RESUMEN
Despite the rapid expansion of the organic cathode materials field, we still face a shortage of materials obtained through simple synthesis that have stable cycling and high energy density. Herein, we report a two-step synthesis of a small organic molecule from commercially available precursors that can be used as a cathode material. Oxidized tetraquinoxalinecatechol (OTQC) was derived from tetraquinoxalinecatechol (TQC) by the introduction of additional quinone redox-active centers into the structure. The modification increased the voltage and capacity of the material. The OTQC delivers a high specific capacity of 327 mAh g-1 with an average voltage of 2.63 V vs Li/Li+ in the Li-ion battery. That corresponds to an energy density of 860 Wh kg-1 on the OTQC material level. Furthermore, the material demonstrated excellent cycling stability, having a capacity retention of 82% after 400 cycles. Similarly, the OTQC demonstrates increased average voltage and specific capacity in comparison with TQC in aqueous Zn-organic battery, reaching the specific capacity of 326 mAh g-1 with an average voltage of 0.86 V vs Zn/Zn2+. Apart from good electrochemical performance, this work provides an additional in-depth analysis of the redox mechanism and degradation mechanism related to capacity fading.
RESUMEN
Hysteresis is a general phenomenon regularly observed in various materials. Usually, hysteretic behavior is an intrinsic property that cannot be circumvented in the nonequilibrium operation of the system. Herein, it is shown that, at least with regard to the hysteretic behavior of phase-separating battery materials, it is possible to enter (deeply) the hysteretic loop at finite battery currents. This newly observed electric response of the electrode, which is inherent to phase-separating materials, is related to its microscopic origin arising from a (significant) share of the active material residing in an intraparticle phase-separated state. This intriguing observation is further generalized by revealing that a phase-separating material can feature (significantly) different chemical potentials at the same bulk lithiation level and temperature when exposed to the same finite current and external voltage hysteresis. Therefore, the intraparticle phase-separated state significantly affects the DC and AC characteristics of the battery. The experimental evidence for entering the intraparticle phase-separated state is supported by thermodynamic reasoning and advanced modeling. The current findings will help advance the understanding, control, diagnostics, and monitoring of batteries composed of phase-separating materials while also providing pertinent motivation for the enhancement of battery design and performance.
RESUMEN
Phase transformations driven by compositional change require mass flux across a phase boundary. In some anisotropic solids, however, the phase boundary moves along a non-conductive crystallographic direction. One such material is LiXFePO4, an electrode for lithium-ion batteries. With poor bulk ionic transport along the direction of phase separation, it is unclear how lithium migrates during phase transformations. Here, we show that lithium migrates along the solid/liquid interface without leaving the particle, whereby charge carriers do not cross the double layer. X-ray diffraction and microscopy experiments as well as ab initio molecular dynamics simulations show that organic solvent and water molecules promote this surface ion diffusion, effectively rendering LiXFePO4 a three-dimensional lithium-ion conductor. Phase-field simulations capture the effects of surface diffusion on phase transformation. Lowering surface diffusivity is crucial towards supressing phase separation. This work establishes fluid-enhanced surface diffusion as a key dial for tuning phase transformation in anisotropic solids.
RESUMEN
Polysulfides are central compounds in lithium-sulfur battery cells. However, the fundamental redox and diffusion properties of polysulfides are still poorly understood. We try to fill this gap by performing an accurate impedance spectroscopy investigation using symmetrical cells consisting of two planar glassy carbon electrodes separated with catholyte-soaked separator. The catholyte contains a mixture of selected polysulfides with predetermined nominal concentrations. Impedance measurements reveal textbook shapes of spectra for most polysulfide compounds or their mixtures. This allows reliable and accurate determination of the rate constant (exchange current density) for a given redox reaction as well as the diffusion coefficient and diffusion length for the rate-determining polysulfide species. Further, it is confirmed that polysulfides tend to disproportionate with time, which significantly changes the chemistry and electrochemistry of the system. Two approaches are proposed for identification of the prevailing redox mechanism in the resulting mixtures. The values of kinetic and transport parameters obtained for different cases of interest are commented on in significant detail. The study provides a solid basis for better understanding of the complex processes in polysulfide mixtures.
RESUMEN
We compare the basic electrochemical performance of a LiMnPO4 battery material with the performance of its much more researched olivine counterpart - LiFePO4. To get a wider picture, we also included another well understood material, LiCoO2. Based on chronopotentiometric (galvanostatic) experiments, we discuss the materials performance in terms of cell energy efficiency and electrode polarization. We propose and justify the use of the "inflection point criterion" for determination of total overpotential (ηtotal). We further demonstrate that the general current-overpotential characteristics can be represented by introducing the total resistance of the cell - Rtotal.We find consistently that whereas in LCO the general current-overpotential characteristics is more or less linear, there is significant deviation from linearity in LiFePO4 and even bigger in LiMnPO4. The phenomenon is discussed in terms of state-of-the art knowledge about phase transformation phenomena in these materials.
RESUMEN
The possibility of preparation of operating rechargeable liquid battery cells based on aluminium and its alloys is systematically checked. In all cases we started from aluminium as the negative electrode whereas as the positive electrode three different metals were tested: Pb, Bi and Sn. Two types of electrolytes were selected: Na(3)AlF(6) -AlF(3) - BaCl(2) - NaCl and Li(3)AlF(3) - BaF(2). We show that some of these combinations allowed efficient separation of individual liquid layers. The cells exhibited expected voltages, relatively high current densities and could be charged and discharged several times. The capacities were relatively low (120 mAh in the case of Al-Pb system), mostly due to unoptimised cell construction. Improvements in various directions are possible, especially by hermetically sealing the cells thus preventing salt evaporation. Similarly, solubility of aluminium in alloys can be increased by optimising the composition of positive electrode.
RESUMEN
Lithium batteries are considered the key storage devices for most emerging green technologies such as wind and solar technologies or hybrid and plug-in electric vehicles. Despite the tremendous recent advances in battery research, surprisingly, several fundamental issues of increasing practical importance have not been adequately tackled. One such issue concerns the energy efficiency. Generally, charging of 10(10)-10(17) electrode particles constituting a modern battery electrode proceeds at (much) higher voltages than discharging. Most importantly, the hysteresis between the charge and discharge voltage seems not to disappear as the charging/discharging current vanishes. Herein we present, for the first time, a general explanation of the occurrence of inherent hysteretic behaviour in insertion storage systems containing multiple particles. In a broader sense, the model also predicts the existence of apparent equilibria in battery electrodes, the sequential particle-by-particle charging/discharging mechanism and the disappearance of two-phase behaviour at special experimental conditions.