Preparation Of Lithium-position-doped Lithium Iron Phosphate By Combustion Method And Surface Carbon Coating Modification

Olivine-type lithium iron phosphate（LiFePO4,LFP）is widely favored by the industry and researchers because of its abundant source of raw materials,environmental friendliness,ideal safety performance,and high cycle stability.Affected by its own structure,the LFP cathode material exhibits low electronic conductivity and Li⁺diffusion coefficient,both of which restrict its electrochemical performance.To improve its electrochemical performance,particle size controlling,ion doping and surface coating are commonly used.Among them,ion doping is considered as promising way,but the optimized doping species and concentration are still unknown.The systematic study of the effects of doping ions and doping ratios on the structure and electrochemical properties of LFP is urgent to promote the development of lithium iron phosphate.In this thesis,lithium site doping,surface carbon coating and electrochemical properties are studied.The specific work is as follows:（1）The doped LFP were prepared by combustion method,using cheap metal nitrates LiNO3,Fe（NO3）3 9H2O and（NH4）2HPO4 as raw materials,citric acid as complexing agent and reducing agent.and the amount of citric acid is n（Critic Acid）:n（LFP）=1.2,the firing system was 5°C/min,heated to 700°C for 10h,cooled naturally.Undoped lithium iron phosphate（marked as N-LFP）and doping ions with different valence states(Na⁺,Sr²⁺,Al³⁺,Zr⁴⁺,Nb⁵⁺)were prepared without introducing protective gas（marked as M-x-LFP,M represents doping element,x represents doping ratio）.The effects of doping valence state and ionic radius on the structural evolution and electrochemical properties were systematically analyzed by means of XRD,XPS,FT-IR,electrochemical testing,etc.The results showed that ions with different radius were doped into the LFP lattice,leading to change slightly on the unit cell parameters but remain its crystal phase.For the same kind of doping ions,a small amount of doping can significantly improve the electrochemical performance of LFP and reduce the polarization phenomenon.The first charge-discharge specific capacity of N-LFP samples is 78.7 m Ah·g^-1.Na-0.03-LFP,Sr-0.04-LFP,Al-0.01-LFP,Zr-0.04-LFP,Nb-0.03 have the best electrochemical performance among the samples with the same doped ions,and the first charge-discharge specific capacities are 117.4 m Ah·g^-1,123.8m Ah·g^-1,127.3 m Ah·g^-1,and 108.7 m Ah·g^-1,130.2 m Ah·g^-1,high-valence ion doping has a greater impact on the electrochemical performance of LFP.M（?）ssbauer spectrum showed that Fe in the N-LFP samples existed in the form of Fe²⁺,which indicated that the combustion method could completely reduce the Fe³⁺in the raw material to Fe²⁺without introducing protective gas,and the prepared product was pure N-LFP.At the same time,the M（?）ssbauer spectra of Nb-0.03-LFP samples were analyzed,and it was found that the doping of high valence ions would cause some amorphous Fe³⁺in the LFP lattice.（2）Based on the optimal ratio of each dopant ion,20 wt%glucose was added to the sample to modify the surface of the sample with carbon coating.The synthesis conditions are the same as above,and high-purity argon gas is introduced as protective gas during sintering to prepare carbon-coated lithium-doped lithium iron phosphate（marked as M-x-LFP@G）.By analyzing the XRD patterns of the samples,the results show that the carbon in the M-x-LFP@G samples exists in an amorphous state,which does not affect the phase of lithium iron phosphate crystals.After the charge-discharge test of the samples,the first charge-discharge specific capacity of the samples after each coating increased:the first charge-discharge specific capacity of the N-LFP@G sample increased from 80.1 m Ah·g^-1 without coating to 89.5 m Ah·g^-1,the first charge-discharge specific capacity of Na-0.03-LFP@G sample was increased from 115.2m Ah·g^-1 without coating to 127.1 m Ah·g^-1,and the first charge-discharge specific capacity of the Sr-0.04-LFP@G sample was increased from 124.3 m Ah·g^-1 without coating to 128.7 m Ah·g^-1.The first charge-discharge specific capacity of the Al-0.01-LFP@G sample increased from 126.9 m Ah·g^-1 without coating to 140.5 m Ah·g^-1,and the first charge-discharge specific capacity of the Zr-0.04-LFP@G sample was increased from 107.4 m Ah·g^-1 without coating to 115.6 m Ah·g^-1.The first charge-discharge specific capacity of the Nb-0.03-LFP@G sample increased from 134.3m Ah·g^-1 without coating to 154.3 m Ah·g^-1.The Nb-0.03-LFP@G sample has the most obvious improvement.Its cycle performance was tested at a current density of 0.2 C.The test results showed that the capacity retention rate after 30 cycles was 97.8%,compared with the uncoated sample,the capacity retention rate of 89.7%has significantly improved,the charge-discharge specific capacity and cycle performance of the coated samples were significantly improved.The Nb-0.03-LFP and Nb-0.03-LFP@G samples were tested by SEM.The results show that the surface carbon coating can reduce the particle size of LFP and prevent the samples from agglomerating,and the crystals are connected by a bridge-like structure.It is beneficial to the wetting of the electrolyte and the transport of Li⁺.