Dipan Kundu

The Emerging Chemistry of Sodium Ion Batteries for Electrochemical Energy Storage

Energy storage technology has received significant attention for portable electronic devices, electric vehicle propulsion, bulk electricity storage at power stations, and load leveling of renewable sources, such as solar energy and wind power. Lithium ion batteries have dominated most of the first two applications. For the last two cases, however, moving beyond lithium batteries to the element that lies below-sodium-is a sensible step that offers sustainability and cost-effectiveness. This requires an evaluation of the science underpinning these devices, including the discovery of new materials, their electrochemistry, and an increased understanding of ion mobility based on computational methods. The Review considers some of the current scientific issues underpinning sodium ion batteries.

A graphene-like metallic cathode host for long-life and high-loading lithium–sulfur batteries

Development of Li–S batteries with long cycle life and high practical capacity is central to enable low-cost, large-scale energy storage. Sulfiphilic cathode materials with strong affinity for lithium (poly)sulfides are a promising new group of candidates to control dissolution/precipitation reactions in the cell, where the improvement of conductivity and the areal sulfur loading is an important objective. Here we report a metallic Co9S8 material with an interconnected graphene-like nano-architecture that realizes this aim. First-principles calculations coupled with spectroscopic evidence demonstrate the synergistic strong dual-interactions of polysulfides with the host. The three dimensional interconnected structure with hierarchical porosity not only manifests up to a factor of 10 increase of cycling stability (fade < 0.045% per cycle over 1500 cycles at C/2) compared to standard porous carbons but also, more importantly, enables a high-loading sulfur electrode with up to 75 wt% sulfur, and up to 4.5 mg cm−2 areal sulfur loading.

A highly active nanostructured metallic oxide cathode for aprotic Li–O2 batteries

The aprotic lithium–oxygen cell is based on the reversible reduction of oxygen on a cathode host to form lithium peroxide, and has received much attention in the last few years owing to its promise to offer increased electrochemical energy density beyond that provided by traditional Li-ion batteries. Carbon has been extensively utilized as a host, but it reacts with Li2O2 to form an insulating layer of lithium carbonate resulting in high overpotentials on charge. Establishing a stable, and conductive interface at the porous cathode is a major challenge that has motivated a search for non-carbonaceous cathode materials. Very few suitable materials have been discovered so far. Here we report on the synthesis of the metallic Magneli phase Ti4O7 with a crystallite size between 10–20 nm, and show that a cathode fabricated from this material greatly reduces the overpotential compared to carbon. Oxidation of lithium peroxide on charge starts just above 3 V, comparable to gold and TiC, and the majority (∼65%) of oxygen release occurs in the 3–3.5 V window vs. Li+/Li as determined by on-line electrochemical mass spectrometry. Ti4O7 is much lighter and lower cost than gold, easy to prepare, and provides a controlled interface. X-ray photoelectron spectroscopy measurements show that a conductive, self-passivating substoichiometric metal oxide layer is formed at the surface which is important for stability.

A Highly Active Low Voltage Redox Mediator for Enhanced Rechargeability of Lithium–Oxygen Batteries

Owing to its high theoretical specific energy, the Li-oxygen battery is one of the fundamentally most promising energy storage systems, but also one of the most challenging. Poor rechargeability, involving the oxidation of insoluble and insulating lithium peroxide (Li2O2), has remained the “Achilles’ heel” of this electrochemical energy storage system. We report here on a new redox mediator tris[4-(diethylamino)phenyl]amine (TDPA), that—at 3.1 V—exhibits the lowest and closest potential redox couple compared to the equilibrium voltage of the Li-oxygen cell of those reported to date, with a second couple also at a low potential of 3.5 V. We show it is a soluble “catalyst” capable of lowering the Li2O2 charging potential by >0.8 V without requiring direct electrical contact of the peroxide and that it also facilitates high discharge capacities. Its chemical and electrochemical stability, fast diffusion kinetics, and two dynamic redox potentials represent a significant advance in oxygen-evolution catalysis. It enables Li–O2 cells that can be recharged more than 100 cycles with average round-trip efficiencies >80%, opening a new avenue for practical Li-oxygen batteries.

