In situ fabrication of Na₃V₂(PO₄)₃ quantum dots in hard carbon nanosheets by using lignocelluloses for sodium ion batteries

1. Introduction

While nanoparticles embedded in a carbonaceous matrix material are promising materials, their assembly method is still a challenge, and the rational design is urgently needed [1]. Nanostructured Na₃V₂(PO₄)₃ (NVP) with NASICON frame structure has been considered as the most promising cathode candidate for sodium ion batteries (SIBs) because of its inherent merits including low cost, long cycle ability, high safety, good thermal stability and high voltage platform [2,3]. However, NVP nanoparticles have the inherent imperfections, such as lower crystallinity, lower conductivity, high specific surface area and the larger ionic radius of sodium, leading to the formation of solid-electrolyte interfaces, undesirable structural change during charge and discharge process, and sluggish kinetics of electrons and Na ions [[4], [5], [6], [7], [8]]. Recently, scientists have made progress in improving the conductivity of NVP through nanocarbon coating [[9], [10], [11]], carbon nanofibers [12], and graphene [13]. Despite this progress, it is still a big challenge to design and fabricate a new nanostructured NVP with both fast charge/discharge rate and high energy density. Quantum dots (QDs) (<10 nm) are nanoscale regions in a crystal structure that can trap electrons and holes, containing only a small number of atoms and having an extremely high surface-to-mass ratio and electron mobility. QDs will have active sites bearing more Na ions because its surface atoms possess higher energy of delocalized electrons than the interior atoms [14,15]. QDs with fascinating physical properties such as size effect, quantum electronic transport, quantum tunneling effect, are expected to harvest the extremely useful electrochemical performances as additional charge carriers via hole traps [16,17]. But little attention has been paid to QDs with superlattice structure (QDs-SL) for enhancing electrochemical performances.

Hard carbon (HC) is basically amorphous carbon material composed of nanoscale domains of rumpled graphite sheets with disordered structure along the c-axis. Its single layers of carbon atoms randomly connect by Van der Waals’ force. It is the most suitable Na-ion host anode material for commercialization of Na-ion batteries because of its advantages of natural abundance, renewability, the expanding interlayer distances and highly disordered porous network structure. Recently, numerous studies regarding the application of HC materials as anodes in SIBs have been reported, exhibiting high reversible capacity (200-300 mA h g^-1), low average sodium storage voltage and excellent cycling stability [18,19]. The progress has been made for synthesizing hard carbon from abundant biomass feed stocks, such as lignocellulosic agricultural waste, sucrose, olive pit, peat moss, cotton, wood, and banana skins [[20], [21], [22]]. Its morphology control can also be achieved by using some processes, such as hydrothermal carbonisation, templating methods or self-assembly [23,24]. HC structure can be modified systematically by heteroatom doping, such as S, N, P or H doping, to dilate the interlayer spacing and increase the defect concentrations, and the structural changes greatly affect Na-ion storage properties [25,26].

Nanobiotechnology is one of the most powerful routes for organizing a nanoscale system with the highest accuracy. The self-assembly nanobiotechnology of nanoparticles with biomaterial systems has been studied for nanomaterial designs with the highest accuracy [27,28]. Biomaterials have the optimal hierarchical structure to adapt to the environment and have been used as bio-template to synthesize various novel functional materials [29,30]. Amongst a range of potential bio-based resources, lignocellulose (LC) is a most abundant and cheap natural biomass resource on earth for production of HC via a pyrolysis process, owing to its high carbon content, chemical functionalization and high surface area [31]. Bamboo fiber is one of the biomass resources, and its features are abundant in natural resources, renewability, biodegradable, avirulent, good processability and unique morphology. Bamboo fiber has been widely utilized for paper production, textile and biocarbon electrode materials [32,33].

