eScience

A perspective of ZnCl₂ electrolytes: The physical and electrochemical properties

Ji Xiulei

2021, 1(2): 99-107. doi: 10.1016/j.esci.2021.10.004

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Molten ZnCl₂ hydrates are ionic liquids at room temperature, which exhibit intriguing physical and electrochemical properties. Continuous efforts have been devoted over several decades to understanding the properties of the molten ZnCl₂ hydrates that have been dubbed as water-in-salt electrolytes recently. The physical properties of molten ZnCl₂ hydrates can be described from the perspectives of ions in their speciation and water molecules regarding their chemical environments. Recently, attention has been given to molten ZnCl₂ hydrates as electrolytes for Zn metal batteries. It was revealed that the physical properties of such electrolytes have rich implications in their electrochemical properties. Therefore, it demands a holistic understanding of the physical and electrochemical properties of molten ZnCl₂ hydrates to design functional electrolytes to serve high-performing Zn metal batteries. This perspective attempts to review the works that described the properties of concentrated ZnCl₂ as an ionic liquid and as an emerging electrolyte. The author also provides a perspective to highlight the needs of future research to circumvent the limits of this electrolyte.

Nitrate additives for lithium batteries: Mechanisms, applications, and prospects

Li Xiang, Zhao Ruxin, Fu Yongzhu, Manthiram Arumugam

2021, 1(2): 108-123. doi: 10.1016/j.esci.2021.12.006

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Lithium-metal batteries (LMBs) are considered as one of the most promising energy storage devices due to the high energy density and low reduction potential of the Li-metal anode. However, the growth of lithium dendrites results in accumulated dead Li and safety issues, limiting the practical application of LMBs. LiNO₃ is a well-known additive in lithium–sulfur batteries to regulate the solid–electrolyte interphase (SEI), effectively suppressing the redox shuttle of polysulfides. Recently, other nitrates have been investigated in various electrolyte and battery systems, yielding improved SEI stability and cycling performance. In this review, we provide an overview of various nitrates, including LiNO₃ for lithium batteries, focusing on their mechanisms and performance. We first discuss the effect of nitrate anions on SEI formation, as well as the cathode–electrolyte interphase (CEI). The solvation behavior regulated by nitrates is also extensively explored. Some strategies to improve the solubility of LiNO₃ in ester-based electrolytes are then summarized, followed by a discussion of recent progress in the application of nitrates in different systems. Finally, further research directions are presented, along with challenges. This review provides a comprehensive understanding of nitrates and affords new and interesting ideas for the design of better electrolytes and battery systems.

Recent advances in micro-supercapacitors for AC line-filtering performance: From fundamental models to emerging applications

Feng Xin, Shi Xiaoyu, Ning Jing, Wang Dong, Zhang Jincheng, Hao Yue, Wu Zhong-Shuai

2021, 1(2): 124-140. doi: 10.1016/j.esci.2021.11.005

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Recently, micro-supercapacitors (MSCs) have undergone major development as next-generation micro-electrochemical energy storage devices for self-powered, integrated, and wearable systems, thanks to their excellent performance capability. In particular, their rapid frequency response characteristics make them potential candidates to replace conventional capacitors and function as alternating current (AC) line filters to rectify pulse energy or as current ripple filters in the kHz range. However, few papers have been published about the associated fundamental device components, architectures, and correct characterization of MSCs applied in filter applications. In addition, it is a huge challenge to achieve a balance between capacitance and frequency response, not yet to be overcome. This review comprehensively summarizes recent advances in MSCs for AC line-filtering, from fundamental mechanisms to appropriate characterization and emerging applications. Special attention is given to progress in microfabrication strategies, electrode materials, and electrolytes for high-frequency MSCs. We also present perspectives and insights into the development of MSCs in different frequency ranges for AC line-filtering applications.

Tailoring the structure of silicon-based materials for lithium-ion batteries via electrospinning technology

Huang Aoming, Ma Yanchen, Peng Jian, Li Linlin, Chou Shu-lei, Ramakrishna Seeram, Peng Shengjie

2021, 1(2): 141-162. doi: 10.1016/j.esci.2021.11.006

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Silicon (Si) is one of the most promising anode materials for the next generation of lithium-ion battery (LIB) due to its high specific capacity, low lithiation potential, and natural abundance. However, the huge variation in volume during the storage of lithium, along with the low conductivity of element, are the main factors hindering its commercial application. Designing micro–nano structures as well as composites of heterogeneous materials have proven to be effective strategies to overcome these issues. Electrospinning technology is an affordable and scalable method for easily constructing a unique hierarchical micro–nano structure while realizing composites of heterogeneous materials. So far, many efforts have been made to solve the problems of Si-based anodes with general electrospinning. This review considers the technical fundamental and design strategies for electrospun Si-based nanofibers, including preparation processes, structural engineering, and lithium storage performance. The structure–performance relationship of various materials and the effects of compositing with heterogeneous materials are explored in detail. Finally, the remaining challenges are discussed, along with directions for future research. This review will provide inspiration for researchers in the design and manufacture of electrospun Si-based nanofibers for LIBs.

