We prepared hard carbon in a sheet-like structure as a high-density anode for sodium-ion batteries.

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Sodium-ion batteries are regarded as one of the most promising energy storage systems, but the choice of anode material is still facing great challenges. Biomass carbon materials were explored for their low cost and wide range of sources. Here, a hard carbon material with a "honeycomb" structure using pine pollen (PP) as a precursor was successfully prepared and applied as an anode. The initial discharge capacity can reach 370 mA h g −1 at a current density of 0.1 A g −1 . After cycling 200 times, the reversible capacity also stabled at 203.3 mA h g −1 with the retention rate of 98%. We further studied the sodium storage mechanism by different methods, especially the Na + diffusivity coefficient (D Na + ) calculated by galvanostatic intermittent titration technique, which was more accurate. Interestingly, the trend of D Na + coincides with cyclic voltammetry curves. Carbonized PP exhibited excellent electrochemical properties because of its three-dimensional structure and larger layer spacing (∼0.41 nm), which reduces the resistance of sodium ions to intercalation and deintercalation.

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Recycling of spent lithium-ion batteries is extremely urgent with their increasing decommission. In this work, eutectic molten salts of LiOH–Li2CO3 used as lithium sources for direct regeneration of LiNi0.5Co0.2Mn0.3O2 were developed. Based on the phase diagram of LiOH and Li2CO3, the effects of different lithium sources on material regeneration have been investigated. The cathode materials regenerated with eutectic molten salts have high capacity, good cycling performance, and rate performance. The discharge capacities during the 1st and 200th cycles at 1 C are 146.3 and 130.3 mA h g–1, respectively, and the capacity retention rate reaches 89.06%. Using the combined X-ray diffraction (XRD), high-resolution transmission electron microscopy (HRTEM), and X-ray photoelectron spectroscopy (XPS) analysis, the original layered structure of spent cathode materials was restored. Therefore, the eutectic molten salt of LiOH–Li2CO3 is feasible for direct regeneration of spent cathode materials.