Preparation And Electrical Property Research Of NaMnO₂ As Cathode Material For Sodium-ion Battery

In this paper,a brief review about the sodium-ion battery was presented firstly.The advantages of the phosphate cathode material and the iron oxide-based anode materials were analyzed,and then puts forward the research emphasis of this paper:the sodium-ion battery electrode material was modified by the material with excellent electrical conductivity,in order to improve the electrochemical properties of the electrodes.The main content of this paper is as follows:（1）P2-type Na_0.66Ni_0.33Mn_0.67-xMo_xO₂（x=0,0.03,0.05,0.07）were prepared using a conventional solid state method and for the first time developed as promising cathode materials for sodium-ion batteries.The XRD patterns show that Mo⁶⁺ions are successfully incorporated into the lattice of the Na-Ni-Mn-O system and the P2-type structure remains unchanged after substitution.The introduction of Mo⁶⁺in the Na-Ni-Mn-O system can significantly improve capacity retention compared to the unsubstituted material during cycling.In addition,an additional charge/discharge profile can be observed between 3.0 and 3.2 V for Mo-substituted samples,demonstrating that Na⁺/vacancy ordering can be suppressed during sodium insertion/extraction.Na_0.66Ni_0.33Mn_0.62Mo_0.05O₂ can deliver an initial capacity of 112mAh/g at 34 mA/g with a high average voltage of 3.6 V and a capacity retention of 87%after 50 cycles.EIS measurements demonstrate that Mo-substitution is an effective way to hinder the increase of inter-particle contact resistance by suppressing any possible irreversible phase transformation found at low sodium contents.（2）P-type layered oxides are promising cathode materials for sodium-ion batteries and a wide variety of compounds have been investigated so far.Nevertheless,detailed studies on how to link synthesis temperature,structure and electrochemistry are still rare.Herein,we present a study on P-type Na_xNi_0.22Co_0.11Mn_0.66O₂ materials,investigating the influence of synthesis temperature on their structure and electrochemical performance.The change of annealing temperature leads to various materials of different morphologies and either P3-type（700^oC）,P3/P2-type（750^oC）or P2-type（800⁹00^oC）structure.Galvanostatic cycling of P3-type materials revealed high initial capacities but also a high capacity fade per cycle leading to a poor long-term cycling performance.In contrast,pure P2-type Na_xNi_0.22Co_0.11Mn_0.66O₂,synthesized at 800^oC,exhibits lower initial capacities but a stable cycling performance,underlined by a good rate capability,high coulombic efficiencies and high average discharge capacity（117 mAh/g）and discharge voltage（3.30 V vs.Na/Na⁺）for 200 cycles.（3）In this study,we focus on P2-Na_2/3Ni_1/3Mn_2/3O₂,which shows a relatively high operating voltage based on a Ni²⁺/Ni⁴⁺redox reaction.However,the discharge capacity of Na//Na2/3Ni1/3Mn2/3O2 cell fades rapidly during cycles by charging to4.5 V;therefore,the available reversible capacity is limited to only 80 mAh/g in a lower potential domain than 3.8 V.In this article,we report the synthesis and electrode performance of titanium substituted Na_2/3Ni_1/3Mn_2/3-xTi_xO2 as novel positive electrode materials for Na-ion batteries.It is found that Na_2/3Ni_1/3Mn_1/2Ti_1/6O₂ delivers127 mAh/g of reversible capacity with 3.7 volts of average discharge voltage with superior cycle life.