Preparation And Electrochemical Performance Of Olivine LiMnPO₄as Cathode Materials For Lithium-ion Batteries

Among the olivine-type lithium transition-metal phosphates LiMPO₄(M=Fe, Mn, Co, orNi), LiMnPO₄has been intensively investigated as promising cathode materials forrechargeable lithium ion batteries owing to their thermal stability, low cost, andenvironmental benignity. This material offers²0%higher energy density than LiFePO₄dueto its higher Li⁺intercalation potential of4.1V (vs. Li⁺/Li). Importantly, the higher operatingvoltage of LiMnPO₄is compatible with most of the currently used liquid electrolytes.However, LiMnPO₄suffers from low electronic and ionic conductivities leading to lowercapacity and poor rate capability, which remain major challenges when used in practicalhigh-power cells. In this work, strategies including control morphology, minimizing particlesize, Mn-site substitution and enhancement of electronic contact between particles through theintroduction of carbon nanotubes additives, have been applied in efforts to improve theelectrochemical performance. The morphology, structure and performance of a series oflithium manganese phosphates were also investigated and discussed in detail.Well-defined three-dimensional hierarchical flower-like LiMnPO₄microspheres weresuccessfully synthesized by a simple one step low-temperature solvothermal method. Thediameter of microspheres ranged from10to20μm. Each flower-like single crystallineconsists of LiMnPO₄sheets self-assembled via a nucleation-dissolution-re-crystallizationprocess. It is found that the presence of diethylene glycol molecules in water plays a key rolein controlling the morphology of the product. Also, glucose not only facilitates the formationof single crystalline LiMnPO₄but also gives rise to a uniform amorphous carbonaceous layeron the LiMnPO₄nanoplates. A possible growth mechanism was also proposed based on theFESEM and XRD characterizations of the products collected at different reaction times. Thelithium storage properties of the LiMnPO₄were investigated as a cathode material for lithiumion battery. Electrochemical tests revealed the morphology dependent electrochemicalproperties of LiMnPO₄materials.LiFe_xMn^1-xPO₄/C (x=0,0.15,0.2) nanocomposites were successfully synthesized through a facile solid state reaction from relatively low cost raw materials. SEM and TEM images haveshown that nano-sized particles of olivine LiFe_xMn^1-xPO₄around100nm in size wereobtained under the described conditions. The carbon decomposed by the sucrose precursor notonly gives rise to an amorphous carbon layer with a thickness of³nm on the surface ofLiFe_0.2Mn_0.8PO₄but also facilitates to bridge the nanoparticles to form a highly-conductive3D network throughout the composites. When evaluated as a cathode material for lithium-ionbatteries, the LiFe_0.2Mn_0.8PO₄/C exhibits the best lithium storage properties with high specificcapacity, superior rate performance and excellent cycling stability. A high discharge capacityof155.9mAh g^-1can be achieved at a current density of0.05C, and even delivering areversible capacity of up to77.8mAh g^-1at2C. The obtained results indicate that theLiFe_0.2Mn_0.8PO₄/C hybrid material has a potential application as cathode material forhigh-power lithium-ion batteries. Moreover, Co-doped LiCo_0.1Mn_0.9PO₄/C microspheres havebeen synthesized using MnCO3microspheres as self-sacrifice templates. Galvanostaticcharge/discharge cycling indicates that this material exhibits a discharge capacity of58.7mAh g^-1at0.05C rate and very stable retention of99%after50cycles, revealing good cyclingstability.Porous LiMnPO₄and LiMnPO₄/MWCNT composite were prepared using a citric acidassisted sol-gel method. The results indicated that fine-sized, well-crystallized olivineLiMnPO₄was synthesized. The interlaced carbon nanotube networks were intimatelyembedded and incorporated into the porous LiMnPO₄particle to forming highly-conductive3D networks. The LiMnPO₄particle and LiMnPO₄/MWCNT composite had rich hierarchicalpores. Detailed analysis showed that average pore size was in the mesoporous range, and aspecific surface area of73.7and69.9m2g^-1, respectively. Compared with the LiMnPO₄particle, the LiMnPO₄/MWCNT composite exhibited much higher specific capacity. Whendischarged at a rate of0.05C and2C, the capacity was108.8,33.2mAh g^-1, respectively.MWCNT effectively improved the electronic conductivity of the hybrid materialsdemonstrated by electrochemical impedance spectroscopy (EIS). The improvedelectrochemical performance of the LiMnPO₄/MWCNT electrode is attributed to the enhanced electrical conductivity caused by tighter binding on the carbon nanotubes with theLiMnPO₄primary particles as well as by the interconnected open pores with a high surfacearea.