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The reliability of electric double-layer capacitors (EDLCs), long overlooked, is important for practical applications. For instance, issues like leakage current (leading to energy loss and heat generation), gassing (increasing internal pressure), and aging (causing capacity fading and cell resistance to rise) greatly affect the applicability of EDLCs. A hierarchical electrode design is developed in the present work to deal with these problems. The hierarchical carbon composites that consist of activated carbon (AC), zero-dimensional carbon nanospheres (CNSs) and one-dimensional CNTs are fabricated using a facile and cost-effective method, which can be easily scaled up for mass production. The charge-discharge properties, operation voltage, temperature profile during charging/discharging, leakage current, gas evolution, and self-discharge of the EDLCs are systematically investigated. The electronic conductivity of the electrodes is found to crucially influence the EDLC overall performance. A hierarchical carbon architecture with an appropriate AC/CNS/CNT constituent ratio can significantly improve charge-discharge capacitances, increase cell reliability, and decrease the aging-related degradation rate.

Recently, high entropy oxide (HEO) materials received compelling research consideration for lithium-ion batteries (LIBs) owing to their exceptional cycling stability, high specific capacity, and exclusive tailorable properties. This unique tailorable aspect of HEO enables the use of infinite metal cation combinations to develop new electrode materials with exciting and unexplored properties. In this study, we developed a series of Co-free spinel-type HEO (HESO) anodes via a facile hydrothermal method. The effects of various elemental combination on phase purity, morphology, crystallinity, microstructures, valence states, and electrochemical charge-discharge properties of the HESO electrodes are systematically studied. For the first time, we demonstrate that the chemical composition of HESOs is crucial to the phase purity, morphology, and oxygen vacancy concentration. The oxygen vacancy concentration seems to have a substantial impact on the rate capability and reversibility of the HEO electrodes. This study demonstrates that an electrochemically inactive spectator element is not necessary for achieving high cyclability, given that the phase purity of the HEO is wisely controlled. The single-phase (CrNiMnFeCu)3O4 shows a great high-rate capability and no capacity fading after 400 cycles. In addition, the charge/discharge behavior was examined using operando X-ray diffraction. According to the operando XRD analysis, a spinel-to-rock salt phase transition is observed for (CrNiMnFeCu)3O4 before the formation of short-range ordered nano-crystals during the first lithiation. The nano-crystalline nature is preserved upon cycling. The the practical feasibility of HEO anode is evaluated by constructing a 4MCu||LiNi0.8Co0.1Mn0.1O2 full cell.

Defect engineering, such as manipulating oxygen vacancies (OVs), which can alter the local structure and electronic properties of metal oxides, has attracted increasing attention for improving the charge-storage performance of electrode materials. In this work, a high-pressure hydrogenation method was developed to fabricate oxygen-deficient TiO2. Two TiO2 phases, namely rutile (TiO2-R) and anatase (TiO2-A), and their hydrogenated phases (denoted with the prefix “H”) are investigated as sodium-ion battery (SIB) anodes. The charge-discharge properties of both phases can be significantly enhanced via the hydrogenation treatment. The introduction of OVs increases the electronic and ionic conductivity of TiO2, and the disordered TiO2 provides more electro-active sites for sodiation/desodiation reactions. For the first time, electrochemcial properties of the H-TiO2-R and H-TiO2-A electrodes are systematically compared. The H-TiO2-A electrode exhibits an excellent high-rate capacity of 100 mAh g–1 at 10000 mA g–1 and great cycling stability of 80% capacity retention after 4500 cycles. The superior performance is well supported by a generalized gradient approximation Perdew-Burke-Ernzerhof density functional calculation. The proposed anode and material modification strategy have great potential for high-power and high-durability SIB applications.

