Spherical Cathode Materials Of Lithium-ion Battery Synthesis And Electrochemical Performance

The energy density of lithium second batteries can be dramatically improved by using thespherical cathode materials which have many advantages such as high tap density, excellent fluidity,dispersivity and so on. Therefore, the research on the synthesis and application of cathode materialswith spherical morphologies is potentially attention.This paper mainly consists of three sections: review, experiments and conclusions. Thestructures and properties of the. current cathode materials, the synthesis methods ofLiMn_xNi_yCo_1-x.yO₂ materials with spherical morphologies and lithium manganates were reviewedin the first section (Chapter 1). The cathode materials with spherical morphologiesLi_1+x[Mn₀.45Co₀.40Ni₀.15]O₂ and Li_{1. 20}Ni_xMn_2.xO₄ were obtained via a carbonate co-precipitationmethod and followed by a calcination treatment, and the synthesis conditions were optimized. Theeffects of reaction conditions on the particle size, morphology and crystallites of the obtainedmateirals were also investigated. In addition, the structure and electrochemical properties of theobtained materials were characterized (Chapter 3,4).By using NH₄HCO₃ as a precipitation agent, the effects of reaction time, reaction temperature,stirring speed and stirring time on the structure and morphologies of the obtained precursor materialswere studied in detail and the optimal synthesis conditions were obtained. Two kinds of particleswith different sizes were prepared by changing the stirring speed on the optimal synthesis conditions.The particle shape was spherical and the particle size distribution was uniform. When the precursormaterials with spherical morphologies were calcined at 800℃, the cathode materials with sphericalmorphologies were obtained, which had high crystallinity and well-ordered layer-structure.Li_1+x[Mn₀.45Co₀.40Ni₀.15]O₂ materials have displayed good cycling performance, despite ofdissatisfying discharge capacity, compared with other LiMn_xNi_yCo_1.x.yO₂ cathode materials, such asLiMr_1/3Ni_1/3Co_1/3O₂ and LiMn₀.4Co₀.2Ni₀.4O₂. At current density of 50, 125 and 250 mA-g"1,respectively, the capacity retention of all the obtainedLi_1+x[Mn₀.45Co₀.40Ni₀.15]O₂ materials between2.8 V and 4.5 V are above 80% of their initial capacities after 50 cycles. Li_1+x[Mn₀.45Co₀.40Ni₀.15]O₂material between 2.8 V and 4.6 V with a current density of 25 mAg～(-1) shows a reversible capacity ofabout 184.5 mAh-g～(-1) and discharge capacity about 173.9 mAh-g～(-1) after 30 cycles.The structure andthe electrochemical performance of the materials Li_1+x[Mn₀.45Co₀.40Ni₀.15]O₂ were characterized byXRD, SEM, AAS, FT-IR, RS, CV and electrochemical testing.On the basis of the optimized synthesis conditions of the spherical cathode materials LiMn_xNi_yCo_1-x.yO₂ , the materials Li_1.20Ni_xMn_2-xO₄ were obtained via a carbonate co-precipitationmethod, which had uniform particle size distribution and an average particle size of about 2μm. Theelectrochemical performances of the materials Li_1.20Ni_xMn_2-xO₄with different doped nickel contentswere also investigated. The results indicated that the sampleLi_1.20Ni_xMn_2-xO₄(x =0.28) delivers thehighest capacity and shows perfect capacity-retention. It delivers a discharge capacity of about 136.6mAh-g(-1) between 3.5 V and 5.0 V at 25 mA-g～(-1) and still keeps 96.9% of stable discharge capacityafter 50 cycles. Surprisingly, it maintains 90.3% of initial discharge capacity between 3.5 V and 5.0V at 250 mA-g～(-1) after 1000 cycles. Even at a current density of 1250 mA-g～(-1), its discharge capacitystill retained 70.7% of that at 25 mA-g～(-1) after 50 cycles. The structural characterization and theelectrochemical performance of the obtained materialsLi_1.20Ni_xMn_2-xO₄ were conducted by XRD,SEM, AAS, FT-IR, CV and charge-discharge experiments testing.