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“Electron Sharing” Mechanism Promotes Co@Co3O4/CNTs Composite as the High Capacity Anode Material of Lithium Ion Battery Yantao Zhao, Wujie Dong, Muhammad Sohail Riaz, Hongxin Ge, Xin Wang, Zichao Liu, and Fuqiang Huang ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.8b15659 • Publication Date (Web): 29 Nov 2018 Downloaded from http://pubs.acs.org on November 29, 2018

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ACS Applied Materials & Interfaces

“Electron Sharing” Mechanism Promotes Co@Co3O4/CNTs Composite as the High Capacity Anode Material of Lithium Ion Battery Yantao Zhao1†, Wujie Dong1†, Muhammad Sohail Riaz1, Hongxin Ge1, Xin Wang1, Zichao Liu1, Fuqiang Huang1, 2* Mr. YT. Zhao, Mr. WJ. Dong, Mr. M.S. Riaz, Mr. HX. Ge, Mr. X. Wang, Mr. ZC. Liu, Prof. FQ. Huang 1

State Key Laboratory of Rare Earth Materials Chemistry and Applications, College

of Chemistry and Molecular Engineering, Peking University, Beijing 100871, P.R. China Prof. FQ. Huang 2State

Key Laboratory of High Performance Ceramics and Superfine

Microstructures, Shanghai Institute of Ceramics, Chinese Academy of Sciences, Shanghai 200050, P.R. China E-mail: [email protected] (FQ. Huang) KEYWORDS： Co@Co3O4/CNTs, arc discharge, high conductivity, lithium ion batteries (LIBs), electron sharing

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ABSTRACT： Hybridization of nanostructured cobalt oxides with carbon nanotubes (CNTs) is considered to be an operative approach to harvest high performance anode material for lithium ion batteries (LIBs). On the other hand,

there are numerous

related works, most of them adopted a “post-combination” strategy, which is not only complicated but also ecologically unpromising for using toxic acid for surface modification

of

CNTs.

Herein,

we

productively

fabricate

Co@Co3O4/CNTs

nanocomposite with excellent conductivity through arc discharge

following low

temperature oxidation in air. As the anode material for LIBs, this nanocomposite shows an exceedingly high reversible capacity of 820 mA h g-1 at a current density of 0.2 A g-1 after 250 cycles, much higher than its theoretical capacity. The rate performance of the material is also outstanding, with a capacity of 760 mA h g-1 after 350 cycles at 1 A g-1 (103 % of the initial capacity) and 529 mA h g-1 after 600 cycles at 2 A g-1, correspondingly. XPS tests are accomplished to disclose the true cause of extra capacity. And for the first time, we propose an "electron sharing" storage mode where extra electrons and Li+ can separate and stored at the interface of cobalt metal/Li2O. This not only gives a reasonable revelation for this unusual capacity

exceeding the

theoretical value, nonetheless also directs the capacitor-like electrochemical behavior of the extra capacity.


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Introduction: Lithium ion batteries (LIBs) are far and wide used in portable electronics, vehicles and large scale energy storage. Though, the LIBs industry is every so often criticized for its sluggish advancement1-2. This is not only due to the restricted energy density and power density of existing LIBs, but also the need for enhanced cycle lifetime and safety2-4. Hence research on innovative electrode materials is crucial for their significant influence on battery performance. The high theoretical capacity of transition metal oxides, coupled with lower price, make them auspicious substitutes to swap traditional graphite-based anode materials2,

4-7.

Amongst

different types of transition metal oxides, Co3O4 has fascinated a particular attention owing to its high theoretical capacity up to 890 mA h g-1(372 mA h g-1 for commercial graphite anode). Unfortunately, just like other transition metal oxides, the cyclic stability and rate performance of Co3O4 are not agreeable since

the large volume changes throughout lithiation/delithiation

and the poor conductivity8-10. To resolve the above problems, the two key points are dispersity and conductivity, which were established by our recent works about some other anode candidates11-14. In the same way, when it comes to Co3O4, the central strategies can be summarized as building nanostructures to improve the dispersity (e.g. nanoparticles, nanorods, nanobelts, nanoplates)4,

8, 15-16

and

introducing conductive carbon materials to advance both the dispersity and conductivity2, 5, 7, 910, 17-24.