In this work, we develop a new technique for controllable fabricating NVP-QDs-SL in H/S-doped hard carbon ultra-thin nanosheets (HCS) using multifunctional lignocelluloses (LCs) from bamboo pulp as a nucleating agent, structural template and abundant hard carbon source. We demonstrate the regulating effect of LCs template on the formation of NVP-QDs-SL and the mechanism for breaking through theoretical capacity of NVP. Electrons and Na ions can be easily transported to the NVP-QDs-SL and have high transport rate due to the lower tunneling barrier and the structural disorder of NVP-QDs-SL. Besides NVP-QDs-SL can be synthesized more easily by self-assemble of LCs. Most importantly, compared to the conventional sol-gel method for NVP synthesis, this new technique has several notable advantages: (1) this technology is a cost effective, more rapid, simple and powerful route with the high accuracy for synthesizing NVP-QDs-SL; (2) the multifunctional lignocellulose template is a most abundant and cheap natural biomass resource on earth for production of HCS; (3) the functional groups contained in LCs can provide lone pair electrons and nucleation sites, selectively adsorb metal ions, regulate and control the formation of NVP-QDs-SL; (4) the metal ions act as carbonization catalysts, induce structural cross-linking of LCs and enhance the formation of HCS in the synthesis; (5) although the conventional sol-gel method showed good stoichiometric control, it is hard to design the lattice structure of QDs-SL as desired. Therefore, compared with the usual Na₃V₂(PO₄)₃ nanoparticles (NVP-NPs), the NVP-QDs-SL/HCS nanocomposites synthesized by this method can display a higher reversible capacity than the theoretical capacity of Na₃V₂(PO₄)₃ at a rate of 0.1 C, fast charge/discharge rate and high cyclic stability due to the synergy of their unique nanostructure (Table S1 in Supporting information). Such a simple and scalable approach may also be applied to other systems for advanced functional materials.

2. Experimental

2.1. Material synthesis

The used reagents include NH₄VO₃ (99%, Tianjin Guangfu Fine Chemical Research Institute Co. Ltd), NaH₂PO₄·2H₂O (99%, Damao Chemical Reagent Factory Co. Ltd), andphosphoric acid (Laiyang Shuangshuang Chemical Co. Ltd.). Bamboo pulp tissue is from GaotangJiamei Paper Industry Co. Ltd. The general synthesis procedure is described as follows. Firstly, 100 ml of deionized water was adjusted with phosphoric acid to pH = 3 and mixed well with 2 g of bamboo pulp tissue. Then it was placed at room temperature for 12 h for the acid hydrolysis of LCs [34]. The purified LCs were obtained by washing with distilled water, centrifugation and drying. Then, according to the stoichiometric ratio of Na₃V₂(PO₄)₃, 0.02 mol NH₄VO₃ and 0.03 mol NaH₂PO₄·2H₂O were dissolved in 100 ml of deionized water and stirred at 80 °C for 15 min and a mixed solution was formed (Eq. (1)). The purified LCs with different additive amount of 1, 2 and 3 g were added to the mixed solution and mixed well to obtain a mixture. Subsequently, the mixture was transferred into a 100 ml hydrothermal reactor, reacts at 180 °C for 24 h, and then cooled naturally to room temperature. The precursor after hydrothermal reaction (Eq. (2)) was dried in air in an oven at 60 °C until balance weight. Finally, the dried precursor samples were calcined at 350 °C for 4 h, then at different temperature (750, 800 and 850 °C) for 8 h in nitrogen atmosphere to get the product (Eq. (3)). The heating rate was kept relatively low, i.e., 3 °C min^-1, to avoid anomaly grain growth. The sintered powder is black, and the samples synthesized at different temperature were marked as NVP-QDs-SL/HCS-750B, NVP-QDs-SL/HCS-800B and NVP-QDs-SL/HCS-850B, respectively. The samples synthesized with different additive amounts of purified LCs at 800 °C were marked as NVP-QDs-SL/HCS-800A (adding 1 g LCs), NVP-QDs-SL/HCS-800B (adding 2 g LCs) and NVP-QDs-SL/HCS-800C (adding 3 g LCs), respectively.

