Reduced Graphene Oxide Anodes

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Energy, Environmental, and Catalysis Applications

Cobalt Hydroxide Carbonate/Reduced Graphene Oxide Anodes Enabled by Confined Step-by-Step Electrochemical Catalytic Conversion Process for High Lithium Storage Capacity and Excellent Cyclability with a Low Variance Coefficient Ya-Qiong Jing, Jin Qu, Wei Chang, Qiu-Yu Ji, Hong-Jun Liu, Ting-Ting Zhang, and Zhong-Zhen Yu ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.9b12088 • Publication Date (Web): 15 Aug 2019 Downloaded from pubs.acs.org on August 18, 2019

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ACS Applied Materials & Interfaces

Cobalt Hydroxide Carbonate/Reduced Graphene Oxide Anodes Enabled

by

Confined

Step-by-Step

Electrochemical

Catalytic

Conversion Process for High Lithium Storage Capacity and Excellent Cyclability with a Low Variance Coefficient Ya-Qiong Jing,a,b Jin Qu,*,a,b Wei Chang,a Qiu-Yu Ji,a Hong-Jun Liu,b Ting-Ting Zhang,b and Zhong-Zhen Yu,*,a,b,c a

State Key Laboratory of Organic-Inorganic Composites, College of Materials Science and

Engineering, Beijing University of Chemical Technology, Beijing 100029, China b

Beijing Key Laboratory of Advanced Functional Polymer Composites, Beijing University of

Chemical Technology, Beijing 100029, China c

Beijing Advanced Innovation Center for Soft Matter Science and Engineering, Beijing

University of Chemical Technology, Beijing 100029, China *E-mail: [email protected] (J. Qu); [email protected] (Z.-Z. Yu)

ABSTRACT: Transition metal carbonates/hydroxides have attracted much attention as appealing anode materials due to their considerable reversible electrochemical catalytic conversion capacity. However, their serious positive or negative trends with cycles caused by the electrochemical catalytic conversion seriously affect their practical applications. Herein, novel one-dimensional cobalt hydroxide carbonate (CHC) nanomaterials are tightly anchored on reduced graphene oxide (RGO) sheets via a facile one-pot hydrothermal synthesis, forming surface confined domains to further restrict the electrochemical catalytic conversion process. 1

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The analysis on the cycled electrodes at varied potentials confirms that the added capacity of CHC arises from the step-by-step reversible reactions of Li2CO3 and LiOH under the electrochemical catalysis of Co metal generated by the conversion reaction of CHC. The reversible reaction of Li2CO3 is followed closely by that of LiOH in the discharge process, while the order is opposite in the charge process. Such a step-by-step electrochemical catalytic conversion process could confine each other to accommodate the volume change and avoid side reactions. The confined effect is further enhanced by limiting the width and length of the CHC which are determined by regulating the nucleation and growth of CHC on the surface of RGO, leading to an extraordinary cyclability. The optimized CHC/RGO hybrid maintains a high reversible capacity of 1110 mA h g-1 after 100 cycles at 0.1 A g-1, much higher than the theoretical value of CHC (506 mA h g-1) on the basis of the recognized conversion reaction. Furthermore, it keeps high reversible capacities of 755 mA h g−1 and 506 mA h g−1 after 200 cycles at 1 and 2 A g-1, respectively, exhibiting a high-rate cyclability with the lowest coefficient of variance of 9.4% among the reported. The confined step-by-step electrochemical catalytic conversion process facilitates high lithium storage capacity and satisfactory cyclability with a pretty low variance coefficient. KEYWORDS: cobalt hydroxide carbonate; electrochemical catalytic conversion; reduced graphene oxide; hybrids; cyclability 1. INTRODUCTION Lithium ion batteries (LIBs) have dominated the market from mobile electronics to electric vehicles by virtue of their high reversible capacity, long cycle life and benign environmental 2


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effect.1-3 Numerous attempts have been made to explore new anode materials with higher theoretical capacity, improved cycling stability and better rate performance, because of the limitation of the low theoretical capacity (372 mA h g-1) of commercialized graphite anodes.4-10 It is reported recently that transition-metal carbonates/hydroxides show much higher reversible capacities of up to 1000 mA h g-1, which surmounts the limitation of conventional conversion reactions and has a great potential for lithium storage on the basis of the electrocatalytic conversion mechanism.11-14 As a typical transition-metal carbonate, CoCO3 has been reported as a high-capacity lithium storage material.14-17 In view of the conversion reaction of CoCO3 and further electrocatalytic conversion reactions: CoCO3 + 2Li+ + 2e− ↔ Li2CO3 + Co and Li2CO3 + (4 + 0.5x) Li+ + (4 + 0.5x) e− ↔ 3Li2O + 0.5LixC2 (x = 0, 1, or 2), the theoretical capacity of CoCO3 can be as high as 1577 mA h g-1.16 Analogously, Co(OH)2 has the potential additional capacity based on the reaction described as LiOH + 2Li+ + 2e− ↔ Li2O + LiH.18 It is thus expected that a larger theoretical value of the transition-metal carbonate hydroxide may be acquired by combining CO32- with OH-, which could convert into Li2CO3 and LiOH during the conversion process. Zhou et al.19 synthesized Co2(OH)2CO3 nanosheets as lithium storage materials. The capacity decreased rapidly to below 600 mA h g-1 during the first 30 cycles and then increased to 800 mA h g-1, accompanied with a V-shape fluctuation during the discharge-charge process for 100 cycles at 0.2 A g-1. The phenomenon of cycling unstability is quite common in electrocatalytic conversion type anode materials.7,11,20,21 As one of the most important concepts in statistics, coefficient of variance is defined as the ratio of the standard deviation to the mean, which is appropriate to judge the cyclability. A high coefficient of 3



