Daniel Gunzelmann

Phosphorus–carbon nanocomposite anodes for lithium-ion and sodium-ion batteries

With the expected theoretical capacity of 2596 mA h g−1, phosphorus is considered to be the highest capacity anode material for sodium-ion batteries and one of the most attractive anode materials for lithium-ion systems. This work presents a comprehensive study of phosphorus–carbon nanocomposite anodes for both lithium-ion and sodium-ion batteries. The composite electrodes are able to display high initial capacities of approximately 1700 and 1300 mA h g−1 in lithium and sodium half-cells, respectively, when the cells are tested within a larger potential windows of 2.0–0.01 V vs. Li/Li+ and Na/Na+. The level of demonstrated capacity is underpinned by the storage mechanism, based on the transformation of phosphorus to Li3P phase for lithium cells and an incomplete transformation to Na3P phase for sodium cells. The capacity deteriorates upon cycling, which is shown to originate from disintegration of electrodes and their delamination from current collectors by post-cycling ex situ electron microscopy. Stable cyclic performance at the level of ∼700 and ∼350–400 mA h g−1 can be achieved if the potential windows are restricted to 2.0–0.67 V vs. Li/Li+ for lithium and 2–0.33 vs. Na/Na+ for sodium half-cells. The results are critically discussed in light of existing literature reports.

Structure and dynamics in an organic ionic plastic crystal, N-ethyl-N-methyl pyrrolidinium bis(trifluoromethanesulfonyl) amide, mixed with a sodium salt

We present for the first time, the solid state phase behaviour of the organic ionic plastic crystal (OIPC) N-methyl-N-ethyl-pyrrolidinium bis(trifluoromethanesulfonyl)amide, [C2mpyr][NTf2], upon mixing with the sodium salt, Na[NTf2]. Whereas the behaviour of OIPCs mixed with lithium salts has been well established, the influence of adding a sodium salt has not previously been reported. The phase diagram is presented for Na[NTf2] compositions between 1 and 50 mol%, and shows a eutectic composition at 15 mol% with a eutectic temperature at 63 °C. In contrast to the lithium doping of the same OIPC, the conductivity does not increase significantly at, or below room temperature, however the material containing 40 mol% Na[NTf2] has an ionic conductivity of 10−4 S cm−1 in the solid state at 60 °C, which is more than 3 orders of magnitude higher than the pure OIPC. Synchrotron XRD, solid-state NMR and SEM all indicate the presence of two distinct phases across all of the compositions studied. One phase is identical to that of the pure [C2mpyr][NTf2] and the other phase is a mixed cation compound, quite distinct from the pure Na[NTf2] material. The higher eutectic temperature (63 °C) in the sodium based system compared to lithium (30 °C) leads to purely solid state conductivity over the entire composition range up to 63 °C.

Ionic transport through a composite structure of N-ethyl-N-methylpyrrolidinium tetrafluoroborate organic ionic plastic crystals reinforced with polymer nanofibres

The incorporation of polyvinylidene difluoride (PVDF) electrospun nanofibres within N-ethyl-N-methylpyrrolidinium tetrafluoroborate, [C2mpyr][BF4] was investigated with a view to fabricating self-standing membranes for various electrochemical device applications, in particular lithium metal batteries. Significant improvement in mechanical properties and ionic conduction was demonstrated in a previous study, which also demonstrated the remarkably high performance of the lithium-doped composite material in a device. We now seek a fundamental understanding of the role of fibres within the matrix of the plastic crystal, which is essential for optimizing device performance through fine-tuning of the composite material properties. The focus of the current study is therefore a thorough investigation of the phase behaviour and conduction behaviour of the pure and the lithium-doped (as LiBF4) plastic crystal, with and without incorporation of polymer nanofibres. Analysis of the structure of the plastic crystal, including the effects of lithium ions and the incorporation of PVDF fibres, was conducted by means of synchrotron XRD. Ion dynamics were evaluated using VT solid-state NMR spectroscopy. ATR-FTIR spectroscopy was employed to gain insights into the molecular interactions of doped lithium ions and/or the PVDF nanofibres in the matrix of the [C2mpyr][BF4] composites. Preliminary measurements using PALS were conducted to probe structural defects within the pure materials. It was found that ion transport within the plastic crystal was significantly altered by doping with lithium ions due to the precipitation of a second phase in the structure. The incorporation of the fibres activated more mobile sites in the systems, but restricted ion mobility with different trends being observed for each ion species in each crystalline phase. In the presence of the fibres a strong interaction observed between the Li ion and the pyrrolidinium ring disappeared and formation of the second phase was prevented. As a result, an increased number of mobile lithium ions are released into the solid solution structure of the matrix, simultaneously removing the blocking effect of the second phase. Thus, ion conduction was remarkably improved within the Li-doped composite compared to the neat Li-doped plastic crystal.

