The 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”.
The Composition of Lithium-ion Battery Materials
Positive electrode materials refer to materials that can provide a source of lithium ionsduring the charging process.
For example:lithium compounds LiCoO₂,LiNiO₂,LiMnO₂,LiNiCoMnO2,and LiFePO4.
Negative electrode materials are materials that provide a receiving site for lithium ionsduring the charging process.
For example:graphite (synthetic or natural),silicon alloys,tin alloys,ect.
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.
Separator is a porous material that blocks the conduction of electrons between thepositive and negative electrodes while allowing the conduction of ions.
For example:PP,PE,non-woven fabric,ect.
The current collector refers to a material carrier that provides a path for material coverageand a bundle of conductors for current.
For examples:copper foil (negative electrode),aluminum foil (for the positive electrode)ect.
The packaging shell refers to the composite material used to package and protect the batterycell.
Examples:aluminum-plastic film,stainless steel shell,and aluminum shell.
The conductive agent refers to a material that can enhance the electronic conductivitybetween active materials and between active materials and the current collector.
Examples:conductive carbon black,carbon nanotubes,etc.
The binder refers to a material that enhances the adhesion between materials and betweenmaterials and the current collector.
Examples:SBR and PVDF,etc.
Additives refer to special substances that enhance the safety and stability of the battery.Examples:film-forming additives and flame retardant additives,etc.
Requirements for the Selection of Positive Electrode Materials
As an ideal positive electrode material for lithium-ion batteries, lithium-ion insertion compounds must meet the following requirements:
(1) Have a high redox potential to ensure the high voltage characteristics of lithium-ion batteries;
Requirements for the Selection of Positive Electrode Materials
(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) Good reversibility during the insertion and extraction process, with minimal changes in material structure during charging and discharging;
(4) Rapid insertion and extraction of lithium ions, with high electronic and ionic conductivity;
(5) Good chemical stability in the electrolyte;
(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.
Classification of Cathode Materials
After process optimization and improvement, the gram capacity of lithium cobalt oxide and ternary materials has been significantly increased.
Direction of Cathode Material Development
- High nickel content and high voltage are the trends for ternary materials development.
- With increasing nickel content, thermal stability and capacity retention rate tend to decrease.
Development Trends of LiNixCoyMnzO2
- 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.
- 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.
- 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.
- 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.
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.
xLi2MnO3∙(1-x)LiMO2
Decay Mechanisms
Explanation of the First Charge Platform:
（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)
（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)
（3）Theory of Π 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)
Common questions in laboratory applications
Company introduction
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We have strong research and development capabilities.If you are interested, please feel free to contact us at any time.
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