Revisiting Electrolyte Kinetics Differences in Sodium Ion Battery: Are Esters Really Inferior to Ethers?

The ether electrolytes usually outperform ester electrolytes by evaluating sodium-ion batteries (SIBs) rate performance, which is a near-unanimous conclusion of previous studies based on an essential configuration of the half-cell test. However, here we find that contrary to consensus, the ester electrolyte shows better Na storage capability than the ether electrolyte in full cells. An in-depth analysis of three-electrode, symmetric cell, and in situ XRD tests indicates that traditional half-cell test results are unreliable due to interference from Na electrodes. In particular, Na electrodes show a huge stability difference in ester and ether electrolytes, and ester electrolytes suffer more severe interference than ether electrolytes, resulting in the belief that esters are far inferior to ether electrolytes. More seriously, the more accurate three-electrode test would also suffer from Na electrode interference. Thus, a "corrected half-cell test" protocol is developed to shield the Na electrode interference, revealing the very close super rate capability of hard carbon in ester and ether electrolytes. This work breaks the inherent perception that the kinetic properties of ester electrolytes are inferior to ethers in sodium-ion batteries, reveals the pitfalls of half-cell tests, and proposes a new test protocol for reliable results, greatly accelerating the commercialization of sodium-ion batteries.