3NaH₂PO₄+2NH₄VO₃+LCs ${80℃ \atop →}$ 2NaVO₃+2NH₄H₂PO₄+NaH₂PO₄+LCs (1)

4NaVO₃+5NH₄H₂PO₄+LCs${180℃ \atop →}$ Na(VO)₂(PO₄)₂+Na₃V₂(PO₄)₃+LCs+5NH₃↑(2)

NaVO₂(PO₄)₂(H₂O)₄+Na₃V₂(PO₄)₃+LCs${800℃ \atop →} $ Na₃V₂(PO₄)₃+HC+3H₂O↑(3)

2.2. Material characterization

The crystal structures of the synthesized samples were analyzed by X-ray diffraction (XRD) employing a CuKα X-ray diffractometer (PANalyticalX’Pert PRO; Netherlands). The XRD patterns were collected over a diffraction angle 2θ range of 10°-70° with a scan rate of 5° min^-1. The property of carbon was performed by a Raman microscopic (Renishaw In-Via) equipping with an Ar⁺ laser (785 nm) at 50× aperture. The morphology and microscopic structure of the samples were characterized by scanning electron microscopy (SEM) (Quanta 200) with SEM-X-ray energy dispersive spectrometry (SEM-EDS) and high-resolution transmission electron microscopy (HRTEM) (Philips Tecnai 20U-TWIN). Fourier transform infrared (FTIR) spectra were performed with a Perkin-Elmer spectrometer (Spectrum One B). Thermogravimetric analysis (TGA) of the samples was performed by a thermal analyzer (TGA1 STAR System) in air at a heating rate of 10 °C min^-1 from ambient temperature to 800 °C. The specific surface area and Barrett-Joyner-Halenda (BJH) pore size distribution were carried out by an automatic surface area analyzer (Micromeritics, Gemini V2380, USA).

2.3. Electrochemical measurements

To make electrodes, 80% active materials were mixed with 10% acetylene black and 10% poly-vinylidene fluoride (PVDF) in N-methylpyrrolidone (NMP) to ensure homogeneity. Then the mixture was coated on aluminum foil with a thickness of about 0.02 mm, dried under the air atmosphere at 60 °C for 3 h and vacuum atmosphere at 120 °C for 12 h and cut into circular strips of 15 mm in diameter with a mass loading of 1.76 mg cm^-2. CR 2032 Coin cells were assembled in a glove box filled with high-purity argon. The electrolyte is consisted of 1 M NaClO₄ dissolved in dimethyl carbonate (DMC)/ethylene carbonate (EC)/fluoroethylene carbonate (FEC) in a volume ratio of 1:1:0.05. Sodium metal and glass microfiber filters were employed as anode and separator, respectively. Before galvanostatic charging/discharging test, the cells were aged for more than 12 h to ensure full absorption of the electrolyte into the electrodes. The galvanostatic charging/discharging tests were carried out in the voltage range of 2.0 V-4.3 V at different current densities on Channels battery analyzer. The voltage range of the cyclic voltammetry (CV) measurements was 2.5-4.5 V and the scanning rate was from 0.2 to 0.6 mV s^-1. The impedance data were recorded in the frequency range from 0.1 MHz to 10 MHz and alternate current signal of 5 mV in amplitude as the perturbation. All the tests were performed at room temperature.

3. Results and discussion

3.1. Fabrication process

LCs-enhanced formation of NVP-QDs-SL was first demonstrated. Fig. 1 shows the process of preparing NVP-QDs-SL/HCS nanocomposites, which relies on the hydroxyl functional groups on lignocellulose biomolecules and the self-assembly of LCs. LCs from bamboo pulp are a renewable resource with the highest natural carbon content and surface activity, which are ideal precursors for biomass carbon materials (Fig. 1(a) and (b)). LCs not only contain lots of hydrophilic functional groups such as phenolic hydroxyl groups, carboxylic acid groups, and methoxyl groups (Fig. 1(c)), but also have many hydrophobic groups (Fig. 1(d)). LCs after acid hydrolysis were cross-linked into spherical particles by the self-assembly reaction in phosphoric acid solution (Fig. 1(e)). These highly hydrophilic groups can provide lone pair electrons and nucleation sites, selectively adsorb metal ions [35]. Therefore, LCs can be used as a carbon source of HCS and a nucleating agent of NVP-QDs-SL during the synthesis. Furthermore, the metal ions act as carbonization catalysts and can enhance the onset of structural ordering, induce structural cross-linking and form cross-linked structures (Fig. 1(f)), owing to its highly cross-linked nature and the oxidation resistance. Thermal degradation of various lignocelluloses is reported, during which the pyrolysis causes their carbonization, and this is a multistep reaction [36]. As shown in Fig. 1(g), the LCs are assembled to stable 2D cross-linked structure, its hydrophilic groups selectively adsorbed VO²⁺, PO₄^3- and Na⁺ ions and NVP crystal nucleus was formed by heterogeneous nucleation during hydrothermal process. Fig. 1(h) and (i) shows schematic diagrams of confined growth process of NVP-QDs-SL in HCS. In the heat treatment process at high temperature, the cross-linked sheet structure is decomposed to form HCS, which can restrict the growth of NVP-QDs-SL, and prevent their aggregation. The NVP-QDs-SL are homogeneously embedded in and anchored on its surface of HCS, which can greatly enhance the electrochemical performance.

