Li-ion batteries have been considered as one of the most promising candidates for energy storage due to their high energy density and cycle stability. As an alternative technology, Na-ion batteries potentially offer a lower cost, safer and more environmental friendly battery system in comparison with Li-ion system. Anode part becomes the main drawback of the commercialization of the Na-ion batteries because typical graphite employed in Li-ion batteries do not intercalate Na+ ions, which is related to the large size of the Na-ion, which is 372% that of a Li-ion, and thus makes it impossible to simply adopt the recent knowledge and strategies developed for high performance Li-ion batteries directly onto Na-ion batteries.
The objectives of the current phase in this project are to develop capable anode and cathode materials for Na-ion batteries which can be used in both organic and aqueous electrolyte, with detailed characterization and electrochemical performance tests. In order to develop synthetic approaches for high-performance anode and cathode materials for aqueous sodium ion battery systems, we currently focus on SnO2 and layered NaMO2 (M=transition metal) to achieve stable electrochemical performance. Both electrodes will be characterized using X-ray diffraction, scanning electron microscopy, transmittion electron microscopy and other advanced techniques, and then tested as half cells in organic electrolyte to exam the electro-chemical performance. The electrochemical performance of both electrodes will be tested in aqueous electrolyte in the next phase of the project.
1- SnO2 coated carbon cloth with surface modification as Na-ion battery anode
In the study of current phase in this project for the anode part, we demonstrate a surface-coating/nanocrystal-active-material-layer/conductive-soft-platform multilayer nanocomposite microfiber electrode by using atomic layer deposition (ALD), hydrothermal synthesis method and carbon cloth (CC) as a soft platform to solve the problems mentioned above. More specifically, we have developed a binder-free multilayer nanocomposite core-shell microfiber electrode consisting of a hydrothermal synthesized SnO2 nanocrystal layer on conductive carbon cloth (SnO2/CC) with surface coatings. Carbon cloth is fabricated by arrays of soft carbon fibers with a uniform diameter. Since the carbon fibers are intrinsically soft and porous, the cycling stability and rate capability of SnO2 for Na-ion batteries can be increased by alleviating: 1) the capacity loss due to electrode pulverization, and 2) the poor rate performance as a result of low electronic conductivity and slow Na-ion transfer kinetics. An ALD Al2O3 coating and a hydrothermal carbon coating are further applied on SnO2/CC electrodes to enhance the cycle life and rate capability. The comparison of impedance and morphology between fresh and cycled electrodes revealed the mechanism of capacity decay and how Al2O3 surface coating can protect the active material layer. The hierarchical core-shell SnO2 anode with designed surface-coating/nanocrystal-active-material-layer/conductive-soft-platform structure described is ideal for high power and large scale sodium ion storage. The hydrothermal synthesis, ALD technology and conductive fiber substrates are scalable for large throughput manufacturing.
2- Layered P2-Na2/3[Ni1/3Mn2/3]O2 as High-Voltage Cathode for Sodium-Ion Batteries
In the current phase of this project, we first explored the electrochemical performance and capacity decay mechanism of Na2/3[Ni1/3Mn2/3]O2 within a high voltage window from 2.5 V to 4.3 V. Based on the discovered degradation mechanism, we applied a surface-coating solution to stabilize the Na2/3[Ni1/3Mn2/3]O2 electrode using a simple wet chemistry method. As illustrated schematically in Figure 1a, the exfoliation phenomena resulting from an unfavorable crystal structure transition was observed in the after-cycling Na2/3[Ni1/3Mn2/3]O2 particles. Furthermore, an Al2O3 surface coating was applied on Na2/3[Ni1/3Mn2/3]O2 particles (Al2O3-Na2/3[Ni1/3Mn2/3]O2) and effectively solved the issues discussed above and enhanced the cycling life. We employed an Al2O3 coating because Al2O3 been shown to be an excellent coating material for mitigating volume expansion and contraction during Li/Na ion insertion and extraction. BecauseP2-Na2/3[Ni1/3Mn2/3]O2 is stable in moist air and, it is compatible with wet chemistry surface coating or other surface modification methods involving water. The comparison between the fresh and cycled electrodes together with the enhanced cycling performance by robust Al2O3 coatingrevealed the mechanism of capacity decay, and further demonstrated that the Al2O3 surface coating can suppress the side reaction and protect the layered metal oxide particles during long cycling within the high voltage window. The designed Al2O3-Na2/3[Ni1/3Mn2/3]O2 cathode is ideal for large scale high power/energy Na-ion storage, not only because of its excellent cycling stability and improved voltage profile, but also due to the scalability of the solid state reaction of Na2/3[Ni1/3Mn2/3]O2 and wet chemistry coating.