Neutron-friendly Li-ion battery coin cell for In Situ 3D visualization of Li plating

Advanced battery characterization using in situ/operando neutron imaging is essential for uncovering degradation modes in lithium-ion batteries (LIBs). However, current LIB designs hinder operando neutron radiography (NR) and in situ micro-computed tomography (N-µCT) for visualizing Li plating near the graphite-separator interface due to strong neutron attenuation by hydrogen-rich components such as trilayer polypropylene–polyethylene–polypropylene (PP-PE-PP) battery separators, electrolyte, and Fe-containing spacers. Here, we present the design, electrochemical testing, and neutron imaging of a neutron-friendly LIB (NFB) optimized for in situ Li detection during extreme fast charging (XFC). Guided by total neutron attenuation cross-sections, path lengths, and material transmission, the NFB enables clear visualization at the graphite–separator interface, where standard LIBs are opaque. Electrochemical testing reveals that both standard and NFB cells exhibit similar voltage and current responses during formation and XFC for up to 50 cycles. However, the NFB shows lower reversibility and specific capacity, likely due to degradation of the Cu-coated Al spacer on the graphite side caused by corrosion or delamination during cycling. Despite these limitations, the NFB achieves stable coulombic efficiency and small cell-to-cell variability. To mitigate spacer degradation, we recommend titanium as a replacement material, owing to its significantly lower solubility in Li, although custom fabrication may be required. Neutron radiography confirms significantly improved transmission at the graphite–separator interface unlike the fully opaque images from standard LIBs. Additionally, our custom cell design enabled simultaneous neutron tomography of multiple coin cells, allowing in situ 3D detection of dead Li following XFC, characterized by spatially heterogeneous accumulation near the graphite edges. Disconnected clusters form, with nodules of varying density concentrated at electrode edges—indicating non-uniform plating behavior likely driven by localized current density hotspots.