Enhanced Rate Capability and Low-Temperature Performance of

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Enhanced rate capability and low temperature performance of Li4Ti5O12 anode material by facile surface fluorination Yixiao Zhang, Ying Luo, Yang Chen, Taolin Lu, Liqin Yan, Xiaoli Cui, and Jingying Xie ACS Appl. Mater. Interfaces, Just Accepted Manuscript • Publication Date (Web): 02 May 2017 Downloaded from http://pubs.acs.org on May 6, 2017

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ACS Applied Materials & Interfaces

Enhanced rate capability and low temperature performance of Li4Ti5O12 anode material by facile surface fluorination

Yixiao Zhanga,c, Ying Luob,c, Yang Chena, Taolin Lub,c, Liqin Yanc, Xiaoli Cuia,*, Jingying Xieb,c,d,*

a

Department of Materials Science, Fudan University, Shanghai 200433, China.

E-mail: [email protected] b

Department of Applied Chemistry, Harbin Institute of Technology, Harbin 150001,

China. E-mail: [email protected] c

Shanghai Power & Energy Storage Battery System Engineering Tech. Co. Ltd.,

Shanghai 200241, China. d

Shanghai Institute of Space Power Sources, Shanghai 200245, China.

Keywords: Li4Ti5O12, anode material, NH4F, surface fluorination, lithium ion batteries

Abstract A commercial Li4Ti5O12 material was modified by NH4F using a facile and dry method at low temperature in air. X-ray diffraction reveals that the fluorination did not change the bulk structure of Li4Ti5O12. X-ray photoelectron spectroscopy demonstrates that LiF was formed at the surface and Ti4+ was partially changed into Ti3+. Microscopic images show that some nanoislands were formed on the surface, 1

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which enlarged the surface area. Consequently, the NH4F modified Li4Ti5O12 material exhibited significantly enhanced capacities and rate capability, even at low temperature. The discharge capacity was increased from 149 to 167 mAh g−1 at 1 C and the capacity retention was increased from 17.8% to 52.0% at 15 C. The capacity retention of NH4F-modified Li4Ti5O12 was greater than that of Li4Ti5O12 at each low temperature point. Additionally, the introduction of F can protect the Li4Ti5O12 material from side reactions with the electrolyte and the atmosphere, enhancing the surface stability and reducing the release of gaseous products. It’s believed that the NH4F-modified Li4Ti5O12 with enhanced electrochemical performance is a promising anode material for lithium ion batteries. Furthermore, this facile surface fluorination strategy is amenable to large scale production.

1. Introduction With the increasing applications of lithium-ion batteries in energy storage and electronic devices, there are more demands to further improve their energy density, safety, rate capability and cycling stability. Owing to a low lithium-intercalation potential of the carbonaceous anode, approaching 0 V (vs. Li/Li+), a safety issue arises from dendritic lithium growth on the carbonaceous anode surface during over-charge processes and long-term cycling.1 Many efforts have been made to find alternatives to carbon-based materials with high safety and good cycling stability.

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Spinel Li4Ti5O12 has been considered as one of the most promising anode candidates for lithium-ion batteries. It possesses several advantages, 1) good Li+ insertion and de-insertion reversibility with negligible structure change, as so called “zero strain” material, 2) high safety and excellent cycle stability, arising from the high discharge plateau of about 1.5 V, which avoids the battery short circuit triggered by the formation of lithium dendrites on the surface of electrodes.2 Unfortunately, the Li4Ti5O12 material has an inherently insulating property due to the empty Ti 3d-states with a band gap of about 2 eV,3 limiting its use in high rate applications. Numerous strategies have been developed to improve the conductivity of Li4Ti5O12, including reducing the particle size, doping using cations or anions and surface modification. The reduction of particle size is usually realized by preparing particles ranging from tens to hundreds of nanometers with uniform morphology, which can shorten the pathways between the conductive carbon and electrochemical reaction sites and between the particles themselves, and enhance the rate capability of Li4Ti5O12 materials.4-6 However, the nanosized particles usually reveal low power tap density and result in low volumetric energy density. Ionic doping method has also proven to be an effective approach to improving the electronic conductivity and/or lithium ion diffusion. Various cations and anions such as Na+, K+, Mg2+, Ni2+, Cu2+, Al3+, V5+ and F- have been successfully doped at the Li+, Ti4+ or O2− sites of Li4Ti5O12.7-13 Despite a 3



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favourable influence on the electrical conductivity for Li4Ti5O12, the interstitial or substituted doping ions in Li4Ti5O12 matrix could always cause local and average structure distortions and further change aspects of the Li4Ti5O12 electrochemical performance such as cycling stability.14 Surface modification is another method to improve the rate capability of Li4Ti5O12 materials. The most commonly-used means include carbon coatings15-18, metal and related oxide coatings,19-21 surface nitridation,22-24 and surface fluorination.25-29 Nakajima’s group used F2 gas, NF3 and ClF3 gas to obtain the surface fluorinated Li4Ti5O12 materials, respectively, which caused increased surface area and improved charge capacities.25,26 Xu et al. calcined commercial Li4Ti5O12 with Al(NO3)3·9H2O and NH4F to obtain an AlF3 modified Li4Ti5O12 with increased discharge capacity and enhanced long-term cycling stability.27 Furthermore, Li et al. mixed the commercial Li4Ti5O12 with Mg(NO3)2·9H2O and NH4F in deionized water and then dried and calcined them to prepare the Mg2+ and F− co-modified Li4Ti5O12.29 It was proven that new impurity phases such as TiO2 and LiF were formed on the Li4Ti5O12 surface and that this improved the capacity and rate capability of the material. Ma et al. synthesized carbon-encapsulated F-doped Li4Ti5O12 by solid state lithiation at high temperatures with LiF as fluoride precursor.30 The results showed that the fluoride doping can both increase the electron conductivity of Li4Ti5O12 through mixed valency (Ti3+ and Ti4+) creation, and the robustness of the structure to repeated lithiation and delithiation. These examples consistently 4


