Functional Metal Oxide Nanostructures Volume 149, science
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//-->Chapter 14Nanostructured Metal Oxidesfor Li-Ion BatteriesJuchen Guo and Chunsheng WangNanoscale and nanostructured metal oxides have drawn tremendous interests fromthe researchers working in the field of energy storage and energy conversiontechnologies in recent years. In this chapter, the state of the art of nano metaloxide materials in Li-ion batteries will be discussed as a comprehensive overview.Lithium-ion battery has been a very important category of rechargeable batteriessince its first commercialization by Sony in 1991. It has been widely used inportable consumer electronics such as laptops, digital cameras, small power tools,etc. However, its potential is not limited to such small devices due to several uniquemerits: Li-ion battery has the highest energy density among all types of recharge-able batteries that are currently on the market. It also has a relatively low self-discharge rate. Because of these virtues, interests in Li-ion batteries keep growingfor defense, aerospace, smart grid system, and automotive applications. To satisfythe demands of these emerging applications, the next generation of Li-ion batteriesmust achieve a holistic and striking advancement from the current technology,specifically in four criteria: energy density, discharging and charging rate (powerdensity), safety feature, and cycle stability. The energy density, power density, andcycle stability of Li-ion batteries are mainly determined by electrode materials andstructures. Enhancement of the safety feature ultimately depends on the develop-ment of nonflammable electrolyte and solid electrolyte to replace the current liquidelectrolyte consisting of highly flammable organic solvents. The use of nano metaloxides (nanoscale or nanostructured) as anode materials, cathode materials, andelectrolyte additives has greatly enhanced the performance of Li-ion batteries dueto their unique chemical and structural properties.J. Guo • C. Wang (*)Department of Chemical and Biomolecular Engineering,University of Maryland, College Park, MD 20742, USAe-mail:cswang@umd.eduJ. Wu et al. (eds.),Functional Metal Oxide Nanostructures,Springer Seriesin Materials Science 149, DOI 10.1007/978-1-4419-9931-3_14,#Springer Science+Business Media, LLC 2012337338J. Guo and C. Wang14.1Classification of Electrode Materials for Li-Ion BatteriesBefore going into further discussion, it is necessary to briefly introduce how Li-ionbattery works. For instance, the most common cathode material is lithium cobaltoxide (LiCoO2) and the anode material is graphite as shown in Fig.14.1.Thetypical electrolyte consists of lithium salts like lithium hexafluorophosphate(LiPF6) or lithium tetrafluoroborate (LiBF4) dissolved in an organic solvent suchas a mixture of propylene carbonate and diethyl carbonate. During the batterycharging process, Li atoms in LiCoO2become ions, migrating to the graphiteanode across the electrolyte and inserting into the gaps between the graphenelayers. The reverse process takes place during the battery discharge: Li atomsstored in the layered graphite become ions, migrating to the LiCoO2cathode andinserting into the layers of octahedral lattices formed by cobalt and oxygen atoms.This type of lithiation/delithiation mechanism is referred asintercalationin whichLi is stored in layer-structured materials such as graphite and LiCoO2. Nowadays,the intercalation mechanism has been generalized to refer to all topotactic reactionsof Li-ions inserting into the interior of the lattice of the host materials of which thestructures are not limited to be layered. Another lithiation/delithiation mechanism isbased on the reversible redox reaction between metal oxide and Li (Li+), and isreferred to asconversionreaction. According to the conversion mechanism,lithiation takes place through the reduction of metal oxide by Li to produce metaland lithium oxide, and delithiation takes place through the oxidation of the formedmetal by lithium oxide. Besides, these two mechanisms, in a few binaryFig. 14.1Schematic of Li-ion battery with graphite anode and LixMO2cathode in the state ofdischarge14Nanostructured Metal Oxides for Li-Ion Batteries339intermetallic AB compounds, Li will reversibly displace A to form LixB, and theformed LixB has a strong structural relationship with the parent AB compound. Thismechanism is referred to asdisplacementreaction. All current metal oxide elec-trode materials for Li-ion batteries can be sorted into these three categories, excepttin dioxide (SnO2)-based materials in which the lithiation/delithiation processcombines conversion and alloying reactions. Therefore, for the purpose of articula-tion, all the metal oxide electrode materials will be discussed with respect to theirdifferent lithiation/delithiation mechanisms.14.2Advantages and Disadvantagesof Nanoelectrode MaterialsThe advantages of nanoelectrode materials come from their nanometer characteris-tic length and their tremendously large surface area. Generally speaking, thesmaller size can shorten the Li-ion/electron transport pathways and enhancephase transformation. The large surface area can also speed up the charge transferreaction kinetics due to the increased contact area with the electrolyte. Enhanced Liinsertion/extraction kinetics can lead to higher rate performance, even novellithiation/delithiation mechanism. Higher surface area can also enhance the capac-ity through the surface Li storage mechanism [1]. Moreover, nanoelectrodematerials can better accommodate the mechanical strain induced by the concomi-tant volume change in the lithiation/delithiation process, thus improving the cyclestability.Unfortunately, the disadvantages of nanoelectrode materials are also from theirnano characteristic scale and large surface area. The nanoscale materials will lowerthe packing density of the electrode thus resulting in a low overall energy density ofthe batteries. Also, the large surface area will promote larger amount of sidereactions at the electrolyte/electrode interfaces. Therefore, the development ofnanoelectrode materials should focus