variance corresponds to a poor cyclability. As the reported anodes with the electrocatalytic conversion usually have high coefficients of variance over 10%, even close to 100%, it is necessary to stabilize the electrocatalytic conversion activities for cycling. The precise control of the electrocatalytic conversion process is an efficient approach to suppress the serious positive or negative trends upon battery cycles. In fact, one advantage of transition-metal hydroxide carbonate is that the electrocatalytic conversion potentials of Li2CO3 and LiOH are exactly different. For a spinel structure anode with the formula of AB2O4, such as CoFe2O4, its successively formed Co/Li2O and Fe/Li2O could serve as the matrix to disperse and confine each other, accommodating the volume change and avoiding side reactions. Analogous to this, the successively formed Li2CO3 and LiOH should have the same functions. In addition, the introduction of nanostructure14,19,22-25 and highly electrically conductive components16,23,26-33 are effective in conquering these electrochemical barriers. It is promising to enlarge the restricted surface area and shorten the lithium ion/electron diffusion path,19,34,35 as well as reduce the interfacial lithium ion/electron transfer resistance and minimize the volume expansion during discharge/charge processes,12,20,36,37 thus ensuring the improvements of capacity and cyclability. However, the two electrocatalytic conversion processes are controlled by the in situ formed Co metal, which means that the electrocatalytic efficiency of Co should be the key factor to regulate the cyclability. We envisage that the two electrocatalytic conversions are restricted to a certain extent. Though it would decrease the reversible capacities, the importance is that it could help to further attenuate the volume change and avoid terrible side reactions, leading to a satisfactory cyclability. Till now, there are rare 4


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studies on the confined step-by-step electrochemical catalytic conversion process to achieve high lithium storage capacity and excellent cyclability with a low variance coefficient in the transition-metal hydroxide carbonate anodes. Nanowires have attracted much attention due to their high length-to-diameter ratios and electronic

properties.38,39

Here,

we

synthesize

cobalt

hydroxide

carbonate

(CHC,

Co2(OH)2CO3) nanowires as anode materials of LIBs. The step-by-step electrocatalytic conversion reactions of Li2CO3 and LiOH in CHC are confirmed by analyzing the cycled electrodes at different voltage states. To further improve electrochemical performances of the CHC nanowires, varied sizes of the CHC nanowires are in situ anchored on the surface of reduced graphene oxide (RGO) with a one-step approach to form separated confined domains. The dosage of GO is the most important factor for forming separated confined domains. Firstly, the nucleation sites of CHC supplied by GO determine the final width and length of CHC, thus forming different confined domains to limit the electrochemical catalytic conversion process. Secondly, RGO could serve as a channel for the flow of electrons during the electrochemical process and a buffer to relieve the volume change preventing the structure collapse during repeated lithiation and delithiation processes. The optimized CHC/RGO (50 wt%) hybrid has an optimal morphology that the CHC nanowires are short in length and narrow in width, and attached uniformly on RGO sheets. The hybrid shows an initial charge capacity of 1168 mA h g-1 and retains a nearly unchanged reversible capacity of 1110 mA h g-1 after 100 cycles at the current density of 0.1 A g-1. Additionally, it exhibits a long cycling performance without obvious fluctuation and retains the capacity of 755 mA h g−1 and 506 mA 5



h g−1 after 200 cycles at the current densities of 1 and 2 A g-1, respectively. Compared to the representative cobalt-oxysalt based anodes,7,10,16,17,19,21 the CHC/RGO (50%) hybrid exhibits the best cyclability with the lowest variance coefficient of 9.4%. 2. EXPERIMENTAL SECTION 2.1. Materials. Cobalt nitrate hexahydrate (Co(NO3)2.6H2O) and urea were purchased from Xilong Scientific (China). Graphite flakes with an average diameter of 13 μm were provided by Huadong Graphite Factory (China). Sodium hydroxide (NaOH) and sulfuric acid (H2SO4, 98%) were supplied by Beijing Chemical Factory (China). All chemicals are analytical grade and were used without further purification. 2.2. Synthesis of CHC Nanowires. 1.25 mmol of Co(NO3)2.6H2O was dissolved in 50 mL of deionized water as solution A, and 1.25 mmol of urea was dissolved in 25 mL of deionized water as solution B. After stirring the solution A for 30 min, the solution B was added dropwise. The mixture was further stirred for 1 h and then transferred into a 100 mL Teflon-lined stainless steel autoclave for hydrothermal synthesis at 150 oC for 2 h. The resultant precipitate was collected by centrifuging, washed with deionized water and ethanol five times, and dried at 60 oC for 12 h. 2.3. Syntheses of CHC/RGO Hybrids. Graphite oxide (GO) was prepared by a modified Hummers method. CHC/RGO hybrids were prepared with different GO contents of 30, 40, 50 and 60 wt% on the basis of the initial amount of GO. In a typical procedure, GO was dispersed in 30 mL of deionized water by ultrasonication for 30 min, and 20 mL of Co(NO3)2.6H2O aqueous solution (62.5 mM) was then added dropwise and stirred for another 30 min to get a 6


