Understanding Why Poly(Acrylic Acid) Works: Decarbonylation and Cross-Linking Provide an Ionically Conductive Passivation Layer in Silicon Anodes

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Abstract

Poly(acrylic acid) (PAA) is commonly used as a binder for fabricating silicon (Si) anode active materials in lithium-ion batteries due to its useful properties including high polar solvent solubility, good rheology, and strong adhesive properties. However, the role and evolution of PAA during electrode fabrication, cycling, and calendar aging are not well understood. In this work, we reveal the evolution of PAA during electrode curing and relate its chemical change to the final electrode properties and performance. These studies are made possible using two types ofin situattenuated total reflectance-infrared Fourier transform (ATR-FTIR) spectroscopy: thermal ATR-FTIR to probe the cross-linking reaction, and ATR-FTIR spectroelectrochemistry of three-dimensional composite electrodes, a unique technique developed herein that probes the solvation dynamics of lithium ions at the silicon anode interface under electrochemical polarization. Specifically, we show that PAA undergoes a thermally-mediated, cross-linking decarbonylation reaction to form an ether-based network polymer. To show the importance of the polyether moieties, we synthesize partially esterified PAA analogues that do not undergo this cross-linking decarbonylation reaction and correlate the degree of cross-linking to half-cell performance metrics. Finally, we unveil the mechanism of the polyether binder performance throughin situATR-FTIR spectroelectrochemistry and show that PAA acts as an interfacial material that conducts lithium-ions, limits solvent molecule access to the Si surface, and stabilizes the electrode against parasitic lithium inventory loss at high state of charge for an extended period of time.

Original languageAmerican English
Pages (from-to)21929-21938
Number of pages10
JournalJournal of Materials Chemistry A
Volume9
Issue number38
DOIs
StatePublished - 14 Oct 2021

Bibliographical note

Publisher Copyright:
© The Royal Society of Chemistry 2021.

NREL Publication Number

  • NREL/JA-5900-80103

Keywords

  • battery
  • lithium-ion
  • silicon
  • solid electrolyte interface
  • spectroelectrochemistry

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