High-performance boron-doped silicon micron-rod anode fabricated using a mass-producible lithography method for a lithium ion battery

Journal of Power Sources 454 (2020) 227931

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High-performance boron-doped silicon micron-rod anode fabricated using a mass-producible lithography method for a lithium ion battery Sungjun Cho 1, Wonsang Jung 1, Gun Young Jung **, KwangSup Eom * School of Materials Science and Engineering (SMSE), Gwangju Institute of Science and Technology (GIST), 123 Chemdangwagi-ro, Buk-gu, Gwangju, 61005, Republic of Korea

H I G H L I G H T S

� The micron rod shaped Si anodes showed a higher rate capability than Si particles. � Structured Si prevents volume expansion, resulting in an enhanced capacity. � This facile process can enable mass-production through repetitive production. � A lightly B-doped anode exhibits a superior performance for lithium ion battery. � In case of heavily B-doped anode, Li atoms are trapped from the outermost side. A R T I C L E I N F O

A B S T R A C T

Keywords: Lithium ion batteries Boron-doped silicon rod anode Laser interference lithography Metal assisted chemical etching Boron doping level Surface kinetics

Although silicon (Si) attracts great attention as a high-capacity anode material in lithium ion batteries (LiBs), a large volume being expanded during charge/discharge (de/lithiation) cycling is a significant problem resulting in a fast capacity fade. To prevent the problem, a variety of Si structures with nano/microscales have been incorporated into the anode, but such structures still have difficulties in terms of mass production. Herein, we present a new way to repetitively produce micron boron (B)-doped Si rods from Si wafer through laser inter­ ference lithography (LIL) in combination with the metal assisted chemical etching (MACE) process, enabling the mass-production of multiple Si rods at low cost. Moreover, the effects of the B-doping level of the produced Si rods on the electrochemical LiB performances are studied in detail. As a result, the lightly B-doped Si rod (~1015 atoms cm 3) anodes exhibit the highest initial capacity of 3524 mAh g 1 and cyclic performance, showing a high average Coulombic efficiency (CE) of 98.1% and a capacity fading rate (per cycle) of 0.11% during 500 cycles. It is due to the highest kinetics of de/lithiation on the surface of the lightly B-doped Si rod attributed to favorable phase transition of Si and diffusion of Li ions.

1. Introduction Owing to the increasing demand for lightweight, high-energy, and long-lifecycle lithium ion batteries (LiBs) in applications such as various electric vehicles (EVs), hybrid electric vehicles (HEVs), and energy storage systems (ESSs), extensive research is being conducted to identify alternative anode materials that improve the capacity and stability of LiBs [1–6]. Among them, silicon (Si) has received great attention due to its high specific theoretical capacity (~4200 mAh g 1), low cost, and non-toxic properties [7,8] However, the biggest obstacle to a practical

use of Si anode is a large volume expansion (300–400% [9–12]) occurring during an electrochemical reaction with Li ions, leading to a significant decrease in capacity and reducing the cyclic stability with cracking and pulverizing. Therefore, many studies have focused on increasing the stability through the fabrication of a variety of structures [2,13–16]. Recently, Si with various morphologies such as porous structures, rods, and tubes have been used as an active high-content (Si dominant) anode material to reduce the stress relaxation and accommodate the volume expansion during de/lithiation in LiBs [17–19]. It is because the

* Corresponding author. ** Corresponding author. E-mail addresses: [email protected] (G.Y. Jung), [email protected] (K. Eom). 1 These authors contributed equally to this work. https://doi.org/10.1016/j.jpowsour.2020.227931 Received 14 January 2020; Received in revised form 12 February 2020; Accepted 19 February 2020 Available online 25 February 2020 0378-7753/© 2020 Elsevier B.V. All rights reserved.

