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Effect of monomer structure on properties of polyimide as LIB separator and its mechanism study
Journal Pre-proof Effect of monomer structure on properties of polyimide as LIB separator and its mechanism study Lei He, Jian-Hua Cao, Tian Liang, Da-Yong Wu PII:
S0013-4686(20)30230-9
DOI:
https://doi.org/10.1016/j.electacta.2020.135838
Reference:
EA 135838
To appear in:
Electrochimica Acta
Received Date: 21 December 2019 Revised Date:
28 January 2020
Accepted Date: 30 January 2020
Please cite this article as: L. He, J.-H. Cao, T. Liang, D.-Y. Wu, Effect of monomer structure on properties of polyimide as LIB separator and its mechanism study, Electrochimica Acta (2020), doi: https://doi.org/10.1016/j.electacta.2020.135838. This is a PDF file of an article that has undergone enhancements after acceptance, such as the addition of a cover page and metadata, and formatting for readability, but it is not yet the definitive version of record. This version will undergo additional copyediting, typesetting and review before it is published in its final form, but we are providing this version to give early visibility of the article. Please note that, during the production process, errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain. © 2020 Published by Elsevier Ltd.
Credit Author Statement Lei He: Investigation, Methodology, Software, Writing - Original Draft. Jian-Hua Cao: Validation. Tian Liang: Investigation. Da-Yong Wu: Conceptualization, Supervision，Funding acquisition and Writing - Review & Editing.
Effect of Monomer Structure on Properties of Polyimide as LIB Separator and Its Mechanism Study Lei Hea,b, Jian-Hua Caoa, Tian Liang a,b, Da-Yong Wua,* a
Technical Institute of Physics and Chemistry, Chinese Academy of Science, 29
Zhong-guan-cun East Road, Haidian District, Beijing 100190, P. R. China b
University of Chinese Academy of Science, Beijing 100049, P. R. China
*
Corresponding to: [email protected]
orcid: 0000-0002-5745-253X Da-Yong Wu orcid: 0000-0002-6023-7515 Lei He Abstract Polyimide (PI) has remarkable thermal stability and mechanical properties, and is considered as an important candidate material for the manufacture of high-security new separators of lithium ion batteries (LIBs). However, different types of PI may exhibit performance differences in battery applications due to their various monomer structures. We synthesize four
PIs
with different
monomers, including 1,2,4,5-pyromellitic
dianhydride-4,4'-oxydianiline
(PMDA-ODA),
3,3',4,4'-biphenyltetracarboxylic
dianhydride-4,4'-oxydianiline
(BPDA-ODA),
dianhydride-4,4'-oxydianiline
(ODPA-ODA)
3,3',4,4'-oxydiphthalic and
3,3',4,4'-benzophenonetetracarboxylic
dianhydride-4,4'-oxydianiline (BTDA-ODA), and electrospin them into nanofiber films. Those PI films are then thoroughly evaluated as the 1
separator of LIB. Results show that the electrochemical window of those four PI films is up to 5.1 V (vs. Li+/Li), their wettability and electrolyte uptake are all related to the surface polarity and porosity. Among the four PI separators, BPDA-ODA shows the best cycle performance (95.8% @0.2C, 25 °C) and rate performance in the NCM811|Li battery system, while the BTDA-ODA is the worst performer in battery applications due to its surface polarity, low electrolyte uptake and the lithiation reaction occurred on the carbonyl groups between two benzene rings, and therefore is not suitable for usage in manufacturing separators. Key words: Polyimide, Lithium ion battery, Electrospinning, Monomer structure 1. Introduction With the development of materials and LIB technologies, the energy density of batteries has increased remarkably. When a battery is charged and discharged with strong current and high power, it will generate heat to raise the temperature, which tests the reliability of separator and threatens battery safety [1-5]. At present, polyolefin separators such as polypropylene (PP) and polyethylene (PE) that are widely used in battery manufacturing are suitable for massive production, but their melting points are relatively low and cannot meet the safety requirements in high-energy-density battery applications [1, 2, 6]. Although the nano-Al2O3 particle coating technology 2
developed in recent years has improved the thermal stability of polyolefin-based separator films, there is still a gap between the expectation and reality [7]. Ways to further improve battery safety include the development of new film-forming materials and technologies, and the use of solid electrolytes [8-11] A lot of researches have been carried out to find better film-forming materials, including PI, Poly(m-phenylene isophthalamide) (PMIA), poly(ether
ether
ketone)
PEEK,
polybenzimidazole
(PBI)
and
polyetherimide (PEI) [12-16]. Among them, PI has attracted great attention from researchers due to its excellent thermal stability, chemical stability and electrochemical properties. Recent studies related to PI separator have been focused on the preparation methods and basic battery application performance. To obtain PI films with a certain porosity and pore size distribution, the feasible methods are electrospinning and phase inversion. Compared with phase inversion method, electrospinning is more feasible and environmentally friendly in preparing porous films. Moreover, electrospun films usually have higher porosity and bigger specific surface area, which result in good performance in battery application [17, 18]. Miao et al. [19] made DuPont's classic polyimide, Kapton (PMDA-ODA) into electrospun film, and tested its application performance as a LIB separator. It did not deform after a high temperature treatment at 150 °C, 3
and had a discharge capacity of 160 mAh g-1 after 100 cycles at 0.2 C, which was better than the reference sample, PP separator. Deng et al. [20] reported that a battery applying PMDA-ODA electrospun film had a capacity retention of 84% after 100 cycles at 0.5 C. The electrospun PMDA-ODA film prepared by Wang et al. [21] had an average diameter of about 600 nm, a porosity of 67%, an electrolyte uptake of 534%, self-extinguishing and good mechanical properties. In general, the battery application performance of electrospun PI separators is better than polyolefin separators in laboratory evaluations. Making composite films with PI and other materials is one of the more in-depth research contents in this field. Studies on preparation of organic/organic
composite
systems
with
PI
include
PI/PVDF
(poly(vinylidene fluoride)), PI/PANI (polyaniline), PI/PET (polyethylene), PE/PI/PE,
PI/PVDF-HFP
(poly(vinylidene
fluoride-co-hexafluoro-
propylene) ) , PI/PEO (polyethylene oxide) films and etc [22-27]. Inorganic particles such as Al2O3, SiO2, TiO2, Li4Ti5O12 etc. are used to prepare an organic/inorganic system with PI [28-31]. As we know, PI is a general term for a family of polymers containing an imide structure in the main chain. It can be synthesized from various monomers, resulting in differences in the properties of PI products. For example, the commercialized products PMDA-ODA (Kapton) and BPDA-ODA (Upilex) are synthesized by different dianhydrides. In contrast, 4
PMDA-ODA has better heat resistance (Tg ≥ 400 °C), while BPDA-ODA has better impact resistance (with a tensile strength more than 400 MPa). In most reports, researchers mainly compared PI or PI composite films with commercial separators, and rarely paid attention to the difference between various PI films. It is believed that the introduction of bridging groups (e.g. -O-, -CO-) in dianhydride will reduce the electron affinity of the dianhydride and also reduces the inter-chain interaction, which resulting in a reduction of the glass transition temperature of polyimide [32]. However, in the face of a large number of monomers that can be selected in the synthesis, we do not know what kind of monomer can construct a PI suitable for LIB applications; nor do we know that some monomer structures will adversely affect the performance of the separator. Therefore, a systematic research is needed in this regard. Here, we plan to synthesize several representative types of PI, including the homo-benzene type polyimide PMDA-ODA (Kapton), the biphenyl type polyimide BPDA-ODA (Upilex), the ODPA-ODA containing ether bond and BTDA-ODA containing carbonyl group, and evaluate their performances as LIB separator film. By analyzing polarity, porosity, wettability, thermal properties, electrochemical properties and battery performances of different PI films, we will investigate the effect of different materials on application performance in order to recognize the regular rule and enrich the research of PI separators. 5
2. Materials and Methods 2.1 Materials The diamine 4,4'-oxydianiline (ODA) was purchased from J&K Chemical (China) and vacuum dried at 70 °C for 24 h. The dianhydrides including
3,3',4,4'-biphenyltetracarboxylic
dianhydride
(BPDA),
1,2,4,5-pyromellitic dianhydride (PMDA), 4,4'-oxydiphthalic anhydride (ODPA), 3,3',4,4'-benzophenonetetracarboxylic dianhydride (BTDA) were purchased from J&K Chemical (China) and vacuum dried at 170 °C for 24 h. The DMAc was purified by a reduced pressure distillation (-0.088 MPa, fraction collected at 69-73 °C). N,N-dimethylacetamide (DMAc, AR), N-methylpyrrolidone (NMP, AR), and dimethyl carbonate (DMC, AR) were purchased from Beijing Chemical Works. LiNi0.8Co0.1Mn0.1O2, Super-P and lithium tablets (thickness = 0.4 mm) were purchased from Shenzhen Kejing Zhida Co., Ltd. The liquid electrolyte (1 M LiPF6 in EC+DEC+DMC (1:1:1, w/w/w)) was purchased from Beijing Chemical Reagent Research Institute. A commercial PP film (Celgard 2400) with a thickness of 25 µm was used for comparison. 2.2 Preparation of different polyimide nanofiber films 2.2.1 Synthesis of different polyamide acid (PAA) PAA was prepared by a condensation reaction between dianhydride and diamine in DMAc (Table 1). The synthesis route is shown in Scheme 1. 6
Taking PMDA-ODA as an example, 2.00 g (0.01 mol) of ODA powder was added into a three-necked flask containing 23.69 g of DMAc, and completely dissolved under a mechanical stirring (300 rpm, 10 min) with an ice bath. Then, 2.18 g (0.01 mol) of PMDA powder was added in 4 batches (50%, 30%, 15% and 5% of the total mass) in 30 min under stirring. As soon as every batch of PMDA powder was completely dissolved, the next batch was added. After adding the last batch of PMDA, mechanical stirring lasted for 210 min to complete the reaction. After a viscosity measurement, the PAA solution was sealed and stored at 4 °C for future use.
