Development of stable and high-power germanium/lithium iron phosphate lithium-ion cells

Despite their impressive features, state-of-the-art (SOTA) lithium-ion batteries (LiBs) do not meet our energy storage demands. Their charging time and the energy they can store per unit mass (specific energy) and volume (energy density), are inadequate when weight and space utilization are to be minimized, like in the automotive and aerospace sectors. These deficiencies are in part due to the use of graphite, a material that can hold a limited charge and support a low current flow, to produce negative electrodes (NEs). For this reason, the optimization of novel materials to supplant graphite is a hot research topic. Germanium (Ge), a material featuring four times the specific capacity and eight times the capacity density of graphite, is ideal for producing lightweight and compact batteries. Moreover, Ge is significantly more stable and supports a higher current flow than competitors like silicon. Still, Ge-based NEs are not ready for the market. Their stability during cycling, for instance, is not on par with that of graphite and needs improvement. Pairing a Ge NE with commercially available positive electrodes (PEs) is a challenge too: off-the-shelf PEs have a significantly lower specific capacity and support a lower current flow than Ge, causing bottlenecks. This thesis concerns the development of solutions to both problems. The stable cycling of Ge-based NEs coupled with lithium iron phosphate-based PEs was enabled with the novel electrolytic formulas described in Chapter III. While cells equipped with a standard electrolyte died off quickly, the cells based on novel electrolytes could cycle indefinitely while maintaining a coulombic efficiency above 99%. Positive electrodes based on lithium-iron phosphate and supporting a high current flow were enabled by introducing polymers able to support ionic and electronic conductivity in the matrix of the electrode. These electrodes, described in Chapter IV, are water-processed and show up to ten times better transport properties and twice the capacity retention than those prepared with a standard binder (PVDF) while maintaining excellent mechanical properties.