Hexagonal BC3 Electrode for a High-Voltage Al-Ion Battery - The

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A Hexagonal BC3 Electrode for High Voltage Al-ion Battery Preeti Bhauriyal, Arup Mahata, and Biswarup Pathak J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.7b02290 • Publication Date (Web): 17 Apr 2017 Downloaded from http://pubs.acs.org on April 26, 2017

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The Journal of Physical Chemistry

A Hexagonal BC3 Electrode for High Voltage Al-ion Battery Preeti Bhauriyal,† Arup Mahata,† Biswarup Pathak, †,#,* †

Discipline of Chemistry, School of Basic Sciences, and #Discipline of Metallurgy Engineering and Materials Science, Indian Institute of Technology (IIT) Indore, Indore, Madhya Pradesh 453552, India Email: [email protected]

Abstract Recent progresses in the field of Al-ion batteries have given directions to look for new electrode materials that can lead towards the enhancement of battery performance. Using the dispersion corrected density functional theory calculations, we have examined the applicability of hexagonal BC3 as a cathode material for Al-ion battery by evaluating its stability, specific capacity and voltage profile diagram of AlCl4 intercalated hexagonal BC3. Our results show that AlCl4 intercalated BC3 compounds are stable. We have found that there is a significant charge transfer from BC3 system to AlCl4 indicating towards the oxidation of BC3 upon intercalation reaction. Several low energy pathways are observed for the diffusion process and it is observed that the AlCl4 diffusion is trouble-free in the two-dimensional plane of BC3 having the diffusion barrier as low as 0.38 eV. Moreover, we have observed that BC3 can provide a higher average voltage 2.41 V and specific capacity of 74.37 mAh/g. These findings suggest that BC3 could be a promising cathode material for Al-ion batteries.

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1. Introduction The reducing reserves of fossil fuels are eventually incapable of dealing with the increasing needs for energy, and the CO2 emissions associated with them are harmful to the environment. This situation results in demands for renewable and clean alternative fuels capable of replacing the current energy sources efficiently.1-3 In this regard, there is considerable interest for energy storage using the rechargeable (secondary) batteries.4-6 The use of rechargeable batteries, which are already widely used for powering electrified vehicles and electronic devices of all kinds, also seems to be particularly relevant with respect to this future scenario. To fulfill such large energy demands, it becomes necessary to look for such energy sources which are highly abundant, recyclable and can provide high energy density, voltage, capacity, cost and safety.7,

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though Li-ion batteries are being explored but the cost and safety issues limit their applications for future aspects.8-13, 14 In this regard, multivalent batteries (Mg,15-17 Zn,18, 19 Al20-28) can emerge as the better options to satisfy the increasing energy demands due to their large natural resources, high energy density and capacity. Among these developing technologies, aluminium batteries have unique advantages. The theoretical volumetric capacity of aluminum metal is 8.0 Ahcm−3, which is 4 times higher than that of Li metal.20-22 Additionally, the higher production, less expensive raw material, less reactivity and easier handling than lithium also make aluminium a promising material for batteries. Despite of these advantages, however, Al-ion batteries faces constant challenges, such as cathode material disintegration, short cycle life, low discharge voltage and low capacity, and researchers are constantly trying to overcome these limitations.23-28 In 2015, Lin et al. developed an ultrafast Al-ion battery using graphite foam as a cathode with an ionic electrolyte 1-ethyl-3-methylimidazolium chloride ([EMIm]Cl) and Al metal anode.29 The battery involves electrochemical deposition and dissolution of Al at the anode side and AlCl4


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intercalation/deintercalation at the cathode side. The mechanism through the battery operates is quite different from the typical rocking chair batteries, and involves participation of electrolyte like in case of dual-ion cell batteries. The reported Al-ion battery showed well-defined discharge voltage plateaus ~ 2.0 V with ultrafast charging/discharging capability. Such ultrafast potential must depend on the choice of cathode material. In our previous study, we have shown that not only graphite-foam, but the pristine natural graphite can also act as a superior cathode forming different AlCl4 intercalation stages maintaining the similar ultrafast diffusion rate, voltage and capacity, which is further independently verified by a recent experimental report on natural graphite cathode for Al-ion battery.30,

