Effect of pore structure of paper-based separators on lithium-ion battery performance

doi:10.21203/rs.3.rs-3336363/v1

Download PDF

Short Report

Effect of pore structure of paper-based separators on lithium-ion battery performance

https://doi.org/10.21203/rs.3.rs-3336363/v1

This work is licensed under a CC BY 4.0 License

Version 1

posted

You are reading this latest preprint version

The two kinds of separators with different pore structure using PET paper as substrate were prepared. The PET ceramic (PC) separator was prepared by filling the macropores of PET paper with ceramic particles. Lyocell fibrillated fiber with a diameter of several hundred nanometers was introduced between PET paper and ceramic particles to produce a PET/Lyocell/ceramic (P/L/C) separator with a three-layer gradient pore structure. Compared with the PE/C microporous separator, the PC and P/L/C paper-based separator both have larger pore size and porosity. However, the P/L/C separator showed better cycle performance than that of PC separator. The large-size blocked areas in PET paper hinder ion transport, and the multilayer gradient pore structure of P/L/C separator can optimize the uniform of ion transport.

PET paper

Lyocell fibrillated fiber

Pore structure

Lithium-ion batteries

The spontaneous combustion accidents of lithium ion batteries occur frequently in recent years, and the safety of battery products becomes more and more important [1, 2]. Separator damage is the direct cause of battery thermal runaway [3]. The main function of separator is to prevent physical contact between anode and cathode materials, while allowing ion transport in the pores [4, 5]. The commercial mainstream polyolefin microporous separators have a relatively uniform pore structure and high mechanical strength. However, the polyolefin microporous separators have poor heat resistance, and the pores of separators may be closed under the action of heat and stress during battery operation, thus affecting the battery life and causing potential safety hazards [6–8]. The uniformity of the separator and the structure stability of separator during the battery life cycle are critical to battery safety [9].

The paper-based separators form porous structures through non-directional interweaving of fibers, which usually have large pore size and porosity, thus achieving excellent rapid charge-discharge performance [10, 11]. The diameter of these fibers is usually less than 10 µm to meet the thickness requirements of separators in lithium-ion batteries. The heat resistant fibers with a small diameter in the paper-based separators mainly are Lyocell fibrillated fibers [12, 13] and ultrafine polyester (PET) fiber [14–18]. Lyocell fiber is a regenerated cellulose fiber with fibrillated properties [19]. Lyocell fibrillated fibers with the diameter from tens of nanometers to tens of microns can be obtained by water swelling and mechanical beating treatment. The paper-based separators made entirely of Lyocell fibrillated fibers has sufficient isolation performance, but low tensile strength. The longitudinal tensile strength of PET paper made by PET fibers is close to that of polyolefin microporous separators. The enhancement of PET paper is achieved through the hot press process, during which the PET bonding fibers soften to increase the contact areas with the PET backbone fibers, and then the PET bonding fibers crystallize during cooling. The diameter of PET fiber is usually below 5 µm, and the pore size of PET paper produced after hot press treatment is tens of microns. The ceramic particles with a particle size of several hundred nanometers are usually used to fill the macropores of PET paper. The PET paper not only has many macropores, but also has many blocked areas with a large size. The ions passing through the PET paper filled with ceramic particles will be affected by the blocked areas of PET paper.

In this paper, we prepared two kinds of separators with different pore structure using PET paper as substrate. The PET ceramic (PC) separator was prepared by filling the pores of PET paper with ceramic particles. A structure control technique was proposed to preserve the pores in PET paper. Lyocell fibrillated fiber with a diameter of several hundred nanometers was introduced between PET paper and ceramic particles to produce a PET/Lyocell/ceramic (P/L/C) separator with a three-layer pore structure. The effect of pore structure of PET paper-based separators on physical properties and cell performance was investigated, compared with polyethylene/ceramic (PE/C) microporous separator.

2.1. Preparation of Lyocell fibrillated fibers

Lyocell fibers were provided by Lenzing, Austria. Lyocell fibers firstly were beated by using PFI mill (PFI mill No. 621, Hamjern Maskin) for 450000 revolutions under the beating pressure with 1 N mm^− 1 and pulp concentration of 10 wt%. The beated fibers were diluted to a concentration of 0.01% and screened through a Dynamic Drainage Jar (DSC75, PRM) with 150 and 300 mesh successively. Lyocell fibrillated fibers with diameters of less than 2 µm can be obtained after screening treatment.