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structured Si can act as a structural buffer space that prevents the cracking of Si and retain their structural integrity and electrical con­ nections, resulting in an enhanced capacity and cycling performance [20]. In particular, the rod-structured Si (Si rod) has the benefit of a direct current pathway, short Li ions diffusion path, and one-dimensional high surface morphology because of their thin wall [12,21–23]. To prepare the structured Si rod, top-down synthesis methods have been introduced to obtain structures from bulk Si such as laser beam ablation [24], metal assisted chemical etching (MACE) [25, 26], and, conversely, a bottom-up synthesis of structures using chemical or vapor precursors, such as vapor liquid solid (VLS) growth [27] and chemical vapor deposition (CVD) [28]. Among these techniques, the MACE technique has the advantage of being able to produce the crys­ talline quality and long vertical structure of Si directly from a bulk Si wafer using simple and inexpensive solutions at room temperature. However, because the mass production of Si rod structures remains difficult to achieve, it is necessary to develop a new facile mass pro­ duction method using MACE. In this context, we propose a modified MACE process combined with laser interference lithography (LIL) enabling the mass-production of Si rod anodes for LiB. Because the metal layer can remain on the roots after blading, the modified MACE process can be carried out again, and the Si rods can be re-produced. This facile process allows us to obtain a large number of Si rods from a single substrate of Si wafer, and hence time and cost savings are expected to be realized. On the other hand, the low electrical conductivity of Si is also one of the chronic problems to be solved for the practical use of high-content Si (Si dominant) as an anode in LiBs. To improve the electrical conductivity of Si anodes, compositing with conductive materials such as carbon and copper [29–31], and/or doping the dopants such as phosphorus (P) and boron (B) [32] have been widely studied. Particularly, the dopants can have an effect on the binding energy of Li with Si, and hence change the properties of Si during the electrochemical processes [33,34] It is typi­ cally expected that a higher electrical conductivity and electrochemical activities are obtained for doped Si with a higher doping level, by the increased contribution of either the electron or hole. However, unlike the commonly doped Si, in which a dopant such as B statically occupies a substitutional site in the Si crystalline structure at room temperature, the de/lithiation process in LiB causes a change from a crystalline to an amorphous structure [35], and dopant B would become rearranged during an electrochemical reaction, such that its electronic structure can be modified. Therefore, questions regarding whether the expected physical properties and enhanced electrical conductivity of doped Si are retained and whether heavy B doping can impact the electrochemical behavior of a Si anode under repetitive electrochemical reaction, such as de/lithiation, should be investigated in detail. Hence, in this work, the doping effect in rod-structured Si anodes as a function of the doping level on the electrochemical LiB performances was studied, by preparing three types of B-doped Si rod anodes (namely, heavily doped Si (~1020 atoms cm 3), lightly doped Si (~1015 atoms cm 3), and undoped Si rod anodes) from commercial Si wafers using a new method using the MACE process combined with LIL.

pattern on the Si substrate. A He–Cd (λ ¼ 325 nm) laser (intensity ¼ 0.7 mW cm 2) was used for the LIL process. The bottom antireflective coating (BARC) layer (Micro-Chemicals, GmbH) was spin-coated at 4500 rpm for 1 min on the Si substrate, followed by soft-baking at 115 � C for 1 min. An AZ 2020 negative tone photoresist (PR) (AZ Electronics Mate­ rials) was mixed with a thinner, using a volume ratio of 1:0.8. The PR layer was spin-coated at 4500 rpm for 1 min, followed by soft-baking at 115 � C for 1 min. Half of the He–Cd laser beam was irradiated onto the PR-coated Si substrate, whereas the other half was irradiated onto the PR-coated Si substrate after a reflection from a Lloyd’s mirror. Two coherent beams were used to produce a periodic interference pattern on the PR-coated Si substrate. The two beams interfered at an angle of 10� and were exposed for 11 s. The sample was rotated 90� and then exposed for 11 s, followed by hard-baking at 115 � C for 1 min. The development process used to remove the unexposed area was applied with an MIF 300 solution for 60 s at room temperature. A dry-etching process with O2 gas plasma (50 SCCM, 20 mTorr, 22 W, 120 s) was conducted to remove the residual PR and underlying BARC layer. Next, Cr films (15 nm, 0.3 Å/s) were deposited onto the sample using an electron beam evaporator. The sample was immersed in EKC 830 for a few seconds to remove the BARC layer, completing the Cr dot pattern array on the Si substrate. To utilize the MACE process, Ag (5 nm, 0.2 Å/s) and Au (10 nm, 0.2 Å/s) films were deposited sequentially onto the Cr dot pattern array on the Si substrate using an electron beam evaporator. 2.2. Repeated metal assisted chemical etching (MACE) process of micro Si rods The micro Si rods were fabricated by immersing a dot patterned Ag/ Au catalytic film onto the Si substrate in a Teflon beaker filled with an etching solution of HF (4.6 M) and H2O2 (0.4 M) for 10 min at room temperature. After the first MACE process, the Si rods produced were cut obliquely with a razor blade to separate them from the Si substrate without touching the metal layer of the roots. To repeat the MACE process on the same substrate, a layer of catalytic metals must remain in the roots of the mother Si substrate. The second MACE process can be repeated because the metal layer remains in the roots. The detachment process was repeated after 10 min of the second MACE process and the Si rods were collected in a vial. It was possible to carry out the MACE process repeatedly because the layers of the catalytic metal were still present on the mother Si substrate. This allowed the production of large quantities of Si rods from a single substrate of Si wafer. 2.3. Preparation of Si electrodes and coin cell assembly As-prepared Si active materials were mixed with conductive carbon graphite, super C and a poly (acrylic) acid (PAA, Sigma Aldrich) binder (5% in water) with a mass ratio of 50:15:15:20. The materials were gently mixed in an agate mortar and stirred using a magnetic stirrer overnight. The slurry was coated on a copper foil, and the electrodes (⌀ ⌀ 10) were dried in a vacuum oven at 80 � C overnight. Si electrodes were assembled using a 2023 coin cell (Welcos). A Li foil (⌀ ⌀ 16, Sigma Aldrich) was used as the counter and reference electrodes and PP separator (2400, Celgard) were applied. A combination of 1 M LiPF6 ethylene carbonate (EC) and diethyl carbonate (DEC) (1:1 wt%), plus fluorine ethylene carbonate (FEC) 10 wt% additive electrolyte (Soulbrain MI Co., Ltd.), was used. All coin cells were assembled in an Ar glove box, the oxygen and humidity levels of which were less than 1 ppm. Undoped Si powder electrodes were fabricated in the same way as 1-μm Si powder (Avention) under the same conditions. The thickness and weight of each electrode were ~20 μm and 0.8–1.0 mg cm-2, respectively.

