Nanoscale Porous Lithium Titanate Anode for Superior High

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Nanoscale Porous Lithium Titanate Anode for Superior High Temperature Performance Pankaj K Alaboina, Yeqian Ge, Md-Jamal Uddin, Yang Liu, Dongsuek Lee, Seiung Park, Xiangwu Zhang, and Sung-Jin Cho ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.6b00895 • Publication Date (Web): 02 May 2016 Downloaded from http://pubs.acs.org on May 6, 2016

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ACS Applied Materials & Interfaces

1

Nanoscale Porous Lithium Titanate Anode for Superior High Temperature Performance

Pankaj K Alaboina†, Yeqian Ge‡, Md-Jamal Uddin†, Yang Liu§, Dongsuek Lee¥, Seiung Park¥, Xiangwu Zhang‡, and Sung-Jin Cho†,∗ †

Joint School of Nanoscience and Nanoengineering, North Carolina Agricultural and Technical State University, Greensboro, NC, 27401, USA

‡

Department of Textile Engineering, Chemistry, and Science, North Carolina State University, Raleigh, NC, 27695, USA

§

Department of Materials Science and Engineering, North Carolina State University, Raleigh, NC, 27695, USA ¥

Panaxetec Inc., Nonsan-si, Chungchenongnamdo, 320-944, South Korea

ACS Paragon Plus Environment


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2 ABSTRACT

In this work, nanoscale porous lithium titanate (LTO) anode material was synthesized by using aqueous spray drying method after ball milling. The size of the LTO nanoparticles was optimized to 200 nm, due to its considerable moisture absorption levels for stable performance and its cooperation to make good quality electrodes found with testing. The electrochemical performance of the synthesized LTO nanoparticles was found to be very stable at high operating temperature (50°C) and high current rate (5C) which was worth noticing than its usual unfavorable behaviors (gas generation and surface phase transitions) at higher temperatures. In the post-analysis on the aged LTO cells, high resolution-transmission electron microscope (HRTEM) and fast fourier transform (FFT) measurements reveal that the LTO phase transitions are maintained to very thin surface level (3-5 nm) even after 500 cycles at 50°C. Moreover, the synthesized LTO material showed stable cycling with a high capacity of 138.74 mA h g-1 at 1C rate and 111.53 mA h g-1 at 5C rate. Furthermore, high columbic efficiency and excellent capacity retention over 500 cycles at 50°C was achieved. The enhanced electrochemical properties can be attributed to the increase in surface area and shortened Li+ diffusion lengths due to the nanoscale primary particle and porous structure of the synthesized LTO particles.

KEYWORDS Lithium titanate anode; aqueous spray drying; nanoscale porous lithium titanate; surface phase transition; high-temperature performance


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3 1. INTRODUCTION In our constant endeavor to improve the batteries, lithium titanium oxide, Li4Ti5O12 (hereafter denoted as LTO), as anode materials hold a true significant position. LTO has a theoretical capacity of 175 mA h g-1 and has a spinel structure (space group Fd3m) which favors high rate charging and discharging. Although graphite has a higher capacity, around 372 mA h g-1, the interest in LTO still remains very high because of its ability for relatively faster lithium intercalations and long cycle life 1. In addition, high reversible capacity, almost no solid electrolyte interphase (SEI) layer, promising high rate capability, and low self-discharge are some of the trademark properties of the LTO material, making them attractive candidates for the high energy applications, especially for electric vehicles sector

2,3

. However, at high

temperatures, such as above 50°C, LTO suffers from surface phase transition that leads to severe gas generation issue, which makes it unsafe and brings the biggest concern to use them for commercial applications. The unstable nature and the serious gassing issue in LTO based batteries at elevated temperatures is not fully understood, but tremendous efforts were made over the recent years to get into the roots of the issue and find a remedy. Initially, it was understood that decomposition of LIPF6 into PF5 component in the presence of moisture promoted gas generation 4. In other works, it was reported that traces of residual moisture in the electrodes and the electrolytes was the primary source of gas generation

