다음세대 리튬이온 배터리용 고에너지 밀도 게르마늄 음극 · 2014. 8. 13. · 다음세대 리튬이온 배터리용 고에너지 밀도 게르마늄 음극 3

Invited reviewAppl. Chem. Eng., Vol. 25, No. 1, February 2014, 1-13http://dx.doi.org/10.14478/ace.2014.1008

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다음세대 리튬이온 배터리용 고에너지 밀도 게르마늄 음극

조이 오콘*⋅이재광*,**⋅이재영*,**,†

*광주과학기술원 환경공학부 전기화학 반응 및 기술실험실, **광주과학기술원 차세대에너지연구소 Ertl 촉매연구센터(2014년 1월 23일 접수)

High Energy Density Germanium Anodes for Next Generation Lithium Ion Batteries

Joey D. Ocon*, Jae Kwang Lee*,**,, and Jaeyoung Lee*,**,†

*Electrochemical Reaction and Technology Laboratory (ERTL), School of Environmental Science and Engineering (SESE),**Ertl Center of Electrochemistry and Catalysis, Research Institute for Solar and Sustainable Energies (RISE),

Gwangju Institute of Science and Technology (GIST), Gwangju 500-712, Republic of Korea(Received January 23, 2014)

리튬이온 배터리는 전기화학 에너지 저장 및 변환 기기에서 가장 높은 수준의 기술력을 기반으로 개발된 셀이며, 여전히 높은 에너지 밀도와 충방전 안정성이 높아서 가장 매력적인 배터리의 부류로서 평가받고 있다. 최근 급속한 대형 에너지 저장 응용시스템의 개발이 이루어지면서 기존의 그래파이트 전극을 대체하기 위한 새로운 음극물질의 개발이 요구되고 있다. 게르마늄과 실리콘은 이론적 에너지 용량이 높아서 다음 세대 리튬 배터리의 적합한 물질로 평가받고 있으며, 특히 게르마늄은 실리콘에 비해 충방전에 따른 부피변화가 상대적으로 적고, 리튬이온의 동력학 거동이 용이하며, 높은 전기전도도 특성이 있다. 본 총설에서는 우선 리튬이온 배터리의 기본 원리를 소개하고, 배터리 특성을 최대한 발휘할 수 있는 이상적인 음극 물질의 구조와 특성을 살펴보고자 한다. 다음 세대 음극물질로 고려되고 있는 게르마늄 복합체가 어떻게 현재의 리튬 배터리를 개선할 수 있을지를 논의하려고 한다. 그리고 최근 시도되고 있는 연구동향에 대한 소개를 끝으로 리튬이온 배터리의 고에너지 밀도화에 대한 참고문헌이 될 수 있기를 바란다.

Lithium ion batteries (LIBs) are the state-of-the-art technology among electrochemical energy storage and conversion cells, and are still considered the most attractive class of battery in the future due to their high specific energy density, high effi-ciency, and long cycle life. Rapid development of power-hungry commercial electronics and large-scale energy storage appli-cations (e.g. off-peak electrical energy storage), however, requires novel anode materials that have higher energy densities to replace conventional graphite electrodes. Germanium (Ge) and silicon (Si) are thought to be ideal prospect candidates for next generation LIB anodes due to their extremely high theoretical energy capacities. For instance, Ge offers relatively lower volume change during cycling, better Li insertion/extraction kinetics, and higher electronic conductivity than Si. In this fo-cused review, we briefly describe the basic concepts of LIBs and then look at the characteristics of ideal anode materials that can provide greatly improved electrochemical performance, including high capacity, better cycling behavior, and rate capability. We then discuss how, in the future, Ge anode materials (Ge and Ge oxides, Ge-carbon composites, and other Ge-based composites) could increase the capacity of today’s Li batteries. In recent years, considerable efforts have been made to fulfill the requirements of excellent anode materials, especially using these materials at the nanoscale. This article shall serve as a handy reference, as well as starting point, for future research related to high capacity LIB anodes, especially based on semiconductor Ge and Si.

Keywords: germanium; lithium batteries; high energy density; nanotechnology; lithium alloying

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† Corresponding Author: Gwangju Institute of Science and TechnologyErtl Center of Electrochemistry and Catalysis, Research Institute for Solar and Sustainable Energies (RISE), Gwangju 500-712, Republic of KoreaTel: +82-62-715-2440, e-mail: [email protected]

pISSN: 1225-0112 @ 2014 The Korean Society of Industrial and Engineering Chemistry. All rights reserved.

1. Introduction

1.1. The Challenges in Energy Storage and Conversion Since the industrial revolution, society’s access to affordable and re-liable energy has been the identifying cornerstone of the increasing prosperity and economic growth. Fossil fuels currently supply most of our energy needs; yet, their inevitable long-term consequences put the survival of humankind’s future generations in question [1-3]. Borro-wing from the words that the Brundtland Commission used to define

