Recently, big news broke out frequently in the domestic power battery industry. First, SAIC entered the power battery sector and married CATL, the second largest power battery company in China and the third largest in the world, to establish a power battery system company. Later, there was news that BYD would be spun off. The power battery department may open its door to supply to all car companies. There are reports that this move may change the global market structure. Moreover, the domestic electric vehicle market has ranked first in the world in terms of production and sales for two consecutive years, with a cumulative sales of more than 1 million units, accounting for more than 50% of the global market. China has surpassed the United States to take the top spot in the electric vehicle market. It can be said that the electric vehicle industry has unlimited prospects and is developing rapidly. The key lies in the improvement of power battery technology.

The development of electric vehicles requires better batteries. The specific energy, lifespan, safety and price of power batteries are crucial to the development of pure electric vehicles. Among them, lithium-ion batteries with the advantages of high specific energy and long life are currently the most practical electric vehicle batteries and are widely used in hybrid vehicles, pure electric vehicles and fuel cell vehicles. The current technical level of commercial power batteries and the goals expected to be achieved in the next 10 years are shown in Figure 1. However, in actual product production, these indicators are often contradictory, and battery-related performance needs to be weighed and considered. Improving battery performance requires taking into account the performance of electrode materials, electrolytes, and separators. At the same time, the follow-up of assembly technology, battery system grouping, and management technology are also crucial. This article aims to summarize the current development achievements of power batteries with lithium-ion batteries as the core from aspects such as battery material technology, single battery design and manufacturing technology, and battery system technology, while looking forward to the future!

Technical indicators of existing power batteries and development goals for the next 10 years

1. Lithium battery material technology

Positive and negative electrode materials

The cathode and cathode material systems of lithium batteries are very rich (Figure 2). At present, the research on cathode materials such as lithium cobalt oxide, lithium manganate, lithium iron phosphate, and lithium nickel cobalt manganese has become mature. The specific capacity of lithium cobalt oxide material is 200-210mA·h/g. Its material true density and electrode plate compacted density are the highest among existing cathode materials. The charging voltage of commercial lithium cobalt oxide/graphite system can be increased by 4.40V. It can already meet the demand for high-volume energy-density soft-pack batteries for smartphones and tablets. Lithium manganate has low raw material cost, simple production process, high thermal stability, good overcharge resistance, high discharge voltage platform and high safety. It is suitable as a low-cost battery for light electric vehicles, but it has problems such as relatively low theoretical capacity, and the possible dissolution of manganese during the cycle, which affects the life of the battery in high-temperature environments. Domestic lithium manganate materials mainly meet the needs of the mobile power supply, power tools and electric bicycle markets, and have a tendency to develop towards the low end. NCM ternary layered cathode materials are mainly used in power batteries. In addition to LiNi1/3Co1/3Mn1/3O2, which each accounts for 1/3 of nickel, cobalt, and manganese, its application in power batteries is relatively mature. LiNi0.5Co0 with higher capacity .2Mn0.3O2 has also entered batch applications and is generally used in electric vehicle batteries mixed with lithium manganate. The energy density of aluminum-doped lithium nickel cobalt oxide (NCA) can be close to high-voltage lithium cobalt oxide batteries. In recent years, electric vehicle manufacturer Tesla has used this computer battery to drive electric vehicles. The material can also be used with lithium manganate Mixed for the manufacture of vehicle power batteries, domestic NCA precursors have formed stable production capacity, and a few companies have completed the development of NCA cathode materials and are in the process of product promotion. Lithium iron phosphate batteries have high safety and long life. At present, nanoscale power materials and high-density lithium iron manganese phosphate materials are developing rapidly. The performance of high-energy and high-power materials tends to be stable, and the cost is further reduced. Gradually meeting the needs of the domestic market and the promotion of new energy vehicles in China at this stage, high-voltage spinel lithium nickel manganese oxide and high-voltage high-specific capacity lithium-rich manganese-based cathode materials are still under development.

Figure 2 Lithium-ion battery electrode material system

Anode material

The negative electrode materials that can be used in power batteries include graphite, hard/soft carbon and alloy materials. Graphite is currently a widely used negative electrode material, and its reversible capacity can reach 360mA·h/g. Amorphous hard carbon or soft carbon can meet the needs of batteries for higher rate and lower temperature applications and is beginning to be used, but it is mainly mixed with graphite. Lithium titanate anode material has optimal rate performance and cycle performance and is suitable for high-current fast-charging batteries, but the battery produced has low specific energy and high cost. Nano-silicon was proposed to be used in high-capacity anodes in the 1990s. Doping a small amount of nano-silicon to increase the capacity of carbon anode materials is a current research and development hotspot. Anode materials that add a small amount of nano-silicon or silicon oxide have begun to enter small batches. In the application stage, the reversible capacity reaches 450mA·h/g. However, due to the volume expansion caused by lithium being embedded in silicon, the problem of reduced cycle life during actual use needs to be further solved.

electrolyte

The electrolyte of lithium-ion batteries is generally a mixture of cyclic carbonate with high dielectric constant and linear carbonate with low dielectric constant. Generally speaking, the electrolyte of lithium-ion battery should meet the requirements of high ionic conductivity (10-3~10-2S/cm), low electronic conductivity, wide electrochemical window (0~5V), and good thermal stability (-40~60℃ ) and other requirements. Lithium hexafluorophosphate and other new lithium salts, solvent purification, electrolyte preparation, and functional additive technology continue to advance. The current development direction is to further increase its operating voltage and improve the high and low temperature performance of the battery. Safe ionic liquid electrolytes and solid electrolytes are under development.

diaphragm

Polyolefin microporous membranes are currently the main variety in the lithium-ion battery separator market due to their excellent mechanical properties, good electrochemical stability and relatively low cost [7]. Including polyethylene (PE) single-layer film, polypropylene (PP) single-layer film and PP/PE/PP three-layer composite microporous film. There are many manufacturers in China that use the dry process for production, and there are many companies that can mass-produce wet process PE separators. As ceramic coating technology is promoted, high temperature and high voltage resistant diaphragms will become the future research and development direction.

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