Synergetic optimization of p-type Ba2+ substitution and carbon nanotubes enwrapping in Na3V2(PO4)3: A superior cathode with high electrochemical and safety performance for half and full cells

Na3V2(PO4)3 (NVP), with a high tap density, is considered a prospective cathode material due to its low cost, ideal specific capacity and cycling stability. However, its low ionic/electronic conductivity has become the main factor hindering its development. In the current work, a dual modification strategy has been proposed to optimize NVP, which is successfully achieved via a facile sol-gel method. The addition of partial Ca2+ with low valence at the V3+ site produces favorable p-type substitution in the pristine NVP bulk, generating beneficial hole carriers in the electronic structure to accelerate the migration rate of Na+. Moreover, the doped Ca2+ with a larger ionic radius (1.03 Å vs. 0.64 Å of V3+) can have a pillar effect to support the cell structure, improving the structural stability of NVP. Meanwhile, the larger radius of Ca2+ contributes to the expansion of the lattice spacing, significantly facilitating the diffusion efficiency of Na+ to optimize the diffusion kinetics. Besides, the evenly coated carbon layers derived from the excess carbon resources combine with the enwrapped carbon nanotubes to construct a highly conductive network to enhance the transportation of electrons. Notably, the modified Ca0.04-NVP@CNTs electrode exhibits a high capacity of 117.4 mA h g-1 at 0.1 C, while that of NVP is only 69.4 mA h g-1. Moreover, it delivers an initial capacity of 110.1 mA h g-1 at 1 C and the mass loss rate per lap is only 0.01%. At 5 C, the initial capacity of Ca0.04-NVP@CNTs is 104.3 mA h g-1 while that of NVP is only 75.9 mA h g-1. Interestingly, it exhibits excellent cycling stability at 50 C; the initial capacity is 75.7 mA h g-1 and the capacity retention is around 99% after 4000 cycles.

Constructing p-type substitution induced by Ca2+ in defective Na3V2-xCax(PO4)3/C wrapped with conductive CNTs for high-performance sodium-ion batteries.

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. 

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

Robyn P. Araujo

Papers

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