|Sketch of the Sn/C/CGPE/ Li2S/C polymer battery. The battery is formed by a Sn/C composite anode, a PEO-based gel polymer electrolyte, and a Li2S/C cathode. PEO=poly(ethylene oxide). Credit: Hassoun and Scrosati. Click to enlarge.|
Researchers at the Università degli Studi di Roma La Sapienza have developed a novel polymer tin sulfur lithium-ion battery that takes advantage of the high theoretical specific energy and energy density of the lithium-sulfur battery chemistry (2,500 Wh kg-1 and 2,800 Wh L-1 respectively, earlier post), while avoiding the shortcomings that have hindered commercialization of this type chemistry.
Rather than taking the more conventional approach of using a sulfur cathode and a lithium metal anode, Jusef Hassoun and Bruno Scrosati have developed a lithium-metal-free battery, using a carbon lithium sulfide composite as the cathode and a tin carbon composite anode. In a paper published online 28 February in the journal Angewandte Chemie International Edition, they report demonstrating a specific energy of the cell on the order of 1,100 Wh kg-1.
Lithium-sulfur cells are based on the electrochemical reaction:
16Li + S8 8Li2S
While Li-ion batteries use a process called intercalation to store lithium ions by inserting the ions between molecules in the electrode, lithium-sulfur batteries rely on a multi-step redox reaction with sulfur that results in a number of stable intermediate sulfide ions. This storage process, in theory, reduces limitations of electrode structure—thus enabling higher capacity in similar volumes.
In the conventional approach, at the negative electrode lithium is dissolved into solution on discharge and plated out on charge. The solubility of the intermediate sulfide ions depends on the solvent used in the electrolyte, and the voltage vs. discharge capacity profile of the cell thus depends on the solvents used.
The practical development of the lithium–sulfur battery has been hindered to date by a series of shortcomings. A major hurdle is the high solubility in the organic electrolyte of the polysulfides Li2Sx (1≤x≤8) that form as intermediates during both charge and discharge processes. This high solubility results in a loss of active mass, which is reflected in a low utilization of the sulfur cathode and in a severe capacity decay upon cycling. The dissolved polysulfide anions, by migration through the electrolyte, may reach the lithium metal anode, where they react to form insoluble products on its surface; this process also negatively impacts the battery operation.
...The key challenge is then to totally renew the chemistry of this battery such as to achieve an advanced configuration that can consistently provide high capacity, a long cycle life, and safe operation. Herein, we report an example of a lithium metal- free new battery version and demonstrate that, to a large extent, it can effectively meet these targets. In contrast to most of the Li–S batteries proposed to date, which are fabricated in the “charged” state, that is, using a carbon–sulfur composite cathode that necessarily requires a lithium metal counter electrode (anode) to assure the 16Li+S8→8Li2S discharge process, we propose to fabricate the battery in the “discharged” state by using a carbon lithium sulfide composite as the cathode.
—Hassoun and Scrosati
Hassoun and Scrosati also replaced the common liquid organic solutions with a gel-type polymer membrane. Since the lithium ions necessary to drive the electrochemical process are provided by the Li2S/C cathode, any material capable of accepting and releasing lithium ions can be chosen as anode to replace lithium metal, they said. They chose a tin/carbon nanocomposite, Sn/C 1:1 in weight. The specific capacity of the improved Sn/C electrode matches that of the Li2S/C electrode, and Sn/C has high chemical stability.
The electrochemical process is basically the conversion of lithium sulfide into sulfur with the release of lithium ions: 2.2Li2S/C→2.2S+C+4.4Li++4.4e-. The lithium ions travel through the electrolyte to reach the anode where they form an alloy with the tin metal: 4.4Li++Sn/C+ 4.4e-→Li4.4Sn+C. The total process is the reversible reaction of the lithium–tin alloy with elemental sulfur to form tin metal and lithium sulfide.
...The reported results show that this innovation is effective in controlling most of the issues that have, to date, prevented practical exploitation of the lithium–sulfur electrochemical system and give rise to a novel tin–lithium sulfide battery that provides a specific energy on the order of 1100 Wh kg-1, a value not previously achieved for a lithium metal-free battery.
—Hassoun and Scrosati
The team notes that “the road to a practical lithium–sulfur battery is still long”; optimization of the electrode morphology and cell structure are needed to further improve the cycle life and the rate capability.
Jusef Hassoun and Bruno Scrosati (2010) A High-Performance Polymer Tin Sulfur Lithium Ion Battery. Angew. Chem. Int. Ed. 49, 1 – 5 doi: 10.1002/anie.200907324
J. Hassoun, A. Fernicola, M.A. Navarra and B. Scrosati (2010) Advanced lithium-ion batteries based on a nanostructured Sn-C anode and an electrochemically stable LiTFSi-Py24TFSI ionic liquid electrolyte J Power Sources 195, 574 - 579 doi: 10.1016/j.jpowsour.2009.07.046