Direct Evidence of a Chemical Conversion Mechanism of Atomic

Article pubs.acs.org/JPCC

Direct Evidence of a Chemical Conversion Mechanism of AtomicLayer-Deposited TiO2 Anodes During Lithiation Using LiPF6 Salt Matthew R. Charlton,† Anthony G. Dylla,‡ and Keith J. Stevenson*,†,‡ †

Materials Science & Engineering Graduate Program, Texas Materials Institute and ‡Department of Chemistry, The University of Texas at Austin, Austin, Texas 78712, United States S Supporting Information *

ABSTRACT: Titanium dioxide has been identified as a prospective anode material for use in lithium ion batteries. The higher lithiation potential of TiO2 versus other common anodes is more electrochemically compatible with most organic electrolytes, thus leading to reduced solid electrolyte interphase formation and overall more stable battery systems. However, in this study TiO2 has exhibited poor cycling stability with common electrolytes containing lithium hexafluorophosphate (LiPF6) salt. Combined electrochemical and spectroscopic analyses have revealed this to be due to the onset of reversible fluorination of the anode and chemical conversion of TiO2 to TiOF2 during lithiation. Comparison of electrochemical cycling of atomiclayer-deposited TiO2 anodes using LiPF6 with and without a hydrofluoric acid scavenger, tributylamine, to cycling using a nonfluorinated lithium perchlorate (LiClO4) salt indicates that the in situ formation of hydrofluoric acid in the electrolyte from decomposition of the PF6− anion alters the lithiation electrochemistry. X-ray photoelectron spectroscopy (XPS) analysis and time-of-flight secondary-ion mass spectrometry (ToF SIMS) depth profiling measurements confirmed the presence of TiF2 and TiOF surface and bulk phases, respectively, in the electrode upon lithiation.

1. INTRODUCTION Failure of many lithium ion (Li-ion) battery systems can be attributed to degradation as a result of solid electrolyte interphase (SEI) formation, a phenomenon caused by electrochemical incompatibility between electrodes and organic electrolytes.1−4 SEI is composed of organic and inorganic decomposition products of the electrolyte that form and deposit on an electrode whose potential is pushed outside the electrochemical stability window of the electrolyte during lithiation.5−10 Most organic electrolytes used in battery applications, including ethylene carbonate (EC), propylene carbonate (PC), diethyl carbonate (DEC), and dimethyl carbonate (DMC), exhibit electrochemical stability between roughly 1.0−4.5 V versus Li+/Li, depending on specific (electro)chemistry and anode and cathode composition.11,12 Although detrimental SEI production can occur at some high-voltage cathode materials,2 it is mainly a challenge for anode materials, such as graphite used in commercial cells, that lithiate well below 1.0 V.8,13−15 The reduction products that compose SEI on graphite have been well-documented and include insulating organics such as esters, ethers, carbonates, and ketones as well as inorganic components such as LiF.6,7,16−18 The deposited layer can stabilize the lithiation process by passivating the electrode and preventing further electrolyte reduction. However, the presence of organics on the surface slows the kinetics of lithiation by requiring Li+ diffusion through the SEI layer before reaching the electrode. More © XXXX American Chemical Society

importantly, many of these insoluble products (LiF, Li2CO3, etc.) irreversibly trap lithium ions leading to overall capacity fade, one of the main failure mechanisms in Li-ion batteries. Higher voltage anode materials have recently been considered as a way to mitigate the issues related to SEI formation. One such material, TiO2, has been particularly promising.19−22 At 1.4−1.8 V, lithiation of TiO2 is better suited for use in most typical electrolytes.19−21 Therefore, electrolyte decomposition can be minimized and SEI reduced, ultimately leading to better cell stability, improved cyclability, and increased capacity retention. The theoretical capacity for bulk anatase TiO2 (168 mAhg−1) is slightly lower than that of graphite (372 mAhg−1), but potential improvement in lifetime performance over current anode options makes TiO2 an attractive material for study.23 In addition to organic solvents, battery electrolytes typically include some form of Li+-containing salt and often additives to improve performance. Most commercial Li-ion battery electrolytes and many battery studies rely on lithium hexafluorophosphate (LiPF6) because of its higher solubility in organic solvents, increased ionic conductivity, better thermal stability, and better overall safety compared to alternatives.11,12 However, the in situ formation of hydrofluoric acid (HF) Received: August 26, 2015 Revised: November 22, 2015