7
Scheme 1. Synthesis of PAAs and PIs. Table 1. Dosage of raw materials and viscosity of PAAs Dianhydride g
Diamine g
Solid content %
DMAc g
viscosity cp
PMDA-ODA
2.18
2.00
15
23.69
4105±143
BPDA-ODA
2.94
2.00
15
27.99
3149±114
ODPA-ODA
3.10
2.00
15
28.90
3753±135
BTDA-ODA
3.22
2.00
15
29.58
2714±127
2.2.2 Electrospinning of PAA nanofiber films and their imidization The PAA nanofibers were prepared by an electrospinning equipment with a single stainless-steel needle nozzle. The nozzle (inner diameter = 0.32 mm) was connected to the positive pole of a DC power supply, and the 8
output was set 16 kV during the electrospinning process. A grounded stainless-steel roller (d = 20 cm, 200 r min-1) located 16 cm away from the horizontally placed nozzle was used as the receiving device. The PAA solution was delivered by a syringe pump at a rate of 1 mL h-1 to the nozzle. The humidity was controlled between RH 55% and 60%, and the temperature was 25 °C. The electrospinning process lasted 4 hours, and the resulting nanofibers accumulated into a film on the collection roller. The obtained film was dried in an oven at 70 ℃ for 1h and then cut into a rectangular of 10 cm × 6 cm and placed in a vacuum oven (vacuum degree: -0.1 MPa). The thermal imidization was completed after a heat treatment. The heating rate was 10 °C min-1, and the temperature was maintained at 300 °C for 60 min. After a natural cooling to room temperature, the PI nanofiber film was obtained. 2.3 Characterization and tests The surface morphology of the electrospun PI nanofiber films was observed by a field emission scanning electron microscope (FE-SEM, S-4800, Hitachi, Japan). Before testing, samples were sprayed with gold using an ion sputtering device (Mc1000, Hitachi, Japan). The imidization of the PI films were confirmed by Fourier Transform Infrared Spectrometer (FT-IR, Excalibur 3100). The heat shrinkage was examined by comparing the two-dimension size 9
of the film before and after heat treatments (150 °C and 250 °C for 30 min, respectively). The glass transition temperature (Tg) was measured by a differential scanning calorimeter (DSC, Mettler with a heating rate of 10 °C min-1 from 100 °C to 400 °C in N2 atmosphere), and the thermal decomposition temperature (Td) was tested by a thermogravimetric analyzer (TGA, TQ-50, TA with a heating rate of 10 °C min-1 from 100 °C to 800 °C in N2 atmosphere). The electrolyte uptake was calculated by using the Eq. (1) with the weights of films before and after soaking them in a liquid electrolyte until saturated in an argon filled glove box (VAC OMNI LAB): θ(%) =
21 1
× 100%
(1)
where M1 and M2 are the weights of the film before and after soaking in the electrolyte, respectively. The contact angles between PI films and electrolyte were measured by using a contact angle analyzer (Data-Physics OCA-20 Apparatus, Germany). The porosity was calculated by using the Eq. (2) [33, 34]: Porosity (%) =
× 100%
(2)
where M is the weight of dry film, M' is the mass of film absorbed n-butanol, V is the volume of dry film, and ρ is the density of n-butanol. 2.4 Electrochemical properties The electrochemical window of the PI and PP films was measured by 10
linear voltammetric scanning (LSV). A stainless steel (SS) sheet was used as a working electrode, and a lithium metal sheet was used as a reference electrode. The films saturated with liquid electrolyte were placed between the stainless-steel sheet and the lithium metal sheet. LSV of the test cells was performed by using a Zennium electrochemical workstation from the open circuit voltage to 6 V with a scan rate of 10 mV s-1. The ionic conductivity of the PI and PP films was evaluated via an electrochemical impedance spectroscopy (EIS) and calculated with the Eq. (3) [35]: η =
(3)
b
where d is the thickness of the film (cm), Rb is the bulk resistance and S is the effective area of the film (cm2). 2.5 Battery assembly and tests In the battery application test of the PI films, we applied CR 2032 coin cells.
The
positive
electrode
was
produced
by
dispersing
LiNi0.8Co0.1Mn0.1O2, Super-P, and PVDF evenly into NMP by a mass ratio of 84: 10: 6 to obtain a mixture with a solid content of 20%. Then the well-dispersed mixture was blade-coated onto an aluminum foil (16 µm). After drying (in an air dry oven for 5 h at 60 °C and in a vacuum oven for 12 h at 120 °C under -0.1 Mpa) and a hot-pressing process (80 °C), a NCM811 cathode plate was obtained with a thickness of 90 µm and a mass loading of 9.1 mg·cm-2. The anode electrode of the cell was lithium metal. The CR2032 cells were assembled in a glove box applying different PI 11
separator films, electrolyte, positive and negative electrode plates, respectively. Subsequently, the cells’ cycling performance was tested in a voltage range 3.0 V– 4.2 V vs. lithium applying a battery cycle tester (CT2001A) at 0.2 C for 200 cycles. And the cells’ rate capability was measured by charging and discharging at various current densities (0.2 C, 0.5 C, 1 C, 2 C, 3 C, 4 C, 5 C, and 10 C). 2.6 Cyclic voltammetry test of BTDA-ODA and BPDA-ODA films A stainless-steel (SS) sheet was used as a working electrode, and a lithium metal sheet was used as a reference electrode. The BTDA-ODA and BPDA-ODA films saturated with liquid electrolyte were sandwiched between SS and Li. Cyclic voltammetric scan (CV) of the test cell was performed by using Zennium electrochemical workstation from 3 V to 4.2 V with a scan rate of 1 mV s-1. 3. Results and Discussion 3.1 Morphology and imidization of PI nanofiber membranes The SEM images of PMDA-ODA, BPDA-ODA, ODPA-ODA, and BTDA-ODA films (Fig. S1a-d) show that their respective fiber diameters are different, which are 210 nm, 130 nm, 130 nm, 200 nm. PAA undergoes an imidization reaction under high temperature and turns into PI. And the differences between PAA and PI in the infrared spectrum are the direct evidence to judge the degree of imidization of PAA. We compared the IR spectra of PAA films before and after heat treatment (Fig. S2). The 12
PMDA-ODA film is chosen as an example (Fig. S2a). The heat-treated samples show absorption at 1780 cm-1, 1720 cm-1, and 720 cm-1. Those peaks are atributed to the characteristic absorptions of asymmetric stretching vibration, symmetric stretching vibration and bending vibration of carbonyl group (C=O) in the PI structure; the absorption peak at 1380 cm-1 is atributed to the stretching vibration of the carbon-nitrogen bond (C-N) in the imide structure [36, 37]. The presence of the above characteristic absorptions confirms the formation of PI. Those absorption peaks present in IR spectrum of the sample before heat treatment, a broad peak at 2800-3400 cm-1 (-COOH), a peak at 1710 cm-1 (vibration of the carbonyl group in -COOH), and a peak at 1660 cm-1 (vibration of carbonyl group in -CONH), disappeared after heat treatment, proving that PAA phase vanished during imidization [38]. The variations of IR spectra of other PAA films before and after heat treatment (Fig. S2b-d) were consistent with that of PMDA-ODA film. The above results can confirm that the PAAs have been transformed into PIs successfully. 3.2 Thermal stability of PI nanofiber films The safety of PI nanofiber films was evaluated from four aspects: heat shrinkage, Tg, Td and flame retardancy. The melting point of PP film is 165 °C, and its shrinkage at 150 °C is more than 70%. However, the PI films have no perceptible shrinkage when they are heated at 150 °C and 250 °C for 30 minutes (Fig. 1). We measured the Tg and Td of the four PI 13
films, and listed the results in Table 2. The results of DSC test (Fig. S3) show that the Tg of all of the four PI films are higher than 267 °C; and the TGA results (Fig. S4) show that the Td (5%) of the four PI films occurs over 500 °C. The above results prove that PI nanofiber films have excellent thermal stability. In subsequent combustion tests (Fig. 1) we found that different PI films behave differently after exposuring to flames. The PMDA-ODA film was quickly carbonized, did not produce an obvious flame; BPDA-ODA and BTDA-ODA were able to be ignited but the flames self-extinguished quickly; ODPA-ODA burned fiercely after contacting with flame; as a comparison, PP quickly shrinked and burned. Therefore, we believe that PMDA-ODA, BPDA-ODA and BTDA-ODA are incombustibility.
