XPS Study of the Interface Evolution of Carbonaceous Electrodes for Li-O₂Batteries during the 1st Cycle

Abstract

The surface chemistry of the commonly employed positive electrode substrate Super C65 carbon was investigated during the 1 st cycle of a Li-O 2 battery with a typical ether electrolyte (0.2 M LiTFSI in Diglyme) by performing in situ online electrochemical mass spectrometry (OEMS), ex situ scanning electron microscopy (SEM), X-ray diffraction (XRD) and X-ray Photoelectron Spectroscopy (XPS). During discharge, a nanometer-thin (< 5 nm) layer of Li 2 O 2 forms homogeneously throughout the electrode before it is passivated at a final specific charge of ∼365 mAh/g. Higher discharge charge rates lead to thinner and more densely packed layers of Li 2 O 2 . Electrolyte decomposition also occurs in parallel, as evidenced by the continuous increase of LiF and sulfur containing species of the degraded LiTFSI salt. On charge, O 2 evolves initially according to a 2e − /O 2 rate, but as the cell over-potentials increase, O 2 evolution rate approaches 4e − /O 2 , thus demonstrating a significant extent of parasitic side-reactions. Unlike the discharge, the charge leads to inhomogeneous electrode reactions causing a continuous removal of Li 2 O 2 with no indication of LiO 2 or Li 2 O intermediates. The decomposition side-products formed during discharge are also removed and the spectra of the pristine electrode are nearly retained at a full charge. Our results show that the analysis of Li-O 2 battery chemistry requires broad approach based on the combination of various electrode surface sensitive techniques, such as XPS and OEMS, in order to provide further fundamental understanding of the mechanisms behind the complex electrochemistry involving the Li and O 2 as well as several other predominant degradation products of the electrode and electrolyte. Non-aqueous electrolyte based Li-O 2 batteries have been a subject of intensive research during the past years, mainly because of their high theoretical specific charge (∼1100 mAh/g Li2O2 ), which provide promising prospects for their implementation in future energy storage systems.1 Several challenges do however remain, of which the most notable are the low round-trip efficiency and poor cycling performance, both partly related to the electrode/electrolyte instability.2,3 On discharge of the Li-O 2 cell, gaseous oxygen dissolves in the electrolyte and is reduced at the positive porous electrode where it combines with Li + (from the negative Li metal electrode) to form solid Li 2 O 2 deposits according to forward reaction of 2Li + O 2 Li 2 O 2 . Upon charge, the reaction is reversed to release the initial reactants Li + and O 2 gas. Despite extensive research on several classes of electrodes and electrolytes (including carbons, gold, carbides, oxides substrates combined with carbonate, sulfoxide, [4][5][6] nitrile, 4,5,7 amide, ionic liquid 8,9 or ether 10-13 based electrolytes, to mention a few), no composition has so far displayed sufficient chemical stability toward the reduced oxygen species to sustain a satisfactory number of discharge/charge cycles.1 The cell failure has been...

Dual Heteroatom‐Doped Carbon Nanofoam‐Wrapped Iron Monosulfide Nanoparticles: An Efficient Cathode Catalyst for Li–O2 Batteries

“…In addition, they contain other bonds, CÀF x (292.2 AE 0.5 eV) and RÀCÀC( 285.1 eV), ( Figure S5 ba nd e). [43,50] The presence of CÀF x bonds in all samples is likely ar esult of the electrolyte interaction with Nafion binder used in this study.T he O1 ss pectra of cycled FeS-C electrode show am ajor peak at 533.2 eV for the CÀOb ond, am inor peak at 532.0 eV for ÀCO 3 or Li 2 CO 3 ,a nd an egligible Li 2 O 2 peak at 531.5 eV,w hich assertst he maximum oxidation of Li 2 O 2 compound even at the 100th cycle (Figure 5g). [44,50] The cycled MC and FeS cathodes ( Figure S5 ca nd f) present ah igh intensity of Li 2 CO 3 at 532.1 AE 0.2 eV,w hich suggestst he cause of increased high overpotential at the 100th cycle.…”

“…The C1ss pectra of the discharged FeS-C cathode (Figure 5f) show am ajor peak at 284.5 eV for CÀCb onds and other minor peaks related to CÀO, C=O, ÀCO 3 ,a nd CF x . [43,44] Similarly,t he discharged MC and FeS cathodes show am ajor peak at 287.0 AE 0.3eVf or C=O, which is possibly the result of the decomposition of ether solvent( TEGDME)o nt heir cathodes urface. The O1s spectra of the dischargedF eS-C, MC, and FeS cathodes how as ignificant peak at 531.5 AE 0.3eVf or Li 2 O 2 formation, accompanied by other species, such as ÀCO 3 ,C =O, and CÀOf unctional groups (Figures 5e and S5 ca nd f).…”

Electrochemical Properties of Large-Discharge-Capacity Air Electrodes with Nickel Foam Sheet Support for Lithium Air Secondary Batteries

“…Some papers 23,27 have reported very large gravimetric discharge capacities of 5,000-10,000 mAh/g-carbon even though the areal loading of air-electrode material was less than 1 mg/cm 2 . Actually, these gravimetric values correspond to rather small areal capacity of less than 2-3 mAh/cm 2 , [28][29][30][31][32][33][34][35] comparable to that of conventional lithium ion batteries. This low loading might result in the low energy density of practical LAB cells.…”

Nanoconfined growth of lithium-peroxide inside electrode pores: a noncatalytic strategy toward mitigating capacity–rechargeability trade-off in lithium–air batteries

“…The single component Li1s spectra at ~ 55 eV and O1s spectra at ~ 532 eV confirm the deposition of Li 2 O 2 as the major DC product. 36 Besides, The O1s spectra show additional peaks at > 532 eV that can be assigned to -CO 3 , -CO or SO 2 species resulting from parasitic reactions or adsorbed electrolyte residues on the deposited Li 2 O 2 and additionally multiple components in the deconvoluted C1s spectra also indicate presence of several functionalities, such as C-C, CH x , CO x etc., in the discharged electrodes. 36 Consistent with the XPS results, the FTIR spectra of the discharged CMK-3 and N-CMK-3 electrodes in Figure S9 identify Li 2 CO 3 and Li-carboxylates to be the parasitic products during DC.…”

“…36 Besides, The O1s spectra show additional peaks at > 532 eV that can be assigned to -CO 3 , -CO or SO 2 species resulting from parasitic reactions or adsorbed electrolyte residues on the deposited Li 2 O 2 and additionally multiple components in the deconvoluted C1s spectra also indicate presence of several functionalities, such as C-C, CH x , CO x etc., in the discharged electrodes. 36 Consistent with the XPS results, the FTIR spectra of the discharged CMK-3 and N-CMK-3 electrodes in Figure S9 identify Li 2 CO 3 and Li-carboxylates to be the parasitic products during DC. There can be different ways for the formation of parasitic side products in Li-Air cells where, in addition to electrolyte and electrode decomposition, the CO 2 (0.04%) present in air can also undergo electrochemical reaction during DC to directly produce Li 2 CO 3 as an undesired product.…”