The negative electrode material refers to the raw material that constitutes the negative electrode in the battery. The negative electrode of lithium-ion battery is made of negative electrode active material carbon material or non-carbon material, binder and additive to make paste glue, which is evenly spread on both sides of copper foil, dried and rolled. The key to the successful fabrication of lithium-ion batteries lies in the preparation of negative electrode materials that can reversibly de/intercalate lithium ions.
What is Lithium Battery Material?
Lithium battery, also known as a secondary battery, refers to a battery containing lithium metal. The earliest "lithium battery" refers to a disposable battery containing lithium metal. However, due to the extremely high energy density of lithium metal in this type of battery, it was later improved to lithium-ion secondary batteries that can be reused by charging and are widely used in various electronic 3C products.
Pure lithium metal is the best anode material since the invention of lithium batteries because it does not have any inactive weight. However, it is easy to form dendritic lithium dendrites during the charging process, resulting in a short circuit inside the battery, causing serious safety concerns. Both carbon and non-carbon actives have been intensively researched towards high performance in recent years, and their characteristics also have a significant impact on battery energy density, especially in the electric vehicle market. Based on the principle of electrochemical energy storage, innovative anode materials have been developed.
Lithium-ion secondary batteries achieve the purpose of storing and discharging electricity by migrating lithium ions between the positive electrode and the negative electrode, respectively, and the negative electrode also plays the role of storing and releasing lithium ions. The ideal anode material has low redox potential, high capacitance (mAh/g or mAh/mL), a stable potential platform, and high safety. The negative electrode material with low redox potential and the positive electrode material with high redox potential can obtain higher energy (Wh) under the same capacitance (Ah). High capacitance can use a smaller amount to meet the positive and negative capacitance ratio requirements and indirectly increase the energy density (Wh/kg).
According to the reaction mechanism of the negative electrode material, it can be divided into intercalation, conversion, and alloy. The capacitance of the negative electrode material is generally higher than that of the positive electrode (130~250 mAh/g), so the development of electrode materials for batteries has been dominated by positive electrode materials for a long time. Although increasing the specific capacitance of the anode material has no direct relationship to the battery's power storage (Wh), it can reduce the weight used and indirectly increase the battery's energy density.
An ideal lithium-ion battery anode should meet the following requirements:
- Has high reversible weight and volume capacity.
- Have the lowest potential for the positive electrode material.
- High-rate charging capability.
- Long cycle life.
- Low cost.
- Excellent anti-abuse ability.
- Environmental compatibility.
Lithium Battery Material Introduction:
The main components of lithium batteries include four major parts: positive electrode material, negative electrode material, electrolyte, and separator. The positive electrode material accounts for 34%, the electrolyte 16%, the separator is about 21%, and the negative electrode material is 15%. The negative electrode material is One of the key factors that determine the performance of lithium batteries.
- Positive electrode material:
Generally, LiMn2O4, LiFePO4, and LiNiCOO2 are used as the main materials, and conductive agents and resin binders are added to the positive active material, and then coated on the aluminum substrate in a thin layer. In terms of structure, lithium cobalt oxide and lithium nickel oxide have very similar structures, while lithium manganese oxide is similar to spinel structure, and has better structural stability under discharge. In terms of advantages and disadvantages, lithium cobalt is the most common, but there is a lack of raw material sources. Lithium nickel has the highest gravimetric energy density, but poor safety. Lithium manganese is the cheapest, but its energy density and high-temperature thermal stability are poor. In addition, lithium iron phosphate also has the main advantages of lithium cobalt, lithium nickel, and lithium manganese, but does not contain precious elements such as cobalt. It has the advantages of low cost, non-toxicity, high power, and high capacity, and meets the demands of safety and environmental protection, has recently become the mainstream material.
- Negative electrode material:
Mainly based on carbon materials, it is divided into graphite series and coke series. Graphite series has high gravimetric energy density and the structure of the material itself has regularity. The negative electrode material has a high irreversible capacitance in the first charge-discharge reaction, but this material can be charged and discharged at a higher C-rate, and the discharge curve of this material is relatively sloping, which is conducive to using voltage to monitor the battery consumption of capacity.
Anode materials are mainly divided into carbon series and non-carbon series. Graphite, artificial graphite, mesophase carbon microspheres, etc. belong to the carbon series. Silicon materials can meet the demand for higher energy density (theoretical gram capacity is 4200mAh/g). With the increase in energy density requirements of power batteries, the development of high nickel ternary batteries is conducive to the popularization of commercial mass production. At present, a new generation of High-capacity silicon series anode materials mainly includes silicon oxide (SiO), silicon carbon, and silicon-based alloys.
