Battery Science & TechnologyIn the state of the art progress in improving the performance of the Lithium Ion Batteries (LIB) full cell, the formulation engineering has received little attention, compared to new structures and new compositions of the electrochemically active material so far. However, much attention is paid to the formulation engineering in the field of battery industry, because it is critically significant in order to manufacture efficient full cells which can be commercialized. Such efforts are quite restricted to access or publication, because they are the proprietary information of the manufacturing companies.
This study is focused on the optimization of full cell formulation factors based on silicon-natural graphite composite negative electrode. The considered factors were composition of active materials, binder, electrolyte and cutoff voltage in a full cell.
The effect of different particle sizes of natural graphite within the composite, which is composed of silicon and natural graphite, was investigated, in which the composite with smaller natural graphite showed the superior electrical conductive network.
The experiment showed that the silicon based composite electrode keeps higher voltage profile than that of natural graphite, especially during delithiation in the half cell. And, the simulation result, computed out of the experimental result, envisioned that it is better to shift cutoff voltage to the lower voltage during discharge in the full cell consisting of silicon–natural graphite composite electrode in order to use the maximum capacity of each electrode, comparing with that of natural graphite electrode. Additionally, a design method of calculating initial discharge capacity in a full cell was investigated.
A statistical analysis method making use of Design of Experiments (DOE) was applied to search for the optimized condition for the weight ratio of binder mixture and the electrolyte kind, which showed that the cycle life of silicon-natural graphite composite electrode with the PAA or CMC binders is superior to that with SBR binder. This experimental result enabled me to argue that those binders, which have mechanically high ‘proportional limit stress’ like PAA and CMC, provide more robust bonds among expansive active materials (Si) which, in turn, can be adhered solidly to current collector substrate, compared with SBR and PVDF which have low ‘proportional limit stress’. Such strong bonds are also formed even between Si and Natural graphite of active materials, which contributed to the preservation of electrical conductivity of a composite negative electrode despite under repeated dimensional changes during cycles.ope