Figure1. Thermal shrinkage and flammability of the PI and PP films. Table 2. Thermal properties of the PI nanofiber films Heat shrinkage sample
30min@150 ºC
30min@250 ºC
14
Tg (ºC)
Td (ºC)
incombustibility
PMDA-ODA BPDA-ODA ODPA-ODA BTDA-ODA
0 0 0 0
0 0 0 0
> 400 273.1 267.1 282.5
527.6 532.5 511.9 523.1
yes yes no yes
3.3 Wettability of PI nanofiber films The wettability with electrolyte is an important performance parameter of a separator film. A separator film with good wettability can fully contact with the electrolyte and uptake more, thereby transporting lithium ions better during the charging and discharging processes of batteries. We determined the wettability and liquid electrolyte uptake of PI nanofiber films by measuring the static contact angle and uptake weight of electrolyte, respectively.
Figure 2. Photographs of static contact angles of PI and PP films with electrolyte.
The images in Fig. 2 show the static contact angles of PMDA-ODA, BPDA-ODA, ODPA-ODA, BTDA-ODA and PP films with electrolyte. They are 21.3°, 0°, 8.6°, 26.7° and 58°, respectively. It indicates that the wettability of all four PI films is much better than that of PP films. And among them, the BPDA-ODA film has the best wettability, while the BTDA-ODA film has the worst. In order to obtain surface polarity data, we used X-ray photoelectron spectroscopy (XPS) to examine the (N+O)/C ratio on the surface of PI films and listed the results in Fig. 3. Generally, 15
the higher (N+O)/C value a material has on its surface, the more polar groups it carries [39]. The (N+O)/C values of PMDA-ODA, BPDA-ODA, ODPA-ODA and BTDA-ODA are 0.33, 0.23, 0.25 and 0.39, respectively. The change trend of (N + O)/C values is consistent with the change of contact angle data. These two sets of data indicate that the polarity of the electrolyte system (1 M LiPF6 in EC+DEC+DMC (1:1:1)) is similar to that of the BPDA-ODA surface, but has significant difference with BTDA-ODA surface.
Figure 3. XPS spectra of different PI films
More meaningfully, the BPDA-ODA film has an electrolyte uptake up to 862% (Table 3), which is over 8 times that of the PP film has. The 16
abundant pore structure of nanofiber films improves their electrolyte uptake. The difference in electrolyte uptake between different PI nanofiber films is related to their wettability with electrolyte. BTDA-ODA film has a poor electrolyte wettability, lower porosity (51.5%) resulting in a relatively low electrolyte uptake.
17
Table 3. Properties of different PI and PP films Sample
Thickness µm
Uptake
%
Contact angle (°)
Porosity %
PMDA-ODA
47
344
21.3
70
BPDA-ODA
51
862
0
86
ODPA-ODA
50
465
8.6
72
BTDA-ODA
46
150
26.7
51
PP
25
102
58
45
3.4 The ionic conductivity and electrochemical window of PI nanofiber films Ionic conductivity is one of the most important performance parameters of a separator film. The separator with a high ionic conductivity is capable to transport lithium ions more efficiently. The width of electrochemical window reflects the electrochemical stability of a separator, and whether it will meet the demand of high voltage, high energy density battery system. The separator films were placed in a “SS|separator|SS” system, and the ionic conductivity and electrochemical window of the separator were measured by the EIS and LSV methods, respectively. The intersection between the Nyquist plots and the real axis obtained by the EIS test is the bulk resistance of the separator film (Fig. 4a) [40]. According to Eq. 3, the ionic
conductivity
of
PMDA-ODA,
BPDA-ODA
ODPA-ODA,
BTDA-ODA and PP films can be calculated as 1.93, 2.24, 1.92, 2.18 and 0.7 mS cm-1, respectively. The electrochemical windows of PMDA-ODA, BPDA-ODA, ODPA-ODA, BTDA-ODA and PP films can be obtained from their LSV curves (Fig. 4b), The electrochemical window of the four 18
PI films is up to 5.1 V. Compared with the PP film (electrochemical window of 4.6V), the PI films exhibit excellent electrochemical stability and can be used in high-voltage batteries.
Figure 4. (a) Nyquist plots and (b) LSV curves of the PMDA-ODA, BPDA-ODA, ODPA-ODA, BTDA-ODA nanofiber films and PP films.
19
Table 4. Ionic conductivity of different PI and PP films Bulk resistance
Ionic conductivity
(Ω)
(mS cm-1)
47
1.29
1.93
BPDA-ODA
51
1.22
2.24
ODPA-ODA
50
1.38
1.92
BTDA-ODA
46
1.13
2.18
PP
25
1.9
0.7
Sample
Thickness (µm)
PMDA-ODA
3.5 Cycling and rate performance of batteries applying PI nanofiber separators PMDA-ODA, BPDA-ODA, ODPA-ODA and BTDA-ODA nanofiber films were used as separator to assemble NCM811/Li CR2032 batteries, and the batteries’ cycling and rate performances were tested. The Fig. 5a gives the results of cycling test. After 200 charge and discharge cycles, the four batteries had different capacity retentions: 147.6 mAh g-1 (95.22%), 150.6 mAh g-1 (95.80%), 143.6 mAh g-1 (93.01%), and 122.4 mAh g-1 (81.5%), respectively. Among them, the capacity decay rate of the BTDA-ODA battery is significantly faster than the other three batteries. In rate performance test (Fig. 5b), the four batteries showed a 5 C discharge capacity of (capacity retention rate) 105.93 mAh g-1 (64.83%), 110.87 mAh g-1 (66.39%), 106.27 mAh g-1 (65.56%) and 99.63 mAh g-1 (60.16%) respectively; and a 10 C discharge capacity of 47.50 mAh g-1 (29.07%), 59.57 mAh g-1 (35.67%), 48.50 mAh g-1 (29.61%) and 36.17 mAh g-1 (21.84%), respectively. Relatively, the battery applying BPDA-ODA separator shows the best cycling and rate performance, while the battery 20
using BTDA-ODA separator is the worst. With increased charge-discharge rates, the performance gap between battery using BTDA-ODA separator and other batteries is getting larger. We judge that the performance difference between the four batteries is caused by the nature of film-forming materials. Further analysis is given in 3.7.
