1.Principle of Lithium-ion Batteries
Note: EC and DEC are electrolyte solvents.In the diagram, the cathode is lithium cobalt oxide,
and the anode is graphite.
With Li+ as the ion conduction medium, the reaction of lithium ion insertion and extraction occurs between the anode and cathode, also known as a "rocking chair battery".
2.
The Composition of Lithium-ion Battery
Materials
2.1.Positive electrode materials refer to materials that can provide a source of lithium ions during the charging process.
For example: lithium compounds LiCoO2, LiNiO2, LiMnO2, LiNiCoMnO2, and LiFePO4.
2.2.Negative electrode materials are materials that provide a receiving site for lithium ions during the charging process.
For example: graphite (synthetic or natural), silicon alloys, tin alloys,etc.
2.3.Electrolyte is a material that provides a conduction medium for lithium ions.For example: organic solutions dissolved with lithium salts such as LiPF6, LiAsF6, etc.
2.4.Separator is a porous material that blocks the conduction of electrons between the positive and negative electrodes while allowing the conduction of ions.For example: PP, PE, non-woven fabric etc.
2.5.The current collector refers to a material carrier that provides a path for material coverage and a bundle of conductors for current.
For examples: copper foil (negative electrode), aluminum foil (for the positive electrode) etc.
2.6.The packaging shell refers to the composite material used to package and protect the battery cell.
Examples: aluminum-plastic film, stainless steel shell, and aluminum shell.
2.7.The conductive agent refers to a material that can enhance the electronic conductivity between active materials and between active materials and the current collector.Examples: conductive carbon black, carbon nanotube,etc.
2.8.The binder refers to a material that enhances the adhesion between materials and between materials and the current collector.
Examples: SBR and PVDF,etc.
2.9.Additives refer to special substances that enhance the safety and stability of the battery.Examples: film-forming additives and flame retardant additives,etc
3.Role and Selection Criteria of Cathode Electrode Materials
3.1.Requirements for the Selection of Cathode Electrode Materials
As an ideal cathode electrode material for lithium-ion batteries, lithium-ion insertion compounds must meet the following requirements:
3.1.1.Have a high redox potential to ensure the high voltage characteristics of lithium-ion batteries;
3.1.2.Allow a large amount of lithium ions to be inserted and extracted,
ensuring the high capacity characteristics of lithium-ion batteries;
Calculation of theoretical capacity: C0 = 26.8n m/M
Co---- theoretical capacity; n---- number of electrons gained or lost in the redox reaction;
m ---- mass of the active material completely reacting; M----molar mass of the active material
Using LiCoO2 as an example:
Co = 96500/M = 96500*1000/3600*98 = 273 mAh/g
LiNiO2has a capacity of 274 mAh/g;
LiMn2O4has a capacity of 148 mAh/g;
LiFePO4 has a capacity of 170 mAh/g.
3.1.3.Good reversibility during the insertion and extraction process, with minimal changes in material structure during charging and discharging;
3.1.4.Rapid insertion and extraction of lithium ions, with high electronic and ionic conductivity;
3.1.5.Good chemical stability in the electrolyte;
3.1.6.Low cost, easy to prepare, and environmentally friendly.
Currently, there are various cathode materials for lithium-ion batteries that are under extensive research, including LiCoO2, nickel-cobalt binary systems, nickel-cobalt-manganese, manganese compounds, and LiFePO4.
4.Classification of Cathode Materials
After process optimization and improvement, the gram capacity of lithium cobalt oxide and ternary materials has been significantly increased.
5.Discussion on the Development Direction of Positive Electrode Materials
5.1.Direction of Cathode Material Development
5.2.High nickel content and high voltage are the trends for NCM materials development.
5.3.With increasing nickel content, thermal stability and capacity retention rate tend to decrease.
5.4.High-manganese lithium-rich solid solution ternary cathode materials
5.4.1.Low-cobalt layered ternary materials: Cobalt is an expensive and scarce resource, and reducing its content can save costs in material production. Currently, materials with cobalt content reduced to 15% have already been put into application.
5.4.2.High-nickel layered ternary materials: The synthesis of high-nickel system materials is conducted under an oxygen atmosphere, which is relatively challenging. Lithium-nickel mixing can easily occur during the synthesis process, affecting the performance of the materials. However, increasing the nickel content can enhance the specific capacity of the materials, making high-nickel products an ideal choice for the future development of large-scale batteries.
5.4.3.Layered nickel-manganese binary materials: In LiNi0.5Mn0.5O2 manganese exists in the form of Mn4+. During charging and discharging, manganese does not participate in electrochemical reactions but stabilizes the crystal structure of the material, resulting in excellent electrochemical performance. However, the synthesis of this material is difficult, and the presence of impurities during synthesis can affect its performance.
