Ultra-high-rate pseudocapacitive energy storage in two-dimensional transition metal carbides

The use of fast surface redox storage (pseudocapacitive) mechanisms can enable devices that store much more energy than electrical double-layer capacitors (EDLCs) and, unlike batteries, can do so quite rapidly. Yet, few pseudocapacitive transition metal oxides can provide a high power capability due to their low intrinsic electronic and ionic conductivity. Here we demonstrate that two-dimensional transition metal carbides (MXenes) can operate at rates exceeding those of conventional EDLCs, but still provide higher volumetric and areal capacitance than carbon, electrically conducting polymers or transition metal oxides.We applied two distinct designs for MXene electrode architectures with improved ion accessibility to redox-active sites. A macroporous Ti3C2Tx MXene film delivered up to 210 F g-1 at scan rates of 10Vs-1, surpassing the best carbon supercapacitors known. In contrast, we show that MXene hydrogels are able to deliver volumetric capacitance of 1,500 F cm-3 reaching the previously unmatched volumetric performance of RuO2

Electrochemical Quartz Crystal Microbalance with Dissipation Real-Time Hydrodynamic Spectroscopy of Porous Solids in Contact with Liquids

Using multiharmonic electrochemical quartz crystal microbalance with dissipation (EQCM-D) monitoring, a new method of characterization of porous solids in contact with liquids has been developed. The dynamic gravimetric information on the growing, dissolving, or stationary stored solid deposits is supplemented by their precise in-operando porous structure characterization on a mesoscopic scale. We present a very powerful method of quartz-crystal admittance modeling of hydrodynamic solid–liquid interactions in order to extract the porous structure parameters of solids during their formation in real time, using different deposition modes. The unique hydrodynamic spectroscopic characterization of electrolytic and rf-sputtered solid Cu coatings that we use for our “proof of concept” provides a new strategy for probing various electrochemically active thin and thick solid deposits, thereby offering inexpensive, noninvasive, and highly efficient quantitative control over their properties. A broad spectrum of applications of our method is proposed, from various metal electroplating and finishing technologies to deeper insight into dynamic build-up and subsequent development of solid-electrolyte interfaces in the operation of Li-battery electrodes, as well as monitoring hydrodynamic consequences of metal corrosion, and growth of biomass coatings (biofouling) on different solid surfaces in seawater

In Situ Tracking of Ion Insertion in Iron Phosphate Olivine Electrodes via Electrochemical Quartz Crystal Admittance

LiFePO₄ is one of most promising cathode materials for lithium-ion batteries (LIBs) due to its superior rate handling ability, moderate cost, low environmental hazards, and safe long-term cyclability. In addition to the electrochemical information on the charge and discharge process, electrochemical quartz crystal admittance (EQCA) of LIB electrodes provides direct access to potential-driven frequency shifts (Δ<i>f</i>_exp) and changes of the resonance peak width (ΔΓ) due to Li-ions insertion/extraction. It is not only possible to monitor mass changes of the electrode, but the two parameters Δ<i>f</i>_exp and ΔΓ also reflect mechano-structural changes caused by hydrodynamic solid–liquid interactions from the operation of a LIB. Applying a suitable model that takes into account such interactions, potential-induced changes of the effective thickness and permeability of the composite electrode have been determined. The latter shows that ion insertion/extraction results in a nonuniform deformation of the electrode. Using EQCA as a unique mechanical probe for insertion-type electrodes, the dynamic effect of the local host environment on Na⁺-ions insertion/extraction has been studied in a mixed solution of Li- and Na-salts. As a highly reliable and quantitative tool, EQCA may enable a broader understanding of coupled electrochemical and mechanical events in LIB during their long-term operation. This includes information about the distortion/deformation of the electrode intercalation particles and the entire composite electrode under polarization. Also, EQCA can help to clarify the role of polymeric binder in the composite electrodes as the factor stabilizing long-term cyclability of Li-ions batteries

Electrochemical Performance of a Layered-Spinel Integrated Li[Ni_1/3Mn_2/3]O₂ as a High Capacity Cathode Material for Li-Ion Batteries

