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Functional Nanostructured Materials (including low-D carbon)

A Tuneable Core Shell Nanowire Active Material for High Capacity Li-Ion Battery Anodes Comprising of PECVD Deposited aSi on Directly Grown Ge Nanowires Killian Stokes, Wil Boonen, Hugh Geaney, Tadhg Kennedy, Dana Borsa, and Kevin M. Ryan ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.9b03931 • Publication Date (Web): 06 May 2019 Downloaded from http://pubs.acs.org on May 6, 2019

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ACS Applied Materials & Interfaces

A Tuneable Core Shell Nanowire Active Material for High Capacity Li-Ion Battery Anodes Comprising of PECVD Deposited aSi on Directly Grown Ge Nanowires Killian Stokes,a Wil Boonen,b Hugh Geaney,a Tadhg Kennedy,a Dana Borsa,b and Kevin M. Ryan a,* a.

Bernal Institute and Department of Chemical Sciences, University of Limerick, Ireland

b.

Meyer Burger B.V., Luchthavenweg 10, NL 5657, Eindhoven, Netherlands

KEYWORDS: coaxial, half-cell, full-cell, prelithiation, preconditioning, ex-situ

ABSTRACT Herein, we report the formation of core@shell nanowires (NWs) comprised of crystalline germanium NW cores with amorphous silicon shells (Ge@aSi) and their performance as a high capacity Li-ion battery anode material. The Ge NWs were synthesized directly from the current collector in a solvent vapor growth (SVG) system and used as hosts for the deposition of the Si shells via a plasma enhanced chemical vapour deposition (PECVD) process utilizing an expanding thermal plasma (ETP) source. The secondary deposition allows for the preparation of Ge@aSi core@shell structures with tuneable Ge:Si ratios (2:1 and 1:1) and superior gravimetric and areal capacities, relative to pure Ge. The binder-free anodes exhibited discharge capacities of up to 2066 mAh/g and retained capacities of 1455 mAh/g after 150 cycles (for the 1:1 ratio). The 2:1 ratio showed a minimal ~5 % fade in capacity between the 20th and 150th cycles. Ex-situ microscopy revealed a complete restructuring of the active material to an interconnected Si1-xGex morphology due to repeated lithiation and delithiation. In full-cell testing, a prelithiation step counteracted first cycle Li consumption, and resulted in a two-fold improvement to the capacity of the prelithiated cell versus the unconditioned full-

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cells. Remarkable rate capability was also delivered where capacities of 750 mAh/g were observed at a rate of 10 C.

INTRODUCTION Si and Ge are among the most promising materials to replace the commercially used graphitic anodes within Li-ion batteries. Upon charging, these materials form Li rich alloys (Li3.75M, where M is Si or Ge), displaying gravimetric capacities that are several multiples (3579 mAh/g for Si and 1384 mAh/g for Ge) of those observed for carbonaceous anodes, that intercalate with Li for a maximum theoretical capacity of 372 mAh/g.1-3 Nanostructured arrangements of these materials have been shown to mitigate the stresses related to volume changes caused by lithiation and delithiation, which has rendered them unusable in bulk form, and has allowed for long-term cycling at high capacities.4-9 In particular, binder-free nanowire (NW) morphologies offer additional benefits of good electrical contact to the current collector, short Li diffusion distances, and allow for the elimination of non-electrochemically active weight from the battery.10-11 Due to possessing the highest theoretical capacities (for Li-alloying materials) and low relative material costs (versus Ge), a pure Si active material anode is desirable. However, appreciable capacity fade over long-term cycling and poor rate performance is still a limitation for Si anodes.12-15 Methods such as coating Si NWs with carbon, Cu, TiO2, or Al2O3, depositing a layer of graphene or TiN on the current collect prior the NW growth and AAO-templated growth16-24 have been somewhat successful in improving the capacity retention over extended cycling, however, these approaches require the addition of electrochemically inactive materials in multi-step processes, resulting in electrodes with reduced capacities. In comparison to Si, Ge anodes possess a higher rate of Li diffusivity at room temperature (400×) and greater


