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Researchers show feasibility of lithium-metal-free anode for Li-air battery; addressing one of three main barriers to Li-air battery development
1 November 2012
|Voltage profiles (A) and initial cycling behavior (B) of the LixSi-O2-C cell. Cycling current: 200 mA g−1carbon. Credit: ACS, Hassoun et al. Click to enlarge.|
Researchers from University of Rome Sapienza (Italy), Hanyang University (Korea) and the Argonne National Laboratory (US) have shown that the highly reactive lithium metal anode typically projected for use in Li-air batteries can be replaced with a lithiated silicon-carbon anode. Although the resulting battery has lower voltage and capacity than a conventional Li-air battery, it offers enhanced safety and an energy density higher than Li-ion batteries.
The elimination of the lithium metal anode addresses one of the major issues affecting the development of the lithium-air battery: the safety hazard of the anode. “To our knowledge,” the team reported in a paper published in the ACS journal Nano Letters, “this is the first report that evidences the feasibility of a lithium-metal-free/air battery and we believe that this breakthrough may contribute to the progress of the lithium-air battery technology providing a step forward for its practical development.”
Li-air batteries are extremely attractive in theory for energy-demanding applications such as electric vehicles due to an energy density, in principle, of of 11,420 Wh kg−1. (Earlier post.) In the most conventional configurations, the researchers noted, the Li-air comprises a lithium metal anode; a separator embedded with a lithium-conducting, nonaqueous electrolyte; and a carbon (with or without catalyst) cathode.
Oxygen reduces at the cathode while lithium oxidizes at the anode, leading to the formation of lithium peroxide: 2Li + O2 ⇆ Li2O2. The reaction sequence is complex, proceeding via a series of steps involving an intermediate O2−• radical anion species.
...there are three main issues that have so far prevented the full development of the lithium air battery; they are: (i) the instability of the electrolytes in the cell environment, (ii) the limited reversibility of the electrochemical process, and (iii) the reactivity of the lithium metal anode.
The electrolyte issue has been addressed by searching media expected to be more stable than the common carbonate organic electrolytes, such as dimethoxy ethane-based solutions, ionic-liquid-based solutions, and poly(ethylene oxide)-lithium salt, for example, PEO-LiCF3SO3 polymer membranes, however, with scarce success. Issue (ii) has been addressed by developing suitable cathode morphologies and also exploring the use of catalysts.
The third issue has not even been considered since, to our knowledge, all of the Li-air works so far reported refer to batteries based on lithium metal as the anode. In this Letter we show that the issue may be successfully addressed by replacing the lithium metal anode by a lithiated silicon in combination with a stable end-capped glyme electrolyte, such as to form a lithium-metal-free, lithium-ion, silicon−oxygen battery.—Hassoun et al.
The lithiated silicon electrode (LixSi) was prepared by contacting spherical nanostructured silicon−carbon composite particles with a lithium foil. The resulting electrode consists of micro-sized carbon particles containing nanosized lithiated silicon particles. Micrographs showed that the electrode is covered by a solid−electrolyte interface (SEI) film induced by the lithiation process.
The electrolyte used was a solution of a lithium triflate salt (LiCF3SO3) in tetraethylene glycol dimethyl ether (TEGDME) adsorbed in a glass fiber separator. This electrolyte is based on a linear, high molecular weight, end-capped glyme solvent, carefully prepared to reduce impurities or residual water down to traces lower than 10 ppm. The stability of the LiCF3SO3-TEGDME electrolyte in the environment of the lithium−oxygen cell has been demonstrated in previous papers.
|Scheme of the full cell. Click to enlarge.|
They first tested the electrochemical behavior of the LixSi electrode in a half lithium cell using the selected LiCF3SO3-TEGDME electrolyte. The electrode exhibited a fully reversible capacity of 780 mAh g−1 at an average voltage of 0.3 V vs Li. They also tested the electrochemical behavior of the O2−C oxygen cathode in a LiCF3SO3-TEGDME electrolyte half lithium cell. The O2−C electrode demonstrated excellent behavior, both in terms of cycling voltage and of limited polarization.
They combined the two LixSi and O2−C electrodes into a complete battery operating at around 3 V. The cell combines the electrochemical processes of the two electrodes and operates with the following reversible reaction:
Assuming an operating discharge voltage of 2.40 V, the theoretical energy density of the complete (anode and cathode) LixSi−O2 battery here reported may be estimated as equal to 980 Wh kg−1, a value considerably higher than that offered by conventional, 3.6 V, graphite anode and lithium cobalt oxide cathode, lithium-ion batteries, assumed as 384 Wh kg−1. We speculate that, by a proper cell design, a valuable a practical energy density can be eventually achieved for our lithium-metal-free LixSi/O2−C battery.
Obviously, the replacement of lithium metal with a lithium metal alloy, where the activity of lithium is necessarily lower than unity, entails a penalty in terms of both voltage and capacity; however, to our opinion, this drawback is favorably counterbalanced by the enhancement in safety, also considering that in the lithium-metal-free version, the battery has still an energy density that may result to be higher than that of common lithium ion batteries.—Hassoun et al.
The team also observed a slight voltage decay upon cycling; the researchers speculate that this is associated with a progressively increase of the LixSi anode overpotential due to attack of oxygen permeating across the electrolyte. They are now attempting to address this issue and will report on their progress in a future paper.
Earlier this year, the researchers from University of Rome Sapienza and Hanyang University also demonstrated a lithium–air battery capable of operating over many cycles with capacity and rate values as high as 5,000 mAh gcarbon−1 and 3 A gcarbon−1, respectively. (Earlier post.)
Jusef Hassoun, Hun-Gi Jung, Dong-Ju Lee, Jin-Bum Park, Khalil Amine, Yang-Kook Sun, and Bruno Scrosati (2012) A Metal-Free, Lithium-Ion Oxygen Battery: A Step Forward to Safety in Lithium-Air Batteries. Nano Letters doi: 10.1021/nl303087j
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