Sankey diagrams are a specific type of flow diagram that illustrates the flow of material, energy, or cost across a system. The width of the bands is proportional to the flow quantity they represent, making it easy to visualize the relative proportions and to track where inputs are consumed and outputs are produced within the system. They are particularly useful in identifying inefficiencies and the primary pathways through which a process operates.
In lithium-ion (Li-ion) cell manufacturing, a Sankey diagram can effectively illustrate the complex series of processes involved in the production of cells. The diagram below shows the flow of materials through the stages of manufacturing an NMC333G lithium-ion cell, prepared by Jinasena et al. (2021). The diagram starts with the active materials, solvents, and other components such as carbon, binder, current collector, separator, and electrolyte. These are combined and processed through various stages such as mixing, coating, drying, calendaring, slitting, and finally stacking/winding followed by electrolyte filling. Each stage is critical in determining the cell's performance characteristics, such as capacity, power output, and cycle life. The flow chart clearly indicates the sequence of processing these materials into a functional battery cell.
Filling the cell with electrolyte and sealing it is the next step, followed by formation, where the cell is charged and discharged several times to form the solid electrolyte interphase (SEI), which is essential for the cell's lifespan and performance. The final product is the cell, ready for assembly into battery packs.
The Sankey diagram also highlights waste streams, such as solvent waste and scrap material, which are the byproducts of the manufacturing process. In this particular model, as you have noted, a solvent recovery unit is absent, which results in all evaporated solvents being classified as waste. This not only presents an environmental concern due to the solvent's contribution to the battery's mass composition but also suggests a potential area for improving the manufacturing process's sustainability. By capturing and reusing solvents, manufacturers can reduce their environmental impact and possibly lower production costs.
By visualizing the process with a Sankey diagram, it becomes apparent where material inputs are not fully converted into product outputs, thus identifying areas for process optimization. Any significant width in the bands representing scrap or waste materials may prompt a review of process efficiency or the implementation of recycling strategies.