Provided herein are methods of forming solid-state ionically conductive composite materials that include particles of an inorganic phase in a matrix of an organic phase. The methods involve forming the composite materials from a precursor that is polymerized in-situ after being mixed with the particles. The polymerization occurs under applied pressure that causes particle-to-particle contact. In some embodiments, once polymerized, the applied pressure may be removed with the particles immobilized by the polymer matrix. In some implementations, the organic phase includes a cross-linked polymer network. Also provided are solid-state ionically conductive composite materials and batteries and other devices that incorporate them. In some embodiments, solid-state electrolytes including the ionically conductive solid-state composites are provided. In some embodiments, electrodes including the ionically conductive solid-state composites are provided.