n-type Doping Eff

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C: Energy Conversion and Storage; Energy and Charge Transport

p- and n-type Doping Effects on the Electrical and Ionic Conductivities of LiTiO Anode Materials 4

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Chi Ho Lee, and Sang Uck Lee J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.8b03995 • Publication Date (Web): 19 Jun 2018 Downloaded from http://pubs.acs.org on June 21, 2018

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The Journal of Physical Chemistry

p- and n-type Doping Effects on the Electrical and Ionic Conductivities of Li4Ti5O12 Anode Materials Chi Ho Lee† and Sang Uck Lee†,‡,*

† ‡

Department of Bionano Technology, Hanyang University, Ansan, 15588, Korea

Department of Chemical and Molecular Engineering, Hanyang University, Ansan, 15588, Korea

*Corresponding author. Email: [email protected]

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Abstract We systematically investigated p- and n-type doping effects on the electrical conductivity of spinel Li Ti O (LTO) by designing theoretically stoichiometric Li Ti O (p-type) and Li Ti O (n-type) because LTO has a non-stoichiometric

(Li) [Li/ Ti/ ]O formula with the Fd3m space group. In this work, we present

evidence that the electronic modification plays a fundamental role in the electrical conductivity of LTO, especially, n-type Li Ti O , which has superior electrical conductivity compared to p-type Li Ti O . We proposed a way to improve the electrical conductivity of pristine LTO by halogen ion doping, Li Ti O Hal (Hal: F, Cl and Br), for an n-type doping effect. However, the substitution of halogen ions can enhance the electrical conductivity by mixing Ti4+ /Ti3+ and impede the Li ion diffusion in the lattice. The larger size of Cl and Br increases the Li ion diffusion energy barrier with van der Waals repulsion. Therefore, our theoretical investigations of the effects of halogen doping on the electrical and ionic conductivities anticipate that the smaller-sized F may be the most promising dopant for improving the performance of LTO.


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1. Introduction There is an increasing demand for the development of sustainable, clean, and efficient energy-storage systems for applications including electric vehicles (EVs) and hybrid electric vehicles (HEVs) based on large-scale lithium ion batteries (LIBs).

1-5

Rechargeable lithium-

ion batteries (LIBs) have been considered one of the most attractive candidates for large-scale applications due to their high energy density, high operating voltage, low self-discharge, and long service life. 6-11 The development of LIBs with high power density, good cyclic stability, and low cost is the key to significantly penetrating the EVs/HEVs market. However, conventional LIBs using carbon/graphite as anodic material are not suitable for high-power applications because they suffer from low operating potential, especially during the charging process, which easily gives rise to dendrites on the surface of the Li metal, especially at high rates. In addition, it has a low Li-ion diffusion coefficient, which limits the performance of LIBs. 3, 12-16 In the search for promising materials with superior performance in terms of lithium ion intercalation/deintercalation and high safety to replace conventional carbon/graphite based anodes, spinel lithium titanium oxide (LTO) is regarded as a promising anode material for LIBs with outstanding safety due to its high working voltage of approximately 1.5V versus Li+/Li with a theoretical capacity of 175 mAh/g.

11, 17-19

Moreover, LTO is known as a zero-

strain material because of its negligible volume variation during Li insertion/extraction, which ensures good reversibility 10-11 and structure stability 20-21 in a long-term cycle. However, LTO suffers from poor electrical and ionic conductivity, which is a major obstacle for large-scale applications. Thus, many approaches to improve the electrical 3 ACS Paragon Plus Environment


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conductivity have been reported, including nano-sized structures, 17, 22-24 surface modification, 25

and metal ion doping such as Mn4+, Al3+, V5+ and Co3+.

26-31

Among various effective

methods, chemical doping of pristine LTO with heteroatoms is an effective way to manipulate its electronic structure and electrochemical properties. Both experimental and theoretical research reveals that the effects of various dopants can improve the electronic effects of LTO. However, the detailed structural interpretation of stoichiometric LTO and tuning the electronic properties of anion doped LTOs are still being studied. In addition, LTO possesses a non-stoichiometric structure when considering the theoretical approach because LTO is one , of the end members of the Li

TM O (0 < " < 1) solid solution phase.

