Nanocomposite solid-state electrolytes for alkali-metal batteries: unveiling the role of PEO-capped TiO2 nanofiller

Specific capacity and energy density of lithium-ion batteries (LIBs) are still clearly below the standards required by the automotive industry for a widespread implementation into electric vehicles. This fact is exacerbated by several degradative processes which induce capacity fading into devices upon few cycles. Therefore, it results necessary to increase the overall battery capacity and, at the same time, to reduce the performance decay. One promising way to increase the specific capacities consists in the substitution of graphite with metallic lithium. The passage to lithiummetal batteries (LMBs) permits to exploit a negative electrode that displays a capacity ten times higher than graphite and a more negative potential.3 Alternatively, to overcome the dependence on lithium also sodium-metal batteries (NMBs) can be considered as a promising high-energy alternative to LIBs. Unfortunately, the exploitation of alkali metal anodes is not as straightforward since dendrites, formed during the plating of ions on the metal anode, can pierce through the separators leading to dangerous short circuits of the whole cell. [1] To tackle this issue many different and specific strategies have been adopted. Particularly, the exploitation of mechanically strong solid-state electrolytes (SSEs) has demonstrated its effectiveness in slowing the growth of dendrites thanks to their high modulus that work as physical obstacle against these protrusions. Since commonly employed polymeric electrolytes are not able to stop the propagation of dendrites, it results necessary to pass to stiffer SSEs that conjugate a sufficient mechanical strength with the good adhesion typical of polymers. Consequently, aim of this thesis was the production, the characterization and the implementation of nanocomposite SSEs into lithium-metal and sodium-metal batteries. In particular, TiO2 NPs functionalized with short chains of PEO5K were embedded into polymeric matrixes, constituted of high Mw PEO4M and conductive salts such as LiTFSI or NaTFSI, in order to increase the resistance of the polymer against dendrite penetration. [2] The functionalization step permitted to encompass ceramic content up to 50% wt% without compromising the homogeneity of the SSE. For intermediate amounts of fillers, both ionic conductivity and mechanical strength resulted enhanced, improving the performances also in full cells and incredibly improving the stability against dendrites. Additionally, a peculiar self-healing behaviour, further investigated in subsequent works, was also observed for symmetric Li/Li cells that resulted able to reinstate cycling operation after dendrite-induced short circuit.