Understanding the Role of Overpotentials in Lithium Ion Conversion Reactions: Visualizing the Interface Downloaded via BUFFALO STATE on July 19, 2019 at 08:41:22 (UTC). See https://pubs.acs.org/sharingguidelines for options on how to legitimately share published articles.
Guennadi Evmenenko,† Robert E. Warburton,‡ Handan Yildirim,‡ Jeffrey P. Greeley,‡ Maria K. Y. Chan,§ D. Bruce Buchholz,† Paul Fenter,§ Michael J. Bedzyk,† and Timothy T. Fister*,§ †
Northwestern University, Evanston, Illinois 60208, United States Purdue University, West Lafayette, Indiana 47907, United States § Argonne National Laboratory, Lemont, Illinois 60439, United States ‡
S Supporting Information *
ABSTRACT: Oxide conversion reactions are known to have substantially higher specific capacities than intercalation materials used in Li-ion batteries, but universally suffer from large overpotentials associated with the formation of interfaces between the resulting nanoscale metal and Li2O products. Here we use the interfacial sensitivity of operando X-ray reflectivity to visualize the structural evolution of ultrathin NiO electrodes and their interfaces during conversion. We observe two additional reactions prior to the well-known bulk, three-dimensional conversion occurring at 0.6 V: an accumulation of lithium at the buried metal/oxide interface (at 2.2 V) followed by interfacial lithiation of the buried NiO/Ni interface at the theoretical potential for conversion (at 1.9 V). To understand the mechanisms for bulk and interfacial lithiation, we calculate interfacial energies using density functional theory to build a potential-dependent nucleation model for conversion. These calculations show that the additional space charge layer of lithium is a crucial component for reducing energy barriers for conversion in NiO. KEYWORDS: battery, reflectivity, reflectometry, nucleation, interfaces potentials are intrinsic and less understood.7 In this study, we determine the origins of this overpotential of the well-known displacement reaction of a metal oxide into nanoscale metal and lithium species. The energy density of these materials, given by the product of their reaction voltage and specific capacity, is shown in Figure 1 in comparison to standard cathode materials (LCO = LiCoO2, LMNO = LiMn1.5Ni0.5O2, LFP = LiFePO4). Despite their lower intrinsic voltages, the theoretical energy density of conversion reactions is often higher than intercalation materials. The significant overpotentials for oxide conversion severely reduce their energy density, as illustrated in Figure 1.8,9 As a result, oxide conversion materials are largely considered to be anodes with limited practical viability. The overpotential, which is especially pronounced during the first discharge, has been attributed to the formation of
L
ithium ion batteries are a key component of many modern technologies but are still predicated on materials capable of intercalating lithium.1,2 While these materials provide improved reversibility and higher voltages than other energy storage technologies, they are intrinsically limited in specific capacity by lithium site density in their crystal structures. Reaching higher lithium content requires a shift from classic insertion materials to the broader class of conversion chemistries where lithium reacts directly with the host material. By removing the crystallographic constraint required for intercalation, conversion reactions, such as those found in a lithium sulfur battery3 or displacement reactions in binary metal oxides4 and fluorides,5 can achieve specific capacities many times higher than materials currently used in lithium ion batteries.6 However, these chemistries universally suffer from poor reversibility, both in their poor Coulombic efficiency and significant overpotentials (η = Eeq − Eexp), which have slowed their commercialization. While irreversible capacity is a known byproduct of the large volume change associated with conversion and can be mitigated by nanoengineering or conductive additives, over© XXXX American Chemical Society
Received: March 13, 2019 Accepted: May 20, 2019
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DOI: 10.1021/acsnano.9b02007 ACS Nano XXXX, XXX, XXX−XXX
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interface and is preceded by significant accumulation of Li+ at this interface. This accumulation is a direct measurement of capacitive lithium storage, which was previously speculated to be a space charge layer giving rise to additional capacity during the first discharge that is ubiquitous for oxide conversion reactions.42,43 Using density functional theory (DFT) calculations, we correlate these effects to the dramatically reduced interfacial energy of Li2O/Ni in the presence of excess lithium at the interface. Similar findings were found for a Ni/NiO/Ni trilayer with improved reversibility. These results help to clarify the fundamental origin of overpotentials in metal oxide conversion reactions and demonstrate that, at least in principle, conversion can occur at the theoretical limit.