In this work, to clear up the non-stoichiometric issue and provide thorough understanding of the electronic properties of LTO, we systematically designed the stoichiometric LTO structure using theoretically designed p-type and n-type LTO structures. Based on a thorough understanding of the electronic properties and doping effects of the designed stoichiometric p- and n-type LTOs, we demonstrated that n-type doping can provide superior electrical conductivity. We also propose a way of improving the electrical conductivity of pristine LTO by halogen ion doping, Li Ti O$ Hal$ (Hal: F, Cl and Br), for representing the n-type doping effect. Finally, we evaluated the effects of halogen atom doping on the electrical and ionic conductivities.


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2. Computational Details. All calculations were performed with the Vienna Ab initio Simulation Package (VASP 5.3.5).

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method

Ab initio calculations were carried out using the projector augmented wave (PAW)

36-37

with the generalized gradient approximation based on the Perdew–Burke–

Ernzerhof (PBE)

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functional including the Hubbard U correlation (GGA+U).

+ U approach has been well tested to predict voltages and phase stability. is used for the 3d states on Ti.

43

41-42

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The GGA

A value of 2.7

A plane-wave cutoff energy of 500 eV was used. Integration

in the Brillouin zone was performed on the basis of the Monkhorst–Pack scheme

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using a k-

point mesh with an interval of 0.05 Å− 1 in each primitive lattice vector of the reciprocal space. Lattice constants and internal atomic positions were optimized until the residual forces became less than 0.04 eV/ ̊A. The barriers for Li diffusion were calculated for the fully lithiated and fully delithiated limits using the nudged elastic band method.

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The diffusion pathways calculated correspond

to one-dimensional (1D) lithium via hops between nearest neighbor Li sites. Calculations were performed on each p-type and n-type LTO containing a single Li vacancy for both delithiated and lithiated states. The lattice parameters were relaxed at the optimized GGA + U lattice parameters of the non-defected structure.



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3. Results and Discussions. 3.1. Designing stoichiometric LTO structures Looking at the LTO structure, the Li Ti O compound is the end member of the

, Li

TM O (0 ≤ " ≤ 1) solid solution with x=1, as shown in Figure 1(a), and it has a

m space group. Within the spinel structure, Li ions occupy cubic spinel structure of the Fd3 all the tetrahedral (Td) 8a site and 1/6 of the octahedral (Oh) 16d sites, while the remaining 5/6

of the 16d sites are taken by Ti ions and all the 32e sites are occupied with O ions, denoted as the [Li ]& [Li Ti ]' [O ] ( formula shown in Figure 1(b). Subsequently, during the

charging process, all 8a Li ions of the [Li ]& [Li Ti ]' [O ] ( change their position from

8a sites to the octahedral (Oh) 16c sites, along with the additional Li ions conserving the cubic spinel structure of Fd3m, denoted as [Li ]) [Li Ti ]' [O ] ( and shown in Figure 1(c). However, considering the formula of the experimentally refined spinel LTO unit cell with the Fd3m

space

group,

the

delithiated

[Li ]& [Li Ti ]' [O ] (

and

lithiated

[Li ]) [Li Ti ]' [O ] ( formulas should be rewritten as a non-stoichiometric form of '

[Li ]& *Li/ Ti/ +

'

[O ] ( and [Li ]& *Li/ Ti/ +

[O ] ( .

