Dual-ion Cell Research Articles

A Direct Real-Time Observation of Anion Intercalation in Graphite Process and Its Fully Reversibility by SAXS/WAXS Techniques.

The process of anion intercalation in graphite and its reversibility plays a crucial role in the next generation energy-storage devices. Herein the reaction mechanism of the aluminum graphite dual ion cell by operando X-ray scattering from small angles to wide angles is investigated. The staging behavior of the graphite intercalation compound (GIC) formation, its phase transitions, and its reversible process are observed for the first time by directly measuring the repeated intercalation distance, along with the microporosity of the cathode graphite. The investigation demonstrates complete reversibility of the electrochemical intercalation process, alongside nano- and micro-structural reorganization of natural graphite induced by intercalation. This work represents a new insight into thermodynamic aspects taking place during intermediate phase transitions in the GIC formation.

Cu3(PO4)2: Novel Anion Convertor for Aqueous Dual-Ion Battery

HighlightsA novel anion electrode Cu3(PO4)2 is proposed at the first time.The reaction mechanism of Cu3(PO4)2 electrode is investigated.The dual-ion cell is constructed by using pretreated Cu3(PO4)2 and Na0.44MnO2.

An anode-free aqueous dual-ion battery

Herein, a novel anode-free aqueous dual-ion cell is developed with expanded graphite as cathode, inactive current collector as anode replacement and inorganic/organic hybrid zinc salts as electrolyte.

Reversible Anion Insertion in Molecular Phenothiazine-Based Redox-Active Positive Material for Organic Ion Batteries.

The increasing demand for rechargeable batteries induces the development of greener and better devices. Significant advances have been made in the last decade together with a renewed interest in organic electrode materials. Thus, stable electron-donating organic materials are candidates for "greener" molecular batteries (metal-free). Herein, we report the design of a monomeric p-type N-substituted phenothiazine salt as an efficient anionic host structure working reversibly in a dual-ion cell configuration using lithium as the negative electrode. Investigation of different electrolyte salts, LiClO4 , LiPF6 , and LiTFSI in PC (propylene carbonate), reveals that lithium 4-(10H-phenothiazin-10-yl) benzoate (LiPHB) exhibits a high operating potential (≈3.7 vs. Li+ /Li) corresponding to a one-electron process with a reversible specific capacity of 86 mAh g-1 in a LiClO4 -based electrolyte, exhibiting an extraordinary cycling stability over 500 cycles at 0.2 C. Such impressive results are rendering LiPHB a promising scaffold for developing next-generation molecular organic batteries.

Kill Two Problems with One Dual-Ion Cell

An electrolyte that conducts both oxygen ions and protons has the potential to enable a solid oxide fuel cell (SOFC) to operate at lower temperatures with a variety of fuels. Now a composite cathode conducting electrons, oxygen ions, and protons shows promise for high-performance SOFC at T

Exploiting cationic vacancies for increased energy densities in dual-ion batteries

Dual-ion Li–Mg batteries offer a potential route to cells that combine desirable properties of both single-ion species. To maximize the energy density of a dual-ion battery, we propose a strategy for achieving simultaneous intercalation of both ionic species, by chemically modifying the intercalation host material to produce a second, complementary, class of insertion sites. We show that donor-doping of anatase TiO2 to form large numbers of cationic vacancies allows the complementary insertion of Li+ and Mg2+ in a dual-ion cell with a net increase in cell energy density, due to a combination of an increased reversible capacity, an increased operating voltage, and a reduced polarization. By tuning the lithium concentration in the electrolyte, we achieve full utilization of the Ti4+/Ti3+ redox couple with excellent cyclability and rate capability. We conclude that native interstitial sites preferentially accommodate Li+ ions, while Mg2+ ions occupy single-vacancy sites. We also predict a narrow range of electrochemical conditions where adjacent vacancy pairs preferentially accommodate one ion of each species, i.e., a [LiTi ​+ ​MgTi] configuration. These results demonstrate the implementation of additional host sites such as cationic sites as an effective approach to increase the energy density in dual-ion batteries.