5–7

. Likewise, some papers reported the reaction between

LTO and electrolyte as the main reason for gassing at high temperature

8,9

. In a recent paper, it

was reported that at higher temperatures (above 50°C), titanium oxide or Ti3+/Ti4+ possibly acts as a catalyst and promotes the chemical reduction reaction of the solvent(organic electrolyte) on the surface of LTO, leading to the severe gassing issue. H2 as major gas component (from traces



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4 of moisture), CO2 and CO (derived from electrolyte decomposition), CH4, and few other gases are released

5–7,9,10

, illustrated in Scheme 1. LTO, when subjected to electrochemical

charge/discharge cycles, was found to promote possibly decarbonylation, decarboxylation, and dehydrogenation of alkoxy group of electrolyte solvents leading to gas generation. Moreover, it was also found that gassing was associated with LTO (111) facets 9,11. The gassing reaction was identified to occur at the surface of LTO (111) plane and to lead to the phase transition of (111) plane to (222) plane and followed by forming (101) plane of anatase TiO2, as illustrated in scheme 1. Also, these interfacial reactions were reported to result in the formation of solid electrolyte interphase (SEI) films covering the LTO surface which later helped to act as a barrier between LTO surface and electrolyte 9. Guo et al. reported a novel Li4Ti5O12–rutile TiO2 (LTO– RTO) hybrid nanowire array electrode that showed 83.6% capacity retention after 500 cycles at 2.5C rate and 60°C. The stable cycling was attributed to the presence of less (111) facets in the synthesized LTO-RTO which restrained gas generation and exhibited ultralong lifetime at 60°C 11

. Treatment with surface modification agents or addition of additives was considered as a

possible simple medication to reduce the catalytic activity of LTO, suppress the interfacial reactions, and solve the gassing issue. For instance, p-toluenesulfonyl isocyanate (PTSI) as electrolyte additive was found to suppress the interfacial reactions

12

. Recently, Liu et al.

synthesized carbon coated LTO nanowires with high crystallinity which exhibited 78% capacity retention at the end of 500 cycles at 1C rate and temperature of 55°C

13

. Similarly, surface

treatment methods, for example, coating LTO with a nanoscale carbon layer 9, zinc oxide (ZnO) 8

, AlF3

14

, or with ion-conductive polyimide layer

15

, were found to reduce the interfacial

reactions effectively and mitigate gassing to a significant extent.


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5 Herein, we report the synthesis of nanoscale porous LTO material and present its stable electrochemical performance at high temperature (50°C) and high charge/ discharge rates (5C rate) without performing any post surface treatments. Simple spray drying method followed by high-temperature calcination was used for synthesis, considering the capability of the synthesis process to be launched for large scale productions. Similar spray drying methods for LTO synthesis has been demonstrated previously by other researchers

16–19

. The significance of the

method discussed here is the synthesis using water, and the final products exhibiting nanoscale and highly porous structures. In addition, the grain size of the LTO particles can be tuned and controlled by the calcination temperature

17

. The synthesis was performed in a total time of

around 12-14 hours and no further treatments like surface coating 8,9,13–15,20 were performed, and the prepared LTO material based cells were found to exhibit stable electrochemical performance. Scheme 1 summarizes the stable performance achieved from the synthesized nanoscale porous LTO. The nanosize of the LTO particles shortens the Li+ diffusion lengths which allows fast diffusivity of Li+ for stable electrochemical performance at elevated temperatures 17,21–23. Hence, there are almost no free electrons on the LTO surface to participate in gassing, and finally, the result is reflected in its stable electrochemical performance. However, there is a trade-off of moisture absorption when scaling down to nanosize. Synthesized porous LTO with smaller nanoparticles would absorb relatively more moisture from the atmosphere than with larger nanoparticles due to the increase in surface area and would require strict moisture control levels. Considering moisture content also as one of the main reasons for LTO’s unstable behavior4,6, the synthesized LTO particles were optimized to 200 nm size (LTO-200). By controlling the calcination temperature the grain size of the LTO particles can be tuned and the LTO particles with grain size’s of 150 nm and 200 nm size were synthesized at 760°C and 850°C calcination



