Nanostructured Materials for Eletrochemical energy conversion and storage devices

Nanostructured Materials for Eletrochemical energy conversion and storage devices

(Parte 1 de 3)

DOI: 10.1002/adma.200800627

Nanostructured Materials for Electrochemical Energy Conversion and Storage Devices**

By Yu-Guo Guo, Jin-Song Hu, and Li-Jun Wan*

1. Introduction

One of the great challenges for today’s information-rich, mobile society is providing high-efficient, low-cost, and environmentally friendly electrochemical energy conversion and storage devices for powering an increasingly diverse range of applications, ranging from portable electronics to electric vehicles (EVs) or hybrid EVs (HEVs).[1,2] As the performance of these devices depends intimately on the properties of their materials, considerable attention has been paid to the research and development of key materials.[1–12] Micrometer-sized bulk materials are reaching their inherent limits in performance and cannot fully satisfy the increasing needs of consumer devices. Therefore, rapid development of new materials with high performance is essential. Nanostructured materials are becoming increasingly important in the field and hence have attracted great interest in recent years. A variety of nanometer size effects have been found in the materials used in electrochemical energy conversion and storage devices, which can be divided into two types: i) ‘trivial size effects’, which rely solely on the increased surface-tovolume ratio and i) ‘true size effects’, which also involve changes of local materials properties. As the coming of ‘nanoionics’[2] has demonstrated an important position in the field, similar to that of nanoelectronics in semiconductor physics, its development may lead to breakthroughs in this field, which holds the key to new generations of clean-energy devices. However, it is beyond the scope of this progress report to give an exhaustive summary of those energy devices that may benefit now or in the future from the use of nanoparticles; rather, we shall limit ourselves to the fields of lithium-based batteries and fuel cells. In particular, we focus on nanostructured electrode materials for rechargeable lithium-ion batteries and nanostructured Pt-based electrocatalysts for direct methanol fuel cells (DMFCs).

One of the greatest challenges for our society is providing powerful electrochemical energy conversion and storage devices. Rechargeable lithium-ion batteries and fuel cells are amongst the most promising candidates in terms of energy densities and power densities. Nanostructured materials are currently of interest for such devices because of their high surface area, novel size effects, significantly enhanced kinetics, and so on. This Progress Report describes some recent developments in nanostructured anode and cathode materials for lithium-ion batteries, addressing the benefits of nanometer-size effects, the disadvantages of ‘nano’, and strategies to solve these issues such as nano/micro hierarchical structures and surface coatings, as well as developments in the discovery of nanostructured Pt-based electrocatalysts for direct methanol fuel cells (DMFCs). Approaches to lowering the cost of Pt catalysts include the use of i) novel nanostructures of Pt, i) new cost-effective synthesis routes, ii) binary or multiple catalysts, and iv) new catalyst supports.

[*] Prof. L.-J. Wan, Prof. Y.-G. Guo, Dr. J.-S. Hu

Beijing National Laboratory for Molecular Sciences (BNLMS) Institute of Chemistry, Chinese Academy of Sciences (CAS) Beijing 100190 (China) E-mail:

[**] This work is supported by the National Natural Science Foundation of

China (Grant Nos. 20673121, 20603041, 50730005, and 20701038), National Key Project on Basic Research (Grant Nos. 2006CB806100 and 2006CB932100), and the Chinese Academy of Sciences. The authors thank the scientific community in the field of energy conversion and storage for laying foundations, Professor Chun-Li Bai for valuable advices, and Professor Hong Li for discussion.

2. Nanostructured Electrode Materials for Lithium-Ion Batteries

Lithium-ion battery currently represents the state-of-the-art technology in small rechargeable batteries because of its many merits (e.g., higher voltage, higher energy density, and longer cycle life) compared with traditional rechargeable batteries such as lead acid and Ni-Cd batteries. Typically, a lithium-ion battery consists of a negative electrode (anode, e.g., graphite), a positive electrode (cathode, e.g., LiCoO2), and a lithium-ionconducting electrolyte (Fig. 1a). When the cell is charged, Li ions are extracted from the cathode and inserted into the anode. On discharge, the Li ions are released by the anode and taken up again by the cathode (Fig. 1a). Although such lithiumion batteries are commercially successful, especially in smallscale devices, these cells are still objects of intense research to enhance their properties and characteristics, which is largely promoted by theincreasingly diverse range ofapplications they need to power, such as next-generation wireless communicationdevices(e.g.,3Gmobilephones,MP4),EVs,HEVs,power tools, uninterrupted power sources (UPS), stationary storage batteries (SSBs), and microchips. Since no single lithium-ion battery type can meet all the demands of such a large variety of applications, different types of batteries with specific properties for certain applications should be considered, including: i) high-energy lithium-ion batteries for modern communication

Y.-G. Guo et al. / Nanostructured Materials for Electrochemical Devices

Prof. Li-Jun Wan received his Ph.D. in Materials Chemistry from Tohoku University of Japan, joined the Institute of Chemistry of Chinese Academy of Sciences (ICCAS) as a professor in 1999, and is currently director of the institute and director of CAS Center for Molecular Science. His research has centered on physical chemistry, with an emphasis on molecular self-assembly, functional nanomaterial, electrochemistry, and scanning probe microscopy.