Figure 5 (a) Charge-discharge cycling and coulombic efficiency of NCM811/Li batteries at 0.2 C. (b) Rate performance of NCM811/Li batteries with different PI 21
separator films.
3.6 Interface resistance of PI nanofiber films For a deeper research and explanation of PI films’ performance in LIBs, we used AC impedance method to examine the change of interface resistance of NCM811||Li half batteries with different PI separators before and after cycling. To interpret EIS data and calibrate errors, we fitted EIS data by the ZSimpWin software. An equivalent circuit model as R(CR)(QR)(CR), which represent ohmic drop, cathode, solid electrolyte interface and anode was established. As shown in Fig. 6, the interface resistance of PMDA-ODA, BPDA-ODA, ODPA-ODA and BTDA-ODA separators after 3 cycles at 0.1 C were 16.13Ω 16.40Ω 12.23Ω 9.00 Ω, respectively. After 100 cycles at 0.2 C, the interface resistance of the them increased by 84.37 Ω 52.73 Ω 85.27 Ω, 105.69 Ω, respectively. During cycling, the solid electrolyte interface (SEI) film continued to grow and consume electrolyte; the interface resistance of the battery increased, and the battery performance gradually declined. Under the same conditions, the battery using a BPDA-ODA separator had the smallest interface resistance change, even better than the widely used PMDA-ODA film [19], while the battery using a BTDA-ODA film had the largest interface resistance change. This result also proves that the discharge capacity of the battery using a BTDA-ODA separator decayed fastest, which is consistent with the results obtained in cycling test. 22
Figure 6. AC impedance spectra of NCM811/Li half-cells after 3 cycles at 0.1C and after 100 cycles at 0.2 C (a) PMDA-ODA, (b) BPDA-ODA, (c) ODPA-ODA, (d) BTDA-ODA. Table 5. Interface resistance of different PI films after 3 cycles at 0.1 C and after 100 cycles at 0.2 C Sample
R3cycles (Ω)
R100cycles (Ω)
∆R (Ω)
PMDA-ODA BPDA-ODA ODPA-ODA BTDA-ODA
16.13 16.40 12.23 9.00
100.50 69.13 97.25 114.69
84.37 52.73 85.27 105.69
Based on the results of various tests, it can be concluded that the battery using the BPDA-ODA separator has the best performance, and the battery using the BTDA-ODA separator has the worst performance. 3.7 Changes occurring on BTDA-ODA film during battery cycling The battery with BTDA-ODA separator has the fastest performance degradation in cycling and other tests. Is this because BTDA-ODA 23
undergoes an electrochemical reaction under the battery working conditions? This is a scientific issue that we need to investigate carefully. We disassembled the batteries after 100 cycles (1 C). The BTDA-ODA film was cleaned with DMC and ethanol, then measured by an ATR-FTIR spectrometer. For comparison, an unused BTDA-ODA film was also measured. At the same time, we compared the IR spectrum of the BTDA-ODA separator with the BPDA-ODA separator, to find out the absorance peak of highly-reactivity ketone carbonyl in BTDA-ODA monomer structure. The monomer structure of BTDA-ODA has one ketone carbonyl between the two benzene rings compared with monomer structure of BPDA-ODA. We compared the FTIR spectra (Fig. 7) of the two different PI separators and found that the absorance peak of the ketone carbonyl group in BTDA-ODA appeared at 1674 cm-1. Taking the absorption peak of the benzene ring at 1500 cm-1 of ODA monomer in BTDA-ODA as an internal standard, we compared the changes in intensity of the carbonyl group's absorption at 1674 cm-1 before and after battery cycling to judge the reaction degree of the ketone carbonyl group [41]. Meanwhile, we also compared the absorption intensities of the carbonyl in the imide structure at 1720 cm-1. The data are listed in Table 6. The results show that after the BTDA-ODA film was used in a battery, the absorption of the carbonyl group in the imide structure did not change, while the absorption of the 24
conjugated ketone carbonyl between two benzene rings was significantly decreased. This indicates that ketone carbonyls were reduced due to the reaction during battery cycling.
Figure 7. (a) FT-IR spectrum of the BPDA-ODA film before cycling test. (b) and (c) FT-IR spectra of the BTDA-ODA film before cycling test. (d) FT-IR spectrum of the BTDA-ODA film after cycling test. Table 6. Relative intensity of FT-IR spectrum of the BTDA-ODA separators Before cycling After cycling Variation (%)
A1674 cm-1 / A1500 cm-1 0.27 0.20 -25.92
A1720 cm-1 / A1500 cm-1 0.92 0.91 -1.08
To confirm that BTDA-ODA film has undergone an electrochemical reaction during the charge and discharge processes of the battery, we assembled the BTDA-ODA film and BPDA-ODA film into a "SS|PI film|Li CR2032 battery with electrolyte, and performed CV scans using a Zennium 25
electrochemical workstation at a rate of 1 mV s-1 between 3V and 4.2V. The voltage range was the same as the condition applied in the battery cycling test. When the forward scanning voltage reached 3.6 V, the battery with BTDA-ODA film produced higher oxidation current than the battery with BPDA-ODA film (Fig. 8a-b). It indicates that the BTDA-ODA film occurred an oxidation reaction on the surface that contacted with the electrode. With more CV scanning cycles, the oxidation current occurred on the BTDA-ODA battery gradually declined, indicating that the intensity of subsequent oxidation reactions decreased. The same phenomenon did not appear when the BPDA-ODA film was measured. Considering the possibility that conjugated carbonyls may undergo a lithiation reaction [42], we speculate that the reaction occurred between the BTDA-ODA film and the electrode is a lithiation reaction. A suggested reaction process is shown in Fig. 8c. Because the conjugated carbonyl group in BTDA-ODA underwent an oxidation reaction, an additional oxidation current generated. When the oxidation current gradually got lower, indicateing that the intensity of the delithiation reaction weakened, while the reduction current stayed unchanged, showing that the intensity of the lithium reaction remained the same. This process led to a decrease in the lithium ion concentration in the electrolyte. Although BTDA-ODA can have a redox reaction at a certain extent, the reaction only occurs on the surface of the film due to its good insulation [43], and it will not lead to a 26
complete failure of the separator quickly. Therefore, after 100 cycles in a battery, the BTDA-ODA still showed an absorption of ketone carbonyl in infrared spectrum. In order to prove that the BTDA-ODA film contains additional lithium element after cycling, we conducted XPS test on the surface of the BTDA-ODA film after 100 battery cycles at 1C rate. Peaks of Li 1s and F 1s were observed in the XPS spectrum. Considering that LiPF6 in the liquid electrolyte may remain on the film surface, we integrated the Li 1s and F 1s peaks to compare the content of Li and F elements. In LiPF6, every Li atom corresponds to 6 F atoms, but the F/Li value in the XPS spectrum is 2.03. This proves that there is additional Li element on the surface of BTDA-ODA film, which is considered to be derived from the lithiation reaction in the cycling progress.
27
Figure 8. (a) Cyclic voltammetry curves of BTDA-ODA separator. (b) Cyclic voltammetry curves of BPDA-ODA separator. (c) Suggested lithiation process of BTDA-ODA. (d) XPS spectrum of BTDA-ODA film after cycling test.
4 Conclusions The four PI nanofiber films, PMDA-ODA, BPDA-ODA,ODPA-ODA and BTDA-ODA were prepared with excellent thermal stability. They show significant differences in terms of wettability, electrolyte uptake and electrochemical properties, because of their different chemical structures. Among them, the BPDA-ODA film has an ionic conductivity of 2.24 mS cm-1 (25 °C), an electrochemical stability window of 5.1 V, and shows the best performance during battery application tests. Relatively, due to the large differences in surface polarity of the material with electrolyte, weak 28
electrolyte uptake, and lithiation reaction would ocurr with the ketone carbonyl groups of BTDA-ODA polyimide under the working condition of batteries, BTDA-ODA is not suitable as a material for manufacturing LIB separator film. Acknowledgements We are grateful for the financial support given by the National Key R&D Program “New Energy Vehicles” Pilot Project (2016YFB0100105) and the Chinese Academy of Sciences Strategic Pilot Science and Technology Special Project XDA09010105.
