Schiff Base as Additive for Preventing Gas Evolution in Li4Ti5O12

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Schiff Base as Additive for Preventing Gas Evolution in Li4Ti5O12 Based Lithium-Ion Battery Jean-Christophe Daigle, Yuichiro Asakawa, Pierre Hovington, and Karim Zaghib ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.7b15112 • Publication Date (Web): 07 Nov 2017 Downloaded from http://pubs.acs.org on November 9, 2017

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ACS Applied Materials & Interfaces

Schiff Base as Additive for Preventing Gas Evolution in Li4Ti5O12 Based Lithium-Ion Battery

Jean‐Christophe Daigleac, Yuichiro Asakawabc, Pierre Hovingtona, and Karim Zaghibac* a

Center of Excellence I transportation electrification and energy storage (CETEES), Hydro‐

Québec, 1800, Lionel‐Boulet blvd., Varennes, Qc., Canada, J3X 1S1 b

Sony Corporation, 1‐7‐1 Konan, Minato‐ku, Tokyo, 108‐0075, Japan

c

Esstalion Technologies Inc., 1804, Lionel‐Boulet blvd., Varennes, Qc., Canada, J3X 1S1

KEYWORDS: Lithium-ion batteries, Lithium titanium oxide, additive, Schiff base, Solidelectrolyte-interphase

ABSTRACT: Lithium titanium oxide (Li4Ti5O12) based electrodes are very promising for longlife cycle batteries. However, the surface reactivity of Li4Ti5O12 in organic electrolytes leading to gas evolution is still a problem that may cause expansion of pouch cells. In this study, we report the use of Schiff base (1,8-diazabicyclo[5.4.0]undec-7-ene (DBU)) as an additive that prevents gas evolution during cell aging by a new mechanism involving the solid-electrolyte-interface (SEI) on the anode surface. The in-situ ring opening polymerization of cyclic carbonates occurs during the first cycles to decrease gas evolution by 9.7 vol% without increasing the internal resistance of the battery.

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Introduction

One of the most promising approaches to limiting climate change is to use greener sources of energy (wind, solar, etc.). However, the electricity output from these sources can fluctuate, thus there is a need for energy storage. The advent of energy storage for wind power, solar plants, and so on, requires a new generation of batteries to optimize the use of these alternate sources of energy. The development of batteries with high rates of charge and discharge is imperative to meet this goal.

Lithium titanium oxide (LTO) is a promising anode material for a high-rate chargedischarge battery.1-4 The use of LTO minimizes the risk of explosion and fire in case of an incident. However, LTO has a very reactive surface and can induce electrolyte degradation with cycling. During battery operation, the lithium titanium oxide and electrolytes (carbonates derivatives) can react with residual water to form CO2, CO, H2, O2 and hydrocarbons. These byproducts cause the pouch cell to expand, which could be a safety issue.5-6 The water is a residual contaminate from the cathode especially with olivine-based cathode materials.7

One of the strategies to prevent electrolyte degradation is to eliminate water from the cathode and anode.7-8 Because the active materials are hydrophilic, the electrodes must be carefully dried. However, this approach has a high-energy cost.2 We identified another strategy that involves a protective coating at the interface on the electrodes. This coating is called SolidElectrolyte-Interphase (SEI) that prevents contact between the electrolyte and the active surface of the electrodes. For example, the decomposition of vinylene carbonate or other compounds as

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an additive in the electrolyte forms a film.9-10 The formation of a shell directly on the active materials before cell assembly is another way to obtain a protection layer on LTO.11

Here, we propose a new and innovative strategy for creating an SEI on the anode surface. We used a Schiff base as an additive in the electrolyte. The 1,8-diazabicyclo[5.4.0]undec-7-ene (DBU) (Scheme 1) promotes in-situ ring opening polymerization (ROP) of propylene carbonate on the surface of LTO in the presence of lithium salt. This strategy was effective for limiting gas generation without increasing the internal resistance of the cell.

