Ab initio study of radiation effects on the Li4Ti5O12 ...

Ab initio study of radiation effects on the Li4Ti5O12 electrode used in lithium-ion batteries Adib Samin, Michael Kurth, and Lei Cao Citation: AIP Advances 5, 047110 (2015); doi: 10.1063/1.4917308 View online: http://dx.doi.org/10.1063/1.4917308 View Table of Contents: http://scitation.aip.org/content/aip/journal/adva/5/4?ver=pdfcov Published by the AIP Publishing Articles you may be interested in Low energy structures of lithium-ion battery materials Li(MnxNixCo1−2x)O2 revealed by first-principles calculations Appl. Phys. Lett. 103, 053903 (2013); 10.1063/1.4817763 Diffusion-induced stresses of electrode nanomaterials in lithium-ion battery: The effects of surface stress J. Appl. Phys. 112, 103507 (2012); 10.1063/1.4767913 Inelastic hosts as electrodes for high-capacity lithium-ion batteries J. Appl. Phys. 109, 016110 (2011); 10.1063/1.3525990 Fracture of electrodes in lithium-ion batteries caused by fast charging J. Appl. Phys. 108, 073517 (2010); 10.1063/1.3492617 Fe-rich and Mn-rich nanodomains in Li 1.2 Mn 0.4 Fe 0.4 O 2 positive electrode materials for lithium-ion batteries Appl. Phys. Lett. 91, 054103 (2007); 10.1063/1.2757587

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AIP ADVANCES 5, 047110 (2015)

Ab initio study of radiation effects on the Li4Ti5O12 electrode used in lithium-ion batteries Adib Samin,a Michael Kurth, and Lei Caoa Nuclear Engineering Program, Department of Mechanical and Aerospace Engineering, The Ohio State University, 201 W 19th Avenue, Columbus, Ohio 43210 USA

(Received 6 February 2015; accepted 30 March 2015; published online 7 April 2015) Lithium-ion batteries are currently in wide use owing to their high energy density and enhanced capabilities. Li4Ti5O12 is a promising anode material for lithium-ion batteries because of its advantageous properties. Lithium-ion batteries could be exposed to radiation occurring in various conditions such as during outer space exploration and nuclear accidents. In this study, we apply density functional theory to explore the effect of radiation damage on this electrode and, ultimately, on the performance of the battery. It was found that radiation could affect the structural stability of the material. Furthermore, the electrode was shown to undergo a transition from insulator to metal, following the defects due to radiation. In addition, the effect of radiation on the intercalation potential was found to be highly dependent on the nature of the defect induced. C 2015 Author(s). All article content, except where otherwise noted, is licensed under a Creative Commons Attribution 3.0 Unported License. [http://dx.doi.org/10.1063/1.4917308]

I. INTRODUCTION

In addition to being the primary energy storage devices for various sectors in the economy,1 lithium-ion batteries are used in many electronic devices and for NASA2 and military applications3 owing to their high energy densities, low operating temperature, enhanced rate capabilities, and superior safety features. However, the performance of these batteries under intense radiation environments is not well understood. Such situations are relevant for robots performing rescue and sampling missions post nuclear explosions. In Fukushima, Japan, in 2011, robots powered by lithium-ion batteries were engineered to perform sampling, recovery, and rescue missions in the high radiation environments, but were found to fail quickly.4 Furthermore, high-energy protons in space may affect the performance of NASA’s electronic devices as well as their battery power sources. Therefore, a careful analysis of the performance of these batteries under high radiation environments is required. Researchers investigating the effects of neutron and gamma irradiation on lithium-ion cathodes, observed a capacity loss in the battery.5 Ratnakumar et al. attempted to characterize the behavior of Lithium-ion cells in high-intensity radiation environments to optimize their performance during planetary exploration missions to Jupiter and its moons.6 The authors used a cumulative radiation dose of up to 25 Mrad in their study and reported that the lithium ion cells exhibited a good tolerance to radiation with only 10% deterioration in their capacity upon radiation exposure for discharges at room temperature. In 2006, Ding et al.7 studied the effects of gamma radiation on lithium-ion batteries by using a Co-60 source with a dose rate of 100 Gy/min for 24 h. The authors found the cell performance to be substantially deteriorated post-irradiation. This degradation in performance was attributed to the defects induced in the electrode and to the carboxyl generation in the electrolyte. Ding et al. further experimentally explored the effects of γ-radiation on the LiCoO2 electrode used in lithium-ion batteries.8 They used a Co-60 source with a dose rate of 100 Gy/min for 24 h, and reported that radiation affects the cobalt valence charge and increases the impedance

a [email protected]; [email protected]

