High Energy Organic Cathode for Sodium Rechargeable Batteries

Organic electrodes have attracted significant attention as alternatives to conventional inorganic electrodes in terms of sustainability and universal availability in natural systems. However, low working voltages and low energy densities are inherent limitations in cathode applications. Here, we propose a high-energy organic cathode using a quinone-derivative, C₆Cl₄O₂, for use in sodium-ion batteries, which boasts one of the highest average voltages among organic electrodes in sodium batteries (∼2.72 V vs Na/Na⁺). It also utilizes a two-electron transfer to provide an energy of 580 Wh kg^–1. Density functional theory (DFT) calculations reveal that the introduction of electronegative elements into the quinone structure significantly increased the sodium storage potential and thus enhanced the energy density of the electrode, the latter being substantially higher than previously known quinone-derived cathodes. The cycle stability of C₆Cl₄O₂ was enhanced by incorporating the C₆Cl₄O₂ into a nanocomposite with a porous carbon template. This prevented the dissolution of active molecules into the surrounding electrolyte

Anti-Site Reordering in LiFePO₄: Defect Annihilation on Charge Carrier Injection

Defects critically affect the properties of materials. Thus, controlling the defect concentration often plays a pivotal role in determining performance. In lithium rechargeable batteries, the operating mechanism is based on ion transport, so large numbers of defects in the electrode crystal can significantly impede Li ion diffusion, leading to decreased electrochemical properties. Here, we introduce a new way to heal defects in crystals by a room-temperature electrochemical annealing process. We show that defects in olivine LiFePO₄, an important cathode material, are significantly reduced by the electrochemical recombination of Li/Fe anti-sites. The healed LiFePO₄ recovers its high-power capabilities. The types of defects in LiFePO₄ and recombination mechanisms are discussed with the aid of first-principles calculations

Critical Role of Oxygen Evolved from Layered Li–Excess Metal Oxides in Lithium Rechargeable Batteries

The high capacity of the layered Li–excess oxide cathode is always accompanied by extraction of a significant amount of oxygen from the structure. The effects of oxygen on the electrochemical cycling are not well understood. Here, the detailed reaction scheme following oxygen evolution was established using real-time gas analysis and ex situ chemical analysis of the surface of the electrodes. A series of electrochemical/chemical reactions involving oxygen radicals constantly produced and decomposed lithium carbonate during cell operation. Moreover, byproducts, including water, affected the cycle life and rate capability: hydrolysis of the electrolyte salt formed hydrofluoric acid that attacked the surface of the electrode. This finding implies that protection of the electrode surface from damage, for example, by a coating or removal of oxygen radicals by scavengers, will be critical to widespread usage of Li–excess transition metal oxides in rechargeable lithium batteries

Highly Stable Iron- and Manganese-Based Cathodes for Long-Lasting Sodium Rechargeable Batteries

The development of long-lasting and low-cost rechargeable batteries lies at the heart of the success of large-scale energy storage systems for various applications. Here, we introduce Fe- and Mn-based Na rechargeable battery cathodes that can stably cycle more than 3000 times. The new cathode is based on the solid-solution phases of Na₄­Mn_<i>x</i>­Fe_3–<i>x</i>­(PO₄)₂­(P₂O₇) (<i>x</i> = 1 or 2) that we successfully synthesized for the first time. Electrochemical analysis and <i>ex situ</i> structural investigation reveal that the electrodes operate via a one-phase reaction upon charging and discharging with a remarkably low volume change of 2.1% for Na₄MnFe₂(PO₄)­(P₂O₇), which is one of the lowest values among Na battery cathodes reported thus far. With merits including an open framework structure and a small volume change, a stable cycle performance up to 3000 cycles can be achieved at 1C and room temperature, and almost 70% of the capacity at C/20 can be obtained at 20C. We believe that these materials are strong competitors for large-scale Na-ion battery cathodes based on their low costs, long-term cycle stability, and high energy density

Toward a Lithium–“Air” Battery: The Effect of CO₂ on the Chemistry of a Lithium–Oxygen Cell

Lithium–oxygen chemistry offers the highest energy density for a rechargeable system as a “lithium–air battery”. Most studies of lithium–air batteries have focused on demonstrating battery operations in pure oxygen conditions; such a battery should technically be described as a “lithium–dioxygen battery”. Consequently, the next step for the lithium–“air” battery is to understand how the reaction chemistry is affected by the constituents of ambient air. Among the components of air, CO₂ is of particular interest because of its high solubility in organic solvents and it can react actively with O₂^–•, which is the key intermediate species in Li–O₂ battery reactions. In this work, we investigated the reaction mechanisms in the Li–O₂/CO₂ cell under various electrolyte conditions using quantum mechanical simulations combined with experimental verification. Our most important finding is that the subtle balance among various reaction pathways influencing the potential energy surfaces can be modified by the electrolyte solvation effect. Thus, a low dielectric electrolyte tends to primarily form Li₂O₂, while a high dielectric electrolyte is effective in electrochemically activating CO₂, yielding only Li₂CO₃. Most surprisingly, we further discovered that a high dielectric medium such as DMSO can result in the reversible reaction of Li₂CO₃ over multiple cycles. We believe that the current mechanistic understanding of the chemistry of CO₂ in a Li–air cell and the interplay of CO₂ with electrolyte solvation will provide an important guideline for developing Li–air batteries. Furthermore, the possibility for a rechargeable Li–O₂/CO₂ battery based on Li₂CO₃ may have merits in enhancing cyclability by minimizing side reactions