Study On Microwave-assisted Synthesis And Modification Of Ni-rich LiNi_0.8Mn_0.2O₂ Materials

As an important portable energy storage device,the working voltage,energy density,output power,cycle life and safety performance of lithium-ion batteries（LIBs）largely depend on the performance of cathode materials.Ni-rich layered materials have high capacity density and lithium insertion/desorption potential,relatively stable cycle performance and low cost,which is one of the focus of current research.There are still some problems in the commercialization of Ni-rich layered materials.Such as the mixing of cations,the precipitation of oxygen in the crystal lattice,and various side reactions on the electrode surface,etc.In order to improve the structure and the stability of lithium storage cycle of Ni-rich cathode material,Ni-rich LiNi_0.8Mn_0.2O₂ materials were prepared through microwave-assisted co-precipitation followed by high-temperature solid-phase method in this work.After the optimal synthesis conditions were determined,the materials were modified by Co,Al doping and Li₂SnO₃ coating respectively.The correlation and mechanism between the structure,morphology and lithium storage performance of the materials were deeply studied and analyzed by scanning electron microscopy（SEM）,X-ray diffraction（XRD）,X-ray photoelectron spectroscopy（XPS）,transmission electron microscopy（TEM）,differential scanning calorimetry（DSC）and other methods of electrochemical testing.The specific studies are as follows:（1）Study on microwave-assisted preparation,optimization and performance of LiNi_0.8Mn_0.2O₂ cathode material.The effects of microwave-assisted co-precipitation reaction time,sintering temperature,calcination time,lithium excess ratio and the amount of reaction solution on the structure,morphology and electrochemical properties have been studied.The results show that the cathode materials,obtained by calcining the mixtures with precursor（which obtained by continue stirring reaction 2 hours after the added of the reaction solution）and 10%excess Li OH at780℃for 14 hours in oxygen atmosphere,have goodα-Na FeO₂ layered structure.The specific discharge capacity at 5C still has 143.90 m Ah?g^-1,with low electrode impedance and good electrochemical reversibility,the optimized particles have a regular spherical morphology,high tap density,low cation mixing degree and good cycle performance,the specific discharge capacity is140.33 m Ah?g^-1 and the capacity retention rate is 78.99%after 100 cycles at 1C in the voltage range between 2.75 and 4.35 V.（2）Study on Co and Al co-doping modification and lithium storage performance of LiNi_0.8Mn_0.2O₂ cathode material.The results show that the c/a values of all samples are greater than 4.9,indicating that the layered structure of the positive electrode material is complete,doping does not affect the structure and morphology of the material.Among them,the LiNi_0.8Mn_0.1Co_0.08Al_0.02O₂has the largest I_（003）/I_（104）value,and the degree of Ni-Li mixing is the smallest,whose capacity retention rate reaches 91.39%after 100 cycles at 1C in the voltage range between 2.75 and 4.35 V,and still owns a specific discharge capacity of 160.03 m Ah?g^-1 at 5C.Its excellent cycle performance is mainly due to the Co and Al doping can effectively inhibit the mixing of cations,and the H2→H3 phase transition and the corresponding volume change during the cycle from destroying the stability of the material structure.The increase in the rate and safety performance of the electrode is attributed to the co-modification which reduces the reaction polarization and charge transfer resistance of the positive electrode material,enhances the Li⁺migration rate,and improves thermal stability.（3）Study on Li₂SnO₃ coating modification and lithium storage performance of LiNi_0.8Mn_0.2O₂ cathode material.The results show that the c/a value of all samples are greater than4.9,with the increase of the amount of addition,the characteristic peak of Li₂SnO₃ gradually become stronger and the（003）peak slightly shifted to smaller 2θ.Among them,2 wt.%SnO₂added（NM@Sn-2）material has the highest I_（003）/I_（104）value,the secondary particles are spherical,and the elements were uniformly distributed,with the size of 10-15μm.Lattice fringe spacing of（003）plane of the NM@Sn-2 has increased,and the apparent Li₂SnO₃ nano-coating layer,proved that Li₂SnO₃ not only exist on the surface,but a proper amount of Sn⁴⁺enter the main lattice of the material.The specific discharge capacity is 149.43 m Ah?g^-1 and the capacity retention rate is 85.16%of NM@Sn-2 after 200 cycles at 1C in the voltage range between 2.8 and 4.3 V,the specific discharge capacity at 5C is still 155.49 m Ah?g^-1.Its excellent electrochemical performance may be attributed to Li₂SnO₃ coating and Sn⁴⁺doping forming a stable lithium ion transport channel,which slows down the side reaction of the interface between the electrode and the electrolyte,the doping of Sn⁴⁺increases the interlayer spacing,improves the Li⁺migration rate,reduces the Ni-Li mixing,and improves the electrochemical reversibility of the cathode material.