Sustainable carbon-sheets and their MnO–C hybrid for Li-ion batteries

Abstract

The future development of large-scale energy store systems requires Li-ion batteries (LIBs) with not only an outstanding electrochemical performance but also sustainability and cost-effectiveness. Herein, unique carbon-sheets (CSs) with rich porosity and high graphitization have been synthesized from sugarcane-stalk for LIBs application. These derived CSs exhibit a superior electrochemical behavior to commercial graphite. Moreover, the performance of CSs can be further upgraded by growing MnO nanoparticles to form a synergetic MnO–C hybrid. As a result, the MnO–C hybrid shows a high capacity, excellent rate capability (354 mA h g⁻¹ at 3 A g⁻¹), and superior cycling performance (814 mA h g⁻¹ at 0.2 A g⁻¹ after 400 cycles).

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To address the challenges of ever-growing energy consumption and the environmental consequences presented by fossil fuels, very innovative, sustainable renewable energy from the sun and/or wind has been considered as the most ideal alternative.^1,2 However, exploitation of this renewable energy sources implies profound changes in the electrical system because such resources are naturally variable and unpredictable.^1,3 In pursuit of a solution to this problem, advanced energy conversion and storage devices have been investigated to ensure the efficient implementation services of renewable energy. Li-ion batteries (LIBs) have become one of the most popular energy storage devices with many outstanding features including high energy density, little self-discharge, and long lifespan. Though LIBs have gained dominance for portable electronic devices, it is still much more expensive for the upcoming large-scale applications (such as electric cars and stationary energy systems).^3–5 In this regard, developing sustainable electrode materials from renewable, inexhaustible and cost-effective resources to achieve satisfactory energy-power performance for LIBs is extremely important.^6–8

Biomass, as a renewable source and natural CO₂ capture products, has attracted much attention in many fields (biofuels, chemicals, polymers, etc.) because of its low cost, rapid regeneration, huge availability, and environmental friendliness.⁹ Moreover, due to their rich variety of microstructures, transformation of biomass into multi-structure porous carbon for LIBs application is becomes a topic of growing interest.^10,11 Sugarcane-stalk, as the by-product of cane sugar industry, has been produced for more than 50 millions tons only in China every year. In consideration of this giant output, how to appropriately deal with the vast of sugarcane-stalk garbage has become one of the overriding concerns in environmental protection. However, the current unreasonable handling such as burned directly and/or landfill not only cause severe environmental disaster (dust, air and water pollution),^12,13 but also make enormous waste of this valuable natural resource. As a result, processing the large amount of sugarcane-stalk into sustainable products without pollution through high-efficiency and cost-effective approaches is seldom encountered, however, it urgently needs reconsidering.

In this work, we report a convenient strategy that combines hydrothermal and carbonization treatments to convert sugarcane-stalk waste into unique carbon-sheets (CSs) with rich porosity and high graphitization for LIBs application. These derived CSs as anode exhibit a better electrochemical behavior in terms of specific energy and rate capability. Furthermore, the performance of CSs can be markedly upgraded by growing MnO nanoparticles on sheets to form a MnO–C hybrid. Benefiting from the high theoretical capacity of MnO and the excellent conductivity of carbon, the MnO–C have been demonstrated as an excellent anode material for LIBs, which exhibit high reversible capacity (814 mA h g⁻¹ after 400 cycles) and excellent rate capability (354 mA h g⁻¹ at 3 A g⁻¹). The success in the evolution of carbon nanosheets from sugarcane-stalk not only opens up the possibility of manufacturing sustainable carbon materials for LIBs, but also sets up an economical approach for preparing carbon-based hybrid that might be available in a wealth of practical applications.