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electrical conductivity (10,000×).25-27 Binder-free Ge NWs have been shown to maintain capacities of 900 mAh/g over 1000 cycles, where the stability was attributed to the transformation of the NW active material to a continuous network of Ge ligaments which remained well connected to the current collector.28-29 Recently, anodes where the active material consisted of Si NWs branches grow from Ge NW stems exhibited high capacities (greater than the theoretical figure for Ge) with improved rate capability performance observed.30 Ex-situ microscopy analysis revealed the incorporation of the Si into the porous network which facilitated boosted battery performance. However, the addition of the Si branches involved a two-stage seed mediated growth process, where a one-step Si deposition process is more desirable. An anode capable of harnessing the respective benefits of both Si (high capacities, low costs) and Ge (high conductivity and lithium diffusivity) would make for a highly attractive battery material. Recently, several approaches combining Si and Ge in the same structure have been reported. These investigations have included the use of alloyed NWs,31-32 core-shell nanostructures,33-35 axial and branched heterostructures,30,

36

crystalline nanotubes,37-38 thin

films,39 and co-sputtered architectures.40-42 For previously reported coaxial architectures an arrangement consisting of a Si core with a Ge shell has been shown to compress material expansion and minimise fracturing due to the mismatched lithiation voltages for Si (0.22 V) and Ge (0.45 V), respectively.35 However, the electrochemical testing of these CVD prepared NWs in this study was limited to 50 cycles and the cycling related composition and morphology changes were not characterized. Here, we report the synthesis of core@shell Ge@aSi NWs, directly from stainless steel current collectors and their electrochemical evaluation as Li-ion battery anodes. The radial coating of as-grown crystalline Ge NWs with Si, enabled the preparation of electrodes with increased gravimetric and areal capacities due to the higher theoretical capacity of Si and the



concomitant densification of the active material. In this study, different Ge:Si mass ratios of 2:1 and 1:1 were investigated and anodes with active mass loadings of 0.296 and 0.412 mg/cm2 were prepared, representing an enhancement of double and quadruple of the areal capacity compared to pure Ge, respectively. This active material arrangement, that combines the capacity boosting effects of a Si shell and the stable capacity retention afforded by the Ge core yielded reversible capacities up to 2066 mAh/g, with 1455 mAh/g retained after 150 cycles (for the 1:1 ratio). Minimal capacity fade was observed for the 2:1 composition, where a ~0.04 % loss in capacity per cycle was observed between the 20th and 150th cycles. The coaxial architectures were also investigated as full-cell electrodes where an electrochemical prelithiation was adopted to provide the cells with a supplementary supply of Li to mitigate irreversible losses that occur during early cycles and resulted in a two-fold improvement to the capacity of the cells versus the unconditioned full-cells tested.

EXPERIMENTAL Chemicals Used All chemicals were used as received. Diphenylgermane (DPG, 97 %) was supplied by Fluorochem and was stored and dispensed from an Ar-filled glovebox. Substrate Preparation Stainless steel (SS, 316) foil was purchased from Pi-Kem Ltd. with a thickness of 0.1 mm. The foil was roughened using P600 grit sandpaper to increase the surface area and improve the contact between the current collector and the active material. From this, substrates of approximately 10 mm x 80 mm were cut and rinsed with acetone. A 1 nm layer of Cu (99.999 %, Kurt J. Lesker) was then thermally evaporated onto these pieces in a glovebox-based evaporation unit. The substrates were stored in the Ar filled glovebox prior to reactions to minimize oxidation. Reaction Setup Ge NW Synthesis. Reactions were carried out in a custom-made Pyrex round bottomed flask containing 7 ml of squalane (Sigma Aldrich, 99%). The growth substrates were placed vertically in the flask, which was attached to a Schlenk line setup via a water condenser. This was then ramped to a temperature of 125 °C using a three-zone furnace. A vacuum of at least 100 mTorr was applied for 1 h to remove moisture from the system. Following this, the system was purged with Ar. The flask was then ramped to the reaction temperature under a constant Ar flow. A water condenser was used to control the HBS reflux and ensure that the reaction was kept under control. At the correct reaction temperature for the growth of the Ge NWs (430 °C) 0.25 mL of DPG precursor was injected, through a septum cap, into the system and the reaction was allowed to proceed for 10 min growth time. When