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Fig. 1.   Fabrication process for NVP-QDs-SL/HCS nanocomposite: (a) photo of natural bamboo bio-template; (b) photos of bamboo pulp tissue; (c) structure schematic and crystal structure of lignocelluloses (LCs) in bamboo pulp tissue; (d) self-assembly schematic of a crosslinked particle of LCs; (e) crosslinked particle after purification; (f) 2D cross-linked structure diagram formed by self-assembly of crosslinked particles; (g) Schematic diagrams of heterogeneous nucleation process of NVP in the 2D cross-linked structure of LCs during hydrothermal reaction; (h, i) schematic diagrams of confined growth process of NVP-QDs-SL in HCS.

3.2. Structural and morphological characterizations

In order to optimize the synthesis conditions, the composition, morphology and microstructure of the different samples were investigated by scanning electron microscopy (SEM), SEM-X-ray energy dispersive spectroscopy (SEM-EDS) and high resolution transmission electron microscopy (HRTEM). The comparisons of morphology, composition, porous structure and microstructure of the samples synthesized with the addition amount of 2 g LCs at different temperature are shown in Figs. S1-S3. Results indicate that the samples synthesized at different temperature have different morphological characterizations of particles and pore structure. The NVP-QDs-SL/HCS-800B sample synthesized with the addition amount of 2 g LCs at 800 °C possesses regular uniform sheet particles (Fig. S1(d)), and the stack of different sheet particles forms stratified structure and hierarchical pores, which enable easier electrolyte penetration and increase the structure stability. Fig. S1(e) shows that many NVP-QDs were uniformity embedded in a HCS in large areas, displaying clear lattice fringes in a NVP-QD (inset in Fig. S1(f)). Therefore, the heat treatment temperature plays a critical role in synthesizing NVP-QDs-SL/HCS. Fig. S4 shows the morphological and composition characterizations of bamboo pulp sample after calcinations at 800 °C, displaying its fine fiber structure constituted by sheets. Fig. S5 shows the morphological characterization of bamboo pulp sample before calcinations, displaying bamboo fiber structure. From the comparison of Figs. S3-S5, it is found that the morphological details of bamboo pulp sample can be preserved well in NVP-QDs-SL/HCS-800B sample. To clearly indicate the thermal decomposition behavior of pristine LCs and NVP-QDs-SL/HCS-800B precursors, the TGA curves under synthesis conditions are shown in Fig. S6. The results show that the decomposition carbonization temperature range (about 360-740 °C) and the initial temperature of the decomposition dehydration (about 360 °C) for NVP-QDs-SL/HCS-800B precursor are higher than that (about 350-700 °C) and that (about 350 °C) of pristine LCs precursor. These indicated that the metal ions induce structural cross-linking of LCs and enhance the formation of HCS in the synthesis. Residual carbon contents of pristine LCs and NVP-QDs-SL/HCS-800B precursors measured by TGA are 11.5% and 9.5%, respectively, which is consistent with the residual carbon content (9.44%) of NVP-QDs-SL/HCS-800B sample in Table S2.

