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͑ a ͒ Voltage profiles from ballistically deposited germanium and ͑ b ͒ evaporated germanium cycled at a rate of C/4. The arrows indicate the charge step of the first cycle.
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Germanium nanocrystals (12 nm mean diam) and amorphous thin films (60-250 nm thick) were prepared as anodes for lithium secondary cells. Amorphous thin film electrodes prepared on planar nickel substrates showed stable capacities of 1700 mAh/g over 60 cycles. Germanium nanocrystals showed reversible gravimetric capacities of up to 1400 mAh/g with 6...
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... peak positions from elemental germanium as well as a number of Li-Ge phases 15,18-20 are shown in Fig. 3c. A plot of the voltage profiles from a 236 g electrode of ballistically deposited nanocrystalline germanium and a 42 g electrode of an evaporated germanium nanofilm are shown in Fig. 4. The most salient feature is the high reversible capacity of the nanostructured electrodes. The evaporated germanium nanofilm accommodates approximately 4.5 lithium atoms per germanium atom, which is slightly higher than the theoretical limit for crystalline Li 22 Ge 5 . The film thickness is estimated to be 60 nm based upon the electrode mass and bulk material density. Thicker films of up to 250 nm ͑ 180 g ͒ were cycled at slower rates with similar results. The ballistically deposited material hosts up to 3.8 lithium atoms per germanium atom. Graphs of the differential capacity, d ͉ x ͉ /dE, are displayed in Fig. 5a and b, where x is the lithium concentration in Li x Ge and E is the cell potential. The peaks are indicative of lithium insertion into equipotential sites. The dual peaks exhibited in these plots suggest that at least two new phases are formed during lithiation. Upon lithium insertion/extraction these new phases are formed at 180/380 mV and 360/500 mV. The disparity in potentials between lithium insertion and extraction is due to the over potential resulting from the constant current, or nonequilibrium state. The actual phase tran- sition energy can be approximated by averaging the charge and discharge values, giving ϳ 280 and ϳ 430 mV at room temperature. Plots of the cycle life for bulk crystalline germanium ͑ grain size р 38 m ͒ and the two types of nanostructured germanium electrodes are displayed in Fig. 6. The nanostructured electrodes were cycled at a rate of approximately C/4 ͑ i.e. , 375 mA/g ͒ between 0 and 1.5 V, whereas the bulk electrode was cycled at a much slower rate ͑ϳ C/ 30 ͒ to maximize the specific capacity. Despite the gentle cycling conditions, bulk germanium exhibited a poor cycle life, with nearly complete capacity loss by the seventh cycle. The evaporated germanium nanofilm exhibits a large first-cycle irreversible capacity with a steady specific capacity of 1.7 Ah/g and no detectable capacity fade over 60 cycles. A similar first-cycle capacity loss is observed with the ballistically deposited germanium. Although the initial stable capacity is similar to that of the evaporated nanofilm 1.4 Ah/g , the ballistically deposited electrode exhibits a constant capacity fade of approximately 0.01 Ah/g per cycle. The rate capabilities of the nanostructured electrodes were also investigated. A 250 nm evaporated germanium film ͑ 180 g ͒ was cycled at a constant discharge rate of 0.5 C and a variable charge rate from 0.5 C to 1000 C. At 0.5 C the cell was cycled between 0 and 1.5 V and the upper limit of the potential was increased by 50-100 mV on each subsequent cycle to account for the over potential associated with the increased cycling rates. A ballistically deposited nanocrystalline film ͑ 314 g ͒ was cycled under similar conditions. Figure 7 shows a plot of the normalized capacity ( Q / Q 0 ) at various discharge rates. The vertical error bars reflect the ...
