Very long cycle life of double-carbon coated sodium vanadium phosphate

Lithium-ion batteries have been widely used in portable electronics for the past few decades. However, traditional lithium is scarce and unevenly distributed, so researchers have been trying to find a cheaper alternative to lithium for batteries. As we all know, sodium is cheap and widely distributed, so its development has a similar electrochemical mechanism to lithium-ion batteries. However, cheaper sodium ion batteries are the development direction of future energy storage batteries. Among the many sodium ion cathode materials, the structure of sodium vanadium phosphate (Na3V2(PO4)3) cathode material has attracted much attention due to its advantages of stable structure, high voltage plateau and good thermal stability. However, the low electron conductance caused by the structure of vanadium phosphate seriously restricts its long cycle life and fast charging and discharging performance (magnification performance).
Recently, Professor Yu Yan’s research group at the University of Science and Technology of China has carried out a systematic study on this problem, and designed a “two-carbon layer” structure of Na3V2(PO4)3, which significantly improves the high rate performance and long cycle life of cathode materials. The results were published in Advanced Energy Materials (DOI: 10.1002/aenm.201402104).
The team successfully embedded carbon coated carbon matrix (CMK-3) into 3D mesoporous carbon matrix (CMK-3) by nano-casting method. In this structure, each nanoparticle is uniformly coated with the “first layer of carbon”, which effectively inhibits the growth of nanoparticle in the process of sintering at high temperature, thus shortening the diffusion distance of ions in the process of charging and discharging. The 3D mesoporous carbon matrix (CMK-3) acts as the “second layer of carbon” to form a 3D nanoscale conductive network, which can not only effectively inhibit the volume change and prevent the agglomeration of materials during the charging and discharging process, but also transport sodium ions and electrons to the surface of each active nanoparticle at a high speed. These two layers of carbon have different structures and properties, which can be synergistic, so that the dynamic advantages of nanostructured electrode materials can be truly used to achieve long cycle life and high rate performance of Na3V2(PO4)3 cathode materials. The test of the battery material shows that the reversible capacity reaches 77% (90 mAhg-1) after 1000 cycles at the discharge current density. At the current density of 5C, the discharge capacity can be maintained at 78 mAhg-1 after 2000 cycles.
The new “two-layer carbon” coating design is not only simple, convenient and effective, but also can be used as a universal method to improve the electrochemical performance of other electrode materials, which is expected to promote the development of energy storage batteries with fast charging and long cycle life.
The above work is supported by the National Nature Foundation of China, the New Century Talents Program, the University of Science and Technology of China Innovation Team Cultivation Fund, the central University Basic Research Funds and Suzhou Nanotechnology Collaborative Innovation Center.

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