Progress and Challenges of Lithium-ion Battery in Electric Vehicle

Recently, Professor TobiasPlacke of the University of Minster in Germany and Professor MartinWinter(communication author) published a synthesis article "Performance and cosofimaterialsforlithium-basedregeablauteotiticebetters" on Natureenergy. This paper reviews the progress and challenges of lithium-ion battery materials in electric vehicles, especially in terms of cost and performance parameters. The production process of positive and negative materials is discussed, and the abundance and cost of materials and the advantages and challenges of different electrolytes to electric vehicles are emphasized. Finally, the energy density and cost of promising chemical batteries are rigorously evaluated, as well as the possibility of achieving electric vehicle propulsion targets.

[Introduction]

The years from 1900 to 1912 were the golden age in the history of electric vehicles. By 1912, the use of electric vehicles in the United States reached 30,000. The power of these electric vehicles was mainly a lead-acid battery (LAB). The battery voltage is about 2V. Due to the poor quality utilization rate and charge-discharge mechanism in LABs, its actual capacity content is only 40 Wh / kg and 90 Wh / l, and Coulomb efficiency and energy efficiency are only 80 % and 70 %, so this type of electric car is Internal combustion engine car replaced. As technology advances and environmental awareness increases, it is essential to reduce vehicle emissions. Electric cars will enter another golden age: in 2016, 160,000 hybrid electric vehicles (PHEVs) will be sold in the United States.

Nickel hydride (NiMH) batteries are the primary choice for hybrid electric vehicles (HEVs). The nominal battery voltage of NiMH batteries is 1.2 V, which can provide a capacity of 80 Wh/kg and 250 Wh/l, but, Its Coulomb efficiency (70 %) and energy efficiency (65 %) are even lower than those of LABs.

Today, PHEVs and electric vehicles (BEVs) use only lithium-ion batteries (LIBs), which offer capacities up to 260Wh/kg and 700Wh/l, as well as higher Coulomb ratios (99%) and energy efficiency (up to 95%). In order to achieve the penetration of the mass market, the US Department of Energy and the Advanced Battery Association estimate that at least 500 kilometers of mileage is required, which is equivalent to the battery capacity of the battery pack needs to reach 235Wh/kg and 500Wh/l, and the battery capacity of the battery unit needs to reach 350Wh/kg. And 750Wh/l, in addition, the cost of the battery pack must be less than 125US$kWh–1.

[Negative material]

Artificial graphite (SGs) and natural graphite (NGs) and amorphous carbon (hard carbon and soft carbon) are more commonly used carbon negative materials. Compared with NGs, SGs have high purity and low volatility. It is usually optimized with a mixture of amorphous and graphed carbon, such as optimizing the ratio of P to E. Currently, in some commercial batteries (such as Panasonic or Hitachi), a small amount of Silicon (mostly SiOx) is added to the carbon electrode to further increase the battery capacity.

In addition, lithium titanate (LTO) is also used in commercial batteries (such as Toshiba's SCiB) due to its low battery voltage (in this case, the formed full-battery voltage) and high power capacity, LTO-based batteries are more suitable for high-power applications, especially in electric buses. Lithium metal is considered to be the most promising negative electrode material in the future, especially in all-solid batteries (ASSBs) using ceramic or polymer electrolytes, which are now used in lithium metal polymer batteries.

Currently, SGs has a market share of 43 per cent and NGs 46 per cent (2016 data), while amorphous carbon accounts for only 7 per cent, which clearly demonstrates the dominance of carbon-based negative polar materials. In contrast, LTO-based and silicon-based negative electrode materials account for only about 2 %.

[Positive material]

Since the commercialization of LIBs, positive Poles have become a bottleneck in the overall capacity of batteries. The key requirements for the positive polar active materials of automotive batteries include: high specific capacity, high discharge potential, high safety, high energy density, rapid battery reaction Kinetics and good stability. At present, the technology is more mature with a layered oxide positive electrode of LiMO2 type, containing transition metals(M) such as nickel, cobalt and manganese (NMC) or nickel, Cobalt and aluminum (NCA) are widely used in automotive battery positive active materials.

Batteries on electric cars

In recent years, the capacity of electric vehicles has continued to increase, and it has been possible to achieve a mileage of 300 km. Due to the market's promotion, a large amount of investment in LIBS research, its energy growth rate is very large. At present, the energy density of cylindrical 18650 batteries can be reached (about 250 Wkg-1 and 670 Wkg-1). In the application of electric vehicles, different shapes of battery structures, such as Prismatic, cylindrical, or bag-like, have been designed and used in specific situations. Most EV batteries are based on graphite negative poles, and recent studies have added a small amount of Silicon to the negative poles. If Silicon can be successfully added to the negative pole, not only will it not shorten the cycle life, but it will further increase the energy density.

At present, NCA, NMC-532 and NMC-622 can be considered the most advanced positive materials, mainly due to their lower volume expansion. lithium iron phosphate (LFP) positive electrode material has been widely used in electric vehicles. The scale of application is currently the largest. This is due to the good stability of LFP and its good cycle life and ratio performance, such as buses and trucks. Has been widely used.

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