The development of new energy storage systems beyond lithium ion batteries is a central task for electric transportation and grid energy storage. Sodium-ion batteries (SIBs) have attracted great interest owing to the natural abundance and low cost of sodium^[1]. However, emerging anode materials for SIBs suffer from reduced cycle stability and limited specific capacity. In contrast to Li and Na, metallic magnesium (Mg) can be safely used as an anode material in Mg-ion batteries (MIBs)^[2]. But, the development of MIBs is hindered by a lack of suitable cathode materials and electrolytes and overall anode-electrolyte-cathode incompatibilities.

In order to fully exploit the cost and safety benefits of SIBs and MIBs, we present herein a hybrid Na/Mg ion battery in which metallic Mg serves as the anode, commercial titanium disulfide (TiS₂) as the cathode, and an electrolyte composed of 1.0 M NaBH₄ + 0.1 M Mg(BH₄)₂ dissolved in diglyme. The stable voltage window of the chosen electrolyte is limited to 0.4 ~ 2.0 V vs. Mg²⁺/Mg, as confirmed by control experiments using a stainless steel current collector. Galvanostatic cycling shows that most of the discharge capacity was released in the voltage window of 0.8~1.2 V (Figure 1a). Good synergetic effect between the anode, cathode and the electrolyte is evidenced by high Columbic efficiency of ~ 100%. Rate capability test demonstrates that the hybrid ion battery shows excellent rate performance (Figure 1b). Cells were initially cycledat 200 mA g^-1 (~ 1 C rate) obtaining a stable discharge capacity of 190 mAh g^-1. Then the current was increased to 500, 1000, 2000 and 4000 mA g^-1. Even at such high rates, the battery cells could deliver stable capacity of 160, 130, 110 and 60 mAh g^-1, respectively. Long term charge-discharge cycling shows that the battery cell could deliver 90 mAh g^-1 after 10000 cycles which is 80 % of its highest capacity at the 10 C rate (Figure 1c).

Elemental analysis of the fully discharged cathode shows that both Na⁺ (~ 90 at.%) and Mg²⁺ (~ 10 at.%) ions are intercalated into the TiS₂ cathode. Meanwhile, the (001) interlayer space is increased from 0.57 nm to 0.66 nm as determined by high transmission electron microscope (HRTEM). On the anode side, nano sized metallic Mg grains are uniformly deposited on the current collector after long term cycling, without formation of any Mg dendrites. The ionic diffusion coefficients of the TiS₂ cathode are determined by galvanostatic intermittent titration technique (GITT), which are 10^-9~10^-10 cm²s^-1 during the whole discharge process. It is believed that the large ionic diffusion coefficients results in high rate capability of the battery. Due to the above excellent electrochemical performance, our hybrid Na/Mg ion battery shows promising potential in stationary energy storage and grid energy storage.

Figure 1. Charge-discharge voltage profiles (a), rate dependent cycling performance (b), and cycling performance of the hybrid Na/Mg ion battery (c).

Acknowledgements

This work was supported by the Ministry of Science and Technology of China under the grant No. 2015CB251103.

References

[1] D. Wang, N. Chen, M. Li, C. Wang, H. Ehrenberg, X. Bie, Y. Wei, G. Chen, F. Du, J. Mater. Chem. A 2015, 3, 8636.

[2] (a) Y. Cheng, L. R. Parent, Y. Shao, C. Wang, V. L. Sprenkle, G. Li, J. Liu, Chemistry of Materials 2014, 26, 4904; (b) D. Aurbach, G. S. Suresh, E. Levi, A. Mitelman, O. Mizrahi, O. Chusid, M. Brunelli, Advanced Materials 2007, 19, 4260.