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6 temperatures, respectively. LTO-200 showed very low moisture absorption levels (around 500 ppm) when compared to LTO with 150 nm particles (LTO-150). The details on the procedure conducted for moisture absorption test are described in the supporting information. Also, the LTO-150 particles were added with carbon nanotube solution (5% CNT solution) in deionized water and mechanically mixed and dried to form LTO-CNT. The moisture absorption levels were found to be low for LTO-200 even when compared to LTO-CNT (Supporting information Figure S1). The low moisture absorption level showed by LTO-200 can be attributed to its relatively low surface area which was confirmed by surface-area measurement using Brunauer Emmett Teller (BET) method, Micromeritics ASAP 2020 (Supporting information Table S1). Fell et al. and Burns et al. reported that the LTO-based anodes do not show significant performance degradations when moisture levels are up to 1000 ppm

6,22

. Therefore, although

LTO-200 showed a moisture absorption of around 500 ppm, they would not cause significant performance degradation and would not require strict moisture control, as illustrated in Scheme 1 6,24

. Moreover, in the slurry viscosity test using Brookfield Viscometer, LTO-200 showed steady

behavior with increasing shear rate (Supporting information Figure S2), indicating good dispersion of the nanoparticles in the slurry which is critical for a good quality electrode, especially when handling nanosized electrode material. Furthermore, LTO-200 proved better again in the adhesion strength test, showcasing high peeling strengths for LTO-200 and LTOCNT when compared to the LTO-150 (Supporting information Figure S3). Considering the results of the moisture absorption investigations, slurry viscosity tests, and the adhesion strength tests, the size of the LTO nanoparticles were optimized to 200nm (LTO-200, hereafter denoted as just LTO) for the best utilization of the properties that come along with nanosize.


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7 Scheme 1. Schematic of the synthesized LTO (Li4Ti5O12) performance with 200 nm particle size. Severe gassing and phase transformation with 1 µm LTO particle at 25°C as reported in Ref. 9 (top row). Synthesized LTO with 200 nm primary particle size showing excellent electrochemical stability at 50°C with minor phase transformation (bottom row).

2. EXPERIMENTAL SECTION Nanoscale Porous LTO Synthesis. In this research, nanoscale and highly porous LTO powder were synthesized by spray drying method from lithium carbonate (Li2CO3) and nanocrystalline TiO2 particles (30-40 nm) as precursors, followed by high-temperature sintering as shown in Figure 1. The stoichiometric amounts of precursors, Li2CO3 (27.25 wt.%, Mitsui Chemical) and anatase type TiO2 (72.75 wt.%, Cosmo Catalysts Co., LTD.), were mixed with deionized water by ball milling (0.3 mm bead size) for 5 h to obtain a slurry; then 3 wt.% of dispersant ammonium polycarboxylate was added and dissolved in the slurry and mixed for another 2 h.



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8

Figure 1. Nanoscale porous LTO (Li4Ti5O12) synthesis using aqueous spray drying. The homogeneous slurry obtained was spray-dried with the inlet and outlet temperatures of 200 and 100°C, respectively. The dried spherical powders were heated at 850°C for 5 h in a designed alumina crucible to obtain the nanoscale porous LTO powder, which was then ground gently for analyses. Material Characterization. Scanning electron microscope (SEM) analysis to study the morphology of the synthesized LTO particles was performed by using Carl Zeiss Auriga-BU FIB FESEM. To estimate the size of the particles, SEM analysis, and particle size distribution measurements were performed. Elemental composition analysis was conducted by using energy dispersive X-ray spectrometer (EDS) Bruker Nano with X-Flash Detector 5030). To study the phase and structure, the X-ray diffraction (XRD) pattern was obtained on a Bruker D8 X-ray diffractometer using Cu k-alpha radiation with a scan rate of 0.02 degrees sec-1 from Bragg angle (2θ) of 10° to 80°.