RESULTS AND DISCUSSION Amorphous Ni and NiO films were grown on annealed Rplane sapphire substrates by pulsed laser deposition (PLD) and measured in a fully immersed, three-electrode cell (Figure S1) at the Advanced Photon Source, sector 12ID-D. The NiO layer was masked on the sides to enable electrical contact to the bottom Ni current collector. Specular X-ray scattering measurements were performed in the Fresnel regime up to a momentum transfer (q) of 1.2 Å−1, providing a vertical spatial resolution of 2.7 Å. Rocking curves measured between each scan (every 0.1 V) showed no change in width (Δθ = 0.04°), suggesting that the lateral structure over the 1 × 3 mm2 footprint of the beam remained homogeneous throughout the reaction. Similarly prepared NiO/Ni interfaces have been shown by TEM to have nearly ideal and uniform interfaces, consistent with the results of the XR analysis of the asdeposited samples.44 Samples were lithiated and delithiated by cyclic voltammetry between 3 and 0.3 V vs Li/Li+ at 0.2 mV/s in 1.0 M LiPF6 in 1:1 ethylene carbonate (EC)/dimethyl carbonate (DMC). Given the extremely small amount of NiO present (0.28 μg), we carefully shielded the working electrode connections to minimize the amount of side reactions with the electrolyte. Fresnel-normalized reflectivity data taken during the lithiation of a 17 Å thick NiO film grown on a 25 Å Ni current collector are shown in Figure 2a. The logarithmic intensity of the XR is denoted by color to highlight structural changes during voltammetry, with data and fits overlaying the plot at select potentials. Data were collected continuously during voltammetry so that each XR scan actually takes place over a 0.09 V range. Fits were performed at potentials where the reflectivity was changing slowly. Changes in the XR data correlate well with the current response of the simultaneously measured cyclic voltammetry, shown in Figure 2b, with structural changes correlating to the two well-defined reduction events centered at 1.7 and 0.6 V. The cyclic voltammograms (CVs) appear to have some capacitive response as well and are shifted to negative currents due to side-reactions related to solid electrolyte interface processes or electrolyte decomposition on the exposed current collector. From the raw data alone, the initiation of the lithation reaction at 1.9 V is observed both with a shift of Kiessig fringes in the XR data to lower q, reflecting the overall vertical expansion of the heterostructure, and with an abrupt increase in scattering power due to heightened interfacial contrast. To quantify these structural changes, we show selected fits to the XR data in Figure 2c. The extracted electron density profiles show that the higher potential phase change is most pronounced at the buried Ni/NiO interface, followed by expansion and lithiation
Figure 1. Comparison of the energy density of intercalation and metal oxide conversion materials. Note that the energy loss in conversion (denoted by arrows) is largely due to the large overpotentials during lithiation, as highlighted for NiO, the subject of this study. Equilibrium (η = 0) potentials are calculated using bulk, experimental enthalpies; experimental capacities and observed lithiation potentials are taken from ref 7.