2.2. Preparation of coating solution

The coating solution was prepared by mixing Al₂O₃ particles (average particle size of 700 nm) and sodium polyacrylate (PAAS) in deionized water, where Al₂O₃/PAAS weight ratio was 100:7. Al₂O₃ particles was dispersed in the deionized water by high speed dispersing machine (GFJ-550A, Shanghai Kestar Coating Machinery Co., LTD) for 30 min. Then a predetermined amount of PAAS was added, and the solution was stirred vigorously for 30 min.

2.3. Preparation of paper-based separators

The P/L/C paper was designed into three-layer structure, namely the PET paper (basis weight of 9.4 g/m²), the Lyocell fiber layer (basis weight of 1.8 g/m²), and the ceramic coating layer (basis weight of 7.3 g/m²). The PET paper (thickness = 16.5 µm Mitsubishi Paper Co., LTD) was placed on the stainless steel wire of the sheet former (No. 1311090, Kumagai Riki Kogyo). Lyocell fibrillated fiber aqueous suspension was added into the sheet former and deposited on the PET paper, then dried and soft calendered at 85 N/mm to obtain the PET/Lyocell (P/L) paper-based materials. Finally, the coating solution is coated on the P/L paper-based materials using rod coater to obtain the P/L/C separator. The PC separator was prepared by impregnating the PET paper with the coating solution. The PE/C microporous separator (thickness = 14.5 µm, Shenzhen Senior Technology Material Co., Ltd) was chosen for comparison.

2.4. Characterization of separators

The morphology of separators was observed by the scanning electron microscope (SEM, G2ProY, Phenom). The thickness of separators was measured by using the thickness tester (No. 251, Lorentzen & Wettre) according to TAPPI T411 om-2010 standard. The porosity φ (%) of separators was estimated using the following equation: φ (%) = (1 - (W₁/ρ₁ + W₂/ρ₂ + W₃/ρ₃)/ T_s) × 100, where W₁, W₂, W₃ were the basis weight of different materials in the separators (g m^− 2), ρ₁, ρ₂, ρ₃ were the density of different materials (PET of 1.40 g cm^− 3, cellulose of 1.50 g cm^− 3, Al₂O₃ of 3.70 g cm^− 3, PE of 0.92 g cm^− 3), and T_s was the thickness of separators (µm). The pore size and pore size distribution of separators were quantitatively determined according to the ASTM standard F316-2019 using a capillary flow porometer (CFP-1500AEX, PMI). The air permeability of separators was evaluated with a Gurley densometer (4100N, Gurley), where Gurley number was expressed as the number of seconds in which 100 ml of gas pass. Tensile strength of separators was tested with a universal testing machine (5565, INSTRON) at a speed of 5 mm min^− 1 with samples in the dimension of 1.5 cm × 4 cm. Electrochemical impedance spectroscopy (EIS) was conducted using impedance analyzer (CHI604E, Chinstruments) at the open circuit voltage with the frequency range of 10^− 2 to 10⁶ Hz. EIS measurements were done on the cells under fully discharged state. The cells were held at least 1 h before EIS measurements to ensure the stability of the baseline voltage. The liquid electrolyte-soaked separators were placed between two stainless steel plates for the measurement of ionic resistance (R_i) of separators at 30°C. A unit cell (2032-type coin) was assembled by sandwiching a separator between a graphite anode (graphite/CMC/SBR = 97.5/1.0/1.5 w/w/w) and a LiNi_0.8Mn_0.1Co_0.1O₂ cathode (LiNi_0.8Mn_0.1Co_0.1O₂/carbon black/PVdF = 95/2/3 w/w/w), and then activated by filling the liquid electrolyte of 1 M LiPF₆ in ethylene carbonate (EC)/diethyl carbonate (DEC) = 1/1 v/v. The diameter of the anode and cathode plate for assembly is 14 mm and 12 mm respectively. The assembly of all the cells was carried out in an argon-filled glove box. The cycle performance and rate discharge capability were evaluated using a battery tester (CT2001A, Land). The cells were cycled with a fixed charge/discharge current density of 1C/1C and 5C/5C at 30°C. The rate performance was performed with the discharge current densities varied from 0.2C to 5C and a constant charge current density of 0.2C. All the cell performance had a constant voltage charging phase with a cut-off current of 0.1C and a voltage range from 3.0 V to 4.2 V. The resistance of the cells at the discharge end state was obtained by EIS measurement at room temperature. The cathode plate, anode plate and liquid electrolyte were provided by Guangzhou Tinci Material Technology Co., Ltd.