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9

Figure 2. (a) Scanning electron microscope (SEM) images of the synthesized nanoscale porous LTO (Li4Ti5O12) at 1K, 5K, and 50K magnification; (b) Energy dispersive X-ray spectroscopy (EDS) confirming the elemental composition; and (c) Particle size distribution showing the average particle size.



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10

Figure 3. X-ray diffraction (XRD) pattern of the synthesized LTO (Li4Ti5O12) nanoparticles matching with JCPDS Card No. 00-049-0207. Electrochemical Characterization. The electrochemical properties were performed using CR2032 coin cells. The ratio of the active material with carbon black (Super C65, Timcal) and polyvinylidene fluoride (PVDF-5130, Solvay) binder dissolved in 1-methyl-2-pyrrolidinone (NMP, ≥99.5%, Aldrich) solvent was 8:1:1. The counter electrode used was lithium metal (99.9%, Aldrich), and Celgard polypropylene (PP) was used as a separator. The electrolyte used was 1 M LiPF6/ethylene carbonate (EC) + dimethyl carbonate (DMC) + diethyl carbonate (DEC) (1:1:1 by volume, MTI Corporation). The 2032 coin cells were assembled in a high purity argonfilled glove box. The assembled batteries were rested for 24 h before testing. Discharge-charge properties with the cutoff potential of 1.2 V-2.8 V (vs. Li/Li+) were measured by using Arbin battery test system (BT2000) battery tester at a temperature of 50°C provided by environmental test chambers (Ransco). The cyclic performance was measured galvanostatically at 1C (175 mA g-1) for 500 cycles and 5C (875 mA g-1) for 100 cycles. For the post-analysis on the aged LTO cell, the battery electrodes were disassembled and washed in dimethyl carbonate


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11 (DMC), then transferred into a JEOL 2010F FEG-TEM for structure and phase characterization. High-resolution transmission electron microscope (HRTEM) and fast fourier transform (FFT) measurements were performed to get a closer look at the surface and check the level of surface phase transitions if occurring, respectively.

3. RESULTS AND DISCUSSION Morphology, Size, and Structure. SEM images in Figure 2(a) reveal that the synthesized LTO particles are highly porous and have almost spherical shape secondary particles which are formed by the association of large number of nearly round shape primary particles. The EDS analysis shows the presence of titanium and oxygen, and considerably matches the elemental composition of the synthesized material to an LTO material as reported in Figure 2(b). The size of the LTO primary particles is approximately 200 nm average and the secondary particle size distribution curves provided an estimated size of secondary particles population falling in the range 2 µm-57 µm diameter which can be interpreted from the reports displayed in Figure 2(a) and Figure 2(c), respectively. Also, the synthesized LTO particles had a high tap density of 1.1 g cm-3 and a surface area of 2.7 m2 g-1 (more details listed in Supporting information Table S1). Figure 3 shows the XRD pattern of the as-prepared LTO material. The pattern can be indexed to a spinel LTO with space group Fd3m showing diffraction peaks at 2θ = 18.36, 30.21, 35.59, 37.23, 43.26, 47.36, 57.21, 62.84, 66.07, 74.36, 75.37, and 79.36°, matching with JCPDS standards Card No. 00-049-0207. Electrochemical Performance at High Temperature and High Rate. The galvanostatic charge–discharge tests at 1C from 1.2 to 2.8 V at a temperature of 50°C were conducted on the Arbin battery test system. In Figure 4(a), the initial charge capacity of LTO was 136.05 mA h g-



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12 1

, and the irreversible capacity in the 1st cycle was 32.17 mA h g-1. The high irreversible capacity

and low charge/discharge efficiency in the 1st cycle can be attributed to the lithium loss in SEI film formation. This is due to high active surface area of the LTO nanoparticles that participate in the surface side reactions to form the SEI. Similar behavior in the initial cycle were reported in the prior works

16,20,21,25–29

. In the next two cycles, the charge-discharge curves of LTO were

nearly repeatable, implying good reversibility. Cycling performance and coulombic efficiency of LTO for 500 cycles are shown in Figure 4(b). From 1st cycle to 32nd cycle, the capacity was increased slightly from 136.05 mA h g-1 to 147.51 mA h g-1, then went to 138.74 mA h g-1 at 500th cycle. The 1st and 500th cycle coulombic efficiency were 80.88% and 100.50%.