Scheme 1. 1,8-diazabicyclo[5.4.0]undec-7-ene (DBU)

Experimental Section

Chemicals. 1,8-diazabicyclo[5.4.0]undec-7-ene (DBU) and N-methyl-2-pyrrolidone (NMP) were purchased from Sigma Aldrich and used without further purification. The lithium iron phosphate (LFP) was purchased from Sumitomo Osaka Cement and the lithium titanium oxide (LTO) from Posco. The PVDF, LIPF6 and the carbonate solvents were obtained from BASF. The carbon Denka Black was from Denka. 3



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Characterisation. FTIR measurements were conducted on a CARY 630 from Agilent. The Gas Chromatography equipped with FID and TCD detectors and HPLC-MS TOF studies were conducted with a Micro GC Refinery Gas Analyser and 6560 Ion Mobility Q-TOF LC-MS, both from Agilent. The NMR analyses were performed on the Varian Inova 300. The chemical shift of deuterated chloroform was used as internal reference. The TGA analysis were performed with a heating rate of 10oCmin-1 from 25-800oC in air in a TGA 550 (TA Instruments). Microstructural characterization was performed using a high-resolution field-emission gun scanning electron microscope coupled with a focussed ion beam using Ga as a liquid metal source (FIB/SEM, Tescan Lyra3). An orthogonal time-of-flight secondary-ion mass spectrometer (TOF-SIMS, TOF-WERK) attached to the FIB/SEM was used for chemical mapping and depth profiling. Ga beam energy of 30 kV was used as the primary ion beam. Both negative and positive ions are recorded using different extractor bias voltage but on a different region. For imaging during TOF-SIMS analysis, 5 kV electrons were used with the sample tilted at 55° in order to orient the sample surface perpendicular the ion beam. Cells Assembly and Electrochemical Measurements. LFP-LTO cells were fabricated as described here. The electrode size is 30 mm x 40 mm. The cathodes consist of LiFePO4 (LFP), carbon black and poly(vinylene difluoride) (PVDF) as binder in a proportion of 90: 5: 5. The slurry was coated on a 15 m aluminium collector by a doctor blade method. The anodes contain Li4Ti5O12 (LTO), carbon black and PVDF as binder in a proportion of 90: 5: 5. The slurry was also coated on a 15 μm aluminium collector by a doctor blade method. The separator is poly(ethylene) based with a 16 m thickness. The electrolyte is composed of 1 molkg-1 LiPF6 with carbonates as solvents. No additive was present in the electrolyte used in the reference studies. Prior to the cycle test, batteries were charged and discharged twice at 0.2 C at 25 °C. xC 4


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is the current that can fully charge/discharge cell capacity in 1/x hour. Charge: CC-CV (Constant Current-Constant Voltage) mode - Voltage: 2.4 V, Current 0.2 C, Cut-off current 0.03 mA Discharge: CC (Constant Current) mode, Cut-off voltage: 0.5 V, Current: 0.2 C. To accelerate gas evolution in the cell, we performed a float test at 2.4 V for 500 hours in the climate chamber with ambient temperature set at 45 °C. The cells were charged at 25 °C to 2.4 V with constant current at 0.2C, followed by constant voltage charge at 2.4 V for 500 hours at 45 °C.

Results and Discussions

Evaluation of a Model System. As already reported, the reaction of a cyclic carbonate in presence of DBU as catalyst initiated by hydroxyl groups is an appealing way to form a poly(carbonate). Examples of organocatalysts and polymerizations are found in the publication from Nederberg et al.12 and Zhang et al.13 This versatile method involves the initiation reaction of DBU and lithium salt as co-catalyst to promote the formation of a linear polymer, as demonstrated by Waymouth group.14-15 Based on these publications, we deem plausible the initiation of polymerization of cyclic carbonate by hydroxyl groups located on the surface of LTO or by the DBU in the lithium-ion cell. The proposed reaction is outlined in Scheme 2.

Scheme 2. Reaction of Cyclic Carbonate Catalysed by DBU on LTO Surface

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This observation is the basis of our hypothesis, with the use of DBU as a good candidate for promoting SEI formation.