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show that the F− from NH4F played an important role in improving the electrochemical performance of Li4Ti5O12 materials. Although one reference29 gives the performance of an NH4F modified sample, preparation using wet processes may be not suitable for Li4Ti5O12, which tends to surface-absorb H2O.31 Hence, a surface modification using only NH4F as additive with a simple dry process is worthy of study. Additionally, NH4F has been successfully applied though a dry process to modify the surface of cathode materials such as LiNi0.6Co0.2Mn0.2O2,32 LiNi0.8Co0.1Mn0.1O2,33 Li-rich layered Li1.2Mn0.54Ni0.13Co0.13O234 and LiNi0.8Co0.15Al0.05O2,35 and has positive effects on the electrochemical performance of these cathodes. In this work, we propose a facile and scalable dry method surface fluorination strategy with NH4F to modified commercial Li4Ti5O12 anode material at low calcinated temperature, which has not been reported elsewhere to the best of our knowledge. Without changing the spinel bulk structure, a fluorine-doped phase appeared on the surface of the particles, which enhanced the electronic conductivity and enlarged the Li4Ti5O12 surface area, resulting in a significantly enhanced capacity, rate capability and low temperature performance.

2. Experimental section 2.1 Material preparation

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The commercial Li4Ti5O12 material (LTO, Shenzhen BTR New Energy Materials Co. Ltd, China) was thoroughly mixed with 1% by weight of NH4F (Sinopharm Chemical Reagent Co. Ltd., China) by grinding in an agate mortar. Then the mixture was heated at 300 °C in air for 2h to obtain the surface fluorinated LTO material (F-LTO). Four more fluorinated LTO materials were also prepared at the increased heat treatment temperatures from 400 to 700 °C, while the material heated at 300 °C presented the highest charge and discharge capacities, as shown in Figure S1, so the further investigations in this paper are based on the fluorinated LTO material heated at 300 °C.

2.2 Material characterization The crystal structures of all the materials were characterized by X-ray diffraction (XRD) using a Bruker D8 Advanced Diffractometer with Cu Kα radiation (V = 40 kV, I = 40 mA) at a scanning rate of 6° min−1. The particle morphology was examined by scanning electronic microscopy (SEM, Hitachi S-4800), transmission electron microscopy (TEM, FEI Tecnai G2 F20) and high resolution transmission electron microscopy (HRTEM). The Brunauer-Emmett-Teller (BET) surface area was determined using a Micromeritics ASAP 2460 instrument. The surface chemical composition was probed using X-ray photoelectron spectroscopy (XPS, RBD upgraded PHI-5000C ESCA system) with a Mg Kα excitation source. Binding energies were calibrated by referencing the C1s peak at 284.6 eV. XPS depth analysis (Thermo Scientific ESCALAB 250Xi) was performed by etching the NH4F modified 6


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LTO powder using Ar-ions. The functional groups on the surface of the materials were characterized using a Spectrum100 FT-IR spectrometer (PerkinElmer).

2.3 Electrochemical measurements All electrochemical measurements were performed using 2016 coin-type cells. The working electrodes were prepared by mixing the active material with carbon black (super P) and poly (vinylidene difluoride) dissolved in N-methyl-2-pyrrolidone with a weight ratio of 8:1:1. The slurry was spread onto Al foil and dried at 80 °C in air to remove the solvent and then dried in a vacuum oven at 100 °C for 12 h. The cells were assembled using lithium foil as both a counter and reference electrode and Celgard 2325 as separator. The electrolyte consisted of a solution of 1.2 M lithium hexafluorophosphate (LiPF6) in ethylene carbonate (EC)/ ethylmethyl carbonate (EMC) (3:7 by volume). The cells were assembled in an argon-filled glove box with concentration of moisture and oxygen below 1.0 ppm. The material loading was around 1.1 mg cm−2 for all samples. Cyclic voltammetry (CV) tests were carried out on an electrochemical workstation (CHI 1000C, CH Instruments) between 1.0 and 3.0 V (vs. Li/Li+). Galvanostatic charge-discharge tests were performed using a LAND CT2001A battery test system and a voltage range of 1.0-2.5 V. In this work, the specific current at a rate of 1 C is equivalent to 175 mA g−1. Low temperature measurements were carried out in a high and low temperature test chamber 7



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(CT4005) and the temperature was allowed to stabilize for 4 h before the measurements. Unless otherwise specified, the electrochemical tests were carried out at room temperature (25 °C).

3. Results and discussion

Figure 1. XRD patterns (a) and enlarged peaks in selected 2θ ranges (b) of LTO and F-LTO. The XRD patterns of LTO and F-LTO materials are shown in Figure 1. In Figure 1a, the F-LTO material exhibits the same diffraction peaks as the LTO material, which can be assigned to the spinel Li4Ti5O12 pattern (JCPDS #49-0207) with an Fd3m space group. It implies that the surface modification using NH4F did not alter the spinel bulk structure. The strongest (111) diffraction peak in the 2θ ranges of 18.0-18.7° is selected for further analysis (Figure 1b). With fluorination, the (111) diffraction peak shows a slight shift toward lower degrees, indicative of the increment in lattice constant. The lattice parameters (a value) calculated by using XRD data with Rietveld refinement are 0.8360 nm and 0.8361 nm for LTO and F-LTO, respectively. 8


