ABSTRACT
Silicon anodes have a high theoretical capacity for lithium storage, but current composite electrode formulations are not sufficiently stable under long-term electrochemical cycling. The choice of polymeric binder has been shown to impact stability and capacity of silicon anodes for electrochemical energy storage. While several promising polymeric binders have been identified, there is a knowledge gap in how various physicochemical properties-including adhesion, mechanical integrity, and ion diffusion-impact electrochemical stability and performance. In this work, we comprehensively investigate the physical properties and performance of a molecular-weight series (3-20 × 106 g/mol) of partially hydrolyzed polyacrylamide (HPAM) in silicon anodes. We quantify the mechanical strength, electrolyte uptake, adhesion to silicon, copper, and carbon, as well as electrochemical performance and stability and find that HPAM satisfies many of the properties generally believed to be favorable, including good adhesion, high strength, and electrochemical stability. HPAM does not show any electrolyte uptake regardless of any molecular weight studied, and thin films of mid- and high-molecular-weight HPAM on silicon surfaces suppress lithiation of silicon. The resulting composite electrodes exhibit an electrochemical storage capacity greater than 3000 mAh/g initially and 1639 mAh/g after 100 cycles. We attribute capacity fade to failure of mechanical properties of the binder or an excess of the solid electrolyte interphase layer being formed at the Si surface. While the highest-molecular-weight sample was expected to perform the best given its stronger adhesion and bulk mechanical properties, we found that HPAM of moderate molecular weight performed the best. We attribute this to a trade-off in mechanical strength and uniformity of the resulting electrode. This work demonstrates promising performance of a low-cost polymer as a binder for Si anodes and provides insight into the physical and chemical properties that influence binder performance.
ABSTRACT
The performance of lithium ion batteries rapidly falls at lower temperatures due to decreasing conductivity of electrolytes and solid electrolyte interphase (SEI) on graphite anode. Hence, it limits the practical use of lithium ion batteries at subzero temperatures and also affects the development of lithium ion batteries for widespread applications. The SEI formed on the graphite surface is very influential in determining the performance of the battery. Herein, a new electrolyte additive, 4-chloromethyl-1,3,2-dioxathiolane-2-oxide (CMDO), is prepared to improve the properties of commonly used electrolyte constituents-ethylene carbonate (EC), and fluoroethylene carbonate. The formation of an efficient passivation layer in propylene carbonate-based electrolyte for MCMB electrode was investigated. The addition of CMDO resulted in a much less irreversible capacity loss and induces thin SEI formation. However, the combination of the three additives played a key role to enhance reversible capacity of MCMB electrode at lower or ambient temperature. The electrochemical measurement analysis showed that the SEI formed from a mixture of the three additives gave better intercalation-deintercalation of lithium ions.
ABSTRACT
Trace water content in the electrolyte causes the degradation of LiPF6, and the decomposed products further react with water to produce HF, which alters the surface of anode and cathode. As a result, the reaction of HF and the deposition of decomposed products on electrode surface cause significant capacity fading of cells. Avoiding these phenomena is crucial for lithium ion batteries. Considering the Lewis-base feature of the N-Si bond, 1-(trimethylsilyl)imidazole (1-TMSI) is proposed as a novel water scavenging electrolyte additive to suppress LiPF6 decomposition. The scavenging ability of 1-TMSI and beneficiary interfacial chemistry between the MCMB electrode and electrolyte are studied through a combination of experiments and density functional theory (DFT) calculations. NMR analysis indicated that LiPF6 decomposition by water was effectively suppressed in the presence of 0.2 vol % 1-TMSI. XPS surface analysis of MCMB electrode showed that the presence of 1-TMSI reduced deposition of ionic insulating products caused by LiPF6 decomposition. The results showed that the cells with 1-TMSI additive have better performance than the cell without 1-TMSI by facilitating the formation of solid-electrolyte interphase (SEI) layer with better ionic conductivity. It is hoped that the work can contribute to the understanding of SEI and the development of electrolyte additives for prolonged cycle life with improved performance.
