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A novel aluminum dual-ion battery
Author’s Accepted Manuscript A Novel Aluminum Dual-ion Battery Erjin Zhang, Wei Cao, Bin Wang, Xinzhi Yu, Longlu Wang, Zhi Xu, Bingan Lu
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To appear in: Energy Storage Materials Received date: 19 September 2017 Revised date: 4 October 2017 Accepted date: 4 October 2017 Cite this article as: Erjin Zhang, Wei Cao, Bin Wang, Xinzhi Yu, Longlu Wang, Zhi Xu and Bingan Lu, A Novel Aluminum Dual-ion Battery, Energy Storage Materials, https://doi.org/10.1016/j.ensm.2017.10.001 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting galley proof before it is published in its final citable form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
A Novel Aluminum Dual-ion Battery Erjin Zhang#1, Wei Cao#1, Bin Wang2, Xinzhi Yu1,3, Longlu Wang1,Zhi Xu3 and Bingan Lu1,3* 1. School of Physics and Electronics, Hunan University, Changsha 410082, P. R. China 2. Physics and Electronic Engineering Department, Xinxiang University, Xinxiang 453003, P.R. China 3. Fujian Strait Research Institute of Industrial Graphene Technologies, Jinjiang, 362200, P. R. China *
Corresponding authors E-mail addresses: [email protected] (B. Lu)
# The authors contributed equally to this work
Acknowledgements This work was financially supported by National Natural Science Foundation of China (Nos. 51672078 and 21473052) and Hunan University State Key Laboratory of Advanced Design and Manufacturing for Vehicle Body Independent Research Project (No. 71676004), Hunan Youth Talents (2016RS3025), and Foundation of State Key Laboratory of Coal Conversion (Grant No. J17-18-903).
Abstract The development of new rechargeable safe battery with high energy density and low cost is one of the most desirable goals for personal electronics and grid storage. Aluminum based rechargeable ion batteries offer the possibilities for safe, high energy density and low cost. Here, we developed a novel aluminum based high-rate capability dual-ion battery with an aluminum anode and a 3D graphene cathode. The battery operated through the electrochemical deposition and dissolution of aluminum at the anode, and intercalation/de-intercalation of ClO4- anions in
the graphene based cathode with a new Al(ClO4)3/Propylene carbonate - Fluoroethylene carbonate electrolyte. The battery exhibited high discharge voltage plateaus (about 1 V), high-rate capacity (101 mA h g-1 at 2000 mA g-1) and long cycle life (more than 400 cycles). More importantly, the battery also showed superior electrochemical properties with fast charge and slow discharge (the battery could be fully charged in 13 minutes and discharged for more than 73 minutes). And the reaction mechanisms of the aluminum based dual-ion battery is also proposed and discussed in detail. KEY WORDS: Aluminum rechargeable ion batteries, dual-ion battery, 3D graphene, deposition and dissolution, electrolyte.
1. Introduction Developing new types of rechargeable battery systems could fuel broad applications from personal electronics to grid storage.[1-4] As one of the most promising next-generation rechargeable batteries, aluminum ion batteries (AIBs) have attracted much attention due to their low cost, environmental benignity, and high charge density (2980 A h kg–1 of Al3+/Al compared to single-electron Li+/Li of 3862 A h kg–1 and Na+/Na of 1166 A h kg–1 for the redox reactions, respectively).[5-8] However, AIBs are generally limited by: 1) Only a limited number of cathode materials can be used to reversibly intercalate and de-intercalate Al3+ ion;[9-11] 2) Low energy density (low cell discharge voltage) and insufficient cycle life with fast capacity decay.[9, 12, 13] Much researchers have been focused on finding new cathode materials for AIBs or new aluminum based ion batteries. For example, Dai and co-workers reported using 3D graphene foam (3DGF) and graphite as the cathode for AIBs, despite the capacity only about 66 mA h
g−1.[14-17] In order to improve the capacity, Dai et. al, Lu et.al, Gao et.al and Jiao et.al reported using Nano-graphite, graphene nanoribbons on highly porous 3DGF, defect-free graphene and Ni3S2@graphene as the cathode, respectively.[18-21] These AIBs shows high capacity and good cycle performance, but the ionic-liquid electrolyte is very expensive. Recently, Dai and co-workers reported a cheap ionic liquid analogue electrolyte for the AIBs, but the energy density is still very low (73 mA h g-1 at 100 mA g-1).[22] Therefore, in order to satisfy the requirements of commercial aluminum based battery, it is crucial to development new aluminum based energy storage system with high energy density. Dual-ion battery (DIB) is a novel type battery developed in recent years, which is safer with high energy density due to the usual high theoretical cell voltage.[23-30] Here, for the first time, we presented a novel aluminum dual-ion batteries (ADIBs), using graphite materials (3DGF and Nano-graphite) as the cathode, Al foil covered with aluminum nanowires (Al-Alnw) as the anode, and a novel electrolyte obtained by dissolve Al(ClO4)3 into carbonate solution. The ADIBs operated through the electrochemical deposition and dissolution of aluminum at the anode, and intercalation/de-intercalation of ClO4- anions at the graphite based cathode. The standard CR2032 coin cell exhibited main discharge voltage plateaus near 1 V, high capacity about 101 mA h g-1 at a current density of 2000 mA g-1, long cycle life (no capacity decay after more than 400 cycles), and outstanding rate performance (158, 132 and 106 mA h g-1 at increasing discharge current densities of 500, 1000 and 2000 mA g-1). In addition, the battery remained 150 mA h g-1 for 150 cycles with fast charge and slow discharge (the battery could be fully charged in 13 min minutes and discharged for more than 73 minutes).
2. Results and discussion As shown in Figure 1a, we designed Al||graphite coin cell, using an Al foil as anode, 3DGF (or Nano-graphite) as cathode and Al(ClO4)3/Propylene carbonate (PC) - Fluoroethylene carbonate (FEC) electrolyte. The energy storage in this novel battery is based on the intercalation of the ClO4- in the cathode materials, and the electrodeposition of Al on the surface of Al anode during the charge process. During discharge process, ClO4- anions return into the electrolyte and Al3+ ions released into electrolyte through electrochemical dissolving of Al foil. As show in Figure 1b, the ADIBs could power a 5 mm-diameter red round light-emitting diode. It is well known, the three-electron redox properties of Al3+/Al could provide high specific capacity for the anode. Therefore, the cathode material is the key factor to determine the capacity of ADIBs. It is also known that the Nano-graphite and 3DGF exhibit typical graphite XRD peak at 2θ ~26.44° (d spacing, 3.36 Å, Figure S1).[31] These graphite materials have generally recognized as ideal candidates for DIBs cathode, which can accommodate
ClO4-
anions
through
reversible
intercalation/de-intercalation.[32-34]Therefore, the graphite materials, especially the 3DGF, has become an important candidate for the cathode of the ADIBs.
Figure 1. Schematic illustration of the rechargeable aluminum dual-ion batteries. a) On the cathode, the ClO4- anions were intercalated/de-intercalated into/from the graphite electrode during the charge/discharge process. On the anode, Al3+ ions electrodeposited on the surface of Al foil and then dissolved back to the electrolyte during charge and discharge process. b) It is a photograph of two Al||3DGF coin cells powered one red LED and internal coin cell design.
3DGF have great electrochemical performance for energy storage due to its huge specific surface area and excellent electrical conductivity.[35, 36] The 3DGF was achieved from a nickel foam template via chemical vapor deposition (Figure S2). The huge specific surface area can greatly decrease the diffusion length for the electrolyte ions, and excellent electrical conductivity which is beneficial to the reversible capacity and cycle life of the ADIBs.[19, 21] For aluminum-based ion batteries, the electrolyte played an important role in influencing battery performance.[10, 37, 38] Based on the principle of energy storage of AIDBs, we designed a novel cheap electrolyte. Figure 2a showed the charge-discharge curves of Al||3DGF coin cell using different carbonate electrolytes with Al(ClO4)3 (Detailed instruction was listed in Table S1).
