A dual-carbon-anchoring strategy to fabricate flexible LiMn2O4 cathode for advanced lithium-ion batteries with high areal capacity

Journal Pre-proof A dual-carbon-anchoring strategy to fabricate flexible LiMn2O4 cathode for advanced lithium-ion batteries with high areal capacity Xiaoliang Yu, Jiaojiao Deng, Xin Yang, Jia Li, Zheng-Hong Huang, Baohua Li, Feiyu Kang PII:

S2211-2855(19)30963-2

DOI:

https://doi.org/10.1016/j.nanoen.2019.104256

Reference:

NANOEN 104256

To appear in:

Nano Energy

Received Date: 1 September 2019 Revised Date:

30 October 2019

Accepted Date: 31 October 2019

Please cite this article as: X. Yu, J. Deng, X. Yang, J. Li, Z.-H. Huang, B. Li, F. Kang, A dual-carbonanchoring strategy to fabricate flexible LiMn2O4 cathode for advanced lithium-ion batteries with high areal capacity, Nano Energy, https://doi.org/10.1016/j.nanoen.2019.104256. This is a PDF file of an article that has undergone enhancements after acceptance, such as the addition of a cover page and metadata, and formatting for readability, but it is not yet the definitive version of record. This version will undergo additional copyediting, typesetting and review before it is published in its final form, but we are providing this version to give early visibility of the article. 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. © 2019 Published by Elsevier Ltd.

A dual-carbon-anchoring strategy to fabricate flexible LiMn2O4 cathode for advanced lithium-ion batteries with high areal capacity Xiaoliang Yu a,b,c‡, Jiaojiao Deng c‡, Xin Yang c, Jia Li c,*, Zheng-Hong Huang a,b,*, Baohua Li c, Feiyu Kang b,c,* a

State Key Laboratory of New Ceramics and Fine Processing, School of Materials

Science and Engineering, Tsinghua University, Beijing 100084, P. R. China b

Key Laboratory of Advanced Materials (MOE), School of Materials Science and

Engineering, Tsinghua University, Beijing 100084, P. R. China c

Engineering Laboratory for Functionalized Carbon Materials, Graduate School at

Shenzhen, Tsinghua University, Shenzhen 518055, P. R. China E-mail: [email protected], [email protected], [email protected]. ‡

These authors contributed equally to this work.

Abstract Lithium transition metal oxides (LTMOs) are important cathode materials in lithium-ion batteries (LIBs). Constructing the robust hybrid of LTMO-flexible substrate is of great significance for developing advanced flexible LIBs. However, currently reported flat noble metal-based flexible cathodes are cost-expensive and show quite low areal capacities. Developing low-cost and nanostructured flexible substrates for LTMO cathodes is highly desirable but still rarely reported. Particularly challenging is preventing flexible substrate corrosion and mitigating/eliminating the severe ion migration at interface during necessary high-temperature annealing process. 1

Herein, carbon nanofibers (CNFs) with truncated conical graphene layers are carefully chosen as flexible substrates for the growth of ultrasmall LiMn2O4 nanocrystals. The highly graphitic structure enables good high-temperature oxidation resistance. The plenty of exposed graphitic edge planes afford unexpected strong anchoring of LiMn2O4, evidenced by both experimental results and theoretical calculations. Moreover, an amorphous carbon layer is simultaneously introduced and coated on LiMn2O4 nanocrystals, which provides another strong outer anchoring like a ‘cargo net’. Such

dual-carbon-anchoring strategy help produce a 1D

LiMn2O4-nanocarbon hybrid with robust interface. As LIB cathode, it owns fast electron conduction, smooth Li+ transportation, good electrochemical stability and especially superior mechanical flexibility, thus enabling a high areal mass loading of 17.7 mg cm-2. The corresponding fabricated flexible LiMn2O4/CNF@C//CNF full cell exhibits a high areal capacity of 2.01 mAh cm-2 as well as good rate capability and cycling stability.

Keywords: flexible lithium-ion batteries; cathode; carbon anchoring; high areal capacity

1. Introduction The accelerated growth in flexible and wearable electronics like deformable displays [1], flexible communication devices [2] and wearable sensors [3] has triggered increasing demands for flexible power sources [4, 5]. Among various energy storage technologies, lithium-ion batteries (LIBs) are quite attractive due to their high 2

energy/power density, long cycle life and environmental benignity [6, 7]. Besides the lithium storage capacity, kinetics and stability, the electrode flexibility is another crucial factor that needs to be considered. Current commercial slurry-coated films with copper/aluminum foils as current collectors cannot withstand repeated bending/deformation because of the rigidity and fragility of the electrode materials. The design of flexible LIBs thus necessitates the advancement of flexible substrates, such as 1D carbon nanotube/nanofiber [8, 9], 2D graphene [10, 11], 3D graphene foam [12, 13] and carbon cloth [14, 15]. Despite the great achievement in manufacturing flexible electrodes, regrettably, current research progress of flexible hybrid cathodes [16, 17] falls far behind that of flexible anodes, especially for the commercial lithium transition metal oxide (LTMO) cathodes [18]. In order to obtain high crystallinity, the synthesis of LTMO normally requests an annealing process at high temperature [19], during which most above-mentioned substrates cannot resist the high-temperature oxidation. To address this issue, on the one hand, some researchers impregnated preformed LTMO slurries into flexible substrates [20-22]. As-fabricated hybrid electrodes inherit the structural flexibility from the substrates but they provide only physical contact between the substrate and active material. Large contact resistance and severe electrode polarization are thus generated during high-power output [23]. On the other hand, some research works synthesized LTMO through hydrothermal reactions at relatively lower temperature [24, 25]. Unfortunately, severe ion migration at LTMO-substrate interface occurs during the solution reaction, leading to quite weak interfacial 3

