Date of Award

Spring 2019

Document Type

Thesis

Degree Name

Master of Science in Mechanical Engineering - (M.S.)

Department

Mechanical and Industrial Engineering

First Advisor

Nadimpalli, Siva P.V.

Second Advisor

Rao, I. Joga

Third Advisor

Chester, Shawn Alexander

Abstract

A study into the particle level mechanics of polymer binder and active material used in composite lithium-ion batteries (LIB) has been conducted. Silicon is highly sought after material that can be used as an active material in a composite anode. Its high theoretical capacity can result in batteries that can store more energy than current LIBs, but high volume expansion of Si during charge/discharge cycles leads to rapid capacity fade and poor cyclic life. Understanding the stress that is generated in the binder and the active material due to the volume expansion has not been fully understood. In this study an idealized composite electrode sample was created to emulate the binder/particle interactions in a commercial composite electrode. The sample was fabricated by etching micro pillars of Si into a crystal Si wafer in a periodic array. Polyvinylidene fluoride (PVDF) and sodium carboxymethyl cellulose (CMC) were then added to the system to form binder bridges between Si pillars. The stress induced in this composite electrode during electrochemical cycling was measured in situ using a multi-beam optical sensor (MOS). A preliminary finite element model was generated which can be used to interpret the stress at the binder/particle interface.

During electrochemical cycling the liquid electrolyte in the LIB reacts with the active particles to form a passivation layer called solid electrolyte interphase (SEI) layer on the electrode surfaces. The location and composition of the SEI can influence the interface properties between binder and active particles which will influence the overall cyclic performance of batteries. However, the effect of binder on the SEI formation has not been fully realized. To understand the effect of binder on the location of the SEI, thin films of PVDF and CMC were spin coated onto crystal Si wafers. The wafers were then cycled to grow a stable SEI layer. X-ray photoelectron spectroscopy (XPS) coupled with depth profile analysis using C-60 ion sputtering was then conducted to analyze the composition of SEI as well as the location of the SEI compounds within the binder. The depth profiling data revealed that SEI forms within the binder but not on the outer layer of the binder. These results can help inform the optimization of Si containing anodes in commercial LIB.

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