- Grid-scale energy storage systems are urgently needed to increase the flexibility and rigidity of the grid for modern society and take full advantage of the renewable green energy resources such as solar energy and wind energy. Na-ion batteries (NIBs) and K-ion batteries (KIBs) have been emerged as one of the most promising solutions for grid-scale energy storage systems. Unfortunately, graphite, which is the commercialized anode in LIBs, does not show meaningful capacity in NIBs, and it shows poor cycling performance in KIBs. Non-graphitic carbon materials have been shown promising electrochemical performance in NIBs and KIBs. However, due to the structural complexity of non-graphitic carbon, the structure-property correlations of non-graphitic carbon anodes for Na-ion and K-ion storage are still not well established. Therefore, in this thesis, I focus on understanding the structure-property correlations of Na-ion and K-ion storage in non-graphitic carbon and improving the Na-ion and K-ion storage performance of non-graphitic carbon anodes.
There had been reports regarding the structure-property correlations of hard carbon anodes in NIBs, where discrepancies still exist. In addition, the capacity of hard carbon anodes in NIBs rarely reaches values beyond 300 mAh/g. Herein, in this thesis, we first applied POx doping on hard carbon to tune its structure, which increases its reversible capacity from 283 to 359 mAh/g. We observe the interlayer d-spacing of the turbostratic nanodomains is expanded and the defect concentration of the doped hard carbon is increased. The structural changes of hard carbon lead to enhanced plateau and slope capacity. Our study demonstrates that Na-ion storage in hard carbon heavily depends on carbon local structures, where such structures, despite being disordered, can be tuned toward unusually high capacities.
Even though our above-mentioned results agree well with our early proposed model, the structure-property correlations of Na-ion storage in hard carbon is still not solidified. Furthermore, how defects affect the slope capacity and what types of defects are beneficial for the slope capacity is still not clear. Therefore, in our following work, we synthesized a series of well-controlled heteroatom doped hard carbons, namely, P-, S- and B-doped hard carbon, and non-doped hard carbon where they show consistently low surface area. We then comprehensively characterized these hard carbons’ structural features and electrochemical performance which allows us to reveal the mechanism of Na-ion storage in hard carbon. Our combined experimental studies and first principles calculations reveal that it is the Na-ion-defect binding that corresponds to the slope capacity, while the Na intercalation between graphenic layers is responsible for the low-potential plateau capacity. In addition, our computational results also revealed that too strong binding between Na-ion and defects will lead to irreversibility. The new understanding provides a new set of design principles to optimize hard carbon anode for Na-ion storage.
In a recent work, guided by our proposed design principles, we synthesized a highly defective hard carbon by microwave heating a low-temperature (650˚C) pre-annealed hard carbon. After a brief microwave treatment, i.e., for 6 seconds, the reversible capacity of the hard carbon was increased from 204 to 308 mAh/g. The microwaved carbon retains a high extent of structural defects after microwaving the low-temperature annealed hard carbon. Such a defective structure exhibits a much higher slope capacity than conventional hard carbon with less low-potential plateau capacity which can reduce the safety concerns. The microwave heating of carbon represents a new direction for tuning structures of hard carbon.
The rate capability of hard carbon has long been underestimated in prior studies that used carbon/Na two-electrode half-cells. Through a three-electrode cell setup, we discover that it is the overpotential of the sodium counter electrode that drives the half-cells to the lower cutoff potential prematurely during hard carbon sodiation, particularly at high current rates, which prevents the hard carbon anode from being fully sodiated. Hard carbon demonstrates a much better rate-capability in this three-electrode setup.
In the last part of this thesis, we studied soft carbon as anode for K-ion storage. In this work, we synthesized a series of soft carbons (SCs) by the pyrolysis of 3,4,9,10-perylenetetracarboxylic dianhydride (PTCDA) at different temperatures. By using polyacrylic acid as the binder, SC obtained at 700˚C (SC700) shows the highest capacity of 354 mAh/g which is the highest capacity of non-graphitic carbons reported so far by accounting the potential between 0-2 V. More importantly, SC700 shows a better cycling stability than SCs obtained at higher temperature, where it is still worse than the cycling performance of hard carbons. Via combined experimental and computational studies, we generate mechanistic insights about the structure-property correlations of K-ion storage in soft carbons.