Transparent Conducting Oxides as Cathodes in LiâO2 Batteries: A

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C: Energy Conversion and Storage; Energy and Charge Transport 2

Transparent Conducting Oxides as Cathodes in Li-O Batteries: A First Principles Computational Investigation Balachandran Gadaguntla Radhakrishnan, and John W. Lawson J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.8b07583 • Publication Date (Web): 29 Jan 2019 Downloaded from http://pubs.acs.org on February 5, 2019

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The Journal of Physical Chemistry

Transparent Conducting Oxides as Cathodes in Li-O2 Batteries: A First Principles Computational Investigation Balachandran Radhakrishnan†,‡ and John W. Lawson∗,† †Thermal Protection Materials Branch, NASA Ames Research Center, Moffett Field, California 94035, USA ‡AMA Inc., Thermal Protection Materials Branch, NASA Ames Research Center, Moffett Field, California 94035, USA E-mail: [email protected] Abstract Li-O2 batteries have traditionally used carbon based electrodes (graphite, buckypaper) as the cathode of choice due to its good electrical conductivity, stability against non-aqueous electrolytes like dimethoxyethane (DME) and ease of handling. But, the carbon cathode also leads to formation of carbonate by-products that increase overpotentials during charging leading to degradation of the cathode and reduction of cyclability. Recent investigations have focused on using metal-oxides like SnO2 , TiO2 as viable cathodes-alternatives in Li-O2 systems. In this paper, we investigate transparent conducting oxides (TCOs) as cathodes with focus on the interface between the TCO surfaces and the discharge product, Li2 O2 , in the Li-O2 battery using first principles computations. We also explore the role dopants can play in achieving a stable electrically conducting cathode that promotes discharge of Li2 O2 . Our results show that attention must be paid to choosing the appropriate surface of these oxides. We

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extend the analysis to suggest possible oxide chemistries that should be investigated as cathodes in Li-O2 batteries.

Introduction Batteries, as energy storage devices, are playing an increasingly critical role in cleaner and more efficient power utilization with applications varying from backup power devices to powering electro-motive vehicles (EMVs). With increasing number of applications, there have been increasing demands for storage devices that can deliver higher specific power. Li-O2 batteries can potentially cater to such demands with estimated theoretical power capacities in the range of 3500 kWh/kg. 1–4 Li-O2 batteries take advantage of using lithium metal as the anode along with a cathode that provides a conductive surface for discharge of the desired product, Li2 O2 , through the electrochemical reversible reaction: 2Li+ + 2 e – + O2 ↔ Li2 O2 , with a realized potential of 2.96 V (vs Li/Li+ ). 5,6 In an ideal scenario, Li2 O2 will be the only discharge product which requires a slightly larger voltage (∼ 3.2 V) for the reverse reaction to occur during the charging process. 7,8 But, parasitic reactions, involving both the electrolyte and carbon-based cathodes lead to the formation of carbonates. 9–11 To recover full capacity during the charging process, the decomposition of carbonates and other side products would require potentials in excess of 4.5 V (vs Li/Li+ ). 5 This, in turn, leads to decomposition of both the electrolyte and the electrode resulting in decreasing performance over cycles.

Carbon based cathodes have been widely used in studies performed on Li-O2 batteries owing to their light weight, ease of handling, availability, and good electrical conductivity (∼ 102−3 S/m). 12 But, the carbon cathode reacts with Li2 O2 in the presence of O2 according to the reaction: 1 Li2 O2 + C + O2 → Li2 CO3 2 The formation of Li2 CO3 has been confirmed via numerous experimental observations of CO2 2


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release during the charging process. 5,13,14 Experiments have also observed the formation of Li2 CO3 from different types of carbon based electrodes: P50, superblack, bucky-paper carbon and is one of the reasons for degradation of battery performance in Li-O2 batteries. 13,15 To address this issue, numerous efforts have centered on testing alternative cathodes that do not react and decompose during the charge-discharge cycle. Noble metals, metal-carbides and metal-oxides, are obvious choices due to their stability against ether-based electrolytes used in the Li-O2 battery. These cathodes have been studied both experimentally as well as using density functional theory computations. 14,16–20 Krishnamurthy et al. 17 investigated the noble metals using DFT computations and found that they catalyze formation of Li2 O rather than Li2 O2 . Geng and Ohno 20 studied adsorption of Li, LiO2 and Li2 O2 on RuO2 , SnO2 and TiO2 (110) surfaces using DFT computations and concluded that Li2 O2 can probably wet these surfaces. Experimentally, TiO2 has been used as a protective coating on carbon cathodes with reasonable success, though further tests have shown that TiO2 leads to formation of other oxides of Ti over the charge-discharge cycle. 16,20 Li-ion battery cathodes such as CoO have also been tested but there is concern of electrochemical Li+ reactions occurring over certain voltage ranges. 21 Recently, Bae et al. 22 used ZnO coatings to cover carbon surface defects. Analysis of the evolved gas during charging process shows reduced amounts of CO2 formed from the cathode. While this is an encouraging sign, cathode degradation continues to be an impediment to achieving cyclability on the order of Li-ion batteries. In this study, we investigate Transparent Conducting Oxides (TCOs) as a class of cathode and/or cathode-coating candidates using first principles computations. TCOs are well established industrially available materials that have found applications in photovoltaics, liquid crystal displays among others due to their ease of synthesis, good electronic conductivity and stability. 23–26 Given their properties and their prior use as electrodes in other applications, TCOs present themselves as promising replacement candidates for carbon based cathodes in Li-O2 batteries. In this study, we sample several well-known TCOs: ZnO, SnO2 , CdO and In2 O3 to study their behavior in a Li-O2 environment. Further, using ZnO as a test-case, we

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also explore the role of dopants on surface adsorption of Li2 O2 and intermediates expected in the Li-O2 batteries. Finally, we extend the study to other oxide cathode candidates by proposing a high-throughput screening strategy to identify materials of interest that can be explored further using more computationally intensive DFT calculations that were performed for the oxides considered in this study.

Methods Density functional theory calculations All density functional theory (DFT) calculations were performed using the Vienna Ab-initio Simulations Package (VASP) 27 within the Projector Augmented Wave (PAW) method. The Perdew-Burke-Ernzerhof (PBE) 28 functional, which is a generalized gradient approximation, was used to model the exchange-correlation functional. A plane-wave cut-off of 520 eV was applied. An electronic energy tolerance of 5e−5 eV was used to relax the crystal structures. The k-point mesh used in all cases was optimized to yield accuracies of 1 meV/atom. Python Materials Genomics (pymatgen) 29 package was used for all input file generation and postprocessing of results. In the case of surface adsorption calculations, the forces on the nuclei were converged to less than 0.02 eV/˚ A.