Ion conduction and phase morphology in sulfonate copolymer ionomers based on ionic liquid–sodium cation mixtures

A series of sulfonate based copolymer ionomers based on a combination of ionic liquid and sodium cations have been prepared in different ratios. This system was designed to improve the ionic conductivity of ionomers by partially replacing sodium cations with bulky cations that are less associated with anion centres on the polymer backbone. This provides more conduction sites for sodium to ‘hop’ to in the ionomers. Characterization showed the glass transition and 15N chemical shift of the ionomers did not vary significantly as the amount of Na+ varied, while the ionic conductivity increased with decreasing Na+ content, indicating conductivity is increasingly decoupled from Tg. Optical microscope images showed phase separation in all compositions, which indicated the samples were inhomogeneous. The introduction of low molecular weight plasticizer (PEG) reduced the Tg and increased the ionic conductivity significantly. The inclusion of PEG also led to a more homogeneous material.

Decoupled ion conduction in poly(2-acrylamido-2-methyl-1-propane-sulfonic acid) homopolymers

As the focus on developing new polymer electrolytes continues to intensify in the area of alternative energy conversion and storage devices, the rational design of polyelectrolytes with high single ion transport rates has emerged as a primary strategy for enhancing device performance. Previously, we reported a series of sulfonate based copolymer ionomers based on using mixed bulky quaternary ammonium cations and sodium cations as the ionomer counterions. This led to improvements in the ionic conductivity and an apparent decoupling from the Tg of the ionomer. In this article, we have prepared a new series of ionomers based on the homopolymer of poly(2-acrylamido-2-methyl-1-propane-sulfonic acid) using differing sizes of the ammonium counter-cations. We observe a decreasing Tg with increasing the bulkiness of the quaternary ammonium cation, and an increasing degree of decoupling from Tg within these systems. Somewhat surprisingly, phase separation is observed in this homopolymer system, as evidenced from multiple impedance arcs, Raman mapping and SEM. The thermal properties, morphology and the effect of plasticizer on the transport properties in these ionomers are also presented. The addition of 10 wt% plasticizer increased the ionic conductivity between two and three orders of magnitudes leading to materials that may have applications in sodium based devices.

Effect of secondary phase on thermal behaviour and solid-state ion conduction in lithium doped N-ethyl-N-methylpyrrolidinium tetrafluoroborate organic ionic plastic crystal

An investigation of the equilibrium phase behaviour and the effect of lithium salt (lithium tetrafluoroborate) on the organic ionic plastic crystal (OIPC), N-ethyl-N-methylpyrrolidinium tetrafluoroborate [C2mpyr][BF4] is described. In addition to the equilibrium phase transformations of the pure OIPC, new transitions were observed after the addition of the lithium salt (10 mol% and 20 mol% LiBF4), which significantly altered the ion conduction behaviour of the material. Attenuated total reflectance (ATR) FTIR spectroscopic results indicated that the molecular interactions between the lithium ion and the OIPC matrix were intensified with increasing lithium salt content. Variable temperature X-ray diffractograms revealed the existence of secondary and ternary phases within the microstructure of lithium ion containing OIPCs in the low temperature structures and during their transformations across the heating scans. Wide-line NMR data were consistent with the results obtained from XRD and DSC observations, indicating environments with varying dynamics for each of the ionic components (C2mpyr+, BF4− and Li+) by increased lithium salt content.