To clear up the non-stoichiometric issue mentioned above, we systematically designed stoichiometric LTO structures by controlling the ion components on the 16d sites of the nonstoichiometric LTO structures. By decreasing (or increasing) the Ti component of '

[Li ]& *Li/ Ti/ +

. [O ] ( , we can design p-type Li Ti

O and n-type

. Li Ti O structures. When we construct p- and n-type LTOs, we compared the relative

stability and determined the most stable structures, shown in Figure 2(a) and (b), among all 6 ACS Paragon Plus Environment

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01 space group of the end members of Figure 1. (a) Cubic spinel structure with a -./ /,9 ; 5 23/4 6784 :5; (< ≤ 4 ≤ 5) solid solution, where TM is transition metal ion, (b) 9 ; delithiated LTO of 235 9 63= :5; with Li ions filled all tetrahedral (Td) 8a sties and 1/6 of

the octahedral (Oh) 16d sties, and (c) lithiated LTO of 23>5 63/.9 :; 5; with Li ions filled = octahedral (Oh) 16c sties instead of tetrahedral (Td) 8a sties. Ti and O ions occupy the 16d and 32e sites, respectively. Gray, purple, sky-blue and red colors indicate Li, TM, Ti+Li and O ions, respectively.

possible configurations because 3 (or 2) Li ions and 13 (or 14) Ti ions can occupy the 16d sites. There are 120 (16C2) and 560 (16C3) configurations for each p- and n-type LTOs. Then, we constructed the stoichiometric LTO structure by combining two p-type LTOs (Li Ti O ) '

and one n-type LTO ( Li Ti O ), that is 3 ? ([Li ]& *Li⁄ Ti⁄ +

[O ] ( ) A

[Li ]& [Li Ti ]' [OB ] ( , as shown in Figure 2(c). Based on the LTO formula, we



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consider

Figure 2. Theoretically designed stoichiometric LTOs, (a) p-type LTO (2355 635/ :/; ), (b) n-

type LTO (235< 6359 :/;) and (c) LTO supercell (23/; 639< :C8 ). Gray, red and sky-blue colors indicate Li, O, Ti, respectively. Blue color shows substitution of Li for Ti.

that the observed LTO properties arise from their average properties. Therefore, it is worth mentioning that if we compare the properties of p- and n-type LTO structures, we can provide insight to control the LTO properties.


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3.2. Voltage profile and electrical conductivity of p- and n-type LTOs In order to evaluate the contribution of p-type Li Ti O and n-type Li Ti O to the experimentally observed properties, especially the voltage profile, we simulated the redox potential difference compared to a pure Li counter electrode for constructing the theoretical voltage profiles of each p- and n-type LTO structure. The redox potential describes the energetic effort required to extract Li ions from the electrodes with the optimized range for electrode materials within the operating voltages. The redox potentials of the p- and n-type LTOs are calculated by the following formula in the range of 0 ≤ " ≤ 1:

V A −[E(Li Ti O ) − E(Li Ti O ) − 8"E(Li)]/8"e

V A −[E(Li Ti O ) − E(Li Ti O ) − 8"E(Li)]/8"e,

(1) (2)

where E(Li Ti O )||E(Li Ti O ), E(Li Ti O )||E(Li Ti O ), and E(Li) denote the total energy of the lithiated||delithiated state of p- and n-type LTO and bulk BCC Li metal, respectively. However, for these equations, we need to define the lithiated structures, Li Ti O and Li Ti O , depending on the ratio of Li in the range of 0 ≤ " ≤ 1. During the charging/discharging process, electrode materials can have one-phase solid solution or a two-phase composite structure of fully lithiated and fully delithiated states because spinel LTO structure can have two different Li sites, tetrahedral (Td) 8a sites and octahedral (Oh) 16c sites in Figure 1(b) and (c), depending on the state of charge. In delithiated LTOs (both p-type Li Ti O and n-type Li Ti O ), all Li ions occupy Td sites and during the lithiation process Li ions change their position from Td to Oh. However, before the fully lithiated state, all Oh sites in LTO cannot be occupied by Li ions. There must 9 ACS Paragon Plus Environment


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be Li occupied Oh sites and empty Oh sites in LTO structure. If the Li occupied and empty Oh sites can be stably mixed in

Figure 3. The relative formation energies of one-phase solid solutions (Erel) and voltage

profile for p-type LTO (2355J4 635/ :/;) and n-type LTO (235

n-type Doping Eff

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