Study on preparation and performance of aqueous binder for lithium iron phosphate electrodes in lithium-ion battery

doi:10.11949/0438-1157.20201085

Abstract

Abstract:

LiFePO₄ is a widely used cathode material for lithium-ion batteries. Polyvinylidenefluoride (PVDF), as a traditional organic solvent soluble binder, is still used in the preparation of LiFePO₄cathode. The water-based binder which can be used in the preparation of LiFePO₄cathode still needs to be further studied. A novel water-based binder PSEHA with different structures for LiFePO₄ cathodes was prepared via copolymerizing polystyrene (St) and 2-ethylhexyl acrylate (2-EHA) with a reactive emulsifier to improve the performance of lithium-ion battery. The influence of binder structure on battery performance was studied. Without unsaturated double bonds, PSEHA binder has good oxidation resistance. Its low swelling ratio can effectively prevent structural damage caused by excessive swelling, and the use of reactive emulsifiers can solve the residue of emulsifier. The LiFePO₄cathode prepared with the optimal binder showed excellent electrochemical stability. After 100 cycles, the LiFePO₄cathode in coin cells maintained stable cycling performance with retention of 96% at the rate of 1 C, while the SBR was only 93.9%. The capacity retention rate of the soft-packed full battery after 170 cycles at 1 C rate is still 98.9%. This new type of water-based binder is of great significance to promote the preparation of LiFePO₄cathode in water-based systems.

Key words: electrochemistry, electrode binder, emulsions, polymers, acrylate binder, reactive emulsifier, LiFePO₄ cathode, lithium-ion battery

摘要：

磷酸铁锂作为锂离子电池正极材料应用广泛。目前在其电极制备中仍采用PVDF油系黏结剂体系，可用于该电极的水性黏结剂仍需进一步研究。通过反应型乳化剂共聚苯乙烯（St）与丙烯酸异辛酯（2-EHA）制备了不同结构的磷酸铁锂正极水系黏结剂PSEHA，探讨了黏结剂对电池性能的影响。PSEHA黏结剂不含不饱和双键，抗氧化性好，较低的溶胀率可以有效防止过度溶胀导致的结构破坏，而反应型乳化剂可以解决乳化剂残留问题。采用所得最优结构黏结剂制备的磷酸铁锂电极表现出优异的电化学稳定性，扣式电池1 C循环100圈后容量保留率仍有96%，而SBR仅有93.9%；软包全电池在1 C倍率下循环170圈后容量保留率仍有98.9%。该新型水性黏结剂对促进磷酸铁锂水性体系制备有重要意义。

关键词: 电化学, 电极黏结剂, 乳液, 聚合物, 丙烯酸酯黏结剂, 反应型乳化剂, 磷酸铁锂正极, 锂离子电池

CLC Number:

TQ 028.8

CAO Jianing, GAO Xiang, LUO Yingwu, SU Rongxin. Study on preparation and performance of aqueous binder for lithium iron phosphate electrodes in lithium-ion battery[J]. CIESC Journal, 2021, 72(2): 1169-1180.

曹佳宁, 高翔, 罗英武, 苏荣欣. 一种用于磷酸铁锂电极的水性黏结剂制备与性能研究[J]. 化工学报, 2021, 72(2): 1169-1180.

Figures/Tables 15

Fig.1 Schematic diagram of pouch lithium cell structure

Fig.2 Synthesis reactions and the chemical structure of the PSEHA

Table 1 Monomer conversion of binders with different monomer ratio

Polymer	Conversion/%
PSEHA-1∶11	99.57
PSEHA-3∶7	99.34
PEHA	99.45

Table 2 Size of PSEHA copolymers

Polymer	Particle size/nm	PDI
PSEHA-1∶11	64.03	0.027
PSEHA-3∶7	69.98	0.022
PEHA	58.8	0.067

Fig.3 FTIR spectra of PSEHA-3∶7

Table 3 Electrolyte uptake of electrodes fabricated by different binders

Binder	Swelling ratio(25℃, 120 h)
PSEHA-1∶11	15.38
PSEHA-3∶7	14.81
PEHA	20.59
SBR	32.86
PVDF	42.31

Fig.4 Peel test of electrodes fabricated by different binders

Fig.5 Schematic diagram of electrodes with different binders before and after cycles

Fig.6 SEM images of electrodes consists of different binders before and after cycles

Fig.7 Electrochemical performances of LiFePO4 cathodes with various binders

Fig.8 EIS spectra of LFP/C electrodes prepared with PSEHA-1∶11 and SBR in frequency range between 10-2 and 105 Hz

Table 4 The resistances of the cells

State	Binder	R/Ω
before cycles	SBR	87.15
before cycles	PSEHA-1∶11	88.21
after cycles	SBR	188.4
after cycles	PSEHA-1∶11	123

Fig.9 Electrochemical performances of the pouch lithium cell with PSEHA-1∶11 binder

Fig.10 Electrochemical stability windows of PSRHA-1∶11 with a scanning rate of 0.5 mV/s

Fig.11 Stability of electrode slurries using PSEHA-1∶11 and SBR

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