rGO nanocomposite anode with large capacitive contribution for high-efficiency and durable lithium storage

Journal Pre-proofs Full Length Article A new CoO/Co2B/rGO nanocomposite anode with large capacitive contribution for high-efficiency and durable lithium storage Jiangmei Yin, Pengxiao Sun, Guangmeng Qu, Guotao Xiang, Peiyu Hou, Xijin Xu PII: DOI: Reference:

S0169-4332(19)33514-7 https://doi.org/10.1016/j.apsusc.2019.144698 APSUSC 144698

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Applied Surface Science

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27 August 2019 22 October 2019 14 November 2019

Please cite this article as: J. Yin, P. Sun, G. Qu, G. Xiang, P. Hou, X. Xu, A new CoO/Co2B/rGO nanocomposite anode with large capacitive contribution for high-efficiency and durable lithium storage, Applied Surface Science (2019), doi: https://doi.org/10.1016/j.apsusc.2019.144698

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A new CoO/Co2B/rGO nanocomposite anode with large capacitive contribution for high-efficiency and durable lithium storage Jiangmei Yin, Pengxiao Sun, Guangmeng Qu, Guotao Xiang, Peiyu [email protected], Xijin [email protected] Functional Micro/nano Materials and Devices Lab, School of Physics and Technology, University of Jinan, 336 Nanxinzhuang West Road, Jinan 250022, P. R. China

Abstract Transition metal oxides (TMOs) with high specific capacities are considered to be alternative anodes for up-to-date lithium-ion batteries. However, the storage mechanism of TMOs-based anodes usually refers to conversion and alloying reactions, which cause the low initial Coulombic efficiency and fast capacity decay. Here, a new proof-of-concept nanocomposite of oxides-borides combined with reduced graphene oxides (CoO/Co2B/rGO) is proposed to regulate the storage mechanism of TMOs-based anodes. Cyclic voltammogram demonstrates the increasing proportion ranging from ~60% to 90% for the capacitive contribution at varied scan rates for this nanocomposite. As expected, the as-prepared CoO/Co2B/rGO nanocomposite exhibits an enhanced initial Coulombic efficiency together with cycling stability probably for the reduced Li+ diffusion contribution

occurred in conversion and alloying reactions. These findings give a new insight into improving the lithium storage property of TMOs-based anodes for LIBs. Keywords: Cobalt-based anode; Nanocomposite; Boride; Lithium storage 1. Introduction Recently, lithium-ion batteries (LIBs) represent the very convincing choice for both electrical vehicles (EVs) and large-scaled electric storage grids.[1-3] Furthermore, LIBs with higher energy/power densities and longer cycle life are urgently needed to meet the potential applications of the devices.[4-6] At present, carbon-based materials are broadly put into use as anodes in LIBs. However, the unstable solid electrolyte interphase (SEI) [7-9] and lithium dendrites are generally formed, which will lead to short circuits or safety issues. Besides, low theoretic capacity (372 mAh g-1) also limits the improvement of energy density of LIBs.[10-13] Thus it is urgent to develop anodes with high capacities and cycle lifetime for the commercialization of LIBs. Transition metal oxides (TMOs), such as Co3O4,[14] SnO2,[15] Fe3O4,[16] NiO[17, 18] and CuO,[19] have been broadly studied as an alternative anode material in the past decade for the high capacity, wide availability. However, the storage mechanism for TMOs-based anodes usually refers to conversion and alloying reactions, which causes the large volume change and further induces the fast capacity decay.[20] Many strategies are devised to cope with this concern, including combined TMOs with conductive carbon or metal materials, constructing metal-organic frameworks (MOFs) or various morphologies on the nanoscale.[21-23]

