Preparation And Performance Of Metal Doped LiMnPO₄/C Nano Composite Cathode Material

Olive LiMnPO₄ was considered as one of cathode materials for LIBs with utmost application prospect due to its safety,environmental compatibility,thermostability,high capacity and low cost.But it had not been applied in industry on a large scale because it suffered from poor electronic conductivity and inferior lithium transportation.Nanocrystallization,metal doping and carbon coating were three important and effective approaches to conquer above-mentioned problems.In the dissertation,the structures,performances and mechanisms of as-prepared LiMnPO₄ materials were investigated,which were modified by Mg and Fe stoichiometric ratio doping,Nb non-stoichiometric ratio doping,microwave sintering process,as well as conducting polymer modifying and coating.Firstly,LiMn_1-xMg_xPO₄/C?x=0,0.03,0.06,0.09?nano composites with different Mg stoichiometric ratio doping were synthesized by a facile hydrothermal method.The grain of as-prepared LiMnPO₄ precursor showed a rodlike shape with a diameter of 200 nm and had an orientated growth,then formed lamellar aggregation.The grains of LiMn_1-xMg_xPO₄ were coated by a carbon layer with a thickness of about 2 nm.The particles of LiMnPO₄/C distributed evenly and had no agglomeration.With Mg doping content increasing,the size of LiMn_1-xMg_xPO₄ grain decreased remarkably and were about 20-30 nm.Moreover,the grains of the as-synthesized materials appeared more and more serious adhesion and their grains'size increased.The discharge capacities of LiMn_1-xMg_xPO₄/C firstly increased and then decreased as Mg doping content increased.Among the four samples,the LiMn_0.94Mg_0.06PO₄/C electrode showed the highest discapacity of 115.8 mAh g^-1 at 0.1 C.Meanwhile the sample presented both the best rate capability and cycling stability with rate capability rention of 93.7%at 1.0 C/0.1 C and a capacity rention of 95.7%after 30 cycles at0.1 C,respectively.Secondly,LiNb_zMnPO₄/C?z=0,0.0011,0.0022,0.0033?nano composites with different Nb non-stoichiometric doping were synthesized by a facile hydrothermal method.Nb doping could suppress the growth of grain and lead to grain refining as well as the increasement of specific surface area.With Nb doping content increasing,the volumes of unit cell of LiNb_zMnPO₄/C firstly decreased and then increased while the boundary between particles became more and more indistinct and presented cracking.Whereas the grains of LiMnPO₄/C nanocomposites without Nb doping were about 20-30 nm via TEM and the boundary between grains seemed very clear.The grains were coated by a carbon layer with a thickness of about 2 nm.With Nb doping content increasing,the discharge capacities of LiNb_zMnPO₄/C electrodes firstly increased and then decreased.Among these samples,the LiNb_0.0022MnPO₄/C electrode showed the highest discapacity of 117.4 mAh g^-1 at 0.1 C.It also presented both the best rate capability and cycling stability with rate capability rention of94.1%at 1.0 C/0.1 C and a capacity rention of 97.3%after 35 cycles at 0.1 C.Thirdly,LiMn_0.75Fe_0.25PO₄/C nano powders were prepared by a facile hydrothermal and ball-milling method,and then calcined.When the calcination temperature was increased from550 to 700?,the grain size of LiMn_0.75Fe_0.25PO₄/C increased markedly,and the electronic conductivities of the coating carbon showed the same growth trends.However,the discharge capacities of LiMn_0.75Fe_0.25PO₄/C decreased with increasing calcination temperature.The EIS testing indicated that the charge-transfer resistances of LiMn_0.75Fe_0.25PO₄/C electrode decreased due to the increasement of conductivities of coating carbon.Thus the growth of grain size contributed much more to the discharge capacities of the as-prepared electrodes than the conductivities of coating carbon.In other words,lithium ion diffusion process was a control step.The powder calcined at 550? exhibited the highest discharge capacity of 131.9mAh g^-1 at a rate of 0.1 C.Meanwhile it also showed the best rate capability and excellent cycling stability with a rate capability rention of 86.0%at 1.0 C/0.1 C and a capacity rention of 99.8%which had scarcely any decay after 50 cycles at 0.1 C.Fourthly,LiMn_0.75Fe_0.25PO₄/C nano composites were synthesized by a facile hydrothermal method,combining with spray drying and microwave sintering process.The LiMnPO₄ precursor showed a rodlike shape with a diameter of about 200 nm and a length of about 1?m and had an orientated array growth,which was much bigger than that of the milled and sintered sample?about 50 nm?.The lattice parameters a,b and c of the microwave sintered sample were all less than those of the ordinary heating sintered sample while the unit cell volume of 299.66?³ for ordinary heating sintered sample contracted to that of299.15?³.The microwave sintering process made rate of temperature increase faster and kept a portion of carbon as stereochemical net structure which led to increasement of porous formation and specific surface area.However the sample via microwave heating sintering presented smaller particle and less agglomeration which resulted in a shorter Li⁺ion diffusion path and promoting in electronic conductivities.Both samples delivered relatively approximate discharge capacities and initial coulombic efficiencies.The microwave sintered sample presented an excellent cyling performance with a discharge capacity of 143.5 mAh g^-1 and a capacity rention of 94.3%after 225 cycles at a rate of 0.1 C.Both CV and EIS showed that the microwave sintered electrode had smaller charge-transfer resistances and a minor polarization and thus resulted in improved electrode kinetics.Finally,the conducting polymers modified LiMn_0.75Fe_0.25PO₄/C nano composites were prepared by a facile hydrothermal method combining with conducting polymer coating and sintering process.The unit cell volume of PPy nanotube modified sample was contracted to299.42?³,which was in agreement with the variation trend of specific surface area measured by BET method.The grain size of PPy nanotube modified sample was about 30 nm and obviously less than those of sucrose coated sample and PANI coated sample.PPy nanotube decorating could inhibit the growth of grain crystal and have a nanocrystallization effect on the particles.The PPy nanotube modified electrode delivered a discharge capacity of 98.1mAh g^-1 and an initial coulombic efficiency of 80.3%.Meanwhile the PPy nanotube modified LiMn_0.75Fe_0.25PO₄/C demonstrated a wonderful rate capability and excellent cycling performance with a capacity rention of 95.7%at 1.0 C/0.1 C as well as a capacity rention of98.5%after 50 cycles at a rate of 0.1 C.The discharge capacity of the sample at 1.0 C was almost twice as the sucrose coated sample.Both CV and EIS proved that the electrode modified by PPy nanotube had a smaller charge-transfer resistance and a minor polarization,and thus resulted in improved electrode kinetics.