Controllable Preparation Of Bismuth-carbon Composites And Their Application In Sodium-based Energy Storage Devices

Sodium-based energy storage devices are expected to achieve large-scale energy storage applications by virtue of their advantages in sodium resources and energy storage mechanism similar to lithium-based energy storage devices.The electrode material as the core part determines the safety,cycle life and energy/power density of the energy storage device.Therefore,it is crucial to design and develop new electrode materials with excellent electrochemical properties to accelerate the commercialization process of sodium-based energy storage devices.Metallic bismuth,known as"green"metal in the field of materials,is considered as a potential electrode material due to its advantages such as large layer spacing,high theoretical capacity,low cost and good sodiophilic properties.Therefore,this paper mainly focuses on the design and construction of Bi/C composites with unique structure,and realizes the perfect combination of bismuth nanoparticles and nitrogen-doped porous carbon by adjusting annealing temperature and introducing nitrogen source.When it is used as the alloy anode material of sodium ion hybrid capacitors and the carrier material of sodium anode for sodium metal battery,the high efficiency of sodium storage and stable ability of sodium deposition/stripping can be realized,respectively.The specific research results are as follows:（1）The nitrogen doped hierarchical porous carbon composites coated with bismuth nanoparticles（Bi@NPC）are synthesized by the chemical blowing method.The proper nitrogen doping is not only conducive to anchoring bismuth nanoparticles,but also improve the conductivity of carbon matrix.By controlling the annealing temperature,it can be found that Bi@NPC-850-U has the best particle size distribution and good electrochemical reactivity at850℃.The rigid carbon skeleton structure can not only effectively relieve the volume expansion of bismuth during the cycle and maintain the stable electrode structure,but also improve the poor electrical conductivity of bismuth-based materials.The evenly distributed bismuth nanoparticles provide sufficient reaction sites and reduce the internal stress caused by volume change.Used as the anodes of sodium ion battery,even at a current density of 10 A g^-1,Bi@NPC-850-U shows a reversible specific capacity of 243.9 m Ah g^-1 after 2000 long cycles.At the same time,it has excellent rate performance,achieving a reversible specific capacity of207.6 m Ah g^-1 at 50 A g^-1.The sodium ion hybrid capacitor consisting of AC cathode and Bi@NPC-850-U anode delivers a high energy density of 94.2 Wh kg^-1 at a power density of200 W kg^-1.At a current density of 1 A g^-1,it exhibits a low capacity decay rate of 0.014%after2400 cycles.（2）The PVP and bismuth nitrate are used as raw materials,N-doped porous carbon with good dispersion of bismuth nanoparticles is synthesized by chemical blowing method at1000℃.Nitrogen doping leads to the introduction of defects in the carbon matrix,and the defect locations together with Na-Bi alloy nanoparticles provide a large number of nucleation sites for sodium deposition.Meanwhile,the sodiophilic bismuth nanoparticles effectively reduce the nucleation barrier and promote the uniform nucleation of sodium.The three-dimensional porous structure not only provides sufficient space for sodium deposition,but also greatly alleviates the volume change of the electrode during the cycle.Due to its advantages in composition and structure,Bi@NPC-1000 substrate effectively inhibits dendrite generation and accumulation of dead sodium,avoids unnecessary loss of sodium and achieves uniform sodium deposition.When it is used as anode of the sodium metal battery,the half battery achieves a stable cycle of 1605 cycles（3210 h）and an average coulomb efficiency of 99.95%under the current density of 1 m A cm^-2 and the cycle capacity of 1 m Ah cm^-2.The overpotential of symmetrical cells assembled with Na–Bi@NPC-1000/Cu electrodes is maintained near 9.2 m V during 2420 h long cycles at 1 m A cm^-2and 1 m Ah cm^-2.Even at 4 m A cm^-2 and 4 m Ah cm^-2,the symmetrical battery can cycle relatively steadily for 410 h.In addition,under the current density of 2 C,the discharge capacity of Na–Bi@NPC-1000||NVP@C full battery reaches to87.7 m Ah g^-1after 800 cycles with a capacity retention rate of 88.5%.