The capacity fading mechanism of high-capacity CrO_x (x ≥ 2.5) cathode material for lithium-ion battery

A series of high-capacity chromium oxides (CrO_x, x ≥ 2.5) were synthesized as cathode for lithium-ion batteries (LIBs) by a step calcination method at about 300 °C. The results of cyclic voltammetry (CV) test and impedance analysis indicated that there is a certain relationship between the structure compositions and electrochemical performance. The initial phase transitioned into two subsequent phases during the circulation process and the higher the ratio of tetrahedrons [CrO₄] to octahedrons [CrO₆], the smaller the potential difference between the two phases. The high ratio of Cr₈O₂₁ was examined as a case study, the structural evolution during the charge/discharge process was investigated utilizing ex-situ X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS), and transmission electron microscope (TEM), showing a sequential phase transition to nanoparticles of LiCrO₂, which exhibit a close-to-amorphous structure, and an irreversible crystalline phase of LiCrO₂ that does not contribute to any capacity. The capacity fading is believed due to the unique phase transition behaviors during Li⁺ insertion/extraction. It is worth noting that Cr₈O₂₁ exhibited a high discharge capacity of 320 mAh g⁻¹ (10 mA g⁻¹) with an impressive capacity retention of 88% after 40 cycles. The present study offers valuable insights into comprehending the capacity fading mechanism of a range of chromium oxides, potentially serving as a means to investigate the structural evolution in metal oxides comprising diverse valence states.

A series of high-capacity chromium oxides (CrO_x, x ≥ 2.5) were synthesized as cathode for lithium-ion batteries (LIBs) by a step calcination method at about 300 °C. This series of compounds was found to switch into [CrO₄] and [CrO₆] during the cell cycle. As a case study of Cr₈O₂₁ with high ratio, ex-situ X-ray diffraction, X-ray photoelectron spectroscopy, and transmission electron microscope show that the phases after transition are the nanoparticles LiCrO₂ close to amorphous and the irreversible phase crystal LiCrO₂ without providing any capacity, which is the main cause of the drastic capacity fading in the previous circulation.