Engineering 1D chain-like architecture with conducting polymer towards ultra-fast and high-capacity energy storage by reinforced pseudo-capacitance

Author’s Accepted Manuscript Engineering 1D Chain-like Architecture with Conducting Polymer Towards Ultra-fast and Highcapacity Energy Storage by Reinforced Pseudocapacitance Peng Ge, Sijie Li, Honglei Shuai, Wei Xu, Ye Tian, Li Yang, Guoqiang Zou, Hongshuai Hou, Xiaobo Ji

PII: DOI: Reference:

www.elsevier.com/locate/nanoenergy

S2211-2855(18)30707-9 https://doi.org/10.1016/j.nanoen.2018.09.062 NANOEN3067

To appear in: Nano Energy Received date: 1 September 2018 Revised date: 22 September 2018 Accepted date: 28 September 2018 Cite this article as: Peng Ge, Sijie Li, Honglei Shuai, Wei Xu, Ye Tian, Li Yang, Guoqiang Zou, Hongshuai Hou and Xiaobo Ji, Engineering 1D Chain-like Architecture with Conducting Polymer Towards Ultra-fast and High-capacity Energy Storage by Reinforced Pseudo-capacitance, Nano Energy, https://doi.org/10.1016/j.nanoen.2018.09.062 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting galley proof before it is published in its final citable form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.

Engineering 1D Chain-like Architecture with Conducting Polymer Towards Ultra-fast and High-capacity Energy Storage by Reinforced Pseudo-capacitance

Peng Ge, Sijie Li, Honglei Shuai, Wei Xu, Ye Tian, Li Yang, Guoqiang Zou, Hongshuai Hou* and Xiaobo Ji

State Key Laboratory of Powder Metallurgy, College of Chemistry and Chemical Engineering, Central South University, Changsha, 410083, China *

Corresponding authors: Prof. Dr Hongshuai Hou Tel: +86-731-88877237; Fax: +86-731-88879616 E-

mail: [email protected] Abstract: Compared to other energy storage types, capacitive energy-storage serves increasingly significant roles in shortening reversible cycling times and enlarging high power than traditional batteries. It still suffers from the low pseudo-capacitive level and short of electrodes, along with low energy density. Considering the great theoretical capacity, here 1D chain-like Co3O4 is prepared though the thermal oxidation of the self-assembled rod-like Co-precursor. Followed by in-situ polymerization of pyrrole monomer, the Co3O4 were encapsulated in the transparent PPy shell. Particle size-tuning, 1D architecture-altering, conducting PPy introduction could effectively broaden the energy distribution of ions, increase the speed of ions directional transferring and improve the conductivity with protecting electrode materials. As Li-storage anodes, Co3O4/PPy delivers a stable capacity of 816.6 mAh g-1 at 1.0 A g-1 after 300 cycles.

1

801.3 mAh g-1 at 5.0 A g-1. The capacity of full-cell still delivers 526 mAh g-1 at 3.0 A g-1 after 50 loops. Supported by detailed kinetic analysis of CV curves, it is confirmed, (1) the nature of Co3O4 approaches to capacitor-like behavior; (2) its electrochemical properties are dominated by capacitive contributions with increased current density as well as cycling. This work provides an in-depth sight on Co3O4, and further improving its pseudo-capacitive behaviors. Graphical Abstrct:

Key words: chain-like Co3O4, conducting polymer, electrochemistry, ultra-fast ions transferring, pseudocapacitance

2

1.cIntroduction Triggered by the limited fossil energy, the lithium-ion battery systems (LIBs) have achieved a plenty of progress, owing to their large energy density with stable electrochemical properties.[1-4] Recently, they have been successfully applied in numerous practical fields, such as the portable electronics and electric vehicle.[5, 6] However, restricted by the low capacity (372 mAh g-1) of the traditional LIBs anodes, the relative weak energy-storage capability still cannot meet the demand of pursuing the higher Li-energy density and the shorter charge/discharge time.[7-10] As known, the traditional materials can be divided into (1) capacitor-like materials, such as MnO2, Nb2O5 and RuO2; (2) battery-type materials, containing SnO2, Sb, Co3S4, etc.[11-13] Note that the capacitive materials as battery anodes could reduce the discharge/charge time, but with unsatisfied energy density.[14-16] Thus, to explore and enhance capacitive behaviors are crucial for traditional battery-type materials with improved capacity. Considering that the candidates could go through the ultra-fast lithiation/delithiation process, the relatively low volume changing is necessary. According to the electrochemical reaction mechanisms, the battery-type materials can be categorized as (1) insertion-type, (2) conversion-type, and (3) alloying-type. Compared to the (2) electrodes, the (1) materials possess the lower capacity and the (3) anodes exhibit the larger volume swelling. Metal-oxides, as the main member of electrode materials based on conversion behaviors, display the fascinated electrochemical properties such as CoO (715 mAh g-1), NiO (715 mAh g-1) and ZnO (658 mAh g-1).[17-19] Among them, Co3O4 delivers a satisfied theoretical Li-storage capacity of 890 mAh g-1, which is mainly ascribed to that 8 electrons could participate in the reversible reaction.[20-22] Through spray pyrolysis manner, the porous hollow spheres Co3O4 displayed a considerable

3

initial capacity of 1417.9 mAh g-1 at 0.176 A g-1 between 0.01 and 3.0V.[23, 24] Moreover, 1D hierarchically porous Co3O4 nanorods obtained from the hydrothermal method after calcination could remain a capacity of 628 mAh g-1 after 350 loops at large current density of 1.0 A g-1.[25] Note that the further improvements of its long lifespan and rate performances are necessary. The nano-engineering of morphology is deemed as one of the most effective stratgies to achieve the anticipated target above.[26] Numerous archtectures have been designed as anodes, including the hollow spheres, 2D sheets, cluters, and nanoparticles.[27] Through the comaprsion of various structures, 1D strucutre could serve as the high pathway, faciliating the rate of electrons transfer at one orientation, which is conductive to enhance the ultra-fast ions storage.[28] Morevoer, the reduced size of particles is regarded as another active way to tailor the characters of surface interface, further boosting the eletrochemical performances. Notably, the decreasing sizes would bring about more active sites, resulting in the adequate infliation of electrolyte and rapid difussion of lithium-ions.[14] Moreover, the introduction of coating layer would effectively protect the electrode materials from the erosion of electrolyte.[29] Also, altering the dominant electrochemical behaviors would have profound effects on the electrochemical properties. The main capacity came from two parts: (1) diffusion-controlled behaviors through insertion, alloying and conversion reaction process; (2) the surface-capacitive behaviors, containing non-faradaic electrical double-layer and faradaic pseudo-capacitive behaviors.[30] Among them, the pseudo-capacitive behavior is dominated by three parts, such as under-potential deposition, surface/near-surface ions reaction, majority ions shuttling. Latest report demonstrates that the surface-dominated behaviors could bring about numerous desirable properties, containing fast charge/discharge progress, long-life stability.[31] Clearly, the enhancement of pseudo-capacitive charge storage would favor the improvement of

