The advent of lithium-ion electrochemical has ushered in a new era of high energy density rechargeable batteries, with applications ranging from portable electronics to electric vehicles.¹ However, several challenges have stymied the implementation of lithium-ion batteries that approach the thermodynamic capacity and energy density limit. This is largely associated with the use of a lithium-metal anode, which under extensive cycling conditions and current densities promotes the formation of dendritic lithium deposits. Lithium dendrites result in the formation of electrochemically “dead” lithium, resulting in poor cycling efficiency.² In addition, dendritic lithium has been implicated in the cell shorting and thermal runaway. To mitigate such problems, alternative energy storage systems have been heavily pursued recently, with efforts largely focused on the development of sodium-ion and magnesium-ion electrochemical cells.³

The implementation of a magnesium metal anode is highly advantageous to lithium, as the volumetric capacity is nearly double (3833 mAh/mL vs. 2062 mAh/mL, respectively) and the deposition morphology is described as non-dendritic.² As such, considerable attention has been focused the development of magnesium electrochemical cells utilizing a magnesium metal anode, high capacity cathode, and suitable electrolyte. One of the leading cathode materials is the Chevrel-phase Mo₆S₈, introduced in 2000 with an experimentally determined specific capacity of 105 mAh/g.⁴ Recently, Sheha described an anatase TiO₂/ reduced graphene oxide (rGO) composite cathode material was used in a magnesium-ion electrochemical cell utilizing a Mg metal anode and polyvinyl alcohol/ MgBr₂/ tetraglyme/ H₂O electrolyte. This cathode was shown to maintain cyclability to fifty cycles. Following this report and the plethora of publications concerning the insertion electrochemistry of anatase-phase titanium dioxide, we decided to investigate the feasibility of TiO₂/ carbon black/ poly tetrafluoroethylene (PTFE) as an electrode for magnesium-ion batteries. 

Herein, we report a study concerning the electrochemistry of Mg/TiO₂ cells as well as a full characterization using spectroscopic and microscopic techniques. Figure 1 shows the voltage profiles of the TiO₂/Mg coin cell with the All Phenyl Complex (APC) electrolyte between 0.05 and 2.2 V at a current density of 5 μA/ mg. During the first discharge, TiO₂exhibits a well-defined plateau at approximately 0.5 V. The first discharge capacity is ~80 mAh/g. The material experiences a large polarization where the charge plateau increases to 1.2 V. The corresponding irreversible capacity in the first cycle is 20  mAh/g, which may arise from the poor conductivity of the electrode as well as the electrolyte side reactions. The close capacity between the discharge and charge process from the second cycle manifests excellent reversibility of the electrode. Figure 1 shows discharge and charge capacities with respect to cycle number at the same current density over 50 cycles. The capacity of the electrode remains at a quite considerable value after 50 cycles, which demonstrates relatively promising cycling stability. 

Following electrochemical galvanostatic/ potentiostatic charge/ discharge experiments, TiO₂ electrodes were carefully removed from electrochemical cells and examined for magnesium insertion mechanism. Following a careful transfer step to mitigate any exposure to oxygen, the TiO₂ electrodes were examined using scanning electron microscopy (Phillips XL30) with an equipped energy dispersive X-ray detector (EDX), synchrotron X-ray diffraction, and ultraviolet-visible spectroscopy. Both the spectroscopic and microscopic characterization confirm that a Mg:Ti ratio of ~0.1:1 is able to reversibly shuttle between the working and counter electrodes in this battery system. 

Acknowledgement

The authors acknowledge the support by the seed funding from Sustainable Power and Energy Center under the Frontier of Innovation Award by Vice Chancellor of Research at UC San Diego.

References

(1) Whittingham, M. S. Chem. Rev. 2004, 104, 4271 – 4301.

(2) Muldoon, J.; Bucur, C. B.; Gregory, T. Chem. Rev. 2014, 114, 11683 – 11720.

(3) Dahbi, M.; Yabuuchi, N.; Kubota, K.; Tokiwa, K.; Komaba, S. Phys. Chem. Chem. Phys. 2014, 16, 15007 – 15028.

(4) Aurbach, D.; Weissman, I.; Goffer, Y.; Levi, E. Chem. Record 2003, 3, 61 – 73.