Direct Proof of the Reversible Dissolution/Deposition of Mn2+/Mn4+ for Mild-Acid Zn-MnO2 Batteries with Porous Carbon Interlayers.

Mild-acid Zn-MnO2 batteries have been considered a promising alternative to Li-ion batteries for large scale energy storage systems because of their high safety. There have been remarkable improvements in the electrochemical performance of Zn-MnO2 batteries, although the reaction mechanism of the MnO2 cathode is not fully understood and still remains controversial. Herein, the reversible dissolution/deposition (Mn2+/Mn4+) mechanism of the MnO2 cathode through a 2e- reaction is directly evidenced using solution-based analyses, including electron spin resonance spectroscopy and the designed electrochemical experiments. Solid MnO2 (Mn4+) is reduced into Mn2+ (aq) dissolved in the electrolyte during discharge. Mn2+ ions are then deposited on the cathode surface in the form of the mixture of the poorly crystalline Zn-containing MnO2 compounds through two-step reactions during charge. Moreover, the failure mechanism of mild-acid Zn-MnO2 batteries is elucidated in terms of the loss of electrochemically active Mn2+. In this regard, a porous carbon interlayer is introduced to entrap the dissolved Mn2+ ions. The carbon interlayer suppresses the loss of Mn2+ during cycling, resulting in the excellent electrochemical performance of pouch-type Zn-MnO2 cells, such as negligible capacity fading over 100 cycles. These findings provide fundamental insights into strategies to improve the electrochemical performance of aqueous Zn-MnO2 batteries.

Synthesis of nanostructured P2-Na2/3MnO2 for high performance sodium-ion batteries.

We report a facile two-step method to synthesize nanostructured P2-Na2/3MnO2via ligand exchange and intercalation of sodium ions into ultrathin manganese oxide nanoplates. Sodium storage performance of the synthesized material shows a high capacity (170 mA h g-1) and an excellent rate performance.

Coordination Polymers for High-Capacity Li-Ion Batteries: Metal-Dependent Solid-State Reversibility.

Electrode materials exploiting multielectron-transfer processes are essential components for large-scale energy storage systems. Organic-based electrode materials undergoing distinct molecular redox transformations can intrinsically circumvent the structural instability issue of conventional inorganic-based host materials associated with lattice volume expansion and pulverization. Yet, the fundamental mechanistic understanding of metal-organic coordination polymers toward the reversible electrochemical processes is still lacking. Herein, we demonstrate that metal-dependent spatial proximity and binding affinity play a critical role in the reversible redox processes, as verified by combined 13C solid-state NMR, X-ray absorption spectroscopy, and transmission electron microscopy. During the electrochemical lithiation, in situ generated metallic nanoparticles dispersed in the organic matrix generate electrically conductive paths, synergistically aiding subsequent multielectron transfer to π-conjugated ligands. Comprehensive screening on 3d-metal-organic coordination polymers leads to a high-capacity electrode material, cobalt-2,5-thiophenedicarboxylate, which delivers a stable specific capacity of ∼1100 mA h g-1 after 100 cycles.

P2 Orthorhombic Na0.7[Mn1-xLix]O2+y as Cathode Materials for Na-Ion Batteries.

P2-type manganese-based oxide materials have received attention as promising cathode materials for sodium ion batteries because of their low cost and high capacity, but their reaction and failure mechanisms are not yet fully understood. In this study, the reaction and failure mechanisms of ß-Na0.7[Mn1-xLix]O2+y (x = 0.02, 0.04, 0.07, and 0.25), α-Na0.7MnO2+y, and ß-Na0.7MnO2+z are compared to clarify the dominant factors influencing their electrochemical performances. Using a quenching process with various amounts of a Li dopant, the Mn oxidation state in ß-Na0.7[Mn1-xLix]O2+y is carefully controlled without the inclusion of impurities. Through various in situ and ex situ analyses including X-ray diffraction, X-ray absorption near-edge structure spectroscopy, and inductively coupled plasma mass spectrometry, we clarify the dependence of (i) reaction mechanisms on disordered Li distribution in the Mn layer, (ii) reversible capacities on the initial Mn oxidation state, (iii) redox potentials on the Jahn-Teller distortion, (iv) capacity fading on phase transitions during charging and discharging, and (v) electrochemical performance on Li dopant vs Mn vacancy. Finally, we demonstrate that the optimized ß-Na0.7[Mn1-xLix]O2+y (x = 0.07) exhibits excellent electrochemical performance including a high reversible capacity of ∼183 mA h g-1 and stable cycle performance over 120 cycles.