2. Experimental section 2.1. Fabrication of dot patterned Ag/Au catalytic film on silicon (Si) substrate The Si substrates with various doping densities (Zhejiang Zhongjing Electronics Co., Ltd.) (where the boron (B) doping density of a Si wafer for heavily and lightly B-doped rod anodes are 1020 atoms cm 3 and 1015 atoms cm 3, respectively) were used for Si rods fabrication. A 2 cm � 2 cm Si substrate (100) was cleaned using acetone, isopropyl alcohol, and deionized water for 15 min each. The Si substrate was immersed in a buffered oxide etchant for 60 s to remove the SiO2 layer. The laser interference lithography (LIL) process was utilized to obtain the Cr dot

2.4. Electrochemical analysis A galvanostatic test was conducted using a WBCS3000 cycler (WonATech). CV and EIS were conducted using a potentiostat (1385, 2

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Fig. 1. Fabrication process and morphological characterization of repetitive production of micro Si rods: (a) Schematic of the fabrication process of repetively producing micro Si rods. FE-SEM topviews and cross-section of (b) deposited Ag/Au on the Cr dot pattern on Si substrate obtained using LIL, (c) the Si substrate after the first MACE, (d) the mother Si substrate after detachment of micro Si rods, and (e) the Si substrate after the second MACE. (f) FE-SEM top-view of fabricated micro Si rods.

Solartron Analytical). EIS profile fitting was conducted using Zview software (Scribner Associates Inc.). For the C-rate test, all Si anode cells were dis/charged within a potential window of 0.05–1.5 V (vs. Li/Liþ) at current rates of 0.05, 0.1, 0.5, 1, and 2C, respectively, and for long-term cyclability tests, the cells were dis/charged at 0.5C under a constant current (CC) and constant voltage (CV) conditions, the cutoff current of which was 1/10 of the test current at 25 � C.

The rate at which the oxidant H2O2 is reduced on the bare metal catalyst surface is extremely slow. Therefore, the etching rate of Si in an etchant solution mixed with H2O2 and HF is negligible. In contrast, if a metallic catalyst such as Ag, Au, or Pt is present on the Si surface, H2O2 receiving electrons from the catalyst surface is reduced (Eq. (1)). At the same time, in the reduction reaction, electron transfer occurs directly at the interface between the metallic catalyst and Si, and Si is oxidized (Eq. (2)). Because the SiO2 layer produced is dissolved as SiF26 using HF, a localized etching of Si occurs (Eq. (3)). As the Si etching progresses, metal particles sink into the Si substrate, which is the main cause of anisotropic etching. When anisotropic etching is conducted for a lengthy period of time, an Si structure with a high aspect ratio is generated. The vertical length of the structure can be controlled based on the etching time. Because Ag, which acts as a catalyst in the MACE process, is easily dissolved in the etchant, and because Au has poor adhesion with a Si substrate, Ag and Au are deposited sequentially on a Cr dot array sub­ strate to carry out the MACE process [36] (Fig. 1a). The detaching process of Si rods is as follows. The produced Si rods are cut obliquely with a razor blade to separate them from the Si substrate. After the detachment process, the detached Si rods are collected. Si rods are again fabricated using the MACE because the catalytic metal layer remains at the roots of the mother Si substrate. This process enables the MACE to be used repeatedly and allows the production of large quantities of Si rods from a single substrate of Si wafer. Fig. 1b–f shows field emission scanning electron microscope (FE-SEM) images of the micro Si rods and Si substrate during repetitive MACE and detaching processes. Fig. 1b shows an FE-SEM top-view and a cross-section of Ag/Au deposited on the Cr dot pattern on a Si-substrate utilized LIL. Each Cr dot is approx­ imately 700 nm in size. As shown in Fig. 1c and Si micro rods are well formed after the first MACE. The detaching process is carried out as follows. The Si rods produced are cut obliquely with a razor blade to separate them from the Si substrate without touching the metal layer of the roots. To repeat the MACE process on the same substrate, a layer of catalytic metals must remain in the roots of the mother Si substrate. FE-SEM images of the mother Si substrate after the detachment process are shown in Fig. 1d. It is possible to execute the MACE process again because the metal layer remains in the roots. After the second MACE process, the Si rods are well formed (Fig. 1e). Many MACE processes are possible because layers of catalytic metal are still present on the mother Si substrate. This allows the production of large quantities of Si rods from a single substrate. Fig. 1f shows the clusters of fabricated micro Si rods detached from the mother Si substrate.

