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Lithium–ion solvation governs the performance of lithium metal anode (LMA) by tuning its interfacial stability. Solvation degree is modulated by adopting fluorinated non-solvating cosolvents (FNSC) to induce anion-rich solvation structure which is beneficial in constructing mechanically stable interface to suppress lithium dendrite. However, FNSC exhibits low cathodic stability owing to their low lowest unoccupied molecular orbital (LUMO) level, aggravating long-term cycling of LMA. We establish that spectroscopically measured Lewis basicity and polarity are critical parameters for designing optimal non-solvating cosolvents. Non-fluorinated non-solvating cosolvents (NFNSC) proposed by our design rule (i.e. anisole, ethoxybenzene and furan) delivered 99.0 % coulombic efficiency over 1400 cycles. In these molecules, the aromatic ring delocalizes oxygen electron pairs and lowers solvation capability, confirmed by electrochemical cycling, Raman spectroscopy, and DFT binding energy calculation. Finally, the quantification of remaining NFNSC in the electrolytes using nuclear magnetic resonance spectroscopy proves their reductive stability for extended cycles.

LiFePO4 (LFP), the attractive commercial cathode materials, exhibits the phase separation between lithium-poor and lithium-rich phase within individual particles due to the wide miscibility gap. The boundaries between such phases develop large stress, lead to cracks within individual particles, and decay the cycle life. There have been lots of interest in investigating the origin of lithium heterogeneity and manipulating the phase boundaries within the particles during cycling. Previously, we revealed that insertion kinetics is dependent on the local lithium composition from which the hysteresis of lithium spatiodynamics originate. [ref.1] In addition, uniformity of ion insertion kinetics at electrolyte-electrode interface governs lithium heterogeneity and spatiodynamics in ether-based organic electrolyte. [ref.1] However, it remains still unclear how the chemical interaction of lithium ions, electrolyte molecules, and solid electrode controls nanoscale ion insertion kinetics. By developing operando scanning transmission x-ray microscopy (STXM) platform which maps lithium composition within individual LFP particles at 50 nm resolution during cycling, we investigated the interfacial lithium transport in various electrolytes: water, carbonates, and ethers. We synthesized LFP particles with platelet morphology and [010] crystallographic axis, which is fast ion diffusion direction, lied parallel to the monochromatic X-ray allowing to visualize lithium transport in the particles. Herein, we discover that aqueous electrolyte greatly accelerates lithium insertion kinetics, completely redirecting lithium heterogenesity and spatiodynamics compared with organic-based electrolyte. Aqueous environment kinetics show higher exchange current density (~700 mAm-2) than organic EC/DMC (~200 mAm-2) and also higher surface diffusion rate which account for the higher degree of intraparticle phase separation within the particles. By extracting kinetics parameters from our result, we experimentally demonstrate that heterogeneity and spatiodynamics in the single particle kinetics is determined by the strong interaction between the electrolyte molecules and the surface of electrode particles. Ref. 1 J. Lim et al. Science (2016) 353 p566 Figure 1