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UCL, UI researchers devise method to optimize magnesium chromium oxide cathode materials; disordered materials

University College London and University of Illinois at Chicago researchers report a new, scalable method for making a material that can reversibly store magnesium ions at high-voltage. The study is published RSC journal Nanoscale.

Chromium oxides with the spinel structure have been predicted to be promising high voltage cathode materials in magnesium batteries. Perennial challenges involving the mobility of Mg2+ and reaction kinetics can be circumvented by nano-sizing the materials in order to reduce diffusion distances, and by using elevated temperatures to overcome activation energy barriers.

Herein, ordered 7 nm crystals of spinel-type MgCr2O4 were synthesized by a conventional batch hydrothermal method. In comparison, the relatively underexplored Continuous Hydrothermal Flow Synthesis (CHFS) method was used to make highly defective sub-5 nm MgCr2O4 crystals. When these materials were made into electrodes, they were shown to possess markedly different electrochemical behavior in a Mg2+ ionic liquid electrolyte, at moderate temperature (110 °C).

The anodic activity of the ordered nanocrystals was attributed to surface reactions, most likely involving the electrolyte. In contrast, evidence was gathered regarding the reversible bulk deintercalation of Mg2+ from the nanocrystals made by CHFS.

—Hu et al.

While it is at an early stage, the researchers say it is a significant development in moving towards magnesium-based batteries. To date, very few inorganic materials have shown reversible magnesium removal and insertion, which is key for the magnesium battery to function.

Lithium-ion technology is reaching the boundary of its capability, so it’s important to look for other chemistries that will allow us to build batteries with a bigger storage capacity and a slimmer design.

Magnesium battery technology has been championed as a possible solution to provide longer-lasting phone and electric car batteries, but getting a practical material to use as a cathode has been a challenge.

—co-lead author, Dr Ian Johnson (UCL Chemistry)

One factor limiting lithium-ion batteries is the anode. Low-capacity carbon anodes have to be used in lithium-ion batteries for safety reasons, as the use of pure lithium metal anodes can cause dangerous short circuits and fires.

In contrast, magnesium metal anodes are much safer, so partnering magnesium metal with a functioning cathode material would make a battery smaller and store more energy.

Previous research using computational models predicted that magnesium chromium oxide (MgCr2O4) could be a promising candidate for Mg battery cathodes.

Inspired by this work, UCL researchers produced a ~5 nm, disordered magnesium chromium oxide material in a very rapid and relatively low temperature reaction.

Collaborators at the University of Illinois at Chicago then compared its magnesium activity with a conventional, ordered magnesium chromium oxide material ~7 nm wide.

The two types of crystals behaved very differently, with the disordered particles displaying reversible magnesium extraction and insertion, compared to the absence of such activity in larger, ordered crystals.

This suggests the future of batteries might lie in disordered and unconventional structures, which is an exciting prospect and one we’ve not explored before as usually disorder gives rise to issues in battery materials. It highlights the importance of seeing if other structurally defective materials might give further opportunities for reversible battery chemistry.

—Professor Jawwad Darr (UCL Chemistry)

We see increasing the surface area and including disorder in the crystal structure offers novel avenues for important chemistry to take place compared to ordered crystals. Conventionally, order is desired to provide clear diffusion pathways, allowing cells to be charged and discharged easily—but what we’ve seen suggests that a disordered structure introduces new, accessible diffusion pathways that need to be further investigated.

—Professor Jordi Cabana (University of Illinois at Chicago)

UCL and the University of Illinois at Chicago intend to expand their studies to other disordered, high surface area materials, to enable further gains in magnesium storage capability and develop a practical magnesium battery.

Funding for the project was provided by the Joint Center for Energy Storage Research, a US Department of Energy Innovation Hub, and the JUICED Energy Hub by the Engineering and Physical Sciences Research Council.

Resources

  • Linhua Hu, Ian D. Johnson, Soojeong Kim, Gene M. Nolis, John W. Freeland, Hyun Deog Yoo, Timothy T. Fister, Liam McCafferty, Thomas E. Ashton,c Jawwad A. Darr and Jordi Cabana (2018) “Tailoring the electrochemical activity of magnesium chromium oxide towards Mg batteries through control of size and crystal structure” Nanoscale doi: 10.1039/C8NR08347A

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