2.5. Material characterization Field-emission scanning electron microscopy (FE-SEM, 2020F, JEOL, Ltd.) was used to observe the morphology. The resistance of various Si electrodes was measured using a 4-point system (M4P302, Keithley In­ struments, Inc.). The surface component analysis was characterized using X-ray diffraction (XRD, Ru–200B, Rigaku) and ToF-SIMS (ION-TOF, GmbH). High-resolution transmission electron spectroscopy (Tecnai, Thermo Fisher Scientific) and energy dispersive spectroscopy (EDAX, AMETEK) were utilized for bright/dark field images and elements line scanning, respectively. X-ray photoemission spectroscopy (K-Alpha, Thermo Fisher Scientific) were used for element depth profiles. 3. Results and discussion 3.1. Production method of Si rods using MACE combined with LIL Fig. 1a shows an overall schematic illustration of the fabrication of micro silicon (Si) rods and the detaching process from the mother Si wafer substrate. Micro Si rods were produced using metal assisted chemical etching (MACE) process combined with laser interference lithography (LIL). Compared to other patterning techniques, the LIL technique has the benefits of easy processing, large area patterning, and the control of many different parameters, such as the diameter or pitch sizes. This technique is also suitable for producing a large quantity of micro Si rods for application on lithium ion batteries (LiBs). A chrome (Cr) dot array on the Si substrate was produced using LIL techniques. To utilize the MACE process, the catalytic metal particles, Ag and Au, were deposited on a Cr dot array substrate (Fig. 1a). The MACE of Si is described through the following reactions (1–3) [25]. H2O2 þ 2Hþ þ 2e → 2H2O

(1)

Si þ 2H2O → SiO2 þ 4Hþ þ 4e

(2)

þ

SiO2 þ 6HF → 2H þ

SiF26

þ 2H2O

(3) 3

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3.3. Electrochemical performances of Si rod anodes

Table 1 Resistance of various Si wafers. Various Si wafers

Based on the dis/charge profiles of the HdSi, LdSi, UdSi rod, and UdSi powder anodes for LiBs (Fig. S3), it was confirmed that the first discharge capacities were 3,377, 3,524, 3,291, and 3976 mAh g 1Si at 0.05C and exhibited the first irreversible capacity losses (ICLs) of 48.1%, 22.3%, 29.6%, and 23.2%, respectively. In particular, it is notable that the LdSi rod shows no substantial variation between the first and fifth de/lithiation profiles (Fig. S1b). Fig. 3 presents the electrochemical performances of HdSi, LdSi, and UdSi rod and UdSi powder anodes for LiBs. As shown in Fig. 3a, the rod-structured Si anodes such as HdSi, LdSi, and UdSi showed a high rate-capability compared to the UdSi powder anode presumably due to a faster ion diffusion into core mate­ rial. Moreover, the doped Si rods showed higher rate-capabilities than undoped Si rods due to the increased electrical conductivity (Table 1). Particularly, when comparing the rate-capabilities of LdSi and HdSi, the LdSi rod showed decreased discharge capacities of 96%, 90%, 83%, and 76% at 0.2, 0.5, 1, and 2C (4.2 A g 1Si), respectively, whereas the HdSi rod showed a decrement of 86%, 75%, 67%, and 60% at 0.2, 0.5, 1, and 2C (4.2 A g 1Si), respectively. Moreover, when applying 1C rate (2.1 A g 1Si) after the rate-capability test to 2C (4.2 A g 1Si), the LdSi rod had the higher recovery capability (1972 mAh g 1Si) than other HdSi, UdSi rod, and UdSi powder anodes of 1,010, 1,076, and 1032 mAh g 1Si, respectively. Furthermore, as shown in the cyclic performance of Fig. 3b, the LdSi rod anode showed the lowest average discharge capacity fading rates per cycle (0.11%) until reaching 500 cycles (HdSi and UdSi rod anodes showed 0.23% and 0.15%, respectively). The lower average coulombic efficiency (CE) of the LdSi rod anode also support the stable cyclic performance (Fig. S4). Interestingly, even though HdSi, LdSi and UdSi rods have the same rod structure, differences in the B doping densities of anodes could largely affect the variation of CE in cyclic performance test. Considering previous literatures concluding that the B doping alleviated pulverization of B-doped Si wafer, and enhanced the stability [38–40], it is speculated that B doping not only improves electrical conductivity but also partially contributes to structural stability.

Sheet resistance 20

3

Heavily doped Si wafer (~10 atoms cm ) Lightly doped Si wafer (~1015 atoms cm 3) Undoped Si wafer (0 atoms cm 3)

