Boosting the rate capability and cycle life of Zr-substituted Na3V2(PO4)3/C enwrapped on carbon nanotubes for symmetric Na-ion batteries

Ultra-Stable Sodium-Ion Battery Enabled by All-Solid-State Ferroelectric-Engineered Composite Electrolytes

Symmetric Na-ion cells using the NASICON-structured electrodes could simplify the manufacturing process, reduce the cost, facilitate the recycling post-process, and thus attractive in the field of large-scale stationary energy storage. However, the long-term cycling performance of such batteries is usually poor. This investigation reveals the unavoidable side reactions between the NASICON-type Na3V2(PO4)3 (NVP) anode and the commercial liquid electrolyte, leading to serious capacity fading in the symmetric NVP//NVP cells. To resolve this issue, an all-solid-state composite electrolyte is used to replace the liquid electrolyte so that to overcome the side reaction and achieve high anode/electrolyte interfacial stability. The ferroelectric engineering could further improve the interfacial ion conduction, effectively reducing the electrode/electrolyte interfacial resistances. The NVP//NVP cell using the ferroelectric-engineered composite electrolyte can achieve a capacity retention of 86.4% after 650 cycles. Furthermore, the electrolyte can also be used to match the Prussian-blue cathode NaxFeyFe(CN)6-z·nH2O (NFFCN). Outstanding long-term cycling stability has been obtained in the all-solid-state NVP//NFFCN cell over 9000 cycles at a current density of 500 mA g-1, with a fading rate as low as 0.005% per cycle.

Unraveling the modified mechanism of ruthenium substitution on Na3V2(PO4)3 with superior rate capability and ultralong cyclic performance

Na3V2(PO4)3(NVP) is an ideal cathode material for sodium ion battery due to its stable three-dimensional frame structure and high operating voltage. However, the low intrinsic conductivity and serious structural collapse limit its further application. In this work, a simultaneous optimized Na3V1.96Ru0.04(PO4)3/C@CNTs cathode material is synthesized by a simple sol-gel method. Specifically, the ionic radius of Ru3+ is slightly larger than that of V3+ (0.68 Å vs 0.64 Å), which not only ensures the feasibility of Ru3+ replacing V3+ site, but also appropriately expands the migration channel of sodium ions in NVP and stabilizes the structure, effectively improving the diffusion efficiency of sodium ions. Moreover, CNTs construct a three-dimensional conductive network between the grains, reducing the impedance at the interface and effectively improving the electronic conductivity. Ex-situ XRD analysis at different SOC were performed to determine the change in the crystal structure of Ru3+doped Na3V2(PO4)3, and the refinement results simultaneously demonstrate the relatively low volume shrinkage value of less than 3 % during the de-intercalation process, further verifying the stabilized crystal construction after Ru3+ substitution. Furthermore, the ex-situ XRD/SEM/CV/EIS after cycling indicate the significantly improved kinetic characteristics and enhanced structural stability. Notably, the modified Na3V1.96Ru0.04(PO4)3/C@CNTs reveals superior rate capability and ultralong cyclic performance. It submits high capacities of 82.3/80.9 mAh g-1 at 80/120C and maintains 71.3/59.6 mAh g-1 after 14800/6250 cycles, indicating excellent retention ratios of 86.6 % and 73.6 %, respectively. This work provides a multi-modification strategy for the realization of high-performance cathode materials, which can be widely applied in the optimization of various materials.

Se-induced defective carbon nanotubes promoting superior kinetics and electrochemical performance in Na3V2(PO4)3 for half and full Na ion cells