Fig. 1a displays the entire evolution procedures from sugarcane-stalk to the carbon-sheets. The recycled sugarcane-stalk is followed by a hydrothermal treatment of KOH solution to dissolve the lignin which parts in the sugarcane-stalk structure, resulting in the only left of cellulose.^12,14 After lignin removal, the cotton-like cellulose is further calcined to achieve the carbon material. Fig. S1† shows the sugarcane-stalk is a thick, three-dimensional stack of carbohydrate sheets. After carbonization, SEM and TEM observations (Fig. 1b–d and S2†) illustrate the yielded carbon products are the dispersed sheet morphology with the thickness of a few hundred nanometers, having a large sheet-structure from several micrometers to even tens micrometers. From the HRTEM image in Fig. 1e, the lattices of the graphite are clearly observed, which demonstrate the high degree of graphitization of the CSs.¹⁵ However, the disordered graphite lattices suggest that it is not absolutely crystalline but the graphite-like amorphous carbon structure.^16,17 Moreover, Fig. S3† show that large quantities of micropores are homogeneously distributed within the sheet, which can enhance the penetrability of electrolytes and the accessible insertion of Li⁺ into the deep structure. The generation of these micropores is originated from the decomposition of the cellulose to carbon and, the activation and corrosion of carbon materials by KOH during the high temperature (6KOH + 2C → 2K + 3H₂ + 2K₂CO₃).¹⁸ This special large sheet-structure with high graphitization and rich porosity would endow the CSs as anode with an outstanding high-rate performance by the fast transferring electrons and the reductive interface resistance, rather than nanoparticle networks through numerous interparticle contacts (Table S1†).

Fig. 1 (a) Photographs displaying the evolution of sugarcane-stalk into carbon-sheets. (b) SEM image, and (c, d) TEM images of the CSs. (e) HRTEM and SAED images of the produced carbon.

In order to demonstrate the high graphitization, XRD (Fig. 2a) and Raman (Fig. 2b) are used to characterize the CSs. Two characteristic peaks with high intensity in XRD pattern which are located at around 25° and 43° correspond to the (002) and (100) planes of hexagonal carbon (JCPDS, 41-1487), respectively,¹⁹ suggesting the as-obtained CSs possesses a partial degree of graphitization with a percentage of 60.8%. As is shown in Fig. 2b, there are two notable Raman shifts of the CSs, the G peak (1597 cm⁻¹) is associated with the E_2g mode of graphite, while the D peak (1334 cm⁻¹) is ascribed to edges, defects, and disordered carbon.^19,20 The intensity ratio of the G to the D (I_G/I_D) is calculated to higher than 1 (∼1.12), indicating a few number of defects and edges in the graphite structure.¹⁵ The results above combined with the presence of broad 2D peak and HRTEM images suggest that the as-obtained CSs possess a high degree of graphitization. To further investigate the porous structure of CSs, the sample is measured using N₂ adsorption–desorption isotherms (Fig. 2c). The strong adsorption below the relative pressure of 0.1 is usually displayed by microporous filling especially for instances of carbon materials.²¹ The specific BET surface area is measured around ∼182 m² g⁻¹. Also, the pore size distribution of CSs is calculated based on the BJH desorption isotherm (inset in Fig. 2c), which shows the existence of micropores below 2 nm, with small mesopores centered among a range of 2–10 nm. As a result, the CSs with excellent electronic good conductivity, rich porosity and large surface area would be not only favorable for Li storage, but also very helpful for growing other functionalized materials to construct advanced hybrid electrodes for LIBs.

Fig. 2 (a) XRD pattern, (b) Raman spectra and (c) N₂ adsorption–desorption isotherms and pore size distribution of the CSs.

Aimed to boost the electrochemical performance of LIBs, we optimize the carbon-sheets by growth of functionalized metal oxides such as MnO to form a MnO–C hybrid material. The choice of MnO as an incorporated material is mainly given by factors like cost, natural abundance and environment friendliness.^22–24 More importantly, the high lithiation plateau of MnO (∼0.5 V vs. Li/Li⁺) can ensure its cycling stability by generating stable passivation layers on the surface, and simultaneously avoids a yield of Li dendrites for safety.²⁵ Fig. 3a shows the XRD pattern of the as-prepared hybrid, the strong diffraction peaks which are located at 34.8°, 40.7° and 59°, assigning to the face-centered cubic MnO (JCPDS, 78-0424). From Raman spectrum (Fig. 3b), a distinct peak at 641 cm⁻¹ belongs to the Mn–O stretching mode of MnO,²⁶ while the peaks located at 1334 and 1598 cm⁻¹ are the fingerprints for the CSs. As a consequence, the successful formation of MnO–C hybrid after post-treatments can be clearly identified according to the above XRD and Raman records. BET measurement (Fig. 3c) demonstrates the interconnected hierarchical pore structure of MnO–C hybrids, especially the micropores and small mesopores around 1–4 nm, with a specific surface area of 275 m² g⁻¹. The higher surface area after MnO growth may results from the increasing interspaces and pore-structure between the MnO nanoparticles. According to the thermogravimetric analysis (TGA, Fig. S4†), the carbon content of MnO–C is evaluated to be about 44.8 wt%. The HRTEM images of MnO–C (Fig. 3d, e and S5†) display the well-dispersed MnO nanoparticles with a uniform size (∼100 nm) which are immobilized within the sheet substrate. A nanoparticle with a clearly observed lattice interplanar spacing of 0.22 nm is shown in the inset of Fig. 3e, corresponding to the (200) plane of a MnO phase, which further demonstrates the successful grown of MnO on carbon sheets.