In order to verify the hypothesis of using DBU in a cell, we evaluated its behavior in a model system, and characterized the results by 1H NMR and HPLC-MS TOF. The model consists of a mixture of propylene carbonate, DBU and LTO heated at 45oC for 24 hours to simplify the analysis. No LIPF6 was added because of the possible formation of hazardous HF. Figure 1a shows the spectrum from the HPLC-MS TOF analysis of the supernatant. The spectrum has two major peaks (PC and DBU) and two minor peaks. The fragments arising from the first minor peak could not be identified. However, the second peak has a major fragment which is clearly associated with a chain started by a DBU with insertion of PC by Ring Opening Polymerization (ROP). This structure was reported by Brown et al., for ROP of lactide with DBU.16 Therefore, hydroxyl groups are not required to initiate the polymerization. Moreover, the 1H NMR spectrum (see Figure S1 in SI) shows signals characteristic of carbonate in small amounts, corroborating the results from HPLC-MS TOF. The LTO recovered from the model system was dried at 120oC under vacuum for 48 hours to ensure the complete removal of DBU and PC. Then the sample was analyzed by FTIR-ATR after drying (See Figure S2 in SI); the FTIR spectrum confirms the presence of poly(propylene carbonate) on the LTO surface by the distinctive peaks associated with poly(carbonate) and DBU. Then, the LTO was washed by chloroform under stirring for 3 hours to remove all the soluble polymers. The amount of polymers remaining was determined by TGA analysis (Figure 1b); the weight loss between

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180oC and 260oC is characteristic of the combustion of homogenous polymers. Thus, the amount is negligible after washing (0.8 wt%), and we assume that initiation of the polymerization is due predominantly to DBU and not LTO.

(a)

x10 7 +ESI TIC Scan Frag=100.0V Esstalion_Daigle_150611_1_POS_LC.d

1.4

1

1 2

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1.2 1

PC and DBU

0.8 0.6 0.4 0.2 0 0.5

(b)

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5.5

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6.5 7 7.5 Counts vs. Acquisition Time (min)

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105

100

Loss weight / wt%

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


95

90 Before washing After washing

85

80

75 200

400

600 o

Temperature / C

Figure 1. a) HPLC-MS TOF of model system. b) TGA thermograms

Electrochemical Results of Cells. With these encouraging results, we made pouch cells with and without 0.5 wt% DBU as additive in the electrolyte. To accelerate gas evolution in the cell, we performed a float test at 2.4 V for 500 hours in the climate chamber with ambient temperature 7



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of 45 °C. The quantity of evolved gas was determined from the volume of the cell before and after the test. The gas species were obtained by gas chromatography (GC). Figure 2 shows the level and distribution of the gases inside the cells after the float test at 45oC for 500 hours. The addition of 0.5 wt% DBU or less in the electrolyte resulted in a decrease of 9.7 vol% of total gases. Specifically, the level of hydrogen, oxygen and propylene decreased but the level of carbon dioxide increased. As reported in the literature concerning the mechanism of decomposition of carbonate solvent with the LTO anode, the production of carbon dioxide usually originates from decomposition of linear carbonate in ether.17

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2.5 2.0 Gas volume / mL

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1.5 1.0 0.5 0.0 Reference

DBU 0.5%

H2 CO CH4 CO2 C2H4 O2 C3H6 C3H8

Figure 2. Volume of gases inside the cells quantified by GC.

The strategy is effective to prevent the generation of explosive gases that are hazardous in case of fire. Although the addition of 0.5 wt% DBU reduces the amount of gas by an appreciable level, we made a compromise between the performance of the cells (retention capacity) and the 9



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ability to minimize gas evolution. To do so, we evaluated the addition of different concentrations of DBU in coin cells utilizing a LFP cathode and LTO anode. Figure 3 shows the results of the charge-discharge of cells for the first and second cycles with an absolute capacity of 2.4 mAh. Based on these results, the use of 0.5 wt% DBU increased the resistance in the cell.

Figure 3. Charge-discharge curves for LFP-LTO coin-cells with DBU additive

This phenomenon is also evident in Figure 4 by the lower retention capacity for this concentration; however 0.25 wt% seems to be a good compromise for the additive in cylindrical cells. The formation of SEI on the LTO anode is probably a key factor for preventing gas evolution. However, the formation of a thicker layer can impede the diffusion of lithium and decrease retention capacity.

