Performance of batteries used in electric and hybrid vehicles

Performance of batteries used in electric and hybrid vehicles

Ni-MH batteries with high power and high energy density are being widely used by heavy vehicles, such as buses. These rechargeable batteries exhibit significant electrical performance and economic advantages.

1. Nickel metal hydride battery packs used in electric and hybrid electric vehicles
According to published automotive articles, nickel metal hydride battery technology has been recognized by almost all major car manufacturers as the highest performance, most reliable and cost-effective energy storage technology for electric vehicles and hybrid electric vehicles. According to Ovonic Battery Company, nickel metal hydride battery technology is highly reliable, maintenance-free and cost-effective, and has the ability to accept peak regenerative braking currents, making it an ideal solution for heavy-duty vehicles. More projects are being developed to achieve significant reductions in weight, size and manufacturing costs. Once these reductions are achieved, nickel metal hydride batteries will be recognized as the only battery supply for electric vehicles and hybrid electric vehicles.
Ovonic has designed and developed nickel metal hydride battery packs to meet the performance requirements of electric vehicles and hybrid electric vehicles from General Motors and other automobile companies. In addition, Ovonic is developing advanced nickel metal hydride batteries with a rated power close to 550W/kg, which is most suitable for hybrid electric buses and trucks. The module-level preliminary life cycle test using a simulated school bus has proved that the cycle life of a module exceeds 1,000 times, which is equivalent to more than 100,000 miles (1 mile=1.609344km) or three years of bus operation. Test data shows that a 108kw·h battery pack composed of nickel metal hydride batteries can provide more than 100 miles of bus service per charge, which proves to be more than 25% higher than lead-acid batteries. Note that two, four or more batteries can be connected in series to meet the voltage and power capacity requirements of a given application.

2. Lithium battery packs and battery systems used in electric and hybrid electric vehicles
In the past 10 years, materials scientists and battery designers have been paying attention to lithium-based battery technology. Materials scientists claim that several lithium-ion batteries exhibit high-quality energy densities exceeding 400w·h/kg, as listed in Table 1. The displayed mass energy density value is sufficient for small electric vehicles and hybrid electric vehicles.

Battery systemsymbolMass energy density/(W.h/kg)
Lithium OxideLiSO21175
Lithium Copper ChlorideLi-CuCl21135
Lithium vanadium oxideLi-V6O13870
Lithium Titanium SulfideLi-TiS2562
Lithium Manganese OxideLi-MnO2432
Table 1 Theoretical mass energy density of lithium-ion rechargeable batteries

There are other lithium-based batteries, but their energy density is not suitable for current electric vehicles and hybrid vehicles. If continue to actively carry out other edge battery research and development activities, this battery will join the ranks of lithium-based batteries in the near future.

Performance of batteries used in electric and hybrid vehicles
lithium-ion battery pack

3. Rechargeable lithium-based thermal battery
Lithium-based thermal batteries have molten salt electrolytes that have excellent performance at temperatures higher than room temperature. In addition to lithium metal polymers, these batteries use lithium or sodium-based negative electrodes. The positive electrode is generally made by loading a mixture of iron sulfide (FeS) and electrolyte into a current collector or packing a material into a honeycomb matrix. Graphite, cobalt sulfide and nickel sulfide are sometimes used in the matrix structure to enhance performance. The battery designer has conducted research on the road traffic application of lithium aluminum iron sulfide (Li-AI/FeS and Li-AI/FeS2) thermal batteries. Although Li-AI/FeS2 provides higher battery voltage and improved electronic conductivity, it has corrosion problems. The use of thicker electrodes can solve the corrosion problem, but slightly increases the manufacturing cost. In a bipolar battery configuration, the positive electrode and the negative electrode are electrically contacted back to back through Mg-O separators or conductive channels. These batteries can meet mass energy density requirements ranging from (80~100) w·h/kg to 200 w·h/kg. Japan has designed and developed large-capacity sodium-sulfur (Na-S) batteries for stationary energy storage applications. These batteries are most suitable for stationary energy sources. Research and development on the application of traction devices is currently being carried out cautiously. The polymer electrolyte that may be used in electric vehicles and hybrid electric vehicles is currently under study.
The sodium nickel chloride used in electric vehicles is the ZEBRA battery. The negative electrode and electrolyte of this particular battery are similar to those used in Na-S batteries, but a metal chloride such as nickel chloride NiCl2 is used instead of sulfur as the positive electrode. This special ZEBRA (Zero Emission Battery Research Activity) was developed by South African battery designers for electric vehicle applications. This battery provides a higher battery voltage of 2.58V. Its resistance to overcharge and overdischarge is remarkable. The battery structure allows several cells to be connected in series, not in parallel, because the battery imbalance is leveled by the electrochemical reaction in the battery. This battery system provides a wide operating temperature range, enhanced safety, excellent design flexibility, high cycle life and low corrosion. According to its design and development activities specifically for electric vehicles and hybrid electric vehicles, ZEBRA batteries are the most cost-effective and ideal choice for electric vehicles.

