Preparation And Electrochemical Properties Of Na_2/3Ni_1/4Mn_3/4O₂ Thin-film Cathode Material By Pulsed Laser Deposition

Lithium ion batteries(LIBs)have been widely used in portable electronic,communication equipment and electric vehicles due to the high energy density,light weight,long cycling life,low self-discharge and non-memory effect.Nevertheless,the increasing price and limited availability of lithium severely impede their applications.On the contrary,sodium with similar electrochemical properties to lithium is highly abundant and inexpensive,attracting much attention as a sustainable candidate to LIBs.Therefore,it is extremely important to explore the suitable host materials for sodium-ion batteries(SIBs).Among them,Na_xM_yMn_1-yO₂(0<x<1,0?y<1,M indicates the transition metal element)with large specific capacity and friendly environment is considered to be a promising cathode.In this thesis,the Na_2/3Ni_1/4Mn_3/4O₂ target was prepared by traditional solid-state method.Then,pulsed laser deposition(PLD)technique was used to fabricate high-performance Na_2/3Ni_1/4Mn_3/4O₂ thin-film cathode material.The influences of the deposition temperature,oxygen partial pressure,deposition time and annealing treatment on the structure and sodium storage properties were studied in detail.Furthermore,the surface coating was deposited on Na_2/3Ni_1/4Mn_3/4O₂thin-film by PLD and it is found that the electrochemical performance can be further improved.The main contents are listed as follows:Firstly,the layered P2-Na_2/3Ni_1/4Mn_3/4O₂ target,calcinated at 750? and 1000?,was synthesized utilizing solid-state reaction.Afterwards,the corresponding thin films were deposited by PLD.The impacts of deposition temperature and oxygen partial pressure on the structure and morphology of films were studied.It demonstrated that the film with highly preferred c-axis orientation is composed of homogeneous nanosized grains.With the temperature increasing,the particle size becomes larger and the crystallinity is enhanced distinctly.Moreover,the higher oxygen partial pressure is able to effectively inhibit the oxygen vacancy and reduce the defects in the film.Secondly,compared to the conventional compacted powder electrode,the Na_2/3Ni_1/4Mn_3/4O₂ thin film electrode with highly preferred orientation along c axis and nano-sized grains has higher electron and ion mobility,and exhibits lager discharge capacity and more stable cycling performance.It has the retention of 91%after 30 cycles at a current density of 13 m A g^-1 between 1.5 and 4.3 V.As the discharge-current density increases to 2080 m A g^-1,the discharge capacity of 98.1 m Ah g^-1 can be maintained,indicating that thin film can keep structural stability at the high voltage.Thirdly,the influences of structure and morphology on the storage performances of Na_2/3Ni_1/4Mn_3/4O₂thin film electrodes were investgated.The results show that the film deposited at 550? for 1 h,with oxygen partial pressure of 65 Pa,followed by in-situ annealing for 1 h,exhibits a high initial discharge capacity of 183.6 m Ah g^-1 and retention of 90%after 130 cycles at the current density of 13 m A g^-1.Besides,the electrochemical properties at different current densities were studied.Fourthly,in order to improve the storage performances,the carbon coating was deposited on Na2/3Ni1/4Mn3/4O2 thin film electrodes.This coating method can not only lower the sensibility when the film was exposed to the air,but prevent the contact and reaction with electrolyte.In addition,it was found that both the electrical conductivity and the rate capability can be also enhanced.The thin film coated by carbon renders robust cycling stability(97%capacity retention at 130 m A g^-1 after 30 cycles).Furthermore,Au-coated stainless steel(Au/SS)substrate was adopted to withstand interdiffusion and reaction between electrode and stainless steel during high temperature depositing,which greatly impromote the rate capability of Na2/3Ni1/4Mn3/4O2 thin film.More impressively,the prototype Na-ion full battery(Na2/3Ni1/4Mn3/4O2//hard carbon)delivers a promising energy density of 190 Wh kg^-1.