Entropy Change Characteristics for Sodium Ion Half/Full Cells Based on Na3V2(PO4)3 and Hard Carbon Materials

Abstract

Understanding the entropy change (ΔS) characteristics of Hard carbon ∣∣ Na3V2(PO4)3 full cell is crucial for its long cycle life and high safety. This work investigated the thermodynamic data of sodium ion half/full cells based on Na3V2(PO4)3 and hard carbon material. The results show that the trend of ΔS for Na ∣∣ Na3V2(PO4)3 exhibits great change at 0%–10% and 90%–100% SOCs (states of charge), and remains constant (≈−14.54 J·mol−1·k−1) in 10%–90% SOCs, which is consistent with the characteristics of two-phase reaction. Whereas the ΔS of Na ∣∣ hard carbon (HC) remains essentially constant (≈8.30 J·mol−1·k−1) in the most Na+ concentration, fluctuating in the range of 3.17–11.71 J·mol−1·k−1. Notably, ΔS shows a negative value (−6.09 J·mol−1·k−1) at x = 0.3 (x in Na x C) and is close to 0 J·mol−1·k−1 at x = 1.0. The HC ∣∣ Na3V2(PO4)3 full cell entropy change is basically constant (≈−19.56 J·mol−1·k−1) in 10%–90% SOCs, and reaches a peak at 60% SOC (−10.75 J·mol−1·k−1), indicating the ΔS of full cell is mainly influenced by Na3V2(PO4)3 electrode. Based on thermodynamic entropy change characteristics, this work aims to provide a reliable reference to the storage, transportation, thermal management, and safety boundary for batteries.