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mixed solution. 1.25 mmol of urea was dissolved in 25 mL of deionized water and the resultant solution was added dropwise to the above mixed solution. The mixture was stirred for 1 h and then transferred into a 100 mL Teflon-lined stainless steel autoclave for hydrothermal reaction at 150 oC for 2 h. The resulting product was collected by centrifuging, washed with deionized water and ethanol for five times, and dried at 60 oC for 12 h. For comparison, GO was treated with a similar hydrothermal process to obtain RGO. 2.4. Characterization. Morphologies and microstructures were observed with a Hitachi S4700 field emission scanning electron microscope (SEM) and a JEOL JEM-3010 transmission electron microscope (TEM). X-ray diffraction (XRD) measurements were conducted using a Rigaku D/Max 2500 X-ray diffractometer with a Cu Kα radiation (λ=0.154 nm) at a generator voltage of 40 kV and a generator current of 40 mA. Thermogravimetric analysis (TGA) measurements were carried out on a TA Instruments Q50 thermogravimetric analyzer at a heating rate of 5 oC min−1 in an air atmosphere. CHC and CHC/RGO hybrids were characterized with a Thermo VG RSCAKAB 250X high-resolution X-ray photoelectron spectroscopy (XPS), a Renishaw inVia Raman microscopy and a Nicolet Nexus 670 Fourier-transform infrared spectroscopy (FT-IR). To confirm the confined step-by-step electrocatalytic conversion process, XPS and TEM measurements were used on the electrodes, which were disassembled from the cell, washed with dimethyl carbonate solvent in an argon-filled glove box, and then dried and sealed under vacuum condition. For the ex-situ XPS, the samples were pretreated by argon-ion sputtering with the etching thickness of 40 nm to expose the inner component. 7



2.5. Electrochemical Performances Measurements. Electrochemical measurements were carried out using coin-type cells assembled in an argon-filled glove box at room temperature. The working electrode composing of 70 wt% of active material, 20 wt% of carbon black (Super-P) and 10 wt% of carboxymethyl cellulose was fabricated by coating these components on a Cu foil current collector and drying at 80 oC in a vacuum oven overnight. The mass loading of the active material is approximately 1.2 ± 0.1 mg cm−2. The capacity of CHC/RGO hybrid was calculated on the whole weight of the active material including CHC and RGO in the hybrid. The cells were assembled using lithium foil as the counter electrode, glass fabric (Whatman GF/A) as the separator, and 1 M LiPF6 dissolved in a mixture of ethylene carbonate (EC), dimethyl carbonate (DMC) and diethyl carbonate (DEC) with a volume ratio of 1:1:1 as the electrolyte. Cyclic voltammograms (CV) were recorded on a CHI 660E electrochemical workstation at a scanning rate of 0.1 mV s-1 between 0.01 and 3.00 V (vs. Li/Li+). Electrochemical impedance spectra (EIS) were performed on the workstation in the frequency of 100 kHz to 0.01 Hz with an AC voltage of 5 mV. The cycling performance and rate capacity of the assembled cells were carried out on a Land CT-2001A battery tester at different current densities and in the voltage range of 0.01 to 3.0 V. 3. RESULTS AND DISCUSSION Figure 1a shows XRD patterns of CHC and CHC/RGO hybrids. Nearly all the diffraction peaks can be well-indexed to crystalline cobalt hydroxide carbonate, which is in accordance with the literature.40 The CHC/RGO hybrids show almost the same diffraction patterns as CHC with major diffraction peaks at 17.5o, 33.8o, 35.5o 39.5o and 47.3o corresponding to (201), 8


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(012), (310), (501) and (601) planes of CHC, respectively. The intensities of the major peaks decrease with the increase of the GO content from 30 to 60 wt%. Concurrently, the characteristic peak of RGO is observed at ~23.9° only in the CHC/RGO (50%) and CHC/RGO (60%) hybrids due to the relatively increased RGO contents in these hybrids. Generally, the combination of RGO component does not change the crystallographic structure of CHC, which is extremely crucial because CHC plays the leading role in contributing to the electrochemical capacity.

Figure 1. (a) XRD patterns of CHC and CHC/RGO hybrids. SEM images of (b) CHC (Inset: TEM image of CHC), (c) CHC/RGO (30%), (d) CHC/RGO (40%), (e) CHC/RGO (50%) (Inset: TEM image of CHC/RGO (50%) hybrid), and (f) CHC/RGO (60%) hybrids.