4

electrochemical properties.[32] As shown by the previous reports, the energy of grains was distributed in under-potential and over-potential. Small particle sizes would broaden the level of energy distribution, where more ions could take part in the redox reaction.[33, 34] Furthermore, the exploring of conducting polymer with electronic activity demonstrated that the existing of rich oxidation construction enables the improvement of capacitive contribution.[35] In this work, 1D subcarbonate was prepared through the hydrothermal reaction, benefitting from self-assembly manners. After pyrolyzation, the chain-like Co3O4 was composed of nanoparticles with the radii of 25 ~ 100 nm. Assisting by the encapsulation with PPy, the target sample Co3O4/PPy displayed a capacity of 816.6 mAh g-1 at 1.0 A g-1 after 300 loops. Even at the large current density of 5.0 A g-1, it showed strong capacitive behaviors, delivering a capacity of 801.3 mAh g-1 just within ~ 9.8 min. 2. Experimental Raw materials: The cobalt acetate (Co(COOH)2) was selected as the cobalt source. The urea ((NH2)2CO) was used as precipitating agent. All the chemical materials were purchased from Merck without any further process. Preparation of Co(CO3)0.35Cl0.20(OH)1.10·1.74H2O (Co-Pr) rod-like materials. The typical hydrothermal reaction was carried out as previous report.[36] Firstly, 0.6 g urea and 2.4 g cobalt sources were dissolved in 120 ml deionized water (DW), then they were transferred into 200 ml polytetrafluoroethylene reaction kettle. After 120 oC for 12h, the pink products were collected and washed with DW and ethyl alcohol for 5 times. Then, they were dried in the vacuum oven at 60 oC for 8 h. The final products were named as Co-precursor (CoPr).

5

Preparation of C-Co3O4, P-Co3O4, 1D Chain-like Co3O4/PPy The Co-Pr, as the raw materials, were placed in the muffle furnace, and oxidized at 600 oC for 2 h with the heating rate of 2 oC. The as-obtained samples were named as P-Co3O4. The contrast sample was prepared through the calcining of cobalt acetate at the same conditions, and the as-prepared sample was termed as C-Co3O4. For preparing 1D Chain-like Co3O4/PPy, 0.24 g Co-Pr was firstly distributed in 80 mL DW, then 8 mg sodium dodecyl sulfate (SDS) was added. After ultrasonication (1 h) and stirring (2 h), 21 µL pyrrole monomers were injected. After stirring for 1h, 4 mL 0.1 M (NH4)S2O8 solution as oxidizing agent was introduced. After stirring for 0.5 h, the fluffy black products were obtained through the suction filtration, accompanying with 3 times washing of DW and ethyl alcohol. After drying, the final sample was denoted as Co3O4/PPy. Materials characterization The detailed crystalline structure was analyzed through the X-ray diffraction (XRD, the scan rate is 5 o min-1, the Cu Kα radiation is about 0.1542 nm), Raman spectra (Renishaw INVIA micro-Raman spectroscopy). The carbon content was defined by TGA (NETZSCH STA 2950). The element characterization of the samples surface was performed by X-ray photoelectron spectroscopy (XPS) with the assistance of Al Kα radiation, and FT-IR (KBr, Vetex70 FTIR Spectroscope). Morphology of the samples were probed through the utilization of field-emission scanning electron microscopy (FESEM, JSM-7600F), the internal structure was explored by the transmission electron microscope (TEM, JEM-2100F). Electrochemical measurements In this work, the CR-2016 cells were taken to analyze the electrochemical properties. The

6

assembling of cell was carried out in the glove box filled with the argon (O2 < 0.5 ppm, H2O < 0.5 ppm). The sodium metal and polypropylene separator (Celgard 2400) were selected as the counter electrode and separator. For obtaining the electrode as anodes, the active materials (CCo3O4 or P-Co3O4), carboxymethyl cellulose sodium (CMC), and Super P were mingled with the ratio of 70 : 15 : 15 (weight). Then, a certain amount of deionized water was added, after stirring for 24 h, the uniform slurry was fabricated. It was homogeneously painted on the copper foil, which was further tailored into the wafer with the radii of 0.7 cm. And the final weight of active materials were about 1.0 ~ 1.5 mg. And the electrolyte is composed of 1M LiClO4 in propylene carbonate (PC) with the additive of 5 vol% fluoroethylene carbonate (FEC). The galvanostatically charge/discharge cycling were performed in the voltage of 0.01 ~ 3.0 V on the Land battery (CT 2001A) and Arbin battery cycler (BT2000). Through the utilization of MULTI AUTOLAB M204 instruments, the cyclic voltammetry (CV) was achieved at different scan rates.

3. Results and discussion 3.1. The physical-chemistry traits of Co-Pr, C-Co3O4, P-Co3O4, Co3O4/PPy. Utilizing the manners of nano-engineering, altering morphology and tailoring the interfacial properties, 1D chain-like Co3O4/PPy was effectively designed. Benefitting from the aforementioned controlling manners, the enhanced electrochemical properties were anticipated. As shown in Fig. 1(a), the existing PPy has the advantage of facilitating the ions shuttling and protecting the electrodes from the electrolyte erosion. Note that its abundant oxidation structure is conductive to enhance surface-controlled behaviors.[35] Meanwhile, the as-obtained 1D construction could promote the rapid rate of Li-ions transfer at one direction, whilst the decreased diameters of grains boosted the broadening of the energy distribution.[37, 38] It is obvious that all of the merits above could have a synergistic effect on improving Li-storage

7

capability. In Fig. 1(b), it is clear that all the peaks at 19o, 31.2o, 36.8o, 44.8o are well corresponding to the planes of (111), (220), (311) and (400). Thus, the XRD patterns of three asprepared samples are assigned to the standard sample Co3O4 (Fd3m (227) space group, JCPDs: 42-1467), indicating the high purity of the as-resulted samples. Upon comparison of the stronger peak intensity of Co3O4, that of PPy was not observed. In the Raman spectra of Fig. 1(c), compared to that of C-Co3O4 and P-Co3O4, the distinct peaks at 1356 cm-1, 1564 cm-1 are viewed, which belongs to the characteristic peaks of antisymmetric C-N vibration with neutral species CC stretching, and C=C vibration in the carbon skeleton.[39] Expected that the functioned PPy is successfully formed with the abundant oxide states, which is crucial for the improvement of pseudo-capacitive contribution.[35] In the inset picture of Fig. 1(d), the residual product is Co3O4 after calcining at air atmosphere. Thus, the content of PPy is defined about 6.57%. And the size distributions were displayed in Fig. 1(e), note that of C-Co3O4 is similar to that of P-Co3O4. Some large particles of C-Co3O4 were found, which is ascribed to the aggregation of small particles during sintering. Furthermore, the size of Co3O4/PPy is slightly increased, resulting from the introduction of PPy. Notably, the largest specific surface area of Co3O4/PPy (14.3 m2 g1

) is revealed in Fig. 1 (f), larger than that of P-Co3O4 (9.4 m2 g-1) and C-Co3O4 (4.3 m2 g-1).