Nano-structured materials have more voids around the nanoparticles which

substantially facilitate the Li+ diffusion and mitigate the negative effects initiated by volume change2. Carbon materials, particularly carbon nanotubes (CNTs),

have good electrical

conductivity, as well as excellent mechanical strength. For that reason, when cobalt oxide is combined with CNTs, the conductivity can be remarkably boosted, and the stress instigated by the volume change can also be alleviated. Even though, there have been a lot of works to associate nano-structured cobalt oxide with CNTs, several of them are relatively complicated and adopt a “post combination” method, which means that CNTs are first synthesized and then combined with cobalt oxides. For example, Gu et al2 lately reported a controllable synthesis of 3 ACS Paragon Plus Environment


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mesoporous peapod-like Co3O4@carbon nanotube arrays as the anode materials for LIBs. In their research, CNTs arrays were generated by using SBA-15 as hard template, and combined with Co3O4 after surface modification. The surface modification and template removal steps used massive acid, which would cause concerns about environmental safety and hinder the practical application. Moreover, the combination between Co3O4 and CNTs is very important for

the cycle stability. Co3O4 particles may fall off from the carbon materials when employing

the “post combination” method while this bonding is stronger in in-situ grown materials25-27. Remarkably, cobalt metal, added with Y, S, or Cr as a promoter are the common catalysts in arc discharge synthesis of CNTs28-33, which means Co/CNTs nanocomposites can be easily synthesized. Though, the preparation of Co3O4/CNTs nanocomposites as the anode materials for LIBs through arc method has rarely been reported. Herein, we fruitfully fabricated Co/CNTs nanocomposites using graphite as carbon source and Co-Na2SO4 mixture as catalyst through arc discharge method. Then Co@Co3O4/CNTs nanocomposites were attained through arc discharge and consequent low temperature oxidation in air (320 C). As the anode material of LIBs, it exhibits exceptionally high reversible specific capacity of 820 mA h g-1 at 0.2 A g-1 after 250 cycles, even higher than the theoretical capacity of the nanocomposites. The rate performance of the material is also remarkable, with a capacity of 760 mA h g-1 after 350 cycles at 1 A g-1 (103 % of the initial capacity) and 529 mA h g-1 after 600 cycles at 2 A g-1, in that order. XPS tests were executed to make known the factual cause of extra capacity. For the first time, we propose an "electron sharing" storage mode where extra electrons and Li+ can separate and stored at the interface of Co/Li2O. This provides us with a reasonable disclosure for this unusual capacity which is exceeding the theoretical value, and also specifies the capacitor-like electrochemical behavior of the extra capacity.

Materials and Methods:


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All chemicals used in this work were analytically pure and commercially available and used without any further purification. Deionized water used was house generated. (Resistivity: 0.860 MΩ·cm at 25 C according to Rex DDSJ-308F Conductivity Meter from INESA Scientific Instrument Co.) Synthesis of Co/CNTs and Co@Co3O4/CNTs: Co/CNTs was well fabricated using arc discharge method. Cobalt metal powder and Na2SO4 powder with mass ratio of 3:1 was first carefully grinded for 15 minutes. Then a graphite tube with an outer diameter of 8 mm and an inner diameter of 6 mm was filled with the

mixture

and adopted as anode for arc discharge. The cathode used was graphite rod with a diameter of 8 mm, both electrodes were fixed on a water-cooled copper pedestal. The chamber was first vacuumed to a pressure below 10 Pa, and then it was filled with a mixed gas of He and H2 (1:1 in molar) to a pressure of 0.08 MPa. The arc discharge was conducted at a direct current of 120 A, and the black soot was collected from the chamber wall after the discharge. In order to remove impurities in the soot, the soot was dispersed in 100 ml of deionized water and sonicated for 1 hour. Afterwards, suction filtration and rinsing with deionized water for three times, the nanocomposite was dried in an oven at 70 C for 6 hours to obtain Co/CNTs. The as-prepared Co/CNTs was further subjected to heat to 320 °C using muffle furnace with a ramping rate of 1 C min−1 and held for 2 hours under air atmosphere to get Co@Co3O4/CNTs. CNTs were also synthesized for comparison with Co@Co3O4/CNTs. 0.5 g Co@Co3O4/CNTs together with 5 mL HNO3 (16 mol L-1) and 30 mL deionized water 5 ACS Paragon Plus Environment


