Characterization and Electrochemical Behaviour of AV4S8 (A=Ga,Ge) Materials as Negative Electrodes of Sodium-Ion Batteries

Lithium-ion batteries have become essential in recent years. Due to both the difficult access and the cost of the alkaline element, a new field of research concerning sodium-ion batteries has recently emerged. Many compounds have been proposed as positive electrodes for Na-ion batteries. However, since sodium does not intercalate into graphite as much as lithium (1), there is a clear need for high capacity materials for negative electrode materials. Intercalation compounds such as Na₂Ti₃O₇ (2) or NaTi₂(PO₄)₃ (3) or Sb-based compounds (4) come usually with a somehow limited capacity (~ 200-400 mAh.g^-1). Here,  we present the possible use of a class of chalcogenide materials, AV4S8 (A=Ga,Ge), as a negative electrode in sodium-ion batteries.

The reactivity of AV₄S₈ with Na leads to a remarkable specific capacity of  800 mAh/g for the first  discharge at a C/20 rate (1 complete charge in 20 hours), followed by a reversible reaction associated with a 500 mAh/g capacity retained after 30 cycles.

Many experiments were performed in order to understand the reaction mechanism of sodium with the AV₄S₈ materials. Electrochemical, in situ X-ray diffraction (Figure 1) and transmission electron microscopy experiments suggest that a smooth conversion mechanism is occurring and that the capacity fade is largely due to reactions with the electrolyte at low potential.

EDX and TEM observations, as well as EELS experiments performed at the vanadium L-edge, indicate that a reduction of the vanadium oxidation state occurs during the discharge process. XAS experiments performed at the Vanadium and Germanium K edges also clearly demonstrate that all the elements are involved during the electrochemical processes.

Synthesis, structural characterization and electrochemical behaviour of AV₄S₈ will be discussed in details in the presentation. Results obtained on the AV₄S₈ phases are very promising and demonstrate that this new class of materials is a good candidate as a negative electrode for Na-ion batteries.

References

(1). M. M. Doeff, Y. Ma, S. J. Visco,L. C. De Jonghe, J. Electrochem.Soc. 140, L169 (1993).

(2). P. Senguttuvan, G.l. Rousse, V. Seznec, J.-M. Tarascon, M.R. Palacin, Chemistry of Materials, 23, 4109-4111 (2011).

(3). S.I. Park, I. Gocheva, S. Okada, J.-i. Yamaki, J. Electrochem. Soc. 158, A1067-A1070 (2011).

(4). A. Darwiche, C. Marino, M.T. Sougrati, B. Fraisse, L. Stievano, L. Monconduit, Journal of the American Chemical Society, 134, 20805-20811 (2012).