Large Polarization of Li - ACS Publications - American Chemical

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Large polarization of LiTiO lithiated to 0 V at large charge/discharge rates Cuiping Han, Yan-Bing He, Shuan Wang, Chao Wang, Hongda Du, Xianying Qin, Zhiqun Lin, Baohua Li, and Feiyu Kang ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.6b04239 • Publication Date (Web): 27 Jun 2016 Downloaded from http://pubs.acs.org on July 3, 2016

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ACS Applied Materials & Interfaces

Large polarization of Li4Ti5O12 lithiated to 0 V at large charge/discharge rates

Cuiping Han,a,b,c Yan-Bing He,a Shuan Wang,a Chao Wang,a,b Hongda Du,a Xianying Qin,a Zhiqun Lin,c,* Baohua Li,a,  and Feiyu Kanga,b

a

Engineering Laboratory for Next Generation Power and Energy Storage Batteries,

Engineering Laboratory for Functionalized Carbon Materials, Graduate School at Shenzhen, Tsinghua University, Shenzhen 518055, China b

Laboratory of Advanced Materials, School of Materials Science and Engineering, Tsinghua

University, Beijing 100084, China c

School of Materials Science and Engineering, Georgia Institute of Technology, Atlanta, GA

30332, USA

Abstract: The ability to enhance the specific capacity of Li4Ti5O12 (LTO) is of practical significance and offers the opportunity to increase the energy density of full LTO-based battery. Widening the discharge cut-off voltage to 0 V is an effective way to increase the capacity of LTO at low current density. However, whether the specific capacity of LTO at large current rates can be enhanced remains largely unaware. Herein, intriguingly, we found that, when cycled down to 0 V (i.e., from 2.5-1.0 V to 2.5-0 V), the LTO exhibited greatly enhanced specific capacity at low rates (< 5 C), while showed a rapid capacity fading and E-mail address: [email protected], and [email protected] 1

ACS Paragon Plus Environment


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greatly increased charge/discharge potential gap at high rates (> 10 C). The decreased Li-ion diffusion caused by an extra Li ion intercalation together with increased internal impedance significantly enhance the polarization and reduce the electrochemical reaction kinetics, which in turn hinders the lithiation reaction from Li4Ti5O12 to Li7Ti5O12 and further to Li9Ti5O12.

Keywords: Batteries, Anode, Capacity, Enhancement, Voltage Range

1. Introduction Lithium ion batteries (LIBs) with both high power density and safety property are promising for applications in electric vehicles and large-scale energy storage stations1-4. Electrode materials with high safety and excellent large current charging/discharging properties are of considerable importance for high-power LIBs. It is well-known that the widely used graphite anode shows relatively poor high-rate charging/discharging and safety performance. Clearly, it is crucial to develop advanced anode materials to improve the properties noted above. The spinel Li4Ti5O12 (LTO) material undergoes an extremely small expansion and contraction during charge/discharge cycling5-7, which therefore presents an extremely long cycling life. Moreover, the spinel structure of LTO provides three-dimensional network-like channels for lithium ion diffusion, thereby rendering excellent lithium ion intercalation/de-intercalation reversibility and structural stability, in particular at high charging/discharging rate (eg.30C)

8-12

. Taken together, LTO has emerged as a promising

anode material for high-rate LIBs. However, LTO has a relative low theoretical capacity of 175 mAhg−1, corresponding to three lithium ions intercalation into LTO in the voltage range of 1.0-2.5 V (equation 1). The ability to enhance the specific capacity of LTO is of practical significance and offers the opportunity to broaden its use in high-power LIBs. It has been shown that widening the discharge cut-off voltage to 0 V is an effective way to increase the capacity of LTO at low 2


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current density13. Two extra lithium ions can be further inserted into Li7Ti5O12 to form Li9Ti5O12, accompanied by the reduction of all Ti4+ to Ti3+ in the compound. The discharging of LTO to 0 V can deliver a theoretical capacity of 293 mAhg−1. More importantly, the lower intercalation potential also facilitates a wider working voltage of full LTO-based battery, thus leading to a higher energy density for rechargeable LIBs. [Li3]8a[LiTi5]16d[O12]32e +3Li+ +3e-→ [Li6]16c[LiTi5]16d[O12]32e

(1)

