The world is moving towards a demand for safer Lithium batteries in which traditionalvolatile liquid electrolytes are replaced by solid-state ones. Methods for fabricating and integrating such solid-state electrolytes (SSEs) in rechargeable batteries have been reported in a large number of literature articles and review papers. SSEs are viewed as a requirement for safe operation of lithium batteries that use metallic lithium as the anode, i.e. Lithium-metal batteries (LMB). SSEs are presently challenged by poor room-temperature ionic conductivity in the bulk and by complex interfacial chemistry and poor ion transport through the interfaces that SSEs form with the LMB anode and cathode. Recently, solid polymer electrolyte (SPE) composed of poly(1,3-dioxolane) (polyDOL) formed inside electrochemical cells by ring-opening polymerization (ROP) of the ethers have received attention for their potential to ameliorate poor interfacial contact between the electrodes and solid electrolyte, enabling a more uniform charge transport. ROP of ethers is unfortunately reversible due to the sensitivity dependence of the equilibrium constant on the relative difference in ring strain in the monomer and polymer. An undesirable consequence is that the ROP of ethers is typically accompanied by large amounts of residual monomer, decreasing the mechanical strength of SPE as well as increasing the potential for parasitic electrochemical side reactions during battery operations. Research summarized in this thesis investigate two potential solutions to this issue: mechanical reinforcement of the in-situ formed SPE using particles with affinity for the growing polyether; and co-polymerization of the ether with multifunctional variants that serve as anchors for the growing chains. Research towards the first solution considers the role of nano- and microsized fillers on the ROP of 1,3-dioxolane. We find that the particles have direct and indirect effects on SPEs. They alter the polymerization kinetics, ionic conduction mechanism, and in the case when nano-sized SiO2 grafted with short chains (aka, PEG-SiO2 hairy nanoparticles (HNPs)) may also introduce co-crystallization of polyDOL with PEG tethers. The co-crystallization creates a uniform structure, increasing room- temperature ionic conductivity to as high as 4 mS/cm. The introduction of micron-sized Li2O, on the other hand, produce rather different effects on particle and electrolyte length scales. At the particle scale, the physical size, hardness, basicity, and gravitational settling tendencies of the particles lead to SPEs with interesting gradient physical properties. The basic Li2O particles for instance retards ROP of DOL in the particle-rich regions of an SPE, producing materials that become progressively more solid-like away from interfaces at which the particle concentration is enlarged by settling. At the electrode scale,Li2O functions an electro-active material and is capable of undergoing reversible redox reactions at sites near the electrode. These processes contribute lithium inside an otherwise closed electrochemical cell, enabling extended cycling of lithium batteries in the most challenging anode-free configurations. Research towards the second solution considers the effects of crosslinking poly(DOL) on the reversibility of the polymerization reaction. It is found that as the degree of crosslinking increases, the residual monomer content at equilibrium falls. And, through straightforward manipulations of the cross-linker content alone, it is possible to create SPEs in a variety of soft matter states, with commensurate gradations in mechanical properties. As a demonstration of the potential benefits of the in-situ formed cross-linked polyDOL SPEs formed inside a conventional ethylene carbonate and dimethyl carbonate (EC/DMC) host, we investigated the electrochemical cycling characteristics of the materials in Li-Cu half cells and in full-cell batteries in which either graphite or metallic lithium are paired with a high-Nickel (NCM811) cathode. These studies show that the materials are mechanically robust with a wide electrochemical stability window, and good oxidative stability.