The batteries exhibited different discharge voltage platforms and discharge capacities. It was worth noticing that the battery with 1 M Al(ClO4)3/PC-FEC (5.5 wt% FEC as additive) electrolyte exhibited a high discharge platform and a high discharge capacity of 114 mA h g-1 over a potential window of 0.6−2.0 V. In order to get better electrochemical performance, we studied the discharge capacity and Coulombic Efficiency (CE) of Al||3DGF battery with different Al(ClO4)3/PC-FEC electrolytes. The concentration of Al(ClO4)3 in PC-FEC electrolyte was 0.5, 1, 1.5 and 2 M, respectively. As shown in Figure S3, the electrolyte with 1M Al(ClO4)3 is colorless and transparent. As the concentration of Al(ClO4)3 increases, the viscosity of the electrolyte increases. The 0.5 M and 1 M Al(ClO4)3/PC-FEC electrolyte have higher mobility and infiltration. It is more conducive to the migration of ions and the transfer of electrons. As shown in Figure 2b, the battery deliver a capacity of 90, 110, 45 and 8 mA h g-1, the corresponding CE is about 60, 61, 77 and 67%, respectively. The 1 M Al(ClO4)3/PC-FEC electrolyte deliver highest capacity and acceptable CE. Therefore, it is believe that the 1 M Al(ClO4)3/PC-FEC electrolyte is the most suitable electrolyte.
Figure 2. Exploration of Electrolyte and Aluminum Electrode. a) Charge and discharge curves of an Al||3DGF coin cell at a current density 2000 mA g-1 with different electrolytes. b) The discharge capacity and Coulombic efficiency of Al||3DGF coin cell using different molar ratio of Al(ClO4)3/PC-FEC electrolyte. c) SEM image of Al foil etching at current density of 450 mA cm-1 for 90 s. d) The Coulombic efficiency of aluminum dual-ion batteries with different Al foil.
In order to achieve high energy density and superior cycle stability, Al-Alnw anode was prepared by direct-current electrochemical etching of Al foil in the solution of 3.8 M H2SO4 and 0.7 M HCl at 70 ℃.[39, 40] The SEM images in Figure S4 showed the morphology of aluminum foil that was etched at various surface current densities and etching times. As shown in Figure S4a, when the Al foil was etched at current density of 300 mA cm-1 for 60 s (mark as (300,60)), the main features on Al foil surface was composed of nanospheres (with ~200 nm in diameter); when
the surface current density increased to 700 mA cm-1 ((700,60), Figure S4b), these nanospheres turned to be nanorods. In addition, when the etching time increased to 120 s, the texture on the surface turned to be pulverized ((300,60), (700,120), Figure S4c,d). In this study, Al-Alnw anode was prepared at current density of 450 mA cm-1 for 90 s (450,90). As shown in Figure 2c, Figure S5a,b, the morphology of the surface became a 3D network structure with nanowire clusters, consisted of aluminum nanowire (with ~8 nm in diameter). The 3D network structure with a huge surface is beneficial for the efficient transportation of electrolyte ions. The reduction of the electrode weight, which increases the energy density of the battery and promotes the dissolution of aluminum. The ADIBs assemblied with different Al foil shown different CE, and the Al-Alnw (450,90) anode delivered a high CE about 80% (Figure 2d). Based on the above results, the Al-Alnw||3DGF battery using 1 M Al(ClO4)3/PC-FEC electrolyte could provide satisfactory electrochemical performance. The electrochemical performance of the Al-Alnw||3DGF coin cell with 1 M Al(ClO4)3/PC-FEC electrolyte was investigated using cyclic voltammetry (CV) and galvanostatic charge/discharge cycling. The Figure 3a showed the CV cure of the battery in the voltage range of 0.6−2.0 V at a scan rate of 0.2 mV s-1. A sharp oxidation peak (attributed to the intercalation of ClO4-) at 1.7 V (vs. Al) and a reduction peak (assigned to the de-intercalation of ClO4-) at 0.95 V (vs. Al) were observed, respectively. The CV results correlated well with the galvanostatic charge-discharge behaviors in Figure 3b. The platform ranging from 1.6 V to 1.9 V, corresponded to the intercalation peak, and the platform during the discharge process corresponded to the de-intercalation peaks.
Figure 3. Electrochemical performance of the aluminum dual-ion battery. a) CV curves at 0.2 mV s-1 in a coin cell. b) Charge and discharge curves at 2000 mA g-1. c) The discharge capacity at different charge cut-off voltage (current density, 500 mA g-1). d) The galvanostatic cycling at a current density of 2000 mA g-1. e) The discharge capacity at different discharge current density with the same charge current (2000 mA g-1). f) The battery charge at 2000 mA g-1 and discharge at 200 mA g-1.
We studied the effect of different cut-off voltages on battery capacity and CE. As shown in Figure 3c, the discharge capacity of Al-Alnw||3DGF battery increased from 110 mA h g-1 to 147 mA h g-1 with the cut-off voltage rising from 1.9 to 2.2 V and the CE kept around 80%. When the
cut-off voltage was set at 2.3 V, the discharge capacity increased to 152 mA h g-1, but the CE gradually decreased to 50%. The result show that higher cut-off voltage may cause decomposition of the electrolyte, thereby reducing the CE. When the cut-off voltage further increased to 2.4 V, the CE rapidly dropped to 30%. So the charge cut-off voltage of 2.2 V is a proper choice for the battery. Figure 3d shows the galvanostatic charge-discharge data for the Al-Alnw||3DGF battery with 1 M Al(ClO4)3/PC-FEC electrolyte. All the capacities were calculated with the weight of the 3DGF. When the battery was charged and discharged at a current density of 2000 mA g-1 between 0.6–2.2 V, it could deliver a capacity of 101 mA h g-1 after 400 cycles. To the best of our knowledge, this is the first time that ADIBs have been made (button battery) and achieved a capacity of 101 mA h g-1 and stably operated for 400 times with 80% CE. More importantly, the capacity of battery had almost no attenuation after 400 cycles, indicating excellent stability. As comparison, the Al-Alnw||Nano-graphite battery was constructed and the galvanostatic cycling was carried out at a current density of 2000 mA g-1 in the voltage of 0.6-2.2 V. The battery derived a reversible capacity of 23 mA h g-1 after 250 cycles (Figure S6). The lower capacity of Al-Alnw||Nano-graphite battery than that of Al-Alnw||3DGF battery indicated that 3DGF could provide higher capacity and better cycle performance for ADIBs. The disparity should be result to the huge specific surface area and excellent conductivity of 3DGF. For practical application, it is desirable for the battery to achieve fast charge and slow discharge. In this study, fast charge and slow discharge measurements were performed at various discharge current densities with the charging current density maintained at 2000 mA g-1. As shown in Figure 3e, the Al-Alnw||3DGF battery exhibited excellent rate capacity and delivered
discharge capacities of 158, 132 and 106 mA h g-1 with increasing discharge current densities of 500, 1000 and 2000 mA g-1, respectively. The capacity recovered to 156 mA h g-1 when the discharge current returned to 500 mA g-1. As shown in Figure 3f, the Al-Alnw||3DGF battery was charged at a current density at 2000 mA g-1 and slowly discharged at 200 mA g-1. The discharge capacity stabilized at 150 mA h g-1 after 150 cycles with high CE about 88%. This indicates that the battery can be continuously discharged for 73 minutes and only need 13 minutes to be fully charged (Figure S7). The outstanding energy storage performance offered a great potential for the practical application of the ADIBs. Based upon the above results, we proposed the following mechanism of the redox reactions of ADIBs during charge/discharge process: For the anode: 𝐴𝑙 3+ + 3𝑒 − ↔ 𝐴𝑙
(1)
For the cathode: 𝐶𝑛 + 𝐶𝑙𝑂4− ↔ 𝐶𝑛 [𝐶𝑙𝑂4 ] + 𝑒 −
(2)
where the n is the molar ratio of carbon atoms to intercalated anions (ClO4-) in the graphite.
