Is There a Ready Recipe for Hard Carbon Electrode Engineering to Enhance Na-Ion Battery Performance?

Abstract

Hard carbon (HC) materials are commonly used as anode materials in Na-ion batteries. In most of the cases, their electrochemical performance is correlated only to their physicochemical properties, and the impact of the electrode additives (binders–conductive agent) and electrolyte is often neglected. In this work, a systematic study is performed to understand the role of electrode/electrolyte engineering on HC initial Coulombic efficiency (iCE), specific capacity, and cycle stability. Four HCs obtained by pyrolysis of several biopolymers, i.e., cellulose (HC-Cell), chitosan (HC-Chs), chitin (HC-Cht), and lignin (HC-Lig), are used. The binder was found to have an important impact on the electrochemical performance, with PVDF resulting in better performance than CMC. The carbon black additive had no significant impact on CMC-based electrochemical performance while it boosted the electrochemical performance of PVDF-based electrodes. For an optimized formulation (PVDF/carbon black), the best HC performance in NaPF6 in 1 EC:DEC was delivered by HC-Cell (83% iCE, 332 mAh g–1 at C/10, and 97% retention). This was attributed to its large graphene interlayer space, high purity, and low surface area. HC-Cht and HC-Chs exhibited similar good electrochemical performance (∼280 mAh g–1) whereas the use of HC-Lig resulted in low iCE and capacity fading overcycling due to the high level of impurities in its structure. This could be overcome by changing the electrolyte salt, by using NaClO4 (76% retention) instead of NaPF6 (52% retention). Based on the obtained results, the electrochemical performance could be correlated with the HC physicochemical properties and binder/conductive additive. It could be demonstrated that careful electrode engineering combined with proper electrolyte selection and tuned HC properties allowed all investigated materials achieving reasonable iCE (up to 83%), high specific capacity (∼280 to 332 mAh g–1), and high-capacity retention (72–97% after 50 cycles).