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... with the data acquisition system and are visible only when the charging times are short. Remarkably, only a moderate capacity loss is observed between 1 and 1000 C. A test of the cycle life at high rates was also performed on the amorphous thin film electrode. In this experiment the 250 nm germanium film was cycled at a discharge rate of 0.5 C and a charge rate of 1000 C. The cell was cycled between 0 and 3.0 V. Figure 8 displays a plot of the charge and discharge capacities over 30 cycles. This figure clearly illustrates the overlap between the high-rate charge step and low-rate discharge step. This indicates that all of the lithium inserted over a 4 h lithiation period is removed in less than 4 s upon delithiation. Li-Ge phases .—According to the Li-Ge phase diagram, a number of phase transitions are expected to occur during lithiation at room temperature. 15-20 Phase transitions are easily identified by plateaus in the voltage profile ͑ Fig. 4 ͒ . The Gibbs phase rule prohibits any variation in the chemical potential ͑ or cell voltage ͒ at fixed temperature when two simultaneous phases are present. In a two-phase re- gion, the potential is constant during changes in lithium concentration as one phase grows at the expense of the other. The voltage profiles of Fig. 4a and b exhibit a reasonably smooth slope on the charge and discharge cycles for both of the nanostructured electrodes. However, even subtle variations in the slope may be indicative of the formation of a new phase. The differential capacity plots of Fig. 5a and b were prepared to accentuate changes in the slope of the potential curves. Peaks in the differential capacity indicate re- gions of the potential where lithium ions are entering nearly equipotential sites. The presence of multiple peaks suggests that a number of different Li-Ge phases are formed during electrochemical lithiation in both the evaporated and ballistically-deposited electrodes. This is in contrast to silicon, which forms amorphous phases upon lithiation at room temperature. 21 The peaks in the Li-Ge diffraction patterns of Fig. 3a and b correspond to a number of crystallographic phases, suggesting that the material is heterogeneous in the lithiated state. The most obvious phases present are LiGe, 15 Li 7 Ge 2 , 18 and Li 15 Ge 4 . 19 Other phases that may be present in small quantities are Li 11 Ge 6 , 16 Li 9 Ge 4 , 17 and Li 22 Ge 5 . 20 In addition, the broad underlying peaks located at 2.2 and 4.5 Å suggest that there may be an amorphous phase present in the lithiated material. However, unlike the silicon system, germanium does not appear to become amorphous during electrochemical lithiation at room temperature. In fact, electrochemical lithiation appears to enhance the crystallinity of the material, which may be attributed to the rapid rate kinetics of lithium in germanium. Surface-electrode interphase .—The large irreversible capacity observed on the initial cycle is likely attributed to the formation of a surface-electrolyte interphase ͑ SEI ͒ . A reaction of lithium with the electrolyte accompanies the initial lithiation of the germanium electrode, forming a passivation layer. This reaction is beneficial when using a carbon electrode because it prevents solvent decomposition and cointercalation during lithiation. Although the lithium alloy electrodes are typically not affected by solvent cointercalation, the presence of a passivation layer may prevent spontaneous solvent decomposition. Despite the first cycle capacity loss, the growth of the SEI layer does not appear to be detrimental to the specific capacity or cycle life. The first cycle capacity loss is ϳ 70% of what was observed in nanostructured Li-Si materials. 11 The lower first- cycle irreversible capacity may be attributed to the lack of a native oxide on germanium, which might otherwise contribute to the SEI if it were reduced by lithium. Cycle life .—The high specific capacities of the nanostructured germanium electrodes were found to be stable for over 50 cycles ͑ Fig. 6 ͒ . Although the high capacities are expected from the high solubility of lithium in germanium, the complete lithiation of germanium has never been observed at room temperature on these time scales. Similarly, the slow kinetics of lithium in germanium are expected to create large stresses within the material, causing the decrepitation of the host. This is clearly observed in the attenuated cycle life of the bulk germanium electrode ͑ Fig. 6 ͒ . Remarkably, there is no capacity loss observed in the amorphous nanofilm over 62 cycles. Similarly, the nanocrystalline electrode exhibits only a slow loss of capacity over 50 cycles. These results indicate that the active germanium particles do not decrepitate significantly during electrochemical cycling. The constant capacity loss observed in the nanocrystalline system is attributed to the spallation of particles off the surface of the current collector resulting from changes in the sample volume by up to 230% during cycling. 22 The stability of the amorphous nanofilm during cycling is surprising. Although this electrode is thin, suggesting rapid lithium transport perpendicular to the film, the electrode is attached to a rigid substrate. The large volume expansion that occurs during lithiation is expected to create large strain gradients as the lithium front propagates in and out of the film. Such strains are more than sufficient to debond the film from the substrate. However, complete decohesion of the film does not occur during cycling; the film remains electrically intact. Despite the electrical continuity, it is ...
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Silicon is of great interest for use as the anode material in lithium-ion batteries due to its high capacity. However, certain properties of silicon, such as a large volume expansion during the lithiation process and the low diffusion rate of lithium in silicon, result in fast capacity degradation in limited charge/discharge cycles, especially at h...
Citations
... 15 Although Ge comes at a high cost, it is an abundant element, and its high price is merely a consequence of the current low demand. 16 The drawback of Ge as anode (like for P, Si, etc.) is a high volume expansion during cycling (greater than 200%); however, this problem might be overcome by microstructural modifications, such as coating with amorphous black carbon, using porous structures, or composite materials. [17][18][19] Thermodynamic stability and average voltages of intermediates during the charge-discharge process are important characteristics of anode materials. ...
