What is energy density?
Energy density is the amount of energy stored per unit of a certain amount of space or mass of material. The energy density of a battery is also the average amount of electrical energy released per unit volume or mass of the battery. The energy density of a battery is generally divided into two dimensions: energy density by weight and energy density by volume.
Battery weight energy density = battery capacity x discharge platform/weight, basic unit is Wh/kg (watt hour/kg)
Battery volumetric energy density = battery capacity x discharge platform/volume, the basic unit is Wh/L (Watt hour/liter)
The higher the energy density of a battery, the more power is stored per unit volume or weight.
What is the energy density of a single cell?
The energy density of a battery often points to two different concepts, the energy density of a single cell and the energy density of a battery system. A cell is the smallest unit of a battery system, M cells make up a module, and N modules make up a pack, which is the basic structure of an automotive power battery. The energy density of a single cell, as the name suggests, is the energy density of a single cell level.
According to “Made in China 2025”, the development plan for power batteries is clear: in 2020, the battery energy density will reach 300 Wh/kg; in 2025, the battery energy density will reach 400 Wh/kg; in 2030, the battery energy density will reach 500 Wh/kg. Here it refers to the energy density at the individual cell level.
What is system energy density?
System energy density refers to the power of the entire battery system after the combination of the individual cells is completed then the weight or volume of the entire battery system. The energy density of a battery system is lower than the energy density of a single cell because the internal battery management system, thermal management system, high and low voltage circuits, etc. take up part of the weight and internal space of the battery system. System energy density = battery system power/battery system weight OR battery system volume
What exactly limits the energy density of lithium batteries?
The chemical system behind the battery is the main reason to blame. Generally speaking, there are four key components of a lithium battery: the positive electrode, the negative electrode, the electrolyte and the diaphragm. The positive and negative electrodes are where the chemical reactions take place, equivalent to the two veins, so their importance is evident. We all know that a pack system with a ternary lithium cathode has a higher energy density than a pack system with a lithium iron phosphate cathode. Why is this?
According to the barrel theory, the height of the water level is determined by the shortest part of the barrel, and the lower limit of the energy density of lithium-ion batteries depends on the cathode material.
The voltage plateau for lithium iron phosphate is 3.2V, while this indicator for ternary is 3.7V. Comparing the two, the energy density is high and low: a difference of 16%. Of course, in addition to the chemical system, the level of production processes such as compaction density, foil thickness, etc., also affect the energy density. Generally speaking, the higher the compaction density, the higher the capacity of the battery in a limited space, so the compaction density of the main material is also seen as one of the reference indicators for the energy density of the battery.