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sustainability, in the context of sustainable energy, our energy sources must meet the needs of this current generation without compromising the ability of future generations to meet their own energy needs [4]. Indeed, our energy in the 21st century must be sustainable, or else, fu-ture generations will face a future devoid of energy security. Including the ill effects of climate change, these major concerns should push us to look for sustainable energy sources with minimal greenhouse gas emissions. The world needs another industrial revolution, one that would provide us with sources of energy that are affordable, acces-sible, sustainable, and environmentally friendly. Over the past decades, significant progress has been achieved in de-veloping alternative technologies to harvest and use clean, sustainable energy (solar energy, wind power, biomass energy, and hydrogen en-ergy) [5]. Nevertheless, these systems require energy storage units due to the intermittent energy generation and consumption. For example, electricity consumption is usually not even and there are peak and off-peak loading vibrations. Storing the excess off-peak electricity and releasing it during the peak period could save a significant amount of energy. Electrochemical energy storage and conversion devices, such as batteries, are attractive technologies for serving these energy-related ef-forts [6]. As a matter of fact, their efficiencies exceed the Carnot limi-tations in mechanical engines because electrochemical processes do not involve conversion of thermal to mechanical energy [7]. In addition to storing energy from renewable sources, portable, entertainment, com-puting, and telecommunication equipment require energy technologies with the best energy densities, and none of the memory effect experi-enced in older battery chemistries and little self-discharge [8]. Future transportation applications, such as plug-in hybrid vehicles, are now looking into using high-energy density batteries for full commercializa-tion [9]. One of the holy grails in electrochemistry and energy research has been the search for new battery chemistries that have power and gravimetric densities comparable to combustion engines. Among electrochemical energy storage and conversion devices, lith-ium ion batteries (LIBs) have been the battery of choice for ubiquitous electronic devices because of their high energy density and decreasing cost. Although all commercial LIBs are currently using conventional graphite anodes, their energy capacity, however, is limited because of the restriction in the lithium intercalation mechanism in graphite, which can only accommodate one lithium ion for every six carbon atoms. This has led to a remarkable interest, of late, on high energy density anodes based on Group IVA elements (Si, Ge, and Sn), although not without major challenges [10]. In this focus review, we summarize the brief history and working principles of LIBs, in order to provide the proper context. We then describe the requirements for more-applicable anode materials and how rationally designed Ge-based anodes can sig-nificantly lead to next generation LIBs with higher energy densities and longer life spans. Although a significant number of reviews have been written on this exciting subject of LIBs, most studies focused on the role of size and morphology (e.g. nanosize effects, one-dimensional nanostructures) and material of choice (e.g. metal oxides and oxysalts, Si-based, Sn-based) and are too broad in scope [10-18, 22]. This article will be the first to provide an in-depth review and discussion on

Ge-based LIB anodes, with particular emphasis on Ge and Ge oxides and the electrochemical lithiation behaviour of Ge nanostructures.

1.2. Brief History and Basic Working Principles of LIBs The main motivation for using Li metal as anode in battery technol-ogy was the fact that it is the most electropositive (ca. 3.4 V vs. SHE) and lightest among metals (molecular weight of 6.94 g mol-1and specif-ic gravity of 0.53 g cm-3). Due to this exceptionally low density, Li has a specific capacity of 3860 mA kg-1, in comparison with a heavy metal such as Pb, which has a specific capacity of 260 Ah kg-1. In ad-dition, another appealing aspect of Li electrochemistry lies in its small ionic radius, which is ideal for diffusion. The use of materials that are highly electropositive in a battery allows an electrochemical cell to op-erate at higher voltages. Hence, with the use of Li anodes, a high volt-age battery with an extremely high energy density is possible. Owing to these characteristics, commercial Li batteries rapidly found applica-tions in almost facets of life. Similar to other innovations, technological development of Li bat-teries resulted from several far-reaching contributions within half a century. A working Li battery was first demonstrated in the 1970s, however, they were only able to develop a primary (non-rechargeable) battery [19]. Eventually, Exxon Company manufactured the first com-mercial secondary Li batteries, which uses Li metal anode, lithium per-chlorate in dioxolane electrolyte, and LiTiS2 cathode [20-31]. Discov-ery of materials that react with Li in a reversible matter, later identified as intercalation compounds, was key to the commercialization of Li batteries. Although the potential of intercalation materials were widely known among the solid-state chemistry circles, it was not a common knowledge to scientists working on battery systems [32]. The work on LiTiS2 cathodes gained prominence, leading to several related studies based on layered manganese oxide. Several attempts were made to convert metallic Li battery into a secondary battery but these efforts failed due to two main issues: 1) dendrites form at the anode during charging, which causes short-circuiting; 2) the high chemical reactivity of metallic Li, which results into poor battery characteristics, poses in-herent risk for thermal runaway reactions that put the safety of these batteries in question [33]. The uneven dendritic growth during charge-discharge cycles presented explosion hazards, limiting the use of these Li systems. Soon enough, Sony introduced a highly efficient secondary Li battery in 1991 following the breakthrough research on LixMO2 (where M is Co, Ni, or Mn) by Goodenough’s group, until to-day this family of layered intercalation compounds are still used almost exclusively [34-35]. Commercialization of Li ion battery, by the joint venture between Asahi Kasei and Toshiba in 1992, marked the birth of the Li battery era that made it possible to store energy at twice the capacity of nick-el-cadmium or nickel-metal hydride batteries. To solve the safety is-sues previously encountered when using Li metal, Li metals were sub-stituted by insertion anode materials. Hence, Li ion batteries can be de-fined as “non-aqueous secondary batteries using transition metal ox-ides containing lithium ion (such as LiCoO2) as a positive electrode and carbonaceous materials (such as graphite) as a negative elec-

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(a)

(b)Figure 1. (a) Typical lithium ion cell configuration. During discharge, graphitic carbon anodes release the electrons (oxidation) and the Li ions insert into the layered metal oxide cathodes (reduction). During charging, the reverse redox reactions occur where Li ion are deintercalated from the cathode and are intercalated into the graphitic layers. (b) Specific capacities and volumetric capacities for selected metal anodes for Li ion batteries. Redrawn from [30].

trode” [23]. Research works on the electrochemical Li intercalation and release in graphite led to this Li ion or rocking-chair technology [24,26-28]. Dealing with ionic-phase Li proved easier as compared to metallic Li, thus making Li ion cells intrinsically safer than Li met-al-based cells. Moreover, the cell reaction is not chemical in nature, leading to a more stable battery characteristic over long charge-dis-charge cycle life without serious degradation from side reactions. In comparison to Li metal-using cells, the C/LiCoO2 cells are produced in a fairly simple and efficient manufacturing process, not requiring any special inert atmosphere since LiCoO2 and graphitic carbon are both stable in air. By providing a cell voltage of around 3.6∼4.0 V and a gravimetric energy density of 120∼150 Wh kg-1, Li ion battery made it possible to operate electronic devices even with a single cell. As previously discussed, in a Li ion cell, Li ions intercalate and de-intercalate reversibly on both the anode and cathode, over hundreds of cycles. In a typical structure, a Li ion cell is composed of a graphite anode (e.g. mesocarbon microbeads, MCMB), a Li metal oxide cathode (e.g. LiMnO2 and LiCoO2), and a Li salt (LiPF6)-containing electrolyte based in mixed organic solvents, along with the separator [29]. A good advantage of graphite and other carbon-based anodes is its capability to allow the insertion/deinsertion reactions close to the potential of Li metal. Moreover, the cyclability and safety are remarkably enhanced with a carbon anode, in contrast with a Li metal anode. As seen in Figure 1(a), which illustrates the Li ion operating principles, the rever-sible insertion and extraction of lithium ions between the two electro-des concurrently occur with the release and gain of electrons. Choosing a particular material for the three major components (anode, cathode, and electrolyte) of a Li ion cell depends on the physical, chemical, electronic, and electrochemical properties, which dictates the cell’s op-erating voltage, energy density, safety, and cycle life.

