Research On The Attenuation Mechanism And Suppression Strategies Of High Nickel Ternary Cathode Materials

The high nickel ternary material LiNi_0.8Co_0.1Mn_0.1O₂has the advantages of high theoretical specific capacity(273 m Ah g^-1),high and stable charging and discharging platform,etc.has an important position in lithium-ion batteries.However,ternary materials still have some problems,such as lithium-nickel mixing,gas production,phase change reaction,microcracks and transition metal dissolution,etc.,which cause the degradation of material performance during the battery cycle,which limits its use in liquid and solid lithium-ion batteries.At present,the attenuation mechanism of high nickel ternary cathode materials is still unclear,and different cut-off voltages also have a greater impact on the attenuation history of electrode materials.In this paper,research on the attenuation mechanism and suppression strategy of NCM811 ternary cathode material is carried out to address the above problems.details as follows:The electrochemical performance and morphological changes of single crystal LiNi_0.8Co_0.1Mn_0.1O₂（SC811）and polycrystalline LiNi_0.8Co_0.1Mn_0.1O₂（PC811）at 4.3,4.5and 4.7 V cut off voltages were compared in liquid batteries.The capacity retention rate of SC811 at 4.3,4.5 and 4.7 V cut-off voltages is higher than that of PC811.The study found that the crystal structure of the material of SC811 remained basically intact after the cycles,and only part of the particle crystal structure produced slip.This is because the movement of the plane is driven during the insertion and extraction of lithium ions,and with the cut-off voltage Rise,this slippage becomes severe.But the PC811particles are broken during the cycles,due to the accumulation of single primary particle stress during the discharge process,which causes the electrolyte to penetrate,and the side reaction is aggravated,which causes the internal resistance to increase.Moreover,during the cycle,PC811 has a more serious irreversible phase change in its crystal structure than SC811.This phase change becomes more serious as the cut-off voltage increases.These reasons cause the cycle performance of PC811 to be lower than SC811.In order to verify whether SC811 has better cycle performance than PC811 in solid electrolyte,a polyethylene oxide（PEO）-based polymer electrolyte with[EO/Li⁺]=18 was prepared.SC811 and PC811 were tested at 4.0,4.1,4.2 and 4.3 V cut-off voltages.At 0.3C current density,the specific discharge capacity of SC811 after 100 cycles at 4.0,4.1,4.2and 4.3 V cut-off voltages is 113.2,114.9,116 and 93.8 m Ah g^-1,while the specific discharge capacity of PC811 is only 91,80,66.5 and 40 m Ah g^-1,all lower than SC811.In order to improve the performance of the solid-state battery,Li₂S was compounded into the PEO matrix to prepare a PEO/Li TFSI-Li₂S composite polymer electrolyte.The addition of Li₂S improves the ionic conductivity,mechanical properties and interface stability of the lithium metal negative electrode of the composite electrolyte.During the cycle,the addition of Li₂S makes the surface of SC811 particles form a more stable CEI film,which makes it have better interface stability with the PEO-based polymer.