Based on the previous reports,[32, 40] the exposed area would enlarge the contacting area with electrolyte, followed by the increasing of electrochemical redox reactions. As presented in Fig. 1(g), the FTIR peaks at 940 cm-1, 1569cm-1, 1627cm-1 were indexed to the vibration of C-C, C-N, C=O bonds, respectively. And the C-C bond came from out-of-plan vibrations, which is the evidential polymeric structure of PPy, further confirming the formation of PPy. The surface composites and valence of Co3O4/PPy were explored in Fig. 2. As expected, the signals of Co2p, O1s, C1s and N1s were clearly observed in the full XPS spectra. In the high-

8

resolution spectra of Co2p, a pair of peaks at ~780 eV, ~795 eV are assigned to Co2p3/2, Co2p1/2.[41] They could be deconvoluted into four peaks: (1) these located at 779.5 eV, 794.2 eV are related to Co3+, (2) those suited at 780.8 eV, 796.4 eV are associated with Co2+.[17, 42] As displayed in Fig. 2(c), the bonds Co-O/Co-O-Co can be found at 531.3 eV. Importantly, in Fig. 2(d), three peaks can be divided into N1s at 399.2 eV, 399.8 eV and 400.3 eV, originated form the quinoidimine (-N=), the benzenoid amine (-NH-) as well as the cationic nitrogen atoms (-N+), indicating the effective polymerization of pyrrole. In Fig. 2(e), the five fitting peaks of C1s were shown, including the β-carbons at 284.3 eV, the α-carbons bonds at 285.1 eV, the C=N bonds at 286.3 eV, the -C=N+/C=O bonds at 287.7 eV and the π-π* bonds at 290.3 eV.[43] According to the afore-mentioned analysis and previous reports,[44, 45] it is concluded that the Co3O4/PPy composite was successfully formed, which could remarkably enhance the Li-storage capability. The physical-chemistry traits of Co-Pr were crucial to obtain the target samples. SEM images at low magnification were presented in Fig. 3(A1, A2), it is clear that the rods were disorderly but loosely heap up, which is conductive to inhibit the aggregation in the calcining progress. And the well-defined rod-like structure was expressed in Fig. 3(A3), which is vital for the uniform obtained chain-like structure. The homogeneous Co-Pr rods were displayed in Fig. 3A1, coming from the self-assembly of cobalt acetate and urea, which exhibit the well-disparity in the ethanol. Moreover, note that no obvious agglomeration plots appeared in the rods, suggesting that the Co ions were well distributed in the structure, which is significant for the Kirkendall effect due to the facilely movement of Co-ions.[46-49] The XRD pattern of Co-Pr agrees well with the standards cobaltous dihydroxycarbonate (Co(CO3)0.35Cl0.20(OH)1.10·1.74H2O, JCPDs: 38-0547), where the existed heter-elements (C, Cl, H) were easily left in the high

9

temperature. And the size distribution of Co-Pr was mainly suited from 0.615 um to 2.5μm, the specific surface area is about 4.58 m2 g-1. TG curves revealed that the Co3O4 would be obtained above 400 oC in the air atmosphere, exhibiting the validity of experiment condition. These advantages of Co-Pr would facilitate the generation of 1D chain-like structures. Note that the morphology-controlling and size-engineering are meaningful for the expected electrochemical properties. The morphology of C-Co3O4 was displayed in Fig. 4(A), which is composed of small particles. This is due to that the by-products (CO2, H2O) could inhibit its aggregation in the sintering process. However, some bulks were still existed, which would bring about less active sites, followed by the unsatisfied capacity. In Fig. 4B, after the oxidation of CoPr, it is anticipated that the structure is very fluffy, favoring the soaking of electrolyte and the rapid rate of ions transfer. Moreover, the chain-like P-Co3O4 rods were obtained, and it is clear that the length of P-Co3O4 kept similar to that of Co-Pr, revealing that the basic morphology was maintained well. And the chain-like rod is composed of particles, showing numerous defects and interfaces of grains.[31] Surprisingly, as displayed in Fig. 4C, note that no crosslinking was discovered, indicating that the conducting PPy was only polymerized on the surface of P-Co3O4. Furthermore, In Fig. 4C3, P-Co3O4 was fully encapsulated in the transparent shell. It is well known that the unbroken layer was very helpful to alleviate the volume swelling and prevent the electrolyte erosion. In addition, the mapping images of Co, O, C, N in Co3O4/PPy sample were exhibited in Fig. 4C4, suggesting that they were uniformly distributed in the composite. Through the analysis of energy dispersive spectrometer (EDS), the C, N elements proved the existing of PPy. And the atomic ratio of Co with O is defined ~ 3 : 4, further revealing the formation of Co3O4.

10

Triggered by the fascinated surface morphology, the detailed internal structure was explored through TEM. In Fig. 5A, the small particles of C-Co3O4 were produced but with some aggregation. Compared with the particles size of C-Co3O4 and P-Co3O4, their similar distribution of particles size is found about 20 ~ 100 nm. According to the Kirkendall effect, the relative motion rates of anions and cations would lead to different morphology.[15] Owing to the high activity and small radius, the oxygen ions rapidly entered into the Co-Pr before the Co ions moved toward outside. Thus, small particles were linked to form 1D chain-like structure, which could supply the diffusion channel for lithium ions, favoring the pseudo-capacitive contributions. In Fig. 5C1, the introduction of PPy did not render the P-Co3O4 cross-linked, surprisingly displaying better mono-dispersity. Fig. 5C4 shown that the thickness of PPy coating layer is about ~20 nm. In Fig. 5C5, it is found that the PPy are tightly attached on the surface of P-Co3O4. Meanwhile, the plane distances are about 0.24, 0.47 nm, indexing to (311), (111) facets of Co3O4 (JCPDs: 43-1003). In the selected area diffraction (SAED), the diffraction spots are related to (111), (311), (511). The afore-mentioned analysis triggered the proposing of the corresponding formed mechanism as shown in Schemes 1. Firstly, the Co2+ and urea were combined to obtain numerous Co-seeds. With the assistance of high temperature and pressure, they are selfassembled at one orient to form 1D rod structure. Based on Kirkendall effect, oxygen ions were introduced from air atmosphere at high temperature. Cobalt ions, before diffusing outwards, have been reacted with O2 to produce Co3O4 particles.[15] And the as-resulted particles were well interconnected to form 1D chain-like structure. Unitizing the sodium dodecyl sulfate (SDS) as dispersing agent, the P-Co3O4 with pyrrole monomers were uniformly distributed in the solution.