2

0.0453 Ω cm 17.108 Ω cm 2 102.99 mΩ cm

2

3.2. Material characterization of Si rod anodes fabricated by MACE combined with LIL Using the MACE process on various Si wafers (Table 1), we fabricated three types of B-doped (p-type) micro Si anodes, namely, heavily doped Si (~1020 atoms cm 3), lightly doped Si (~1015 atoms cm 3), and undoped Si rod anodes, respectively. Fig. 2 shows the structural and morphological characterization of the heavily doped Si rod (HdSi rod), lightly doped Si rod (LdSi rod), and undoped doped Si rod (UdSi rod) anodes. As shown in the TEM image of Fig. 2a, the as-prepared Si rods have a thickness of approximately 2–4 μm. The rods were used as active materials for the electrodes after mixing with carbon conducting addi­ tives and a poly (acrylic) acid (PAA) binder (Fig. 2b). The X-ray diffraction (XRD) patterns of as-prepared electrodes containing HdSi, LdSi, and UdSi rod indicate that all Si rods have a crystalline structure (Fig. 2c). All diffraction peaks of HdSi, LdSi, and UdSi rod anodes can be readily indexed to the Si phase, indicating well fabricated Si anodes without oxidation and contamination. When comparing the three types of anodes, the Si diffraction peak of pristine UdSi rod is the largest, and the Si diffraction peak of pristine HdSi rod is relatively small with two peaks of 32.2� and 70.7� at 2θ (Fig. S1). This indicates that B-doping causes a degradation of the degree of order in a crystal lattice of Si (Fig. S2) [37]. Furthermore, the ToF-SIMS of as-prepared HdSi and LdSi rod anodes (Fig. 2d) could confirm the minute existence of B elements in HdSi and LdSi rod anodes. Although the quantitative comparison was impossible, but it could observe that the intensity of the B element peak of the HdSi rod anode is higher than that of the B element peak of the LdSi rod anode, leading to a fact that the HdSi rod anode has more B dopants than the LdSi rod anode.

Fig. 2. Characterization of the B-doped micro Si rod anodes: (a) TEM image of LdSi rod, (b) SEM image of LdSi rod anode, (c) XRD pattern of HdSi, LdSi, and UdSi rod anodes, (d) ToF-SIMS spectra of HdSi and LdSi rod anodes. For XRD profiles, all values are normalized absolutely with the highest value. For ToF-SIMS spectra, all values are normalized with the carbon value (cþ). 4

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Fig. 3. (a) Rate capability of HdSi, LdSi, and UdSi rod and UdSi powder anodes. Each electrode is cycled once at 0.05C and 5 times at 0.1, 0.2, 0.5, 1 and 2C, respectively (1C ¼ 4.2 A g 1). (b) Discharge capacity of HdSi, LdSi, and UdSi rod and UdSi powder anodes at 2.1 A g 1 (cyclic performance). All Si anodes are dis/ charged within a potential window of 0.05–1.5 V (vs. Li/Liþ) with constant current (CC) and constant voltage (CV) condition for which the cutoff current is 1/10 of the test current. A long-term galvanostatic tests are conducted after the rate test. Cyclic voltammograms (CVs) for (c) HdSi, (d) LdSi, and (e) UdSi rod anodes, which are cycled 15 times with a scan rate of 0.05 mV s 1 and within a potential window of 0.05–1.5 (V vs. Li/Liþ). For CV profiles, all values are absolutely normalized with the highest value.

Structural stability is also proved by electrochemical impedance spectroscopy (EIS) in a long term galvanostatic test (Fig. S5). Overall, the various Si rods anodes have stable RSEI value for 200 cycles. Inter­ estingly, also in this analysis, not only RSEI values of LdSi rod are the smallest values compared to those of HdSi and UdSi rod but also the standard variation of RSEI of LdSi rod, 4.58, is the smallest compared to that of HdSi and UdSi rod, 23.3 and 15.8, respectively. This result ex­ hibits the tendency which is consistent with the result of CE analysis (Fig. S4). For further investigation, cyclic voltammetry (CV) measure­ ments were conducted as a function of the scan rates for 15 cycles to compare the current peak shifts and variation of the current densities (Fig. 3c–e and Fig. S6). As Si active materials are being lithiated, Si forms amorphous Li silicides, a-LixSi (x ¼ 0–0.2), as well as large Si–Si clusters and a Si-extended network, at 0.25–0.30 V [41–43]. Next, a-LixSi (x ¼ 0–0.2) is more lithiated, forming small Si clusters and a-LixSi (x ¼ 2.0–3.5) at ~0.10 V [41]. For CV of HdSi rod at 0.05 mV s 1, the current densities gradually increased in the initial 8 cycles. However, after 8 cycles, the current densities gradually decreased. Cyclic voltammograms (CVs) asymptotically converged to a certain curve as their current densities decreased. Notably, for a reduction reaction, the current den­ sity peak (~0.15 V) was substantially suppressed and shifted to a lower voltage (Fig. 3c). Meanwhile, for the CV of LdSi rod at 0.05 mV s 1, the current densities gradually increased within 10 cycles, and CVs asymptotically converged to the curve for which the current densities are maximal. After 10 cycles, LdSi rod was reversibly de/lithiated at the maximal curve, which exhibited the most stable cycles (Fig. 3b). For the CV of UdSi rod, and similarly for HdSi rod, the current densities grad­ ually increased within 5 cycles. The CVs of UdSi rod converged to a suppressed curve. In particular, the cathodic current peak (~0.17 V) was significantly shifted to ~0.05 V, and thus it converged with a cathodic current peak (~0.05 V), was thus not discernible (Fig. 3e). When considering that lithiation was easier for HdSi rod at a much higher potential (Fig. S7), this could imply that de/lithiation in HdSi rod

