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Abstract
AbstractThe coupling of nickel‐rich LiNi0.8Mn0.1Co0.1O2 (NMC811) cathodes with high‐capacity silicon–graphite (Si–Gr) anodes is one promising route to further increase the energy density of lithium‐ion batteries. Practically, however, the cycle life of such cells is seriously hindered due to continuous electrolyte degradation on the surfaces of both electrodes. In this study, tris(trimethylsilyl) phosphite (TMSPi) is introduced as an electrolyte additive to improve the electrochemical performance of the NMC811/Si–Gr full cells through formation of protective surface layers at the electrode/electrolyte interfaces. This is thought to prevent the surface fluorination of the active materials and enhance interfacial stability. Notably, TMSPi is shown to significantly reduce the overpotential and operando X‐ray diffraction (XRD) confirms that an irreversible “two‐phase” transition reaction caused by the formed adventitious Li2CO3 layer on the surface of NMC811 can transfer to a solid‐solution reaction mechanism with TMSPi‐added electrolyte. Moreover, influences of TMSPi on the cathode electrolyte interphase (CEI) on the NMC811 and solid electrolyte interphase (SEI) on the Si–Gr are systematically investigated by electron microscopy and synchrotron‐based X‐ray photoelectron spectroscopy which allows for the nondestructive depth‐profiling analysis of chemical compositions and oxidation states close to the electrode surfaces.
Highlights
The coupling of nickel-rich LiNi0.8Mn0.1Co0.1O2 (NMC811) cathodes with highcapacity silicon–graphite (Si–Gr) anodes is one promising route to further increase the energy density of lithium-ion batteries
It is demonstrated that the TMSPi additive can sufficiently limit the continuous change of cathode electrolyte interphase (CEI) and solid electrolyte interphase (SEI) chemistries resulting from the decomposition of organic solvents and achieve good cycling stability and high Coulombic efficiency
The influence of 2 wt% TMSPi additive in the baseline electrolyte (1 m LiPF6 in ethylene carbonate (EC):DEC) on the electrochemical performance of the NMC811/Si–Gr full cells was investigated in pouch cells by performing different measurements to determine electrochemical response, structure, surface morphology and depth-dependent composition
Summary
TMSPi has a lower oxidation stability than the solvent ethylene carbonate (EC) in the electrolyte, meaning it will be oxidized before EC and contribute to the passivating CEI formed at the cathode surface.[37,40,41] To confirm this, linear sweep voltammetry (LSV) was performed with the baseline. A slightly increased first-cycle Coulombic efficiency of ≈87% is obtained, and relatively higher Coulombic efficiency is retained in following cycles with the TMSPi electrolyte It indicates that the decomposition of TMSPi on the NMC811 cathode and the Si–Gr anode during the first cycles promotes the formation of stable CEI and SEI, reducing the cell polarization and improving Coulombic efficiency. The rapid capacity fading and decreased Coulombic efficiency of the full cells with the baseline electrolyte are mainly due to the largely increased cell impedance, which is caused by the formation of unstable and lowly conducting CEI and SEI, inducing continuous consumption of electrolyte on both cathode and anode sides. The operando XRD study suggests that the intrinsic phase transition mechanism of NMC811 in a cell could be influenced by the electrode/ electrolyte interactions induced by electrolyte additive
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