Research On Synthesis And Properties Of Li-V-O System Cathode Materials For Lithium Ion Secondary Batteries

Nowadays, the threats of limited oil storage and global warming arisingfrom the fast development of modem society and economy have become a greatincentive to develop renewable energy generation and storage systems.Rechargeable lithium-ion batteries （LIBs） are considered the most promisingenergy storage technology for portable electronics, electric vehicles, hybridelectric vehicles, smart grid because of their high operation voltage, high energydensity, long life, small size, light weight, easy design configurations, etc. Theperformance of rechargeable batteries depend on both the characteristics of theactive materials contained in the cathode and anode and on the integrationtechnologies of the battery components, and among them, the performance ofcathode materials is the most important.Compared with other cathode materials for lithium ion batteries, lithiumvanadium oxides belonged to Li-V-O system have been extensively studied overthe past two decades due to its attractive electrochemical properties such as highspecific discharge capacity, good rate capacity, long cycle life, good safety,facile preparation, low cost and rich reserves of raw materials. Among them, thelayered lithium trivanadate (Li_1+xV₃O₈) has very attractive characteristics such ashigh specific energy （above300mAh/g）, good rate capability and long life due totheir unique crystal structure and outstanding structural stability. This makes it isconsidered to be a promising candidate cathode material for lithium secondarybatteries. The preparation methods and conditions, crystallinity, size andmorphology of Li_1+xV₃O₈have significant influences on its electrochemicalproperties such as discharge capacity and cycle stability.In this thesis, we introduced improved sol-gel process and microwavehydrothermal assisted low temperature solid state reaction to prepare Li_1+xV₃O₈cathode materials, and studied the effects of technological conditions on thephase, morphology and electrochemical performances of the as synthesized products. Furthermore, NH4V₃O₈microcrystallites with different morphologieshave been synthesized via microwave hydrothermal method for the first time.The main contents are summarized as follows:（1） On the synthesis of ultrasonic improved sol-gel process by using citricacid as chelating agent, pure LiV₃O₈with rod-like morphology can be preparedat400℃for6hours; the morphology of the product changes to flat-like and thesize of the product becomes larger with the calcination temperature increasing.On the synthesis of LiV₃O₈via microwave assisted sol-gel, LiV₃O₈can beprepared under microwave power of540and720W for30min, at a power of900W, a high temperature is generated which results in the Li_0.3V₂O₅impuritycaused by decomposition of some LiV₃O₈.（2） We studied the effects of different chelating agents on theelectrochemical performance of LiV₃O₈prepared via sol-gel process. TG-DSCanalysis indicates that the crystallization temperature of LiV₃O₈is around320℃,which is much lower than those prepared by other three chelating agents. Theproducts prepared with oxalic and tartaric acid show thin sheet-like or rod-likemorphologies, small particle size and uniform distribution. The product preparedby oxalic, tartaric, citric and malic acid delivers an initial discharge capacity of304.4,296.8,268.7and275.3mAh/g, respectively. After20cycles, they remaindischarge capacity of250.2,237.6,198.5and206.8mAh/g, respectively. Theproduct prepared with oxalic acid exhibits the best electrochemical performance.（3） Pure LiV₃O₈can be synthesized at300℃for2hours via microwavehydrothermal assisted sol-gel process using LiOH, NH₄VO₃and oxalic acid asraw materials. Calcination temperature has obvious effect on the morphologyand electrochemical performances of the product. The product prepared at300℃gives the highest initial discharge capacity of311.3mAh/g; the product preparedat350℃exhibits the best cycle stability which can remain a capacity of271.6mAh/g after10cycles. The electrochemical performance of the productdecreases with the increase of calcination temperature.（4） We introduced microwave hydrothermal assisted low temperature solidstate process to prepare LiV₃O₈. The product prepared with V₂O₅showsimpurities. But when NH₄VO₃is used as raw materials, pure LiV₃O₈can beobtained at the same calcination temperature. With the temperature increasing, the morphology of the product changes from thin sheet-like to rod-like, theparticle size changes from about100-500nm to several micrometers, and theinitial discharge capacity decreases. The product prepared at350℃shows thehighest initial discharge capacity of288.1mAh/g, while the product prepared at500℃exhibits the best cycle stability which can remain82.7%of its initialcapacity after20cycles. Polyvinylpyrrolidone （PVP） has no obvious influenceon the phase and morphology of the product but decrease its performance.（5） NH4V₃O₈microcrystallites have been synthesized via microwavehydrothermal reaction using NH₄VO₃as raw materials for the first time. Resultsshow that pH value has significant influence on the morphology of the products,it changes from irregular sheet, leaf, stretched six party flakes to six party flakeswhile pH changes from2.0to4.5at an interval of0.5. The crystallinity of theproduct becomes higher with the increase of microwave hydrothermaltemperature. Polyethylene glycol （PEG4000） makes the morphology of theproduct become incomplete while PVP has no obvious effect on it.