An Advanced Na–FeCl2 ZEBRA Battery for Stationary Energy Storage Application

Sodium‐metal chloride batteries, ZEBRA, are considered one of the most important electrochemical devices for stationary energy storage applications because of its advantages of good cycle life, safety, and reliability. However, sodium–nickel chloride (Na–NiCl₂) batteries, the most promising redox chemistry in ZEBRA batteries, still face great challenges for the practical application due to its inevitable feature of using Ni cathode (high materials cost). Here, a novel intermediate‐temperature sodium–iron chloride (Na–FeCl₂) battery using a molten sodium anode and Fe cathode is proposed and demonstrated. The first use of unique sulfur‐based additives in Fe cathode enables Na–FeCl₂ batteries can be assembled in the discharged state and operated at intermediate temperature (<200 °C). The results presented demonstrate that intermediate‐temperature Na–FeCl₂ battery technology could be a propitious solution for ZEBRA battery technologies by replacing the traditional Na–NiCl₂ chemistry.     

Electrochemical Energy Storage with a Reversible Nonaqueous Room‐Temperature Aluminum–Sulfur Chemistry

A reversible room‐temperature aluminum–sulfur (Al‐S) battery is demonstrated with a strategically designed cathode structure and an ionic liquid electrolyte. Discharge–charge mechanism of the Al‐S battery is proposed based on a sequence of electrochemical, microscopic, and spectroscopic analyses. The electrochemical process of the Al‐S battery involves the formation of a series of polysulfides and sulfide. The high‐order polysulfides (S_x^2?, x ≥ 6) are soluble in the ionic liquid electrolyte. Electrochemical transitions between S₆^2? and the insoluble low‐order polysulfides or sulfide (S_x ^2?, 1 ≤ x < 6) are reversible. A single‐wall carbon nanotube coating applied to the battery separator helps alleviate the diffusion of the polysulfide species and reduces the polarization behavior of the Al‐S batteries.     

Energy Storage Data Reporting in Perspective—Guidelines for Interpreting the Performance of Electrochemical Energy Storage Systems

Due to the tremendous importance of electrochemical energy storage, numerous new materials and electrode architectures for batteries and supercapacitors have emerged in recent years. Correctly characterizing these systems requires considerable time, effort, and experience to ensure proper metrics are reported. Many new nanomaterials show electrochemical behavior somewhere in between conventional double‐layer capacitor and battery electrode materials, making their characterization a non‐straightforward task. It is understandable that some researchers may be misinformed about how to rigorously characterize their materials and devices, which can result in inflation of their reported data. This is not uncommon considering the current state of the field nearly requires record breaking performance for publication in high‐impact journals. Incorrect characterization and data reporting misleads both the materials and device development communities, and it is the shared responsibility of the community to follow rigorous reporting methodologies to ensure published results are reliable to ensure constructive progress. This tutorial aims to clarify the main causes of inaccurate data reporting and to give examples of how researchers should proceed. The best practices for measuring and reporting metrics such as capacitance, capacity, coulombic and energy efficiencies, electrochemical impedance, and the energy and power densities of capacitive and pseudocapacitive materials are discussed.     

Gel Polymer Electrolytes for Electrochemical Energy Storage

With the booming development of flexible and wearable electronics, their safety issues and operation stabilities have attracted worldwide attentions. Compared with traditional liquid electrolytes, gel polymer electrolytes (GPEs) are preferred due to their higher safety and adaptability to the design of flexible energy storage devices. This review summarizes the recent progress of GPEs with enhanced physicochemical properties and specified functionalities for the application in electrochemical energy storage. Functional GPEs that are capable to achieve unity lithium‐ion transference number and offer additional pseudocapacitance to the overall capacitance are carefully discussed. The smart GPEs with self‐protection, thermotolerant, and self‐healing abilities are particularly highlighted. To close, the future directions and remaining challenges of the GPEs for application in electrochemical energy storages are summarized to provide clues for the following development.     

Liquid Metal Electrodes for Energy Storage Batteries

The increasing demands for integration of renewable energy into the grid and urgently needed devices for peak shaving and power rating of the grid both call for low‐cost and large‐scale energy storage technologies. The use of secondary batteries is considered one of the most effective approaches to solving the intermittency of renewables and smoothing the power fluctuations of the grid. In these batteries, the states of the electrode highly affect the performance and manufacturing process of the battery, and therefore leverage the price of the battery. A battery with liquid metal electrodes is easy to scale up and has a low cost and long cycle life. In this progress report, the state‐of‐the‐art overview of liquid metal electrodes (LMEs) in batteries is reviewed, including the LMEs in liquid metal batteries (LMBs) and the liquid sodium electrode in sodium‐sulfur (Na–S) and ZEBRA (Na–NiCl₂) batteries. Besides the LMEs, the development of electrolytes for LMEs and the challenge of using LMEs in the batteries, and the future prospects of using LMEs are also discussed.