Rocking‐chair based lithium‐ion batteries (LIBs) have extensively applied to consumer electronics and electric vehicles (EVs) for solving the present worldwide issues of fossil fuel exhaustion and environmental pollution. However, due to the growing unprecedented demand of LIBs for commercialization in EVs and grid‐scale energy storage stations, and a shortage of lithium and cobalt, the increasing cost gives impetus to exploit low‐cost rechargeable battery systems. Dual‐ion batteries (DIBs), in which both cations and anions are involved in the electrochemical redox reaction, are one of the most promising candidates to meet the low‐cost requirements of commercial applications, because of their high working voltage, excellent safety, and environmental friendliness compared to conventional rocking‐chair based LIBs. However, DIB technologies are only at the stage of fundamental research and considerable effort is required to improve the energy density and cycle life further. We review the development history and current situation, and discuss the reaction kinetics involved in DIBs, including various anionic intercalation mechanism of cathodes, and the reactions at the anodes including intercalation and alloying to explore promising strategies towards low‐cost DIBs with high performance.

Sodium-based dual-ion batteries (Na-DIBs) show a promising potential for large-scale energy storage applications due to the merits of environmental friendliness and low cost. However, Na-DIBs are generally subject to poor rate capability and cycling stability for the lack of suitable anodes to accommodate large Na+ ions. Herein, we propose a molecular grafting strategy to in-situ synthesize tin pyrophosphate nanodots implanted in N-doped carbon matrix (SnP2O7@N-C), which exhibits a high fraction of active SnP2O7 up to 95.6 wt% and a low content of N-doped carbon (4.4 wt%) as the conductive framework. As a result, this anode delivers a high specific capacity ∼400 mAh g−1 at 0.1 A g−1, excellent rate capability up to 5.0 A g−1, and excellent cycling stability with a capacity retention of 92% after 1200 cycles under a current density of 1.5 A g−1. Further, pairing this anode with an environmentally friendly KS6 graphite cathode yields a SnP2O7@N-C||KS6 Na-DIB, exhibiting an excellent rate capability up to 30 C, good fast-charge/slow-discharge performance, and long-term cycling life with a capacity retention of ∼96% after 1000 cycles at 20 C. This study provides a feasible strategy to develop high-performance anodes with high-fraction active materials for Na-based energy storage applications.

Dual‐ion batteries (DIBs) inherently suffer from limited energy density. Proposed here is a strategy to effectively tackle this issue by employing locally ordered graphitized carbon (LOGC) cathodes. Quantum mechanical modeling suggests that strong anion–anion repulsions and severe expansion at the deep‐charging stage raise the anion intercalation voltage, therefore only part of the theoretical anion storage sites in graphite is accessible. The LOGC interconnected with disordered carbon is predicted to weaken the interlaminar van der Waals interactions, while disordered carbons not only interconnect the dispersed nanographite but also partially buffer severe anion–anion repulsion and offer extra capacitive anion storage sites. As a proof‐of‐concept, ketjen black (KB) with LOGC was used as a model cathode for a potassium‐based DIB (KDIB). The KDIB delivers an unprecedentedly high specific capacity of 232 mAh g−1 at 50 mA g−1, a good rate capability of 110 mAh g−1 at 2000 mA g−1, and excellent cycling stability of 1000 cycles without obvious capacity fading.

Owing to the low cost of sodium/potassium resources and similar electrochemical properties of Na+/K+ to Li+, sodium‐ion batteries (SIBs) and potassium‐ion batteries (KIBs) are regarded as promising alternatives to lithium‐ion batteries (LIBs) in large‐scale energy storage field. However, traditional organic liquid electrolytes bestow SIBs/KIBs with serious safety concerns. In contrast, quasi‐/solid‐phase electrolytes including polymer electrolytes (PEs) and inorganic solid electrolytes (ISEs) show great superiority of high safety. However, the poor processibility and relatively low ionic conductivity of Na+ and K+ ions limit the further practical applications of ISEs. PEs combine some merits of both liquid‐phase electrolytes and ISEs, and present great potentials in next‐generation energy storage systems. Considerable efforts have been devoted to improving their overall properties. Nevertheless, there is still a lack of an in‐depth and comprehensive review to get insights into mechanisms and corresponding design strategies of PEs. Herein, the advantages of different electrolytes, particularly PEs are first minutely reviewed, and the mechanism of PEs for Na+/K+ ion transfer is summarized. Then, representative researches and recent progresses of SIBs/KIBs based on PEs are presented. Finally, some suggestions and perspectives are put forward to provide some possible directions for the follow‐up researches.

Aqueous Zn‐based hybrid energy storage devices (HESDs) exhibit great potential for large‐scale energy storage applications for the merits of environmental friendliness, low redox potential, and high theoretical capacity of Zn anode. However, they are still subjected to low specific capacities since adsorption‐type cathodes (i.e., activated carbon, hard carbon) have limited capability to accommodate active ions. Herein, a hierarchical porous activated carbon cathode (HPAC) is prepared via an in situ two‐step activation strategy, different from the typical one‐step/postmortem activation of fully carbonized precursors. The strategy endows the HPAC with a high specific surface area and a large mesoporous volume, and thus provides abundant active sites and fast kinetics for accommodating active ions. Consequently, pairing the HPAC with Zn anode yields an aqueous Zn‐based HESD, which delivers a high specific capacity of 231 mAh g−1 at 0.5 A g−1 and excellent rate performance with a retained capacity of 119 mAh g−1 at 20 A g−1, the best result among previously reported lithium‐free HESDs based on carbon cathodes. Further, the aqueous Zn‐based HESD shows ultra‐long cycling stability with a capacity retention of ≈70% after 18 000 cycles at 10 A g−1, indicating great potential for environmentally friendly, low‐cost, and high‐safety energy storage applications.

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Schematically deciphering the electrochemical mechanism of the electrochromic NiOx thin film based on the Li+-ion electrolytes.

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