For further investigating fine microstructure and crystalline characteristics of NVP-QDs-SL/HCS-800B sample, TEM and HRTEM images with different magnifications are displayed in Fig. 2. For comparison, TEM images of NVP-QDs-SL/HCS-750B and NVP-QDs-SL/HCS-850B samples are shown in Fig. S7, illustrating that they all have uneven aggregate particle morphology and nanostructure. It is clear that the NVP-QDs-SL/HCS-800B sample has the lamellar structure (Fig. 2(a)). Fig. 2(b) and (c) shows that many of the spots with sizes of 2-10 nm were uniformity embedded in HCS in large areas, whose good dispersibility can greatly enhance the electrochemical performance. The mesoporous structure of the NVP-QDs-SL/HCS-800B sample is elucidated by the nitrogen adsorption-desorption isotherms (Fig. S8(a) and (b)), which can also be observed by TEM image in Fig. S8(c). The corresponding size distribution analysis in Fig. S8(d) confirms the evenly distribution and uniform particle size of the NVP-QDs-SL with an average particle size of 7.8 nm (Fig. S8(e)). The brighter area in Fig. 2(c) corresponds to monolayer of HCS. Those spots in Fig. 2(d) and (e) have the well-resolved lattice fringes with the lattice-fringe distance of 0.42 and 0.62 nm, corresponding to the (110) and (012) lattice planes of the Na₃V₂(PO₄)₃. This indicates that LCs can control nucleation and growth of NVP-QDs-SL in the synthesis process. Effective electron transport is another important requirement for kinetics, since insertion/extraction of Na ions must be accompanied by electrons transfer to keep the charge balance. Fig. 2(f) shows the disordered structure of HCS, which is identical with the result in Fig. 3(e). Fig. 2(g)-(i) clearly display that different types of SLs in NVP-QDs have different growth directions or different lattice plane. This could be attributed to self-assembling of LCs and the local variations in nanocrystal concentration during NVP-QDs growth. These results show that the NVP-QDs-SL were created by self-assembling of metal ions and LCs. In the synthesis, self-assembling of LCs can influence the nucleation and growth of NVP-QDs (Fig. S9), and the right addition amount (adding 2 g LCs) of LCs can induce its cyclical arrangement and form superlattice structure (Fig. S9(c) and (d)). The superlattice structure has abundant reactive sites of electron and Na⁺ in the quantum wells due to quantum confinement and size effects.

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Fig. 2.   Fine microstructure characterization of NVP-QDs-SL/HCS-800B sample: (a) low magnification TEM image, showing multilayered ultra-thin sheets; (b, c) high magnification TEM images, showing the heterostructures of large-scale NVP-QDs embedded in HCS; (d, e) enlarged images, showing NVP-QDs embedded in HCS and lattice structure of monocrystalline (insets); (f) an enlarged image, showing disordered hard carbon structure (insets); (g-i) enlarged images, showing NVP-QDs-SL with different lattice spacing (insets) embedded in HCS.

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Fig. 3.   (a) XRD pattern of the precursor sample before calcinations, (b) XRD patterns for the samples synthesized at the different temperature, (c) XRD patterns of bamboo pulp after calcination (the black line) and before calcination (the green line), (d) Raman spectrum of NVP-QDs-SL/HCS-800B sample, (e) Raman spectrum deconvoluted by three Gaussian peaks in 1100-1800 cm^-1 (inset), (f) SEM image of bamboo pulp before calcinations and (g) TEM image of bamboo pulp after calcinations, showing five layers of hard carbon.

