Metallic lithium (Li) has great potential as an anode material for high-energy-density batteries due to its high specific capacity. However, the uncontrollable dendritic lithium growth on the metallic lithium surface limits its practical application owing to the instability of the solid electrolyte interphase (SEI). A tailored SEI composition/structure can mitigate or inhibit the lithium dendrites' growth, thereby enhancing the cyclability of the Li-metal anode. In this work, excellent cycling stability of lithium metal anodes was achieved by utilizing a novel dual-salt electrolyte based on lithium bis(fluorosulfonyl) imide (LiFSI) and lithium difluorobis(oxalato) phosphate (LiDFBOP) in carbonate solvents. By combining surface/microstructural characterization and computations, we reveal that the preferential reduction of LiDFBOP occurs prior to LiFSI and carbonate solvents and its reduction products (Li 2 C 2 O 4 and P−O species) bind to LiF, resulting in a favorable compact and protective SEI on the Li electrodes. It was found that the improved oxidative stability was accompanied by reduced corrosion of the current collector. A Li/Li symmetrical cell with a designed dual-salt electrolyte system exhibits stable polarization voltage over 1000 h of cycle time. In addition, the LiFSI− LiDFBOP advantage of this dual-salt electrolyte system enables the Li/LiFePO 4 cells with significantly enhanced cycling stability. This work demonstrates that constructing a tailored SEI using a dual-salt electrolyte system is vital for improving the interfacial stability of lithium metal batteries.
High-voltage
cathodes provide a promising solution to the energy
density limitation of currently commercialized lithium-ion batteries,
but they are unstable in electrolytes during the charge/discharge
process. To address this issue, we propose a novel electrolyte additive,
pentafluorophenyltriethoxysilane (TPS), which is rich in elemental
F and contains elemental Si. The effectiveness of TPS has been demonstrated
by cycling a representative high-voltage cathode, LiNi0.5Mn1.5O4 (LNMO), in 1.0 M LiPF6–diethyl
carbonate/ethylene carbonate/ethyl methyl carbonate (2/3/5 in weight).
LNMO presents an increased capacity retention from 28 to 85% after
400 cycles at 1 C by applying 1 wt % TPS. Further electrochemical
measurements combined with spectroscopic characterization and theoretical
calculations indicate that TPS can not only construct a robust protective
cathode electrolyte interphase via its oxidation during initial lithium
desertion but also scavenge the detrimental hydrogen fluoride (HF)
present in the electrolyte via its strong combination with the species
HF, F–, and H+, highly stabilizing LNMO
during the charge/discharge process. These features of TPS provide
a new solution to the obstacle in the practical application of high-voltage
cathodes not limited to LNMO.
Electrolyte additives have been successfully applied for the performance amelioration of lithium-ion batteries, especially under high voltage, which are based on the protective interphases on anode and cathode. Many additives have been proposed but less knowledge is available on the relationship between additive molecule structure and the interphase stability. In this work, we uncover the significance of the additive molecule structure in constructing a stable and robust interphase by comparing the effects of two similar additives, trimethyl borate (TMB) and tripropyl borate (TPB), on the performance of a layered lithiumrich oxide cathode (LRO) under a high voltage (4.8 V). Electrochemical measurements combined with physical characterizations and theoretical calculations demonstrate that TMB and TPB exhibit similar oxidative activity and both can build protective cathode interphases on LRO but they yield different cyclic stability improvement for LRO. The B-containing species derived from the TMB oxidation are more stable, yielding a more robust interphase than those from the TPB oxidation. This established relationship paves a road to design electrolyte additives more efficiently for high-voltage batteries.
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