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This was opposite to that expected by Vegard’s rule for the substitution of O2− for the smaller F− ion, and it could be assumed to originate from the partial reduction of Ti4+ to the larger Ti3+ while charge compensation related to the substitution of oxygen by fluoride ions took place in the structure.36

Figure 2. SEM images of LTO (a, c) and F-LTO (b, d); TEM and HRTEM images of LTO (e, g) and F-LTO (f, h). 9



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Figure 2a-d show the SEM images of LTO and F-LTO. It is observed that the F-LTO material has a similar particle size distribution to the LTO. While the TEM images in Figure 2e and f show that the surface of the F-LTO material is rougher than that of the LTO. The LTO particles have smooth edges, as shown in Figure 2e. However, the edges of the F-LTO particles are rougher and some nanoparticles are visible on the surface as shown in Figure 2f. This indicates that a reaction between the LTO particles and NH4F may change their surface structure. Differences are more clearly seen in the HRTEM images in Figures 2g and h. In Figure 2g, it can be seen that the LTO material has a well-defined crystal structure and the lattice spacing is measured to be 0.484 nm, corresponding to the (111) crystal planes of spine LTO. The typical lattice fringes can be distinguished in Figure 2h for the F-LTO material. However, the surface is rough and the lattice arrangement shows less long range order, indicating that NH4F modifying could change the surface structure. Moreover, the nanoparticles on the surface of F-LTO also emerge distinct lattice fringes, and the lattice distance is measured to be 0.484 nm, indexing to the (111) planes of spinel LTO phase. It is proposed that the islands of Li4Ti5O12 formed at the surface may be the result of being etched away from the bulk.37 The surface structures of LTO and F-LTO materials were examined by using the Brunauer-Emmett-Teller (BET) nitrogen (N2) adsorption method. The specific surface areas of LTO and F-LTO were calculated to be 3.86 and 4.20 m2 g-1, respectively. As expected, the rougher surface leads to a higher surface area. 10


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Figure 3. XPS survey spectra of LTO and F-LTO (a); Ti 2p XPS spectra of LTO and F-LTO (b); F 1s XPS spectra of F-LTO (c); XPS survey spectra of F-LTO with vary Ar-ion etching time (d); Atomic percent of F 1s in XPS spectra of F-LTO with etching depth (e); FT-IR spectra of LTO and F-LTO (f). In order to investigate the chemical composition on the surface of the F-LTO material, XPS and FT-IR were performed. 11



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The general XPS spectra of LTO and F-LTO are shown in Figure 3a. In addition to the signals for Li, C, O and Ti elements in both samples, the distinct fluorine can be found in F-LTO. The high-resolution XPS spectra of Ti 2p of LTO and F-LTO materials are shown in Figure 3b. Both samples have two peaks at 464.1 eV and 458.4 eV, which correspond to the Ti 2p1/2 and Ti 2p3/2 binding energy of Ti4+, respectively. Additionally, there is a small shoulder at around 456.2 eV in F-LTO, which can be attributed to Ti3+, indicating that Ti4+ is partially reduced to Ti3+.38-40 The F 1s spectrum at 685.7 eV (Figure 3c) can be assigned to LiF, which reveals the formation of LiF after fluorination. As a consequence, it is proposed that NH4F modification process caused F− to attack the surface atoms of the LTO and to partially substitute O2− to form LiF. In addition, charge compensation related to the substitution of oxygen by fluoride ions took place, resulting in the conversion of Ti from a higher-valence state (+4) to a lower-valence state (+3) and subsequently bringing on the improvement of electronic conductivity.36,41 The distribution of the added F in F-LTO was measured using XPS after Ar-ion etching and the result is shown in Figure 3d. The etching rate was 0.33 nm s−1, and so the corresponding etching depths at 15, 65, 165 and 365 s are 4.95 nm, 21.45 nm, 54.45 nm and 120.45 nm. This is much smaller than the particle size, which is more than 500 nm as shown in Figure 2b and d. It can be seen that the intensity of the F peak tends to decrease with etching. Meanwhile, the atomic percent of F decreases from 4.89% to 1.72%, as shown in Figure 3e, indicating that it is mainly distributed in the outer region of the F-LTO material rather than in the bulk, which has been shown 12


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to prevent side reactions between the LTO material and an electrolyte or air.35 The surface characteristics of LTO and F-LTO materials were also examined using an FT-IR spectrometer. The FT-IR spectra of both materials are plotted in Figure 3f. Both samples show the same peaks at 1044 and 1637 cm−1, respectively. The band at 1044 cm−1 could be attributed to LTO.42 The band at 1637 cm−1 is attributed to a bending mode of absorbed water.43,44 Additionally, the LTO material shows two more peaks at 1501 and 1439 cm−1, which correspond to asymmetric CO stretching modes and are attributed to Li2CO3.31 It’s recognized that the fresh Li4Ti5O12 particle tends to absorb H2O and CO2 in the air, leading to the formation of amorphous Li2CO3 at the surface surface.31 However, when modified with NH4F, the peaks at 1501 and 1439 cm−1 almost disappeared, indicating that the presence of LiF on the surface of the F-LTO was likely to prevent the reactions between the F-LTO and CO2 to produce carbonate ions.