To investigate the crystal structure, synthesis mechanism and the nature of the carbon formed in the NVP-QDs-SL/HCS sample, XRD and Raman spectra were carried out as shown in Fig. 3. Fig. 3(a) shows the XRD pattern of the precursor sample before calcination, and its diffraction peaks well match the XRD data of Na(VO)₂(PO₄)₂(H₂O)₄ crystals (PDF#70-4607), LCs and NVP. Fig. 3(c) is the XRD pattern of bamboo pulp after calcination (the black line), showing two broad diffraction peaks at about 12°-32° and 39°-48°, which is in good agreement with the diffraction peaks of the hard carbon material [19]. By contrast with the XRD pattern of pure LCs (the green line in Fig. 3(c)), the (101) peak of LCs in the precursor sample become a sharp peak, has moved to small diffraction angle and its intensity markedly increased, which implies that LCs formed 2D cross-linked structure during hydrothermal reaction and proves its formation mechanism (Fig. 1(d)-(f)). Na(VO)₂(PO₄)₂(H₂O)₄ has laminar structure with interlaminar spacing of 0.63 nm, both Na(VO)₂(PO₄)₂(H₂O)₄ and NVP have good crystallinity, which confirm the heterogeneous nucleation mechanism in Fig. 1(g). The (024) and (211) peaks of NVP in the precursor sample show that NVP- QDs-SL are preferred growth in the two directions of (024) and (211) during hydrothermal reaction. The XRD patterns of the samples synthesized with different calcination temperature are shown in Fig. 3(b). XRD data of three samples all show characteristic diffraction peaks of Na₃V₂(PO₄)₃ (PDF#62-0345) [37,38]. The NVP-QDs-SL/HCS-800B sample synthesized at 800 °C has the strongest diffraction peaks, the higher carbon content (9.438 wt%) and the highest tap density (1.11 g cm^-1) (Table S2). The lattice volume (V), crystallinity and strain of (113) crystal face increase with the rise of synthesized temperature. The carbon content and tap density of NVP-QDs-SL/HCS-850B sample synthesized at 850 °C decreased, which was caused by the oxidation of HCS due to the high calcination temperature, causing more pores to form. Furthermore, excessive calcination temperature also leads to the particle aggregation and abnormal grain growth of NVP-QDs-SL (Fig. S1(g)-(i)). So the electrochemical performances of NVP-QDs-SL/HCS-850B sample obviously decrease (Fig. 4(a) and (b)). The results show that the synthesis temperature has a strong impact on the electrochemical performance of final NVP-QDs-SL/HC composites.

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Fig. 4.   Electrochemical property characterizations of different samples: (a) charge-discharge curves at 0.2 C; (b) capacity retentions at 10 C of the different cathodes synthesized with 2 g LCs at different temperature; (c) capacity retentions of the different cathodes synthesized with different additive amounts of LCs at 800 °C at 10 C; (d) charge-discharge curves of the NVP-QDs-SL/HCS-800B cathode synthesized with 2 g LCs at 800 °C at different rate; (e) rate capability of NVP-QDs-SL/HCS-800B cathode; (f) CV curves of NVP-QDs-SL/HCS-800B cathode at different scanning rate; (g) capacity retention and coulombic efficiency of the NVP-QDs-SL/HCS-800B cathode for 3000 cycles at 20 C.

Fig. 3(d) and (e) shows the typical and deconvoluted Raman spectra of NVP-QDs-SL/HCS-800B sample. Characteristic bands located at around 1350 and 1580 cm^-1 are attributed to the D-band (representing the disordered hard carbon) and G-band (representing graphitized carbon) [39], respectively. The value of the peak intensity ratio of the D to G band (I_D/I_G) is 2.34, indicating a relatively high degree of disordered hard carbon, which could enhance electronic conductivity and Na storage [40]. From the deconvolution of the Raman spectrum in Fig. 3(e), we found that the fraction of disordered hard carbon in the residual carbon is about 67.75%. According to the analysis result of elemental analysis, accurate hard carbon content in NVP-QDs-SL/HCS-800B sample is about 9.44% in weight percent (Table S2). Fig. 3(f) is a SEM image of bamboo pulp before calcinations, showing fine fiber structure constituted by sheets. Fig. 3g shows a TEM image of bamboo pulp after calcinations, showing five-storeyed HCS and its disordered nanostructure. The flexible HCS offers more electrons for the redox reactions occurring in NVP-QDs-SL, not only ensuring high conductivity and the fast ion transportation, but also buffering the volume expansion/contraction of NVP-QDs-SL.

To optimize the synthesis conditions, the crystal structure of the different synthetic samples was evaluated and compared by XRD and FT-IR spectroscopy measurements (see Fig. S10 and detailed analysis). The structural model of chemical bond linkages among C—S, N—H and O—H in HCS network is shown in Fig. S10(c). The FTIR results indicate that there are in situ nanocomposite linkages between NVP-QDs and HCS network, which further verifies the nanocomposite structure of NVP-QDs /HCS.

