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As the leading anode material for sodium-ion batteries (SIBs), hard carbon (HC) still faces the puzzle of low initial Coulombic efficiency (ICE) in achieving commercialization. From the perspective of precursors, the low ICE has been attributed to the large specific surface area and porosity produced by the rapid decomposition of polymers during the carbonization. Therefore, increasing the cross-linking degree of precursors will be an effective shortcut to improve the ICE. Herein, a facile pre-oxidation tactic was successfully employed to tailor the cross-linking degree of phenolic resin precursors to precisely control the specific surface area of the obtained HC. As the pre-oxidation time is increased, the optimal HC with the lowest specific surface area shows an ICE elevated by 22.2% (from 62.5 to 84.7%) compared to the original pre-oxidation HC and delivers a high reversible capacity of 334.3 mAh g–1 at 20 mA g–1. Besides, the pre-oxidation also introduces abundant carbonyl groups, which increase the disorder degree of HC and supply abundant adsorption sites of Na+, thus enhancing the rate performance. When matched with a layered O3-NaNi1/3Fe1/3Mn1/3O2 cathode, the full cell achieves an energy density of ca. 256.2 Wh kg–1 with superior rate performance. This work sheds light on the positive effect of pre-oxidation in elevating the ICE of HC and provides effective guidance to achieve a high ICE for other HC materials.

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.

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