Preparation And Electrochemical Performance Studies Of Chromium Oxide As Cathode Material For Lithium Batteries

The multi-electron transfer high-valent transition metal chromium oxides CrOx(Cr₈O₂₁ and Cr₂O₅)have a high theoretical mass ratio capacity?over 580 mAh/g?and are two times of the ternary high nickel materials(LiNi_0.8Mn_0.1Co_0.1O₂).It also has a high operating voltage?3.0 V vs.lithium?and is a very promising cathode material for lithium batteries that combines high specific capacity and high voltage.However,the preparation process of Cr₈O₂₁ and Cr₂O₅ is not mature,and the electrochemical performance?discharge specific capacity and cycle performance?of chromium oxide is greatly affected by the preparation conditions.Secondly,the reason for the first irreversible capacity loss and the discharge mechanism of chromium oxide as the positive active material of lithium battery are not clear.Aiming at these two problems,the effects of pyrolysis temperature and heating rate on the electrochemical performance of chromium oxide were studied,and the preparation parameters of chromium oxide were optimized.Furthermore,the ex-situ XRD and XPS were used to study the structure evolution and chromium valence state during charging and discharging,and revealed the mechanism of chromium oxide charging and discharging.Through the thermogravimetric test of chromium trioxide,it was found that there are two stable intermediate phases in the process of thermal decomposition of CrO₃,namely Cr₈O₂₁ and Cr₂O₅.This provides a theoretical basis for the experimental preparation of high-valence chromium oxide.In order to optimize the preparation conditions of transition metal chromium oxide,CrO₃ was calcined at 325?,350? and 375? for 2h,respectively,and the heating rates were 3?/min and 5?/min,respectively.The electrochemical properties of the obtained products Cr₈O₂₁ and Cr₂O₅ were tested.The results show that the first discharge mass specific capacity of the chromium oxide material decreases with increasing temperature and heating rate.The first constant current?10mA/g?discharge specific capacity of the product obtained by pyrolysis CrO₃ at 3?/min at 325? for 2 h was the highest,reaching 369.3 mAh/g,and the average discharge voltage exceeded 3.0 V.However,the capacity retention of the CrOx/Li battery increases as the pyrolysis temperature and the heating rate increase.After 40 cycles,the capacity retention rate of the product obtained by pyrolysis CrO₃ at 5?/min at 375? for 2 h was the highest,reaching 84.70%.The rest capacity is 190 mAh/g.The reason for this phenomenon is that at low temperatures,chromium oxide remains in the chromium oxide crystal lattice,and at high temperatures,chromium trioxide is completely converted into a product.The first discharge specific capacity of Li-CrOx batteries is up to 365 mAh/g,but the second only is 290 mAh/g.By ex-situ XRD analysis,after the first discharge,the phase of chromium oxide completely disappeared,replaced by the phase of LiCrO₂ and the phase of Li_xCrO_?3-x?/2.The Li_xCrO_?3-x?/2 phase remained stable during the subsequent cycle,and the?104?peak of LiCrO₂ showed a significant intensity change during charge and discharge:the?104?peak of lithium chromate was significantly weakened after the first charge.After recharging,the?104?peak is enhanced.It represents that the content of LiCrO₂ changes continuously with charge and discharge.Therefore,the first irreversible capacity loss of the lithium chromium oxide battery is due to an irreversible phase transition,resulting in a decrease in capacity.In the subsequent cycles,LiCrO₂ undergoes reversible extraction and intercalation of lithium.