Mechanical Activation of Graphite for Na-Ion Battery Anodes: Unexpected Reversible Reaction on Solid Electrolyte Interphase via X-Ray Analysis.

Although sodium-ion batteries (SIBs) offer promising low-cost alternatives to lithium-ion batteries (LIBs), several challenges need to be overcome for their widespread adoption. A primary concern is the optimization of carbon anodes. Graphite, vital to the commercial viability of LIBs, has a limited capacity for sodium ions. Numerous alternatives to graphite are explored, particularly focusing on disordered carbons, including hard carbon. However, compared with graphite, most of these materials underperform in LIBs. Furthermore, the reaction mechanism between carbon and sodium ions remains ambiguous owing to the structural diversity of disordered carbon. A straightforward mechanical approach is introduced to enhance the sodium ion storage capacity of graphite, supported by comprehensive analytical techniques. Mechanically activated graphite delivers a notable reversible capacity of 290.5 mAh·g-1 at a current density of 10 mA·g-1. Moreover, it maintains a capacity of 157.7 mAh·g-1 even at a current density of 1 A·g-1, benefiting from the defect-rich structure achieved by mechanical activation. Soft X-ray analysis revealed that this defect-rich carbon employs a sodium-ion storage mechanism distinct from that of hard carbon. This leads to an unexpected reversible reaction on the solid electrolyte surface. These insights pave the way for innovative design approaches for carbon electrodes in SIB anodes.

Polythiophene-Wrapped Olivine NaFePO4 as a Cathode for Na-Ion Batteries.

The surface of olivine NaFePO4 was modified with polythiophene (PTh) to develop a high-performance cathode material for use in Na-ion batteries. The Rietveld refinement results of the prepared material reveal that PTh-coated NaFePO4 belongs to a space group of Pnma with lattice parameters of a = 10.40656 Å, b = 6.22821 Å, and c = 4.94971 Å. Uncoated NaFePO4 delivers a discharge capacity of 108 mAh g(-1) at a current density of 10 mA g(-1) within a voltage range of 2.2-4.0 V. Conversely, the PTh-coated NaFePO4 electrode exhibits significantly improved electrochemical performance, where it exhibits a discharge capacity of 142 mAh g(-1) and a stable cycle life over 100 cycles, with a capacity retention of 94%. The NaFePO4/PTh electrode also exhibits satisfactory performance at high current densities, and reversible capacities of 70 mAh g(-1) at 150 mA g(-1) and 42 mAh g(-1) at 300 mA g(-1) are obtained compared with negligible capacities without coating. The related electrochemical reaction mechanism has been investigated using in situ X-ray absorption spectroscopy (XAS), which revealed a systematic change of Fe valence and reversible contraction/expansion of Fe-O octahedra upon desodiation/sodiation. The ex situ X-ray diffraction (XRD) results suggest that the deintercalation in NaFePO4/PTh electrodes proceeds through a stable intermediate phase and the lattice parameters show a reversible contraction/expansion of unit cell during cycling.