11

The oxidizing agent rendered the monomer polymerized to PPy at the surface of the basic PCo3O4 to prepare the target sample 1D chain-like Co3O4/PPy. 3.2. The exploring of the as-resulted samples on electrochemical properties of half/full-cell. The interesting development of morphology would induce the evolution of electrochemical properties as shown in Fig. 6. The cycling performance was tested at 1.0 A g-1 in the potential rage from 0.01V to 3.0V. The initial discharge /charge capacities of the as-prepared samples were 1069.4/771.3 mAh g-1 of C-Co3O4 with coulombic efficiency (CE, 72.1%), 1071.9/871.3 mAh g-1 of P-Co3O4 with CE (81.3%) and 1114.5/865.9 mAh g-1 of Co3O4/PPy with CE (77.6%). The first irreversible capacity mainly came from the side reactions and the formation of solid electrolyte interphase film (SEI).[50] Note that P-Co3O4 sample exhibited the highest Li-storage capacity, which is mostly ascribed to its adequate active sites. In comparison, the target sample Co3O4/PPy delivered the largest discharge capacity, and it is conjected that the introduction of PPy rendered more ions absorbed on the surface without desorption. After 300 loops, the residual capacity is remained in order of Co3O4/PPy (816.6 mAh g-1) > P-Co3O4 (802.9 mAh g-1) > CCo3O4 (453.7 mAh g-1). The considerable capacity of Co3O4/PPy is resulted from the unique architectures and significant coating layer. Fig. 6b displayed galvanostatic discharge and charge profiles of the as-obtained samples. The similar platforms of them were observed, revealing the same phase transformation. In the inset pictures of Fig. 6b, the small platforms referring to insertion process of P-Co3O4 and Co3O4/PPy were found, which is beneficial for the improvement of Li-storage capacity. The differential curves dQ/dV at different cycles were displayed at gradually cycling. The discharged peak located at ~ 1.3V came from the reaction (Co3O4 + 8e- + 8Li+ → 3Co + 4 Li2O), and the charged peak at ~ 2.0V referred to the reversible reaction (3Co + 4Li2O - 8e-→ Co3O4 + 8Li+).[51] With cycling, it is found that the ~ 1.3V peak

12

was gradually fading away, whilst the new peak at ~ 1.4V was enhanced. Meanwhile, the intensity of ~ 2.0V charged peak was reduced and then shifted to the low potential. Based on the previous reports,[51-53] it can be conjected to two reasons: (1) Co3O4 samples were gradually transformed into CoO; (2) the reaction was dominated by the adsorbed reaction, that is to say, by pseudo-capacitive behaviors. Moreover, their rate capability was explored in Fig. 6e at stepwise current of 0.1, 0.5, 2.0 and 3.0 A g-1. The Li-storage capacity of Co3O4/PPy could retain 1105.2, 1047.5, 936.5 and 839.4 mAh g-1, better that those of other samples. Surprisingly, even at large current of 5.0 A g-1, the capacity still reaches up to 801.3 mAh g-1, which is attributed to the protection and conducting of PPy layer. Compared with their platforms at various current densities in Fig. S1, it is viewed that the curves were gradually sloped, indicating that the electrochemical behaviors were mainly controlled by near-surface redox reactions. Furthermore, the change of Co3O4/PPy curves was relatively smaller than those of C-Co3O4 and P-Co3O4. With the ultra-fast charge/discharge of ions at 5.0 A g-1 after 200 loops, Co3O4/PPy can still deliver a highest capacity of 702.7 mAh g-1. Through the analysis of platforms at different cycles in Fig. S11, it is found that the electrochemical behaviors belong to the capacitive behaviors. That is much higher than that of P-Co3O4 (508.1 mAh g-1) and C-Co3O4 (220.8 mAh g-1), perhaps resulted from that the protection of effectively existed PPy, which were further demonstrated by ex-situ SEM in Fig. S7. Moreover, it is found that the target samples of Co3O4/PPy could stabilize the capacities of 822 mAh g-1 after 300 loops at 2.0 A g-1, 800 mAh g1

after 800 loops at 3.0 A g-1 and 533 mAh g-1 after 1200 loops at 4.0 A g-1, greater than previous

reports as displayed in Fig. 6f and Table S1. In Fig. S13, Ragone plots of previous reports were displayed, it is found that the target samples Co3O4/PPy deliver energy density (960 Wh Kg-1) and power density (1.67 W Kg-1).[54] According to the above-mentioned analysis, it is

13

concluded that: (1) the redox reaction would be dominated by surface-controlled behaviors in the cycling; (2) the introduction of PPy could improve the conductivity and defend the electrode materials. As known, LiFePO4 has been commercially used as cathodes due to its ultra-stable electrochemical properties and available potentials in Fig. S3, so it is selected as cathodes of the full-cell in the work. In Fig. S4, it is noted that the capacity of cathode was stabilized with ~100 mAh g-1 at 3.0 A g-1, which is crucial for the matching of electrode materials. Fig. 6i schematically verified the charge-discharge process of the full cell with Co3O4/PPy vs. LiFePO4. Based on the mass of Co3O4/PPy in Fig. S5, the initial charge/discharge capacitates can achieve the capacity of 871/1430 mAh g-1. The cycling properties were displayed in Fig. 6g, showing that the capacity can be kept 526 mAh g-1 at 3.0 A g-1 after 50 cycles. Even after 120 cycles, the capacity of the full-cell still remain 472 mAh g-1 in Fig. S10. Through the 2nd, 3rd discharge/charge profiles, the electrochemical stability was exhibited with the CE of 97%. Different from the platforms of half-cell in Fig. 6b, the smooth profiles of full-cell were found, mainly derived from the voltage gap between LiFePO4 and Co3O4/PPy. Those results above confirmed that Co3O4/PPy is a promising anode material for LIBs. 3.3. The detailed anaylsis of kiectic properties and the change during cycling Owing to the fascinated electrochemical performances, more attentions have been devoted to the exploring of detailed kinetic through CV curves as displayed in Fig. 7. The CV curves of the as-prepared samples were shown in Fig. 7a-c and S7, and it is obvious that their shapes have hardly changed, indicating that their redox reaction could be stabilized at different scan rates. Note that the broad oxygen peaks were centered in the range of 1.0-3.0V (Peak 1 at 2.0V), the wide reduction peaks were distributed between 0.5 and 1.5V (Peak 2 at 0.75V).[55] Previous

14

reports demonstrated that the wide peak mainly resulted from capacitive behaviors.[14, 38] It is explained that particle size-tuning and morphology-altering could favor the increased surface defects with more active sites. Considering their growing of width, it is confirmed that the energy distribution would be broadened, further bringing about more redox reaction on the surface/near-surface. Fig. 7d exhibited the first CV curves at 0.1 mV s-1. And their sharp peaks suited at ~ 0.75V were observed, mostly derived from the formation of SEI films and irreversible side reactions, matching well with the analysis of discharge/charge profiles.[56, 57] Notably, the broad peak of Co3O4/PPy was distinguished, verifying the enhancement of pseudo-capacitive behaviors. Moreover, according to the Equation. 1, the diffusion coefficient (D) of lithium-ions was calculated. Herein, the n is the number of electrons transferring (8), A is the contact area of electrolyte/electrode, v is the scan rate, and C is selected as the concentration of electrolyte. In Fig. 7e, the value (Peak1, 2) of Ip/v1/2 is fitting to 1.6, -2.8 for C-Co3O4, 1.8, 2.6 for P-Co3O4, 2.5, and -3.1 for Co3O4/PPy. Thus, the D value of C-Co3O4 is about 0.53×10-13, 1.63×10-13 cm2 s-1, that of P-Co3O4 is calculated about 0.68×10-13, 1.41×10-13 cm2 s-1, whilst that of Co3O4/PPy is defined about 1.3×10-13, 2.0×10-13 cm2 s-1. Obviously, the target sample Co3O4/PPy showed the largest average D (~ 1.65 ×10-13 cm2 s-1), boosting the diffusion of lithium ions. Herein, the in-depth kinetic analysis of Co3O4/PPy was investigated in details. In comparison of the results in Fig. 6(e), the small polarization of redox peaks agreed well with the minor platforms dropping. And the capacity vs. charge/discharged times were plotted in Fig. 7(f), which revealed that the charge-storage capacity achieved 1083.5 mAh g-1 after ~639 min. The value has a difference with the theoretical capacity (890 mAh g-1), which may be attributed to the emerging of new reaction sites of surface.[58] More significantly, it is surprising that the capacity still reach up to 795.3 mAh g-1, about ~ 90% of theoretical capacity, just consuming 5.5