1.3. The Search for New LIB Anodes In order to fully evaluate the merits and demerits of a particular anode candidate, it will be of interest to look at the properties of an ideal anode and compare it with that of conventional graphite elec-trode. There are five main criteria in the search for new anode materi-als in LIBs, and they are as follows [16]: (1) An ideal anode material should be based on light elements and compounds that can accommodate high number of Li ions and allow reversible lithiation/delithiation reaction in order to get stable energy capacities. Although graphite is a light material, its energy capacity is limited to just 372 mAh h-1 because six carbon atoms can only accom-modate one Liion (LiC6). (2) An ideal anode material’s potential should be close to Li metal’s redox potential so as to maintain an overall working potential of at least 4 V. With respect to Li metal, graphite exhibits a potential of 0.15∼0.25 V. (3) An ideal anode material should be insoluble and unreactive to the components of the electrolyte. In graphitic anodes with ethylene carbonate-based electrolytes, electrolyte decomposition during initial cycles eventually results in making the cell operate under kinetic stability. The reaction induces the formation of a solid electrolyte inte-

face (SEI), a protective film at the anode surface that acts as a barrier to prevent further electrolyte degradation. Despite the benefits of the SEI in the cell operation, however, it hampers the diffusion of Li ions and results in metallic Li plating at the surface. (4) An ideal anode should have good electronic and Li ionic con-ductivity to minimize cell resistance for electronic and ionic conduction during cell operation. Although graphite in itself is a semiconductor, LiC6 shows excellent metallic conductivity and Li ion diffusivity. (5) An ideal anode material should be relatively inexpensive and en-vironmentally safe. This criterion made graphite the best material of choice in making LIBs commercially available.

Since the commercialization of graphite-based LIBs, a considerable number of materials have received great attention as potential alter-native anodes [30]. Generally, these materials can be divided according to the Li ion storage mechanisms: (1) intercalation-deintercalation mechanism (e.g. transition metal oxides and other compounds with two-dimensional structure), (2) lithium alloying (e.g. Si, Ge, Sn), and


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Figure 2. In situ TEM investigations of the electrochemical lithiation and delithiation phenomena in Ge NWs and Ge NPs. (a) Isotropic swelling of Ge NW during lithiation, characterized by expansion in both axial and radial directions, as shown in the in situ TEM images taken before (i) and after lithiation (ii). Close up images of the Ge NW tip before (iii) and after (iv) lithiation, showing the large volume change. EDPs of pristine crystalline Ge and (vi) amorphous Li-Ge alloy, signifying the phase tranformation after lithiation [35]. (b) Nanoscale pore formation in Ge observed in both in situ and ex situ cycling experiments. Nanoporous Ge after delithiation in a cell using an ionic liquid electrolyte (i) or using a Li2O solid electrolyte (ii). (iii) Nanoporous Ge resulting from cycling in a conventional half cell set-up and investigated using ex situ TEM [45]. (c) Crystalline and amorphous phases formed during lithiation-delithiation cycling in Ge NPs. (i-ii) Pristine Ge NP showing a single crystalline EDP pattern, (iii-iv) Lithiated Ge NP with the EDP showing the transformation to an amorphous lithiated phase (a-LixGe), (v-vi) Fully delithiated Ge NP with the EDP showing an amorphouse Ge phase (a-Ge), (vii-viii) Fully lithiated Ge NP with the EDP showing a crystalline lithiated phase c-Li15Ge4 [46].

(3) redox or conversion reaction with Li (e.g. nanosized CoO).

2. Germanium as LIB Anode

Despite Ge’s prohibitively high cost relative to the more widely available and cheaper Si, Ge has garnered increased interest these days due to a number of valid reasons. Although Ge (1600 mAh g-1) has a much lower gravimetric capacity than Si (4200 mAh g-1), their volu-metric capacities are comparable with one another (Ge, 7366 Ah L-1; Si, 8334 Ah L-1). As illustrated in Figure 1(b), Ge has one of the high-est energy densities among candidate anode materials. In addition, Ge exhibits better electrochemical kinetics than Si, arising from two orders of magnitude higher Li ion diffusivity and four orders of magnitude higher electronic conductivity [31,32]. If this outstanding kinetics trans-lates to a highly improved anode performance, it can lead to LIBs with higher energy and power densities, especially in applications where cost is not an issue. Analogous to Si, Ge undergoes a volume change of 370%, albeit at a smaller volume change than Si (400%). The huge volume expansion during lithiation leads to pulverization and cracking, and represents the most critical challenge in the use of Ge and Si as LIB anode materials. In fact, recent works in LIBs have proposed many rationally designed anodes, according to the material’s chemical composition and structure, in order to address the inherent issue of large volume variations, unstable surfaces/interfaces, and breakage of electron transport pathways during cycling. Wu et al. categorized four design considerations for anode materials based on Group IVA ele-ments: (1) anode materials should have the right number of voids to minimize volume expansions, (2) anode materials should be based on a “plum-pudding” structure to buffer volume variations, (3) anode ma-terials should facilitate electron and lithium transport across the struc-tures, and (4) anode materials should provide a highly stable SEI [10]. Although Ge lithiation has been known to occur in three distinct lithiated crystalline phases, ultimately forming Li15Ge4, in depth under-standing on the microscopic mechanism during the electrochemical re-action is not yet clear [32,33].