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were placed in a glass beaker and stirred for 1 hour. Then the liquid was transferred to a Teﬂon-lined stainless steel autoclave and reacted at 120 °C for 12 hours. After suction filtration and rinsing with deionized water for three times, the nanocomposites was dried in an oven at 70 C for 6 hours to obtain CNTs. Material Characterization: Powder X-ray diffraction (PXRD) patterns were collected using a Bruker D2 Focus diffractometer with monochromatized Cu Kα radiation (λ = 1.5418 Å) at a scanning rate of 6 min−1 within the 2θ range of 10°– 80°. Images and semi quantitative energy dispersive X-ray spectroscopy (EDS) analyses were acquired using a Phenom Pro scanning electron microscope (SEM) equipped with a PGT energy-dispersive X-ray analyzer. Thermogravimetric analysis (TGA) of Co/CNTs and Co@Co3O4/CNTs were both conducted at a heating rate of 10 C min-1 in the temperature range of 25 C–850 C under air atmosphere using a Thermal Analysis Q600SDT. The specific surface area of both samples were calculated with Brunauer-EmmettTeller (BET) method using isotherms collected by an Accelerated Surface Area & Porosimetry system (ASAP2020, Micrometer). The morphologies and microstructures were characterized by transmission electron microscope (TEM) combined with a selected area electron diffraction (SAED) using a JEM−2100 electron microscope (JEOL Ltd.) working at 200 kV. Raman spectra were obtained using a Thermal Fisher Micro Raman imaging spectrometer (DXRxi) with a laser excitation wavelength of 532 nm. X-ray photoelectron spectroscopy (XPS) measurements were performed on an Axis Ultra imaging photoelectron spectrometer (Kratos Analytical Ltd.) using a monochromatized Al Kα anode. The peak of delocalized sp2hybridized carbon (284.8 eV) which appeared on every sample due to atmospheric contamination was used to calibrate all XPS data. Electrochemical measurements:


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An N-methyl pyrrolidinone (NMP) slurry composed of 80 wt% active material (Co@Co3O4/CNTs, CNTs, and Co3O4), 10 wt% acetylene black, and 10 wt% polyvinylidene fluoride (PVDF) was first achieved by magnetic stirring for 12 hours. Then a copper foil was uniformly coated with the slurry and dried at 70 C for 12 hours in vacuum oven later it was cut into disk electrodes with a diameter of 14 mm and further used as electrode for coin cells. Lithium foil (China Energy Lithium Co., Ltd.) was used as counter and reference electrode and glass fiber (Whatman) as separator. The electrolyte used for the coin cell was 1 M LiPF6 dissolved in a mixture of dimethyl carbonate (DMC) and ethylene carbonate (EC) (50:50 w/w). A recirculating argon glovebox with the moisture and oxygen contents below 1 ppm was adopted to assemble the coin cells. The cycle performance and rate performance tests of cells are all carried out within a voltage window of 0.01~3 V (vs. Li+/Li) using LAND-CT2001C test system. Electrochemical impedance spectroscopy (EIS), and cyclic voltammetry (CV) of the assembled coin cells were performed on an automated electrochemical workstation (CHI760E). For EIS tests, the frequency was set between 0.01 Hz to 105 Hz and the potential was set to be the very potential acquired using open circuit potential test. As for CV tests, the voltage window were set from 0.01 to 3.00 V (vs. Li+/Li) with a scanning rate of 0.3 mV S-1. Results and discussion:

Figure 1. Illustration of synthesis: (a) illustration of arc discharge; (b), (c): Co/CNTs and Co@Co3O4/CNTs.



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As shown in Figure 1, Co@Co3O4/CNTs has been successfully produced by the arc discharge and subsequent low temperature oxidation in air (320 C). The thermos gravimetric analysis (TGA) of Co/CNTs (Figure S1) shows the oxidation of cobalt begins at 275 °C and the oxidation of CNTs starts at 392 °C. The oxidation temperature and time were thus set to 320 °C and 2 hours at which only cobalt was partially oxidized to sustain its exceptional conductivity. This fabrication approach is not only simple, but also avoids using toxic acid in post-combination method2.

