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 ...

To circumvent the imbalances of electrochemical kinetics and capacity between Li storage anodes and capacitive cathodes for lithium-ion capacitors (LICs), we herein demonstrate an efficient solution by boosting the capacitive charge-storage contributions of carbon electrodes to construct a high-performance LIC. Such a strategy is achieved by the in situ and high-level doping of nitrogen atoms into carbon nanospheres (ANCS), which increases the carbon defects and active sites, inducing more rapidly capacitive charge-storage contributions for both Li storage anodes and PF storage cathodes. High-level nitrogen-doping-induced capacitive enhancement is successfully evidenced by the construction of a symmetric supercapacitor using commercial organic electrolytes. Coupling a pre-lithiated ANCS anode with a fresh ANCS cathode enables a full-carbon LIC with a high operating voltage of 4.5 V and high energy and power densities thereof. The assembled LIC device delivers high energy densities of 206.7 and 115.4 Wh kg at power densities of 0.225 and 22.5 kW kg, respectively, as well as an unprecedented high-power cycling stability with only 0.0013% capacitance decay per cycle within 10 000 cycles at a high power output of 9 kW kg.

Supercapacitors represent an important energy storage system for high power applications with carbon electrodes acting as the key component. Design and synthesis of advanced carbon materials with supercapacitive activity (capacitance and rate capability) and stability have been the focus of supercapacitor developments. This study reports a high-performance carbon electrode by in situ doping boron atoms into nanoporous carbon particles, which is achieved by a continuous spraying assisted coassembly process. Boron doping leads to extra oxygen graft into carbon surface, enabling both enhanced wettability and durability for the fabricated carbon electrodes. Density functional theory calculations further suggest that boron doping enhances electrolyte ion penetration and interactions with carbon surface, leading to the improved capacitances and rate capability.The constructed symmetric aqueous supercapacitor exhibits all-round performance improvements including high energy density (9.1 Wh kg −1 ) and power density (24.1 kW kg −1 ) as well as ultralong cycling life (100 000 cycles). This work for the first time provides insights into the role of boron doping in enhancing both supercapacitive activity and stability of carbon materials for high-performance supercapacitors.

Oxygen-containing groups in carbon materials have been demonstrated to be effective in the anodic sodium-ion storage process; however, the effect of specific oxygen-containing groups on the sodium-ion storage in the carbon framework remains to be explored. Based on a mechanochemical process (exemplified by ball milling in the presence of dry ice), a selectively modified cellulose-derived hard carbon (BHC-CO2) with a high oxygen content of 19.33 at. % and carboxyl-dominant groups was prepared in this work. The fabricated BHC-CO2 anode exhibits excellent electrochemical performance with a high reversible capacity of 293.5 mA h g–1 at a current density of 0.05 A g–1, two times as high as that of the oxygen-deficient BHC-CO2-H2 anode, demonstrating the significant role of oxygen-containing groups in enhancing the Na+ storage. Moreover, the BHC-CO2 anode has an excellent high-rate cycling stability with a specific capacity of 80.0 mA h g–1 even after 2000 cycles at 1 A g–1. Qualitative analyses of capacitive effect combined with density functional theory calculations further reveal that carboxyl groups introduced by the mechanochemical process facilitate Na+ adsorption on the carbon surface, enhancing the capacitive Na+ storage process and thus greatly improving the capacity. This work demonstrates the role of carboxyl on Na+ storage by carbonaceous materials and provides theoretical guidance for the oxygen functional group modification of carbon materials to enhance the sodium-ion storage.

No abstract