Jan Brückner

Nanocasting Hierarchical Carbide-Derived Carbons in Nanostructured Opal Assemblies for High-Performance Cathodes in Lithium–Sulfur Batteries

Silica nanospheres are used as templates for the generation of carbide-derived carbons with monodisperse spherical mesopores (d=20-40 nm) and microporous walls. The nanocasting approach with a polycarbosilane precursor and subsequent pyrolysis, followed by silica template removal and chlorine treatment, results in carbide-derived carbons DUT-86 (DUT=Dresden University of Technology) with remarkable textural characteristics, monodisperse, spherical mesopores tunable in diameter, and very high pore volumes up to 5.0 cm3 g(-1). Morphology replication allows these nanopores to be arranged in a nanostructured inverse opal-like structure. Specific surface areas are very high (2450 m2 g(-1)) due to the simultaneous presence of micropores. Testing DUT-86 samples as cathode materials in Li-S batteries reveals excellent performance, and tailoring of the pore size allows optimization of cell performance, especially the active center accessibility and sulfur utilization. The outstanding pore volumes allow sulfur loadings of 80 wt %, a value seldom achieved in composite cathodes, and initial capacities of 1165 mAh gsulfur(-1) are reached. After 100 cycle capacities of 860 mAh gsulfur(-1) are retained, rendering DUT-86 a high-performance sulfur host material.

A lithium–sulfur full cell with ultralong cycle life: influence of cathode structure and polysulfide additive

Lithium–sulfur batteries are highly attractive energy storage systems, but suffer from structural anode and cathode degradation, capacity fade and fast cell failure (dry out). To address these issues, a carbide-derived carbon (DUT-107) featuring a high surface area (2088 m2 g−1), high total pore volume (3.17 cm3 g−1) and hierarchical micro-, meso- and macropore structure is applied as a rigid scaffold for sulfur infiltration. The DUT-107/S cathodes combine excellent mechanical stability and high initial capacities (1098–1208 mA h gS−1) with high sulfur content (69.7 wt% per total electrode) and loading (2.3–2.9 mgS cm−2). Derived from the effect of the electrolyte-to-sulfur ratio on capacity retention and cyclability, conducting salt is substituted by polysulfide additive for reduced polysulfide leakage and capacity stabilization. Moreover, in a full cell model system using a prelithiated hard carbon anode, the performance of DUT-107/S cathodes is demonstrated over 4100 cycles (final capacity of 422 mA h gS−1) with a very low capacity decay of 0.0118% per cycle. Application of PS additive further boosts the performance (final capacity of 554 mA h gS−1), although a slightly higher decay of 0.0125% per cycle is observed.

Synthesis of highly electrochemically active Li2S nanoparticles for lithium–sulfur-batteries

Carbothermal reduction of lithium sulfate below its melting point was used to produce sub-micron sized lithium sulfide particles which retain the morphology of the source particle. Using a lithium polysulfide-doped ether electrolyte, significantly enhanced activation of lithium sulfide could be accomplished in battery cells, achieving high discharge capacities up to 1360 mA h gsulfur−1 at a 0.1C rate. Showing an economically viable and scalable reaction routine for future lithium–sulfur-batteries.

High Area Capacity Lithium-Sulfur Full-cell Battery with Prelitiathed Silicon Nanowire-Carbon Anodes for Long Cycling Stability

We show full Li/S cells with the use of balanced and high capacity electrodes to address high power electro-mobile applications. The anode is made of an assembly comprising of silicon nanowires as active material densely and conformally grown on a 3D carbon mesh as a light-weight current collector, offering extremely high areal capacity for reversible Li storage of up to 9 mAh/cm2. The dense growth is guaranteed by a versatile Au precursor developed for homogenous Au layer deposition on 3D substrates. In contrast to metallic Li, the presented system exhibits superior characteristics as an anode in Li/S batteries such as safe operation, long cycle life and easy handling. These anodes are combined with high area density S/C composite cathodes into a Li/S full-cell with an ether- and lithium triflate-based electrolyte for high ionic conductivity. The result is a highly cyclable full-cell with an areal capacity of 2.3 mAh/cm2, a cyclability surpassing 450 cycles and capacity retention of 80% after 150 cycles (capacity loss <0.4% per cycle). A detailed physical and electrochemical investigation of the SiNW Li/S full-cell including in-operando synchrotron X-ray diffraction measurements reveals that the lower degradation is due to a lower self-reduction of polysulfides after continuous charging/discharging.

Sulfur: an intermediate template for advanced silicon anode architectures

The lithium–sulfur chemistry provides a next generation battery technology on the verge of commercialization with significantly increased specific energy. However, the cycle life mainly suffers from dendrite and continuous SEI formation in lithium anodes inducing active material and electrolyte depletion. Here, we report on a silicon–carbon composite anode as a stable alternative anode for safe Li–S cells. Well-defined sulfur coatings generate a shell for a silicon core (Si@S) to further form a carbon shell (Si@S@sucrose). After sulfur removal the void structure (Si@void@C) allows to compensate the mechanical stress imposed by the huge volume change during the lithiation of the silicon. In this case, sulfur is not only used as a low cost and high capacity cathode material but also as a template to create free volume. It is easily removed during the pyrolysis and no acid leaching steps are required. In half cell tests vs. lithium a high capacity of 2270 mA h gSi−1 (690 mA h g−1) was achieved in the 10th cycle and the reversible lithiation of the silicon particles could be ensured for more than 50 cycles. The prelithiated Si–C anode with a high areal capacity of 2 mA h cm−2 was successfully matched with a sulfur cathode in a SLS full cell on coin cell and on pouch cell levels. A high capacity of about 807 mA h gsulfur−1 (2nd cycle) was reached with a low lithium excess of only 76% compared to 2000% lithium excess in state-of-the-art Li–S cells.