Figure 4. Structural and spectroscopic evolution of electrode materials. a) Ex-situ XRD patterns of Nano-graphite in various charged and discharged states at a current density of 2000 mA g-1 after 20 cycles. b) Ex situ Raman spectra of Nano-graphite cathode in pristine and various charged and discharged states after 200 cycles, showing ClO4- intercalate/de-intercalate into/from Nano-graphite. c and d) SEM images with different magnification for Al-Alnw after 200 cycles.
To illustrate the mechanism of the ADIBs, ex-situ X-ray diffraction (XRD) and ex-situ Raman of Nano-graphite in various charged and discharged states were performed. As shown in Figure 4a, with increasing charge voltage from 1.6 V to 2.0 V, the peak (002) of the graphite shifted gradually from 26.44° to 26.4°. Analysis of the peak shift suggested the spacing between adjacent
graphitic layers increased, indicating that the ClO4- ions were intercalated between graphene layers.[41, 42]Full discharging led to the graphite peak (002) returned to 26.44°, which suggested the interlayer space was recover to its original value due to the de-intercalation of ClO4- ions. In addition, the Raman spectra of Nano-graphite cathode at pristine, various charged and discharged status were shown in Figure 4b. For the original Nano-graphite, the D band at 1350 cm-1 was assigned to be the ring-breathing mode from sp2 carbon rings, associated with the presence of structural defects, disorders and decoration with hetero atoms in the carbon materials. And the G band at 1578 cm-1 was due to the planar configuration sp2 bonded carbon with bond-stretching motion.[19, 43, 44] The ratio of D peak intensity to G peak intensity (ID/IG) for pristine Nano-graphite is 0.217. It was found that the ID/IG value increased to 0.393 when the battery was charged to 1.6 V. When the battery was charged to 2.0 V, the ID/IG value increased to 0.527. The value changes suggested that the ClO4- anions intercalated into graphite layer and the intercalation increased the structural defects and disorders on both edges and basal planes of graphite during the charging process.[19, 45, 46] In addition, when the battery was discharged to 1.2 V, the value of ID/IG decreased from 0.884 to 0.807. When full discharge to 0.6 V, the value of ID/IG reduced to 0.733. The recovery of value (ID/IG) revealed that the ClO4- ions de-intercalated out from the graphite layers during the discharge process. Unexpected, the value of ID/IG (0.733) was larger than the initial value (0.217), probably caused by irreversible defects and decoration with hetero atoms in graphite after the de-intercalated of most ClO4- ions. However, the increment of defects in graphite allowed more anions to be embedded into the disordered graphite layers. These anions could be trapped in these disordered sites during the discharge processes, which may lead to lower CE (about 80%). Furthermore, we performed Raman tests on 3DGF electrodes
for different states (Figure S8). It is clear that the value of ID/IG increased from 0.318 to 0.448 when the battery charged from 1.6 V to 2.0 V. When the battery discharged to 0.6 V, the value of ID/IG reduced to 0.349. Although the value of ID/IG still has some irreversible increase, the results show that the 3DGF cathode has batter reversibility. Figure 4c,d showed the SEM images with different magnifications for Al-Alnw electrode after 200 cycles. There were no stacks or gullies in the surface of Al-Alnw electrode after constant current charge and discharge (Figure 4c). It proves that the deposition and dissolution of Al on the anode was uniform and there was no presence of Al dendrites. Therefore, the battery with Al-Alnw anode shows excellent cycle stability. Furthermore, when examined with high magnification SEM image (as shown in Figure 4d), the aluminum nanowire clusters disappeared and the aluminum nano-spheres appeared on the surface. The nano-spheres are composed of nanowire that covered with aluminum thin film. The result indicated that the Al-Alnw electrode dissolved into electrolyte during the discharge process. And the Al3+ ion deposited on the Aluminum nanowires anode during the charge process to form a thin layer. The regrowth of Al thin layer on the nanowire surface result in the more stable nano-sphere morphology with nanowires backbone. The special structure makes the battery possess superior cycle stability and higher CE. We also performed calculations based on the density functional theory (DFT) to investigate ClO4- ion intercalation in the graphite layers. In the calculation, the 4×4×2 supercell of graphite with periodic structure (Figure 5a) was used. After geometry optimization, the lattice parameters were listed in Table S2 and were compared with other experimental data.[47, 48] From Table S2, it was clear that the calculated data agreed well with experimental ones. The ClO4- was inserted in 4×4×2 supercell of graphite (G+ClO4-) (Figure 5b). The bond length of ClO4- is 1.453 Å and the
distance between the neighboring oxygen atoms is 2.373 Å (Figure S9a). With the ClO4- inserted to the graphite, the space between oxygen atom and its adjacent carbon atom is less than 0.5 Å (the layer spacing of graphite is 3.3 Å). Thus, the model is not reasonable. Then, the ClO4- was changed from tetrahedral structure to planar quadrangle geometry (Figure S9b),[49] and the distance between the neighboring oxygen atoms decreased to 2.179 Å. Then, the planar quadrangle geometry ClO4- anion was inserted in graphite. After geometry optimization (Figure 5c), the cell parameters were listed in Table S2. It is found that there is no change for lattice constant a, only lattice constant c increased from 13.37 Å to 16.04 Å. The increment of 2.67 Å corresponds well to the layer space of crystal plane (001). Furthermore, the volume increased to 225.36 Å3. It was notable that two oxygen atoms broke away from ClO4- and bonded with carbon atoms. ClO2 was left between graphite layer. From the charge density of electron density of ClO4-, graphite and ClO4- inserted in graphite (Figure 5d,e and f), it was clear that the electron density of two oxygen atoms overlapped with four carbon atoms. The bonding between atoms was further analyzed from electronic density states and electronic population. Figure 5g showed the partial density of states (PDOSs). From Figure 5g, it was found that from -10 eV to -5 eV the electronic states were strongly hybridized by the s states and p states of Cl and O to form Cl-O bond of ClO4-. On the other hand, for graphite, C2s and C2p states represented the covalent bonding from -20 eV to -5 eV, corresponding to the C-C bond. However, there was no variation for PDOSs of ClO4being inserted in graphite.
Figure 5. Theoretical simulation of ClO4- intercalation in 3D graphene foam Cathode. a) 4×4×2 supercell of graphite. b and c) ClO4- inserted in 4×4×2 supercell of graphite before geometry optimization and after geometry optimization. d, e and f) Electron density of ClO4-, graphite and ClO4- inserted in graphite. g) The artial density of states of ClO4-, graphite and G+ClO4-. 0 eV is fermi level (green dash line).
The electronic population is widely used to analyze electronic structure, and to assess the covalent or ionic nature of a bond as it provides an objective criterion for bonding between atoms.[50] The higher the value of population is, the higher the bond covalency is. Otherwise the iconicity is high. From Table S3, it is found that O-Cl and C-C are covalent for ClO4- and graphite. With ClO4- inserted into graphite, the value of electronic population of C-C bonds changed between 0.93 and 1.17, which was mainly due to the O atom bonded with C atom and caused the distortion of C atom near O atom (Figure 5c). Thus, the lengths of C-C bonds also changed between 1.391 Å and 1.458 Å. Furthermore, the length and iconicity of O-Cl bond increased. The covalency of C-O bond is stronger than that of the O-Cl bond but weaker than that of the C-C bond. The ClO4- intercalation energy (Ei) is defined as 𝐸𝑖 = 𝐸𝐶𝑙𝑂4− +𝐺 − 𝐸𝐶𝑙𝑂4− − 𝐸𝐺 , where 𝐸𝐶𝑙𝑂4− +𝐺 and 𝐸𝐺 are total energies of ClO4- intercalated and graphite, respectively, while 𝐸𝐶𝑙𝑂4− is the energy of ClO4-. After calculation, it was found that 𝐸𝑖 = 0.4 eV. Furthermore, the de-insertion voltage can be defined as Ei/e. Thus, the de-insertion voltage should be around 0.4 V, which agreed well with experimental data.