- Isolation diaphragm:
Placed in the positive and negative plates, it is a microporous and porous film. The material is mainly PP and PE. Its function is to close or block the channel. It is used to isolate the positive and negative plates, prevent short circuits, and make the ions pass through and have the function of maintaining the electrolyte. The so-called closing or blocking function is that the abnormal temperature rises of the battery blocks or blocks the pores as ion channels so that the battery stops the charging and discharging reaction. The separator can effectively prevent the abnormal heating of the battery caused by excessive current caused by an external short circuit. The separator is divided into nonwoven fiber mats, microporous polymeric membranes, and inorganic composite membranes.
- Nonwoven fiber mats: Made of natural or synthetic fibers, usually with a porosity of 60%~80%, a pore size of 20~50um, and a thickness of 100~200um, the diameter of the fiber determines the film thickness and surface flatness, if the fiber diameter is close to the thickness, there can only be one layer of fibers. When two or more such fibers are adjacent to each other, there may be regional open spaces in the structure, which will not be able to effectively prevent the short circuit of the positive and negative electrodes. Currently used in nickel-cadmium, nickel-metal hydride batteries.
- Microporous polymeric membranes: The porosity is about 40% and the film thickness is about 20um. When the battery is abnormally high temperature, the porous polymer film starts at the softening point temperature due to the density difference between the crystalline state and the amorphous state. Shrinkage is generally used in commercial lithium batteries.
- Inorganic composite membranes: Made of nano-particle inorganic metal oxides, combined with Sol-Gel technology on non-woven fiber mats, with excellent thermal stability and dimensional stability, mainly used for large lithium batteries. Such as electric vehicles and power tools.
The function of the electrolyte is to conduct lithium ions and isolate the positive and negative electrodes from direct contact. To dissolve the lithium salt of the electrolyte component, it must have a high permittivity and a solvent with good compatibility with lithium ions, that is, a low-viscosity organic solution that does not hinder the movement of ions. Within the range, it must be in a liquid state, with a low freezing point and a high boiling point. Electrolyte refers to an ionic conductor that can move ions when a chemical reaction occurs between the positive and negative electrodes and is mainly responsible for the conduction of ions in the entire electrochemical reaction. Electrolytes are further divided into liquid electrolytes, polymer electrolytes, and solid electrolytes. Currently, liquid electrolytes and polymer electrolytes can be commercialized and are mainly used in 3C products. As for solid electrolytes, they are still in the experimental stage. Lithium hexafluorophosphate is the core raw material of the electrolyte, accounting for about 50% of the cost of the electrolyte.
- Safety valve:
To ensure the safety of the use of lithium-ion batteries, a safety device that cuts off abnormal current is generally provided through the control of external circuits or inside the battery. The safety valve is a one-time non-repairable ruptured membrane. Once it enters the working state, it protects the battery and stops it from working, so it is the last means of protection for the battery.
The Development of Lithium Batteries:
At present, the biggest controversy about lithium batteries is their stable safety. The problem is caused by the increase of the internal temperature of the battery, including improper heating of the battery, overcharging, a short circuit caused by the contact between the positive and negative materials, etc. When the battery is dropped or collided, it is easy to cause a short circuit. In addition, the long-term violent vibration of the battery will also affect the battery. Do not disassemble the battery pack at will, especially the soft-packed battery, which is easy to cause internal damage during disassembly.
When the internal temperature of the battery continues to rise and cannot be suppressed, the separator used to separate the positive and negative materials will begin to melt and break, resulting in a large amount of current short circuit, and then the battery will heat up rapidly, and the positive electrode will be triggered when the temperature rises to 180°C. The material decomposes and generates a lot of heat, which makes the temperature of the battery rise sharply in an instant, and finally produces a thermal explosion, ejecting a large amount of gas, causing dangers. Such as combustion and explosion, and the activation of energy released by the positive electrode is the key to the safety of the entire battery. Therefore, lithium iron phosphate or ternary nickel-manganese-cobalt series are used, because lithium iron phosphate with an olivine structure has strong structural properties. And it is not easy to cause crystal damage when encountering voltage or excessive ambient temperature. At the same time, phosphate groups bound by ionic bonds also have the advantage that they are not easily broken to generate oxygen, but the low charge-discharge platform is one of the disadvantages.
Research Status and the Prospect of Anode Technology for High-energy Battery:
In recent years, to realize the practical application of Li metal anode, while considering the high cost of limited excess Li metal foil as an anode, and the serious safety hazard of a large amount or even excess Li in Li metal batteries (LMB), Anode-free Lithium Metal Battery (AFLMB). It is new hope for creating next-generation lithium-ion secondary batteries at low cost while maintaining high energy density.
At present, the energy density of commercial lithium-ion batteries has appeared a technical bottleneck, and the highest energy density specification achieved by existing commercial materials is less than 300 Wh/kg. The specific capacitance of practical anode materials is higher than that of cathode materials. Only by developing cathode materials with higher specific capacitance, supplemented by metal alloy anode materials to reduce weight, can there be a chance to create a breakthrough of more than 1.5 times the existing specifications. In addition, the biggest challenges to commercialization are still battery life and safety.