With the increasing requirements for the capacity of lithium-ion batteries for electrical equipment, there is a growing expectation for the increase in energy density of lithium-ion batteries. In particular, various portable devices such as smart phones, tablet computers, and notebook computers have put forward higher requirements for lithium ion batteries with small size and long standby time. Also in other electrical equipment, such as energy storage equipment, power tools, electric vehicles, etc., we are constantly developing lithium-ion batteries with lighter weight, smaller size, higher output voltage and higher power density, so we develop high energy density. Lithium-ion batteries are an important research and development direction for the lithium battery industry.
Background for the development of a high voltage lithium ion batteryIn order to design high-energy density lithium-ion batteries, in addition to continuous optimization of space utilization, improve the compaction density and gram capacity of the battery positive and negative materials, use high-conductivity carbon nano-polymer and polymer binder to improve the positive and negative electrodes. In addition to the active substance content, increasing the operating voltage of a lithium ion battery is also one of the important ways to increase the energy density of the battery.
The cut-off voltage of the lithium-ion battery is gradually transitioning from the original 4.2V to 4.35V, 4.4V, 4.45V, 4.5V and 5V. Among them, the 5V nickel-manganese lithium-ion battery has excellent characteristics such as high energy density and high power. One of the important directions for the development of new energy vehicles and energy storage in the future. With the continuous development of power supply R&D technology, lithium-ion batteries with higher voltage and higher energy density will gradually step out of the laboratory and serve consumers.
Application status of two high voltage lithium ion batteriesGenerally speaking, a high-voltage lithium-ion battery refers to a battery with a charge cut-off voltage higher than 4.2V. For example, a lithium-ion battery used on a mobile phone has a cut-off voltage of 4.2V to 4.3V, 4.35V, and then 4.4V. (Millet mobile phone, Huawei mobile phone, etc.). At present, 4.35V and 4.4V lithium-ion batteries have been matured in the market, and 4.45V and 4.5V have also begun to be favored by the market, and will gradually mature.
At present, manufacturers of mobile phones and other digital electronic products at home and abroad are moving in the direction of high-voltage lithium-ion batteries. High-voltage and high-energy-density lithium-ion batteries will have more market space in high-end mobile phones and portable electronic devices. Cathode materials and electrolytes are key materials for improving the high voltage of lithium ion batteries. The use of modified high voltage lithium cobalt oxide and high voltage ternary materials will be more mature and common.
High-voltage lithium-ion batteries have reduced safety performance during use, so they are not used in power vehicles. At present, the battery cathode materials used in power vehicles are mainly ternary materials and lithium iron phosphate. In order to improve the energy density to meet the demand, generally choose high-nickel cathode materials such as 811NCM and NCA, high-capacity silicon-carbon anodes or improve the utilization of battery space to improve its energy density and endurance.
The performance of high-voltage lithium-ion batteries is mainly determined by the structure and properties of active materials and electrolytes. The positive electrode material is the most critical core material, and the matching effect of electrolyte is also very important. The following mainly analyzes the current research and application status of high voltage cathode materials.
1. Research status of high pressure lithium cobalt oxide materialsThe most widely studied high-voltage cathode material currently in use is lithium cobaltate, which has a two-dimensional layer. The structure, α-NaFeO2 type, is more suitable for the insertion and extraction of lithium ions. The theoretical energy density of lithium cobaltate is 274 mAh/g, which has the advantages of simple production process and stable electrochemical properties, so the market share is high. In practical applications, only a part of lithium ions can be reversibly embedded and extracted, and the actual energy density is about 167 mAh/g (operating voltage is 4.35 V). Increasing the operating voltage can significantly increase its energy density. For example, increasing the operating voltage from 4.2V to 4.35V can increase the energy density by about 16%.
However, when lithium ions are intercalated and removed from the material at high voltage, the structure of lithium cobaltate changes from trigonal to monoclinic. At this time, the lithium cobaltate material no longer has the ability to intercalate and deduct lithium ions. At the same time, the particles of the positive electrode material are loosened and fall off from the current collector, resulting in an increase in internal resistance of the battery and deterioration in electrochemical performance.
At present, the modification of lithium cobaltate cathode material mainly improves the crystal structure stability and interface stability of the material from the aspects of doping and coating.
At present, lithium cobalt oxide high-voltage materials have been used in batches in high-energy-density batteries. For example, high-end mobile phone battery manufacturers have higher and higher requirements on battery performance, which are mainly reflected in higher requirements on energy density, such as carbon as a negative electrode. The energy density of 4.35V mobile phone battery is about 660Wh/L, and the 4.4V mobile phone battery has reached 740Wh/L, which requires the cathode material to have higher compaction density, higher air volume, and high voltage. The material structure at high voltage has better stability. However, lithium cobalt oxide electrode materials have the disadvantages of lack of cobalt resources and high price. In addition, cobalt ions have certain toxicity, and these defects limit their wide application in power batteries.
2. Research status of ternary materialsIn order to reduce the amount of cobalt and improve the safety of the battery, researchers began to work on layered ternary high voltage materials (LiNixCoyMn1-x-yO2 or LiNixCoyAl1-x-yO2). In this type of ternary material, nickel (Ni) element plays a role in providing capacity, cobalt (Co) can reduce the mixing of lithium (Li) and Ni, and manganese (Mn) or aluminum (Al) can improve the layered material. The structural stability improves the safety of the battery. This type of battery is mainly used for general conventional digital batteries, such as: charging treasure, business backup battery, etc., as a substitute for lithium cobalt oxide, to improve the price competitiveness of the battery, the ratio of nickel-cobalt-manganese is 5:2:3 common.