Chemical and Electrochemical Stability Any cathode candidate must be stable against chemical attack by the discharge product: Li2 O2 . The candidate should also be stable against Li+ attack in the operating voltage range (2.0 to 4.5 V, accounting for both discharge and charging environments). This is to avoid a mixed Li-ion and Li-O2 electrochemical behavior and formation of other side-products that can lead to degradation of the cathode. To assess the first chemical stability requirement, we construct a pseudo-binary phase diagram 30 between the cathode, C, and Li2 O2 wherein possible reaction products are probed under thermodynamic equilibrium assumption, given 4


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by the equation:

∆E(C, Li2 O2 ) = min Ex.C−(1−x).Li2 O2 − (x.EC − (1 − x).ELi2 O2 ) 0≤x≤1

(1)

wherein, ∆E(C, Li2 O2 ) is the reaction energy, normalized with respect to number of atoms, E(x.C−(1−x).Li2 O2 ) is the equilibrium phase energy with composition: x.C − (1 − x)Li2 O2 , EC is the energy of the cathode candidate and ELi2 O2 is the energy of Li2 O2 . The Materials Project database containing DFT computed energies of chemistries was used to perform the above analysis. 31 More detailed analysis of this method can be found in literature that focuses on cathode-electrolyte interfaces in Li-ion and Na-ion batteries. 30,32,33 To assess the second requirement, electrochemical stability against Li+ over the operating voltage range, we utilize the grand potential phase diagram approach. Here, Li+ is assumed to the main mobile species with the system being open with respect to Li. The grand potential is then given by: φ = E − µLi NLi

(2)

wherein, E is the total DFT computed energy, µLi is the chemical potential of Li and NLi is the number of lithium atoms in the system. The grand potential approach has been successfully used in predicted interface phase equilibria in batteries previously and we refer to literature 32,34,35 for detailed discussions of the method. Phase equilibria is computed at 4.5 V as most experiments use that value as the cut-off voltage during charging process.

Dopant formation energy In order to determine whether a dopant in ZnO is interstitial or substitutional, we calculate the dopant formation energy, Ef , given by:

Ef = Edoped − Eundoped −

5

X

µ i Ni


(3)


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wherein, Edoped is the DFT computed energy of the doped structure, Eundoped is the energy of the pristine undoped structure and µi and Ni are the chemical potential and number of dopants respectively. In the case of doping ZnO with a neutrally charged species X, we entertained two possibilities:

48ZnO + X = Zn47 XO48 + Zn

(4a)

48ZnO + X = Zn48 XO48

(4b)

The former equation represents substitutional doping while the latter represents interstitial doping. All unique dopant positions were evaluated using enumerator implemented in pymatgen. To avoid numerical errors associated with scaling energies, the same supercell was used for both doped and pristine structures.

Wulff Shape Oxygen-cathodes are conducting electrodes that are expected to provide surfaces favorable for Li2 O2 deposition and avoid any side-reactions. To this end, we construct the Wulff shape, the equilibrium crystal shape, to identify surfaces that interact with intermediates of the LiO2 electrochemical reactions. In calculating these Wulff shapes, no surface reconstructions were considered. Pymatgen module was used to construct the Wulff shape in this study. 36 The surface energy of any metal oxide, Mx Oy , is computed using the relation:

γsurf =

NM Nm 1 [Eslab (NM NO ) − Ebulk (Mx Oy ) + ( y − NO )µO ] 2A x x

(5)

wherein, µO is the potential of oxygen, Eslab (NM NO ) is the energy of the surface slab containing NM atoms of M and NO atoms of oxygen. Ebulk (Mx Oy ) is the bulk energy per formula unit with the simulation cell oriented along the basis vectors of the surface slab. The oriented unit cell energies are computed with k-point mesh density matching those of

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the surface slab.

Adsorption studies For a given surface of the cathode, we need to determine whether it is thermodynamically favorable for the formation of Li2 O2 . To evaluate this capability, we adopt the approach of Krishnamurthy et al. 17 , wherein, the protonic equations of water synthesis were replaced with lithium ions to evaluate the possibility of formation of Li2 O2 /Li2 O. In the electrochemical process, two possibilities are considered: 1. A two e- process leading to the formation of Li2 O2 2Li+ (solv) + O2 (g) + 2e− + ∗ → LiO2∗ + Li+ (solv) + e−

(6a)

Li+ (solv) + LiO2∗ + e− → Li2 O2 (s) + ∗

(6b)

2. A four e- process leading to the formation of Li2 O 4Li+ (solv) + O2 (g) + 4e− + ∗ → LiO2∗ + 3Li+ (solv) + 3e−

(7a)

3Li+ (solv) + LiO2∗ + 3e− → Li2 O(s) + O∗ + 2Li+ (solv) + 2e−

(7b)

2Li+ (solv) + Li2 O(s) + O∗ + 2e− → Li2 O(s) + LiO∗ + Li+ (solv) + e−

(7c)

Li+ (solv) + Li2 O(s) + LiO∗ + e− → 2 Li2 O(s) + ∗

(7d)

We evaluate both possibilities on surfaces of interest identified in each cathode candidate. The notation: ‘*’, refers to the site of adsorption on the surface of the cathode material. The free energy, ∆G = ∆H − T ∆S, for the above reactions are computed under the following assumptions: 1. ∆S = 0 for solids. 7



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2. The anode is in thermodynamic equilibrium with the Li+ salt containing electrolyte. Thus, Li+ (solv) + e− ↔ Li(s). This relation is used to compute the free-energy of Li+ (solv) in equations listed above. 3. Chemical potential of oxygen gas accounting for entropy corrections: µO = 4.94 eV. Based on the above assumptions, the potential realized due to the electrochemical reaction: 2Li+ + 2e− + O2 ↔ Li2 O2 is calculated as eU = 2.83 eV using DFT computed energies. Various works in literature have made corrections to either µO and/or µLi+ to obtain values closer to the experimentally observed 2.96 eV. 8,17,37 While numerically differing from experimental observations, the qualitative observations made in this study will not be affected by the deviations of the realized voltage, eU . Detailed discussions on variations in this value can be found elsewhere in literature mentioned above. We note that bulk values of energies are used for Li2 O2 (s) and Li2 O(s).