29
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34
Declaration of interests ☒ The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper. ☐The authors declare the following financial interests/personal relationships which may be considered as potential competing interests:
S0013-4686(20)30230-9
DOI:
https://doi.org/10.1016/j.electacta.2020.135838
Reference:
EA 135838
To appear in:
Electrochimica Acta
Received Date: 21 December 2019 Revised Date:
28 January 2020
Accepted Date: 30 January 2020
Please cite this article as: L. He, J.-H. Cao, T. Liang, D.-Y. Wu, Effect of monomer structure on properties of polyimide as LIB separator and its mechanism study, Electrochimica Acta (2020), doi: https://doi.org/10.1016/j.electacta.2020.135838. This is a PDF file of an article that has undergone enhancements after acceptance, such as the addition of a cover page and metadata, and formatting for readability, but it is not yet the definitive version of record. This version will undergo additional copyediting, typesetting and review before it is published in its final form, but we are providing this version to give early visibility of the article. Please note that, during the production process, errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain. © 2020 Published by Elsevier Ltd.
Credit Author Statement Lei He: Investigation, Methodology, Software, Writing - Original Draft. Jian-Hua Cao: Validation. Tian Liang: Investigation. Da-Yong Wu: Conceptualization, Supervision，Funding acquisition and Writing - Review & Editing.
Effect of Monomer Structure on Properties of Polyimide as LIB Separator and Its Mechanism Study Lei Hea,b, Jian-Hua Caoa, Tian Liang a,b, Da-Yong Wua,* a
Technical Institute of Physics and Chemistry, Chinese Academy of Science, 29
Zhong-guan-cun East Road, Haidian District, Beijing 100190, P. R. China b
University of Chinese Academy of Science, Beijing 100049, P. R. China
*
Corresponding to: [email protected]
orcid: 0000-0002-5745-253X Da-Yong Wu orcid: 0000-0002-6023-7515 Lei He Abstract Polyimide (PI) has remarkable thermal stability and mechanical properties, and is considered as an important candidate material for the manufacture of high-security new separators of lithium ion batteries (LIBs). However, different types of PI may exhibit performance differences in battery applications due to their various monomer structures. We synthesize four
PIs
with different
monomers, including 1,2,4,5-pyromellitic
dianhydride-4,4'-oxydianiline
(PMDA-ODA),
3,3',4,4'-biphenyltetracarboxylic
dianhydride-4,4'-oxydianiline
(BPDA-ODA),
dianhydride-4,4'-oxydianiline
(ODPA-ODA)
3,3',4,4'-oxydiphthalic and
3,3',4,4'-benzophenonetetracarboxylic
dianhydride-4,4'-oxydianiline (BTDA-ODA), and electrospin them into nanofiber films. Those PI films are then thoroughly evaluated as the 1
separator of LIB. Results show that the electrochemical window of those four PI films is up to 5.1 V (vs. Li+/Li), their wettability and electrolyte uptake are all related to the surface polarity and porosity. Among the four PI separators, BPDA-ODA shows the best cycle performance (95.8% @0.2C, 25 °C) and rate performance in the NCM811|Li battery system, while the BTDA-ODA is the worst performer in battery applications due to its surface polarity, low electrolyte uptake and the lithiation reaction occurred on the carbonyl groups between two benzene rings, and therefore is not suitable for usage in manufacturing separators. Key words: Polyimide, Lithium ion battery, Electrospinning, Monomer structure 1. Introduction With the development of materials and LIB technologies, the energy density of batteries has increased remarkably. When a battery is charged and discharged with strong current and high power, it will generate heat to raise the temperature, which tests the reliability of separator and threatens battery safety [1-5]. At present, polyolefin separators such as polypropylene (PP) and polyethylene (PE) that are widely used in battery manufacturing are suitable for massive production, but their melting points are relatively low and cannot meet the safety requirements in high-energy-density battery applications [1, 2, 6]. Although the nano-Al2O3 particle coating technology 2
developed in recent years has improved the thermal stability of polyolefin-based separator films, there is still a gap between the expectation and reality [7]. Ways to further improve battery safety include the development of new film-forming materials and technologies, and the use of solid electrolytes [8-11] A lot of researches have been carried out to find better film-forming materials, including PI, Poly(m-phenylene isophthalamide) (PMIA), poly(ether
ether
ketone)
PEEK,
polybenzimidazole
(PBI)
and
polyetherimide (PEI) [12-16]. Among them, PI has attracted great attention from researchers due to its excellent thermal stability, chemical stability and electrochemical properties. Recent studies related to PI separator have been focused on the preparation methods and basic battery application performance. To obtain PI films with a certain porosity and pore size distribution, the feasible methods are electrospinning and phase inversion. Compared with phase inversion method, electrospinning is more feasible and environmentally friendly in preparing porous films. Moreover, electrospun films usually have higher porosity and bigger specific surface area, which result in good performance in battery application [17, 18]. Miao et al. [19] made DuPont's classic polyimide, Kapton (PMDA-ODA) into electrospun film, and tested its application performance as a LIB separator. It did not deform after a high temperature treatment at 150 °C, 3
and had a discharge capacity of 160 mAh g-1 after 100 cycles at 0.2 C, which was better than the reference sample, PP separator. Deng et al. [20] reported that a battery applying PMDA-ODA electrospun film had a capacity retention of 84% after 100 cycles at 0.5 C. The electrospun PMDA-ODA film prepared by Wang et al. [21] had an average diameter of about 600 nm, a porosity of 67%, an electrolyte uptake of 534%, self-extinguishing and good mechanical properties. In general, the battery application performance of electrospun PI separators is better than polyolefin separators in laboratory evaluations. Making composite films with PI and other materials is one of the more in-depth research contents in this field. Studies on preparation of organic/organic
composite
systems
with
PI
include
PI/PVDF
(poly(vinylidene fluoride)), PI/PANI (polyaniline), PI/PET (polyethylene), PE/PI/PE,
PI/PVDF-HFP
(poly(vinylidene
fluoride-co-hexafluoro-
propylene) ) , PI/PEO (polyethylene oxide) films and etc [22-27]. Inorganic particles such as Al2O3, SiO2, TiO2, Li4Ti5O12 etc. are used to prepare an organic/inorganic system with PI [28-31]. As we know, PI is a general term for a family of polymers containing an imide structure in the main chain. It can be synthesized from various monomers, resulting in differences in the properties of PI products. For example, the commercialized products PMDA-ODA (Kapton) and BPDA-ODA (Upilex) are synthesized by different dianhydrides. In contrast, 4
PMDA-ODA has better heat resistance (Tg ≥ 400 °C), while BPDA-ODA has better impact resistance (with a tensile strength more than 400 MPa). In most reports, researchers mainly compared PI or PI composite films with commercial separators, and rarely paid attention to the difference between various PI films. It is believed that the introduction of bridging groups (e.g. -O-, -CO-) in dianhydride will reduce the electron affinity of the dianhydride and also reduces the inter-chain interaction, which resulting in a reduction of the glass transition temperature of polyimide [32]. However, in the face of a large number of monomers that can be selected in the synthesis, we do not know what kind of monomer can construct a PI suitable for LIB applications; nor do we know that some monomer structures will adversely affect the performance of the separator. Therefore, a systematic research is needed in this regard. Here, we plan to synthesize several representative types of PI, including the homo-benzene type polyimide PMDA-ODA (Kapton), the biphenyl type polyimide BPDA-ODA (Upilex), the ODPA-ODA containing ether bond and BTDA-ODA containing carbonyl group, and evaluate their performances as LIB separator film. By analyzing polarity, porosity, wettability, thermal properties, electrochemical properties and battery performances of different PI films, we will investigate the effect of different materials on application performance in order to recognize the regular rule and enrich the research of PI separators. 5
2. Materials and Methods 2.1 Materials The diamine 4,4'-oxydianiline (ODA) was purchased from J&K Chemical (China) and vacuum dried at 70 °C for 24 h. The dianhydrides including
3,3',4,4'-biphenyltetracarboxylic
dianhydride
(BPDA),
1,2,4,5-pyromellitic dianhydride (PMDA), 4,4'-oxydiphthalic anhydride (ODPA), 3,3',4,4'-benzophenonetetracarboxylic dianhydride (BTDA) were purchased from J&K Chemical (China) and vacuum dried at 170 °C for 24 h. The DMAc was purified by a reduced pressure distillation (-0.088 MPa, fraction collected at 69-73 °C). N,N-dimethylacetamide (DMAc, AR), N-methylpyrrolidone (NMP, AR), and dimethyl carbonate (DMC, AR) were purchased from Beijing Chemical Works. LiNi0.8Co0.1Mn0.1O2, Super-P and lithium tablets (thickness = 0.4 mm) were purchased from Shenzhen Kejing Zhida Co., Ltd. The liquid electrolyte (1 M LiPF6 in EC+DEC+DMC (1:1:1, w/w/w)) was purchased from Beijing Chemical Reagent Research Institute. A commercial PP film (Celgard 2400) with a thickness of 25 µm was used for comparison. 2.2 Preparation of different polyimide nanofiber films 2.2.1 Synthesis of different polyamide acid (PAA) PAA was prepared by a condensation reaction between dianhydride and diamine in DMAc (Table 1). The synthesis route is shown in Scheme 1. 6
Taking PMDA-ODA as an example, 2.00 g (0.01 mol) of ODA powder was added into a three-necked flask containing 23.69 g of DMAc, and completely dissolved under a mechanical stirring (300 rpm, 10 min) with an ice bath. Then, 2.18 g (0.01 mol) of PMDA powder was added in 4 batches (50%, 30%, 15% and 5% of the total mass) in 30 min under stirring. As soon as every batch of PMDA powder was completely dissolved, the next batch was added. After adding the last batch of PMDA, mechanical stirring lasted for 210 min to complete the reaction. After a viscosity measurement, the PAA solution was sealed and stored at 4 °C for future use.