Figure 4. Cyclic performance (a) and the initial charge-discharge curves (b) of LTO and F-LTO at 1 C. Figure 4a presents the cyclic performance of LTO and F-LTO materials with current density of 1 C at room temperature. Both materials show good capacity 13



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retention (higher than 99%) up to 100 cycles. However, the F-LTO material delivers an initial capacity of 167 mAh g−1, much higher than that of LTO material (149 mAh g−1), owing to a higher BET surface area and improved contact between the electrode and electrolyte, leading to enhanced surface electrochemical reactivity.26,45 The charge and discharge curves of LTO and F-LTO in the first cycle are shown in Figure 4b. The two materials show similar shapes for discharge curves, while the F-LTO material shows a relatively lower charge profile, leading to a smaller polarization and indicating enhanced electrochemical kinetics.38

Figure 5. Electrochemical properties of LTO and F-LTO at various current rates from 0.2 to 15 C at ambient temperature: Rate performance (a) and the capacity retention (b); Charge and discharge curves of the LTO (c) and F-LTO (d).

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Figure 5a shows the rate performance of the LTO and F-LTO materials at various current rates from 0.2 C to 15 C at room temperature. The LTO and F-LTO materials deliver similar initial discharge capacities of 173 and 175 mAh g−1 at 0.2 C, respectively. As the current rates increased from 0.5 C to 15 C, the differences of capacity between LTO and F-LTO become larger. Compared to the reversible capacity of 0.2 C, as shown in Figure 5b, the capacity retention of LTO is 95.2%, 90.7%, 80.5%, 49.1%, 28.6% and 17.8% corresponding to the current densities of 0.5, 1, 2, 5, 10, and 15 C, respectively. The F-LTO material, however, exhibits larger retention values of 97.3%, 95.9%, 93.9%, 86.9%, 71.1% and 52.0%. Moreover, comparing the charge and discharge curves of the two materials at various current rates in Figure 5c and d, the F-LTO material reveals elongated charge and discharge plateaus, and a relatively smaller polarization, indicating a higher utilization efficiency and superior electrode kinetics for F-LTO. In addition, a larger discharge capacity for F-LTO at various current densities could also be attributed to an increased specific surface area and enhanced electrochemical kinetics.

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Figure 6. CV curves of LTO (a) and F-LTO (b) at various scan rates between 0.1 and 1.6 mV s−1 in a potential window of 1-3 V (vs. Li/Li+); (c) and (d) are the corresponding relationship of the peak current (ip) and the square root of scan rate (v1/2). To verify the enhanced electrochemical kinetics, cyclic voltammetry (CV) was carried out after 100 cycles at 1C to study the oxidation/reduction processes of the electrode reactions and to estimate the Li+ diffusion coefficient (DLi+), as shown in Figure 6a and b, respectively. It can be seen that both the peak current and the area under the redox peaks of both materials increase with increasing scan rate. The results fit a linear relationship between the peak current ip and the square root of the scan rate v for both the LTO and F-LTO materials, as shown in Figure 6c and d. This suggests 16


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that diffusion controls the lithium ion intercalation/deintercalation processes. The peak current can be expressed by the Randles-Sevcik equation:46,47 ip=(2.69×105)n3/2ADLi+ 1/2CLi+v1/2

(1)

where n is the stoichiometric number of electrons involved in the electrode reaction; A is the area of the electrode (1.54 cm2 in this case); CLi+ is the bulk concentration of lithium ions (4.37 × 10−3 mol cm−3), and DLi+ is diffusion coefficient of lithium ions (in cm2 s−1). The DLi+ of LTO and F-LTO can be calculated from the slope of the ip vs. v1/2 curve according to this equation (K values, as shown in Figure 6c and d). The diffusion coefficients for lithiation and delithiation of the F-LTO are calculated to be 6.51 × 10−8 and 3.15 × 10−8 cm2 s−1, respectively. These are much larger than those for LTO (2.38 × 10−8 cm2 s−1 for lithiation and 1.47 × 10−8 cm2 s−1 for delithiation process). The larger DLi+ values indicate faster lithium-ion transfer for F-LTO, which can be mainly attributed to the surface Ti3+ enhancing the electron conductivity and reducing the lithium ions diffusion barrier.48

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Figure 7. Electrochemical properties of LTO and F-LTO at various temperature from 25 to −20 °C at 1 C: cyclic performance (a); capacity retention (b); charge and discharge curves of the LTO (c) and F-LTO (d).

The low temperature electrochemical properties of LTO and F-LTO materials were further investigated and the results are shown in Figure 7. Figure 7a gives the cyclic performances of the two materials at different temperatures ranging from 25 °C to −20 °C at the current density of 1 C between 1-2.5 V (vs. Li/Li+). With the temperatures decreasing, the charge and discharge capacities of both the materials appear to be expectedly reduced. While the cycle stabilities for both the materials are excellent at various temperature, with coulombic efficiency of almost 100%. It also can be obviously seen that F-LTO material presents higher charge and discharge 18


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capacities at all temperatures. The reversible discharge capacities of F-LTO are 157.6, 143.3, 130.0 and 100.0 mAh g−1 at 25 °C, 0 °C, −10 °C and −20 °C, respectively. While the LTO sample exhibits only 147.6, 102.9, 81.1 and 59.0 mAh g−1, respectively. The higher low temperature capacities for F-LTO obtained at 1 C indicate that the surface fluorination is beneficial for improving the electrochemical performance of Li4Ti5O12 at low temperature. Correspondingly, Figure 7b shows that the low temperature capacity retentions of F-LTO, compared to the reversible capacity at 25 °C, are 90.9%, 82.6% and 63.3%, respectively, and the charge and discharge capacities of F-LTO material decrease more slowly with the temperature dropping than that of LTO. Additionally, the charge and discharge profiles of F-LTO appear to be always longer than that of LTO, as shown in Figure 7c and d. Further comparing with Figure 7c and d, it can be found that the F-LTO present a relatively smaller polarization at various low temperature, indicating a more superior electrode kinetics at low temperature for F-LTO. Consistent with the results drawn from the CV and rate measurements, the improved electrochemical properties are mostly likely due to the enlarged specific surface area, the faster lithium ion transfer and the enhanced electronic conductivity from the Ti3+ on the surface.49

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Figure 8. SEM images of the fresh electrodes (a, c) and the electrodes after cycling at low temperatures (b, d); HRTEM images of the electrodes after cycling at low temperatures (e, f); (a, c and e) for the LTO sample and (b, d and f) for the F-LTO sample.