becomes unfavorable as the number of cycles increases. However, in several initial cycles, this factor was not dominant for the capacity loss because HdSi rod showed a similar de/lithiation behavior when considering the CVs of HdSi rod (Fig. 3c–d). Even though electro­ chemical responses as a function of scan rate were scrutinized in detail (Fig. S6), the tendency that a significant capacity loss at the initial cycles for HdSi rod is needed to be elucidated by various electrochemical analyses. 3.4. Effects of B doping level in Si rod anodes on the initial lithiation/ delithiation behaviors For that, galvanostatic intermittent titration technique (GITT) mea­ surements was conducted but it could not show discernible differences between HdSi and LdSi rod (Fig. S8). This tendency appeared to be highly dependent on the rate because the phenomenon was clearly manifested and saturated in the abrupt lowering of the potential, as found in the potentiostatic intermittent titration technique (PITT) measurements (Fig. 4). Apparent differences were observed from the current responses when static potential (0.1–0.4 V) was applied. In addition, considering the shape of overall current densities profiles as a function of time it was notable that HdSi rod current profile was shifted to higher voltages. For HdSi rod, when 0.25 V static voltage was applied, reduction reaction occurred. On contrary, for LdSi and UdSi rod, when 0.25 V static voltage was applied, delithiation occurred, which means that the electrode potential of LdSi and UdSi rod is between 0.15–0.20 V (Fig. 4b and c) and of HdSi rod is positioned between 0.25–0.3 V (Fig. 4a). This also implies that LdSi and UdSi rod are more favorable to be used as anode for lithium ion battery than HdSi rod considering electrode potential. Notably, the significant differences appeared when 0.3 V above static potentials were applied. For HdSi rod, when normalizing current densities at 0.4 and 0.45 V with the current density at 0.35 V, current densities at 0.4 and 0.45 V, respectively, is 40 and 28% 5

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Fig. 4. Potentiostatic intermittent titration technique (PITT) of (a) HdSi, (b) LdSi, and (c) UdSi rod anodes during delithiation. PITT was conducted with 0.05 V voltage steps. Constant voltages were applied for 10 min and all cells were relaxed for a 1 h. All values were normalized with absolute highest value of the anodic current.

reduced values compared to the current density at 0.35 V, respectively (Fig. 4a). In contrast, for LdSi rod, current densities at 0.35 and 0.40 V normalized with the current density at 0.30 V, current densities at 0.35 and 0.40 V, respectively, is 12 and 23% reduced values compared to the current density at 0.35 V, respectively (Fig. 4b). Similarly, for UdSi rod,

current densities at 0.4 and 0.45 V normalized with the current density at 0.35 V, current densities at 0.4 and 0.45 V, respectively, is 20 and 19% reduced values compared to the current density at 0.35 V, respectively (Fig. 4c). Those results were consistent with the fact that de/lithiation in HdSi rod is unfavorable at high scan rate CVs and high current densities 6

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diffusion, resulting in a high over-potential and inferior kinetics. The B mass ratios of HdSi (~1020 atoms cm 3) and LdSi (~1015 atoms cm 3) are 7.7 � 10 2 and 7.7 � 10 7 wt%, respectively, which are extremely small relative to the density of Si (2.32 g cm 3) and molar mass of B (10.8 g mol 1). Nevertheless, interestingly, doping elements not only had a significant impact on the electrical conductivity of the Si active materials but also substantially affected the electrochemical perfor­ mance of the Si anode (Fig. 3). An appropriate B-doping through ball milling was observed to improve the rate capability of the Si/C com­ posite [46]. The doping element and its density appear to play a critical role in the electrochemistry of doped Si anodes. It is clearly reasonable to conclude based on our results that there is an optimal doping density for an enhanced capacity and rate capability.

Table 2 Resistance of various Si electrodes. Various Si electrodes

Sheet resistance 20

3

Heavily doped Si rod (~10 atoms cm ) electrode Lightly doped Si rod (~1015 atoms cm 3) electrode Undoped Si rod (0 atoms cm 3) electrode Undoped Si powder (0 atoms cm 3) electrode

10.93 kΩ cm 2 78.40 kΩ cm793.10 kΩ cm 2 6399.20 kΩ cm 2

in galvanostatic test. At the microscopic level, it is supposed that for B-doped Si, the electrons in the valence band are excited by the thermal energy at room temperature. For p-type Si, doped B which is an impurity contributes to the acceptor level formation between the valence band and the con­ duction band. Thus, excitation forms multiple holes and a greater number of holes is generated with greater B-doping in Si, which is consistent with the measured electrical conductivities of various Si an­ odes as a function of the B doping density (Table 2) [40,44,45]. In addition, the ionization of electrons from the B atoms leaves negatively charged B atoms in substitutional sites in Si. Presumably, the negatively charged B atoms can trap the positively charged Li ions when Li ions are being de/lithiated at above a certain B doping density where the trap­ ping effect exceeds the enhanced electrical conductivity. Trapped Li ions in the vicinity of the negatively charged B atoms might block Li ions

