Effect Of Charge System On The Electrochemical Properties Of Lithium - Rich Lithium - Based Cathode Materials

The lithium-rich manganese material has high discharge capacity, which has received much attention. However, this material is limited by its high initial irreversible capacity and poor rate capability. That’s because the cut-off voltage of the lithium-rich manganese is higher than that of other cathode materials of lithium-ion batteries. A high voltage facilitates the side reaction between electrolyte and cathode material. The SEI film is a passive coating on the surface of cathode material, which is also a good conductor of Li-ion. It prevent the further reaction between electrolyte and cathode material. Therefore, controlling the stability of SEI will improve the electrochemical performance of cathode materials.In this paper, the Li1.13Ni0.27Mn0.54Al0.06O2 cathode material was synthesized by solid state reaction at the temperature of 900℃, using NiSO4·6H2O、MnSO4·H2O and Al2(SO4)3·18H2O as starting materials. The relationship between pre-cycling treatment and electrochemical performance was studied. Studies have shown that the initial irreversible capacity is made up of formation of SEI film, activation of Li2MnO3 and the decomposition of electrolyte. When the cut-off voltage of pre-cycling is between 4.3～4.6 V, the initial irreversible capacity of the material is among 106.9-116.07 mAhg-1, which is 31% less than that of un-precycled one (119.3 mAh·g-1). The stepwise pre-cycling treatment can effectively reduce the initial irreversible capacity. It can also improve the capacity retention ability and rate properties. The irreversible capacity in the voltage range of 4.8-2.0 V could be reduced from 155.1 to 64 mAh·g-1 after stepwise pre-cycling treatment. Its capacity retention rate is 94.4%. And it still has 81.5 mAh·g-1 at 10C.Analysis by the electrochemical impedance spectra (EIS) indicate the impedance of SEI is reduced and the electrochemical performance of lithium-rich manganese material is improved. Data showed that a low impedance SEI film was formed when the voltage of pre-cycling is between 4.3 and 4.6 V. At the same time, the impedance of the SEI did not increase much after 50 cycles. A stable SEI film on the surface of cathode material obtained in pre-cycling treatment improves the capacity retention ability because, this SEI film can effectively prevent the side effect between electrolyte and cathode material as the cycling voltage is higher than 4.5 V.Transmission electron microscopy (TEM and STEM) were applied to analyze the microstructure of the materials. SEI film on the surface of cathode material were observed, the thickness of which is less than 10 nm. Fine traces were also found on the surface of cycled material under TEM and STEM. This may be the mark of the crystal structure transformation of this material during cycling.