Date Published: January 20, 2018
Publisher: John Wiley and Sons Inc.
Author(s): Jing Zhao, Xu Yang, Ye Yao, Yu Gao, Yongming Sui, Bo Zou, Helmut Ehrenberg, Gang Chen, Fei Du.
Polyanionic Na3V2(PO4)2F3 with a NASICON‐type structure is heralded as a promising cathode material for sodium‐ion batteries due to its fast ionic conduction, high working voltage, and favorable structural stability. However, a number of challenging issues remain regarding its rate capability and cycle life, which must be addressed to enable greater application compatibility. Here, a facile and effective approach that can be used to overcome these disadvantages by introducing an aqueous carboxymethyl cellulose (CMC) binder is reported. The resulting conductive network serves to accelerate the diffusion of Na+ ions across the interface as well as in the bulk. The strong binding force of the CMC and stable solid permeable interface protect the electrode from degradation, leading to an excellent capacity of 75 mA h g−1 at an ultrahigh rate of 70 C (1 C = 128 mA g−1) and a long lifespan of 3500 cycles at 30 C while sustaining 79% of the initial capacity value. A full cell based on this electrode material delivers an impressive energy density as high as 216 W h kg−1, indicating the potential for application of this straightforward and cost‐effective route for the future development of advanced battery technologies.
Energy storage plays an important role in enabling a decarbonized society by storing electricity from sustainable sources such as wind and solar power. The past two decades have witnessed considerable technological advancements for rechargeable lithium‐ion batteries (LIBs), which are currently the storage technology of choice for portable electronic devices and electric vehicles; however, cost and safety issues remain two major obstacles that limit the use of LIBs in the smart grid. Taking into account the raw material abundance and battery cost as well as the similar electrochemical storage mechanism, room‐temperature sodium‐ion batteries (SIBs) based on Na‐ion shuttling between positive and negative electrodes are generally regarded as a promising choice for grid storage.[[qv: 1a–d]] Nevertheless, SIB technology must be further explored to obtain batteries that are more economical, safer, and show a high‐rate capability and longer durable life.[[qv: 2a–c]]
We have demonstrated a facile and effective route to improve both the rate capability and cycle stability of NVPF by using an aqueous CMC binder. A conductive network consisting of CMC–Super P chains provides long‐range charge transfer pathways and a porous architecture. The electrochemical characterizations showed that Na‐ion diffusion in the bulk material and at the interface is accelerated due to improved electrochemical kinetics. Furthermore, the SPI interfacial film induced by the CMC binder limits the resistance increase and maintains the integrity of the electrode. Thus, the electrode enables an excellent capacity of 75 mA h g−1 at an ultrahigh rate of 70 C (1 C = 128 mA g−1) and a long lifespan of 3500 cycles at 30 C rate with capacity retention of 79%. Finally, a full battery cell assembly using hard carbon as the anode exhibits an energy density of 216 Wh kg−1, which suggests the potential for application in future scalable energy storage systems. The strategy we have proposed here is believed to be widely applicable to other cathode materials for sodium‐ion batteries.
Material Synthesis: The Na3V2(PO4)2F3@C nanocomposite was prepared by a facile sol–gel method following the previous work.4 Typical preparation processes are described as follows: stoichiometric amounts of NH4VO3, NaF, and NH4H2PO4 (Sigma‐Aldrich, 99.9%) with a molar ratio of 2:3:2 were dissolved in deionized water. Then, a saturated citric acid [HOC(COOH)(CH2COOH)2] solution was added into the above solution until the ratio of vanadium:citric acid equaled 5:4. Next, the obtained solution was evaporated at 80 °C, dried at 120 °C for 12 h, and ground to form a precursor. Finally, the precursor was preheated at 300 °C for 4 h and sintered at 650 °C for 8 h under a nitrogen atmosphere with intermediate grinding to obtain the NVPF@C nanocomposite.
The authors declare no conflict of interest.