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Figure 4. Charge-discharge efficiency for coin-cells with different concentrations of DBU

Then, we decided to add 0.2 wt% DBU in 26650 cylindrical cells. After several cycles at room temperature at 1C, a float test was performed at 45oC, 2.4 V of charging voltage (approximately 3.9 V vs. Li/Li+ for cathode, 1.5 V vs. Li/Li+ for anode) for 1400 hours. Figure 5 shows the current leakage during the test. The reference cell shows a sudden current rise, indicating the occurrence of undesirable side reactions. By contrast, there is no significant current rise with DBU, thus we concluded that DBU suppresses the side reactions. 0.03

Charge Current / A

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Figure 5. Current during the float test at 45 °C. Green line: DBU, Blue line: reference

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Post-Mortem Analysis of the Anode. We observed only solids on the surface of the anode after disassembly of the pouch cells. Also, based on the formation of carbon dioxide during the float test, we surmise the formation of poly(ether) instead of poly(carbonate). Moreover, there is no residue of DBU in the leachate after washing the negative electrode with deuterated chloroform (used for analysis by NMR). We attribute the absence of DBU to the formation of the insoluble polymeric film.

The proportion of polymer on the surface of the anode was determined by thermogravimetric analysis (TGA) under air as shown in Figure 6. We can divide the spectra in different sections of the temperature range, which are due to the degradation of different components. We believe that the first section between 30-60oC (1.6 wt%) is connected with the evaporation of HF. This section is only visible with the curve for the anode plus the additive, which we attribute to trapped hydrogen fluoride. A second section between 250-600oC corresponds to the degradation of the polymer. This shows that the anode with the additive has 0.9 wt% more polymer according to the weight loss. The oxidation of LTO occurs at a temperature above 800oC and involves the reaction of hydroxyl groups located on the surface of LTO, which are responsible for the degradation of electrolyte. We suggested previously the possible initiation of polymerization by these groups, however, our results from the model were not in agreement with this supposition. We now suspect that a small proportion of the polymerization is initiated by the hydroxyl group. This mechanism is confirmed by the reduction of anode oxidation in the presence of the additive; we observed a decrease of 1.2 wt% at temperatures higher than 800oC. However, electrolyte degradation occurs to form the SEI on the

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LTO in both cases; thus we presume that the addition of DBU is responsible for most of the coating formation on the surface of LTO. This reaction is initiated by hydroxyl groups that are present on the particle surface and stabilised by DBU.

100 98

Loss Weight / Wt%

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96 94 92 90 88 Anode with additive Reference anode 86 84 0

200

400

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Temperature / oC Figure 6. TGA thermograms of the anodes from Li-ion batteries

In order to confirm the nature of the film on the anode surface, the formation of poly(ether), probably poly(propylene oxide) (PPO), was confirmed by analysis in a FTIR equipped with ATR-diamond. The spectrum (see Figure S3 in SI) does not show any band from

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the vibration of carbonyl group (1750 cm-1), thus we can discard the presence of poly(propylene carbonate) that is observed for the model system.

The full characterization of the electrode surface was done by scanning electron microscopy (SEM) coupled with time-of-flight secondary-ion mass spectroscopy (TOF-SIMS), which confirmed the deposition of a polymer layer on the anode surface. Table 1 lists the major fragments and the composition of the chemical species in the SEI of both samples that were observed by TOF-SIMS. Table 1: Major fragments detected by TOF-SIMS Reference Positive m/z ID 7 Li

DBU

Negative m/z ID 16 O

Positive m/z ID 7 Li

Negative m/z ID 16 O

33

Li2F

17

OH

31

Li2OH

17

OH

48

Ti

24

C2

33

Li2F

19

F

64

TiO

25

C2H

37

Li3O

24

C2

32

C2H7Li

48

Ti

25

C2H

36

COLi

58

C3H6O

31

P

42

COLi2

59

C3H7O

35

NH2F

45

LiF2

64

TiO

38

Li2C2

85

Li4F3

45

LiF2

Significantly, Li2F+ was detected in both samples. The intensity of the sample with DBU was 3.5 times higher, suggesting DBU enhances the formation of LiF. We believe that also the fragment corresponding to NH2F- is probably related with the trapping of HF by the DBU. For the reference, a fragment at 59 was detected in a very small proportion (not included in the Table 1), and a fragment at 32 was observed that is an organic solid without carbonyl group. Carbon

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monoxide derivative fragments were detected in the reference, which was in agreement with the presence of Li2CO3 in the SEI, as reported in the literature.9

