Haijian
Huang
a,
Long
Pan
a,
Xi
Chen
a,
Elena
Tervoort
a,
Alla
Sologubenko
b and
Markus
Niederberger
*a
aLaboratory for Multifunctional Materials, Department of Materials, ETH Zürich, Vladimir-Prelog-Weg 5, 8093 Zürich, Switzerland. E-mail: markus.niederberger@mat.ethz.ch
bLaboratory for Nanometallurgy, Department of Materials, ETH Zürich, Vladimir-Prelog-Weg 5, Zürich, 8093, Switzerland
First published on 24th September 2019
Compared to the progress made in high-rate anodes, the exploration of cathode materials with comparable performance remains a challenge in the fabrication of high power Li-ion storage devices. Here a cathode material with fast Li-ion storage is reported, which is composed of Ni0.25V2O5·nH2O nanobelts with a layered structure and reduced graphene oxide. Operando X-ray diffraction and ex situ X-ray photoelectron spectroscopy results reveal an electrochemical process with high structural reversibility during charging/discharging. Kinetics analysis based on cyclic voltammetry measurements demonstrates fast Li-ion storage in the cathode material. These positive properties lead to high rate capability and cycling stability of the composite. Even at a high mass loading of 11.3 mg cm−2, the material still offers high-performance Li-ion storage, indicating its great promise for practical applications. Furthermore, we report for the first time the combination of such a high-rate cathode with a surface redox pseudocapacitive anode made of N-doped reduced graphene oxide foam for a fast-charging Li-ion storage full-cell device with a long lifespan. The high performance of the full cell suggests that the concept of using materials with fast Li-ion storage in both the positive and negative electrodes represents a promising strategy to develop high-power and high-energy Li-ion storage devices.
One of the challenges for the development of high-rate cathodes lies in discovering materials with fast Li+ diffusion kinetics during operation. Conventional faradaic LIB cathodes possess Li+ intercalation channels, whereas the slow diffusion rate will lead to prolonged charging time and thus limits the power density.15–18 On the other hand, non-faradaic cathodes, e.g., activated carbon (AC), intrinsically offer fast charge/discharge capability, because the capacitive charge storage in this type of cathode is based on fast physical adsorption/desorption processes.19–22 The high rate capability of AC as the cathode resulted in the emergence of high-power supercapacitors (SCs) and Li-ion hybrid supercapacitors (Li-HSCs).11,23–30 As the capacitive storage mechanism is a surface effect, the energy storage capability of AC is limited by its accessible surface area. Therefore, the high power density of SCs and HSCs comes at the cost of low energy density. In this context, developing high-energy and high-power Li-ion storage devices is still challenging.
Layered metal vanadates have been proven to show good electrochemical performance in rechargeable batteries due to their open-framework crystal structure, which leads to fast ion diffusion kinetics.31–35 In a recent report, we demonstrated that fast Na-ion insertion/extraction in layered metal vanadates leads to high-rate Na-ion storage performance.36 However, so far this type of material has rarely been studied as high-power LIB cathodes. Here we report the synthesis of Ni0.25V2O5·nH2O/reduced graphene oxide (referred to as NVO@rGO) as a new type of LIB cathode with high rate capability for LIBs. The layered crystal structure of Ni0.25V2O5·nH2O consists of V2O5 layers pillared by NiO6 octahedra in between. Ni0.25V2O5·nH2O has a large interlayer spacing (d001 = 10.3 Å), which is able to provide spacious channels for fast Li+ intercalation/extraction. Ni0.25V2O5·nH2O was further integrated with reduced graphene oxide (rGO) sheets, forming a conducting network to facilitate electron transport. The synergetic effects between the two constituents, one offering channels for fast Li+ storage and the other one good electronic conductivity, lead to the high rate capability of the composite material when working as the LIB cathode within the voltage range of 2.0–4.3 V. Moreover, it is worth noting that for practical applications, the electrodes usually have about 10 mg cm−2 of mass loading. However, in most of the studies on Li-ion battery cathodes, the electrochemical performances were tested at low mass loadings,37–41 which results in low areal capacities. In our study, due to the favourable Li-ion storage kinetics and the conducting network, the composite material is able to deliver high-performance Li-ion storage even at a high mass loading, indicative of its large application perspective.