Figure 6. Chemical probing of a 3D graphene foam cathode by HRTEM. a and b) EDS mapping images for C, Cl and O elements of 3D graphene foam charged to 2.0 V and discharged to 0.6 V, respectively. c, d and e) HRTEM images of the 3D graphene foam cathodes in different states (Pri-graphene, charged-2.0 V and discharged-0.6 V). f, g and h) Contrast profiles along the arrows indicate interlayer spacing of corresponding samples.
Energy dispersive spectrometer (EDS) were also used to probe the chemical nature of the intercalated species in 3DGF. Figure 6a,b showed the EDS mapping images for C, Cl and O in the
3DGF cathode after charged to 2.0 V and discharged to 0.6 V, respectively. When the battery charged to 2.0 V, apparently, the Cl and O atoms uniformly distributed over the entire 3DGF (Figure 6a), confirming ClO4- intercalation. Only a few Cl and O atoms were observed when fully discharged to 0.6 V (Figure 6b). The result revealed the most ClO4- de-intercalated from the graphite layer in the Al-Alnw||3DGF battery. Note that most of the ClO4- anions have removed from the graphite layer, but there still have a small amount of Cl and O elements. A portion of the elements may belong to the electrolyte remained on the cathode surface. The other part should be attributed to the formation of C-O bonds and C=O bonds between O atoms (the decomposition of ClO4- in charging process) and adjacent C atoms. This was consistent with the results of XRD, Raman spectroscopy and FTD calculations.
Figure 6c, d and e showed high-resolution transmission electron microscopy (HRTEM) images for 3DGF at pristine state, charged to 2.0 V and discharged to 0.6 V, respectively. The average interlayer spacing of the pristine 3DGF was 0.35 nm (Figure 6c,f), which is consistent with previous reports.[14, 35, 36] When the battery was charged to 2.0 V, the interlayer spacing increased to 0.49 nm (Figure 6d,g). The expansion may due to intercalation of ClO4- anions. There were some distortions in the HRTEM image of 3DGF shown in Figure S10b. These distortions and defects probably related to intercalation of ClO4- anions. On the contrary, the interlayer spacing reduced to 0.4 nm after full discharged to 0.6 V. The recovery indicated that the ClO4- ions were de-intercalated from the graphite layers with some ClO4- anions left behind (Figure 6e,h). The nearly reversible interlayer spacing change proved the
intercalation/de-intercalation of anions (ClO4-) in the graphite layers during the charging/discharging process, respectively. 3. Conclusion In summary, we have developed a new rechargeable ADIBs for the first time. The battery used Al-Alnw as anode, 3DGF as cathode, and an electrolyte with 1 M Al(ClO4)3 in PC and 5.5 wt% FEC as additive. The battery displayed a capacity of 101 mA h g-1 (based on the mass of 3DGF) and long cycling life up to 400 cycles at current density of 2000 mA g-1. The new mechanism of the ADIBs was proposed and explored. During the charging process, Al3+ electrodeposited to the Al-Alnw anode, and anions (ClO4-) intercalated into the 3DGF cathode. Conversely, Al-Alnw anode was oxidized and Al3+ migrated back to the electrolyte, and the anions (ClO4-) de-intercalated from the cathode during the discharging process. The rechargeable ADIBs is believed to be one of the practical aluminum-base ion batteries and may lead further research efforts on other aluminum ion batteries. However, the CE of ADIBs is still very low, find the cause and solve this problem is one of our future research focus.
4. Experimental Section Calculations Method: The present calculations were performed based on the density functional theory (DFT).[51, 52] Ultrasoft pseudopotentials were used to describe the interaction of ionic core and valence electrons. Valence states were considered in this study corresponding to Cu 3s2p6d104s1, S 3s2p4 and Na 2p63s1. The generalized gradient approximation (GGA) of Perdew–Burke–Ernzerh method parameterized by Perdew[53,
54]
was used to calculate the
exchange and correlation terms. Brillouin-zone integrations were performed using Monkhorst and Pack k-point meshes.[55] During the calculation, the 750 eV for cutoff energies and 3×3×2 for
the numbers of k-point can ensure the convergence for the total energy. All the calculations were considered converged when the maximum force on the atom was below 0.03 eV Å−1, maximum stress was below 0.05 GPa, and the maximum displacement between cycles was below 0.001 Å. Preparation of electrolytes: First, the Al(ClO4)3·9H2O (≥ 98%) annealed to clean crystal water and transferred to glove box immediately. After that, Al(ClO4)3 was dissolved in different carbonate solvents as needed. Preparation of 3D graphene foam: The nickel foams were heated to 1000 °C in a horizontal tube furnace under Ar and H2 and annealed for 10 min. Then, a small amount of methane (CH4) was introduced into the reaction tube at ambient pressure and the flow rate of CH4 was 10 sccm. After 10 min flow of the reaction gas mixture, the samples were rapidly cooled to room temperature at a rate of 300 °C min−1 under Ar and H2. Then the samples were drop-coated with a poly(methyl methacrylate) (PMMA) solution, and baked at 110 °C for 0.5 h. And then, these samples were put into HCl solution to completely dissolve the Ni foam to obtain the PMMA/graphene. Finally, the pure graphene foam was obtained after removing the PMMA. Preparation of Nano-graphite flake electrode: The Nano-graphite slurry was prepared by mixing Nano-graphite powder and PVDF (10 %) in N-methyl pyrrolidone. After that, the slurry was cast on a Cu foil and dried at 60 °C for 24 h. The Cu foil was then etched by immersing the sample into an iron chloride solution (0.4 g ml-1) to form a Nano-graphite film. Finally, the Nano-graphite film was rinsed with deionized water to remove the residual FeCl3, and dried at 60 °C for 12 h. Preparation of aluminum foil covered with aluminum nanowire clusters (Al-Alnw): The material used in the direct current (DC) etching experiments was a commercial aluminum foil (99.998 wt%) for high-voltage electrolytic capacitors. Platinum sheet was used as the counter electrode. DC
etching was carried out in a solution of 3.8 M H2SO4 with addition of 0.7 M HCl at constant current density of 450 mA cm-2 with the temperature of 70 °C for 90 s. The temperature, current density and etch period can be adjusting to obtain different aluminum foil. Material characterizations: The morphology of the as-prepared materials was characterized by field emission scanning electron microscopy (FE-SEM, Carl Zeiss, ΣIGMA HD, Germany). Energy Dispersive Spectroscopy (EDS) mapping that obtained on a Titan G2 60-300 TEM with an EDS attachment probe the chemical nature of the intercalated species in 3DGF cathode. High-resolution TEM (Titan G2 60-300), Raman spectroscopy (WITec, alpha 300R, using a 532 nm wavelength yttrium aluminum garnet (YAG) laser) and XRD (Philips X’pert diffractometer, Cu-Kα radiation, λ = 0.15406 nm, 2θ range 5−80°) were employed to characterize the crystal structure of the sample and study the changes of graphite cathodes during charge and discharge process. Electrochemical measurements: Prior to assembling the Al dual-ion battery in the glove box, all components were heated under vacuum at 60 °C for more than 24 h to remove residual water. The standard CR2032 coin cell was constructed using a 3D graphene foam (~0.15−0.6 mg) cathode and an Aluminum nanowires foil (~50 mg) anode, which separated by cellulose paper separator to prevent shorting. The cyclic voltammetry measurements were performed using a CHI660e electrochemical workstation at scan rate of 0.2 mV s-1 over the range of 0.6−2.0 V.