interaction. This would significantly reduce the charge transfer capability and overall flexibility of the hybrid electrode [26]. Moreover, the crystallinity is not high after such low-temperature synthesis, inducing poor electrochemical stability [27]. Up to date, only a limited number of high-temperature oxidation-resistant substrates have been reported for the fabrication of flexible LTMO cathodes. For instance, Lu et al. employed Au substrate to grow LiCoO2 nanowire arrays [28]. Xia’s group reported Pt-coated Ti foil as the substrate to grow 3D LiMn2O4 nanowall arrays [29] and Au-coated stainless steel as flexible substrate to grow porous LiCoO2 nanosheet arrays [30], respectively. Nevertheless, such noble metal-based substrates are quite heavy and expensive, resulting in reduced specific capacity and increased fabrication cost of the whole electrode. Moreover, the flat metal substrates with low surface area offer only a few growth sites for LTMO. It would cause two severe concerns at practically necessary high areal mass loading. First, it provides few electron conduction channels, leading to sluggish charge transfer at high mass loading [31]. Second, the active material far away from the interface lacks the support from the substrate, making it quite fragile during repeated bending/deformation process. Due to these obstacles, the above-mentioned flexible hybrids can only be fabricated into thin-film electrodes with low areal capacities of < 0.3 mAh cm-2, which cannot satisfy the high energy requirement for most flexible power sources. From the above discussions, one can find that the development of flexible LTMO cathodes still stays at quite early stages. Exploration of appropriate nanostructured substrates combined with design innovation towards robust hybrid interface is highly 4

desirable. Graphite materials can generally withstand the high temperature corrosion over 700 °C [32]. Herein, graphitic hollow carbon nanofibers (CNFs) with truncated conical graphene layers are carefully chosen as the flexible substrate to load carbon-coated ultrasmall LiMn2O4 nanocrystals (designated as LiMn2O4@C/CNF). The highly graphitic structure ensures robust high-temperature oxidation resistance during the annealing process. The CNF walls expose plenty of graphitic edge planes, which provide strong anchoring of LiMn2O4 nanocrystals. And the introduced carbon-coating layer offers an additional strong outer anchoring. As a result of the synergy of the dual carbon anchoring, a robust LiMn2O4-CNF hybrid has been produced. Such smart hybrid architecture exhibits superior lithium storage properties by virtue of their advantageous structural features. First, the intimate contact between LiMn2O4 nanocrystal and CNF ensures smooth electron conduction along the 1D conductive substrate, thus promotes fast charge transfer even at high power delivery. Second, the ultrasmall LiMn2O4 particle size allows rapid ion diffusion through the whole electrode. Third, the thin carbon-coating layer protects the LiMn2O4 from the electrochemical erosion and thus enhances the cycling stability. Fourth, the robust interface helps the hybrid electrode to well inherit the flexibility from CNFs. As a result, the LiMn2O4@C/CNF cathode delivers a high specific capacity of 126 mAh g-1 with 81 mAh g-1 maintained at 20 C and 81% capacity retention after prolonged 1000 cycles at 1C. And it demonstrates excellent flexibility even when the areal loading is increased to as high as 17.7 mg cm-2. The design strategy proposed here provides a new option to develop a variety of other flexible LTMO cathodes. 5

2. Experimental section 2.1 Preparation of LiMn2O4@C/CNF and the flexible film CNFs with truncated conical graphene layers were used as received (Applied Sciences, Inc.). CNTs were prepared according to a reported method [33]. 30 mg of CNF or CNT was fully dispersed in 24 ml of ethanol and 80 mg of Mn(CH3COO)2•4H2O was added. The mixture was transferred into a Teflon-lined stainless steel autoclave (100 ml) and kept at 160 °C for 3 h to obtain Mn3O4/CNF and Mn3O4/CNT composites. Mn3O4 was converted into LiMn2O4 by a solid-state reaction at high temperature. Typically, Mn3O4/CNF and LiOH•H2O were mixed at Mn/Li molar ratio of 2:1.05 using water/ethanol mixture (1:3 v/v) as solvent. After drying, it was annealed in air at 470 °C for 2h to get LiMn2O4/CNF. LiMn2O4@C/CNF sample was prepared by a similar process except for the introduction of sucrose during the mixing process. The mass ratio of sucrose/Mn3O4 was controlled to be 1:6. LiMn2O4@C/CNT was prepared by the same process. The powder samples were firstly probe sonicated for 2 hours. After resting the solution for one night, the top two thirds of the dispersion were picked up carefully and vacuum filtrated to obtain the flexible films. For the preparation of LiMn2O4@C/CNF and LiMn2O4@C/CNT films, additional 10 wt% super-long carbon nanotubes were introduced. 2.2 Materials characterization Scanning electron microscopy (SEM, HITACH S4800) was used to investigate the morphology of as-prepared materials. Transmission electron microscopy (TEM, 6

Tecnai G20, 200 kV) was used to examine the inner microstructure. Energy-dispersive X-ray spectroscopy (EDS) was evaluated by the EDS spectrometer attached on the TEM machine. N2 adsorption isotherms were measured by using a volume adsorption apparatus (autosorb-1) at 77 K. The specific surface area was calculated by Brunauer-Emmett-Teller (BET) method, and pore size distributions (PSD) were obtained according to the density functional theory (DFT). Thermogravimetric analysis (TGA) was carried out on a TA 2950 instrument by heating the samples in air from room temperature to 800 °C at a heating rate of 10 °C min-1. X-ray diffraction (XRD) patterns were obtained by a powder XRD (Rigaku D/max 2500/PC using Cu Kα radiation with λ = 1.5418 Å). Raman spectra were measured by a LabRAM HR Raman microscope with a 514 nm laser. The manganese average oxidation state in the LiMn2O4 samples was determined by potentiometric titration. The detailed method was reported in a previous literature [34]. The manganese dissolution was assessed by soaking samples in electrolyte solution at 55 °C for 80 hours and determining the amount of Mn into the electrolyte by inductively coupled plasma-atomic emission spectrometer (ICP-AES, Thermo Fisher). 2.3. Computational Method All the spin-polarized density functional theory (DFT) calculations were performed using the Vienna Ab Initio Simulation Package (VASP) with the projected augmented wave method (PAW) and Perdew-Burke-Ernzerhof (PBE) functional within generalized gradient approximation (GGA) [35-38]. The valence electrons were expanded by plane-wave basis with a cutoff energy at 650 eV. All the atoms were 7

