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Flexible Binder for S@pPAN Cathode of Lithium Sulfur Battery- part 1

Mar 31 , 2023

Flexible Binder for S@pPAN Cathode of Lithium Sulfur Battery- part one

LI Tingting, ZHANG Yang, CHEN Jiahang, MIN Yulin, WANG Jiulin. Flexible Binder for S@pPAN Cathode of Lithium Sulfur Battery. Journal of Inorganic Materials, 2022, 37(2): 182-188 DOI:10.15541/jim20210303

Abstract

Sulfurized pyrolyzed poly(acrylonitrile) (S@pPAN) composite as cathode material of Li-S battery realizes a solid-solid conversion reaction mechanism without dissolution of polysulfides. However, its surface and interface characteristics influence the electrochemical performance significantly, and there are also obvious volume changes during electrochemical cycling. In this study, single-walled carbon nanotubes (SWCNT) and sodium carboxymethyl cellulose (CMC) were used as binder for S@pPAN cathode to regulate the surface of S@pPAN and alleviate volume changes during charging and discharging. At a current density of 2C, capacity retention rate of the batteries after 140 cycles was 84.7%, and a high specific capacity of 1147 mAh∙g-1 can still be maintained at a high current density of 7C. The ultimate tensile strength for the film of the composite binder increases by 41 times after adding SWCNT, and the composite binder guarantees a more stable electrode interface during operation, thereby effectively improves the cycle stability of the as sembled lithium-sulfur batteries.
Keywords: lithium-sulfur battery, S@pPAN cathode, sodium carboxymethyl cellulose; binder, stable interface

Traditional lithium-ion batteries have the advantages of simple preparation process and convenient use, but the problems of low energy density (generally less than 250 Wh∙kg-1) and high cost are still prominent. Lithium-sulfur batteries have a higher theoretical specific energy density (2600 Wh∙kg-1), and are considered to be the next generation of secondary rechargeable batteries with great development potential. Moreover, elemental sulfur has the advantages of abundant reserves, low cost, and a theoretical specific capacity of 1672 mAh·g-1. However, the traditional elemental sulfur positive electrode will have a large volume change (about 80%) and electrode powdering during the charging and discharging process, resulting in shortened battery life. And it will generate soluble polysulfides, resulting in a shuttle effect, which eventually leads to a series of problems such as low utilization of active materials and poor cycle stability of the battery. In order to reduce the impact of the shuttle effect on battery performance, researchers have developed many sulfur-based composite cathode materials to improve the performance of lithium-sulfur batteries. Such as carbon-sulfur composite materials, conductive polymers and composite materials formed by metal oxides and sulfur. Single-walled carbon nanotubes (SWCNTs) are a general-purpose additive with the advantages of low density, light weight, and good electrical conductivity. In this study, sodium carboxymethyl cellulose was modified by adding SWCNT to enhance the toughness and ultimate tensile strength of the binder. The application of this composite binder (denoted as SCMC) in lithium-sulfur batteries with S@pPAN as the cathode material can significantly improve the cycle stability of the battery.

Experimental method

1.1 Material preparation

Weigh a certain amount of polyacrylonitrile (Mw=1.5×105, Aldrich) and elemental sulfur according to the mass ratio of 1:8, add an appropriate amount of absolute ethanol as a dispersant, and mix them evenly in a sealed agate ball mill jar. After ball milling for 6 hours, it was dried in a blast oven at 60 °C. After drying, grind the block mixture well. Then a certain amount of mixed powder was weighed and placed in a quartz boat, and the temperature was raised to 300 °C in a tube furnace under a nitrogen protective atmosphere, and kept for 6.5 h to obtain a S@pPAN black powder with a sulfur mass fraction of 41%. Weigh 20 mg SWCNT into a sample bottle, and then add 0.5 mg·mL-1 sodium dodecylbenzenesulfonate (SDBS). After ultrasonic treatment for 10 hours, CMC (Mw=7×105, Aldrich) was added to the SWCNT suspension (the mass ratio of CMC and SWCNT was 2:1) and stirred for 2 hours to obtain SCMC, and its solid content mass fraction is 1%。In addition, the CMC used in the control experiment is exactly the same as the CMC used in the above SCMC synthesis without other treatment. Dissolve CMC in deionized water, the mass fraction of CMC is 1%, and the sample is labeled as CMCP.

1.2 Electrode preparation and battery assembly

S@pPAN, Super P and bonding slurry (SCMC or CMCP) were weighed according to the mass ratio of 8:1:1. Put it in a polytetrafluoroethylene tank for ball milling for 2 hours, and the mass of the bonded slurry is calculated according to the mass of the solid phase component. The slurry was coated on the carbon-coated aluminum foil with a film applicator, and after drying at room temperature, it was cut into ϕ12 mm discs with a microtome, and dried in a blast oven at 70 °C for 6 hours. After pre-drying, the pole piece was processed with a tablet press under a pressure of 12 MPa to reduce the thickness of the pole piece and increase the compaction density of the pole piece, and then continue to vacuum dry at 70 °C for 6 hours. After the temperature of the vacuum oven dropped to room temperature, the pole piece was quickly transferred to the glove box for weighing and set aside. The active material loading per unit area of the cathode in this study is about 0.6 mg∙cm-2. The electrodes based on SCMC and CMCP are denoted as S@pPAN/SCMC and S@pPAN/CMC, respectively.

1.3 Electrochemical performance test

A 2016-type button battery was assembled in the order of positive electrode case, positive electrode sheet, separator, and lithium sheet. The electrolyte is 1 mol L-1 LiPF6 ethylene carbonate (EC)/dimethyl carbonate (DMC) (volume ratio 1 : 1) solution + mass fraction 10% fluoroethylene carbonate (10% FEC), The diaphragm is a polyethylene (PE) diaphragm.

Use the Xinwei battery test system to conduct constant current charge and discharge tests on the assembled batteries. The battery was allowed to stand for 4 h before cycling to fully infiltrate the separator and electrodes with the electrolyte. The charge-discharge cut-off voltage ranged from 1.0 to 3.0 V, and a constant temperature of 25 °C was maintained during cycling. The long-term cycle test was carried out at 2C current density, and the rate performance of the battery was tested at 0.5C, 1C, 3C, 5C, and 7C current density. Cyclic voltammetry (CV) was performed on a CHI 760E electrochemical workstation with a scan rate of 1 mV s-1. The specific capacity is calculated based on the active component sulfur.

1.4 Physical Properties Characterization

X-ray photoelectron spectroscopy (XPS) was used to analyze the surface elements of lithium sheets after battery cycling, and the sample preparation was completed in a glove box. The XRD spectrum of the S@pPAN material was tested by X-ray diffractometer (XRD).
The stress-strain curve of the adhesive was tested with a dynamic thermomechanical analyzer (DMA Q850). The sample preparation process is as follows: drop CMCP and SCMC on the surface of a flat and clean polytetrafluoroethylene plate, put it in a blast oven at 55 °C for 8 h to form a film, and cut it into strips for testing, respectively denoted as CMC film and SCMC membrane.
The cycled electrodes were washed three times with an appropriate amount of DMC solvent in a glove box to remove residual electrolyte on the surface, and dried naturally. The morphology of the samples was observed by electron microscopy (SEM).

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