→→→ (1)

Of late, thanks to advances in nanobattery assembly within a trans-mission electron microscopy (TEM), it is now possible to directly probe the reaction kinetics and microstructural changes of Ge nano-wires (Ge NWs) with near-atomic resolution in real-time [34,35]. This development is undoubtedly important in bringing into light the funda-mental aspects of lithiation and delithiation in order to be able to ra-tionally design better anode materials. In addition, this has allowed comparison between observations performed using ex situ electroche-mical experiments and in situ TEM electrochemical experiments. As an example, the formation of the intermediate Li-M and Li15M4 (where M= Si or Ge) phases that were observed in conventional electrochemical cells have been verified in situ via advanced TEM electrochemical ex-periments [36-44]. In fact, in situ TEM investigations allow the identi-fication of unknown Li-Ge phases, which were visible in the cyclic

voltammograms but remained unknown. Furthermore, electron dif-fraction patterns (EDPs) taken before and after lithiation provide suffi-cient evidence on the transformation of a single crystalline Ge to an amorphous Li-Ge phase, before eventually forming a crystalline Li15Ge4 phase. A particular feature of Ge lithiation that makes Ge more interesting as an anode material is its isotrophic lithiation, as shown in Figure 2(a). Despite having a similar crystal structure and growth direction



along with Si, Ge NWs exhibit nearly isotrophic response, an extremely opposite behavior to Si NWs’ highly anisotrophic behavior [35]. During lithiation, the pristine Ge nanowire expands equally in both axial and radial directions, whereas Si NWs swell preferentially on one direction, eventually resulting to cracking. The isotrophic Ge lithiation can lead to a highly reversible capacity because it avoids fracture and prevent loss of electrical contact in between Ge NWs and also with the current collector. Another property of lithiation in Ge NWs, which is relevant to un-derstanding its anodic characteristics, is the reversible nanopore for-mation during lithiation-delithiation cycles, as shown in Figure 2(b) [35]. Thereby proving that delithiation in Ge, which causes extreme mechanical stress due to the large volume variation, does not result in-to cracking. The formation of porous Ge structure validates an earlier observation made using ex situ techniques, showing the strong con-sistency between in situ and ex situ methods [45]. Pore formation was found to occur from the combination of vacant sites, previously occu-pied by Li ions, and has similarity in structure with porous materials after dealloying. This unique structure - oddly not observed in Si - en-hances the mechanical robustness of the anode via facile stress relaxa-tion, leading to fast lithiation/delithiation rates. Building upon the results obtained with Ge NWs, Liang et al. re-ported on the highly reversible expansion and contraction of Ge NPs during lithiation-delithiation using in situ TEM, in combination with chemomechanical modeling [46]. The results confirmed the two-step phase transformation of Ge NPs during lithiation, from amorphous Ge to amorphous Li-Ge alloys, and finally to crystalline Li15Ge4, with the particle volume increasing c.a. 260%. Moreover, the nanosized Ge NPs (100 nm to submicron) exhibited highly reversible expansion and con-traction at fast cycling, in contrast to Si NPs where the anisotrophy of the lithiation strain led to fracture (see Figure 2(c))[47]. This particular behaviour of Ge NPs offers substantial potential in the development of both high capacity and high-rate anodes, although maintaining a stable SEI during large-strain cycling remains a challenge. The authors argue, however, that the crystallography-dependent anisotrophy can be avoid-ed in Si by using amorphous materials, mitigating the mechanical deg-radation of the electrode materials during cycling.

3. Ge and Ge Oxide Anodes

Generally, the large theoretical capacity of Ge anode materials can be fully taken advantage if pure Ge or Ge oxides are used. Ge and Ge oxides, in many different forms such as nanoparticles (NPs), films, three-dimensional porous structures, nanotubes (NTs), and nanowires (NWs), have been studied as anode materials for LIBs.

3.1. Ge NPs Graetz et al., for instance, investigated Ge nanocrystals (12 nm mean diameter), which were prepared by gas-phase ballistic deposition, and demonstrated a stable capacity of up to 1400 mAh g-1 with 60% ca-pacity retention after 50 cycles, as displayed in Figure 3(a) [31]. They attributed the enhanced capacity, rate capability (up to 1000C), and cy-

cle life of Ge NPs over bulk germanium from the increased surface area, short diffusion lengths of the active material and the absence of defects in the nanomaterial. A huge improvement in capacity retention was achieved using butyl-capped amorphous Ge NPs, which exhibited an initial capacity of 1470 mAh g-1 and a capacity retention of 98% over 30 cycles. In addition, the anode material had excellent rate capa-bilities, with just 2% capacity loss even at a cycling rate of 5C [48]. Butyl groups that capped the Ge NPs - acting as surface stabilizer - decreased the rate of electrolyte decomposition on the particle surface and prevented serious aggregation of the Ge NPs during lithiation-delithiation, as evidenced by the reformation of the Ge-Ge metallic bond-ing according to X-ray Absorption Spectroscopy (XAS). Recently, a very stable, high capacity cycling performance over 2500 deep cycles at various C rates (1C, 5C, 10C) using slurry-case LIB anodes from commercial Ge nanopowder was reported [49]. Polydispersed commer-cial Ge nanopowder, along with the use of fluorinated ethylene carbo-nate (FEC)-based electrolyte, demonstrated a capacity near 700 mAh g-1 at 10C rate over 500 cycles. The significant improvement in cycling stability resulted from the highly stable SEI/Ge interface, as a con-sequence of the different surface films containing lithium fluoride, al-koxy, and polycarbonate species [50,51]. Aside from metallic Ge, GeO2 is an equally interesting material of choice because it has a theoretical gravimetric capacity of 1100 mAh g-1 and a volumetric capacity of 4653 mAh cm-3 [50]. Amorphous GeO2 NPs, prepared using surfactant-assisted hydrothermal process and uti-lizing a FEC-based electrolyte, exhibited a stable capacity higher than 600 mAh g-1 at 0.2C for up to 500 cycles, with 96% capacity retention (Figure 3(b)) [52]. The use of an FEC-based electrolyte resulted in a qualitatively different but highly stable SEI and Li2O matrix. Although several studies have already reported the use of particles in the nano-scale, the effect of quantum confinement to the electrochemical per-formance was rarely mentioned. To this end, Son et al. looked into the critical size of GeO2 NPs for LIB anode applications. They controlled the nanoparticle sizes by adjusting the reaction rate, reaction temper-ature and reactant concentration. Unexpectedly, particles smaller than 2 nm exhibited inferior electrochemical performance, arising from the low electrical conductivity due to quantum confinement effect and higher propensity to aggregate [53]. Contrary to the conventional no-tion that the smaller particles the better they are as anode materials, nanoparticles of diameters c.a. 6 nm displayed the best electrochemical characteristics over smaller particles, as displayed in Figure 3(c). Com-paring the cycle-dependent lithiation/delithiation in Ge NPs and GeO2 NPs, a recent study showed that a metastable tetragonal (ST12) phase Ge NPs were always produced, becoming the dominant phase after on-ly a few cycles and completely replacing the original phase [54]. In this study, the highly crystalline Ge NPs and GeO2 NPs, which were synthesized via gas phase laser photolysis, displayed excellent cycling performances, maintaining their capacities of c.a. 1100 mAh g-1 after 100 cycles.