The morphology characteristics of separators was elucidated. Fig. 1(a) shows the PET fiber diameter is several microns, and the pore size of the PET paper formed by PET fiber interweaving is tens of microns. The blocked areas in PET paper where the fibers are tightly interwoven can be seen, and these areas may affect ion transport. Fig. 1(b) shows that the PET paper consists of several layers of fibers tightly stacked together. Fig. 1(c) shows that the surface of the PC separator is covered by ceramic particles. Fig. 1(d) shows that the ceramic particles penetrate the interior of the PET paper, the pores of PET paper are almost completely filled with ceramic particles. In order to prevent short circuit of cells, a large number of ceramic particles are embedded in the macropores of PET paper. This results in a significant increase in the weight of separator, which is not conducive to improving the mass energy density of cells. Fig. 1(e) shows the PET paper side surface of P/L/C separator. The Lyocell fibrillated fibers are interwoven on the PET paper to form a mesh filter cake. The diameter of Lyocell fibrillated fiber is mainly several hundred nanometers, which can achieve uniform coverage of PET paper at a low dosage. Fig. 1(f) shows the cross section of the P/L/C separator. It can be seen that the P/L/C separator has a three-layer pore structure. The ceramic particles did not penetrate into the interior of the PET paper, and the internal pore space of the PET paper is fully preserved.

Fig. 2 shows the properties of these three kinds of separators. From Fig. 2(a), we can see that the PE/C separator has the minimum basis weight and ceramic coating weight in these three separators, which are 11.7 g m^-2 and 5.1 g m^-2 respectively. The coating weight of P/L/C paper-based separator is 7.3 g m^-2, far lower than that of PC paper-based separator of 24.8 g m^-2. It is attributed that ceramic particles can be coated very thinly on the fibrillated fiber layer rather than filling inside the macropores of thick PET paper. Fig. 2(b) shows that the thickness and porosity of PE/C separator are the smallest in these three separators, which are 14.5 μm and 41.0%, respectively. The thickness of P/L/C separator is 20.6 μm, lower than that of PC separator of 23.9 μm. The porosity of P/L/C separator is 52.0%, larger than that of PC separator of 43.9%. It is attributed to the low ceramic coating amount in P/L/C separator and the retention of pore space in the PET paper. Compared with the PC separator, the P/L/C separator has lower basis weight and thickness, which is conducive to improving the mass density and volume density of batteries. Fig. 2(c) shows that the maximum and average pore size of PE/C separator are lower than 100 nm. The maximum and average pore size of the P/L/C separator are 445 nm and 188 nm respectively, which is slightly larger than the pore size of the PC separator. Fig. 2(d) shows the PE/C separator has the largest Gurley value of 214 s due to its pore size of less than 100 nm, but the smallest ionic impedance (1.43 Ω) due to its lower thickness and excellent pore connectivity [20]. The Gurley value and ionic impedance of P/L/C separator are 25.4 s and 2.16 Ω. The Gurley value and ionic impedances of these two paper-based separators are close. For the separators prepared by the same method, the ionic impedance of separators usually decreases as the Gurley value of separators decreases [21,22]. The Gurley value of PET paper-based separators is smaller than that of PE/C microporous separator, but its impedance is greater than that of PE/C microporous separator. This is mainly due to the fact that many blocked areas with a large size in PET paper increases the ionic impedance of separator. Fig. 2(e) shows the Gurley value of PET paper is very low, but the impedance of PET paper (1.7Ω) is greater than that of PE microporous separator (1.1Ω). For the separators prepared by different methods, it is necessary to evaluate the ion transport performance of separators in combination with pore connectivity [20]. Fig. 2(f) shows the strength of paper-based separators is lower than that of PE/C microporous separator. The machine direction (MD) strength of paper-based separators is close to 1000N/m, and it can meet the assembly requirement of batteries. The cross direction (CD) strength of paper-based separators is only 225 N/m. Increasing the strength can be achieved by increasing the pressure and temperature of hot press, but it will further increase the area of the blocked areas.