Hard carbon (HC), probably the most potential anode for sodium-ion batteries, currently encounters three major obstacles, which are low first-cycle coulombic efficiency, unsatisfactory charge–discharge kinetics, and insufficient cyclic stability. Our developed electrolyte can effectively improve these properties. The concept of using moderately concentrated electrolyte is proposed to deal with the limitations of high cost, low conductivity, and poor viscosity (or wettability) of superconcentrated electrolytes. The crucial roles of ethylene carbonate in the moderately concentrated electrolyte are explored in this study. With the formation of well-balanced robust inorganic–organic solid–electrolyte interphase, the first-cycle as well as steady-state coulombic efficiency of the HC electrode significantly improved to 85% and >99.9%, respectively. This electrolyte significantly improved the charge−discharge capacities at high current rates. Furthermore, after 500 sodiation−desodiation cycles, 95% of the initial capacity can be maintained. The proposed electrolyte design strategies are expected to be applicable to other battery electrodes to boost their energy-storage performance.

Sodium-ion batteries (NIBs) are considered as next-generation energy storage devices to substitute lithium-ion batteries in large-scale energy storage systems for smart grids and electric vehicles. Pursuing low-cost, resource-abundant, and high-performance anodes are decisive to the future growth and expansion of NIBs. Recently, several suitable cathode materials for NIBs have been successfully developed and investigated. However, the major hurdle is to obtain appropriate anode materials with reversible electrochemistry and fast kinetics for NIBs. Herein, we have successfully developed a new hard carbon (HC) anode derived from abundant sugarcane bagasse (SB) as the biomass precursor. We synthesized SB hard carbon (SB-HC) under various carbonization temperatures. SB has a unique balance of lignin, cellulose, and hemicellulose, which avoids full graphitization of the pyrolyzed carbon but guarantees an adequately ordered carbon structure for Na-ion transport. When compared to HC derived from waste apple biomass, which is rich in pectin and has less cellulose compared to SB, SB‐HC is less defective and has lower oxygen content. As a result, SB‐HC has a higher first‐cycle charge/discharge coulombic efficiency and enhanced cycling stability. In addition, SB‐HC has a distinctive flake‐like morphology, which can reduce Na+ diffusion length, resulting in outstanding high‐rate charge-discharge performance. The effects of pyrolysis temperature on electrochemical properties of both HC anodes are comprehensively studied.

Electrolyte optimization plays a key role in deciding energy density (ED) and power density (PD) of supercapacitors. Increasing cell voltage is an effective way to improve ED and PD. Therefore, an electrolyte with a large potential window is highly desired. Ionic liquid (IL) electrolytes, characterized by wide potential windows, excellent thermal stability, intrinsic ionic conductivity, non-volatility, non-flammability, and low environmental concerns, are potential electrolytes and are used for micropore-and mesopore-rich activated carbon (ACmicro and ACmeso) supercapacitors. IL electrolytes consisting of various cations of EMI+ (1-ethyl-3-methyl imidazolium), PMP+ (N-propyl-N-methyl pyrrolidinium), and BMP+ (N-butyl-N-methyl pyrrolidinium) and various anions of TFSI– (bis(trifluoromethyl sulfonyl) imide), BF4 – (tetrafluoroborate), and FSI– (bis(fluorosulfonyl)imide) are explored. The electrolyte viscosity and conductivity and ion transport properties at the ACmicro and ACmeso electrodes are studied. Also, the capacitance, cycling stability, and rate capability of the ACmicro and ACmeso electrodes are systematically investigated, and post-mortem material analyses are conducted. This study shows the influence of IL composition on charge–discharge capacitances of the ACmicro electrodes is more pronounced than that for the ACmeso electrodes. The FSI-based IL electrolytes, whose electrochemical properties are dependent on cation, are found to be highly promising. Uniting EMI+ with FSI– results in low viscosity of electrolyte and high ion transport, optimizing the electrode capacitance and rate capability. Interchanging EMI+ with PMP+ enhances the cell voltage (to 3.5 V) and maximum ED (to 42 Wh kg−1) of the ACmicro cell at the cost of cycling stability.