LiMPO₄（M=Fe,Mn） Nanocrystals:Hydrothermal Synthesis And （De）lithiation Mechanism

Since the industrial revolution,the rapid development of science and technology has enriched the humans’ life,whereas at the same time,fossil energy consumption and CO2 emissions increase year by year,resulting in global warming and critical ecological environment issues.New energy vehicles are evolving quickly to enable in the realisation of carbon peaking and carbon neutrality targets.As the power source of new energy vehicles,power batteries attracted much attention.It is very important to develop the power batteries with low price,abundant resources and excellent safety for new energy vehicles.LiFePO4 with olivine structure is favored because of its advantages such as low cost,high safety and environmental friendliness.However,LiFePO4 is insufficient to meet today’s high-power demand due to its low electronic conductivity(<10-9 S cm-1)and low lithium-ion diffusion coefficient(10-16～10-14 cm2 s-1).Based on the market demand for fast dis/charging of new energy vehicles,it is urgent to improve the electrochemical performance of LiFePO4.Current strategies mainly include control of crystal size and orientation by improving conditions,surface coating,doping,defect control,etc.In addition,the theoretical energy density of LiFePO4(586 Wh kg-1)is increasingly unable to meet the demand for range.In contrast,LiMnxFe1-xPO4(0≤x≤1)with higher theoretical energy density(700 Wh kg-1)has been recognized as the next generation ideal cathode material for LiFePO4 development,however,the inherent Jahn-Teller effect of Mn3+ leads to MnⅢO6 octahedra severe distortion,which hinders the Li+diffusion.In this paper,based on the composition and working principle of Li-ion batteries,combined with the crystal structure and(de)lithiation mechanism of LiMnxFe1-xPO4,the high-performance strategy of LiMnxFe1-xPO4 is discussed in depth,which provides a platform for the industrial production of high-performance LiMnxFe1-xPO4.The main points of this paper are as follows:(1)Based on the advanced knowledge of the hydrothermal synthesis mechanism for LiFePO4,the morphology of LiFePO4 crystal is modulated.In this paper,the effects of different synthesis conditions,such as the order of material addition,precursor morphology,heating method and pH on the morphology and electrochemical properties of LiFePO4 were investigated in detail.It was indicated that the average particle size of LiFePO4 synthesized by material addition order of LiOH→H3PO4→FeSO4 was significantly smaller than that of LiFePO4 synthesized by addition order of FeSO4→H3PO4→ LiOH during the hydrothermal synthesis of LiFePO4.Based on the result,Li3PO4 with uniform and fine grain morphology was synthesized via the homogeneous co-precipitation method by modulation of the precursor morphology to increase its dissolution rate,which further reduced the grain size of LiFePO4.Using rapid heating characteristics of microwave,increase the heating rate,and thus shorten the nucleation window time,thereby increasing the nucleation rate to reduce grain size.In addition,increasing the pH as well as the concentration of the Li3PO4 precursor also allowed the synthesis of LiFePO4 nanocrystals with smaller average grain size.(2)In order to improve the electrical conductivity of LiFePO4,a bio-inspired coating method is proposed to prepare carbon-coated LiFePO4 nanocrystals in this paper.Tannic acid(TA)as a plant polyphenol,is capable of coating hydrothermally grown LiFePO4 nanocrystals by means of simple wet impregnation and drying.Upon subsequent mild pyrolysis in an inert atmosphere,the TA coating on LiFePO4 nanocrystals transformed into a homogeneous and compact carbon film.Introducing conductive carbon film on LiFePO4 nanocrystals definitely increases the electrical conductivity of the LiFePO4/C composite.Whereas,the reaction kinetics study by potentiostatic intermittent titration technique reveal that excessively thick carbon film limits the Li+diffusivity of the LiFePO4/C composite during(de)lithiation.As a result,there must be a balance between electrical conductivity and Li+ diffusivity to achieve required electrochemical properties.Herein,upon coating a carbon film with moderate thickness,the TA-derived carbon-coated LiFePO4 nanocrystals exhibit superb electrochemical properties for high-rate lithium-ion batteries.(3)An interesting observation was noted by electrochemical measurements on LiMnxFe1-xPO4 solid solution.With the increase of Mn content,the energy density of LiMnxFe1-xPO4 solid solution shows a non-monotonic variation with rising followed by decreasing,the LiMn0.5Fe0.5PO4 has the highest energy density and the minimum degree of polarization.In addition,the relaxation times distribution of the electrochemical impedance spectroscopy at the redox peak potentials of LiMnxFe1-xPO4 indicates that LiMn0.5Fe0.5PO4 has excellent Li+diffusivity.Therefore,it can be deduced that the hopping distance of small polaron between Fe2+/3+or Mn2+/3+ is an important factor affecting the electrochemical properties of LiMnxFe1-xPO4.Whereas,the ideal Li+diffusivity couldn’t be obtained either too short or too far distance of small polariton hopping.In situ X-ray diffraction measurement indicated that as the Mn content increased to LiMn0.5Fe0.5PO4,the phase transition characteristics of LiMnxFe1-xPO4 solid solution gradually transitioned from two-phase transition to single-phase transition with more activated particles and smaller miscibility gap.