Lithium-ion capacitors are a novel type of energy storage device, distinct from supercapacitors and lithium-ion batteries. Compared to supercapacitors, they offer significant advantages such as high power density, high capacitance, and long cycle life, making them promising for applications in new energy vehicles, solar energy, and wind energy. Their operating principles also differ from those of lithium-ion batteries and supercapacitors.
1. Operating mechanism of lithium-ion batteries
Lithium-ion batteries are among the fastest-growing rechargeable batteries, following nickel-cadmium and nickel-metal hydride batteries. Both the positive and negative electrode materials in lithium-ion batteries are compounds capable of reversibly inserting and extracting lithium. Before assembly, at least one electrode material must be in a lithium-intercalated state. For example, transition metal oxides such as LiCoO2, LiNiO2, and LiMn2O4 can be used as positive electrodes, while carbon materials, metal oxides, or alloys can be used as negative electrode materials.
Lithium-ion batteries work by using graphite as the negative electrode and LiCoO2 as the positive electrode. During charging, lithium ions are released from the positive electrode material and migrate to the negative electrode through the electrochemical potential gradient under the influence of the electrolyte. To maintain charge balance, the positive electrode releases an equal number of electrons to the external circuit for use by the negative electrode. Once the lithium ions reach the negative electrode, they embed themselves in the negative electrode material's crystal lattice and gain electrons to reform into lithium ions. During discharging, this process is reversed. In other words, lithium ions leave the negative electrode lattice and recombine at the positive electrode to form LiCoO2.
The reaction equation represents the charging and discharging process of the battery:
Positive electrode reaction:
Lithium cobalt oxide LiCoO2⇌Li₁-xCoO2+xe+xLi+
Negative electrode reaction:
Li++xe-+nC⇌Li(x)C(n)
Battery reaction:
When LiCoO2 is combined with an n-fold number of carbon atoms, it will react to produce Li1-xCoO2 and LixCn.
Besides the oxidation-reduction reaction, the working principle of lithium-ion batteries also involves electrochemical insertion-extraction reactions. During charging and discharging, lithium-ion batteries utilize the electrolyte and lithium ions (Li+) as the energy exchange medium to perform insertion and extraction between the positive and negative electrodes, thus achieving energy conversion. Compared to other types of batteries, lithium-ion batteries have many advantages, such as high energy density, high average output voltage, high charging efficiency, low self-discharge efficiency, good safety performance, and long cycle life.
2. A supercapacitor is an electronic device capable of rapidly storing and releasing energy. Its working principle is based on the electrochemical reaction between electrode materials, rather than the accumulation of an electric field as in traditional capacitors.
When a supercapacitor is charged, charge concentrates on the electrode surface and is transferred to ions via the electrolyte. These ions are accelerated by the potential difference between the negative and positive electrodes, forming a double-layer capacitor. This double-layer capacitor enables the supercapacitor to store a large amount of charge and provide a large current in a short time.
When electrical energy needs to be released, the charge is quickly released back into the electrolyte. This discharge method can be reused because the ions circulate in the electrolyte and can withstand the effects of high-speed discharge.
Supercapacitors are widely used in energy storage and conversion systems due to their high current density and long cycle life, especially in applications requiring rapid charging and discharging.
Supercapacitors typically consist of electrodes, an electrolyte, a current collector, and a separator. In terms of their working principle, when the capacitor is charging, electrons flow from the external power source to the negative electrode, passing through the electrolyte solution, causing the positive and negative electrodes to acquire positive and negative charges, respectively. Simultaneously, positive and negative ions in the solution move along the electrode surfaces, opposing the charges on the electrode surfaces to form a symmetrical electric double layer. When the capacitor needs to discharge, electrons flow from the negative electrode to the positive electrode through the load. At this time, the positive and negative ions move away from the electrode surfaces and return to the electrolyte solution, and the electric double layer of the capacitor disappears. As can be seen from the above description, double-layer capacitors utilize the electric double layer between the electrodes and the electrolyte to store charge. The charging and discharging processes are physical processes and do not trigger any electrochemical reactions, thus exhibiting stable performance, fast charging speed, long cycle life, high power density, and excellent high and low temperature adaptability.
3. The principle of a lithium-ion capacitor can be explained as follows: In an ionic dielectric, positively charged lithium ions are embedded in the gaps between the electrodes. When the capacitor is charging, the lithium ions move from the positive electrode to the negative electrode, causing charge to accumulate between the electrodes, thus storing energy. When the capacitor needs to output energy, the lithium ions move in the opposite direction, releasing the charge and driving the external circuit. Due to its high energy density and long cycle life, lithium-ion capacitors are widely used in medical devices, energy storage stations, aerospace, and other fields.
The team at Hiromoto and Fuji Heavy Industries proposed the working principle of lithium-ion capacitors. The positive electrode material of a lithium-ion capacitor is activated carbon, which possesses double-layer capacitance; while the negative electrode material is intercalated carbon, which allows for lithium-ion insertion and extraction. The electrolyte is a lithium salt electrolyte. During charging, lithium ions leave the surface of the positive electrode material, pass through the electrolyte and separator, and then insert into the crystal lattice of the negative electrode material. During discharging, lithium ions are extracted from the crystal lattice of the negative electrode material, pass through the electrolyte, and return to the surface of the positive electrode material, forming a double-layer capacitance. Because the potential of the negative electrode is lower after lithium insertion, lithium-ion capacitors have the characteristics of high operating voltage and energy density and power density between lithium-ion batteries and supercapacitors.
4. What are the advantages of lithium-ion capacitors compared to lithium-ion batteries and supercapacitors?
Comparison of capacity, voltage, and self-discharge.