Fig. 3 shows that the PE/C separator has a contact angle of 68.4° on the PE side and a contact angle of 73.5° on the ceramic coating side. The P/L/C separator has a smaller contact angle of 44.4° on the PET paper side as compared to PC separator (72.8°). It is attributed to the Lyocell fibrillated fibers wrapped around the PET fibers on the PET paper side. The surface of Lyocell fibers contains a large number of hydroxyl groups, which is conducive to electrolyte infiltration.

Fig. 4(a) shows the cycle performance at 1C and 30 °C. All the separators have similar discharge capacity at 1^st cycle, but the P/L/C separator delivers excellent discharge capacity retention of 91.1% after 100 cycles, as compared to the PE/C separator (74.1%) and the PC separator (65.8%). Fig. 4(b) shows the rate discharge performance at 30 °C. The capacity of P/L/C separator is larger than that of PE/C and PC separator under different current density, and the difference is greater with the increase of current density. Fig. 4(c) shows the cycle performance at 5C and 30 °C. The P/L/C separator has larger discharge capacity of 108.6 mAh g^-1 at 1st cycle, as compared to the PE/C separator (52.5 mAh g^-1) and the PC separator (40.6 mAh g^-1). After 100 cycles, the P/L/C and PC paper-based separators have similar capacity retention (＞79%), and the PE/C separator has a lower capacity retention of 71.6%.

In order to attain a more comprehensive understanding of the cell performance of different separators, we analyze the EIS of the cells. Fig. 4(d) shows the EIS of the cells after 100 cycles at 1C and 30 °C. The EIS profiles compose of two semicircles and an oblique line. The intersection of the curve and the horizontal axis in the high-frequency region represents bulk resistance (R_b) of the cells, which reflects the overall ohmic resistance of the separator, electrolyte, electrodes and shell body. The first semicircle appearing in the high-frequency region stands for the resistance (R_s) of solid electrolyte interphase (SEI) associated with the migration of lithium ions through the interface of active materials. The second semicircle situating in the high-to-medium-frequency region is connected with the charge-transfer resistance (R_ct), and the oblique line in the low-frequency region represents the Warburg resistance (W) of the active materials which is associated with the Li⁺ diffusion in the bulk phase [23]. The R_b of the PE/C separator is 4.27 Ω, which is lower than that of the P/L/C separator (5.35 Ω) and the PC separator (5.63 Ω). The R_b difference is consistent with the difference of the ionic resistance (Fig. 2(d)), indicating the ionic resistance has a great influence on R_b. After fitting with the equivalent circuit in Fig. 4(d), the R_sof the P/L/C separator is 9.2 Ω, lower than that of the PE/C separator (9.9 Ω) and the PC separator (33.8 Ω). The R_ct of the P/L/C separator is 15.3 Ω, lower than that of the PE/C separator (21.3 Ω) and the PC separator (27.4 Ω). The PE/C separator acquires larger impedances due to smaller pore sizes and porosity after 100 cycles. The inhomogeneity of ion transport in the PC separator causes the SEI film of the negative electrode to grow unevenly. This further reduces the ion transport performance of the negative electrode, resulting in a positive feedback process that leads to a sharp decline in battery capacity (Fig. 4(a)). The size and uniformity of pores have an important effect on the impedance of ion transport. Fig. 4(e) shows the EIS of the cells after 100 cycles at 5C and 30 °C. The Rs of P/L/C separator is 24.7 Ω after fitting with the equivalent circuit, which is lower than that of PE/C separator (28.2 Ω) and PC separator (69.3 Ω). The Rct of P/L/C separator is 31.4 Ω, lower than that of PE/C separator (34.9 Ω) and PC separator (89.4 Ω). Both Rs and Rct are markedly increased and almost overlapped at 5C in comparison to 1C. This observation can be explained by an accelerated and uneven SEI film growth on the graphite surface during longtime charge/discharge processes at a high current rate [24-26].