9



The presence of RGO is not only to improve the lithium ion reaction kinetics and promote the dispersion of CHC nanowires, but also to regulate the nucleation and growth of CHC. The rich negative functional groups of GO could adsorb positive cobalt ions to form C-O-Co bonds as the chemical sites for the nucleation of CHC, thus controlling the morphology and microstructure of the hybrids. Figure 1b-f show SEM images of CHC and CHC/RGO hybrids with different RGO contents. It is seen that large quantities of CHC nanowires with 100 nm in width and a few microns in length tend to stack randomly (Figure 1b). At the low GO dosage of 30%, dense CHC nanowires with short length (0.8-1.5 µm) and narrow width (70-80 nm) are packed among RGO sheets (Figure 1c). At a higher GO dosage of 40%, many small CHC nanowires are anchored on the RGO sheets, while some isolated CHC nanowires are observed around the RGO sheets (Figure 1d). The width and length of the CHC nanowires are further shortened to 40-60 nm and 600-900 nm, respectively. When the dosage of GO increases to 50%, almost all of the CHC nanowires are uniformly dispersed on RGO sheets and there are some voids existing among individual CHC nanowires to form separated domains (Figure 1e). Remarkably, the CHC nanowires have smaller sizes with 20-40 nm in diameter and 200-500 nm in length in the hybrid, which indicates that the RGO substrate not only effectively prevents CHC nanowires from aggregating, but also reduces the size of nanowires, leading to a shorter transport route of lithium ions/electrons and a rapid equilibrium for the electrocatalytic conversion process. The lattice fringes in two regions with spacings of 2.53 Å and 1.99 Å (Figure 1e, inset) could be assigned to (310) and (601) planes of CHC, respectively, which are consistent with the XRD results. Further increase of the GO dosage to 60% causes more 10


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exposure of the RGO surface, and the CHC becomes sparse dots instead of nanowires (Figure 1f). At a certain amount of cobalt salt, more dosage of GO would uniformly adsorb the cobalt ions, then decrease the amount of CHC per RGO sheet. It means that inadequate GO could not well anchor and disperse CHC nanowires, while excessive GO would produce superfluous chemical sites that make the shape of CHC not nanowire-like. It is also obvious that the interfacial interaction could be tuned by constructing different amounts of interfacial bonds, which could affect the confined effect of every separated domain. To characterize the changes in composition and microstructure of the hybrids, Figure 2a shows FT-IR spectra of CHC, RGO and CHC/RGO hybrids. The peak of CHC at 3500 cm-1 is ascribed to the stretching vibration of O-H, indicating the presence of the Co-OH layer, while the peak at 3380 cm-1 verifies the interconnection between CO32- and O-H groups.40,41 The following characteristic peaks of ν(OCO2) (1514 cm-1), ν(CO3)/C−O (1385 cm-1), ν(C=O) (1068 cm-1), δ(CO3) (831 cm-1), δ(OCO) (742 cm-1), and ρ(OCO) (669 cm-1) demonstrate the existence of CO32- in the synthesized CHC nanowires.19,42 The two absorption bands at 962 and 515 cm-1 can be indexed to δ(Co–OH) bending mode and ρw(Co–OH).40 The spectrum of RGO is taken for comparison. It is seen that the absorption bands at 3439 and 1630 cm-1 correspond to O-H stretching vibration and C=C stretching vibration, respectively. The peak at 1400 cm-1 relates to O–H bending vibration from hydroxyl groups.43,44 The weak peaks around 1211 and 1105 cm-1 imply the presence of epoxide and alkoxy C-O bonds,43,45 owing to the residual oxygen-containing groups of RGO. For the CHC/RGO hybrids, nearly all the peaks are in

11



accordance with neat CHC. Note that the peak at ~1105 cm-1 is diminished, indicating the further reduction of RGO.

Figure 2. (a) FT-IR spectra and (b) Raman spectra of CHC, RGO and CHC/RGO hybrids. (c) TGA curves of CHC and CHC/RGO hybrids. (d) C 1s XPS spectra of CHC, GO and CHC/RGO (50%) hybrid. (e) O 1s XPS spectra of CHC, GO and CHC/RGO hybrids. Raman spectra of CHC, RGO and CHC/RGO hybrids are characterized to further confirm the structural change during the hydrothermal process (Figure 2b). RGO and CHC/RGO hybrids display two distinct peaks at approximately 1357 and 1600 cm-1 relating to D band and G band, respectively. The intensity ratio of D band and G band (ID/IG) is used to represent the disorder of carbon materials.43,46 All the CHC/RGO hybrids have higher ID/IG values than that of RGO, suggesting that the combination of CHC with RGO introduces structural defects to make RGO sheets more disordered. These defects could bring more lithium storage sites, 12


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benefiting better electrochemical performances of the CHC/RGO hybrids. In addition, the peaks at 483, 522, 690 and 1069 cm-1 observed in the spectra of CHC and CHC/RGO, correspond to the Raman active modes of CHC,47,48 further proving the formation of CHC/RGO hybrid. The RGO contents in the hybrid are estimated on the basis of the TGA results (Figure 2c). The mass loss of neat CHC mainly results from dehydroxylation and decarbonation,42,49 while the distinct mass loss of CHC/RGO is attributed to the combustion of RGO, and simultaneous removal of hydroxyl and carbonate anions. Their mass losses are 28.6%, 34.4%, 48.1% and 56.5% for CHC/RGO (30%), CHC/RGO (40%), CHC/RGO (50%), and CHC/RGO (60%), respectively. To further study the chemical compositions of the hybrids, Figure S2a shows wide-survey XPS spectra of CHC and CHC/RGO (50%) hybrid, proving the presence of carbon, oxygen, and cobalt elements. The XPS spectra of Co 2p (Figure S2b) present two main peaks of Co 2p3/2 (781.2 eV) and Co 2p1/2 (797.2 eV), along with matching satellite peaks of 785.5 and 802.8 eV, indicating the characteristic of Co2+.17,50 In the C 1s XPS spectra (Figure 2d), the peaks at 284.6, 285.7, 287.5 and 289.6 eV are related to C–C, C–O, C=O and CO32-, respectively.14,15,17 The peaks for the oxidized groups decrease obviously from GO to CHC/RGO (50%) hybrid, indicating the gradual reduction of GO to RGO during the hydrothermal synthesis. As shown in Figure 2e, the two peaks of CHC at 531.9 and 531.2 eV are attributed to the binding of oxygen to carbonate and hydroxyl units, respectively.49 Compared to bare CHC and GO, the new peak at 531.5 eV in the CHC/RGO hybrid is attributed to the possible formation of Co–O–C bond,50,51 revealing the strong chemical 13