Lithium-ion capacitors have high output density, but lower energy density than lithium-ion batteries. Compared to the 2-8Wh/L capacity of double-layer capacitors, the energy density of a single lithium-ion capacitor is 10-15Wh/L, which is about twice that of the latter.
In terms of voltage, lithium-ion capacitors can reach a maximum voltage level of 4V, which is quite close to that of lithium-ion batteries, but significantly higher than that of double-layer capacitors. At the same time, lithium-ion capacitors also exhibit less self-discharge than both of the above.
Security
Because lithium-ion batteries use a cathode containing lithium oxide, the cathode contains a large amount of lithium. This lithium can form lithium dendrites and puncture the separator. The cathode also contains oxygen, a significant ignition source. Once a short circuit occurs, the battery rapidly undergoes a thermal decomposition reaction, leading to combustion due to the reaction with the electrolyte. In contrast, lithium-ion capacitors use activated carbon as the cathode. Even in the event of an internal short circuit, the carbon reacts with the negative electrode but not with the electrolyte, making them theoretically safer than lithium-ion batteries.
Long lifespan
To achieve a long lifespan in lithium-ion batteries, certain limits need to be placed on their depth of charge and discharge. While this reduces usable capacity, it effectively extends the battery's actual lifespan. In contrast, double-layer capacitors operate solely on the adsorption or removal of ions from the electrolyte, thus possessing a long lifespan. However, this alone is insufficient to significantly extend actual lifespan. Furthermore, even with a reduction in the positive electrode potential, the voltage of a lithium-ion battery cell does not drop substantially, thus ensuring capacity.
High temperature resistance
In high-temperature environments, the electrolyte and positive electrode are prone to oxidation and decomposition. Therefore, under high-temperature conditions, it may be necessary to lower the potential of the positive electrode. However, with a lower potential, the overall voltage drops, and the capacity cannot be guaranteed. Furthermore, lithium-ion batteries are difficult to de-voltage, thus posing a safety hazard. Only lithium-ion capacitors can be located away from the oxidation and decomposition region, utilizing a higher positive electrode potential, thus exhibiting superior high-temperature performance.
5. Current Status of Application and Industrialization of Lithium-ion Capacitors
Lithium-ion capacitors are an emerging type of electrochemical energy storage device with broad application prospects and potential. They store and release electrical energy by the migration of lithium ions between the positive and negative electrodes, and feature high energy density, long cycle life, and fast charge and discharge capabilities. They are widely used in electric vehicles, renewable energy storage, smart grids, and other fields.
Currently, the industrialization of lithium-ion capacitors has made significant progress. Many companies and research institutions both domestically and internationally are actively investing in research and development and production, driving breakthroughs and innovations in lithium-ion capacitor technology. In terms of application areas, the electric vehicle market is one of the most promising and in-demand sectors for lithium-ion capacitors. With the increasing prevalence of electric vehicles and the growing demand for high energy density and fast charging, lithium-ion capacitors are expected to become an important component of electric vehicle energy storage systems.
Furthermore, renewable energy storage is another important application area for lithium-ion capacitors. With the rapid development of renewable energy sources such as solar and wind power, energy storage has become a key issue in addressing intermittent power supply and balancing energy demand. Lithium-ion capacitors can quickly respond to and balance energy demand, supporting a stable supply of renewable energy.
Smart grids are another important application area for lithium-ion capacitors. Smart grids require energy storage and regulation capabilities to cope with energy fluctuations and changes in grid demand. The high energy density and fast response characteristics of lithium-ion capacitors make them ideal energy storage devices for smart grids.
In conclusion, lithium-ion capacitors have broad application prospects in fields such as electric vehicles, renewable energy storage, and smart grids. With continuous technological advancements and industrialization, lithium-ion capacitors are expected to play an increasingly important role in energy storage.
The upstream of the lithium-ion capacitor industry mainly includes positive and negative electrode raw materials, electrolytes, separators, perforated current collectors, and elemental lithium electrodes; the midstream includes lithium-ion capacitor cells of various shapes and specifications and their system integration modules; and the downstream mainly applies to end-market demand. Currently, the Japanese market has been initially opened, and subsequent global expansion is planned, including in areas such as wind power generation, LED lighting, solar power generation, and hybrid vehicles.
Currently, these industries are mainly controlled by foreign companies. For example, Coca-Cola and ACT of Japan respectively possess activated carbon and nanofiber carbon technologies; while Kanebo, Kureha Chemical, and ATEC of Japan possess technologies for anode materials such as polybenzene and hard carbon. Furthermore, Ferro and Honeywell dominate the electrolyte field, while NKK of Japan monopolizes the separator market, and three Japanese metal companies monopolize porous current collectors. Domestically, only a few companies are currently developing activated carbon for positive electrodes and hard carbon for negative electrodes in lithium-ion capacitors.
Supercapacitors and lithium batteries each have their advantages and disadvantages. Supercapacitors are batteries that operate based on physical principles, while lithium batteries are primarily chemical batteries made using chemical principles. Therefore, they are essentially two different technologies: supercapacitors store charge physically, while lithium batteries work by converting chemical energy into electrical energy.
In terms of usage, supercapacitors have lower internal resistance, allowing them to release larger currents instantaneously. Unlike all chemical batteries, supercapacitors are unaffected by temperature. At low temperatures, the capacity of chemical batteries decreases and their internal resistance increases. However, because supercapacitors are implemented using purely physical principles, they have a longer lifespan.
However, supercapacitors present challenges in battery management due to their extremely low internal resistance, and their safety is even more precarious than that of lithium batteries. Their disadvantages include lower energy density and higher price.
