Cathode Coatings for Magnesium Ion Batteries
Authors: Hawthorne Krista L , Vaughey John T. , Han Binghong , Kim Soojeong , Fister Timothy T 
Published: February 2020

Over the past 25 years, significant effort has gone into perfecting lithium ion battery chemistries, including materials coatings, electrolytes, and electrolyte additives to improve safety, capacity, and cyclability. [1,2] By comparison, the area of non-lithium-based batteries is far less explored due to the relative novelty of the technology. Magnesium ion batteries in particular are promising as a next-generation energy storage technology due to the forecasted lower costs. The best cathode materials to date have been metal sulfides and have been demonstrated to achieve high cycleability; however, their capacity and voltage are relatively low compared to state of the art Li ion batteries. One of the challenges in demonstrating high voltage MV-based systems is identifying a cathode capable of desolvating Mg2+ at the cathode-electrolyte interface in order to allow Mg2+
insertion. [3] Additionally, magnesium electrolytes can become quite complicated, with highly solvated ions, large anions, and a variety of both solvents and ion pairs (i.e. MgxClx species). For most electrolyte/cathode pairs, the interaction between electrolyte and cathode surface is not well understood. In the case of the Chevrel Mo6S8 cathode and APC electrolyte, Wan and Prendergast calculated that Mg-Cl species actually interact quite favorably with Mo6S8, which facilitates breaking the Mg-Cl bond and Mg2+ insertion. [4]
The cathode-electrolyte interface of magnesium cathode materials is what facilitates desolvation and Mg2+ insertion. By modifying the surface of these cathodes, improvements in performance and lifetime of magnesium ion batteries could be achieved. Materials such as metal sulfides and metal nanoparticles have been demonstrated to interact with the solvated Mg complex and improve the insertion rates of Mg2+into a host lattice. As the Chevrel Mo6S8 was calculated to favorably interact with magnesium electrolytes, it is possible that other transition metal sulfides will provide the same type of interactions and facilitate Mg2+ insertion into cathode materials. Here we focus on Prussian blue type cathode materials, as they have a relatively high voltage and have demonstrated issues with reaching their full theoretical capacity due to desolvation and ion pairing issues. [5] Coatings of Ag­2S and MnS were found to decrease the accessible capacity by about 40% (Figure 1), with similar behavior found in two different coating methods. As cathodes and electrolytes are highly particular in their interactions, coatings may also need to be tailored to the specific cathode/electrolyte system.
Figure 1. Charge/discharge curves for Ni[Fe(CN)6] with Mg(TFSI)2 in propylene carbonate electrolyte, with Ag2S and MnS coatings. Cycled at 10 mA/g with a BP2000 carbon anode.
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Figure 1