Lithium-sulfur (Li-S) batteries are one of the most attractive candidates for the next generation of high-energy rechargeable Li batteries because of their high specific energy at a working voltage of ca. 2.2 V. For the assembly of Li-S batteries, metallic Li as negative electrode is commonly paired with S as positive active material due to their high theoretical capacities (Li, 3860 mAh g^-1; S, 1672 mAh g^-1). However, the use of metallic Li deals with serious safety hazards as fire or explosion of the battery, thus considering its substitution by high capacity anodes as an interesting approach.

In the recent years, metal hydrides (MH) have been proposed as breakthrough negative electrode materials for lithium ion batteries (LiBs) owing to their high theoretical Li storage capacities, and suitable working potentials of 0.1-1.0 V vs. Li⁺/Li. MgH₂ and TiH₂ exhibit high theoretical capacities of 2038 mAh g^-1 and 1074 mAh g^-1 with average potentials of 0.5 and 0.16 V vs. Li⁺/Li, respectively. These hydrides exhibit excellent performance in half all-solid-state (ASS) cells with LiBH₄ as solid electrolyte (SE) and metallic Li as anode. However, their implementation as negative active materials in complete cells has not been yet accomplished.

It is necessary to find suitable lithium-based positive materials with similar theoretical capacities and chemical compatibility with the electrolyte to pair them. In this context, the Sulphur/Lithium sulfide (S/Li₂S) redox couple appears to be an ideal candidate. Indeed, it has been shown to offer an excellent compatibility and performance in ASS batteries using LiBH₄ as solid electrolyte (~800 mAh g^-1 after 50 cycles). From this inspiring background, we decided to evaluate the performance of MH anodes with the S/Li₂S couple integrated in an ASS battery.

—López-Aranguren et al.

The team used a a hydride-based nanocomposite 0.8MgH2-0.2TiH₂ as the anode material and a Li₂S composite as the cathode. LiBH₄ was used as the solid electrolyte due to its high Li⁺ conductivity of 10^-3 S cm^-1 at 120 ˚C.

Charge/discharge profiles at different C rates (C/50, C/20 and C/50, five cycles each) of the complete ASS battery and schematic representation of the discharge redox reaction. López-Aranguren et al. Click to enlarge.

The redox reaction for this cell can be written as:

This is, to the best of our knowledge, the first high capacity all-solid-state battery using metal hydrides as anode. The compatibility of other solid electrolytes with high Li⁺ conductivity at lower temperatures such as Li_m(BH₄)_nX (X = Cl, Br, I) is envisaged for this redox system. Further studies on the cathode and anode composites will be investigated in order to improve the overall performance of the battery.

—López-Aranguren et al.

The research was funded from the European Union’s Seventh Framework Programme for research technological development and demonstration under grant agreement Nº 607040 through the Marie Curie ITN ECOSTORE project.

Resources

• Pedro López-Aranguren, Nicola Berti, Anh Ha Dao, Junxian Zhang, Fermín Cuevas, Michel Latroche, Christian Jordy (2017) “An all-solid-state metal hydride – Sulfur lithium-ion battery” Journal of Power Sources, Volume 357, Pages 56–60 doi: 10.1016/j.jpowsour.2017.04.088