The morphology changes in the LTO and F-LTO electrodes after cycling at low temperatures were also investigated. SEM images of fresh electrodes are displayed in Figure 8a and b, while Figure 8c and d depict the SEM images of the electrodes after cycling at low temperatures. It is obvious that solid electrolyte interface (SEI) films were formed on these electrodes after cycling, in agreement with the results in the 20


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previous literatures, attributed to the intrinsic reaction between the Li4Ti5O12 electrodes and electrolyte solution, generally relating to the production of gases.42,50-52 The SEI film on the F-LTO electrode, however, is denser and smoother, and it is thinner (Figure 8e and f), which confirms that the introduced fluorine can partly suppress chemical reactions between the Li4Ti5O12 electrode and the electrolyte. This is likely to reduce gaseous by-products.

Figure 9. Illustration of the formation process for NH4F modified LTO (not drawn to scale). Combining the electrochemical measurements with the XRD results, SEM and TEM images, BET results, XPS and FT-IR spectra, a possible mechanism for the formation process of F-LTO is proposed in Figure 9. Firstly, F− attacks the surface atoms of LTO and then chemically reacts with Li+ at the surface to form LiF, which is effective in reducing the side reactions between Li4Ti5O12 particles and the electrolyte or air. Simultaneously, Ti4+ at the surface was partially reduced to Ti3+ to compensate the charge balance, which can enhance the electronic conductivity of LTO. The reaction between NH4F and LTO material also created an etching effect, bringing out some nanoscale LTO particles on the surface of the large LTO particles, which subsequently increased the BET surface area. 21



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4 Conclusions In this work, a commercial LTO material was modified by NH4F using a facile and dry method in air. Material characterizations reveal that the modification process did not change the spinel bulk structure, but modified the surface composition of pristine LTO material. XPS demonstrated that LiF was formed at the surface after NH4F modified, which simultaneously triggered partial Ti4+ reduction to Ti3+ so that the electronic conductivity of the LTO material was enhanced. Moreover, the reaction between the NH4F and LTO etched the surface and enlarged the BET surface area, which is improved the charge and discharge capacities. As a consequence, the F-LTO material exhibited significantly enhanced rate capability and low temperature performance. The initial discharge capacity of F-LTO was 175 mAh g−1 at 0.2 C with a capacity retention of 52.0% even at high current density of 15 C. The capacity retention of F-LTO was greater than that of LTO at low temperature, i.e. 63.3% (100 mAh g−1) for F-LTO vs. 40.0% (59.0 mAh g−1) for LTO at −20 °C. In addition, the introduction of F can protect the LTO material from side reactions with the electrolyte and atmosphere, enhancing the surface stability and reducing the release of gaseous by-products. It is believed that LTO surface modified by NH4F is a promising anode material for lithium ion batteries and this simple modification strategy should be amenable to large scale production.

ASSOCIATED CONTENT Supporting Information. Cyclic performance of the raw Li4Ti5O12 and the 22


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fluorinated Li4Ti5O12 materials at different heat treated temperatures. The Supporting Information is available free of charge on the ACS Publications website at http://pubs.acs.org.

AUTHOR INFORMATION Corresponding Authors *Email: [email protected]. *Email: [email protected]. Author Contributions The manuscript was written with the contribution of all authors. All authors have given approval to the final version of the manuscript.

Acknowledgements Authors Liqin Yan, Yixiao Zhang and Ying Luo received funding from National 863 Program Grant 2014AA052202. Author Jingying Xie received funding from National Natural Science Foundation of China Grant 21373137. Authors Jingying Xie, Yixiao Zhang, Ying Luo and Liqin Yan received funding from Shanghai Science and Technology Engineering Research Center Program Grant 15DZ2282000.

References (1) Zhu, G. N.; Wang, Y. G.; Xia, Y. Y. Ti-Based Compounds as Anode Materials for Li-Ion Batteries. Energ. Environ. Sci. 2012, 5, 6652. 23



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

(2) Zhang, B.; Liu, Y.; Huang, Z.; Oh, S.; Yu, Y.; Mai, Y. W.; Kim, J. K. Urchin-like Li4Ti5O12-Carbon Nanofiber Composites for High Rate Performance Anodes in Li-ion Batteries. J. Mater. Chem. 2012, 22, 12133. (3) Ouyang, C. Y.; Zhong, Z. Y.; Lei, M. S. Ab initio Studies of Structural and Electronic Properties of Li4Ti5O12 Spinel. Electrochem. Commun. 2007, 9, 1107-1112. (4) Wang, J.; Zhao, H.; Li, Z.; Wen, Y.; Xia, Q.; Zhang, Y.; Yushin, G. Revealing Rate Limitations in Nanocrystalline Li4Ti5O12 Anodes for High-Power Lithium Ion Batteries. Adv. Mater. Interfaces 2016, 3, 1600003. (5) Lu, J.; Nan, C.; Peng, Q.; Li, Y. Single Crystalline Lithium Titanate Nanostructure with Enhanced Rate Performance for Lithium Ion Battery. J. Power Sources 2012, 202, 246-252. (6) Liu, Z.; Zhang, N.; Sun, K. A Novel Grain Restraint Strategy to Synthesize Highly Crystallized Li4Ti5O12 (∼20 nm) for Lithium Ion Batteries with Superior High-Rate Performance. J. Mater. Chem. 2012, 22, 11688. (7) Liu, Z.; Sun, L.; Yang, W.; Yang, J.; Han, S.; Chen, D.; Liu, Y.; Liu, X. The Synergic Effects of Na and K Co-Doping on the Crystal Structure and Electrochemical Properties of Li4Ti5O12 as Anode Material for Lithium Ion Battery. Solid State Sci. 2015, 44, 39-44. (8) Chen, C. H.; Vaughey, J. T.; Jansen, A. N.; Dees, D. W.; Kahaian, A. J.; Goacher, T.; Thackeray, M. M. Studies of Mg-Substituted Li4−xMgxTi5O12 Spinel Electrodes ( 0