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Graphical abstract
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PII: DOI: Reference:
S2405-8297(17)30468-3 https://doi.org/10.1016/j.ensm.2017.10.001 ENSM223
To appear in: Energy Storage Materials Received date: 19 September 2017 Revised date: 4 October 2017 Accepted date: 4 October 2017 Cite this article as: Erjin Zhang, Wei Cao, Bin Wang, Xinzhi Yu, Longlu Wang, Zhi Xu and Bingan Lu, A Novel Aluminum Dual-ion Battery, Energy Storage Materials, https://doi.org/10.1016/j.ensm.2017.10.001 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting galley proof before it is published in its final citable form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
A Novel Aluminum Dual-ion Battery Erjin Zhang#1, Wei Cao#1, Bin Wang2, Xinzhi Yu1,3, Longlu Wang1,Zhi Xu3 and Bingan Lu1,3* 1. School of Physics and Electronics, Hunan University, Changsha 410082, P. R. China 2. Physics and Electronic Engineering Department, Xinxiang University, Xinxiang 453003, P.R. China 3. Fujian Strait Research Institute of Industrial Graphene Technologies, Jinjiang, 362200, P. R. China *
Corresponding authors E-mail addresses: [email protected] (B. Lu)
# The authors contributed equally to this work
Acknowledgements This work was financially supported by National Natural Science Foundation of China (Nos. 51672078 and 21473052) and Hunan University State Key Laboratory of Advanced Design and Manufacturing for Vehicle Body Independent Research Project (No. 71676004), Hunan Youth Talents (2016RS3025), and Foundation of State Key Laboratory of Coal Conversion (Grant No. J17-18-903).
Abstract The development of new rechargeable safe battery with high energy density and low cost is one of the most desirable goals for personal electronics and grid storage. Aluminum based rechargeable ion batteries offer the possibilities for safe, high energy density and low cost. Here, we developed a novel aluminum based high-rate capability dual-ion battery with an aluminum anode and a 3D graphene cathode. The battery operated through the electrochemical deposition and dissolution of aluminum at the anode, and intercalation/de-intercalation of ClO4- anions in
the graphene based cathode with a new Al(ClO4)3/Propylene carbonate - Fluoroethylene carbonate electrolyte. The battery exhibited high discharge voltage plateaus (about 1 V), high-rate capacity (101 mA h g-1 at 2000 mA g-1) and long cycle life (more than 400 cycles). More importantly, the battery also showed superior electrochemical properties with fast charge and slow discharge (the battery could be fully charged in 13 minutes and discharged for more than 73 minutes). And the reaction mechanisms of the aluminum based dual-ion battery is also proposed and discussed in detail. KEY WORDS: Aluminum rechargeable ion batteries, dual-ion battery, 3D graphene, deposition and dissolution, electrolyte.
1. Introduction Developing new types of rechargeable battery systems could fuel broad applications from personal electronics to grid storage.[1-4] As one of the most promising next-generation rechargeable batteries, aluminum ion batteries (AIBs) have attracted much attention due to their low cost, environmental benignity, and high charge density (2980 A h kg–1 of Al3+/Al compared to single-electron Li+/Li of 3862 A h kg–1 and Na+/Na of 1166 A h kg–1 for the redox reactions, respectively).[5-8] However, AIBs are generally limited by: 1) Only a limited number of cathode materials can be used to reversibly intercalate and de-intercalate Al3+ ion;[9-11] 2) Low energy density (low cell discharge voltage) and insufficient cycle life with fast capacity decay.[9, 12, 13] Much researchers have been focused on finding new cathode materials for AIBs or new aluminum based ion batteries. For example, Dai and co-workers reported using 3D graphene foam (3DGF) and graphite as the cathode for AIBs, despite the capacity only about 66 mA h
g−1.[14-17] In order to improve the capacity, Dai et. al, Lu et.al, Gao et.al and Jiao et.al reported using Nano-graphite, graphene nanoribbons on highly porous 3DGF, defect-free graphene and Ni3S2@graphene as the cathode, respectively.[18-21] These AIBs shows high capacity and good cycle performance, but the ionic-liquid electrolyte is very expensive. Recently, Dai and co-workers reported a cheap ionic liquid analogue electrolyte for the AIBs, but the energy density is still very low (73 mA h g-1 at 100 mA g-1).[22] Therefore, in order to satisfy the requirements of commercial aluminum based battery, it is crucial to development new aluminum based energy storage system with high energy density. Dual-ion battery (DIB) is a novel type battery developed in recent years, which is safer with high energy density due to the usual high theoretical cell voltage.[23-30] Here, for the first time, we presented a novel aluminum dual-ion batteries (ADIBs), using graphite materials (3DGF and Nano-graphite) as the cathode, Al foil covered with aluminum nanowires (Al-Alnw) as the anode, and a novel electrolyte obtained by dissolve Al(ClO4)3 into carbonate solution. The ADIBs operated through the electrochemical deposition and dissolution of aluminum at the anode, and intercalation/de-intercalation of ClO4- anions at the graphite based cathode. The standard CR2032 coin cell exhibited main discharge voltage plateaus near 1 V, high capacity about 101 mA h g-1 at a current density of 2000 mA g-1, long cycle life (no capacity decay after more than 400 cycles), and outstanding rate performance (158, 132 and 106 mA h g-1 at increasing discharge current densities of 500, 1000 and 2000 mA g-1). In addition, the battery remained 150 mA h g-1 for 150 cycles with fast charge and slow discharge (the battery could be fully charged in 13 min minutes and discharged for more than 73 minutes).
2. Results and discussion As shown in Figure 1a, we designed Al||graphite coin cell, using an Al foil as anode, 3DGF (or Nano-graphite) as cathode and Al(ClO4)3/Propylene carbonate (PC) - Fluoroethylene carbonate (FEC) electrolyte. The energy storage in this novel battery is based on the intercalation of the ClO4- in the cathode materials, and the electrodeposition of Al on the surface of Al anode during the charge process. During discharge process, ClO4- anions return into the electrolyte and Al3+ ions released into electrolyte through electrochemical dissolving of Al foil. As show in Figure 1b, the ADIBs could power a 5 mm-diameter red round light-emitting diode. It is well known, the three-electron redox properties of Al3+/Al could provide high specific capacity for the anode. Therefore, the cathode material is the key factor to determine the capacity of ADIBs. It is also known that the Nano-graphite and 3DGF exhibit typical graphite XRD peak at 2θ ~26.44° (d spacing, 3.36 Å, Figure S1).[31] These graphite materials have generally recognized as ideal candidates for DIBs cathode, which can accommodate
ClO4-
anions
through
reversible
intercalation/de-intercalation.[32-34]Therefore, the graphite materials, especially the 3DGF, has become an important candidate for the cathode of the ADIBs.
Figure 1. Schematic illustration of the rechargeable aluminum dual-ion batteries. a) On the cathode, the ClO4- anions were intercalated/de-intercalated into/from the graphite electrode during the charge/discharge process. On the anode, Al3+ ions electrodeposited on the surface of Al foil and then dissolved back to the electrolyte during charge and discharge process. b) It is a photograph of two Al||3DGF coin cells powered one red LED and internal coin cell design.
3DGF have great electrochemical performance for energy storage due to its huge specific surface area and excellent electrical conductivity.[35, 36] The 3DGF was achieved from a nickel foam template via chemical vapor deposition (Figure S2). The huge specific surface area can greatly decrease the diffusion length for the electrolyte ions, and excellent electrical conductivity which is beneficial to the reversible capacity and cycle life of the ADIBs.[19, 21] For aluminum-based ion batteries, the electrolyte played an important role in influencing battery performance.[10, 37, 38] Based on the principle of energy storage of AIDBs, we designed a novel cheap electrolyte. Figure 2a showed the charge-discharge curves of Al||3DGF coin cell using different carbonate electrolytes with Al(ClO4)3 (Detailed instruction was listed in Table S1).
