Beyond Chevrel Phase Mo6S8 : High Energy Density Mg Battery Cathodes

Rechargeable magnesium batteries have recently gained more and more attention for a possible candidate of post lithium-ion batteries, because of i) potentially high energy density ((3.0V x 2e^- for Mg) / (4.0V x 1e^- for Li) = 1.5), ii) low cost (high clarke number; Mg (1.93) vs. Li(0.006)) and iii) intrinsic safety (no dendrite growth for Mg, while needle-like dendrite for Li).  In 2000, a great success of Mg battery was achieved by Prof. Aurbach in the system comprising Mg metal anode, Chevrel phase Mo₆S₈cathode and Grignard based electrolyte [1].  The prototype Mg battery can work very well over >2000 cycles.  Guided by this discovery, intensive researches on anode, cathode and electrolyte materials have accelerated all over the world, to maximize the advantages of the Mg battery system.

Although there have many hurdles to be overcome in the Mg batteries, the biggest hurdle is that the cathode energy density stays much lower than expected.  The Chevrel phase Mo₆S₈ is the only reported cathode material with good cyclability, but due to low operation voltage (1.2 V vs. Mg) and low capacity (122 mAh/g), it is theoretically difficult to realize the Mg battery which is competitive with lithium-ion batteries in term of energy density.  Therefore, cathode materials having high operation voltage and high capacity are strongly desired.  Previously, we worked on MnO₂ cathodes and tried to understand its reaction mechanisms.  Although MnO₂ cathodes show higher average voltage (1.5 V vs. Mg) and high initial capacity (280 mAh/g) [2], they faced certain restrictions (i.e. structural deformation, thermodynamically preferred conversion reaction and resistive shell formation (Figure 1)) during magnesium ion intercalation into the host structure, resulting in a severe capacity fading during cycling [3,4].  Based on this knowledge, we have been currently researching solutions to such issues while continuously trying to find new cathode candidates.  For example, CaFe₂O₄ phase MgMn₂O₄ [5], carbon cluster fullerene (C₆₀) [6], amorphous vanadium oxide [7], and so on.

In this presentation, we will discuss about key challenges in improving the cathode performances of these materials and in further enhancing the energy density of Mg batteries.

References    [1] D. Aurbach et al., Nature, 407 (2000) 724.  [2] R. Zhang et al., Electrochem. Commun., 23 (2012) 110.  [3] C. Ling et al., submitted for publication (2014).  [4] T. Arthur et al., submitted for publication (2014).  [5] C. Ling et al., Chem. Mater., 25 (2013) 3062.  [6] R. Zhang et al., submitted for publication (2014).  [7] T. Arthur et al., submitted for publication (2014).