Structural Regulation And Performance Optimization Of P2-type Oxide Cathode Materials For Sodium Ion Batteries

With the vigorous development of clean energy such as wind and solar energy,large-scale energy storage devices are urgently needed to build a new smart grid system.Until now,lithium-ion battery（LIB）is the most promising electrochemical energy storage system.However,due to the insufficient and uneven distribution of lithium resources,the cost of LIB is increasing and it is difficult to meet the demand of large-scale energy storage.As for the sodium ion battery（SIB）which has a similar electrochemical mechanism to LIB,it has a strong advantage in cost due to the rich and widely distributed sodium resources,so SIB is very suitable for large-scale energy storage.As an important part of SIB,the research on cathode materials plays a decisive role in the practical use of SIB.Among various kinds of cathode materials,P2-type layered oxide ones have high capacity,and their unique Na sites of triangular prism which connect by face to face provide the fast Na ion transport channels.However,the phase transformation of this kind of materials is easy to occur during the cycling process,resulting in a large volume change,which affects the electrochemical performance.In addition,the extra capacity of P2-type layered oxide cathodes provided by the insertion of extra Na ions in the process of discharge is difficult to use in practical application,and part of the reaction in high voltage range is usually unstable,thus the remaining available capacity is too low.Moreover,the cost of materials must be considered to better adapt to large-scale commercial applications.In view of above problems,the research of this dissertation is focused on structural regulation,increasing specific capacity,reducing material costs and so on,the main study contents are as follows:First,a series of cathode materials with different compositions were designed to control the distribution of Na ions,and then to control the de-intercalation order of Na ions.As a result,the cathode material with uniform distribution of residual Na ions in the lattice after charging was obtained.The optimized cathode material Na_2/3Mn_1/2Ni_1/6Co_1/3O₂ keeps P2 phase structure when it is cycled in a wide voltage range of 1.5-4.5 V（when it is charged into 4.5 V,there are 0.17 Na ions in the material）,and the volume change is only 1.9%during the whole cycle.Subsequently,various in-situ and ex-situ tests were used to determine the main factors which affect the structural evolution of P2 layered oxide cathode material:the number and distribution of Na ions which are remained in lattice during cycling.Then,due to the unstable high voltage capacity and difficult utilization of low voltage capacity,the capacity of P2-type layered oxide cathode materials is too low after limiting the voltage window to 2.0-4.25 V.In order to improve the specific capacity,the Ni content was increased and part of Ni²⁺ions were oxidized to+3,so that the distribution of Na ions is more uniform and the Na ion transport channels are expanded.Based on this strategy,synthitic Na_0.67Mn_0.45Ni_0.22Co_0.33O₂ shows a specific capacity of 113 m A h g^-1 at a current density of 50m A g^-1,and this material has excellent rate performance and cycle stability.At the high current density of 1 A g^-1,this material still has a specific capacity of about 100 m A h g^-1,and the capacity retention rate is 80%after 1000 cycles.The introduction of Ni³⁺ions not only improves the specific capacity,but also has excellent structural stability,and the volume change is only 0.6%during cycling.Finally,as the previous materials contain a large amount of Co,in order to reduce the cost of materials,bimetallic elements were used to replace the expensive Co.Optimized cathode material of Na_0.67Mn_0.53Ni_0.30Mg_0.085Ti_0.085O₂ still has a specific capacity of 118 m A h g^-1,which is equivalent to that of the materials with Co element.Moreover,the median discharge voltage of this material is increased form 3.21 V to 3.59 V,which increases the energy density.In addition,the lack of Co makes the oxygen react and provide capacity in high voltage range,and the combined action of Mg and Ti makes the oxygen redox reaction in this material stable.