Batteries using multivalent ions—such as Mg²⁺ and Ca²⁺ (e.g., earlier post)—are promising candidates for achieving higher energy density than available with current Li-ion technology. As an example, Mg, being divalent and denser, is theoretically capable of delivering a higher volumetric energy-density (3833 mAh cm^-3) than Li (2061 mAh cm^-3).

However, finding electrode materials that reversibly store and release these multi-valent cations is a major challenge for enabling such multi-valent battery technology

The Berkeley Lab and MIT team estimated the insertion voltage; capacity; thermodynamic stability of charged and discharged states; and the intercalating ion mobility and then used these properties to evaluate promising directions.

Our calculations indicate that the Mn₂O₄ spinel phase based on Mg and Ca are feasible cathode materials. In general, we find that multivalent cathodes exhibit lower voltages compared to Li cathodes; the voltages of Ca spinels are ~ 0.2V higher than those of Mg compounds (versus their corresponding metals), and the voltages of Mg compounds are ~1.4 V higher than Zn compounds; consequently, Ca and Mg spinels exhibit the highest energy densities amongst all the multivalent cation species.

The activation barrier for the Al³⁺ ion migration in the Mn₂O₄ spinel is very high (~1400 meV for Al³⁺ in the dilute limit); thus, the use of an Al based Mn spinel intercalation cathode is unlikely. Amongst the choice of transition metals, Mn-based spinel structures rank highest when balancing all the considered properties.

—Liu et al.

Resources

• Miao Liu, Ziqin Rong, Rahul Malik, Pieremanuele Canepa, Anubhav Jain, Gerbrand Ceder and Kristin Persson (2014) “Spinel Compounds as Multivalent Battery Cathodes: A Systematic Evaluation Based on ab initio Calculations” Energy Environ. Sci., doi: 10.1039/C4EE03389B