Revealing the Thermodynamics of Magnesium and Lithium Ion Insertion Chemistry in Ultrafast Discharging Hybrid Rechargeable Batteries

We present a combined experimental and theoretical strategy to design dual-salt Mg²⁺/Li⁺ batteries with very high electrochemical performance.^[1] In this work, we first investigate the (co)-insertion mechanisms of Mg²⁺ and Li⁺ ions into the Mo₆S₈ cathode by using density functional theory (DFT) calculations together with the Nernst relation. We show that there exists a threshold for the Li⁺ activity in the electrolyte above which lithiation is preferred instead of magnesiation. Based on the DFT results, to achieve high electrochemical performance, we suggest an approach to control the insertion chemistry by varying the Li salt content of the electrolyte. Using this approach, we have carefully revealed the ion insertion chemistry for the Mg-metal anode/Mo₆S₈cathode hybrid Mg/Li batteries with all-phenyl complex (APC) and LiCl electrolytes.

Our experimental results confirm that at low Li⁺ activities in the electrolyte, Mg²⁺ insertion dominates and the battery can cycle only below 80 mAh g^-1, whereas at high Li⁺ activities, fast Li⁺ ionic transport in the entire intercalation range leads to higher voltage, capacity and rate capability. We find that the Mg/Li hybrid rechargeable battery with an APC 0.2 M and LiCl 0.5 M dual-salt electrolyte has a remarkable electrochemical performance. This hybrid battery with optimized insertion chemistry achieves 93.6% capacity retention at 20 C and 87.5% at 30 C at room temperature. Our combined computational and experimental study suggests an effective route to develop new dual-salt hybrid systems with controlled insertion chemistry.

Acknowledgement

This work has been supported by The Dow Chemical Company and Northwestern-Argonne Institute of Science and Engineering (NAISE).

References

[1] J.-H. Cho, M. Aykol, S. Kim, J.-H. Ha, C. Wolverton, K. Y. Chung, K.-B. Kim, and B. -W. Cho, J. Am. Chem. Soc. 2014, 136, 16116.