Highly Defective Hard Carbon Anode for Na-Ion Batteries By Microwave Heating

Hard carbon has been considered as one of the most promising anode materials for Na-ion batteries (NIBs). As low cost is one of the major advantages of NIBs over Li-ion batteries (LIBs), the relative high cost of hard carbon has been the bottleneck for the commercialization of NIBs. In addition, a large portion of the capacity for conventional hard carbon is contributed from the low-voltage plateau part, which increases the risk of dendrite formation, thus causing a safety concern. Guided by the mechanism of Na-ion storage in hard carbon we proposed, where the sloping capacity is correlated to the structural defects, we design a new hard carbon by a microwave treatment on a low-temperature annealed hard carbon. The microwave treatment enhances its conductivity by forming a heterogeneous carbon structure, where there are highly ordered large nanodomains formed under microwave as conducting conduits and importantly during the short microwave treatment, i.e ., 6 seconds, the carbon structure still retains its high extent of structural defects, as demonstrated by neutron scattering and pair distribution function results. Such a structure exhibits short plateau Na-ion storage but high sloping capacity, where surprisingly the Na-ion diffusivity in the microwave-treated carbon is higher than conventional hard carbon formed by high temperature annealing. Notably, the microwave treatment is much energy efficient in preparation of hard carbon than conventional annealing. This work supports our prior proposed mechanism and points out a new direction for hard carbon preparation as NIB anodes.