What is battery energy density?

What is battery energy density?

Under the condition that the same energy consumption, the volume and weight of the battery pack are strictly limited, the single maximum driving range of a new energy vehicle mainly depends on the energy density of the battery. So, what is the battery energy density?

Energy density is how much energy stored in unit space or unit mass of matter. The energy density of a battery is how much the electric energy released by the average unit volume or mass of the battery, which is generally divided into two dimensions: weight energy density and volume energy density.

The weight energy density of the battery can be simply calculated with this formula: Nominal voltage (V) * Rated capacity (Ah) / Battery weight (kg) = specific energy or energy density (Wh/kg).

The energy density of different types of rechargeable batteries is as follows:

  • Energy density of Lead acid battery ranges between 50-70 Wh/kg;
  • Energy density of Nickel-cadmium battery ranges between 50-80 Wh/kg;
  • Energy density of Nickel-metal hydride battery ranges between 60-140 Wh/kg;
  • Energy density of Lithium-ion battery ranges between 150-300 Wh/kg;

Lead-acid batteries have a low energy density. If they are used to drive a family car for more than 200km, it requires nearly 1 ton of batteries, which is too heavy to be used as a power source for electric vehicles. Another reason is that Pb is toxic, not environmentally friendly, and the cycle performance of lead-acid batteries is poor. Whereas, the energy density of lithium-ion batteries is about 150~300Wh/kg, which is much higher than that of lead-acid batteries, likewise with the cycle performance, so lithium-ion batteries are the best choice for the development of new energy electric vehicles.

Currently, there are two main technical routes for high-energy-density lithium batteries on the market: Economical LiFePO4 batteries & Mid-to-high-end Lithium Nickel Manganese Cobalt Oxide(NMC) batteries. In 2015, LiFePO4 batteries were the mainstream of the market. At that time, the energy density of most LiFePO4 battery systems on the market was around 70-90Wh/kg, while the energy density of NMC batteries was much higher, reaching 130Wh/kg. In order to quickly open the passenger car market that is sensitive to driving range, the Chinese government first proposed to take battery energy density as a reference indicator in the new energy vehicle subsidy policy in 2016. The higher the energy density, the more subsidies. The market structure of LiFePO4 batteries and NMC batteries began to change, and major car companies began to replace NMC batteries on a large scale. Since June 2019, with the withdrawal of subsidies and the high production cost of NMC lithium batteries, the LiFePO4 batteries have returned to be the main energy solution in the market. To adapt to market development, all large battery manufacturers have started a two-line strategy of LiFePO4 + NMC. Now the LiFePO4 battery has reached an energy density of 210Wh/kg.

What limits the energy density of lithium batteries

There are four key parts of a lithium battery: anode, cathode, electrode and diaphragm, which all affect the battery’s energy density. And the electrodes are the places where chemical reactions occur. The key to improving the energy density of batteries is to develop new electrodes materials and improve production processes.

From the above we can know that the energy density of lithium batteries composed of LiFeO4 and the ternary material Ni Co Mn are very different. Different ratios of Ni, Co, and Mn in ternary materials will also cause differences in battery performance. The higher the proportion of Ni, the higher the specific capacity of the battery. The high Ni positive cathode system batteries currently being promoted have a mass energy density between 240-300Wh/kg (volume energy density 560Wh/L-650Wh/L).

The mainstream anode material in the lithium battery market is mainly graphite (carbon-based material), but the current energy storage of carbon-based materials is close to the theoretical upper limit. The specific capacity of silicon-based anode materials can reach 4200mAh/g, which is much higher than the theoretical specific capacity of graphite anodes of 372mAh/g. With the introduction of silicon carbon anode, the mass energy density of the battery cell will be upgraded to 300-400Wh/kg (volume energy density 630Wh/L-750Wh/L), thus becoming a powerful substitute for graphite anode.

For lithium-ion batteries, high energy density means that there are more active Li+ per unit volume, and more probably to form sharp lithium dendrites. It is easy to penetrate the diaphragm and cause the battery to short-circuit or explode. Therefore, when pursuing high energy density , we also need to strengthen security protection measures:

  1. Improve the anode and cathode material, add flame retardant to the electrolyte, etc.;
  2. Pay attention to the use condition of the battery, strengthen the protective structure, flame-retardant design, over-current protection, etc.;
  3. Continuously upgrade BMS system control.
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