The batteries exhibited different discharge voltage platforms and discharge capacities. It was worth noticing that the battery with 1 M Al(ClO4)3/PC-FEC (5.5 wt% FEC as additive) electrolyte exhibited a high discharge platform and a high discharge capacity of 114 mA h g-1 over a potential window of 0.6−2.0 V. In order to get better electrochemical performance, we studied the discharge capacity and Coulombic Efficiency (CE) of Al||3DGF battery with different Al(ClO4)3/PC-FEC electrolytes. The concentration of Al(ClO4)3 in PC-FEC electrolyte was 0.5, 1, 1.5 and 2 M, respectively. As shown in Figure S3, the electrolyte with 1M Al(ClO4)3 is colorless and transparent. As the concentration of Al(ClO4)3 increases, the viscosity of the electrolyte increases. The 0.5 M and 1 M Al(ClO4)3/PC-FEC electrolyte have higher mobility and infiltration. It is more conducive to the migration of ions and the transfer of electrons. As shown in Figure 2b, the battery deliver a capacity of 90, 110, 45 and 8 mA h g-1, the corresponding CE is about 60, 61, 77 and 67%, respectively. The 1 M Al(ClO4)3/PC-FEC electrolyte deliver highest capacity and acceptable CE. Therefore, it is believe that the 1 M Al(ClO4)3/PC-FEC electrolyte is the most suitable electrolyte.
Figure 2. Exploration of Electrolyte and Aluminum Electrode. a) Charge and discharge curves of an Al||3DGF coin cell at a current density 2000 mA g-1 with different electrolytes. b) The discharge capacity and Coulombic efficiency of Al||3DGF coin cell using different molar ratio of Al(ClO4)3/PC-FEC electrolyte. c) SEM image of Al foil etching at current density of 450 mA cm-1 for 90 s. d) The Coulombic efficiency of aluminum dual-ion batteries with different Al foil.
In order to achieve high energy density and superior cycle stability, Al-Alnw anode was prepared by direct-current electrochemical etching of Al foil in the solution of 3.8 M H2SO4 and 0.7 M HCl at 70 ℃.[39, 40] The SEM images in Figure S4 showed the morphology of aluminum foil that was etched at various surface current densities and etching times. As shown in Figure S4a, when the Al foil was etched at current density of 300 mA cm-1 for 60 s (mark as (300,60)), the main features on Al foil surface was composed of nanospheres (with ~200 nm in diameter); when
the surface current density increased to 700 mA cm-1 ((700,60), Figure S4b), these nanospheres turned to be nanorods. In addition, when the etching time increased to 120 s, the texture on the surface turned to be pulverized ((300,60), (700,120), Figure S4c,d). In this study, Al-Alnw anode was prepared at current density of 450 mA cm-1 for 90 s (450,90). As shown in Figure 2c, Figure S5a,b, the morphology of the surface became a 3D network structure with nanowire clusters, consisted of aluminum nanowire (with ~8 nm in diameter). The 3D network structure with a huge surface is beneficial for the efficient transportation of electrolyte ions. The reduction of the electrode weight, which increases the energy density of the battery and promotes the dissolution of aluminum. The ADIBs assemblied with different Al foil shown different CE, and the Al-Alnw (450,90) anode delivered a high CE about 80% (Figure 2d). Based on the above results, the Al-Alnw||3DGF battery using 1 M Al(ClO4)3/PC-FEC electrolyte could provide satisfactory electrochemical performance. The electrochemical performance of the Al-Alnw||3DGF coin cell with 1 M Al(ClO4)3/PC-FEC electrolyte was investigated using cyclic voltammetry (CV) and galvanostatic charge/discharge cycling. The Figure 3a showed the CV cure of the battery in the voltage range of 0.6−2.0 V at a scan rate of 0.2 mV s-1. A sharp oxidation peak (attributed to the intercalation of ClO4-) at 1.7 V (vs. Al) and a reduction peak (assigned to the de-intercalation of ClO4-) at 0.95 V (vs. Al) were observed, respectively. The CV results correlated well with the galvanostatic charge-discharge behaviors in Figure 3b. The platform ranging from 1.6 V to 1.9 V, corresponded to the intercalation peak, and the platform during the discharge process corresponded to the de-intercalation peaks.
Figure 3. Electrochemical performance of the aluminum dual-ion battery. a) CV curves at 0.2 mV s-1 in a coin cell. b) Charge and discharge curves at 2000 mA g-1. c) The discharge capacity at different charge cut-off voltage (current density, 500 mA g-1). d) The galvanostatic cycling at a current density of 2000 mA g-1. e) The discharge capacity at different discharge current density with the same charge current (2000 mA g-1). f) The battery charge at 2000 mA g-1 and discharge at 200 mA g-1.
We studied the effect of different cut-off voltages on battery capacity and CE. As shown in Figure 3c, the discharge capacity of Al-Alnw||3DGF battery increased from 110 mA h g-1 to 147 mA h g-1 with the cut-off voltage rising from 1.9 to 2.2 V and the CE kept around 80%. When the
cut-off voltage was set at 2.3 V, the discharge capacity increased to 152 mA h g-1, but the CE gradually decreased to 50%. The result show that higher cut-off voltage may cause decomposition of the electrolyte, thereby reducing the CE. When the cut-off voltage further increased to 2.4 V, the CE rapidly dropped to 30%. So the charge cut-off voltage of 2.2 V is a proper choice for the battery. Figure 3d shows the galvanostatic charge-discharge data for the Al-Alnw||3DGF battery with 1 M Al(ClO4)3/PC-FEC electrolyte. All the capacities were calculated with the weight of the 3DGF. When the battery was charged and discharged at a current density of 2000 mA g-1 between 0.6–2.2 V, it could deliver a capacity of 101 mA h g-1 after 400 cycles. To the best of our knowledge, this is the first time that ADIBs have been made (button battery) and achieved a capacity of 101 mA h g-1 and stably operated for 400 times with 80% CE. More importantly, the capacity of battery had almost no attenuation after 400 cycles, indicating excellent stability. As comparison, the Al-Alnw||Nano-graphite battery was constructed and the galvanostatic cycling was carried out at a current density of 2000 mA g-1 in the voltage of 0.6-2.2 V. The battery derived a reversible capacity of 23 mA h g-1 after 250 cycles (Figure S6). The lower capacity of Al-Alnw||Nano-graphite battery than that of Al-Alnw||3DGF battery indicated that 3DGF could provide higher capacity and better cycle performance for ADIBs. The disparity should be result to the huge specific surface area and excellent conductivity of 3DGF. For practical application, it is desirable for the battery to achieve fast charge and slow discharge. In this study, fast charge and slow discharge measurements were performed at various discharge current densities with the charging current density maintained at 2000 mA g-1. As shown in Figure 3e, the Al-Alnw||3DGF battery exhibited excellent rate capacity and delivered
discharge capacities of 158, 132 and 106 mA h g-1 with increasing discharge current densities of 500, 1000 and 2000 mA g-1, respectively. The capacity recovered to 156 mA h g-1 when the discharge current returned to 500 mA g-1. As shown in Figure 3f, the Al-Alnw||3DGF battery was charged at a current density at 2000 mA g-1 and slowly discharged at 200 mA g-1. The discharge capacity stabilized at 150 mA h g-1 after 150 cycles with high CE about 88%. This indicates that the battery can be continuously discharged for 73 minutes and only need 13 minutes to be fully charged (Figure S7). The outstanding energy storage performance offered a great potential for the practical application of the ADIBs. Based upon the above results, we proposed the following mechanism of the redox reactions of ADIBs during charge/discharge process: For the anode: 𝐴𝑙 3+ + 3𝑒 − ↔ 𝐴𝑙
(1)
For the cathode: 𝐶𝑛 + 𝐶𝑙𝑂4− ↔ 𝐶𝑛 [𝐶𝑙𝑂4 ] + 𝑒 −
(2)
where the n is the molar ratio of carbon atoms to intercalated anions (ClO4-) in the graphite.