Electrical conductivity The electrical conductivity, σαβγ , is evaluated using the semi-classical Boltzmann theory equations: −1 3 σαβγ (i, k) = e3 .τ(i,k) .εγuv .vα(i,k) vv (i, k)Mβu

(8)

−1 wherein, the inverse mass tensor, Mβu , is derived from DFT computed band structures using

the equation: −1 Mβu (i, k) =

and vα (i, k) =

1 (i,k) ¯ ∂kα h

1 ∂ 2 (i,k) h ¯ 2 ∂kβ ∂ku

(9)

is the group velocity, τ is the relaxation time and εγuv is the Levi-Civita

symbol. A constant relaxation time of 1e − 14 s is found to be a reasonable approximation in previous computations. 38 The inverse mass and the group velocity are obtained by numerically computing the curvature and slope of the band structures respectively, which requires a finer k -mesh than those used for regular band structure calculations. Band structures used

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for computing the inverse mass tensor were obtained from hybrid functional (HSE06) 39 DFT computations with a uniform k -point mesh density of 200 points per atom. BoltzTraP was used to compute the mass tensor and conductivity from these band structures. 38 Pymatgen modules were used to plot the eigenvalues of the conductivity tensor that provides a measure of the electrical conductivity of the chemistry. 40 Further discussion of the numerical method used to calculate the transport computations can be found in Hautier et al. 40 and Madsen and Singh 38 .

Results Chemical and Electrochemical stability To assess the chemical stability of the cathodes, we constructed a pseudo-binary phase diagram of cathode with Li2 O2 using Equation 1. Table 1 lists possible products that are formed when in contact with Li2 O2 . Of the candidates considered here, we see that ZnO, In2 O3 and CdO are chemically stable against Li2 O2 while SnO2 forms Li2 SnO3 . Figure S1 presents the pseudo-phase diagrams representing possible reactions between Li2 O2 and the cathodes of interest. Any cathode candidate, in addition to being stable against Li2 O2 , Table 1: Chemical and electrochemical stability of cathode candidates. Chemical stability was ascertained by constructing a pseudo-binary phase diagram of the cathode and Li2 O2 . 30 The electrochemical stability was analyzed using a grand potential phase diagram. 32 The same analysis is extended to predict stable chemistries at 4.5 V(vs Li/Li+ ) Chemistry ZnO SnO2 CdO In2 O3

Reaction Products with Li2 O2 Li2 SnO3 -

Electrochemical stability range (V) (vs Li/Li+ ) ≥ 1.36 ≥ 1.98 ≥ 1.72 ≥ 1.57

Products formed at 4.5 V (vs Li/Li+ ) ZnO, Li, O2 SnO2 , Li, O2 CdO, Li, O2 In2 O3 , Li, O2

should also not react with Li+ over the potential range at which Li-O2 battery operates. To assess this requirement, we also evaluated the electrochemical stability of the cathode using Equation 2. Table 1 also tabulates the electrochemical stability range of these oxides. We also consider the products that are formed at 4.5 V (vs Li/Li+ ) as such high voltages are 9



encountered during charging process. Table 1 shows that the oxides considered in this study are stable at the operating voltages of interest (2- 4.5 V). While SnO2 chemically reacts with the Li2 O2 to form Li2 SnO3 , it eventually decomposes to SnO2 , Li and O2 at the operating voltages thereby preserving the cathode.

Wulff Shape The Wulff shape of ZnO, In2 O3 , SnO2 and CdO are given in Table 2. Table 2 also lists the percentage of coverage of different surfaces on the Wulff shape. In the case of ZnO, the polar surfaces were included as it is well known from the literature that these surfaces are stable. 23,41 The stability of these surfaces has been attributed to charge redistributions from the surface to the bulk. 41 In the case of In2 O3 and SnO2 , due to the large computational cost associated with the number of atoms involved, the maximum miller index is restricted to 1. For CdO and ZnO, surfaces with maximum miller index of 2 were considered. The computed surface energies are listed in Table S1. The equilibrium crystal structure of ZnO is composed of two surfaces: (0001) and (10¯10) of which the former surface is polar with an oxygen rich surface. Polar surface energies are influenced by the chemical potential of O2 under which they are synthesized. Hence, the equilibrium crystal structure can be altered under different oxygen environments leading to formation of needle like structures or film-like structures. The Wulff shape of SnO2 is predominantly occupied by (110) and (101) surfaces while the In2 O3 and CdO Wulff shapes are mostly composed of (111) and (110) surfaces respectively. The arguments made for ZnO equilibrium crystal structure variations can also be extended to the other oxides considered here. But, for the purposes of this study, the surfaces mentioned above will be considered as those interacting with the discharge products and intermediates in the Li-O2 electrochemistry.

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Table 2: Wulff shape of cathode candidates. The equilibrium crystal shapes were constructed using surface energy values given in Table S1. Miller indices to the maximum of 2 were considered for the chemistries ZnO and CdO while miller indices to restricted to 1 for In2 O3 and SnO2 due to computational cost. No surface reconstructions were considered in this study. Chemistry

Wulff shape

Surface coverage

• (0001) : 20.83% ZnO

• (10¯10) : 79.17%

• (110) : 100%

CdO

• (111) : 97% In2 O3

• (100) : 3%

• (110) : 42.79% • (101) : 34.88% SnO2

• (001) : 18.5% • (100) : 3.78%

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Surface adsorption studies Cathodes in Li-O2 battery systems are expected to provide a surface conducive to the formation of Li2 O2 . The feasibility of different products formed is assessed by comparing the free energies of different reactions, listed in Equations 6 and 7, on each of the surfaces listed in Table 2. Figure 1 shows the free energies of adsorption of different intermediates in a 2/4 e – process for each of the oxides considered in this study.

Figure 1: Free energies of adsorption of intermediate products on surfaces of oxides identified in Wulff shapes (Table 2). Both the 2 e− and 4 e− reactions, given by Equations 6 and 7 were considered in this study to determine possible thermodynamic paths for formation of either Li2 O(s)and Li2 O2 (s). The realized potential, eU , in the electrochemical reactions was fixed at 2.83 eV. The 2 e− process in Equation 6 is represented by the the first 2 electrons consumed leading to the formation of LiO2* and Li2 O2 (s) respectively. The 4 e− process in Equation 7 is represented by the 4 electrons consumed leading to formation of LiO2* , O* + Li2 O(s), LiO* + Li2 O(s) and 2 Li2 O(s) respectively. The free energies of adsorption also determine the deviation from the equilibrium voltage which, in effect, is a measure of the thermodynamic overpotential. Table S2 in Supplementary tabulates the thermodynamic overpotentials ( ∆GLiO2 ) in the reaction pathways leading to formation of Li2 O2 separately for the sake of convenience. The formation of Li2 O is possible if 12


