The identification of a solid-state electrolyte that fulfills all the requirements to enable a competitive bulk-type all-solid-state battery remains a challenging task. The electrolyte should display high ionic conductivity, high thermal stability, good processability and importantly a large electrochemical stability window that ideally spans the redox potential of the conducted metal ion. Several electrolytes within the class of complex hydrides display such a favorable combination of battery relevant properties. Here, we will discuss our recent results for Li-, Na- and Mg-ion conductors.

We identified several compositions within the lithium-amide borohydride (Li(BH₄)_1-x(NH₂)_x) system that display near room temperature conductivities of >6 mS/cm, comparable to the ones of typical carbonate-based liquid electrolytes used in commercial lithium-ion batteries. An Li₄Ti₅O₁₂ half-cell based on such an electrolyte displays >60% capacity retention at 3.5 mA/cm² (5C) and stable cycling for 400 cycles.[1] The mixed closo-borate Na₂(B₁₂H₁₂)_0.5(B₁₀H₁₀)_0.5 displays a conductivity of 0.9 mS/cm at 20 °C, excellent thermal stability of up to 300 °C and an electrochemical stability window of 3 V that enables reversible plating of sodium.[2] We will also report on the preparation and characterization of magnesium ethylenediamine borohydride (Mg(en)₁(BH₄)₂), which displays a very high conductivity of 6x10^-5 S/cm at 70 °C for a magnesium-ion solid-state electrolyte.[3]

[1] Y. Yan, R.-S. Kühnel, A. Remhof, L. Duchêne, E. Cuervo Reyes, D. Rentsch, Z. Łodziana, C. Battaglia, Adv. Energy Mater., accepted.

[2] L. Duchêne, R.-S. Kühnel, D. Rentsch, A. Remhof, H. Hagemann, C. Battaglia, Chem. Commun. 2017, 53, 4195.

[3] E. Roedern, R.-S. Kühnel, A. Remhof, C. Battaglia, Sci. Rep. 2017, 7, 46189.