on the direction of the optimized propertiesbalancing the advantages and disadvantages.14.3Nanometal Oxide Anode Materials14.3.1 Intercalation Metal OxidesMaterials with a layered structure (graphite for anode and LiCoO2for cathode) arethe natural choice as the Li host material [2], but not the only ones. Materials withtunneled structures (such as spinel) can also be used as Li storage hosts withintercalation mechanism. Among them, Li4Ti5O12and TiO2are the two mostintensively studied metal oxide anode materials.340J. Guo and C. WangThe concept of using the B2O4framework of an AB2O4(A¼Li) spinel materialas host structure for Li-ion storage was originally proposed by Thackeray et al. inthe 1980s [3]. Lithium titanium oxide, Li4Ti5O12, a ceramic material having adefect tunneled ([Li1/3Ti5/3]O4) structure, was initially proposed as an anode mate-rial by Colbow et al. in 1989 [4] and tested by Ferg et al. [5] and Ohzuku et al.[6] in the early 1990s. Li4Ti5O12can be lithiated over the composition range ofLi4+xTi5O12(0<x<3) at a potential of about 1.55 V versus Li/Li+, and itstheoretical lithiation capacity is 175 mAh gÀ1. Despite its moderate lithiationcapacity, the particular advantage of Li4Ti5O12comparing to other spinel anodeswas that it is a “zero-strain” intercalation material. The defect Li1/3Ti5/3O4spinelframework exhibits minimal volume change during Li-ion insertion and extractionso that the crystal structure is better retained, thus resulting in better cycle life.Another characteristic of Li4Ti5O12is its lithiation voltage of 1.55 V versus Li/Li+,which is considered to have two-faced effects. On one hand, the 1.55 V lithiationvoltage is higher than the decomposition voltage of the organic solvents in theelectrolyte. Therefore, using Li4Ti5O12as the anode material can eliminatethe formation of solid electrolyte interface (SEI) film, which a considerable costefficiency factor. Also, the higher lithiation voltage significantly reduces the possi-bility of the lithium metal plating at the anode, so that the safety can be enhanced.On the other hand, using Li4Ti5O12anode sacrifices the full cell working voltagebecause of its higher lithiation voltage compared to graphite (0.1–0.2 V versus Li/Li+).Since pure Li4Ti5O12is an electric insulator, the advantage of nanoscaleLi4Ti5O12is the extraordinary enhancement of Li insertion/extraction kinetics.The mean Li-ion diffusion time in an ideal anode particle can be approximatelyexpressed using the following equation, assuming Fickian diffusion:L2t¼2DwhereLis the diffusion distance andDis the Li-ion diffusivity in the material.Based on this equation, the advantage of nanoscale electrode material is obvious:the resultant short diffusion distance can reduce the diffusion time significantly. Forinstance, if the particle size is reduced to 100 nm from 1mm,Li-ion diffusion timecan be decreased 100 times. Another advantage of nanoscale electrode materials forcharge transfer kinetics enhancement is their large surface area which results inlarge contact surface between the electrode and electrolyte. As an electricallyinsulating material, the electronic conductivity of Li4Ti5O12increases during thelithiation reaction from the outer surface directing inward, which is not criticallyproblematic for lithiation, since the Li+/eÀtransport takes place at the outer layeranyway. During delithiation, as Li is being extracted, the conductivity starts todecrease from the outer layer of the Li4Ti5O12particle. Therefore, the delithiationprocess has worse kinetics than lithiation. Fast separation of Li+and eÀis criticalto achieve fast charge/discharge rate, which can be achieve by reducing the Li+/eÀtransport pathway using nanoscale Li4Ti5O12materials.14Nanostructured Metal Oxides for Li-Ion Batteries341Fig. 14.2(a) TEM image of the Li4Ti5O12nanorods, (b) rate capacity test of the Li4Ti5O12nanorods at different C rates [7]Because of these advantages, nanoscale Li4Ti5O12anode materials have beenextensively studied. Among them, Kim and Cho [7] reported the synthesis andelectrochemical performance of Li4Ti5O12nanorods. The diameter of the reportedLi4Ti5O12nanorods is about 100 nm as shown in Fig.14.2a.The notable merit ofthis material is its very promising discharge rate capacity. As shown in Fig.14.1b,the reversible first discharge capacity was 165 mAh gÀ1under a cycling rate of 0.l C(16 mA gÀ1), and no capacity fading was observed up to 30 cycles between 1 and2.5 V. At rates of 0.5 C, the first capacity at 0.5 C was identical to that at 0.1 C. Verysmall capacity decreases with increasing current were observed at 5 and 10 C(1,600 mA gÀ1) rates, the capacity retention was 95 and 93%, showing 157 and155 mAh gÀ1, respectively. As a comparison, the electrochemical performance ofLi4Ti5O12particles with 700 nm diameter is distinctly worse. Though this cannot beused as a direct evidence of the superiority of nanorods over nanoparticles due totheir different characteristic sizes, it clearly demonstrates the significant advantageof nanoscale materials by enhancing charge transfer kinetics.In light of the great promise of Li4Ti5O12as Li intercalation anode materials,researchers naturally began to investigate titanium dioxides as candidates of anodematerials because of their higher theoretical lithiation capacity. The Li intercalationreaction to TiO2can be generally expressed as the following reaction:xLiþþTiO2þxeÀ$LixTiO2Full lithiation should lead to the formation of lithium titanium oxide with a formulaof LiTiO2(x¼1) with 335 mAh gÀ1theoretical capacity. This reaction takes placein the voltage range 1.5–1.8 V. Therefore, like Li4Ti5O12, using TiO2as the anodematerial can avoid anode passivation and also enhance the safety feather. Theinvestigation on TiO2was actually not a recent idea: it started in the 1980s andcontinued in the 1990s [8–11]. However, the sluggish performance of the earlierTiO2materials had merely attracted lukewarm attention. The TiO2research reallytook off in virtue of the development of nanotechnology. To date, four types oftitanium dioxides have been reported to have lithiation capacity: are rutile,
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