A

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The Journal of Physical Chemistry C through a reaction between the PF6− anion and any water in the system can occur and affect oxide electrode performance.24 Researchers have commonly used lithium perchlorate (LiClO4) as a replacement for LiPF6 in initial studies with TiO2, even though LiClO4 poses a risk of explosion.25 Although this HF/oxide reaction process has not yet been directly observed in TiO2, it has previously been identified in the V2O5 cathode system, where self-catalytic formation of HF led to a conversion reaction in the cathode during cycling.26 One potential method to address this concern has been to add thin sacrificial layers of oxide that preferentially react with the HF and protect the active material, which Xiong and coworkers demonstrated using V 2 O 5 coatings on LiNiO 2 cathodes.27 However, the question still remains as to how the presence of HF affects the lithiation of these oxide materials from a mechanistic standpoint. In this study, we report a detailed chemical and electrochemical analysis of the lithiation of TiO2 thin films deposited using atomic layer deposition (ALD) with various electrolytes to investigate the effects of HF on the charge-transfer processes. ALD provided a uniform model interface that facilitated the coupling of multiple high-resolution, spatially resolved spectroscopic techniques with electrochemical data, which enabled the elucidation of a chemical conversion mechanism of TiO2 to TiOF2 during lithiation in the presence of LiPF6 electrolyte.

averaged. Modeling of the optical data was carried out using WVASE ellipsometry analysis software (J. A. Woolham Co.). 2.2. Cyclic Voltammetry. Electrochemical cycling of 25 nm thick TiO2 anodes was carried out (CH Instruments CH660D Potentiostat) inside an Ar filled glovebox (MBraun) in a Teflon open-air flood cell. Electrolytes used for sample sets 1, 2, and 3 were prepared as follows: 1 M LiPF6 in 1:1 EC/ DEC (BASF), 50 μL of tributylamine (TBA, Sigma-Aldrich) added to 1 M LiPF6 in 1:1 EC/DEC, and 2.624 g of LiClO4 (Sigma-Aldrich) added to 25 mL of EC/DEC (BASF). Lithium metal was used as the counter and quasi-reference electrodes. CV cycling was carried out at 5 mVs−1. Each electrode designated “delithiated” underwent 3 cycles from 3.4−1.0 V (ending at 3.4 V), was then held at 3.4 V, and rinsed three times in fresh EC/DEC. Each electrode designated “lithiated” underwent 3.5 cycles from 3.4 V−1.0 V (ending at 1.0 V), then was held at 1.0 V, and rinsed three times in fresh EC/DEC. All electrodes underwent a 10 min soft bake in an argon-filled glovebox immediately after rinsing at 40 °C. 2.3. Spectroscopic Analysis. All samples were transferred from the glovebox to the X-ray photoelectron spectroscopy (XPS) and time-of-flight secondary ion mass spectrometry (ToF SIMS) analysis chambers using a reduced oxidation (RoX) interface as described elsewhere.18 The RoX interface prevents sample exposure to ambient air and moisture in the air during transfer. All XPS measurements were taken using a Kratos Axis Ultra DLD XPS system with an Al Kα source. Survey scans of each sample were collected from 0 to 1200 eV binding energy with 1.0 eV resolution, followed by highresolution scans of the C 1s, O 1s, F 1s, and Ti 2p regions at 0.1 eV resolution and 1000 ms dwell time. High-resolution scans of the N 1s region were taken of samples cycled with TBAcontaining electrolyte. High-resolution scans were also taken of the Cl 2p region of the samples cycled in LiClO4 electrolyte. All spectra were charge-corrected relative to the aromatic C 1s component at 284.7 eV binding energy. Analysis of XPS spectra was carried out with CasaXPS software (version 2.3.15, Casa Software Ltd.). Spectra from high-resolution scans were used to estimate atomic composition of the electrode. Fits of decoupled components were made using the sum of multiple Voigt functions composed of 30% Lorentzian and 70% Gaussian profiles optimized using the Simplex method. ToF SIMS analysis was carried out using a TOF.SIMS 5 (ION-TOF GmbH), with a mass resolution better than 8000 (m/Δm). Bi32+ (0.9 pA) accelerated at 30 keV and Cs+ (65 nA) accelerated at 1 keV were used as the primary analysis gun and the secondary sputtering gun, respectively. The depth profiling of the TiO2 electrodes was carried out in static mode where the sputtering gun was engaged for 1.0 s over a 300 × 300 μm2 area. The analysis gun then rastered over 100 × 100 μm2 area centered in the sputtered square. Profiling proceeded into the sample until a strong signal from the Si− ion was detected. Negative ion mode was used to detect secondary ions. Analysis of ToF SIMS spectra was carried out using proprietary IONTOF software (version 6.3).