fully relaxed by conjugate gradient algorithm until the Hellmann-Feynman force on each atom was smaller than 0.01 eV. The binding energy (BE) is obtained by calculating the energy difference between the summation of LiMn2O4 (E1) and graphene (E2) and LiMn2O4/graphene system (Etotal) (i.e. E = Etotal - E1 - E2). 2.4. Electrochemical measurements The slurry-coated electrodes were prepared by mixing the active material, carbon black and polyvinylidene difluoride at a weight ratio of 80:10:10 with N-Methyl pyrrolidone (NMP) as the solvent. The resulting cathode and anode slurries were coated onto aluminum and copper foils, respectively. The electrode film was dried at 120 °C under vacuum for 24 h and then punched into a disk with a diameter of 12 mm. The areal mass loading of active materials is controlled to be about 2 mg cm-2. 2032 coin cells were assembled using above electrodes and lithium foil as a counter electrode. For fabrication of the flexible full battery, firstly the anode film was pre-lithiated by charging/discharging the carbon film half cell for 3 cycles at 0.1 C and then discharging to 0.01 V. Each electrode was cut into a rectangle of 40 × 30 mm. The cathode and anode films were pasted with narrow aluminum and nickel strips on edge, respectively. The full cell was fabricated and sealed by aluminium plastic. The electrolyte was LiPF6 (1 M) in a mixed solvent of ethylene carbonate (EC)/diethyl carbonate (DEC) (volume ratio = 1:1). A LAND battery tester (Jinnuo Electronics Co., Wuhan, China) was used to perform the galvanostatic charge-discharge tests. Cyclic voltammetry (CV) and electrochemical impedance spectroscopy (EIS) tests were performed in VSP-300 electrochemical interface. The CV tests were conducted at a 8

scan rate of 0.1 mV s-1. EIS tests were performed in frequency range of 0.1 Hz to 100k Hz with an amplitude of 5 mV. The cathode, anode and full cell were tested in the potential ranges of 3.0-4.3 V, 0.01-3 V and 2.0-4.3 V, respectively.

3. Results and discussions 3.1. Synthesis and Structure Characterization The synthesis process of the 1D hybrid is illustrated in Scheme 1. First, CNFs with truncated conical graphene layers are used as substrates to grow untrasmall Mn3O4 nanocrystals through a facile solvothermal reaction. Subsequent LiOH-assisted annealing procedure converts the Mn3O4/CNF composite into LiMn2O4/CNF through the reaction: 8Mn3O4+12LiOH+5O2→12LiMn2O4+6H2O. Some Mn3O4 nanocrystals are detached from the CNF substrate (Scheme 1a). Sucrose, as a carbon source with low pyrolysis temperature, is further introduced during the annealing process to generate the target LiMn2O4@C/CNF product (Scheme 1b). LiMn2O4 nanocrystals are firmly attached on 1D CNF substrate due to the dual carbon-anchoring effect as shown in the inset of Scheme 1. The exposed graphitic edge planes on CNF walls work like pins to strongly anchor LiMn2O4 nanocrystal. The carbon coating layer serves like a cargo net to provide another strong outer anchoring. Carbon nanotubes (CNTs) with aligned basal planes along the wall are also used as the substrate to produce the carbon-coated LiMn2O4-CNT hybrid (designated as LiMn2O4@C/CNT). After the same synthesis process, the LiMn2O4 nanocrystals are mostly detached from the CNT substrate (Scheme 1c).

9

Scheme 1. Synthesis procedures of LiMn2O4/CNF (a), LiMn2O4@C/CNF (b) and LiMn2O4@C/CNT (c). The inset on the right illustrates the dual carbon-anchoring strategy. The comparison between (a) and (b) reveals the importance of the outer carbon anchor. And the comparison between (b) and (c) demonstrates the significant role of the inner carbon anchor. In this research, CNF works as both flexible substrate and inner carbon anchor for LiMn2O4 nanocrystals. The microstructures of CNF and CNT were firstly characterized by SEM, TEM, XRD and Raman measurements. Fig. 1a and Fig. S1a-c show the SEM images of CNF and CNT. They both demonstrate one dimensional fiber-like morphology with length of over tens of micrometres. And the diameter of CNF is obviously larger than that of CNT. The magnified SEM image in the inset of Fig. 1a reveals the inner hollow feature of CNF. The TEM images in Fig. 1b, c and Fig. S1c give more detailed inner structure information of two samples. Both carbon materials demonstrate the characteristic morphology of a hollow core and a graphitic carbon wall. The thicknesses of the graphitic carbon wall can be evaluated to be about 10

18 nm and 8 nm, respectively for CNF and CNT. And it is intriguing that, the graphite basal planes in CNF stacked at about 15 ° from the longitudinal axis of the fiber (Fig. 1c), while the graphite basal planes of CNT are parallel to the longitudinal axis. Such unique microstructure of CNF endows plenty of graphitic edge planes exposed along the outer surfaces of the nanofibers and may bring about intriguing physiochemical properties. The XRD patterns of two samples in Fig. 1d demonstrate the characteristic peak at 26 º, which corresponds to the (002) plane of the graphitic carbon. The full width at half maximum (FWHM) of two peaks can be evaluated to be 0.73 º and 1.7 º and the crystal sizes of CNF and CNT can be calculated by using the Scherrer equation [39] Dhkl = K*λ/( β*cosθ)

