Atom Probe Tomography of the LiFePO₄-Electrolyte Interface Enabled by Thin Film Electrodes

Atom probe tomography (APT) can yield three-dimensional tomographic images at atomic-scale resolution and low-AMU elements such as Li are readily observed, making it a powerful tool for exploring battery materials interfaces. However, it is difficult to prepare APT specimen tips containing the interface of interest starting with typical particle-based battery electrodes. Here we demonstrate a methodology for reliable APT imaging of battery interfaces in which a thin film electrode geometry is used to provide well-controlled planar interfaces that are ideal for APT sample preparation and imaging. LiFePO₄ (LFP) thin film electrodes, synthesized using pulsed laser deposition (PLD), were studied as an example system, with standard Li-salt electrolytes. For the results to be applicable to conventional particulate electrodes, it is important to obtain representative thin film structure and electrochemical characteristics. Thus, the effects of PLD conditions including substrate temperature, substrate crystallinity, target composition, and deposition time (number of laser pulses) on the thin film’s crystallographic texture, morphology, and electrochemical performance were studied. Optimized LFP film showed good crystallinity with low-C-rate capacity of ∼90 mAh g⁻¹. Initial APT three-dimensional imaging of the LFP/electrolyte interface shows an ∼10 nm cathode-electrolyte interphase layer that is enriched in F and Li.