During the second sodiation (Figure ), there is a noticeable reduction in the slope capacity region for all HCs, indicating that sodiation in this region is more irreversible in comparison to the plateau region. This irreversibility is attributed to sodium adsorption during the first cycle at highenergy surface defect sites, where strong interactions between sodium and defect sites lead to sodium ion trapping, preventing their subsequent extraction. Filling the high-energy surface defect sites during the first cycle results in only one gradient starting at lower potentials during the second sodiation. Additionally, electrolyte reduction at moderate potentials (~0.75 V vs. Na/Na + ) initiates the formation of a stable SEI during the first cycle, contributing significantly to irreversible capacity. The contributions from the slope and the plateau for the first and second cycles are quantified in Figure . The materials are ranked based on their performance, with HC-1400 exhibiting the highest initial capacity, followed by HC-1100, and then HC-2000. However, HC-2000 demonstrates the highest reversibility, followed by HC-1400, and then HC-1100. This ranking is consistent with HCs synthesized at lower temperatures having higher defect densities and presenting a higher initial irreversible capacity. The performance differences exhibited by HC produced at different pyrolysis temperatures originate in their distinct microstructures and local chemical environments. To gain insight into the local order of the HCs with increasing pyrolysis temperature, X-ray total scattering measurements were performed to determine the pair distribution function (PDF). The PDFs (G(r), where r is the interatomic distance) derived from the Fourier-transformation of the corrected and normalized Xray total scattering data are shown in Figure (c) for the three HCs. Consistent with prior observations, the peak positions closely match those observed from graphene layers. The lack of long-range order in HCs is evident from the damping of the PDF peaks with increasing r. The average coherence length of the graphitic domains is determined from this decay (Table ). Qualitatively, longer-range ordering at higher temperatures is apparent from increased correlations at larger r. Furthermore, the absence of peaks in the PDF corresponding to graphene-sheet interlayer atomic correlations suggests significant turbostratic and positional disorder from one graphene sheet to the next. Further insight into the structural ordering is obtained by fitting structural models to the XPDF profiles. Here, we employ a simple small-box model consisting of a graphite unit cell with an expanded interlayer spacing to simulate the lower density of HC compared to graphite. High r correlations from the graphite are smoothed out by representing the diminishing long-range order with a spherical shape dampening function 𝛾(𝑟, 𝐿 𝑐 ) = 1 - where Lc is the average coherence length and r is the interatomic distance. This dampening function is applied only when r < 𝐿 𝑐 . The lack of interlayer atomic correlations due to disorder between layers is simulated with artificially large interlayer atomic displacement parameters while maintaining the microscopic density at the measured value. The in-plane interatomic distance (a), interlayer distance (d002), Lc, and anisotropic displacement parameters (U11 and U33, corresponding to in-plane and out-of-plane atomic displacements, respectively) are each refined. To maintain symmetry of the graphene layers, the following constraints are imposed: U22 = U11 and U12 = ½ U11. The resulting best-fits are included in Figure (c) with the experimental data and the parameters are reported in Table . Overall, we find good agreement between the measured and simulated G(r) profiles.