Direct Integration of Calcium Nitrate Tetrahydrate PCM Into an Oscillating Heat Pipe for Passive Lithium‐Ion Battery Thermal Management

ABSTRACT Fast‐charging lithium‐ion batteries (LIBs) generate substantial Joule heating and electrochemical polarization, causing rapid temperature rise, accelerated degradation, and increased thermal‐runaway risk. Conventional active cooling systems consume parasitic power, whereas standalone phase‐change materials (PCMs) exhibit poor thermal conductivity, and oscillating heat pipes (OHPs) lack sufficient latent‐heat buffering during transient high‐rate operation. This study proposes a fully passive battery thermal management system (BTMS) that directly integrates calcium nitrate tetrahydrate (CaNT) PCM as both the latent‐heat storage medium and working fluid within a multi‐turn closed‐loop OHP. The hybrid design combines latent heat absorption with self‐sustained two‐phase oscillatory heat transport, enabling efficient thermal regulation without external energy input. The system was experimentally evaluated using a four‐cell 26980 cylindrical LIB module at 0.5 C, 1 C, and 1.5 C discharge rates, supported by thermal characterization, energy and exergy analyses, and a techno‐economic assessment. Compared with a DI‐water‐filled OHP, the CaNT‐PCM system reduced thermal resistance by up to 30% at the highest heat load, while maintaining cell‐to‐cell temperature differences below the 5°C industry threshold under all operating conditions (maximum 4.2°C). Temperature‐drop efficiency relative to an uncooled module reached 10%–14% at 0.5 C and 1 C and 8%–13% at 1.5 C, consistently outperforming the DI‐water baseline. Exergy analysis demonstrated lower thermodynamic irreversibility and higher second‐law efficiency across all discharge rates. A model‐based techno‐economic assessment predicted return on investment increasing from 38% to 48.4%, assuming a 10‐year service life. These results demonstrate that directly charging CaNT‐PCM into an OHP provides an effective, energy‐efficient, and economically attractive passive BTMS for high‐rate LIB applications.