Lithium-ion batteries are composed of positive and negative electrode sheets, diaphragm, electrolyte, and auxiliary materials such as case lugs. The battery electrode coating can be seen as a composite material, consisting of three main parts: (1) particles of living matter; (2) a constituent phase of conductive agents and binders mixed with each other; and (3) pores, which need to be filled with electrolyte. Battery diaphragm is also porous structure, on the one hand, isolated from electrons, on the other hand pores need to be filled with electrolyte allowing the passage of lithium ions. Therefore, the electrolyte volume of lithium-ion batteries is mainly the need to fill the pores inside the pole piece and diaphragm, the pore volume is the volume of electrolyte usage, i.e.
electrolyte volume = positive electrode pore volume + negative electrode pore volume + diaphragm pore volume
And the pore volume of the pole piece and diaphragm is calculated as follows
The pore volume of the pole piece = (length × width × thickness of each pole piece coating) × number of pieces × porosity
Diaphragm pore volume = area of the diaphragm × thickness × porosity
Considering that in addition to the cells, there are remaining spaces inside the shell that have not been filled (these spaces can also be calculated based on the cell design), electrolyte will also remain in these places, i.e.
Actual electrolyte volume = volume of all pores + volume of residual electrolyte
Hard shell battery has more residual volume and the actual electrolyte volume is much larger than the theoretical value, soft pack battery has average residual space and moderate residual electrolyte volume, cylindrical battery has high internal space utilization and less residual electrolyte volume.
The thickness and porosity of the diaphragm will be provided by the material manufacturer, for example, 25 microns thickness and 49% porosity. According to the cell design or directly disassemble the battery to measure the length and width of the diaphragm, the area of the diaphragm is calculated, so that the amount of electrolyte needed for the diaphragm can be calculated.
As for the parameters of the pole piece, the length, width and thickness of the coating (except foil) can be measured according to the actual pole piece. The porosity of the pole piece is calculated as
where the average density of the coating and the compacted density of the coating are
And where
In practical working experience, the ductility of copper foil is 0%, and aluminum foil ductility is about 1%. The true densities of common materials are shown in the table (Ref. 1).
