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St. Andrews team identifies TiC as a promising cathode for Li-air batteries

Researchers at the University of St. Andrews in Scotland report in a paper in the journal Nature Materials that titanium carbide (TiC) may represent a viable, stable cathode for rechargeable lithium-air batteries.

Li-air batteries are receiving intense interest because of their extremely high theoretical specific energy. However, the team, led by Dr. Peter Bruce, notes that the cathodes for lithium-air batteries are “a serious problem.” The basic mechanism of the Li-air (Li-O2) battery requires highly reversible formation and decomposition of Li2O2 at the cathode on cycling. Although carbon is ubiquitously used as the basis of the cathode, its use in an Li-O2 battery is problematic, they note.

Carbon decomposes during oxidation of Li2O2 on charging above 3 V (owing to attack by intermediates of Li2O2 oxidation) and it actively promotes electrolyte decomposition on discharge and charge, rendering it unsuitable for aprotic Li–O2 cells. For example, in a recent detailed study of carbon cathodes in Li–O2 cells, 16% of the products at the cathode on the fifth discharge were Li2CO3; of which 10% was from direct decomposition of carbon and the remainder from decomposition of the electrolyte, promoted by the carbon electrode. Importantly, the proportion of these side reactions increases on cycling. The Li2CO3, formed by the side reactions, deposits on the cathode, leading to electrode passivation, resulting in severe polarization, capacity fading on cycling and premature cell death.

Identifying a suitable alternative cathode to carbon is one of the greatest challenges. Recently, we reported that a nanoporous gold cathode when combined with an electrolyte based on dimethyl sulphoxide (DMSO) exhibits better stability. However, the high mass of an all-gold cathode destroys the key advantage of high specific energy offered by Li-O2 cells over Li ion; nanoporous gold is also too expensive and difficult to fabricate as an electrode.

Here we report that a cathode based on TiC can overcome the disadvantages of carbon and nanoporous gold. It reduces greatly side reactions at the electrolyte/cathode interface (associated with electrode and electrolyte degradation) compared with carbon, and is more stable even than nanoporous gold. It delivers reversible Li2O2 formation/decomposition on discharge/charge. TiC is also fourfold lighter than nanoporous gold, is projected to be less expensive and is easier to handle and form into an electrode.

—Thotiyl et al.

In their paper, they show that a TiC-based cathode reduces greatly side reactions (arising from the electrolyte and electrode degradation) compared with carbon and exhibits better reversible formation/decomposition of Li2O2 even than nanoporous gold.

TiC demonstrated a capacity retention of >98% after 100 cycles (compared with 95% for nanoporous gold). Evidence from Fourier transform infrared (FTIR); powder X-ray diffraction (PXRD); chemical analysis; and differential electrochemical mass spectrometry (DEMS) demonstrated that the reaction at the cathode is overwhelmingly Li2O2 formation/decomposition.

TiC is also four times lighter, of lower cost and easier to fabricate than nanoporous gold.

They suggest that its stability may originate from the presence of TiO2 (along with some TiOC) on the surface of TiC.

Resources

  • Muhammed M. Ottakam Thotiyl, Stefan A. Freunberger, Zhangquan Peng, Yuhui Chen, Zheng Liu and Peter G. Bruce (2013) “A stable cathode for the aprotic Li–O2 battery”, Nature Materials doi: 10.1038/NMAT3737

Comments

HarveyD

More research will soon find the most appropriate material and lower cost method to mass produce improved long lasting cathode and electrolyte for high energy density Li-Air batteries.

Affordable improved batteries will be mass produced by the end of the current decade. By 2020 or so, lighter long range BEVs (800+ Km with 1000+ Wh/Km batteries) will become a reality.

Brotherkenny4

"Li-air batteries are receiving intense interest because of their extremely high theoretical specific energy." How so? Or, on what basis? Are we talking about the cathode energy? Does it include the carbon or TiC, and what about the air? Where does that come in? How is the air delivered, and do you include the mass or volume assiciated with the air pump, plenum etc. in the calculation of what is battery? Please you great researchers of Li-air, someone tell us the facts, not the spin. I have no problem with you looking into using this technology but you continually misrepresent the reality. How can I know that you are honestly pursuing solutions if your disingenuous regarding the rationale for doing the work. I suspect, that the technology may be able to be a low cost solution that may be viable for stationary applications, but I hardly think the energy density will ever meet transportation needs. It is my presumption that Li-air is only energy dense if your discussing the cathode energy density and only if you assume the Li2O2 is the cathode and don't include the none storage support material, and don't even think about the volume of air space and delivery apparatus. If someone can logically and factually discuss how my assumption is incorrect, I would actually appreciate it. Don't waste your time if your going to spew falsehoods.

HealthyBreeze

@Brotherkenny4,

Your concern for the relative benefit when considering total system is reasonable. Air-flow management will to some degree offset the lightweighting gains of LiO2. There have also been traditional concerns with water content in air fouling the reaction. Some membranes have been proposed to keep water vapor out, let O2 through, and they may or may not allow enough O2 through.

That said, the theoretical energy density of Li02 is sufficiently high enough, that for a relatively large battery the system weight, volume and cost of an airflow management system should still produce more range for less weight than current generation commercial batteries.

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