Yu-Guo Guo received his Ph.D. in Chemistry from ICCAS under the supervision of Prof. Chun-Li Bai and Prof. Li-Jun Wan. From 2004 to 2007 he worked with Prof. Joachim Maier at the Max Planck Institute for Solid State Research at Stuttgart (Germany) first as a Guest Scientist and then a Staff Scientist. He joined ICCAS as a professor in 2007. His current research interests are centered on the nanostructured materials for advanced energy conversion and storage devices, the sizedependent properties of energy materials, as well as ion/electron transport in nanoscaled systems.

Jin-Song Hu received his Ph.D. in Chemistry (2005) from ICCAS with Prof. Chun-Li Bai and Prof. Li-Jun Wan as his supervisors. He joined ICCAS as an assistant professor in 2005 and was prompted as an associate professor two years later. His current scientific interests are focused on functional nanomaterials for environmental remediation, energy system and electronics.

Adv. Mater. 2008, 20, 2878–2887 2008 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim 2879

devices; i) high-power lithium-ion batteries for HEVs, EVs and power tools; and i) long-cycle-life lithium-ion batteries for UPS and SSBs. Many anode and cathode materials with appropriate properties have been considered for different types of lithium-ion batteries (Table 1). However, commercial batteries are mostly based on micrometer-sized electrode materials, which are limited by their kinetics, lithium-ion intercalation capacities, and structural stability. The performance of currently available lithium-ion batteries can only meet the requirements of these different applications to some degree; challenges still remain, naturally, in developing new electrode materials for high energy density, high power density (viz., higher rates), longer cycle life, and improved safety. The development of nanostructured electrode materials is considered to be the most promising avenue towards overcoming the current limits and achieving these goals.[1–12] However, it is necessary first of all to know how nanomaterials impact on the performance of the lithium-ion batteries and what kinds of mechanisms these nanomaterials exhibit. Here, we address the benefits of nanometer size effects and the disadvantages of using ‘nano’, as well as strategies for solving these issues and fulfilling the nanomaterials’ potential.

2.1. Benefits of Nanometer Size Effects

Nanomaterials can play a large role in improving the performance of lithium-ion batteries, because in nanoparticle systems the distances over which Liþ must diffuse are dramatically decreased; the nanoparticles can quickly absorb and store vast numbers of lithium ions without causing any deterioration in the electrode; and nanoparticles have large surface areas, short diffusion lengths, and fast diffusion rates along their many grain boundaries. Much enhanced capacities, high rate performance, capacity retention abilities, and many novel Li storage systems have been found to benefit from nanometer size effects.

2.1.1. Enhanced Lithium Storage Kinetics

Lithium-ion batteries are amongst the most promising candidates for applications in EVs, HEVs, and power tools in terms of energy density, while the achievement of high power density is hindered by kinetic problems in the electrode materials, i.e., the slow Liþ and e diffusion. For solid-state diffusion of Li in electrode materials, the mean diffusion (or storage) time, teq, is determined by the diffusion coefficient, D, and the diffusion length, L, according to the following formula:

Two approaches are, at present, gaining momentum in resolving the kinetic problems of electrode materials. One approach is increasing D by doping the electrode materials with foreign atoms. Although mixed conduction is thus improved, only limited rate-performance enhancement can be achieved, and sometimes the introduction of heteroatoms can result in unstablecrystalstructure. Analternativeapproachisdecreasing L, which has been realized by nanostructuring of electrode materials.[3] For example, a reduction of L from 10mm (the typical size of commercial electrode materials) to 100nm reduces, for a material with D¼10 10cm2s 1 (the typical value of electrode materials), the teq from 5000 to 0.5s. The effects are so remarkable that the most extensive research work over the years has followed this direction.

