From microstructure to sodium-storage mechanism: Structure-performance insights into hard carbon

doi:10.1016/j.metadv.2026.04.004

Abstract

Abstract:

Hard carbon is the most promising anode material for sodium-ion batteries, yet its complex disordered structure has long obscured the actual sodium-ion storage behaviors. This review focuses on the critical structure-performance correlation that governs electrochemical performance. It examines how key microstructural features—including pseudo-graphitic domains, interlayer spacing, defects, heteroatom doping, and multi-scale pore architecture—collectively determine sloping/plateau capacities, initial Coulombic efficiency, and rate capability. The review highlights recent advances in decoupling these complex relationships through advanced characterization, predictive descriptors, and machine learning. Looking into the future, it is necessary to shift from empirical optimization methods toward quantifiable and predictable design principles for developing high-performance hard carbon anodes.

Key words: Sodium-ion batteries, Hard carbon, Microstructure, Structure-performance correlation, Sodium storage mechanism

Lin Wang, Yue Yu, Zihe Wei, Liang Zhou. From microstructure to sodium-storage mechanism: Structure-performance insights into hard carbon[J]. Metals Advances, 2026, 44: 27-43.

Figures/Tables 7

Fig. 1. Schematic illustration of the unclear black box of structure-performance correlation in hard carbon for SIBs.

Fig. 2. The structural model of hard carbon keeps evolving with the emergence of advanced characterization methods. (a) Franklin first proposed the concept of hard carbon in 1951. Reproduced with permission [25]. Copyright 1951, Royal Society of Chemistry. (b) Ban et al. first proposed the ribbon-like model in 1975. Reproduced with permission [26]. Copyright 1975, WILEY-VCH. (c) Bonijoly et al. first identified turbostratic elements in 1982. Reproduced with permission [27]. Copyright 1982, Elsevier. (d) Townsend et al. first proposed the curved graphitic sheet model in 1992. Reproduced with permission [28]. Copyright 1992, American Physical Society. (e) Harris et al. first identified fullerene-like elements in 1997. Reproduced with permission [29]. Copyright 1997, Taylor & Francis. (f) Stevens and Dahn first proposed the house of cards model in 2000. Reproduced with permission [30]. Copyright 2000, ECS-The Electrochemical Society. (g) Terzyk et al. first proposed the fullerene-like curved graphene model in 2007. Reproduced with permission [31]. Copyright 2007, Royal Society of Chemistry. (h) Chu et al. proposed the topological structural elements of the pores and defects in hard carbon in 2023. Reproduced with permission [32]. Copyright 2023, WILEY-VCH.

Fig. 3. Four classic sodium storage mechanism models. (a) "Intercalation-Filling" [30]. Reproduced with permission [37]. Copyright 2015, American Chemical Society. (b) "Adsorption-Intercalation". Reproduced with permission [36]. Copyright 2012, American Chemical Society. (c) "Multistage Process". Reproduced with permission [37]. Copyright 2015, American Chemical Society. (d) "Adsorption-Filling". Reproduced with permission [38]. Copyright 2016, WILEY-VCH.

Fig. 4. (a) Structural schematic diagram of the microcrystalline graphitic domain in hard carbon. Reproduced with permission [43]. Copyright 2020, WILEY-VCH. (b) Schematic illustration of the intrinsic/extrinsic defects in hard carbon. (c) TEM images and corresponding SAED patterns showing the d002 shrinks as the annealing temperature rises. (d) La and Lc vary with the annealing temperature. Reproduced with permission [51]. Copyright 2023, WILEY-VCH. (e) Differential PDFs obtained by subtracting PDFs taken during an operando experiment from the PDFs obtained at the start of each electrochemical process. Reproduced with permission [53]. Copyright 2021, American Chemical Society. (f) Hard-carbon defects and a schematic of topological defect structures in the carbon matrix promoting charge-carrier transport. Reproduced with permission [54]. Copyright 2024, WILEY-VCH. (g) Heteroatom doping promotes the rate performance of hard carbon. Reproduced with permission [51]. Copyright 2023, WILEY-VCH.

Fig. 5. (a) Schematic illustration of typical pore classifications (pore connectivity and pore architecture) in hard carbon. (b) The evolution of pore structure and carbon matrix during the pyrolysis process of biomass-derived hard carbon [52]. (c) The pore entrance diameter (PED) and the pore body diameter in hard carbon. Reproduced with permission [57]. Copyright 2022, Oxford University Press. (d) Multi-gas adsorption isotherms and corresponding pore size distribution curves. Reproduced with permission [58]. Copyright 2021, Royal Society of Chemistry. (e) SAXS and WAXS characterization of hard carbon. Reproduced with permission [59]. Copyright 2019, Elsevier. (f) The He true density as complementary tool for closed-pore quantification. Reproduced with permission [60]. Copyright 2019, WILEY-VCH. (g) HAADF-STEM electron tomography reconstructs the three-dimensional spatial distribution of closed pores within hard carbon. Reproduced with permission [61]. Copyright 2025, American Chemical Society.

Fig. 6. (a) Conventional view and new coupling view of structure-performance correlation in hard carbon. (b) Unlocking the sodium-storage behavior in hard carbon by TMS using D2O and H2SO4 titration agents to quantitatively analyze the sodium-storage mechanism. (c) The specific capacity contribution of different sodium-storage stages in hard carbon at 0.02 V cutoff (according to the D2O/H2SO4-based TMS results) and corresponding Na+ evolution schematic. Reproduced with permission [70]. Copyright 2024, American Chemical Society. (d) A coupled structure-performance description model: the properties (P) of hard carbon as a superposition of properties (Pi) of local structures. Reproduced with permission [71]. Copyright 2025, Royal Society of Chemistry. (e) A prediction rule for the LPP of hard carbon in SIBs—the substitution index (Δ) links precursor structure to closed pore formation and LPP behavior. Reproduced with permission [72]. Copyright 2025, WILEY-VCH. (f) Machine learning-assisted thermomechanical coupling fabrication of hard carbon for SIBs. Reproduced with permission [73]. Copyright 2024, Elsevier.

Fig. 7. Future research directions for decoupling the structure-performance correlation of hard carbon for SIBs.

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