15

min. It can be concluded that (1) the smaller particles size reduced the ions diffusion paths; (2) 1D architecture quickened the rate of ions shuttling at one orient; (3) the coating PPy effectively improved its conductivity. In Fig. 7g, h, in the cell, the natural traits of the charge-capacitive behaviors were explored in details. Utilizing the Trasatti analysis, the diffusion-controlled and capacitivedominated were quantified. According to Eqs. 2, 3, Q(v), Qcapacitive and Qtotal referred to the measured galvanostatic charge transferring capacity, the capacitive storage capacity of doublelayer with surface/near/surface redox reactions and the total Li-storage capacity, respectively. α is about the constant, and v is the sweep rate. According to Eqs. 2, when the v-value accessed to infinitely great, the diffusion-dominated behaviors could be nearly eliminated, which would induce the extrapolated largest capacitive-storage. Through the intercept of the fitting linear, it is calculated that the capacity can achieve up to 974 mAh g-1. Alternatively, with Eqs. 3, at the condition that the value of v approached to 0, that is to say, the charge time → ∞, it is confirmed that the sites can be reacted near completely. Through the linear fitting between 1/ Q(v) and v1/2, the value of Qtotal can be conjected to 1167 mAh g-1. Note that the ratio of Qcapacitive/Qtotal can be up to 83%, intimately originated from the effect of nano-engineering and unique constructer. Ip = 2.69×105 n3/2AD1/2v1/2CLi+

(1)

Q(v) = Qcapacitive + α(v-1/2)

(2)

1/ Q(v) = 1/ Qtotal + α(v1/2)

(3)

i = avb

(4)

log (i) = blog(v) + log(a)

(5)

I(V)v-1/2 = k1v1/2 + k2

(6)

16

Moreover, benefitting from the Dunn’s work, the electrochemical properties mainly controlled by diffusion or capacitive behaviors can be determined.[32, 33] In the Equation. 4, when the b is close to 1, the capacitive behaviors is deemed as the dominated electrochemical actions, and the b approaches to 0.5, the capacity is mainly controlled by the diffusion process. As shown in Fig. 7e, the b-values of the line were about 0.78, 0.86, revealing that the pseudocapacitive contribution is the primary contribution of the capacity. Through, the well linear relation of i/v1/2 with v1/2 is exhibited in Fig. 7i, which is vital for calculating k1 (slope) and k2 (intercept).[49] In the Fig. 7k, the pink part is the capacity contribution of capacitive behaviors, whilst the part of diffusion effect is associated with the blue. At the scan rate of 0.9 mV s-1, the surface-controlled contribution reached up to 90.1%, larger than that of other samples as displayed in Fig. S9. Although the obvious blue is deemed as the conversion reaction dominated by diffusion contribution, the data still revealed that the almost total capacity from the capacitive behaviors (containing surface, near-surface, intercalation pseudo-capacitance). In addition, as shown in Fig. 7l, the ratio of capacitive increases with the enlargement of scan rate, such as 82.5% at 0.1 mV s-1, 84.2% at 0.3 mV s-1 , 85.9% at 0.5 mV s-1 and 87.7% at 0.7 mV s-1. This increased ratio is due to that the lacking of time renders fewer ions in-depth participating in the redox reaction, but reacted on the surface/near-surface. The strong capacitive behaviors is conductive to the enhanced rate capability, which mainly originates from the unique structure with shuttling paths and the reducing sixe with broadening energy levels.[31-33] The long-term CV curves were tested at 0.5 and 5.0 mV s-1 to explore the phase transform and electrochemical behaviors as exhibited in Fig. 8. According to the previous report,[51] a couple of Peak 1, 2 were related to conversion reactions, and a pair of Peak 3, 4 referred to insertion/extraction reactions in Fig. 8a. Moreover, the Co3O4 would be decomposed into CoO

17

and Li2O, and the main conversion is gradually fading, which was demonstrated in Fig. S6. Note that the Peak 3, 4 were still remained, revealing that the internal electrochemical properties in the cell were hardly affected during cycling. The sharp peak was related to the diffusion-insertion process, which agreed well with the platform in Fig. 6. After 400 cycling, the reaction was controlled by the adsorbed redox reaction, and the shape of CV curves tended to be that of capacitor-like materials, showing the capacitive behaviors. At 5.0 mV s-1, the wide peaks were displayed, where the shape has similar changing process at 0.5 mV s-1. And, the stability of shape from 401st to 600th effectively suggested that the reaction was denominated by the capacitive behaviors. In order to explore the internal resistance of the as-obtained samples, the electrochemical impedance spectroscopy (EIS) were tested at different cycling in Fig. 9. For the as-obtained samples, the pristine EIS and fitting curves were exhibited in Fig. 9D, and the relative equivalent-circuit diagram were delivered in Fig. 9F. In Fig. S12, the τ (τ = 1/f) is displayed, and it is found that of Co3O4/PPy is about 0.934s, much smaller than that of P-Co3O4 (1.667s), indicating the faster rate of ions transfer.[59] Moreover, Co3O4/PPy displayed the maximum phase angle (~ 78.9), strongly verified its relatively high capacitive behavior.[54, 60] Among them, Rs is associated with the nature impedance of the battery, Rf with CPE1 referred to SEI films, whilst Rct with CPE2 corresponded to the impedance of transfer resistance. It is known that Ws is related to the resistance of ions diffusion, which is the crucial standard to evaluate the speed of electrons shuttling. Co3O4/PPy displayed the relative larger resistance, derived from the inactivity of electrode materials, which is demonstrated by the rapidly reducing resistance in Fig. 9C1. Obviously, in Fig. 9A1-C1, their impedance was reduced with cycling, which is ascribed to the CE (< 100%). Some lithium ions were left in the electrode materials, followed by the

18

improved conductivity. Note that the Nyquist plots of Co3O4/PPy in Fig. 9C1 shown smaller fluctuations than those of C-Co3O4, P-Co3O4, revealing that the stable reactions of the target sample, which confirms that the existing PPy could stabilize the internal electrochemical behaviors. As known, the slope of the fitting linear between ω1/2 and Z″ is of importance for the diffusion resistance, and it is clear that their slopes were decreasing, which is significant for the electrochemical reactions. ω = 2πf