2. EXPERIMENTAL SECTION 2.1. Electrode Synthesis and Characterization. Electrodes were composed of 1 mm thick quartz slides (Technical Glass Products) as a rigid substrate, a conductive carbon film formed from pyrolysis of spin-coated photoresist (AZ 1518, Clariant Corporation) films28 acting as a current collecting layer, and an atomic-layer-deposited TiO2 top film as the electrochemically active layer. Carbon films (10 nm thick) used in electrodes for electrochemical and spectroscopic analysis were achieved by spin-coating dilute photoresist in a 1:3 ratio of photoresist to 1-methoxy-2-propanol acetate solvent. ALD was carried out in a Savannah S100 deposition system (Cambridge NanoTech). Immediately before deposition, the carbon surface was subjected to a 15 s exposure to air plasma (Harrick Plasma Cleaner, PDC-32G, Ithaca, NY) in order to further oxygenate the surface and promote oxide film nucleation.29 TiO2 was deposited at 200 °C using tetrakis dimethylamido titanium (TDMAT) and water vapor (H2O) as the Ti- and O-containing precursors, respectively, with highpurity N2 as the both the purge and carrier gas. Each ALD cycle consisted of a 0.1 s TDMAT pulse and a 0.015 s H2O pulse, with 10 s N2 purges between each pulse, resulting in a hydroxylated surface. After ALD, the layered electrodes were annealed in air at 400 °C for 1 h. Raman spectroscopy was carried out with a Renishaw In Via microscope system using the backscattering configuration and a 514.5 nm Ar laser as the excitation source. Grazing incidence Xray diffraction (GI-XRD) (Bruker Nokius AXS D8 Advance) was carried out on a TiO2 electrode (deposited with 750 ALD cycles) using a Cu Kα radiation source (1.54 Å) at an angle of incidence of 0.5°. Patterns were background-subtracted using Eva software (Bruker Corporation). Spectroscopic ellipsometry (J. A. Woolham M3000 Spectroscopic Ellipsometer) was carried out at a 45−80° angle of incidence in 5° increments. Five sets of measurements were taken from each electrode and

3. RESULTS AND DISCUSSION 3.1. Film Characterization. Titania thin film electrodes were deposited by ALD onto conductive carbon substrates. Figure 1 shows the Raman spectra of 6, 12, and 19 nm thick films before and after annealing. The peaks at 150, 205, 403, 514, and 636 cm−1, observed in all six samples, are consistent with the characteristic Raman modes for anatase TiO2.30,31 B

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because it was found that incorporation of Ar+ cations in the film artificially reduces the Ti4+ to Ti3+ and complicates interpretation of the results. 3.2. Cyclic Voltammetry. To compare the effects of the different electrolyte chemistries on the lithiation properties of these TiO2 electrodes, mainly the negative aspects of hydrofluoric acid formation, electrodes were cycled using three different electrolyte mixes. LiPF6 in EC/DEC was chosen as the standard electrolyte because of its common use in commercial batteries and other experimental investigations. Electrodes were also cycled in a second electrolyte containing 2000 ppm TBA added to the standard as a HF scavenger. Previously, the presence of TBA in solution has been shown to decrease the F− concentration by almost half.26 The third electrolyte, containing LiClO4 salt in place of LiPF6, was chosen as a non-fluorine-containing control. In preparation for the XPS and ToF SIMS analyses, discussed below, a total of four electrode samples were cycled in each electrolyte, with excellent repeatability. Samples cycled in electrolyte containing LiPF6 alone, LiPF6 with TBA, and LiClO4, will henceforth be referred to as sets 1−3, respectively. Figure 3 shows the resulting CVs

Figure 1. Raman spectra of TiO2 films with characteristic modes labeled. Green, red, and blue lines represent 6, 12, and 19 nm thick films, respectively. Spectra from as-deposited and annealed films are shown in bottom and top panels, respectively.

Small peaks in the spectra from the as-deposited samples at 435 and 610 cm−1 indicate that the deposited films contained some rutile character, possibly stemming from small metastable regions that formed in the film during the low-temperature deposition. Annealing at 400 °C was sufficient to completely transform the films to anatase. The XRD pattern of a 38 nm thick annealed film is shown in Figure 2. The deposited films were XRD amorphous. After

Figure 2. GI-XRD pattern of ALD TiO2 anode. Red lines indicate the standard XRD pattern for the anatase phase with associated crystal orientations noted.

annealing, the films were nanocrystalline anatase32,20 with average crystallite diameter of 14 ± 3 nm for all film thicknesses tested, on the basis of the Scherrer relation.33,34 Film thicknesses were determined via spectroscopic ellipsometry. Films deposited using between 125 and 750 ALD cycles were analyzed, and a growth per cycle rate (GPC) of 0.50 Å/cycle was determined. Conducting tip AFM was used to measure the film surface quality. A 25 nm thick film exhibited a root-mean-square surface roughness of 0.76 ± 0.04 nm. Annealing increased the surface roughness by 54% to 1.17 ± 0.08 nm. XPS confirmed that Ti in annealed ALD TiO2 is composed almost entirely of Ti4+,35−37 with an O/Ti atomic ratio of 2.00:1. High-resolution spectra of the Ti 2p, O 1s, C 1s, and F 1s transitions are shown in Figure S1. Spectra also indicated an adventitious layer composed of 71.0, 23.8, and 5.2 atom % carbon, oxygen, and fluorine, respectively, on the surface of the electrodes, which is typical of materials stored in air. The binding energy of the TiO2 oxygen was centered at 529.9 eV.36,37 Oxygen attributed to surface contamination and surface hydroxide was shifted up by more than 1.2 eV and is easily neglected when calculating the O/Ti ratio. Deconvolved components of the O 1s spectrum can be seen in Figure S1b. Argon sputtering to clean the surface was not carried out