3.3. Electrochemical performance evaluation

Galvanostatic charge/discharge, cyclic voltammetry (CV) and electrochemical impedance spectroscopy (EIS) experiments were employed to evaluate electrochemical performances of the different cathodes. Different samples were used to fabricate a cathode and then assembled to a half cell with sodium metal as a counter electrode in the electrolyte (1 M NaClO₄/EC/DMC/FEC) for electrochemical property characterization and measurements. The tap density of different samples is shown in Table S2. Fig. 4(a) shows the charge-discharge curves of samples synthesized at different temperatures at 0.2 C. Fig. 4(b) shows the capacity retentions of samples synthesized at different temperatures at 10 C, it is obvious that the sample synthesized at 800 °C has the highest discharge capacity of about 96.6 mA h g^-1 and a capacity retention rate of 94.5% after 100 cycles, demonstrating that the available annealing temperature has a crucial effect on the electrochemical properties. We also investigated the effect of additive amounts of LCs in the synthesis on the electrochemical properties. Fig. 4(c) shows that the NVP-QDs-SL/HCS-800B cathode has the best capacity retention at 10 C. After 100 cycles, the capacity retention of NVP- QDs-SL/HCS-800B is 94.6%, higher than that of NVP-QDs-SL/HCS-800A (88.4%) and NVP-QDs-SL/HCS-800C (82.7%), which derives from the optimal degree of carbon content. The rate performance of NVP-QDs-SL/HCS-800B is shown in Fig. 4(d) and (e), respectively. The NVP-QDs-SL/HCS-800B cathode exhibits a high discharge capacity of 149.2 mA h g^-1 at 0.1 C (Fig. 4(d)), and the discharge capacities of 86.3, 83.0, 77.6 and 69.2 mA h g^-1 are still remained at rates of 2, 5, 10 and 20 C, respectively. It is obvious that the charge/discharge capacities at 0.2 C are much lower than that of 0.1 C due to the inherent imperfections of NVP nanoparticles, such as lower crystallinity, high specific surface area and the larger ionic radius of sodium, leading to the formation of solid-electrolyte interfaces, undesirable structural change and so on [[4], [5], [6], [7], [8]]. Moreover, the reversible capacity at 0.1 C is far beyond the theoretical value for Na₃V₂(PO₄)₃, which can be attributed to the collective and synergetic effect of NVP-QDs-SL and HCS. TheNVP-QDs-SL not only has abundant Na storage sites and optimized electronic structure for charge transfer because its surface atoms possess higher energy of delocalized electrons than the interior atoms [14,15], but also could provide a large electrode/electrolyte interface area and the fast diffusion path of Na⁺ and electrons. Therefore, the size effect, quantum electronic transport and quantum tunneling effect of NVP-QDs-SL can harvest the unique electrochemical performances as additional charge carriers via hole traps [16,17]. More importantly, H/S-doped HCS can not only serve as a stable scaffold for more Na ion storage sites, act as a conductive network for better electronic conductivity, but also could ensure the fast ion transportation because of its great conductivity and high specific surface area for the redox reactions. Moreover, its mesoporous layered structure could be fully infiltrated by the electrolyte and could buffer volume change for NVP-QDs-SL during charge and discharge process. Because HC materials as anodes in SIBs can exhibit the reversible capacity of 200-300 mA h g^-1 [18,19], so the H/S-doped HCS in the composite cathode also has the certain capacity contribution (about 31.6 mA h g^-1) in SIBs. Even at a higher current density of 50 and 100 C after 31 and 36 cycles, the discharge capacities of 55 and 40 mA h g^-1 are obtained (Fig. 4(e)), highlighting its remarkable rate performances. Although the discharge capacity of the NVP-QDs-SL/HCS-800B gradually decreases with the increasing current rate (Fig. 4(e)), an ultra-high coulombic efficiency close to 100% and the capacity retention rate of 56% have been achieved at the high current rate of 20 C after 3000 cycles, implying its excellent long cycling life (Fig. 4(g)). Fig. 4(f) shows the CV curves of NVP-QDs-SL/HCS-800B electrode at different scanning rate to further investigate its excellent cycling stability. With the increase of scanning rates, it can be seen that the potential polarization does not obviously increase, indicating good reversibility of redox reaction and good electronic conductivity.

