Modifying Polymer-Based Hard Carbons for Enhanced Understanding of Sodium Storage Properties

Numerous theories on the storage of sodium in hard carbons can be found in the literature. In particular, understanding the contributions of (heteroatom) doping, porosity and graphitic layers is of great importance. Much of the current literature on hard carbons uses different biomaterials as precursors in order to improve the sustainability of the synthesis. However, large differences in the composition of the starting materials and the presence of foreign atoms make a systematic investigation of the storage process extremely difficult. Therefore, a hard carbon material based on furan resins was used in this work to obtain a well-defined starting material that can be modified systematically. This allows a detailed investigation of the effects caused by those modifications. Modifications can be achieved by directly adding various template materials to the polymerization process. Both doping with heteroatoms by adding template chemicals such as boric acid and the introduction of graphitic layers by adding high-purity graphite during polymerization were investigated. Addition of boron and phosphorus heteroatoms led to an increase in reversible capacity up to 240 mAh g-1 from 150 mAh g-1 for carbonization temperatures as low as 650°C. Formation of a potential plateau was observed for boron doping, which is usually found at higher carbonization temperature surpassing 1000°C. Utilizing Raman and XRD spectroscopy, crystallite size was found to be decreasing with phosphorus doping, while boron doping led to an increase. This increase could be a possible explanation for the observed capacity plateau. The addition of high-purity carbon samples, such as KS6L graphite or graphene, allowed for a targeted introduction of single- and multi-layer graphitic domains without alteration of additional properties. Increasing graphite content up to 5 wt-% during polymerization resulted in enhanced electrochemical characteristics, including increased sodium intercalation into the lattice and shift of the sloping proportion to lower potentials. Further increase up to 10 wt-% led to a vanishing of the plateau potential and a lowering of the reversible capacity. The results indicate that the interlayer spacings has now decreased below a threshold, which prohibits the sodium from intercalation between the graphitic layers. Additionally, doping Hard Carbons with graphene revealed a capacity decrease at content ratios as low as 2 wt-%, emphasizing the significance of both the stacking and interlayer spacing of the graphitic lattice, as well as the chemical composition, for efficient sodium storage. In summary the results presented provide an enhanced understanding into the sodium storage properties of Hard Carbons, which help improve the design of synthesis processes for more cost-efficient and high-performance carbon materials. Figure 1