(Invited) Electrochemical Investigation of Hard Carbon Electrodes for Sodium-Ion Batteries Using a Coaxial Ring Reference Electrode

Abstract

Sodium-ion batteries (NIBs) are an attractive alternative to lithium-ion batteries (LIBs) because of the natural abundance of sodium resources [1]. Owing to their similar working principles to LIBs, NIBs also offer the advantage of utilizing the same manufacturing protocols and methodology as LIBs, and as such minimal additional capital costs are required for existing LIB manufacturers to switch to NIB technology.Since the sodium storage mechanism of hard carbon (HC) was demonstrated by Stevens and Dahn in 2000 [2], it has become the state-of-the-art anode material for NIBs due to its good cycling stability, low working potentials, small voltage hysteresis and relatively low cost. However, the storage mechanism for high performance is not fully understood due to the complexity of the HC structure. The prevalent problem of under-estimated capacity of HC in half-cell tests indicates that this conventional test method requires improvement. Furthermore, half-cell studies are generally carried out in two-electrode configuration, where the counter electrode (CE) acts also as reference electrode (RE). This provides reliable results only if the passage of current does not significantly perturb the potential of the CE [3].In this study, we present an in-depth investigation of the electrochemical behavior of a range of HC-based working electrodes (WE) in three-electrode half-cell configuration, with a pure Na metal CE and a concentric Na metal coaxial ring RE. The use of a separate RE allows simultaneous data acquisition for both WE and CE, during galvanostatic cycling and electrochemical impedance spectroscopy measurements.Using this approach, the electrochemical performance, including cycling stability, rate capability and electrochemical impedance, is measured in a reliable manner to assess the capacity and cyclability of HC. In addition, the effect of four different types of electrolyte solution on the electrochemical behavior is analysed and discussed to inform electrolyte selection from a full-cell perspective (Figure 1). Reference s : [1] C. Vaalma, D. Buchholz, M. Weil, S. Passerini, Nat. Rev. Mat., 3 (2018) 18013.[2] J. R Dahn, D. A. Stevens, J. Electrochem. Soc., 4 (2000) 147.[3] R. Raccichini, M. Amores, G. Hinds, Batteries., 5 (2019) 12. Figure 1