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∆G(O∗ +Li2 O) ≤ ∆GLi2 O2 or if ∆GLiO∗ +Li2 O ≤ ∆GLi2 O2 . Either of these steps demand breaking of the O-O bond to be thermodynamically favorable. In the case of ZnO, both (10¯10) and (0001) surfaces offer a thermodynamically easier path to formation of Li2 O2 with the free-energy overpotentials much smaller than those required for formation of Li2 O (Figure 1). It should be noted that the (0001) surface is more preferable due to better adsorption of LiO*2 (∆G = 0.23eV ) than (10¯10) surface (∆G = 0.85eV ). The Wulff shapes of CdO and In2 O3 are dominated by (110) and (111) surfaces respectively. While CdO (Figure 1) behaves in a similar manner to ZnO (10¯10), the In2 O3 (111) surface favors formation of Li2 O. The intermediate LiO*2 adsorption is highly favorable (∆G = −2.7 eV) and subsequent steps requiring less than 0.5 eV. It is more probable that In2 O3 favors forming Li2 O rather than Li2 O2 given that ∆GO∗ +Li2 O < ∆GLi2 O2 as well as ∆GLiO∗ +Li2 O < ∆GLi2 O2 . In the case of SnO2 , we performed the adsorption studies on 2 surfaces (110) and (101). Adsorption studies show that both surfaces have nearly identical interactions with the adsorbed specie with differences within numerical tolerance errors encountered in this study. SnO2 (110) adsorbs LiO*2 with the least free energy required of all the oxides (∆G = −0.006 eV). But, ∆G(LiO∗ +Li2 O) = 0.28 eV, required for the intermediary that would result in likely formation of Li2 O, is not large enough to be insurmountable. This portends formation of a mix of Li2 O and Li2 O2 with the former requiring voltages larger than 5 eV for recharge. Further, we also calculated the free energy of adsorption of Li2 O2* on these oxide surfaces to understand their wettability. Table S3 in Supplementary tabulates the adsorption energy of Li2 O2* . In general, ∆GLi2 O2∗ follows ∆GLiO2∗ with In2 O3 showing higher adsorption energy than the rest of the oxides considered in this study. Both LiO2* and Li2 O2* have similar inclinations of wetting any given surface with the exception of (0001) surface of ZnO. The observed trend is also in agreement with the surface wetting study performed by Geng and Ohno 20 on (110) surface of RuO2 , TiO2 and SnO2 . Henceforth, favorability of LiO2* adsorption should be sufficient to predict wetting of any cathode surface of interest by Li2 O2* .

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Effect of doping ZnO Cathodes in Li-O2 batteries are expected to be current carriers. While pristine TCOs can offer relatively better stability against a variety of organic solvents (ethers, mostly) that are used as electrolytes, they suffer from large bandgaps. For example, ZnO has a band gap of 2-3 eV. 42 Traditionally, TCOs have been doped using either p- or n-type dopants to alter the optical bandgap for use in photovoltaic applications. Here, we explore the effect of n-type dopants on adsorption behavior using ZnO as a test case. We consider the following dopants - Al, Ga, Ge, In, Si, and Sn in ZnO at %∼2 at. wt. The dopant concentration was chosen closer to industrially available doped ZnO chemistries while within computational cost limits. We also note that similar or even higher dopant concentrations have been achieved in experiments. 43–45 Dopants can either substitute Zn forming Zn(1-y) Xy O or occupy interstitial sites forming ZnXy O which can be represented by Equation 4. To evaluate the most stable configuration, we evaluated both possibilities and calculated the dopant formation energies, as per Equation 3. Table 3 lists the dopant formation energies of the six dopants in interstitial as well as substitutional cases. Based on the formation energies of all the dopants considered in this study, substitutional dopants are more stable than interstitial dopants. Al and Si can be doped easily into ZnO while Ga doping is still possible to achieve with some thermodynamic penalty. Further analysis will be restricted to these three doped chemistries that are found to be thermodynamically feasible. Using the semi-classical Boltzmann equations, we computed the electronic conductivity of pristine as well as doped ZnO. BoltzTrap tool was used to compute the conductivities. Figure 2 shows the conductivities (average of the eigenvalues of conductivity tensor) of ZnO and %∼2 at. wt. Al, Ga and Si doped ZnO. The figures show that the electronic conductivity of ZnO increases from 10−3 S/m to 106 S/m on account of the dopants. It should be noted that graphitic carbon has a modest electronic conductivity of around 100 S/m. 12 To understand the dopant effects on adsorption, we repeated the adsorption studies presented in previous section (See Figure 1) with dopants substituting the farthest bulk Zn 14


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Figure 2: Electronic conductivity as a function of fermi energy computed using semi-classical Boltzmann equations. HSE-DFT computed band structure was used as input for calculating the curvature of band structures in these calculations. The plot shows the variation of the average of the eigenvalues of the conductivity tensor (Equation 8) as a function of fermi energy. BoltzTrap package was used to compute the electronic conductivity with post processing of results performed using pymatgen.

Table 3: Dopant formation energy of n-type dopants in ZnO. Formation energies were computed using Equation 3. Pristine cells were scaled to match the doped structures so as to avoid numerical errors associated with energy scaling. Substitutional dopant 48 ZnO + X → Zn47 XO48 +Zn Doped Chemistry Dopant Formation Energy (eV) Zn47 AlO48 -1.75 Zn47 GaO48 0.83 Zn47 GeO48 2.56 Zn47 InO48 1.97 Zn47 SiO48 -1.608 Zn47 SnO48 4.15

Interstitial dopant 48 ZnO + X → Zn48 XO48 Doped Chemistry Dopant Formation Energy (eV) Zn48 AlO48 2.153 Zn48 GaO48 4.417 Zn48 GeO48 5.502 Zn48 InO48 5.182 Zn48 SiO48 2.536 Zn48 SnO48 7.52

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atom (See Figure S2) in surface slabs from the adsorbed species (LiO*2 , LiO* , O* ) to avoid any short-order interactions between the dopant and the adsorbed species. Figure 3 shows the free-energies of adsorption on both (10¯10) as well as (0001) surface in the presence of dopants (Al, Ga and Si).

All the dopants lower the adsorption energy of LiO*2 adsorption on both (10¯10) and (0001) surfaces (∆G ≤ 0 eV representing energetically favorable adsorption on the surface). Further, the thermodynamic path to formation of Li2 O2 still remains more favorable compared to formation of Li2 O + O* on both the surfaces. In the case Al and Ga doping, it is more likely to form Li2 O2 than O* + Li2 O on both (10¯10) and (0001) surfaces. But, ∆GLiO∗ +Li2 O < ∆GLi2 O2 on (10¯10) for all 3 dopants points to possible formation of both Li2 O as well as Li2 O2 . In contrast, the (0001) surface favors formation of Li2 O2 compared to Li2 O for all the dopants considered in this study.