A high-performance dual-ion cell utilizing Si nanosphere@graphene anode

The growing global demands of high-safety, low-cost and high working voltage energy storage devices trigger great interests in novel batteries beyond state-of-the-art lithium-ion battery. Herein, we firstly fabricate a novel rechargeable dual-ion cell with graphite cathode and Si nanosphere@graphene (Si-NS@G) composite anode in a LiPF6-based organic electrolyte. The graphene layers anchored onto Si nanospheres surface can accommodate the volume expansion and improve the electrical conductivity of Si nanospheres. The cell shows stable long-term cycling and excellent rate performance with a high discharge capacity of 100 mA h g−1 at the current density of 100 mA g−1 after 1000 cycles. Moreover, the cell operates a high working voltage of >4.2 V, enabling a higher energy density. Our result may open a new paradigm for novel electrochemical cell technologies and applications.

Enabling bis(fluorosulfonyl)imide-based ionic liquid electrolytes for application in dual-ion batteries

In this work, we present a comprehensive study on the effect of adding different conductive salt additives including LiPF6, LiBF4 and LiDFOB, as well as the fluorinated solvent additive methyl difluoroacetate (MDFA) to a bis(fluorosulfonyl)imide (FSI)-based ionic liquid (IL) electrolyte, i.e. Pyr14FSI/LiFSI, to protect the Al current collector (ACC) from anodic dissolution and, thus, enable reversible charge/discharge cycling in a high performance dual-ion cell. Chronocoulometry and scanning electron microscopy measurements were conducted to evaluate the specific ACC passivation ability of each electrolyte. Furthermore, the influence of these additives on anion intercalation behavior into the graphite positive electrode with special emphasis on the Coulombic efficiency (CE), reversible capacity, as well as capacity retention is presented. Overall, we can show that the addition of small amounts of LiPF6, LiBF4 and MDFA (0.5 wt%) into the FSI-based IL electrolyte significantly increases the overall cell performance, whereas LiDFOB as electrolyte additive deteriorates the dual-ion cell performance. In addition, an excellent cycling performance for 1000 cycles is obtained for the Pyr14FSI electrolyte having 5 wt% LiPF6, displaying an average reversible capacity of 40 mAh g−1, a CE exceeding 98% and a capacity retention of 91%, which has not been reported so far.

A Comprehensive Study of the Intercalation Behavior of Anions from Ionic-Liquid Based Electrolytes into a Graphite Positive Electrode for Dual-Ion Batteries

The increasing demand for large scale energy storage capabilities causes a big field of research to make the battery systems safer and environmentally benign. A promising approach is the replacement of conventional volatile and highly flammable organic solvent-based electrolytes by ionic liquids (ILs) as the main component of the electrolyte. ILs exhibit excellent properties such as a broad electrochemical stability window, a high thermal stability, non-flammability and a high ionic conductivity.[1] A further prevention of pollution is the application of carbon-based, transition metal-free electrode materials. Recently, “dual-ion” and “dual-graphite” cells were introduced which are composed of a lithium-incorporating anode and an anion incorporating graphite-based cathode.[2][3] During charge the ions intercalate into the electrode active materials and are released back into the electrolyte during discharge (see Figure 1). Since the cathode potential reaches and may even exceed 5 V vs. Li/Li+, electrolytes that exhibit a high stability vs. oxidation are necessary. For that reason, ILs with their versatile properties were introduced and investigated. The state of the art electrolyte for this system is composed of a solution of lithium bis(trifluoromethanesulfonyl) imide (LiTFSI) in N-butyl-N-methylpyrrolidinium bis(trifluoromethanesulfonyl) imide (Pyr14TFSI). In order to increase the specific capacity of this system, one main strategy is to focus on the intercalation of smaller anions than TFSI into the graphite structure. In this context, the major challenge is the synthesis of ILs or mixtures of ILs with organic electrolytes consisting of small anions to provide an electrochemically stable electrolyte. Interestingly, not only the size and shape of the intercalated anions plays a major role in view of onset potential for anion uptake as well as discharge capacity. Instead, also more complex coherences such as ion association, self-aggregation of ions as well as solvent co-intercalation have to be taken into consideration. In this work, we investigate the electrochemical intercalation of anions with different shapes and sizes from IL based electrolytes into a graphite-based cathode. Against this background, the influence of side chain modifications within the anion structure as well as the influence of the used cation within the conductive salt (Li+/K+) on the intercalation behavior was studied using different electrochemical analysis techniques. Therefore, in this contribution we will give new insights to the question if the anion size is the main issue to tailor the reversible capacity for anion intercalation. Furthermore, the influence of lithium plating behavior in different electrolyte systems on the overall cell performance as well as the effect of adding small amounts of different additives such as foreign cations and different conductive salts into the baseline electrolyte is discussed. Figure 1: Schematic illustration of the operating principle of a "dual-ion cell”.[2] 1] A. Lewandowski, A. Swiderska-Mocek, Journal of Power Sources, 194 (2009) 601-609. [2] T. Placke, P. Bieker, S.F. Lux, O. Fromm, H.W. Meyer, S. Passerini, M. Winter, Zeitschrift für Physikalische Chemie, 226 (2012) 391-407. [3] K. Beltrop, P. Meister, S. Klein, A. Heckmann, M. Grünebaum, H.-D. Wiemhöfer, M. Winter, T. Placke, Electrochimica Acta, 209 (2016) 44-55. Figure 1

