(Battery Division Student Research Award Sponsored by Mercedes-Benz Research & Development) Energy Storage with the Abundant Divalent Metal Batteries
Authors: Hou Singyuk , Ji Xiao , Chen Long , Borodin Oleg , Wang Chunsheng 
Published: November 2022

The transitions to electric vehicles and renewable power sources are not only a change of personal lifestyles but also tactical approaches to a sustainable future for us and our offspring, in which the energy supply will no longer be at the expense of natural resource depletion and environmental damage. The cornerstones of this sensational transition are rechargeable batteries, particularly lithium-ion batteries (LIBs). And it is not difficult to imagine that the consumption and demand for LIBs will skyrocket in the foreseeable future if EVs dominate the market. However, the massive growth of the LIBs industry will lead to another resource crisis due to the scarcity of materials used in LIBs, such as lithium and cobalt1.

While battery recycling could partially relieve these issues, it is desirable to find abundant and cheap substitutes, especially for the regions where lithium resources are less accessible. Magnesium (Mg) and calcium (Ca) are among the most abundant elements in the earth's crust (>1000 times more abundant than lithium)2. More importantly, they store charges by two electron-transfer reactions and have fairly low redox potentials3. When appropriate high-potential cathodes are paired with Mg or Ca metal, the resulting rechargeable magnesium and calcium batteries (RMBs and RCBs) could render energy densities comparable to or even higher than LIBs4. However, these systems are always typified by irreversibility. The underlying reason is that it is much more difficult to desolvate and transfer the divalent Mg2+ or Ca2+ than Li+ at the electrolyte-electrode interface and inside electrode materials, and the extra energy required for reaction sometimes causes undesirable parasitic reactions such as electrolyte decomposition and irreversible phase transition in the intercalating cathodes (especially high-voltage oxide cathodes) 5-7. These issues have plagued the practicality of RMBs for over a decade.

In this study, we found that the charge transfer kinetics involving Mg2+ and Ca2+ in both transitional metal oxide cathodes and the metal anodes can be significantly enhanced with methoxylethyleneamine chelants in the solvation sheath8. A peculiar observation is that the methoxylethyleneamine chelants demonstrate a much higher affinity toward the Mg2+ than the ether solvent as demonstrated by the Nuclear Magnetic Resonance (NMR) results, requiring higher energy to remove them from the solvation sheath. However, the large desolvation energies do not translate to more sluggish reaction kinetics; on the contrary, low overpotentials were observed. With a combined effort of the surface chemistry analysis with X-ray photoelectron spectroscopy, solvation sheath characterization with NMR and Raman spectroscopy, electrochemical modeling with Marcus-Chidsey kinetics for interfacial electron transfer, and quantum chemistry calculations, we have identified that the cause of such unconventional behavior is that the charge transfer reactions occur without desolvation. This process is significantly facilitated by the polarizable structures of the chelants in the solvation sheath. By boosting the main reaction kinetics, the undesirable side reactions are suppressed without the solid electrolyte interface as in LIBs, thus opening up a completely different electrolyte design approach to stabilize high-energy RMBs and RCBs. Furthermore, this achievement could help reduce reliance on mining raw materials to build LIBs and provide cheaper and safer options for future energy storage.

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