Li­[Ni_1/3Mn_2/3]­O₂ was synthesized by a self-combustion reaction (SCR), characterized by X-ray diffraction (XRD), scanning electron microscopy (SEM), transmission electron microscopy (TEM), and Raman spectroscopy, and studied as a cathode material for Li-ion batteries at 30 °C and 45 °C. The structural studies by XRD and TEM confirmed monoclinic Li­[Li_1/3Mn_2/3]­O₂ phase as the major component, and rhombohedral (LiNiO₂), spinel (LiNi_0.5Mn_1.5O₄), and rock salt Li_0.2Mn_0.2Ni_0.5O as minor components. The content of the spinel phase increases upon cycling due to the layered-to-spinel phase transition occurring at high potentials. A high discharge capacity of about 220 mAh g^–1 is obtained at low rate (C/10) with good capacity retention upon cycling. However, LiNi_0.5Mn_1.5O₄ synthesized by SCR exhibits a discharge capacity of about 190 mAh g^–1 in the potential range of 2.4–4.9 V, which decreases to a value of 150 mAh g^–1 after 100 cycles. Because of the presence of the spinel component, Li­[Ni_1/3Mn_2/3]­O₂ cathode material exhibits part of its capacity at potentials around 4.7 V. Thus, it can be considered as an interesting high-capacity and high-voltage cathode material for high-energy-density Li-ion batteries. Also, the Li­[Ni_1/3Mn_2/3]­O₂ electrodes exhibit better electrochemical stability than spinel LiNi_0.5Mn_1.5O₄ electrodes when cycled at 45 °C

The Charge Storage Mechanisms of 2D Cation-Intercalated Manganese Oxide in Different Electrolytes

2D ion-intercalated metal oxides are emerging promising new electrodes for supercapacitors because of their unique layered structure as well as distinctive electronic properties. To facilitate their application, fundamental study of the charge storage mechanism is required. Herein, it is demonstrated that the application of in situ Raman spectroscopy and electrochemical quartz crystal microbalance with dissipation monitoring (EQCM-D), provides a sufficient basis to elucidate the charge storage mechanism in a typical 2D cation-intercalated manganese oxide (Na0.55Mn2O4 center dot 1.5H(2)O, abbreviated as NMO) in neutral and alkaline aqueous electrolytes. The results reveal that in neutral Na2SO4 electrolytes, NMO mainly displays a surface-controlled pseudocapacitive behavior in the low potential region (0-0.8 V), but when the potential is higher than 0.8 V, an intercalation pseudocapacitive behavior becomes dominant. By contrast, NMO shows a battery-like behavior associated with OH- ions in alkaline NaOH electrolyte. This study verifies that the charge storage mechanism of NMO strongly depends on the type of electrolyte, and even in the same electrolyte, different charging behaviors are revealed in different potential ranges which should be carefully taken into account when optimizing the use of the electrode materials in practical energy-storage devices

Classical and Quantum Modeling of Li and Na Diffusion in FePO₄

Lithium diffusion in olivine phosphates has been widely studied both experimentally and theoretically. However, nuclear quantum effects (NQEs) of the Li ions have not been accounted for in theoretical studies thus far. In the current work, we compared Li and Na diffusion in Li_0.25FePO₄ and Na_0.25FePO₄ by computing density functional theory based classical diffusion barriers in conjunction with NQEs for the Li and Na ions. The NQEs are computed using a novel three-dimensional wave function method based on a path integral formulation. The calculations of both the potential and free energy diffusion barriers suggest that Li diffusion is faster than Na diffusion, in agreement with recent experiments. The NQEs for lithium ions in Li_0.25FePO₄ are higher than those for sodium ions in Na_0.25FePO₄. Although the contribution of NQEs to the computed Li and Na ion diffusion rates is rather small, the quantum behavior of the Li ions is unusual. Indeed, we observe a reduction in the computed diffusion rate for Li ions due to quantization. We ascribe this effect to the ability of FePO₄ to tightly bind the Li ions in the transient tetrahedral transition state, which reduces the classical diffusion barrier but also enhances quantum confinement