Coupled strain, heat, and electrochemistry fingerprints: A multi-physics in-situ and operando optical Fiber Bragg Grating sensor-based lithium-ion battery diagnostics

Understanding, at the multi-physics level, how electrochemical, mechanical, and thermal effects interact during battery operation is vital for advancing performance and safety. In this work, we extend established incremental and differential diagnostic methodologies by employing spatially distributed, multiplexed FBG sensors to capture the evolution of strain and temperature across multiple regions of a commercial NMC111/graphite pouch cell. By deriving Incremental Strain Analysis (ISA) and Incremental Temperature Analysis (ITA) curves ( d S / d V , d T / d V ) and directly comparing them with conventional Incremental Capacity Analysis (ICA) ( d Q / d V ), we show how electrochemical staging and phase transitions manifest in co-localized mechanical and thermal signals and how these signatures evolve with C-rate, temperature, and spatial position. We present a spatially resolved thermo-mechanically discriminated operando framework that reveals local heterogeneities and coupled multi-physics signatures not visible in voltage-based analysis alone. This establishes a direct connection between mechano-thermal phenomena and electrode-stage transitions in NMC111/graphite lithium-ion cells. Furthermore, staging-event-based SOC recalibration derived from a reproducible coupled ISA/ITA peak shows that while the staging voltage shifts with C-rate and temperature due to polarization, the mapped SOC ( ≈ 18 –20%) during charge and SOC ( ≈ 22 –23%) during discharge remain tightly clustered. This confirms the staging transition as a stable thermodynamic anchor enabling correction of Coulomb-counting drift.