Nonequilibrium Phase Transitions in Amorphous and Anatase TiO2

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Non-Equilibrium Phase Transitions in Amorphous and Anatase TiO Nanotubes Andrea Auer, Dominik Steiner, Engelbert Portenkirchner, and Julia Kunze-Liebhäuser ACS Appl. Energy Mater., Just Accepted Manuscript • DOI: 10.1021/acsaem.7b00319 • Publication Date (Web): 18 Apr 2018 Downloaded from http://pubs.acs.org on April 19, 2018

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ACS Applied Energy Materials

Non-Equilibrium Phase Transitions in Amorphous and Anatase TiO2 Nanotubes Andrea Auer,† Dominik Steiner,† Engelbert Portenkirchner,† and Julia Kunze-Liebhäuser†,* †

Institute of Physical Chemistry, Leopold-Franzens-University Innsbruck, Innrain 52c,

Innsbruck 6020, Austria KEYWORDS. Li-ion intercalation, self-organized anodic TiO2 nanotubes, anode material, phase transition, bulk lithiation

ABSTRACT.

The electrochemical lithiation/ delithiation behavior of self-organized amorphous and anatase titanium dioxide (TiO2) nanotubes (NTs) is analyzed by means of electrochemical impedance spectroscopy (EIS) and X-ray photoelectron spectroscopy (XPS). The bulk lithiation properties are governed by the different phase transitions in amorphous and anatase TiO2. While in the case of amorphous nanotubes the phase transition only leads to a thermodynamic limitation of the bulk Li content, it additionally limits the lithiation kinetics for the anatase case. This kinetic constraint is found to originate from under-lithiation of the anatase TiO2-x bulk caused by the instant first phase transition during lithium insertion. Together with the surface lithiation properties, it leads to different lithiation characteristics. Amorphous nanotubes are characterized

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by a reversible surface chemistry and thus pseudocapacitive lithiation/ delithiation behavior. As a result, amorphous TiO2 nanotubes show higher overall capacities due to the contribution of surface lithiation, higher capacity retention, higher rate capability, and higher coulombic efficiencies at high C-rates, even though at the lowest applied lithiation potential of 1.1 V, slightly more lithium is inserted into the bulk of anatase TiO2-x nanotubes under quasi steadystate conditions.

1. Introduction: For several years, research on anode materials in lithium (Li) ion batteries has been addressed to titanium dioxide (TiO2), which turned out to provide important advantages in terms of cost effectiveness, safety and environmental compatibility.1,2 The potential for Li intercalation into TiO2 lies between 1.2 and 2.0 V versus Li, which is very desirable with regard to safety, since most organic electrolytes do not strongly decompose in this potential range. The theoretical capacity of TiO2 with amorphous or anatase structure is 335 mAh g-1, which corresponds to 1 mole inserted Li per mole TiO2.1 Reported capacities usually correspond to less than 1 mole Li per mole TiO2 (x ≤ 1 in LixTiO2), where full lithiation can only be achieved in particles smaller than 10 nm in diameter3,4 and in nanostructured anatase.4–8 Full lithiation for bulk materials is understood to be prevented through changes of diffusion coefficients during lithium intercalation.9,10 Upon lithiation to Li0.55TiO2, anatase is observed to undergo a tetragonal to orthorhombic phase transition. This composition is also most frequently reported as the maximum electrochemical insertion limit of Li into bulk anatase, although concentrations as high as x=0.6 have been reported.11–13


