Structural Evolution And Kinetics Of Nickel-Rich Cathode Materials During Charge/Discharge Processes

The rapid expansion of electric vehicles demands higher energy density and safety,longer cycle life and lower cost of power batteries.Cathode materials play a key role for the battery performance.Compared to traditional cathode materials such as LiCoO2,LiFePO4 and LiMn2O4,Nickel-rich cathode materials LiNixCoyMnzO2(x?>0.6)have been arising people's attention due to its higher energy density,of which the commercialization have been carrying on.To further increase the energy density,we need increase either the Nickel content or the cut-off voltage.However,both would cause unstable structure change and seriously impact the capacity retention and safety.The key to solving performance degradation is to figure out the structure evolution during the whole delithiation process.Additionally,understanding the kinetics during lithiation/delithiation also helps to improve the rate performance,which is important for fast charge and discharge.Herein,we studied two representative Nickel-rich materials LiNi0.6Co0.2Mn0.2O2(NCM622)and LiNi0.8Co0.1Mn0.1O2(NCM811).Solid State NMR and in situ XRD were combined to investigate the long range and local structure during charge/discharge.Meanwhile,in combination with the bulk structure change and observation of the interface by TEM,we give a reasonable explanation for the dynamic results measured by GITT and EIS.Although NCM622 and NCM811 behave different degradation rate at the same cut-off voltage,the degradation rate converge when extracting the same amount of lithium.Hence it is the amount of extracted lithium instead of the Nickel content that accounts for the performance degradation.During charge,NCM undergoes H1-H2-H3 phase transformation.When the specific capacity is higher than 220 mAh/g,which means more than 80%lithium is extracted,the formation of H3 phase will cause a sharp contraction of cell volume.The volume change is more than 7%when charged to 4.7V.Crack of the particles as well as the severe side reactions happens after long cycle.Therefrom,H3 phase is the main reason for the capacity degradation of NCM at high voltage.We also give an explanation for the H1/H2/H3 phase transformation by means of ss-NMR and in situ XRD for the first time.The pristine structure of NCM is H1 phase.During charge process,the migration rate is different for lithium located in different environment.The lithium surrounding Mn4+ extract priorly to form H2 phase.While at the end of charge when H3 is formed,Li+ would stay away from Ni4+ to form H3-1 phase.Finally,when all the transition metal ions are quadruple valence,the residual lithium randomly rearrange to form H3-2 phase.The lithium diffusion coefficient measured by GITT is 10-8?10-9 cm2·s-1 and 10-7?10-11 cm2·s-1 for NCM811 during the first charge and discharge respectively,which means the different process for charge/discharge.But the lithium diffusion coefficient measured by EIS is 10-8?10-12 cm2·s-1.This value is in relation with the lithium content despite charging or discharging.The reason for the different result of these two methodes is that the result measured by GITT includes the influence of concentration polarization.The value of lithium diffusion coefficient is influenced by the number of lithium vacancy and the thickness of Li layer space.During the early charge and the end of discharge,the diffusion rate shows a minimum because diffusion of lithium is hindered due to the narrow Li layer space and lack of lithium vacancy.At the end of charge and early discharge,the lithium vacancy is abundant while the Li layer space decrease abruptly.Hence the lithium diffusion coefficient is decreased.For the GITT result,there is a slight difference between charge and discharge due to the concentration polarization.When the material is at stable H2 phase.The lithium migration between octahedral sites is more influenced by phase transformation and layer spacing.Larger laver spacing is benefit for fast lithium diffusion.The trend of film resistance and charge transfer resistance are also in consistent with lithium diffusion rate.The passivation layer,loose CEI layer and poor contact between particles when it charged to high voltage will hinder the charge transfer process,leading to the increase of film resistance and charge transfer resistance.This research revealed the bulk structure and interfacial evolution as well as the kinetics of Nickel-rich cathode materials.We think this work is beneficial for further understanding the capacity and rate performance degradation mechanism and also provides theoretical basis for further modification.