Figure 2. (a) XRD patterns of as-prepared CNTs, Co/CNTs and Co@Co3O4/CNTs; (b) Nitrogen adsorption/desorption isotherms of Co/CNTs and Co@Co3O4/CNTs; (c) Raman spectra of Co/CNTs and Co@Co3O4/CNTs; (d) Conductivity of CNT, Co/CNTs and Co@Co3O4/CNTs; (e)(f) TEM and SAED images of Co@Co3O4/CNTs.

The phase of Co@Co3O4/CNTs and other samples are determined by XRD patterns. As shown in Figure 2a, the peak at 26.6° agrees to characteristic (004) facet of CNTs (PDF#261080, a = b = 2.456 Å, c = 13.392 Å, α = β = 90°, γ= 120°). All three samples have solid 8 ACS Paragon Plus Environment

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CNTs signals, representing that the structure of the carbon nanotubes is retained after oxidation and HNO3 treatment. The XRD pattern of Co/CNTs also has peaks at 44.2° and 51.5° which are ascribed to (111) and (200) facet of Cobalt (PDF#15-0806, a = b = c = 3.5447 Å, α = β= γ = 90°). As for Co@Co3O4/CNTs, there are not only the above-mentioned peaks of the CNTs and Co, but also peaks of the Co3O4 with strong intensities (31.3° and 36.8°, attributed to (220) and (311) facet of Co3O4 (PDF#43-1003, a = b = c = 8.084 Å, α = β = γ = 90°). Nitrogen adsorption-desorption isotherms at 77 K (as shown in Figure 2b) were used to calculate the specific surface area of Co/CNTs and Co@Co3O4/CNTs with BET method. The surface area of Co/CNTs was 156.63 m2 g-1 and decreased to 144.85 m2 g-1 after oxidation which was primarily triggered by oxidation of amorphous carbon formed in arc discharge30, 34. The Raman spectra of CNTs, Co/CNTs and Co@Co3O4/CNTs shown in Figure 2c has three peaks at around 1346 cm−1, 1585 cm−1 and 2681 cm−1 belonging to D band, G band and 2D band of CNTs, respectively35-36. ID/IG of Co/CNTs is 0.547, which means that A1g in-plane breathing vibration caused by disorder and defects in the edge of sp2 domain is quite intense. It is advantageous for lithium ion transfer as the layer spacing is greater than graphite. The ID/IG of Co@Co3O4/CNTs (0.677) is even higher than that of Co/CNTs, which might be cause of the surface oxidation of CNTs. Though, the ID/IG of CNTs decreased to 0.495, indicative of lower disorder and fewer defects. This should be a result of HNO3 treatment which could also remove some lower graphitized carbon during the acid etching of Co and Co3O4. The samples for conductivity tests were tablets prepared under a pressure of 18 MPa. As shown in Figure 2d, the conductivity of Co@Co3O4/CNTs has a significant improvement equated to Co3O4. The original conductivity of Co3O4 is just 7.1 × 10-4 S m-1 though the conductivity of Co/CNTs and Co@Co3O4/CNTs are 33.5 S m-1 and 7.6 S m-1. The morphologies and structures were further investigated by TEM images (See in Figure S2 and Figure 2e, f). CNTs can be evidently understood in the TEM images of Co/CNTs and Co@ Co3O4/CNTs and most of them were multiwall carbon nanotubes. In addition, the particle size of the cobalt nanoparticles before oxidation is within 100 nm, and most of them are even lower than 20 nm. The particle size of the partially oxidized particles also did not increase knowingly, and most of them are still below 9 ACS Paragon Plus Environment


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20 nm. The TEM and SAED images of Co@ Co3O4/CNTs in Figure 2f also shows that there are Co, Co3O4 and CNTs in the nanocomposites which is in accordance with XRD patterns. Moreover, the TEM image also indicates that Co and Co3O4 exist in the form of a core-shell structure. Since Co3O4 is formed by subsequent oxidation, it is quite natural to exist in such a form. XPS spectra (Figure S3) shows that Co transformed from mainly metallic Co to metallic Co and

Co3O4 after oxidation, which is also in accordance with SAED results in Figure 2f.

Figure 3. Elemental maps of Co@Co3O4/CNTs nanocomposite: (a) TEM image of Co@ Co3O4/CNTs nanocomposite; (b), (c), (d): EDS maps of C, Co, O.

