Poly (acrylic Acid Sodium) Grafted Carboxymethyl Cellulose As a High Performance Polymer Binder for Silicon Anode in Lithium Ion Batteries

The design of novel binder systems is required for the high capacity silicon (Si) anodes which usually undergo huge volume change during the charge/discharge cycling. Here, we introduce a poly (acrylic acid sodium)-grafted-carboxymethyl cellulose (NaPAA-g-CMC) copolymer as an excellent binder for Si anode in lithium ion batteries (LIBs). The NaPAA-g-CMC copolymer was prepared via a free radical graft polymerization method by using CMC and acrylic acid as precursors. Unlike the linear, one-dimensional binders, the NaPAA-g-CMC copolymer binder is expected to present multi-point interaction with Si surface, resulting in enhanced binding ability with Si particles as well as with the copper (Cu) current collectors, and building a stable solid electrolyte interface (SEI) layer on the Si surface. The NaPAA-g-CMC based Si anode shows much better cycle stability and higher coulombic efficiency than those made with the well-known linear polymeric binders such as CMC and NaPPA.A 1 g of CMC power was dissolved in 50 mL distilled water form a transparent sticky solution, and then the solution was put in a 250 mL three-necked flask equipped with a mechanical stirrer. After being purged with nitrogen for 6 h to remove oxygen, 2.5 ml of acrylic acid monomer and the initiator (NH)SO/NaHSO, 0.1:0.03 g) were added. The solution was then heated to 55 C under continuous stirring with rpm of 270 for 2h to produce PAA--CMC. The nitrogen atmosphere was maintained throughout the reaction process. Finally, the PAA--CMC was neutralized by sodium hydroxide solution to PH 6.CR2016-type coin cells were assembled in an argon-filled glove box with oxygen and water contents less than 1 ppm, using lithium-foil as counter electrode and a polymer as a separator. The electrolyte was 1M LiPF dissolved in the mixture of dimethyl carbonate (DMC) and ethylene carbonate (EC) (1:1 in volume ratio), plus 10 wt% fluoroethylene carbonate (FEC) as additive. The working electrode consisted of 60 wt% Si (the Si nanoparticles was obtained from Alfa Aesar, and the diameter ranged from 50100 nm. The SEM, TEM and the particle size analysis of the pristine Si nanoparticles were shown in , ), 20 wt% conducting agent (Super P) and 20 wt% binders. The loading weight of pure Si on the copper foil is about 0.45 mg cm. The electrochemical performances were tested on a LAND battery test system (Wuhan, China) at room temperature, in the voltage range of 0.011.2 V.The morphologies of the samples were characterized by field emission scanning electron microscopy (SEM, Carl Zeiss Ultra 55). Fourier transform infrared (FTIR) spectra were measured on Perkin-Elmer 70 using KBr pieces. The transmission electron microscopy (TEM) images were obtained from JEOL 2100. The particle size was measured from Brookhaven BI-90 plus.For 180 peel test, the electrodes without Super P were first prepared. The mass ratio of Si : binder was 4: 1, and the thickness of each electrode materials was about 50 m. Before the peel test, the electrode film was cut into a 30 mm wide and 60 mm long specimen, and the KingTiger tape was then attached to the specimen. This tape was peeled off using a high-precision material mechanical testing machine (Shanghai Hengyi Testing Machine Company, Shanghai, China). The peel strength was monitored while the force-displacement plot was recorded.