The discovery of new electrode materials is key to realizing safe and efficient electrochemical energy storage systems essential to enabling future green energy technologies. Beyond conventional intercalation chemistry, reaction of lithium with sulfur and oxygen (so-called “Li-air” batteries) have the potential to provide 2 to 5 times the energy density of current commercial systems. However, both Li/S and Li/O₂ systems suffer from cycling performance issues that impede their commercial applications: Li/O₂ cycling is limited by electrolyte decomposition and large cell polarization; Li/S suffers from the low conductivity of S and the solubility of intermediary polysulfide species during cycling.

Here we explore the potential of selenium, a d-electron containing member of group 16 with high electrical conductivity, as an electrode material for rechargeable batteries. We show that Se and mixed Se_xS_y represent an attractive new class of cathode materials with promising electrochemical performance in reactions with both Li and Na ions. Notably, unlike existing Na/S batteries that only operate at high temperature, these new Se and Se_xS_y electrodes are capable of room temperature cycling against Na. Accordingly, Se not only provides opportunities for developing new high performance rechargeable batteries, including mixed chalcogenide systems but also has the potential to enhance our fundamental understanding of batteries.

—Abouimrane et al.

The team built coin cells using carbon nanotube-containing composite Se and SeS₂ electrodes (Se−C and SeS₂−C) and metallic Li and Na. Among their findings were:

• The systems exhibited strong cycle life, with repeated cycling up to 100 cycles.

• The Li/Se−C system sustained a reversible capacity of ∼500 mAh g⁻¹ for >25 cycles at low current density (10 mA g⁻¹, ∼C/60), which reduced to ∼300 mAh g⁻¹ at higher current density (50 mA g⁻¹, ∼C/12) with a small fade for 100 cycles.

• For Na/Se−C, a lower capacity was observed with an excellent cycle life (265 mAh g⁻¹ at 50 mA g⁻¹)

• Extension of the cycling potential up to 4.6 V did not adversely impact the electrochemical performance of Li/Se−C, which sustained a capacity of 280 mAh g⁻¹ over 80 cycles (100 mA g⁻¹, ∼C/6). This allows for use of high potential windows, unlike for Li/S, where charging beyond 3.6 V disables any further cycling.

The Selenium-containing electrodes offer several advantages over the widely studied sulfur systems. Some of these are:

1. Se has electric conductivity, approximately 20 orders of magnitude greater than S. This facilitates cycling with Na at room temperature, while Na/S operation is limited to elevated temperatures (300−350 °C).

2. Despite a lower theoretical gravimetric capacity, the high density of Se allows for volumetric capacities that are comparable to S.

3. The Se systems provide higher output voltages than S and, accordingly, higher energy densities, a key advantage in commercial applications. For Li/Se, the output voltage is at least 0.5 V higher for Li/S.

4. For Na/Se, the theoretical capacity exceeds that observed for Na/S at room temperature.

From a practical standpoint, the toxicity of Se is comparable to S and other common electrode elements (LD50 [median lethal dose]: Se ∼6.2 g; S ∼8.4 g; Co ∼6.7 g; Ni ∼5.0 g), and it is included, in trace quantities, in supplements and personal care items. While the lower abundance (and higher cost) of Se compared to S may impede large scale commercialization, this could be largely offset by using Na rather than Li, and/or by using mixed Se_x S_y systems.

—Abouimrane et al.

Not only does the Se electrode show promising electrochemical performance with both lithium and sodium anodes, but the additional potential for mixed Se_xS_y systems allows for tunable electrodes, combining the high capacities of S-rich systems with the high electrical conductivity of the d-electron containing Se.

A preliminary study of a Se_xS_y material showed higher theoretical capacities than the Se alone, with improved performance and conductivity compared to S. For Li/SeS₂−C, the discharge capacity is 30% greater than Li/Se−C in the range 0.8 to 4.6 V (512 vs 394 mAh g⁻¹ after 30 cycles, 50 mA g⁻¹ current density).

SeS₂−C can also be cycled with Na at room temperature, with a capacity of 288 mAh g⁻¹ sustained over 30 cycles.

As Se and S are infinitely miscible, with many readily available solid solutions (e.g., Se₅S, Se₅S₂, Se₅S₄, SeS, Se₃S₅, SeS₂, SeS₇), Se_xS_y materials represent a broad class of new battery electrodes with higher theoretical capacities than Se alone (675−1550 mAh g⁻¹ for systems above) with improved conductivity (and room temperature cycling) compared to S alone. Systems with even lower Se proportions, i.e. SeS₂₀, can be easily prepared.

In the current drive to discover and optimize materials for electrochemical energy storage, this new class of room temperature Li- and Na-based Se_xS_y rechargeable batteriespaves the way for new, promising opportunities to enable high energy batteries for transportation and grid applications.

—Abouimrane et al.

Resources

• Ali Abouimrane, Damien Dambournet, Karena W. Chapman, Peter J. Chupas, Wei Weng, and Khalil Amine (2012) A New Class of Lithium and Sodium Rechargeable Batteries Based on Selenium and Selenium–Sulfur as a Positive Electrode. Journal of the American Chemical Society doi: 10.1021/ja211766q