⩽ x ⩽ 1 ) for Lithium Batteries. J. Electrochem. Soc. 2000, 148, A102-A104. (9) Lin, C.; Lai, M. O.; Lu, L.; Zhou, H.; Xin, Y. Structure and High Rate 24


Page 24 of 41

Page 25 of 41

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


Performance of Ni2+ Doped Li4Ti5O12 for Lithium Ion Battery. J. Power Sources 2013, 244, 272-279. (10) Bai, X.; Li, T.; Wei, C.; Sun, Y. K.; Qi, Y. X.; Zhu, H. L.; Lun, N.; Bai, Y. J. Enhancing the Long-Term Cyclability and Rate Capability of Li4Ti5O12 by Simple Copper-Modification. Electrochim. Acta 2015, 155, 132-139. (11) Zhao, H.; Li, Y.; Zhu, Z.; Lin, J.; Tian, Z.; Wang, R. Structural and Electrochemical Characteristics of Li4−xAlxTi5O12 as Anode Material for Lithium-Ion Batteries. Electrochim. Acta 2008, 53, 7079-7083. (12) Yu, Z.; Zhang, X.; Yang, G.; Liu, J.; Wang, J.; Wang, R.; Zhang, J. High Rate Capability and Long-Term Cyclability of Li4Ti4.9V0.1O12 as Anode Material in Lithium Ion Battery. Electrochim. Acta 2011, 56, 8611-8617. (13) Zhao, Z.; Xu, Y.; Ji, M.; Zhang, H. Synthesis and Electrochemical Performance of F-doped Li4Ti5O12 for Lithium-Ion Batteries. Electrochim. Acta 2013, 109, 645-650. (14) Yi, T. F.; Yang, S. Y.; Xie, Y. Recent Advances of Li4Ti5O12 as a Promising Next Generation Anode Material for High Power Lithium-Ion Batteries. J. Mater. Chem. A 2015, 3, 5750-5777. (15) Zhu, Z.; Cheng, F.; Chen, J. Investigation of Effects of Carbon Coating on the Electrochemical Performance of Li4Ti5O12/C Nanocomposites. J. Mater. Chem. A 2013, 1, 9484. (16) Long du, H.; Jeong, M. G.; Lee, Y. S.; Choi, W.; Lee, J. K.; Oh, I. H.; Jung, H. G. Coating Lithium Titanate with Nitrogen-Doped Carbon by Simple Refluxing for 25



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

High-Power Lithium-Ion Batteries. ACS Appl. Mater. Interfaces 2015, 7, 10250-10257. (17) Jung, H. G.; Kim, J.; Scrosati, B.; Sun, Y. K. Micron-Sized, Carbon-Coated Li4Ti5O12 as High Power Anode Material for Advanced Lithium Batteries. J. Power Sources 2011, 196, 7763-7766. (18) Zhu, G. N.; Liu, H. J.; Zhuang, J. H.; Wang, C. X.; Wang, Y. G.; Xia, Y. Y. Carbon-Coated Nano-Sized Li4Ti5O12 Nanoporous Micro-Sphere as Anode Material for High-Rate Lithium-Ion Batteries. Energ. Environ. Sci. 2011, 4, 4016. (19) Ji, G.; Ma, Y.; Ding, B.; Lee, J. Y. Improving the Performance of High Capacity Li-Ion Anode Materials by Lithium Titanate Surface Coating. Chem. Mater. 2012, 24, 3329-3334. (20) Wang, Y. Q.; Gu, L.; Guo, Y. G.; Li, H.; He, X. Q.; Tsukimoto, S.; Ikuhara, Y.; Wan, L. J. Rutile-TiO2 Nanocoating for a High-Rate Li4Ti5O12 Anode of a Lithium-Ion Battery. J. Am. Chem. Soc. 2012, 134, 7874-7879. (21) Han, C.; He, Y. B.; Li, H.; Li, B.; Du, H.; Qin, X.; Kang, F. Suppression of Interfacial Reactions Between Li4Ti5O12 Electrode and Electrolyte Solution via Zinc Oxide Coating. Electrochim. Acta 2015, 157, 266-273. (22) Park, K. S.; Benayad, A.; Kang, D. J.; Doo, S. G. Nitridation-Driven Conductive Li4Ti5O12 for Lithium Ion Batteries. J. Am. Chem. Soc. 2008, 130, 14930-14931. (23) Wan, Z.; Cai, R.; Jiang, S.; Shao, Z. Nitrogen- and TiN-Modified Li4Ti5O12: One-Step Synthesis and Electrochemical Performance Optimization. J. Mater. Chem. 2012, 22, 17773. 26


