Decoupling Sodium Metal Artifacts from Hard Carbon Electrochemistry in Sodium-Ion Batteries

Abstract

Hard carbon (HC) is the benchmark anode for commercial sodium-ion batteries, yet widespread misconceptions regarding its electrolyte-dependent kinetics continue to hinder its practical development. Currently, the field champions ether-based electrolytes over conventional carbonates, based almost entirely on conventional Na||HC half-cell testing. Here, we demonstrate that these comparisons may often be flawed due to chemical artifacts originating from the highly reactive Na-metal counter electrode. We establish an artifact-free, symmetric hard carbon (HC||HC) cell geometry to isolate intrinsic HC behavior. Combined with complementary NMR spectroscopy, we investigate the underlying mechanisms of Na-ion storage in HC. Galvanostatic cycling and temperature-dependent impedance analyses reveal that while ether electrolytes initially form a low-impedance solid electrolyte interphase (SEI), progressive degradation limits long-term cycling. Variations in the Na+ storage mechanism cause large solvent-dependent changes in the impedance and activation energy barriers. Thus, while ether electrolytes hold promise for high-rate applications, they suffer from poor long-term cycling. Ultimately, these findings correct misinterpretations in the literature, prompting a re-evaluation for Na storage mechanisms and guiding electrolyte selection for practical Na-ion batteries.