In order to obtain more detailed chemical composition information, EDS mapping tests were also performed. Figure 2a shows the Co@ Co3O4/CNTs nanocomposite selected for EDS mapping. It can be seen from Figure 3b-d that the elemental distribution of cobalt and oxygen is more consistent, and there is a significant difference with the distribution of carbon. This 10 ACS Paragon Plus Environment

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result indicates that oxidation mainly occurs on cobalt rather than carbon nanotubes, which is consistent with previous XRD, TEM, and SAED results.

Figure 4. (a) Cyclic voltammogram (CV) of Co@Co3O4/CNTs at a scan rate of 0.3 mV s-1 between 0.01 and 3.00 V vs. Li/Li+; (b) Charge-discharge voltage profiles for the 1st, 2nd, 10th , 50th, 100th, 250th cycles within voltage range of 0.01 and 3.00 V vs. Li/Li+ at 0.2 A g-1; (c) Electrochemical impedance spectra (Nyquist plots) of Co@Co3O4/CNTs at full charged state, with a simulation curve and equivalent circuit diagram inset; (d) Charge and discharge capacities vs. cycle numbers of CNTs, commercial Co3O4 and Co@Co3O4/CNTs at 0.2 A g-1; (e) Charge and discharge capacities vs. cycle numbers of Co@Co3O4/CNTs at 1 A g-1 and 2 A g-1 with initial 10 cycles at 0.2 A g-1. 11 ACS Paragon Plus Environment


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The electrochemical properties and lithium storage performance of as-prepared Co@Co3O4/CNTs are shown in Figure 4. The first 5 consecutive CV curves given in Figure 4a provide us an approach to the charge and discharge details of Co@Co3O4/CNTs. There is one typical peak at around 0.75 V in the cathodic sweep of first cycle which is somewhat usual in transition metal oxides and generally attributed to the reduction of oxides to metal dispersed in Li2O matrix and the formation of solid electrolyte interlayer (SEI)6, 37-38. This peak in first cathodic sweep was substituted by two close peaks at 0.92, 1.24 V later. This phenomenon indicates that the reduction of cobalt oxide has undergone a multi-step process in the following cycles. The peaks in all anodic sweeps are maintained at around 2.11 V which is corresponding to the oxidation of Co to oxides. These results are also consistent well with the charge-discharge voltage profiles shown in Figure 4b. The plots shown in Figure 4c was electrochemical impedance spectra (Nyquist plots) of Co@Co3O4/CNTs acquired at full charged state. The equivalent circuit diagram and the simulation curve of the spectra was also inset. The linear part of the plots possesses a slope of 2.78 (78°) which means that the resistance caused by lithium ion diffusion is quite small. This is also consistent with the resistance obtained by simulation (Re = 1.2 Ω, Rf = 111.6 Ω, Rcf = 90.2 Ω). Therefore, it is quite reasonable to expect an excellent cycle and rate performance.


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Figure 5. Performance of Co@Co3O4/CNTs and other literature works at different current densities.

The cycle performance of CNTs, commercial Co3O4 and Co@Co3O4/CNTs are shown in Figure 4d, and all these tests were conducted at 0.2 A g-1. Co@Co3O4/CNTs displayed an initial reversible capacity of 747 mA h g-1 and increased to 1035 mA h g-1 (139 % of initial reversible capacity) after 80 cycles. This increase in capacity can be attributed to the activation of the active material triggered by phase transformation for crystalline structure to amorphous structure and the resulting enhanced lithium ion transportation6, 39. Then the reversible capacity slowly decreased and harvested an extremely high capacity of 820 mA h g-1 after 250 cycles, which is 109 % of the initial reversible capacity.