3.5. Effects of B doping level in Si rod anodes on the electrochemical kinetics To investigate not only the effect of the heavy B doping on the ca­ pacities but also how such heavy B doping affects the kinetics of de/ lithiation in Si, all Si anodes were dis/charged with constant current and constant voltage (CC/CV) conditions based on the fact that dQ/dV profiles of the charging of Si anodes depend on the discharge cutoff voltage [41]. Discharge dQ/dV profiles of Si rod anodes were very similar to the discharge dQ/dV profiles of Si when the discharge cutoff

Fig. 5. Differential capacity profiles of dis/charge of (a) HdSi, (b) LdSi, and (c) UdSi rod anodes. The first dis/charge was performed at 0.05 C-rate and sequential 5 cycles at 0.1C-rate, respectively, with potential window of 0.05–1.5 V (vs. Li/Liþ). 7

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Fig. 6. Electrochemical impedance spectroscopy (EIS) profiles of (a) HdSi, LdSi, and UdSi rod anodes during de/lithiation. (b) Linear fitting of Warburg diffusion impedance during de/lithiation. All cells were charged and discharged at 0.01 C-rate. Each EIS was measured at 1.5, 1.0, and 0.5 V (vs. Li/Liþ) after resting a cell until its absolute OCV variation (|dv/dt|) is smaller than 1.66 10 6 V s 1 at each step. 20 points ranging from 89 to 10 mHz were selected for the fitting.

voltage was lower than 50 mV (Fig. 5 and Fig. S9) [41]. In addition, charge dQ/dV profiles of HdSi rod was similar to combining charge dQ/dV profiles after discharging to the cutoff voltage of 45 and 30 mV (Fig. 5a and Fig. S9a). By contrast, charge dQ/dV profiles of LdSi and UdSi rod were neither similar to charge dQ/dV profile after discharging to the cutoff voltage of 45 mV nor to that after discharging to the cutoff voltage of 60 mV, even though the discharge cutoff voltage was 50 mV (Fig. 5b,c and Figs. S9b and c). This finding was observed because in the galvanostatic test, all cells were dis/charged in CC/CV conditions. Lithium-silicide is further lithiated from the a-Li3.75Si phase to the c-Li3.75Si phase because the further phase transition potential is near ~50 mV [47–50]. This finding was observed because a broad anodic current peak (~0.27 V) was manifested when the a-LixSi (x ¼ 2.0–3.75) phase was delithiated, which means that the a-LixSi (x ¼ 2.0–3.75) phase was retained even after fully lithiation by applying a constant voltage (50 mV). This difference in the charge dQ/dV profiles is possibly ascribed to B doping and its critical density which could affect the phase transitions of lithium-silicides. Heavy B doping possibly hinders the lithium-silicide de/alloying kinetics, namely, the phase transitions of lithium-silicides. In EIS analysis (Fig. 6), diffusion coefficients [51,52] derived from

Table 3 Diffusion coefficients as a function of dis/charge cut off voltages. Points

HdSi rod

Discharge cut off voltage 1.0 (V) Discharge cut off voltage 0.5 (V) Charge cut off voltage 0.5 (V) Charge cut off voltage 1.0 (V) Charge cut off voltage 1.5 (V)

7.59ⅹ10 7.81ⅹ10 2.67ⅹ10 3.38ⅹ10 4.95ⅹ10

LdSi rod 16 16 13 15 16

2.19ⅹ10 2.93ⅹ10 4.39ⅹ10 3.32ⅹ10 1.23ⅹ10

UdSi rod 17 16 13 15 15

2.99ⅹ10 4.96ⅹ10 2.08ⅹ10 5.00ⅹ10 1.92ⅹ10

18 16 13 15 15

the fitting of the EIS profiles of HdSi, LdSi and UdSi rods showed that at initial lithiation (at 1.0 V and 0.5 V discharge cutoff voltages), the slopes of LdSi and UdSi rod derived from linear fitting of EIS profiles were larger than that of HdSi rod, meaning that LdSi and UdSi rods have smaller diffusion coefficients than HdSi rod (Table 3). However, at the initial charge cutoff voltage of 0.5 V, all the diffusion coefficients of Si rods greatly increased 1000 times and the order of the value was changed. It is notable that the diffusion coefficients of LdSi and UdSi are higher than that of HdSi at final charge cutoff voltage of 1.5. From the results, it is supposed that rearrangements of B or configuration changes of Si and B in HdSi rod occur by way of decreasing the diffusivity, 8

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Fig. 7. (a) High-angle annular dark-field scanning TEM (HAADF-STEM) images of pristine HdSi and LdSi rod anodes and corresponding EDS line scanning. (b) TEM (HAADF-STEM) images of the first delithiated HdSi and LdSi rod anodes and corresponding EDS line scanning. All Si rods are in the delithiated state after the first lithiation and delithiation.

whereas LdSi and UdSi became more favorable to ion diffusion.

