Challenges in Measuring Transport Parameters of Carbonate-based Electrolytes

Numerical simulations are a powerful tool for the development and improvement of Li-ion batteries. Modeling the mass transport of the involved electrolytic solutions requires precise determination of the corresponding electrolyte parameters. In this work, we attempt to measure the conductivity, the diffusion coefficient, the transference number and the thermodynamic factor for a system of 0.5$\,$M LiPF$_6$ dissolved in a blend of ethylene carbonate and ethyl methyl carbonate (EC:EMC, 3:7 weight) at 20$\,${\deg}C and 50$\,${\deg}C. Applying galvanostatic polarization experiments to symmetrical metal Li | electrolyte + separator | Li metal cells reveals, however, a potential response qualitatively deviating from theoretical expectations. Impeded diffusion processes indicate the presence of additional, undesired porous structures on the Li electrodes, preventing a reliable evaluation of the electrolyte parameters. To spectrally resolve the diffusive processes, we conduct very-low-frequency impedance spectroscopy. The impedance in fact exhibits multiple interfering diffusive features. In our measurements, an explicit identification of the impedance for the sole diffusion through the separator is however not feasible. Therefore, the authors doubt that polarizing experiments using Li metal electrodes yield accurate parameters for electrolytes.Comment: 18 pages, 12 figure

Identification of the Underlying Processes in Impedance Response of Sulfur/Carbon Composite Cathodes at Different SOC

For lithium-sulfur batteries, porous carbon/sulfur composite cathodes are the primary solution to compensate the non-conductive nature of sulfur. The composition and structure of this class of cathodes are crucial to the electrochemical performance, achieved energy density and the stability of the cell. Electrochemical impedance spectroscopy is employed to investigate and correlate the electrochemical performance of lithium-sulfur batteries to the composition and microstructure of differently fabricated carbon/sulfur composite cathodes. A transmission line model is applied to identify different underlying electrochemical processes appearing in the impedance response of a range of porous carbon/sulfur cathodes. The integration of a lithium ring serving as a counter electrode coupled with advanced wiring has allowed an artifact-free recording of the cathode impedance at different states of charge with the aim to investigate the evolution of impedance during discharge/charge and the kinetics of charge transfer depending on the infiltration method and the utilized carbon host. It is shown that impedance response of this class of cathodes is highly diverse and the plausible underlying processes are discussed in details. To this end, quasi-solid-state and various polysulfide-based charge transfer mechanisms are identified and their time constants are reported

Identification of the underlying processes in impedance response of sulfur/carbon composite cathodes at different SOC

For lithium-sulfur batteries, porous carbon/sulfur composite cathodes are the primary solution to compensate the non-conductive nature of sulfur. The composition and structure of this class of cathodes are crucial to the electrochemical performance, achieved energy density and the stability of the cell. Electrochemical impedance spectroscopy is employed to investigate and correlate the electrochemical performance of lithium-sulfur batteries to the composition and microstructure of differently fabricated carbon/sulfur composite cathodes. A transmission line model is applied to identify different underlying electrochemical processes appearing in the impedance response of a range of porous carbon/sulfur cathodes. The integration of a lithium ring serving as a counter electrode coupled with advanced wiring has allowed an artifact-free recording of the cathode impedance at different states of charge with the aim to investigate the evolution of impedance during discharge/charge and the kinetics of charge transfer depending on the infiltration method and the utilized carbon host. It is shown that impedance response of this class of cathodes is highly diverse and the plausible underlying processes are discussed in details. To this end, quasi-solid-state and various polysulfide-based charge transfer mechanisms are identified and their time constants are reported

Corrosion Study of Current Collectors for Magnesium Batteries

The transition to renewable energy requires a significant amount of low-cost energy storage systems. Regarding batteries, magnesium provides a highly abundant raw material which is less sensitive to air in comparison to lithium, crucial to the mass production and safety. Promising candidates for intercalation materials on the cathode side are Prussian green FeFe(CN)6 with a electrochemical potential of around 0,9 V vs. Mg or the Chevrel phase Mo6S8 which shows a high specific capacity of around 120 mAh/g. Magnesium perchlorate-based electrolytes provide a practicable solution for fundamental work in the early stage of cathode research, yet are not compatible with Mg metal due to corrosion. Therefore, the organo-metallic all phenyl complex (APC) based electrolyte is a potential candidate for magnesium full cells. However, both systems contain highly reactive chloride species which cause severe corrosion of the current collector. In this work, potential materials for current collectors (carbon coated Al and Ni) are investigated applying linear sweep voltammetry, chronoamperometry and electrochemical impedance spectroscopy. A graphite based current collector is identified as the most promising candidate due to its high corrosion resistivity of 2 V vs. Mg/Mg2+ and low areal density, which helps to increase the energy density of practical Mg batteries

Magnesium Anode Protection by an Organic Artificial Solid Electrolyte Interphase for Magnesium-Sulfur Batteries

In the search for post-lithium battery systems, magnesium–sulfur batteries have attracted research attention in recent years due to their high potential energy density, raw material abundance, and low cost. Despite significant progress, the system still lacks cycling stability mainly associated with the ongoing parasitic reduction of sulfur at the anode surface, resulting in the loss of active materials and passivating surface layer formation on the anode. In addition to sulfur retention approaches on the cathode side, the protection of the reductive anode surface by an artificial solid electrolyte interphase (SEI) represents a promising approach, which contrarily does not impede the sulfur cathode kinetics. In this study, an organic coating approach based on ionomers and polymers is pursued to combine the desired properties of mechanical flexibility and high ionic conductivity while enabling a facile and energy-efficient preparation. Despite exhibiting higher polarization overpotentials in Mg–Mg cells, the charge overpotential in Mg–S cells was decreased by the coated anodes with the initial Coulombic efficiency being significantly increased. Consequently, the discharge capacity after 300 cycles applying an Aquivion/PVDF-coated Mg anode was twice that of a pristine Mg anode, indicating effective polysulfide repulsion from the Mg surface by the artificial SEI. This was backed by operando imaging during long-term OCV revealing a non-colored separator, i.e. mitigated self-discharge. While SEM, AFM, IR and XPS were applied to gain further insights into the surface morphology and composition, scalable coating techniques were investigated in addition to ensure practical relevance. Remarkably therein, the Mg anode preparation and all surface coatings were prepared under ambient conditions, which facilitates future electrode and cell assembly. Overall, this study highlights the important role of Mg anode coatings to improve the electrochemical performance of magnesium–sulfur batteries

