Polysaccharides as a binders in lithium-ion and sodium-ion cells

Lithium-ion cells are currently the primary source of power for portable electronic devices such as smartphones and laptops. The global lithium-ion battery market size was valuted at 32.91 billion U. S. dollars in 2019 and 34.18 billion U. S. dollars in 2020. Most of these systems contain organic compounds that are difficult to utilize. Environmentally friendly materials should be substituted for these elements from green chemistry perspective. The scientific project included the use of water-soluble polysaccharides as binders in the electrodes of the secondary cells to eliminate organic soluble binders. Additionally, the kinetic parameters of the processes occurring during the cyclic operation of the cell (charge transfer resistance, activation energy of the charge transfer reaction reduction of lithium and sodium ions, and the exchange current density) were determined. The obtained results showed that compounds from the group of polysaccharides, i.e. locust bean gum (LBG), gum tragacanth (GT), karayah gum (KG), and gum arabic (GA), are an excellent alternative to polyvinylidene fluoride (PVDF) soluble in organic solutions, which is unfriendly to the environment. Results obtained during the galvanostatic charge/discharge of the Li | 1 M LiPF₆ system in EC: DMC (50:50 wt.) | MoS₂ at 50 mA g^-1 current regime showed high capacity values during the first lithiation step for all analyzed systems: 1405 mAh g^-1 (for MoS₂-LBG), 1100 mAh g^-1 (MoS₂-GA), 1050 mAh g^-1 (MoS₂ – GT), 861 mAh g^-1 (MoS₂ – KG). An electron microscope was used to examine the electrode morphology after preparation and after lithiation of the cell. After cell operation, locust bean gum has been found to significantly reduce the commonly occurring volume expansion effect of electrodes. As a result, water-soluble compounds can act as binders in secondary cells. Various electrode materials of metal oxides and sulfides with water-soluble binders were tested as part of further research. Then, the exchange current density j₀ was determined for the systems with locust bean gum as a binder. These results demonstrate the charge transfer rate during the cycle of the cell. This process is fast in the system with tin sulfide and tin oxide as electrode, amounting to 1.97 · 10^-6 and 1.79 · 10^-5 A cm², respectively. In parallel, sodium-ion cells with 0.8 NaPF₆ in EC:DMC (50:50 wt.) as electrolyte were tested using selected electrode materials and a binder. Kinetic studies have shown that the exchange current density values range from 1.3 · 10^-6 A cm² for the CuS-LBG system to 7.37 · 10^-6 A cm² for SnS-LBG. It is interesting to note that the data obtained for the same systems with polyvinylidene fluoride as a binder are of the same order of magnitude 10^-6 A cm². This confirms the usefulness of polysaccharides as secondary cell binders.