Figure 3. Three CV cycles of TiO2 anodes in electrolytes containing (a) in LiPF6 only, (b) LiPF6 with TBA, and (c) LiClO4. Black, red, and blue lines are the first, second, and third scans, respectively.

taken from one sample from each set. Generally, the lithiation peak can be seen at about 1.6 V, whereas the delithiation peak is found between 2.3 and 3.0 V, depending on the sample. Of the three mixes, set 1 exhibited the most unique electrochemical behavior. TiO2 lithiation at 1.6 V was preceded by a reduction around 2.1 V, which became much less prominent after the first cycle. Recently, Louvain et al. demonstrated that lithiation of TiOF2 occurs at about the same potential (2.3 V).38 The presence of the peak at 2.1 V in Figure 3a suggests that the TiO 2 films might have spontaneously reacted with the electrolyte to chemically form a TiOF2 surface layer prior to cycling. Reversible conversion back to TiO2 during the first delithiation would then cause the subsequent lithiation at 2.1 V to be negligible. Two distinct delithiation potentials were observed at 2.6 and 2.9 V. As cycling progressed, both peaks shifted to become more positive by about 0.1 V, suggesting an increase in the charge transfer resistance of the system, possibly as a result of increased structural disorder in the electrode material after multiple phase changes. C

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The Journal of Physical Chemistry C Sets 2 and 3 exhibited strikingly similar behavior with lithiation and delithiation potentials at 1.6 and 2.3 V, respectively. The CVs are shown in Figure 3b,c, respectively. Both exhibited a prominent secondary reduction peak at 1.25 V, which could be an indicator of the lithium-concentrationdependent phase change. Previous studies have shown that a peak at about 0.4 V lower than the primary lithiation peak during insertion can be associated with the phase transition from tetragonal TiO2 to orthorhombic in order to accommodate greater than 0.5 Li per TiO 2 as lithiation progresses.39−41 3.3. Surface Spectroscopic Chemical Analysis. Figure 4 shows the Ti 2p spectra of the six samples cycled in three

Figure 5. O 1s and F 1s XPS spectra in both the lithiated and delithiated states of anodes cycled in electrolyte with only LiPF6.

Figure 6. O 1s and F 1s XPS spectra in both the lithiated and delithiated states of anodes cycled in electrolyte with LiPF6 and 2000 ppm TBA.

showing fits for individual chemical components for all samples can be found in Figures S3−5. These data were used to calculate atomic ratios for all main elemental constituents of the surfaces of the samples, summarized in Table S1. To calculate the O/Ti ratios of the films themselves, only the O 1s component centered at 529.8 eV,36,37 associated with the bulk TiO2, was compared with the Ti signal. The set 2 samples exhibited near-stoichiometric O/Ti ratios at 1.95:1 and 1.97:1 for the delithiated and lithiated, respectively. Set 1 samples were oxygen-deficient at 1.72:1 and 1.87:1, respectively. In addition to the TiO2 peak at 529.8 eV, three other components were consistently present in the four LiPF6containing samples. The two peaks identified above 532 eV have been attributed to various organic functionalities, including carbonates, esters, ethers, and ketones often found in oligomeric SEI components.7,18,42 The binding energy of the third peak at 531.2 eV, however, is too low to originate from one of these organics. On the basis of results reported by Le and co-workers, this peak might indicate the presence of a titanium oxy-fluoride species at the electrode surface.43 Though lithium is expected to be a major component of these systems, it is often difficult to detect at depth in XPS because of its low photoelectric cross section of analysis. In this study, Li was primarily found by XPS in the form of LiF, a surface product and common inorganic component of SEI, with a Li 1s binding energy of about 55.8 eV.44 The only sample that exhibited Li in another form was the lithiated sample in set 2. It exhibited a second small constituent (about 20% of total Li) centered at 54.5 eV, likely from Li+ inserted in the TiO2 lattice. The strong peaks in the F 1s spectra at 685.0 eV support LiF formation. In the absence of TBA, cycling in LiPF6 resulted in

Figure 4. Ti 2p XPS spectra from TiO2 anodes lithiated and delithiated (dark and light lines, respectively) in varying electrolytes.

