Revealing solid electrolyte interphase formation through interface-sensitive Operando X-ray absorption spectroscopy.

The solid electrolyte interphase (SEI) that forms on Li-ion battery anodes is critical to their long-term performance, however observing SEI formation processes at the buried electrode-electrolyte interface is a significant challenge. Here we show that operando soft X-ray absorption spectroscopy in total electron yield mode can resolve the chemical evolution of the SEI during electrochemical formation in a Li-ion cell, with nm-scale interface sensitivity. O, F, and Si K-edge spectra, acquired as a function of potential, reveal when key reactions occur on high-capacity amorphous Si anodes cycled with and without fluoroethylene carbonate (FEC). The sequential formation of inorganic (LiF) and organic (-(C=O)O-) components is thereby revealed, and results in layering of the SEI. The addition of FEC leads to SEI formation at higher potentials which is implicated in the rapid healing of SEI defects and the improved cycling performance observed. Operando TEY-XAS offers new insights into the formation mechanisms of electrode-electrolyte interphases and their stability for a wide variety of electrode materials and electrolyte formulations.

Solvent-Induced Bond-Bending Isomerism in Hexaphenyl Carbodiphosphorane: Decisive Dispersion Interactions in the Solid State.

Previous reports in the literature describe that the crystallization of hexaphenyl carbodiphosphorane (CDPPh) from a variety of solvents gives a "bent" geometry for the P-C-P moiety as the solid-state molecular structure. However, a linear structure is observed when CDPPh is crystallized from benzene. Here, we report detailed spectroscopic and theoretical studies on the linear and bent structures. X-ray powder diffraction examinations show a phase transition of linear CDPPh upon the loss of co-crystallized benzene molecules, which is accompanied by the bending of the P-C-P unit. Studies on the linear and bent structures (i.e., X-ray powder diffraction, solid-state NMR, UV-vis spectroscopy, and IR spectroscopy) show significant differences in their properties. Investigations of the solid-state structures with density functional theory-based methods (PBE-D3) point toward subtle dispersion effects being responsible for this solvent-induced bond-bending isomerism in CDPPh.

Double donation in trigonal planar iron-carbodiphosphorane complexes - a concise study on their spectroscopic and electronic properties.

We present the syntheses of trigonal planar coordinated Fe(ii) carbodiphosphorane (CDPR) complexes, starting from iron(ii)-bis(trimethylsilylamide) [Fe{N(SiMe3)2}2] and hexaphenyl-(CDPPh) and sym-dimethyltetraphenyl-carbodiphosphoranes (CDPMe), respectively. Both complexes [CDPPh-Fe{N(SiMe3)2}2] (1) and [CDPMe-Fe{N(SiMe3)2}2] (2) were examined in solution and in the solid state. 1 shows a dissociation equilibrium in solution which we monitored by variable temperature 1H-NMR spectroscopy. Magnetic measurements of 1 and 2 yielded a high spin configuration (S = 2) for both complexes. Quantum chemical calculations were performed to analyze the bonding situation in compound 1.

NMR Study of the Degradation Products of Ethylene Carbonate in Silicon-Lithium Ion Batteries.

Ethylene carbonate (EC) is the most widely used electrolyte solvent in lithium ion batteries, but it fails to form a stable passivation layer on materials such as Si and Li metal, which will enable the long-term cycling of the next-generation high-capacity lithium ion batteries containing these anode materials. High concentrations of soluble degradation products are detected in the electrolyte after prolonged cycling, but the chemical structures of these species remain unclear. Here, we used 1D, 2D, and diffusion NMR techniques combined with mass spectrometry to analyze electrolyte-containing 13C-labeled EC, and we report on the formation of a series of linear oligomers consisting of ethylene oxide and carbonate fragments with methoxide end groups as the major soluble degradation products of EC. Oligomers with methoxide terminals are likely to have weak interaction toward the electrode; thus, they easily detach from the electrode and are unable to passivate the surface, which may explain the origin of the capacity fade for high-capacity Si-based or Li metal anodes.

Hexaphenyl carbodiphosphorane adducts of heavier group 15 element trichlorides: syntheses, properties and reactivities.

Herein, we present a series of hexaphenyl carbodiphosphorane (CDPPh) adducts of heavier group 15 trichlorides ECl3 (E = P-Bi). The reaction with PCl3 yields the known salt [CDPPh-PCl2][Cl] ([1][Cl]), the heavier element trichlorides ECl3 (E = Sb (4), Bi (5)) give the neutral adducts CDPPh-ECl3 which were characterised crystallographically and spectroscopically. The reaction of CDPPh with AsCl3 does not yield CDPPh-AsCl3 (2), but in the presence of GaCl3 the corresponding salt [CDPPh-AsCl2][GaCl4] ([3][GaCl4]) is formed. DFT (density functional theory) calculations were carried out to examine the molecular frontier orbitals in 1+-5. Additional reactivity studies revealed an intramolecular electrophilic aromatic substitution (SEAr) in 1+, which represents an excellent starting point for further selective C-P bond formation reactions.

Understanding Fluoroethylene Carbonate and Vinylene Carbonate Based Electrolytes for Si Anodes in Lithium Ion Batteries with NMR Spectroscopy.

Fluoroethylene carbonate (FEC) and vinylene carbonate (VC) are widely used as electrolyte additives in lithium ion batteries. Here we analyze the solid electrolyte interphase (SEI) formed on binder-free silicon nanowire (SiNW) electrodes in pure FEC or VC electrolytes containing 1 M LiPF6 by solid-state NMR with and without dynamic nuclear polarization (DNP) enhancement. We find that the polymeric SEIs formed in pure FEC or VC electrolytes consist mainly of cross-linked poly(ethylene oxide) (PEO) and aliphatic chain functionalities along with additional carbonate and carboxylate species. The formation of branched fragments is further confirmed by 13C-13C correlation NMR experiments. The presence of cross-linked PEO-type polymers in FEC and VC correlates with good capacity retention and high Coulombic efficiencies of the SiNWs. Using 29Si DNP NMR, we are able to probe the interfacial region between SEI and the Si surface for the first time with NMR spectroscopy. Organosiloxanes form upon cycling, confirming that some of the organic SEI is covalently bonded to the Si surface. We suggest that both the polymeric structure of the SEI and the nature of its adhesion to the redox-active materials are important for electrochemical performance.

Identifying the Structural Basis for the Increased Stability of the Solid Electrolyte Interphase Formed on Silicon with the Additive Fluoroethylene Carbonate.

To elucidate the role of fluoroethylene carbonate (FEC) as an additive in the standard carbonate-based electrolyte for Li-ion batteries, the solid electrolyte interphase (SEI) formed during electrochemical cycling on silicon anodes was analyzed with a combination of solution and solid-state NMR techniques, including dynamic nuclear polarization. To facilitate characterization via 1D and 2D NMR, we synthesized 13C-enriched FEC, ultimately allowing a detailed structural assignment of the organic SEI. We find that the soluble poly(ethylene oxide)-like linear oligomeric electrolyte breakdown products that are observed after cycling in the standard ethylene carbonate-based electrolyte are suppressed in the presence of 10 vol% FEC additive. FEC is first defluorinated to form soluble vinylene carbonate and vinoxyl species, which react to form both soluble and insoluble branched ethylene-oxide-based polymers. No evidence for branched polymers is observed in the absence of FEC.