it in the future.
Aluminum ion batteries and
At present, rechargeable batteries using lithium metal as resource for the electrode are mainstream, but as demand increases because of uneven distribution of resources and buried reserves of lithium, concerns arise about the long-term stable supply. Development of rechargeable batteries made with elements in place of lithium has been carried out in many countries. Aluminum ion batteries are rechargeable batteries, and they charge and discharge using the movement of aluminum ions between the positive and negative electrodes. Since there is a rich resource of aluminum in the world, if making rechargeable batteries using aluminum becomes practical, then it may be possible to store energy at a lower cost than using the present lithium ion batteries. Electrolyte will also be able to use non-flammable materials, thus increasing safety. In addition, depending on the type of electrolyte, there is the possibility of use at temperatures of about 250–300°C. Green Science Alliance makes battery prototypes for aluminum ion batteries having a capacity of about 50 mAh/g with carbon-based positive electrode material (laminate cells). These prototypes have stable cycle characteristics, having little deterioration even over 50 cycles.
At the same time, research and development is being done on aluminum–air batteries that has second theoretical cell capacity next to lithium air batteries, which have the largest in capacity among batteries. An aluminum–air rechargeable battery conversion is being considered, but a reduction reaction to aluminum metal in the electrolyte is not confirmed; the present situation is that conversion to a rechargeable battery has not been confirmed. In relation to the aluminum ion battery, joint research and development aimed at practical use is being carried out with Osaka University.
Zinc–air rechargeable battery
Metal–air batteries use the oxygen in the air as their positive electrode and metal as their negative electrode. They use oxygen by taking it from the air outside into the interior. Because there is no need to have a positive electrode active material included within the battery, it can be made lighter and so it has advantages over other batteries in both weight and volume. Metals elements that can serve as negative electrodes in air batteries include zinc, aluminum, magnesium, lithium, silicon, and iron. Lithium–air batteries that use lithium for the negative electrode are reported to have the greatest capacity among these batteries. However, zinc–air batteries have a long history and there is a good world reserve of zinc available, and so they can be manufactured at a lower cost than lithium ion batteries, and thus they show promise. In addition, they have a theoretical battery capacity five times greater than that of lithium ion battery, making them attractive. They are in use as primary batteries in hearing aids, film cameras, railway signaling equipment, and the like. However, zinc in zinc–air batteries deposits in the form of dendrites after repeated charging and discharging, and this is problematic for a rechargeable battery. In addition, the catalysts that can suppress charge and discharge deterioration are precious metals such as platinum, which are highly priced, and so there are characteristics making it unsuitable for use as a rechargeable battery. Green Science Alliance is carrying out research and development in cooperation with Kyoto University to find a substitute material for platinum and to modify the electrolyte to make a practical zinc–air rechargeable battery that is cheap and stable.
Solid oxide fuel cell (SOFC)
A fuel cell is a device for converting the chemical energy in fuel directly into electricity and heat. There are four main types of fuel cells: phosphoric acid fuel cell (PAFC), molten carbonate fuel cell (MCFC), polymer electrolyte fuel cell (PEFC), and SOFC. Fuel cell batteries emit virtually none of the characteristic air pollutants such as nitrogen oxides and sulfur oxides, which is a great advantage. Fuel cells produce electricity by ions passing through and moving in electrolyte. In PAFCs and PEFCs, hydrogen ions pass through electrolytes and at the cathodes to form a compound with oxygen and water, so that the fuel is limited to hydrogen. With SOFCs, oxygen in the air turns to ions at the cathode; as it passes through the electrolyte, carbon monoxide and hydrogen can be used as a fuel. At present, there are not many places where hydrogen can be obtained easily as a gas or a liquid; this prevents the application of fuel cells. Accordingly, the use of improved natural gas or propane is considered a solution under the existing conditions. SOFCs can use carbon monoxide directly, and so there is no need for a treatment process to reduce carbon monoxide to a very small amount as with PEFCs, and there is the advantage of the system staying simple. With SOFCs, there is the advantage that high-quality exhaust heat can be obtained and can be used as energy, at the same time.
