The buried interface between the bulk electrode material and the solid electrolyte interphase (SEI) in cycled Li-ion battery anodes is suggested to incorporate an electric potential gradient. This suggestion is based on photoelectron spectroscopy (PES) results from different anode materials that all show relative binding energy shifts between the components of the SEI and the active anode. Implications of this electric potential gradient on binding energy reference points in PES as well as on charge-transfer kinetics in Li-ion batteries are discussed. Specifically, we show that the separation of surface layer and bulk material spectral contributions (depth profiling) is crucial for consistent data interpretation. We conclude that previous interpretations of lithiation as cause for changes in PES spectra may need to be revised.
Environmentally friendly and cost‐effective Li‐ion cells are fabricated with abundant, non‐toxic LiFePO4 cathodes and iron oxide anodes. A water‐soluble alginate binder is used to coat both electrodes to reduce the environmental footprint. The critical reactivity of LiPF6‐based electrolytes toward possible traces of H2O in water‐processed electrodes is overcome by using a lithium bis(oxalato)borate (LiBOB) salt. The absence of fluorine in the electrolyte and binder is a cornerstone for improved cell chemistry and results in stable battery operation. A dedicated approach to exploit conversion‐type anodes more effectively is also disclosed. The issue of large voltage hysteresis upon conversion/de‐conversion is circumvented by operating iron oxide in a deeply lithiated Fe/Li2O form. Li‐ion cells with energy efficiencies of up to 92 % are demonstrated if LiFePO4 is cycled versus such anodes prepared through a pre‐lithiation procedure. These cells show an average energy efficiency of approximately 90.66 % and a mean Coulombic efficiency of approximately 99.65 % over 320 cycles at current densities of 0.1, 0.2 and 0.3 mA cm−2. They retain nearly 100 % of their initial discharge capacity and provide an unmatched operation potential of approximately 2.85 V for this combination of active materials. No occurrence of Li plating was detected in three‐electrode cells at charging rates of approximately 5C. Excellent rate capabilities of up to approximately 30C are achieved thanks to the exploitation of size effects from the small Fe nanoparticles and their reactive boundaries.
The electrolyte solution of NaBOB in TEP is a low-cost, fluorine-free and flame-retardant electrolyte with ionic conductivity of 5 mS/cm, recently discovered to show promises for sodium-ion batteries. Here, the abilities of this electrolyte to effectively form a solid electrolyte interphase (SEI) was augmented with five common electrolyte additives of fluoroethylene carbonate (FEC), vinylene carbonate (VC), prop-1-ene-1,3-sultone (PES), 1,3,2-dioxathiolane 2,2-dioxide (DTD) and tris(trimethylsilyl)phosphite (TTSPi). Full-cells with electrodes of Prussian white and hard carbon and industrial mass loadings of >10 mg/cm2 and electrolyte volumes of <5 mL/g were used. X-ray photoelectron spectroscopy (XPS) and pressure analysis were also deployed to investigate parasitic reactions. Cells using electrolyte additives of PES, PES + DTD and PES + TTSPi (3 wt%) showed significantly increased performance in terms of capacity retention and initial Coulombic efficiency as compared to additive-free NaBOB–TEP. The best cell retained 80% discharge capacity (89 mAh/g) after 450 cycles, which is also significantly better than reference cells using 1 M NaPF6 in EC:DEC electrolyte. This study sheds light on opportunities to optimize the NaBOB–TEP electrolyte for full-cell sodium-ion batteries in order to move from low-mass-loading lab-scale electrodes to high mass loading electrodes aiming for commercialization of sodium-ion batteries.
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