However, even with the use of nanoscale materials, the large volume changes associated with charging and discharging cause the fragmentation of the anode material, [2] decreasing their capacities, and cycling lifetimes. The coating and doping of anode nanomaterials are among the various methods proposed to address this problem. [3] However, these methods are complex and expensive, and the overlayer coated on the anode nanomaterials may hinder the fast transport of carrier ions. Another approach based on the reaction between an anode material and electrolyte has also been reported. [4] This method induces the spontaneous restructuring of a bulk anode material into a 3D porous nanostructure during battery cycling with a glyme-based electrolyte [4b-e] . Although the short diffusion distance in thus-formed nanostructure is beneficial for improving the rate performance of the anode, its energy capacity still rapidly decreases with increasing current rate (C-rate). [3a,b,5] This indicates that a strategy based on reducing the diffusion distance alone is not sufficient to simultaneously improve the energy capacity, rate capability, and cycling stability of alloying anodes.To determine another variable governing the kinetics of carrier-ion diffusion at high C-rates, it is necessary to investigate why the practical capacity of the anode decreases with It is challenging to develop alloying anodes with ultrafast charging and large energy storage using bulk anode materials because of the difficulty of carrierion diffusion and fragmentation of the active electrode material. Herein, a rational strategy is reported to design bulk Bi anodes for Na-ion batteries that feature ultrafast charging, long cyclability, and large energy storage without using expensive nanomaterials and surface modifications. It is found that bulk Bi particles gradually transform into a porous nanostructure during cycling in a glyme-based electrolyte, whereas the resultant structure stores Na ions by forming phases with high Na diffusivity. These features allow the anodes to exhibit unprecedented electrochemical properties; the developed Na-Bi half-cell delivers 379 mA h g −1 (97% of that measured at 1C) at 7.7 A g −1 (20C) during 3500 cycles. It also retained 94% and 93% of the capacity measured at 1C even at extremely fast-charging rates of 80C and 100C, respectively. The structural origins of the measured properties are verified by experiments and first-principles calculations. The findings of this study not only broaden understanding of the underlying mechanisms of fast-charging anodes, but also provide basic guidelines for searching battery anodes that simultaneously exhibit high capacities, fast kinetics, and long cycling stabilities.The ORCID identification number(s) for the author(s) of this article can be found under https://doi.org/10.1002/adma.202201446.
The authors reveal the mechanisms of degradation of capacity, charge voltage, and discharge voltage of commercially‐available high‐nickel cathode material when it is cycled without a voltage margin by two different charge protocols: constant‐current charging and constant‐current, constant‐voltage charging. With repeated constant‐current charging, the cathode material changes to a non‐periodic cation‐mixed state, which causes a relatively low voltage degradation, whereas during constant‐current, constant‐voltage charging, the cathode material changes from a layered structure to a periodic cation‐mixed spinel‐like phase, with consequent severe voltage decay. This decay results from a reduction in the equilibrium electrode potential and an increase of overpotential which are aggravated in a periodic cation‐mixed state. The findings provide insights into the use of excess Li without charge‐voltage margin in high‐Ni cathode materials.
In article number 1903658, Wonyoung Chang, Kyung Yoon Chung, Sang‐Young Lee and co‐workers present DNA‐wrapped MWCNT as an eco‐friendly chemical activation strategy for overlithiated layered oxides (OLO) cathode materials. The Li4Mn5O12‐type spinel nanolayers are formed on an OLO surface through a cation exchange reaction of Na+–Li+. The spinel nanolayers significantly improve the charge‐discharge kinetics, cyclability, and thermal stability. This unique behavior is comprehensively investigated by in‐depth structural/electrochemical characterization.
Sodium-rich metallic Na x+z has received significant attention as a low-cost alternative to the conventional electrode materials used in Li-ion batteries. However, the poor cyclability of Na x Cl remains a major challenge to its practical application. Here, a simple method is developed for improving the electrochemical performance of Na x Cl by controlling the upper limit of cut-off voltage. It is demonstrated that additional Na-vacancy defects can be introduced in the NaCl structure during the high-voltage activation process at 4.5 V. The structure then accommodates more sodium ions during the next discharge, resulting in increased capacity. At the same time, Cl-ions released by NaCl decomposition are oxidized to form Cl-based organic species at the active material interfaces. This plays a crucial role in protecting the NaCl electrode from undesired side reactions at high voltage. In short, this control of the charging protocol helps to induce more vacancies in the NaCl structure, as well as form stable interphases on the electrode surface, contributing to the increased capacity and enhanced cycle stability. This study will help in exploring a new approach for developing low-cost and high-capacity electrode material, which can potentially be applied in future energy-storage systems.
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