Tobias Placke

Dual-graphite cells based on the reversible intercalation of bis(trifluoromethanesulfonyl)imide anions from an ionic liquid electrolyte

Recently, dual-ion cells based on the anion intercalation into a graphite positive electrode have been proposed as electrochemical energy storage devices. For this technology, in particular electrolytes which display a high stability vs. oxidation are required due to the very high operation potentials of the cathode, which may exceed 5 V vs. Li/Li+. In this work, we present highly promising results for the use of graphite as both the anode and cathode material in a so-called “dual-graphite” or “dual-carbon” cell. A major goal for this system is to find suitable electrolyte mixtures which exhibit not only a high oxidative stability at the cathode but also form a stable solid electrolyte interphase (SEI) at the graphite anode. As an electrolyte system, the ionic liquid-based electrolyte mixture Pyr14TFSI-LiTFSI is used in combination with the SEI-forming additive ethylene sulfite (ES) which allows stable and highly reversible Li+ ion and TFSI− anion intercalation/de-intercalation into/from the graphite anode and cathode, respectively. By addition of ES, also the discharge capacity for the anion intercalation can be remarkably increased from 50 mA h g−1 to 97 mA h g−1. X-ray diffraction studies of the anion intercalation into graphite are conducted in order to understand the influence of the electrolyte additive on the graphite structure and on the cell performance.

Lithium ion, lithium metal, and alternative rechargeable battery technologies: the odyssey for high energy density

Since their market introduction in 1991, lithium ion batteries (LIBs) have developed evolutionary in terms of their specific energies (Wh/kg) and energy densities (Wh/L). Currently, they do not only dominate the small format battery market for portable electronic devices, but have also been successfully implemented as the technology of choice for electromobility as well as for stationary energy storage. Besides LIBs, a variety of different technologically promising battery concepts exists that, depending on the respective technology, might also be suitable for various application purposes. These systems of the “next generation,” the so-called post-lithium ion batteries (PLIBs), such as metal/sulfur, metal/air or metal/oxygen, or “post-lithium technologies” (systems without Li), which are based on alternative single (Na+, K+) or multivalent ions (Mg2+, Ca2+), are currently being studied intensively. From today’s point of view, it seems quite clear that there will not only be a single technology for all applications (technology monopoly), but different battery systems, which can be especially suitable or combined for a particular application (technology diversity). In this review, we place the lithium ion technology in a historical context and give insights into the battery technology diversity that evolved during the past decades and which will, in turn, influence future research and development.

Dual-ion Cells Based on Anion Intercalation into Graphite from Ionic Liquid-Based Electrolytes

Abstract Electrochemical energy storage systems using graphite as both the negative and the positive electrode have been proposed as “dual-graphite cells”. In this kind of electrochemical system, the electrolyte cations intercalate into the negative electrode and the electrolyte anions intercalate into the positive electrode, both based on graphite, during the charging process. On discharge, cations and anions are released back into the electrolyte. So far, the systems proposed in literature are primarily based on Li+ and PF6- intercalation/de-intercalation into/from graphite from non-aqueous organic solvent based electrolytes. As the positive electrode potential during charging always exceeds 4.2 V vs. Li/Li+, the organic electrolyte starts to decompose at these highly oxidizing conditions resulting in insufficient discharge/charge efficiencies. The replacement of organic solvent by ionic liquids (ILs) leads an increased stability of the electrolyte towards oxidation and thus to remarkably higher efficiencies as well as an increased cycling stability. In fact, ionic liquids provide extended anodic electrochemical stability and in addition, no solvent co-intercalation occurs in parallel to anion intercalation at high potentials. Here, we present highly promising results for “dual-ion cells” based on a graphite cathode and an ionic liquid based electrolyte, namely N-butyl-N-methylpyrrolidinium bis(trifluoromethanesulfonyl)imide (Pyr14TFSI). As the compatibility of this IL with graphite anodes is poor, alternative anodes such as metallic lithium or lithium titanate (Li4Ti5O12, LTO) are used. Consequently, the “dual-graphite” cell is renamed to “dual-ion” cell. In addition, the calculation of the specific energy of these systems will be in the focus of the discussion.

Alternative electrochemical energy storage: potassium-based dual-graphite batteries

In this contribution, we report for the first time a novel potassium ion-based dual-graphite battery concept (K-DGB), applying graphite as the electrode material for both the anode and cathode. The presented dual-graphite cell utilizes a potassium ion containing, ionic liquid (IL)-based electrolyte, synergetically combining the extraordinary properties of potassium, graphite and ILs in terms of cost effectiveness, sustainability and safety. The IL electrolyte shows a very stable cycling performance in combination with the graphite anode at a so far not reported reversible capacity of ≈230 mA h g−1. A highly reversible capacity of >42 mA h g−1 (with respect to the graphite cathode) even at a current of 250 mA g−1, and a Coulombic efficiency (CE) exceeding 99% in a potential range from 3.4 V to 5.0 V vs. K/K+ represent the corner pillars of this innovative battery technology. The very promising electrochemical performance is further emphasized by a capacity retention of 95% after 1500 cycles. Furthermore, the electrochemical formation of a stage-1 potassium graphite intercalation compound (K-GIC) from an IL electrolyte, resulting in a stoichiometry of KC8 is presented in this work for the first time. The presented results shed new light on an alternative energy storage technology, especially in view of stationary (“grid”) energy storage by employing environmentally friendly, abundant and recyclable materials.

