Flameless Combustion Synthesis And Modification Of LiMn₂O₄as Cathode Materials For Lithium Ion Battery

In this paper, the principle and the development of lithium ion batteries and theresearch of cathode materials was reviewed. The preparation method, the capacitydecay reason and modification research of LiMn₂O₄was introduced with emphasis.On the basis of this, a new synthesis method named molten-salt flamelesscombustion synthesis, is proposed. The influence of different factors upon thestructure, morphology and electrochemical performance were investigated.Meanwhile, the influence of different doping elements and doping contents onLiM_xMn_2-xO_4-yQ_ywere discussed.Firstly, use the lithium acetate （lithium nitrate）, manganese acetate （manganesenitrate） as raw materials to synthesize the cathode materials with different malorratio of NO₃-/Ac-at500oC for3hours. The samples are very puff and uniformitywhen the n（NO₃-/Ac-）=1or3/2, which we thought synthesized by flamelesscombustion. When the n（NO₃-/Ac-）＜1, the samples aren’t synthesized by flamelesscombustion totally, and the samples are synthesized by flame combustion when then（NO₃-/Ac-）＞3/2. The particles size are the biggest of all the samples synthesizedwith the n（NO₃-/Ac-）=1, are120-200nm. It was found that the LiMn₂O₄phase isformed when the melting molten-salt is calcined at280oC, and the pure LiMn₂O₄isabtained over400oC. And the main phase LiMn₂O₄were abtained with differentmalor ratio of NO₃-/Ac-, the impurity is Mn3O4and Mn₂O₃when the n（NO₃-/Ac-）＜7/3or n（NO₃-/Ac-）＞7/3. The pure LiMn₂O₄was abtained when the n（NO₃-/Ac-）=7/3or3/7, and had the highest initial specific capacity is115mAh·g^-1at0.1C, after20cycles the capacity retention rate was85.7%. Thus, we choose the n（NO₃-/Ac-）=1tobe as the flameless combustion condition.And then, the effects on calcinations temperature, time, second calcinationstime and the ratio of Li/Mn were investigated. The crystallinity is developed with theincreasing temperature, the LiMn₂O₄was decomposed to impurities over700oC;The increase of calcinations time can reformed the micro-morphology and improvedthe discharged capacity; The frequencies of Mn-O bonds does not change with the variety of calcinations temperature and time. The samples had the goodelectrochemistry performance synthesized for12h or9h at lower temperaturerange（400-600oC） or high temperature（700oC） respectively. The initial specificcapacity is116.1and104.1mAh·g^-1which synthesized at500oC for12h and700oCfor9h, respectively. Take the sample to be calcanated second time for different time,which obtained at500oC for3h with n（Li/Mn）=1/2and n（NO₃-/Ac-）=1. It was foundthat the electrochemistry performance developed at6h, the specific capacity is111.4mAh·g^-1, raised13.7%, hold for longer time is not more effectively. Therefore,the optimization condition for flameless combustion is500oC,12h, secondcalcination6h.Cation-doping spinel LiM_xMn_2-xO₄（M=Li,Mg,Co,Cu,Ni,Al,Cr,Fe） and anion-doping spinel LiMn₂O₄-yQy（Q=F,Cl） were synthesized by the method introducedpreviously, the best amount of the ion doped into spinel was fixed. Except Fe doped,all the cation-doping samples cell parameter decrease, micro-morphology reform, thecapacity and circle performance improve. The highest capacity is138mAh·g^-1whenthe amount of Mg equal to0.02, the capacity is more than100mAh·g^-1at5C rate. Thecell parameter decrease at first and then increase of anion-doping samples. Lessdoped（y≤0.1） samples capacity get improved, though the cycle performance does notreform. The highest capacity is138mAh·g^-1when the amount of F equal to0.02, butisn’t suitable for big electric current charge and discharge.According to the results of single-doping research, we prepared multi-dopingspinel LiM_xMn_2-xO_4-yQ_y（M=Li,Mg,Al,Co,Ni; Q=F）. The results show that thecapacity and cycle performance of the Li,Al-doping and Co,Ni-doping samples ishigher the single-doping samples. Whereas, the cycle performance get a littleincrease of the Mg,F-doping samples, and the discharge capacity reduce a lotcompared with single-doping samples.