In addition to the poor cycling stability, TMOs-based anodes also show a very low initial Coulombic efficiency (ICE) because of the special reaction mechanism and the thick SEI formed at initial cycle.[24, 25] Note that the low ICE will consume a large amount of Li+ derived from cathode and further decreases the energy density and electrochemical properties of full cells. Many efforts, such as adding prelithiation additives (e.g. Li donors or transition metals), have been made to mitigate the problem for TMOs-based anodes.[26-28] But the preparation of prelithiation additives is complicated in that the compounds are very sensitive to moisture and oxygen. Overall, it is still a great challenge to prepare TMOs-based anodes with large specific capacity, superior cycle stability together with high ICE. Recently, the 2D graphene (oxides) with superior mechanical strength and electron conductivity

are

applied

for

the

performance

improvement

of

batteries

electrodes.[29-31] In particular, graphene oxides (GO) with good dispersibility in water solution are widely utilized to anchor TMOs primary grains and build reduced graphene oxide (rGO) based composite.[32] The rGO sheets can create a mechanical cushion to limit the volume change of TMOs-based anodes during discharge/charge processes. Moreover, TMOs primary grains on rGO surfaces or between the rGO layers will induce high lithium-storage capacity for the synergistic effects created by interfacial interactions. Therefore, the rational design and simple preparation of the rGO-based composite are crucial for enhancing the electrochemical properties of TMOs-based anodes.

It is well known that the reversible capacity of TMOs-based anodes is attributed to the capacitive and diffusion contribution.[33] Note that the diffusion contribution is related to the conversion and alloying reactions that generally cause the large volume change. The contribution of capacitive comparable to that of intercalation effectively offsets the poor kinetics of Li-ion transport and hence significantly improves the capacity and rate performance.[26] Then, the improvement of capacitive contribution will be beneficial to improve the ICE and cycling stability of TMOs-based anodes. Considering transitional metal borides have exhibited superior electrochemical properties in the field of electrocatalysis and supercapacitor because of their high reactivity and conductivity.[34-35] Besides, the formation of biphasic composite can prevent the growth of crystal growth, which is helpful for the preparation of nano-sized primary grains and further improves capacitive contribution. In this work, a new, proof-of-concept CoO/Co2B/rGO nanocomposite is proposed to regulate the storage mechanism of TMOs-based anodes. We develop a facile and mild route to prepare the designed CoO/Co2B/rGO nanocomposites, as shown in Scheme 1, in which rGO sheets with large surface area can provide sufficient anchor sites for nanoparticles and buffer the volume expansion upon cycling. Cyclic voltammogram demonstrates of the increasing capacitive contribution from ~60% to 90% at varied scan rates for this nanocomposite. As expected, the as-prepared CoO/Co2B/rGO nanocomposite exhibits an enhanced initial Coulombic efficiency together with cycling stability probably for the reduced Li+ diffusion contribution occurred in conversion and alloying reactions. These findings give a new insight into

improving the lithium storage property of TMOs-based anodes for LIBs. 2. Experimental Section 2.1 Materials Cobaltous chloride (CoCl2·6H2O), sodium borohydride (NaBH4), KOH together with ethanol were obtained from Sinopharm Chemical Reagent Co., Ltd. Graphene oxide (rGO) 2 mg mL-1, were purchased from Suzhou Hengqiu Technology Co., Ltd. 2.2 Materials synthesis First, 80 mL of 0.4 M CoCl2·6H2O aqueous solution was obtained under stirring and Ar purging, and kept at 0 °C for 10 min. Then 100 mL solution of graphene oxide was mixed with the solution and maintained at 0 °C via an ice-bath. 1 M NaBH4 added to 40 mL KOH solution (0.1 M), which was separately deaerated and flushed with Ar. The solution of CoCl2·6H2O was then slowly added into solution. It is worth noting that the process of adding sodium borohydride should be slow, otherwise vigorous foam will occur. The formation of dark precipitate was observed instantly. The precursors were collected by centrifugation. The collected precursors were subjected to pyrolysis at 100 °C for 1 h and then 400 °C for 2 h under Ar atmosphere. 2.3 Characterization method X-ray diffraction (XRD, D/MAX2500 V, λ = 1.5418 Å) and X-ray photoelectron spectroscopy (XPS, ESCALAB250) were used to characterize the structures and the chemical states. The Scanning electron microscopy (SEM, JEOL JSM-6330F) and

