Interest in rechargeable magnesium batteries has increased due to a number of factors. Magnesium is low-cost, safe and environmentally benign. When used as an anode, Mg has a low standard electrode potential and fast deposition/stripping kinetics with nearly 100% reversibility without formation of dendritic structures—the last being a major problem with a pure Li metal anode.

Magnesium also has a much higher theoretical energy density (gravimetric: 2205 mAh g^-1, volumetric: 3832 mAh cm^-3) compared with typical anode materials because of its divalent energy storage characteristics, the PNNL team noted.

Over the past few years, substantial progress in rechargeable Mg batteries has been demonstrated, but their practical application is still facing great challenges, largely due to limitations of cathode materials. … As an alternative to develop cathode materials for intercalating Mg²⁺ ions, here we desire to fabricate batteries by combining Mg and Li electrochemistry.

… It should be noted that a similar design has been discussed in a previous study [Yagi et al.], but it appears that prototypes fabricated in that work have some limitations (for example, low coulombic efficiency and short cyclic life), and therefore it is not evident that the design is indeed practical.

—Cheng et al.

(a) Typical cycling stability of hybrid cells tested at 10 C for 3000 cycles; (b) comparison of the charge–discharge profiles for the first cycle and the 3000th cycle, showing minimal changes upon repeated charge–discharge operations. Cheng et al. Click to enlarge.

Mo₆S₈ can undergo intercalation reactions with either Li⁺ or Mg²⁺ ions; reactions with Li⁺ ions have better kinetics. The PNNL team’s results showed that Li⁺ ion intercalation was the dominant reaction when both Mg²⁺ and Li⁺ ions were present; this is the fundamental basis for the design of Mg–Li hybrid batteries.

Electrochemical testing showed that the cell had a specific capacity of 126 mAh g^-1 at 0.1 C—very close to the theoretical capacity of Mo₆S₈ (128.8 mAh g^-1), calculated based on the mass of Mo₆S₈. The capacity had a slight decrease with increase of the C-rate, but was able to maintain 105 mAh g^-1 at 15 C (83% retention).

The hybrid cell was very stable, with close to 100% coulombic efficiency for each cycle.

The results discussed above demonstrate that the hybrid cells designed here could combine the advantages of Mg and Li electrochemistry and have outstanding rate performance and cycling stability. Practically, however, the performance of hybrid cells will depend strongly on their assembly methods (in particular, active material loadings relative to amounts of electrolytes) due to their unique operating principle. In particular, the electrolyte should be able to supply sufficient Li⁺ ions for the cathode electrochemical reaction and intercalation kinetics, since the Mg–Li battery was assembled at the charged state.

… the hybrid batteries will have minimum requirements on the amount of electrolytes for optimal performance. The volume of electrolytes could be reduced by developing novel electrolytes, such as the solvent-in-salt type electrolyte developed recently for a Li–S battery. In principle, it is possible to adopt most Li cathode materials to develop Mg–Li hybrid battery. However, developing electrochemically stable Mg–Li dual salt electrolytes is an immediate technical hurdle, since most known Mg electrolytes have stable electrochemical windows less than 3 V. Research on developing new electrolytes to increase the voltage of hybrid batteries is underway.

—Cheng et al.

Resources

• Yingwen Cheng, Yuyan Shao, Ji-Guang Zhang, Vincent L. Sprenkle, Jun Liu and Guosheng Li (2014) “High performance batteries based on hybrid magnesium and lithium chemistry,” Chem. Commun., 50, 9644-9646 doi: 10.1039/C4CC03620D

• A. Yagi, T. Ichitsubo, Y. Shirai, S. Yanai, T. Doi, K. Murase and E. Matsubara (2014) “A concept of dual-salt polyvalent-metal storage battery ,” J. Mater. Chem. A 2, 1144–1149 doi: 10.1039/C3TA13668J