Text|Yanagihachihara
Editor|Yanagihachihara
Tantalum-based perovskites are a new type of anode material for lithium-ion batteries, which are composed of titanium-oxygen octahedral and tantalum-containing oxygen octahedron, with excellent electrochemical performance and structural stability. The material has the following characteristics: high capacity and high rate charge and discharge performance, able to store more charge and achieve fast charge and discharge; At the same time, it has low volume expansion and excellent cycle stability, effectively reducing the attenuation of battery capacity and the loss of cycle life.
Let's explore the role of tantalum perovskites as anode materials for lithium-ion batteries!
The key to improving the performance of lithium-ion batteries
The latest lithium-ion batteries utilize the process of inserting and disengaging lithium ions in the structural matrix of the electrode to achieve energy storage. During insertion, lithium ions are embedded in specific locations in the electrode structure, which is usually achieved in different ways, depending on the electrode material used. For example, silicon materials can be inserted into lithium-ion through an alloying process.
During the detachment process, lithium ions are released from the electrode material, a process that involves the transfer of ions in the electrode and the flow of equivalent electrons in external circuits.
Lithium-ion batteries are a reversible chemical reduction-oxidation reaction that enables the storage and release of chemical energy. During the redox reaction, energy is released while electrons and ions are exchanged between the positive and negative electrodes to form more stable and promising compounds.
The electrochemical characteristics of lithium-ion batteries are influenced by several factors, including the electrode material, the type of electrolyte (i.e., voltage window), and the geometry of the battery. These factors have a huge impact on charging capacity. For example, lithium-ion batteries with lithium iron phosphate or lithium cobalt oxide as cathode materials are often used in handheld devices.
To ensure high performance, high-quality cathode, anode and electrolyte materials need to be selected and a precise preparation process performed. This includes ensuring that the purity, particle size and crystallinity of the material meet the requirements, as well as preparing the electrode material using appropriate methods, including the coating or deposition of the positive and negative electrodes. This ensures uniformity, compactness and good charge-discharge performance of the electrode material.
By performing cycle performance tests, the capacity attenuation, cycle stability, and electrochemical performance of the battery can be evaluated. This includes charging and discharging cycles and cyclic voltammetry tests to evaluate the performance of the battery in actual use.
In order to improve the stability and cycle life of the battery, the interface between the electrode and the electrolyte can be optimized. This can be achieved with additives or coatings to reduce electrolyte breakdown and side reactions on the electrode surface. In addition, various structural characterization techniques, such as scanning electron microscopy, transmission electron microscopy, and X-ray diffraction, allow the microstructure of battery materials and electrodes to be analyzed to understand their crystal structure and interface characteristics.
Common anode materials include graphite and other carbon compounds, each with its advantages. For example, lithium cobaltate-based electrodes have a higher storage capacity, while lithium iron phosphate has better safety performance.
It should be noted that some lithium-ion batteries may have safety risks. For example, lithium-metal-based batteries use lithium metal as an electrode, which can produce hydrogen gas and cause an explosion when in contact with water. In addition, some commonly used electrolytes contain flammable organic compounds and toxic lithium salts. Therefore, corresponding safety measures need to be taken during battery design and manufacturing.
In lithium-ion batteries, there are three basic functional components, they are: positive electrode, also known as cathode, negative electrode, also known as anode. These electrodes are responsible for the transfer of electrons during battery charging and discharging.
The electrolyte acts as a medium that facilitates the movement of lithium ions between the cathode and anode, it is an ionic conductor, usually consisting of a mixture of organic solvents and lithium salts, the electrolyte allows ion flow while preventing direct contact of the electrodes, which can lead to short circuits.
Initially, lithium metal was used as a cathode material for early lithium-ion batteries. However, due to safety concerns and short circuits and battery failures caused by lithium metal being prone to dendrite formation, lithium metal has been replaced by lithium compounds as the choice of cathode material.
In lithium-ion batteries, carbon is usually found in the form of graphite as the most commonly used anode material. Currently, research and development is underway to find materials that can provide higher charge capacity to replace graphite. Despite silicon's potential for higher energy storage capacity, it faces the challenge of volume expansion during charge and discharge cycles.
In lithium-ion batteries, cathode materials typically use phosphates, such as LiFePO4, or transition metal oxides, such as the lithium metal cobalt oxide series. These materials provide the necessary redox reactions during battery operation.
Currently, the anode active material for lithium-ion batteries is usually graphite. However, ongoing research is mainly focused on finding alternative materials to improve the energy storage capacity and performance of lithium-ion batteries.
To physically separate the cathode and anode and allow the transport of lithium ions, polyethylene or polypropylene separators are widely used as separators. The use of this diaphragm helps prevent direct contact between the electrodes, thus avoiding the occurrence of short circuits.
The electrolyte used in lithium-ion batteries is a mixture of organic solvents that are usually combined with lithium salts (such as lithium hexafluorophosphate, LiPF6) so that lithium ions can move inside the battery.
At the positive electrode: LiCoO2 Li1-xCoO2 + xLi+ + xe- During charging, lithium ions are detached from the LiCoO2 ↔ cathode material, resulting in a decrease in lithium content, a process accompanied by the release of electrons (e-).
At the anode: xLi+ + xe- + 6C ↔ LixC6 At the same time, lithium ions and electrons from the cathode bind to the graphite anode, resulting in lithium being embedded in the graphite structure.
