Preparation And Properties Of Phosphate Coated LiNi_1/3Co_1/3Mn_1/3O₂Materials

As a cathode material of lithium-ion battery, LiNi_1/3Co_1/3Mn_1/3O₂has highercapacity, lower cost and less toxic than LiCoO2. But it will take place side reactionsbetween LiNi_1/3Co_1/3Mn_1/3O₂and electrolyte during the charge-discharge process.The composition and structure of LiNi_1/3Co_1/3Mn_1/3O₂will be changed. Theresistance of the electrode reaction will increase. Surface coating is a good methodto solve the problem, because the coated layer can separate LiNi_1/3Co_1/3Mn_1/3O₂andelectrolyte. In this thesis, a method to coat phosphate on the surface ofLiNi_1/3Co_1/3Mn_1/3O₂has been proposed and4kinds of phosphate, AlPO₄, FePO4,Co3（PO4）2and Mg3（PO4）2, were coated on LiNi_1/3Co_1/3Mn_1/3O₂. The method issimple, low cost, suitable for industrial production, and can significantly improvethe cycle stability of LiNi_1/3Co_1/3Mn_1/3O₂.There are two steps in this method, a liquid phase reaction process and a solidphase reaction process. In order to coat2wt.%AlPO₄on LiNi_1/3Co_1/3Mn_1/3O₂, fourkinds of liquid-phase reaction processes were tried. PVP was used to promote thecoating process. The result of TEM test shows this method is an effective way toobtain a uniform thin AlPO₄layer on the surface of LiNi_1/3Co_1/3Mn_1/3O₂. Thecalcinations temperature of solid phase reaction was studied, the result shows thatthe coated product which was calcined at600℃for10h has better electrochemicalperformance than5h.The influence of AlPO₄contents on the properties of LiNi_1/3Co_1/3Mn_1/3O₂wasstudied. Four contents of AlPO₄were coated on LiNi_1/3Co_1/3Mn_1/3O₂,0.5%,1%,2%,3%. The result of TEM test shows that all the coated layers have a good crystallinityand a uniform thickness, about several tens of nanometers. The physicalcharacterization shows that the products with1%,2%content of AlPO₄have alower extent of nickel lithium disorder and a more uniform thickness of coated layer,which are both conducive to the diffusion of Li ions. Electrochemical tests alsoshow that LiNi_1/3Co_1/3Mn_1/3O₂which is coated1%or2%AlPO₄has good cycleperformances. Their capacity retentions after150cycles at0.1C rate were90.1%,89.3%, respectively, higher than75.2%of the uncoated LiNi_1/3Co_1/3Mn_1/3O₂. Thecapacity retentions after500cycles at0.5C were80.1%,76.3%, respectively,higher than64.5%of the uncoated LiNi_1/3Co_1/3Mn_1/3O₂.When coating FePO4, Co3（PO4）2and Mg3（PO4）2on LiNi_1/3Co_1/3Mn_1/3O₂, thecontent was chosen as1%, for its best electrochemical performance of the AlPO₄coated products. The TEM test confirms that the method of coating AlPO₄is also effective in coating FePO4, Co3（PO4）2and Mg3（PO4）2. The XRD result shows thatafter coating the three phosphates, LiNi_1/3Co_1/3Mn_1/3O₂has a better layered structureand a lower extent of nickel lithium disorder than before. Electrochemical testsreveal that coating1%content of phosphate is able to inhibit the increase of chargetransfer resistance of the electrode reaction during charge-discharge process,thereby increasing the cycle stability of LiNi_1/3Co_1/3Mn_1/3O₂. After300cycles at0.5C, the capacity retentions of LiNi_1/3Co_1/3Mn_1/3O₂coated1%FePO4,1%Co3（PO4）2,1%Mg3（PO4）2were82.1%,82.3%,83.5%, respectively, significantly higher thanthe71.0%of pristine LiNi_1/3Co_1/3Mn_1/3O₂.