interaction between CHC and RGO. According to the estimated peak area percentages of Co–O–C, the bond proportions of CHC/RGO (30%), CHC/RGO (40%), CHC/RGO (50%) and CHC/RGO (60%) are 0.6%, 2.5%, 19.1% and 14.9%, respectively. Combining with the discussion in Figure 1, these results further confirm that the size and morphology of CHC are determined by the interfacial bonds serving as the nucleation sites for the growth of CHC. Moreover, the tunable interfacial interaction could block the distribution and growth of CHC to form separated confined domains, thus accommodating the volume change and avoiding side reactions. Especially, it could further confine the two electrochemical catalytic conversion reactions of Li2CO3 and LiOH to retain a stable reversibility and a good cyclability. Figure 3a-c shows representative charge/discharge curves of CHC, CHC/RGO (50%) hybrid and RGO. Their first discharge curves are different from those of other cycles, because of the formation of SEI films and the electrolyte degradation.7,16 The initial discharge/charge capacity of CHC is 1695/1022 mA h g-1 with a Coulombic efficiency of 60 % (Figure 3a, S3a). Nevertheless, the reversible capacity decreases to only 60 mA h g-1 after 100 cycles (Figure 3e), owing to the intrinsic poor electrical conductivity (Figure 3d). Compared to neat CHC, the first discharge/charge capacity of the CHC/RGO (50%) hybrid is 1628/1168 mA h g-1, and the subsequent discharge/charge curves are overlapping (Figure 3b). The enhanced cyclability might depend on a strong synergistic effect between RGO and CHC. Differently, the RGO delivers the lowest initial discharge/charge capacity of 376/84 mA h g-1 with a low coulombic efficiency of 22 %, and no obvious voltage plateau is observed in discharge/charge curves (Figure 3c). However, proper amount of RGO could help improve the electrochemical 14


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performances. Figure 3d shows EIS curves of CHC and CHC/RGO hybrids, which generally consist of semicircle shapes in the high to the medium frequency region and inclined lines in the low frequency region. The diameter of the semicircle is assigned to the charge transfer resistance of the electrode/electrolyte interface, whereas the slope of the inclined line corresponds to the diffusion resistance of lithium ions in the electrodes.23,46 The CHC/RGO (50%) reveals the smallest semicircle diameter than others, implying the lowest charge transfer resistance. It also shows the highest slope value of the inclined line, indicating the fastest lithium ion diffusion rate. The results demonstrate that the RGO sheets effectively improve the conductivity of the hybrids and facilitate the lithium ion diffusion.

Figure 3. Discharge/charge curves of (a) CHC, (b) CHC/RGO (50%), and (c) RGO at the current density of 0.1 A g-1 in a voltage window of 0.01-3.0 V (vs. Li/Li+). (d) Nyquist plots of CHC and CHC/RGO hybrids. (e) Cycling performances of CHC and CHC/RGO hybrids at the 15



current density of 0.1 A g-1, and Coulombic efficiency of CHC/RGO (50%) hybrid. (f) Rate performances of CHC and CHC/RGO hybrids at various rates of 0.1–2 A g−1. Figure 3e presents the cycling performances of CHC/RGO hybrids with different RGO contents at a current density of 0.1 A g-1. All CHC/RGO hybrids exhibit much better cycling abilities than neat CHC. At low RGO contents, CHC/RGO (30%) and CHC/RGO (40%) deliver the first discharge/charge capacities of 1752/1186 and 1660/1202 mA h g-1, respectively. However, they suffer from a reversible capacity fading and reach ~130 and ~602 mA h g-1 after 100 cycles, respectively. This is because the small amount of RGO plays a less efficient role in the distribution of CHC nanowires, and the effective chemical bonding between CHC and RGO. The CHC/RGO hybrid with 50 wt% of RGO exhibits the best electrochemical performance, and its reversible specific capacity stabilizes at 1110 mA h g-1 after 100 cycles, with a capacity retention rate of 95%. On the basis of the cycling performances at the low current density, the capacity is highly competitive among the representative Co-based anode materials (Table S1).7,10,13-17,19,21 Such excellent cycling performance of the CHC/RGO (50%) corresponds to an optimal morphology with a forceful synergistic effect between CHC nanowires and RGO sheets in the composite. The well separated nanowires called confined domains could effectively stabilize the two electrochemical catalytic conversion, while suitable RGO conductive component and interfacial bonds improve electron transfer rate and enhance the lithium ion reaction kinetics effectively. When the RGO content increases to 60 wt%, the initial discharge/charge capacity is 1559/1091 mA h g-1, and the reversible capacity declines to 838 mA h g-1 after 100 cycles. 16