electrolytes. Spectra taken of electrodes from sample sets 1−3 are shown in the top, middle, and bottom pairs, respectively, with dark-colored lines representing the lithiated samples and light-colored lines representing the delithiated samples. The peaks centered at 458.6 and 464.4 eV are consistent with Ti 2p3/2 and Ti 2p1/2 of titanium in the Ti4+ oxidation state, respectively.35−37 A downward shift of 2.1 eV from these positions is associated with reduction to Ti3+.36 Fitting of these peaks has indicated that all three delithiated samples are at least 96% in the Ti4+ state, comparable to that of the pristine electrode at 97.6%. Results from the lithiated samples showed that the electrode cycled in LiPF6 alone showed no significant Ti4+ reduction, whereas the electrodes from the set 2 and 3 trials showed a 20.4 and 31.3% Ti4+ to Ti3+ reduction, respectively. Spectra normalized by maximum intensity are presented in Figure S2a,b for clarity. In traditional insertion of Li+ ions into an oxide lattice, the transition metal center accepts an electron and is reduced to accommodate the insertion of a Li+ cation into the structure. Although results from tests with the two alternative electrolyte mixes yield seemingly traditional results, the apparent lack of Ti4+ reduction in set 1 suggests that an alternative lithiation mechanism is involved. This is consistent with the CV data discussed earlier. Figures 5 and 6 show the O 1s and F 1s spectra taken of sets 1 and 2, respectively. Ti 2p, O 1s, C 1s, F 1s, and Cl 2p spectra D

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Figure 8 presents a more detailed look at some of the prevalent anions found at the surface, specifically Li−, LiF−, and

an almost 30% contribution of F signal to the total composition of the sample surfaces, yet adding 2000 ppm of TBA reduced F signal by more than two-thirds, the effects of which are mainly reflected by the reduction of LiF formation. Small amounts (less than 8% of total fluorine) of fluorocarbon compounds, likely polyvinylidene fluoride, were detected on the surfaces of the electrodes, represented in the F 1s spectra by components centered around 688.1 eV.7,45 Two other major components were detected, centered at 685.8 and 686.5 eV, identified as substitutional fluorine in TiO2, leading to TiOxFy and TiF2 formed at the surface of the electrode, respectively. Consequently, introduction of electronegative fluorine into the TiO2 structure might increase the binding energy of Ti3+ in TiOF sufficiently to become indistinguishable from Ti4+ in TiO2 in the Ti 2p XPS spectra, explaining the apparent lack of reduction in anodes lithiated using LiPF6. To test whether the unmodified LiPF6-based electrolyte was spontaneously reacting with the electrodes, an additional sample was prepared for XPS analysis by exposing a pristine TiO2 film to the electrolyte for the time equivalent to three CV cycles, then washing. The resulting F 1s spectrum, found in Figure S6, included components found in the pristine sample (Figure S1d) indicating physisorbed fluorocarbons. The presence of a third prominent peak centered at 685.8 eV, consistent with TiOxFy species, supports chemical conversion of the electrode upon electrolyte exposure. ToF SIMS was used to further interrogate the chemical nature of the surfaces of the electrodes. Figure 7 shows the

Figure 8. ToF SIMS depth profiles of Li-cycled TiO2 electrodes showing selected ion traces. Sets 1 and 2 were cycled in LiPF6containing electrolyte without and with TBA additive, respectively.

TiOF−, which elucidated some key differences between sets 1 and 2. The lithiated sample cycled in standard LiPF6 electrolyte exhibits two regions of concentrated Li: at the surface in the LiF layer, and deeper in the top layer of the TiO2. The latter region is not found in the delithiated state. Additionally, a layer of TiOxFy material was detected at a distinctly different depth from the LiF surface in both samples of set 2. This data provides evidence that the surface of the TiO2 was fluorinated upon cycling. The Li−, LiF−, and TiOF− profiles of the samples of set 2 tracked together, suggesting that the formation of a thinner SEI and any surface TiOxFy formation was confined to the top few atomic layers of the TiO2 which would be expected for a system with a lower concentration of HF available to react with the electrode surface. 3.4. Proposed Conversion Reaction. Electrochemical data indicates that the electrodes in set 1 underwent lithiation chemistry different from that in sets 2 and 3. The reaction undergone by set 1 likely involved significant chemical fluorination of the electrode surface before electrochemical lithiation proceeded as a result of HF formation in the electrolyte. ToF SIMS analysis showed that fluorine penetrated into the TiO2 electrode in those samples and that a layer of TiOxFy exists at the surface of the TiO2 beneath the SEI. XPS spectra showed evidence that TiOF and TiF2 are present. It is reasonable to deduce that a conversion reaction is occurring in tandem with traditional insertion. Although F− is not likely to enter the electrode at reducing potential, HF can freely diffuse. If HF and Li+ coinsertion occurs, then the acid can react with the TiO2, consequently releasing oxygen. The released O2− can then react with the free protons (or hydroxyls) to reform water

Figure 7. ToF SIMS depth profiles of anion markers in a pristine TiO2 anode. Sputter time is analogous to depth from the electrode surface.