Unfortunately, to date, most of the works have focused on the voltage range of 1.0-2.5 V, the study on the electrochemical characteristics of LTO below 1.0 V is comparatively few. The reason that LTO can be discharged to 0 V is attributed to its strong covalent bonding between Ti and O, which ensures a high thermodynamic stability of LTO during extralithiation14-15. The excellent structural stability of LTO imparts a good cycling performance under small current density over a broad voltage range and a negligible volume change 16-17. Table S1 summarizes some of the reported capacities of LTO anodes discharged below 1.0V. It is seen that by extending the voltage range of LTO, the specific capacities under relatively small current densities were greatly enhanced. However, it is also notable that when discharging to 0V, the capacities of LTO under large current densities (>10C) have not yet been reported. The electrochemical characteristics of LTO at large current densities when discharging to 0V remains largely unknown. Clearly, it is of key importance to systematically scrutinize the effect of cut-off potential below 1.0 V on the electrochemical behaviors of LTO, especially at high current densities. Herein, we report the exploration on the enhancement of specific capacity of LTO at various rates enabled by extending the discharge cut-off voltage to below 1.0 V. The LTO electrodes were prepared by hydrothermal reaction, followed by calcinations. Subsequently, they were examined over different voltage ranges (i.e., 1.0-2.5 V, 0.5-2.5 V, and 0-2.5 V, respectively) and different charge/discharge current densities. The effects of cut-off discharge voltage on the electrochemical performance, structural and phase stability, lithium ion and 3



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electron transportation of LTO were systematically investigated. The LTO discharged to 0 V exhibited markedly enhanced capacity at low current density (< 5 C), while displayed a rapid capacity fading at high current density(> 10 C). The charge/discharge curves presented a large plateau loss at 1.5 V and large increase of the charge/discharge potential gap over the entire rates with widened discharge voltage range (i.e., from 1.0-2.5 V to 0-2.5 V). The increase of the internal resistance and decrease of Li-ion diffusion caused by extra lithium intercalation suggested an enhanced polarization and reduced electrochemical reaction kinetics, which resulted in the capacity fading of LTO electrode when discharged to 0 V.

2. Experimental Section 2.1. Synthesis of LTO Material LTO used in this study was prepared by hydrothermal reaction, followed by heat treatment. Typically, 5.9 mL tetrabutyl titanate (Ti(OC4H9)4, AR) was drop-wisely added into a

60

mL

saturated

solution

of

cetyltrimethyl

ammonium

bromide

(CTAB,

C 16 H33 (CH3 )3 NBr, AR) at ambient temperature (~298 K) under ultrasonication. After complete hydrolysis, a liquid with white suspensions was obtained. The lithium hydroxide solution prepared by dissolving 0.611 g lithium hydroxide (LiOH·H 2 O, AR) in 20 mL purified water was then added to the above suspension. The prepared mixture solution was then transferred into a 150 mL Teflon-lined stainless steel autoclave and maintained at 180oC for 24 h. White precipitate was obtained after the hydrothermal treatment. After drying in an oven at 90oC, the obtained precursor materials were calcinated at 700oC for 7 h in Ar. 2.2. Materials Characterization The phase composition was characterized by X-ray diffraction (XRD, Rigaku D/max 2500/PC using CuKα radiation with λ=1.5418 Å). The morphologies and structures were

4


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examined by a field emission scanning electron microscopy (FE-SEM, HITACH S4800) at 5.0 kV.

2.3. Electrochemical Characterization Electrochemical performance of as-prepared LTO with different cut-off voltages was examined by CR2032-type coin cells assembled in an Ar filled glove box (Mbraum). The coin cells used the as-prepared LTO as cathode materials, lithium foil as anode, and polypropylene (Celgard 2500, Celgard Inc., USA) as separator. The cathode was consisted of 80 wt% the asprepared LTO, 10 wt% acetylene black and 10 wt% polyvinylidene fluoride (PVDF) binder. The cathode slurry was uniformly coated on a copper foil current collector and dried in a vacuum oven at 110oC for 10 h to obtain cathode. The mass loading of LTO was between 2.0~4.0 mg cm-2. The electrolyte employed was a 1M LiPF6 solution in ethylene carbonate (EC) / diethyl carbonate (DEC) / ethyl methyl carbonate (EMC) (volume ratio: 1:1:1). The assembled cells were galvanostatically charged and discharged at various rates over different voltage ranges, that is, 1.0-2.5 V, 0.5-2.5 V, 0-2.5 V (vs. Li+/Li) using a Land 2001A battery testing system under room temperature. These cells are here after referred to LTO-1, LTO-0.5 and LTO-0, respectively. The electrochemical working station (VMP3) was used to measure the cyclic voltammograms (CV) and electrochemical impedance spectrum (EIS) of cells. The CV curves were recorded at scanning rates of 0.1 mVs−1, 0.5 mVs−1, 1.0 mVs−1 and 2.0 mVs−1, respectively. The EIS spectra were measured point-by-point along the discharge and charge processes. The coin cells were kept at open circuit for 2 hrs prior to the EIS measurement at open circuit voltage. In the EIS measurement, the frequency range is 10-2-105Hz and the ac oscillation is 5mV. To examine the structural evolution of LTO during the discharge/charge process, the cells that were first discharged to 1.0V and 0V and then first charged (i.e., recharged back) to 2.5V were disassembled in the glove box. To investigate the structural 5



