No abstract

Owing to the energy crisis and environmental pollution, developing efficient and robust electrochemical energy storage (or conversion) systems is urgently needed but still very challenging. Next-generation electrochemical energy storage and conversion devices, mainly including fuel cells, metal-air batteries, metal-sulfur batteries, and metal-ion batteries, have been viewed as promising candidates for future large-scale energy applications. All these systems are operated through one type of chemical conversion mechanism, which is currently limited by poor reaction kinetics. Single atom catalysts (SACs) perform maximum atom efficiency and well-defined active sites. They have been employed as electrode components to enhance the redox kinetics and adjust the interactions at the reaction interface, boosting device performance. In this Review, we briefly summarize the related background knowledge, motivation and working principle toward nextgeneration electrochemical energy storage (or conversion) devices, including fuel cells, Zn-air batteries, Al-air batteries, Li-air batteries, Li-CO 2 batteries, Li-S batteries, and Na-S batteries. While pointing out the remaining challenges in each system, we clarify the importance of SACs to solve these development bottlenecks. Then, we further explore the working principle and current progress of SACs in various device systems. Finally, future opportunities and perspectives of SACs in next-generation electrochemical energy storage and conversion devices are discussed.

Li-metal is considered as the most promising anode material to advance the development of next-generation energy storage devices owing to its unparalleled theoretical specific capacity and extremely low redox electrochemical potential. However, safety concerns and poor cycling retention of Li-metal batteries (LMBs) caused by uncontrolled Li dendrite growth still limit their broad application. Herein, liquid polydimethylsiloxane (PDMS) terminated by -OCH 3 groups is proposed as a graftable additive to reinforce the anode dendrite suppression for LMBs. Such a grafting triggers the formation of a conformal hybrid solid electrolyte interphase (SEI) with increased fractions of LiF and Li-Si-O-based moieties, which serve as a rigid barrier and ionic conductor for uniform Li-ion flow and Li-mass deposition. The grafting protected anode endows Li/Li symmetric cells with a long lifetime over 1800 h with a much smaller voltage gap (≈25 mV) betweenLi plating and stripping, than the naked anode. The coulombic efficiency values for Li/Cu asymmetric cells in carbonate electrolyte can reach up to 97% even at a high current density of 3 mA cm −2 or high capacity up to 4 mAh cm −2 . The liquid PDMS additive shows advantage over solid siloxane additives with poor grafting ability in terms of Li surface compaction and SEI stabilization.

Figure 3. Inorganic materials for lithium-sulfur batteries: a) Schematic illustration of CNT@TiO 2-x regulation of LiPS redox. Reproduced with permission. [51] Copyright 2019, Wiley-VCH Verlag. b) Layered LiTiO 2 for the protection of Li 2 S cathodes against dissolution. Reproduced with permission. [58] Copyright 2018, Royal Society of Chemistry. c)The roles of the Chevrel-phase Mo 6 S 8 in the hybrid Mo 6 S 8 /S 8 cathode. Reproduced with permission. [61] Copyright 2019, Springer Nature. d) The MoN-C-MoN trilayer architecture enabled high-performance lithium-sulfur. Reproduced with permission. [68] Copyright 2020, John Wiley and Sons Ltd. e) Ti 3 C 2 T x entrapping the polysulfides by forming a sulfate complex protective barrier. Reproduced with permission. [75] Copyright 2018, Wiley-VCH Verlag. f) Cycle life of TiO 2 -Ni 3 S 2 /rGO electrode with 3.92 mg cm À2 sulfur loading. Reproduced with permission. [76] Copyright 2020, Wiley-Blackwell. g) cobalt in nitrogen-doped graphene (Co-N/G) as a single-atom catalyst toward high-performance lithium-sulfur batteries. Reproduced with permission. [79] Copyright 2019, Wiley-Blackwell. h) The natural Vermiculite is used as a sulfur host in lithium-sulfur batteries. Reproduced with permission. [81]