3.2. Ge Films In the seminal paper of Graetz et al. in 2004, they investigated the


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Figure 3. Representative morphologies and the cycling capacities for various Ge and GeO2 anodes. (a) TEM images of ballistically depo-sited Ge nanocrystals and amorphous Ge thin film along with the cycling behaviour during charge-discharge (iii) [31]. (b) SEM images of GeO2 NPs prepared using different ratios (w/w) of GeCl4/1,2-di-aminopropane (i-iii) and the reversible capacities of representative GeO2 NP electrode cycled at 0.2C in two electrolyte types (ii) [52]. (c) High resolution TEM images of GeO2 NPs with three increasing sizes (i) (~2 nm, ~6 nm, and ~25 nm) before (top) and after cycling (bottom), and the corresponding cycle performances at charge-discharge rates of 0.2C (ii) and 1C (iii), voltage profiles of the 6 nm GeO2 NPs between 0.005 and 1.5 V at 0.2C rate (iv), and rate capability of the NPs from 0.2 to 10C [53].

potential of amorphous thin film electrodes prepared on planar nickel (Ni) substrates. In comparison with Ge nanocrystals (1400 mAh g-1 af-ter 50 cycles), the Ge thin films (60∼250 nm thick) showed stable ca-pacities of 1700 mAh g-1 after 60 cycles, having a crystalline phase in the fully lithiated state [31]. Of late, nanostructured ion beam-modi-fied Ge film electrodes, were found to exhibit excellent specific gravi-metric capacities as LIB anode [55]. After depositing amorphous Ge films (200∼240 nm thick) on Ni foil substrates, it was then implanted with Ge+ at room temperature with an energy of 260 keV to a dose of 1.0 × 1016 cm-2, producing nanoporous electrodes. The charge-dis-charge capacity of the nanoporous Ge films remained high (1352 mAh g-1 initial capacity), at 1260 mAh g-1 after 25 cycles, in contrast with the as-deposited film’s fast degradation to only 200 mAh g-1 after 25 cycles. Analogous to the aggregation of NPs during cycling, the inter-digitated network of strands within the porous Ge was lost after cycling. The exact role, however, of ion beam mixing on the anode

characteristics was not determined due to the concurrent changes in the morphology. Following this initial work, ion beam modification was performed on Ge films, without changing their morphology. This has resulted in the improved adhesion of the Ge film on the current collec-tor, exhibiting a stable capacity of 1500 mAh g-1 after 25 cycles for cycling rates of 0.2C∼1.6C [56]. Although the film was transformed into a rough, cracked, and porous layer, the electrical contact between the active material and the current collector remained intact.

3.3. Three-dimensional Porous Ge In order to accommodate the large volume strains during cycling, engineering anodes with a large void fraction is one of the most effec-tive strategies. This approach is ilustrated in a number of recent studies utilizing porous Ge NPs or a nanoporous Ge network [57-61], which were prepared from simple (e.g. mechanochemical synthesis) to of-ten-complicated synthesis (e.g. silica and polystyrene (PS) templating) techniques. First of such efforts, 0D and 3D porous Ge NP assemblies were prepared by etching a thermally annealed physical mixture of SiO2 and ethyl-capped Ge gels at 800 °C. Depending on the weight ra-tio of the Ge precursor, either a 0D or 3D porous Ge NPs (pore size of around 200 nm and a wall thickness of 20 nm) are formed, and they showed capacities of 1162 and 1415 mAh g-1, respectively, after 100 cycles at a rate of 1C (Figure 4(a)) [57]. Long-range ordering of thin- walled porous Ge NPs induced the very stable cycling performance, with only 2% capacity loss after 100 cycles. Ge inverse opals with unique wall microstructures, synthesized using silica opal templates and grown directly on the current collector, were tested recently as LIB anode as displayed in Figure 4(b) [60]. The two types of Ge inverse opal structures - with porous wall and with dense wall - exhibited cou-lombic efficiencies over 96% after the first cycle, but the Ge inverse opal electrode with porous walls (1387 mAh g-1 first discharge ca-pacity) displayed a much higher retention after 100 cycles, at 88.9% compare to electrode with dense walls (1299 mAh g-1 first discharge capacity) at 80.5%. It is believed that the voids within the 3D nano-structures lessened the impact of the large mechanical stress and strain during cycling, in addition to the reduced internal resistance. In a sim-ilar study, Ge NPs were electrodeposited at room temperature in PS templates to form a 3D ordered mesoporous Ge with a highly crystal-line phase. The porous Ge anode displayed a reversible capacity of 1024 mAh g-1 and retained a capacity of 844 mAh g-1 after 50 cycles at a rate of 0.2C. Without the aid of any template, however, a recent work reported a very stable capacity of an amorphous hierachical po-rous GeOx, with capacities of 1728, 1575, 728 mAh g-1, for 0.05C, 0.2, and 0.5C, respectively, after 600 cycles [61]. The authors argue that the superior performance of the porous GeOx anode arises from the synergy of the material’s four characteristics: small primary particles, porous structure, amorphous state, and the incorporation of oxygen.