A dual–ion battery using diamino–rubicene as anion–inserting positive electrode material

A novel and non-polymeric anion-inserting electrode material has been designed and prepared for promoting research on molecular ion rechargeable batteries: 5,12-diaminorubicene (DARb). The apolar core structure of a rubicene molecule has been coupled to two amino-groups for producing an original conjugated primary diamine exhibiting low affinity for polar solvents such as common carbonate-based battery electrolytes. The electrochemical reactivity of this organic molecule has been probed in a dual-ion cell configuration (vs. Li) using six different electrolyte formulations in terms of solvent (PC, EC-DMC) and lithium salt (LiPF6, LiClO4, LiTFSI). This diamino-rubicene material systematically showed a reversible electroactivity and promising performances when using 1M LiPF6 in EC:DMC (1:1vol.%) as the electrolyte, such as an average potential of ~3.4V vs. Li+/Li0, an initial capacity of 115mAh·g−1 and a good capacity retention over 60cycles without any optimization.

In-Situ Raman Study of the Intercalation of Bis(trifluoromethylsulfonyl)imid Ions into Graphite inside a Dual-Ion Cell

Anion intercalation into graphite is a central process for energy storage in dual-ion battery cells. Electrochemical investigations show a strong kinetic hindrance of the intercalation process in the first charging cycle and less kinetic hindrance in subsequent cycles. In order to obtain information about the structure and properties of the graphite electrode during cycling and about the origin of the kinetic hindrance, we carried out an in-situ Raman spectroscopy study during the first and second charging/discharging cycle of a dual-ion cell. This cell consisted of a metallic lithium anode, a graphite cathode and a Pyr1,4TFSI/LiTFSI mixture as electrolyte. We show that the TFSI− anion intercalation is not fully reversible, implying that TFSI− anions remain inside the graphite matrix after completion of the first charging/discharging cycle. This is in contrast to the reversible intercalation/deintercalation of cations, like Li+ ions. Remarkably, the TFSI− intercalation leads to enhanced Raman signals, also in stark contrast to Li+ intercalation. We discuss the in-situ Raman spectra in terms of staging phenomena, mechanical strain formation, electronic charge densities, and defects. Furthermore, we show that intercalation-induced defects are self-healing over time. Our results suggest that there is an anti-correlation between the kinetic barrier for TFSI− intercalation in graphite and the number of defects.