Lithium–germanium binary compounds are promising anode materials for secondary lithium-ion batteries due to their high capacity, low operating voltage, and high electronic conductivity of lithiated Ge. For their successful application in batteries, it is essential to know the temperature stability of different Li–Ge phases and the variation of their ionic conductivity depending on the operating temperatures of the batteries. This work aims to comprehensively study the thermodynamic stability and ionic conductivity in Li–Ge binary compounds using a combination of first-principle computations and machine-learning interatomic potentials. We calculated convex hulls of the Li–Ge system at various temperatures and a temperature–composition phase diagram was obtained, delineating stability fields of each phase. Our calculations show that at temperatures higher than 590 K, LiGe undergoes a I41/a–P4/mmm transition, which leads to a change in the ionic conductivity. We show that all stable and metastable Li–Ge compounds have high ionic conductivity, but LiGe and Li7Ge12 have the lowest lithium diffusion. Trajectories of diffusion and Ge arrangements depend on lithium concentration. Based on advanced theoretical approaches, this study provides insights for the development of Li–Ge materials in lithium-ion and lithium-metal battery applications.
... The theoretical Li-storage capacity of a graphite anode, however, is limited to 372 mAh g −1 , and the development of highercapacity anode materials to enable further downsizing and higher energy efficiency of LIBs is an active research area. 1,2) Si [3][4][5][6] and Ge [7][8][9][10] are promising candidates for next-generation anode materials because they have theoretical Li-storage capacities of 4200 and 1624 mAh g −1 , respectively. 11,12) However, Si and Ge form alloys with Li during the charging process, resulting in a ∼400% volume expansion. ...
... We have focused on Ge, which has advantages over Si for use in LIBs, including 400 times higher Li + -ion diffusivity 9) and 10 4 times higher electrical conductivity. 10,14) A promising approach for realizing high-capacity Ge anodes is fabricating a Ge nanostructure on a Cu current collector using nanoparticles, 7,15) where the nanomaterial can uniformly expand during Li alloying because of the large surface-to-volume ratio, reducing physical stress and suppressing material pulverization. ...
To realize high-capacity Ge anodes for next-generation Li⁺-ion batteries, a multilayer anode with a C(top)/Ge(middle)/C(bottom) structure was developed, where nanostructured amorphous Ge (a-Ge) and amorphous-like carbon films with a grain size of 10–20 nm were deposited sequentially by high-pressure Ar sputtering at 500 mTorr. Compared with the a-Ge anode, the C(top)/a-Ge(middle)/C(bottom) multistacking layer anode showed improved capacity degradation for repeated lithiation/delithiation reactions and achieved a high capacity of 910 mAh g⁻¹ with no capacity fading after 90 cycles at a C-rate of 0.1.
... At the same time, Ge has higher electronic conductivity than Si. Furthermore, the diffusion coefficient of Li + ions in Ge is 400 times higher than in Si [8,9]. However, as with other high-capacity materials, the main disadvantage of germanium is its huge (over 350%) volumetric expansion during lithium insertion/extraction. ...
This work demonstrates the possibility of electrochemical formation of Ge-Sn-O nanostructures from aqueous solutions containing germanium dioxide and tin (II) chloride at room temperature without prior deposition of fusible metal particles. This method does not require complex technological equipment, expensive and toxic germanium precursors, or binding additives. These advantages will make it possible to obtain such structures on an industrial scale (e.g., using roll-to-roll technology). The structural properties and composition of Ge-Sn-O nanostructures were studied by means of scanning electron microscopy and X-ray photoelectron spectroscopy. The samples obtained represent a filamentary structure with a diameter of about 10 nm. Electrochemical studies of Ge-Sn-O nanostructures were studied by cyclic voltammetry and galvanostatic cycling. Studies of the processes of lithium-ion insertion/extraction showed that the obtained structures have a practical discharge capacity at the first cycle ~625 mAh/g (specific capacity ca. 625 mAh/g). However, the discharge capacity by cycle 30 was no more than 40% of the initial capacity. The obtained results would benefit the further design of Ge-Sn-O nanostructures formed by simple electrochemical deposition.
