Materials Design for High Safety Sodium Ion Battery
DOI: 10.1002/aenm.202000974 and the relatively high cell cost raises concerns on the sustainable development of LIBs, especially in the large scale energy storage area, which put specific requirements on the price cost, safety, and durability of the battery.[1] In addition to the concern over potential shortage of lithium, the incidents associated with fires and explosions of state of the art LIBs are stimulating advanced strategies and new safe alternatives in recent years. Sodium ion batteries (SIBs) with identical internal components and working principles with LIBs, have been proposed as one of the most promising nextgeneration energy storage technology because of the evident advantages of lowcost and worldwide abundance of charge carriers.[2] Besides, the cost of SIBs could be further reduced by use of Co/Ni free cathode materials[3] and aluminum current collector on the anode side since sodium does not alloy with aluminum.[4] In addition to the economic benefits, the configuration of SIBs offers a potentially safe way for batteries storage and transportation. Since Al current collector does not dissolve into electrolyte at a voltage of 0 V, shipping and storing SIBs which contain no energy (a fully discharged state) is potentially feasible.[5] Moreover, Dahn's group investigated the thermal stability of positive materials for SIBs and found that the desodiated Na0.5CrO2 cathode was less reactive than Li0FePO4 in nonaqueous electrolyte at elevated temperatures.[6] Robinson et al. found that the self heating rate in a Na ion pouch cell is significantly slower than that in a commercial LiCoO2 (LCO) pouch cell and the thermal runaway process is less exothermic for Na ion cells, indicating that SIBs could be a potentially safer option compared with LIBs.[7] However, the larger and heavier Na ions have poor kinetic characteristic in the host structure during insertion reaction process, so it may lead to rapid degradation of the host materials with exothermic reaction.[8 10] In addition, the higher solubility of solid electrolyte interphase (SEI) of SIBs resulting from lower Lewis acidity of sodium complex, indicates that the incomplete coverage of electrode may further lead to undesired side reactions, accelerating heat generation. The cathode materials reported so far, roughly including oxides, polyanions, organics, Prussian blue and its analogues, which have poor electronic/ionic conductivity, will bring issues to thermal diffusion as well.[11] So far, nonaqueous liquid electrolyte is still the primary option for SIBs because of wide electrochemical stable window, high ionic conductivity, and rapid mass transfer at the electrolyteelectrode interface, yet giving rise to safety hazards.[12] Recent Sodium ion batteries, with their evident superiority in resource abundance and cost, are emerging as promising next generation energy storage systems for large scale applications, such as smart grids and low speed electric vehicles. Accidents related to fires and explosions for batteries are a reminder that safety is prerequisite for energy storage systems, especially when aiming for grid scale use. In a typical electrochemical secondary battery, the electrical power is stored and released via processes that generate thermal energy, leading to temperature increments in the battery system, which is the main cause for battery thermal abuse. The investigation of the energy generated during the chemical/electrochemical reactions is of paramount importance for battery safety, unfortunately, it has not received the attention it deserves. In this review, the fundamentals of the heat generation, accumulation, and transportation in a battery system are summarized and recent key research on materials design to improve sodium ion battery safety is highlighted. Several effective materials design concepts are also discussed. This review is designed to arouse the attention of researcher and scholars and inspire further improvements in battery safety.