(6)

Zw = Rs + RSEI + Rct + σω-1/2

(7)

D = R2T2/2S2n4F4C2σ2

(8)

Based on Eqs. 6-8, the diffusion coefficients would be defined. Obviously, the D of Co3O4/PPy is much larger than those of C-Co3O4 and P-Co3O4 as shown Table 1, and it is due to the introduction of high conductivity PPy. Moreover, the resistance values of Rs + Rct + R SEI at various cycling were displayed in Fig. 9G. It is found that the R value tended to be similar, indicating that their resistance is stable as displayed Table 2. Thus, it is concluded that the existing PPy and 1D structure facilitated the improvement of conductivity, which is significant for improving the electrochemical properties. 4. Conclusion In summary, the uniform rod-like Co-Pr was successfully prepared through the selfassembly strategy due to the inducing of temperature and pressure. By the thermal-chemical manner and Kirkendall effect, the chain-like P-Co3O4 is obtained, followed by the in-situ polymerization of pyrrole monomer, further forming 1D chain-like Co3O4/PPy with the thickness of ~ 20 nm and the content of 6.57 %. The target sample of Co3O4/PPy displays the smaller size

19

of particles, accompanying with more defects, active sites and energy distribution. Meanwhile, incorporating with PPy, the electrode materials with enhanced conductivity were positively protected from the erosion of electrolyte. Utilized as LIBs anodes, Co3O4/PPy delivers a chargestorage capacity of 816.6 mAh g-1 at 1.0 A g-1 after 300 loops. Even at the large current density of 3.0 A g-1, the capacity still reach up to 800 mAh g-1 after 800 cycles And the capacity of full cell still display a capacity of 526 mAh g-1 after 50 loops at 3.0 A g-1. More importantly, with the assistance of the detailed kinetic analysis, the natural traits of materials is capacitor-like battery system, demonstrating that the strong pseudo-capacitive behaviors is mainly rational and the capacitive contribution could reach up to 90.1% at 0.9 mV s-1. Benefitting from the analysis of long-term CV curves, it is confirmed that the as-resulted Co3O4 could turn into CoO with Li2O, and the electrochemical behaviors are gradually dominated by capacitive redox reaction such as surface/near-surface and intercalation pseudo-capacitance. This work is anticipated to provide the new understanding on the capacitive properties of Co3O4 and shed light on the fundamental reason of high-rate performances.

ACKNOWLEDGMENT This work was financially supported by Young Elite Scientists Sponsorship Program by CAST (2017QNRC001),

National

Key

Research

and

Development

Program

of

China

(2017YFB0102003, 2018YFB0104204), National Natural Science Foundation of China (51622406, 21673298 and 21473258), National Postdoctoral Program for Innovative Talents (BX00192), China Postdoctoral Science Foundation (2017M6203552), Innovation Mover Program of Central South University (2017CX004, 2018CX005), Hunan Provincial Natural Science Foundation of China (2018JJ3633), Postgraduate Electronic Design Competition of

20

China (502241802) and the Fundamental Research Funds for the Central Universities of Central South University (2018zzts013 and 2018zzts369) and National Mittal Student Innovation Program (201810533258).

Appendix A. Supporting information Supplementary data associated with this article can be found in the online version at doi:10.1016/j.nanoen.

REFERENCES [1] V. Augustyn, P. Simon, B. Dunn, Energy Environ. Sci., 7 (2014) 1597-1614. [2] B. Wang, W. Al Abdulla, D. Wang, X.S. Zhao, Energy Environ. Sci., 8 (2015) 869-875. [3] G. Yang, H. Song, H. Cui, C. Wang, J. Mater. Chem. A, 3 (2015) 20065-20072. [4] B. Wang, X.-Y. Lu, C.-W. Tsang, Y. Wang, W.K. Au, H. Guo, Y. Tang, Chem. Eng. J., 338 (2018) 278-286. [5] C. Luo, Y. Xu, Y. Zhu, Y. Liu, S. Zheng, Y. Liu, A. Langrock, C. Wang, ACS Nano, 7 (2013) 8003-8010. [6] G. Zhou, D.-W. Wang, L.-C. Yin, N. Li, F. Li, H.-M. Cheng, ACS Nano, 6 (2012) 32143223. [7] H. Kim, H. Kim, Z. Ding, M.H. Lee, K. Lim, G. Yoon, K. Kang, Adv. Energy Mater., 6 (2016). [8] H. Hou, L. Shao, Y. Zhang, G. Zou, J. Chen, X. Ji, Adv. Sci, 4 (2017).

21

[9] M. Wang, Y. Yang, Z. Yang, L. Gu, Q. Chen, Y. Yu, Nitrogen Dual-Doping, Advanced Science, 4 (2017). [10] H. Hou, C.E. Banks, M. Jing, Y. Zhang, X. Ji, Adv Mater, 27 (2015) 7861-7866. [11] X. Zhou, L.-J. Wan, Y.-G. Guo, Adv. Mater., 25 (2013) 2152-2157. [12] K. Zhang, Z. Hu, X. Liu, Z. Tao, J. Chen, Adv. Mater., 27 (2015) 3305-3309. [13] Z. Hu, Q. Liu, S.L. Chou, S.X. Dou, Adv Mater, (2017). [14] P. Ge, H. Hou, C.E. Banks, C.W. Foster, S. Li, Y. Zhang, J. He, C. Zhang, X. Ji, Energy Storage Materials, (2018). [15] P. Ge, C. Zhang, H. Hou, B. Wu, L. Zhou, S. Li, T. Wu, J. Hu, L. Mai, X. Ji, Nano Ener., 48 (2018) 617-629. [16] Q.-Z. Zhang, D. Zhang, Z.-C. Miao, X.-L. Zhang, S.-L. Chou, Small, 14 (2018). [17] X.-l. Huang, R.-z. Wang, D. Xu, Z.-l. Wang, H.-g. Wang, J.-j. Xu, Z. Wu, Q.-c. Liu, Y. Zhang, X.-b. Zhang, Adv. Funct. Mater., 23 (2013) 4345-4353. [18] G. Zhang, W. Li, K. Xie, F. Yu, H. Huang, Adv. Funct. Mater., 23 (2013) 3675-3681. [19] S. Zhu, J. Li, X. Deng, C. He, E. Liu, F. He, C. Shi, N. Zhao, Adv. Funct. Mater., 27 (2017). [20] S. Kalasina, N. Phattharasupakun, M. Suksomboon, K. Kongsawatvoragul, M. Sawangphruk, Electrochim. Acta., 283 (2018) 1125-1133. [21] S. Kalasina, N. Phattharasupakun, M. Sawangphruk, J. Mater. Chem. A., 6 (2018) 36-41. [22] X. Wu, Z. Han, X. Zheng, S. Yao, X. Yang, T. Zhai, Nano Ener., 31 (2017) 410-417. [23] H. Du, K. Huang, M. Li, Y. Xia, Y. Sun, M. Yu, B. Geng, Nano Research, 11 (2018) 14901499. [24] Y. Zhu, Y. Chu, J. Liang, Y. Li, Z. Yuan, W. Li, Y. Zhang, X. Pan, S.-L. Chou, L. Zhao, R. Zeng, Electrochim. Acta., 190 (2016) 843-851.