Furthermore, conventional strategies for improving the ionic diffusion kinetics of electrode materials usually involve nanoscaling and complex nanostructural design to shorten the diffusion distance,11,42–48 which often complicates large-scale manufacturing. Even without downsizing the particles, anode materials of niobium oxide49 and niobium tungsten oxides50 have been reported to possess fast ion storage kinetics mainly related to the crystal structure rather than to the diffusion length. However, analogous properties of cathode materials have been overlooked. In our study, it is found that NVO@rGO shows fast Li+ intercalation kinetics even when the diffusion length is of the order of micrometers, indicating its great promise in practical applications.
To produce a high-power and high-energy LIB full cell, the NVO@rGO cathode is combined with an anode composed of N-doped reduced graphene oxide (N-rGO) foam. Cyclic voltammetry (CV) analysis indicates surface redox pseudocapacitance in the N-rGO foam, which results in a high rate performance of the anode material in half-cell tests. Prototype LIB full cells were fabricated from NVO@rGO as the cathode and N-rGO as the anode. To the best of our knowledge, this is the first time that a surface redox pseudocapacitive anode is combined with a cathode with fast Li-ion storage in a full cell configuration. The fast Li+ storage on both sides is expected to improve the rate capability of the full cells without compromising the energy density.
Scheme 1 Schematic of the preparation of NVO@rGO and its advantages as a high-rate Li-ion battery cathode. |
Interestingly, GO was partially reduced under microwave radiation during the reaction. As shown in the X-ray photoelectron spectroscopy (XPS) data (Fig. S1a and b†), a significant decrease in the C–O/C–C (C1s) signal ratio was observed for NVO@rGO compared with pristine GO. Such reduced GO sheets with the restoration of the sp2 hybridization of carbon facilitate the formation of a conductive network between the NVO nanobelts, enabling enhanced rate capability as discussed below. Thermogravimetric analysis (TGA) (Fig. S2†) reveals ∼0.9 molecules of lattice water per formula unit in NVO. In addition, the weight loss difference between NVO@rGO and NVO indicates a carbon content of 7% in the NVO@rGO composite. The X-ray diffraction (XRD) results of NVO and NVO@rGO (Fig. 1a) show that there is no phase change with the introduction of reduced graphene oxide in the composite. The XRD peaks of both samples can be indexed to monoclinic Ni0.25V2O5·nH2O51 (ICSD PDF no. 04-009-5930) although the pattern of our sample is dominated by (00l) reflections due to the highly anisotropic shape of the nanobelts. The same observation was reported for Zn0.25V2O5·nH2O with a similar structure.33,52Fig. 1b shows the crystal structure of NVO, where NiO6 octahedra connect the V2O5 layers. The open framework of the structure creates spacious and stable pathways for fast Li+ diffusion in the ab plane along the [00] and [010] directions, which will be discussed in detail below.
The morphology of NVO and NVO@rGO was studied by scanning electron microscopy (SEM) and transmission electron microscopy (TEM). The SEM overview image in Fig. 1d reveals the 1-dimensional morphology of NVO with anisotropic particles that are tens of micrometers long and 100–500 nm wide. The structures are nearly transparent to the electron beam, indicating a thin nanobelt-like shape. The thickness of the NVO nanobelts is determined to be ∼35 nm by atomic force microscopy (AFM) measurements (Fig. 1c), further corroborating the belt-like morphology with a high width to thickness aspect ratio. Fig. 1e shows a representative TEM image of a typical nanobelt. The high-resolution TEM (HRTEM) image of a selected area in the nanobelt is presented in Fig. S3a.† The corresponding fast Fourier transform (FFT) acquired from Fig. S3a† (inset in Fig. 1e) reveals well-resolved spots, typical for a single-crystalline material. The evaluation of the FFTs (Fig. S3b†) confirms the monoclinic C12/m1 structure of the Ni0.25V2O5·nH2O nanobelts51 and reveals that their growth direction is [020]. The enlarged HRTEM image in Fig. 1f reveals well-defined lattice fringes with a spacing of 0.59 nm corresponding to the (200) planes, further confirming that the nanobelts grow along the b direction. It is interesting to note that, as discussed above, Li+ diffuses into the structure along the [00] and [010] directions, i.e., along the growth direction of the nanobelts, which results in long diffusion paths of several micrometers. Nevertheless, the Li+ intercalation rates are still high due to the spacious structure of the NVO host lattice, as discussed below. In addition, energy dispersive spectroscopy (EDS) analyses using an atomic number sensitive high-angle annular dark-field detector (HAADF) confirm the co-existence of Ni, V, and O inside the NVO nanobelts (Fig. 1g–j). The resulting atomic ratio of Ni to V is about 1:8 (Table S1†), which is in line with the structural formula of Ni0.25V2O5·nH2O. The SEM image of NVO@rGO (Fig. 1k) reveals a composite architecture with NVO nanobelts embedded in the rGO sheets. TEM images at low (Fig. 1l) and at higher magnification (Fig. 1m) prove the interconnected composite structure of NVO@rGO with the nanobelts embedded in the rGO sheets. Although not all NVO nanobelts are similarly well connected with the rGO nanosheets (a less connected example is shown in Fig. S4†) due to the low content of rGO sheets, the rGO in the composite is able to create a conductive network and the interaction between these two components facilitates fast and efficient electron transfer in the composite nanostructure. This feature is especially beneficial for achieving high-rate capability, as discussed below.
