| The lithiun ion batteries(LIBs)have been generally identified as a kind of environmental friendly secondary batteries,which are now applied in varieties portable electronic equipment.With the development of technology and expanded range of application,capacity density of lithium ion batteries needs to be more excellent.Among various kinds of cathode materials,the LiNixCoyMnzo2(NCM,x+y+z=1)is considered to be one of the most potential cathode,drawing more and more attention for its excellent specific capacity and energy density.With typical a-NaFeO2 layered structure,the NCM possesses favorable behaviors of LiNiO2,LiCoO2 and LiMnO2 at the same time associating with the high capacity of nickel(Ni),excellent rate capability of cobalt(Co)and outstanding stability and safety of manganese(Mn).The capacity of ternary materials is affected by the content of element Ni,therefore the NCM with high percent of Ni is favorable for the development of cathode in the future.However,an excess percent of element Ni may cause a serious of problems during the charge and discharge process of lithium ion batteries.For example,the Ni(0.69A)may occupy the ionic site of Li(0.76A)during the charge/discharge procedure for its ion radius is extremely similar to the Li.And the Ni4+in NCM tends to convert to more stable Ni2+ along with O2 releasing during delithiating process with the temperature rising,which is often followed by a reaction between the NCM and electrolyte.Then the highly delithiated NCM may lead to a thermal runaway of the whole battery because of the heat releasing from the reaction between O2 and organic solvent.Therefore,it is significant to improve the thermal stability of cathode materials by a simple and economical method.Based on the objectives above,physical blending is adopted to prepare the composites.Specific research contents include the following two parts.(1)In the chapter 3,the blend cathode materials are prepared by mixing the NCM523 with the LFMP/C material through a ball-milling method without any damage to the micro structure and crystal structure.The electrochemical and thermal characters have suggested that the blend materials exhibit not only excellent electrochemistry property offered by the NCM523 but also high thermostability provided by the LFMP.The blend cathode materials can combine the advantages of both LFMP and NCM523 together.And there is an obvious synergistic effect of thermal behavior between the LFMP and NCM523.The existence of LFMP reduces the total released-heat.And there is an interchange of Li+and H+ between NCM and HF without any generation of H2O,resulting in the disappearance of exothermic peaks at 207 C which is generated from the reaction between delithiated LFMP and HF.Additionally,the pouch cells with NCM-LFMP as cathodes present better safety performance than NCM when tested by Accelerating Rate Calorimetry(ARC)respectively.(2)In the chapter 4,we prepared the composite NCM622@LFP via a simple high-speed rotating method in order to combine the excellent capacity of NCM622 and outstanding thermal stability of nano-LFP.The coating material nano-LFP separates NCM622 and electrolyte,and reduces the contacting points between them,improving the electrochemical performance and thermal properties.The discharge specific capacity and capacity retention of NCM622 and NCM622@LFP are 151.06mAh/g,88.3%and 158.18mAh/g,93.8%,respectively.Meanwhile the max heat flow and total released-heat of NCM622@LFP with electrolyte is 1.872mW/mg and 1028.3J/g,both much lower than that of NCM622.It can be summarized that the thermal procedure of composite is flat and there is no intense heating release during the DSC test.The synthetic methods involved in this paper is economical and can be applied in industrial manufacture easily,meanwhile has significant effects on improving thermal performance of cathodes. |
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