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Mechanochemical synthesis using CeCl 3 -MBH 4 (M = Li, Na or K) mixtures are investigated and produced a new compound, LiCe(BH 4 ) 3 Cl, which crystallizes in a cubic space group I4̅ 3m, a = 11.7204(2) Å. The structure contains isolated tetranuclear anionic clusters [Ce 4 Cl 4 (BH 4 ) 12 ] 4− with a distorted cubane Ce 4 Cl 4 core, charge-balanced by Li + cations. Each Ce atom coordinates three chloride ions and three borohydride groups via the η 3 −BH 3 faces, thus completing the coordination environment to an octahedron. Combination of synchrotron radiation powder X-ray diffraction (SR-PXD), powder neutron diffraction and density functional theory (DFT) optimization show that Li cations are disordered, occupying 2/3 of the 12d Wyckoff site. DFT calculation indicates that LiCe(BH 4 ) 3 Cl is stabilized by higher entropy rather than lower enthalpy, in accord with the disorder in Li positions. The structural model also agrees well with the very high lithium ion conductivity measured for LiCe(BH 4 ) 3 Cl of 1 × 10 −4 Scm −1 at T = 20°C. In situ SR-PXD reveals that the decomposition products consist of LiCl, CeB 6 and CeH 2 . The Sieverts measurements show that 4.7 wt % H 2 is released during heating to 500°C. After rehydrogenation at 400°C and p(H 2 ) = 100 bar for 24 h an amount of 1.8 wt % H 2 is released in the second dehydrogenation. The 11 B MAS NMR spectra of the central and satellite transitions for LiCe(B(D/H) 4 ) 3 Cl reveal highly asymmetric manifolds of spinning sidebands from a single 11 B site, reflecting dipolar couplings of the 11 B nuclear spin with the paramagnetic electron spin of the Ce 3+ ions.

Light weight and cheap electrolytes with fast multi-valent ion conductivity can pave the way for future high-energy density solid-state batteries, beyond the lithium-ion battery.