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In contrast to this, several studies14–16 report extraordinarily high capacities of up to 810 mAh g-1 on the first discharge of amorphous TiO2, in LiPF6 in ethylene carbonate (EC) /dimethyl carbonate (DMC) electrolyte, which corresponds to Li2.42TiO2.14 Upon continuous cycling, this capacity gradually decreases to 120 – 230 mAh g-1 after 50 cycles and C-rates from 10C to C/10.14,17 The origin of this irreversible capacity loss has been explained by side reactions where Li+ ions are consumed due to the reaction with H2O/OH species adsorbed at the TiO2 surface to Li2O.14,18 TiO2, in form of self-organized and well-oriented nanotube (NT) arrays,20 is expected to promote one dimensional electronic and ionic conduction due to short diffusion pathways for electrons and Li-ions. The simple way of nanostructuring by anodic oxidation of the parent Ti metal in fluoride containing electrolytes20 makes self-organized TiO2 NTs highly attractive as model anode material in Li-ion battery applications. Thermal treatment at 400 °C under argon atmosphere, converts the as-grown amorphous anodic TiO2 NTs to oxygen-deficient anatase TiO2-x (x < 2) NTs.21 The presence of oxygen vacancies is believed to enhance the chargetransfer properties of these electrodes and to facilitate the phase transition during Li-ion intercalation/de-intercalation, which leads to higher Li-ion intercalation capacity and rate capability when compared to stoichiometric anatase NTs.8,22–24 Self-organized, amorphous TiO2 NTs frequently show electrochemical lithiation capacities larger than 1 mole Li per mole TiO2 (x > 1 in LixTiO2) upon the first discharge,16,19,25,26 which is not the case for anatase TiO2 NTs.8,17,19,27 In Li-ion battery research, electrodes consisting of self-organized TiO2 NT arrays are considered model anodes, because either their nanotubular morphology or bulk structure can be adjusted for the study of different phenomena through changes of the anodization and annealing conditions.


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We recently reported on the influence of the surface film formation on both amorphous and anatase NT arrays with proceeding lithiation of the active material.19 Although it could be shown that the higher reversibility of the surface chemistry is crucial for the higher overall capacity of the amorphous TiO2 NTs, the role of the different bulk lithiation processes, accompanied by different phase transitions, in amorphous and anatase TiO2 materials, is mostly unknown. The aim of this paper is therefore to verify and to fundamentally understand the Li to Ti ratios in the bulk found at different stages of electrochemical lithiation of amorphous TiO2 and anatase TiO2-x NTs. The intrinsic thermodynamic and kinetic properties of the Li intercalation process in the two different NT arrays are determined and compared, using ex-situ emersion X-ray photoelectron spectroscopy (XPS) and in-situ electrochemical impedance spectroscopy (EIS), cyclic voltammograms (CVs) and galvanostatic cycling techniques. For this purpose, thin-walled amorphous TiO2 NTs are investigated and compared to anatase TiO2-x NTs with the same geometry. 2. Results and Discussions Scanning electron microscopy was performed to image the morphology of both amorphous and anatase NT samples and the micrographs with the corresponding dimensions are shown in Figure S1. The apparent identical morphology of the two different structures allows for an accurate and direct comparison of the electrochemical behavior.

Cyclic voltammetry and galvanostatic

cycling was used to explore the electrochemical lithiation/ delithiation properties of both materials. Figure 1 depicts CVs and galvanostatic discharge curves of amorphous TiO2 and anatase TiO2-x NTs. For amorphous NTs a broad symmetrical wave (~1.8 V) can be observed in


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the CV, which points to pure pseudocapacitive behavior, where the lithium insertion/extraction kinetics are not limited by solid-state diffusion of the Li ions.28 Therefore, higher diffusion coefficients are expected for amorphous TiO2 NTs, which causes higher Coulombic efficiency and rate capability. The higher disorder and the larger number of defects in amorphous TiO2 can cause the higher Li-diffusion coefficients.17 An additional peak at around 1.1 V is indicative for a phase transition of a fraction of the active amorphous material to cubic Li2Ti2O4.16,19 This phase transition can also be observed by a plateau in the galvanostatic lithiation curve and appears to be irreversible, since it is only visible in the first cycle. In comparison, for anatase TiO2-x NTs, a sharp peak pair appears in the CV at 1.72/2.03 V. This peak pair represents a phase transition from a Li poor LixTiO2 (0.01 < x ≤ 0.21)29 phase with anatase structure to the orthorhombic Lititanate (Li~0.55TiO2) phase; their positions are in good agreement with those reported in the literature.8,13,29-30 Furthermore for the case of anatase NTs, a second, much smaller peak pair is visible at ~1.46/1.74 V, representing a second phase transition from Li~0.55TiO2 to fully lithiated LiTiO2, which can only proceed in crystallites smaller than 10 nm.3,4,8,19 In the galvanostatic discharge curve of anatase, these phase transitions appear as potential plateaus.


