Decomposition kinetics of anhydrous and moisture exposed LiPF6 salts by thermogravimetry

Designing Nonflammable Liquid Electrolytes for Safe Li-Ion Batteries

Li-ion batteries are essential technologies for electronic products in the daily life. However, serious fire safety concerns that are closely associated with the flammable liquid electrolyte remains a key challenge. Tremendous effort has been devoted to designing nonflammable liquid electrolytes. It is critical to gain comprehensive insights into nonflammability design and inspire more efficient approaches for building safer Li-ion batteries. This review presents current mechanistic understanding of safety issues and discusses state-of-the-art nonflammable liquid electrolytes design for Li-ion batteries based on molecule, solvation, and battery compatibility level. Various safety test methods are discussed for reliable safety risk evaluation. Finally, the challenges and perspectives of the nonflammability design for Li-ion electrolytes are summarized.

Molecular Influences on the Quantification of Lewis Acidity with Phosphine Oxide Probes

Gutmann-Beckett-type measurements with phosphine oxide probes can be used to estimate effective Lewis acidity with ³¹P nuclear magnetic resonance spectroscopy, but the influence of the molecular structure of a given probe on the quantification of Lewis acidity remains poorly documented in experimental work. Here, a quantitative comparison of triethyl (E), trioctyl (O), and triphenyl (P) phosphine oxides as molecular probes of Lewis acidity has been carried out via titration studies in MeCN with a test set of six mono- and divalent metal triflate salts. In comparison to E, the bulkier O displays a similar range of chemical shift values and binding affinities for the various test metal ions. Spectral linewidths and speciation properties vary for individual cation-to-probe ratios, however, confirming probe-specific properties that can impact the data quality. Importantly, P displays a consistently narrower dynamic range than both E and O, illustrating how electronic changes at phosphorus can influence the NMR response. Comparative parametrizations of the effective Lewis acidities of a broader range of metal ions, including the trivalent rare earth ions Y³⁺, Lu³⁺, and Sc³⁺ as well as the uranyl ion (UO₂²⁺), can be understood in light of these results, providing insight into the fundamental chemical processes underlying the useful approach of single-point measurements for quantification of effective Lewis acidity. Together with a study of counteranion effects reported here, these data clarify the diverse ensemble of factors that can influence the measurement of Lewis acid/base interactions.

Solvent-Diluent Interaction-Mediated Solvation Structure of Localized High-Concentration Electrolytes

The latest developments of localized high-concentration electrolytes (LHCEs) shed light on stabilizing the high-energy-density lithium (Li) metal batteries. It is generally considered that the nonsolvating diluents introduced into the LHCEs improve the viscosity and wettability of high-concentration electrolytes (HCEs) without changing their inner solvation structures, thus maintaining the highly coordinated contact ion pairs (CIPs) and ionic aggregates (AGGs) of the precursor HCEs with limited free solvent numbers and high Coulombic efficiency (CE) of Li metal anodes. Herein, we show an unexpected effect of the diluent amount on the solvation structures of the LHCEs: as the diluent amount increases, the proportions of free solvent molecules and CIPs rise up simultaneously. The latter is probably due to the partial splits of the AGGs via the dipole-dipole interactions between the diluent and solvent molecules. Accordingly, a moderately diluted LHCE shows the best Coulombic efficiency of Li metal anodes (99.6%), compared with the precursor HCE (97.4%) or highly diluted LHCE (99.0%). This work reveals a new criterion of the LHCE chemical formulation for the designing of advanced electrolytes for high-energy-density batteries.

Effect of short and long range order on crystal structure interpretation: Raman and powder X-ray diffraction of LiPF₆

The structure of LiPF6 has been probed using Raman scattering as well as pXRD and the results are compared and contrasted. The conventional Bragg angle scattering pXRD determines that dry LiPF6 crystallizes in a trigonal structure (Space Group R-3 (148)), while Raman data suggests that the observed structure is close to cubic (Space Group Fm-3m (225)), similar to NaPF6 and KPF6. DFT heat capacity calculations using ab-initio methods for both R-3 and Fm-3m structures show better correlation with the Raman observations. The differences between Raman and pXRD data in LiPF6 appear to be in common with that observed in other materials that are close to phase transitions at the temperature where order/disorder and phase transition processes are known to occur. In these circumstances, Raman spectroscopy emerges as a more sensitive probe of local structural changes than pXRD, which is known to probe the overall average long range order of crystalline materials. The results are interpreted through the relationship between the local symmetry, internal energy and the heat capacity.

A Gel-Polymer Sn-C/LiMn0.5Fe0.5PO4 Battery Using a Fluorine-Free Salt

Safety and environmental issues, because of the contemporary use of common liquid electrolytes, fluorinated salts, and LiCoO2-based cathodes in commercial Li-ion batteries, might be efficiently mitigated by employing alternative gel-polymer battery configurations and new electrode materials. Herein we study a lithium-ion polymer cell formed by combining a LiMn0.5Fe0.5PO4 olivine cathode, prepared by simple solvothermal pathway, a nanostructured Sn-C anode, and a LiBOB-containing PVdF-based gel electrolyte. The polymer electrolyte, here analyzed in terms of electrochemical stability by impedance spectroscopy (EIS) and voltammetry, reveals full compatibility for cell application. The LiBOB electrolyte salt and the electrochemically delithiaded Mn0.5Fe0.5PO4 have a higher thermal stability compared to conventional LiPF6 and Li0.5CoO2, as confirmed by thermogravimetric analysis (TGA) and by galvanostatic cycling at high temperature. LiMn0.5Fe0.5PO4 and Sn-C, showing in lithium half-cell a capacity of about 120 and 350 mAh g(-1), respectively, within the gelled electrolyte configuration are combined in a full Li-ion polymer battery delivering a stable capacity of about 110 mAh g(-1), with working voltage ranging from 2.8 to 3.6 V.

Safer Electrolytes for Lithium-Ion Batteries: State of the Art and Perspectives

Lithium-ion batteries are becoming increasingly important for electrifying the modern transportation system and, thus, hold the promise to enable sustainable mobility in the future. However, their large-scale application is hindered by severe safety concerns when the cells are exposed to mechanical, thermal, or electrical abuse conditions. These safety issues are intrinsically related to their superior energy density, combined with the (present) utilization of highly volatile and flammable organic-solvent-based electrolytes. Herein, state-of-the-art electrolyte systems and potential alternatives are briefly surveyed, with a particular focus on their (inherent) safety characteristics. The challenges, which so far prevent the widespread replacement of organic carbonate-based electrolytes with LiPF6 as the conducting salt, are also reviewed herein. Starting from rather "facile" electrolyte modifications by (partially) replacing the organic solvent or lithium salt and/or the addition of functional electrolyte additives, conceptually new electrolyte systems, including ionic liquids, solvent-free, and/or gelled polymer-based electrolytes, as well as solid-state electrolytes, are also considered. Indeed, the opportunities for designing new electrolytes appear to be almost infinite, which certainly complicates strict classification of such systems and a fundamental understanding of their properties. Nevertheless, these innumerable opportunities also provide a great chance of developing highly functionalized, new electrolyte systems, which may overcome the afore-mentioned safety concerns, while also offering enhanced mechanical, thermal, physicochemical, and electrochemical performance.