Influence of the Graphite Morphology on Anion Intercalation

The redox-amphoteric character of graphitic carbon makes it possible to intercalate a broad range of anions and cations. These graphite intercalation compounds (GICs) are divided into two types: donor-type GICs, which host cations and acceptor-type GICs, which are formed by anions. A well-known application for a donor-type GIC is lithium-intercalated graphite (LiC6) as anode material in lithium-ion batteries. Besides this example, also acceptor-type GICs are known as active material in batteries. In this context, Placke et al.1 presented the “dual-ion cell”, that also deals with anion intercalation into the graphite structure. During charge anions and cations are inserted into the respective electrode and are released back into the electrolyte during discharge. In the charged state the cathode reaches a potential of 5 V (vs. Li/Li+). In order to avoid electrolyte degradation, an ionic liquid mixture is chosen as electrolyte. It is composed of N-butyl-N-methyl-pyrrolidinium bis(trifluoromethanesulfonyl) imide (Pyr14TFSI) and lithium bis(trifluoromethanesulfonyl) imide (LiTFSI). The focus of this work is to investigate the influence of different graphite characteristics on the intercalation of the bis(trifluoromethanesulfonyl) imide anion into the graphite structure. For that purpose, nitrogen adsorption measurements were performed in order to determine the porosity of the graphite samples and X-ray diffraction was carried out to identify the crystallite size. Both structural parameters are correlated to the electrochemical performances. 1. Placke, T.; Bieker, P.; Lux, S. F.; Fromm, O.; Meyer, H. W.; Passerini, S.; Winter, M., Dual-ion cells based on anion intercalation into graphite from ionic liquid-based electrolytes. Zeitschrift für Physikalische Chemie 2012, 226, 391-407.

Investigation of Lithium-Ion Diffusion and Irreversible Processes in Dual-Ion Cell

In 2012 Placke et al. proposed so called „dual-ion cells“ as a new type of energy storage system. They reported an extraordinary cycling stability with a capacity retention of more than 99% after 500 cycles. Even though the achievable cell voltage in this kind of cells is about 4.6 V, the specific energy is still quite low compared to common lithium-ion batteries. The reason for this is a fundamental difference from conventional lithium-ion batteries, since not only the electrodes, but also the electrolyte acts as active material. During charging, lithium cations are reduced and deposited at a metallic lithium anode, while the anions are intercalated into graphite. During discharging, anions and cations are released back into the electrolyte (see figure 1).[1] Therefore the electrolyte acts as ion source, and the concentration hereby limits the energy density. As the cell voltage of “dual-ion cells” is very high the electrolyte needs to have a wide electrochemical window. For this reason, typically ionic liquid based electrolytes are used for this application. However, the concentration of lithium salts in ionic liquids is strongly limited due to solubility, increasing viscosity and decreasing conductivity.[2] During cycling the concentration underlies changes due to the extraction of lithium salt. This fact gives the possibility to investigate fundamental processes during charging and discharging by measuring the electrolyte resistance. In this work we therefore use electrochemical impedance spectroscopy in 2- and 3-electrode setup to gather information about diffusion processes in the electrolyte and about the reversibility of the processes during charging and discharging. Figure 1: Functionality of a dual-ion cell (derived from [1]). [1] T. Placke, P. Bieker, S. F. Lux, O. Fromm, H.-W. Meyer, S. Passerini, M. Winter, Zeitschrift für Physikalische Chemie, 226, 391 (2012). [2] T. Frömling, M. Kunze, M. Schönhoff, J. Sundermeyer, B. Roling, Journal of Physical Chemistry B, 112, 12985 (2008). Figure 1

Supercritical fluid chromatography interface for a differentially pumped dual-cell Fourier transform mass spectrometer

A Fourier transform mass spectrometer used as a detector and identifier for supercritical fluid chromatography demonstrates low nanogram detection limits and a pressure-limited resolution in excess of 10,000 at m/z 128 for the molecular ion of naphthalene. Supercritical carbon dioxide is introduced directly into the source of a dual differential pumped trapped ion cell generating pressures in the low 10/sup -4/ Torr range with sample detection occurring in the analyzer cell at pressures a factor of 100 lower. Ionization is accomplished by charge exchange with carbon dioxide to yield spectra exhibiting 70-eV-like fragmentation patterns. Supercritical fluid chromatography/Fourier transform mass spectrometry data for three test samples, a six-component mixture of substituted aromatic compounds and polyaromatic hydrocarbons, a five-component barbiturate mixture, and a seven-component pesticide mixture, are presented.