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Figure 1. CVs (scan rate: 0.05 mV s-1) of A) amorphous TiO2 NTs and B) anatase TiO2-x NTs. Galvanostatic lithiation (1st cycle) of C) amorphous TiO2 NTs and D) anatase TiO2-x NTs to 1.1 V with C/20. Figure 2 shows X-ray diffractograms of amorphous TiO2 and anatase TiO2-x NTs after lithiation to 1.1 V. In both cases, the Ti metal substrate peaks are clearly identifiable. In case of anatase NTs, the peaks labeled A(101) (2θ = 25.2°) , A(004) (2θ = 37.9°) , A(200) (2θ = 47.8°) , and A(105)/A(211) (2θ = 54.9°) indicate that the active oxide material has anatase structure (Figure 2B). A small peak at 2θ = 43.8° (Figure 2A,C) that only occurs after lithiation of amorphous TiO2, is indicative for the presence of the cubic Li2Ti2O4 (pdf card no. 01-077-1389) phase.16


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Figure 2. X-ray diffractograms of A) amorphous TiO2 NTs after lithiation to 1.1 V and B) anatase TiO2-x NTs after annealing at 450 °C. C) Comparison of pristine and lithiated amorphous TiO2 NTs. Prolongated cycling at different C-rates is depicted in Figure 3. Amorphous TiO2 NTs show higher capacity, higher capacity retention, higher rate capability, and higher coulombic efficiencies at high C-rates in comparison to anatase TiO2-x NTs.


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Figure 3. Galvanostatic charge-discharge curves of A) amorphous TiO2 NTs and B) anatase TiO2-x NTs recorded at different C-rates (C/20, C/2, 1.5C and 7.5C) between 3.0 and 1.1 V. Gravimetric capacities (squares) and corresponding coulombic efficiencies (circles) of C) amorphous TiO2 NTs and D) anatase TiO2-x NTs as a function of the cycle number, measured at different C-rates. Closed squares correspond to discharge (lithiation) and open squares to charge (delithiation) cycles. It has been previously shown that surface film formation is an important factor for the better lithiation/ delithiation performance of amorphous over anatase TiO2 NTs.19 Bulk lithiation has been found to strongly depend on the applied potential as well. Since the lithiation of TiO2 is accompanied by the reduction of Ti4+ to Ti3+, the Ti3+ fraction, detectable with XPS, is a measure for the degree of lithiation of the oxide bulk. Figure 4 shows high resolution XPS spectra of amorphous TiO2 NTs and anatase TiO2-x NTs that allow to deduce the amount of lithiated active material through evaluation of the Ti3+ fraction after lithiation at different potentials. Bulk lithiation is partially reversible for amorphous and anatase NTs, however the irreversibilities are locally much more pronounced for anatase NTs.19


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Figure 4. High resolution XPS spectra (Ti 2p signal) of A) amorphous TiO2 NTs and B) anatase TiO2-x NTs. Grey lines show the individual component fits and the red and dark grey lines the total fits of the spectra. Galvanostatic discharge curves (1st cycle) for C) amorphous TiO2 NTs and D) anatase TiO2-x NTs (C/20). The points in C) and D) mark the positions of the XPS measurements. The overall intensity of the Ti 2p signal depicted in Figure 4 decreases with decreasing potential due to the increasing amount of a surface film formed on top of the active material, confirming the findings in ref. 19. At the same time, the Ti3+ fraction increases which shows an increasing Li content in the TiO2 bulk. To quantify this observation and to compare the bulk lithiation


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properties of amorphous and anatase NTs, the Ti3+ and the Ti4+ fractions are plotted as functions of the potential, which is depicted in Figure 5.

Figure 5. Ti3+ (closed circles) and the Ti4+ (open circles) fractions of amorphous (red) and anatase (black) NTs plotted versus the lithiation potentials of amorphous (red, upper x-axis) and anatase (black, lower x-axis) NTs. ‘pristine’: NT composition prior to exposure to electrolyte; ‘OCP’: NT composition at the open circuit potential (OCP); ‘3.0 V’: NT composition after full lithiation (1.1 V) and full delithiation (3.0 V). Solid lines: guides for the eye without specific physical meaning. The Ti3+ fraction for both pristine materials is practically zero, while after contact with the electrolyte at OCP, a small amount of Ti3+ (2.7 % for amorphous, 0.7 % for anatase NTs) is measured. For amorphous TiO2 NTs, the Ti3+ fraction steadily increases with decreasing potential until it reaches a value of 40.1 % when the potential reaches the plateau. This value stays constant until 1.1 V, without showing a further increase. Therefore, in amorphous NTs, in