7
Scheme 1. Synthesis of PAAs and PIs. Table 1. Dosage of raw materials and viscosity of PAAs Dianhydride g
Diamine g
Solid content %
DMAc g
viscosity cp
PMDA-ODA
2.18
2.00
15
23.69
4105±143
BPDA-ODA
2.94
2.00
15
27.99
3149±114
ODPA-ODA
3.10
2.00
15
28.90
3753±135
BTDA-ODA
3.22
2.00
15
29.58
2714±127
2.2.2 Electrospinning of PAA nanofiber films and their imidization The PAA nanofibers were prepared by an electrospinning equipment with a single stainless-steel needle nozzle. The nozzle (inner diameter = 0.32 mm) was connected to the positive pole of a DC power supply, and the 8
output was set 16 kV during the electrospinning process. A grounded stainless-steel roller (d = 20 cm, 200 r min-1) located 16 cm away from the horizontally placed nozzle was used as the receiving device. The PAA solution was delivered by a syringe pump at a rate of 1 mL h-1 to the nozzle. The humidity was controlled between RH 55% and 60%, and the temperature was 25 °C. The electrospinning process lasted 4 hours, and the resulting nanofibers accumulated into a film on the collection roller. The obtained film was dried in an oven at 70 ℃ for 1h and then cut into a rectangular of 10 cm × 6 cm and placed in a vacuum oven (vacuum degree: -0.1 MPa). The thermal imidization was completed after a heat treatment. The heating rate was 10 °C min-1, and the temperature was maintained at 300 °C for 60 min. After a natural cooling to room temperature, the PI nanofiber film was obtained. 2.3 Characterization and tests The surface morphology of the electrospun PI nanofiber films was observed by a field emission scanning electron microscope (FE-SEM, S-4800, Hitachi, Japan). Before testing, samples were sprayed with gold using an ion sputtering device (Mc1000, Hitachi, Japan). The imidization of the PI films were confirmed by Fourier Transform Infrared Spectrometer (FT-IR, Excalibur 3100). The heat shrinkage was examined by comparing the two-dimension size 9
of the film before and after heat treatments (150 °C and 250 °C for 30 min, respectively). The glass transition temperature (Tg) was measured by a differential scanning calorimeter (DSC, Mettler with a heating rate of 10 °C min-1 from 100 °C to 400 °C in N2 atmosphere), and the thermal decomposition temperature (Td) was tested by a thermogravimetric analyzer (TGA, TQ-50, TA with a heating rate of 10 °C min-1 from 100 °C to 800 °C in N2 atmosphere). The electrolyte uptake was calculated by using the Eq. (1) with the weights of films before and after soaking them in a liquid electrolyte until saturated in an argon filled glove box (VAC OMNI LAB): θ(%) =
21 1
× 100%
(1)
where M1 and M2 are the weights of the film before and after soaking in the electrolyte, respectively. The contact angles between PI films and electrolyte were measured by using a contact angle analyzer (Data-Physics OCA-20 Apparatus, Germany). The porosity was calculated by using the Eq. (2) [33, 34]: Porosity (%) =
× 100%
(2)
where M is the weight of dry film, M' is the mass of film absorbed n-butanol, V is the volume of dry film, and ρ is the density of n-butanol. 2.4 Electrochemical properties The electrochemical window of the PI and PP films was measured by 10
linear voltammetric scanning (LSV). A stainless steel (SS) sheet was used as a working electrode, and a lithium metal sheet was used as a reference electrode. The films saturated with liquid electrolyte were placed between the stainless-steel sheet and the lithium metal sheet. LSV of the test cells was performed by using a Zennium electrochemical workstation from the open circuit voltage to 6 V with a scan rate of 10 mV s-1. The ionic conductivity of the PI and PP films was evaluated via an electrochemical impedance spectroscopy (EIS) and calculated with the Eq. (3) [35]: η =
(3)
b
where d is the thickness of the film (cm), Rb is the bulk resistance and S is the effective area of the film (cm2). 2.5 Battery assembly and tests In the battery application test of the PI films, we applied CR 2032 coin cells.
The
positive
electrode
was
produced
by
dispersing
LiNi0.8Co0.1Mn0.1O2, Super-P, and PVDF evenly into NMP by a mass ratio of 84: 10: 6 to obtain a mixture with a solid content of 20%. Then the well-dispersed mixture was blade-coated onto an aluminum foil (16 µm). After drying (in an air dry oven for 5 h at 60 °C and in a vacuum oven for 12 h at 120 °C under -0.1 Mpa) and a hot-pressing process (80 °C), a NCM811 cathode plate was obtained with a thickness of 90 µm and a mass loading of 9.1 mg·cm-2. The anode electrode of the cell was lithium metal. The CR2032 cells were assembled in a glove box applying different PI 11
separator films, electrolyte, positive and negative electrode plates, respectively. Subsequently, the cells’ cycling performance was tested in a voltage range 3.0 V– 4.2 V vs. lithium applying a battery cycle tester (CT2001A) at 0.2 C for 200 cycles. And the cells’ rate capability was measured by charging and discharging at various current densities (0.2 C, 0.5 C, 1 C, 2 C, 3 C, 4 C, 5 C, and 10 C). 2.6 Cyclic voltammetry test of BTDA-ODA and BPDA-ODA films A stainless-steel (SS) sheet was used as a working electrode, and a lithium metal sheet was used as a reference electrode. The BTDA-ODA and BPDA-ODA films saturated with liquid electrolyte were sandwiched between SS and Li. Cyclic voltammetric scan (CV) of the test cell was performed by using Zennium electrochemical workstation from 3 V to 4.2 V with a scan rate of 1 mV s-1. 3. Results and Discussion 3.1 Morphology and imidization of PI nanofiber membranes The SEM images of PMDA-ODA, BPDA-ODA, ODPA-ODA, and BTDA-ODA films (Fig. S1a-d) show that their respective fiber diameters are different, which are 210 nm, 130 nm, 130 nm, 200 nm. PAA undergoes an imidization reaction under high temperature and turns into PI. And the differences between PAA and PI in the infrared spectrum are the direct evidence to judge the degree of imidization of PAA. We compared the IR spectra of PAA films before and after heat treatment (Fig. S2). The 12
PMDA-ODA film is chosen as an example (Fig. S2a). The heat-treated samples show absorption at 1780 cm-1, 1720 cm-1, and 720 cm-1. Those peaks are atributed to the characteristic absorptions of asymmetric stretching vibration, symmetric stretching vibration and bending vibration of carbonyl group (C=O) in the PI structure; the absorption peak at 1380 cm-1 is atributed to the stretching vibration of the carbon-nitrogen bond (C-N) in the imide structure [36, 37]. The presence of the above characteristic absorptions confirms the formation of PI. Those absorption peaks present in IR spectrum of the sample before heat treatment, a broad peak at 2800-3400 cm-1 (-COOH), a peak at 1710 cm-1 (vibration of the carbonyl group in -COOH), and a peak at 1660 cm-1 (vibration of carbonyl group in -CONH), disappeared after heat treatment, proving that PAA phase vanished during imidization [38]. The variations of IR spectra of other PAA films before and after heat treatment (Fig. S2b-d) were consistent with that of PMDA-ODA film. The above results can confirm that the PAAs have been transformed into PIs successfully. 3.2 Thermal stability of PI nanofiber films The safety of PI nanofiber films was evaluated from four aspects: heat shrinkage, Tg, Td and flame retardancy. The melting point of PP film is 165 °C, and its shrinkage at 150 °C is more than 70%. However, the PI films have no perceptible shrinkage when they are heated at 150 °C and 250 °C for 30 minutes (Fig. 1). We measured the Tg and Td of the four PI 13
films, and listed the results in Table 2. The results of DSC test (Fig. S3) show that the Tg of all of the four PI films are higher than 267 °C; and the TGA results (Fig. S4) show that the Td (5%) of the four PI films occurs over 500 °C. The above results prove that PI nanofiber films have excellent thermal stability. In subsequent combustion tests (Fig. 1) we found that different PI films behave differently after exposuring to flames. The PMDA-ODA film was quickly carbonized, did not produce an obvious flame; BPDA-ODA and BTDA-ODA were able to be ignited but the flames self-extinguished quickly; ODPA-ODA burned fiercely after contacting with flame; as a comparison, PP quickly shrinked and burned. Therefore, we believe that PMDA-ODA, BPDA-ODA and BTDA-ODA are incombustibility.
