BEYOND LI ION: ELECTRODE MATERIALS FOR SODIUMAND MAGNESIUM-ION BATTERIES
Nanomaterials for Energy Conversion and Storage
to develop and install renewable energy harvesting technologies . However, their successful implementation will be dependent on reliable and robust storage devices since harvesting solar and wind energy is inherently intermittent and the majority of consumption targets cannot be readily tethered to the grid. As energy storage devices, batteries possess high portability, high energy density, high Coulombic efficiency, and long cycle life. They are ideal power sources for portable devices,
... mobiles, and backup power supplies; accordingly, batteries power nearly all of our mobile electronics and are used to improve the efficiency of hybrid electric vehicles [4, 5] . Unfortunately, considerable improvements in performance are still required in order to meet the demands of advanced portable devices and achieve energy sustainability (e.g., through smart grid and electric vehicle technologies)  . These enduring needs have driven intensive research investments. While efforts have been successful, there is still significant room for improvement regarding the development and understanding of electrode materials  . The overall capacity and potential cycling window of many electrode materials are limited to prevent degradation over long term cycling. In addition to exploring new electrode materials, there have been strong efforts to improve those that are already utilized. Expense reduction is a priority as approximately 23% and 8% of the overall battery pack costs stem from the respective cathode and anode active materials alone  . An alkali-ion battery consists of several electrochemical cells connected in parallel and/or in series to provide a designated current or voltage. Each electrochemical cell has two electrodes separated by an electrolyte that is electrically insulating but ionically conductive. During discharge, when the alkali-ion battery operates as a galvanic cell, alkali ions exit the negative electrode (typically carbon) and insert themselves into the positive electrode while electrons The need for economical and sustainable energy storage drives battery research today. While Li-ion batteries are the most mature technology, scalable electrochemical energy storage applications benefit from reductions in cost and improved safety. Sodium-and magnesium-ion batteries are two technologies that may prove to be viable alternatives. Both metals are cheaper and more abundant than Li, and have better safety characteristics, while divalent magnesium has the added bonus of passing twice as much charge per atom. On the other hand, both are still emerging fields of research with challenges to overcome. For example, electrodes incorporating Na + are often pulverized under the repeated strain of shuttling the relatively large ion, while insertion and transport of Mg 2+ is often kinetically slow, which stems from larger electrostatic forces. This review provides an overview of cathode and anode materials for sodium-ion batteries, and a comprehensive summary of research on cathodes for magnesium-ion batteries. In addition, several common experimental discrepancies in the literature are addressed, noting the additional constraints placed on magnesium electrochemistry. Lastly, promising strategies for future study are highlighted.