Synthesis And Characterizations Of LiMn₂O₄/LiFePO₄and Li[Ni_1/3Co_1/3Mn_1/3]O₂/LiFePO₄Blend Cathodes For Li-ion Batteries

Great efforts have been made on the lithium-ion batteries since SONY companyfirstly applied lithium-ion batteries for the commercial use in1991. Nowadays, themost popular cathode materials are layered LiCoO₂and Li[Ni_1/3Co_1/3Mn_1/3]O₂, spinelLiMn₂O₄, and olivine LiFePO₄. Recently, blend electrode materials, which containstwo or more electrodes, have attracted wide attention, due to the superior performancethan single-type electrode. LiMn₂O₄and LiFePO₄are competitive andcomplementary to each other as cathode materials, especially for the electric vehiclesor hybrid electric vehicles. However, LiMn₂O₄undergoes a gradual capacity fadingduring charge-discharge cycling because of the dissolution of Mn²⁺and the structuraldistortion by the Jahn-Teller effect of Mn³⁺ions. One of the main problems of LiFePO₄is the low electronic conductivity and slow lithium ion diffusion across the LiFePO₄/FePO4boundary. Therefore, in the first part of this work, we selectedLiMn₂O₄/LiFePO₄as a blend electrode in order to compensate their respectivedisadvantageous and improve its electrochemical performances:Firstly, we successfully prepared LiMn₂O₄/LiFePO₄blend cathodes using asimple blending technique with five different mass ratios. Structural preoperty,particle size and particle surface morphology were examined by XRD and SEM.When the mass ratio of LiMn₂O₄: LiFePO₄equalled to1:1, the nano-sized LiFePO₄powders not only uniformly adhered to the micron-sized LiMn₂O₄particlesbut also effectively filled into the cavities of the LiMn₂O₄space as a close-packedstate. We also found that carbon-coated LiFePO₄had a higher electron conductivityand was two orders of magnitude larger than that of LiMn₂O₄. Then we studied theelectrochemical properties of the blend cathodes by charge-discharge cycling, cyclicvoltammetry （CV） and electrochemical impedance spectroscopy （EIS）. This blendmaterial had a high discharge capacity and good cycle stability, because the special surface morphology could induce a good electrical contact and tap densitySecondly, we prepared LiMn₂O₄/LiFePO₄blend cathodes with mass ratio of1:1,using different blending techniques, i.e. hand-milling and ball-milling for5h/120rpmand10h/rpm. XRD showed that the samples did not change the originalcrystalstructures after hand milling or ball milling. While SEM indicated thathand-milling method was easy to get a uniform dispersion of the blend materials, butball-milling methods seriously damaged to the uniformity of the blend materials. Inparticular, when the ball-milling energy reached200rpm/10h, the spindle-shapedmorphology of LiFePO₄was destroyed and bonded on LiMn₂O₄particle surface withthe result of LiMn₂O₄particles reunion. We also studied the electrochemicalproperties of the material by charge-discharge cycling, CV and EIS. It was shown thatLiMn₂O₄/LiFePO₄blend cathodes using hand-milling method had a higher capacityand cycle stability in comparison with other methods. While ball-milling methodwould reduce electrode capacity and cycle resistance, and the higher milling energy,the more capacity fading. Because ball-milling would result in a tight wrap on theLiMn₂O₄particle surface by LiFePO₄, isolate the contact between LiMn₂O₄particlesand the electrolyte, reduce the actual reaction area of LiMn₂O₄, and lose the contactbetween the particles both from the physical and the electronic aspects. Thus all theseeffects would result in a increase of the interfacial impedance dramatically.Layered LiNi_1/3Co_1/3Mn_1/3O₂material showed a high reversible capacity, a lowcost and moderate thermal stability because it combines the advantages of Ni, Co andMn. However, the low electronic conductivity limited the rate capability and capacityretention rate of LiNi_1/3Co_1/3Mn_1/3O₂. Padhi et al. developed a method ofcarbon-coated LiFePO₄and was considered to have a good application prospect. Thecarbon-coating on the particle surface would enhance the electronic conductivity ofthe LiFePO₄material, and the nanoparticles benefited to Li⁺ion diffusion. Therefore,in the latter part of this paper, we choose LiNi_1/3Co_1/3Mn_1/3O₂/LiFePO₄blendcathodes in order to improve the rate capability, cycle characteristics and thermalstability of LiNi_1/3Co_1/3Mn_1/3O₂material by the surface conductivity of particles:Firstly, we synthesized LiNi_1/3Co_1/3Mn_1/3O₂material using a co-precipitationmethod, then prepared LiNi_1/3Co_1/3Mn_1/3O₂/LiFePO₄blend material by mechanical ball milling. XRD showed that LiNi_1/3Co_1/3Mn_1/3O₂is a layered structure. Afterblending with LiFePO₄, the crystal structure of LiNi_1/3Co_1/3Mn_1/3O₂did not change.SEM pictures showed that LiFePO₄nanoparticles uniformly and completely filledinto the uneven spherical particles of LiNi_1/3Co_1/3Mn_1/3O₂surface, when the contentwas20wt%, and the morphology was what we expected. Thanks to such kind ofsurface morphology, the blend cathode had a initial discharge capacity of178mAh/gand a capacity retention rate of100%when it charged to4.4V at C/4rate. Rateperformance test showed that LiNi_1/3Co_1/3Mn_1/3O₂/LiFePO₄blend cathodes exhibitedexcellent cycle stability which was significantly better than singleLiNi_1/3Co_1/3Mn_1/3O₂cathode. Then, CV and EIS measurement showed that LiFePO₄wrapped on the particle surfaces of LiNi_1/3Co_1/3Mn_1/3O₂, avoided the direct contactbetween LiNi_1/3Co_1/3Mn_1/3O₂and the electrolyte, and inhibited the unwanted sidereaction between the electrolyte and LiNi_1/3Co_1/3Mn_1/3O₂with the results of thereduced impedance of the SEI film. On the other hand, the surface carbon layer of LiFePO₄benefited to both electron conductivity and lithium ion insertion/extractionand reduced the charge transfer resistance. Hence, DSC measurement showed that theexothermic peak of the electrode moved to the high-temperature region, and therelease of heat was also reduced. Therefore, the safety performance ofLiNi_1/3Co_1/3Mn_1/3O₂/LiFePO₄blend cathodes could be improved.At last, the electrochemical performance analysis ofLiNi_1/3Co_1/3Mn_1/3O₂/LiFePO₄blend cathodes in the voltage window of2.5⁴.8Vfound that discharge capacity, cycle stability and rate capability ofLiNi_1/3Co_1/3Mn_1/3O₂/LiFePO₄blend cathodes had also been enhanced. CV and EISanalysis showed that LiFePO₄particles avoided the direct contact betweenLiNi_1/3Co_1/3Mn_1/3O₂and electrolyte, suppressed the unwanted side reaction betweenthe electrolyte and LiNi_1/3Co_1/3Mn_1/3O₂, and resisted the growth of the SEI film at thesame time. The surface carbon layer of LiFePO₄benefited to both electronconductivity and lithium ion insertion/extraction, and reduced the charge transferresistance. DSC measurement showed that safety performance ofLiNi_1/3Co_1/3Mn_1/3O₂/LiFePO₄blend cathodes had been improved in high voltage.In sum, this work provided a comprehensive understanding on the preparation, structural and electrochemical performances of two types of blend cathode materials:LiMn₂O₄/LiFePO₄and LiNi_1/3Co_1/3Mn_1/3O₂/LiFePO₄, and offered a necessarytheoretical and technical guidance for the research of blend cathodes.