Investigation into the Surface Chemistry of Li4Ti5O12 Nanoparticles

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Investigation into the Surface Chemistry of LiTiO Nanoparticles for Lithium Ion Batteries 4

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Yongqing Wang, Jing Zhao, Jin Qu, Fang-Fang Wei, Wei-Guo Song, Yu-Guo Guo, and Baomin Xu ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.6b07902 • Publication Date (Web): 12 Sep 2016 Downloaded from http://pubs.acs.org on September 16, 2016

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ACS Applied Materials & Interfaces

Investigation

into

the

Surface

Chemistry

of

Li4Ti5O12 Nanoparticles for Lithium Ion Batteries ∥

Yongqing Wang,*,†,‡ Jing Zhao,§ Jin Qu, Fangfang Wei,

⊥

Weiguo Song,# Yu-guo Guo*, # and

Baomin Xu*,‡ †

Department of Chemical Engineering, School of Chemistry & Chemical Engineering, Sun Yat-

Sen University, Guangzhou 510275, China ‡

Department of Materials Science and Engineering, South University of Science and Technology

of China, Shenzhen, 518055, China. §

Center for Gene and Cell Engineering, Institute of Biomedicine and biotechnology, Shenzhen

Institutes of Advanced Technology, Chinese Academy of Sciences, Shenzhen 518055, China ∥

State Key Laboratory of Organic-Inorganic Composites, College of Materials Science and

Engineering, Beijing University of Chemical Technology, Beijing 100029, China ⊥

Anshun University, 25 Xue Yuan Road, Anshun Economic Development Zone, Guizhou,

China, and #Beijing National Laboratory for Molecular Sciences (BNLMS), Institute of Chemistry, Chinese Academy of Sciences (CAS), Beijing, 100190, China KEYWORDS lithium-ion battery, swelling, Li4Ti5O12, Lewis acid, Carbon dioxide

ACS Paragon Plus Environment

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ABSTRACT: Elucidating surface chemistry of Li4Ti5O12 anode material plays a critical role in solving gas evolution in Li4Ti5O12-based lithium ion batteries. Herein we propose a CO2 cycloaddition reaction to study the surface chemistry of Li4Ti5O12 nanoparticles. Through the reaction, bare Li4Ti5O12 nanoparticles were demonstrated to have extensive Lewis-acid sites, i.e. dangling Ti bonds or hydroxyl groups. Lewis-acid site is considered to be able to initiate the decomposition of electrolyte solvents and may also serve as one of the main reasons for gas evolution. TiNx coating layer is used to cover up the Lewis-acid site and is able to decrease yield of the cycloaddition reaction to some extent. These findings may provide a simple yet very effective way to evaluate surface chemistry and gas evolution in other lithium ion batteries, not limited to Li4Ti5O12-based batteries.

1. Introduction From electronic applications to power large electric vehicles (EVs) and smart grid, lithium ion batteries (LIBs) have gained great success since 1990.1-4 Spinel Li4Ti5O12 (LTO) compound, as one of the promising anode materials for lithium ion batteries(LIBs),5-8 shows excellent cyclic performance, making it an excellent choice for high power applications such as electric vehicles and energy storage stations (ESSs).9-11 However, LTO-based LIBs suffer from severe gassing problem during charging/discharging cycles and even at storage which further leads to serious swelling, potential safety hazard at elevated temperature and limit its applications.12-17 Generally, the reason for gassing in graphite-based LIBs are attributed to reductive decomposition of electrolyte solvent at a potential of ca. 0.7 V vs Li+/Li and below, which is meanwhile accompanied with the formation of solid electrolyte interphase (SEI) film.18-20 Scheme 1a illustrates the likely gassing process in graphite case. The gassing evolution in


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graphite case mainly occurred in the initial discharging process and gradually decreased in the subsequent ones.18 However, the gassing mechanism in LTO-based batteries is quite different. LTO anodes are usually operated at a potential of above 1.0 V vs Li+/Li,21-24 at which potential electrolyte solvents are considered to be electrochemically stable25. Lewis-acid species exist in LTO batteries are usually considered to be mainly responsible for gassing evolution12,16,17,21. But there is little knowledge about how LTO particles act as Lewis-acid. Amine et al. analyzed the gas species gathered from LTO-based LIBs after cycling16, and they found that the ionization product PF5 and trace amount of water in electrolyte solution, both of which are weak Lewis-acid species, play important roles in gassing reaction, however they did not describe the unique role of LTO particles. Gu and Hu et al. proposed that the dangling Ti bonds should be responsible for gassing problem in LTO-based LIBs21, which can also be considered to be a kind of Lewis-acid active center on the surface of LTO nanoparticles, but they did not show direct experiment evidence to confirmed such speculations.