3.4. Ge NWs and Ge NTs Ge nanowires (Ge NWs) and nanotubes (Ge NTs) are another class of well-studied morphology for LIB anode application, with their ad-vantage of being able to alleviate the stress by the sufficient space be-



Figure 4. Representative morphologies and the cycling capacities for various Ge and GeO2 anodes. (a) SEM image (i) of the 3D porous Ge NP assembly prepared by etching a thermally annealed physical mixture SiO2 and ethyl-capped Ge gels at 800 ℃ and the cycling performance (ii) at a rate of 1C in coin-type half cells [57]. (b) Top view SEM images of the Ge inverse opal with porous walls (i) and with porous walls (ii), and the electrochemical characteristics of both Ge inverse opal structures at a rate of 0.2C [60]. (c) Comparison between a traditional battery electrode and a NW electrode design consisting of NWs grown on top of the current collector (i), TEM image and SAED pattern of Ge NW before and after cycling (ii), and the cycling life at a rate of 0.05C [62]. (d) TEM image of the Ge NT (i) and the plot of capacity versus cycle number in three Li ion cells (ii) [66].

tween adjacent NWs and NTs during lithiation and delithiation. In ad-dition, these almost 1D materials can be directly connected to the cur-rent-collecting substrate, decreasing the cell resistance and enabling ro-bust electrical contact. Cui et al. first proposed the use of Ge NWs, which were fabricated using vapor-liquid-solid (VLS) growth on the metallic current collector, as LIB anodes (Figure 4(c)). Electrochemical cyling of the Ge NWs showed good stability at ca. 1000 mAh g-1 over 20 cycles at a rate of 0.05C [62]. Ge NWs remained intact during cy-cling, maintaining their structure and good electron conductivity via facile strain relaxation and allowing Li ions to diffuse quickly due to the short distance. Succeeding studies looked at different nanowire preparation techniques, such as direct electrodeposition of crystalline Ge NW film electrodes from an aqueous solution of GeO2, solution grown Ge NWs using FEC-based electrolyte to improve the cycling ca-pacities, and a tin-catalyzed Ge NWs in a solvent-vapor system for a dual-cyling anode (tin and germanium) [63-65]. These promising Ge

NW anodes were able to maintain at least 1000 mAh g-1 in as many as 50 cycles. Following the Kirkendall effect that was discovered in 2004, Ge NTs were prepared using a high-yielding synthesis technique and tested for its electrochemical properties as LIB anode. It exhibited exceptionally high rate capability of up to 40C, while maintaining a ca-pacity of more than 1000 mAh g-1 over 400 cycles. The increase in void space improved the capacity retention of Ge NTs over Ge NWs by more than 20%, likely resulting from the absence of non-uniform expansion in the regions of the crystalline Ge with different Li con-centrations. The hollow space inside the amorphous Ge NTs acted as a buffer to accommodate the mechanical strain during lithiation-delithiation, as shown in Figure 4(d) [66].

4. Germanium-carbon Composite Anodes

Despite the huge capacity of Si and Ge-based anodes, the large vol-ume change leads to pulverization and capacity loss in bulk electrodes. Of late, many strategies have been implemented to mitigate this con-cern, including the use of nanosized and nanostructured materials, and the combination of the active material with carbon or with an inactive matrix. Of these methods, a widely used approach to improve the elec-tron transport and maintain the electrical contact in electrodes is the use of conductive additives such as carbon materials to form compo-sites. This approach had been successfully applied to enhance the cy-cling performance of anodes based on high-volume-change anodes such as Si and Ge. To date, the selection of carbon materials used as additive to Ge for LIB anodes include graphene, carbon nanotubes (CNT) and carbon nanofibers (CNF), mesoporous carbon, and carbon coatings on Ge.

4.1. Ge-graphene Composites Although graphene has had a relatively short history, it has been at the forefront of thousands of researches on future disruptive tech-nologies in almost all facets of life. The parent form of all graphitic carbon, graphene has excellent thermal properties, ballistic electron mobility, high surface area, and high Young’s modulus, which makes it ideal to suppress the mechanical stress during lithiation-delithiation cycles, prevent particle aggregation, and maintain the electron-conduct-ing channels [67,68]. In fact, a number of studies using graphene, in combination with Ge, have reported enhanced electrochemical perform-ance of the LIB anodes [69-79]. One of the earliest reports was the combination of Ge NPs and gra-phene, which was synthesized under mild conditions and using gra-phene oxide (GO) derived from sugarcane bagasse. Although the Ge NP-graphene nanocomposite’s capacity retention reached 90% of the second cycle capacity of 532 mAh g-1, it was only tested for 15 cycles [69]. A capacity retention of 84.9% after 400 cycles in Li ion half cells was reported lately by Ren et al. using a low pressure thermal evapo-ration method to deposit crystalline Ge particles (200 nm) onto gra-phene sheets. They demonstrated that the large irreversible capacity loss during the first cycle is not really an intrinsic characteristic of Ge-based anodes [76]. Succeeding reports on Ge-graphene composites


공업화학, 제 25 권 제 1 호, 2014

Figure 5. Representative morphologies and the cycling capacities for various Ge-carbon composite anodes. (a) SEM image (i) and the cycling performance (ii) of Ge-graphene nanocomposite using a low- pressure thermal evaporation approach [70]. (b) TEM image (i) and the cycling performance (ii) of the self-assembled hexahedral-like GeO2cluster nanostructures after carbon coating [82]. (c) TEM image of the Ge-carbon core shell nanoparticles (i) and Ge-carbon nanoparticle- graphene nanocomposite (ii) and the cycling performance [84]. (d) TEM image (i) and the cycling performance (ii) of hierarchically porous Ge-modified carbon materials, synthesized via facile hydro-thermal method [88].