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the present case, the highest measurable capacity related to bulk lithiation at 1.1 V is limited to 135.2 mAh g-1 (Li0.40TiO2). After delithiation at 3.0 V, 6.1% of Li remains inside the amorphous TiO2 NT bulk. In case of anatase TiO2-x NTs, almost no Li enters the bulk until the beginning of the phase transition. With the start of the phase transition, the Ti3+ fraction rapidly increases from 0.8 to 31.1 %. It then undergoes only small changes (from 31.1 to 33.7 %) and stays practically constant until 1.4 V. After lithiation to 1.1 V, a further increase of the Ti3+ fraction to 50.5 % is observed. Therefore, in anatase NTs, in this study, the highest measurable capacity related to bulk lithiation at 1.1 V is limited to 167.5 mAh g-1 (Li0.51TiO2). After delithiation at 3.0 V, 11.3 % of Li remain inside the anatase TiO2-x NT bulk. It is remarkable that almost no change in Li content occurs during and shortly after the phase transitions in both materials. This is most likely due to an instant phase boundary movement which has been reported for anatase TiO2, causing a non-equilibrium, and thus under-lithiation in the material’s bulk.31 The fact that also for amorphous NTs and its phase transition to cubic Li2Ti2O4 the same effect is visible indicates a similar behavior. The non-equilibrium has been reported to strongly depend on the particle size of the electrode. Smaller particle sizes lead to faster kinetics due to the absence of coexisting phases and no need of ionic transport in between the particles.31 Therefore, it is assumable that in amorphous nanostructured systems such phase transitions are kinetically favored. For a better understanding of the observed phenomena, the electrical properties of the two NT systems during lithiation and delithiation are investigated with EIS in the potential range from 3.0 to 1.1 V. Figure 6 shows some exemplary Nyquist plots and the two equivalent circuits (EEC) used to fit the EIS data.32,33 A detailed description of the EECs used can be found in ref. 19 and 33. Additional Nyquist and Bode plots are given in Figure S2 and Figure S3 in the SI.


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Figure 6. Nyquist plots for A) amorphous TiO2 NTs and B) anatase TiO2-x NTs. The open circles show the experimental data, while the lines represent the fits. Equivalent circuits used for fitting the EIS data for C) amorphous TiO2 NTs and D) anatase TiO2-x NTs. Rs: solution resistance, Rf: surface film resistance, Cf: corresponding capacitance (modelled with a constant phase element (CPE)), Rfdiff, Cfdiff: diffusion resistance and capacitance in the surface film, Cdl: double layer capacitance of the surface film/electrode interface, Rct: charge transfer resistance, Zw: Warburg element, Cint: low-frequency internal chemical capacitance.


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Figure 7. Electrical parameters fitted from the EIS data of amorphous TiO2 NTs (red) and anatase TiO2-x NTs (black): A) charge transfer resistance Rct, B) zoom of Rct for amorphous NTs C) double layer capacitance Cdl, D) internal charge capacitance Cint for lithiation (closed circles) and delithiation (open circles). Solid lines: guides for the eye without specific physical meaning. Figure 7 depicts the electrical parameters extracted from fitting the impedance data. The charge transfer resistance Rct corresponds to a transfer of Li+ ions from the surface film to the TiO2 NT and vice versa. In case of anatase TiO2-x NTs, Rct is highly potential dependent and shows a minimum at ~1.90 – 1.82 V where the phase transition takes place. In case of amorphous TiO2 NTs, lower Rct values than for anatase NTs are recorded, and a broader minimum at ~2.27 – 1.60 V is measured. The curve is more symmetrical which indicates a higher reversibility of the


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lithiation/ delithiation reaction (see Figure 7B). The double layer capacitance Cdl goes through a maximum which is approximately located in the potential range where Rct is minimal. It is much higher in case of amorphous TiO2 NTs. Figure 7c depicts the potential dependence of the internal chemical capacitance Cint, which accounts for the accumulation of Li+ ions in the bulk.34,35 This plot of the internal capacitance Cint versus potential resembles the lithiation/ delithiation CVs for both anatase and amorphous NTs. For anatase TiO2-x NTs, a sharp peak pair at 1.78/1.86 V at the first phase transition is measured, while a small peak at 1.51 V is recorded at the second phase transition. Even for the quasi stationary conditions of the EIS experiment, a small hysteresis remains.33 The origin of this hysteresis can be interpreted as an effect arising from the characteristics of a many-particle electrode.36,37 The charge measured for the electrochemical lithiation process from Cint corresponds to a composition of Li0.03TiO2 (integration until 1.4 V) which is not in agreement with XPS (Li0.33TiO2). This is most likely due to the previously discussed instant phase boundary movement causing non-equilibrium and thus under-lithiation,31 which leads to a non-accurate determination of the lithiation charge from EIS. For amorphous TiO2 NTs, a broad peak at 1.34 – 2.49 V is observed. The lithiation/ delithiation process appears to be completely reversible (no hysteresis), and the composition derived from the charge (integration until 1.4 V) of Li0.19TiO2 is in reasonable agreement with that measured with XPS (Li0.15TiO2) at that potential. This shows that extracted charges are only accurate in case of a quasi-stationary state which is only reached for amorphous NTs at the respective potentials of interest. In case of anatase NTs, the expected Li content can only be reached by further lithiation after the phase transition due to the under-lithiation caused by the non-equilibrium. This effect is so pronounced that it leads to errors when the degree of bulk lithiation is extracted from charge measurements, which is not observed for amorphous NTs. At the same time, it is possible to