Structural Effect on Electrochemical Performance of Ordered Porous Carbon Electrodes for Na-Ion Batteries.

Ordered meso- or macro-porous carbons (OMCs) were applied as anodes in Na ion battery (NIB) systems. Three different block copolymers (BCPs) enabled us to control the pore sizes (6, 33, and 60 nm) while maintaining the same 2-D hexagonal structure. To exclude other effects, the factors including precursors, particle sizes, and degrees of graphitization were controlled. The structures of OMCs were characterized by nitrogen physisorption, Raman spectroscopy, X-ray analyses (XRD and SAXS), and microscopies (TEM and SEM). With a galvanostatic charge/discharge, we confirmed that OMC electrode with medium pore size (OMC-33) exhibited a higher reversible capacity of 134 mA h g(-1) (at 20th cycle) and faster rate capability (61% retention, current densities from 50 to 5000 mA g(-1)) than those of OMC-6, and OMC-60 electrodes. The high performance of OMC-33 is attributed to the combined effects of pore size and wall thickness which was supported by charge/discharge and electrochemical impedance spectroscopy (EIS) analyses.

SnSe alloy as a promising anode material for Na-ion batteries.

SnSe alloy is examined for the first time as an anode for Na-ion batteries, and shows excellent electrochemical performance including a high reversible capacity of 707 mA h g(-1) and stable cycle performance over 50 cycles. Upon sodiation, SnSe is changed into amorphous NaxSn nanodomains dispersed in crystalline Na2Se, and SnSe is reversibly restored after desodiation.

Synthesis of ordered mesoporous phenanthrenequinone-carbon via π-π interaction-dependent vapor pressure for rechargeable batteries.

The π-π interaction-dependent vapour pressure of phenanthrenequinone can be used to synthesize a phenanthrenequinone-confined ordered mesoporous carbon. Intimate contact between the insulating phenanthrenequinone and the conductive carbon framework improves the electrical conductivity. This enables a more complete redox reaction take place. The confinement of the phenanthrenequinone in the mesoporous carbon mitigates the diffusion of the dissolved phenanthrenequinone out of the mesoporous carbon, and improves cycling performance.

Abnormal excess capacity of conjugated dicarboxylates in lithium-ion batteries.

Lithium-ion batteries (LIBs) are considered to be key energy storage systems needed to secure reliable, sustainable, and clean energy sources. Redox-active organic compounds have been proposed as interesting candidates for electrode materials for the next-generation LIBs because of their flexible molecular design, recyclability, and low production cost. Despite wide interest, a molecular-level understanding of the electrochemical lithiations/delithiations of those materials remains rudimentary. We synthesized a set of π-conjugated dicarboxylates and discovered unprecedented excess capacities for inverse-Wurster-type nonfused aromatic compounds (dilithium terephthalate and dilithium thiophene-2,5-dicarboxylate). Molecular structural investigations based on solid-state CP/MAS (13)C NMR combined with the stable isotope labeling method and ex situ X-ray diffraction were carried out to elucidate the origin of the excess reversible capacity. Interestingly, an open-chain-type dilithium muconate did not show an analogous behavior, signifying the key role played by the cyclic moiety in the electrochemical reaction.

An amorphous red phosphorus/carbon composite as a promising anode material for sodium ion batteries.

An amorphous red phosphorus/carbon composite is obtained through a facile and simple ball milling process, and its electrochemical performance as an anode material for Na ion batteries is evaluated. The composite shows excellent electrochemical performance including a high specific capacity of 1890 mA h g(-1), negligible capacity fading over 30 cycles, an ideal redox potential (0.4 V vs. Na/Na(+)), and an excellent rate performance, thus making it a promising candidate for Na ion batteries.

Sodium terephthalate as an organic anode material for sodium ion batteries.

Disodium terephthalate and its various derivatives are synthesized via simple acid-base chemistry for anode materials in Na ion batteries. They show excellent electrochemical performance, including little capacity fading over 90 cycles, ideal redox potential, and excellent rate performance, making them promising candidates for Na ion batteries.