Martin Kolek

A Step toward High-Energy Silicon-Based Thin Film Lithium Ion Batteries

The next generation of lithium ion batteries (LIBs) with increased energy density for large-scale applications, such as electric mobility, and also for small electronic devices, such as microbatteries and on-chip batteries, requires advanced electrode active materials with enhanced specific and volumetric capacities. In this regard, silicon as anode material has attracted much attention due to its high specific capacity. However, the enormous volume changes during lithiation/delithiation are still a main obstacle avoiding the broad commercial use of Si-based electrodes. In this work, Si-based thin film electrodes, prepared by magnetron sputtering, are studied. Herein, we present a sophisticated surface design and electrode structure modification by amorphous carbon layers to increase the mechanical integrity and, thus, the electrochemical performance. Therefore, the influence of amorphous C thin film layers, either deposited on top (C/Si) or incorporated between the amorphous Si thin film layers (Si/C/Si), was characterized according to their physical and electrochemical properties. The thin film electrodes were thoroughly studied by means of electrochemical impedance spectroscopy, Raman spectroscopy, X-ray photoelectron spectroscopy, and atomic force microscopy. We can show that the silicon thin film electrodes with an amorphous C layer showed a remarkably improved electrochemical performance in terms of capacity retention and Coulombic efficiency. The C layer is able to mitigate the mechanical stress during lithiation of the Si thin film by buffering the volume changes and to reduce the loss of active lithium during solid electrolyte interphase formation and cycling.

Ultra-high cycling stability of poly(vinylphenothiazine) as a battery cathode material resulting from π–π interactions

Organic cathode materials are promising candidates for a new generation of “green batteries”, since they have low toxicity and can be produced from renewable resources or from oil. Especially suitable are organic redox polymers that can be reversibly oxidized and reduced. Because of their often-insulating nature, however, many redox polymers have limited rate capabilities. Their cycling stabilities, which are of high importance for the long cycle-life of a battery cell, rarely exceed 1000 cycles. Here, we present a new concept for redox polymers as cathode materials, in which the oxidized states are stabilized through π–π interactions between redox-active groups. We found that due to these interactions poly(3-vinyl-N-methylphenothiazine) showed excellent cycling stability (after 10 000 cycles at a 10C rate, 93% of the initial capacity was retained) in addition to a high rate capability because of supramolecular hole transport. We propose this concept to be used in the future design of redox polymers for batteries.