Na3V2(PO4)3 (NVP), with unique Na super ionic conductivity (NASICON) framework, has become an prospective cathode material. However, the low electronic conductivity and poor structural stability limit its further development. Currently, the optimized carbon nanotubes (CNTs) by selenium doping are utilized to modify NVP system for the first time. Notably, the introduction of selenium in CNTs promotes to generate more defects, resulting in abundant active sites for the de-intercalation of Na+ to achieve more pseudocapacitance. Moreover, the newly formative C-Se bonds possess much stronger bond energy than the original CC (586.6 KJ mol-1 vs 377.4 KJ mol-1) bonds. The structure arrangement of the original CNTs is significantly improved by the doped selenium element, indicating that an enhanced carbon skeleton could be obtained to sustain the structural stability of NVP system. Furthermore, the excess selenium can be doped into the bulk of NVP crystal to replace of partial oxygen. Due to the larger ionic of Se2- (1.98 Å vs 1.4 Å of O2-), the VSe6 group has larger framework, which provides a broadened pathway for Na+ migration to improve the kinetic characteristics. Accordingly, the modified NVP@CNTs:Se = 1:1 sample exhibits superior rate capability and cyclic performance. It reveals high capacities of 78.6 and 76.5 mAh/g at 20 and 60C, maintaining 65.4 and 53.8 mAh/g after 5000 and 7000 cycles with high capacity retention of 84.49 % and 70.32 %, respectively. The assembled NVP@CNTs:Se = 1:1//CHC full cell delivers a high value of 153.6 mAh/g, suggesting the optimized sample also behaves excellent application potentials.

Outstanding long cycle stability provide by bismuth doped Na3V2(PO4)3 enwrapped with carbon nanotubes cathode for sodium-ion batteries

Na3V2(PO4)3 (NVP), possessing good ionic conduction properties and high voltage plateau, has been deemed as the most prospective material for sodium ion batteries. However, the weak intrinsic electronic conductivity has hindered its further commercialization. Herein, an ingenious strategy of Bi3+ substitution at V3+ site in NVP system is proposed. The ionic radius of Bi3+ is slightly larger than that of V3+, which can further expand the crystal structure inside the NVP, thus accelerating the migration of Na+. Meanwhile, the appropriate amount of carbon coating and carbon nanotubes (CNTs) enwrapping construct an effective three-dimensional network, which provides a conductive framework for electronic transfer. Furthermore, the introduction of CNTs also inhibit the agglomeration of active grains during the sintering process, reducing the particle size and shortening the diffusion path of Na+. Comprehensively, the conductivity, ionic diffusion ability and structural stability of the modified Na3V2-xBix(PO4)3/C@CNTs (0 ≤ x ≤ 0.05) sample are significantly improved. The Na3V1.97Bi0.03(PO4)3/C@CNTs sample obtains a reversible capacity of 97.8 mAh g-1 at 12C and maintains a value of 80.6 mAh g-1 after 9000 ultra-long cycles. As for the super high rate at 80C, it exhibits a high capacity of 84.34 mAh g-1 and retains a capacity of 73.34 mAh g-1 after 6000 cycles. The superior electrochemical performance is derived from the enhancement of the crystal structure by Bi3+ doping and the highly conductive network consisting of carbon coating layers and enwrapped CNTs.

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 Ti₃C₂T_x 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, K_0.1Na_2.95V_1.95Co_0.05(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 400^th 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 1^st cycle and 66.6 and 53.4 mA h g^-1 at the 1000^th 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.

Simultaneous defect regulation by p-n type co-substitution in a Na3V2(PO4)3/C cathode for high performance sodium ion batteries

The Na₃V₂(PO₄)₃ (NVP) cathode is deemed to be a promising candidate for sodium ion batteries due to its strong structural stability and high theoretical capacity. Nevertheless, its poor intrinsic conductivity restricts further development. To overcome these shortcomings, a dual modification strategy of Mn²⁺/Ti⁴⁺ co-substitution is proposed for the first time. Significantly, Mn doping can efficiently accelerate the transmission speed of electrons by introducing beneficial holes derived from the low valence state of +2, presenting the classical p-type doping modification. Moreover, the presence of Mn²⁺ with a larger ionic radius can support the crystal to stabilize the Na superionic conductor (NASICON) framework of the NVP system. Ti⁴⁺ is introduced for perfect charge compensation. Accordingly, the addition of Ti⁴⁺ can generate excess electrons due to the n-type substitution, which contributes to the favorable electronic conductivity. In addition, conductive carbon nanotubes (CNTs) are utilized to construct an efficient network to improve the rate capability of the NVP composite. Meanwhile, CNTs can inhibit particle growth and thus reduce particle size, shortening the transport path of Na⁺ and promoting the diffusion of Na⁺. Comprehensively, the optimized Na₃V_2-xMn_xTi_x(PO₄)₃/C@CNTs (x = 0.15) deliver high capacities of 70.3 and 68.2 mA h g^-1 at 90C and 180C, maintaining 58 and 53.8 mA h g^-1 after 1000 cycles with high capacity retention of 82.5% and 78.9%.

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.