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stability of LTO as a funciton of cycle numbers, the fully charged coin cells after designated cycles were disassembled in the glove box for further examination. The disassembled LTO electrode was rinsed using dimethyl carbonate (DMC) to remove the electrolyte from the electrode surface, and was dried before examination. The structure of the electrode was characterized by X-ray diffraction (XRD, Rigaku D/max 2500/PC using CuKα radiation with λ=1.5418 Å). The surface morphology of the electrode was examined on FE-SEM (HITACH S4800) at 5 kV. The X-ray photoelectron spectroscopy (XPS) was conducted with a Physical Electronics PHI5802 instrument using X-rays magnesium anode (monochromatic Kα X-rays at 1253.6 eV) as the source. The C 1s region was used as a reference and was set at 284.8 eV. Fourier transform infrared spectroscopy (FTIR, Nicolet IS10) was used to evaluate the structure and compositions of solid electrolyte interface (SEI) films on the cycle-tested LTO electrode in the near infrared (NIR) region (600-2000 cm-1).

3. Results and Discussion The XRD pattern of as-prepared LTO suggested the success in synthesizing phase-pure and well-crystallized spinel LTO (JCPDS card No. 49-0207, space group Fd3m) (Figure 1a). The peaks at 2θ= 18.3o, 30.2o, 35.5o, 37.2o, 43.2o, 47.3o, 57.2o, 62.8o and 66.0o can be ascribed to the (111), (220), (311), (222), (400), (331), (333), (440) and (531) planes of cubic LTO, respectively. No peaks for TiO2 and Li2TiO3 were observed. The SEM measurement and low magnification TEM image revealed that the particle size of LTO was in the range of 100-250 nm (Figure 1b and Figure S1a). High resolution TEM image also confirmed the formation of well-crystalline LTO particles. The lattice fringe has a spacing of 0.483nm, corresponding to the (111) planes of LTO (Figure S1b). The LTO was cycled from 0.1C to 20C over different voltage ranges, that is, from 2.5 to 1.0 V, 0.5 V and 0 V, respectively. The corresponding rate cycling performance and charge/discharge curves are shown in Figure 2. It is clear that the specific capacity of LTO 6


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depended largely on the charge/discharge rate and the discharge cut-off potential. With the increase of charge/discharge rate from 0.1 C to 20 C, LTO cycled from 1.0 to 2.5 V (i.e., sample LTO-1) showed the least capacity fading and perfect charge/discharge flat plateau (Figure 2b). The capacities at 0.1 C, 1 C, 10 C and 20 C were 176 mAhg-1, 161.3 mAhg-1, 147.2 mAhg-1 and 131.7 mAhg−1, respectively, indicating an excellent rate performance. When the cut-off voltage decreased to 0.5 V (i.e., sample LTO-0.5), the corresponding capacities at 0.1 C, 1 C, 10 C and 20 C are 234.9 mAhg-1, 163.4 mAhg-1, 143.3 mAhg-1 and 117.6 mAhg-1, respectively (Figure 2c). Clearly, the specific capacity had a slight improvement at low current rates of 0.1 C, 0.5 C and 1 C, while it decreased and was even lower than that of LTO-1 when the charge/discharge rates were higher than 20 C. When the LTO was discharged to 0 V (i.e., sample LTO-0), the specific capacities at lower current rates (

Large Polarization of Li - ACS Publications - American Chemical

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