Bulk Doping And Interface Engineering Of P2-type Layered Cathode Materials For Sodium-Ion Batteries

Nowadays,lithium-ion batteries(LIBs)have been commercialized and widely used as power sources of automobiles and portable electronic devices,but the limitation of natural lithium source makes them unsuitable for large-scale energy storage system.In this scenario,sodium-ion batteries(SIBs)have attracted ever-increasing attention in view of similar rocking-chair working mechanism as lithium-ion batteries,intrinsic advantages of sodium source including large reserves and no geographical global restriction.The P2-type layered cathode material of sodium ion battery has the characteristics of high theoretical capacity,easy preparation and environmental friendliness.However,this type of material will undergo phase transitions during the charge and discharge process,which contributes to multiple charging or discharging platforms corresponding to Na+/vacancy ordering and the phase transitions from P2 to O2.Na+/vacancy ordering will affect the rate of insertion and extraction.Moreover,the phase transitions from P2 to O2 will cause irreversible changes in the structure of the material.Thus,the cycle life and rate capabilities are greatly affected.Therefore,this thesis focuses on improving the electrochemical performance of layered cathode materials for sodium ion batteries from the perspective of bulk structure doping and interface engineering.First,inert metal doping is used to improve the crystal structure change of the material during charging and discharging from the bulk phase and enhance the stability of the crystal structure.In addition,the in-situ film formation of electrolyte additives improves the stability of the interface between the material and the electrolyte and achieves better electrochemical cycle performance of the material,which is characterized by various in-situ technical means.The following are the research contents of this thesis:1.The bulk structure of the material was modified by lithium doping.The influence of lithium substitution on the electrochemical performance of coppercontaining Na0.6Mn0.8Cu0.2O2 composites is investigated.It is found by XRD that the composite materials with lithium content of 0.15 and 0.2 will form a Na-P2/Li-O3 intergrowth phase structure.Moreover,excellent cycling stability is achieved when lithim content is 0.15.The capacity retention will reach 82.8%over 200 cycles,which is pronouncedly higher than that of bulk NCM.The rate capabilities are also enhanced after Li incorporation.In-situ EIS characterization and in-situ XRD research results show that lithium doping can reduce the electrode/electrolyte interface resistance when Na ions are intercalated and extracted.Furthermore,unexpected polarization and crystal structure changes are also alleviated,keeping the structure of the material stable during charging and discharging.2.Li doping can effectively alleviate the phase transition of the material and improve the electrochemical cycling performance.However,its capacity has been greatly reduced at a small rate,which is not conducive to the use of the material in the actual storage process.In order to improve the capacity of the material,Ni element was introduced into the material system considering its high redox activity.The materials with different ratios of Cu/Ni elements were synthesized and studied.It is found that the addition of Ni increases the specific capacity of the material but reduces its cycle stability.In-situ EIS characterization reveals that the increase of Ni element will lead to larger resistance during charging and discharging.The film resistance and charge transfer resistance on the surface of the material will also increase accordingly.The insitu XRD characterization indicates that when there is too much Ni in the material,significant change in the unit cell parameter c value will occur during the charge and discharge process,resulting in a large change in the unit cell volume,final structural damage and then poor cyclic performance.Therefore,when the Ni element is introduced,its capacity and cycle performance need to be considered comprehensively in order to optimize the overall performance of the material.3.In order to further improve the cycle stability of the battery,succinic anhydride(SA)was used as a film-forming additive which can work together with FEC in PCbased electrolyte to boost the lifespan of NLCNM cathode in SIBs.The addition of SA in the FEC-containing electrolyte can mitigate the decomposition of electrolyte due to the higher HOMO value.The formed interphase layer with the presence of SA is more uniform and stable on the cathode,which prevents continuous decomposition of electrolyte and preserves the crystal structure of the cathode after repeated Na+insertion/extraction.In addition,NaF derived from the decomposition of FEC in the interphase layer is favorable to the diffusion of Na ions.As a result,a capacity retention of 87.2%can be achieved in the Na/NLCNM cell with dual additives after 400 cycles at 1 C rate,better than the systems with sole additive or without additive.The synergistic effect between FEC and SA has a positive impact on the properties of interphase layer in terms of composition,structure and conductivity,which significantly influences the cycling stability of the cells.Moreover,the CO2 generation of the cells with SA is dramatically alleviated,which is important for the battery safety as well as future practical application.In summary,from the perspective of bulk structure doping and interface engineering,this thesis improves the cycle stability of the layered cathode material of sodium ion batteries and increases the energy density of the battery.In-situ XRD,exsitu and in-situ EIS,in-situ OEMS and other technologies have been employed and combined effectively to characterize,explore and explain the mechanism of the material in the modification process.