Changes in Structure and Ionic Resistance of Lithium-Ion Battery Graphite Electrodes – Part I: Impact of Formation and SEI
Jonas L. S. Dickmanns, Lennart Reuter, Robert Morasch, Filippo Maglia, Roland Jung, Hubert A. Gasteiger
Abstract
With the interest in achieving higher gravimetric and volumetric energy densities in lithium-ion batteries, increasing the electrode loading and reducing its thickness—and consequently porosity—while maintaining low tortuosity is desirable. Achieving high power density requires a thorough understanding of these structural parameters of thick graphite electrodes and their changes during the formation of the solid-electrolyte-interphase (SEI). Graphite electrodes with loadings of <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" overflow="scroll"> <mml:mrow> <mml:mo>∼</mml:mo> </mml:mrow> <mml:mn>10</mml:mn> <mml:mspace width="0.25em"/> <mml:msup> <mml:mrow> <mml:mrow> <mml:mi mathvariant="normal">mg</mml:mi> </mml:mrow> <mml:mspace width="thickmathspace"/> <mml:mrow> <mml:mi mathvariant="normal">cm</mml:mi> </mml:mrow> </mml:mrow> <mml:mrow> <mml:mo>−</mml:mo> <mml:mn>2</mml:mn> </mml:mrow> </mml:msup> </mml:math> and pristine porosities ranging from 20–50% underwent formation at 45 °C in LP57 + 2% VC electrolyte. The ionic pore resistance after formation was quantified in non-blocking conditions using a μ-reference electrode and measuring electrochemical impedance spectroscopy at −5 °C. Upon formation, the ionic resistance of the graphite electrodes increased by up to 25%, which was quantitatively related to specific changes in electrode thickness, porosity, and tortuosity. The change in electrode thickness depended on the pristine porosity and increased by up to ∼11%. Mercury intrusion porosimetry revealed a decrease in porosity for initially high-porosity electrodes and an increase for those with low pristine porosity. The tortuosity of the electrodes increased by up to ∼10% upon formation. The thickness and density of the SEI were determined using a combination of pycnometry and mass-gain measurements, yielding 16–20 nm and ∼ <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" overflow="scroll"> <mml:mn>1.07</mml:mn> <mml:mspace width="0.25em"/> <mml:msup> <mml:mrow> <mml:mrow> <mml:mi mathvariant="normal">g</mml:mi> </mml:mrow> <mml:mspace width="0.25em"/> <mml:mrow> <mml:mi mathvariant="normal">cm</mml:mi> </mml:mrow> </mml:mrow> <mml:mrow> <mml:mo>−</mml:mo> <mml:mn>3</mml:mn> </mml:mrow> </mml:msup> </mml:math> , respectively.