2158-3226/2015/5(4)/047110/7

5, 047110-1

© Author(s) 2015

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of LiCoO2 in electrochemical cells, thus leading to a decrease in its electrochemical capacity. Recently, Li4Ti5O12 has been investigated as a promising anode material for lithium-ion batteries, due to its several advantageous properties. Li4Ti5O12 has been identified as a zero-strain anode material.9 The material exhibits minimal change in cell volume upon intercalation and de-intercalation (less than 1%), which helps maintain its structural stability,10 and it also has good thermal stability.11 In addition, the charge discharge plateau of Li4Ti5O12 is approximately 1.55 V, which is higher than the reduction decomposition potential of most organic solvents;12 therefore, it is a safer option compared to the graphite anode.13 In this study, we investigate the radiation effects on the lithium titanium oxide electrode and determine its potential impacts on the battery performance. At the anode, oxidation reactions occur during discharge. One mole of Li4Ti5O12 accommodates three moles of Li+ during the charging process: Li 4Ti 5O12 + 3Li + + 3e−

Discharge



Charge

Li 7Ti 5O12.

¯ space group (No. 227). The unit cell contains eight formula units Li4Ti5O12 belongs to the Fd 3m of (Li)[Li 1/3Ti 5/3]O4. The Li atoms occupy all the tetrahedral 8a sites and 1/6th of the 16d sites; the Ti atoms occupy the other 5/6th of the 16d sites; and the O atoms occupy the 32e tetrahedral sites. Upon lithiation, lithium ions on the 8a sites are moved to the 16c sites and the additional lithium ions occupy the remaining 16c sites, thereby changing the LTO structure from spinel to rocksalt.14 The structural parameters of Li4Ti5O12 are shown in Table I. There have been several investigations using density functional theory (DFT) ab initio calculations to determine the structural parameters, electronic properties, and average intercalation potentials for lithium titanate.16–18 A 1 × 1 × 3 supercell was used for both Li4Ti5O12 and Li7Ti5O12, resulting in the Li32Ti40O96 and Li56Ti40O96 structures used for evaluation. This study verified and used the reported values18 of the most energetically stable arrangement for the random Li occupations of the 16d sites. Moreover, because radiation particles transfer their energies to the solid lattice structure directly or indirectly, supplying enough energy can knock atoms of the target material out of position, leading to vacancies and interstitials. In this manner, vacancy defects were introduced to the perfect cell structures to simulate the effects of radiation damage and to analyze the possible effects on the electronic and structural properties. II. COMPUTATIONAL DETAILS

The ab initio calculations were performed under the DFT framework and generalized gradient approximation using the plane wave pseudopotential method as implemented in the Quantum Espresso package.19 The generalized gradient approximation (GGA)20 was used for the exchange correlation functionals as parametrized by Perdew-Burke and Ernzerhof (PBE).21 Furthermore, Vanderbilt ultrasoft pseudopotentials22 were used to describe the core electrons and nuclei. A 1 × 1 × 3 supercell (a total of 168 atoms for Li4Ti5O12 and 192 atoms for Li7Ti5O12) and a Gaussian smearing with a small broadening width of 0.003 Ry were employed in all simulations. The wave functions and the electron density were expanded in a plane wave basis with cutoff energies of 60 Ry and 600 Ry, respectively. The Brillouin zone was sampled by a 3 × 3 × 1 Monkhorst-Pack grid. The total energy of the system was converged to an accuracy better than 10−4 eV/atom, with respect to the k-point mesh and energy cutoff. In our calculation, we allowed the atomic positions and the cell volume to relax until the forces at each atom converged to within 1.5 × 10−3 eV/Å. TABLE I. Structural Parameters of Li4Ti5O12.15 Atom

Position

Atomic Coordinates x = y = z

Occupancy

Li

8a

0

1.00

Li

16d

0.625

0.16

Ti

16d

0.625

0.84

O

32e

0.3878

1.00

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FIG. 1. Crystal structure of Li4Ti5O12 along the [100] crystal vector after removing (1) a 16d Li atom, and (2) a 32e O atom. These two structures were used for defect calculations in this study.

