Hierarchically structured materials for lithium batteries

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Hierarchically structured materials for lithium batteries

This content has been downloaded from IOPscience. Please scroll down to see the full text. 2013 Nanotechnology 24 424004 (http://iopscience.iop.org/0957-4484/24/42/424004) View the table of contents for this issue, or go to the journal homepage for more

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IOP PUBLISHING

NANOTECHNOLOGY

Nanotechnology 24 (2013) 424004 (7pp)

doi:10.1088/0957-4484/24/42/424004

Hierarchically structured materials for lithium batteries Jie Xiao, Jianming Zheng, Xiaolin Li, Yuyan Shao and Ji-Guang Zhang Energy and Environment Directorate, Pacific Northwest National Laboratory, Richland, WA 99352, USA E-mail: [email protected]

Received 12 February 2013, in final form 9 April 2013 Published 25 September 2013 Online at stacks.iop.org/Nano/24/424004 Abstract The lithium-ion battery (LIB) is one of the most promising power sources to be deployed in electric vehicles, including solely battery powered vehicles, plug-in hybrid electric vehicles, and hybrid electric vehicles. With the increasing demand for devices of high-energy densities (>500 Wh kg−1 ), new energy storage systems, such as lithium–oxygen (Li–O2 ) batteries and other emerging systems beyond the conventional LIB, have attracted worldwide interest for both transportation and grid energy storage applications in recent years. It is well known that the electrochemical performance of these energy storage systems depends not only on the composition of the materials, but also on the structure of the electrode materials used in the batteries. Although the desired performance characteristics of batteries often have conflicting requirements with the micro/nano-structure of electrodes, hierarchically designed electrodes can be tailored to satisfy these conflicting requirements. This work will review hierarchically structured materials that have been successfully used in LIB and Li–O2 batteries. Our goal is to elucidate (1) how to realize the full potential of energy materials through the manipulation of morphologies, and (2) how the hierarchical structure benefits the charge transport, promotes the interfacial properties and prolongs the electrode stability and battery lifetime. (Some figures may appear in colour only in the online journal)

1. Introduction

with solely nanoparticles will shorten the ion diffusion path and increase the power rate during the electrochemical process [10]. However, these nano-scaled materials also lead to more side reactions at the electrode and electrolyte interface. They also reduce the pack density and energy density of the electrode materials. Therefore, significant efforts have been made to pack nano-scaled materials into micron-sized secondary particles. Hierarchically structured materials integrate the advantages of nano-materials and micro/macromaterials to exhibit both high power rate and high packing/energy density. One of the main differences between hierarchical structures and single-scale nano-structures is that, in the former, continuous ion and electron pathways have been built up and maintained during repeated cycling [11]. In the single-scale nano-materials, however, the nano-domains often become isolated after cycling. As a consequence, the efficient flow of ions and electrons in the whole electrode composed of pure nano-materials may be interrupted, especially at high current densities. This will lead to fast fading of

Many natural materials exhibit interesting structures with more than one length scale, which is called hierarchy. These hierarchical structures play an important role in determining the properties of bulk materials [1, 2]. Different functions can be expressed in the multi-scale structures for various purposes [3, 4]. In addition, hierarchical structures exhibit many advantages over single-scale structures and, therefore, are of particular interest to the materials science community [5]. In a recently published review, Li et al [1] identified many applications of hierarchically structured materials, including solar cells [6], water splitting [2], fuel cells [7], batteries, and supercapacitors [8]. For applications such as batteries, electrode materials with well-defined hierarchical structures contain a significantly increased surface area, increasing the number of reactive sites in the material [9]. More importantly, novel multi-functional materials may be designed to exhibit very different properties in different length scales. For example, battery electrodes 0957-4484/13/424004+07$33.00

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c 2013 IOP Publishing Ltd Printed in the UK & the USA