utilized different techniques to form the composites. One of the studies reported a method for the high yield synthesis of free-standing Ge NPs and using it to make hybrid nanostructures of Ge NP and reduced gra-phene oxide (RGO) that showed interesting light sensitivity properties and excellent cycling capability as LIB anode (1000 mAh g-1 after 50 cycles) [72]. In another study, nanosized GeOx/RGO, synthesized via a single-step reduction synthesis, delivered a high reversible capacity of 1600 mAh g-1 at a rate of 0.1C and a capacity of 410 mAh g-1 even at 20C [73]. Low temperature chemical vapor deposition (CVD) was also used to create vertically alligned graphene-amorphous GeOx sand-wich nanostructures, showing a stable capacity of 1008 mAh g-1 for 100 cycles at 0.33C [78]. Besides Ge NPs, Ge NW-graphene composites were also developed as negative electrode materials in LIBs. For example, the cycling be-

haviour of the binder-free graphene-supported Ge NW depended on the Ge loading and the discharge-charge rate, with the Ge lithiation domi-nating at high Ge loading (> 50% w/w) and low cycle rates (< 0.1C) [71]. Although the composite exhibited improved performance at a re-versible capacity of 800-1400 mAh g-1 for a current rate of 0.05C, it was only tested at 20 cycles. Meanwhile, graphene encapsulated Ge NWs, synthesized in situ in an arc discharge process, showed a much better electrochemical performance or 1400 mAh g-1 capacity after 50 cycles at 1.6C [77].

4.2. Carbon Coated Ge, Ge-porous Carbon, Ge-CNT, and Ge-CNF Composites

Another approach to improve the reversibility of LIB anodes based on Ge is the use of carbon coating and porous carbon to act as struc-tural buffers during Li insertion/deinsertion, which was demonstrated by a significant number of reports [74, 80-86]. For example, a recent study reported a facile method to produce Ge/carbon nanostructures by carbon coating and reduction of the Ge oxide precursor, forming two different self-assembled nanostructures; clustered and non-clustered structures [82], as shown in Figure 5(b). Only the clustered Ge nano-structure within the hollow carbon shell, however, showed exception-ally high rate capacity up to 40C. The capacity retention of the clus-tered (896 mAh g-1) Ge-C nanostructures was 74% and 63%, respec-tively. A similar approach of simultaneous carbon coating and reduc-tion of the Ge precursor reported the synthesis of mesoporous and hol-low Ge-carbon nanostructure using a one-pot ultrasonication method. The hollow ellipsoidal Ge-carbon composite exhibited highly improved cycling stability (100% capacity retention after 200 cycles at 0.2C) and better rate capability (805 mAh g-1 at 20C). Aside from being an active material for lithium storage, Ge was also found to have a catalytic ef-fect in promoting the decomposition of LiO2 in a GeO2/Ge/carbon nanocomposite anode [86]. Furthermore, another promising strategy for solving the aggregation issue when using Ge NP is the combination of the coating strategy with the use of graphene to wrap the NPs, forming a mixed conducting 3D network. Demonstrating this double protection approach, Xue et al. and Li et al. demonstrated, separately, the much improved specific ca-pacity, cycling performance, and rate capability of Ge@C/RGO nano-composites, arising from the combined buffer and structural stability of the electrode [84,85]. In addition to carbon coating, combining Ge nanostructures and porous carbon had been shown to improve the cy-cling retention of the Ge-based anodes [87,88]. For instance, Ge NW-porous carbon composite anode displayed better stability and high-er specific capacity, indicating a synergistic effect between the two components [87]. A nanocomposite consisting of hierarchically porous Ge and modified carbon, synthesized via a facile hydrothermal method followed by annealing, had shown superior electrochemical perform-ance, mainly due to the increase in electrode conductivity and buffer-ing to accommodate the volume changes during cycling [88], as dis-played in Figure 5(d). Another disruptive approach for electrode design in LIB in the use of free-standing anode to eliminate the use of binders and inactive cur-



rent collectors. DiLeo et al. for example, demonstrated in a series of studies the use of free-standing carbon nanotube (CNT) electrodes in combination with high capacity Ge [89-91]. CNT electrodes are thought to be viable anode materials because they are light-weight, has robust mechanical and electrical properties, and has the ability to support high energy anode materials [92]. Recently, carbon nanofibers (CNF) were also used as composite material, combining it with highly dispersed Ge NWs via CVD and incorporating Ge into electrospun CNF as bind-er-free electrodes [93,94].

5. Other Ge-based Composite Anodes

5.1. Si-Ge Combination of Si and Ge, both leading candidates to replace the carbon anodes in LIBs, has been suggested in recent years. The Si-Ge anodes can be in the form of layered Si-Ge, Si-Ge alloys, physical mixture of Si-Ge, and alloys in combination with other metals like copper (Cu), nickel (Ni), and molybdenum (Mo) [95-102]. Si and Ge are miscible over the entire range of composition and terminal phases in lithiation have similar Li contents (Li15Si4 and Li15Ge4), structures, and lattice constants [100,103,104]. Si-Ge and Si-Ge-Mo multilayer an-odes, prepared by magnetron sputtering, were reported lately [95-97]. For instance, Mo dispersed homogeneously in the Si and Ge matrix prevented the active material from aggregating, resulting on the en-hancement of the capacity and cycling performance, with the Si0.41 Ge0.34Mo0.25 composite film showing long cycleability of 870 mAh g-1 over 100 cycles [96]. Meanwhile, Si-Ge multilayered anodes onto Cu current collector exhibited a stable reversible capacity of 1559 mAh g-1 after 100 cycles [97]. Nanostructured Si1-xGex thin films, with x = 0, 0.25, 0.5, 0.75, 1, were recently tested for their electronic, electro-chemical, and optical properties [100]. The nanoporous Si1-xGex films with reproducible structure and porosity, synthesized via evaporative deposition in high vacuum at normal or glancing angles, showed the typical trade-off between the specific capacity (Si) and rate capability (Ge), with the Si and Ge pure anodes as the two extremes. Although 1D nanostructures, such as NWs and NTs, have been ex-plored for their application in LIB anodes, nanotube heterostructures have received little attention because of the complexity in the synthesis. Recently, a vertically aligned Si and Ge-based nanotube heterostructure arrays, synthesize by a template-assisted CVD, displayed improved electrochemical performances over analogous homogeneous Si anodes [98]. Using high capacity Si and Ge as inner and outer material layers, respectively, the maximum hoop strain in the NTs decreased relative to NTs based on Si alone and the activation energy for Li diffusion lowered. The reduction of the mechanical stress - confirmed also by theoretical mechanics modeling - led to a first charge capacity of 1544.6 mAh g-1 at a rate of 0.2C with a capacity retention of 85% af-ter 50 cycles, higher than that of Si NTs’ 82% capacity retention. Furthermore, Si1-xGex and NixSiy-SiGe core-shell NW array electrodes have been reported recently and showed improved electrochemical per-formance due to good adhesion and contact between the core-shell NWs and current collector, better strain accommodation and faster