Nanostructured Metal Carbides for Aprotic Li-O2 Batteries: New Insights into Interfacial Reactions and Cathode Stability.

The development of nonaqueous Li-oxygen batteries, which relies on the reversible reaction of Li + O2 to give lithium peroxide (Li2O2), is challenged by several factors, not the least being the high charging voltage that results when carbon is typically employed as the cathode host. We report here on the remarkably low 3.2 V potential for Li2O2 oxidation on a passivated nanostructured metallic carbide (Mo2C), carbon-free cathode host. Online mass spectrometry coupled with X-ray photoelectron spectroscopy unequivocally demonstrates that lithium peroxide is simultaneously oxidized together with the Li(x)MoO3-passivated conductive interface formed on the carbide, owing to their close redox potentials. The process rejuvenates the surface on each cycle upon electrochemical charge by releasing Li(x)MoO3 into the electrolyte, explaining the low charging potential.

The Nature and Impact of Side Reactions in Glyme-based Sodium-Oxygen Batteries.

Although Na-O2 batteries have a low overpotential and good capacity retention, degradation reactions of glyme-based electrolytes are the primary reason for inefficiency in cell performance. The discharge capacity is accounted for through analysis of the side-products. Although sodium superoxide is the primary product (90 % theoretical), quantitative and qualitative evaluation of the side-products (using (1) H NMR, iodometric titration, and on-line mass spectrometry) shows the presence of sodium acetate (∼3.5 %), and three-fold less sodium formate, methoxy (oxo)acetic anhydride, and sodium carbonate. Our reaction mechanism proposes two paths for their formation. Because the side-products are not fully removed during oxidation, they accumulate on the cathode upon cycling. Resting the cell at open circuit potential during discharge results in consumption of the superoxide through the reaction with diglyme, which greatly increases the fraction of side products, as also confirmed by ex situ reaction studies. These findings have implications in the search for more stable electrolytes.

A low dimensional composite of hexagonal lithium manganese borate (LiMnBO3), a cathode material for Li-ion batteries

The ultrasonic nebulized spray pyrolysis technique has been applied to synthesize amorphous nanospheres, which are further transformed into nano h-LiMnBO3 with an average crystallite size of ∼14 nm. A composite electrode of nano h-LiMnBO3 with reduced graphite oxide and amorphous carbon delivers a high first discharge capacity of 140 mA h g−1 at C/15 rate within 4.5–2.0 V and retains a discharge capacity of 110 mA h g−1 at the 25th cycle. The dissolution of Mn into the electrolyte and the instability of the highly delithiated phases during cycling are suggested as the reasons, which limit the cycling stability of h-LiMnBO3. An improved cycling stability at higher capacities is expected via the combination of the particle size reduction, conductive network formation and the metal site doping strategies.

Aqueous vs. nonaqueous Zn-ion batteries: consequences of the desolvation penalty at the interface

Zinc ion batteries using metallic zinc as the negative electrode have gained considerable interest for electrochemical energy storage, whose development is crucial for the adoption of renewable energy technologies, as zinc has a very high volumetric capacity (5845 mA h cm−3), is inexpensive and compatible with aqueous electrolytes. However, the divalent charge of zinc ions, which restricts the choice of host material due to hindered solid-state diffusion, can also pose a problem for interfacial charge transfer. Here, we report our findings on reversible intercalation of up to two Zn2+ ions in layered V3O7·H2O. This material exhibits very high capacity and power (375 mA h g−1 at a 1C rate, and 275 mA h g−1 at an 8C rate) in an aqueous electrolyte compared to a very low capacity and slow rate capabilities in a nonaqueous medium. Operando XRD studies, together with impedance analysis, reveal solid solution behavior associated with Zn2+-ion diffusion within a water monolayer in the interlayer gap in both systems, but very sluggish interfacial charge transfer in the nonaqueous electrolyte. This points to desolvation at the interface as a major factor in dictating the kinetics. Temperature dependent impedance studies show high activation energies associated with the nonaqueous charge transfer process, identifying the origin of poor electrochemical performance.