We also compared the first charge/discharge capacities of the different cathodes at the low current rate of 0.2 C (Fig. 5(a)). Compared to the usual NVP nanoparticles synthesized without adding LCs at 800 °C (NVP-NPs) (Fig. S11), the NVP-QDs-SL/HCS-800B cathode shows the better first discharge capacity of 149.2 mA h g^-1 at a discharge platform of 3.40 V, which is higher than the theoretical capacity of NVP (117.6 mA h g^-1). But the NVP-NPs cathode only has the first discharge capacity of 101.5 mA h g^-1. Extraordinarily, at the high current rate of 10 C, the NVP-QDs-SL/HCS-800B cathode delivered the higher discharge specific energy retention than that of the NVP-NPs cathode and has a discharge specific energy of 273.9 mW h g^-1 and retention rate of 93.5% after 100 cycles (Fig. 5(c)). The CV curves of both the NVP-QDs-SL/HCS-800B and NVP-NPs cathodes in Fig. 5(b) show a pair of well-defined redox peaks, which is attributed to the reversible transformation of V³⁺/V⁴⁺ accompanied with the insertion and extraction reaction of Na-ion from the NVP lattice matrix. Observably, it is noted that the redox peaks of NVP-QDs-SL/HCS-800B have become more sharp and symmetrical than that of NVP-NPs cathodes. By calculation, the diffusion coefficient (D_Na) values of sodium ion in NVP-QDs-SL/HCS-800B and NVP-NPs cathodes were 3.27 × 10^-11 and 2.675 × 10^-12 cm² s^-1, respectively (Fig. S12(a) and (b)). These results indicate that NVP-QDs-SL/HCS-800B has higher D_Na value and better electrochemical reversibility of the electrode reaction, which benefits from its superior nanostructure. Fig. 5(d) shows EIS curve comparison of the NVP-QDs-SL/HCS-800B and NVP-NPs cathodes. The charge-transfer resistance (semicircle diameter R_ct = 245.4 Ω) of NVP-QDs-SL/HCS-800B cathode is far less than that (R_ct = 954.7 Ω) of NVP-NPs cathode, indicating that the synergistic effect of NVP-QDs-SL and HCS accelerates sodium ions movement velocities and improves high rate performance.

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Fig. 5.   Comparison of electrochemical properties of NVP-QDs-SL/HCS-800B and NVP-NPs cathodes: (a) initial charge-discharge curves at 0.1 C; (b) CV curves at a scanning rate of 0.2 mV s^-1 vs Na/Na⁺; (c) discharge specific energy retention comparison of the different electrodes for 100 cycles at 10 C; (d) EIS curve comparison of the different samples at the fully uncharged/undischarged state.

4. Conclusion

In summary, we have developed a new assembly method of Na₃V₂(PO₄)₃ quantum dots with superlattice structure (NVP-QDs-SL) by using multifunctional lignocelluloses (LCs) as the control template, and the source of hard carbon sheet (HCS). In this design, we highlight the advantages of nanobiotechnology and the synergy of unique nanostructure enhancing electrochemical properties. Firstly, the fabrication of NVP-QDs-SL/HCS nanocomposite is very facile and easy to be realized, which can be applied for large-scale production. The study of synthesis mechanism shows that the self-assembly of multifunctional LCs can control the formation of NVP-QDs-SL and make them evenly embed in HCS structure. Second, detailed electrochemical analysis results for sodium-ion batteries show that NVP-QDs-SL with the quantum tunneling and confinement effects result in more storage sites of Na⁺ ions and easier transfer kinetics. Third, the hierarchical pores and the stratified structure enable easier electrolyte penetration and H/S-doped HCS has high electronic conductivity for both facile mass transfer and charge transfer of NVP-QDs-SL. Fourth, the synergy effect of electrochemical reactions in the NVP-QDs-SL/HCS electrode has been highlighted in detail for the first time. This work provided a reliable way to fabricate quantum dots with superlattice structure by natural biomass templates.

Acknowledgements

This work was supported financially by the National Natural Science Foundation of China (Nos. 51672139, 51472127 and 51272144) and the Projects Supported by the Key Laboratory of Pulp and Paper Science and Technology of Ministry of Education (No. KF2016-01).

Appendix A. Supplementary data

Supplementary data associated with this article can be found, inthe online version, at https://doi.org/10.1016/j.jmst.2019.06.002.