Advances of entropy-stabilized homologous compounds for electrochemical energy storage

Abstract

Recently, high-entropy materials (HEMs) have gained increasing interest in the field of energy storage technology on account of their unique structural characteristics and possibilities for tailoring functional properties. Herein, the development of this class of materials for electrochemical energy storage have been reviewed, especially the fundamental understanding of entropy-dominated phase-stabilization effects and prospective applications are presented. Subsequently, critical comments of HEMs on the different aspects of battery and supercapacitor are summarized with the underlying principles for the observed properties. In addition, we also summarize their potential advantages and remaining challenges, which will ideally provide some general guidelines and principles for researchers to study and develop advanced HEMs. The diversity of material design contributed by the entropy-mediated concept provides the researchers numerous ideas of new candidates for practical applications and ensures further research in the emerging field of energy storage.

Introduction

The explosive energy demand in the future would reach 28 TW by the year 2050 and most of the value is given rise due to the massive consumption of fossil fuels [1], [2]. More importantly, the concentration increase of greenhouse gases due to the combustion of fossil fuels has resulted in severe environmental issues, such as global warming and ocean acidification. Although the widely used renewable energies can alleviate the energy shortage to a certain extent, the volatile energy supply will inevitably hinder their practical application. Therefore, more efficient energy storage systems are essential in the near future, which largely depends on the development of functional materials [3], [4], [5].

Recently, high-entropy materials (HEMs) based on the entropy-stabilization strategy have received increasing attention towards advanced functional applications. The library of HEMs includes alloys [6], [7], oxides [8], [9], [10], [11], [12], [13], oxyfluorides [14], [15], borides [16], carbides [17], [18], [19], nitrides [20], sulfides [21], and phosphides [22], which prefer for single-phase solid solutions. By means of tailoring their constituent elements, HEMs give rise to attractive features and the functional properties, such as environmental protections [23], hydrogen storage [24], [25], energy storage [26], [27], thermoelectricity applications[28], and catalytic processes [29], [30], [31]. HEMs, as a novel family of energy storage materials, show infinite potential on account of the severe lattice distortion (strain) promoted by large entropy. For example, rock-salt type high-entropy oxides (HEOs) can be utilized as lithium storage anodes with enhanced cycling performance and higher efficiency than those of traditional oxides [9], [14], [15]. Moreover, layered structure of HEOs were studied as high-performance cathodes towards sodium/lithium storage owing to the entropy stabilization effect [10], [32], [33]. In addition, HEMs also manifest excellent performance in supercapacitors, which have large capacity and wide potential. Consequently, high entropy compounds can lead a new development direction in the electrochemical energy storage, and exploring the mechanism can further expand their application.

Although there are several reviews related to the functional applications of HEMs, the recent work in the field of energy storage has not been summarized comprehensively. Meanwhile, the effects of entropy-stabilization strategy on the certain performance also are not outlined explicitly. In this review, we summarize an intensive overview of HEMs in rechargeable batteries and supercapacitors (Fig. 1). Finally, we systematically extract the potential advantages and challenges of the development of HEMs, accompanied by designing new materials of the high-entropy family, which will inspire scientists to develop advanced HEMs for energy storage.