The N-rGO foam was prepared by a hydrothermal process using urea as the source for the nitrogen dopant. A typical photograph of monolithic N-rGO foam on a dandelion flower is presented in Fig. S5,† illustrating the low density of the foam. High-resolution C1s XPS spectra in Fig. S6† reveal the significantly decreased intensity of the C–O signal in N-rGO compared with pristine GO, indicating the reduction of GO in the N-rGO foam. Furthermore, the N1s XPS spectrum confirms the successful doping of N in the rGO sheets. As shown in Fig. S7,† the envelope of the N1s peak can be deconvoluted into three components that are assigned to pyridinic N (398.7 eV), pyrrolic N (400.0 eV) and graphitic N (401.5 eV), respectively.53,54 The reduction of GO together with N doping is well-known means to optimize the electronic properties of such materials.55,56 N2 gas sorption analysis demonstrates the coexistence of mesoporosity and macroporosity with pore sizes between 5 and 75 nm (Fig. S8†). SEM images (Fig. S9†) clearly show the hierarchical porous structure of the foam. The high porosity with a surface area of 154 m2 g−1 (Fig. S10†) leads to more surface active sites on the sheet-like structure of N-rGO (Fig. S11†), which is especially important for improving the rate capability as discussed below.
To gain better insight into the electrochemical mechanism of the NVO@rGO electrodes, operando XRD investigation, ex situ high-resolution XPS analysis and CV measurements were performed. The operando XRD results are illustrated in Fig. 3a–d. A broad peak centered around 25°, which stems from the operando cell as confirmed by the XRD pattern of the blank cell (Fig. S14†), is found for all patterns. Apart from this, three dominating reflections, which are indexed to the (001), (003) and (004) crystal planes of NVO, are observed. The exclusive (00l) set of reflections provides detailed information on the evolution of the a–b interlayer spacing, i.e., on the c parameter, during Li+ intercalation/extraction. Specifically, upon discharging to 2.0 V, a slight shift of the (00l) reflections to higher 2θ positions is observed, which corresponds to a decrease of the interplanar spacing from 10.3 to 9.8 Å. The small contraction of the NVO lattice is likely due to the reinforced electrostatic attraction between the inserted Li+ and the VOx layers. A continuous intensity decrease of the (00l) is also observed, which might be the result of an uneven distribution of Li-ions between the layers, leading to the broadening and weakening of the peaks. During charging back to 4.3 V, the structural evolution follows the reverse process of discharge. The intensity and the peak positions of the charged electrodes are almost the same as those of the pristine electrodes, indicating the good structural reversibility of the cathode. The high reversibility can be attributed to the NiO6 octahedra, which work as mechanically stable pillars between the VOx layers, efficiently stabilizing the layered crystal structure during charging/discharging. In addition, the high structural stability makes it possible to maintain the morphology of the NVO nanobelts in the composite for 250 cycles at 15C, as confirmed by the ex situ SEM measurements (Fig. S15†).
To further explore the Li+ storage mechanism in NVO@rGO, ex situ high-resolution XPS spectra of the pristine electrodes, the discharged electrodes, and the charged electrodes were collected. As shown in Fig. 3e, no Li signal is observed in the pristine NVO cathode as expected. However, the Li1s peak emerges when the NVO electrodes were discharged to 2 V, demonstrating the intercalation of Li+ into the NVO structure. After charging back to 4.3 V, the Li signal nearly disappears again, clearly indicating that most of the intercalated Li ions are extracted during charging. The V2p spectra of the discharged and charged electrodes are shown in Fig. 3g. Relative to the original V2p state of the pristine electrodes (see Fig. 3f), the intensity of V3+ strongly increased upon discharging, reflecting the reduction of V during Li+ intercalation. After recharging to 4.3 V, the V3+ signal intensity decreases again and the pristine V2p spectrum is almost completely regenerated, indicating that the initial chemical state of V is restored upon extraction of Li+. The XPS results also underline the reversibility of the electrochemical processes, which is in good agreement with the high coulombic efficiency of the cathode as discussed above.