interfaces between the metal and Li2O products.7 This energy barrier can involve components related to strain imposed by significant volume change upon conversion, transport of electrons and Li ions through the complex network of nanoparticles formed during lithiation, and the intrinsic nucleation barrier required for phase separation. Many groups have improved the overall reversibility and kinetics of conversion by nanoengineering10−12 or with conductive additives,13,14 butwith a few exceptions15the overpotential appears to be intrinsic. In order to understand the origin of the overpotentials and possible contributions from interfaces, nanoscale operando characterization techniques are sorely needed. The spatial resolution of transmission electron microscopy (TEM) has been used to visualize the nucleation and evolution of a conversion reaction.16−18 However, the electrochemical conditions of in situ TEM are often ill-defined and the complex, three-dimensional nature of conversion can obscure the atomic scale properties of the interface. Here we take an alternative route by studying the lithiation of ultrathin ( Eeq (see Figures 2c and 3b). Incorporation of a single lithium monolayer between the Ni(111) substrate and the amorphous Li2O reduces the calculated interfacial energy to approximately 0.41 J/m2. We note that the exact coverage of excess lithium is not known, and the value of one monolayer assumed in the
Figure 4. Schematic of the interfacial and bulk conversion processes during the first lithiation including (a) lithium accumulation, (b) nucleation of conversion products at the buried Ni/NiO interface, (c) formation of Ni and Li2O layers at the buried interface, and (d) bulk three-dimensional conversion. For reference, an electron density profile (similar to those measured in Figure 2) is shown with each picture. In each density profile, the original film is denoted by the black dashed line, and changes in the density profile at each step are highlighted by the representative color of the reacting species.
electronically insulating NiO to the charged Ni surface, forming a solid-state electric double layer. This layer is not visible by XR until ∼2.5 V, possibly due to the finite energy barrier to migrate through the ultrathin NiO. At the equilibrium potential for conversion (1.9 V), we see a sharp transition in the XR data that we associate with the sudden nucleation of a conversion reaction at the Ni/NiO interface (including growth of the bottom nickel layer and subnanometer thick Li2O layer at its interface with the current collector). However, the CVs in each case do not show a similar discontinuity. The more gradual changes in the interfacial structure (near the 1.7 V peak in the CV) suggest that NiO redox may occur more gradually as the Li+ space charge layer is consumed to form a Li2O-rich layer that also blocks interfacial sites for Ni reduction. The formation of this layer appears to create a barrier for interaction between Li+ and e− species, limiting the extent of reaction to 0). The negative interfacial component of ΔG leads to prediction of a barrierless nucleation reaction, as shown in Figure 5c,d. This result implies that the formation of the Li2O/Li/Ni interface is likely to be facile upon introduction of an electrochemical bias, as is also suggested by the interfacial conversion observed by XR at high potentials. We note that the presence of excess lithium appears to be consistent with the formation of a space charge layer that catalyzes the initial conversion reaction at the buried Ni/NiO interface. In addition, we expect that conversion at interfacial defects (e.g., steps, kinks) on the nickel surface is likely to reduce nucleation barriers and overpotentials even further, as previously suggested.49 These barriers are consistent with the observed conversion processes in the region E > 0.6 V and underline the importance of excess lithium in reducing interfacial energies between Ni- and Li2Orich regions of the conversion product. Furthermore, the consumption of the Li+ to form Li2O in this region is likely why this phenomenon is self-limiting until bulk conversion is observed near 0.6 V.
