Entropy Change Characteristics for Sodium Ion Half/Full Cells Based on Na<sub>3</sub>V<sub>2</sub>(PO<sub>4</sub>)<sub>3</sub> and Hard Carbon Materials
Fanglin Wei, Pengzhao Li, Qiaoping Zhang, Guosheng Shao, Jing Mao
Abstract
Understanding the entropy change (Δ S ) characteristics of Hard carbon ∣∣ Na 3 V 2 (PO 4 ) 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 Na 3 V 2 (PO 4 ) 3 and hard carbon material. The results show that the trend of Δ S for Na ∣∣ Na 3 V 2 (PO 4 ) 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 Δ S 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, Δ S 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 ∣∣ Na 3 V 2 (PO 4 ) 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 Δ S of full cell is mainly influenced by Na 3 V 2 (PO 4 ) 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.