Page 26 of 41

Page 27 of 41

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


(24) Li, X.; Lin, H. C.; Cui, W. J.; Xiao, Q.; Zhao, J. B. Fast Solution-Combustion Synthesis

of

Nitrogen-Modified

Li4Ti5O12

Nanomaterials

with

Improved

Electrochemical Performance. ACS Appl. Mater. Interfaces 2014, 6, 7895-7901. (25) Nakajima, T.; Ueno, A.; Achiha, T.; Ohzawa, Y.; Endo, M. Effect of Surface Fluorination and Conductive Additives on the Electrochemical Behavior of Lithium Titanate (Li4/3Ti5/3O4) for Lithium Ion Battery. J. Fluorine Chem. 2009, 130, 810-815. (26) Utsunomiya, H.; Nakajima, T.; Ohzawa, Y.; Mazej, Z.; Žemva, B.; Endo, M. Influence of Conductive Additives and Surface Fluorination on the Charge/Discharge Behavior of Lithium Titanate (Li4/3Ti5/3O4). J. Power Sources 2010, 195, 6805-6810. (27) Xu, W.; Chen, X.; Wang, W.; Choi, D.; Ding, F.; Zheng, J.; Nie, Z.; Choi, Y. J.; Zhang, J. G.; Yang, Z. G. Simply AlF3-Treated Li4Ti5O12 Composite Anode Materials for Stable and Ultrahigh Power Lithium-Ion Batteries. J. Power Sources 2013, 236, 169-174. (28) Li, W.; Li, X.; Chen, M.; Xie, Z.; Zhang, J.; Dong, S.; Qu, M. AlF3 Modification to Suppress the Gas Generation of Li4Ti5O12 Anode Battery. Electrochim. Acta 2014, 139, 104-110. (29) Li, W.; Wang, H.; Chen, M.; Gao, J.; Li, X.; Ge, W.; Qu, M.; Wei, A.; Zhang, L.; Liu, Z. The Reaction Mechanism of the Mg2+ and F− Co-Modification and Its Influence on the Electrochemical Performance of the Li4Ti5O12 Anode Material. Electrochim. Acta 2016, 188, 499-511. (30) Ma, Y.; Ding, B.; Ji, G.; Lee, J. Y. Carbon-Encapsulated F-Doped Li4Ti5O12 as a High Rate Anode Material for Li+ Batteries. Acs Nano 2013, 7, 10870-10878. 27



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

(31) Gao, Y.; Wang, Z.; Chen, L. Stability of Spinel Li4Ti5O12 in Air. J. Power Sources 2014, 245, 684-690. (32) Yue, P.; Wang, Z.; Li, X.; Xiong, X.; Wang, J.; Wu, X.; Guo, H. The Enhanced Electrochemical Performance of LiNi0.6Co0.2Mn0.2O2 Cathode Materials by Low Temperature Fluorine Substitution. Electrochim. Acta 2013, 95, 112-118. (33) Yue, P.; Wang, Z.; Wang, J.; Guo, H.; Xiong, X.; Li, X. Effect of Fluorine on the Electrochemical Performance of Spherical LiNi0.8Co0.1Mn0.1O2 Cathode Materials via a Low Temperature Method. Powder Technol. 2013, 237, 623-626. (34) Li, L.; Chang, Y. L.; Xia, H.; Song, B. H.; Yang, J. R.; Lee, K. S.; Lu, L. NH4F Surface Modification of Li-Rich Layered Cathode Materials. Solid State Ionics 2014, 264, 36-44. (35) Zhu, L.; Liu, Y.; Wu, W.; Wu, X.; Tang, W.; Wu, Y. Surface Fluorinated LiNi0.8Co0.15Al0.05O2 as a Positive Electrode Material for Lithium Ion Batteries. J. Mater. Chem. A 2015, 3, 15156-15162. (36) Huang, S.; Wen, Z.; Gu, Z.; Zhu, X. Preparation and Cycling Performance of Al3+ and F− Co-Substituted Compounds Li4AlxTi5−xFyO12−y. Electrochim. Acta 2005, 50, 4057-4062. (37) Liu, H.; Qian, D.; Verde, M. G.; Zhang, M.; Baggetto, L.; An, K.; Chen, Y.; Carroll, K. J.; Lau, D.; Chi, M.; Veith, G. M.; Meng, Y. S. Understanding the Role of NH4F and Al2O3 Surface Co-modification on Lithium-Excess Layered Oxide Li1.2Ni0.2Mn0.6O2. ACS Appl. Mater. Interfaces 2015, 7, 19189-19200. (38) Xia, Q.; Jabeen, N.; Savilov, S. V.; Aldoshin, S. M.; Xia, H. Black Mesoporous 28


Page 28 of 41

Page 29 of 41

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


Li4Ti5O12−δ Nanowall Arrays with Improved Rate Performance as Advanced 3D Anodes for Microbatteries. J. Mater. Chem. A 2016, 4, 17543-17551. (39) Cai, Y.; Huang, Y.; Jia, W.; Wang, X.; Guo, Y.; Jia, D.; Sun, Z.; Pang, W.; Guo, Z. Super High-Rate, Long Cycle Life of Europium-Modified, Carbon-Coated, Hierarchical Mesoporous Lithium-Titanate Anode Materials for Lithium Ion Batteries. J. Mater. Chem. A 2016, 4, 9949-9957. (40) Xia, T.; Zhang, W.; Murowchick, J. B.; Liu, G.; Chen, X. A Facile Method to Improve the Photocatalytic and Lithium-Ion Rechargeable Battery Performance of TiO2 Nanocrystals. Adv. Energy Mater. 2013, 3, 1516-1523. (41) Zhang, Q.; Lu, H.; Zhong, H.; Yan, X.; Ouyang, C.; Zhang, L. W6+& Br− Codoped Li4Ti5O12 Anode with Super Rate Performance for Li-ion Batteries. J. Mater. Chem. A 2015, 3, 13706-13716. (42) Wu, K.; Yang, J.; Liu, Y.; Zhang, Y.; Wang, C.; Xu, J.; Ning, F.; Wang, D. Investigation on Gas Generation of Li4Ti5O12/LiNi1/3Co1/3Mn1/3O2 Cells at Elevated Temperature. J. Power Sources 2013, 237, 285-290. (43) Snyder, M. Q.; DeSisto, W. J.; Tripp, C. P. An Infrared Study of the Surface Chemistry of Lithium Titanate Spinel (Li4Ti5O12). Appl. Surf. Sci. 2007, 253, 9336-9341. (44) Nugroho, A.; Kim, S. J.; Chang, W.; Chung, K. Y.; Kim, J. Facile Synthesis of Hierarchical Mesoporous Li4Ti5O12 Microspheres in Supercritical Methanol. J. Power Sources 2013, 244, 164-169. (45) Wagemaker, M.; Mulder, F. M. Properties and Promises of Nanosized Insertion 29