However, the capacity of commercial Co3O4

decays faster, with only 455 mA h g-1 after 250 cycles, which is just 59 % of the initial capacity (769 mA h g-1). We also carried out oxidation at 320 °C for 4 h and 390 °C for 2 h (Figure S5). The products were still Co@Co3O4/CNTs composites according to the XRD patterns, but the content of Co3O4 increased comparing with the one oxidized at 320 °C for 2 h. The specific capacities of both samples (at 0.2 A g-1) are relatively lower than the one oxidized at 320 °C for 2 h, which might be caused by poorer conductivity after oxidation for long time or at high temperature. These results shows that the nanocomposite we built is extraordinary and this strategy can effectively improve the performance of Co3O4. The cycle performance of CNTs was also tested. The reversible capacity of CNTs is not high but quite stable which only attenuated by 1.5 % after 250 cycles from 393 mA h g-1 to 387 mA h g-1. In addition to the excellent cycle performance, the material's rate performance is also outstanding. As shown in Figure 4e, the capacity of Co@Co3O4/CNTs at 1 A g-1 after 350 cycles was 760 mA h g-1, which is 103 % of the initial capacity. And the capacity at 2 A g-1 was 453 mA h g-1 after 350 cycles and maintained 529 mA h g-1 after 600 cycles (Figure S4). As we can see from Figure 5, this composite harvest larger capacity especially at high current densities. According to the TGA results (Figure S1), even if Co was fully oxidized to Co3O4, the theoretical capacity of 13 ACS Paragon Plus Environment


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the composite is less than 720 mA h g-1. In fact, the material contains a lot of metallic cobalt, which makes its theoretical capacity even smaller. This result is very reasonable, but it also raises the question of why the nanocomposite’s capacity far exceeds the theoretical capacity. In order to clarify this problem, it is necessary to understand the redox mechanism of Co3O4 during lithiation and delithiation. As reported by researchers, there are two types of redox mechanisms for Co3O4 anode called direct and partial redox mechanism3. The direct redox mechanism can be summarized as follows40-41: Lithiation/delithiation: Co3O4 + 8Li+ + 8e- ↔ Co + 8Li2O

(1)

The partial redox mechanism proposed by Liu et al can also be summarized as follows42: Lithiation in first cycle: Co3O4 + 8Li+ + 8e- → Co + 8Li2O

(2)

Delithiation in fist cycle: Co + Li2O → CoO + 2Li+ + 2e-

(3)

Lithiation/delithiation after first cycle: CoO + 2Li+ + 2e- ↔ Co + Li2O

(4)

Comparing the above two mechanisms, it can be seen that the main difference is Co3O4 is only present in the lithiation process of first cycle in the partial redox mechanism. Based on this understanding, it is possible to determine the exact redox mechanism if we can track down the valence of Co element during electrochemical process. Thus we conducted XPS tests of the anode disks at different cut-off potentials using inert gas-protected injection system. To prevent oxidation in air, the disassembly of the battery at different cut-off potentials and the loading of the electrode disks into the inert gas protected injection rod were all performed in the glove box.


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Figure 6. XPS spectrums of electrode disks at different cut-off potentials in the cathodic sweep of 2nd cycle. (a), (b), (c) and (d) are spectrums of Co 2p at cut-off potentials of 3, 1.273, 0.929 and 0.01 V, respectively. (e), (f), (g) and (h) are spectrums of Li 1s at cut-off potentials of 3, 1.273, 0.929 and 0.01 V, respectively. All XPS data was calibrated using the peak of delocalized sp2-hybridized carbon (284.8 eV) which appeared on every sample due to atmospheric contamination.

As the electrode disks were assimilated by disassembly of batteries in the cathodic sweep of 2nd cycle, the XPS spectrums signify different degree of lithiation which increase from 3 V to 0.01 V. The six peaks shown in Figure 6a located at 778.5, 779.8, 784.3, 793.5, 795.2 and 802.5 eV are assigned to the 2p3/2 of Co, Co3O4, satellite and

2p1/2 of Co, Co3O4, satellite,

respectively. In contrast to peaks shown in Figure 6b-d, the relative intensity of Co3O4 gradually decreased and even disappeared with increasing degree of lithiation while the relative intensity of Co greatly enhanced. Changes of Co 3p peaks appeared in Li 1s spectrums (Figure 6e-h) are also in line with this result. The peaks of Co 2p and Li 1s during charge (Figure S6) are also consistent with this. These evidences indicate that Co@Co3O4/CNTs follows a direct redox mechanism and even achieve complete reduction of Co3O4.