Scheme 1. a) Gas production and the formation of SEI film in Li-ion battery; b) the formula of cycloaddition reaction. Here, we developed a CO2 cycloaddition reaction26,27 to investigate the surface chemistry (Lewis-acid activity) of LTO nanoparticles (LTO NPs). As shown in scheme 1b, the reaction is


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exactly one of the inverse reactions involved in the gassing evolution in a LIB and is activated by Lewis-acid catalyst. Through substituting LTO NPs for the Lewis acid described in the reaction and analyzing the yield of the propylene carbonates (PC) by gas chromatography/mass spectrometry (GC/MS), we found that LTO NPs show apparently catalytic activity toward the CO2 cycloaddition, which indicate the existence of abundant Lewis-acid centers on the surface of LTO NPs. To confirm our speculation, TiNx coated LTO NPs (TiNx@LTO) were investigated as contrast.28 The result shows TiNx coating layer is able to cover up the Lewis-acid center and decrease PC yield of the reaction. In other way, the existence of Lewis-acid center on the surface of LTO NPs should also be responsible for gas evolution in LTO-based LIBs. Coating strategies may be used to change surface chemistry and further used to suppress gas production to some extent in lithium ion batteries. These findings may also indicate the feasibility to evaluate surface chemistry of electrode material as well as gas evolution in lithium ion batteries through the CO2 cycloaddition reaction. 2. Experimental Section 2.1 Materials fabrication: The Li4Ti5O12 nanoparticles (LTO NPs) were fabricated by a sol– gel process. Li metal (4mM) was dissolved into ethanol (50ml) in glove box filled with Ar atomosphere so as to form lithium ethoxide solution. Pluronic F-127 (0.5 g) and tetrabutyl titanate (5mM) were then added to the prior solution in the open air. The mixture was then dried in a rotary evaporator to remove excess ethanol in the solution until a viscous brown solution was obtained. Pour the solution into a round dish preloaded with 5 mL of ethanol/water (v:v=1:1) mixture solution. A white LTO precursor will be obtained, which was dried at 100 °C in an oven overnight. LTO NPs was obtained by calcining the precursor at 700 °C for 10 h in a tube furnace


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in air. TiNx-coated LTO NPs were obtained by calcining LTO NPs at 700 °C in NH3 atmosphere for 1 h. 2.2 Cycloaddition reactions: The cycloaddition reactions were carried out in a 100 mL stainless high-pressure Parr 4560 reactor equipped with mechanical stirrer and Parr 4848 controller (Parr Instrument Co.). Firstly, LTO, KI (1 mM) and propylene epoxide (100 mM) were sealed to the Parr reactor. Open the inlet and outlet valves of the reactor, pass into CO2(99.99 %) through the inlet valves for 30 seconds to remove the remaining air in the Parr reactor and close the outlet valve until the pressure in the reactor reaches up to 2.0 MPa, then stop gassing into the reactor and close the inlet valve. Raised the temperature rapidly to 125°C and keep stirring for 1 hr, then cooled the reactor to room temperature and slowly release the inner pressure through the outlet valve. The liquid products were collected and analyzed by GC (Agilent Technologies, 6890N) and GC-MS (SHIMADZU, GCMS-QP2010s). 1HNMR spectra were measured on Bruker DMX-400 spectrometer. Chemical shifts of NMR were reported in ppm relative to the singlet of CDCl3 at 7.26 ppm. 2.3 Electrochemical characterization: electrochemical measurements were performed using coin-type cells assembled in an argon-filled glove box. For preparing the working electrode, a mixture of active material, super-P (SP), and poly (vinyldifluoride) (PVDF, Aldrich) at a weight ratio of 80:10:10 were pasted on a pure aluminum foil. The loading mass of active materials is about 2-3 mg cm−2. Pure lithium foil was used as a counter electrode. A celgard-2400 porous polypropylene was used as a separator. The electrolyte consisted of a solution of 1 M LiPF6 in ethylene carbonate /dimethyl carbonate (EC:DMC, 1:1, v%). Galvanostatic tests of the assembled cells were carried out in the voltage range of 1.0 – 3.0 V (vs. Li+/Li). The specific capacities were calculated based on the mass of active materials.


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2.4 Structural Characterization. High-resolution transmission electron microscopy (HRTEM) was performed using a Tecnai G2 20S-TWIN transmission electron microscope (TEM) worked at an accelerating voltage of 200 kV. EDX elemental mappings were recorded by a JEOL 2100F transmission electron microscope operated at 200 kV. The phase and the crystallographic structure of the NPs were characterized by powder X-ray diffraction (XRD) using a Regaku D/Max-2500 diffractometer equipped with a Cu Kα1 radiation (λ = 1.54056 Å). 3. Results and Discussion

Figure 1. a) XRD patterns of bare LTO and TiNx coated LTO, b) HRTEM of TiNx@LTO and c) elemental mapping of the corresponding N, O, and Ti EDX maps of TiNx@LTO Figure 1a shows the XRD patterns obtained from materials with and without TiNx coating layer. All these patterns can be well indexed as spinel Li4Ti5O12 (JCPDS Card No. 49-0207, space group Fd-3m (227), a=8.36Å). Small amounts of rutile-TiO2 appear in the TiNx@LTO NPs. All the XRD patterns were normalized based on the intensity of (111) Bragg diffraction line. Figure 1b shows the HRTEM images of LTO NPs with TiNx coating layer. The observed and


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calculated d-spacings from the HRTEM images were measured to be 0.48 nm, which matches well with the d-spacing of {111} facets of spinel LTO. Figure 1c shows the elemental distribution of TiNx@LTO NPs obtained by transmission electron microscopy (TEM). Nitrogen, oxygen and titanium element distribution signals are precisely mapped and displayed on the right side of Figure 1c. According to the image, nitrogen signals uniformly dotted in the entire field of view, together with the HRTEM image shown in Figure 1b, TiNx@LTO NPs exhibits a uniform 5nm of TiNx coating layer on LTO NPs.

