Asymmetric Imides as Electrolyte Additive for Lithium‐Ion Batteries with NCM111 Cathode

The synthesis, spectroscopic and electrochemical characterization of Li[N(SiMe3)(SO2RF)] (RF=CF3, n‐C4F9) as well their behavior as electrolyte additive in lithium ion batteries (LIBs) is reported. The lithium salts were obtained by deprotonation of the corresponding acids HN(SiMe3)(SO2RF) with n‐butyllithium in n‐pentane. The electrochemical investigations suggested potential as additives for LIBs. Thus, NCM111/graphite cells (NCM111=Li[Ni0.33Co0.33Mn0.33]O2) with LP57 as electrolyte (LP57=1.0 M LiPF6 in EC/EMC 3 : 7) were built to test the performance. Cells with Li[N(SiMe3)(SO2RF)] as additives show coulombic efficiencies of over 99.6 %, less capacity fading over 55 cycles and a significantly lower cell impedance built up

Lithium bis(2,2,2-trifluoroethyl)phosphate Li[O2P(OCH2CF3)2]: A high voltage additive for LNMO/graphite cells

The fluorinated phosphate lithium bis (2,2,2-trifluoroethyl) phosphate (LiBFEP) has been investigated as a film-forming additive employed to passivate the cathode and hinder continuous oxidation of the electrolyte. Cyclic voltammetry (CV) and linear sweep voltammetry coupled with online electrochemical mass spectrometry (LSV-OEMS) on a conductive carbon electrode (i.e., a C65/PVDF composite) showed that LiBFEP decreases electrolyte oxidation (CV and LSV) and LiPF6 decomposition at high potentials. Incorporation of LiBFEP (0.1 and 0.5 wt%) into LiPF6 in ethylene carbonate (EC)/ethyl methyl carbonate (EMC) (3:7 wt) results in improved coulombic efficiency and capacity retention for LNMO/graphite cells. Ex-situ surface analysis of the electrodes suggests that incorporation of LiBFEP results in the formation of a cathode electrolyte interface (CEI) and modification of the solid electrolyte interface (SEI) on the anode. The formation of the CEI mitigates electrolyte oxidation and prevents the decomposition of LiPF6, which in turn prevents HF-induced manganese dissolution from the cathode and destabilization of the SEI. The passivation of the cathode and stabilization of the SEI is responsible for the increased coulombic efficiency and capacity retention

Free volume in ionic liquids: a connection of experimentally accessible observables from PALS and PVT experiments with the molecular structure from XRD data

In the current work, free volume concepts, primarily applied to glass formers in the literature, were transferred to ionic liquids (ILs). A series of 1-butyl-3-methylimidazolium ([C4MIM](+)) based ILs was investigated by Positron Annihilation Lifetime Spectroscopy (PALS). The phase transition and dynamic properties of the ILs [C4MIM][X] with [X](-) = [Cl](-), [BF4](-), [PF6](-), [OTf](-), [NTf2](-) and [B(hfip)(4)](-) were reported recently (Yu et al., Phys. Chem. Chem. Phys., 2012, 14, 6856-6868). In this subsequent work, attention was paid to the connection of the free volume from PALS (here the mean hole volume, ) with the molecular structure, represented by volumes derived from X-ray diffraction (XRD) data. These were the scaled molecular volume V-m,V-scaled and the van der Waals volume V-vdw. Linear correlations of at the "knee'' temperature ((T-k)) with V-m,V-scaled and V-vdw gave good results for the [C4MIM](+) series. Further relationships between volumes from XRD data with the occupied volume V-occ determined from PALS/PVT (Pressure Volume Temperature) measurements and from Sanchez-Lacombe Equation of State (SL-EOS) fits were elaborated (V-occ(SL-EOS) approximate to 1.63 V-vdw, R-2 = 0.981 and V-occ(SL-EOS) approximate to 1.12 V-m,V-scaled, R-2 = 0.980). Finally, the usability of V-m,V-scaled was justified in terms of the Cohen-Turnbull (CT) free volume theory. Empirical CT type plots of viscosity and electrical conductivity showed a systematic increase in the critical free volume with molecular size. Such correlations allow descriptions of IL properties with the easily accessible quantity V-m,V-scaled within the context of the free volume

An Artificial SEI Layer Based on an Inorganic Coordination Polymer with Self‐Healing Ability for Long‐Lived Rechargeable Lithium‐Metal Batteries

Upon immersion of a lithium (Li) anode into a diluted 0.05 to 0.20 M dimethoxyethanesolutionof the phosphoric acid derivative (CF 3 CH 2 O) 2 P(O)OH (HBFEP), anartificial solid electrolyte interphase (SEI) is generated on the Li-metal surface. Hence,HBFEP reacts on the surface to the corresponding Li salt (LiBFEP), which is a Li-ionconducting inorganic coordination polymer. This film exhibits -due to the reversiblybreaking ionic bonds- self-healing ability upon cycling-induced volume expansion of Li.The presence of LiBFEP as the major component in the artificial SEI is proven by ATRIRand XPS measurements. SEM characterization of HBFEP-treated Li samplesreveals porous layers on top of the Li surface with at least 3 μm thickness. Li-Lisymmetrical cells with HBFEP-modified Li electrodes show a three- to almost fourfoldcycle-lifetime increase at 0.1 mA·cm –2 in a demanding model electrolyte thatfacilitates fast battery failure (1 m LiOTf in TEGDME). Hence, the LiBFEP-enrichedlayer apparently acts as a Li-ion conducting protection barrier between Li and theelectrolyte, enhancing the rechargeability of Li electrodes