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With the increasing demand for lithium battery capacity in electrical equipment, people's expectations for improving the energy density of lithium batteries are becoming higher and higher. Especially for various portable devices such as smartphones, tablets, laptops, etc., higher requirements have been put forward for lithium batteries with small size and long standby time. Similarly, in other electrical devices such as energy storage devices, power tools, electric vehicles, etc., lithium batteries with lighter weight, smaller size, higher output voltage and power density are constantly being developed. Therefore, developing high-energy density lithium batteries is an important research and development direction in the lithium battery industry.

1、 Background of High Voltage Lithium Battery Development

In order to design high-energy density lithium batteries, in addition to continuously optimizing their space utilization, improving the compaction density and gram capacity of the positive and negative electrode materials, and using highly conductive carbon nanoparticles and polymer adhesives to increase the content of active materials in the positive and negative electrodes, increasing the operating voltage of lithium batteries is also an important way to increase battery energy density.

The cut-off voltage of lithium batteries is gradually transitioning from the original 4.2V to 4.35V, 4.4V, 4.45V, 4.5V, and 5V. Among them, 5V nickel manganese lithium batteries have excellent characteristics such as high energy density and high power, and will be one of the important directions for the development of new energy vehicles and energy storage fields in the future. With the continuous development of power supply research and development technology, higher voltage and higher energy density lithium batteries will gradually leave the laboratory and serve consumers in the future.

2、 The current application status of high-voltage lithium batteries

The commonly referred to high-voltage lithium battery refers to a battery with a single charging cut-off voltage higher than 4.2V, such as the lithium battery used in mobile phones, whose cut-off voltage has developed from 4.2V to 4.3V, 4.35V, and then to 4.4V (Xiaomi phones, Huawei phones, etc.). At present, 4.35V and 4.4V lithium batteries have been maturely used 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 batteries for mobile phones and other digital electronic products both domestically and internationally are moving towards high-voltage lithium batteries. Lithium batteries with high voltage and energy density will have a larger market space in high-end mobile phones and portable electronic devices. Positive electrode materials and electrolytes are key materials for improving the high voltage of lithium batteries, among which the use of modified high-voltage lithium cobalt oxide and high-voltage ternary materials will become more mature and widespread.

As the voltage increases, some safety performance of high-voltage lithium batteries may decrease during use, so they have not been widely used in power vehicles yet. At present, the main positive electrode materials used in power vehicles are still ternary materials and lithium iron phosphate. In order to improve energy density and meet the demand, high nickel positive electrode materials such as 811NCM and NCA, high-capacity silicon carbon negative electrode, or increasing the utilization rate of battery space are generally chosen to enhance its energy density and endurance.

3、 Current Status of Important Materials and Processes for High Voltage Lithium Batteries

The performance of high-voltage lithium batteries is determined by the structure and properties of the active material and electrolyte, among which the positive electrode material is the most critical core material, and the matching use of the electrolyte is also very important. The following is an important analysis of the current research and application status of high-voltage positive electrode materials.

1. Research status of high-pressure lithium cobalt oxide materials

The most widely researched and applied high-voltage positive electrode material currently is lithium cobalt oxide, which has a two-dimensional layered structure. Structure, α - NaFeO2 type, more suitable for lithium ion insertion and extraction. The theoretical energy density of lithium cobalt oxide is 274mAh/g, and it has the advantages of simple production process and stable electrochemical properties, resulting in a high market share. In practical applications, only a portion of lithium ions in cobalt oxide materials can be reversibly inserted and removed, with an actual energy density of approximately 167mAh/g (operating voltage of 4.35V). Raising its operating voltage can significantly increase its energy density, for example, increasing the operating voltage from 4.2V to 4.35V can increase its energy density by about 16%. However, multiple insertion and extraction of lithium ions from the material under high voltage can cause the structure of lithium cobalt oxide to transition from the trigonal crystal system to the monoclinic crystal system. At this point, the lithium cobalt oxide material no longer has the ability to insert and extract lithium ions. At the same time, the particles of the positive electrode material become loose and detach from the current collector, resulting in an increase in the internal resistance of the battery and a deterioration in its electrochemical performance.

At present, the modification of lithium cobalt oxide cathode materials is mainly focused on improving the crystal structure stability and interface stability of the material from two aspects: doping and coating.

At present, lithium cobalt oxide high-voltage materials have been widely used in high-energy density batteries. For example, high-end mobile phone battery manufacturers have increasingly high requirements for battery performance, which is mainly reflected in the higher requirements for energy density. For example, 4.35V mobile phone batteries with carbon as the negative electrode require an energy density of about 660Wh/L, and 4.4V mobile phone batteries have reached about 740Wh/L. This requires positive electrode materials to have higher compaction density, higher empty capacity, and better stability in material structure under high voltage and high voltage. However, lithium cobalt oxide electrode materials have disadvantages such as limited cobalt resources and high prices. In addition, cobalt ions have certain toxicity, which limits their wide application in power lithium batteries.

2. Research Status of Ternary Materials

In order to reduce the amount of cobalt used and improve the safety performance of batteries, researchers have begun to focus on the research of layered ternary high-voltage materials (LiNixCoyMn1-x-yO2 or LiNixCoyAl1-x-yO2). In this type of ternary material, nickel (Ni) plays a role in supplying capacity, cobalt (Co) can reduce the mixing of lithium (Li) and Ni, and manganese (Mn) or aluminum (Al) can improve the structural stability of layered materials, thereby enhancing the safety performance of batteries. This type of battery is important for general conventional digital batteries, such as power banks, business backup batteries, etc. It is considered a substitute for lithium cobalt oxide to improve the price competitiveness of batteries, with a nickel cobalt manganese ratio of 5:2:3 being the most common. In the field of power vehicles, many manufacturers are trying to improve energy density by increasing the working voltage of individual lithium batteries and adding nickel content to ternary materials. However, the industry is still in the development stage and there are no bulk products available. This is important because currently, power lithium batteries must first meet the high safety, consistency, low cost, and long lifespan of the battery, and increasing capacity is not the primary issue.

