With the continuous improvement of the capacity requirements of lithium-ion batteries by electrical equipment, people expect more and more energy density of lithium ion batteries. In particular, portable devices such as smart phones, tablet computers and notebook computers have put forward higher requirements for the lithium ion batteries with small size and long standby time.
With the continuous improvement of the capacity requirements of lithium-ion batteries by electrical equipment, people expect more and more energy density of lithium ion batteries. In particular, portable devices such as smart phones, tablet computers and notebook computers have put forward higher requirements for the lithium ion batteries with small size and long standby time. The development of lithium ion batteries with light quality, smaller volume, higher output voltage and higher power density is also being developed in other electrical equipment, such as energy storage equipment, electric tools, electric vehicles and so on. Therefore, the development of lithium ion batteries with high energy density is the important research and development direction of the lithium battery industry.
The background of the development of a high voltage lithium ion battery
In order to design a lithium ion battery with high energy density, in addition to the continuous optimization of its space utilization, the density and capacity of the positive and negative electrode materials are improved. The high conductive carbon nanometers and polymer adhesives are used to improve the positive and negative active substances. The working voltage of the lithium ion battery is also increased. One of the important ways of energy density.
The cut-off voltage of lithium ion batteries is gradually transition from the original 4.2V to 4.35V, 4.4V, 4.45V, 4.5V and 5V, of which 5V Ni Mn Li ion batteries have excellent characteristics such as high energy density and high power, which will be one of the important directions in the future development of new energy vehicles and energy storage fields. With the continuous development of power R & D technology, the future lithium ion batteries with higher voltage and higher energy density will gradually come out of the lab and serve consumers.
Application status of two high voltage lithium-ion batteries
Generally speaking, the high voltage lithium ion battery is the battery with the cut-off voltage of the monomer charging higher than 4.2V, for example, the lithium ion battery used on the cell phone, the cut-off voltage is developed from 4.2V to 4.3V, 4.35V, and then to the 4.4V (millet cell phone, HUAWEI cell phone, etc.). At present, 4.35V and 4.4V lithium-ion batteries have been mature in the market, and 4.45V and 4.5V have begun to be favored by the market, and will gradually mature.
At present, domestic and foreign manufacturers of mobile phones and other digital electronic products are moving towards the direction of high voltage lithium ion batteries. Lithium ion batteries with high voltage and high energy density will have more market space in high-end mobile phones and portable electronic devices. Cathode materials and electrolyte are the key materials to improve the high voltage of lithium ion batteries. The use of modified high voltage lithium cobalt and high voltage three yuan materials will be more mature and universal.
With the increase of voltage, some high voltage lithium ion batteries will reduce their safety performance during use, so they are not yet used in power cars. At present, battery cathode materials used in power cars are mainly composed of three yuan material and lithium iron phosphate. In order to improve the energy density to meet demand, the energy density and endurance of high nickel cathode materials such as 811NCM and NCA, high capacity silicon carbon negative or increase of battery space utilization are generally chosen.
Progress in the main materials and technology of three high voltage lithium ion batteries
The performance of high voltage lithium-ion batteries is mainly determined by the structure and properties of active materials and electrolytes. The positive material is the key core material, and the matching effect of electrolyte is also very important. The following is the analysis of the current research and application of high voltage cathode materials.
1, research status of high pressure lithium cobaltate materials
At present, the most widely used high-voltage cathode material is lithium cobalt oxide, which has two dimensional lamellar structure. The structure, alpha -NaFeO2 type, is more suitable for the insertion and removal of lithium ions. The theoretical energy density 274mAh/g of lithium cobalt oxide has the advantages of simple production process and stable electrochemical properties, so the market share is relatively high. In practical applications, only part of the lithium ion can be reversibly embedded and removed, and its actual energy density is about 167mAh/g (working voltage 4.35V). Raising its working voltage can significantly increase its energy density, such as raising the working voltage from 4.2V to 4.35V, and its energy density can be increased by about 16%.
However, at high voltage, the lithium ion is embedded and removed from the material many times, which will change the structure of lithium cobaltate from three square crystal system to monoclinic system. At this time, lithium cobaltate no longer has the ability to embed and remove lithium ion, while the particles of the cathode material loose and fall off the fluid, causing the battery internal resistance to become larger. The electrochemical performance is worse.
At present, the modification of lithium cobaltate cathode material mainly improves the crystal structure stability and interface stability from 2 aspects: doping and coating.
At present, lithium cobalt high voltage materials have been used in high energy density batteries, such as the higher demand for battery performance by high-end cell phone battery manufacturers, which is mainly reflected in the higher demand for energy density. For example, the energy density of 4.35V cell phone battery with carbon as negative electrode needs to be around 660Wh/L, and 4.4V cell phone battery has reached At about 740Wh/L, this requires that the cathode materials have higher compaction density, higher air volume, and better stability in the material structure under high pressure and high voltage. But lithium cobalt electrode materials have some disadvantages, such as lack of cobalt resources and high price. In addition, cobalt ions have some toxicity. These defects restrict their wide application in power batteries.
Research status of 2 and three yuan materials
In order to reduce the amount of cobalt and improve the safety performance of the battery, the researchers began to work on the study of the layered three element high voltage materials (LiNixCoyMn1-x-yO2 or LiNixCoyAl1-x-yO2). In this kind of three element material, the nickel (Ni) element plays a role in providing capacity, and cobalt (Co) can reduce the mixing of lithium (Li) and Ni. Manganese (Mn) or aluminum (Al) can improve the structural stability of the layered material, thus improving the safety performance of the battery. This kind of battery is mainly used in general conventional digital batteries, such as charging treasure, business standby battery and so on. It is regarded as a substitute for lithium cobalt, and the price competitiveness of the battery is improved. The ratio of nickel and cobalt to manganese is 5: 2: 3 is the most common.
