Mg Battery Anodes Based on p-Block Elements: High Performances and Electrochemical Mechanisms

Since their first commercial version in early 90’s Li‑ion Batteries (LiB) rapidly flooded the market of portable energy storage. Today they power most of our electronic devices. However lithium resources are not widely spread on Earth and dramatic extraction costs are expected. Moreover despite great improvements in capacity delivery and lifetime, LiB might near their limits in the future.

Therefore developing alternative systems gathering high energy density and moderate cost is crucial.^[1] As Mg is light and divalent, high theoretical capacity is available.  It is also abundant and cheap; however, it reacts with conventional electrolytes, and first prototypes of rechargeable Magnesium-ion Batteries (MiB) have to use air-sensitive and narrow-electrochemical-window organohaloaluminate-based electrolytes. ^[2]

Shifting from Mg metal to new anode materials that can fit with conventional electrolytes might be a solution to allow intensive search for performant positive cathode materials and to finally propose high-energy MiB prototypes. Toyota has pointed the way by showing that some specially designed p‑block elements such as Bi, Sn or Sb may electrochemically alloy with Mg and therefore acts as good anode materials.^[3,4,5]

Greatly inspired by these pioneering works, we have undertaken the preparation of negative electrode using p-block elements alone (Bi, Sn, In, etc.) or alloyed together (Bi_1‑xSb_x) and/or with a transition metal (NiBi₃). In comparison with electrodeposited or nanosized materials, in this work a special effort was made in designing simple electrode formulation especially using easy-made ball-milled sample powders. This communication aims to present our preliminary investigations in this new field of research.

In case of Bi, we managed to prepare electrode formulations exhibiting performant electrochemical performances and shows impressive resistance at high current rates.  

Figure 1: Rate capability of ball-milled Bi based negative electrodes.^[7]

In parallel with performances quests, we were also interested in investigating the electrochemical mechanism that occurs during the battery cycling. For the first time, the electrochemical alloying Mg₃Bi₂ during discharge was investigated by X-ray diffraction using a specially design in situ cell. A biphasic transition from Bi to Mg₃Bi₂was evidenced here without any amorphization.

Fig. 2: Electrochemical reversible alloying of Bi with Mg followed by in situ X-ray diffraction.^[7]

Other p-block elements are also electrochemically active versus Mg, like In. Indeed the formation of MgIn was evidenced by ex situ XRD, corresponding to a high capacity of 470mAh/g. However until now Bi seems to exhibit a unique affinity which still needs to be understood.

Fig. 3: Electrochemical behavior of In electrode and XRD showing MgIn formation at the end of the discharge.^[8]

Removing magnesium metal from the batteries leads to rechargeable Mg-ion batteries using conventional electrolytes. However it also involves that either the cathode or the anode material should initially contains Mg. Following this idea and using the same synthesis route, Mg₃Bi₂as well as other Mg-alloys were prepared by ball-milling and evaluated in optimized formulations as MiB negative electrodes. 

References:

[1] Van Noorden. Nature 2014

[2] Aurbach et al. Nature. 2000

[3] Arthur et al. Electrochem. Commun. 2012

[4] Singh et al. Chem. Commun. 2013

[5] Muldonn et al. Chem. Rev. online

[6] Shao et al. Nano Lett. 2013

[7,8] Murgia et al. in prep.