In the literature, two different structures have been proposed for Li4Ti5O12 depending on how the lithium atoms occupy the 16d sites: the first was proposed by Ouyang et al.16 in 2007, where the arrangement of the Li atoms partially occupying the 16d sites was optimized using cubic spinel-type cells, and the second was proposed by Tsai et al.18 in 2014. In order to determine the energetically preferable structure to use for our study, we ran simulations using both types of structures. Our results indicated that the structure proposed by Tsai et al.18 has a lower energy, and therefore was used for the remainder of this study. Two different kinds of point defects were introduced to investigate the effects of radiation on the electrode. In one defect calculation, a 16d lithium atom was removed (Defect 1), and in another calculation, a 32e oxygen atom was removed (Defect 2). The defects were introduced to the spinel Li4Ti5O12 and are shown in Figure 1. Following the introduction of the defects, the atomic positions and the cell volume were relaxed. Finally, after self-consistency was achieved, these structures were lithiated by removing the lithium atoms from 8a sites and filling the 16c sites with lithium atoms. The new structure was again relaxed with respect to the atomic positions and the cell volume. III. RESULTS AND DISCUSSION A. Structural properties

The lattice parameters obtained after the relaxation of atomic positions and cell volume are summarized in Table II. In all simulations, the experimental values were used as an initial estimate for the cell parameters and the cell volume was then allowed to relax. It can be seen that our results for the perfect structure are in good agreement with those reported by Tsai et al.18 and also the experimental results. Most importantly, the cell volume decreases when lithium intercalates the structure by less than 1%, and this has been experimentally verified in many studies. Furthermore, the small change in cell volume upon lithiation is one of the advantageous properties that make this material a good candidate for use as an anode material. TABLE II. Equilibrium lattice parameters for Li4Ti5O12 and Li7Ti5O12 in Angstroms (Å). Lattice Constant

Li4Ti5O12

Li7Ti5O12

Experimental Values14

8.3595

8.3538

Ab initio (Tsai et al.)18

8.4257

8.3609

This work (Perfect Structure)

8.4032

8.3354

Defect 1 (16d Li deleted)(This work)

8.4037

8.3359

Defect 2 (32e O deleted) (This work)

8.4181

8.2778

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AIP Advances 5, 047110 (2015)

Upon the introduction of the Lithium point defect to mimic the effects of radiation, there is little change in the cell lattice parameter for the Li4Ti5O12 or the lithiated structure. However, and in the case of the O atom deletion, there is an observable change in the cell parameter when the structure is lithiated. The change in the lattice constant after discharge is greater than 1%, indicating noticeable radiation effects on the structural parameters and stability of the lattice in the case of Oxygen deletion. The material clearly loses one of its outstanding properties, which is its negligible volume change after discharge, upon irradiation. Upon examination of the atomic positions for Li4Ti5O12, the oxygen defect structure has a maximum displacement of 0.626 angstroms and an average of 0.0625 angstroms compared to the perfect structure and the lithium defect results in a 0.103 maximum displacement, with an average of 0.0170 angstrom. Finally, it was found that the formation energies for the Lithium vacancy and the Oxygen vacancy were (1.97 eV) and (7.45 eV) respectively. Both of these numbers are well within the range of the energy introduced to the cell in the case of radiation exposure and indicate that the defects being studied are physically meaningful. B. Intercalation potential

The average intercalation potential can be obtained from an ab initio calculation using the formula:23 ∆G V= − nF where, n is the number of electrons being transferred during the intercalation process, F is the Faraday constant, and ∆G is the Gibbs free energy (∆G = ∆E + P∆V − T∆S). The changes in energy due to volume and entropy are negligible during the intercalation process23 as the change in internal energy dominates the process. Therefore, the intercalation potential can by approximated as: ∆E nF The change in internal energy ∆E of intercalation between Li4Ti5O12 and Li7Ti5O12 in this case can be calculated using the relation: V≈ −

∆E = Et ot [Li7Ti5O12] − Et ot [Li4Ti5O12] − 3E BCC [Li] where Et ot [Li7Ti5O12] and Et ot [Li4Ti5O12] are the total energies of Li7Ti5O12 and Li4Ti5O12, respectively, and E BCC [Li] is the total energy of metallic lithium in a body-centered cubic unit cell. Our results are summarized in Table III and show a reasonable agreement with experiment and are more consistent with Tsai’s calculations. It is well-known that DFT underestimates the intercalation potentials of metal oxides compared to experimental observations.24 Because the structure proposed by Tsai et al.18 was used in this study, similar results can be expected. Possible reasons for the difference include the calculation method (pseudopotential plane wave method vs. projector augmented wave method), differences in the cutoff energies (60 Ry vs. 45 Ry), and differences in the convergence criteria for the forces (1.5 × 10−3 eV/Å vs. 1 × 10−4 eV/Å) and the pseudopotentials used in the two studies. The post-radiation intercalation potential obtained for the damaged cells significantly vary with the type of defect the radiation causes. From our results, it is apparent that the post-radiation intercalation potential for oxygen-deficient structure is smaller compared to its value for the perfect TABLE III. Intercalation potentials in volts from the Li4Ti5O12 to the Li7Ti5O12 phases taken from theoretical and experimental studies including this one. Study