Nanotechnology 24 (2013) 424004

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the capacities in the electrode composed of nano-structured materials, especially for the thick electrodes required for practical use. Other advantages of hierarchical structures over nano-structures include continuous ‘fuel (such as oxygen in Li–O2 batteries)’ delivery through meso-scale channels, free space to tolerate volume expansion (which is critical for materials such as silicon that will expand more than 300% in volume during the lithium injection process), and increased tap density without increasing the cell impedance [5, 12]. In hierarchical structures, the concurrent construction of nano-scale electrochemical reaction regions and meso-scale (or even longer length) regions provides unique advantages to minimize the resistance raised from different levels and will be the focus of this review article. Other important topics related to hierarchical materials, for example synthesis methodologies, are also very critical since they substantially determine the final structure as well as the activities/performance of the materials. In a recent review article that covered this area well, the authors extensively discussed the approaches employed to prepare hierarchically structured materials using templatebased assembly or through self-assembly, for different applications [1]. Therefore, in this short review, we will only focus on the discussion of hierarchical structure-derived electrochemical properties, which in turn provides clues for the rational design of energy materials for lithium batteries. During recent years, significant progress has been made in the development of Lithium-ion batteries (LIBs), even though many barriers must still be overcome before large-scale market penetration of electric vehicles powered by LIBs. The goal of this paper is to analyze the properties of hierarchical structured materials and to address the following questions. (1) How to understand the electrochemical reactions/interactions at the nano- and/or meso-scales in LIB and Li–O2 battery systems, and (2) how to correlate the hierarchical structure of the electrode with the performance of LIB and Li–O2 batteries from the chemical and electrochemical perspectives. The fundamental understanding of the properties of materials with hierarchical morphologies might provide some clues in this research area and accelerate R&D advances in these batteries. Exploration of the relationship between the hierarchical structure and the material properties will advance our fundamental understanding of the electrochemical processes occurring on different length scales, and also will provide guidance toward the synthesis of new materials with novel properties that may satisfy the energy/power density requirements of next generation energy storage systems.

oxide with a layered structure, has been developed and commercialized first for LIB applications [13]. LiCoO2 containing hierarchical structures (e.g. meso-porous nanowire, nanofiber, etc) was first reported by Bruce et al [14]. The hierarchical structured material exhibited increased discharge capacity, which was attributed to the enhanced contact area between the electrode and the electrolyte and the shortened lithium-ion diffusion length. Another promising high-energy cathode is a lithium-rich, manganese-rich (LMR) composite material with a typical composition of xLi2 MnO3 –(1 − x)LiMO2 (M = Ni, Co, Mn). The Li2 MnO3 component is used to stabilize the electrode structure and enhance the discharge capacity of the electrode. During the initial formation cycle, lithium in Li2 MnO3 is extracted concomitant with the release of oxygen (a net loss of Li2 O, typically at 4.6–4.8 V) to form a layered MnO2 component [15]. However, the release of oxygen during the first charge also leads to a large irreversible capacity loss (ICL) during the first cycle [16]. An exemplary approach to address the aforementioned issue through synthesis is the concentration-gradient cathode material based on a layered lithium–nickel–cobalt–manganese oxide developed by Amine et al [17]. As shown in figure 1, each particle has a nickel-rich central bulk to provide high capacity, and a manganese-rich outer layer to improve its thermal stability. Figure 1(a) is the scanning electron microscopy (SEM) image of an as-prepared particle with core–shell secondary structures, confirmed by electron-probe x-ray microanalysis (EPMA) results (figure 1(b)). The concentration of nickel decreases while manganese and cobalt increases as the surface is approached (figure 1(c)). High capacity and improved thermal stability are achieved through this combination (figure 1(d)). Their recent work upgraded this core–shell layered composite into the fully gradient composition; that is, the linear concentration changes of nickel and manganese from the center to the outer layer of each particle (figure 1(d)) to fully harvest the benefits from this hierarchical structure and minimize the structural mismatch and the difference in volume change between the core and the shell (figure 1(f)) [18]. The use of hierarchical structures in other cathode materials also has been investigated. For example, LiMn2 O4 , with the advantages of low-cost, environmental benignity and intrinsic safety is prepared into double-walled hollow microspheres, which are comprised of nanoparticles with diameters ranging between 200 and 400 nm [19]. A high reversible capacity, superior rate capability and excellent cycling stability are observed from these microspheres, owing to the short lithium-ion diffusion lengths in the nano-building blocks, and the void core and space between the inner and outer shells, that accommodate the volume expansion/contraction during lithium-ion insertion/extraction processes. High-voltage spinel LiNi0.5 Mn1.5 O4 also can be constructed into spherical particles (∼10 µm size), each of which consists of many fine crystalline grains with an average grain size of about 140 nm (figure 2) [20]. In addition to the superior electrochemical performance of as-prepared high-voltage spinel, the tap density of these hierarchically structured particles reaches 2.03 g cm−3 , which

2. Electrode materials with hierarchical structures for LIBs 2.1. Hierarchical structures in cathode materials Hierarchically structured cathode materials have been extensively investigated to develop high-energy and high-power LIBs because of the kinetic advantage provided by the hierarchical structure. LiCoO2 , which is a transition metal 2


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Figure 1. (a) SEM and (b) EPMA line scans of Li[Ni0.64 Co0.18 Mn0.18 ]O2 . The nickel concentration decreases and the cobalt and manganese concentrations increase towards the surface. (c) A schematic diagram of a layered-composite particle with a nickel-rich core surrounded by a concentration-gradient outer layer. (d) Comparison of the cycling abilities of Li[Ni0.8 Co0.1 Mn0.1 ]O2 , Li[Ni0.46 Co0.23 Mn0.31 ]O2 and concentration-gradient material cycled between 3.0 and 4.4 V at 55 ◦ C. The former two can be considered as the core and shell of the concentration-gradient material, respectively. (e) A schematic diagram of the full concentration-gradient (FCG) cathode recently developed at Argonne National Laboratory. (f) Cycling performance comparison of inner composition (IC), outer composition (OC), and FCG cathode materials. (Reprinted with permission from Amine et al [17, 18]. Copyright 2012 Nature Publishing Group.)

