Mauro Pasta

The application of transition metal fluorides as energy-dense cathode materials for lithium ion batteries has been hindered by inadequate understanding of their electrochemical capabilities and limitations. Here, we present an ideal system for mechanistic study through the colloidal synthesis of single-crystalline, monodisperse iron(II) fluoride nanorods. Near theoretical capacity (570 mA h g-1) and extraordinary cycling stability (>90% capacity retention after 50 cycles at C/20) is achieved solely through the use of an ionic liquid electrolyte (1 m LiFSI/Pyr1,3FSI), which forms a stable solid electrolyte interphase and prevents the fusing of particles. This stability extends over 200 cycles at much higher rates (C/2) and temperatures (50 °C). High-resolution analytical transmission electron microscopy reveals intricate morphological features, lattice orientation relationships and oxidation state changes that comprehensively describe the conversion mechanism. Phase evolution, diffusion kinetics and cell failure are critically influenced by surface-specific reactions. The reversibility of the conversion reaction is governed by topotactic cation diffusion through an invariant lattice of fluoride anions and the nucleation of metallic particles on semicoherent interfaces. This new understanding is used to showcase the inherently high discharge rate capability of FeF2.

Salinity gradients are a vast and untapped energy resource. For every cubic meter of freshwater that mixes with seawater, approximately 0.65 kW h of theoretically recoverable energy is lost. For coastal wastewater treatment plants that discharge to the ocean, this energy, if recovered, could power the plant. The mixing entropy battery (MEB) uses battery electrodes to convert salinity gradient energy into electricity in a four-step process: (1) freshwater exchange; (2) charging in freshwater; (3) seawater exchange; and (4) discharging in seawater. Previously, we demonstrated a proof of concept, but with electrode materials that required an energy investment during the charging step. Here, we introduce a charge-free MEB with low-cost electrodes: Prussian Blue (PB) and polypyrrole (PPy). Importantly, this MEB requires no energy investment, and the electrode materials are stable with repeated cycling. The MEB equipped with PB and PPy achieved high voltage ratios (actual voltages obtained divided by the theoretical voltages) of 89.5% in wastewater effluent and 97.6% in seawater, with over 93% capacity retention after 50 cycles of operation and 97-99% over 150 cycles with a polyvinyl alcohol/sulfosuccinic acid (PVA/SSA) coating on the PB electrode.

© 2018 Elsevier Inc. Prussian blue analogs have significant promise as active materials for the next generation of battery electrodes with improved cycle life and rate capability. Their useful electrochemical properties include two independent redox centers per unit cell; a nanoporous, open framework for rapid ion conduction; high stability during ion (de)insertion; and structural and electrochemical tunability for diverse applications. Here we share insights into how control of the five main crystallographic features (two transition-metal ions, the inserting ion, defects, and water) imparts control over the ion-insertion reaction. We then identify five key opportunities to expand our understanding of these materials, including the role of water in their ion conduction, modeling, synthesis methods, use as anode materials, and technoeconomics. Further research in these areas will accelerate the development of new, high-performing battery electrodes. In this work, we offer our perspective on a class of compounds known as Prussian blue analogs, a group of versatile cyano-coordination polymers. Originally commercialized in the dye industry, they have more recently been used in diverse energy technologies including energy storage, electrocatalysis, and thermal power generation. We address their role as components of the next generation of batteries with improved cycle life and power capability. Through a review of achievements from early electrochemical experiments to crystallographic breakthroughs to development of state-of-the-art electrodes, we outline how control over the crystal structure allows these materials’ electrochemistry to be tuned for diverse uses. Despite recent advances, there remain conspicuous and fundamental questions about the structure-electrochemistry relationship that we identify. We adumbrate investigations using density functional theory calculations, synchrotron X-ray techniques, electrochemical tests, and technoeconomic analyses to answer these questions. Batteries are a form of electrochemical energy storage that allows electrical energy to be stored and transported. Prussian blue analogs (PBAs) are a family of materials that can enable the next generation of batteries with higher rate capability and longer cycle lives. Furthermore, controlling the chemical composition of these materials allows their crystal structure and electrochemistry to be tuned to target specific battery applications. We also identify critical research fronts in PBA science.