Figure 4. Structural and spectroscopic evolution of electrode materials. a) Ex-situ XRD patterns of Nano-graphite in various charged and discharged states at a current density of 2000 mA g-1 after 20 cycles. b) Ex situ Raman spectra of Nano-graphite cathode in pristine and various charged and discharged states after 200 cycles, showing ClO4- intercalate/de-intercalate into/from Nano-graphite. c and d) SEM images with different magnification for Al-Alnw after 200 cycles.
To illustrate the mechanism of the ADIBs, ex-situ X-ray diffraction (XRD) and ex-situ Raman of Nano-graphite in various charged and discharged states were performed. As shown in Figure 4a, with increasing charge voltage from 1.6 V to 2.0 V, the peak (002) of the graphite shifted gradually from 26.44° to 26.4°. Analysis of the peak shift suggested the spacing between adjacent
graphitic layers increased, indicating that the ClO4- ions were intercalated between graphene layers.[41, 42]Full discharging led to the graphite peak (002) returned to 26.44°, which suggested the interlayer space was recover to its original value due to the de-intercalation of ClO4- ions. In addition, the Raman spectra of Nano-graphite cathode at pristine, various charged and discharged status were shown in Figure 4b. For the original Nano-graphite, the D band at 1350 cm-1 was assigned to be the ring-breathing mode from sp2 carbon rings, associated with the presence of structural defects, disorders and decoration with hetero atoms in the carbon materials. And the G band at 1578 cm-1 was due to the planar configuration sp2 bonded carbon with bond-stretching motion.[19, 43, 44] The ratio of D peak intensity to G peak intensity (ID/IG) for pristine Nano-graphite is 0.217. It was found that the ID/IG value increased to 0.393 when the battery was charged to 1.6 V. When the battery was charged to 2.0 V, the ID/IG value increased to 0.527. The value changes suggested that the ClO4- anions intercalated into graphite layer and the intercalation increased the structural defects and disorders on both edges and basal planes of graphite during the charging process.[19, 45, 46] In addition, when the battery was discharged to 1.2 V, the value of ID/IG decreased from 0.884 to 0.807. When full discharge to 0.6 V, the value of ID/IG reduced to 0.733. The recovery of value (ID/IG) revealed that the ClO4- ions de-intercalated out from the graphite layers during the discharge process. Unexpected, the value of ID/IG (0.733) was larger than the initial value (0.217), probably caused by irreversible defects and decoration with hetero atoms in graphite after the de-intercalated of most ClO4- ions. However, the increment of defects in graphite allowed more anions to be embedded into the disordered graphite layers. These anions could be trapped in these disordered sites during the discharge processes, which may lead to lower CE (about 80%). Furthermore, we performed Raman tests on 3DGF electrodes
for different states (Figure S8). It is clear that the value of ID/IG increased from 0.318 to 0.448 when the battery charged from 1.6 V to 2.0 V. When the battery discharged to 0.6 V, the value of ID/IG reduced to 0.349. Although the value of ID/IG still has some irreversible increase, the results show that the 3DGF cathode has batter reversibility. Figure 4c,d showed the SEM images with different magnifications for Al-Alnw electrode after 200 cycles. There were no stacks or gullies in the surface of Al-Alnw electrode after constant current charge and discharge (Figure 4c). It proves that the deposition and dissolution of Al on the anode was uniform and there was no presence of Al dendrites. Therefore, the battery with Al-Alnw anode shows excellent cycle stability. Furthermore, when examined with high magnification SEM image (as shown in Figure 4d), the aluminum nanowire clusters disappeared and the aluminum nano-spheres appeared on the surface. The nano-spheres are composed of nanowire that covered with aluminum thin film. The result indicated that the Al-Alnw electrode dissolved into electrolyte during the discharge process. And the Al3+ ion deposited on the Aluminum nanowires anode during the charge process to form a thin layer. The regrowth of Al thin layer on the nanowire surface result in the more stable nano-sphere morphology with nanowires backbone. The special structure makes the battery possess superior cycle stability and higher CE. We also performed calculations based on the density functional theory (DFT) to investigate ClO4- ion intercalation in the graphite layers. In the calculation, the 4×4×2 supercell of graphite with periodic structure (Figure 5a) was used. After geometry optimization, the lattice parameters were listed in Table S2 and were compared with other experimental data.[47, 48] From Table S2, it was clear that the calculated data agreed well with experimental ones. The ClO4- was inserted in 4×4×2 supercell of graphite (G+ClO4-) (Figure 5b). The bond length of ClO4- is 1.453 Å and the
distance between the neighboring oxygen atoms is 2.373 Å (Figure S9a). With the ClO4- inserted to the graphite, the space between oxygen atom and its adjacent carbon atom is less than 0.5 Å (the layer spacing of graphite is 3.3 Å). Thus, the model is not reasonable. Then, the ClO4- was changed from tetrahedral structure to planar quadrangle geometry (Figure S9b),[49] and the distance between the neighboring oxygen atoms decreased to 2.179 Å. Then, the planar quadrangle geometry ClO4- anion was inserted in graphite. After geometry optimization (Figure 5c), the cell parameters were listed in Table S2. It is found that there is no change for lattice constant a, only lattice constant c increased from 13.37 Å to 16.04 Å. The increment of 2.67 Å corresponds well to the layer space of crystal plane (001). Furthermore, the volume increased to 225.36 Å3. It was notable that two oxygen atoms broke away from ClO4- and bonded with carbon atoms. ClO2 was left between graphite layer. From the charge density of electron density of ClO4-, graphite and ClO4- inserted in graphite (Figure 5d,e and f), it was clear that the electron density of two oxygen atoms overlapped with four carbon atoms. The bonding between atoms was further analyzed from electronic density states and electronic population. Figure 5g showed the partial density of states (PDOSs). From Figure 5g, it was found that from -10 eV to -5 eV the electronic states were strongly hybridized by the s states and p states of Cl and O to form Cl-O bond of ClO4-. On the other hand, for graphite, C2s and C2p states represented the covalent bonding from -20 eV to -5 eV, corresponding to the C-C bond. However, there was no variation for PDOSs of ClO4being inserted in graphite.
Figure 5. Theoretical simulation of ClO4- intercalation in 3D graphene foam Cathode. a) 4×4×2 supercell of graphite. b and c) ClO4- inserted in 4×4×2 supercell of graphite before geometry optimization and after geometry optimization. d, e and f) Electron density of ClO4-, graphite and ClO4- inserted in graphite. g) The artial density of states of ClO4-, graphite and G+ClO4-. 0 eV is fermi level (green dash line).
The electronic population is widely used to analyze electronic structure, and to assess the covalent or ionic nature of a bond as it provides an objective criterion for bonding between atoms.[50] The higher the value of population is, the higher the bond covalency is. Otherwise the iconicity is high. From Table S3, it is found that O-Cl and C-C are covalent for ClO4- and graphite. With ClO4- inserted into graphite, the value of electronic population of C-C bonds changed between 0.93 and 1.17, which was mainly due to the O atom bonded with C atom and caused the distortion of C atom near O atom (Figure 5c). Thus, the lengths of C-C bonds also changed between 1.391 Å and 1.458 Å. Furthermore, the length and iconicity of O-Cl bond increased. The covalency of C-O bond is stronger than that of the O-Cl bond but weaker than that of the C-C bond. The ClO4- intercalation energy (Ei) is defined as 𝐸𝑖 = 𝐸𝐶𝑙𝑂4− +𝐺 − 𝐸𝐶𝑙𝑂4− − 𝐸𝐺 , where 𝐸𝐶𝑙𝑂4− +𝐺 and 𝐸𝐺 are total energies of ClO4- intercalated and graphite, respectively, while 𝐸𝐶𝑙𝑂4− is the energy of ClO4-. After calculation, it was found that 𝐸𝑖 = 0.4 eV. Furthermore, the de-insertion voltage can be defined as Ei/e. Thus, the de-insertion voltage should be around 0.4 V, which agreed well with experimental data.
