Study On Synthesis And Modification Of LiMnPO₄Material For Li-ion Battery

Lithium ion batteries have been widely used in mobile phones, camera, electric toolsand notebook computers, etc. because of its high working voltage, light, high specific energy,no memory effect, no pollution and long cycle life. Nowdays, LiFePO₄has been graduallyindustrialized. However, the potential of LiFePO₄was only3.4V³.7V relative to lithium.The potential of LiMnPO₄can improve reach4.1V⁴.5V with30%higher energy densitythan that of LiFePO₄. Besides, the raw materials used for synthetize are cheap andenvironment friendly. As a result, LiMnPO₄has been concerned as a new generation ofpositive material. In this paper, we synthesized LiMnPO₄by high temperature solid phasemethod, by different synthesis conditions. Glucose coating and magnesium doping werestudied to improve the performance of LiMnPO₄.Firstly, the reaction mechanism of synthesizing LiMnPO₄by raw materials Li2CO3,MnCO3and NH4H2PO₄was studied by DSC-TG analysis. The condition that synthesizedLiMnPO₄was determined by optimizing the ratio of Li/Mn, calcinations temperature, carbonmodifcation and magnesium doping amount.Research shows the lithium volatilized underhigh temperature, so we decided the ratio is Li:Mn=1.02:1, LiMnPO₄synthesized under lowtemperature had mixed phase, so we decided the preliminary calcinations temperature is750℃, Glucose coating had obviously improved the electrochemical performance fromalmost no discharge specific capacity up to40mAh/g and doping magnesium could alsoimprove the discharge specific capacity from40mAh/g to100mAh/g.The influence of different calcination temperature, calcination time, the adding quantityof glucose and the content of doping magnesium on the structure, morphology, and thedischarge specific capacity and cycle performance of LiMnPO₄was also studied. We foundthe crystallize size grew up if the calcination temperature and the calcination time were toohigh or too long separately. The crystallinity of the sample was not good if the calcinationtemperature and the calcination time were too low or too short separately. The coating effectwas not obvious to the discharge specific capacity when the glucose was not enough.However, the coating layer would be too thick and reduced the electrochemical performance of LiMnPO₄if the glucose amount was too much. The internal conductivity and the speed oflithium ion diffusion would be improved if the magnesium doping is too little, while theelectrical performance of LiMnPO₄would be reduced when the magnesium doping is toomuch due to the less amount of active substance. By contrast we found the sample ofLiMn_0.98Mg_0.02PO₄/C （40%wt） under750℃-10h had good electrochemical performance, thefirst discharge specific capacity was100mAh/g at0.05C, and remained in73mAh/g aftercycling30times. And the first discharge specific capacity was80mAh/g at0.2C, andremained in43mAh/g after cycling30times.Finally, we tested the material through the cycle volt ampere and AC impedance. Resultsshow that the internal impedance of LiMn_0.98Mg_0.02PO₄/C was large with obvious polarizationphenomenon. The reversibility of this materied was poor. So the cycle performance droppedquickly, especially the cycle performance at0.2C fell even quicker.