Figure1. Thermal shrinkage and flammability of the PI and PP films. Table 2. Thermal properties of the PI nanofiber films Heat shrinkage sample
30min@150 ºC
30min@250 ºC
14
Tg (ºC)
Td (ºC)
incombustibility
PMDA-ODA BPDA-ODA ODPA-ODA BTDA-ODA
0 0 0 0
0 0 0 0
> 400 273.1 267.1 282.5
527.6 532.5 511.9 523.1
yes yes no yes
3.3 Wettability of PI nanofiber films The wettability with electrolyte is an important performance parameter of a separator film. A separator film with good wettability can fully contact with the electrolyte and uptake more, thereby transporting lithium ions better during the charging and discharging processes of batteries. We determined the wettability and liquid electrolyte uptake of PI nanofiber films by measuring the static contact angle and uptake weight of electrolyte, respectively.
Figure 2. Photographs of static contact angles of PI and PP films with electrolyte.
The images in Fig. 2 show the static contact angles of PMDA-ODA, BPDA-ODA, ODPA-ODA, BTDA-ODA and PP films with electrolyte. They are 21.3°, 0°, 8.6°, 26.7° and 58°, respectively. It indicates that the wettability of all four PI films is much better than that of PP films. And among them, the BPDA-ODA film has the best wettability, while the BTDA-ODA film has the worst. In order to obtain surface polarity data, we used X-ray photoelectron spectroscopy (XPS) to examine the (N+O)/C ratio on the surface of PI films and listed the results in Fig. 3. Generally, 15
the higher (N+O)/C value a material has on its surface, the more polar groups it carries [39]. The (N+O)/C values of PMDA-ODA, BPDA-ODA, ODPA-ODA and BTDA-ODA are 0.33, 0.23, 0.25 and 0.39, respectively. The change trend of (N + O)/C values is consistent with the change of contact angle data. These two sets of data indicate that the polarity of the electrolyte system (1 M LiPF6 in EC+DEC+DMC (1:1:1)) is similar to that of the BPDA-ODA surface, but has significant difference with BTDA-ODA surface.
Figure 3. XPS spectra of different PI films
More meaningfully, the BPDA-ODA film has an electrolyte uptake up to 862% (Table 3), which is over 8 times that of the PP film has. The 16
abundant pore structure of nanofiber films improves their electrolyte uptake. The difference in electrolyte uptake between different PI nanofiber films is related to their wettability with electrolyte. BTDA-ODA film has a poor electrolyte wettability, lower porosity (51.5%) resulting in a relatively low electrolyte uptake.
17
Table 3. Properties of different PI and PP films Sample
Thickness µm
Uptake
%
Contact angle (°)
Porosity %
PMDA-ODA
47
344
21.3
70
BPDA-ODA
51
862
0
86
ODPA-ODA
50
465
8.6
72
BTDA-ODA
46
150
26.7
51
PP
25
102
58
45
3.4 The ionic conductivity and electrochemical window of PI nanofiber films Ionic conductivity is one of the most important performance parameters of a separator film. The separator with a high ionic conductivity is capable to transport lithium ions more efficiently. The width of electrochemical window reflects the electrochemical stability of a separator, and whether it will meet the demand of high voltage, high energy density battery system. The separator films were placed in a “SS|separator|SS” system, and the ionic conductivity and electrochemical window of the separator were measured by the EIS and LSV methods, respectively. The intersection between the Nyquist plots and the real axis obtained by the EIS test is the bulk resistance of the separator film (Fig. 4a) [40]. According to Eq. 3, the ionic
conductivity
of
PMDA-ODA,
BPDA-ODA
ODPA-ODA,
BTDA-ODA and PP films can be calculated as 1.93, 2.24, 1.92, 2.18 and 0.7 mS cm-1, respectively. The electrochemical windows of PMDA-ODA, BPDA-ODA, ODPA-ODA, BTDA-ODA and PP films can be obtained from their LSV curves (Fig. 4b), The electrochemical window of the four 18
PI films is up to 5.1 V. Compared with the PP film (electrochemical window of 4.6V), the PI films exhibit excellent electrochemical stability and can be used in high-voltage batteries.
Figure 4. (a) Nyquist plots and (b) LSV curves of the PMDA-ODA, BPDA-ODA, ODPA-ODA, BTDA-ODA nanofiber films and PP films.
19
Table 4. Ionic conductivity of different PI and PP films Bulk resistance
Ionic conductivity
(Ω)
(mS cm-1)
47
1.29
1.93
BPDA-ODA
51
1.22
2.24
ODPA-ODA
50
1.38
1.92
BTDA-ODA
46
1.13
2.18
PP
25
1.9
0.7
Sample
Thickness (µm)
PMDA-ODA
3.5 Cycling and rate performance of batteries applying PI nanofiber separators PMDA-ODA, BPDA-ODA, ODPA-ODA and BTDA-ODA nanofiber films were used as separator to assemble NCM811/Li CR2032 batteries, and the batteries’ cycling and rate performances were tested. The Fig. 5a gives the results of cycling test. After 200 charge and discharge cycles, the four batteries had different capacity retentions: 147.6 mAh g-1 (95.22%), 150.6 mAh g-1 (95.80%), 143.6 mAh g-1 (93.01%), and 122.4 mAh g-1 (81.5%), respectively. Among them, the capacity decay rate of the BTDA-ODA battery is significantly faster than the other three batteries. In rate performance test (Fig. 5b), the four batteries showed a 5 C discharge capacity of (capacity retention rate) 105.93 mAh g-1 (64.83%), 110.87 mAh g-1 (66.39%), 106.27 mAh g-1 (65.56%) and 99.63 mAh g-1 (60.16%) respectively; and a 10 C discharge capacity of 47.50 mAh g-1 (29.07%), 59.57 mAh g-1 (35.67%), 48.50 mAh g-1 (29.61%) and 36.17 mAh g-1 (21.84%), respectively. Relatively, the battery applying BPDA-ODA separator shows the best cycling and rate performance, while the battery 20
using BTDA-ODA separator is the worst. With increased charge-discharge rates, the performance gap between battery using BTDA-ODA separator and other batteries is getting larger. We judge that the performance difference between the four batteries is caused by the nature of film-forming materials. Further analysis is given in 3.7.
