Design of High-Performance Defective Graphite-Type Anodes for Sodium-Ion Batteries

Intercalation of graphite with alkali metal ions, such as Li+, K+, Rb+, and Cs+ ions, has long been reported over past few decades. One exceptional candidate is Na+ ions in which they have very small solubility in graphite in the form of C70Nax (x ≤ 1), attributed to the positive formation energies. In this paper, we systematical design a series of high-defect graphite-type anodes through the ball-milling process. To our surprise, the electrochemically Na+ storage of the highly defective graphite was found to have a 3-fold increment, comparing to the pristine graphite. The hexagonal space group P63mmc was progressively converted into the rhombohedral space group R3̅m, attributed to the mechanical gliding during the ball-milling process. In additional large amount of atomic defects were found in two positions in the P63mmc space group. The average grain sizes of the particles would decrease with increasing the grinding time. The defective graphite samples disintegrated into the nanoscale flakes with enlarged interlayer spacings which could further promote the secondary formation of defects in the crystal structure. Some of the regions had highly distorted lattices in defective graphite. All these positive changes lead to a dramatic increase in the adsorption and intercalation of Na+ ions into the graphite host. The capacity of the high defect graphite was found to be 128.7 mAh g–1 at 0.1 A g–1, with a superior capacity retention over 8000 cycles at 5 A g–1. This work sheds some light on the design prospective of synthesizing defective graphite-type anodes for sodium-ion batteries.