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The flexible layered structure of graphite intercalates

AlCl4 anions during charging with the expansion of interlayer spacing of host graphite gallery and the diffusion of AlCl4 is very fast through the large interlayer spaces leading to ultrafast charging rate.30-34 Significant research work has been done to further understand the system and enlarge the battery performance.35-37 However, the discharge voltage (~ 2.0 V) is quite low compared to Li-ion batteries. Therefore, for the better advancement of electrochemical properties of Al-ion battery, it is important to search for other flexible layered materials for high voltage Al-ion battery that can serve as cathode material without having any negative effect on the performance of battery. One of such potential candidate is BC3 that is in the great interest to material scientist and physicist since it is reported.38 It has been reported that BC3 is a layered material with graphite like structure with the similar appealing electrochemical properties39-42 but with increased conductivity.43 From the time of its synthesis, a lot of experimental and theoretical studies have been done to illustrate the structural and electronic properties of BC3 and the AB stacked BC3 structure is found to be more stable having two boron atoms at 1, 3 positions of the hexagon.44-48



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In field of batteries, BC3 has become interesting electrode material from the realization of the first synthesized Li intercalated BC3 structure.49 After that many experimental and theoretical studies have been done to investigate the electrochemical properties of BC3 electrode for metal ion intercalation.48-54 It has been well established that BC3 can serve as a potential candidate for metal-ion batteries involving positive metal-ion intercalation that provides higher intercalation capacity due to its relatively light mass, higher energy density as well as better voltage stability compared to graphite electrode.52-56 However, BC3 is still not explored for the intercalation study of anion intercalated batteries. As BC3 has a similar stacking pattern like graphite, so it should also be explored towards the intercalation of anions to form different stages of anion intercalation as graphite does.45 Therefore, we have investigated the efficiency of BC3 for Al-ion battery as a cathode material replacing the typical graphite cathode, where AlCl4 anion intercalates and deintercalates in the cathode. In this current work, the staging mechanism is used to investigate the intercalation behavior of AlCl4 into bulk BC3, which is a characteristic feature of graphite-like layered structures. Here, we have examined the detailed properties of AlCl4 intercalated BC3, including the geometric structures, binding energies, charge transfer, electronic properties, voltage study and diffusion by van der Waals-corrected density functional theory (DFT) calculations. The thermal stability of the system is examined using the Ab Initio Molecular Dynamics (AIMD)57 at a temperature range of 300-600 K. Our results show that BC3 could be one of the promising cathode candidates that can be used in Al-ion batteries to enhance its efficiency other than graphite.


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2. Computational Details We have performed density functional theory calculations by using the Projector augmentedwave (PAW) method to treat interactions between ion cores and valance electrons as implemented in the Vienna Ab initio Simulation Package (VASP).58-60 The exchange-correlation potential is described by using the generalized gradient approximation of Perdew-BurkeErnzerhof (GGA-PBE)61 and a 470 eV cutoff for plane-wave basis set is adopted for the overall calculations. The Brillouin zone is sampled with a 6 × 6 × 6 Monkhorst-Pack grid for unit cell and 2 × 2 × 2 Monkhorst-Pack grid for supercell calculations. Energy minimization is performed with a tolerance of 10-3 eV. The atomic and lattice positions are fully relaxed until the forces on each atom is less than 0.02 eV/Å. We have used the DFT-D3 approach for the correction of van der Waals interactions for potential energy and interatomic forces.62 The DFT-D3 level of theory is reliable enough to correlate with the real experimental senerio, because the results obtained in our previous theoretical study have been independently justified by a recent experimental report.30,31 Moreover, the accuracy of DFT-D3 method has also been checked and it shows negligible difference from the experimental values.33 For the calculation of density of states (DOS), the Brillouin zone is sampled with a k-point grid of 11 × 11 × 11. The Ab Initio Molecular Dynamic Simulation (AIMD) is performed using the canonical ensemble with fixed volume, temperature and particle number.57 AIMD simulations are performed at 300-600 K with the time step of 1 femtosecond for 5 picosecond time steps. Temperature control is achieved by Nóse thermostat model. A 2 × 2 × 1 supercell of 48 carbon atoms and 16 B atoms is used for both DOS and AIMD calculations. Bader charge analysis63-69 is performed with the help of the Henkelman programme using near-grid algorithm refine-edge method to understand the charge transfer process between the atoms. We have calculated the 5 ACS Paragon Plus Environment