... Consequently, great efforts have been made in designing Ge-based nanomaterials that can buffer volume changes during cycling. These include nanocrystalline Ge thin film [3], Ge nanowires [4][5][6][7], nanotubes [8,9], mesoporous Ge [10], and Ge-based composites [11,12]. The main drawbacks of these Ge-based nanomaterials are their agglomeration, resulting in an uneven formation of solid electrolyte interphase (SEI) during cycling. ...
... 41 These techniques are used alone or in combination with one another and can be used to fabricate individual electrodes, electrolytes, current collectors, and packaging. Even though the vapor-phase deposition methods (thermal evaporation, 42 radio-frequency magnetron sputtering, 43 chemical vapor deposition (CVD), 44 pulse laser deposition (PLD), 45 and atomic layer deposition (ALD) 46 ) can provide conformal coatings on 3D architectures, most of them require complex and costly instrumentation and facilities. ...
Li-ion microbatteries are the frontline candidates to fulfill the requirements of powering miniature autonomous devices. However, it still remains challenging to attain the required energy densities of > 0.3mWh/cm-2µm-1 in a planar configuration. To overcome this limitation, 3D architectures of LIMBs have been proposed. However, most deposition techniques are poorly compatible with 3D architectures because they limit the choice of current collectors and selective deposition of the active materials. Electrodeposition was suggested as an alternative for rapidly and reproducibly depositing active materials under mild conditions, and with controlled properties. However, despite the huge potential, electrodeposition remains underexplored for LIMB cathode materials, partly due to challenges associated with the electrodeposition of Li-ion phases. Herein, we review advances in the electrodeposition of Li-ion cathode materials with the main focus set on the direct, one-step deposition of electrochemically active phases. We highlight the merits of electrodeposition over other methods and discuss the various classes of reported materials, including layered transition metal oxides, vanadates, spinel, and olivines. We offer a perspective on the future advances for the adoption of electrodeposition processes for the fabrication of microbatteries to pave the way for future research on the electrodeposition of cathode materials.
... While Si anodes have a considerably high theoretical capacity (e.g., 3580 mAh g -1 for Li 15 Si 4 ), the high-rate capacity and cycle characteristics are limited by their low electrical and ionic conductivities [12][13][14][15] . Ge anodes, although inferior to Si in theoretical capacity (e.g., 1625 mAh g -1 for Li 4.4 Ge), exhibit good rate performance and cycle characteristics owing to their 100 times higher electrical conductivity (σ), 400 times higher Li + diffusivity than Si, and lower volume expansion than Si [16][17][18][19] . www.nature.com/scientificreports/ ...
SiGe is a promising anode material for replacing graphite in next generation thin-film batteries owing to its high theoretical charge/discharge capacity. Metal-induced layer exchange (LE) is a unique technique used for the low-temperature synthesis of SiGe layers on arbitrary substrates. Here, we demonstrate the synthesis of Si 1− x Ge x ( x = 0–1) layers on plastic films using Al-induced LE. The resulting SiGe layers exhibited high electrical conductivity (up to 1200 S cm ⁻¹ ), reflecting the self-organized doping effect of LE. Moreover, the Si 1− x Ge x layer synthesized by the same process was adopted as the anode for the lithium-ion battery. All Si 1− x Ge x anodes showed clear charge/discharge operation and high coulombic efficiency (≥ 97%) after 100 cycles. While the discharge capacities almost reflected the theoretical values at each x at 0.1 C, the capacity degradation with increasing current rate strongly depended on x . Si-rich samples exhibited high initial capacity and low capacity retention, while Ge-rich samples showed contrasting characteristics. In particular, the Si 1− x Ge x layers with x ≥ 0.8 showed excellent current rate performance owing to their high electrical conductivity and low volume expansion, maintaining a high capacity (> 500 mAh g –1 ) even at a high current rate (10 C). Thus, we revealed the relationship between SiGe composition and anode characteristics for the SiGe layers formed by LE at low temperatures. These results will pave the way for the next generation of flexible batteries based on SiGe anodes.
... Still, the need for increased power and energy densities and longer cycle life is unwavering. With the lattice structure and redox kinetics limiting the maximum achievable capacity of intercalation-based active materials, alloying [1][2][3] and conversion [4] type materials have emerged as alternative high-capacity materials. While conversion electrodes often require a large overpotential to drive the Li reaction, alloying-type electrodes such as silicon, germanium, and tin become the choice for the near-term viable technology due to their high theoretical specific capacity (∼990 − 4000 mAh g −1 ), favorable voltage window (∼0.3 − 0.6 V versus Li/Li + ), abundant raw material availability, relatively low cost, and pre-existing material processing techniques [2,[4][5][6][7]. ...