High sulfur content in microporous carbon aerogels for lithium-sulfur batteries

Lithium-Sulfur (Li-S) batteries are currently one of the attractive systems among next generation of the batteries offering high theoretical specific capacity of 1675 mAh/g and high specific energy density of 2500 Wh/Kg. In addition, sulfur is abundant on earth and inexpensive. These advantages of Li-S battery make it the promising candidate for the realization of cost and performance- efficient electro-mobility. However, the commercialization of Li-S batteries is challenged by the capacity loss induced by the so-called polysulfide shuttle effect. The encapsulation of the active material in the cathode matrix is one of the many strategies impeding the polysulfide shuttle effect [1]. Due to the adjustable microporous structure and pore size distribution, carbon aerogels are highly promising material to be used as user-defined porous matrix for sulfur. Starting from organic resorcinol-formaldehyde (RF) aerogels, recently developed carbon aerogels exhibit porous structure with huge porosity up to 97 %, high surface area about 500-2000 m²/g, large micro pore volume about 0.1-0.6 cm³/g, and good electrical conductivity [2,3]. Moreover, flexibility of carbon aerogels allows elimination of crack formation during volume change of sulfur. In the presented study, we synthesized and investigated highly microporous carbon aerogel as conductive matrix embedding up to 32 wt-% of sulfur for cathodes in Li-S batteries [4]. New aerogel based battery exhibits high cyclic stability and high specific capacity of 1000 mAh/g(S) after 150 cycles. The innovative gas-phase sulfur infiltration of the carbon aerogels and the resulting confinement of the short sulfur-chains in the microspores (< 2 nm) are demonstrated employing complementary characterization techniques such as TGA, XPS, and elemental analysis. It is shown that S-infiltrated microporous carbon aerogel cathodes are indeed able to suppress the polysulfide shuttle effect, resulting in longer cycle stability of the cell in both ether and carbonate-based electrolyte

Highly porous carbon aerogels for electrochemical applications

Carbon aerogels are highly-porous, electrically conductive materials with tunable micro- and mesoporosity as well as surface area and surface modification. Their properties are dictated from sol-gel process of their organic precursors and carbonization conditions. Due to their well adjustable structural properties and high electrical conductivity, they became very attractive in many fields of electrochemical applications. Energy and power-storage capability of supercapacitors are associated with the physical and chemical characteristics of carbon-based electrodes. Cyclic voltammetry and galvanostatic charge/discharge measurements demonstrate the good electrochemical performance of the supercapacitor. A specific capacitance of 21.8 F·g−1 at 2 A·g−1 and cycle durability of 87% over 10,000 cycles was observed due to the presence of dual mesopores. Ultramicroporous (pores smaller 1 nm) carbon aerogels as conductive matrices embedding sulfur for cathode application in lithium–sulfur batteries are able to suppress the polysulfide shuttle effect, maintaining 80% (about 1000 mA·h·g(S)−1) and 70% (about 800 mA·h·g(S)−1) of the initial discharge capacity after 200 cycles at a rate of 0.3C in carbonate and ether-based electrolytes, respectively. The mesoporous carbon aerogels can be considered as promising cathode catalysts for application in polymer electrolyte membrane fuel cells. The platinum free carbon aerogels contain catalytic active Fe-Nx sites and demonstrate suitable microstructure for enhanced electrocatalytic performance. Carbon aerogels with different structural (specific surface area, pore volume and pore sizes), chemical (chemical structure) and physical (density, electrical conductivity, mechanical properties) properties applied for several electrochemical applications will be presented on the conference

Influences of mechanical and structural properties of carbon aerogels on specific capacity in lithium-sulfur batteries

Carbon aerogels (CA) are highly porous, electrically conductive materials with tunable micro- and mesoporosity, as well as surface area and envelope density. Their properties are defined by the sol-gel process of their organic precursors and the carbonization conditions. Due to their well tunable properties they are very attractive as a cathode material for lithium-sulfur batteries. One of the challenges in lithium-sulfur batteries is the capacity loss induced by the polysulfide shuttle effect. Ultramicroporous (pores smaller 1 nm) carbon aerogels as conductive matrices embedding sulfur are able to suppress the polysulfide shuttle effect, maintaining 80% (about 1000 mA·h·g(S)−1) and 70% (about 800 mA·h·g(S)−1) of the initial discharge capacity after 200 cycles at a rate of 0.3C in carbonate and ether-based electrolytes, respectively. The conversion reaction of sulfur and the formation of Li2S cause a large volume expansion, which results in a reduction of electron transport paths and thus a decrease in kinetics. Flexible carbon aerogels with tailored microstructure can accommodate volume expansion due to their ability for reversible deformation up to a certain degree during cycling. In this work, we synthesized and investigated highly microporous CAs with different degree of flexibility and particle sizes to study their influences on the capacity in lithium-sulfur batteries. The investigations show that the mechanical and structural properties play a key role in the performance of lithium-sulfur batteries