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

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 a multidimensional porous structure of K/Co co-substituted Na3V2(PO4)3/C attached on the lamellar Ti3C2Tx MXene substrate for superior sodium storage property.

Na3V2(PO4)3 (NVP) materials have emerged as prospective cathodes for sodium-ion batteries (SIBs). However, its weak intrinsic conductivity has limited deeper research. Herein, we adopt the strategy of simultaneous K/Co co-substitution and Ti3C2Tx MXene (MX) introduction to optimize NVP. The K/Co co-substitution brings about the synergetic effect of NVP framework stabilization. Doping Co2+ generates beneficial holes and accelerating electronic conductivity. The MX plates are stacked at random to form a porous construction, increasing the contact areas to provide more active sites for Na+ shuttling and buffering the volume change. Furthermore, the lamellar MX and the carbon layers form efficient conductive networks that increase electron migration. Notably, K0.1Na2.95V1.95Co0.05(PO4)3@MX (KC05@MX) exhibited an initial capacity of 116 mA h g-1 under 1 C with an extraordinary retention of 86.8% at the 400th cycle. It realized high performance under 20 C and 50 C, and the outputs were 93.5 and 82.4 mA h g-1 at the 1st cycle and 66.6 and 53.4 mA h g-1 at the 1000th cycle, respectively, with slight capacity loss at 0.028% and 0.035%. Furthermore, the Bi2Se3//KC05@MX asymmetric full cell expressed great electrochemical properties, indicating the superior practical application prospect of KC05@MX.

Constructing hierarchical heterojunction structure for K/Co co-substituted Na3V2(PO4)3 by integrating carbon quantum dots.

Na3V2(PO4)3 (NVP) has been widely adopted as cathode in sodium ion battery devices. Nevertheless, the weak intrinsic conductivity and serious structural collapse limit the further development. Herein, a simultaneous modified strategy of doping K/Co and integrating carbon quantum dots (CQD) is proposed. Substituting K+ is beneficial to afford amount of Na+ transport within the stabled and expanded lattice. The introduction of Co2+ generates beneficial hole carriers to improve conductivity. Furthermore, the bonding of conductive CQD guides to obtain nano-sized NVP grains, reducing the pathway for ionic migration to accelerate the diffusion capability. Importantly, a unique p-n type heterojunction construction is established in the interface between CQD (n-type) and NVP (p-type). This heterojunction structure enhances the mobility of electrons owing to the free pathways, in which the electrons transport in a relatively lower energy level without the scatter and collision of anions dopants. Ultimately, K0.1Na2.95V1.95Co0.05(PO4)3@CQD exhibits with the best energy output level. It's initial capacity under 5C is 109.8 mA h g-1 and the retention is 87.6% after cycle 400 cycles. Even at 20 and 50C, its output is 93.5 and 82.6 mA h g-1 for 1st and 66.6 and 52.1 mA h g-1 for 1000th cycle, respectively. Finally, an asymmetric full cell test confirms its application practically.

Activating the Extra Redox Couple of Co2+/Co3+ for a Synergistic K/Co Co-Substituted and Carbon Nanotube-Enwrapped Na3V2(PO4)3 Cathode with a Superior Sodium Storage Property.

Na3V2(PO4)3 (NVP) materials have emerged as a promising cathode for sodium ion batteries (SIBs). Herein, NVP is successfully optimized by dual-doping K/Co and enwrapping carbon nanotubes (CNTs) through a sol-gel method. Naturally, the occupation of K and Co in the Na1 sites and V sites can efficiently stabilize the crystal cell and provide the expanded Na+ transport channels. The existence of tubular CNTs could restrict the crystal grain growth and effectively downsize the particle size and provide a shorter pathway for the migration of electrons and ions. Moreover, the amorphous carbon layers combined with the conductive CNTs form a favorable network for the accelerated electronic transportation. Furthermore, the ex situ XPS characterization reveals that an extra redox reaction pair of Co2+/Co3+ is successfully activated at the high voltage range, resulting in superior capacity and energy density property for KC0.05/CNTs composites. Comprehensively, the optimized KC0.05/CNTs electrode exhibits a distinctive electrochemical property. It delivers an initial reversible capacity of 119.4 mA h g-1 at 0.1 C, surpassing the theoretic value for the NVP system (117.6 mA h g-1). Moreover, the KC0.05/CNT electrode exhibits the initial capacity of 113.2 mA h g-1 at 5 C and 105.8 mA h g-1 at 10 C, and the maintained capacities at 500 cycles are 105.8 and 100.8 mA h g-1 with outstanding retention values of 96.6 and 95.3%. Notably, it releases capacities of 99.8 and 84.5 mA h g-1 at 50 and 100 C, and the capacity retention values at 2500 cycles are 66.2 and 58.8 mA h g-1, respectively. What is more, the KC0.05/CNTs//Bi2Se3 asymmetric full cell exhibits a high capacity of 191.4 mA h g-1 at 2.65 V, with the energy density being as high as 507 W h kg-1, demonstrating the eminent practical application potential of KC0.05/CNTs in SIBs.

A Superior Polymer Electrolyte with Rigid Cyclic Carbonate Backbone for Rechargeable Lithium Ion Batteries.

The fabricating process of well-known Bellcore poly(vinylidene fluoride-hexafluoropropylene) (PVdF-HFP)-based polymer electrolytes is very complicated, tedious, and expensive owing to containing a large amount of fluorine substituents. Herein, a novel kind of poly(vinylene carbonate) (PVCA)-based polymer electrolyte is developed via a facile in situ polymerization method, which possesses the merits of good interfacial compatibility with electrodes. In addition, this polymer electrolyte presents a high ionic conductivity of 5.59 × 10-4 S cm-1 and a wide electrochemical stability window exceeding 4.8 V vs Li+/Li at ambient temperature. In addition, the rigid cyclic carbonate backbone of poly(vinylene carbonate) endows polymer electrolyte a superior mechanical property. The LiFe0.2Mn0.8PO4/graphite lithium ion batteries using this polymer electrolyte deliver good rate capability and excellent cyclability at room temperature. The superior performance demonstrates that the PVCA-based electrolyte via in situ polymerization is a potential alternative polymer electrolyte for high-performance rechargeable lithium ion batteries.