Lanzi [27] proved that the pore size of the electrode is mainly in the micron level, and the ion transport is mainly determined by the larger pore, because the larger pore carries almost all the current from one side of the electrode to the other side. The utilization rate of lithium ion transmission through small size pores is reduced at higher current density, resulting in a larger overpotential [28]. The pore size has a more significant impact on the initial specific capacity under the current density of 5C. The PE/C separator has the smallest pore size and porosity, so it obtains a lower initial specific capacity at 5C (Fig. 4(b)). The initial capacity of PC separator is lower than that of PE/C separator at 5C, although its pore size and porosity of PC separator are larger than that of the PE/C separator. Fig4. (f) shows the transport route simulation of lithium ions in the paper-based separators. The macropores in the PET paper of P/L/C separator are preserved. When ions quickly bypass the blocked areas in the PET paper, they pass through the Lyocell layer and ceramic layer with smaller pore size and uniform thickness on the micron scale. The ion transport is evenly distributed due to the limitation of pores with smaller pore size. The gradient change of pore size in the separators is beneficial to optimize the uniform of ion transport. In addition, the P/L/C separator has the largest pore size and porosity in these three separators, so it gets the maximum initial capacity at the current density of 5C. The macropores in the PET paper of PC separator are filled with ceramic particles. The pore size in the PET paper filled with ceramic particles is the same as the pore size of the ceramic coating. When ions pass through the blocked areas in the PET paper of PC separator, it is difficult to form uniform distribution of ion transport because the pore size is not changed. The polarization is more pronounced at a current density of 5C, which allows the PC separator to achieve the lowest initial capacity.

The PET paper not only has macropores, but also has the large-size blocked areas. In order to ensure the isolation performance of PET paper-based separators, we use two pore structure design methods to decrease the pore size of separators. Ceramic particles are used to fill macropores in the PET paper of PC separator, and the uniform transmission of ions through PC separator is poor. The P/L/C separator adopts gradient pore structure design, Lyocell layer as the intermediate transition layer. It enables the pores in the PET paper of P/L/C separator are preserved. The ion transport uniformity is improved after the ions are transferred from the PET paper with a larger pore size to the ceramic layer with a smaller pore size. Compared with the PC separator, the P/L/C separator shows better cycle performance at the current density of 1C and 5C.

Author contribution

YW and RY wrote the manuscript text; GH offered funding acquisition.

Funding This work was supported by postdoctoral program of Shanxi University (203521032).

Data availability There is no supporting information in this article.

Ethical approval This article does not involve human or biological research and has no moral or ethical issues.

Conflict of interest The authors declare no competing interests.