(1),

where Dhkl is the crystal size in the direction vertical to the (hkl) lattice planes, K is the Scherrer constant, λ is the wavelength of the X-rays, β is FWHM of diffraction peak and θ is the Bragg angle. The crystal size of CNF and CNT are thus determined to be 13.5 nm and 5.8 nm, respectively, which accord well with the thickness values from the TEM observation. The Raman spectra in Fig. 1e further clarifies the structural complexity of CNF and CNT. Both CNF and CNT demonstrate characteristic G band at about 1580 cm-1 and 2D band at about 2680 cm-1 for sp2 carbons. In addition, the disorder in the graphitic structure has been considered to result in the generation of D band at about 1350 cm-1 with the intensity proportional to the defect levels [40]. It can be observed that no distinctive D band can be seen in CNF suggesting its highly graphitic structure. In order to evaluate the thermal stability of two carbon materials, TGA was conducted in air atmosphere and the resulting TGA curves have been shown in Fig. 1f. The high-temperature oxidation resistance of CNF 11

is superior to that of CNT. For CNF, the weight loss mainly occurs in the high temperature range over 650 °C, much higher than 500 °C of CNT. Such phenomenon can be ascribed to the highly graphitic structure of CNF and thicker graphitic carbon wall. The high-temperature oxidation resistance assures its application as the substrate for growth of LiMn2O4. The porosity properties of two samples are evaluated by nitrogen adsorption-desorption tests (Fig. S1d). Both samples show type IV isotherms with little nitrogen adsorption in the low pressure region and abrupt uptake in the high pressure region, indicating the absence of micropores. Through the BET method, the specific surface areas of CNF and CNT are determined to be 23 m2 g-1 and 153 m2 g-1, respectively.

Fig. 1. Microstructure characterizations of CNF and CNT. (a) SEM image of CNF. The inset shows the high-resolution image. TEM (b) and high-resolution TEM (c) image of CNF. XRD patterns (d), Raman spectra (e) and TGA curves (f) of CNF and CNT. Mn3O4 nanoparticles are then grown on CNF or CNT substrate through

12

solvothermal reactions. Fig. S2 and Fig. S3 illustrate the microstructures of Mn3O4/CNF and Mn3O4/CNT composites. It can be found that ultrasmall Mn3O4 nanoparticles are uniformly anchored on CNF or CNT substrate and there are no obvious aggregations within the carbon matrices. From the XRD pattern in Fig. S2b, we can see that all the diffraction peaks can be well assigned to spinel Mn3O4 except for the graphite (002) peak centered at 26 º. The broadening of the peaks can be attributed to the small particle size. The TEM images in Fig. S2c, d and Fig. S3b further confirm the above evaluation. The Mn3O4 nanocrystals demonstrate ultrasmall particle size of about several nanometers. After high-temperature conversion reaction with LiOH, Mn3O4 is converted into LiMn2O4. Typical SEM images (Fig. 2a and d) demonstrate perfect morphology inheritance of the 1D hybrid structure from Mn3O4/CNF to LiMn2O4/CNF and LiMn2O4@C/CNF. The magnified SEM images in the inset of Fig. 2a and d reveal a minor morphological difference. In the LiMn2O4/CNF hybrid, a small number of LiMn2O4 nanoparticles are detached from the surface of the CNF substrate (emphasized in the red dashed circle). By contrast, in the carbon-coated LiMn2O4@C/CNF hybrid, the CNF surface has been completely enwrapped by LiMn2O4 nanoparticles. The TEM images in Fig. 2b and e further confirm the above observation. For both samples, it can be seen that plenty of ultrasmall LiMn2O4 nanoparticles are attached on the CNF walls. Different from the fully and uniformly covered surface of LiMn2O4@C/CNF, LiMn2O4/CNF exposes some naked CNF walls (also highlighted in the red circle). This comparison indicates the improved anchoring 13

force by the outer carbon-coating layer. High-resolution TEM (HRTEM) images are further collected to manifest the atomic-scale structural information of LiMn2O4/CNF and LiMn2O4@C/CNF. Fig. 2c reveals the crystalline feature of each LiMn2O4 nanoparticle. The lattice fringe of d=0.473 nm corresponds to the (111) plane of spinel LiMn2O4. Fig. 2f demonstrates a typical region with LiMn2O4 nanocrystal sandwiched between the graphitic carbon wall and outer carbon-coating layer. It can be seen that the carbon-coating layer is amorphous and quite thin with a thickness of 1-2 nm. To confirm the existence of the carbon-coating layer, EDS equipped with the TEM machine has been measured. Fig. 2g and h exhibit the elemental mapping of C, O and Mn. For LiMn2O4/CNF, C element locates only in the central region of the nanofiber, while O and Mn elements are homogeneously distributed through the whole nanofiber region. By contrast, for LiMn2O4@/CNF, the C element shows a hierarchical distribution. In the central region of nanofiber it shows a high intensity while in the outer region it shows a weak intensity. This is a good evidence for the uniform and thin-layer carbon coating. The carbon-coating layer, on the one hand provides strong outer anchoring of LiMn2O4 nanocrystals and on the other hand protects the LiMn2O4 nanocrystal from the electrochemical corrosion [41]. The morphology of LiMn2O4@C/CNT is shown in Fig. S4. Almost all LiMn2O4 nanocrystals are detached from the CNT substrate and form aggregates. The comparison between LiMn2O4@C/CNT and LiMn2O4@C/CNF indicate the importance of the inner carbon anchoring. Only with the synergistic effect of inner and outer dual anchoring, the robust interface can be obtained. 14

It is also found that the solid state reaction at an appropriate temperature is quite important for mitigating the ion migration and acquiring the robust interface. For reference, a hydrothermal lithiation method was also employed (see the experimental details under Fig. S5). It can be found that the nanocrystal morphology of the Mn3O4 has not been maintained. After hydrothermal lithiation, LiMn2O4 grow into large polyhedron crystals with particle size of hundreds of nanometers (Fig. S5a) and polycrystalline feature (Fig. S5c, d). All the LiMn2O4 polyhedrons are detached from the CNF substrate, resulting in only physical contact between LiMn2O4 and CNF (Fig. S5b). It can be deduced that in the solution reaction system, metal ions can migrate more freely and the inner carbon anchor cannot alleviate such severe migration. The calcination temperature is controlled at a moderate value of 470 oC on the one hand to assure the complete conversion of Mn3O4 to LiMn2O4 and on the other hand to prevent the excessive oxidation of the outer amorphous carbon layer.