Green Science Alliance is researching and developing positive electrodes, negative electrodes, and solid electrolyte. SOFC materials are generally oxides, and we have a rich experience and track record in synthesizing them. These can be turned into inks and pastes in our company, and so we can do everything from synthesis to printing ink or paste conversion for SOFC materials within our company. We are also planning to start considering small-scale SOFCs sometime.
PEFC : Polymer Electrolyte Fuel Cell
As fuel cells use only oxygen and hydrogen, the product from the reaction is just water and so they are very clean electricity-generation devices. Compared with the SOFC described above, the PEFC has a low operating temperature. Hence, it is getting attention as suitable for application over a wide range of household uses, particularly for vehicles.
In most cases with PEFC electrolytes, fluorine cation-exchange membranes are generally used. These have a thickness in the range 20–50 µm, as well as a structure where an electrode having a platinum or platinum ruthenium alloy catalyst supported on carbon is adhered to the film on the fuel electrode side. And an electrode with platinum supported on carbon is adhered on the air electrode side. The membrane and electrode made into one unit is called a membrane electrode assembly. Researchers are considering the setting of the size of these precious metal catalysts used in the fuel cells at several nanometers and making the specific surface area as large as possible in order to reduce the amount used.
One reason why PEFCs have been attracting much attention in recent years is the output density. If the output density exceeds 1 kW/L, then it can adequately compete with a gasoline engine. With its high-efficiency motive force with a low burden on the environment, it is being researched and developed as a fuel cell for cars; some practical applications are practically used. With further cost reductions and reductions in size, it would advance further in its use as a power source in homes.
Green Science Alliance is doing research and development, along with Kyoto University, into replacing the platinum, or part of it, being used in electrodes by using materials such as those derived from metal–organic structures or a carbon alloy type. It would be a great advantage to replace these precious metals not only because of their high cost but also because of concern about their availability.
Perovskite and quantum dot solar cells
Solar cells are devices for converting the energy of the sun’s light directly into electricity. There are many types, depending on the semiconductor used as the basic element, but at present most of the mass-produced solar cells are divided into silicon-type solar cells and compound-type solar cells. These solar cells are robust and do not break easily, and they are highly efficient at converting light energy. But they are not widely spread throughout the world because the material and manufacturing costs are high, making them problematic. Accordingly, next-generation-type solar cells such as perovskite solar cells or quantum dot solar cells are explored as new solar cells. Perovskite solar cells are a new type of solar cells using a crystal structure material called perovskite (NH3CH3PbI3). In the most recent research, high conversion efficiencies, comparable to those of silicon-type solar cells and compound-type solar cells, are starting to be obtained. A great attraction is that they have the potential to be cheaper than the silicon-type and compound-type solar cells. Because they can be manufactured at a low temperature, it is possible to use plastic as a base substrate. Hence, they can be made as flexible solar cells as a power source, in places where it has been difficult to install solar cells previously with general type of solar cell.
In the current mainstream solar cell as single-junction type using silicon-base material, photons having energy above the band gap in sunlight, they turned into heat after being absorbed, and photons with energy below the band gap cannot pass through and cannot undergo photoelectric conversion. This is one factor limiting the efficiency of energy conversion. Another next-generation cell that can deliver higher energy efficiency and is attracting attention is the quantum dot solar cell. It traps electrons in quantum dots, thus causing quantum effects such as the quantum size effect. It is therefore possible to increase the conversion efficiency by using a light effectively that could not be absorbed in the previous solar batteries, and so deliver a theoretical efficiency of 75%. Hence, a conversion efficiency higher than that of existing solar cells using semiconductors is possible, and much is anticipated from them.
Green Science Alliance is promoting research and development together with manufacturing and sales of electrode material, photosensitizing material, electrolyte material, and others for use in these solar cells.