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terminating the reaction, the furnace was opened and the setup was allowed to cool to room temperature before removing the NW coated substrates. Amorphous Si Coating. aSi coatings were deposited on the (10 mm x 80 mm) Ge NW covered substrates by plasma-enhanced chemical vapour deposition (PECVD) using an expanding thermal plasma (ETP) source on the FLEx XL platform. The ETP deposition exceeds the speed of conventional plasma systems (10×) making it an attractive system for Si deposition. The setup consisted of a cascaded arc plasma source and a low-pressure deposition chamber. The FLEx XL platform, equipped with multiple ETP sources to ensure uniform coatings. By varying process parameters such as precursor flows, source current, process pressure, the target thickness of the amorphous Si layer could be tuned. The plasma source was used to create reactive species for the dissociation of SiH4 and the system was operated on high flows of Ar-H2 mixture leading to pressures of 0.2–0.6 bar when the plasma is ignited. The plasma emanates from the cascaded arc source through a nozzle and expands supersonically into the deposition chamber which is typically at a pressure of 0.1–0.3 mbar. The substrate temperature is accurately controlled in the range of 200–500 ºC.

Electrochemical Measurements Mass loadings were determined by the careful weighing of the substrates post Cu evaporation, after Ge NW growth and after aSi deposition using a Sartorius Ultra-Microbalance SE2 (repeatability ± 0.25 μg). From the 10 mm x 80 mm substrate, 1 cm2 anodes were prepared and used for electrochemical characterization. The current applied (1 C) was evaluated using the entire Li+ active mass of the anode which was then used in the calculation of the gravimetric (mAh/g) and areal capacities of the anodes (mAh/cm2). The electrochemical performance was evaluated by assembling two electrode Swagelok type cells in an Ar filled glovebox. Half Cells. The cells consisted of binder-free Ge@aSi core@shell NWs on a SS current collector (6 mm x 6 mm pieces were cut out) as the working electrode, Li foil as the counter and reference electrode, a Celgard separator and an electrolyte solution of 1 M LiPF6 in ethylene carbonate/diethyl carbonate (1:1 v/v) + 3 % vinylene carbonate (Sigma Aldrich, 99 %). Galvanostatic measurements were carried out using a Biologic MPG-2 in a potential range of 0.01 – 1.0 V versus Li/Li+. Full Cells. All specific capacities are given in terms of the anode mass for the various tests. Set up is the same as in half cells except that the Li foil is replaced with LCO cathodes (NEI Corporation, mass loading = 6.2 mg/cm2). Galvanostatic measurements were carried out in a potential range of 2.8 – 3.9 V. Prelithiated Full Cells. Ge@aSi NW electrodes were initially assembled in a half-cell configuration where they were lithiated (at a C/10 rate) to 0.011 V. A positive current was applied and the anode was allowed to partially delithiated until the voltage reached the range of 0.3 to 0.4 V before being disassembled and reassembled in a full-cell against an LCO cathode. The OCV was typically between 2.4 and 2.6 V and the galvanostatic measurements were carried out between 2.8 and 3.9 V. Electrochemical Impedance Spectroscopy. EIS was performed using an Autolab PGSTAT100 potentiostat, applying an AC voltage with an amplitude of 10 mV over the frequency range: 1 MHz to 0.01 Hz. Material Characterization Scanning electron microscopy (SEM) analysis was performed on a Hitachi SU-70 system operating between 5 and 20 kV. The substrates required no prior treatment before SEM analysis. For transmission electron microscopy (TEM) analysis the NWs were removed from the growth substrate through sonication before being drop cast onto a lacy carbon TEM grid. TEM analysis was conducted at 200 kV on a JEOL JEM-2100F field emission microscope equipped with a Gatan Ultrascan CCD camera and EDAX Genesis EDS detector. The respective mass of Ge and Si was determined through measurement using a Sartorius Ultra-Microbalance SE2 (repeatability ± 0.25 μg), in conjunction with



EDX analysis. Raman spectroscopy was carried out using a Horiba Labram 300 spectrometer system equipped with a 532 nm laser.

RESULTS AND DISCUSSION

Figure 1. Schematic summarising the two-step process for preparing Ge@aSi core@shell NWs where SVG grown Ge NWs are subsequently coated with an amorphous Si layer by PECVD, utilizing an ETP source. A schematic detailing the protocol used to prepare the coaxial Ge@aSi NWs is outlined in Figure 1. The Ge NWs were synthesized through a previously reported solvent vapour growth (SVG) method where a substrate consisting of 1 nm of thermally evaporated Cu evaporated onto stainless steel was placed in the vapour phase of a high boiling point solvent.4344

At temperatures above 430 ºC, the diphenylgermane (DPG) precursor was injected, with NW

growth occurring via the vapor-solid-solid (VSS) method, facilitated by in-situ formation of