Gong YZ, Xie S, Ping XK, Li GS,·He JX (2022) The influence of overcharge and discharge rate on the thermal safety performance of lithium‑ion battery under low air pressure. Ionics 28: 4653–4665
Chen W, Jiang JC, Wen JF (2021) Thermal runaway induced by dynamic overcharge of lithium‑ion batteries under different environmental conditions. J Therm Anal Calorim 146:855-863
X.N. Feng, M.G. Ouyang, X. Liu, L.G. Lu, Y. Xia, X.M. He (2018) Thermal runaway mechanism of lithium ion battery for electric vehicles: A review. Energy Storage Mater 10:246-267
Chen HZ, Wang ZC, Feng YT, Cai SY, Gao HP, Wei ZZ, Zhao Y (2023) Cellulose-based separators for lithium batteries: Source, preparation and performance. Chem Eng J 471:144593
Wu MX, Yang CY, Xia HH, Xu JQ (2021) Comparative analysis of different separators for the electrochemical performances and long-term stability of high-power lithium-ion batteries. Ionics 27:1551–1558
Zhai P, Liu KX, Wang ZY (2021) Multifunctional separators for high-performance lithium ion batteries. J Power Sources 499:229973
Roth EP, Doughty DH, Pile DL (2007) Effects of separator breakdown on abuse response of 18650 Li-ion cells. J Power Sources 174:579-583
Peabody C, Arnold CB (2011) The role of mechanically induced separator creep in lithium-ion battery capacity fade. J Power Sources 196:8147-8153
Orendorf CJ (2012) The role of separators in lithium-ion cell safety. Electrochem Soc Inte 21:61-65
Huang XS (2014) Performance evaluation of a non-woven lithium ion battery separator prepared through a paper-making process. J Power Sources 256:96-101
Wang ZH, Xiang HF, Wang LJ (2018) A paper-supported inorganic composite separator for high-safety lithium-ion batteries. J Membrane Sci 553:10-16
Wang Y, Luo JR, Chen L, Long J, Hu J, Meng L (2021) Effect of fibrillated fiber morphology on properties of paper-based separators for lithium-ion battery applications. J Power Sources 482:228899
Long J, Wang XW, Zhang HF, Hu J, Wang Y (2016) A nano-based multilayer separator for lithium rechargeable battery. Int J Electrochem Sci 11:6552-6563
Liu LX, Wang ZH, Xie YS, Peng Y, Dong JH, Chow YT, Zhang GH, Zhu LY, Wang XQ, Xu D (2020) Zirconia/polyethylene terephthalate ceramic fiber paper separator for high-safety lithium-ion battery. Ionics 26:6057-6067
Li DN, Xu H, Liu Y, Jiang YS, Li FF, Xue B (2019) Fabrication of diatomite/polyethylene terephthalate composite separator for lithium-ion battery. Ionics 25:5341-5351
Li WB, Li XZ, Xie XH, Yuan AB, Xia BJ (2017) Effect of drying temperature on a thin PVDF-HFP/PET composite nonwoven separator for lithium-ion batteries. Ionics 23:929-935
Wu YS, Yang CC, Luo SP, Chen YL, Wei CN, Lue SJ (2017) PVDF-HFP/PET/PVDF-HFP composite membrane for lithium-ion power batteries. Int. J. Hydrogen Energ 42:6862-6875
Choi ES, Lee SY (2011) Particle size-dependent, tunable porous structure of a SiO₂/poly(vinylidene fluoride-hexafluoropropylene)-coated poly(ethylene terephthalate) nonwoven composite separator for a lithium-ion battery. J Mater Chem 21:14747-14754
Okugawa A, Sakaino M, Yuguchi Y, Yamane C (2020) Relaxation phenomenon and swelling behavior of regenerated cellulose fibers affected by water. Carbohyd Polym 231:115663
Lagadec MF, Zahn R, Müller S, Wood V (2018) Topological and network analysis of lithium ion battery components: the importance of pore space connectivity for cell operation. Energy Environ Sci 11:3194-3200
Hantel MM, Armstrong MJ, DaRosa F (2017) Characterization of Tortuosity in Polyetherimide Membranes Based on Gurley and Electrochemical Impedance Spectroscopy. J Electrochem Soc 164(2):A334-A339
Arora P, Zhang ZM (2004) Battery separator. Chem Rev 104(10):4419-4462
Schmidt A, Smith A, Ehrenberg H (2019) Power capability and cyclic aging of commercial, high power lithium ion battery cells with respect to different cell designs. J Power Sources 425:27-38
Zhang PC, Yuan T, Pang YP, Peng CX, Yang JH, Ma ZF, Zheng SY (2019) Influence of current density on graphite anode failure in lithium-ion batteries. J Electrochem Soc 166:A5489-A5495
Schmidt A, Smith A, Ehrenberg H (2019) Power capability and cyclic aging of commercial, high power lithium ion battery cells with respect to different cell designs. J Power Sources 425:27-38
Matasso A, Wong D, Wetz D, Liu F (2015) Effects of high-rate cycling on the bulk internal pressure rise and capacity degradation of commercial LiCoO₂ cells. J Electrochem Soc 162:A885-A891
Lanzi O, Landau U (1990) Effect of pore structure on current and potential distributions in a porous electrode. J Electrochem Soc 137:585-593
Chandrasekaran R (2014) Quantification of contributions to the cell overpotential during galvanostatic discharge of a lithium-ion cell. J Power Sources 262:501-513

No competing interests reported.

Download PDF

Version 1

posted

You are reading this latest preprint version

Effect of pore structure of paper-based separators on lithium-ion battery performance

Status:

Version 1

Abstract

Figures

1. Introduction

2. Experimental

2.1. Preparation of Lyocell fibrillated fibers

2.2. Preparation of coating solution

2.3. Preparation of paper-based separators

2.4. Characterization of separators

3. Results and discussion

4. Conclusions

Declarations

References

Additional Declarations

Status:

Version 1