depth profile of the major constituents of a pristine electrode. The horizontal axis, representing sputter time, is analogous to depth from the surface, whereas the vertical axis is normalized (by maximum value) ion intensity. The layered electrodes exhibited excellent structural order, with each material interface being coherent and ordered, as illustrated by the sharp transitions from the surface organic layer down through the TiO2 electrode, the carbon conductive layer, and finally into the quartz substrate. Trace levels of contamination from common elements including F, Na, N, and Cl were detected at each interface as well, stemming from exposure to air between each processing step. Depth profiles for each cycled electrode can be found in Figure S7. In set 1 and 2 samples, significant F− penetration into the bulk of the TiO2 was detected. The intensity of Li− at the surface in set 2 was about 20% that of the corresponding samples in set 1, reflecting the reduced presence of LiF in those samples (F− in all samples saturated the detector at max counts at the surface). Results from set 3 confirmed that Cl− was confined to the surface. E

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molecules in a self-catalyzing manner. This phenomenon can be summarized in the following three chemical reactions: n TiOOH + nHF → TiOFn + H 2O 2

(1)

TiOFn + x H 2O → TiOFn − x (OH)x + x HF

(2)

TiO2 + 2HF + e− + Li+ → LiTiOF2 + H 2O

(3)

4. CONCLUSIONS TiO2 electrodes were cycled in three different electrolytes followed by high-resolution surface and depth profiling chemical analysis. Lithiation in an electrolyte containing LiPF6 salt exhibits a self-catalyzing partial chemical conversion charge storage mechanism related to in situ formation of HF and subsequent attack of the electrode material to form lithiated titanium oxy-fluoride via a cochemical conversion reaction and insertion mechanism. Addition of 2000 ppm TBA as an HF scavenger is sufficient to inhibit the effect of HF formation. Anode cycling stability and performance increases dramatically upon suppression of chemical conversion reactions of TiO2 anodes to form LiTiOF2 phases. ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.jpcc.5b08298. High-resolution XPS spectra of a pristine planar TiO2 electrode surface, intensity-normalized spectra comparing the Ti 2p transitions of lithiated and delithiated electrodes, high-resolution deconvolved spectra of selected elements taken from electrodes that were delithiated and lithiated in each of three different electrolyte mixes, and surface compositions of each electrode sample as determined by XPS analysis. (PDF)

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REFERENCES

(1) Aurbach, D.; Zinigrad, E.; Cohen, Y.; Teller, H. A Short Review of Failure Mechanisms of Lithium Metal and Lithiated Graphite Anodes in Liquid Electrolyte Solutions. Solid State Ionics 2002, 148, 405−416. (2) Patil, A.; Patil, V.; Wook Shin, D.; Choi, J.-W.; Paik, D.-S.; Yoon, S.-J. Issue and Challenges Facing Rechargeable Thin Film Lithium Batteries. Mater. Res. Bull. 2008, 43, 1913−1942. (3) Srinivasan, V. Batteries for Vehicular Applications. In Physics of Sustainable Energy : Using Energy Efficiently and Producing It Renewably; Hafemeister, D. W., Ed.; AIP Conference Proceedings, Berkeley, California, March 1−2, 2008; American Institute of Physics: Melville, NY, 2008. (4) Wagner, M. R.; Albering, J. H.; Moeller, K.