In Situ DRIFTS Analysis of Solid‐Electrolyte Interphase Formation on Li‐Rich Li_1.2Ni_0.2Mn_0.6O₂ and LiCoO₂ Cathodes during Oxidative Electrolyte Decomposition

Abstract

In situ diffuse reflectance infrared Fourier‐transformed spectroscopy (DRIFTS) investigations have been made to examine solid‐electrolyte interphase (SEI) formation on lithium‐rich Li1.2Ni0.2Mn0.6O2 (LLNMO) and LiCoO2 cathodes during first‐ and second‐cycle charging and discharging. This DRIFTS technique allows us to clarify SEI formation with different charging voltages. Both cathodes revealed the formation of the same surface species during first‐cycle charging, initially including ethylene carbonate (EC) adsorption, and SEI species, for example, ROCOF, RCOOR, Li2CO3, ROCO2Li, and PFx, are formed above the onset potential, namely 4.0 and 4.5 V for LiCoO2 and LLNMO, respectively. The onset potentials correspond to the upper limit of the reversible redox potential range for transition‐metal couples (e.g. Co3+/Co4+ in LiCoO2 and Ni2+/Ni4+ in LLNMO), which account for the intrinsic instability of these cathode materials. Such results suggest the participation of intermediate reactive oxygen species in SEI formation. SEI species continue to form during the discharge process when the potential is scanned cathodically below 3.6 and 4.0 V for LiCoO2 and LLNMO, respectively. Similar SEI species are also observed during the second cycle charge–discharge over LLNMO, where additional oxidized species such as carboxylate (−COO−) and CO2 are also found during charging. With the exception of PFx, all of the observed SEI species can be attributed to the oxidative decomposition of the organic solvent, EC. Finally, possible reaction mechanisms related to the oxidative decomposition of EC are discussed.

Surface Degradation and Chemical Electrolyte Oxidation Induced by the Oxygen Released from Layered Oxide Cathodes in Li−Ion Batteries

“…The chemical oxidation of the EC by the lattice oxygen (O 2− ) or oxygen radicals is also reported to form acyl fluoride species (ROCOF), carboxylate (HCO 2 Li) and alkyl carbonate (ROCO 2 Li) . But it can also generate CO 2 gas and water as suggested by Jung et al .…”

“…Besides the chemical reactions triggered by the O‐species of the layered oxides or the salt by‐products (i. e. HF), electrochemical oxidation of the electrolyte is still a likely degradation mechanism that we cannot fully disregard . Unfortunately, the discrimination between chemical and electrochemical processes is challenging, because they both form similar by‐products in terms of electrolyte oligomers/polymers and gas compounds ,. However, even if it exists, the electrochemical oxidation of the solvents on the cathode does not lead to the migration or diffusion of the by‐products to the anode, as shown by the absence of a surface layer on LTO cycled against the LNMO cathode at 5 V vs. Li + /Li.…”

Improving Stability of LiCoO2 Cathode by Using Lithium Bis(Trifluoroborane)‐5‐Cyano‐2‐(Trifluoromethyl) Benzimidazolide as Additive

“…With the 2 % additive, in situ DRIFTS reveals the following differences in SEI formation comparing to that without the additive: (i) no band indicating the formation of ester species, (ii) a relatively low PF x band intensity comparing to other SEI species, and (iii) a relatively thin SEI layer. The thin SEI is indicated by the ∼2 orders of magnitude lower intensity of SEI species comparing to the EC intensity at OCV (Figure ), while the intensity of SEI was only ∼1 order of magnitude lower than OCV intensity when in the absence of additive . This thin SEI film is attributed to suppression of electrolyte solvent decomposition by the additive.…”

“…These SEI species formed along delithiation from 4.2 V to 4.5 V as indicated by the similar difference spectra. Table presents the comparison of identified SEI species over LiCoO 2 using 1 M LiPF 6 /EC+DEC and 0.5 M Li[CBTBI]/EC+DEC and with 2 % Li[CBTBI] additive in 1 M LiPF 6 /EC+DEC at OCV and during the first cycle charging. Using 2 % additive the formation of SEI species such as RCOOF, ROCO 2 Li, Li 2 CO 3 and PF x were similar with the species identified when using the reference electrolyte1 M LiPF 6 /EC+DEC …”

“…Without additive, an onset potential of SEI formation at 4.0 V was noted corresponding to the upper bound of delithiation potential window of LiCoO 2 when oxidative species evolved . In the presence of 2 % Li[CBTBI] additive, the onset potential of SEI formation was at 4.2 V, obviously shifted indicating the additive‐stabilized LiCoO 2 surface.…”