State-of-the-art rechargeable batteries are mainly based on conventional lithium intercalation chemistry, using lithium transition metal oxides as cathode material with typical capacities of 120−160 mA·h/g^-1. The low energy density and/or high cost of these cathode materials have limited their large-scale production and application in Li ion batteries. Discovery of new cathode materials with higher energy density is, thus, a key to realizing more efficient energy storage systems. Recently, lithium−sulfur (Li/S) and lithium−oxygen (Li/O₂) cells have been demonstrated to possess the potential to provide 2−5 times the energy density of conventional Li ion cells. However, both Li/S and Li/O₂ cells suffer from poor cycling performance, which had impeded their commercial utilization.

For instance, the cyclability of Li/O₂ cells is limited by severe electrolyte decomposition and large cell polarization under deep discharge/charge conditions, while Li/S cells suffer from the solubility of intermediate lithium polysulfide species during cycling, which causes the so-called redox shuttle effect and, thus, poor cyclability.

—Cui et al.

Selenium has higher electrical conductivity compared to sulfur and high theoretical gravimetric capacity (678 mA·h g^-1) and volumetric capacity (3,268 mA·h/cm³). In their 2012 paper they we reported that Se represented an attractive cathode material for not only rechargeable lithium ion batteries but also sodium batteries.

There are fundamental issues with the use of selenium that need to be clarified, they noted. For example, they said:

• Charge and discharge voltages are evolving in the Li/Se system during the initial cycles in the carbonate-based electrolyte. Significant polarization occurs once the charge and discharge voltages are stabilized after five cycles, which leads to a low roundtrip efficiency.

• The Coulombic efficiency is quite low during the first 20 cycles. The mechanisms or underlying reasons for this unsatisfactory performance are still not well understood due to inadequate characterization of the battery materials during electrochemical cycling.

• The effects of organic electrolytes on the (de)lithiation process (i.e., both lithiation and delithiation) of the Li/SeS_x cells, if any, are still not clear, nor are the effects of the sulfur content in SeS_x.

In the study, they adopted an ether-based electrolyte in the Li/SeS_x cell to investigate its effect; results showed significantly improved cell performance in terms of voltage profile and Coulombic efficiency.

The voltage profiles of these cells indicate that complete lithiation of selenium to Li₂Se is occurring through the formation of intermediate phases, i.e., Li₂Se_n. This behavior differs from the single-phase transition in carbonate-based electrolyte, as reported earlier. This result clearly suggests that cell performance highly depends on the nature of the electrolyte.

—Cui et al.

To examine electrochemical performance in the ether-based electrolyte, they tested active cathode materials containing Se, SeS₂, and SeS₇ combined with carbon against Li metal anodes between 0.8 and 4.0 V. A discharge capacity of 350, 571, and 833 mA·h g^-1 for the Li/Se, Li/SeS₂, and Li/SeS₇ cells, respectively, was maintained for more than 50 cycles.

Capacity increased with increasing S content in the composites due to its contribution to the overall capacity. The capacity faded a little for all three cells tested. The Coulombic efficiency was nearly 100% for the Li/Se and Li/ SeS₂ cells. The Li/SeS₇ cell showed a relatively low Coulombic during the initial 20 cycles, although much higher capacity.

Using the X-ray Science Division (XSD) beamline 11-ID-C at the Advanced Photon Source, the team carried out in situ synchrotron high-energy x-ray diffraction (HEXRD) studies and complementary, selenium K-edge x-ray absorption near-edge structure (XANES) analysis to observe the chemical changes that take place in these novel electrode materials as they charge and discharge a battery.

These measurements, which were undertaken at more than 12-keV energy, were also done in transmission mode on the XSD bending-magnet beamlines 9-BM-C and 20-BM-B. This technique allowed the team to hone in on the changing chemistry of the selenium atoms in the electrode and how they shift between crystalline and non-crystalline phases as current and lithium ions flow through the experimental battery’s ether-based electrolyte. Raman microscopy at Argonne’s Center for Nanoscale Materials provided additional information about the Li₂Se that was observed on the Li anode of the charged cells.

The team discovered that it is the chemical composition of the electrolyte that seems to have the most impact on the changes that take place. The researchers suggest it might be possible to tune the efficiency of a battery based on these new composites by optimizing the electrolyte and so improve battery performance still further.

The x-ray studies and analysis of the electrochemistry of the electrode as it operates also allowed the team to suggest a plausible chemical mechanism for the processes involved in discharging the battery.

The composite electrode is reduced to form lithium polyselenide with more than four selenium atoms per lithium atom; additional discharging to lower voltage leads to chemical species containing two lithium ions per selenium atom. Charging involves the reverse process. This mechanism is first proposed and experimentally proven by the team, and it is similar to that seen in experimental lithium-sulfur electrodes.

Resources

• Yanjie Cui, Ali Abouimrane, Jun Lu, Trudy Bolin, Yang Ren, Wei Weng, Chengjun Sun, Victor A. Maroni, Steve M. Heald, and Khalil Amine (2013) “(De)Lithiation Mechanism of Li/SeSx (x = 0−7) Batteries Determined by in situ Synchrotron X‑ray Diffraction and X‑ray Absorption Spectroscopy,” J. Am. Chem. Soc. 135, 8047 doi: 10.1021/ja402597g

• 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,” J. Am. Chem. Soc. doi: 10.1021/ja211766q