Effects Of Doping On Physical And Electrochemical Properties In LiFePO₄Cathode Materials For Lithium-ion Batteries

The olivine LiFePO₄now stands as a competitive candidate of cathode material forthe next generation of a green and sustainable lithium-ion battery system due to its longlife span, abundant resources, low toxicity, and high thermal stability. However, poorintrinsic electronic and ionic conductivity hamper its extensive application in high-ratelithium ion batteries. Attempts have been devoted to optimizing the material for superiorelectrochemical performance, such as modifying the surface of LiFePO₄with conductivenanostructures and doping alien ions in the lattice, etc.In this study, I have reviewed the development and challenges for the LiFePO₄cathode material, including the defects in the crystal and the mechanism of lithiuminsertion/extraction. My study is focused on controlling the morphology, particle size andimpurities of LiFePO₄as well as its doped and coated derivatives. The major goal is tounderstand the enhancement mechanism of the electrochemical performance for LiFePO₄which is helpful to provide meaningful insights on the rationale of future design ofLiFePO₄based high power lithium batteries with high charge/discharge capacities andwith better cycle performance. The main results and new findings in this work aresummarized as follows:（1） Phase-pure Na⁺and Cl-co-doped LiFePO₄has been successfully synthesized viasolid-state reaction. Rietveld refinements and XPS evidently confirm that the Na⁺and Cl-ions are co-doped into the olivine structure. The electrochemical measurementsdemonstrate that the co-doped material exhibits not only enhanced initial capacity andcoulomb efficiency, but also high rate capability, indicative of a remarkably improvedelectrochemical performance as compared with the pristine and the doped LiFePO₄/C withseparate Na⁺and Cl-doping. Therefore, Na⁺and Cl-co-doping is an effective way to attainhigh capacity and high rate capability for the phosphate positive materials.（2） Phase-pure LiFePO₄and metal-ion doped, LiFe_0.95M|₍0.05_PO₄powders （M=Na,Mg, Zn, Mn, Ni, Al, V） have been successfully synthesized via solid-state reaction. It isfound that LFP composite shows the initial discharge capacities of150,147,140,132 and117mAh g-1at0.1,0.2,1,2and5C, respectively. Moreover, a charge/discharge rateof1C still offers a discharge capacity of129mAh g-1after100cycles. In comparison toundoped LiFePO₄, the V and Ni-doped LiFePO₄/C presented a higher rate capability thanthe pristine one did. It indicates that there is a doping effect in the redox reaction, LFVPexhibits the discharge median potential of3.1855V converts the higher capacity117.1mAh g-1to the superior energy density at5C-rate.In spite of a detailed characterization, a clear correlation between the compositionsof the various samples with their electrochemical performances remains difficult. Overallelectrochemical performance is a combined effect of lattice parameters, electronicconductivity and Li+ion diffusion coefficient, which in turn kinetically alters the Li-cationand electron availability in these materials. Nevertheless, some general trends are visible:（i） dopant can course the corresponding median potential changed,（ii） the dopantsincrease the electronic conductivity meanwhile decrease Li+ion diffusion coefficient, theimproves electronic conductivity did not greatly affect the electrochemical performance,（iii） Li+“effective” ionic diffusion plays a more important role of the improved rateperformances （iv） doping with alien atoms may induce structural distortion, resulted in thecyclic degradation of LiFePO₄. The study might provide helpful insights on the rationaleof future design of LiFePO₄based high power lithium batteries with high charge/dischargecapacities and with better cycle performance.（3） LiFePO₄/C samples doped with different titanium content were prepared via solidstate reaction process. Combined with XRD, CV curves and charge/discharge profiles, it isshown that titanium can be doped into the host lattice when the doping level of titanium islow （x≤0.05）; whereas excessive titanium can form titanium phosphate impurities likeTiP2O7and LiTi2（PO₄）3at high doping level （0.07≤x≤0.2）. The electrochemicalperformance can be greatly improved by appropriate amount of titanium incorporation.LiFe0.9Ti0.1PO₄/C sample shows excellent reversible capacity and rate capability, probablybecause of the enhanced electronic conductivity and electrode kinetics due to the titaniumphosphate modification caused by Ti doping. Our experiment provides a clear feature oftitanium incorporation in LiFePO₄, which is helpful to understand the enhancementmechanism of the electrochemical performance for LiFePO₄. （4） Impurity phases can be effectively controlled by off-stoichiometry andNa-incorporation in LiFePO₄. By creation of Fe and P deficiency, LiFe0.9（PO₄）_0.95/Cconsists of well-recognized LiFePO₄accompanied with impurity phases Li₄P₂O₇andLi₃PO₄. Li₄P₂O₇impurity can be suppressed by Na incorporation. The Li₄P₂O₇and Li₃PO₄impurities show different effects on electrochemical performance. Existence of Li₄P₂O₇and a small amount of Li₃PO₄can improve the capacity and rate capability. However, highcontent of Li₃PO₄makes the electrochemical properties poor. The Li₄P₂O₇-involvedLiFe0.9P0.95O₄/C cathode shows high polarization and low capacity retention. TheLi₃PO₄-involved1%Na-doped LiFe0.9P0.95O₄/C exhibits the best capacity, rate capabilityand cyclability, which is ascribed to its fast kinetics and low charge-transfer resistance.Our results indicate that small amounts of cationic doping and off-stoichiometry are usefulto control the impurity phases and hence to optimize the electrochemical performance forthe olivine phosphate cathode materials.（5） The careful electrochemical experiments on Lix（MnyFe1-y）PO₄have led to the twohitherto unidentifed unique features in this system:（i） a systematic up-shift of the redoxpotential for both Fe³⁺/Fe²⁺and Mn³⁺/Mn²⁺by0.1V;（ii） an opposite kinetic effect,reduced polarization in the Fe³⁺/Fe²⁺plateau while increased polarization in theMn³⁺/Mn²⁺plateau upon increasing Mn content. Substitution of Fe²⁺by Mn²⁺in LiFePO₄may be a practical way to improve the electrode kinetics, and hence to enhance ratecapability and energy density. However, a linear loss in capacity was observed withincreasing Mn content （y） in LiMnyFe1-yPO₄, which was explained by a gradual decreaseof electronic conductivity due to the Mn substitution.In order to enhance rate capability and energy density of LiFePO₄-based cathodematerials, we substitute20at%Mn for Fe to achieve LiFe_0.8Mn_0.2PO₄/C composite.Synthesis condition has been optimized. Electrochemical measurements show that thecapacity of LFMP/C strongly depends on the synthesis condition, i.e., mianly the sinteringtemperature. The composite sintered at600C exhibits the best performance. It shows theinitial discharge capacities of160,156,147,144,133and122mAh g^-1at0.1,0.2,1,2,5and10C, respectively. At5C, the LFMP/C（600） cathode delivers an average specificdischarge capacity of132mAh g^-1, i.e., an average specific energy density of414Wh Kg^-1 in100cycles. We attribute the excellent performance of LFMP/C（600） to high crystallinity,small and uniform particle size and high specific surface area. We believe that LiFePO₄doped with small amount of Mn can deliver an excellent electrochemical performance,especially a high energy density, which benefits for practical applications.