15

Fig. 2. The SEM images of LiMn2O4/CNF (a) and LiMn2O4@C/CNF (d). The insets show the high resolution images. TEM (b) and HRTEM (c) image of LiMn2O4/CNF. TEM (e) and HRTEM (f) image of LiMn2O4@C/CNF. Dark field STEM images of LiMn2O4/CNF (g) and LiMn2O4@C/CNF (h) and the corresponding elemental mapping of C, Mn and O.

The crystalline feature and phase purity of LiMn2O4/CNF, LiMn2O4@C/CNF and LiMn2O4@C/CNT are clarified by XRD. The diffraction peaks can be well assigned to (111), (311), (400), (511), (440) planes of spinel phase LiMn2O4 (JCPDS No. 35-0782) (Fig. 3a). The broaden diffraction peaks can be ascribed to the small particle 16

size of LiMn2O4 nanocrystals. The characteristic peak centered at 26 ° can be attributed to the carbon matrices. Notably, the diffraction patterns of LiMn2O4/CNF and LiMn2O4@C/CNF are quite similar and no wide diffraction peak for the amorphous carbon can be found. This is because the outer carbon-coating layer is too thin and the diffraction intensity is too low. TGA curves are tested and shown in Fig. 3b. The weight ratios of LiMn2O4 in three hybrids can be determined to be 68.8%, 68.1% and 76.7%, respectively. In order to investigate the interface bonding between CNF and LiMn2O4 nanocrystals, X-ray photoelectron spectroscopy (XPS) survey spectra of CNF and LiMn2O4/CNF are measured as shown in Fig. 3c and d. In the full XPS spectra (Fig. 3c), CNF only shows a pronounced C 1s peak while LiMn2O4/CNF reveals the presence of O and Mn signals. Fig. 3d shows the high-resolution C 1s XPS spectra of two samples. The spectrum of CNF dominantly consists of C-C peak and satellite peak. By contrast, the spectrum of LiMn2O4/CNF shows a pronounced C-O peak (see the peak assignment summarized in Table S1). The manganese average oxidation state can be evaluated to be 3.505 and 3.509, respectively for LiMn2O4/CNF and LiMn2O4@C/CNF. They are very close to the stoichiometric LiMn2O4. In order to evaluate whether such C-O bonding arises from the C-O-Mn bonding or from the oxidation of carbon substrate during high-temperature annealing, a reference sample is prepared with CNF undergoing the same LiOH-assisted annealing process. It can be found that the annealed CNF sample exhibits very similar spectrum with CNF, showing quite weak C-O peak (Fig. S6). Thus it can be deduced that it is the C-O-Mn bonding that mainly causes the C-O peak in Fig. 3d. Such C-O-Mn bonding can be 17

formed between the dangling bond of carbon and O during the LiMn2O4 nucleation. To offer more insight into the strong anchoring effect of LiMn2O4 by CNF, here we investigate the binding behaviours of LiMn2O4 on different carbon matrices (CNF and CNT) by employing DFT calculations (Fig. 3e and 3f). To make a trade-off between the computational cost and the reliability of simulated models, CNT and CNF were simulated by using the graphene basal plane and graphene edge plane, respectively. The (111) slab model of LiMn2O4, which was mostly observed from our TEM images, were adopted from Kim et al’ s research work [42], in which the Li-terminated reconstructed (111) facet was identified to be the most stable surface under ambient and high temperature. For the graphene basal plane, a √21 ×√21 supercell of graphene which exhibits only 3.55% lattice mismatch with the LiMn2O4 (111) surface was constructed and then adsorbed on the LiMn2O4 (111) surface. The binding energy between the the graphene basal plane and the LiMn2O4 (111) surface was calculated to be 0.026 eV per carbon atom, indicating that LiMn2O4 nanocrystals may not anchor the CNT energetically. By contrast, the binding energy between the LiMn2O4 (111) surface and the graphene edge plane is determined to be -0.929 eV per carbon atom, suggesting that spontaneous binding between LiMn2O4 and CNF is energetically favourable. Fig. 3f also shows the strong interaction between the dangling carbon atoms of graphene edge and the oxygen atoms on the LiMn2O4 substrate, which accords well with the result of XPS test. Therefore, CNF is beneficial for a more stable structure of the hybrid, which may resist the high-temperature corrosion and relieve the mechanical stress during repeated bending/deformation processes. 18

Fig. 3. XRD patterns (a) and TGA curves (b) of three samples. (c) XPS spectra of CNF and LiMn2O4/CNF.

