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.
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