Fig. 3 (a) XRD pattern, (b) Raman spectra, (c) N₂ adsorption–desorption isotherms and pore size distribution, (d, e) TEM images of the MnO–C.

The electrochemical performance of the derived CSs and MnO–C hybrid is investigated as anode in LIBs, respectively. Cyclic voltammetry (CV) is initially used to evaluate the Li storage behavior between 0.01 and 3.00 V vs. Li/Li⁺ at a scanning rate of 0.2 mV s⁻¹ (Fig. 4). In the first catholic sweep of CSs (Fig. 4a), the wide region below 0.5 V is attributed to complex phase transitions caused by Li intercalation into carbon, such as irreversible trap of Li⁺ in lattices, electrolyte decomposition and solid electrolyte interface (SEI) layer formation.^12,14 However, this Li-intercalation reaction becomes highly reversible in the following cycles, and the corresponding reversible Li-deintercalation peak launches at ∼0.18 V. Fig. 4b shows representative CV curves of the MnO-based materials. After the first irreversible phase transformation, the reduction peak and the anode peak center at 0.34 and 1.32 V could be ascribed to the formation of Mn and Li₂O and the oxidation of Mn⁰ to Mn²⁺, respectively.²⁷ These processes can be expressed by the following reactions

MnO + 2Li⁺ + 2e⁻ ⇆ Mn + Li₂O

Fig. 4 CV curves of (a) CSs and (b) MnO–C at the scan rates of 0.2 mV s⁻¹.

Moreover, the nearly overlap curves in the following cycles of CSs and MnO–C, respectively, suggesting a high reversibility and good stability of the both electrodes.

Galvanostatic charge–discharge tests at 0.2 A g⁻¹ have been conducted to estimate the long-term cyclic behavior of CSs and MnO–C. As is seen in Fig. 5a, the CSs electrode exhibits an initial discharge and charge capacities of 776 and 480 mA h g⁻¹, respectively. For MnO–C, a higher initial discharge capacity up to 996 mA h g⁻¹ and charge capacity of 655 mA h g⁻¹ are revealed, with an upper initial coulombic efficiency of 65.7% from the 61.8% of CSs. It should be noted that the capacity of both electrodes becomes reversible after the initial few cycles and shows a rising in subsequent steps. The specific capacity of CSs and MnO–C rises to 531 mA h g⁻¹ after 200 cycles and 814 mA h g⁻¹ after 400 cycles, respectively, which exhibits an excellent cycling performance of both electrodes. To investigate this capacity-rise phenomenon, the charge–discharge profiles of different cycles are examined (Fig. 5b). When comparing the charge curve of 50^th with 200^th cycle of CSs, no much change of the cycling profiles is observed but that a slope of 1.4–3.0 V appears to enhance the specific capacity, which may be ascribed to structural defects developed by the repeated Li insertion/deinsertion process that provides more active sites available for Li faradic capacitive reactions.^12,19 For MnO–C, after long-term cycling, a slope between 1.5 and 2.1 V originated from the Mn²⁺ oxidate to Mn⁴⁺ can be clearly observed.^28,29 Therefore, this new electrochemical oxidation reaction of Mn²⁺ which is associated with defect developing of carbon contributes to the capacity-rise of MnO–C hybrid.