Figure 4. (a) The first three galvanostatic cycles of LTO (Li4Ti5O12) and

(b) Cycling

performance and coulombic efficiency of LTO at 1C (175 mA g-1) from 1.2 to 2.8 V at a temperature of 50°C for 500 cycles (loading 3.14 mg cm-2); (c) Cycling performance and coulombic efficiency of LTO at 5C (875 mA g-1) from 1.2 to 2.8 V at a temperature of 50°C for 100 cycles.


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13 The galvanostatic charge–discharge tests at 5C from 1.2 to 2.8 V at a temperature of 50°C for 100 cycles were also shown in Figure 4(c). The charge capacity at the 1st and the 100th cycle was 98.12 mA h g-1 and 111.53 mA h g-1, and coulombic efficiency was 79.15% and 100.02%, respectively. Although the temperature was high (50°C), and also at faster charging rate (5C) with a very high loading of 3.14 mg cm-2, the synthesized LTO showed stable performance during long term cycling. For instance, the 115.53 mA h g-1 capacity at the end of the 100th cycle at 5C and 50°C reported is high compared to the previously reported ~110 mA h g-1 at 5C and 55°C with carbon coated LTO nanowires 13.This is a great electrochemical performance and very stable behavior of LTO-anode based cells at high temperature (50°C) and high rate (5C) operation, unlike relatively low performance and unstable behaviors which were reported in the prior works considering similar conditions

4,20,30

.In addition, it should be noted that all this was

achieved without performing any additional post surface treatments on the synthesized LTO anode material. In Figure 4(b) and 4(c), the initial columbic efficiency of LTO is very low and reaches close to 100% with extended cycling. The coulombic efficiency of LTO in the 1st cycle can be very low depending on the particle size, loading, and other factors. Especially the nanosized active material can lead to a very low 1st cycle coulombic efficiency 20,31. The synthesized LTO was a spray-dried, porous, and nanosized material with high surface area. Thus, there were more active sites on the anode surface during the first cycle charging, and the charge capacity was much higher (136.05 mA h g-1). However, during the first cycle discharge, the amount of lithium ions de-intercalated from LTO was lower due to SEI formation which resulted in low coulombic efficiency and high irreversibility for the 1st cycle. Similar results have been reported by other researchers

20,31

. Another important factor to consider is the electrode loading. The LTO-anode



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14 based cells were built with very high electrode loading of 3.14 mg cm-2. Due to high loading, with extended cycling, the lithium ions get intercalated into new active sites of LTO anode and the wettability of the anode by the electrolyte gets improved, resulting in a slight increase in capacity up to few initial cycles, as observed in Figure 4(b) and 4(c) cycling. However, after the active material utilization is saturated there is no further increase in the capacity in the later cycles. Similar behavior were reported in prior works

20,31,32

. However, Synthesized LTO

possessed excellent cycling ability at 50°C both at 1C and 5C, demonstrated a promising anode material for lithium ion batteries at high temperature and high rate. Surface Phase Transitions. To investigate the surface phase transitions, synthesized LTO based cells were cycled at 1C (175 mA g-1) in the potential window 1.2 to 2.8 V at a temperature of 50°C. The LTO electrode after completing 1st cycle was disassembled and washed with dimethyl carbonate (DMC), and then transferred into a high-resolution TEM (HRTEM) for the structural phase transition study. Figure 5(a) is an HRTEM image of an LTO particle after 1 full cycle. The surface of the LTO particle is relatively clean, indicating the formation of the solid electrolyte interphase (SEI) layer is minor on the LTO during the cycle. It is interesting to note that the crystal structure of the surface (shell) was changed in comparison with that in the interior (core), showing a phase transformation occurred at the surface within a 3-5 nm layer. The phase boundary is marked by a thick dashed line to guide the eye in Figure 5(a) and 5(d). Fast fourier transform (FFT) of the interior area agrees with the spinel structure of LTO (space group Fd3m) with [11-2] zone axis (Figure 5(b)) while the FFT of the shell shown in Figure 5(c) exhibits that the d-spacings were changed to half of the corresponding d-spacings in LTO, indicating the formation of a new crystal structure at the surface area. HRTEM also characterized the LTO electrode after 500 cycles. Figure 5(d) shows an HRTEM image of an LTO particle after 500


