Julia Maibach

Electric Potential Gradient at the Buried Interface between Lithium-Ion Battery Electrodes and the SEI Observed Using Photoelectron Spectroscopy

The buried interface between the bulk electrode material and the solid electrolyte interphase (SEI) in cycled Li-ion battery anodes is suggested to incorporate an electric potential gradient. This suggestion is based on photoelectron spectroscopy (PES) results from different anode materials that all show relative binding energy shifts between the components of the SEI and the active anode. Implications of this electric potential gradient on binding energy reference points in PES as well as on charge-transfer kinetics in Li-ion batteries are discussed. Specifically, we show that the separation of surface layer and bulk material spectral contributions (depth profiling) is crucial for consistent data interpretation. We conclude that previous interpretations of lithiation as cause for changes in PES spectra may need to be revised.

Conformal and highly luminescent monolayers of Alq3 prepared by gas-phase molecular layer deposition.

The gas-phase molecular layer deposition (MLD) of conformal and highly luminescent monolayers of tris(8-hydroxyquinolinato)aluminum (Alq3) is reported. The controlled formation of Alq3 monolayers is achieved for the first time by functionalization of the substrate with amino groups, which serve as initial docking sites for trimethyl aluminum (TMA) molecules binding datively to the amine. Thereby, upon exposure to 8-hydroxyquinoline (8-HQ), the self-limiting formation of highly luminescent Alq3 monolayers is afforded. The growth process and monolayer formation were studied and verified by in situ quartz crystal monitoring, optical emission and absorption spectroscopy, and X-ray photoelectron spectroscopy. The nature of the MLD process provides an avenue to coat arbitrarily shaped 3D surfaces and porous structures with high surface areas, as demonstrated in this work for silica aerogels. The concept presented here paves the way to highly sensitive luminescent sensors and dye-sensitized metal oxides for future applications (e.g., in photocatalysis and solar cells).

The Li–S battery: an investigation of redox shuttle and self-discharge behaviour with LiNO3-containing electrolytes

The polysulfide redox shuttle and self-discharge behaviour of lithium–sulfur (Li–S) cells containing the electrolyte additive LiNO3 has been thoroughly explored by a range of electrochemical and surface analysis techniques on simple Li–S (i.e., not specifically optimised to resist self-discharge) and symmetrical Li–Li cells. Despite the relatively effective passivation of the negative electrode by LiNO3, fully charged cells self-discharged a quarter of their capacity within 3 days, although in the short-term cells can be recharged without any noticeable capacity loss. The processes governing the rate and reversibility of self-discharge in these cells have been investigated and explained in terms of the reactions of polysulfides occurring at both electrodes during idle conditions.

Surface Layer Evolution on Graphite During Electrochemical Sodium-tetraglyme Co-intercalation

One obstacle in sodium ion batteries is the lack of suitable anode materials. As recently shown, the most common anode material of the state of the art lithium ion batteries, graphite, can be used for sodium ion storage as well, if ether-based electrolyte solvents are used. These solvents cointercalate with the sodium ions leading to the highly reversible formation of ternary graphite intercalation compounds (t-GIC). In order for the solvent cointercalation to work efficiently, it is expected that only a very thin surface layer forms during electrochemical cycling. In this article, we therefore present the first dedicated study of the surface layer evolution on t-GICs using soft X-ray photoelectron spectroscopy. This technique with its inherent high surface sensitivity and low probing depth is an ideal tool to study the underlying interfacial reactions during the sodiation and desodiation of graphite. In this report, we apply this approach to graphite composite electrodes cycled in Na half cells with a 1 M sodium bis(fluorosulfonyl)imide/tetraethylene glycol dimethyl ether (NaFSI/TEG-DME) electrolyte. We have found a surface layer on the cycled electrodes, mainly composed of salt decomposition products and hydrocarbons, in line with irreversible capacity losses observed in the electrochemical cycling. Although this surface layer does not seem to block cointercalation completely, it seems to affect its efficiency resulting in a low Coulombic efficiency of the studied battery system.

Impact of processing on the chemical and electronic properties of phenyl-C61-butyric acid methyl ester

For the comparison of solution-processed to evaporated materials in organic optoelectronic devices, phenyl-C61-butyric acid methyl ester (PCBM) has been claimed to be a suitable material. However, we ascertained differences between spin-coated and vacuum sublimed thin films. In this contribution, we thoroughly investigate the effects of thermal evaporation of PCBM in a strongly interdisciplinary approach, applying physical characterization techniques such as photoelectron (PES) and infrared (IR) spectroscopy in combination with further chemical analysis using thermal gravimetric analysis (TGA), differential scanning calorimetry (DSC) and ultra-performance liquid chromatography-coupled mass spectrometry (UPLC-MS), as well as proton nuclear magnetic resonance spectroscopy (1H-NMR). The methods were applied to thin films prepared by solution-based deposition techniques and by thermal evaporation. Additionally, comparison to an evaporated C60 film and the crucible residues is carried out. Changes in the IR spectrum of the PCBM films already indicate a change in the molecular structure of PCBM. The UPLC chromatogram of the redissolved organic film proves the formation of several molecular species, including bare C60. However, the effect of degradation on the electronic properties was found to be limited, as an almost unchanged ionization potential of 6.1 eV was determined with UPS for both the solution processed as well as the evaporated films. Also bulk-heterojunction (BHJ) solar cells fabricated using pure and thermally treated PCBM showed the same J–V characteristics under illumination.