Facile Synthesis and Lithium Storage Properties of a Porous NiSi2/Si/Carbon Composite Anode Material for Lithium-Ion Batteries

In this work, a novel, porous structured NiSi2/Si composite material with a core-shell morphology was successfully prepared using a facile ball-milling method. Furthermore, the chemical vapor deposition (CVD) method is deployed to coat the NiSi2/Si phase with a thin carbon layer to further enhance the surface electronic conductivity and to mechanically stabilize the whole composite structure. The morphology and porosity of the composite material was evaluated by scanning electron microscopy (SEM), transmission electron microscopy (TEM), and nitrogen adsorption measurements (BJH analysis). The as-prepared composite material consists of NiSi2, silicon, and carbon phases, in which the NiSi2 phase is embedded in a silicon matrix having homogeneously distributed pores, while the surface of this composite is coated with a carbon layer. The electrochemical characterization shows that the porous and core-shell structure of the composite anode material can effectively absorb and buffer the immense volume changes of silicon during the lithiation/delithiation process. The obtained NiSi2/Si/carbon composite anode material displays an outstanding electrochemical performance, which gives a stable capacity of 1272 mAh g(-1) for 200 cycles at a charge/discharge rate of 1C and a good rate capability with a reversible capacity of 740 mAh g(-1) at a rate of 5C.

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.

One-step synthesis of novel mesoporous three-dimensional GeO2 and its lithium storage properties

Novel mesoporous three-dimensional GeO2 was successfully synthesized by a facile one-step synthesis method followed by mixing with graphene using a spray drying process. The well-dispersed mesoporous GeO2 demonstrates a bean-like morphology (b-GeO2) with a particle size of 400 to 500 nm in length and 200 to 300 nm in diameter, in which mesopores with an average size of 3.6 nm are distributed. The b-GeO2 without any additional conductive surface layer shows a high reversible capacity for lithium storage of 845 mAh g−1 after 100 cycles, with nearly no capacity fading. When graphene was employed to be mixed with GeO2via a spray drying method, the electrochemical performance is further significantly improved. The b-GeO2/graphene composite electrode gives a higher de-lithiation capacity of 1021 mAh g−1, and the capacity retention is measured to be as high as 94.3% after 200 charge–discharge cycles for constant current cycling at 0.2 C, as well as an excellent rate performance, even displaying a reversible capacity of 730 mAh g−1 at a rate of 5 C.

Anodic Behavior of the Aluminum Current Collector in Imide-Based Electrolytes: Influence of Solvent, Operating Temperature, and Native Oxide-Layer Thickness

The inability of imide salts to form a sufficiently effective passivation layer on aluminum current collectors is one of the main obstacles that limit their broad application in electrochemical energy-storage systems. However, under certain circumstances, the use of electrolytes with imide electrolyte salts in combination with the aluminum current collector is possible. In this contribution, the stability of the aluminum current collector in electrolytes containing either lithium bis(trifluoromethanesulfonyl) imide (LiTFSI) or lithium fluorosulfonyl-(trifluoromethanesulfonyl) imide (LiFTFSI) as conductive salt was investigated by electrochemical techniques, that is, cyclic voltammetry (CV) and chronocoulometry (CC) in either room-temperature ionic liquids or in ethyl methyl sulfone. In particular, the influence of the solvent, operating temperature, and thickness of the native oxide layer of aluminum on the pit formation at the aluminum current collector surface was studied by means of scanning electron microscopy. In general, a more pronounced aluminum dissolution and pit formation was found at elevated temperatures as well as in solvents with a high dielectric constant. An enhanced thickness of the native aluminum oxide layer increases the oxidative stability versus dissolution. Furthermore, we found a different reaction rate depending on dwell time at the upper cut-off potential for aluminum dissolution in TFSI- and FTFSI-based electrolytes during the CC measurements; the use of LiFTFSI facilitated the dissolution of aluminum compared to LiTFSI. Overall, the mechanism of anodic aluminum dissolution is based on: i) the attack of the Al2 O3 surface by acidic species and ii) the dissolution of bare aluminum into the electrolyte, which, in turn, is influenced by the electrolytes dielectric constant.

Synthesis and electrochemical performance of surface-modified nano-sized core/shell tin particles for lithium ion batteries

Tin is able to lithiate and delithiate reversibly with a high theoretical specific capacity, which makes it a promising candidate to supersede graphite as the state-of-the-art negative electrode material in lithium ion battery technology. Nevertheless, it still suffers from poor cycling stability and high irreversible capacities. In this contribution, we show the synthesis of three different nano-sized core/shell-type particles with crystalline tin cores and different amorphous surface shells consisting of SnOx and organic polymers. The spherical size and the surface shell can be tailored by adjusting the synthesis temperature and the polymer reagents in the synthesis, respectively. We determine the influence of the surface modifications with respect to the electrochemical performance and characterize the morphology, structure, and thermal properties of the nano-sized tin particles by means of high-resolution transmission electron microscopy, x-ray diffraction, and thermogravimetric analysis. The electrochemical performance is investigated by constant current charge/discharge cycling as well as cyclic voltammetry.

In situ X-ray diffraction study on the formation of α-Sn in nanocrystalline Sn-based electrodes for lithium-ion batteries

In situ X-ray diffraction (XRD) was performed to study the formation of the α-Sn structure in nanocrystalline Sn-based electrodes during electrochemical lithium insertion and extraction at room temperature. Therefore, pure β-Sn nanoparticles were synthesised and further processed into electrodes. The lithiation and de-lithiation process of the β-Sn nanoparticles follows the formation of discrete lithium–tin phases which perfectly fits the voltage plateaus in the charge/discharge diagram. However, unlike bulk electrodes, where no α-Sn is formed, we observed the formation of the semiconducting α-modification at 870 mV vs. Li within the first de-lithiation process. This observation explains earlier reports of an increasing internal resistance of such an electrode. Additionally, our study supports earlier suggestions that predominantly small tin crystallites are transformed from the β-Sn phase into the α-Sn phase, while larger crystallites retain their metallic β-Sn structure.