METHODS AND EXPERIMENTAL Film Growth. Nickel/nickel oxide bilayer and trilayer thin films were grown by PLD on the 10 × 3 mm2 R-face surfaces of 1 mm thick sapphire α-Al2O3 (102) substrates (CrysTec GmbH, Germany) using a PLD/MBE 2300 (PLD Products) system in the Northwestern University Pulsed Laser Deposition Facility. The system employed a 248 nm KrF excimer laser with a 25 ns pulse operating at 5 Hz. The laser was focused to a 1.5 × 3.5 mm2 spot size on the targets, which were rotated at 5 rpm to prevent localized heating. The target− substrate separation was fixed at 6 cm. Nickel was deposited from a metallic nickel target at a laser energy of 300 mJ per pulse at the chamber base pressure of ∼5 × 10−7 Torr. There was a 30% energy loss along the optical train to yield an energy density at the target of 4 J/(pulse × cm2). Nickel oxide was deposited from a dense hot-pressed nickel oxide target at an energy of 200 mJ per pulse in a deposition ambient of 5 × 10−4 Torr ultra-high-purity (99.994%) oxygen. There was a 30% energy loss along the optical train to yield an energy density at the target of 2.7 J/(pulse × cm2). The thickness of each deposited layer was controlled by adjusting the number of laser pulses: nickel deposited at 0.01 Å/pulse; NiO deposited at 0.1 Å/
CONCLUSIONS This study enhances the understanding and visualization of the nanoscale interfacial reactions in lithium ion conversion reactions through the use of model NiO thin-film electrodes. We identified an interfacially controlled reaction step in the conversion mechanism, in addition to the known bulk conversion processes. We developed a simple model to E
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bulk where EDFT are the total DFT energy of an interfacial total , ni, and gi model, the number of formula units of compound i, and the per formula unit bulk free energy of compound i, respectively, for an interface with interfacial area A. Structures that included an extra layer of lithium were treated using bulk metallic lithium as reference, since analysis of interfacial electron density suggested a metallic nature for nearly all of the excess lithium in this layer. We used an amorphous Li2O reference state, as described further in the SI.
pulse. The bottom nickel layer of each heterostructure was masked prior to deposition of subsequent layers to form a 3 × 3 mm2 electrical contact with the spring-loaded electrode of the electrochemical cell.32 X-ray Reflectivity. The operando X-ray reflectivity experiments were performed with an X-ray energy of 20.00 keV at the 12ID-D Advanced Photon Source station at Argonne National Laboratory. The X-ray beam was defocused and collimated to 1.0 × 0.18 mm2, and the scattered X-ray pattern was acquired with a Pilatus 100k detector. Full reflectivity data scans were measured in 15 min (every 0.09 V) and were collected repeatedly during electrochemical cycling. Data were analyzed by subtracting the background (linear fit in the sample-χ direction) in each Pilatus image. Reflectivity was normalized by the incident intensity and beam spill-off, which was proportional to q. The electrochemical cell32 had separate lithium metal counter and reference electrodes, and the samples were fully immersed in a 1 M solution of LiClO4 in a 1:1 ratio by volume of EC and DMC. A CHI760E electrochemical workstation was used for electrochemical control of lithiation. Ex situ XR studies of test samples were carried out at a Rigaku ATXG diffractometer (NU X-ray Diffraction Facility) with E = 8.04 keV (λ = 1.54 Å) X-rays collimated to a 0.1 × 2.0 mm2 spot. All XR measurements were performed at ambient laboratory temperature, which ranged between 20 and 25 °C. XR analysis used Motofit52 with a multiple-slab model that included a sapphire substrate, Ni and NiO layers, and an electrolyte (operando) or air (ex situ experiments). Structural parameters for sapphire and the electrolyte were fixed, whereas the parameters for the buffer and active layers (electron density, interface roughness, and layer thickness) were allowed to vary. The electron densities were initially estimated based on the chemical composition of the multilayer electrode components. As discussed in the SI, the errors in the structural parameters and their covariances were used to calculate the uncertainty for the overall density profile (i.e., as a function of height). In most cases, the errors in derived electron density are actually smaller than the line-width used in the plots. Individual