transmission electron microscope (TEM, JEOL-2100) were used to characterize the morphologies and crystalline states. Elemental analysis was performed using ICP-AES. 3. Results and discussion XRD result for CoO/Co2B/rGO in Figure 1 (a) presents the peaks of CoO (PDF#43-1004, space group: Fm-3m (225)) and Co2B (PDF#25-0241), space group: 14/mcm (140)), manifesting the coexistence of the CoO and Co2B. ICP-AES measurements show that the mass proportions of Co, C and B are 48.7 wt.%, 5.14 wt.% and 6.3 wt.%, respectively. SEM images (Figure 1 (b, c)) show the agglomeration of rGO and numerous primary grains. The experiment adopted a typical procedure, according to the partitioning effect of the graphene nanosheets, the introduction of graphene nanosheets into the composite can help to inhibit aggregation and hinder the growth of the nanoparticles. The morphology and structure of the as-prepared samples were studied by SEM, and the recorded images are displayed in Figure 1 (b, c). The porosities among graphene nanosheets not only provide spaces for the confined growth of CoO/Co2B nanoparticles but also cushion the volume expansion and shorten the ion diffusion pathway. As evidenced by the Figure 1(b), the SEM images clearly show that the CoO/Co2B nanoparticles grow firmly on the surface of graphene nanosheets with diameter in the range of several tens of nanometers. Meanwhile, the graphene nanosheets form an electrically conductive network treated as promoting the charge transfer highway during charge/discharge cycles. And, small particle sizea are the key to improving

electrochemical performance because they shorten the distance of diffusion of Li+ ions in the solid phase. In other words, the CoO/Co2B/rGO hybrid nanostructures enabled rapid electron transfer to the active material during repeated cycling. To further confirm the existence of rGO and the size of primary grains, TEM analysis is utilized, as seen in Figure 1 (d-e). The rGO sheet can be clearly observed and the size of primary grains is 20-60 nm in Figure 1 (d). HRTEM image of CoO/Co2B/rGO is given in Figure 1(e). The spacing distances of 0.257 nm and 0.237 nm correspond well to (200) of CoO and (211) Co2B, and the crystalline nature are further confirmed by FFT as shown in Figure 1 (f). Elemental mappings in Figure 1 (g) confirm the homogenous distributions of Co, O, B and C in the CoO/Co2B/rGO nanocomposites. The surface composition and the elemental valence states of the CoO/Co2B/rGO nanocomposites are studied by XPS.

The exsistence of Co, B, O and C is confirmed

by survey spectrum in Figure 2(a). For Co 2p in Figure 2(b), two main spin-orbit lines value of Co 2p1/2 and Co 2p3/2 reaches 15.4 eV, manifesting the coexistence of Co3+ and Co2+ in the CoO/Co2B/rGO.[36] Peaks at 797.5 eV and 781.4 eV can be ascribed to Co 2p1/2 and Co 2p3/2 of Co2+.[37, 38] Peaks centering at 783.1 eV and 798.6 eV are of Co 2p1/2 and Co 2p3/2 for Co3+.[39, 40] Two apparent peaks at 188.2 and 192.05 eV in Figure 2(c) correspond to elemental and oxidized B 1s levels, respectively, verifying the existence of Co-B and B-O.[40-42] Peak of C 1s spectrum is shown in Figure 2(d). The peak at 284.8 eV is due to C-C, which is ascribed to the calibrated carbon during testing. The C 1s XPS spectra at 286.18 eV (C-C bonds) and at 289.88 eV(C-O species) in rGO are also observed.[43] The peak of O 1s at 531.7 eV is Co-O