Figure 2. (a) SEM and (b) TEM images of hierarchically structured LiNi0.5 Mn1.5 O4 microspheres. (Reprinted with permission from Gao et al [20]. Copyright 2010 The Electrochemical Society.)

is a significantly improved value compared to nanoparticles. For vehicle electrification, a higher tap density means a higher volumetric energy density, an important criterion that most pure nanoparticles cannot satisfy. Applications of similar hierarchical structures also have been reported for LiFePO4 [21], LiMnPO4 [22] and LiV3 O8 [23].

understand how a hierarchical structure promotes the kinetics of the alloying and de-alloying processes between lithium and silicon. Silicon has long been known to exhibit a very high theoretical capacity of ∼4000 mAh g−1 [27]. However, the use of silicon anodes in lithium-ion batteries has been limited by the large volume changes of silicon, which lead to pulverization of silicon particles and unstable electrode/electrolyte interfaces [28]. Therefore, quick increases in cell impedance and fast capacity fading are often observed in silicon anodes. Recently, single-crystal silicon nanowires have demonstrated the capability to accommodate the large volume changes [29]. The quick capacity decrease of silicon is effectively alleviated through this approach. However, similarly to other nano-materials, the loading of silicon in the form of nanowires or nanotubes is very limited; otherwise, quick capacity degradation still

2.2. Hierarchical Si-based composite anode There are many examples of anode materials composed of hierarchical structures, for example, hierarchically porous Li4 Ti5 O12 [24], urchin-like natural graphite/carbon nanofiber hybrid material, [25], spherical multi-deck cage particles of Li2 O–CuO–SnO2 [26], etc. Considering the fact that a silicon-based anode exhibits the highest specific capacity and is one of the most promising anode materials for high-energy battery applications, we will focus on silicon as a platform to 3


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Figure 3. (a) Schematic of Si–C nano-composite synthesized through hierarchical bottom-up assembly. (b) Discharge capacity and Coulombic efficiency of the silicon–carbon-composite electrode versus cycle number. (c) Schematic diagram and TEM image of the TMV-enabled silicon anode. (d) Cycling stability of the silicon-composite anode at 1 C rate. Panels (a) and (b) are reprinted with permission from Yushin et al [30], copyright 2010 Nature Publishing Group; (c) and (d) are reprinted with permission from Wang et al [32], copyright 2010 American Chemical Society.

occurs. A hierarchically structured silicon-based anode may provide solutions to high volumetric energy density of the thick electrode as well as LIBs. Yushin’s group [30] used a bottom-up approach to develop a hierarchically structured silicon–carbon nanocomposite anode. In their work, silicon was incorporated into porous carbon black by chemical vapor deposition. Figure 3(a) is a schematic diagram of the synthesis concept. Silicon nanoparticles (3.6 V) and generates Li2 CO3 -like species, which further complicate this system [37, 38]. Although it is still challenging to find a stable electrolyte that can exclusively form Li2 O2 [39], manipulation on the air electrode side may help alleviate the aforementioned issues. All of these reaction products, including the desired reaction product, Li2 O2 , and undesired byproducts such as Li2 CO3 /LiCOO–R/LiOH formed in side reactions, form an insulating passivation layer on the surface in the air electrode, for which the hierarchically structured electrode presents significant advantages. Williford and Zhang [40] proposed an interconnected bimodal-pore system in an air electrode (i.e. the integration of one macro-pore system and one meso-pore/micro-pore system). Based on their modeling results, it is expected that bimodal-pore hierarchical electrodes (materials) are most promising in terms of energy density and power density: the first meso-pore/micro-pore system serves as the place for storage of Li–O2 discharge products, and the second macro-pore system allows efficient oxygen transport during discharge/charge. The products do not clog the