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Although there is still separated CHC, too high content of RGO with low cycling capacity causes the suppressed capacity (Figure S3b). Figure 3f shows rate performances of CHC and CHC/RGO hybrids at various rates of 0.1–2 A g-1. Similar to the cycling performances, the CHC/RGO (50%) exhibits a higher rate performance than those of CHC and other CHC/RGO hybrids. Its reversible capacities are 1128, 993, 846, 710 and 577 mA h g-1 at increased current densities of 0.1, 0.2, 0.5, 1, to 2 A g-1, respectively. When the current density returns to 0.5 and 0.1 A g-1, the specific capacity recovers to 829 and 1053 mA h g-1 with recovery rates of 98.0% and 93.4%, respectively. These results confirm the advantage of the separated confined domain structure. As described above, the largest discharge capacities of CHC (1695 mA h g-1) and CHC/RGO (50%) hybrid (1628 mA h g-1) are far more than the theoretical value (506 mA h g-1) on the basis of the Reaction 1, but very close to the exceptional capacity (1645 mA h g−1) according to the Reactions 2 and 3. These multistep reactions can be described as the following: Co2(OH)2CO3 + 4Li+ + 4e− ↔ 2Co + Li2CO3 + 2LiOH

(1)

Li2CO3 + (4 + 0.5x)Li+ + (4 + 0.5x)e− ↔ 3Li2O + 0.5LixC2 (x = 0, 1, or 2)

(2)

LiOH + 2Li+ + 2e− ↔ Li2O + LiH

(3)

To clarify these electrochemical redox reactions, the representative first charge and second discharge profiles with the matching differential capacity versus voltage (dQ/dV) curves are depicted in Figure 4a-b. The first charging profile is divided into several parts (A-B, B-C and C-D) according to the main peaks at 1.14 and 2.10 V in the corresponding dQ/dV curve. Likewise, on the basis of reverse reactions at the voltage positions of 0.95 and 1.50 V in 17



the second discharging dQ/dV curve, the second discharging profile is marked as D-E, E-F, F-G and G-H parts. To better illustrate lithium storage mechanisms of the CHC/RGO (50%), XPS spectra are used to analyze the chemical bonds of elements and estimate the contents of each bonds in cycled anodes at critical electrochemical states (A-H). In the C 1s XPS spectra (Figure 4c), the point A represents the beginning of initial charge, namely, the completion of initial discharge process, where CO32- is particularly concentrated in Li2CO3 at 289.1 V. As the first charging process from voltage state A to D, the peak position of CO32- gradually returns to 289.6 V, which exactly agrees on CO32- in the pristine CHC/RGO (50%) hybrid (Figure 2d). The opposite shift of the CO32- peak from 289.6 to 289.1 V can be observed during the subsequent discharging process from voltage state E to H (Figure 4d). This is a significant evidence for conversion reaction and its reversibility (Reaction 1). In addition, at the critical electrochemical state A, the carbon content of CO32- is the lowest (15.1%), and conversely the peaks at 284.6, 285.7 and 287.5 V related to C–C, C–O and C=O exhibit the highest carbon content of 58.4%, 24.8% and 4.4%, respectively. With the progress of the first charging from electrochemical state A to D, the carbon content of CO32- increases to 28.6%, while those of C–C, C–O and C=O gradually decrease to 42.8%, 23.2% and 2.6%, respectively. Oppositely, it can be found in the following discharge process from electrochemical state E to H, the carbon content of CO32- gradually falls back to 13.4%, along with the carbon content of C–C, C–O and C=O rising to 57.4%, 24.6% and 4.6%, respectively. These changes of the carbon content during a full cycle prove the existence of electrocatalytic conversion reaction (Reaction 2), that

18


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is, CO32- in CHC can reversibly transform to low chemical valence state of carbon species (LixC2, x = 0, 1 or 2).

Figure 4. (a) First charge and second discharge profiles of CHC/RGO (50%) hybrid. (b) Corresponding dQ/dV curves of CHC/RGO (50%) hybrid. C 1s XPS spectra during (c) first charge process (corresponding to A-D) and (d) second discharge process (corresponding to 19