electron transport [99,102].

5.2. Other Ge-based Compounds Although Cu has been the common current collector in LIB re-search, it has been applied also as an inactive additive to Ge anodes, as mentioned in studies discussed previously [105-109]. For example, Cu-Ge core-shell NW arrays have been been used as high-rate capa-bility LIB anodes. The 3D nanostructure, synthesized by directly de-positing Ge on pre-synthesized Cu NW arrays via radio frequency sputtering, showed a high capacity of 1419 mAh g-1 after 40 cycles at a rate of 0.5C and 734 mAh g-1 after 80 cycles at an extremely high rate of 60C [107]. Aside from NWs, Ge-Cu NPs, composed of amor-phous Ge and orthorhombic Cu3Ge, fabricated via a single-step inert gas condensation method were found to have good performance during cycling, with reversible capacities of 843, 773, 680, and 516 mAh g-1 at 0.2, 0.5, 1, and 2C, respectively[109]. In addition, copper meta-germanate (CuGeO3) nanorods that were synthesized using a low tem-perature hydrothermal process, with diameters in the range of 40∼70 nmand lengths from 250 to 350 nm, demonstrated a first charge ca-pacity of 924 mAh g-1 and 690 mAh g-1 after 50 cycles [108]. Further-more, anodes based on the combination of high capacity elements, in-active elements, and carbon were reported, such as the case of Ge-Cu3Ge-C composite prepared using pyrolysis and a high energy mechanical milling process and single crystalline CuGe3ONW-RGO composite via a one-pot hydrothermal synthesis [75,105]. To date, various inactive and active matrix buffers have been pro-posed, including those Cu and S as discussed earlier, and also Sn, Ti, Zn, Fe, Co Ba, Sr, and Ba, in the hope that they could enhance the reversibility of Ge-based LIB anodes [110-119]. Germanium sulfides, for example, both GeS2 and GeS4 and in NP and glass-like forms, were investigated as LIB anodes [112,106,117]. Similar to the combination of Si and Ge, Sn-Ge alloys, in the form of nanocrystals and flake-like ribbons, have been reported as well and have shown attractive rate ca-pability and capacity retention for fast charge-discharge applications [118,119]. Another interesting class of materials are hybrid mi-cro-nanostructures, such as the reported free-standing Ge-Ti film nano-structures in membrane-like tubular form, which showed excellent re-versible capacity and cycling performance due to enhanced electronic transport and stabilized structure [115]. Metal germanate NWs (based on Ca, Sr, and Ba) synthesized using a low-cost, high yield hydro-thermal process have been put forward as well, showing superb lithium storage capabilities, cycling reversibilities, and rate capabilities [114].

6. Conclusions and Outlook

The prospects of high capacity anodes for next generation LIBs are definitely bright because of the existence of many materials of various compositions and morphologies that have been studied and proven to have excellent Li storage properties. For instance, Ge-based anodes of-fer a considerable increase in capacity relative to conventional graphite electrodes, better electron transport properties and faster Li insertion/ deinsertion kinetics than Si, and have favorable isotrophic expansion


공업화학, 제 25 권 제 1 호, 2014

during cycling. The main challenge, however, lies on how to solve the material and technical issues on the large volume changes that take place during lithiation-delithiation cycling, in addition to aggregation in the case of NPs and the stabilization of SEI. Most of the studies that have been reported so far dealt on the synthesis techniques of different Ge anodes and looked at the effect on the electrochemical properties of these materials in LIBs. A small minority, however, have focused into the lithiation mechanism and how the increased fundamental un-derstanding can translate into rationally designed LIB anode materials based on Ge. As seen in most papers reported in this review, ap-proaches that have been widely successful for Si anodes can be applied as well to Ge due to their fundamental similarities. We believe that more fundamental studies are needed, whether it may come from den-sity functional theory (DFT) modeling, molecular modeling, or more combinative techniques like mechanochemical modeling, are needed in order to better understand the lithiation-delithiation phenomena and its effect on the electrochemical performances of Ge anode materials. The advent also of advanced in situ techniques can complement the use of modeling methods in order to better elucidate the changes that occur in the anode at operationally relevant conditions. Another important is-sue to think about also is the cost associated with using Ge anodes and the scalability of most synthesis procedures proposed recently. Although there are many research challenges that remain, Ge anodes, particularly nanostructured Ge and 3D porous Ge, have the potential to be high en-ergy density anodes for the next generation LIB in the future.

Acknowledgments

This work was supported by the Core Technology Development Program for Next-generation Energy Storage of the Research Institute for Solar and Sustainable Energies (RISE), GIST. J. D. Ocon gratefully acknowledges the ERDT Faculty Development Program of the Univer-sity of the Philippines Diliman and the Department of Science and Technology (DOST).

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다음세대 리튬이온 배터리용 고에너지 밀도 게르마늄 음극 · 2014. 8. 13. · 다음세대 리튬이온 배터리용 고에너지 밀도 게르마늄 음극 3