In situ electrochemical surface modification for high-voltage LiCoO2 in lithium ion batteries

High-voltage LiCoO2 has been revisited to improve the energy density of lithium ion batteries. LiCoO2 can deliver the reversible capacity of about 200 mA h g(-1) when the upper cut-off voltage increases to 4.55 V (vs. Li/Li+). However, the high upper cut-off voltage causes the severe failures of LiCoO2 such as structural degradation, electrolyte decomposition, and Co dissolution. Various surface-modified LiCoO2 materials have been introduced to suppress electrolyte decomposition and Co dissolution, thereby leading to the improved electrochemical performance. Most of the coated LiCoO2 materials are obtained through a conventional coating process such as sol-gel synthesis, which is complex and high-cost. In this paper, the in situ electrochemical coating method is introduced as a simple and low-cost coating process, where the electrolyte additive of Mg salts is electrochemically decomposed to form a MgF2-based coating layer on the LiCoO2 surface. LiCoO2 electrochemically coated with MgF2 suppresses Co dissolution in electrolytes, resulting in excellent electrochemical performance such as high reversible capacity of 198 mA h g(-1) and stable cycle performance over 100 cycles in the voltage range between 3 and 4.55 V (vs. Li/Li+) at 45 degrees C. The formation mechanism of MgF2 is also demonstrated through ex situ XPS and XANES analyses.

Contrasting Miscibility of Ionic Liquid Membranes for Nearly Perfect Proton Selectivity in Aqueous Redox Flow Batteries

Ion-selective membranes are widely used in various energy storage applications. However, conventional polymer-based ion-selective membranes, such as Nafion, are not perfectly ion-selective, leading to a significant permeation of undesirable ionic species. As the imperfect ion-selectivity of membranes leads to the failure of energy storage devices, much effort is devoted to improving membrane ion-selectivity. Herein, an immiscible liquid-state membrane is introduced for aqueous redox flow batteries. As hydrophobic ionic liquids are immiscible with aqueous catholyte and anolyte solutions, they separate the two without crossover. This property renders them suitable for use as a membrane for aqueous redox flow batteries. In addition, ionic liquids with long side alkyl chains, such as 1-hexyl-3-methylimidazolium bis(trifluoromethylsulfonyl)imide (HMIM-TFSI), are miscible with sulfuric acid, whereas transition metal sulfates remain insoluble in HMIM-TFSI. For this reason, HMIM-TFSI is selectively permeable to protons and remains impermeable to transition metal cations. As a result, the HMIM-TFSI membrane is almost perfectly proton-selective, leading to negligible permeability of unfavorable ionic species, such as transition metal cations. Eventually, the HMIM-TFSI membrane shows the excellent electrochemical performance of vanadium redox flow batteries, such as negligible self-discharge over 2800 h, high Coulombic efficiency (& AP;99%), and stable capacity retention over 100 cycles. The contrasting miscibility of the hydrophobic ionic liquid membrane, which is miscible with sulfuric acid but immiscible with aqueous catholyte and anolyte solutions, gives rise to an almost perfect proton-selectivity, leading to negligible permeability of unfavorable ionic species, such as transition metal cations. As a result, the HMIM-TFSI membrane shows the excellent electrochemical performance of vanadium redox flow batteries.imageN

Scalable Solid-State Synthesis of Self-Assembled Si Nanoparticles in Spherical Carbons through Relative Miscibility for Li-Ion Batteries

Nanosized Si-based materials have been extensively investigated because of their high gravimetric capacity and stable cycle performance. However, the tap density of nanosized materials is poor, leading to poor volumetric capacity. In this regard, micrometer-sized Si nanoparticles and carbon composites have been introduced to improve the volumetric energy density of Li-ion cells. However, most synthesismethods for these Si/C composites are complex, and thus, only a few methods among them are scalable for mass production. Herein, a scalable solid-state synthesis through self-assembly due to the relative miscibility of hydrophobic and hydrophilic precursors is introduced to obtain micrometer-sized porous carbon spheres containing nanosized Si particles. The self-assembly synthesis uses hydrophilic Si/SiO2 core-shell nanoparticles, hydrophilic phenolic resins, and hydrophobic fumed silica. Because phenolic resin melts and Si/SiO2 core-shells are miscible, the Si/SiO2 core-shells are embedded in the phenolic resins. Immiscible phenolic resin melts and fumed silica lead to the formation of spherical resins. Eventually, the self-assembled micrometer-sized Si/C composite spheres are obtained after heating and HF etching. The tap density of the self-assembled Si/C spheres is much higher than that of the bare Si nanoparticles. In addition, the self-assembled Si/C composite shows excellent cycle performance because of voids in the composite

