A team from France-based leading battery company Saft and Université Paris Est reports on solid-state batteries operating at 120 °C prepared with metal hydride MH nanocomposites xMgH2+(1-x)TiH2 used as active materials for the positive electrode; metallic Li as the negative electrode; and LiBH4 as the solid electrolyte in a paper in the Journal of Power Sources.
For the study, the molar content x of the MH nanocomposites ranged from 0.2 to 0.8. Mg-rich nanocomposites offered higher specific capacity (more than 1700 mAh g−1 at C/50 regime for x = 0.7), whereas Ti-rich ones exhibited better cycle-life (ca. 100% capacity retention after 10 cycles for x = 0.2).
In comparison to equivalent liquid electrolyte cells operated at room temperature, the solid-state cells showed improved properties such as a coulombic efficiency above 98% and a rate capability of around 50% of delivered capacity at 1C-rate.
To increase the current specific energy of LiBs, researchers are addressing their efforts to develop new materials with enhanced capacity. Metals or semimetals such as aluminum, tin and silicon are attractive candidates because of their large capacity through alloying-type reactions, e.g. over 3600 mAh g−1 for Li15Si4 alloy. However, the large volume changes occurring during reversible lithiation leads to loss of electrical contact, limiting their cycle-life. This issue has been largely solved by nanostructuration and composite formation. For instance, Si-C nanocomposites show capacity exceeding 1000 mAh g−1 for more than 200 cycles.
Alternatively, through a conversion reaction scheme, nanostructured materials based on transition metal oxides have been demonstrated to be promising anodes. Fe2O3-based materials can deliver a reversible capacity of 800 mAh g−1, with working potential of ∼1.0 V (vs Li/Li+). Irreversibility of the conversion reaction and volume expansion concerns in these materials have been successfully improved. However, metal oxides face, as intrinsic issue, a large voltage hysteresis (i.e. polarization) observed between discharge and charge, which severely diminishes the round-trip efficiency of the electrode.
In 2008, metal hydrides (MHy) were proposed as high capacity anode materials for LiBs. … In a previous publication, we built for the first time a complete SSB metal hydride – sulfur Li-ion cell. We evidenced the excellent compatibility of this redox couple and its outstanding electrochemical performance. An interesting approach to enhance the individual properties of the two hydrides MgH2 and TiH2 is to combine them to form a nanocomposite. The addition of TiH2 to MgH2 improves the MgH2 electrochemical behavior, electronic conductivity, and hydrogen transport.—Dao et al.
The conversion reaction for metal hydrides is written as:
MHy + yLi+ + ye− ↔ M + yLiH
Metal hydrides such as MgH2 and TiH2 with y = 2 can provide theoretical specific capacities of 2038 and 1074mAh g−1, with low working potentials of 0.55 and 0.22 V vs. Li/Li+, respectively and low polarization.
The purpose of the Saft study was to examine the electrochemical performance of the nanocomposites as a function of molar ratio x to clarify the synergetic effects between MgH2 and TiH2 in a solid-state cell.
The team noted that future efforts are still needed, particularly in terms of cycle-life, to make these composite metal hydride materials suitable for commercial applications.
Anh Ha Dao, Nicola Berti, Pedro López-Aranguren, Junxian Zhang, Fermín Cuevas, Christian Jordy, Michel Latroche (2018) “Electrochemical properties of MgH2 – TiH2 nanocomposite as active materials for all-solid-state lithium batteries,” Journal of Power Sources, Volume 397, Pages 143-149 doi: 10.1016/j.jpowsour.2018.07.028.