In the power car, many manufacturers are trying to improve the energy density, which is mainly to increase the working voltage of the single lithium ion battery and increase the nickel content in three yuan material, but at present, the industry is still in the development stage, and there is no batch product. This is mainly due to the high safety, consistency, low cost and long life of batteries.
The main problem of the three element material is that with the increase of nickel content, the alkalinity of the material becomes stronger and the requirement for the process and environment of the battery is getting higher and higher. At the same time, the thermal stability of the material is reduced and the oxygen is released during the cycle process, which causes the structural stability of the material to be worse. The matching of solution is also higher. Therefore, the three electrode materials have higher limitations in popularization and application.
3, research status of manganese based cathode materials
Lithium manganate is a typical spinel type positive material. The theoretical energy density is 148mAh/g. The energy density of lithium manganese dioxide is lower than lithium cobalt and three yuan material. It has the advantages of cheap price, high thermal stability, friendly environment and easy preparation. It is expected to be widely used in energy storage batteries and dynamic batteries.
In the power battery, the application of lithium manganate in the domestic comparison of three yuan material and lithium phosphate lithium is not wide enough, mainly limited to its low energy density and poor cycle life, resulting in short range of battery life and low service life. The cyclic performance of lithium manganate, especially at high temperature (55 C), has been criticized. Its main influencing factors are divided into 3 aspects:
(1) the dissolution of the surface Mn3+. Due to the current lithium salts of six fluoro phosphate (LiPF6) used in the conventional electrolyte, the electrolyte itself contains a certain amount of hydrofluoric acid (HF) impurities. The trace water in the battery system will lead to the decomposition of LiPF6 to produce HF. The existence of HF will corrode the lithium manganese acid (LiMn2O4) and cause the dissolving of Mn3+, 2Mn3+ (Gu Xiang) to Mn4+ ( Solid phase) +Mn2+ (Rong Yexiang). The Mn3+ content of the surface of the material is higher than that of the bulk phase at the end of discharge and high rate discharge, which aggravates the dissolution of Mn3+ on the surface of the material.
(2) Taylor effect of ginger. In the process of battery discharge, especially in the case of over discharge, the Li1+ [Mn2] O4 produced on the surface of the material is unstable, and the structure of the material is transformed from cubic phase to the Quartet phase, and the original structure is destroyed, so the cycle performance of the material becomes worse.
(3) high oxidation of Mn4+. At the end of charging or overcharging, the Mn4+ of highly depolarized Li1+ Delta [Mn2] O4 material is stronger.
Oxidation, which can oxidize and decompose organic electrolyte, will deteriorate the cycle performance of batteries. At present, the energy density of most lithium manganese dioxide batteries is less than 100mAh/g, only 400~500 times at normal temperature cycle and 100~200 times in high temperature cycle, which can not meet the demand of production. But in fact, the battery system of Nissan, which accounts for nearly 20% of the global electric vehicle sales, is the lithium manganese dioxide battery, which has a range of about 200km.
Although the performance of lithium manganate battery is restricted by the structure of the material itself, as long as it solves the disadvantages of low energy density and poor cycling performance, it still has a very wide application space in the future power battery field.
In order to improve the energy density and cycling performance of lithium manganese dioxide electrode materials, some researchers improve the voltage of cathode materials by doping modification, such as LiMxMn2-xO4 [(M= chromium (Cr), Fe, Co, Ni, copper (Cu)] 5V high voltage cathode material, of which the study of nickel manganese high pressure material LiNi0.5Mn1.5O4 is the most extensive. . The discharge capacity of Ni Mn high pressure material is up to 130mAh/g, the platform can reach about 4.7V, the energy density is higher than the energy density of lithium cobaltate under the conventional working voltage, and there is no Mn3+ effect of ginger Taylor.
When the working voltage is raised to about 5V, the nickel and manganese high pressure material is compared with the traditional lithium cobalt carbonate, lithium manganate, three yuan and iron lithium. It has the advantages of high gram capacity, high discharge platform, high safety performance and high performance ratio. It has a great advantage in battery pack allocation, but its high temperature performance and cycle performance need to be improved. From now on, it is still only in the small batch production stage of the steel shell battery, the doping modification of the Ni Mn high pressure material and the surface coating work still have a long way to go.
4. Research status of high voltage electrolyte
Although high voltage lithium ion battery has great contribution in improving battery energy density, there are still many problems. With the increase of the energy density, the density of the positive and negative poles is relatively large, the wettability of the electrolyte is poor, and the liquid content is reduced. The minimum guaranteed liquid volume will lead to poor battery cycling and storage performance. In recent years, with the continuous emergence and application of high voltage cathode materials, conventional carbonate and lithium six fluoro phosphate system will be decomposed in voltage batteries above 4.5V, poor cycling performance, poor performance of high temperature performance and so on, which can not fully meet the requirements of high voltage lithium ion electric pool. Therefore, it is very important to study the electrolyte system that matches these high voltage cathode materials.
In view of the poor wettability of electrolyte caused by high pressure density, the electrolyte design is constantly screening solvents with high oxidation potential and small viscosity to meet the performance requirements of high voltage solid batteries. In addition, additives or fluorine solvents which can improve the wettability of electrolyte are also used to improve the effect.