bonds in the lattice of cobalt oxides in Figure 2(e).[37, 44] Galvanostatic discharge/charge tests of CoO/Co2B/rGO composite as anode in half cells between 0.01 to 3 V (vs. Li/Li+) are measured, as presented in Figure 3. In the first discharge, the voltage drops rapidly to 1.2 V with a plateau, corresponding to the reduction of CoO into Co and the produce of Li2O. Initial discharge/charge capacities of 889.1/669.3 mA h g-1 with a high ICE of 75.28% at 0.1 C are delivered for CoO/Co2B/rGO in Figure 3(a), in which the capacity loss is induced by the SEI layer formation on the of anode surfaces. A reversible capacity (681.6 mA h g−1) with the Coulombic efficiency of ~95% is achieved in the second cycle. The outstanding stability for Li+ intercalation/deintercalation is probably due to the synergistic effect of biphasic composite and rGO in Figure 3(b). The large reversible capacity (∼574.6 mAh g−1) and superior retention of capacity (∼99%) are maintained after 250 cycles at 0.5C. The charge/discharge curves from 5th to 250th cycle at 0.5C are displayed in Figure 3(c). The sloping plateau in discharge at 1.8-0.9 V is caused by the generation of Li2O and the reduction of CoO into Co. Two sloping plateaus at ca. 1.2 and 2.3 V in charge processes are ascribed to oxidation reactions of Co and Li2O. Furthermore, sloping curves to the cutoff voltage of 0.01 V indicate the lithium insertion/extraction to graphene. Besides, discharge plateau gradually moves to 1.6-1.1 V on cycling, implying the homogeneous distribution of amorphous Co embedded in the Li2O matrix in the discharge processes. The ICE of this work and recent reports is compared, as shown in Figure 3(d).[25, 43, 45-48] The results demonstrate that our designed CoO/Co2B/rGO nanocomposite has a higher ICE compared with these

reports. The proposed strategy is a simple and feasible approach to improve the ICE of TMOs-based anodes. Notably, the CoO/Co2B/rGO electrode here presents better cycling stability compared to the recent reports as shown in Figure 3(e). [21, 49-61] The rate properties of CoO/Co2B/rGO nanocomposite as anode are evaluated in Figure 4. Reversible capacities around 681, 639, 614, 533, 476, 399, and 276 mAh g-1 are achieved at the rates of 0.1 C to 10 C in Figure 4(a), indicating the good high-rate capability. The CoO/Co2B/rGO anode can recover the initial reversible capacity when the rate returns from 10C to the initial 0.1C. The charge/discharge curves (Figure 4(b)) from 0.1C to 10C exhibit the increase of the charge voltages, while the discharge voltages decrease with the increasing rates, manifesting electrochemical polarizations (especially at high rates). Note that the nano-sized primary grains shorten the Li+ migration path, and the carbon and boride framework will promote fast electron transfer. Then we believe that the impressive rate properties are due to the formation of nanostructure and conductive framework. Typical CVs of the CoO/Co2B/rGO electrode in Figure 5(a) exhibit the main redox couple at ~2.45/1.5 V (vs. Li/Li+), relating to Li+ insertion/extraction in CoO. The characteristic peaks and shape can still maintain even at high scan rates above 0.5 mV s-1. As the scan rates increase, the shift of the main anodic peak is slight while the main cathodic peak moves to lower potential gradually. The b-value are calculated to be 1.189 and 1.062 for the cathodic and anodic peak, respectively (Figure (5b)), suggestive of a dominant capacitive contribution taken place in CoO/Co2B/rGO electrode during cycling. Herein, a fast Li+ insertion/extraction (high rate property)

together with extended cycling life will be obtained. Normally, two separate reaction mechanisms (surface capacitive effect, diffusion-controlled Li+ insertion) are expressed for the current response at a fixed potential.To

