Abstract:

Cr₈O₂₁ is considered as a potential cathode material due to its high specific capacity and low cost. However, Cr₈O₂₁ can be only applied in primary batteries due to the large first-cycle irreversible capacity and the poor cycle stability. Currently, Cr₈O₂₁ is usually synthesized under high pressure or normal pressure under oxygen atmosphere. This procedure is dangerous and prone to generate impurity. Herein, the pure-phase Cr₈O₂₁ was prepared by two-step pyrolysis of CrO₃ under air atmosphere. The electrochemical performance of the as-prepared Cr₈O₂₁ cathode was investigated. X-ray photoelectron spectroscopy (XPS) and X-ray diffraction (XRD) were used to investigate the electrochemical reaction mechanism of Cr₈O₂₁. The as-prepared Cr₈O₂₁ delivers a high initial discharge specific capacity of 400.4 mAh/g and a specific energy of 1218 Wh/kg at 0.1 C. The reversible specific capacity is 304.4 mAh/g and a reversible capacity retention of 88.7% is achieved after 100 cycles, exhibiting good electrochemical performance and cycling stability. The crystal structure of Cr₈O₂₁ consists of two [CrO₆] sublattice units and one [CrO₄] sublattice unit, in hich the [CrO₄] is located between the two [CrO₆] sublattice units. XPS tests reveal that the redox reaction of Cr₈O₂₁ involving three-electron transfer between Cr⁶⁺ and Cr³⁺ occurs during charging and discharging process. XRD tests indicate that the sublattice of [CrO₄] in Cr₈O₂₁ transfer to amorphous after the intercalation of Li-ions which corresponding to the first plateau at the initial discharge curve. Furthermore, at the second discharge plateau, the microcrystals of LiCrO₂ are generated. During the subsequent cycling, LiCrO₂ is electrochemically oxidized and reduced reversibly, suggesting the good rechargeable capability of Cr₈O₂₁.

Key words: lithium battery, Cr₈O₂₁, two-step pyrolysis, chromium-based oxides cathode