Enhancing Li-ion capacity and rate capability in cation-defective vanadium ferrite aerogels via aluminum substitution.

Cation-defective iron oxides have proven to be effective Li-ion charge-storage hosts in nonaqueous electrolytes, particularly when expressed in disordered, nanoscale forms such as aerogels. Replacing a fraction of Fe sites in ferrites with high-valent cations such as V5+ introduces cation-vacancy defects that increase Li-ion capacity. Herein, we show that compositional substitution with electroinactive Al3+ further increases Li-ion capacity by 30% when incorporated within a disordered VFe2Ox aerogel, as verified by electrochemical tests in a two-terminal Li half-cell. We use electroanalytical techniques to show that both Al-VFe2Ox and VFe2Ox aerogels exhibit many of the hallmarks of pseudocapacitive materials, including fast charge-discharge and surface-controlled charge-storage kinetics. These disordered, substituted ferrites also provide the high specific capacity expected from battery-type electrode materials, up to 130 mA h g-1 for Al-VFe2Ox. Our findings are discussed in the context of related Li-insertion hosts that blur the distinctions between battery-like and capacitor-like behavior.

In†Operando Calorimetric Measurements for Activated Carbon Electrodes in Ionic Liquid Electrolytes under Large Potential Windows.

This study aims to investigate the effect of the potential window on heat generation in carbon-based electrical double layer capacitors (EDLCs) with ionic-liquid (IL)-based electrolytes using in operando calorimetry. The EDLCs consisted of two identical activated-carbon electrodes with either neat 1-butyl-1-methylpyrrolidinium bis(trifluoromethane-sulfonyl)imide ([Pyr14 ][TFSI]) electrolyte or 1.0 m [Pyr14 ][TFSI] in propylene carbonate (PC) as electrolyte. The instantaneous heat generation rate at each electrode was measured under galvanostatic cycling for different potential windows ranging from 1 to 4 V. First, the heat generation rates at the positive and negative electrodes differed significantly in neat IL owing to the differences in the ion sizes and diffusion coefficients. However, these differences were minimized when the IL was diluted in PC. Second, for EDLC in neat [Pyr14 ][TFSI] at high potential window (4 V), a pronounced endothermic peak was observed at the beginning of the charging step at the positive electrode owing to TFSI- intercalation in the activated carbon. On the other hand, for EDLC in 1.0 m [Pyr14 ][TFSI] in PC at potential window above 3 V, an endothermic peak was observed only at the negative electrode owing to the decomposition of PC. Third, for both neat and diluted [Pyr14 ][TFSI] electrolytes, the irreversible heat generation rate increased with increasing potential window and exceeded Joule heating. This was attributed to the effect of potential-dependent charge redistribution resistance. A further increase in the irreversible heat generation rate was observed for the largest potential windows owing to the degradation of the PC solvent. Finally, for both types of electrolyte, the reversible heat generation rate increased with increasing potential window because of the increase in the amount of ion adsorbed/desorbed at the electrode/electrolyte interface.

Electrochemical and Spectroscopic Analysis of the Ionogel-Electrode Interface.

Ionogels, pseudo-solid-state electrolytes consisting of an ionic liquid electrolyte confined in a mesoporous inorganic matrix, have attracted interest recently due to their high ionic conductivity and physicochemical stability. These traits, coupled with their inherent solution processability, make them a viable solid electrolyte for solid-state battery systems. Despite the promising properties of ionogels, there have been very few investigations of the electrode-ionogel interface. In the present study, X-ray photoelectron spectroscopy, Raman spectroscopy, and electrochemical measurements were utilized to probe the surface reactions occurring at the electrode-ionogel interface for several electrode materials. Our results indicate that the sol acidity initiates breakdown of the organic constituents of the sol and reduction of the transition metals present in the electrode materials. This chemical attack forms an organic surface layer and affects the electrode composition, both of which can impede Li+ access. By modifying the silica sol-gel reaction via a two-step acid-base catalysis, these interfacial reactions can be avoided. Results are shown for a LiCoO2 electrode in which a high Li-ion capacity and stable cycling were achieved.

A Metal-Organic Framework with Tetrahedral Aluminate Sites as a Single-Ion Li+ Solid Electrolyte.

We demonstrate the synthesis of the first anionic aluminum metal-organic framework (MOFs) constructed from tetrahedral AlO4 sites. Al-Td-MOF-1 was obtained in a simple two-step synthesis by condensation of 1,4-dihydroxybenzene and lithium aluminum hydride into an amorphous aluminate framework before applying a solvothermal treatment under basic conditions to obtain the crystalline Al-Td-MOF-1 with a chemical composition of Li[Al(C6 H4 O2 )2 ]. The overall Al-Td-MOF-1 structure consists of one-dimensional chains of alternating edge-sharing AlO4 and LiO4 tetrahedral sites describing unidirectional pore channels with a square window aperture of ≈5×5 Å2 , best described topologically as a uninodal 6-coordinated snp rod net. Al-Td-MOF-1 features the highest Li+ loading reported to date for a MOF (2.50 wt %) and proved to be an effective single-ion solid electrolyte. An ionic conductivity of 5.7×10-5  S cm-1 was measured for Al-Td-MOF-1 and the beneficial contribution of crystallinity was evidenced by an 8-fold increase in conductivity between the disordered and crystalline material.