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CHAPTER -VII

SUMMARY

7.1 Conclusions

7.2 Scope of the further work on surface modified cathode materials

7.3 Possible technological applications of the surface modified metal oxidenanostructures

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CHAPTER - VIISUMMARY

7.1 ConclusionsPresent chapter consolidate all the experimental results obsenied in the present

investigation of synthesis and characterization of nanocrystalline LiCoO2, LiM nz 04 andLiNio5Co05V04 powders by combustion process using various fuels as well as fuelcombinations and their surface modification by Dy2O3 for secondary lithium batteryapplications. Also, it summaries the results of synthesis and surface modification ofnanocrystalline NiFe204 and Z r 0 2powders for various applications.

Effect of various fuels in the combustion process for the synthesis ofnanocrystalline layered L iC o0 2 powders was investigated. According to the fuel natureand fuel combination, the above investigation was carried out under four differentcategories of processes and named as,i) Direct carboxylic acid, using three different carboxylic acids such as citric acid, tartaricacid and poly acrylic acid,ii) Pechini process using three different polyhydroxy alcohols named ethylene glycol,glycerol and polyvinyl alcohol,iii) Acryl am ide, glycine and urea assisted polymeric citrate process,iv) Ammonium carboxylates assisted process using ammonium acetate, ammonium citrateand ammonium tartarate.

A systematic analysis was carried out using FTIR, XRD, TGI DTA and SEM toidentify the effect of various fuels in the combustion process in order to optimize thesuitable fuel condition for the formation of organic free phase pure nanocrystalline L ie 002

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powders. From the above analysis, it is found that organic fiee phase pure nanocrystallineLiCoOz powders were prepared fiom (i) Citric acid assisted process (with MICA ratio of1 1) (ii) Ethylene glycol assisted Pechini processes (with M iCA= 1:1 and MiEG = 1: ) , (iii)Acrylamide assisted polymeric citrate process (with M /CA= l:l and M/ACR = 1: 1) and(iv) ammonium citrate assisted combustion process (with MIAC = 1:1) and the crystallitesize was respectively found to be 38, 34, 16, and 24 nm. The Acrylamide assistedpolymeric citrate process (with M/CA=l: 1 and MIACR = 1: 1) have an advantage amongothers, which yields lower crystallite size as 16 nm and hence. it is optimized for thesynthesis of nanocrystalline L iC o0 2 powders for further studies.

Similarly, effect of various fuels and experimental conditions were investigated forthe synthesis of nanocrystalline LiMn204 and L iN io .j C ~ 0 5 V 0 4owders. In this, twodifferent combustion processes were used for the synthesis of nanocrystalline LiMnzO4powders, which are given below,

0 Urea assisted polymeric citrate process using citric acid and urea under aceticconditionNitric acid assisted Pechini process using citric acid and ethylene glycol- effect ofthe nitric acid addition was investigatedA systematic study was carried out to investigate the above through FTIR, XRD,

TGIDTA, SE M and TEM analysis and the following conclusions were made. From theTG/ DTA results o f polymeric intermediates for the synthesis of LiM nz04, it is observedthat the ignition temperature and duration of combustion reaction were decreased withincrease of urea content. Lowest ignition temperature was observed at 255 OC forpolymeric intermediates prepared with W U= 1 2. Phase pu re organic free nanocrystallineLiM n204 powder was synthesized by calcining th e polymeric intermediate prepared withMI U= 1 2 and the crystallite size is found to be 19nrn.

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In nitric acid assisted Pechini process for the s,ynthesis of nanocrystalline LiMn204powders, addition of nitric acid to the starting solution inhibits the precipitation of metalcitrates and motivated the better polymerization through the esterifidation between citricacid and ethylene glycol. Large expansion of the polymeric resin was observed for thePechini process with nitric acid addition, whereas, Pechini process without nitric acidaddition results hard solid mass. The phase pure nanocrystalline LiM n204 powders wereobtained at 450 OC and the smallest crystallite size is found to be 16 nm for the Pechiniprocess with nitric acid addition. From both the investigations the lowest crystallite size of16 nrn was obtained using Pechini process with nitric acid addition, hence it i s optimized.

Also, the following two different combustion processes were investigated for thesynthesis of nanocrystalline L iN ia ,5 C ~o ,j 04owders,

0 Citric acid assisted sol-gel combustion process using n itric acid as catalyst.Pechini process using citric acid and different ethylene glycol precursors such asethylene glycol, polyethylene glycol- 400 & polyethylene glycol- 4000)The effect of citric acid amount in citric acid assisted sol-gel combustion process

was investigated for the synthesis of nanocrystalline LiNio.jCoo.sV04powders relativelylower temperature at 450 OC. Addition of more citric acid (M/CA= 1:2 & MICA= 1.3)caused the porous foamy intermediates (dried gels). However, fiom FTIR results, theintermediates prepared with MICA = 1:2 and 1:3 exhibit organic residuals in the finalproducts due to the poor combustion. Organic free phase pure LiNio.jCoo.jV04 powder wasprepared with MICA = 1:l and the crystallite size was found to be 89 nrn. Also, th e effectof various ethylene glycol precursors (EG, PEG-400 and PEG-4000) were investigated forthe synthesis of nanocrystalline LiNi0.5C00.5V04owders b y Pechini process. FTIR, XRD,DSC and SEM results confirmed that the addition of polyethylene glycols (PEG-400 andPEG-4000) to citric acid decreases the porosity of polymeric intermediates, which caused

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the poor combustion with long duration and leaves organic impunties in the preparedLiN io~C oo5V04 powders. The lowest crystallite siz e was found to be 49 nrn forLiNio COO 5V04 powders prepared by EG assisted process at relatively lower temperature(450C for 12 hours). From both the investigations, the lowest crystallite size was found tobe 49 nrn for the Pechini process with ethylene glycol and hence it is optimized for thefurther investigation.