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15 cycles. FFT of the interior area illustrated in figure 5(e) confirms the LTO spinel structure (space group Fd3m) with [01-1] zone axis, and the FFT of the surface area shown in Figure 5(f) also exhibits the d-spacings were changed to half of the corresponding d-spacings in LTO. The surface layer with this new phase has a thickness of 3-5 nm (Figure 5(d)) after 500 cycles, which is comparable to the surface layer on LTO after 1 cycle (Figure 5(a)). The new phase at the surface with d-spacings of only the half of the corresponding d-spacings in LTO suggests the unit cell of this new phase is only 1/8 of that of LTO, resulting from the rearrangement of Li, Ti and O atoms in the LTO crystal. The phase transition is the transformation of LTO (111) plane to (222) plane and followed by forming (101) plane of anatase TiO2 9. In addition to the Ti4+ ions participation in chemical reduction reactions, Li+ and O2- ions associated with (111) plane were claimed to be taken out from the LTO to participate in the phase transition reactions giving rise to gassing 9. The recently developed revolving scanning transmission electron microscopy (RevSTEM) technique can be used to determine the locations of Li, Ti, and O atoms, which can be used to understand the phase at the surface and lattice distortions from lithium insertion/extraction in LTO 33.



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16

Figure 5. High-resolution TEM (HRTEM) characterization of LTO (Li4Ti5O12) after 1 cycle and 500 cycles. (a) HRTEM image of a LTO particle after 1 cycle, showing a phase transformation at the surface (the boundary was marked by red dashed line); (b) Fast Fourier transform (FFT) of the image (a) in the interior (core) area, confirming the LTO structure with [11-2] zone axis; (c) FFT of the image (a) in the surface (shell) area, where the d-spacings were changed to half of the corresponding d-spacings in LTO; (d) HRTEM image of an LTO particle after 500 cycles, a phase boundary near the surface was marked by red dashed-line; (e) FFT of the image (d) in the interior (core) area, confirming the LTO structure with [01-1] zone axis; (f) FFT of the image (d) in the surface (shell) area, where the d-spacings were changed to half of the corresponding dspacings in LTO. The phase transformation and gas generation could be attributed to the reaction of LTO electrode with electrolyte9 and the presence of residual moisture in the electrodes and


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17 electrolyte6. The LTO when subjected to electrochemical charge/discharge cycles promotes decarbonylation, decarboxylation, and dehydrogenation of electrolyte solvents leading to surface phase transition 9. LTO with 200 nm primary particles has optimum moisture absorption levels, around 500 ppm (Supporting information Figure S1), which was not high enough to cause any performance degradation

6,24

. The gas generation study can also be performed and quantified by

building pouch cells or preparing gauge pressure cells incorporated with gas chromatographymass spectrometry (GC-MS). However, this would be a broad study and requires separate extensive research. Phase transformation, gas generation, and electrochemical stability at high temperature are all mutually associated 4–10,14,24,29,34. In the post-analysis results using HRTEM, it is worth noticing that the thickness of the surface layer on LTO after 500 cycles is almost the same as that after 1 cycle (3-5 nm). This is considerable evidence indicating that the gassing reactions or the side reaction on the LTO during battery cycles are minor, and the nanoscale porous lithium titanate anode is very stable at high temperature, which is in good agreement with the stable cyclability shown in the coin cell results.