quantify

total

capacitive

and

diffusion

contribution

in

this

CoO/Co2B/rGO electrode, the following Equations are generally used:[62, 63] i(V) = 𝑘1v + 𝑘2𝑣1/2 (1) 𝑖(𝑉)/𝑣1/2 = 𝑘1𝑣1/2 + 𝑘2 (2) Where i (V) is the total current response at a given potential V, 𝑘1v and 𝑘2𝑣1/2 refer the current contributions from capacitive effect and insertion process.[64, 65] 𝑘1 and 𝑘2 are determined from 𝑖𝑣 ―1/2 and 𝑣1/2 with eq. 2. This methodology can distinguish the currents arising from Li+ insertion and those from capacitive effects. From Figure 5(c), the proportion of the capacitive contribution is calculated to be 89.2%. Figure 5(d) confirms that the capacitive contribution gradually increases with increment of scan rate, indicating a large capacitive contribution in this CoO/Co2B/rGO nanocomposite electrode. 4. Conclusions In summary, a new, proof-of-concept CoO/Co2B/rGO nanocomposite is proposed to regulate the storage mechanism of TMOs-based anodes. As expected, the cyclic voltammogram of CoO/Co2B/rGO demonstrates that the proportion of capacitive contribution in the total charge storage ranges from ~60% to 90% at varied scan rates for this nanocomposite. CoO/Co2B/rGO nanocomposites possess an initial capacity of 937.3 mAh g-1 with high ICE of 75.28%. Moreover, the CoO/Co2B

coupled with interacted carbon network provide efficient channels for electrons and Li+ diffusion, resulting an excellent rate performance(693.2, 656.1 597.8, 531.8, 469.7, 383.9 and 280.9 mAh g-1 at 0.1, 0.2, 0.5, 1.0, 2.0, 5.0 and 10.0 C, respectively) and long cycling stability (574.6 mAh g-1 at 0.5C after 250 cycles). These findings give a new insight into improving the lithium storage property of TMOs-based anodes for LIBs. Overall, the CoO/Co2B/rGO has been presented as a promising candidate for LIB anode.

Acknowledgment This work was financially supported by the National Natural Science Foundation of China (51672109), the Natural Science Foundation of Shandong Province for Excellent Young Scholars (ZR2016JL015) and the Shandong Provincial Natural Science Foundation, China (ZR2017BEM010).

Conflicts of interests: none

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Fig. 1. (a) XRD, (b,c) SEM of the as-prepared CoO/Co2B/rGO, (d) TEM, (e) HRTEM, (f) the corresponding fast Fourier transform (FFT) and inverse FFT results, and (g) the corresponding elemental mapping images of the as-prepared CoO/Co2B/rGO. Fig. 2. XPS spectra of the CoO/Co2B/rGO nanocomposite: (a) survey spectrum, (b)

Co 2p, (c) O 1s, (d) B 1s and (e) C 1s. Fig. 3. (a) initial two charge/discharge curves at 0.1 C, (b) cycle life of capacity at 0.5 C, (c) the continuous charge/discharge curves from 5th to 250th cycle at 0.5C, (d) the comparison of initial Coulombic Efficiency with previous reported works, (e) the comparison of the cycle performance of cobalt-based as anode in the present work and previous work in literatures. Fig. 4. (a) rate charge/discharge capacities and (b) charge/discharge curves of the CoO/Co2B/rGO anode at different rates. Fig. 5. (a) CV curves of the CoO/Co2B/rGO electrode at scanning rates of 0.1, 0.2, 0.4, 0.6, 0.8 and 1 mV s−1, and (b) corresponding dependence of peak currents scan rate, (c) capacitive contribution and diffusion contribution at 1.0 mV s-1 of the CoO/Co2B/rGO, (d) normalized contribution ratio of capacitive at different scan rates.

Scheme

1.

Schematic

diagram

CoO/Co2B/rGO nanocomposite.

of

preparation

processes

of