Figure. 2 a) GC/MS chromatograms in methanol and b) 1HNMR spectrum in CDCl3 of the products collected from the cycloaddition reaction using LTO and KI as catalyst, the unknown in NMR spectrum is denoted as star (*). To investigate the effect of surface chemistry of LTO NPs for LTO-based LIBs, LTO NPs together with KI was used to catalyze the cycloaddition reaction.26.27 The liquid product of the cycloaddition reaction was analyzed by GC/MS and 1HNMR. The collected chromatogram (Figure 2a) displays one distinct peak of propylene carbonate (PC) retained less than 18 min. 1

HNMR was carried out in order to pinpoint characteristic signals (Figure 2b) of propylene

carbonate. Both GC/MS and

1

HNMR results indicate that LTO NPs can catalyze the

cycloaddition reaction with excellent selectivity.


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Table 1. Cycloaddition of CO2 and PO under different conditions Li4Ti5O12 [mg ] 50 0 50

KI [mM] 0 1 1

PC Yield [%] 0 ～1.4% ～34.5%

Table 1 shows the PC yields of cycloaddition reaction under different activator. A propylene carbonate (PC) yield of only 0 % was obtained when using LTO NPs alone as catalyst while only 1.4% for KI. These two results indicate that neither KI nor LTO alone are inactive towards the cycloaddition reaction. However, the yield of PC reached 34.5% when adding LTO and KI together into the reactor. As reported, the cycloaddition reaction is catalyzed by Lewis-acid and nucleophile groups.26,27 Lewis-acid is used to activate the epoxide and nucleophile groups KI help to open the ring of the epoxide. Besides traditional chemical substance called Lewis-acid, hydroxyl groups stabilized on the surface of inorganic NPs have also been able to activate the epoxide. For example, Song et al. reported that Fe(OH)3 NPs together with KI increase PC yields effectively, because Fe(OH)3 NPs has large amount of hydroxyl groups on its surface Based on these findings, we proposed a solid/liquid interfacial hydrogen-bond-assisted mechanism for the cycloaddition reaction catalyzed by LTO NPs. As shown in Figure 3, the mechanism is similar to that described for a heterogeneous catalytic system26 with Fe(OH)3 and KI. In our case, we take the dangling Ti bonds on the surface of the LTO NPs for example. The dangling Ti bonds act as a Lewis-acid center which can activate the epoxide by forming a Ti-O


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coordination bond. The activated epoxide then undergoes ring opening and the intermediate can be stabilized by nucleophilic attack of I- anion from KI. The oxygen anion of the ring-opened epoxide then interacts with CO2 to form an alkyl carbonate anion, which is transformed into a cyclic carbonate by intra-molecular substitution of I- anion in the subsequent step. It should be noted that whether the dangling Ti bonds or the hydroxyl groups on LTO surface play a role as Lewis-acid center is not important, because both of them are able to catalyze the cycloaddition reaction. There is a key difference between the forward and backward reactions of the CO2 cycloaddition. In the forward reactions, high activation energy is needed to open the threemembered epoxide (PO) rings, nucleophile groups KI help to open the PO ring and stabilize the unstable oxypropyl intermediate. However, in the backward reactions, it was a five-membered PC ring to be opened, the activation energy is much less than that to open a three-membered PO rings, KI is not required to stabilize unstable oxypropyl groups.

Figure 3. Illustration of cycloaddition process between CO2 and epoxide at interface of LTO nanoparticles assisted by KI. In order to confirm our speculation further, the effect of surface chemistry of TiNx@LTO NPs on cycloaddition reaction was investigated as contrast. Table 2 shows the results of cycloreaction activated by TiNx@LTO NPs. A propylene carbonate (PC) yield of less than 4% was obtained


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when using TiNx@LTO as activator. These results clear indicate that coating method with TiNx is effective to decrease the amount of Lewis-acid center, i.e. the dangling Ti bond on LTO surface. Considering it is mainly the Lewis-acid species be responsible for gas evolution in LTO batteries, yet coating strategy can be regarded as an effective way to suppress gassing problem, however, whether or not coating is an effective way depends on whether it can decrease the amount of Lewis-acid center on the surface of electrode material. Table 2. Effect of different LTO NPs on the yield of PC. Particles [50 mg] bare LTO TiNx@LTO

PC Yield [%] 34.5 %

Investigation into the Surface Chemistry of Li4Ti5O12 Nanoparticles

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