The planar view micrograph of the electrode in the reference test (Fig. 7a) still shows the initial LTO surface. However, with the presence of DBU, we can clearly notice a thick layer on the LTO (Fig. 7 (b). This layer has a thickness of around 350 nm. The protective layer is confirmed by the presence of organic fragments (C3H7O+) connected to propylene oxide (PO) on the anode surface. Also, titanium is detected at a greater depth compared with the LTO reference, which suggests the presence of a coating on the LTO. Figure 7a shows the SEM images and Figure 7b shows the depth profile of m/z = 48 (Ti+) and the fragment m/z = 59 is associated with propylene oxide (PO+) (Figure 7c). [The original sentence seems to contradict the comment #5 on page 2 by Reviewer 1. Please check.]

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Figu ure 7. a) SEM M images, b)) depth profiile of m/z = 448 (Ti+), c) ffragment m//z = 59 (PO+)

There T was a large propo ortion of flu uoride locatted on the eelectrode suurface and inn the polymer. The fluorid de is associaated with LiF F because thhe proportioon and distriibution matcch the distributiion of lithiu um at the LTO L surface. Moreover,, fragments associated with Li2F+ were found (T Table 1 and Figure F 8), an nd the propo ortion of Li2F + is more iimportant with DBU preesent. We specu ulate that thee salt reacteed with the DBU D and proomoted the fo formation off SEI.14

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Figure F 8. On the left is th he fragment for m/z =7 ((Li+) and thee right is the fragment foor m/z = 19 (F-)

These T results led to a hyp pothesis for a proposed m mechanism ((Scheme 3) tthat describees the formation n of the prottective layerr on the anod de. In the firrst step, proppylene carboonate degraddes to produce CO2 and thee cationic an nd radical forrms of poly((propylene ooxide) (PPO)). Polymerizzation of fragments of propylene oxide is initiated by b DBU, cattalysed by liithium salt, tto form the m major product or o by hydro oxyls groupss located on n the LTO ssurface (form ms minor prroduct), andd then

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stabilised by DBU. This cation reacts rapidly with LTO or PPO to form a stable layer at the surface of LTO.12, 16

Scheme 3. Proposed mechanism for the formation of SEI with DBU in a Li-ion battery

Conclusion

In conclusion, we demonstrated the efficient use of Schiff Base (DBU) as additive in a lithium-ion battery with LTO anode. A SEI layer that minimizes the evolution of dangerous gases such as HF, propylene and oxygen was obtained with DBU in the electrolyte. The DBU plays a critical role to induce the polymerization of cyclic carbonate in the electrolyte. This

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approach is attractive because the film formed on the surface is ionically conductive and limits an increase in the cell resistance after extensive aging.

ASSOCIATED CONTENT The supporting information is available free of charge. Additional FTIR,NMR spectra, and Table are available.

AUTHOR INFORMATION Corresponding Author *Email: [email protected] Notes The authors declare no competing financial interest

ACKNOWLEDGEMENT This works was supported by Hydro-Québec and Sony.

References

1. Wu, K.; Yang, J.; Zhang, Y.; Wang, C.; Wang, D., Investigation on Li4Ti5O12 Batteries Developed for Hybrid Electric Vehicle. J.Appl. Electrochem. 2012, 42 (12), 989‐995. 2. Guerfi, A.; Sévigny, S.; Lagacé, M.; Hovington, P.; Kinoshita, K.; Zaghib, K., Nano‐particle Li4Ti5O12 Spinel as Electrode for Electrochemical Generators. J. Power Sources 2003, 119 (1), 88‐94. 3. Sun, X.; Radovanovic, P. V.; Cui, B., Advances in Spinel Li4Ti5O12 Anode Materials for Lithium‐ Ion Batteries. New J. Chem. 2015, 39 (1), 38‐63. 4. Yuan, T.; Tan, Z.; Ma, C.; Yang, J.; Ma, Z.‐F.; Zheng, S., Challenges of Spinel Li4Ti5O12 for Lithium‐ Ion Battery Industrial Applications. Adv. Energy Mater. 2017, 7 (12), 1601625.

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651x431mm (96 x 96 DPI)


Schiff Base as Additive for Preventing Gas Evolution in Li4Ti5O12

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