Characterization techniques for lithium metal anodes at multiple spatial scales

Abstract

Conventional lithium-ion batteries with graphite anode have gradually ceased satisfying demand for the rapid development of modern electric commodities, such as portable electronic devices and electric vehicles. Therefore, metallic lithium is considered the ultimate alternative anode material for future high-energy-density lithium batteries because of its excellent properties, including the highest theoretical capacity and lowest potential of available materials as well as its low density. However, research on lithium metal anodes in traditional liquid batteries has encountered impediments. Numerous studies have shown that lithium dendrites, dead lithium, solid electrolyte interphase problems, and the correlating safety hazards are the main hindrances to the practical application of liquid-based lithium metal batteries. For solidstate batteries, the challenges of lithium metal anodes continue to grow. Studies on the mechanical, thermal, chemical, and electrochemical stability of solid-state electrolyte and lithium metal anode indicate that, unlike early recognition, solid-state lithium metal batteries remain far from commercialization. Unexpected issues like lithium growth along crystal boundaries, mixed-conductivity interphase generation, and interfacial contact losses have emerged that complicate the solid-state lithium metal battery. To achieve practically applicable lithium metal anodes, it is necessary to deepen our understanding of the basic scientific issues. This review systematically discusses the electrode behaviors of lithium metal and the corresponding electrode characterization techniques at multiple spatial scales. First, the basic science and technology issues of lithium metal anodes at different scales are reviewed. Lithium electrodeposition behaviors from the atomic to the macroscale are divided into ion transportation, deposition, nucleation, crystallization, expansion and growth. Various issues are also categorized among different characteristic scales. Second, advanced characterization techniques for all spatial scales are reviewed in light of recent works. Finally, the technical characteristics of various characterization techniques from the atomic to macroscale are analyzed. Features and possible directions of improvement of various characterization techniques used to examine lithium metal anodes in solid-state batteries are highlighted. In situ observation has become a common requirement for battery characterization as it can connect macroscale phenomena to microscale mechanisms. Meanwhile, non-damaging detection techniques have faced growing demand because of the urgent need to understand the complete actual reactions at the bulk and interfaces of solid-state electrolytes and lithium anodes. The combination of techniques for different scales should provide comprehensive information to characterize lithium metal anodes and identify reasonable mechanisms for their behaviors.