Discussion ZnO From the computational analysis, the ZnO chemistry presents itself as a promising candidate for cathode in Li-O2 systems. The Wulff shape of ZnO is dominated by (0001) and (10¯10) surfaces under standard room temperature conditions. Experiments have used different oxidizing environments to get thin films of ZnO dominated by (0001) or thin-needle like crystals dominated by (10¯10) surface. Further, substrate assisted growth can also be used to obtain surfaces of interest. Both the chemical and electrochemical analysis points towards a stable cathode that can remain non-reactive against attacks from Li2 O2 as well as Li+ over the voltage range at which Li-O2 batteries operate. It should be noted that ZnO has been previously used as anodes in Li-ion batteries at voltages lower than 2 V. The Li-O2 batteries are operated in the range of 2.5 - 4.5 V range and use of ZnO would not result in mixed

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(a) Doped ZnO: (10¯10)

(b) Doped ZnO: (0001)

Figure 3: Free energies of adsorption on ZnO (10¯10) and (0001) surfaces when doped with Al, Ga and Si dopants. The surface slabs have the same dopant concentration of ∼ 2 % as that of the bulk system used for conductivity calculations.The dopants were introduced at the farthest position from the adsorbed species to reduce any short range interactions. The volume of the doped slabs were fixed as the dopants induced volumetric relaxations are assumed to be negligible. The realized potential, eU , in the electrochemical reactions was fixed at 2.83 eV. 17



Li-ion/Li-O2 electrochemical reactions as is possible in candidates such as CoO2 . The inert nature of the ZnO has been confirmed in experiments performed by Bae et al. 22 wherein, ZnO coated carbon cathodes fared far batter than uncoated carbon cathodes with reduced evolution of CO2 gas and other parasitic products. Further, based on the adsorption free energies from Figure 1, (0001) of ZnO offers a better surface for adsorption of LiO*2 than (10¯10) surface of ZnO with much lower overpotential. Formation of Li2 O is highly unlikely due to the high free energy cost associated with splitting of the O−O bonds as is shown in Figure 1. But, pristine ZnO also suffers from large impedance indicated by the low conductivity shown in Figure 2. This could lead to lower yields of Li2 O2 whose discharge is already limited by its own impedance. This can be remedied by using dopants such Al, Ga or Si with conductivities reaching 103 − 106 S/m as is shown in Figure-2. The semi-classical computations are in agreement with experiments reporting 103−5 S/m. Dopants, in addition to remedying the impedance issues, also affect the adsorption energies of intermediates. Figure 3 shows that adsorption energies of all the intermediates are lowered in the presence of dopants. In particular, Si doping of ZnO reduced the overpotential from 0.85 eV to -0.3 eV on (10¯10) surface. On the other hand, the (0001) surface strongly binds LiO*2 in the presence of dopants while maintaining viable paths to formation of Li2 O2 . This might lead to slightly larger recharging potentials compared to pristine ZnO, a penalty paid to achieve good electrical conductivity. Our doping studies indicate that care should be taken in cathode surfaces that should be synthesized for favorable discharge products. Further analysis on the role of dopant concentrations and substrate assisted crystal growths could pave the way for a stable ZnO based cathode. Concerns regarding increase in weight of cathodes leading to decrease in specific power of Li-O2 batteries are well placed. However, such optimizations should take a back seat and can be addressed once the parasitic reactions in Li-O2 batteries have been suppressed or completely eliminated.

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CdO, In2 O3 and SnO2 Chemical and electrochemical analysis of CdO and In2 O3 point towards non-reactive interfaces with Li2 O2 . Also, both these chemistries do not react with Li+ over the voltage range at which Li-O2 battery is operated. SnO2 , on the other hand, reacts with Li2 O2 to form Li2 SnO3 . But, electrochemical analysis points towards decomposition of Li2 SnO3 to the parent cathode material (SnO2 ) at 2 V. In the case of SnO2 , the (110) and (101) surfaces have also been observed in experiments and in fact, have been investigated computationally for Li-O2 cathode coatings previously by Geng and Ohno 20 with focus on the O−O bond distances on (110) surface of SnO2 . In2 O3 Wulff construction has been restricted to surfaces of maximum miller index 1. This is due to the large simulations cells that were encountered on surfaces of higher miller indices. Wulff shape of CdO is completely constructed from (110) surfaces due to its much lower surface energy compared to other surfaces considered. Experimental observations confirm that these surfaces are observed in the chemistries.

Based on the adsorption studies, the behavior of CdO is similar to ZnO with high overpotentials and favorable thermodynamic path to formation of Li2 O2 rather than Li2 O. Given the small bandgap of CdO, doping could be easily used to increase the electronic conductivity of the material. We can also expect similar effects of doping on adsorption energies given that both Cd and Zn belong to the same group in the periodic group. However, the toxic nature of Cd should be considered against testing this material in experiments and applications that can reach the public domain.

In2 O3 and SnO2 provide favorable adsorption surfaces for LiO*2 . But, they also create possible thermodynamic paths to formation of Li2 O which are harder to decompose. Indeed this is confirmed in the study by Beyer et al. 46 wherein, doped SnO2 was used as a cathode. Beyer et al. 46 showed that while use of SnO2 does not show any decomposition of the cathode, the discharge product contains both Li2 O2 and Li2 O with the later requiring more 19



than 5 V to decompose on charging.

Other oxides of interest The analysis performed on the four TCOs present a set of characteristics that are expected of any cathode/cathode-coating in a Li-O2 battery. The cathode must be chemically nonreactive with the discharge product, Li2 O2 , it should be electrochemically stable against Li+ over the voltage range of Li-O2 battery operation and it should, preferably, promote formation and deposition of Li2 O2 rather than Li2 O. The question of electronic conductivity can be remedied using dopants as shown in the case of ZnO with further benefits of reducing the overpotential. The role of promoting Li2 O2 requires substantial computational investigation involving identifying surfaces of the cathode and adsorption studies of different intermediates in the electrochemical reaction and should be considered as a final step in the evaluation of the cathode-candidate. But, the former stability criteria can be easily checked using a high-throughput screening of the Materials Project database. Table 4 shows a list of oxides, screened using stability criteria, that can potentially be explored as a cathode in Li-O2 batteries. All the chemistries mentioned below do not react with Li2 O2 and with some electrochemical stability above 3 V. The identified chemistries can be broadly classified into alkali and alkaline earth metal oxides, rare earth metal oxides, and transition metal oxides. MgO, while stable against Li2 O2 , is an insulator with a very large band gap that could prove harder to be rectified using doping strategies. The superoxides: SrO2 , BaO2 , CsO2 , while being good candidates are susceptible to Li+ attack at lower operating voltages of Li-O2 system and cannot effectively be used as cathode candidates. Of the transition metal oxides: Y2 O3 (yttria), CeO2 , and La2 O3 can be used as a cathode with dopants to overcome impedance issues. Yang et al. 47 tested using CeO2 as a cathode coating with reasonable success. Further studies on surface adsorption on these three oxides could prove more beneficial. The rare earth metal oxides shown in Table 4 could potentially be tested as coatings though computational investigations of these oxides 20


