Influence Of Nanostructuring On Electrochemical Performance Of Titania-Based Electrodes And Liquid Electrolytes For Rechargeable Lithium-Ion Batteries
Das, Shyamal Kumar
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The present thesis deals with the beneficial influence of nanostructuring on electrochemical performance of certain promising electrode and electrolyte materials for lithium-ion batteries (LIBs). Electrochemical performances of chosen electrodes and electrolytes have been presented in a systematic and detailed manner via studies related to both transport and lithium storage. Titanium dioxide (TiO2) or titania, a promising non-carbonaceous anode material for LIBs was chosen for the study. As part of the study, variety of nanostructured titania were synthesized. In general, all materials exhibited high lithium storage ( theoretical value for lithium storage in titania) and some of them showed exemplary rate capability, typically desired for modern lithium-ion batteries. Studies related to performance of these materials and mechanistics of lithium storage and kinetics are presented in Chapters 2-5. “Soggy sand” electrolyte, a promising soft matter electrolyte for LIBs was studied on the electrolyte side. Ion transport, mechanical strength and electrochemical properties of “soggy sand” electrolytes synthesized via dispersion of various surface chemically functionalized silica particles dispersed in model as well as LIB relevant electrolytes were studied in this thesis. Extensive physico-chemical and battery performance studies of “soggy sand” electrolytes are discussed in Chapters 6-8. A brief discussion of the contents and highlights of the individual chapters are described below: Chapter 1 briefly discusses the importance of electrochemical power sources as a viable green alternative to the combustion engine. Various facets of rechargeable LIBs, one of the most important electrochemical storage devices, are presented following the general discussion on electrochemical power devices. The importance of nanostructuring of electrodes with special emphasis on anodes for high lithium storage capacities and rate capabilities are also discussed in the opening chapter. The various advantages and disadvantages of the most commonly used electrolytes in LIB i.e. the liquid electrolytes are also discussed in Chapter 1. Suggestions for improvement of the physico-chemical properties of liquid electrolytes especially via nanostructuring (demonstrated via dispersions of fine oxide particles in liquid electrolytes in Chapters 6-8) using the concept of Heterogeneous doping are discussed in detail. A brief description on the importance of rheology for comprehension of soft matter microstructure is also provided in this chapter. Chapter 2 discusses composite of anatase titania (TiO2) nanospheres and carbon grown and self-assembled into micron-sized mesoporous spheres via a solvothermal synthesis route as prospective anode for rechargeable lithium-ion battery. The morphology and carbon content and hence the electrochemical performance are observed to be significantly influenced by the synthesis parameters. Synthesis conditions resulting in a mesoporous arrangement of an optimized amount of carbon and TiO2 exhibited the best lithium battery performance. The first discharge cycle capacity of carbon-titania mesoporous spheres (solvothermal reaction at 150 oC at 6 h, calcination at 500 oC under air, BET surface area 80 m2g-1) was 334 mAhg-1 (approximately 1 Li) at current rate of 66 mAg-1. High storage capacity and good cyclability is attributed to the nanostructuring (i.e. mesoporosity) of TiO2 as well as due to formation of a percolation network of carbon around the TiO2 nanoparticles. The micron-sized mesoporous spheres of carbon-titania composite nanoparticles also show good rate cyclability in the range (0.066-6.67) Ag-1. The electrochemical performance of the mesoporous carbon-TiO2 spheres has been compared with nonporous TiO2 spheres, normal mesoporous TiO2 and bulk TiO2. Implications of nanostructuring and conductive carbon interface on lithium insertion/removal capacity and insertion kinetics in nanoparticles of anatase polymorph of titania is discussed in Chapter 3. Sol-gel synthesized nanoparticles of titania (particle size ~ 6 nm) were hydrothermally coated ex situ with a thin layer of amorphous carbon (layer thickness: 2-5 nm) and calcined at a temperature much higher than the sol-gel synthesis temperature. The carbon-titania composite particles (resulting size 10 nm) displayed immensely superior cyclability and rate capability (higher current rates 4 Ag-1) compared to unmodified calcined anatase titania. The conductive carbon interface around titania nanocrystals enhances the electronic conductivity and inhibits crystallite growth during electrochemical insertion/removal thus preventing detrimental kinetic effects observed in case of un-modified anatase titania. The carbon coating of the nanoparticles also stabilized the titania crystallographic structure via reduction in the accessibility of lithium ions to the trapping sites. This resulted in decrease in the irreversible capacity observed in case of nanoparticles without any carbon coating. Chapter 4 discusses the morphology and electrochemical performance of mixed crystallographic phase titania nanotubes and nanosheets for prospective application as anode in rechargeable lithium-ion batteries. Hydrothermally grown nanotubes/nanosheets of titania (TiO2) and carbon/silver-titania (C/Ag-TiO2) comprise a mixture of both anatase and TiO2(B) crystallographic phases. The first cycle capacity (at current rate = 10 mAg-1) for bare TiO2 nanotubes was 355 mAhg-1 (approximately 1.06 Li), which is higher than both the theoretical capacity (335 mAhg-1) as well as reported values for pure anatase and TiO2(B) nanotubes. Higher capacity is attributed to a combination of presence of mixed crystallographic phases of titania as well as trivial size effects. The surface area of bare TiO2 nanotubes was very high being equal to 340 m2g-1. Surface modification of the TiO2 nanotubes via amorphous carbon and Ag nanoparticles resulted in significant improvement in battery performance. The first cycle irreversible capacity loss can be minimized via effective coating of the surface. Carbon coated TiO2 nanotubes showed superior performance than Ag nanoparticle coated TiO2 nanotubes in terms of long term cyclability. Unlike Ag nanoparticles which are randomly distributed over the TiO2 nanotubes, the