Marco-Tulio F. Rodrigues

Tuning the Electrochemical Reactivity of Boron- and Nitrogen-Substituted Graphene

The structural modification of nanomaterials at the atomic level has the potential to generate tailor-made components with enhanced performance for a variety of tasks. The chemical versatility of graphene has been constantly employed to fabricate multi-functional doped 2D materials with applications encompassing energy storage and electrocatalysis. Despite the many reports on boron- and nitrogen-doped graphenes, the possible synergy that arises from combining these electronically complementary elements has yet to be fully understood and explored. The techniques used for the fabrication of these nanomaterials are reviewed, along with the most recent reports on the benefits of B, N singly doping and co-doping in the electrocatalysis for oxygen reduction reactions and for energy storage in supercapacitors and lithium secondary batteries. The investigation of bulk co-doped materials has intrinsic limitations in fully understanding the real role of heteroatoms in the above applications. Ultimately, the design and creation of substituted monolayers with controlled compositions might hold the key for carbon-based energy-related applications.

Quasi-Solid Electrolytes for High Temperature Lithium Ion Batteries.

Rechargeable batteries capable of operating at high temperatures have significant use in various targeted applications. Expanding the thermal stability of current lithium ion batteries requires replacing the electrolyte and separators with stable alternatives. Since solid-state electrolytes do not have a good electrode interface, we report here the development of a new class of quasi-solid-state electrolytes, which have the structural stability of a solid and the wettability of a liquid. Microflakes of clay particles drenched in a solution of lithiated room temperature ionic liquid forming a quasi-solid system has been demonstrated to have structural stability until 355 °C. With an ionic conductivity of ∼3.35 mS cm(-1), the composite electrolyte has been shown to deliver stable electrochemical performance at 120 °C, and a rechargeable lithium battery with Li4Ti5O12 electrode has been tested to deliver reliable capacity for over several cycles of charge-discharge.

Fast vortex-assisted self-assembly of carbon nanoparticles on an air-water interface.

In this work a self-assembly technique is presented, allowing the fast formation of carbon black thin films. It consists in the controlled addition of a stable carbon materials dispersion over the water surface, disturbed by a vortex. The vortex, although not essential for the film formation, was found to drastically improve film homogeneity. A physical chemical study concerning how several parameters could be used to tune film properties was also conducted. The self-assembled films, which can be picked up in any hydrophilic substrate, showed a good electrical conductivity and a high optical transparency. As an application example, films about 200 nm thick were employed as supercapacitor electrodes.

The curious case of positive current collectors: corrosion and passivation at high temperature

In the evaluation of compatibility of different components of cell for high-energy and extreme-conditions applications, the highly focused are positive and negative electrodes and their interaction with electrolyte. However, for high-temperature application, the other components are also of significant influence and contribute toward the total health of battery. In present study, we have investigated the behavior of aluminum, the most common current collector for positive electrode materials for its electrochemical and temperature stability. For electrochemical stability, different electrolytes, organic and room temperature ionic liquids with varying Li salts (LiTFSI, LiFSI), are investigated. The combination of electrochemical and spectroscopic investigations reflects the varying mechanism of passivation at room and high temperature, as different compositions of decomposed complexes are found at the surface of metals.

In Situ Lithiated Reference Electrode: Four Electrode Design for In-operando Impedance Spectroscopy

Extending operating voltage of Li-ion batteries results in higher energy output from these devices. High voltages, however, may trigger or accelerate multiple processes responsible for long-term performance decay. Given the complexity of physical processes occurring inside the cell, it is often challenging to achieve a full understanding of the root causes of this performance degradation. This difficulty arises in part from the fact that any electrochemical measurement of a battery will return the combined contributions of all components in the cell. Incorporation of a reference electrode can solve part of the problem, as it allows the electrochemical reactions of the cathode and the anode to be individually probed. A variation in the voltage range experienced by the cathode, for example, can indicate alterations in the pool of cyclable lithium ions in the full-cell. The structural evolution of the many interphases existing in the battery can also be monitored, by measuring the contributions of each electrode to the overall cell impedance. Such wealth of information amplifies the reach of diagnostic analysis in Li-ion batteries and provides valuable input to the optimization of individual cell components. In this work, we introduce the design of a test cell able to accommodate multiple reference electrodes, and present reference electrodes that are appropriate for each specific type of measurement, detailing the assembly process in order to maximize the accuracy of the experimental results.

2D material integrated macroporous electrodes for Li-ion batteries

Three-dimensionally structured architectures are known to improve the performance of electrodes used in Li ion battery systems. In addition, integration of select 2D materials into 3D structures, for enhancing both electrical conductivity and electrochemical activity, will prove advantageous. Here a scalable one-step chemical vapor deposition technique is demonstrated for the controlled etching and simultaneous graphene growth on stainless steel substrates resulting in a 3D micro-mesh architecture that is ideal for high rate/high capacity electrodes; the graphene coated 3D stainless steel current collector is used with an MoS2 electrode material for demonstrating high stability and rate capacity in Li-ion batteries.