Figure 5 (a) Charge-discharge cycling and coulombic efficiency of NCM811/Li batteries at 0.2 C. (b) Rate performance of NCM811/Li batteries with different PI 21
separator films.
3.6 Interface resistance of PI nanofiber films For a deeper research and explanation of PI films’ performance in LIBs, we used AC impedance method to examine the change of interface resistance of NCM811||Li half batteries with different PI separators before and after cycling. To interpret EIS data and calibrate errors, we fitted EIS data by the ZSimpWin software. An equivalent circuit model as R(CR)(QR)(CR), which represent ohmic drop, cathode, solid electrolyte interface and anode was established. As shown in Fig. 6, the interface resistance of PMDA-ODA, BPDA-ODA, ODPA-ODA and BTDA-ODA separators after 3 cycles at 0.1 C were 16.13Ω 16.40Ω 12.23Ω 9.00 Ω, respectively. After 100 cycles at 0.2 C, the interface resistance of the them increased by 84.37 Ω 52.73 Ω 85.27 Ω, 105.69 Ω, respectively. During cycling, the solid electrolyte interface (SEI) film continued to grow and consume electrolyte; the interface resistance of the battery increased, and the battery performance gradually declined. Under the same conditions, the battery using a BPDA-ODA separator had the smallest interface resistance change, even better than the widely used PMDA-ODA film [19], while the battery using a BTDA-ODA film had the largest interface resistance change. This result also proves that the discharge capacity of the battery using a BTDA-ODA separator decayed fastest, which is consistent with the results obtained in cycling test. 22
Figure 6. AC impedance spectra of NCM811/Li half-cells after 3 cycles at 0.1C and after 100 cycles at 0.2 C (a) PMDA-ODA, (b) BPDA-ODA, (c) ODPA-ODA, (d) BTDA-ODA. Table 5. Interface resistance of different PI films after 3 cycles at 0.1 C and after 100 cycles at 0.2 C Sample
R3cycles (Ω)
R100cycles (Ω)
∆R (Ω)
PMDA-ODA BPDA-ODA ODPA-ODA BTDA-ODA
16.13 16.40 12.23 9.00
100.50 69.13 97.25 114.69
84.37 52.73 85.27 105.69
Based on the results of various tests, it can be concluded that the battery using the BPDA-ODA separator has the best performance, and the battery using the BTDA-ODA separator has the worst performance. 3.7 Changes occurring on BTDA-ODA film during battery cycling The battery with BTDA-ODA separator has the fastest performance degradation in cycling and other tests. Is this because BTDA-ODA 23
undergoes an electrochemical reaction under the battery working conditions? This is a scientific issue that we need to investigate carefully. We disassembled the batteries after 100 cycles (1 C). The BTDA-ODA film was cleaned with DMC and ethanol, then measured by an ATR-FTIR spectrometer. For comparison, an unused BTDA-ODA film was also measured. At the same time, we compared the IR spectrum of the BTDA-ODA separator with the BPDA-ODA separator, to find out the absorance peak of highly-reactivity ketone carbonyl in BTDA-ODA monomer structure. The monomer structure of BTDA-ODA has one ketone carbonyl between the two benzene rings compared with monomer structure of BPDA-ODA. We compared the FTIR spectra (Fig. 7) of the two different PI separators and found that the absorance peak of the ketone carbonyl group in BTDA-ODA appeared at 1674 cm-1. Taking the absorption peak of the benzene ring at 1500 cm-1 of ODA monomer in BTDA-ODA as an internal standard, we compared the changes in intensity of the carbonyl group's absorption at 1674 cm-1 before and after battery cycling to judge the reaction degree of the ketone carbonyl group [41]. Meanwhile, we also compared the absorption intensities of the carbonyl in the imide structure at 1720 cm-1. The data are listed in Table 6. The results show that after the BTDA-ODA film was used in a battery, the absorption of the carbonyl group in the imide structure did not change, while the absorption of the 24
conjugated ketone carbonyl between two benzene rings was significantly decreased. This indicates that ketone carbonyls were reduced due to the reaction during battery cycling.
Figure 7. (a) FT-IR spectrum of the BPDA-ODA film before cycling test. (b) and (c) FT-IR spectra of the BTDA-ODA film before cycling test. (d) FT-IR spectrum of the BTDA-ODA film after cycling test. Table 6. Relative intensity of FT-IR spectrum of the BTDA-ODA separators Before cycling After cycling Variation (%)
A1674 cm-1 / A1500 cm-1 0.27 0.20 -25.92
A1720 cm-1 / A1500 cm-1 0.92 0.91 -1.08
To confirm that BTDA-ODA film has undergone an electrochemical reaction during the charge and discharge processes of the battery, we assembled the BTDA-ODA film and BPDA-ODA film into a "SS|PI film|Li CR2032 battery with electrolyte, and performed CV scans using a Zennium 25
electrochemical workstation at a rate of 1 mV s-1 between 3V and 4.2V. The voltage range was the same as the condition applied in the battery cycling test. When the forward scanning voltage reached 3.6 V, the battery with BTDA-ODA film produced higher oxidation current than the battery with BPDA-ODA film (Fig. 8a-b). It indicates that the BTDA-ODA film occurred an oxidation reaction on the surface that contacted with the electrode. With more CV scanning cycles, the oxidation current occurred on the BTDA-ODA battery gradually declined, indicating that the intensity of subsequent oxidation reactions decreased. The same phenomenon did not appear when the BPDA-ODA film was measured. Considering the possibility that conjugated carbonyls may undergo a lithiation reaction [42], we speculate that the reaction occurred between the BTDA-ODA film and the electrode is a lithiation reaction. A suggested reaction process is shown in Fig. 8c. Because the conjugated carbonyl group in BTDA-ODA underwent an oxidation reaction, an additional oxidation current generated. When the oxidation current gradually got lower, indicateing that the intensity of the delithiation reaction weakened, while the reduction current stayed unchanged, showing that the intensity of the lithium reaction remained the same. This process led to a decrease in the lithium ion concentration in the electrolyte. Although BTDA-ODA can have a redox reaction at a certain extent, the reaction only occurs on the surface of the film due to its good insulation [43], and it will not lead to a 26
complete failure of the separator quickly. Therefore, after 100 cycles in a battery, the BTDA-ODA still showed an absorption of ketone carbonyl in infrared spectrum. In order to prove that the BTDA-ODA film contains additional lithium element after cycling, we conducted XPS test on the surface of the BTDA-ODA film after 100 battery cycles at 1C rate. Peaks of Li 1s and F 1s were observed in the XPS spectrum. Considering that LiPF6 in the liquid electrolyte may remain on the film surface, we integrated the Li 1s and F 1s peaks to compare the content of Li and F elements. In LiPF6, every Li atom corresponds to 6 F atoms, but the F/Li value in the XPS spectrum is 2.03. This proves that there is additional Li element on the surface of BTDA-ODA film, which is considered to be derived from the lithiation reaction in the cycling progress.
27
Figure 8. (a) Cyclic voltammetry curves of BTDA-ODA separator. (b) Cyclic voltammetry curves of BPDA-ODA separator. (c) Suggested lithiation process of BTDA-ODA. (d) XPS spectrum of BTDA-ODA film after cycling test.
4 Conclusions The four PI nanofiber films, PMDA-ODA, BPDA-ODA,ODPA-ODA and BTDA-ODA were prepared with excellent thermal stability. They show significant differences in terms of wettability, electrolyte uptake and electrochemical properties, because of their different chemical structures. Among them, the BPDA-ODA film has an ionic conductivity of 2.24 mS cm-1 (25 °C), an electrochemical stability window of 5.1 V, and shows the best performance during battery application tests. Relatively, due to the large differences in surface polarity of the material with electrolyte, weak 28
electrolyte uptake, and lithiation reaction would ocurr with the ketone carbonyl groups of BTDA-ODA polyimide under the working condition of batteries, BTDA-ODA is not suitable as a material for manufacturing LIB separator film. Acknowledgements We are grateful for the financial support given by the National Key R&D Program “New Energy Vehicles” Pilot Project (2016YFB0100105) and the Chinese Academy of Sciences Strategic Pilot Science and Technology Special Project XDA09010105.
29
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Declaration of interests ☒ The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper. ☐The authors declare the following financial interests/personal relationships which may be considered as potential competing interests: