Study On The Optimization Of P2 Type Na_xCoO₂ Materials And Its Capacitive Deionization Performance

Capacitive deionization(CDI)has attracted great attention as a potential alternative technology for desalination due to its advantages of high efficiency,low energy consumption,low cost,and environmental friendliness.P2-type NaxMO2(M=Mn,Co,V,etc.)has become the preferred material for Faradaic electrode materials due to its open layered structure and rapid ion transport properties.Among all,layer structural NaxCoO2(NCO)is composed of the CoO2,layers and Na+ions in the interlayer,which has various excellent properties and has been demonstrated to be a viable electrode material of CDI.However,the relatively low capacity resulted from poor electrical conductivity and the structural instability make it still cannot meet the requirements of high performance.In this thesis,based on the tuning Na content,doping and n-alkylamine intercalation strategy,as well as a variety of advanced characterization techniques,we have realized the modification of structure,and the controlling of layer spacing,electronic and ionic conductivity and desalination performance.Moreover,we have systematically elaborated the underlying mechanism of the adsorption/desorption of Na+ ions in the CDI process using the multiple advanced characterization techniques combined with the density functional theory calculations.The main research and results are as follows:1.P2-type layered Na0.71CoO2 was successfully synthesized by sol-gel method,and used as a model system for the first time to explore the effect of Na+/vacancy ordering on desalination performance.An asymmetrical FDI device was developed where NaxCoO2 as the Faradic electrode material and AC act as the Cl-storage electrode material.For x=0.50,this configuration yields an ultrahigh desalination capacity(65.7 mg g-1)and an excellent charge efficiency(96.5%)in 529 mg L-1 NaCl solution at 1.2 V.The experimental results reveal that the desalination capacity depends on the range of reversible adjacent intermediate phases region and the desalination rate is affected by the sodium ions mobility in the interlayer determined by the Na+/vacancy ordering pattern between the CoO2 slabs.The excellent desalination capacity and rate are encouraging.2.We develop an asymmetrical FDI device assembled by Ca2+-decorated NaxCoO2(x?0.71,y?0.05)as the Faradaic negative electrode and activated carbon as the positive electrode.Na0.27Ca0.03CoO2·0.6H2O was synthesized via a facile sol-gel and chemical oxidation method,which delivered a desalination capacity of 45.6 mg g-1 and a charge efficiency close to 1,and an inappreciable capacity fading was observed after 50 cycles.It is found that the presence of Ca2+residing in the face-sharing sites helps to maintain the layered structure and promotes efficient deintercalation of Na+by anchoring the CoO2 slabs,which results in its unexpected desalination capacityandgoodcyclability.Moreover,electrochemical quartz-crystal microbalance(EQCM)was successfully used to reveal Faradaic intercalation mechanism.3.Zn-doped NaxCoO2 has been developed for the enhanced capacitive deionization(CDI)properties.The partial Co3+ substituted by Zn2+ can induce the appearance of electronic holes and effectively improve the electrical conductivity,which consequently is beneficial to the enhancement of the desalination performance.Compared with the pristine material,the capacity retention of Na0.71Co0.99Zn0.01O2 improved from 88.6%to 98.3%over 50 cycles,indicating that the inactive Zn2-ions can also prevent the irreversible interlayer-gliding from alleviating the structure destruction.Additionally,in situ Raman was employed for the first time to investigate the mechanism,demonstrating that Na+ions were reversibly inserted/extracted into/out of interlayers,along with c-axis length decreased and expanded back completely.4.The intercalation of n-alkylamine with different alkyl chain length in the structure of layered Na0.71CoO2(NCO)for application in capacitive deionization(CDI)is reported.The amine intercalation via acid-base reaction routes allows to expand layer spacing of NCO,increase the active site of Na+ions storage,reduce the diffusion energy barrier of Na+ions,improve electronic conductivity and suppress the structural deformation of NCO upon the de/sodiation process.Benefiting from these features,the n-propylamine-intercalated NCO delivered a high capacity of 74.0 mg g-1 and comparable energy consumption of 0.55 kWh kg-1 in 500 mg L-1 NaCl solution at 1.2V.A combination of mechanism analyses and density functional theory calculations demonstrated the synergistic effects of electrical and ionic conductivity account for the enhanced properties.Our work provides a strategy for engineering the interlayer to enable high capacity and cycling stability,which offers guidance for the design of preintercalated electrode materials.