Figure 6. Chemical probing of a 3D graphene foam cathode by HRTEM. a and b) EDS mapping images for C, Cl and O elements of 3D graphene foam charged to 2.0 V and discharged to 0.6 V, respectively. c, d and e) HRTEM images of the 3D graphene foam cathodes in different states (Pri-graphene, charged-2.0 V and discharged-0.6 V). f, g and h) Contrast profiles along the arrows indicate interlayer spacing of corresponding samples.
Energy dispersive spectrometer (EDS) were also used to probe the chemical nature of the intercalated species in 3DGF. Figure 6a,b showed the EDS mapping images for C, Cl and O in the
3DGF cathode after charged to 2.0 V and discharged to 0.6 V, respectively. When the battery charged to 2.0 V, apparently, the Cl and O atoms uniformly distributed over the entire 3DGF (Figure 6a), confirming ClO4- intercalation. Only a few Cl and O atoms were observed when fully discharged to 0.6 V (Figure 6b). The result revealed the most ClO4- de-intercalated from the graphite layer in the Al-Alnw||3DGF battery. Note that most of the ClO4- anions have removed from the graphite layer, but there still have a small amount of Cl and O elements. A portion of the elements may belong to the electrolyte remained on the cathode surface. The other part should be attributed to the formation of C-O bonds and C=O bonds between O atoms (the decomposition of ClO4- in charging process) and adjacent C atoms. This was consistent with the results of XRD, Raman spectroscopy and FTD calculations.
Figure 6c, d and e showed high-resolution transmission electron microscopy (HRTEM) images for 3DGF at pristine state, charged to 2.0 V and discharged to 0.6 V, respectively. The average interlayer spacing of the pristine 3DGF was 0.35 nm (Figure 6c,f), which is consistent with previous reports.[14, 35, 36] When the battery was charged to 2.0 V, the interlayer spacing increased to 0.49 nm (Figure 6d,g). The expansion may due to intercalation of ClO4- anions. There were some distortions in the HRTEM image of 3DGF shown in Figure S10b. These distortions and defects probably related to intercalation of ClO4- anions. On the contrary, the interlayer spacing reduced to 0.4 nm after full discharged to 0.6 V. The recovery indicated that the ClO4- ions were de-intercalated from the graphite layers with some ClO4- anions left behind (Figure 6e,h). The nearly reversible interlayer spacing change proved the
intercalation/de-intercalation of anions (ClO4-) in the graphite layers during the charging/discharging process, respectively. 3. Conclusion In summary, we have developed a new rechargeable ADIBs for the first time. The battery used Al-Alnw as anode, 3DGF as cathode, and an electrolyte with 1 M Al(ClO4)3 in PC and 5.5 wt% FEC as additive. The battery displayed a capacity of 101 mA h g-1 (based on the mass of 3DGF) and long cycling life up to 400 cycles at current density of 2000 mA g-1. The new mechanism of the ADIBs was proposed and explored. During the charging process, Al3+ electrodeposited to the Al-Alnw anode, and anions (ClO4-) intercalated into the 3DGF cathode. Conversely, Al-Alnw anode was oxidized and Al3+ migrated back to the electrolyte, and the anions (ClO4-) de-intercalated from the cathode during the discharging process. The rechargeable ADIBs is believed to be one of the practical aluminum-base ion batteries and may lead further research efforts on other aluminum ion batteries. However, the CE of ADIBs is still very low, find the cause and solve this problem is one of our future research focus.
4. Experimental Section Calculations Method: The present calculations were performed based on the density functional theory (DFT).[51, 52] Ultrasoft pseudopotentials were used to describe the interaction of ionic core and valence electrons. Valence states were considered in this study corresponding to Cu 3s2p6d104s1, S 3s2p4 and Na 2p63s1. The generalized gradient approximation (GGA) of Perdew–Burke–Ernzerh method parameterized by Perdew[53,
54]
was used to calculate the
exchange and correlation terms. Brillouin-zone integrations were performed using Monkhorst and Pack k-point meshes.[55] During the calculation, the 750 eV for cutoff energies and 3×3×2 for
the numbers of k-point can ensure the convergence for the total energy. All the calculations were considered converged when the maximum force on the atom was below 0.03 eV Å−1, maximum stress was below 0.05 GPa, and the maximum displacement between cycles was below 0.001 Å. Preparation of electrolytes: First, the Al(ClO4)3·9H2O (≥ 98%) annealed to clean crystal water and transferred to glove box immediately. After that, Al(ClO4)3 was dissolved in different carbonate solvents as needed. Preparation of 3D graphene foam: The nickel foams were heated to 1000 °C in a horizontal tube furnace under Ar and H2 and annealed for 10 min. Then, a small amount of methane (CH4) was introduced into the reaction tube at ambient pressure and the flow rate of CH4 was 10 sccm. After 10 min flow of the reaction gas mixture, the samples were rapidly cooled to room temperature at a rate of 300 °C min−1 under Ar and H2. Then the samples were drop-coated with a poly(methyl methacrylate) (PMMA) solution, and baked at 110 °C for 0.5 h. And then, these samples were put into HCl solution to completely dissolve the Ni foam to obtain the PMMA/graphene. Finally, the pure graphene foam was obtained after removing the PMMA. Preparation of Nano-graphite flake electrode: The Nano-graphite slurry was prepared by mixing Nano-graphite powder and PVDF (10 %) in N-methyl pyrrolidone. After that, the slurry was cast on a Cu foil and dried at 60 °C for 24 h. The Cu foil was then etched by immersing the sample into an iron chloride solution (0.4 g ml-1) to form a Nano-graphite film. Finally, the Nano-graphite film was rinsed with deionized water to remove the residual FeCl3, and dried at 60 °C for 12 h. Preparation of aluminum foil covered with aluminum nanowire clusters (Al-Alnw): The material used in the direct current (DC) etching experiments was a commercial aluminum foil (99.998 wt%) for high-voltage electrolytic capacitors. Platinum sheet was used as the counter electrode. DC
etching was carried out in a solution of 3.8 M H2SO4 with addition of 0.7 M HCl at constant current density of 450 mA cm-2 with the temperature of 70 °C for 90 s. The temperature, current density and etch period can be adjusting to obtain different aluminum foil. Material characterizations: The morphology of the as-prepared materials was characterized by field emission scanning electron microscopy (FE-SEM, Carl Zeiss, ΣIGMA HD, Germany). Energy Dispersive Spectroscopy (EDS) mapping that obtained on a Titan G2 60-300 TEM with an EDS attachment probe the chemical nature of the intercalated species in 3DGF cathode. High-resolution TEM (Titan G2 60-300), Raman spectroscopy (WITec, alpha 300R, using a 532 nm wavelength yttrium aluminum garnet (YAG) laser) and XRD (Philips X’pert diffractometer, Cu-Kα radiation, λ = 0.15406 nm, 2θ range 5−80°) were employed to characterize the crystal structure of the sample and study the changes of graphite cathodes during charge and discharge process. Electrochemical measurements: Prior to assembling the Al dual-ion battery in the glove box, all components were heated under vacuum at 60 °C for more than 24 h to remove residual water. The standard CR2032 coin cell was constructed using a 3D graphene foam (~0.15−0.6 mg) cathode and an Aluminum nanowires foil (~50 mg) anode, which separated by cellulose paper separator to prevent shorting. The cyclic voltammetry measurements were performed using a CHI660e electrochemical workstation at scan rate of 0.2 mV s-1 over the range of 0.6−2.0 V.
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