-C.; Besenhard, J. O.; Winter, M. XRD Evidence for the Electrochemical Formation of Li +(PC)yCn- in PC-Based Electrolytes. Electrochem. Commun. 2005, 7, 947−952. (5) Yeo, J.-S.; Park, T.-H.; Seo, M.-H.; Miyawaki, J.; Mochida, I.; Yoon, S.-H. Solid Electrolyte Interphase Formation Behavior on WellDefined Carbon Surfaces for Li-Ion Battery Systems. Electrochim. Acta 2012, 77, 111−120. (6) Arreaga-Salas, D. E.; Sra, A. K.; Roodenko, K.; Chabal, Y. J.; Hinkle, C. L. Progression of Solid Electrolyte Interphase Formation on Hydrogenated Amorphous Silicon Anodes for Lithium-Ion Batteries. J. Phys. Chem. C 2012, 116, 9072−9077. (7) Verma, P.; Maire, P.; Novák, P. A Review of the Features and Analyses of the Solid Electrolyte Interphase in Li-Ion Batteries. Electrochim. Acta 2010, 55, 6332−6341. (8) Kong, F.; Kostecki, R.; Nadeau, G.; Song, X.; Zaghib, K.; Kinoshita, K.; McLarnon, F. In Situ Studies of SEI Formation. J. Power Sources 2001, 97−98, 58−66. (9) Goodenough, J. B.; Park, K.-S. The Li-Ion Rechargeable Battery: A Perspective. J. Am. Chem. Soc. 2013, 135, 1167−1176. (10) Kaskhedikar, N. a.; Maier, J. Lithium Storage in Carbon Nanostructures. Adv. Mater. 2009, 21, 2664−2680. (11) Scrosati, B., Abraham, K., Van Schalkwijk, W., Hassoun, J., Eds. Lithium Batteries: Advanced Technologies and Applications; Wiley: Hoboken, NJ, 2013. (12) Xu, K. Nonaqueous Liquid Electrolytes for Lithium-Based Rechargeable Batteries. Chem. Rev. 2004, 104, 4303−4417. (13) Endo, M.; Kim, C.; Nishimura, K.; Fujino, T.; Miyashita, K. Recent Development of Carbon Materials for Li Ion Batteries. Carbon 2000, 38, 183−197. (14) Flandrois, S.; Simon, B. Carbon Materials for Lithium-Ion Rechargeable Batteries. Carbon 1999, 37, 165−180. (15) Billaud, D.; Henry, F. X.; Willmann, P. Dependence of the Morphology of Graphitic Electrodes on the Electrochemical Intercalation of Lithium Ions. J. Power Sources 1995, 54, 383−388. (16) Aurbach, D. The Surface Chemistry of Lithium Electrodes in Alkyl Carbonate Solutions. J. Electrochem. Soc. 1994, 141, L1−L3. (17) Dedryvère, R.; Gireaud, L.; Grugeon, S.; Laruelle, S.; Tarascon, J.-M.; Gonbeau, D. Characterization of Lithium Alkyl Carbonates by X-Ray Photoelectron Spectroscopy: Experimental and Theoretical Study. J. Phys. Chem. B 2005, 109, 15868−15875. (18) Schroder, K. W.; Celio, H.; Webb, L. J.; Stevenson, K. J. Examining Solid Electrolyte Interphase Formation on Crystalline Silicon Electrodes: Influence of Electrochemical Preparation and Ambient Exposure Conditions. J. Phys. Chem. C 2012, 116, 19737− 19747. (19) Berger, T.; Monllor-Satoca, D.; Jankulovska, M.; Lana-Villarreal, T.; Gómez, R. The Electrochemistry of Nanostructured Titanium Dioxide Electrodes. ChemPhysChem 2012, 13, 2824−2875. (20) Dambournet, D.; Belharouak, I.; Amine, K. Tailored Preparation Methods of TiO2 Anatase, Rutile, Brookite: Mechanism of Formation and Electrochemical Properties. Chem. Mater. 2010, 22, 1173−1179. (21) Dylla, A. G.; Lee, J. A.; Stevenson, K. J. Influence of Mesoporosity on Lithium-Ion Storage Capacity and Rate Performance of Nanostructured TiO2(B). Langmuir 2012, 28, 2897−2903.