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discrete Cu3Ge seeds. A SEM image of the as-grown Ge NWs is shown in Figure 2a with a higher resolution image seen in Figure 2b. The Ge NW diameters varied between 40 and 80 nm where typical lengths were > 5 µm with NWs as long as 10 µm observed. X-ray diffraction (XRD) analysis (supporting information, Figure S1) gave reflections consistent with cubic Ge with remaining peaks corresponding to the stainless steel current collector. No peaks were identified for the Cu3Ge seeds due to the low masses present in the array. The amorphous Si shell was deposited on the surface of the Ge NWs via a plasma enhanced chemical vapor deposition (PECVD) process, that utilized an expanding thermal plasma (ETP) process, using silane gas (SiH4). Varying the process parameters allowed for control over the Ge to Si mass ratio where quicker deposition rates resulted in higher Si content electrodes. The ratios investigated in this study were (Ge:Si) 2:1 and 1:1 with typical mass loadings of 0.296 and 0.412 mg/cm2, respectively, and SEM images of Ge@aSi NWs (Ge:Si 2:1 wt. %) shown in Figure 2c and 2d. Additionally, SEM images of the higher mass loading 1:1 ratio are presented in supporting information as Figure S2.The Si deposition process utilizes a remote plasma system (the plasma production, plasma transport, and deposition are geometrically separate) which enables independent variation of plasma parameters allowing for higher deposition rates to be easily accessible.45 Cross-sectional SEM images showing the Ge@aSi NWs on the underlying stainless steel current collector can be seen in supporting information Figure S3. Figure 2e shows DFSTEM image of an uncycled Ge@aSi NW with related elemental maps for Ge and Si presented as (i) and (ii) respectively. In the DFSTEM image, both the Ge core and Si shell are well defined with a clear interface between them. The associated EDX elemental maps confirm this, showing no obvious sign of mixing. Raman spectroscopy conducted on the Ge@aSi NWs gave signals that corresponded to Ge – Ge, Si – Si and Ge – Si bonding,46-47 suggesting the amorphous Si deposition process stimulated bonding at the interfaces between



the Ge and Si layers (supporting information, Figure S4). This ensures good electrical contact between the Ge and Si, which is important for dual cycling of both the core and the shell.48

Figure 2. (a,b) Low and high magnification SEM images of Cu3Ge seeded Ge NWs. (c,d) Low and high magnification SEM images NWs after the layer of amorphous Si has been deposited (e) DFSTEM image of a Ge@aSi NW with associated EDX elemental maps corresponding to (i) Ge and (ii) Si. Ge:Si ratio = 2:1.


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Figure 3. (a) Charge and discharge capacities and coulombic efficiencies for the two different compositions of Ge@aSi NWs. (b) Rate capability testing for each composition at C/10, C/5, C/2, C, 2C, 5C, 10C and C/10. (c) Voltage profiles corresponding to the fist charge and discharge for the compositions. (d) Relative capacity retention for the anodes over the different rates. (e) Differential capacity plots related to the initial charge and discharge for each of the composition.

The electrochemical properties of the Ge@aSi NWs were examined through galvanostatic cycling in a two electrode Swagelok cell cycled in the voltage range of 0.01 – 1 V versus Li/Li+ in a 1 M LiPF6 in ethylene carbonate/diethyl carbonate (1:1 v/v) + 3 wt. % vinylene carbonate (VC) electrolyte and Li metal foil was used as the counter/reference electrode. Electrodes of two different Ge to Si mass ratios were evaluated: 2:1 and 1:1, which were determined through EDX and weighing (supporting information, Figures S5 and S6). The entire Li active mass was used in calculating the currents applied. The maximum theoretical capacities for the 2:1 and 1:1 ratios were determined to be: 2149 mAh/g and 2580 mAh/g (gravimetric) and 0.64 mAh/cm2 and 1.02 mAh/cm2 (areal), respectively which compared



