Sodium-Ion Storage in Electrochemically Enhanced Hard Carbons

摘要

The performance of batteries is critically dependent on the electrode, thus the development of electrode materials is at the heart of advancement for Na-ion batteries (NIBs). In Li-ion batteries (LIBs), carbonaceous materials have played a pivotal role as an anode material. The low cost, abundance and availability of large-scale processing systems for carbon materials has allowed LIBs to become commercially successful, and, accordingly, the nature of lithium-ion storage in various types of carbonaceous material is relatively well known. However, despite the similar practical advantages of NIBs, the nature of sodium-ion storage in carbonaceous materials remains relatively poorly understood. Moreover, while chemical differences between lithium and sodium result in fundamentally different electrochemical responses in NIBs, and while different carbon-based anodes show distinct electrochemical properties with sodium, the correlation between carbon local structure and sodium ion storage has not been investigated extensively to date. Herein, we demonstrate the direct interplay between carbon local orderings and their diverse electrochemical properties in NIBs using morphologically identical carbon platform with different carbon local structures. It is shown that sodium ion storage behaviors vary among surface-driven physisorption and chemisorption, diffusion-controlled insertion and nanoclustering of the metallic state, and the expression of these mechanisms is highly dependent on the local carbon structure. In particular, the nanoclustering of sodium metal within the carbon material is directly visualized, which is the first direct observation of metallic sodium storage, to the best of our knowledge, in carbon-based electrodes. These three different mechanisms result in distinct working voltages, specific capacities and power capabilities. The straightforward dependency of electrochemical properties, such as potential and capacity, on the carbon structure found in this work provides guidelines for tailoring carbon structures to tune the electrochemical performance of carbonaceous materials for NIBs. Thus, the findings in this work not only provide a better understanding of sodium ion storage behavior in carbonaceous materials but also offer a design strategy for electrode optimization in various sodium-based electrochemical devices.