Solid state ionics – Selected topics and new directions

Abstract

Solid state ionics has grown to be one of the most important directions of science, combining emerging interdisciplinary technologies for many applications. This paper begins with the brief history of solid state ionics, followed by detailed discussions on scientific problems, state-of-art theoretical and experimental methods, and promising research directions in this field. The practical applications of principles and knowledge in solid state ionics are also summarized.

Introduction

The term “Ion” was invented by William Whewell (1794–1866) [1], who was a polymath and an expert in philosophy, theology, astronomy, mechanics, architecture and etc.. William believed that ancient Greek words can express durable concepts in scientific areas, and “ion” originates from ancient Greek word as well as the mythological hero “Ion” or tribe of Ionions who migrated from north-western Thessaly to Peloponnese and eventually to Attica [2]. In ancient Greek language, the term “ion” stands for “going” or “moving” [1].

Michael Faraday (1791–1867) respected William so much that he requested from Willaim a terminology to describe his own electrolysis experiments [3]. William Whewell then proposed to Faraday various terms, including “cathode”, “anode” and “ion”. Michael Faraday⁴ thus introduced the term “ion” into science for the first time (1834) in a paper entitled “Experimental Researches in Electricity”, as shown in Fig. 1. In this paper, Faraday proposed to use “ion” to represent “cations” and “anions” when they need to be discussed together [4] (see Fig. 2).

In the early 1960s, Professor Takehiko Takahashi [5] at Nagoya University in Japan, who is an expert in applied electro-chemistry and ionic motion in solid electrolytes (SE), defined the new discipline of “solid state ionics (SSI)” for the first time. Solid-state ionics is a new interdisciplinary subject involving physics, chemistry, materials and engineering. It studies the scientific and technical issues of ion conduction and mixed conduction of ions and electrons in solids. It involves the thermodynamics, mechanics, interfacial phenomena for crystal defects, ion transport and ion exchange. The content includes the design, synthesis, characterization of structure and performance of ion conductors, ion–electron mixed conductors, and microscopic mechanism of ion transport and its application in new high-energy batteries, fuel cells and other solid-state ion devices. The term “solid state ionics” indicates the importance of ionic motion in solids in analogy to the term “solid state electronics” [1]. After 6 decades’ development, SSI has grown to be an important field of study that is combined with emerging interdisciplinary technology to produce new knowledge and applications. The knowledge of SSI has also been developed to predict or elongate the lifetime of materials and to improve the performance of devices.

In spite of the importance of SSI, this field has not become common knowledge. This paper aims to set out the significance of SSI with respect to the present scientific and societal problems, providing a comprehensive overview for the benign development of SSI in the future. More specifically, this paper elaborates on the following four topics in SSI: 1) important scientific problems, 2) theoretical and experimental methods, 3) promising research directions, and 4) applications of SSI devices for multiple purposes. Moreover, the answers to the above topics are complemented by more complete analyses and discussions in each section for the sake of other researchers interested in this field.