The actual behavior of lithium-ion batteries is influenced by factors such as specific electrode materials, electrolyte composition, temperature, and other operating conditions, and graphite remains the most suitable candidate for anode electrodes due to its excellent properties, such as a low and flat potential window compared to pure lithium metals, as well as good cycle life and low cost.
However, when LiCoO2 represents the active material of the cathode, graphite can only insert one Li ion out of every six carbon atoms to form LiC6, and thus produce a reversible equivalent capacity of 372 mAh g-1.
Compared with graphite, the diffusion rate of lithium ions in the disordered carbon layer is between 10-12 and 10-6 cm2/s, while the diffusion rate of graphite is between 10-9 and 10-7 cm2/s, thus resulting in a lower power density of the battery.
Recently, there has been a need to replace graphite anode materials with materials with better and enhanced capacity characteristics and power density, although pure lithium metal has a very high capacity in anode materials, but safety concerns limit the continued use of pure lithium metal as an anode material in rechargeable batteries, because dendrite formation on lithium metal results in a short interval between the cathode and anode.
Therefore, the realization of lithium-ion batteries with improved energy and power density faces an important challenge, that is, choosing the right anode materials, they can provide high capacity, facilitate the diffusion of lithium ions into the anode, while having a good cycle life and not in the process of discharge in the lithium-ion battery, the right side shows the layered structure of the Li1-xCoO2 material, and the left side represents the anode of the graphite sheet. During charging, lithium ions migrate from the cathode (positive electrode) to the anode (negative electrode).
The source of energy storage in lithium-ion batteries
The cathode acts as a carrier of lithium ions during discharge, collecting lithium ions that migrate from the other electrode through the electrolyte, a good cathode material on the one hand must be stable enough to accommodate lithium ions without reacting with it, so that the material structure is permanently altered or distorted, but on the other hand it is not so stable that it reduces the chance of lithium ion acceptance, resulting in an irreversible process, similar to the case of disposable batteries.
In-depth studies of intercalation materials have shown that they are the most suitable for this purpose, since lithium ions can carry themselves in the vacancies of the material without significant changes in the structural characteristics, and the intercalation process can be easily achieved by applying almost the same voltage in the opposite direction.
Lithium cobalt oxide LiCoO2 was the first commercially produced lithium-ion battery by Sony as a cathode material, and in addition to the very high cost of cobalt and its known dangers and safety issues, lithium cobalt oxide was the best choice at the time.
Through in-depth research, new LCOs that can be used as alternative cathode materials have been discovered, and new improved cathode materials have been formed by adding manganese, aluminum and nickel. Materials such as lithium-nickel-cobalt-aluminum oxide and lithium-manganese-cobalt oxide enhance the thermal stability of the structural layer of the cathode material, although cobalt remains a part of the structure and there are obstacles.
As a cathode material, the specific energy density of LiFePO4 is about 586Wh/kg, which is not significantly improved compared with lithium cobalt oxide LiCoO2, and the electrochemical properties of the material are not the best at first, but a lot of research work has been carried out to improve this characteristic.
Pure LiFePO4 has very low electronic conductivity, about 10^-9 S/cm, which is very low, and later studies have shown that carbon coatings can significantly improve their electronic conductivity if the particle size is small enough and the size distribution is narrow.
Due to its volumetric energy density slightly lower than lithium cobalt oxide LiCoO2, as well as its reasonable cost, long lifetime, and environmental friendliness, all of these properties make it an ideal candidate for novel cathodes.
Lithium manganate LiMn2O4 is an interesting example as an early candidate for cathode materials for lithium-ion batteries, which was initially considered a promising cathode material, but found that manganese dissolution is an inherent problem that cannot be solved, which is thought to be due to the "average oxidation state".
When investigating the possibility of overcoming the dissolution problem, a new cathode material called lithium manganese nickelate was obtained from the same material, which exhibited very promising properties, from raising the intercalation potential to about 4.7V instead of about 4.0V of LMO material, a high potential that made LMNO material the first in the history of cathode materials.
In the study, it is shown that orthosilicate Li2MSiO4 materials have great potential as a new "polyoxygenated anion" cathode for lithium-ion batteries, at the reversible potential, that is, the extraction and insertion of lithium ions during discharge/charging, they provide two lithium ions per chemical formula unit, corresponding to a theoretical capacity of about 330mAh/g, which is quite high, this capacity is twice higher than olivine LiMPO4 type materials.
Among the orthogonal crystal system Li2MSiO4, Li2FeSiO4 is the most promising material because silicon and iron are abundant and significantly less expensive, however, the inherent low electronic conductivity associated with silicon is a major disadvantage, the electronic conductivity of Li2FeSiO4 is calculated as 6×10^-14 S·cm^-1, and the corresponding specific capacity is found to be 165mAh/g, with the ability to remove 1Li+/Li2FeSiO4.
After several cycles, the capacity is reduced to 140mAh/g, and according to the study, during the cycle, it has a side reaction with some impurity phases that coexist with the material, so that the potential is reduced from more than 3V of the initial charging curve to about 2.9V.
Studies have shown that tantalum-based perovskites show good cycle stability and electrochemical performance in lithium-ion batteries, they have high lithium ion intercalation and intercalation potential, which is very important for improving the energy density and cycle life of batteries, in addition to these, tantalum-based perovskites also show excellent conductivity and ion diffusion properties, which is conducive to rapid ion transport and charge-discharge processes.