Fig. 1

(a) Multiple sodium storage mechanisms of hard carbon[11]; (b) Sodium insertion into carbon nanosheets and subsequent pore filling[12]; (c) Typical Potential and Capacity Curve of Hard Carbon in Sodium Half-cell[13]; (d) Schematic illustration of the Na-ion storage in hard carbons according to the “adsorption-intercalation” mechanism[14]; (e) Typical potential vs. capacity profile for hard carbons in sodium cells with the associated parameters of interest, such as the irreversible capacity, IR drop, and Na plating[12]"

Fig. 2

Main structural characteristics of hard carbon in SIBs [15]"

Fig. 3

(a) Structure of graphitic crystallites[18]; (b) Schematics of hard carbon structures obtained at 700 and 2000 ℃ and their structural parameters[19]; (c) Theoretical energy cost for Na and Li ions insertion into carbon as a function of carbon interlayer distance[16]; (d) Schematic illustration of the evolution of the microstructure, sodium storage mechanism and behavior with the pyrolysis temperature of HCs[17]"

Fig. 4

(a) DFT calculation of chemical adsorption energy of sodium atoms on hard carbon with different structures [24]; (b) DFT calculation of intrinsic defect adsorption energy of sodium atoms[20]; (c) Hard carbon materials prepared by introducing defects rich in pyridine nitrogen during the heat treatment process by adding a nitrogen source[21]; (d) Hard carbon materials prepared by doping multiple elements [22]; (e) Synthesis of carboxyl rich hard carbon by ball milling method [23]"

Fig. 5

Normalized HC pore size distributions (continuous line) of all HCs at different filling degree along the plateau capacity[28]"

Fig. 6

(a) Hard-carbon formation scheme as a function of different temperatures[34]; (b) TEM images, specific surface area, and ICE of HC materials derived from bamboo powder at different temperatures[33]"

Fig. 7

(a) Introducing small molecules to repair defects[38]; (b) Soaking Aluminium chloride inhibits electrolyte decomposition[39]; (c) Schematic diagram of the route for reducing hard carbon through hydrogen gas[48]"

Fig. 8

(a) The behavior of SEI in different pores on the electrolyte accessible surface of the anode[49]; (b) Schematic diagram of commercial carbon molecular sieve[56], Schematic diagram of the formation mechanism of solid electrolyte interface between (c) typical porous carbon and (d) screened carbon after 5 cycles [32]; (e) The insertion mechanism of sodium in topological carbon[50]"

Fig. 9

(a) Mn2+ regulates the carbon layer structure of hard carbon materials; (b) The reversible capacity and ICE change of hard carbon modified by manganese chloride of different concentrations; (c) The reversible capacity and ICE change of hard carbon modified by various metal chloride salts at the optimal concentration; (d) The reversible capacity and ICE of manganese chloride modified hard carbon at different carbonization temperatures with optimal concentration[53]; (e) Schematic illustration of the synthesis for Zn-HC; (f) HRTEM images of Zn-HC and HC electrodes[6]"

Fig. 10

SEI comparison of (a) ether-; (b) ester-based electrolytes[55]"

Fig. 11

(a) Schematic diagram of pre-sodiation[60]; (b) Schematic illustration of the direct-contact method [61]; (c) Pre-soaking sodium powder to improve ICE[66]"