Xiaohua Ma

Abstract Lithium (Li)‐ion batteries (LIB) have governed the current worldwide rechargeable battery market due to their outstanding energy and power capability. In particular, the LIB's role in enabling electric vehicles (EVs) has been highlighted to replace the current oil‐driven vehicles in order to reduce the usage of oil resources and generation of CO 2 gases. Unlike Li, sodium is one of the more abundant elements on Earth and exhibits similar chemical properties to Li, indicating that Na chemistry could be applied to a similar battery system. In the 1970s‐80s, both Na‐ion and Li‐ion electrodes were investigated, but the higher energy density of Li‐ion cells made them more applicable to small, portable electronic devices, and research efforts for rechargeable batteries have been mainly concentrated on LIB since then. Recently, research interest in Na‐ion batteries (NIB) has been resurrected, driven by new applications with requirements different from those in portable electronics, and to address the concern on Li abundance. In this article, both negative and positive electrode materials in NIB are briefly reviewed. While the voltage is generally lower and the volume change upon Na removal or insertion is larger for Na‐intercalation electrodes, compared to their Li equivalents, the power capability can vary depending on the crystal structures. It is concluded that cost‐effective NIB can partially replace LIB, but requires further investigation and improvement.

To evaluate the potential of Na-ion batteries, we contrast in this work the difference between Na-ion and Li-ion based intercalation chemistries in terms of three key battery properties—voltage, phase stability and diffusion barriers. The compounds investigated comprise the layered AMO2 and AMS2 structures, the olivine and maricite AMPO4 structures, and the NASICON A3V2(PO4)3 structures. The calculated Na voltages for the compounds investigated are 0.18–0.57 V lower than that of the corresponding Li voltages, in agreement with previous experimental data. We believe the observed lower voltages for Na compounds are predominantly a cathodic effect related to the much smaller energy gain from inserting Na into the host structure compared to inserting Li. We also found a relatively strong dependence of battery properties on structural features. In general, the difference between the Na and Li voltage of the same structure, ΔVNa–Li, is less negative for the maricite structures preferred by Na, and more negative for the olivine structures preferred by Li. The layered compounds have the most negative ΔVNa–Li. In terms of phase stability, we found that open structures, such as the layered and NASICON structures, that are better able to accommodate the larger Na+ ion generally have both Na and Li versions of the same compound. For the close-packed AMPO4 structures, our results show that Na generally prefers the maricite structure, while Li prefers the olivine structure, in agreement with previous experimental work. We also found surprising evidence that the barriers for Na+ migration can potentially be lower than that for Li+ migration in the layered structures. Overall, our findings indicate that Na-ion systems can be competitive with Li-ion systems.

Monoclinic α- NaMnO2 is re-investigated electrochemically as a positive electrode material for sodium ion batteries. About 0.85 Na can be deintercalated from NaMnO2 and 0.8 Na be intercalated back during potentiostatical intermittent charge and discharge. Galvanostatical cycling between 2.0 V and 3.8 V gives 185 mAh/g discharge capacity for the first cycle at C/10 rate and 132 mAh/g remains after 20 cycles. Charge and discharge curves are significantly different indicating more hysteresis than is typical for lithium intercalation compounds. We also explain the remarkable difference between layered LiMnO2 and NaMnO2 upon alkali removal.

This review provides a new prospective on the role of the state-of-the-art polymers of intrinsic microporosity (PIMs) in key energy-intensive membrane-based gas separations including O2/N2, H2/N2, H2/CH4, CO2/CH4, H2S/CH4, C2H4/C2H6, and C3H6/C3H8 applications. A general overview on the gas separation properties of novel PIM materials developed in the past 15 years is presented with updated performance maps on the latest pure-gas 2015 O2/N2, H2/N2, and H2/CH4 permeability/selectivity upper bounds. Specifically, functionalized ladder PIMs and polyimides of intrinsic microporosity (PIM-PIs) are discussed targeting at high-performance, plasticization-resistant membranes for demanding acid gas (CO2 and H2S) removal from CH4 in natural gas and olefin/paraffin separations. Experimental CO2/CH4 performance data of nearly 70 polymeric membrane materials available in the literature were gathered and plotted for the first time on the Robeson plot, from which a mixed-gas 2018 CO2/CH4 upper bound was proposed to provide guidance for future membrane materials development. A number of PIMs have demonstrated outstanding performances in O2/N2, H2/N2, and H2/CH4 separations, and several functionalized PIMs have shown great promises in CO2/CH4 separation under realistic mixed-gas conditions. The potential of PIMs materials and their derivatives for H2S/CH4, C2H4/C2H6, and C3H6/C3H8 separations are underexplored and significant efforts are needed to develop stable and high-performance materials under mixed-gas conditions. Ultimately, fabricating PIMs materials into defect free, inexpensive thin-film composite or integrally-skinned asymmetric membranes is paramount to their successful large-scale commercialization.