A convenient approach for a controlled and high-yield synthesis of copper-deficient Cu 3−x P (0.1 < x < 0.7) is reported that makes use of ionic liquids with highly nucleophilic "naked" halide anions. Halide anions drastically enhance the reactivity of the white phosphorus precursor and kinetically disfavour the formation of phosphorus-rich side products. Cu 3−x P shows a high degree of tolerance for cation vacancies without mayor structural reorganisation, as evidenced by X-ray diffraction and solid-state nuclear magnetic resonance spectroscopy. Measurements of the electric properties reveal that Cu 3−x P is a bad metallic p-type conductor. The resistivity is composition-dependent and displays a distinct anomaly from a phase transition, leading to the discovery and structural characterisation of two hitherto unknown low temperature polymorphs. Electrochemical evaluation of copper-deficient Cu 3−x P as anode material for lithium ion batteries reveals a drastic change in the cycling mechanism leading to an increase of the initial capacities by about 70 %. This work gives a comprehensive insight into the chemical and structural features of copper-deficient Cu 3−x P and should lead to an improved understanding of its properties, not only for battery applications.

The electrochemical characteristics of silicon diphosphide (SiP) as a new anode material for future lithium-ion batteries (LIBs) are evaluated. The high theoretical capacity of about 3900 mA h g (fully lithiated state: LiSi + LiP) renders silicon diphosphide as a highly promising candidate to replace graphite (372 mA h g) as the standard anode to significantly increase the specific energy density of LIBs. The proposed mechanism of SiP is divided into a conversion reaction of phosphorus species, followed by an alloying reaction forming lithium silicide phases. In this study, we focus on the conversion mechanism during cycling and report on the phase transitions of SiP during lithiation and delithiation. By using ex situ analysis techniques such as X-ray powder diffraction, formed reaction products are identified. Magic angle spinning nuclear magnetic resonance spectroscopy is applied for the characterization of long-range ordered compounds, whereas X-ray photoelectron spectroscopy gives information of the surface-layer species at the interface of active material and electrolyte. Our SiP anode material shows a high initial capacity of about 2700 mA h g, whereas a fast capacity fading during the first few cycles occurs which is not necessarily expected. On the basis of our results, we conclude that besides other degradation effects, such as electrolyte decomposition and electrical contact loss, the rapid capacity fading originates from the formation of a low ion-conductive layer of LiP. This insulating layer hinders lithium-ion diffusion during lithiation and thereby mainly contributes to fast capacity fading.

We report on the facile synthesis of porous carbons based on a biopolymer lignin employing a two-step process which includes the activation by KOH in various amounts under an inert gas atmosphere. The resulting carbons are characterized with regard to their structural properties and their electrochemical performance as an active material in double-layer capacitors using for the first time an ionic liquid (EMIBF4) as the electrolyte for this type of carbon material to enhance storage ability. A capacitance of more than 200 F g–1 at 10 A g–1 is achieved for a carbon with a specific surface area of more than 1800 m2 g–1. One of the most crucial factors determining the electrochemical response of the active materials was found to be the strong surface functionalization by oxygen-containing groups. Furthermore, the sulfur content of the carbon precursor lignin does not result in a significant amount of sulfur-containing surface functionalities which might interact with the electrolyte.