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In addition to the changes in peak intensity, the emergence of new peak and shifts of binding energy after complete lithiation are also important. There is a new peak of Co 2p at 770.1 eV in Figure 6d which is significantly lower than that of metallic Co (778.2 eV, typically). Besides, as shown in Figure 6e-h, the binding energies of Li 1s are 56.9, 56.6, 56.6 and 55.9 eV and all peaks are attributed to Li+ in Li2O. It can be noted that the binding energy of Li+ at 0.01 V is lower than the binding energy at other potentials. The lower binding energy of cobalt and lithium at 0.01V means a higher electron cloud density around the nucleus, that is, both cobalt and lithium further obtained more electrons. These extra electrons obtained by Co/Li2O should be the source of extra capacity. Stimulatingly, similar beyond theoretical capacity phenomena are also present in other materials such as RuO2 anode material. Joachim Maier and co-workers43 studied RuO2 anode whose specific capacity could be 100 mA h g-1 higher than the theoretical capacity and proposed a novel energy storage mode called “job-sharing” mode. This job-sharing energy storage mode reveals that a heterojunction composed of an electron conductor and an ionic conductor can store additional lithium by separate electrons and ions into electron and ion conductors. Their subsequent work further confirmed this storage mechanism and provided theoretical and experimental evidence44-47. It was even found that hydrogen can be stored at the heterojunction interface through a similar mode45. On the other hand, our work here indicates that electrons are not only stored in the metallic Co but also in the Li+.

Figure 7. Schematic illustration of anode electrochemical process during discharge


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As stated above, the core-shell structured Co@Co3O4 is in upright contact with the highly conductive CNTs network, which is also one of the advantages of the in situ synthetic strategy. Thus, a good electron path is formed between the current collector and Co@Co3O4, which significantly eases the electrochemical process. As shown in Figure 7, Co@Co3O4 is first reduced to Co and Li2O during discharge. In general, the electrochemical reaction ends here because Co cannot alloy with Li and store more energy. But the XPS results (Figure 5d) indicates that the reaction is still going on and Co together with Li2O continues to get more electrons, which have never been observed or noticed before. Since the net charge is only existing on the surface of one object, the electrons should be positioned at the interface of the heterojunction. As explained by the “job-sharing” mechanism, electrons are principally located on the electron conductor side43, which is pretty natural. In our system, electron conductor Co is more electronegative than Li, thus it has the ability and tendency to accept partial electrons from Li. And the counter ion (Li+) is similarly present on Li2O side of the interface to balance the charge. The electrons located at the interface not only prominently reduce the Co 2p binding energy of some surface cobalt atoms, but also lithium ions in the vicinity of the interface are inevitably affected. This means that although the electrons are theoretically located on the side of the electron conductor at the interface, the lithium ions in the formed electric double layer still have a certain attraction to electrons. This also indicates that the previous “job-sharing” mechanism appears to be relatively rough and lacks microscopic details. Electrons are not only located on the cobalt surface; they are actually more similar to an “electron sharing’’ model or “metal-Li surface alloy”. As shown in Figure 7, the extra capacity is dependent on the formation of an electric double layer at a relative low potential. It also shows that the electrochemical behavior of the extra capacity is similar to that of a typical capacitor, which is consistent with the results presented by charge-discharge voltage profiles shown in Figure 4b. The formation of such an electric double layer may be closely related to the good electrical conductivity of the material. Lots of literatures reported the exceeding-theoretical capacity phenomena of transition metal oxides48-56 (Table S1), but little of them explained the origin of 17 ACS Paragon Plus Environment


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the extra capacity, the “electron sharing’’ model or “metal-Li surface alloy” may provide a novel vision. Conclusion: In summary, although cobalt oxide has a very high theoretical capacity, its poor conductivity and large volume change during lithiation/delithiation limits its practical application. Enhancing the dispersity and conductivity are the two effective and universal strategies for improving the performance of anode candidates. Herein, we demonstrate a Co@Co3O4/CNTs nanocomposite via a facile arc discharge and subsequent low temperature oxidation in air (320 C). Unlike the “post combination” synthetic methods reported in the literature, the combination of Co3O4 and CNTs is stronger and the conductivity is better in this in-situ grown nanocomposite. It harvests excellent cycle performance over commercial Co3O4 with an initial capacity of 747 mA h g-1 (0.2 A g-1) and remains at 820 mA h g-1 after 250 cycles (109 % of the initial capacity). The rate performance of this nanocomposite is also admirable, with a capacity of 760 mA h g-1 after 350 cycles at 1 A g-1 and 529 mA h g-1 after 600 cycles at 2 A g-1, respectively. Considering that the theoretical capacity is only 524 mA h g-1, such a result is quite satisfactory and raises questions about its mechanism at the same time. XPS tests are performed to reveal the true cause of extra capacity. This is for