22

[25] X. Li, X. Tian, T. Yang, Y. Song, Y. Liu, Q. Guo, Z. Liu, J. Alloys Compd., 735 (2018) 2446-2452. [26] P. Ge, H. Hou, X. Ji, Z. Huang, S. Li, L. Huang, Mater. Chem. Phys., 203 (2018) 185-192. [27] Y. Shi, X. Pan, B. Li, M. Zhao, H. Pang, Chem. Eng. J., 343 (2018) 427-446. [28] H. Fan, H. Yu, Y. Zhang, J. Guo, Z. Wang, H. Wang, N. Zhao, Y. Zheng, C. Du, Z. Dai, Q. Yan, J. Xu, Energy Storage Materials, 10 (2018) 48-55. [29] Q. Sun, Z.J. Wang, Z.J. Zhang, Q. Yu, Y. Qu, J.Y. Zhang, Y. Yu, B. Xiang, ACS Appl. Mater. Interfaces., 8 (2016) 6303-6308. [30] D. Chao, P. Liang, Z. Chen, L. Bai, H. Shen, X. Liu, X. Xia, Y. Zhao, S.V. Savilov, J. Lin, Z.X. Shen, ACS Nano, 10 (2016) 10211-10219. [31] T. Brezesinski, J. Wang, S.H. Tolbert, B. Dunn, Nat Mater, 9 (2010) 146-151. [32] C. Chen, Y. Wen, X. Hu, X. Ji, M. Yan, L. Mai, P. Hu, B. Shan, Y. Huang, Nat Commun, 6 (2015) 6929. [33] D. Chao, C. Zhu, P. Yang, X. Xia, J. Liu, J. Wang, X. Fan, S.V. Savilov, J. Lin, H.J. Fan, Z.X. Shen, Nat Commun, 7 (2016) 12122. [34] Y.L. Ding, Y.R. Wen, P.A. van Aken, J. Maier, Y. Yu, Small, 11 (2015) 2011-2018. [35] V. Brânzoi, L. Pilan, F. Brânzoi, , Electroanalysis, 21 (2009) 557-562. [36] H. Li, X. Qian, C. Zhu, X. Jiang, L. Shao, L. Hou, J. Mater. Chem. A, 5 (2017) 4513-4526. [37] H. Hou, M. Jing, Z. Huang, Y. Yang, Y. Zhang, J. Chen, Z. Wu, X. Ji, ACS Appl. Mater. Interfaces., 7 (2015) 19362-19369. [38] A. Eftekhari, M. Mohamedi, Mater. Today Energy, 6 (2017) 211-229. [39] V.S. Simi, A. Satish, P.S. Korrapati, N. Rajendran, Applie. Sur. Sci., 445 (2018) 320-334.

23

[40] H. Wan, Y. Liu, H. Zhang, W. Zhang, N. Jiang, Z. Wang, S. Luo, H. Arandiyan, H. Liu, H. Sun, Electrochim. Acta., 281 (2018) 31-38. [41] C. Yan, Y. Zhu, Y. Li, Z. Fang, L. Peng, X. Zhou, G. Chen, G. Yu, Adv. Funct. Mater., 28 (2018) 1705951. [42] K. Zhang, M. Park, L. Zhou, G.-H. Lee, W. Li, Y.-M. Kang, J. Chen, Adv. Funct. Mater., 26 (2016) 6728-6735. [43] D.-H. Nam, M.-J. Kim, S.-J. Lim, I.-S. Song, H.-S. Kwon, J. Mater. Chem. A, 1 (2013) 8061. [44] N. Ballav, R. Das, S. Giri, A.M. Muliwa, K. Pillay, A. Maity, Chem. Eng. J., 345 (2018) 621-630. [45] Y. Wang, X. Wu, W. Zhang, C. Luo, J. Li, J. Magn. Magn. Mater, 443 (2017) 358-365. [46] Z. Fei, S. He, L. Li, W. Ji, C.-T. Au, Chem Commun, 48 (2012) 853-855. [47] Z. Yang, M. Walls, I. Lisiecki, M.-P. Pileni, , Chem. Mater., 25 (2013) 2372-2377. [48] X. Yu, B. Qu, Y. Zhao, C. Li, Y. Chen, C. Sun, P. Gao, C. Zhu, , Chemistry-a European Journal, 22 (2016) 1638-1645. [49] G. Xiao, Y. Zeng, Y. Jiang, J. Ning, W. Zheng, B. Liu, X. Chen, G. Zou, B. Zou, Small, 9 (2013) 793-799. [50] W. Kang, Y. Zhang, L. Fan, L. Zhang, F. Dai, R. Wang, D. Sun, ACS Appl Mater Interfaces, 9 (2017) 10602-10609. [51] Q. Yang, C. Feng, J. Liu, Z. Guo, Appli. Sur. Sci., 443 (2018) 401-406. [52] L. Cao, Q. Kang, J. Li, J. Huang, Y. Cheng, Appli. Sur. Sci., 455 (2018) 96-105. [53] C.-L. Zhang, B.-R. Lu, F.-H. Cao, Z.-L. Yu, H.-P. Cong, S.-H. Yu, J. Mater. Chem. A, 6 (2018) 12962-12968.

24

[54] B. Purty, R.B. Choudhary, A. Biswas, G. Udayabhanu, Mater. Chem. Phys., 216 (2018) 213-222. [55] C. Yan, C. Lv, Y. Zhu, G. Chen, J. Sun, G. Yu, Adv Mater, 29 (2017). [56] H. Xue, Z. Na, Y. Wu, X. Wang, Q. Li, F. Liang, D. Yin, L. Wang, J. Ming, J. Mater. Chem. A, 6 (2018) 12466-12474. [57] M. Liang, M. Zhao, H. Wang, F. Wang, X. Song, J. Energy. Storage, 17 (2018) 311-317. [58] C. Wu, Y. Jiang, P. Kopold, P.A. van Aken, J. Maier, Y. Yu, Adv Mater, 28 (2016) 72767283. [59] C. Pan, H. Gu, L. Dong, J. Power Sources., 303 (2016) 175-181. [60] H. Wang, L. Ma, M. Gan, T. Zhou, J. Alloys Compd., 699 (2017) 176-182.

Peng Ge received his Master degree in College of Chemistry, Chemical and Environmental Engineering, Henan University of Technology. He is now pursuing his Ph.D. degree at College of Chemistry and Chemical Engineering, Central South University. His research focuses on the synthesis and application of nanomaterials for electrochemical energy storage and conversion.

25

Sijie Li received her B.S. degree in The College of Chemistry from Xiangtan University in 2016. She is currently working toward the Master degree in Central South University and her current research involves the transitional metal chalcogenides as anode for lithium ion battery and sodium ion battery.