... With the lattice structure and redox kinetics limiting the maximum achievable capacity of intercalation-based active materials, alloying [1][2][3] and conversion [4] type materials have emerged as alternative high-capacity materials. While conversion electrodes often require a large overpotential to drive the Li reaction, alloying-type electrodes such as silicon, germanium, and tin become the choice for the near-term viable technology due to their high theoretical specific capacity (∼990 − 4000 mAh g −1 ), favorable voltage window (∼0.3 − 0.6 V versus Li/Li + ), abundant raw material availability, relatively low cost, and pre-existing material processing techniques [2,[4][5][6][7]. However, due to the nature of alloying reactions, the electrodes inherently undergo significant volume changes (up to ∼300%) upon (de)lithiation [8]. ...
Mechanical failure and its interference with electrochemistry are a roadblock in deploying high-capacity electrodes for Li-ion batteries. Computational prediction of the electro-chemo-mechanical behavior of high-capacity composite electrodes is a significant challenge because of (i) complex interplay between mechanics and electrochemistry in the form of stress-regulated Li transport and interfacial charge transfer, (ii) thermodynamic solution non-ideality, (iii) non-linear deformation kinematics and material inelasticity, and (iv) evolving material properties over the state of charge. We develop a computational framework that integrates the electrochemical response of batteries modulated by large deformation, mechanical stresses, and dynamic material properties. We use silicon as a model system and construct a microstructurally resolved porous composite electrode model. The model concerns the effect of large deformation of silicon on charge conduction and electrochemical response of the composite electrode, impact of mechanical stress on Li transport and interfacial charge transfer, and asymmetric charging/discharging kinetics. The study captures the rate-dependent, coupled electro-chemo-mechanical behavior of high-capacity composite electrodes that agrees well with experimental results.
... Germanium (Ge) is one of the promising anode materials for high-capacity lithium-ion batteries, due to its excellent volumetric capacity, high electrical conductivity (two orders of magnitude higher than Si), and fast lithium-ion diffusion rate (four orders of magnitude higher than Si) [1][2][3][4]. As a group-IV element, Ge also has high electron and hole mobilities (about two and four times higher than silicon), which make it an ideal candidate as a channel material for the next-generation field-effect transistor (FET) with fast switching speed. ...
One-dimensional germanium (Ge)-related nanostructures including core–shell nanowires and nanotubes with high specific surface area show enhanced performance in energy storage and electronic devices, and their structural control is important for further improving their performance and stability. In this work, we fabricated vertically formed ZnO/Ge core–shell nanowires with different shell thicknesses. The dependence of morphology, crystallinity, and internal stress of the nanowires on the shell growth time and temperature was investigated. By applying the wet-etching method to the ZnO/Ge core–shell heterojunction nanowires, we demonstrated the Ge nanotube fabrication and stress relaxation in Ge after ZnO core removal.
... The sputtering was performed at a power of 100 W under 1.33 Pa pressure of Ar. Earlier work has shown that Ge sputtered under such condition is amorphous [51]. The Ti and Ge film thicknesses after sputtering were characterized using a Tencor P15 profilometer (KLA Corp., Milpitas, CA, USA). ...
Deformation and stress in battery electrode materials are strongly coupled with diffusion processes, and this coupling plays a crucial role in the chemical and structural stability of these materials. In this work, we performed a comparative study of the mechanical characteristics of two model materials (lithiated and sodiated germanium (Ge)) by nanoindentation. A particular focus of the study was on indentation size effects and harnessing them to understand the chemo-mechanical interplay in these materials. While the quasi-static measurement results showed no significant size dependence, size effects inherent in the nanoindentation creep response were observed and utilized to investigate the deformation mechanism of each material. Supplemented by computational chemo-mechanical modeling, we found that lithiated Ge creeps through a stress-gradient-induced diffusion (SGID) mechanism but a model combining the SGID and conventional shear transformation deformation (STD) mechanisms was needed to capture the creep behavior of sodiated Ge. Broadly, this work reveals the importance of stress-diffusion coupling in governing the deformation of active electrode materials and provides a quantitative framework for characterizing and understanding such coupling.
... Since germanium-based anodes show excellent rate performances with good cycling stability due to their large Li + ion diffusivity as well as high electronic conductivity, it is believed that they could meet the ever-growing demands of high energy and power density energy storage devices [52]. Thus, in the extension of MPs research, germanium phosphides were also investigated for battery applications because of their high theoretical capacity (1,033 mAh g −1 ) and relatively large initial coulombic efficiency. ...