A high pressure x-ray photoelectron spectroscopy experimental method for characterization of solid-liquid interfaces demonstrated with a Li-ion battery system

We report a methodology for a direct investigation of the solid/liquid interface using high pressure x-ray photoelectron spectroscopy (HPXPS). The technique was demonstrated with an electrochemical system represented by a Li-ion battery using a silicon electrode and a liquid electrolyte of LiClO4 in propylene carbonate (PC) cycled versus metallic lithium. For the first time the presence of a liquid electrolyte was realized using a transfer procedure where the sample was introduced into a 2 mbar N2 environment in the analysis chamber without an intermediate ultrahigh vacuum (UHV) step in the load lock. The procedure was characterized in detail concerning lateral drop gradients as well as stability of measurement conditions over time. The X-ray photoelectron spectroscopy (XPS) measurements demonstrate that the solid substrate and the liquid electrolyte can be observed simultaneously. The results show that the solid electrolyte interphase (SEI) composition for the wet electrode is stable within the probing time and generally agrees well with traditional UHV studies. Since the methodology can easily be adjusted to various high pressure photoelectron spectroscopy systems, extending the approach towards operando solid/liquid interface studies using liquid electrolytes seems now feasible.

Adiponitrile–Lithium Bis(trimethylsulfonyl)imide Solutions as Alkyl Carbonate-free Electrolytes for Li4Ti5O12 (LTO)/LiNi1/3Co1/3Mn1/3O2 (NMC) Li-Ion Batteries

Recently, dinitriles (NC(CH2 )n CN) and especially adiponitrile (ADN, n=4) have attracted attention as safe electrolyte solvents owing to their chemical stability, high boiling points, high flash points, and low vapor pressure. The good solvation properties of ADN toward lithium salts and its high electrochemical stability (≈6 V vs. Li/Li+ ) make it suitable for safer Li-ions cells without performance loss. In this study, ADN is used as a single electrolyte solvent with lithium bis(trimethylsulfonyl)imide (LiTFSI). This electrolyte allows the use of aluminium collectors as almost no corrosion occurs at voltages up to 4.2 V. The physicochemical properties of the ADN-LiTFSI electrolyte, such as salt dissolution, conductivity, and viscosity, were determined. The cycling performances of batteries using Li4 Ti5 O12 (LTO) as the anode and LiNi1/3 Co1/3 Mn1/3 O2 (NMC) as the cathode were determined. The results indicate that LTO/NMC batteries exhibit excellent rate capabilities with a columbic efficiency close to 100 %. As an example, cells were able to reach a capacity of 165 mAh g-1 at 0.1 C and a capacity retention of more than 98 % after 200 cycles at 0.5 C. In addition, electrodes analyses by SEM, X-ray photoelectron spectroscopy (XPS), and electrochemical impedance spectroscopy after cycling confirming minimal surface changes of the electrodes in the studied battery system.

The band energy diagram of PCBM–DH6T bulk heterojunction solar cells: synchrotron-induced photoelectron spectroscopy on solution processed DH6T:PCBM blends and in situ prepared PCBM/DH6T interfaces

Applying high resolution synchrotron-induced photoelectron spectroscopy (SXPS), the electronic properties of a bulk heterojunction (BHJ) model solar cell consisting of phenyl-C61-butyric-acid-methyl-ester (PCBM) as an acceptor and α,ω-dihexylsexithiophene (DH6T) as a donor are investigated. This donor material can be prepared via UHV thermal evaporation and solution based techniques. Samples prepared by either technique show identical core levels and valence band spectra proving the equivalency of the resulting electronic properties. The formation of the PCBM/DH6T interface is studied in an in situ experiment based on stepwise evaporation of DH6T onto PCBM. The deposition of donor–acceptor mixed solutions leads to phase separated bulk heterojunction layers with a donor cap. The combination of SXPS measurements on a series of ex situ prepared blend films from solutions with varying donor–acceptor concentrations with in situ interface formation experiments enables deriving the band diagram across the bulk heterojunction and into the donor cap. Band bending of up to 0.3 eV is induced in the DH6T cap layer and a dipole of 0.26 eV forms at the PCBM:DH6T bulk heterojunction. The direction of the band bending leads to hole accumulation on the donor side of the interface, which may increase recombination with transferred electrons in the acceptor and thereby negatively affects the device performance.

Capacity fading mechanism of tin phosphide anodes in sodium-ion batteries

Tin phosphide (Sn4P3) is here investigated as an anode material in half-cell, symmetrical, and full-cell sodium-ion batteries. Results from the half-cells using two different electrolyte salts of sodium bis(fluorosulfonyl)imide (NaFSI) or sodium hexafluorophosphate (NaPF6) show that NaFSI provides improved capacity retention but results from symmetrical cells disclose no advantage for either salt. The impact of high and low desodiation cut-off potentials is studied and the results show a drastic increase in capacity retention when using the desodiation cut-off potential of 1.2 V as compared to 2.5 V. This effect is clear for both NaFSI and NaPF6 salts in a 1 : 1 binary mixture of ethylene carbonate and diethylene carbonate with 10 vol% fluoroethylene carbonate. Hard X-ray photoelectron spectroscopy (HAXPES) results revealed that the thickness of the solid electrolyte interphase (SEI) changed during cycling and that SEI was stripped from tin particles when tin phosphide was charged to 2.5 V with NaPF6 based electrolyte.