Abstract The Nobel Prize in Chemistry 2019 recognized the importance of Li-ion batteries and the revolution they allowed to happen during the past three decades. They are part of a broader class of electrochemical energy storage devices, which are employed where electrical energy is needed on demand and so, the electrochemical energy is converted into electrical energy as required by the application. This opens a variety of possibilities on the utilization of energy storage devices, beyond the well-known mobile applications, assisting on the decarbonization of energy production and distribution. In this series of reviews in two parts, two main types of energy storage devices will be explored: electrochemical capacitors (part I) and rechargeable batteries (part II). More specifically, we will discuss about the materials used in each type of device, their main role in the energy storage process, their advantages and drawbacks and, especially, strategies to improve their performance. In the present part, electrochemical capacitors will be addressed. Their fundamental difference to batteries is explained considering the process at the electrode/electrolyte surface and the impact in performance. Materials used in electrochemical capacitors, including double layer capacitors and pseudocapacitive materials will be reviewed, highlighting the importance of electrolytes. As an important part of these strategies, synthetic routes for the production of nanoparticles will also be approached (part I).

Abstract In the second part of the review on electrochemical energy storage, the devolvement of batteries is explored. First, fundamental aspects of battery operation will be given, then, different materials and chemistry of rechargeable batteries will be explored, including each component of the cell. In negative electrodes, metallic, intercalation and transformation materials will be addressed. Examples are Li or Na metal batteries, graphite and other carbonaceous materials (such as graphene) for intercalation of metal-ions and transition metal oxides and silicon for transformation. In the positive electrode section, materials for intercalation and transformation will be reviewed. The state-of-the-art on intercalation as lithium cobalt oxide and nickel containing oxides will be approached for intercalation materials, whereas sulfur and metal-air will also be explored for transformation. Alongside, the role of electrolyte will be discussed concerning performance and safety, with examples for the next generation devices. Finally, a general future perspective will address both electrochemical capacitors and batteries.

Neste trabalho, foi estudado o comportamento eletroquímico de Mg e Fe em líquidos iônicos (ILs). Para melhorar o entendimento do comportamento de Mg em IL baseado no ânion bis(trifluormetanosulfonamida) ([Tf2N]), nós realizamos uma série de experimentos de voltametria cíclica. Os resultados mostraram deposição/dissolução de Mg irreversível em alta concentração de água (ca. 1300 ppm, 50 mmol L-1) e uma reversibilidade muito baixa (7.3%) em uma concentração moderada de água (ca. 65 ppm, 5 mmol L-1). A formação de um filme na superfície do eletrodo e a presença de Mg foi confirmada por microscopia eletrônica de varredura-espectrometria de energia dispersiva de raios-x (MEV-EDS). A irreversibilidade do processo indica a formação de um filme passivante. Como a presença de água impacta a reversibilidade do processo, foram realizados estudos de deposição/dissolução de Fe utilizando microeletrodos para avaliar como a água modifica a reversibilidade do processo e a difusão dos íons utilizando dois IL diferentes. A água tem um papel importante na reversibilidade da deposição/dissolução de Fe sendo a reação menos reversível na mistura seca. A difusão do íon de Fe também é modificada, já que a esfera de coordenação do íon Fe tem um grande impacto na presença e ausência de água, mas isto depende da habilidade de coordenação do cátion do IL.

In this work, the electrochemical behavior of Mg and Fe in ionic liquids (IL) were studied. We performed a series of cyclic voltammetry experiments to improve the understanding of Mg behavior in an IL containing the bis(trifluoromethanesulfonylimide) ([Tf2N]) anion. The results show an irreversible deposition/dissolution of Mg at a high water concentration (ca. 1300 ppm, 50 mmol L-1) and very low reversibility (7.3%) at a moderate water concentration (ca. 65 ppm, 5 mmol L-1). The formation of a film on the electrode surface and the presence of Mg were confirmed by scanning electron microscopy and energy-dispersive X-ray spectroscopy (SEM-EDS). The process irreversibility indicates the formation of a passivating film. Because the presence of water affects the reversibility of the process, studies of Fe deposition/dissolution were conducted in two different ILs and with microelectrodes to evaluate how the water modifies the reversibility and the diffusion of ions. Water plays an important role in the reversibility of Fe deposition/dissolution being that deposition is less reversible when water is absent. The Fe diffusion is also modified because the Fe ion coordination sphere is strongly affected by the presence or absence of water; the Fe diffusion was also shown to depend on the coordination ability of the cation.