Trigonal Na4Ti5O12 Phase as an Intercalation Host for Rechargeable Batteries

Trigonal Na4Ti5O12 with a tunnel-structured three-dimensional framework was first examined as an anode material for Li ion batteries. The nanosized Na4Ti5O12 was synthesized at 600 degrees C using the solid state method with carbon-coated nanosized TiO2 anatase. Carbon layers on TiO2 play an important role of inhibiting the growth of Na4Ti5O12 particles. Li ions are reversibly de/intercalated in the Na4Ti5O12 structure, and this shows the reversible capacity of ca. 100 mA h g(-1) with no capacity fading over 100 cycles. During the repeated charge and discharge, the ion exchange between Na ion and Li ion happens to form LixNa4-xTi5O12, and this causes an increase of the relative tunnel size due to smaller Li ion, resulting in decreasing polarization of voltage profiles. Also, trigonal Na4Ti5O12 was additionally examined as an anode for Na ion batteries.close9

Charge carriers in rechargeable batteries: Na ions vs. Li ions

We discuss the similarities and dissimilarities of sodium- and lithium-ion batteries in terms of negative and positive electrodes. Compared to the comprehensive body of work on lithium-ion batteries, research on sodium-ion batteries is still at the germination stage. Since both sodium and lithium are alkali metals, they share similar chemical properties including ionicity, electronegativity and electrochemical reactivity. They accordingly have comparable synthetic protocols and electrochemical performances, which indicates that sodium-ion batteries can be successfully developed based on previously applied approaches or methods in the lithium counterpart. The electrode materials in Li-ion batteries provide the best library for research on Na-ion batteries because many Na-ion insertion hosts have their roots in Li-ion insertion hosts. However, the larger size and different bonding characteristics of sodium ions influence the thermodynamic and/or kinetic properties of sodium-ion batteries, which leads to unexpected behaviour in electrochemical performance and reaction mechanism, compared to lithium-ion batteries. This perspective provides a comparative overview of the major developments in the area of positive and negative electrode materials in both Li-ion and Na-ion batteries in the past decade. Highlighted are concepts in solid state chemistry and electrochemistry that have provided new opportunities for tailored design that can be extended to many different electrode materials for sodium-ion batteries.close492

Si-Encapsulating Hollow Carbon Electrodes via Electroless Etching for Lithium-Ion Batteries

Remarkable improvements in the electrochemical performance of Si materials for Li-ion batteries have been recently achieved, but the inherent volume change of Si still induces electrode expansion and external cell deformation. Here, the void structure in Si-encapsulating hollow carbons is optimized in order to minimize the volume expansion of Si-based anodes and improve electrochemical performance. When compared to chemical etching, the hollow structure is achieved via electroless etching is more advanced due to the improved electrical contact between carbon and Si. Despite the very thick electrodes (30 approximate to 40 m), this results in better cycle and rate performances including little capacity fading over 50 cycles and 1100 mA h g1 at 2C rate. Also, an in situ dilatometer technique is used to perform a comprehensive study of electrode thickness change, and Si-encapsulating hollow carbon mitigates the volume change of electrodes by adoption of void space, resulting in a small volume increase of 18% after full lithiation corresponding with a reversible capacity of about 2000 mA h g1.close141

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 desodiationclose0

P2 Orthorhombic Na-0.7[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 beta-Na-0.7[Mn1-xLix]O2+y (x = 0.02, 0.04, 0.07, and 0.25), alpha-Na0.7MnO2+y, and, beta-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 beta-Na-0.7[Mn1-x.Li-x]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 beta-Na-0.7[Mn1-x.Li-x]O2+y (x = 0.07) exhibits excellent electrochemical performance including a high reversible capacity of similar to 183 mA h g(-1) and stable cycle performance over 120 cycles