favourably with previously published studies involving Si/Ge composites as Li-ion battery anode active materials (supporting information, Table 1). The Ge@aSi NW covered substrates were used directly as the working electrodes and cycled at a rate of C/5 for 150 cycles, with the charge and discharge capacities presented in Figure 3a. The initial discharge capacities for the 2:1 and 1:1 (Ge:Si) ratios were 1478 and 1996 mAh/g respectively, with the related voltage profiles shown in Figure 3c. The discharge capacity for the 1:1 anode increased to 2066 mAh/g by the 10th cycle. Additional voltage profiles of the 10th, 50th, 100th and 150th cycles in supporting information, S7. These Li extraction capacities were below the maximum theoretical capacities calculated for each ratio, with this effect likely due to a native oxide layer on the material which formed due to exposure to air during processing steps. The oxides of Ge and Si irreversibly cycles with Li and results in reduced capacities.39 All initial discharge capacities are above the maximum theoretical capacity for Ge (1384 mAh/g) showing the capacity boosting effects of the Si shell. After 150 cycles, the anodes exhibited capacities of 1357 (2:1) and 1455 mAh/g (1:1) which corresponded to a 90 % and 73 % retention of their initial capacities respectively. Furthermore, in the case of the 2:1 ratio, a ~5 % fade in capacity was observed between 20th and 150th cycle, showing exceptional stability for the material after initial cycles. The minimal capacity fade (0.04 % loss in capacity per cycle) from the 20th cycle onwards observed for the 2:1 composition suggests it is the optimal composition to utilize the boosted capacities afforded by the amorphous Si shell, while also retaining the noteworthy stability associated with the Ge NW core. The rate capability properties of the Ge@aSi NWs was investigated by charging and discharging the material for 5 cycles at rates of C/10, C/5, C/2, C, 2C, 5C, 10C and back to C/10 (Figure 3b). The more Si rich electrode exhibited higher capacities at the slower rates (C/10 to C). At higher currents, 2:1 Ge;Si electrode (with higher Ge ratios) began


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outperforming the heavier samples. This finding agrees well with our previous study on Si1xGex

alloy NWs where a rate dependent tipping point existed for gravimetric capacity, with Ge

rich anodes outperforming Si rich anodes at higher rates.30, 39 After cycling at 10 C, the rate was returned to C/10 where both anodes returned to approximately 96 % of their initial capacities Figure 3d). Electrical impedance spectroscopy (EIS) was conducted on both the 2:1 and 1:1 (Ge:Si) ratios. The impedance for the 1:1 (Ge:Si) material was observed to be greater than that for the 2:1 ratio and this higher resistance is attributed to the increased mass loading of poorly conductive Si active material (Figure S8). DCPs for the first C/10 rate capability charge and discharge for the 2:1 (Ge:Si) (green) and 1:1 (magenta) ratios of the Ge@aSi NWs is shown in Figure 3e. Lithiation peaks for the material were observed at 0.30 V, 0.19 V and 0.08 V and are intermediate peaks to those observed for pure Si (0.23 V and 0.08 V) and pure Ge (0.36 V, 0.20 V and 0.10 V), confirming dual cycling of Si and Ge in the electrodes.32, 49-50 The delithiation peaks can be viewed at 0.61 V, 0.46 V and 0.29 V. With the most intense peaks seen at 0.46 V, overlapping with the broad delithiation peaks for Ge and Si. The dual cycling of Si and Ge was also confirmed through cyclic voltammetry (supporting information, S9).



Figure 4. Low and high magnification SEM images of Ge@aSi morphology (Ge:Si ratio = 2:1) after: (a,b) 1 cycle (c,d), 20 cycles (e,f), and 100 cycles.


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Figure 5. TEM and DFSTEM images showing the evolution of the Ge@aSi morphology after (a,b) 1 cycle (c,d), 20 cycles (e,f), and 100 cycles. The corresponding elemental maps are presented as (i) Ge and (ii) Si. Ratio: (Ge:Si) = 2:1

Ex-situ electron microscopy was conducted on the NWs, after different numbers of cycles, in order to gain insight into the structural changes occurring to the active material. The SEM images in Figure 4 show that there was little change to the representative wires for the 2:1 (Ge:Si) ratio after 1 cycle (Figure 4 a – b), the material became progressively more textured (Figure 4 c – d) before forming an interconnected network after approximately 100 cycles (Figure 4 e – f). Higher magnification SEM images of the restructured material after 100 cycles have been added to supporting information (Figure S10). This gradual restructuring, which has been previously reported for other Ge and Si NW containing electrodes, is caused by a combination of pore formation and lithium assisted electrochemical welding and results in the formation of an active material comprised of nanometre-sized ligaments.1, 30, 51, 52 The ability