of the LdSi rod anode, respectively. This notable difference implies that the remaining Li content in HdSi rod is higher than that of LdSi rod even after being fully delithiated. Because the remaining Li on the Si surface was likely to form Li oxides and solid electrolyte interphase (SEI) con­ taining Li and oxygen during the first lithiation and delithiation, the oxygen counts also seem to be higher in the HdSi rod anode. Further­ more, an X-ray photoemission spectroscopy (XPS) depth profile analysis for B-doped Si rod anodes was conducted, as shown in Fig. 8. It was confirmed that the HdSi rod had the highest atomic weight percent of Li at more than 60 at.% from the outermost side, and with an increase in etching time, the amount gradually decreased, which indicates that a number of Li atoms were trapped from the outermost side (Fig. 8a). This suggests a strong correlation in which B-doped can be rearranged and migrated into the surface of Si rods, as mentioned in previous study [53]. However, for LdSi rod, regardless of the etching time, a continuous low Li content of about 14 at.% was observed, presumably owing to the

3.6. Effects of B doping level in Si rods on the Li ion surface diffusion (STEM/EDS/XPS) As shown in Fig. 7, scanning transmission electron microscope (STEM)/energy dispersive spectroscopy (EDS) line scanning along the cross-section of a rod for B-doped Si anodes was conducted to prove our assumption that Li ions in a HdSi rod can be trapped by a large count of B dopants. Fig. 7a shows that through the surface and center of Si rods, the ratio of Si to oxygen for pristine LdSi rod with a relatively low B con­ centration was higher than that for prisitine HdSi. In the case of fully delithiated HdSi rod (Fig. 7b), the counts (maximum value was normalized to 100) for Si and oxygen were 51 and 46 on the surface of the HdSi rod anode, respectively. Whereas the fully delithiated LdSi rod exhibited a higher Si and lower oxygen counts of 90 and 9 on the surface

Fig. 8. X-ray photoelectron spectroscopy (XPS) depth profiles of (a) HdSi and (b) LdSi rod anodes showing Li 1s, C 1s, and Si 2p satellite (sat.) peaks. All Si rods are in the delithiated state after the first lithiation and delithiation. 9

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stable and uniform remaining Li species such as Li2O, LiF, and Li2CO3 (Fig. 8b) [54,55]. It is in good agreement with STEM/EDS result, based on the assumption that Li ions are easily trapped by a large number of B dopants on the surface.

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4. Conclusions Micron silicon (Si) rods were fabricated through a new method using the MACE process combined with LIL followed by a detachment of the Si rods from the substrate of Si wafer, which can enable mass-production through repetitive production. The micron rod shaped Si anodes showed a higher rate-capability and capacity retention than powdery Si particles. These characteristics are attributed to an increase in the surface-to-volume ratio, and a short radial Li ions diffusion path, leading to alleviated pulverization during the de/lithiation process. Among the various micron Si rod anodes with different boron (B) doping levels, the lightly B-doped Si rod anode (~1015 atoms cm 3) showed the highest capacity and most superior stability, with a high initial CE of 98.1% and high cyclic stability showing a 0.11% capacity fading rate during 500 cycles. Through a CV analysis, it was confirmed that the lightly B-doped Si rod have a low over-potential, implying high kinetics for the de/ lithiation process during LiB cycles. Whereas, the heavy B doping (~1020 atoms cm 3) hinders the de/lithiation kinetics because a number of Li atoms are trapped from the outermost side of the Si rods even after being fully delithiated. Declaration of competing interest The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper. CRediT authorship contribution statement Sungjun Cho: Conceptualization, Methodology, Investigation, Writing - original draft. Wonsang Jung: Investigation, Data curation, Methodology, Writing - original draft. Gun Young Jung: Supervision, Writing - review & editing. KwangSup Eom: Supervision, Writing review & editing. Acknowledgements This work was supported by the National Research Foundation of Korea (Basic Science Research Program (NRF-2017R1C1B2010814 and NRF-2019R1A2B5B01070640)) funded by the Ministry of Science and ICT, and the GIST Research Institute (GRI) grant funded by the GIST in 2020. Appendix A. Supplementary data Supplementary data to this article can be found online at https://doi. org/10.1016/j.jpowsour.2020.227931. References [1] J. Shi, D. Xiao, M. Ge, X. Yu, Y. Chu, X. Huang, X. Zhang, Y. Yin, X. Yang, Y. Guo, L. Gu, L. Wan, High-capacity cathode material with high voltage for Li-ion batteries, Adv. Mater. 1705575 (2018) 1–8. [2] Y. Jin, S. Li, A. Kushima, X. Zheng, Y. Sun, J. Xie, J. Sun, W. Xue, G. Zhou, J. Wu, F. Shi, R. Zhang, Z. Zhu, K. So, Y. Cui, J. Li, Self-healing SEI enables full-cell cycling of a silicon-majority anode with a coulombic efficiency exceeding 99.9%, Energy Environ. Sci. 10 (2017) 580–592. [3] Z. Liu, Q. Yu, Y. Zhao, R. He, M. Xu, S. Feng, S. Li, L. Zhou, L. Mai, Silicon oxides: a promising family of anode materials for lithium-ion batteries, Chem. Soc. Rev. 48 (2019) 285–309. [4] N. Kim, S. Chae, J. Ma, M. Ko, J. Cho, Fast-charging high-energy lithium-ion batteries via implantation of amorphous silicon nanolayer in edge-plane activated graphite anodes, Nat. Commun. 8 (2017) 1–10.

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