Tools for characterizing performance degradation in lithium-ion batteries

Sammanfattning: Lithium-ion batteries have enabled vast societal changes, ranging in scale from the adoption of personal electronics to electromobility and grid-scale, renewable energy storage. However, all applications face performance fade over time, observed as losses of battery capacity and power. This gradual degradation is most often due to electrochemical aging processes inside the cell, including phenomena causing a loss of cyclable lithium (e.g., lithium plating, growth of the solid­‑electrolyte interphase or SEI), a loss of active material (e.g., particle cracking), and/or a loss of ionic or electronic conductivity. In the compiled works, many individual batteries have been aged and analyzed to better understand the conditions contributing to aging in different cell designs. The cells studied include lab-built pouch cells, commercial cylindrical cells (with electrodes LiNixMnyCo1‑x‑yO2‑LiMn2O4/C6 and LiNixCoyAl1‑x‑yO2/C6‑SiOx), and larger automotive-grade prismatic cells (LiNixMnyCo1‑x‑yO2/C6).Complementary in situ and post mortem methods are developed, with relevance for both battery research and battery control systems. Excellent characterization can often be achieved by a combination of differential voltage and incremental capacity analyses. Obtained from a simple, slow cycle, the derivatives of the voltage profile reveal many features that can be tracked over aging. This thesis particularly develops these techniques for blended electrodes, deconvoluting the aging of individual components. Dynamic performance is resolved with a novel polarization factor, impedance spectroscopy, and tools based on current pulses/interruptions. Finally, a protocol based on nuclear magnetic resonance spectroscopy is developed, enabling fast and direct quantification of lithium plating and SEI on harvested battery components. With such tools, we can improve how batteries are used and monitored, paving the way for efficient research and safer, more reliable batteries.