It has been found that electrode materials inactive towards Li insertion may become active when ‘‘going nano’’. For example, rutile TiO2 has very sluggish Li diffusion along the ab-plane (Dab 10 15cm2s 1), which is why Li insertion into rutile is usually reported to be negligible, viz., ‘inactive’ towards Li insertion. However, nanometer-sized rutile TiO2 (10nm 40nm) is able to reversibly accommodate Li up to

Li0.5TiO2 (168mAhg 1) at 1–3V versus Liþ/Li with excellent capacity retention on cycling.[13] This is mainly related to the drastic decrease of the diffusion time teq from ca. four years for 10mm rutile (assume the mean square displacement need to diffusion along the ab-plane Lab¼5mm) to ca. 2min for 10nm rutile (Lab¼5nm). Nanometer-sized electrode materials may not only increase the electroactivity towards Li insertion but also enhance the high rate capability (high power), as has been observed for many anode materials. The high rate capability results directly from the transport advantages of the fine particle size, such as shorter transport distances for both e and Liþ transport as well as a larger electrode/electrolyte contact area resulting

Y.-G. Guo et al. / Nanostructured Materials for Electrochemical Devices

Table 1. Summary of cathode and anode materials used for different types of lithium-ion batteries.

Types Cathodes Anodes

High energy LiNixCoyM1 x yO2 (layered) [a] Si, Sn, Sb

LiMn2 xMxO4 (spinel) [b] MOx [d] MFx [c] graphite

High power LiMn2 xAlxO4þd (spinel) hard carbon

Long cycle life LiFePO4 (olivine) Li4Ti5O12 LiMn2 xAlxO4þd graphite

[a]M¼Mn, Al, and Cr. [b]5V systems, M¼Ni, Cu, and Cr. [c]M¼Cu, Ni, and Fe. [d]M¼Fe, Co, Ni, Cr, Mn, Cu, and Sn.

Figure 1. Schematic illustration of a) a lithium-ion battery and b) a direct methanol fuel cell.

from the larger surface area. The former makes full Lidiffusion possible within a short storage time, i.e., at high charge/ discharge rates, and the latter greatly reduces the specific current density of the active material. For example, the above- mentioned nanometer-sized rutile TiO2 with a specific surface area of ca. 110m2g 1 also exhibits an excellent high rate performance (100mAhg 1 at 10C and 70mAhg 1 at 30C, where 1C¼336mAg 1), which makes it a promising anode material for high-power lithium-ion batteries.[13] These findings have encouraged people to reinvestigate materials that were thought to be electrochemically inactive in bulk form due to poor electronic and Liþ conductivity, but that could present improved electrochemical performance at the nanoscale. More examples should present themselves in due course.

2.1.2. Enhanced Structural Stability

Since structural transition to thermodynamically undesir- able structures can only occur if the particle radius rp is larger than the critical nucleation radius rc for that phase, it is possible to eliminate such transitions by using nanoparticles with rp>rc. Thus, small particles would more easily accommodate the structural changes occurring during the cycling process where

Li is inserted and extracted. For example, layered LiMnO2 suffers from structural instability during cycling and as a result, exhibits significant capacity fade. As a way to overcome such difficulties, nanocrystalline structures have attracted increasing attention, since the lattice stress caused by Jahn–Teller distortion can be accommodated more easily, and hence they exhibit much higher Li-intercalation capacity than their conventional crystalline counterparts.[4]

In nanoparticles the charge accommodation occurs largely at or very near the surface and the smaller the particles are, the larger the portion of these constituent atoms at the surface. This fact reduces the need for diffusion of Liþ in the solid phase, greatly increasing the charge and discharge rate of the cathode as well as reducing the volumetric changes and lattice stresses caused by repeated Li insertion and expulsion.

2.1.3. New Lithium-Storage Mechanisms

Another benefit of nanostructured electrode materials is that they can lead to new Li-storage mechanisms, affording high capacities,rechargeability,andgeneralapplicabilitytoarangeof battery systems. One such new mechanism is referred to as a ‘conversion’mechanism,[14]firstfoundintransitionmetaloxides, followedbyfluorides,sulfides,andnitrides.[5–7]Themechanisms are mainly related to reversible in situ formation and decom- position of LiyX (where X¼O, S, F, or N) upon Li uptake and release, which can be described by the following equation:

where M¼Fe, Co, Ni, Cr, Mn, Cu, and so on. Usually, reversible capacities in these systems, which have been demonstrated as innovative high energy anode materials for lithium-ion batteries, are in the range of 400–1100mAhg 1.I t is reported that electrodes made of CoO nanoparticles can deliver a specific capacity of 700mAhg 1 with 100% capacity retention for up to 100charge/discharge cycles and high recharging current rates.[14]

(Parte 1 de 3)