(d) High-resolution C 1s XPS spectra of CNF and

LiMn2O4/CNF. The optimized structure in the DFT calculation, showing the interactions of LiMn2O4 with graphene basal plane (e) and with graphene edge plane (f). The carbon (C), lithium (Li), mangnese (Mn), and oxygen (O) atoms are denoted by gray, green, purple, and red spheres, respectively. 19

2.2. Evaluation of Electrochemical Property In

order

to

evaluate

the

lithium

storage

properties

of

LiMn2O4/CNF,

LiMn2O4@C/CNF and LiMn2O4@C/CNT, half cells with three hybrids as working electrodes and lithium foil as the counter electrode are fabricated. Fig. 4a shows their cyclic voltammetry (CV) curves for the first cycle at a low scan rate of 0.1 mV s-1. Three electrodes demonstrate quite similar profiles. The well-resolved two pairs of redox peaks suggest that the lithium storage in LiMn2O4 cathodes is a two-stage process. Such peak differentiation has been reported to be ascribed to the distinct energy levels of two different intercalated Li+ states [43]. Moreover, in comparison with LiMn2O4@C/CNT, the two oxidation current peaks of LiMn2O4/CNF and LiMn2O4@C/CNF cathodes shift left, and meanwhile their two reduction current peaks shift right, which corresponds to smaller electrode polarization [44]. This phenomenon can be mainly ascribed to the more robust hybrid interfaces of two CNF-based cathodes, which provide more smooth channels for enhanced charge transfer. The charge/discharge curves in Fig. 4b confirm the above evaluation. There are two charge plateaus and two discharge plateaus, again confirming the two-stage Li+ intercalation/de-intercalation process. It is noteworthy that the plateaus are not as flat as those of the bulk LiMn2O4 electrode [45]. This can be mainly attributed to the ultrasmall LiMn2O4 nanocrystal size and the resulting large surface area. It significantly enhances the capacity contribution from pseudocapacitive lithium storage [46], which will be discussed more in detail in the following section. The initial

discharge

capacities

of

LiMn2O4/CNF, 20

LiMn2O4@C/CNF

and

LiMn2O4@C/CNT cathodes can be evaluated to be 123, 126 and 118 mAh g-1. The large reversible capacities indicate that the lithium storage sites are highly accessible in those electrodes. Rate performance is important for assessing the high-power output ability of LIB electrodes. Herein the specific capacities of three cathodes at gradually increased current rates are evaluated and plotted in Fig. 4c. LiMn2O4@C/CNF delivers a high reversible capacity of 126 mAh g-1 at a low current rate of 0.2C and as the current rates increase. As the current rates increase to 0.5C, 1C and 5C, the specific capacities show slight capacity reduction to 122, 115 and 103 mAh g-1, respectively. At a high current rate of 20C, the LiMn2O4@C/CNF cathode still maintains a high specific capacity of 81 mAh g-1, far surpassing 56 mAh g-1 of LiMn2O4/CNF and 21 mAh g-1 of LiMn2O4@C/CNT. When the current rate returns to 0.2C, the specific capacity of LiMn2O4@C/CNF also recovers back to 125 mAh g-1. Even when compared with those of previously reported advanced LiMn2O4 nanostrucutres, such high rate performance is still among the best (see Table S2) [47-50]. The internal resistances of three cathodes are examined by electrochemical impedance spectroscopy (EIS) and the resulting Nyquist plots are shown in Fig. 4d. The sloping line in the low-frequency range can be associated with the Li+ diffusion process through the electrode. The semicircle in the high-middle frequency range generally corresponds to the charge-transfer process and the intercept of the semicircle with the real axis stands for the electronic resistance (Rs). An equivalent circuit is obtained by fitting the experimental curves as shown in the inset of Fig. 4d. 21

The electronic resistances of LiMn2O4/CNF and LiMn2O4@C/CNF (0.16 Ohm and 0.15 Ohm) are much lower than that of LiMn2O4@C/CNT (0.29 Ohm), indicating that the intimate contact of LiMn2O4 nanocrystals with the carbon substrate could enhance the

electron

conduction

capability.

The

charge

transfer

resistance

of

LiMn2O4@C/CNF can be determined to be 128 Ohm, much lower than those of LiMn2O4/CNF (162 Ohm) and LiMn2O4@C/CNT (290 Ohm), revealing the accelerated charge transfer rate. In addition, the steeper incline in the low frequency region of LiMn2O4/CNF and LiMn2O4@C/CNF cathodes suggests more rapid diffusion of lithium ions. In order to better understand the electrochemistry in the LiMn2O4@C/CNF cathode, CV tests at various scan rates are carried out (Fig. 4e), which has been reported to be an efficient method to evaluate the lithium storage kinetics [51]. In general, the peak current (i) and scan rate (v) obey the law: i = aνb,

(2)

where a and b are constants. The b value of 0.5 corresponds to a fully diffusion-controlled behaviour, while the b value of 1.0 suggests an ideally capacitive lithium storage process [52]. For LiMn2O4@C/CNF, the b values for A1 and C1 peaks are calculated to be 0.93 and 0.92, respectively (Fig. 4f). They are quite close to 1, thus revealing the capacitive nature of the redox processes. The relationships of i and ν for LiMn2O4/CNF and LiMn2O4@C/CNT cathodes are also plotted in Fig. S7 and the corresponding b values are summarized in Table S3. LiMn2O4/CNF shows b values of 0.83 and 0.85, indicating enhanced contribution from diffusion-controlled 22

process. And LiMn2O4@C/CNT displays b values of 0.74 and 0.78, which suggests a more mixed behaviour. The capacity contribution from capacitive process and diffusion-controlled process can further be quantitatively determined by using the following equation: i(V)=k1v+ k2v1/2,

(3)

where k1 and k2 are constants at a given voltage (V). The current can thus be separated into a capacitive component and a diffusion-controlled counterpart. From Fig. S8 we can see that the fraction of current contributed by capacitive process can be determined to be 84 % for LiMn2O4@C/CNF at a scan rate of 1 mV s-1, which is slightly higher than 75% for LiMn2O4/CNF and much higher than 58% for LiMn2O4@C/CNT.