From the above results, the following conclusions were made,Nature of the fuels and fuel combinations play major role in the formation ofpolymeric intermediates and influence in the microstructure, foaming nature andthermal behaviors, which plays major role in their physiochemical properties of thefinal products.

o Among the various fuels used in the present study, a high porous foamy polymericintermediates were formed only in the four processes, which are polymeric citrateprocess, ethylene glycol & polyvinyl alcohol assisted Pechini processes andacrylarnide assisted polymeric citrate process.From TGJDTA, FTIR, XRD and SEM analysis of the polymeric intermediatesobtained fi-om various processes, it is found that the intermediate with foamy natureexhibits complete decomposition of the organic derivatives during the calciningprocess.Hence, it is necessary to choose the precursor chemicals, which is having the abilityto form the foamy intermediate with porous microstructure. Such a foamyintermediate with porous microstructure may lead to the formation of organic free,homogenous, ultra fine and non agglomerated free particles o f final product.

Effect of calcining temperature on electrochemical performance of synthesizednanocrystalline LiMn204cathode powders were investigated and the highest capacity was

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observed for the lithium battery fabricated using LiMn204 powder calinced at 750 OC.Present investigation ascribed that the organic free cathode powders with orderedcrystallinity are necessary for better electrochemical performance, which can be achievedby either choosing proper fuel as well as calcining procedure.

Novel polymeric resin process is developed and investigated for the coating of metaloxides over nanoparticles using metal nitrate, polyacrylic acid and ethylene glycol. Thecomplete process was investigated through TGIDTA, XRD, FTIR and SEM analysis.Coating of thin Dy203 layer over the nanocrystalline L iCoOz, L iM nr 04 andLiNi05Co05V04 powders was confirmed by FTIR and SEM (EDS) analysis. The newlydeveloped polymeric resin process is a simple, cost effective and can be used as apromising process for the coating of variety of metal oxides over the nanostructuredmaterials for the development of various types of technologically important materials.Furthermore, the newly developed polymeric resin process and its chemistry can be scaledup to the industry for large scale production of various types of novel surface activematerials relatively at lower temperature and shorter time.

Two different types of surface modifications of metal oxides (NiFe204 and Zr02)were successfully investigated using sol-gel and polymeric resin processes for differentapplications. Nanocrystalline spinel NiFez04 powder of 14 nm was synthesized byethylene glycol assisted polyacrylic acid process at 450 OC. Coating of S i0 2 hin layer overnanocrystalline NiFe204powder was carried out using sol-gel process. XRD, FTIR, SEMand TEM analysis confirmed the coating of SiO2 layer over NiFe204 powders. From XRDanalysis, it is confirmed that the coated S i0 2 layer was in amorphous phase. VSM analysisconfirms the superparamagnetic behavior for both bare and S i0 2 coated nanocrystallineNiFez04 powders.

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Nanocrystalline ZrO:! powders were synthesized by novel acrylamide assistedpolymeric citrate combustion process at lower temperature of 600 'C. The lourestcrystallite size of the obtained ZrO? powders with two phases, such as monoclinic (m-ZrOz) and tetragonal (t-Zr02) was found to be 12 nm (m-ZrO?) and 14nm (t-ZrO2) for theZrOz powders prepared with M/CA ratio of 1:3 and hence it is optimized. Polymeric resinprocess was successfully exploited for the coating of M o o 3 layer over nanocrystallineZrOz powders at 500 O C . Coating of M oo3 layer is confirmed by FTIR, XRD and SEM-EDS analysis. Furthermore, the newly developed polymeric resin process and its chemistrycan be used for the developm ent of wide range of surface enhanced metal oxides relativelyat lower temperature and shorter time.

7.2 Scope of the further work on surface modified cathode materialsThere has been a significant research work over a decade in the area of surface

modified cathode materials in order to enhance the performance of rechargeable lithiumbatteries due to the huge requirement of portable energy devices. Synthetic approaches(physical and chemical) towards the fabrication of surface modified nanocrystallinecathode materials received more attention to meet the requirements such as uniform andhomogeneous coating. In order to investigate the effect of surface modified cathodematerials in lithium battery application, various techniques such as electrochemicalimpedance spectroscopy, accelerating colorimetry, atomic force microscopy,electrochemical force microscopy, etc., are can be used.

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7.3 Possible technological applications of the surface modified metal oxidenanostructures

Recently, surface modified nanostructures have received tremendous scientificimportant because of their wide range of potential applications. Some of the possibletechnological applications of surface modification of metal oxide nanostructures are givenbelow,

0 The surface modified magnetic nanoparticles found wide range of applications suchas magnetic resonance contrast-enhancing media, therapeutic agent in cancertreatment (e.g., hyperthermia), drug delivery, cell separation, and proteinimmobilization.Surface modification of ceramic nanostructures (A1203, ZrOl, C e0 2 , etc.) withmetals such as Ni, Co, and Ag has received intensive attention since the coating ofmetal layer acts as a protective against corrosion and wear, as well as to improvethe wetting of ceramics.Surface modified nanocrystalline metal oxides (A1203, Zr02, CeO2, etc.) withSO^^-, WO,, M o o g , etc., are found applications in many fields such as catalysis,

restorative dentistry, high temperature ceramics, polymer nano composites, microelectronics, etc., either in pure state or its modified state with other metal oxides.