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Table 4: Potential oxide cathodes for Li-O2 batteries identified based on chemical and electrochemical stability criteria. Chemistry BeO MgO K 2 O2 CaO SrO2 BaO2 CsO2 Y2 O3 La2 O3 CeO2 Sm2 O3 Gd2 O3 Tb2 O3 Dy2 O3 Er2 O3 Tm2 O3 Lu2 O3 HgO

Electrochemical stability range (V) (vs Li/Li+ ) ≥0 ≥ 0.64 ≥ 2.4 ≥0 ≥ 2.89 ≥ 2.68 ≥ 2.88 ≥0 ≥0 ≥ 1.22 ≥0 ≥0 ≥0 ≥0 ≥0 ≥0 ≥0 2.45

Notes Toxic Insulator Prone to Li+ attack Promising candidate with doping Prone to Li+ attack Prone to Li+ attack Conductor Promising candidate with doping Promising candidate with doping Used as coating 47 Rare earth mineral Rare earth mineral Rare earth mineral Rare earth mineral Rare earth mineral Rare earth mineral Rare earth mineral Toxic

will prove to be expensive at this point of time. It should be noted that our interest in oxides as replacement to carbon cathode is fueled by their stability against degradation in ether type of electrolytes that dominate the Li-O2 batteries. Any new class of electrolytes should trigger stability checks against the oxide-cathodes. Additional considerations should be given to the relative permittivity of the electrolyte while computing adsorption energies of different intermediates in the electrochemical reaction. Oxides are heavier than graphitic carbon by an order of magnitude. Any resulting reduction in specific power capacities of Li-O2 batteries can be mitigated by optimizations such as ALD techniques or substrate induced crystal growth techniques. For example, Bae et al. 22 used ALD to coat CNT cathodes with ZnO, thereby, limiting the cathode mass increase to 15%. We also note that specific capacity of Li-O2 batteries are limited by the thickness of Li2 O2 which is determined by various factors such as electrolyte, discharge current and induced defects in deposition product. 4,22,37,48 Optimization of specific capacity to realize the full potential of Li-O2 batteries can be pursued once a stable, recyclable electrochemistry devoid of parasitic reactions is achieved. Oxide based cathode/cathode-coatings, based on our computations, can prove to be an important factor in this direction.

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Conclusion In this study, we evaluated the performance of Transparent Conducting Oxides (TCOs) as potential cathodes in Li-O2 battery using first principles computations. Our study shows that of the chemistries considered ZnO offers a promising option for a stable cathode. In addition, doping of TCOs could favorably reduce the overpotentials involved in the discharge of Li2 O2 . We also present a set of characteristics: non-reactivity with Li2 O2 , electrochemically stability against Li+ over the range of operating voltage, electrical conductivity and a thermodynamic favorability of formation of Li2 O2 , that should be evaluated for any potential candidate. Our results show that for the same chemistry, different surfaces can promote formation of different oxides of Li: Li2 O2 or Li2 O. Finally, using a high-throughput screening, we identify CsO2 , CeO2 , Y2 O3 and La2 O3 as possible candidates that can be evaluated as a cathode in Li-O2 batteries.

Acknowledgement The authors acknowledge funding from the NASA ARMD (Aeronautics Research Mission Directorate) CAS (Convergent Aeronautics Solutions) project.

Supporting Information Available This material is available free of charge via the Internet at http://pubs.acs.org/.

References (1) Imanishi, N.; Luntz, A. C.; Bruce, P. The Lithium Air Battery; 2014; p 310. (2) Abraham, K. M.; Jiang, Z. A Polymer Electrolyte Based Rechargeable Lithium/Oxygen Battery. Journal of the Electrochemical Society 1996, 143, 1–5. 22


Page 22 of 38

Page 23 of 38 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


(3) Aurbach, D.; McCloskey, B. D.; Nazar, L. F.; Bruce, P. G. Advances in understanding mechanisms underpinning lithium-air batteries. Nature Energy 2016, 1, 1–11. (4) Lim, H.-D.; Lee, B.; Bae, Y.; Park, H.; Ko, Y.; Kim, H.; Kim, J.; Kang, K. Reaction chemistry in rechargeable Li-O2 batteries. Chem. Soc. Rev. 2017, 46, 2873–2888. (5) McCloskey, B. D.; Speidel, A.; Scheffler, R.; Miller, D. C.; Viswanathan, V.; Hummelshøj, J. S.; Nørskov, J. K.; Luntz, A. C. Twin Problems of Interfacial Carbonate Formation in Nonaqueous Li-O2 O2 Batteries. J. Phys. Chem. Lett. 997. (6) McCloskey, B. D.; Bethune, D. S.; Shelby, R. M.; Mori, T.; Scheffler, R.; Speidel, A.; Sherwood, M.; Luntz, A. C. Limitations in Rechargeability of Li-O2 Batteries and Possible Origins. J Phys Chem Lett 2012, 3, 3043–3047. (7) Liu, R.; Lei, Y.; Yu, W.; Wang, H.; Qin, L.; Han, D.; Yang, W.; Zhou, D.; He, Y.; Zhai, D.; et al., Achieving Low Overpotential Lithium-Oxygen Batteries by Exploiting a New Electrolyte Based on N,N’-Dimethylpropyleneurea. ACS Energy Letters 2017, 2, 313–318. (8) Hummelshøj, J. S.; Luntz, A. C.; Nørskov, J. K. Theoretical evidence for low kinetic overpotentials in Li-O2 electrochemistry. Journal of Chemical Physics 2013, 138 . (9) Mahne, N.; Schafzahl, B.; Leypold, C.; Leypold, M.; Grumm, S.; Leitgeb, A.; Strohmeier, G. A.; Wilkening, M.; Fontaine, O.; Kramer, D.; Slugovc, C.; Borisov, S. M.; Freunberger, S. A. Singlet oxygen generation as a major cause for parasitic reactions during cycling of aprotic lithium-oxygen batteries. Nature Energy 2017, 2, 1–9. (10) McCloskey, B. D.; Scheffler, R.; Speidel, A.; Bethune, D. S.; Shelby, R. M.; Luntz, A. C. On the efficacy of electrocatalysis in nonaqueous Li-O2 batteries. Journal of the American Chemical Society 2011, 133, 18038–41.