The process is initiated when HF reacts with the hydroxylterminated oxide surface before lithiation begins. After cycling begins, electrochemical reduction and oxidation of the electrode drives the incorporation and release of F− ions into and out of the electrode. eq 3 is a summary of the entire conversion process. It is likely that TiO2 in LiPF6/EC/DEC is an example of a system with altered lithiation chemistry from mixed insertion and chemical conversion mechanisms.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This material is based upon work supported as part of the program “Understanding Charge Separation and Transfer at Interfaces in Energy Materials (EFRC:CST)”, an Energy Frontier Research Center funded by the U.S. Department of Energy, Office of Science, Office of Basic Energy Sciences under award no. DE-SC0001091. K.J.S. acknowledges the Welch Foundation (Grant F-1529). F

DOI: 10.1021/acs.jpcc.5b08298 J. Phys. Chem. C XXXX, XXX, XXX−XXX

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The Journal of Physical Chemistry C (22) Hardwick, L. J.; Holzapfel, M.; Novák, P.; Dupont, L.; Baudrin, E. Electrochemical Lithium Insertion into Anatase-Type TiO2: An In Situ Raman Microscopy Investigation. Electrochim. Acta 2007, 52, 5357−5367. (23) Dylla, A. G.; Henkelman, G.; Stevenson, K. J. Lithium Insertion in Nanostructured TiO(2)(B) Architectures. Acc. Chem. Res. 2013, 46, 1104−1112. (24) Barlowz, C. G. Reaction of Water with Hexafluorophosphates and with Li Bis(perfluoroethylsulfonyl)imide Salt. Electrochem. SolidState Lett. 1999, 2, 362−364. (25) Vincent, C. A.; Scrosati, B. Modern Batteries: An Introduction to Electrochemical Power Sources; E. Arnold: London, 1997. (26) Wu, J.; Membreno, N.; Yu, W.; Wiggins-Camacho, J. D.; Flaherty, D. W.; Mullins, C. B.; Stevenson, K. J. Influence of Hydrofluoric Acid Formation on Lithium Ion Insertion in Nanostructured V2O5. J. Phys. Chem. C 2012, 116, 21208−21215. (27) Xiong, X.; Wang, Z.; Yan, G.; Guo, H.; Li, X. Role of V2O5 Coating on LiNiO2-Based Materials for Lithium Ion Battery. J. Power Sources 2014, 245, 183−193. (28) Kostecki, R.; Schnyder, B.; Alliata, D.; Song, X.; Kinoshita, K.; Kotz, R. Surface Studies of Carbon Films from Pyrolyzed Photoresist. Thin Solid Films 2001, 396, 36−43. (29) Loussaert, J. A.; Fosdick, S. E.; Crooks, R. M. Electrochemical Properties of Metal-Oxide-Coated Carbon Electrodes Prepared by Atomic Layer Deposition. Langmuir 2014, 30, 13707−13715. (30) Mathews, N. R.; Morales, E. R.; Cortés-Jacome, M. A.; Toledo Antonio, J. A. TiO2 Thin Films − Influence of Annealing Temperature on Structural, Optical and Photocatalytic Properties. Sol. Energy 2009, 83, 1499−1508. (31) Orendorz, A.; Brodyanski, A.; Lösch, J.; Bai, L. H.; Chen, Z. H.; Le, Y. K.; Ziegler, C.; Gnaser, H. Phase Transformation and Particle Growth in Nanocrystalline Anatase TiO2 Films Analyzed by X-Ray Diffraction and Raman Spectroscopy. Surf. Sci. 2007, 601, 4390−4394. (32) Powder Diffraction File 01-078-2486, International Centre for Diffraction Data, 1961. (33) Patterson, A. The Scherrer Formula for X-Ray Particle Size Determination. Phys. Rev. 1939, 56, 978−982. (34) Scherrer, P. Determining the Size and the Internal Structure of Colloidal Particles by Means of X-Rays. Göttingen News Life 1918, 2, 98−100. (35) Kuznetsov, M. V.; Zhuravlev, J. F.; Zhilyaev, V. A.; Gubanov, V. A. XPS Study of the Nitrides, Oxides and Oxynitrides of Titanium. J. Electron Spectrosc. Relat. Phenom. 1992, 58, 1−9. (36) Sodergren, S.; Siegbahn, H.; Rensmo, H.; Lindstrom, H.; Hagfeldt, A.; Lindquist, S. Lithium Intercalation in Nanoporous Anatase TiO2 Studied with XPS. J. Phys. Chem. B 1997, 101, 3087− 3090. (37) Lange, F.; Schmelz, H.; Knözinger, H. An X-Ray Photoelectron Spectroscopy Study of Oxides of Arsenic Supported on TiO2. J. Electron Spectrosc. Relat. Phenom. 1991, 57, 307−315. (38) Louvain, N.; Karkar, Z.; El-Ghozzi, M.; Bonnet, P.; Guérin, K.; Willmann, P. Fluorination of Anatase TiO2 towards Titanium Oxyfluoride TiOF2: A Novel Synthesis Approach and Proof of the Li-Insertion Mechanism. J. Mater. Chem. A 2014, 2, 15308. (39) Wagemaker, M.; Borghols, W. J. H.; Mulder, F. M. Large Impact of Particle Size on Insertion Reactions. A Case for Anatase Li(x)TiO2. J. Am. Chem. Soc. 2007, 129, 4323−4327. (40) Cava, R. J.; Murphy, D. W.; Zahurak, S.; Santoro, A.; Roth, R. S. The Crystal Structures of the Lithium-Inserted Metal Oxides Li0.5TiO2 Anatase, LiTi2O4 Spinel, and Li2Ti2O4. J. Solid State Chem. 1984, 53, 64−75. (41) Zakharova, G. S.; Jahne, C.; Popa, A.; Taschner, C.; Gemming, T.; Leonhardt, A.; Buchner, B.; Klingeler, R. Anatase Nanotubes as an Electrode Material for Lithium-Ion Batteries. J. Phys. Chem. C 2012, 116, 8714−8720. (42) Beamson, G.; Briggs, D. High Resolution XPS of Organic Polymers: The Scienta ESCA300 Database; Wiley: Chichester, U.K., 1992.

(43) Le, T. K.; Flahaut, D.; Martinez, H.; Pigot, T.; Nguyen, H. K. H.; Huynh, T. K. X. Surface Fluorination of Single-Phase TiO2 by Thermal Shock Method for Enhanced UV and Visible Light Induced Photocatalytic Activity. Appl. Catal., B 2014, 144, 1−11. (44) Kanamura, K.; Tamura, H.; Shiraishi, S.; Takehara, Z.-I. XPS Analysis for the Lithium Surface Immersed in Γ-Butyrolactone Containing Various Salts. Electrochim. Acta 1995, 40, 913−921. (45) Aurbach, D.; Markovsky, B.; Shechter, A.; Ein-Eli, Y.; H, C. A Comparative Study of Synthetic Graphite and Li Electrodes in Electrolyte Solutions Based on Ethylene Carbonate-Dimethyl Carbonate Mixtures. J. Electrochem. Soc. 1996, 143, 3809−3820.

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DOI: 10.1021/acs.jpcc.5b08298 J. Phys. Chem. C XXXX, XXX, XXX−XXX