Such

capacitive-dominated

lithium

storage

endows

LiMn2O4@C/CNF with superior high-rate capability. The

cycling

stability

of

three

cathodes

are

examined

by

prolonged

charging/discharging at 1C for 1000 cycles as shown in Fig. 4g. LiMn2O4@C/CNF demonstrates a superior high-rate cyclability. It retains high specific capacities of 92 mAh g-1 over 1000 cycles at 1C with capacity decay of 0.019% per cycle. Moreover, the Coulombic efficiency retains over 99% from the 7th cycle, indicating the high reversibility of the electrochemical reaction. By contrast, LiMn2O4/CNF and LiMn2O4@C/CNT cathodes only maintain 61 mAh g-1 and 56 mAh g-1 over 1000 cycles, respectively. The manganese dissolution contents were evaluated to be 0.82wt% and 0.29 wt% for LiMn2O4/CNF and LiMn2O4@C/CNF, respectively by ICP-AES. This comparison suggests that the outer carbon coating layer could significantly 23

reduce the Mn dissolution, thus enhancing the cycling stability.

Fig. 4. Electrochemical properties of three hybrids as LIB cathodes. (a) CV curves, (b) initial charge/discharge curve at 0.2C, (c) specific capacities at various rates, (d) EIS spectra tested at 4 V. (e) CV curves for the LiMn2O4@C/CNF cathode with the anodic 24

(A1 and A2) and cathodic (C1 and C2) peaks labelled. (f) b-values determination of anodic (A1) and cathodic (C1) peaks with sweep rate from 0.1 mV s-1 to 2 mV s-1. (g) Cycling stability test at 1C for 1000 cycles. Herein the lithium storage property of CNFs as LIB anode is also investigated as shown in Fig. S9. In a wide potential range (0.01-3 V), CNF shows a quite similar specific capacity (345 mAh g-1) with graphite (344 mAh g-1), which is much higher than that of CNT (257 mAh g-1). In the practically necessary low voltage domain of 0.01-1 V, CNF maintains 81.2% of the specific capacity in the wide potential range, which is close to 94.2% for graphite and much higher than 49.9% for CNT. The capacity comparison between CNF and reported carbon nanofiber materials has been summarized in Table S4. It can be seen that although a lot of works claims quite high capacity, such advantage would be much weakened when the voltage window was set in the low potential range (0-1 V). At a high current rate of 3.2C, CNF still maintains a high specific capacity of 140 mAh g-1, much higher than 78 mAh g-1 of CNT and 50 mAh g-1 of graphite electrodes. The superior rate capability can be ascribed to its abundant exposed edge planes, which may provide a high lithium ion diffusivity through the carbon nanofibers [53]. Moreover, after prolonged 1000 cycles at 1C, CNF still maintains 95.9% of the initial specific capacity, indicating its excellent cycling stability. Therefore, CNF could be an appropriate anode in flexible LIB.

2.3. Assembling and Electrochemical Behavior of Flexible LIB Free-standing and flexible LiMn2O4@C/CNF and CNF films are fabricated by vacuum filtration. To enhance the electronic conductivity and flexibility of 25

LiMn2O4@C/CNF, 10 wt% superlong CNTs are introduced [54]. The carbon nanofibers were interwoven into a network (Fig. S10), leading to a free-standing film with a high flexibility. Fig. 5a and b demonstrate the robust flexibility of CNF and LiMn2O4@C/CNF films. Even after repeated bending at 180°, as-obtained CNF-based films still demonstrate good mechanical integrity. Typical low-magnified SEM images of the electrode after repeated bending show no observable cracks, again revealing the good robustness and flexibility (Fig. S11). A flexible full cell has been constructed

as

schematically

illustrated

in

Fig.

5c.

The

free-standing

LiMn2O4@C/CNF and CNF electrodes are laminated onto a polypropylene separator and then sealed with aluminum plastic in an Ar-filled glove box. The optical image of the soft package battery is shown in Fig. 5d. According to the specific capacities of CNF (280 mAh g-1) and LiMn2O4 (126 mAh g-1), the areal mass loading of CNF and LiMn2O4 were controlled to be about 8.8 mg cm-2 and 17.7 mg cm-2, respectively, to make the capacity of anode a little excessive. It is notable that a pre-lithiation process is necessary to improve the initial Coulombic efficiency of the full cell [55], which has been described in detail in the experimental section. The full cell demonstrates a high flexibility with no observable structural failure after repeatedly bending process at various bending modes and it can power a brown LED (Fig. 5e-h).

26

Fig. 5. Flexibility demonstration of CNF film (a) and LiMn2O4@C/CNF film (b). (c) Configuration of the flexible LIB. (d) Optical image of the soft package battery. (e-h) The optical images showing a brown LED lighted by the flexible LIB with different bending modes. Fig. 6a shows the charge/discharge curves of LiMn2O4@C/CNF//CNF under flat and bent states. It is noteworthy that the specific capacity is calculated based on the mass of the LiMn2O4 cathode. Since the CNF anode shows a very low working voltage like graphite, the full cell here shows similar potential plateaus with those of LiMn2O4 cathode. Such high operating voltage is quite favorable for high energy storage. It can also be found that negligible electrode polarization occurred after bending, indicating no internal resistance change. And the capacity decay of the bent cell in comparison with the flat cell is less than 1%. The excellent cyclic stability of the flexible full cell 27

under flat and bent states was revealed in Fig. 6b. The flat cell maintains 99.6% of the capacity over 20 cycles, and 98.7% after another 20 cycles under bent states. When the bent cell was recovered to the flat state, the specific capacity also increases back to the initial value. The high-rate performances of the flexible full cells are further investigated. Fig. 6c shows the areal capacities of LiMn2O4@C/CNF//CNF and LiMn2O4/CNF//CNF at various current densities. LiMn2O4@C/CNF//CNF shows a high areal capacity of 2.01 mAh cm-2 at 0.5 mA cm-2 and maintains 1.24 mAh cm-2 at 6 mA cm-2, while LiMn2O4/CNF//CNF only remains 0.73 mAh cm-2 at 6 mA cm-2. The areal capacity and rate performance are superior to those of most previously reported systems as summarized in Table S5 [21, 24, 28-30, 56, 57]. Fig. 6d shows the cycling stability of LiMn2O4@C/CNF//CNF under a bent state. Over prolonged 500 cycles at 2 mA cm-2, it still delivers a high specific capacity of 1.58 mAh cm-2 with quite low capacity decay of 0.019% per cycle. By contrast, LiMn2O4/CNF//CNF only remains an areal capacity of 1.58 mAh cm-2 with a capacity decay of 0.053% per cycle. The high areal capacity, excellent rate capability and cycling stability demonstrate the high potential for practical applications.