Honglei Shuai received his Master degree in College of Chemistry, Chemical and Environmental Engineering, Xinyang Normal University. He is now pursuing his Ph.D. degree at

26

College of Chemistry and Chemical Engineering, Central South University. His research focuses on the synthesis and application of nanomaterials for electrochemical energy storage and conversion.

Wei Xu received his Master degree in College of Science, Huazhong Agricultural University. He is now pursuing his Ph.D. degree at College of Chemistry and Chemical Engineering, Central South University. His research focuses on the theory and application of single particle collision method for battery materials.

Ye Tian received Phd degree in school of physics and optoelectronics, Xiangtan University. Now He is working in collage of chemistry and chemical engineering, Central South University as a post-doctor, researching on the design and fabrication of nano-sized energy storage materials.

27

Li Yang received her Ph.D. degree in College of Chemistry and Chemical Engineering, Nanjing University. She is now pursuing postdoctor at College of Chemistry and Chemical Engineering, Central South University. Her research focuses on the synthesis and application of nanomaterials for electrochemical energy storage and conversion.

Guoqiang Zou is an Associate Professor at the College of Chemistry and Chemical Engineering, Central South University. He received his Ph.D. at Central South University in 2018. His current research interests are MOF-derived materials for electrochemical energy storage devices.

28

Hongshuai Hou is an Associate Professor at the College of Chemistry and Chemical Engineering, Central South University. He received his Ph.D. at Central South University in 2016. His current research interests are electrochemistry and key materials for electrochemical energy storage devices.

Xiaobo Ji is a “Shenghua” Professor at Central South University and a Fellow of the Royal Society of Chemistry, specializing in the research and development of batteries and supercapacitor materials and their systems. He received his Ph.D. in Electrochemistry in 2007 under the supervision of Prof. Richard Compton at the University of Oxford and undertook postdoctoral work at MIT with Prof. Donald Sadoway.

29

Figures and Captions Figure 1. The schematic diagram of lithium shuttling (a), power XRD patterns (b), Raman spectra (c), Size distribution (e), nitrogen adsorption/desorption isotherms (f), FT-IR curves (g) of C-Co3O4, P-Co3O4 and Co3O4/PPy, the TGA and DSC curves of Co3O4/PPy (d). Figure 2. For 1D chain-like Co3O4/PPy: the full XPS spectra (a), high-resolution XPS spectra Co2p (b), O1s (c), N1s (d) and C1s (e). Figure 3. For 1D rod-like Co-Pr: SEM images (A1 - A3), TEM images (B1 - B3), XRD patterns (C1), Size distribution (C2), N2 adsorption-desorption isotherm (C3), TGA and DSC curves (C4). Figure 4. SEM images of C-Co3O4 (A1 - A3), P-Co3O4 (B1 - B3) and 1D chain-like Co3O4/PPy (C1 - C3), the corresponding elemental Mapping images of Co, O, C, N in Co3O4/PPy sample (C4), EDS spectra of Co3O4/PPy (C5). Figure 5. TEM images of C-Co3O4 (A1 - A3), P-Co3O4 (B1 - B3) and Co3O4/PPy (C1 - C3); For 1D chain-like Co3O4/PPy: HRTEM images (C4, C5) and SAED image (C6). Figure 6. For three as-obtained samples in the half-cell: the lithiation/delithiation cycling performance at 1.0 A g-1 (a), the initial charge/discharge platforms (b), the rate capability (d), the cycling properties at large current density at 5.0 A g-1 (e), differential curves dQ/dV of Co3O4/PPy at different cycling (c). The comparison of previous works on Li-storage capacity. In the full-cell, the cycling properties of Co3O4/PPy vs. LiFePO4 (g), the corresponding charge/discharge profiles (h), the schematic illustration of the full-cell (i). Figure 7. The CV curves at different sweep rates of C-Co3O4 (a), P-Co3O4 (b) and 1D chain-like Co3O4/PPy (c). For the as-prepared samples : the CV curves at first cycling at 0.1 mV s -1 (d), the linear relation between peak current Ip and square root of various scan rate v1/2 (e). Li-storage capacity vs. charge/discharge time of Co3O4/PPy (f). Detailed kinetic exploring through Trasatti

30

analysis: v → ∞, indicating the more diffusion-controlled behaviors were inhibited, the maximum theoretical capacitive storage capacity is calculated about 974 mAh g-1 (g), v → 0, indicating that the diffusion-controlled redox reaction can take place adequately, the Qtotal is calculated about 1167 mAh g-1 (h), the linear relation of I/v1/2 vs. v1/2 (i), log(i) vs. log(v) (j), capacitive storage (pink), diffusion contribution (blue) and the corresponding ratio at 0.9 mV s-1 (k), the capacitive ratios at stepwise scan rates (I). Figure 8. The long-term CV curves of 1D chain-like Co3O4/PPy at 0.5 mV s-1 (a - c), 21st 200th (a), 201st - 400th (b), 401st - 600th (c). At 5.0 mV s-1 (d - f), 21st - 200th (d), 201st - 400th (e), 401st - 600th (f). Figure 9. The Nyquist plots, the liner relation between ω1/2 and Z″ at various cycling of C-Co3O4 (A1, A2), P-Co3O4 (B1, B2) and 1D chain-like Co3O4/PPy (C1, C2). For the as-obtained samples: the pristine EIS and fitting curves (D) and relative equivalent-circuit diagram (F), the diffusion coefficients at different cycling (E), the resistance values of Rs + Rct + R SEI at various cycling (G). Scheme 1. The schematic diagram of 1D chain-like Co3O4/PPy.

Table. 1 The value of DLi+ for the target samples at different cycles.

cycles DLi+

C-Co3O4

6th

9th

12th

15th

18h

21st

24th

0.29

0.58

0.78

1.19

1.48

1.84

2.46

31

P-Co3O4

0.85

2.46

3.92

6.70

10.4

11.4

14.52

Co3O4/PPy

5.18

10.66

17.72

24.55

29.91

38.33

38.49

(DLi+, ×10-14 cm2 s-1)

Table 2. The value of Rs + Rct + RSEI for the target samples at different cycles. 1st

6th

9th

12th

15th

18h

21st

24th

C-Co3O4

98

50

36

27

23

21

19

18

P-Co3O4

70

43

29

28

30

27

26

27

Co3O4/PPy

260

33

26

24

22

22

22

22

cycles Rtotal

(Rtotal, Ω)

32

Figure 1

33

Figure 2

34

Figure 3

35

Figure 4

36

Figure 5

37

Figure 6

38

Figure 7

39

Figure 8

40

Figure 9

41

Scheme 1

42

Highlights 1. 2. 3. 4. 5. 6.

The chain-like Co3O4 was obtained through Kirkendall effect. The sample was in-situ encapsulated into PPy. The pseudo-capacitive nature of Co3O4 was studied. Co3O4/PPy showed the good cycling stability (800 mAh g-1 after 800 cycles). The capacity of full-cell reached to 532 mAh g-1 at 3.0 A g-1. The in-depth kinetic analysis was effectively performed.

43