E-H) of CHC/RGO (50%) hybrid. (e) A comparison of the estimated peak area percentages of CO32-, C–C, C–O and C=O at different voltage states from A to H. Figure 4e provides a comparison of the relative area percentages of C 1s peaks at different electrochemical states, representing the reversible electrochemical conversion process of CHC during a full delithiation-lithiation cycle. The carbon content of CO32- shows the most obvious change in the electrochemical regions of B-C (12.4%) and F-G (7.3%). Simultaneously, the C-C bond also has evident carbon content changes of 12.5%, and 7.2% in the electrochemical regions of B-C and F-G, respectively. However, the carbon contents of C–O and C=O have a small change of 1.6% and 1.8% in the first charge stage of A-D, respectively, corresponding to 1.4% and 2.0% in the following discharge stage of D-H, respectively. The dominant changes of B-C and F-G voltage regions indicate that the reversibly reduction of CO32- mainly relates to C-C bond based species, preferring to occur in lower voltage regions corresponding to the first charging plateau (1.14 V) and the second discharging plateau (0.95 V). According to the difference (1.7%) between the carbon content of CO32- at the electrochemical state A (15.1%) and H (13.4%), the reduction of CO32- can be partially irreversible, leading to the loss of capacity during the initial several cycles (Figure 3e). Figure 5a and b shows five main peaks at 55.4, 55.0, 54.5, 54.3 and 53.6 eV related to Li2CO3, LixC2, LiOH, LiH and Li2O, respectively.18, 54 Almost no Li 1s peak is detected at the voltage points of D and E, resulting from the end and start of the conversion reaction of CHC/RGO (50%) hybrid, respectively. As shown in Figure 5c, with the first charging process from voltage state A to C, the contents of Li2O, LiH and LixC2 taper off from 20.1% to 1.4%, 20


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18.7% to 2.7% and 12.1% to 2.7%, respectively; Whereas the contents of LiOH and Li2CO3 increases from 8.9% and 40.2% to 35.5% and 57.7%, respectively. Strikingly, LiOH has a major change during the region of A-B (24.9%), and a little change (1.7%) during the B-C region. On the contrary, the content change of Li2CO3 primarily occurs in the voltage stage of B-C (15.1%) rather than A-B (2.4%).

Figure 5. XPS spectra of Li 1s during (a) the first charge process (corresponding to A-D) and (b) the second discharge process (corresponding to E-H) of CHC/RGO (50%) hybrid. (c) A comparison of the estimated peak area percentages of Li2CO3, LixC2, LiOH, LiH and Li2O at different voltage states from A to H. 21



As a reverse process from F to H, the contents of Li2O, LiH and LixC2 gradually build up to 17.8%, 16.0% and 17.8%, respectively, and synchronously, the contents of LiOH and Li2CO3 decrease to 10.2% and 38.2%, respectively. The prime peak content changes of LiOH and Li2CO3 appear in the regions of G-H (23.1%) and F-G (18.9%), respectively. Clearly, the electrocatalytic conversion reactions of Li2CO3 and LiOH (Reactions 2 and 3) take place step-by-step in the voltage states (A-C and F-H). The electrocatalytic conversion reaction of Li2CO3 mainly occurs in the voltage platform regions of B-C and F-G, while that of LiOH tends to be in the lower voltage regions of A-B and G-H. XRD is used to characterize the crystal phases of materials (Figure S5). However, the XRD pattern of the CHC/RGO (50%) hybrid at various voltage states (points A, D, and H) turns to broad peaks undergoing first charge and second discharge process. It is difficult to distinguish the crystal phases of charge/discharge products based on the XRD results. The aforementioned electrochemical process is further confirmed by high-resolution TEM observation (Figure 6a-c). When the initial discharging is completed (voltage state A), Figure 6a shows the discharged product with clear lattice fringes. The lattice distances are 2.11 Å for Co (100), 2.81 Å for Li2CO3 (002), 2.68 Å for Li2O (111), 2.57 Å for LiC (032), and 3.60 Å for Li2C2 (011), indicating that the product of the conversion reaction (Li2CO3) can be decomposed to Li2O and Li2C2 (or LiC) with deep discharge. Besides, the RGO thin layer with a lattice spacing of 3.39 Å is seen on the edge of the crystal particles. As the first charging ends up to voltage state D, the CHC are observed (Figure 6b), exhibiting two sets of lattice fringes with the spacings of 2.27 and 2.41 Å, assigned to (414) and (501) planes of CHC, respectively. Due to the partial 22


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irreversibility in CHC, metallic Co with a lattice distance of 2.02 Å is also observed. After the second discharging to voltage state H, there is no CHC observed (Figure 6c), which is reduced to Li2O and Li2C2 again, with the remaining of intermediate of Li2CO3. To make the observed lattice distance more credible, several high-resolution TEM images are taken for analysis, and clear regions are chosen to get the lattice distances (Figure S6). These results support the supposition on the exceptional capacity of CHC. The extra capacity results from the reversible transformation of anions (CO32- and OH-) under the electrochemical catalysis of Co metal generated by the conversion reaction of CHC (Figure 6d).

Figure 6. TEM images of CHC/RGO (50%) hybrid at various voltage states: (a) point A, (b) point D, and (c) point H. (d) Illustration of the electrochemical catalytic conversion process of CHC/RGO (50%) hybrid. Cycling performances of CHC/RGO hybrids during the electrochemical steps of (e) A-B, (f) B-C, and (g) C-D. 23