23



(11) Khetan, A.; Pitsch, H.; Viswanathan, V. Solvent Degradation in Nonaqueous Li-O2 Batteries: Oxidative Stability versus H-Abstraction. Journal of Physical Chemistry Letters 2014, 5, 2419–2424. (12) Marinho, B.; Ghislandi, M.; Tkalya, E.; Cor E., K.; de With, G. Electrical conductivity of compacts of graphene, multi-wall carbon nanotubes, carbon black, and graphite powder. Powder Technology 2012, 221, 351–358. (13) McCloskey, B. D.; Valery, A.; Luntz, A. C.; Gowda, S. R.; Wallra, G. M.; Garcia, J. M.; Mori, T.; Krupp, L. E. Combining Accurate O2 and Li2 O2 Assays to Separate Discharge and Charge Stability Limitations in Nonaqueous Li-O2 Batteries. J. Phys. Chem. Lett. 2013, 4, 2989–2993. (14) Ottakam Thotiyl, M. M.; Freunberger, S. A.; Peng, Z.; Chen, Y.; Liu, Z.; Bruce, P. G. A stable cathode for the aprotic Li-O2 battery. Nature Materials 2013, 12, 1050–1056. (15) McCloskey, B. D.; Bethune, D. S.; Shelby, R. M.; Mori, T.; R., S.; Speidel, A.; Sherwood, M.; Luntz, A. C. Limitations in Rechargeability of Li-O2 Batteries and Possible Origins. J Phys Chem Lett 2012, 3, 3043–3047. (16) Adams, B. D.; Black, R.; Radtke, C.; Williams, Z.; Mehdi, B. L.; Browning, N. D.; Nazar, L. F. The importance of nanometric passivating films on cathodes for Li-Air batteries. ACS Nano 2014, 8, 12483–12493. (17) Krishnamurthy, D.; Hansen, H. A.; Viswanathan, V. Universality in Nonaqueous Alkali Oxygen Reduction on Metal Surfaces: Implications for Li-O2 and Na-O2 Batteries. ACS Energy Letters 2016, 1, 162–168. (18) Ma, S.; Wu, Y.; Wang, J.; Zhang, Y.; Zhang, Y.; Yan, X.; Wei, Y.; Liu, P.; Wang, J.; Jiang, K.; et al., Reversibility of Noble Metal-Catalyzed Aprotic Li-O2 Batteries. Nano Letters 2015, 15, 8084–8090.

24


Page 24 of 38

Page 25 of 38 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


(19) Kundu, D.; Black, R.; Adams, B.; Harrison, K.; Zavadil, K.; Nazar, L. F. Nanostructured Metal Carbides for Aprotic Li-O2 Batteries: New Insights into Interfacial Reactions and Cathode Stability. The Journal of Physical Chemistry Letters 2015, 6, 2252–2258. (20) Geng, W. T.; Ohno, T. Li2 O2 wetting on the (110) surface of RuO2 , TiO2 , and SnO2 : An initiating force for polycrystalline growth. Journal of Physical Chemistry C 2015, 119, 1024–1031. (21) Shang, C.; Dong, S.; Hu, P.; Guan, J.; Xiao, D.; Chen, X.; Zhang, L.; Gu, L.; Cui, G.; Chen, L. Compatible interface design of CoO-based Li-O2 battery cathodes with longcycling stability. Scientific Reports 2015, 5, 1–7. (22) Bae, Y.; Ko, D.-H.; Lee, S.; Lim, H.-D.; Kim, Y.-J.; Shim, H.-S.; Park, H.; Ko, Y.; Park, S. K.; Kwon, H. J.; et al., Enhanced Stability of Coated Carbon Electrode for LiO2 Batteries and Its Limitations. Advanced Energy Materials 2018, 1702661, 1702661. (23) Wang, Z. L. Zinc oxide nanostructures: Growth, properties and applications. Journal of Physics Condensed Matter 2004, 16 . (24) Cui, J.; Wang, A.; Edleman, N. L.; Ni, J.; Lee, P.; Armstrong, N. R.; Marks, T. J. Indium Tin Oxide Alternatives - High Work Function Transparent Conducting Oxides as Anodes for Organic Light-Emitting Diodes. Advanced materials 2001, 13, 1476– 1480. (25) Minami, T. New n-type transparent conducting oxides. MRS bulletin 2000, 25, 38–44. (26) Lany, S.; Zunger, A. Dopability, intrinsic conductivity, and nonstoichiometry of transparent conducting oxides. Physical Review Letters 2007, 98, 045501. (27) Kresse, G.; Furthm¨ uller, J. Efficient iterative schemes for ab initio total-energy calcu-

25



lations using a plane-wave basis set. Physical review. B, Condensed matter 1996, 54, 11169–11186. (28) Perdew, J. P.; Burke, K.; Ernzerhof, M. Generalized Gradient Approximation Made Simple. Physical Review Letters 1996, 77, 3865–3868. (29) Ong, S. P.; Richards, W. D.; Jain, A.; Hautier, G.; Kocher, M.; Cholia, S.; Gunter, D.; Chevrier, V. L.; Persson, K. A.; Ceder, G. Python Materials Genomics (pymatgen): A robust, open-source python library for materials analysis. Computational Materials Science 2013, 68, 314–319. (30) Richards, W. D.; Miara, L. J.; Wang, Y.; Kim, J. C.; Ceder, G. Interface Stability in Solid-State Batteries. Chemistry of Materials 2016, 28, 266–273. (31) Jain, A.; Ong, S. P.; Hautier, G.; Chen, W.; Richards, W. D.; Dacek, S.; Cholia, S.; Gunter, D.; Skinner, D.; Ceder, G.; et al., Commentary: The Materials Project: A materials genome approach to accelerating materials innovation. APL Materials 2013, 1, 011002. (32) Zhu, Y.; He, X.; Mo, Y. First principles study on electrochemical and chemical stability of solid electrolyte-electrode interfaces in all-solid-state Li-ion batteries. J. Mater. Chem. A 2016, 4, 3253–3266. (33) Tang, H.; Deng, Z.; Lin, Z.; Wang, Z.; Chu, I.-H.; Chen, C.; Zhu, Z.; Zheng, C.; Ong, S. P. Probing Solid-Solid Interfacial Reactions in All-Solid-State Sodiumion Batteries with First Principles Calculations. Chemistry of Materials 2017, acs.chemmater.7b04096. (34) Ong, S. P.; Wang, L.; Kang, B.; Ceder, G. Li-Fe-P-O2 Phase Diagram from First Principles Calculations. Chemistry of Materials 2008, 20, 1798–1807.