effective homogeneous carbon coating forms an efficient percolation network for the conducting species thus exhibiting better battery performance. The C-TiO2 and Ag-TiO2 nanotubes showed a better rate capability i.e. higher capacities compared to bare TiO2 nanotubes in the current range 0.055-2 Ag-1. Although titania nanosheets retains mixed crystallographic phases, the lithium battery performance (first cycle capacity = 225 mAhg-1) is poor compared to TiO2 nanotubes. It is attributed to lower surface area (22 m2g-1) which resulted in lesser electrode/electrolyte contact area and inefficient transport pathways for Li+ and e-. Implications of iron on electrochemical lithium insertion/removal capacity of iron (Fe3+) doped anatase TiO2 is discussed in Chapter 5. Iron doped anatase TiO2 nanoparticles with different doping concentrations were synthesized by simple sol-gel method. The electrochemistry of anatase TiO2 is observed to be a strong function of concentration of iron (Fe3+). A high 1st cycle discharge capacity of 704 mAhg−1 (2.1 mol of Li) and 272 mAhg−1 (0.81 mol of Li) at the 30th discharge cycle with Coulombic efficiency greater than 96% has been observed for 5% iron (Fe3+) doped TiO2 at a current density of 75 mAg−1. Additional increase in the iron (Fe3+) concentrations deteriorates the lithium storage of TiO2. An improvement in lithium storage of more than 50% is noticed for 5% iron (Fe3+) doped TiO2 compared to pure anatase TiO2 which shows an initial discharge capacity of 279 mAhg−1. The anomalous lithium storage behavior in all the iron (Fe3+) doped TiO2 has been accounted, in addition to homogeneous Li insertion in the octahedral sites, on the basis of formation of metallic Fe and Li2O during initial lithiation process and subsequent heterogeneous interfacial storage between Fe and Li2O interface. Chapter 6 discusses in a systematic manner the crucial role of oxide surface chemical composition on ion transport in “soggy sand” electrolytes. A “soggy sand” electrolytic system comprising of aerosil silica functionalized with various hydrophilic and hydrophobic moeities dispersed in lithium perchlorate ethylene glycol solution ( = 37.7) was used for the study. Detailed rheology studies show that the attractive particle network in case of the composite with unmodified aerosil silica (with surface silanol groups) is most favorable for percolation in ionic conductivity as well as rendering the composite with beneficial elastic mechanical properties. Though weaker in strength compared to the composite with unmodified aerosil particles, attractive particle networks are also observed in composites of aerosil particles with surfaces partially substituted with hydrophobic groups. However, ionic conductivity is observed to be dependent on the size of the hydrophobic moiety. No spanning attractive particle network was formed for aerosil particles with surfaces modified with stronger hydrophilic groups (than silanol) and as a result no percolation in ionic conductivity was observed. The composite with hydrophilic particles was a sol contrary to gels obtained in case of unmodified aerosil and partially substituted with hydrophobic groups. Chapter 7 also discusses the influence of oxide surface chemical composition but additionally the role of solvent on ion solvation and ion transport of “soggy sand” electrolytes. Compared to the liquid electrolyte in Chapter 6, a lower dielectric constant liquid electrolyte was employed for the study in this chapter. A “soggy sand” electrolyte system comprising of dispersions of hydrophilic/hydrophobic functionalized aerosil silica in lithium perchlorate-methoxy polyethylene glycol solution ( = 10.9) was employed for the study. Static and dynamic rheology measurements again showed formation of an attractive particle network in case of the composite with unmodified aerosil silica (i.e. with surface silanol groups) as well as composites with hydrophobic alkane groups. While particle network in the composite with hydrophilic aerosil silica (unmodified) were due to hydrogen bonding, hydrophobic aerosil silica particles were held together via van der Waals forces. The network strength in the latter case (i.e. for hydrophobic composites) were weaker compared with the composite with unmodified aerosil silica. Both unmodified silica as well as hydrophobic silica composites displayed solid-like mechanical strength. However, this time around no enhancement in ionic conductivity compared to the liquid electrolyte was observed in case of the unmodified silica. This is attributed to the existence of a very strong particle network which leads to the “expulsion” of all conducting entities from the interfacial region between adjacent particles. The ionic conductivity for composites with hydrophobic aerosil particles displayed ionic conductivity as a function of the size of the hydrophobic chemical moiety. No spanning attractive particle network was observed for aerosil particles with surfaces modified with stronger hydrophilic groups (than silanol). The composite resembled a sol and no percolation in ionic conductivity was observed. Chapter 8 describes the influence of dispersion of uniformly sized mono-functional or bi-functional (“Janus”) particles on ionic conductivity in lithium battery solutions and it’s implications on battery performance. Mono-functionalized (hydrophilic or hydrophobic) and bi-functionalized Janus (hydrophilic and hydrophobic) particles form physical gels of varying strength over a wide range of concentration (0.1 0.4; , oxide volume fraction). While the composites with mono-functionalized particles display shear thinning typical of gels (due to gradual breaking up spanning particle network held together by hydrogen/van der Walls force), the bi-functionalized “Janus” particles exhibit both complementary properties of gel and sol. The latter observation is interpreted in terms of existence of both hydrogen and van der Waals force arising out of the particle arrangement which get perturbed under the influence of external shear. Composites with homogeneous hydrophilic surface group show the highest ionic conductivity whereas the homogeneous hydrophobic surfaces exhibit superior electrode/electrolyte interface stability and battery cyclability. The Janus particles did not show any enhancement in ionic conductivity however, battery performance is highly satisfactory taking intermediate values between the homogeneously functionalized hydrophilic and hydrophobic particle composites.