Ionically Crosslinked Chitosan-Tripolyphosphate Binder for Silicon Anode in Lithium Ion Batteries

Lithium-ion batteries have become the choice for portable electronics and EVs because of their high energy density and reasonable cost. To meet today’s demand of high capacity lithium-ion batteries one has to look beyond the commonly used graphite-based anodes. Silicon is capable of electrochemically alloying with Li to have high charge capacity but its high volumetric expansion on lithiation often results in low rate capability and limited cycle life. Binders play an important role in the cycling stability of the batteries. They keep active materials in electrical contact, and are critical to assembling high-quality electrodes, regardless of the particle size. An ideal binder should provide best elasticity to the anode composite and should accommodate the expansion of the active material during the lithiation/delithiation process. It has been found that using different biopolymers like cellulose, carboxymethyl cellulose, sodium alginate, etc. as binders can give a better cycling stability than polyvinylidene fluoride, a commonly used binder. This research focuses on using Chitosan, the second most abundant biopolymer found in nature, and its derivative Chitosan-tripolyphosphate (TPP), as binders for Silicon (Si) anode. Specifically, 2% Chitosan solution was prepared in 1% acetic acid solution. The ionically cross-linked hybrid gel polymer was prepared by adding 14 ml of 4 mg/ml solution of Sodium tripolyphosphate (TPP) solution dropwise into 35 ml of 2% Chitosan gel under mechanical stirring at 300 rpm for 5 hours. Anode slurries were prepared by mixing active material Si nanoparticles with conductive agents super P and carbon nanofibers (CNF), and binder gel solutions in a ball mill (Zheng Xian QM-3SP04, Nanjing University) at 400 rpm for 6 hours. The mass ratio for Si:C:Binder (Chitosan or Chitosan-TPP) was 4:4:2. For Si:C:CNF:Chitosan-TPP, it was 6:1.5:0.5:2. The anode slurries were then coated on copper foil and vacuum dried for coin cells preparation. Pure lithium metal was employed as the counter electrode, Celgard 2325 film as separator and 1M LiPF 6 in EC: DMC (1:1) as electrolyte. The cells were charged and discharged at constant rate of 0.1C (1C = 4200 mA/g) between 0.01 and 1 V. Figure 1 shows the cycling performance of Si anodes using different binders. The initial discharge capacities for anodes using Chitosan and Chitosan-TPP binders were around 1400 mAh/g and the discharge capacity retention was around 55% and 70% respectively, after 500 cycles. Initial discharge capacity for anode using CNF as conducting agent with higher amount of Si was around 2100 mAh/g and the discharge capacity retention was around 82% after 500 cycles. The large drop in discharge capacity in the initial cycles is caused by the incomplete delithiation during the initial delithiation cycles, during which the particle size is decreased and the electrode resistance, predominantly contact and SEI resistance increases and inhibits Li extraction. Upon subsequent cycling, a stable SEI layer is formed and the Li alloys with the partially delithiated Si, thus producing a stable discharge capacity in the later cycles. The improved cycling performance of the Si anode is attributed to the formation of the chemical bonds and interactions between the Si active material and binder molecules. X-Ray photoelectron spectroscopy (XPS) was employed to determine the spectral shifts in the Carbon and Nitrogen present in the binder molecules. As shown in Figure 2, the C 1s XPS spectrum of the Chitosan-TPP shows three obvious characteristic peaks corresponding to C-C and C-H bonds (283.8 eV), C-O bond (285.7 eV) and C=O bond (283.0 eV). The N 1s XPS spectrum also showed the strong N-H bond (399 eV) in Chitosan-TPP. As expected, pristine Si powder does not show any signs of C and N atoms on the surface. Comparing with Chitosan-TPP, the binding energy of C-H and C-O of the Si-C-Chitosan-TPP mixture increased from 283.8 to 284.2 eV and 285.7 to 287 eV, respectively, while the N 1s signal shifted from 399 to 400.8 eV. In spite of careful purification by washing with water for four times, the Si-C-Chitosan-TPP still shows strong C 1s and N 1s signals in XPS spectra, indicating that a large amount of Chitosan still remains on the surface of Si nanoparticles. This implies that the -OH, -CH 2 OH and -NH 2 groups of C-chitosan were bound to the hydroxylated Si surface through formation of abundant number of hydrogen bonds between the hydroxylated Si surface and Chitosan. The above results show us that Chitosan and its ionically crosslinked derivative can be ideal binders for Si to be used as an active material for anode in Li-ion batteries. Figure 1