26


Page 26 of 38

Page 27 of 38 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


(35) Ong, S. P.; Mo, Y.; Richards, W. D.; Miara, L.; Lee, H. S.; Ceder, G. Phase stability, electrochemical stability and ionic conductivity of the Li10±1 MP2 X12 (M = Ge, Si, Sn, Al or P, and X = O, S or Se) family of superionic conductors. Energy Environ. Sci. 2013, 6, 148–156. (36) Tran, R.; Xu, Z.; Radhakrishnan, B.; Winston, D.; Sun, W.; Persson, K. A.; Ong, S. P. Surface energies of elemental crystals. Scientific Data 2016, 3, 1–13. (37) Mccloskey, B. D.; Scheffler, R.; Speidel, A.; Girishkumar, G.; Luntz, A. C. On the Mechanism of Nonaqueous Li-O2 Electrochemistry on C and Its Kinetic Overpotentials Some Implications for Li-Air Batteries. 2012, (38) Madsen, G. K.; Singh, D. J. BoltzTraP. A code for calculating band-structure dependent quantities. Computer Physics Communications 2006, 175, 67–71. (39) Heyd, J.; Scuseria, G. E.; Ernzerhof, M. Hybrid functionals based on a screened Coulomb potential. The Journal of Chemical Physics 2003, 118, 8207. (40) Hautier, G.; Miglio, A.; Waroquiers, D.; Rignanese, G. M.; Gonze, X. How does chemistry influence electron effective mass in oxides? A high-throughput computational analysis. Chemistry of Materials 2014, 26, 5447–5458. (41) Noguera, C. Polar oxide surfaces. Journal of Physics-Condensed Matter 2000, 12, R367–R410. (42) Janotti, A.; Van De Walle, C. G. Fundamentals of zinc oxide as a semiconductor. Reports on Progress in Physics 2009, 72 . (43) Schumann, J.; Eichelbaum, M.; Lunkenbein, T.; Thomas, N.; Alvarez Galvan, M. C.; Schlgl, R.; Behrens, M. Promoting strong metal support interaction: Doping ZnO for enhanced activity of Cu/ZnO: M (M= Al, Ga, Mg) catalysts. ACS catalysis 2015, 5, 3260–3270. 27



(44) Liu, C.; Meng, D.; Pang, H.; Wu, X.; Xie, J.; Yu, X.; Chen, L.; Liu, X. Influence of Fe-doping on the structural, optical and magnetic properties of ZnO nanoparticles. Journal of Magnetism and Magnetic Materials 2012, 324, 3356–3360. (45) Jin, Z.; Fukumura, T.; Kawasaki, M.; Ando, K.; Saito, H.; Sekiguchi, T.; Yoo, Y.; Murakami, M.; Matsumoto, Y.; Hasegawa, T.; et al., High throughput fabrication of transition-metal-doped epitaxial ZnO thin films: A series of oxide-diluted magnetic semiconductors and their properties. Applied Physics Letters 2001, 78, 3824–3826. (46) Beyer, H.; Metzger, M.; Sicklinger, J.; Wu, X.; Schwenke, K. U.; Gasteiger, H. A. Antimony Doped Tin Oxide: Synthesis, Characterization and Application as Cathode Material in Li-O2 Cells: Implications on the Prospect of Carbon-Free Cathodes for Rechargeable Lithium-Air Batteries. Journal of The Electrochemical Society 2017, 164, A1026–A1036. (47) Yang, C.; Wong, R. A.; Hong, M.; Yamanaka, K.; Ohta, T.; Byon, H. R. Unexpected Li2 O2 Film Growth on Carbon Nanotube Electrodes with CeO2 Nanoparticles in Li-O2 Batteries. Nano Letters 2016, 16, 2969–2974. (48) Aetukuri, N. B.; McCloskey, B. D.; Garci´a, J. M.; Krupp, L. E.; Viswanathan, V.; Luntz, A. C. Solvating additives drive solution-mediated electrochemistry and enhance toroid growth in non-aqueous Li-O2 batteries. Nature Chemistry 2015, 7, 50–56.

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Graphical TOC Entry

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Wulff shape of cathode candidates. The equilibrium crystal shapes were constructed using surface energy values given in Table~\ref{ext1-tbl:surf}. Miller indices to the maximum of 2 were considered for the chemistries \ce{ZnO} and \ce{CdO} while miller indices to restricted to 1 for \ce{In2O3} and \ce{SnO2} due to computational cost. No surface reconstructions were considered in this study. 203x279mm (90 x 90 DPI)


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Free energies of adsorption of intermediate products on surfaces of oxides identified in Wulff shapes (Table ~\ref{tbl:wulff}). Both the 2 $e^-$ and 4 $e^-$ reactions, given by Equations~\ref{eqn:2e} and ~\ref{eqn:4e} were considered in this study to determine possible thermodynamic paths for formation of either \ce{Li2O}(s)and \ce{Li2O2}(s). The realized potential, $eU$, in the electrochemical reactions was fixed at $2.83$ eV. The 2 $e^-$ process in Equation~\ref{eqn:2e} is represented by the the first 2 electrons consumed leading to the formation of \ce{LiO2^*} and \ce{Li2O2}(s) respectively. The 4 $e^-$ process in Equation~\ref{eqn:4e} is represented by the 4 electrons consumed leading to formation of \ce{LiO2^*}, \ce{O^*} $+$ \ce{Li2O}(s), \ce{LiO^*} + \ce{Li2O}(s) and 2 \ce{Li2O}(s) respectively. 264x179mm (72 x 72 DPI)


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Electronic conductivity as a function of fermi energy computed using semi-classical Boltzmann equations. HSE-DFT computed band structure was used as input for calculating the curvature of band structures in these calculations. The plot shows the variation of the average of the eigenvalues of the conductivity tensor (Equation~\ref{eqn:cond}) as a function of fermi energy. BoltzTrap package was used to compute the electronic conductivity with post processing of results performed using pymatgen. 291x185mm (72 x 72 DPI)



Free energies of adsorption on \ce{ZnO} $(10\bar{1}0)$ and $(0001)$ surfaces when doped with Al, Ga and Si dopants. The surface slabs have the same dopant concentration of $\sim$ 2 \% as that of the bulk system used for conductivity calculations.The dopants were introduced at the farthest position from the adsorbed species to reduce any short range interactions. The volume of the doped slabs were fixed as the dopants induced volumetric relaxations are assumed to be negligible. The realized potential, $eU$, in the electrochemical reactions was fixed at $2.83$ eV. 264x179mm (72 x 72 DPI)


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Free energies of adsorption on \ce{ZnO} $(10\bar{1}0)$ and $(0001)$ surfaces when doped with Al, Ga and Si dopants. The surface slabs have the same dopant concentration of $\sim$ 2 \% as that of the bulk system used for conductivity calculations.The dopants were introduced at the farthest position from the adsorbed species to reduce any short range interactions. The volume of the doped slabs were fixed as the dopants induced volumetric relaxations are assumed to be negligible. The realized potential, $eU$, in the electrochemical reactions was fixed at $2.83$ eV. 264x179mm (72 x 72 DPI)



For Table of Contents Only 875x460mm (72 x 72 DPI)


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Transparent Conducting Oxides as Cathodes in LiâO2 Batteries: A

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