The reversibility of every electrochemical step (A-B, B-C and C-D) of CHC/RGO hybrids is investigated (Figure 6e-g). Nearly all the hybrids exhibit good cyclability, except the CHC/RGO (30%) with a sharp decline with cycles. Interestingly, the reversible capacities resulting from the transformation of LiOH are not obviously related to the RGO contents (40-60%) (Figure 6e), as those of Li2CO3 and CHC do, which may be because the electrochemical catalytic conversion of LiOH is much easier than the other two. Therefore, the designs of microstructure and components are beneficial for the electrochemical conversions of Li2CO3 and CHC than that of LiOH (Figure 6f, g). Compared to other works with graphene as the conductive substrate, our original intention is that the generated Co–O–C bonds between CHC and RGO serve as nucleation sites for determining the final width and length of CHC, thus forming separated confined domains to limit the electrochemical catalytic conversion process of Li2CO3 and LiOH to obtain a good cyclability. Here, the CHC/RGO (50%) with well separated CHC nanowires could well confine these electrochemical conversions to a certain extent due to the strong interfacial interaction (Figure 2e). Thus, the confined effect would further attenuate the volume change and avoid terrible side reactions, leading to a satisfactory cyclability. As long-term and high-rate cyclic stability is an important indicator for evaluating electrochemical properties of electrode materials, the CHC and CHC/RGO hybrids are tested at high rates of 1 and 2 A g-1 for 200 cycles (Figure 7a and b). At the current density of 1 A g-1, the reversible capacities of CHC, CHC/RGO (30%), CHC/RGO (40%), CHC/RGO (50%) and CHC/RGO (60%) are kept at 52, 399, 551, 755 and 485 mA h g−1 after 200 cycles, 24


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respectively. Similarly, at 2 A g-1, the reversible capacities of CHC, CHC/RGO (30%), CHC/RGO (40%), CHC/RGO (50%) and CHC/RGO (60%) retain at 85, 105, 355, 506 and 360 mA h g−1 after the 200 cycles, respectively. Obviously, the CHC/RGO (50%) hybrid exhibits the best cycling performance without distinct fluctuation during the 200 cycles, fully proving that a stable electrocatalytic conversion process can be actualized with the confined effect.

Figure 7. High-rate performances of CHC and CHC/RGO hybrids at the current densities of (a) 1 A g-1 and (b) 2 A g-1. (c) Comparison of coefficients of variance between CHC/RGO (50%) hybrid and representative cobalt-oxysalt based anodes reported in the literature. It has been reported that many cyclic curves involve a large fluctuation with increasing the number of cycles. Some reversible capacities suffered from a gradual decline,15,17 while others kept a gradual rising.7,10,19,52,53 Both cases could severely impact the cycle life and safety of batteries. As one of the most important concepts in statistics, the coefficient of variance is innovatively used to compare high-rate cyclic stability of the CHC/RGO (50%) hybrid with those of previously reported cobalt-oxysalt based anodes at the same current density of 1 A g-1. 25



Coefficient of variance is defined as the ratio of the standard deviation to the mean, and the higher the number is, the poorer the cyclic stability is. As shown in Figure 7c, the coefficient of variance of CHC/RGO (50%) is 9.4% belonging to the low intensity (below 10%), which is lower than those of other typical anodes.7,10,16,17,19,21 In a word, the confined domain effect with strong interfacial interaction results in the improved cyclability. 4. CONCLUSIONS Separated CHC nanowires as the confined domains are carefully anchored on the surface of RGO due to the tunable strong interfacial interaction by a facile one-pot hydrothermal synthesis. The step-by-step electrochemical catalytic conversion of Li2CO3 and LiOH are investigated and confirmed in detail with ex situ XPS and TEM. In the discharge process, the conversion of Li2CO3 occurs at 0.95 V followed by the conversion of LiOH at 0.01V, while the order is reversed in the charge process. The step-by-step electrochemical catalytic procedure could restrict the two conversions for each other, relieve the volume change and avoid serious side reactions. Moreover, the separated CHC nanowires as the confined domains could further boost the confined effect, because of the strong interfacial interaction. Additionally, appropriate quantity of flexible RGO sheets could well disperse CHC nanowires and reduce their size, so as to expose more lithium storage sites and shorten ions/electrons transport path. The introduced interfacial bonds between CHC and RGO significantly enhance the electron transfer rate and lithium ion reaction kinetics, thus accelerating the electrocatalytic conversion process. As a result, the optimized CHC/RGO (50 wt%) hybrid maintains a high reversible capacity of 1110 mA h g-1 after 100 cycles at the current density of 0.1 A g-1, much higher than 26


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the theoretical value of CHC (506 mA h g-1). Furthermore, it retains the reversible capacity of 755 and 506 mA h g−1 after 200 cycles at the current densities of 1 and 2 A g-1, respectively, and exhibits a high-rate cyclability with the coefficient of variance of 9.4%, which is much smaller than reported. This work offers a valuable thought for designing electrocatalytic conversion type anodes with long cyclability, rather than simply targeting the exceptional capacity. ASSOCIATED CONTENT Supporting Information TEM images of CHC/RGO hybrids; survey scan and Co 2p XPS spectra of CHC and CHC/RGO; Coulombic efficiencies of CHC and CHC/RGO hybrids; cycling performances of RGO; CV curves of CHC and CHC/RGO; comparison of electrochemical performances. The Supporting Information is available free of charge on the Internet. AUTHOR INFORMATION Corresponding authors: E-mail: [email protected] (J. Qu), [email protected] (Z.-Z. Yu) ACKNOWLEDGMENTS Financial support from the National Natural Science Foundation of China (51402012, 51533001),

the

National

(2016YFC0801302),

and

Key State

Research Key

and

Laboratory

Development of

(oic-201801002) is gratefully acknowledged. REFERENCES 27


Program

Organic-Inorganic

of

China

Composites


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