urgently demanded. Lithium ion batteries (LIBs) have been widely utilized in portable electronic devices due to their large energy densities, high efficiency, and light weight; [1] however, uneven distribution and insufficient lithium resource on earth make LIBs not sufficiently affordable in future energy storage systems, especially in the large-scale energy storage scenarios. As a promising alternative, sodium ion batteries (SIBs) have recently gained considerable interests owing to the abundant sodium resource and the similar rockingchair electrochemistry mechanism to LIBs. [2] However, the differences in size and ionization environment between the Na and lithium atoms have brought major challenges in the development of SIBs, among which the failure of commercial graphite anode to insert Na + is the most critical one due to the larger ionic radius of Na atom. [3] To this end, extensive explorations have been carried out to screen promising anode candidates for SIBs which include titanium-based oxides, [4,5] transition metal borates, [6] metal sulfides, [7,8] carbon materials [9][10][11] as well as their composite structures. [12,13] Among these, carbonaceous materials have attracted most attentions because of their cost-effective preparations, highly tunable physicochemical structures, and negligible environmental harms. [14] To reveal the structure-performance relationships, various Na + storage mechanisms in carbonaceous anodes have been proposed, which can be summarized as the individual or synergetic improvements on multiple sodiation processes including insertion, adsorption, and ultramicropore filling. [15][16][17][18] Correspondingly, engineering the multiscale nanostructures in carbon materials has been the research focus which can be refined as follows: i) expanding interlayer spacing between graphene layers to allow for large-radius Na + insertion (for instance, hard carbon, [19] and expanded graphite [20] ); ii) optimizing pore network to facilitate Na + transportation or storage (for example, hard carbon with closed pore [21,22] and ultramicroporous carbon [9,23] ); iii) introducing heteroatoms (e.g., S, P, B, N, F-doped [11,[24][25][26][27] or codoped carbons [28][29][30] ) or defects (e.g., defect-rich soft carbon [31] ) into carbon lattice to induce capacitive adsorption or reactions; iv) constructing favorable 3DOxygen-containing groups in carbon materials have been shown to affect the carbon anode performance of sodium ion batteries; however, precise identification of the correlation between specific oxygen specie and Na + storage behavior still remains challenging as various oxygen groups coexist in the carbon framework. Herein, a postengineering method via a mechanochemistry process is developed to achieve accurate doping of (20.12 at%) carboxyl groups in a carbon framework. The constructed carbon anode delivers all-round improvements in Na + storage properties in terms of a large reversible capacity (382 mAg −1 at 30 mA g −1 ), an excellent rate capability (153 mAg −1 at 2 A g −1 ) as well ...