Cyclic voltammetry (CV) measurements at various sweep rates were carried out (Fig. 3h) to evaluate the electrochemical kinetics of the cathode material during charging/discharging. In the inset of Fig. 3h, the plot of normalized capacity (Q) vs. sweep rate−1/2 (v−1/2) shows that the capacity does not vary significantly with the sweep rates increasing from 0.1 to 5 mV s−1, indicating that the non-diffusion limited processes are independent of the scan rate. The linear decrease of Q with v−1/2 observed at a sweep rate >10 mV s−1 reflects a rate-limited diffusion process. Similar observations were reported for other intercalation pseudocapacitive materials, e.g., T-Nb2O5 (ref. 49) and partially bonded graphene–TiO2.61 Possible reasons for the rate-limited process at high sweep rates are the increase of the ohmic contribution and/or the diffusion constraints/limitations.49 The total contribution of the non-diffusion limited processes at a certain sweep rate can be quantified based on eqn (1):
i = k1v + k2v1/2 | (1) |
The electrochemical performance of N-doped reduced graphene oxide (N-rGO) foam is also first tested in the half-cell configuration within the voltage range of 0.01–2 V. As shown in Fig. S16,† the N-rGO foam shows a high rate capability with a capacity decrease of only 29.1% at 4.5 A g−1 compared with that at 1.05 A g−1. In the long-cycling tests, a capacity of 160 mA h g−1 is achieved at 4.5 A g−1 after 500 cycles, with an average coulombic efficiency of close to 100% (Fig. S17†). When cycled at an even higher current rate of 7.5 A g−1, the N-rGO anode still delivers a capacity of 131 mA h g−1 after 500 cycles (see Fig. S18†), further supporting its high rate capability and cycling stability. The Li+ storage kinetics of the N-rGO is also investigated by CV measurements. Fig. S19† shows the CV curves at various sweep rates ranging from 0.1–5 mV s−1. Based on eqn (1), the relative contributions from pseudocapacitive processes are accessible for different sweep rates. For instance, as shown in Fig. S20a,† 85% of the total stored charge can be attributed to surface redox pseudocapacitive effects at 5 mV s−1. The pseudocapacitive contributions at various rates are illustrated in Fig. S20b.† The results clearly indicate the dominant pseudocapacitive charge storage mechanism in the N-rGO anode.
The performance of our full cell is compared with that of some reported high-energy LIB full cells and high-power Li-HSCs, which represent the two typical Li-ion storage devices. As shown in the Ragone plots in Fig. 5, the NVO@rGO‖N-rGO full cells are not only able to deliver an energy density (168 W h kg−1 at 209 W kg−1) comparable to that of high-performance LIB full cells (e.g., graphene‖LiFePO4,64 Sn–C‖Li[Ni0.45Co0.1Mn1.45]O4,65 Si/graphene‖LiNi1/3Mn1/3Co1/3O2,66 graphene‖LiNi0.5Mn1.5O4,67 and Li4Ti5O12–Li2Ti3O7‖LiFePO4 (ref. 68)), but additionally our full cell device exhibits an impressively high power-output (17194 W kg−1 with 32 W h kg−1 retained), which outperforms some recently reported most advanced Li-HSCs (e.g., pillared Ti3C2 MXene‖AC,25 Li3VO4‖AC,69 NixFeyOz@rGO aerogel‖AC,11 m-Nb2O5/C‖AC,70 and TiO2@rGO‖AC62). The successful combination of high energy density and high power density within one device can be ascribed to the fast Li-ion storage of the two materials used as the cathode and anode, both able to drive high rate and charge storage capability. The well-balanced kinetics between the two electrodes thus leads to the high power output of the full cells. Compared with AC, the cathode of NVO@rGO shows much higher charge storage capability, which finally provides the high energy density of the full cells.
Fig. 5 Ragone plots (energy density vs. power density) of NVO@rGO‖N-rGO LIB full cells developed in this work in comparison to some advanced Li-HSCs and LIB full cells reported in the literature. |
Footnote |
† Electronic supplementary information (ESI) available: Experimental section, Fig. S1–S21, and Tables S1 and S2. See DOI: 10.1039/c9ta08000g |
This journal is © The Royal Society of Chemistry 2019 |