5.4.4.5V spinel-structured nickel-manganese binary materials: Among them, LiNi0.5Mn1.504 has received the most attention. With the maturing technology of lithium titanate anodes, which exhibit stable structures, combining high-performance 5V battery materials with lithium titanate anodes can result in battery systems with consistently stable voltage cycling.
The optimization of 5V high-voltage nickel-manganese materials requires the cooperation of electrolytes.
5.5.High-manganese lithium-rich solid solution ternary cathode materials
On June 5, 2009, BASF signed a cooperation agreement with Argonne National Laboratory of the U.S. Department of Energy, aiming to industrialize thex Li[Li1/3Mn2/3]O2·(1-x)LiMO2(M= Mn, Ni, Co) cathode material on a large scale. This strategic move by the world's largest chemical company is based on the belief that this material will occupy a dominant position in the future market for lithium-ion battery cathode materials.
5.7.Decay Mechanisms
Explanation of the First Charge Platform:
5.7.1.Theory of Oxygen Loss
During the extraction of Li+, the oxidation reaction of the O element occurs, and the O atoms leave the material lattice and enter the electrolyte. (J.R. Dahn)
5.7.2.Theory of Proton Exchange
The capacity associated with the 4.5V platform is due to the exchange of lithium ions with protons generated by the decomposition of the electrolyte. (P.G. Bruce)
5.7.3.Theory of Î Bonding
Between Mn and O, π bond is formed. The three d-electrons of Mn4+ enter the antibonding t*2g orbital, with oxygen acting as the electron donor, resulting in the corresponding 4.5V platform. (Y.S. Hong)
6.Common questions in laboratory applications
7.Canrd Brief Introduce
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8.
Q & A
In this Q&A session, Dr. Ke answered the questions raised by everyone one by one.
Black Black: "Is the high pressure of lithium cobalt oxide due to the large particle size? Is the high voltage achieved through coating?"
Dr. Ke:
"The high pressure of lithium cobalt oxide is mainly due to two reasons: first, the true density of lithium cobalt oxide is high, reaching 5.2 g/cc; second, lithium cobalt oxide is mostly large single-crystal particles, not secondary particles. Therefore, using a combination of small and large particles can achieve high compaction. Currently, high voltage is mainly achieved through surface coating of the material."
Black Black: "The reason for the nanomization of lithium iron phosphate is to improve electronic conductivity, right?"
Dr. Ke:
"The main reason for the nanomization of lithium iron phosphate is to shorten the lithium-ion diffusion path and improve ion conductivity. The improvement of electronic conductivity is achieved through carbon coating on the material surface."
Angel is offline: "What is generally used for low-temperature battery cathodes?"
Dr. Ke:
"I haven’t done a comparison of different materials for this, but from a rate performance perspective, lithium cobalt oxide is better because it has better conductivity."
10: "Does a low first-cycle efficiency affect subsequent charge/discharge capacity?"
Dr. Ke:
"A low first-cycle efficiency leads to lower discharge capacity."
Angel is offline: "Is the capacity of a battery determined by the first-cycle efficiency?"
Dr. Ke:
"Actually, the first-cycle efficiency affects the capacity of the battery."
10: "So what can be done to improve the first-cycle efficiency?"
Dr. Ke:
"It is necessary to adjust the electrolyte formulation, passivate the cathode surface, reduce electrolyte consumption, and minimize lithium-ion loss."
Angel is offline: "I still don’t understand first-cycle efficiency. I heard that both the anode and cathode have first-cycle efficiency. How is the overall battery’s first-cycle efficiency calculated?"
Dr. Ke:
"Both the anode and cathode have first-cycle efficiency. The overall first-cycle efficiency is calculated using the 'weakest link' theory, where the lowest first-cycle efficiency determines the overall efficiency."
10: "Will using the anode's electrolyte on the cathode significantly affect the battery's capacity?"
Dr. Ke:
"If the surface is not passivated, the electrolyte will continue to react with the surface, causing active lithium loss."
Angel is offline: "Will the second-cycle efficiency be lower than the first?"
Dr. Ke:
"The efficiency of the second cycle will generally be higher than the first."
Angel is offline: "Does first-cycle efficiency affect capacity but not determine it?"
Dr. Ke:
"Generally, the material's capacity is determined by its structure, which gives the theoretical capacity. The actual capacity depends on the amount of lithium extracted or inserted during the charge/discharge process. Additionally, we know that during the first cycle, both the anode and cathode form a passivation film, which consumes lithium ions. So, what we are discussing more here is the loss of lithium ions during this process."
Qinghai: "Does the separator conduct electricity or not?"
Dr. Ke:
"The separator does not conduct electrons, but it does conduct ions."
Angel is offline: "Is the separator coated on one side or both sides?"
Dr. Ke:
"Currently, the mainstream is single-sided coating because it doesn’t adhere well to the negative electrode side."
Angel is offline: "Then it should be coated on the positive electrode side?"
Dr. Ke:
"Yes, it should be on the side facing the positive electrode."
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