The important issue with ternary materials is that as the nickel content increases, the alkalinity of the material becomes stronger, and the requirements for battery manufacturing processes and environments become increasingly high; At the same time, the thermal stability of the material decreases, and oxygen is released during the cycling process, resulting in a deterioration of the structural stability of the material; In the charging state, nickel has strong oxidizing properties, which also puts higher demands on the compatibility of the electrolyte. So the promotion and use of ternary electrode materials have high limitations.

3. Research Status of Manganese based Positive Electrode Materials

Lithium manganese oxide is a typical spinel structure positive electrode material, with a theoretical energy density of 148mAh/g reported in literature. Its energy density is lower than that of lithium cobalt oxide and ternary materials. It has the characteristics of low price, high thermal stability, environmental friendliness, and easy preparation, and is expected to be widely used in energy storage batteries and power lithium batteries.

In terms of power lithium batteries, lithium manganese oxide is not widely used in China compared to ternary materials and lithium iron phosphate, mainly due to its low energy density and poor cycle life, resulting in short battery range and low service life. The cycling performance of lithium manganese oxide, especially at high temperatures (55 ℃), has been criticized, and its important influencing factors can be divided into three aspects: ① dissolution of surface Mn3+. Due to the fact that the lithium salt currently used in conventional electrolytes is lithium hexafluorophosphate (LiPF6), the electrolyte itself contains a certain amount of hydrofluoric acid (HF) impurities. Trace amounts of water in the battery system can cause the decomposition of LiPF6, resulting in the formation of HF. The presence of HF can corrode lithium manganese oxide (LiMn2O4) and cause the dismutation and dissolution of Mn3+, resulting in 2Mn3+(solid phase) → Mn4+(solid phase)+Mn2+(solution phase). At the end of discharge and under high rate discharge conditions, the Mn3+content on the material surface is higher than that in the bulk phase, which exacerbates the dissolution of Mn3+content on the material surface The Jiang Taylor Effect. During the discharge process of batteries, especially in the case of over discharge, Li1+δ [Mn2] O4 generated on the surface of the material is thermodynamically unstable. At the same time, the material structure undergoes a transition from cubic to tetragonal phase, and the original structure is destroyed, resulting in a deterioration of the material's cycling performance The high oxidation property of Mn4+. In the late stage of charging or overcharging, Mn4+in highly delithiated Li1+δ [Mn2] O4 material exhibits strong

Oxidative properties can oxidize and decompose organic electrolytes, deteriorating the cycling performance of batteries. At present, the vast majority of lithium manganese oxide batteries have an energy density of less than 100mAh/g, and can only achieve 400-500 cycles at room temperature and 100-200 cycles at high temperature, which cannot meet the needs of mass production. But in fact, the battery system of Nissan Leaf, which accounts for nearly 20% of global electric vehicle sales, uses lithium manganese oxide batteries, with a range of about 200km.

Although the performance of lithium manganese oxide batteries is limited by the structure of the material itself, as long as its low energy density and poor cycling performance are addressed, it still has a very broad application space in the field of power lithium batteries in the future.

In order to improve the energy density and cycling performance of lithium manganese oxide electrode materials, some researchers have increased the voltage of positive electrode materials through doping modification, such as LiMxMn2-xO4 [(M=chromium (Cr), iron (Fe), Co, Ni, copper (Cu)] 5V high-voltage positive electrode materials, among which the nickel manganese high-voltage material LiNi0.5Mn1.5O4 has been widely studied. The discharge specific capacity of nickel manganese high-voltage materials is as high as 130mAh/g, with a plateau of about 4.7V. The energy density is higher than that of lithium cobalt oxide under conventional operating voltage, and there is basically no Jangteler effect of Mn3+. When the working voltage is increased to around 5V, nickel manganese high-voltage materials have advantages over traditional lithium cobalt oxide, lithium manganese oxide, ternary and lithium iron, such as high capacity, high discharge platform, high safety performance and rate performance. It has significant advantages in the assembly of battery packs, but its high-temperature performance is poor and its cycling performance still needs improvement. From the current application perspective, it is still in the stage of small-scale production of steel shell batteries, and there is still a long way to go for doping modification and surface coating of nickel manganese high-voltage materials.

4. Research Status of High Voltage Electrolytes

Although high-voltage lithium batteries have made significant contributions to improving the energy density of batteries, they also have many problems. As the energy density increases, the compaction density of the positive and negative electrodes is generally higher, resulting in poorer electrolyte wettability and reduced retention capacity. Low holding capacity can lead to poor cycling and storage performance of the battery. In recent years, with the continuous emergence and application of high-voltage positive electrode materials, conventional carbonate and lithium hexafluorophosphate systems have undergone degradation, poor cycling performance, and poor high-temperature performance in batteries with voltages above 4.5V, which can no longer fully meet the requirements of high-voltage lithium batteries. Therefore, studying the electrolyte system that matches these high-voltage positive electrode materials is of great significance.

In response to the problem of poor electrolyte wettability caused by high voltage density, the electrolyte design is constantly screening solvents with high oxidation potential and low viscosity to meet the performance requirements of high-voltage batteries. In addition, additives or fluorinated solvents that can improve the wettability of the electrolyte are also being used, and the effect is quite significant.

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