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reach a higher lithium content in the anatase bulk (Li0.51TiO2), after lithiation at 1.1 V compared to the amorphous system (Li0.40TiO2). This shows that under-lithiation during the phase transition takes place in amorphous TiO2 NTs as well, but since the corresponding phase transition is irreversible and thus only effective in the first cycle, it does not kinetically hinder the lithiation reaction of the amorphous NTs. 3. Summary and Conclusions In summary, amorphous TiO2 nanotubes show higher overall capacities due to the contribution of surface lithiation, higher capacity retention, higher rate capability, and higher coulombic efficiencies at high C-rates. At the same time, at the lowest applied lithiation potential of 1.1 V, slightly more lithium is detected in the anatase TiO2-x NT bulk after lithiation under quasi steadystate conditions. This observation can be understood considering the different phase transitions in amorphous and in anatase TiO2. The internal chemical capacitance Cint, extracted from EIS, together with the XPS results of the bulk lithiation properties show that in both systems the lithium content stays constant during the phase transitions. Comparison of the compositions extracted from Cint (Figure 7c) and from XPS (Figure 5) reveals that in the potential range of interest only anatase TiO2-x NTs can reach their expected Li content (Li0.5TiO2), while the amorphous NTs only reach a composition of Li0.40TiO2. The reason for this is expected to be the lower possible lithiation potential that can be applied after the phase transition in anatase compared to that applicable to amorphous NTs. In the frame of this study, it was not possible to detect the Li content in the bulk of amorphous TiO2 NTs at lower potentials after the phase transition, e.g., 0.8 V, because the thick surface films deposited on top of the active material prohibit XPS measurements of the TiO2 bulk.


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We conclude that in the case of amorphous NTs, the phase transition only leads to a thermodynamic limitation of the bulk Li content, whereas it additionally limits the lithiation kinetics for anatase NTs. Reason for this is most likely the instant first reversible phase transition during lithium insertion in the anatase TiO2-x bulk leading to under-lithiation of the material. In addition, the lithiation reaction of amorphous NTs has been shown to be governed by highly reversible surface lithiation and thus pseudocapacitive lithiation/delithiation behavior,19 which is confirmed in this study. The lower Rct can be expected to lead to faster Li insertion/extraction and Li diffusion and thus to faster kinetics measured during prolonged cycling at different Crates.

4. Experimental A detailed description of all the experimental procedures is given in the SI.

ASSOCIATED CONTENT Supporting Information. SEM images of amorphous and anatase NTs, Nyquist and Bode plots of the impedance data and a detailed description of the experimental methods.

AUTHOR INFORMATION Corresponding Author * Corresponding author: Prof. Julia Kunze-Liebhäuser, e-mail: [email protected], tel.: +43512-507-58013 Author Contributions


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The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript. Funding Sources J.K.-L. and E.P. thank the DFG (project KU 2397/3 1) and the Austrian Science Foundation (FWF, project P29645-N36) for financial support.

ACKNOWLEDGMENT J.K.-L. and E.P. thank the DFG (project KU 2397/3 1) and the Austrian Science Foundation (FWF, project P29645-N36) for financial support.

ABBREVIATIONS CV: cyclovoltammogram EIS: electrochemical impedance spectroscopy NT: nanotube Rct: charge transfer resistance TiO2: titanium dioxide XPS: X-ray photoelectron spectroscopy

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Nonequilibrium Phase Transitions in Amorphous and Anatase TiO2

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