second macro-pore system, which secures oxygen transport into the inner regions of the electrode and improves the utilization efficiency of pores and surface [41]. The beauty of the dual-pore hierarchical electrode concept was recently demonstrated experimentally using a specially managed graphene air electrode. Xiao et al [5] successfully constructed hierarchically porous air electrodes with functionalized graphene sheets (FGS) using a colloidal micro-emulsion approach (figure 4). The three-dimensional air electrodes consist of interconnected pore channels on both the microand nano-meter length scales. It is clearly seen that graphene aggregates into loosely packed, ‘broken-egg’ structures with large interconnected tunnels; the ‘shells’ of the ‘broken eggs’ consist of numerous nano-scale pores in direct communication with the large tunnels/large pores. During discharge, the robust large tunnels/large pores function as ‘highways’ to supply the oxygen to the interior carbon while the small pores on the walls are the ‘exits’ that provide tri-phase regions for oxygen reduction reaction and Li–O2 discharge product storage. Furthermore, the FGS contains functional groups such as carboxyl, epoxy, hydroxyl, and defects. Carbon materials with functional groups/defects are usually more electro-catalytically active [42]. These oxygen functional groups/defects on graphene can accelerate oxygen reduction in Li+ -containing conditions [43]. The hierarchically porous graphene air electrode shows a discharge capacity of 15 000 mAh g−1 carbon; this translates to a specific area capacity of 20–30 mAh cm−2 carbon in the pouch cell design [5]. The importance of the hierarchically structured carbon electrode was also revealed by several other reports. Zhang and co-workers [44] developed a free standing, hierarchically porous carbon electrode from graphene oxide gel. The graphene oxide is the key in forming this hierarchically porous carbon which is related to the 3D gel framework of graphene oxide and the proper surface acidity via its intrinsic COOH groups on graphene oxide. This hierarchically porous carbon electrode displays a high discharge capacity of 11 060 mAh g−1 (0.2 mA cm−2 , 280 mA g−1 ). The rate performance is also significantly improved; a high capacity of 2020 mAh g−1 can be obtained at 2 mA cm−2 (2.8 A g−1 ). Manipulation of the hierarchical structure of carbon electrodes can not only lead to higher energy density, but also 5


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mismatch and enhanced electrode mechanical strength during repeated cycling have been demonstrated in the unique hierarchical structures for different battery chemistries. From a practical perspective, design of electrodes with hierarchical structures also addresses some of the critical challenges associated with nano-materials, such as low tap density and rapid increase of the electrode impedance, but still maintains the advantages of the nano-scale phenomenon. More effort should be devoted to using these hierarchical structures to gain fundamental information from different levels of scale, which, in turn, will guide the rational design of electrodes for energy storage and conversion.

provides new tools for fundamental understanding of Li–O2 reactions. Recently, Shao-Horn et al studied all-carbonnanofiber electrodes for high-energy rechargeable Li–O2 batteries [45]. Carbon fibers were grown on a ceramic porous substrate as a binder-free oxygen electrode [45]. The energy density of this oxygen electrode is four times greater than that of the state-of-the-art lithium intercalation cathode materials. This improvement can be attributed to the low carbon packing in the grown carbon-fiber electrodes which leads to highly efficient utilization of the available carbon and the void volume for Li2 O2 formation. The all-nanofiber electrode structure allows the direct observation of Li2 O2 growth and disappearance during the discharge/charge process, providing valuable information in understanding the Li–O2 reaction process which is fundamentally important for development of Li–O2 batteries. Mai and co-workers [46] reported hierarchical mesoporous perovskite La0.5 Sr0.5 CoO2.91 (LSCO) nanowires for the lithium air battery. LSCO nanowire with a diameter around 150 nm is composed of interconnected porous nanorods, which form a second level of meso-pores in the nanowires. The surface of the nanowire is rough and porous. The LSCO nanorods are closely attached to each other at the atomic level. This unique structure is believed to provide good physical contact between the nanorods and an increased oxygen pathway and is beneficial for electronic conduction. The LSCO nanowire presented high activity for oxygen reduction. When tested in a Li-air cell, the first discharge capacity can be as high as 11 059 mAh g−1 . Because of the higher density, the high specific capacity of the perovskite-based air electrode is probably more meaningful for volumetric energy storage, which is believed to be more important than specific energy density (based on mass) for transportation applications. These few examples have shown the importance of the hierarchically structured air electrode in the lithium air battery. It can significantly improve the specific capacity and rate performance [45–47]. However, significant challenges still exist. The performance improvement is clearly seen for the first discharge, but there are significantly less or no reported values for sequential discharge/charge. This is largely related to the instability of electrolytes. The air electrode which has higher activity for oxygen reaction probably also aggravates the instability issue of electrolytes [48]. Recently, concern about the involvement of the carbon electrode in side reaction has been raised [39, 49]. Searching for an alternative to a carbon-based electrode with hierarchical structures will be critical for high performance rechargeable lithium air batteries.

Acknowledgments We gratefully acknowledge the support provided by the US Department of Energy’s Office of Energy Efficiency and Renewable Energy, Office of Vehicle Technologies.

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4. Conclusions In this paper, we discuss the design and application of novel electrodes using hierarchically structured materials, especially for applications in LIBs and Li–O2 batteries. The microstructural properties of the hierarchical electrodes have been promoted to facilitate reaction mechanisms during the electrochemical process. Control of phase nucleation and growth, accelerated charge transfer, minimum structural 6


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