Fig. 5 (a) Long-term cyclic performance, (b) charge–discharge profiles, (c) rate performance, and (d) EIS spectra in the discharged state of the CSs and MnO–C.

To further examine the rate-capability, discharge capacities at various current rates from 0.1 to 3 A g⁻¹ are investigated. As is shown in Fig. 5c, a reversible capacity of 479 mA h g⁻¹ for CSs and 646 mA h g⁻¹ for MnO–C is realized at 0.1 A g⁻¹, respectively. With the continuously increasing rates, the discharge capacities of both electrodes show a regularly decreasing. Note that when the current jumps to very high of 3 A g⁻¹, the CSs electrode still retains a capacity of 136 mA h g⁻¹ while 354 mA h g⁻¹ of MnO–C electrode remains higher. Even when the current suddenly jumps back to 0.1 A g⁻¹, the fully recovered or even surpassing values are able to deliver to both electrodes. Comparing with some other carbonaceous materials and MnO/C hybrid, the CSs and MnO–C with the largest loading shows a better rate capability and rate-retention (Table S2†). Electrochemical impedance spectroscopy (EIS, Fig. 5d) recorded in the frequency range of 0.1 MHz to 10 mHz at the discharge state of the 10^th cycle. The measuring data are represented as symbols, while the continuous lines are fitted data according to the equivalent circuit shown in the inset of Fig. 5d. The data fitted reveal the electrode of MnO–C has a charge transfer resistance (R_ct) of ∼36 Ω, which substantially higher than that of CSs (∼18 Ω), evidencing that the loading of MnO in the pores impedes the electrolyte infiltration and charge transport capability of the carbon substrate. However, the similar low-frequency slope angle of both electrodes indicates an approximate Li⁺ diffusivity along the inter-space of carbons. Even after long-term cycling, the R_ct does not change a lot or show a decreasing (Fig. S6†), suggesting the both electrodes possess a fast faradaic process and a high activation energy for Li⁺ diffusion.

The excellent electrochemical performance of the CSs can be attributed to the unique structure of large sheet-structure, high degree of graphitization, and rich porosity. The large sheet-structure with high graphitization could provide abundant electronic conducting phase for the fast transfer electrons directly, which effectively reduces the interface resistance of the CSs rather than in particle networks through interparticle contacts. Moreover, the porous structure with a large electrode/electrolyte interfaces would offer low-resistant diffusion pathways for rapid Li⁺ transport, which finally results in an effective ambipolar diffusion of Li⁺ and e⁻ into/out the CSs structure. For MnO–C hybrid, the outstanding cyclability and rate-capability are mainly ascribed to the synergy effects between MnO and the carbon matrix. The nanostructured MnO ensures the delivery of a high capacity for the hybrid electrode, while the carbon sheet not only functions as the robust scaffold to anchor the MnO for excellent mechanical stability, but also as electronic conductor to enhance the electrochemical activity of the hybrid.

In summary, sustainable carbon-sheets material transform from sugarcane-stalk have been achieved by a simple approach. The prepared material (CSs) shows unique sheet-structure with a large surface area, rich porosity, and high degree graphitization. When evaluated as anode of LIBs, the CSs show a superior electrochemical performance to practical graphite. Moreover, the electrochemical behavior can be further optimized by growing MnO nanoparticle on the CSs surface. The as-designed MnO–C hybrid with great synergetic effects exhibits a high rate capability and excellent cycling stability. This work provides a sustainable way to prevent sugarcane-stalk from environmental pollution, and more importantly develops advanced carbon-based materials for practical use from LIBs to other fields like adsorbents, catalysis, etc.

Acknowledgements

This work was supported by the National Basic Research Program of China (973 Program) (2013CB934001), National Natural Science Foundation of China (51274017), National 863 Program of China (2013AA050904), International S&T Cooperation Program of China (2012DFR60530), Shanghai Academy of Space Technology (SAST201467) and Academic Excellence Foundation of BUAA for PhD Students.

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Footnote

† Electronic supplementary information (ESI) available: Experimental details, SEM, HRTEM, TGA, and electrochemical characterization. See DOI: 10.1039/c6ra15411e

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