In order to realize the promise of superior energy density, the solid electrolytes in solid-state batteries have to synergize fast ionic conductivity with a low contribution to the overall cell mass. While the dead mass of the “separator” solid electrolyte layer can be minimized by reducing its thickness to the extent limited by operational safety and processability, the catholyte within the inevitably thick composite cathodes will constitute 80-90% of the total electrolyte volume in high energy density cells. Thus, in the absence of fast-ion conducting high energy density cathode materials, low density catholytes are of key importance to maximize the energy density of all-solid-state batteries. Additional constraints for fast-ion conductors include electrochemical and electromechanical compatibility as well as interface wettability with the electrode materials. Soft, compressible electrolytes not only facilitate densification, they also increase both the effective interfacial contact area and prolong cycle life by retaining close interfacial contact with breathing electrodes.

Here we discuss two design strategies to realize such compressible low-density fast-ion conductors. The first strategy involves ultrathin ceramic-in-polymer Composite Solid Electrolytes (CSEs). We developed CSEs combining Li-doped polyacrylonitrile with various ceramic electrolytes (Li_1.5Al_0.5Ge_1.5(PO₄)₃, LiTa₂PO₈ etc.) with and without plasticizers. Optimized composites reach Li⁺ conductivities of 0.1 – 1 mS·cm^-1 at densities < 2 g cm^-3 and retain wide electrochemical windows. Temperature-dependent analysis of Li⁺ paths shows that the ceramic filler induces a percolating network of fast ion-conductive interphases below the glass transition of the bulk polymer. Interfacial resistance is drastically reduced by spin coating and in situ UV-curing the precursor slurry on Li anode and composite cathode, respectively. Symmetric cells cycled with up to 2 mA·cm^-2 over 500h retain a stable overpotential and no dendrite growth is observed. LFP/CSE/Li cells achieve a maximum specific discharge capacity of 148 mAh·g^-1 and retain 89% of that after 200 cycles. Stable room temperature cycling is demonstrated for cells with a cathode mass loading of 6 mg·cm^-2. Strong binding and three-dimensional architecture of the in situ-formed interfaces is crucial for the favourable cycling stability.[1]

An alternative all-ceramic design strategy is to exploit the correlation between fast ionic conductivity, density and glass forming ability in glass-ceramic oxides[2] and oxyhalides[3]. The higher number of monovalent light halides required to balance the charges of typically heavier high valent glass-former cations reduces the overall density and turns the material less brittle. Compared to close-packed halides, oxyhalides typically have lower coordination numbers, creating free volume for ionic motion. In view of their favorable oxidation stability yet limited reduction stability, oxyhalides are in particular suitable to function as catholytes. The recent report[3] of fast room temperature ionic conductivity exceeding 10 mS cm^-3 in LiMOCl₄ (M = Nb, Ta) exemplifies the strong potential of the so far hardly explored class of crystalline oxyhalides. Our redetermination of LiNbOCl₄ (density 2.7 g cm^-3) finds a simpler and more plausible, Li-disordered, tetragonal crystal structure with smaller unit cell than the originally reported orthorhombic phase. The revised structure model [4] is consistent with DFT simulations. Molecular dynamics simulations utilizing our bond-valence related embedded-atom method type forcefield softBV-EAM reveal the ion transport mechanism, the origin of the temperature dependent activation energy and its relation to the rotational mobility of (NbOCl₄^–)_n polyanions therein. The simulations thus indicate that LiNbOCl₄ is in a plastic-crystalline state at room temperature, which explains its favorably high compressibility and formability of LiNbOCl₄.