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activation barriers using the climbing image nudged elastic band (CI-NEB)70 method as implemented in Henkelman programme. The minimum energy paths (MEP) are initialized by inserting six image structures between fully optimized initial and final structural geometries and the energy conversion criteria of each image is set to 10-3 eV. Zero-point energy corrections have been included in all the calculations, which is calculated as: ZPE = ∑ constant and is vibrational frequency.

, where h is Planck’s

3. Results and Discussion 3.1. Geometric Structures and Binding Sites The layered BC3 structure shows different stacking patterns. The lowest energy structures that differ in energy at less than 5 meV/atom are AB and AA stacked BC3 layered structures.56 Therefore, we have first determined the most stable stacking of AlCl4 intercalated BC3 by calculating the total energies of the AA and AB stacked BC3 having AlCl4 intercalant. The obtained results show that the most stable stacking for AlCl4 intercalated BC3 is AB stacking with the relative energy difference of 0.06 eV (Figure 1a-b). Moreover, it is also observed that both AA and AB stacked BC3 layers show lateral shifting on AlCl4 intercalation, this could be due to strong interaction between intercalated AlCl4 and BC3 layers and it can be concluded that the orderly stacked BC3 layers are unfavorable and changes to lateral shifted layers upon AlCl4 intercalation (Figure S1, Supporting Information). In this study, we have considered the AB stacked BC3 structure having in-plane B atoms at 1, 3-positions to carry out all the further investigations. The obtained interlayer distance (di) is 3.36 Å which is consistent with the


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previously reported experimental (3.35 Å)38 value, however the difference in our and previous theoretical studies (3.44 Å)71 can be mostly due to different relaxation procedures used. Further, to investigate the intercalation of AlCl4 into BC3, it is important to be clarified about the geometry of AlCl4 after intercalation, whether it is planar or tetrahedral.30,

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determine the probable geometry of AlCl4 into BC3, we have considered both tetrahedral and planar AlCl4 geometries and studied their intercalation behavior. We have found that the tetrahedral geometry of AlCl4 is more stable than planar AlCl4 with 0.68 eV energy difference. Moreover, the planar geometry of AlCl4 changes to quasiplanar AlCl4. The structural stability of intercalated tetrahedral AlCl4 to planar AlCl4 is in accordance with the gas phase calculations where tetrahedral AlCl4 is 0.88 eV more stable than planar one. In fact, the tetrahedral geometry of AlCl4 is also justified by recent theoretical and experimental reports in graphite cathode.30-33



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Figure 1: Optimized unit cell of BC3 (a) top, and (b) side view. Optimized AlCl4 intercalated geometries: (c) quasiplanar, and (d) tetrahedral. Optimized structures (top view and side view) of tetrahedral AlCl4 intercalation at eight different intercalation sites: S1, S2, S3, S4, S5, S6, S7 and S8. RE represents the relative energy difference (in eV) of different intercalating sites with respect to S3 site. Al, Cl, B and C atoms are shown by green, red, orange and blue spheres, respectively. 8 ACS Paragon Plus Environment

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Next, we have examined the possible intercalation sites for AlCl4. Eight representative binding sites are available that are shown in Figure 1 with their relative energy differences and among these sites, S3 site is most stable with -0.81 eV binding energy. This clearly indicates toward the stability of AlCl4 intercalated BC3 system and the strong interaction between intercalated AlCl4 and BC3 layers which is also reflected by lateral shifting of BC3 layers upon AlCl4 intercalation. In addition to this, we have observed that the tetrahedral AlCl4 gets slightly distorted on intercalation into BC3 having the

Hexagonal BC3 Electrode for a High-Voltage Al-Ion Battery - The

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