the first time, we propose an

"electron sharing" storage mode where extra electrons and Li+ can separate and store at the interface of Co/Li2O. This not only gives a reasonable disclosure for this unusual capacity exceeding the theoretical value, but also indicates the capacitor-like electrochemical behavior of the extra capacity. Though been rough and require further exploration, this energy storage mode may also exist with other nanocomposite systems (e.g. iron oxide/carbon, etc.), providing us a novel vision for the widespread exceeding-theoretical capacity phenomena. ASSOCIATED CONTENT


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Supporting Information. The following files are available free of charge. TGA of Co/CNTs; TEM images of Co/CNTs with different resolution; XPS spectra of Co/CNTs and Co@Co3O4/CNTs powders; rate performance plots of Co@Co3O4/CNTs at 2 A g-1; XRD pattern, CV curve, and cycling performance (0.2 A g-1) of Co@Co3O4/CNTs synthesized at 320 C for 4 h and 390 C for 2 h; XPS spectrums of electrode disks at 2.113 V (vs. Li/Li+) in the anodic sweep of 2nd cycle; XPS spectrums of electrode disks at 2.113 V (vs. Li/Li+) in the anodic sweep of 2nd cycle; the capacity table of Co@Co3O4/CNTs and other related materials (PDF). AUTHOR INFORMATION Corresponding Author *E-mail: [email protected] ORCID Fuqiang Huang: 0000-0001-7727-0488 Author Contributions †Yantao Zhao and Wujie Dong contribute equally to this work. The authors declare no competing financial interest. The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript. Notes The authors declare no competing financial interest. 19 ACS Paragon Plus Environment


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ACKNOWLEDGMENT This work was supported by the National Key R&D Program of China (Grant No. 2016YFB0901600), National Science Foundation of China (Grant No. 51402334, 51502331), Science and Technology Commission of Shanghai (Grant No. 14520722000), and Key Research Program of Chinese Academy of Sciences (Grant No. KGZD-EW-T06). REFERENCES (1) Tarascon, J. M.; Armand, M., Issues and Challenges Facing Rechargeable Lithium Batteries. Nature 2001, 414, 359. (2) Dong, G.; Wei, L.; Fei, W.; Hans, B.; Bernd, S.; Wolfgang, S.; Claudia, W.; Yongyao, X.; Dongyuan, Z.; Ferdi, S., Controllable Synthesis of Mesoporous Peapod-like Co3O4@Carbon Nanotube Arrays for HighPerformance Lithium-Ion Batteries. Angew. Chem. Int. Ed. 2015, 54 (24), 7060-7064. (3) Chae, B. M.; Oh, E. S.; Lee, Y. K., Conversion Mechanisms of Cobalt Oxide Anode for Li-Ion Battery: In Situ X-ray Absorption Fine Structure Studies. J. Power Sources 2015, 274, 748-754. (4) Do, J. S.; Weng, C. H., Electrochemical and Charge/Discharge Properties of the Synthesized Cobalt Oxide as Anode Material in Li-Ion Batteries. J. Power Sources 2006, 159 (1), 323-327. (5) Hsieh, C. T.; Lin, J. S.; Chen, Y. F.; Teng, H., Pulse Microwave Deposition of Cobalt Oxide Nanoparticles on Graphene Nanosheets as Anode Materials for Lithium Ion Batteries. J. Phys. Chem. C 2012, 116 (29), 1525115258. (6) Huang, G.; Zhang, F.; Du, X.; Qin, Y.; Yin, D.; Wang, L., Metal Organic Frameworks Route to in Situ Insertion of Multiwalled Carbon Nanotubes in Co3O4 Polyhedra as Anode Materials for Lithium-Ion Batteries. ACS Nano 2015, 9 (2), 1592-1599. (7) Huang, Z. X.; Wang, Y.; Wong, J. I.; Shi, W. H.; Yang, H. Y., Synthesis of Self-assembled Cobalt Sulphide


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