Materials scientists describe the evolution of polymer electrolytes in three different stages:
①The first generation electrolyte is based on the combination of high molecular weight polyethylene oxide (PEO) polymer matrix material and lithium salt;
②The second generation electrolyte is based on the combination of modified PEO structure and lithium salt;
③The third-generation electrolyte is formed by capturing a low-molecular-weight lithium salt solution in an organic solvent of high-molecular-weight substances.

The electrolyte can be made in the form of a solid film, thereby eliminating the need for separator elements. The combination of a very thin electrolyte with a thin electrode structure allows high-speed performance of the electrode and improves the eternal morphology of lithium. This possibility of greater inherent safety combined with improved rate capability makes polymer electrolyte battery systems a viable option for high-performance batteries. The main advantages of polymer electrolyte batteries can be summarized as follows.
①Stable electrolyte materials provide non-volatile, solid materials.
②The wide electrochemical window allows the cathode with high energy capacity.
③Low electrode load produces better lithium cycle life.
④Flexible form factors provide efficient and compact packaging. It provides an ideal choice for applications where weight and volume are the most critical requirements.

4. Unique design features of polymer electrolyte batteries
The design of polymer electrolyte batteries is based on thin-film technology components, which contain a very large area of ​​electrolyte layer and electrode layer, with a thickness ranging from 20 to 200 μm. This technology minimizes the impedance of the electrolyte, thereby increasing the electrode dynamics. In addition, the all-solid-state structure allows the most compact size, possibly resulting in the most efficient electrochemical performance. The solid-state design demonstrates high energy efficiency and a cycle life of more than 100 cycles for small batteries even at 100°C. The high energy density of the lithium-based system is due to the low atomic mass of the lithium element and the high reactivity with most cathode materials. Materials scientists claim that the energy density of lithium-based batteries is 2-10 times higher than that of lead-acid batteries. However, these batteries also have problems with charging and performance degradation. The specific power level of the first-generation battery is 100~200W/kg, which has been increased to 400W/kg. Simulation research and experimental verification of energy and power capacity based on the design parameters of the battery are necessary to demonstrate the performance of polymer batteries. Due to some safety issues, rechargeable lithium polymer electrolyte batteries have not been commercialized even after 2005. Research is needed to improve the electrolyte, interface behavior, electrode structure, and time-dependent effects of multi-cell and single-cell cycles.

5. Lithium metal polymer batteries for electric vehicle applications
This special battery was originally developed to be used as a backup power source. The latest research and development activities seem to indicate that the design structure of the battery can be modified to meet the application requirements of electric vehicles and hybrid electric vehicles. The battery uses lithium as the negative electrode, vanadium oxide as the positive electrode, and polymer solution as the electrolyte. The conductivity of the electrolyte is less than 0.0001/(Ω·cm) when the temperature is greater than 40°C. As a backup power source for communication applications, it provides the most satisfactory battery performance. The battery can be connected to the bus, and the voltage provided is 24V or 48V, which is the most ideal voltage for communication applications. The battery capacity is only lost 1% per year, even in the environment or the storage temperature is as high as 60 ℃, the battery life is more than 10 years. The battery test shows that the battery can operate reliably for at least 12 years during the floating period and the backup period.

6. The influence of ambient temperature on the capacity of rechargeable batteries
When considering charging a battery, time is extremely important. If you wait until the battery is fully discharged, in some cases, it may take several hours to charge the battery to its maximum capacity. In addition, the reduction in battery capacity strictly depends on the ambient temperature. However, the rate of reduction is entirely based on the density of the electrolyte. It is obvious from the data listed in Table 2 that the battery capacity varies with the percentage of ambient temperature.

Temperature/oFbattery capacity/%
-10 (ice)9
Table 2 The percentage change of battery capacity with ambient temperature

The data given in Table 2 shows that when the temperature approaches the freezing point, the capacity of the battery decreases at a faster rate.