In the power car, there are many manufacturers in trial use. The way to increase the energy density is mainly to increase the working voltage of the single-cell lithium-ion battery and increase the nickel content in the ternary material. However, the industry is still in the development stage, and there is no batch. product. This is mainly because the current power battery must first meet the high safety, consistency, low cost and long life of the battery. The increase of capacity is not the primary problem.
The main problem of ternary materials is that as the nickel content increases, the alkalinity of the material becomes stronger, and the requirements for the battery manufacturing process and environment are higher and higher; at the same time, the thermal stability of the material is lowered, and oxygen is released during the cycle. The structural stability of the material is deteriorated; in the state of charge, nickel has strong oxidizing property, and higher requirements are placed on the matching of the electrolyte. Therefore, the ternary electrode material has higher limitations in promotion and use.
3. Research status of manganese-based cathode materialsLithium manganate is a typical spinel-type positive electrode material. The theoretical energy density is 148 mAh/g, and its energy density is lower than that of lithium cobaltate and ternary materials. It has low cost, high thermal stability and environmental friendliness. It is easy to prepare and so on, and it is expected to be applied on a large scale in energy storage batteries and power batteries.
On the power battery, the application of lithium manganate in domestic comparison with ternary materials and lithium iron phosphate is not wide enough, mainly due to its shortcomings of low energy density and poor cycle life, resulting in short battery life and long service life. Low problem. The cycle performance of lithium manganate, especially the high temperature (55 ° C) cycle performance has been criticized, the main influencing factors are divided into three aspects:
1 Surface Mn3+ dissolution. Since the lithium salt used in the conventional electrolyte is lithium hexafluorophosphate (LiPF6), the electrolyte itself contains a certain amount of hydrofluoric acid (HF) impurities, and trace amounts of water in the battery system may cause decomposition of LiPF6 to produce HF, and the presence of HF may erode. Lithium manganate (LiMn2O4) causes disproportionation and dissolution of Mn3+, 2Mn3+ (solid phase)→Mn4+ (solid phase)+Mn2+ (solution phase). At the end of discharge and large rate discharge conditions, the Mn3+ content on the surface of the material is higher than that of the bulk phase, which aggravates the dissolution of Mn3+ on the surface of the material.
2 ginger Taylor effect. During the discharge process of the battery, especially in the case of overdischarge, Li1+δ[Mn2]O4 formed on the surface of the material is thermodynamically unstable, and the material structure changes from cubic to tetragonal, and the original structure is destroyed. The cycle performance of the material deteriorates.
High oxidizability of 3Mn4+. Mn4+ in the highly delithiated Li1+δ[Mn2]O4 material is strong at the end of charge or overcharge
Oxidation, which can oxidize and decompose organic electrolytes, and deteriorate the cycle performance of the battery. At present, most lithium manganate batteries have an energy density of less than 100 mAh/g, a normal temperature cycle of only 400 to 500 times, and a high temperature cycle of only 100 to 200 times, which cannot meet the mass production requirements. But in fact, Nissan's battery system, which accounts for nearly 20% of global electric vehicle sales, is a lithium manganate battery that has a cruising range of about 200km.
Although the performance of the lithium manganate battery is limited by the structure of the material itself, as long as it has the disadvantages of low energy density and poor cycle performance, it still has a very broad application space in the field of power batteries in the future.
In order to improve the energy density and cycle performance of lithium manganate electrode materials, some researchers have increased the voltage of the cathode material by doping modification, such as LiMxMn2-xO4 [(M=chromium (Cr), iron (Fe), Co, Ni, copper (Cu)] 5V high voltage cathode material, among which nickel-manganese high-pressure material LiNi0.5Mn1.5O4 is the most widely studied. Nickel-manganese high-pressure material has a specific discharge capacity of 130mAh/g, platform can reach 4.7V, and energy density is high. The energy density of lithium cobaltate at normal operating voltages, and substantially no ginger Taylor effect of Mn3+.
When the working voltage is increased to about 5V, the nickel-manganese high-pressure material has the advantages of high gram capacity, high discharge platform, high safety performance and high rate performance compared with traditional lithium cobaltate, lithium manganate, ternary and iron-lithium. It has great advantages in the assembly of the battery pack, but its high temperature performance and cycleability still need to be improved. From the current application, it is still only in the small-scale production stage of steel shell batteries, and the doping modification and surface coating work of nickel-manganese high-pressure materials still have a long way to go.
4. Research status of high voltage electrolyteAlthough high-voltage lithium-ion batteries have contributed greatly to increasing the energy density of batteries, there are still many problems. As the energy density increases, the compaction density of the positive and negative electrodes is generally large, the electrolyte wettability is deteriorated, and the liquid retention amount is lowered. Low fluid retention can result in poor battery cycling and storage performance. In recent years, with the continuous emergence and application of high-voltage cathode materials, conventional carbonate and lithium hexafluorophosphate systems have decomposed in voltage batteries above 4.5V, poor cycle performance, poor high-temperature performance and other battery performance, which can not fully meet the high requirements. Requirements for voltage lithium-ion batteries. Therefore, it is of great significance to study the electrolyte system matching these high-voltage cathode materials.
In view of the problem of poor wettability of electrolyte caused by high-pressure solid density, the electrolyte design is constantly screening for solvents with high oxidation potential and low viscosity to meet the performance requirements of high-voltage batteries. In addition, it is also improved by using an additive or a fluorinated solvent which can improve the wettability of the electrolyte, and the effect is also remarkable.
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