to form this interconnected network is beneficial for Li alloying materials, as once formed an improved stability during Li-ion cycling has been observed. 28, 32, 36 Despite the high degree of restructuring, the SEM images show little active material delamination from the current collector, which can be related to the impressive capacity retention seen during long-term cycling of the material. SEM images of 1:1 ratio active material after 1, 20 and 100 cycles are presented in supporting information as Figures S11 and S12. The ex-situ SEM of the 1:1 (Ge:Si) ratio (Figure S12) revealed a greater amount of active material delamination when compared to the 2:1 material (Figure 4) after 100 cycles. This is due to the reduced void space between NWs resulting in material loss and subsequent capacity fade observed on the galvanostatic cycling profile (Figure 3a) for this composition. The morphology for the 2:1 ratio material remained better connected to the current collector as verified by ex-situ analysis (and exhibited little capacity fade) suggesting that this is the optimal ratio in this study. The relative drop in capacity retention for the 1:1 material suggests that any improvement (in terms of increasing the mass loading) will have to be through increasing surface area of the current collector.53 The compositional changes occurring during the restructuring of the active material were also analysed, by TEM and DFSTEM. Figures 5a and 5b are TEM and DFSTEM images of a Ge@aSi NW after 1 cycle. The morphology remains more or less the same as the uncycled structure. However, the formation of nanopores is evident on the Ge core which are a result of Li extraction.54-55 There are no obvious vacancies on the Si shell at this point and this could be because the Ge NWs converted from crystalline to amorphous, while the Si is already amorphous. After 20 cycles, along with the typical NW morphology becoming significantly more textured, inter-mixing between the Ge core and the Si shell has become apparent. Figure 5e shows images taken of the typical morphology after 100 cycles with the restructured material appearing porous and composed of ligaments of approximately 10 nm in diameter.


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The EDX elemental mapping, corresponding to the area imaged in 5f, shows that the material is no longer core/shell in nature and has formed a S1-xGex alloy from repeated cycling with Li. TEM for 1:1 ratio after 1, 20 and 100 cycles as well as EDX elemental mapping which shows the material also restructured to a Si1-xGex alloy by 100 charge/discharge cycles, is available in supporting information (Figure S13).

Figure 6. (a) Comparison of the charge, discharge capacities and coulombic efficiencies of directly assembled and prelithiated full-cells for the 2:1 (Ge:Si) ratio, cycled against a LCO cathode. (b) Comparison of the charge, discharge capacities and coulombic efficiencies of directly assembled and prelithiated full-cells for the 1:1 (Ge:Si) ratio, cycled against a LCO cathode. (c) Rate capability testing for each 2:1 ratio when used directly in a full-cell and when prelithiated prior to assembly. (d) Rate capability testing for each 1:1 ratio when used directly in a full-cell and when prelithiated prior to assembly. Rates tested were: C/10, C/5, C/2, C, 2C, 5C, 10C and C/10.



Most studies investigating the performance of Li alloying material anodes are typically conducted in half-cells, where the electrode is cycled against a Li-foil counter/reference electrode. Within these geometries, there is a major excess supply of Li ions where any Li consumed through SEI formation or any other irreversible processes that occur during the early cycles will not affect the measured capacity of the cell. This is not the case in real world Li-ion batteries where there is a finite inventory of Li, and any irreversible processes (e.g. SEI formation, Li trapping and loss of active material) will likely result in a loss in capacity.56-61 The performance of Ge@aSi NWs as full-cell anodes versus fixed capacity lithium cobalt oxide (LCO) cathodes within a two-electrode configuration was also investigated. The anode performance was contrasted with i) anodes directly used and ii) anodes that were prelithiated before assembling in batteries. A galvanostatic prelithiation step was implemented as a means of gauging the impact of providing these electrodes with an additional supply of Li to reduce the effects of the Li depletion processes that occur over initial cycles.62 Some of these approaches have involved inducing electrical shorting with Li foil,63-64 applying stabilized lithium metal powder (SLMP),65-66 the use of LixSi–Li2O core–shell nanoparticles or LixSi nanoparticles as prelithiation agents,67-68 hydrothermal treatment with LiCl,69 and reacting with n-butyllithium.70 Galvanostatic cycling (C/5) for pristine anodes and their equivalent prelithiated fullcell counterpart is presented as Figures: 6a (2:1) and 6b (1:1). The 1st cycle CEs exhibited for the directly used anodes in full cell configurations were poor at 38 and 45 % respectively, due to SEI formation and Li trapping, and resulted in extensive consumption of the total Li afforded by the cathode. Following this, there was a rapid capacity fade over the first 20-30 cycles, when compared to their half-cell counterparts, and is likely due to continued SEI formation and subsequent Li depletion. Discharge capacities of 565 and 744 mAh/g were retained after 100 cycles, which were approximately 42-45 % of their initial capacities. For the galvanostatic


