Optimized Synthesis And Electrochemical Performance Of Layered Cathode Materials For Sodium Ion Batteries

The demands for efficient and sustainable energy-storage systems become more and more urgent,leading to an improvement of lithium-ion batteries.However,the increasing price of lithium resources has limited the development of lithium-ion batteries.Sodium-ion batteries have been regarded as one of the most promising energy storage devices to replace lithium-ion batteries due to their wide range of raw materials,large reserves and low cost.Cathode materials play an important role in determining the electrochemical performance of SIBs.Among them,layered oxides Ni/Co/Mn based Na_0.67Ni_0.17Co_0.17Mn_0.66O₂ is the most promising ones for application because they have a high theoretical capacity and cost little.However,there are still some problems to be overcome for the positive cathode materials of layered P2-type sodium-ion batteries.For example,during the insertion/extraction process of Na⁺,forming a typical O2-type structural transition,which greatly damages the structural stability and limits cycle life.Except for the phase transition problem,the cathode material is also vulnerable to electrolyte corrosion,which damage crystal structure and cycle stability.And the average working voltage is low,which is also one of the problems.This work is to prepare Ni/Co/Mn based P2-type sodium-ion battery cathode material through a controlled synthesis process,study the modification method of element doping and surface coating,and prepare sodium-ion battery cathode material with high capacity and long life.（1）In this work,we modified P2-type Na_0.67Ni_0.17Co_0.17Mn_0.66O₂ by a new strategy of Na-Site Ca²⁺substitution and（transition metal site）Ni-Co-Mn-Site Mg²⁺substitution with co-precipitation method followed by solid-state reaction.The co-doped Na_0.64Ca_0.03(Ni_0.17Co_0.17Mn_0.66)_0.9Mg_0.1O₂shows an impressive initial discharge capacity of 158 m Ah/g,and the capacity after 100 cycles reaches 140 m Ah/g.And the findings suggests Na_0.67Ni_0.17Co_0.17Mn_0.66O₂ was successfully optimized.（2）Herein,spherical P2-type cathode Na_0.67(Ni_0.17Co_0.17Mn_0.66)_0.9Ti_0.05Mg_0.05O₂has been designed and the critical roles of Ti and Mg on the electrochemical performance of the cathodes are systematically investigated.Na_0.67(Ni_0.17Co_0.17Mn_0.66)_0.9Ti_0.05Mg_0.05O₂ exhibits better electrochemical performance than that of the undoped Na0.₆₇Ni_0.17Co_0.17Mn_0.66O₂in the electrochemical processes.It delivers an initial reversible capacity of 151 m Ah/g（2-4.5 V）with an average voltage of 3.8 V and exhibits a high capacity retention of 87.7%after 300 cycles at 100 m A/g.The improved electrochemical performance benefits from the Ti/Mg co-substitution,Ti improves the average voltage while Mg and Ti significantly mitigate the undesired P2→O2 phase transition of the cathode,and these two elements jointly promote the development of the electrochemical performance.（3）MgO layer could effectively prevent the surface from corroding by HF and promote migration of Na⁺.MgO-coated Na_0.67(Ni_0.17Co_0.17Mn_0.66)_0.9Mg_0.05Ti_0.05O₂exhibits improved electrochemical performance than the raw material.It delivers 142.6m Ah/g initial discharge capacity and maintains 90.6%at the high current density of 100m A/g within 2-4.5V,which has been obviously enhanced than that of Na0.67Ni0.17Co0.17Mn0.66O2.