28

Fig. 6. (a) Charge/discharge curve of LiMn2O4@C/CNF//CNF flexible cell. (b) Cyclic performance of the battery under flat and bent states. (c) Rate performance and cycling stability (d) of the flexible cells.

4. Conclusion In summary, a dual-carbon-anchoring strategy has been proposed to fabricate 1D LiMn2O4@C/CNF hybrid with robust interface for flexible LIBs. CNFs with truncated

conical

graphene

layers

are

employed

as

high-temperature

oxidation-resistant substrates for growth of ultralsmall LiMn2O4 nanocrystals. They provide smooth 1D electron conduction and charge transfer pathways and endow the hybrid with robust flexibility. More importantly, CNF exposes plenty of graphitic edge planes, affording strong inner anchoring of LiMn2O4 evidenced by both experimental results and DFT calculations. A strong outer anchoring is further realized by an amorphous carbon layer coated on LiMn2O4, which works like a ‘cargo net’. The synergy of the dual carbon anchoring induces several desirable structural 29

merits

including

high

electronic

conductivity,

rapid

ion

diffusion,

good

electrochemical stability and most importantly excellent flexibility. As LIB cathode, LiMn2O4@C/CNF delivers a high specific capacity of 126 mAh g-1, maintains 81 mAh g-1 at 20 C and 81% capacity retention after prolonged 1000 cycles at 1C. Flexible LiMn2O4/CNF@C//CNF full cell has been fabricated with a high areal capacity of 2.01 mAh cm-2 as well as good rate capability and cycling stability. The design strategy proposed here can be expected to extend to a variety of other flexible LTMO cathodes.

Acknowledgements The authors gratefully thank the National Natural Science Foundation of China (Grant No. 51672151), National Key Basic Research Program of China (No. 2014CB932400) for the financial support.

Appendix A. Supplementary material Supplementary data associated with this article can be found in the online version.

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35

Xiaoliang Yu was born in 1989, received his B.S. and PhD from Tsinghua University in 2011 and 2016, respectively. His research interest is focused on the synthesis of carbon-based nanomaterials and their applications in various energy storage applications including supercapacitors, lithium-ion batteries, lithium sulfur batteries and lithium metal batteries.

Jiaojiao Deng was born in 1990, received her B.S. (2014) from Northwestern Polytechnical University and PhD (2019) from Tsinghua University in China, respectively. Currently, she is working as a postdoctor in Tsinghua Shenzhen International Graduate School. Her research is focused on designing functional micro-/nanostructures of transition metal oxides for advanced energy storage.

Xin Yang obtained his bachelor degree from Northwestern Polytechnical University in 2016. Then He obtained his master degree from Tsinghua University in 2019. His research interest is to theoretically investigate the mechanism of energy strorage and conversion for low-dimensional materials.

Jia Li obtained his Ph.D. degree from Tsinghua University in 2009. Then he was a 36

Postdoctoral Research Fellow in Fritz-Haber Institute of MPG in Berlin, Germany from 2009 to 2010. He is currently an associate professor of Graduate School at Shenzhen, Tsinghua University. His research interest is applying first-principles methods to study the relationship between the structure and the performance of energy storage and conversion for two-dimensional materials.

Zheng-Hong Huang was born in 1970, received his BS (1992) degree from Wuhan University of Technology in China, MS (1995) degree in Zhejiang University in China and PhD (2002) degree in engineering from Tsinghua University in China. He now is an associate professor in School of Materials Science and Engineering, Tsinghua University. His research interests focus on the synthesis of carbon materials and their applications in energy storage/conversion and environmental protection.

Baohua Li received his Ph.D. degree in Chemical Technology from Institute of Coal Chemistry (ICC), Chinese Academy of Science (CAS) in 2003. He is currently a full professor, and director of Division of Energy and Environment of Graduate School at Shenzhen, Tsinghua University. His research interests focus on the carbon materials for energy storage such as electrode materials for super capacitor and lithium ion battery.

Feiyu Kang was born in 1962, received his B.S. from Tsinghua University in 1985 and his Ph.D. from Hong Kong University of Science and Technology in 1997. He now is a full professor in School of Materials Science and Engineering, and also a dean in Graduate School at Shenzhen, Tsinghua University. His research is focusing on nanocarbon materials, graphite, thermal conductive materials, lithium ion battery, 37

super-capacitors, electric vehicles, porous carbon and adsorption, indoor air clearing and water purification.

38

Carbon nanofibers (CNFs) with truncated conical graphene layers are used as flexible substrate for LiMn2O4 cathode. The exposed graphitic edge planes on CNF walls and another introduced amorphous carbon layer provide strong inner and outer anchoring respectively. The dual-carbon-anchoring strategy help produce a 1D LiMn2O4-nanocarbon hybrid with robust interface. As-fabricated flexible LiMn2O4 cathode exhibits quite high areal capacity as well as good rate capability and cycling stability.

Declaration of interests ☒ The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper. ☐The authors declare the following financial interests/personal relationships which may be considered as potential competing interests: