Prof. Dr. Volker Presser

Abstract In recent years, numerous studies have explored ways to overcome the low intrinsic electrical conductivity of lithium titanate (Li4Ti5O12, LTO) for energy storage with lithium-ion batteries. These approaches almost exclusively considered element doping and elaborate LTO-carbon nanocomposites, whereas simple adjustment of the defect concentration remains largely unexplored. In our study, we tune the Ti3+/Ti4+ concentration of a commercial LTO nanopowder through oxygen vacancy formation during thermal annealing in hydrogen atmosphere. We investigate the impact of the treatment on material properties like energy band structure, electrical conductivity, crystallinity, phase distribution, surface chemistry, and particle morphology, and correlate these parameters to the electrochemical performance. At optimum treatment conditions, the intrinsic electrical conductivity can be greatly improved, while circumventing LTO phase transformations or amorphization. This enables the reduction of the carbon concentration to 5 mass%, while yielding a high electrode capacity of about 70 mAh/g (82 mAh/g based on active mass) at ultrahigh C-rates of 100C. When combined with an activated carbon/lithium manganese oxide composite cathode, an excellent energy and power performance of 70 Wh/kg and 47 kW/kg were obtained (82 Wh/kg and 55 kW/kg based on active mass), while maintaining 83 % of its energy ratings after 5000 cycles at 10C (78 % after 15000 cycles at 100C).

Synthesis of high surface area carbon materials with hierarchical pore structure is reported. Combined salt templating with ZnCl2 and hard templating with SBA-15 is used to produce ordered mesoporous and microporous hard–salt-templated carbons (OM-HSTCs) from simple sucrose as carbon precursor. OM-HSTCs achieve specific surface areas of more than 2600 m2 g−1 and total pore volumes up to 2.2 cm3 g−1. In comparison to purely hard-templated ordered mesoporous carbons, the additional salt template leads to high micropore volume and provides control over the size/distribution of micro- and mesopores and over the carbon microstructure. This method combines carbonization and the formation of well-defined micropores in one step and is more versatile in terms of resulting pore structure than previously reported routes toward ordered mesoporous/microporous carbons. When applied as electrode materials in electric double-layer capacitors with 1 m tetraethylammonium tetrafluoroborate in acetonitrile organic electrolyte, OM-HSTCs combine high gravimetric capacitance (133 F g−1 at 0.1 A g−1) resulting from high micropore volume with high capacitance retention under high-power conditions (126 F g−1 at 40 A g−1), exceeding the purely microporous or purely ordered mesoporous reference materials.

Abstract Lignin-derived carbon is introduced as a promising electrode material for water desalination by using capacitive deionization (CDI). Lignin is a low-cost precursor that is obtained from the cellulose and ethanol industries, and we used carbonization and subsequent KOH activation to obtain highly porous carbon. CDI cells with a pair of lignin-derived carbon electrodes presented an initially high salt adsorption capacity but rapidly lost their beneficial desalination performance. To capitalize on the high porosity of lignin-derived carbon and to stabilize the CDI performance, we then used asymmetric electrode configurations. By using electrodes of the same material but with different thicknesses, the desalination performance was stabilized through reduction of the potential at the positive electrode. To enhance the desalination capacity further, we used cell configurations with different materials for the positive and negative electrodes. The best performance was achieved by a cell with lignin-derived carbon as a negative electrode and commercial activated carbon as a positive electrode. Thereby, a maximum desalination capacity of 18.5 mg g−1 was obtained with charge efficiency over 80 % and excellent performance retention over 100 cycles. The improvements were related to the difference in the potential of zero charge between the electrodes. Our work shows that an asymmetric cell configuration is a powerful tool to adapt otherwise inappropriate CDI electrode materials.

The MicroJet reactor technique is an excellent continuous method to produce spherical and homogeneous organically modified silica (ORMOSIL) particles in a large scale (10-15 g min-1). We applied this method to manufacture polyorganosilsesquioxanes with different ratios of phenyl and vinyl functional groups, which were later pyrolyzed to obtain silicon oxycarbides. Such polymer-derived ceramic (PDC) materials are highly suited as precursor for carbide-derived carbon (CDC) synthesis. Chlorine etching of PDC at high temperatures removed silicon and oxygen, yielding the formation of nanoporous carbon. Pure poly(phenyl-silsesquioxane) spheres lost their shape during the thermal process by undergoing further condensation reactions. Yet, the spherical shape was conserved during thermal processing after adding vinyl functionalities. The ratio of vinyl and phenyl functionalities controlled the pore structure and the total CDC yield, enabling an increase from 2 mass% to 22 mass%. The total pore volume varied between 1.3-2.1 cm3 g-1 and the specific surface area between 2014-2114 m2 g-1. The high surface area and large pore volume makes these materials attractive for high power supercapacitor electrodes. The specific capacitance of the best sample at low rates in 1 M tetraethylammonium tetrafluoroborate in acetonitrile was 116 F g-1 (at 5 mA g-1) and still 80 F g-1 at very high rates (at 100 A g-1).

Novolac-derived nanoporous carbon beads were used as conductive matrix for lithium-sulfur battery cathodes. We employed a facile self-emulsifying synthesis to obtain sub-micrometer novolac-derived carbon beads with nanopores. After pyrolysis, the carbon beads showed already a specific surface area of 640 m2 g−1 which was increased to 2080 m2 g−1 after physical activation. The non-activated and the activated carbon beads represent nanoporous carbon with a medium and a high surface area, respectively. This allows us to assess the influence of the porosity on the electrochemical performance of lithium-sulfur battery cathodes. The carbon/sulfur hybrids were obtained from two different approaches of sulfur infiltration: melt-infusion of sulfur (annealing) and in situ formation of sulfur from sodium thiosulfate. The best performance (∼880 mAh gsulfur−1 at low charge rate; 5th cycle) and high performance stability (>600 mAh gsulfur−1 after 100 cycles) were found for the activated carbon beads when using melt infusion of sulfur.

Intercalation-induced dimensional changes in a composite battery electrode (comprising a polymeric binder) are one of the major factors limiting electrode cycling performance. Since electrode performance is expressed by the quantities averaged over its entire surface area (e.g., capacity retention, Faradaic efficiency, rate capability), significant efforts have been made to develop a methodology allowing its facile mechanical diagnostics at the same areal scale. Herein we introduce such a generic methodology for a highly sensitive in situ monitoring of intrinsic mechanical properties of composite battery electrodes. The gravimetric, dimensional, viscoelastic, and adhesive changes in the composite electrodes caused by Li-ions intercalation are assessed noninvasively and in real time by electrochemical quartz-crystal microbalance with dissipation monitoring (EQCM-D). Multiharmonic acoustic waves generated by EQCM-D penetrate into thin porous electrodes comprising either rigid or a soft binder resulting in frequency and dissipation changes quantified by analytical acoustic load impedance models. As a first demonstration, we used a composite LiFePO4 (LFP) electrode containing either polyvinylidene dichloride (PVdF) or Na carboximethyl cellulose (NaCMC) as rigid and viscoelastic binders, respectively, in aqueous electrolytes. The intercalation-induced volume changes of LFP electrode were evaluated from a hydrodynamic correction to the mass effect of the intercalated ions for PVdF, and both components of the effective complex shear modulus (i.e., storage and loss moduli) in case of NaCMC binder have been extracted. The sliding friction coefficients for large particles bound at their bottom to the quartz crystal surface (a measure of the adhesion strength of binders) has also been evaluated. Tracking the mechanical properties of the composite electrodes in different environments and charging/cycling conditions in a self-consistent manner provides all necessary conditions for an optimal selection of the polymeric binders resistant to intercalation-induced volume changes of intercalation particles.

