Advanced Bismuth-Based Anode Materials for Efficient Potassium Storage: Structural Features, Storage Mechanisms and Modification Strategies

Abstract

Potassium-ion batteries (PIBs) are considered as a promising energy storage system owing to its abundant potassium resources. As an important part of the battery composition, anode materials play a vital role in the future development of PIBs. Bismuth-based anode materials demonstrate great potential for storing potassium ions (K+) due to their layered structure, high theoretical capacity based on the alloying reaction mechanism, and safe operating voltage. However, the large radius of K+ inevitably induces severe volume expansion in depotassiation/potassiation, and the sluggish kinetics of K+ insertion/extraction limits its further development. Herein, we summarize the strategies used to improve the potassium storage properties of various types of materials and introduce recent advances in the design and fabrication of favorable structural features of bismuth-based materials. Firstly, this review analyzes the structure, working mechanism and advantages and disadvantages of various types of materials for potassium storage. Then, based on this, the manuscript focuses on summarizing modification strategies including structural and morphological design, compositing with other materials, and electrolyte optimization, and elucidating the advantages of various modifications in enhancing the potassium storage performance. Finally, we outline the current challenges of bismuth-based materials in PIBs and put forward some prospects to be verified.

1. Introduction

There is a growing demand for energy storage devices for environmental and developmental reasons [1]. Over the past few decades, lithium-ion batteries (LIBs) have dominated the market for portable electronic devices and electric vehicles due to their high energy density, long cycle life and environmental friendliness [2]. However, the scarcity and uneven distribution of lithium resources hinder the sustainable development of LIBs [3]. Sodium-ion batteries (SIBs) and potassium-ion batteries (PIBs) have attracted extensive attention due to the similar physicochemical properties to LIBs and abundant resources [4]. It is worth noting that three advantages of PIBs over SIBs have been demonstrated: (1) The redox potential of K/K+ is −2.93 V (vs. SHE), which is closer to Li/Li+ (−3.04 V) than Na/Na+ (−2.71 V), so PIBs may exhibit higher voltage. This could result in a wider voltage window and higher energy density for PIBs [5]. (2) Another advantage of PIBs is the low Lewis acidity of K+ in organic solvents, which leads to small solvated K+ [6] and low desolvation energy [7]. Thus, the transmission conductivity and quantity of solvated K+ are much greater than that of Li+ and Na+. (3) K+ can be electrochemically embedded in and deintercalated from the interlaminar space of graphite, whereas the insertion and extraction of Na+ can only occur in the co-intercalation of ether molecules. As a result, the specific capacity of SIBs in graphite (110 mAh g−1) is much lower than that of PIBs (279 mAh g−1) [2].

However, the development of PIBs currently faces some challenges. Firstly, the large ionic radius of the K+ results in a large structural change of the electrode during the potassiation/depotassiation process [8]. Secondly, the slow kinetics of K+ limits the rate performance of PIBs. Finally, metallic potassium directly used as an anode has potential safety hazard [9]. In 2015, commercial graphite was used in PIBs and measured that the reversible capacity reached 273 mAh g−1. However, the graphite expansion of 61% is observed during the potassium storage process, which results in rapid capacity decay and low-rate performance of PIBs [10]. Thus, a lot of works have been devoted to developing more advanced and efficient anode materials to effectively improve the performance of PIBs, such as carbonaceous materials, conversion materials, alloying materials, metal oxides and organic materials [11].