Intercalation Potential (Volts)

Experiment9,26

1.55

Tsai et al.18

1.41

This work (Perfect Structure)

1.26

Defect 1 (16d Li deleted)(This work)

1.33

Defect 2 (32e O deleted) (This work)

1.09

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AIP Advances 5, 047110 (2015)

FIG. 2. Total density of states for (a) the unaltered Li4Ti5O12 (b) the undamaged Li7Ti5O12 (c) the Li4Ti5O12 with a 16d Li atom removed (d) the Li7Ti5O12 with a 16d Li atom removed (e) the Li4Ti5O12 structure with a 32e O removed (f) the Li7Ti5O12 structure with a 32e O removed. The Fermi Energy was aligned to 0 eV in the energy scale.

structure, which further implies a degradation in the battery’s performance and capabilities. This is consistent with previous experimental findings.25 However, the structure in which a 16d Li atom was removed displays an enhanced discharge capacity. The differences in the calculated intercalation potentials for the two types of defects may be due to the observed distortions in the lattice after radiation in the case where an oxygen atom was removed, which decreases the Li mobility and causes deterioration in the battery’s discharge capacity in the LTO structure; however, the atomic positions were not significantly affected in the case where a 16d lithium atom was removed. Furthermore, it provides an additional site for the intercalating lithium ions to occupy, and therefore, enhances the battery’s discharge capacity and performance in the latter. C. Electronic Structures

Li4Ti5O12 is known to be an insulator and its band gap has been measured experimentally by various groups; some of the recent reported values are 3.8 eV,27,28 3.1 eV,29 and 1.8 eV.30 We

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AIP Advances 5, 047110 (2015)

obtained a band gap of 2.8 eV that is in reasonable agreement with experiment even though it is well known that DFT calculations underestimate the bandgap. This result also agrees well with Tsai’s ab initio study18 that reported a value of 2.3 eV. After the introduction of point defects to mimic the effects of high energy radiation, we observed Li4Ti5O12 undergo an insulator-to-metal transition with the Fermi energy shifting to the conduction band. This is consistent with previous experimental studies31–33 reporting an improved electrical conductivity after introducing oxygen defects through heating, in a reducing atmosphere. This may be attributed to the reduction of some Ti4+ ions to Ti3+ ions. However, a known complication under these circumstances is that the partially reduced titanium in LTO is unstable in air, and reoxidation to its original state eventually leads to a reduced electrical conductivity.34,35 The total density of states (DOS) plots for the various structures examined in this study are reported in Figure 2. IV. CONCLUSIONS

Understanding the radiation effects on lithium-ion batteries is important for a wide variety of applications. In this study, we conducted density functional theory (DFT) calculations to investigate the effects of radiation damage on Li4Ti5O12 anode used in lithium-ion batteries. We introduced two different point defects to a Li4Ti5O12 supercell. It was found that the deletion of an Oxygen atom had a significant effect on the cell parameters change, implying that radiation damage could affect the structural stability of the material. Furthermore, it was determined that the electrode underwent an insulator-to-metal transition post radiation, which is in agreement with previous experimental observations. Finally, it was concluded that the effects on the intercalation potentials were highly dependent on the nature of the introduced defect. For the oxygen deficient structure, the intercalation potential was notably reduced, indicating a reduction in the battery’s capacity, whereas, the removal of a 16d Li atom enhanced the intercalation potential. This could be explained by the distortions in the lattice that impede lithium mobility in the oxygen-deficient structure, and the increase in the number of available sites for the diffusing lithium atoms in the structure where a 16d Li atom was removed.

ACKNOWLEDGMENTS

This research is being performed using funding received from the U.S. Defense Threat Reduction Agency’s (DTRA) research grant (HDTRA1-13-0012). Additionally, this work was supported in part by an allocation of computing time from the Ohio Supercomputer Center. 1

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