Designing safer lithium-based batteries with nonflammable electrolytes: A review

Zhang Shichao, Li Siyuan, Lu Yingying

2021, 1(2): 163-177. doi: 10.1016/j.esci.2021.12.003

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Lithium-based batteries have had a profound impact on modern society through their extensive use in portable electronic devices, electric vehicles, and energy storage systems. However, battery safety issues such as thermal runaway, fire, and explosion hinder their practical application, especially for using metal anode. These problems are closely related to the high flammability of conventional electrolytes and have prompted the study of flame-retardant and nonflammable electrolytes. Here, we review the recent research on nonflammable electrolytes used in lithium-based batteries, including phosphates, fluorides, fluorinated phosphazenes, ionic liquids, deep eutectic solvents, aqueous electrolytes, and solid-state electrolytes. Their flame-retardant mechanisms and efficiency are discussed, as well as their influence on cell electrochemical performance. We conclude with a summary of future prospects for the design of nonflammable electrolytes and the construction of safer lithium-based batteries.

High-performance Zn battery with transition metal ions co-regulated electrolytic MnO₂

Chuai Mingyan, Yang Jinlong, Wang Mingming, Yuan Yuan, Liu Zaichun, Xu Yan, Yin Yichen, Sun Jifei, Zheng Xinhua, Chen Na, Chen Wei

2021, 1(2): 178-185. doi: 10.1016/j.esci.2021.11.002

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Electrolytic MnO₂/Zn batteries have attracted extensive attention for use in large-scale energy storage applications due to their low cost, high output voltage, safety, and environmental friendliness. However, the poor electrical conductivity of MnO₂ limits its deposition and dissolution at large capacities, which leads to sluggish reaction kinetics and drastic capacity decay. Here, we report a theory-guided design principle for an electrolytic MnO₂/Zn battery co-regulated with transition metal ions that has improved electrochemical performance in terms of deposition and stripping chemistries. We start with first-principles calculations to predict the electrolytic effects of regulating transition metal ions in the deposition/stripping chemistry of the MnO₂ cathode. The results indicate that with the simultaneous incorporation of strongly electronegative Co and Ni, the MnO₂ cathode tends to possess more active electron states, faster charge-transfer kinetics, and better electrical conductivity than either MnO₂ regulated with Co or Ni on their own, or pristine MnO₂; hence, this co-regulation is beneficial for the cathode solid/liquid MnO₂/Mn²⁺ reactions. We then fabricate and demonstrate a novel Co²⁺ and Ni²⁺ co-regulated MnO₂/Zn (Co–Ni–MnO₂/Zn) battery that yields significantly better electrochemical performance, finding that the synergistic regulation of Co and Ni on MnO₂ can significantly increase its intrinsic conductivity and achieve high rates and Coulombic efficiencies at large capacities. The aqueous Co–Ni–MnO₂/Zn battery exhibits a high rate (10C, 100 mA cm^–2), high Coulombic efficiency (91.89%), and excellent cycling stability (600 cycles without decay) at a large areal capacity of 10 mAh cm^–2. Our proposed strategy of co-regulation with transition metal ions offers a versatile approach for improving the electrochemical performance of aqueous electrolytic MnO₂/Zn batteries in large-scale energy storage applications.

Polypyrrole as an ultrafast organic cathode for dual-ion batteries

Sun Tao, Sun Qi-Qi, Yu Yue, Zhang Xin-Bo

2021, 1(2): 186-193. doi: 10.1016/j.esci.2021.11.003

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Organic electrode materials based on the chemical bond cleavage/recombination working principle usually produce unimpressive reaction kinetics and stability. In this work, polypyrrole (PPy) is investigated as an ultrafast (87% retention at 20 A g⁻¹) and stable (83% retention across 3000 cycles) cathode material in PPy|graphite dual-ion batteries. The fast intrinsic reaction kinetics, coupled with a capacitance-dominated mechanism, enable PPy to bypass the sluggish chemical bond rearrangement process. Electrochemically induced secondary doping improves the ordered aggregation of polymer chains and thus has a profound impact on anion diffusion and electrical conductivity. The excellent rate capability presented here changes our understanding of organic electrode materials and could prove useful for designing ultrafast rechargeable electrochemical devices.