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prelithiation experiments, the anodes were fully charged in a half-cell configuration against Lifoil before being partially delithiated (to 0.3 – 0.4 V) and then reassembled as full-cells. The plots presented in Figure 6 begin with the first full-cell charge and discharge with voltage profiles corresponding to the prelithiation cycle available in supporting information, S14. The anode materials exhibited first cycle CEs of 84 % and 75 %, which were significant improvements over the values observed for the anodes directly incorporated into full-cells (i.e. without prelithiation). Long term capacities of 1226 (2:1) and 1020 (1:1) mAh/g were retained after 100 cycles, representing noteworthy enhancements over their non-prelithiated counterparts. Voltage profiles of the 1st, 2nd, 10th, 25th, 50th and 100th charge and discharge cycles for the full-cells and prelithiated full-cells are available in supporting information (Figure S15 and S16). Rate capability plots of the pristine anode based full-cells compared to prelithiated equivalents can be seen in Figure 6c and 6d with the same rates investigated as with half-cells (Figure 3c) and the prelithiation voltage profile is available in supporting information, S17. The capacities observed at each rate were higher for the prelithiated electrodes over the directly assembled full-cells, and likely due to irreversible Li consumption during the first cycle limiting the possible capacity for the cells. At the fastest rate tested (10C), discharge capacities as high as 750 mAh/g were observed which was approximately a 4x improvement over the equivalent half-cell test. Interestingly, all full-cells saw a much improved rate capability performance over their half-cell counterparts at the faster rates analysed, and is likely due to improved lithiation kinetics of the LCO over the Li metal foil.32 For the pre-lithiated electrodes, > 90 % of the initial capacities were regained at C/10 after impressive cycling during the rate testing (at C/10, C/5, C/2, C, 2C, 5C, 10C), while the untreated full-cells only regained ~74 % of their initial capacity (shown as % capacity retention in supporting information, Figure S18).



This highlights that irreversible Li consumption in full-cells can be mitigated through prelithiation.

CONCLUSIONS By utilising dense arrays consisting of pre-grown Ge NW as scaffolds for secondary amorphous Si deposition, core@shell Ge@aSi anode active materials were prepared. The highly tuneable Si deposition step allowed for precise control over the active material compositions, with enhanced gravimetric and areal capacities over the pure Ge anode. When electrochemically tested as Li-ion battery anodes, the material retained high reversible capacities, up to 1455 mAh/g (1:1), after 150 cycles. A 2:1 Ge:Si ratio was determined to be the optimal material (versus the 1:1) ratio as it exhibited exceptional capacity retention, of approximately 90 % after 150 cycles. This observation was further explained by the identification of reduced material delamination for the optimal composition despite the extensive restructuring that occurred. When the Ge@aSi NWs were tested as full-cell anodes, it was found that a prelithiation step greatly improved the specific capacities, long-term cycling stability and rate capability of the electrodes. Overall, the results show that core shell structures combining bottom up grown Ge nanowires with top down plasma coating of Si is an effective route to high capacity Li-ion batteries, exhibiting long-term cycling stability and impressive rate capability performance.

ASSOCIATED CONTENT Supporting Information The Supporting Information is available on the ACS Publications website at DOI: XXXXX. Additional characterization of the active materials by XRD, SEM, Raman and Quantitative EDX as well additional electrochemical characterization of the material through cyclic


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voltammetry, electrical impedance spectroscopy, voltage profiles and percentage capacity retention.

AUTHOR INFORMATION Corresponding Author * E-mail: [email protected] Notes The authors declare no competing financial interest. ACKNOWLEDGEMENTS This work was supported by Science Foundation Ireland (SFI) under the Principal Investigator Program under Contract Nos. 16/IA/4629 and 11-PI-1148. The work was also supported by SFI under Grant No. SFI 16/M-ERA/3419. K.S. thanks the Irish Research Council for funding through the Government of Ireland Postgraduate Scheme. H.G. acknowledges Enterprise Ireland under Contract No. CF20144014. We thank Iobhar Stokes-Rodriguez for help in preparing the schematics.

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A Tuneable Core Shell Nanowire Active Material ... - ACS Publications

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