In this study, atomic layer deposition (ALD) is employed to synthesize hybrid electrode materials of especially tailored mesoporous carbon and vanadium oxide. The highly conformal and precise character of ALD allowed for depositing up to 65 mass % of vanadium oxide inside the 5–20 nm mesopores of the carbon particles, without substantially obstructing internal surface area. The deposited phase was identified as orthorhombic V2O5, and an increasing crystalline order was detected for higher mass loadings. Employing the hybrid material as lithium and sodium intercalation hosts at a rate of 0.5C yielded specific capacities of 310 and 250 mAh/g per V2O5, respectively, while showing predominantly pseudocapacitive behavior, that is, capacitor-like voltage profiles. C-rate benchmarking revealed a retention of about 50% of the maximum capacity for both lithium and sodium at a high rate of 100C. When testing for longevity in lithium-containing electrolyte, a steadily increasing capacity was observed to 116% of the initial value after 2000 cycles. In sodium electrolyte, the capacity faded to 75% after 2000 cycles, which represents one of the most stable performances for sodium intercalation in the literature. Homogeneously distributed vanadium oxide that is locally confined in the tailored carbon mesopores was identified as the reason for enhanced cyclability and rate behavior of the hybrid material.

Atomic layer deposition has proven to be a particularly attractive approach for decorating mesoporous carbon substrates with redox active metal oxides for electrochemical energy storage. This study, for the first time, capitalizes on the cyclic character of atomic layer deposition to obtain a highly conformal and atomically controlled decoration of carbon onions with alternating stacks of vanadia and titania. The addition of 25 mass% TiO2 leads to an expansion of the VO2 unit cell, thus greatly enhancing lithium intercalation capacity and kinetics. Electrochemical characterization revealed ultrahigh discharge capacity of up to 382 mAh[middle dot]g-1 of the composite electrode (554 mAh[middle dot]g-1 per metal oxide) with an impressive capacity retention of 82 mAh[middle dot]g-1 (120 mAh[middle dot]g-1 per metal oxide) at a high discharge rate of 20 A[middle dot]g-1 or 52 C. Rigorous stability benchmarking showed superior stability over 3,000 cycles when discharging to a reduced potential of -1.8 V vs. carbon. These capacity values are among the highest reported for any metal oxide system, while in addition, supercapacitor-like power performance and longevity are achieved. On a device level, high specific energy and power of up to 110 Wh[middle dot]kg-1 and 6 kW[middle dot]kg-1, respectively, were achieved when employing the hybrid material as anode versus activated carbon cathode.

The study presents the synthesis of vanadium oxide/carbon onion hybrid materials. Flower-like vanadium oxide nanostructures nucleate on carbon onion nanoparticles under hydrothermal conditions, forming a highly intertwined network. By varying the amount of added carbon onions during the synthesis, the number of possible nucleation sites can be adjusted, resulting in the preferential growth of vanadium dioxide in either P21/c or C2/m space group. When employed as a lithium intercalation electrode, P21/c VO2 exhibits capacitor-like (pseudocapacitive) lithium intercalation, whereas C2/m VO2 shows battery-like intercalation peaks with a maximum capacity of 127 mA h g-1. By selecting an optimum ratio and thereby combining both intercalation mechanisms, enhanced kinetics with discharge capacities of 45 mA h g-1 and 29 mA h g-1 at high rates of 50 A g-1 and 100 A g-1 (equal to 394C and 788C) are obtained. This behavior can be translated to a device level by using the material as anodes in asymmetric supercapacitors with activated carbon cathodes, yielding a maximum specific energy of 45 W h kg-1 and a high power of 58 kW kg-1, while longevity over 5000 charge/discharge cycles is demonstrated.

Intercalation-induced dimensional changes of composite battery electrodes containing either a stiff or a soft polymeric binder is one of the many factors determining the cycling performance and ageing. Herein, we report dimensional changes in bulk composite electrodes by in situ electrochemical dilatometry (eD) combined with electrochemical quartz-crystal microbalance with dissipation monitoring (EQCM-D). The latter tracks the mechanical properties on the level of the electrode particle size. Lithium iron phosphate (LiFePO4, LFP) electrodes with a stiff binder (PVdF) and a soft binder (NaCMC) were investigated by cycling in lithium sulfate (Li2SO4) aqueous solution. The electrochemical and mechanical electrode performances depend on the electrode cycling history. Based on combined eD and EQCM-D measurements we provide evidence which properties are preferred for a binder used for a composite Li-ion battery electrode.