Search

We report that the presence of superstructure ordering serves as an important building block in securing the long-term stability of the oxygen redox activity in O2-type lithium-rich layered oxides.

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. more...

Localized high-concentration electrolyte (LHCE) has been adopted to derive resilient inorganic-rich solid-electrolyte interphase (SEI) layer for lithium-metal anodes (LMA) and increase the ionic conductivity of high-concentration electrolyte (HCE). While the ideal diluent molecules used in LHCE should be miscible in electrolyte system, it should not solvate lithium-ion effectively. To date, systematic investigation to reveal the design rule of diluent molecules remain poorly investigated. In addition, the choice of diluents is largely limited to fluorinated organic molecules which are expensive and environmentally hazardous and unstable at the reductive potential. Because of poor cathodic stability, fluorinated diluents (FDs) have failed to achieve cycle lives longer than 500 cycles in lithium metal half-cell. Herein, we establish the design rule of diluents for the ideal LHCE, which allows to derive three non-fluorinated diluents (NFDs) systematically. We present that two solvatochromic parameters, HBA basicity (β) and normalized electronic transition energy (ET N), unravel the generalization rule for the lithium-ion solvation capability and miscibility of diluents. Our design rule discovers three NFDs: anisole, ethoxybenzene, and furan. These NFDs have the unique resonance structures which effectively delocalize the oxygen lone pair electrons and thus enhance non-solvating characteristics, confirmed by the β value and the density functional theory (DFT) calculation. In addition, nuclear magnetic resonance (NMR) spectroscopy studies confirm that our NFDs remains stable and unreacted after long-term cycling. Finally, NFD-containing electrolytes achieve superior cyclability up to 1400 cycles, the record for LHCE to date. The results obtained in this study may pave the way for designing advanced electrolyte for next generation LMA. more...

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 more...

We report the bonding structure of the selenophene molecules adsorbed on the Si(100)-2 × 1 surface at 300 K, and its evolution upon annealing investigated by adopting core-level photoemission spectroscopy, near-edge X-ray absorption fine structure (NEXAFS), and ab initio calculations. The Si 2p, C 1s, Se 3d core-level spectra measured at two temperatures, 300 and 350 K, are consistently interpreted in terms of the two major structures suggested by theory, a twisted (T) 2,5-dihydroselenophene (T-DHS) and a T-deseleniumization where the selenium atom is dissociated from the selenophene ring. We find a significantly enhanced deseleniumization of selenophene molecules by mild thermal annealing indicating that these two equally abundant structures at 300 K become a single uniform phase of the T-deseleniumization structures at 350 K by overcoming a relatively low dissociation energy barrier between the two structures. In addition, we obtain an average tilt angle of a selenophene ring at 300 K from our NEXAFS sp... more...