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Enhanced Electrochemical Properties of LiFePO4 Cathode Using Waterborne Lithiated Ionomer Binder in Li-ion Batteries with Low Amount Shu Huang, Jianguo Ren, Rong Liu, Yang Bai, Xiaolong Li, Youyuan Huang, Min Yue, Xueqin He, and Guohui Yuan ACS Sustainable Chem. Eng., Just Accepted Manuscript • DOI: 10.1021/ acssuschemeng.8b01532 • Publication Date (Web): 29 Aug 2018 Downloaded from http://pubs.acs.org on September 5, 2018
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Enhanced Electrochemical Properties of LiFePO4 Cathode Using Waterborne Lithiated Ionomer Binder in Li-ion Batteries with Low Amount Shu Huanga, Jianguo Renb, Rong Liua, Yang Baia, Xiaolong Lia, Youyuan Huangb, Min Yueb, Xueqin Heb, Guohui Yuana,* a School
of Chemistry and Chemical Engineering, Harbin Institute of Technology, 92 West Dazhi Street, Nan Gang District, Harbin, 150001, P. R. China
b
Shenzhen BTR New Energy Materials Inc., High-Tech Industrial Park, Xitian, Gongming Town, Guangming New District, Shenzhen, 518000, P. R. China
* Corresponding Author (Guohui Yuan) E-mail addresses:
[email protected]. Supporting Information
ABSTRACT The poor conductivity of olivine structure LiFePO 4 led to inevitable hindered electrochemical performances were restricted their uses in Li-ion batteries. To overcome the problem, introduction of conductive agents bind with LiFePO 4 active material are universal solutions. In this paper, a novel waterborne lithiated ionomer binder (PSBA-Li) for Li-ion batteries were originality designed and assembled with a low addition of 1.5% of LiFePO4 cathode showing a high areal capacity of 2.0 mAh cm-2. During lithiation/delithiation processes, PSBA-Li provided more Li+ ions resulted in enhanced conductivity, led to better performances. After 200 cycles, PSBA-Li-cathode based battery maintained stable cycling performances with 1
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retentions of 108.5%, 103.6% and 114.9% at the rate of 0.5C, 1C and 2C respectively, in contrast of commercial used PVDF-cathode based with retention of only 7% (0.5C). The rate performance of the PSBA-Li-cathode is higher than that of the PVDF-cathode, which shows almost no capacity at high rates. For contrastive analysis, SEM indicates a well integrity PSBA-Li-cathode structure after cycling. On the contrary, active materials of PVDF-cathode separate from the aluminum foil after cycling. It is believed that the PSBA-Li has a significant potential for further application for LiFePO4 cathode in Li-ion batteries. Keywords: Lithiated Ionomer Binder, LiFePO4 electrode, enhanced cycling stability, limited addition, Lithium ion battery
INTRODUCTION Lithium ion batteries (LIBs) as an enabling energy storage technology have occupy a high market share of emerging applications, including electric vehicles (EVs) and plug-in hybrid electric vehicles (PHEVs)
1, 2.
LiFePO4, as a promising cathode
material, has been focused for its considerably low-cost and eco-environment 3. However, lack of electronic conductivity of LiFePO 4 materials restricts their potential application in high-performance cathodes 4. To overcome the problems, many efforts have been taken on amending of materials structures. One of the approaches is to control the size and morphology of LiFePO4 particles to increase the pathway of Li+ 5, 6,
another common approach is to introduce conductive media such as carbons to
enhance the conductivity 7. However, addition of conductive media affects the 2
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capacity of LiFePO4 cathodes, which in return, lowered overall properties and application prospects of LIBs 8. For recent researches, the polymer binders which is one of the most vital parts of the LIBs system have attracted ever-increasing attention, thank for their efficient functions on electrochemical performances 9. Polymer binders, are used to bond active materials and conductive additives onto the current collector which could directly affect performances of electrodes. Polyvinylidene fluoride (PVDF), as a traditional binder, has been widely used for cathodes and anodes in LIBs because of the electrochemical stability, adhesive and electrolyte absorption ability 10, 11. However, the solvents use for dissolving PVDF are usually strong polarized, such as N-methyl pyrrolidone (NMP), which is considerably toxic and costly 12, 13. Moreover, binders occupy 10-30%wt within the LIBs system 14, 15
(Table 1), and such amount of binder may impacts the overall energy density in
consequence restrict the electrochemical performances. In recent years, waterborne binders have intensively studied due to the elevated cycling and rate performances 17,
16,
in addition they are eco-friendly and cost-effectively 18, 19. In order to improve the
conductivity of LiFePO4 cathodes, many works have been focused on conductive polymers binders
20,21,
such as poly(ethylene oxide)-block-poly(acrylonitrile)
(PEO-b-PAN) copolymer 14 and poly(3,4-ethylenedioxythiophene) (PEDOT) 22, which showed enhanced long-term cycle life. Despite improved performances, recent literatures of waterborne binders are still limited by the followings: (1) the addition amount of binders and conductive additives still occupy a large weight in the cathodes (Table 1). The large addition content decreases the energy density of electrodes, 3
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hinders overall performances. (2) capacity retentions should be further elevated for the further application prospects. In order to minimize the usage of binder in LIBs system, as well as maintain cycling and rate performances of LiFePO 4 cathodes, a waterborne lithiated ionomer binder (PSBA-Li) was prepared through polymerization. There are more Li+ attached to molecule chains to enhance conductivity and transfer at the interface between the electrode and electrolyte to shorten the pathway of Li+ to particle surfaces. Moreover, the addition amount of the PSBA-Li binder and conductive additive is only 1.5% and 3% which is much lower than recent researches (Table 1). We demonstrate that LiFePO4 cathodes fabricated with PSBA-Li binder can present outstanding cycling and rate properties compared to PVDF-based cathodes. Surface and cross-sectional morphology verifies the integrity of PSBA-Li-cathodes without cracks and gaps before and after cycling. Therefore, the PSBA-Li binder offers a promising binder-design viewpoint for further application of LiFePO4 cathodes in LIBs.
EXPERIMENTAL SECTION Synthesis of the lithiated ionomer binder Initially,
styrene
(Alfa
Aesar),
n-butyl
acrylate
(Alfa
Aesar),
2-arcylamido-2-methylpropane sulfonic acid copolymer (AMPS), methacrylic acid (Alfa Aesar) and lithium hydroxide monohydrate (Alfa Aesar) were dissolved in deionized water at the room temperature with 250r/min for 1 h. Subsequently, ammonium persulphateto (Alfa Aesar) was added in the mixture as the initiator. Then, 4
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the polymerization was taken place at 70oC with 200r/min for 6 h in nitrogen atmosphere. During the process, ammonium persulphate was stepwise added to the mixture. At last, slowed the stir speed and cooled down the PSBA-Li emulsion. The un-lithiated binder (PSBA-H, PSBA-Li without LiOH) and polyvinylidene fluoride (PVDF) binder were used as comparative samples. Characterization of the ionomer binder The Fourier-transform infrared (FT-IR) spectrum was recorded in the range of 400-4000 cm-1 (Bruker tensor 27, Germany). Thermogravimetric analysis (TGA) of pure binder films were performed with a TGA instrument (DSC 200F3, Germany). Preparation of electrode slurries and Li-ion half cells assembling configuration Initially, CMC (Ashland, USA) was dissolved in deionized water and stirred at 800r/min for 2h at 50oC resulted in uniform solution (1.5%wt). Subsequently, the stoichiometric PSBA-Li binder, LiFePO4 (BTR,