Ultrathin salt-free polymer-in-ceramic electrolyte for solid-state sodium batteries

Tang Bin, Zhao Yibo, Wang Zhiyi, Chen Shiwei, Wu Yifan, Tseng Yuming, Li Lujiang, Guo Yunlong, Zhou Zhen, Bo Shou-Hang

2021, 1(2): 194-202. doi: 10.1016/j.esci.2021.12.001

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The practical energy density of solid-state batteries remains limited, partly because of the lack of a general method to fabricate thin membranes for solid-state electrolytes with high ionic conductivity and low area-specific resistance (ASR). Herein, we use an ultrahigh concentration of a ceramic ion conductor (Na₃SbS₄) to build an ion-conduction "highway", and a polymer (polyethylene oxide, 2 wt%) as a flexible host to prepare a polymer-in-ceramic ion-conducting membrane of approximately 40 μm. Without the use of any salt (e.g., NaPF₆), the resulting membrane exhibits a threefold increase in electronic ASR and a twofold decrease in ionic ASR compared with a pure ceramic counterpart. The activation energy for sodium-ion transport is only 190 meV in the membrane, similar to that in pure ceramic, suggesting ion transport predominantly occurs through a percolated network of ion-conducting ceramic particles. The salt-free design also provides an opportunity to suppress dendritic metal electrodeposits, according to a recently refined chemomechanical model of metal deposition. Our work suggests that salt is not always necessary in composite solid-state electrolytes, which broadens the choice of polymers to allow the optimization of other desired attributes, such as mechanical strength, chemical/electrochemical stability, and cost.

Confining ultrafine tin monophosphide in Ti₃C₂T_x interlayers for rapid and stable sodium ion storage

Tang Jiayong, Peng Xiyue, Lin Tongen, Huang Xia, Luo Bin, Wang Lianzhou

2021, 1(2): 203-211. doi: 10.1016/j.esci.2021.12.004

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Phase separation in conversion/alloying-based anodes easily causes crystal disintegration and leads to bad cycling performance. Tin monophosphide (SnP) is an excellent anode material for sodium ion battery due to its unique three-dimensional crystallographic layered structure. In this work, we report the in situ growth of ultrafine SnP nanocrystals within Ti₃C₂T_x MXene interlayers. The MXene framework is used as a conductive matrix to provide high ionic/electrical transfer paths and reduce the Na⁺ diffusion barrier in the electrode. In situ and ex situ measurements reveal that the synergy between small SnP crystal domains and the confinement provided by the MXene host prevents mechanical disintegration and major phase separation during the sodiation and desodiation cycles. The resultant electrode exhibits fast Na⁺ storage kinetics and excellent cycling stability for over 1000 cycles. A full cell assembled with this new SnP-based anode and a Na₃V₂(PO₄)₃ cathode delivers a high energy density of 265.4 Wh kg⁻¹ and a power density of 3252.4 W kg^–1, outperforming most sodium-ion batteries reported to date.

Ammonium-ion batteries with a wide operating temperature window from −40 to 80 ℃

Yan Lei, Qi Ya-e, Dong Xiaoli, Wang Yonggang, Xia Yongyao

2021, 1(2): 212-218. doi: 10.1016/j.esci.2021.12.002

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Ammonium-ion batteries are promising solutions for large-scale energy storage systems owing to their cost-effectiveness, safety, and sustainability. Herein, we propose an aqueous ammonium-ion battery based on an organic poly(1, 5-naphthalenediamine) anode and an inorganic Prussian blue cathode in 19 M (M: mol kg⁻¹) CH₃COONH₄ electrolyte. Its operation involves a reversible coordination reaction (C=N/C–N– conversion) in the anode and the NH₄⁺ insertion/extraction reaction in the cathode, along with NH₄⁺ acting as the charge carrier in a rocking-chair battery. Benefiting from the fast kinetics and stability of both electrodes, this aqueous ammonium-ion battery shows an excellent rate capability and long cycle stability for 500 cycles. Moreover, an energy density as high as 31.8 Wh kg⁻¹ can be achieved, based on the total mass of the cathode and anode. Surprisingly, this aqueous ammonium-ion battery works well over a wide temperature range from −40 to 80 ℃. This work will provide new opportunities to build wide-temperature aqueous batteries and broaden the horizons for large-scale energy storage systems.

2021 Vol. 1, No. 2