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

Materials for Li-Ion Batteries. Acc. Chem. Res. 2013, 46, 1206. (46) Yan, B.; Li, M.; Li, X.; Bai, Z.; Yang, J.; Xiong, D.; Li, D. Novel Understanding of Carbothermal Reduction Enhancing Electronic and Ionic Conductivity of Li4Ti5O12 Anode. J. Mater. Chem. A 2015, 3, 11773-11781. (47) Chen, Y.; Ma, X.; Cui, X.; Jiang, Z. In situ Synthesis of Carbon Incorporated TiO2 with Long-Term Performance as Anode for Lithium-Ion Batteries. J. Power Sources 2016, 302, 233-239. (48) Zou, H.; Liang, X.; Feng, X.; Xiang, H. Chromium-Modified Li4Ti5O12 with a Synergistic Effect of Bulk Doping, Surface Coating, and Size Reducing. ACS Appl. Mater. Interfaces 2016, 8, 21407-21416. (49) Peng, L.; Zhang, H.; Fang, L.; Zhang, Y.; Wang, Y. Novel Peapoded Li4Ti5O12 Nanoparticles for High-Rate and Ultralong-Life Rechargeable Lithium Lon Batteries at Room and Lower Temperatures. Nanoscale 2016, 8, 2030-2040. (50) He, Y. B.; Li, B.; Liu, M.; Zhang, C.; Lv, W.; Yang, C.; Li, J.; Du, H.; Zhang, B.; Yang, Q. H.; Kim, J. K.; Kang, F. Gassing in Li4Ti5O12-Based Batteries and Its Remedy. Sci. Rep. 2012, 2, 913. (51) He, Y. B.; Liu, M.; Huang, Z. D.; Zhang, B.; Yu, Y.; Li, B.; Kang, F.; Kim, J. K. Effect of Solid Electrolyte Interface (SEI) Film on Cyclic Performance of Li4Ti5O12 Anodes for Li Ion Batteries. J. Power Sources 2013, 239, 269-276. (52) Kitta, M.; Akita, T.; Maeda, Y.; Kohyama, M. Study of Surface Reaction of Spinel Li4Ti5O12 During the First Lithium Insertion and Extraction Processes Using Atomic Force Microscopy and Analytical Transmission Electron Microscopy. 30


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Langmuir 2012, 28, 12384-12392.

31



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Table of Contents

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Figure 1. XRD patterns (a) and enlarged peaks in selected 2θ ranges (b) of LTO and F-LTO. 56x28mm (600 x 600 DPI)



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Figure 2. SEM images of LTO (a, c) and F-LTO (b, d); TEM and HRTEM images of LTO (e, g) and F-LTO (f, h). 119x179mm (300 x 300 DPI)


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Figure 3. XPS survey spectra of LTO and F-LTO (a); Ti 2p XPS spectra of LTO and F-LTO (b); F 1s XPS spectra of F-LTO (c); XPS survey spectra of F-LTO with vary Ar-ion etching time (d); Atomic percent of F 1s in XPS spectra of F-LTO with etching depth (e); FT-IR spectra of LTO and F-LTO (f). 171x202mm (300 x 300 DPI)



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Figure 4. Cyclic performance (a) and the initial charge-discharge curves (b) of LTO and F-LTO at 1 C. 70x27mm (300 x 300 DPI)


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Figure 5. Electrochemical properties of LTO and F-LTO at various current rates from 0.2 to 15 C at ambient temperature: Rate performance (a) and the capacity retention (b); Charge and discharge curves of the LTO (c) and F-LTO (d). 134x100mm (300 x 300 DPI)



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Figure 6. CV curves of LTO (a) and F-LTO (b) at various scan rates between 0.1 and 1.6 mV s-1 in a potential window of 1-3 V (vs. Li/Li+); (c) and (d) are the corresponding relationship of the peak current (ip) and the square root of scan rate (v1/2). 145x129mm (300 x 300 DPI)


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Figure 7. Electrochemical properties of LTO and F-LTO at various temperature from 25 to -30 °C at 1 C: Cyclic performance (a); Capacity retention (b); Charge and discharge curves of the LTO (c) and F-LTO (d). 115x89mm (300 x 300 DPI)



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Figure 8. SEM images of the fresh electrodes (a, c) and the electrodes after cycling at low temperatures (b, d); HRTEM images of the electrodes after cycling at low temperatures (e, f); (a, c and e) for the LTO sample and (b, d and f) for the F-LTO sample. 142x161mm (300 x 300 DPI)


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Figure 9. Illustration of the formation process for NH4F modified LTO (not drawn to scale). 42x15mm (300 x 300 DPI)


Enhanced Rate Capability and Low-Temperature Performance of

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