RESUMO
Vinylene carbonate (VC) is a widely used electrolyte additive in lithium-ion batteries for enhanced solid electrolyte interphase formation on the anode side. However, the cathode electrolyte interphase (CEI) formation with VC has received a lot less attention. This study presents a comprehensive investigation employing advanced in situ/operando-based Raman and X-ray absorption spectroscopy (XAS) to explore the effect of electrolyte composition on the CEI formation and suppression of surface reconstruction of LixNiyMnzCo1-y-zO2 (NMC) cathodes. A novel chemical pathway via VC polymerization is proposed based on experimental results. In situ Raman spectra revealed a new peak at 995 cm-1, indicating the presence of C-O semi-carbonates resulting from the radical polymerization of VC. Operando Raman analysis unveiled the formation of NiO at 490 cm-1 in the baseline system under ultrahigh voltage (up to 5.2 V). However, this peak was conspicuously absent in the VC electrolyte, signifying the effectiveness of VC in suppressing surface reconstruction. Further investigation was carried out utilizing in situ XAS compared X-ray absorption near edge structure spectra from cells of 3 and 20 cycles in both electrolytes at different operating voltages. The observed shift at the Ni K-edge confirmed a more substantial reduction of Ni in the baseline electrolyte compared to that in the VC electrolyte, thus indicating less CEI protection in the former. A sophisticated extended X-ray absorption fine structure analysis quantitatively confirmed the effective suppression of rock-salt formation with the VC electrolyte during the charging process, consistent with the operando Raman results. The in situ XAS results thus provided additional support for the key findings of this study, establishing the crucial role of VC polymerization in enhancing CEI stability and mitigating surface reconstruction on NMC cathodes. This work clarifies the relationship between the enhanced CEI layer and NMC degradation and inspires rational electrolyte design for long-cycling NMC cathodes.
RESUMO
The present article entails a novel concept of storing extra energy in a multifunctional polymer electrolyte membrane (PEM) beyond the storage capacity of a cathode, which is achieved by so-called "prelithiation" upon simply deep discharging to a low potential range of a lithium-metal electrode (i.e., -0.5 to 0.5 V). This unique extra energy-storage capacity has been realized recently in the PEM consisting of polysulfide-co-polyoxide conetworks in conjunction with succinonitrile and LiTFSI salt that facilitate complexation via ion-dipole interaction of dissociated lithium ions with thiols, disulfide, or ether oxygen of the conetwork. Although ion-dipole complexation may increase the cell resistance, the prelithiated PEM provides excess lithium ions during oxidation (or Li+ stripping) at the Li-metal electrode. Once the PEM network is fully saturated with Li ions, the remaining excess ions can move through the complexation sites at ease, thereby affording not only facile ion transport but also extra ion-storage capacity within the PEM conetwork. Of particular interest is that the lithiated polysulfide-co-polyoxide polymer network-based PEM exhibits a high conductivity of 1.18 × 10-3 S/cm at ambient, which can also store extra energy with a specific capacity of about 150 mAh/g at a 0.1C rate in the PEM voltage range of 0.01-3.5 V in addition to 165 mAh/g at 0.2C of an NMC622 (nickel manganese cobalt oxide) cathode (i.e., 2.5-4.6 V) with a Coulombic efficiency of approximate unity. Moreover, its Li-metal battery assembly with an NMC622 cathode exhibits a very high specific capacity of â¼260 mAh/g at 0.2C in the full battery range of 0.01-5 V, having a higher Li+ transference number of 0.74, suggestive of domination by the lithium cation transport relative to those (0.22-0.35) of organic liquid electrolyte lithium-ion batteries.
