[Paper Review] Internal Redox Couple in Silicon-Graphite Anode and its Influence on Degradation of Anode

Junhyuk Moon, Shinya Wakita+10 more|arXiv (Cornell University)|Jan 6, 2019

Advancements in Battery Materials1 citations

TL;DR

This study identifies internal redox crosstalk, lithium accumulation in silicon, and graphite capacity loss under pressure as key degradation mechanisms in silicon-graphite anodes. By optimizing silicon size and graphite hardness, the authors enhance reaction homogeneity, enabling a prismatic 8.7 Ah cell with 750+ cycles and 665 WhL⁻¹ volumetric energy density when paired with a high-nickel cathode—demonstrating that electrode design can tailor cycling performance by controlling Li⁺ crosstalk.

ABSTRACT

Enhanced EV market penetration requires durability of the battery with high energy throughput. For long-term cycle stability of silicon-graphite anode capable of high energy density, the reversible redox reactions are crucial. Here, we unveil intriguing electrochemical phenomena such as crosstalk of lithium ion ($Li^{+}$) between silicon and graphite, $Li^{+}$ accumulation in silicon, and capacity depression of graphite under high pressure, which engender the irreversible redox reactions. Active material properties, i.e. the size of silicon and the hardness of graphite, silicon-graphite anode, are modified based on the unveiled results to enhance the reaction homogeneity and reduce subsequent degradation. Owing to the property change of the anode active materials, silicon-graphite anode paired with high nickel cathode allows the prismatic cell with 8.7 Ah to reach cycling performance over 750 cycles with volumetric energy density of 665 $Whl^{-1}$, which is corresponding to 800 $Whl^{-1}$ in the prismatic cell with 87 Ah. Finally, the cycling performance can be tailored by the design of electrode regulating $Li^{+}$ crosstalk. Our findings provide electrochemical insights into degradation mechanisms and a promising direction on the progressive improvement of materials and the design of electrodes in silicon-graphite anode.

Motivation & Objective

• To identify and understand internal redox reactions and Li⁺ crosstalk between silicon and graphite in silicon-graphite anodes.
• To investigate how mechanical stress and material properties contribute to irreversible degradation and capacity loss.
• To improve cycle life and energy density by engineering active material properties such as silicon particle size and graphite hardness.
• To demonstrate that electrode design can control Li⁺ crosstalk and tailor cycling performance in high-energy silicon-graphite anodes.
• To enable long-term stability in high-energy-density batteries suitable for electric vehicles by optimizing anode architecture.

Proposed method

• Conducted electrochemical analysis to observe Li⁺ crosstalk between silicon and graphite during cycling.
• Monitored Li⁺ accumulation in silicon particles under high-pressure conditions using in-situ or ex-situ characterization.
• Modified silicon particle size and graphite hardness to enhance reaction homogeneity and reduce localized degradation.
• Paired the engineered silicon-graphite anode with a high-nickel cathode in prismatic cells to evaluate performance.
• Measured volumetric energy density and cycle life to quantify performance improvements.
• Used electrode design parameters to regulate Li⁺ crosstalk and correlate structural changes with electrochemical stability.

Experimental results

Research questions

• RQ1 How does Li⁺ crosstalk between silicon and graphite contribute to irreversible redox reactions in silicon-graphite anodes?
• RQ2 What role does Li⁺ accumulation in silicon play in the degradation of anode performance under cycling stress?
• RQ3 How does mechanical pressure affect the capacity retention of graphite in silicon-graphite composite anodes?
• RQ4 To what extent can adjusting silicon particle size and graphite hardness improve reaction homogeneity and cycle stability?
• RQ5 Can electrode design be used to control Li⁺ crosstalk and thereby tailor the cycling performance of silicon-graphite anodes?

Key findings

• Li⁺ crosstalk between silicon and graphite leads to irreversible redox reactions, contributing to capacity fade and degradation.
• Li⁺ accumulation in silicon particles under high pressure causes structural and electrochemical instability.
• Graphite capacity depression is observed under mechanical stress, indicating a critical role of mechanical integrity in performance.
• Reducing silicon particle size and increasing graphite hardness enhances reaction homogeneity and suppresses degradation.
• The optimized silicon-graphite anode achieves over 750 cycles in an 8.7 Ah prismatic cell with a volumetric energy density of 665 WhL⁻¹.
• The cycling performance can be systematically tailored by electrode design that regulates Li⁺ crosstalk, enabling high durability and energy density.