4. CONCLUSIONS The synthesized nanoscale porous LTO material exhibited stable electrochemical performance at high temperature (50°C) and high rate (5C). The stable electrochemical properties can be attributed to the shortened Li+ diffusion lengths due to the nanoscale size of the primary particles (~200 nm optimized size). The synthesized LTO particles with 200 nm size has considerable moisture absorption levels (~500ppm), which was not high enough to cause any performance degradation

6,24

, and also the nanosize favors high rate Li+ diffusion, leaving almost no free

electrons on the surface of LTO to participate in further parasitic surface phase reactions 9. High



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18 columbic efficiency (~100%), and excellent capacity retention at 1C over 500 cycles and 5C rate over 100 cycles at 50°C were demonstrated. Even after 500 cycles at 1C rate and high temperature 50°C, the phase transitions at the surface were significantly low (3-5 nm) indicating minor interfacial side reactions and thereby showcasing enhanced stability.

ASSOCIATED CONTENT Supporting Information Moisture absorption test (Figure S1), slurry viscosity test (Figure S2), and adhesion strength test (Figure S3) for the size optimization of the synthesized LTO nanoparticles.

AUTHOR INFORMATION Corresponding Author ∗

E-mail: [email protected]

Notes The authors declare no competing financial interest.

ACKNOWLEDGMENTS The authors gratefully acknowledge material support from Panaxetec Inc., South Korea, high technical guidance from the North Carolina State University, USA, and a startup financial support from Joint School of Nanoscience and Nanoengineering, NC, USA.

REFERENCES (1)

Han, X.; Ouyang, M.; Lu, L.; Li, J. Cycle Life of Commercial Lithium-Ion Batteries with


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Abstract Graphic: For Table of Contents Only



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Scheme 1. Schematic of the synthesized LTO (Li4Ti5O12) performance with 200 nm particle size. Severe gassing and phase transformation with 1 µm LTO particle at 25°C as reported in Ref. 9 (top row). Synthesized LTO with 200 nm primary particle size showing excellent electrochemical stability at 50°C with minor phase transformation (bottom row). 172x97mm (300 x 300 DPI)


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Figure 1. Nanoscale porous LTO (Li4Ti5O12) synthesis using aqueous spray drying. 114x64mm (300 x 300 DPI)



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Figure 2. (a) Scanning electron microscope (SEM) images of the synthesized nanoscale porous LTO (Li4Ti5O12) at 1K, 5K, and 50K magnification; (b) Energy dispersive X-ray spectroscopy (EDS) confirming the elemental composition; and (c) Particle size distribution showing the average particle size. 114x64mm (300 x 300 DPI)


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Figure 3. X-ray diffraction (XRD) pattern of the synthesized LTO (Li4Ti5O12) nanoparticles matching with JCPDS Card No. 00-049-0207. 57x32mm (600 x 600 DPI)



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Figure 4. (a) The first three galvanostatic cycles of LTO (Li4Ti5O12) and (b) Cycling performance and coulombic efficiency of LTO at 1C (175 mA g-1) from 1.2 to 2.8 V at a temperature of 50°C for 500 cycles (loading 3.14 mg cm-2); (c) Cycling performance and coulombic efficiency of LTO at 5C (875 mA g-1) from 1.2 to 2.8 V at a temperature of 50°C for 100 cycles. 57x32mm (600 x 600 DPI)


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Figure 5. High-resolution TEM (HRTEM) characterization of LTO (Li4Ti5O12) after 1 cycle and 500 cycles. (a) HRTEM image of a LTO particle after 1 cycle, showing a phase transformation at the surface (the boundary was marked by red dashed line); (b) Fast Fourier transform (FFT) of the image (a) in the interior (core) area, confirming the LTO structure with [11-2] zone axis; (c) FFT of the image (a) in the surface (shell) area, where the d-spacings were changed to half of the corresponding d-spacings in LTO; (d) HRTEM image of an LTO particle after 500 cycles, a phase boundary near the surface was marked by red dashed-line; (e) FFT of the image (d) in the interior (core) area, confirming the LTO structure with [01-1] zone axis; (f) FFT of the image (d) in the surface (shell) area, where the d-spacings were changed to half of the corresponding d-spacings in LTO. 114x75mm (300 x 300 DPI)


Nanoscale Porous Lithium Titanate Anode for Superior High

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