layer parameters and their errors are reported in the SI. First-Principles Calculations. Spin-polarized DFT calculations were performed using the Vienna ab Initio Software Package (VASP).53−55 The generalized gradient approximation (GGA) of Perdew−Burke−Ernzerhof (PBE) was used as the exchange− correlation functional.56 Γ-centered k-point grids were used for Brillouin zone sampling, where the product of the number of k-points and the corresponding lattice vector was ∼30 Å. The projectoraugmented wave method57,58 was used to treat the effective core potentials, with Li 1s electrons treated explicitly as valence in all calculations. For interfacial models between Ni and amorphous lithium (a-Li2O), the Kohn−Sham wave functions were expanded in a plane-wave basis set up to kinetic energy of 400 eV. All models containing NiO (interfacial models between NiO/a-Li2O and Ni/ NiO) used a kinetic energy cutoff of 520 eV. Calculations containing NiO used the DFT+U method59−62 with an effective U value of 6.2 eV applied to the Ni 3d electrons.63 A U value of 4.0 eV was applied to the 3d states of metallic Ni for Ni/NiO interfacial models to accurately reproduce experimental formation enthalpies for NiO and LiNiO2 as in previous work.64,65 Further details are provided in the SI. Interfacial energies were calculated for interfaces between Ni/aLi2O, NiO/a-Li2O, and Ni/NiO. For models containing amorphous lithium, an ensemble of structures is constructed through sampling a bulk supercell of a-Li2O. The a-Li2O supercell was generated from a melt and quench approach within classical molecular dynamics simulations performed using the DL_POLY code66 (see SI for further details). Crystalline Ni/NiO interfaces were made using an in-house lattice matching algorithm to minimize strain between Ni(111) and NiO(100). We sampled several lattice matching ratios about the minimum strain model output from the algorithm to ensure the interfacial energy is well optimized. Interfacial energies, γ, between two compounds, α and β, were evaluated using the general formula γ=
1 DFT [Etotal − (nαgαbulk + nβ gβbulk )] 2A
ASSOCIATED CONTENT S Supporting Information *
The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsnano.9b02007. Further details related to the sample cell, data analysis procedures, and theoretical calculations; results for all parameters extracted from fits to the reflectivity (PDF)
AUTHOR INFORMATION Corresponding Author
*E-mail: fi
[email protected]. ORCID
Guennadi Evmenenko: 0000-0002-6580-2237 Handan Yildirim: 0000-0002-3060-7896 Jeffrey P. Greeley: 0000-0001-8469-1715 Maria K. Y. Chan: 0000-0003-0922-1363 Paul Fenter: 0000-0002-6672-9748 Michael J. Bedzyk: 0000-0002-1026-4558 Timothy T. Fister: 0000-0001-6537-6170 Author Contributions
G.E., T.T.F., M.J.B., and P.F. performed the XR measurements and analyzed the data. D.B.B grew the thin films. R.E.W., H.Y., M.K.Y.C., and J.P.G. performed the DFT and MD calculations. All authors were involved in the writing of the manuscript. Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS This work was supported by the Center for Electrochemical Energy Science, an Energy Frontier Research Centre funded by the U.S. Department of Energy, Office of Science, Office of Basic Energy Sciences, under award number DE-AC0206CH11. Use of the Advanced Photon Source and the Center for Nanoscale Materials, both Office of Science user facilities operated for the DOE, Office of Science by Argonne National Laboratory, was supported by the U.S. DOE under Contract No. DE-AC02-06CH11357. Use of the Pulse Laser Deposition and X-ray Diffraction Facilities at Northwestern University are supported by the MRSEC under National Science Foundation grant DMR-1720139. REFERENCES (1) Li, M.; Lu, J.; Chen, Z. W.; Amine, K. 30 Years of Lithium-Ion Batteries. Adv. Mater. 2018, 30, 24. (2) Whittingham, M. S. Lithium Batteries and Cathode Materials. Chem. Rev. 2004, 104, 4271−4301. (3) Bruce, P. G.; Freunberger, S. A.; Hardwick, L. J.; Tarascon, J. M. Li-O-2 and Li-S Batteries with High Energy Storage. Nat. Mater. 2012, 11, 19−29. (4) Poizot, P.; Laruelle, S.; Grugeon, S.; Dupont, L.; Tarascon, J. M. Nano-Sized Transition-Metaloxides as Negative-Electrode Materials for Lithium-Ion Batteries. Nature 2000, 407, 496−499.
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DOI: 10.1021/acsnano.9b02007 ACS Nano XXXX, XXX, XXX−XXX
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DOI: 10.1021/acsnano.9b02007 ACS Nano XXXX, XXX, XXX−XXX
