The two biggest issues confronting humanity right now are the energy crisis and environmental degradation. For hundreds of years, the exploitation and excessive consumption of non-renewable conventional fossil fuels like coal, oil and natural gas have caused a series of grave problems. To overcome these concerns, countries have committed to developing clean renewable energy [1,2]. However, the intermittence, volatility and instability of renewable energy generation greatly hinder its efficient development and application. Efficient energy storage systems emerge as the times require, which effectively buffer this fluctuation to achieve stable energy storage and output [3]. Because of its high energy density and excellent cycling performance, lithium-ion batteries (LIBs) have become a popular choice for grid energy storage, electronic consumer products, and electric vehicles in recent decades. However, lithium metal is difficult to explore, the production cost is high, and most of them employ organic electrolytes that are combustible and poisonous, so there are great potential safety risks and environmental contamination [4]. These problems seriously hinder its development and application in the field of large-scale energy storage. As a result, people have started looking for alternative energy storage battery systems that can take the place of lithium-ion batteries and have the advantages of being inexpensive, safe, high capacity, and environmentally friendly [5].

A growing number of academics have started researching aqueous zinc ion battery (AZIBs) since Shoji et al. published their findings on a rechargeable Zn-MnO2 aqueous battery in 1988. Compared with lithium metal, Zn metal is abundant in the earth's crust (about 7 × 10−5), which is about 3.5 times of lithium reserves (about 2 × 10−5) [6]. The redox potential of zinc is relatively low (−0.76 V vs. SHE), and the zinc atom loses two electrons during the electrochemical reaction, which can carry more charges than single-electron lithium ion, thereby obtaining higher battery power density and theoretical capacity (820 mAh g−1) [7]. Furthermore, the aqueous electrolyte used in AZIBs has two orders of magnitude higher conductivity than the organic electrolyte used in LIBs. It is also cheaper, easier to encapsulate, and safer than the organic electrolyte used in LIBs [8]. It is worth noting that AZIBs also often use hydrogel electrolytes to balance ion transfer, expand the electrochemical stability window, and achieve highly reversible zinc plating/stripping [9]. Therefore, AZIBs are considered to be the most ideal “candidate” for LIBs, and are anticipated to emerge as the most promising new batteries in future electrochemical energy storage [10]. At present, there are five main energy storage mechanisms of MnO2-based AZIBs: Zn2+ insertion/extraction, H+ and Zn2+ co-insertion/extraction, chemical conversion reaction, dissolution/deposition reaction and hybrid reaction mechanism [[11], [12], [13]]. The general consensus is that, in accordance with the charge-discharge mechanism, zinc ions are reversibly deposited/dissolved on the zinc anode, while the ion de-intercalation electrochemical reactions on the cathode material become the key to determine the effectiveness of AZIBs.

Recently, vanadium-based compounds, manganese-based compounds, Prussian blue analogues, organic-based compounds and other materials have been the primary cathode materials for AZIBs [[14], [15], [16]]. Fig. 1 compares the benefits and drawbacks of typical cathode materials used in AZIBs. Vanadium-based materials have the advantages of high capacity and cost-effectiveness, but their practical application is hindered by the problems of vanadium dissolution, slow diffusion kinetics of Zn2+ ions, and low operating voltage [[17], [18], [19]]. In contrast, manganese-based compounds with tunnel/layered crystal structures exhibit strong zinc ion storage capacity. The multivalent state and high redox potential of Mn make it have relatively high theoretical capacity and discharge voltage platform, which has an extensive application prospect [20]. However, the prone dissolution of Mn ions during the charging and discharging, causing the cathode structure to collapse and leading to a decrease in capacity, as well as the poor conductivity of the MnO2 material itself have limited the development of the Mn-based cathode materials. Consequently, researchers have put up a series of strategies to solve the problem of Mn-based cathode materials [21]. For example, the Mn-based cathode materials are compounded with other materials to give full play to the advantages of each material and improve its performance in various aspects. The guest pre-intercalation strategy can provide a broad and stable internal space for zinc ions by pre-intercalation anions/cations in manganese-based cathode materials, and effectively stabilize its crystal structure [22]. In addition, the construction of defect engineering causes crystal defects by destroying the regularity and integrity of the crystal structure of Mn-based materials, which not only provides active intermediates and ions in chemical reactions to occupy and react, but also promotes the directional movement of ions and electrons [23,24]. Among many methods, the strategy of pre-intercalation metal cations, organic molecules and other guests into the manganese-based cathodes is the most simple and efficient. This method has unique superiorities in controlling the interplanar spacing, optimizing the electronic band structure, improving the reaction kinetics and cycling stability [25]. On the one hand, the intercalation of metal ions or organic molecules can significantly improve the electronic conductivity of manganese-based materials, promote the rapid diffusion of zinc ions, and enhance the charge transport capacity, thereby improving the rate performance and cycling stability of batteries. On the other hand, pre-embedded ions or molecules can form a more stable structure with manganese-based materials, inhibit the dissolution of Mn2+ ions, reduce the structural collapse of materials during charging and discharging, and thus prolong the service life of batteries.

It is worth noting that some reviews on the research progress of Mn-based oxide materials have appeared nowadays, but the majority of these studies concentrate on the challenges, reaction mechanism, and various optimization strategies of manganese dioxide-based AZIBs. There is no systematic and comprehensive description of the method of employing guest pre-intercalation to enhance the electrochemical performance of manganese-based cathode materials. For example, these reviews are focus on different MnO2 energy storage mechanisms and describe how to improve the electrochemical performance of Mn-based cathodes from the perspectives of surface engineering, defect engineering, electrolyte modification and heteroatom doping [[26], [27], [28]]. However, according to our knowledge, there are few reports on the application of guest pre-intercalation method for different Mn-based materials. These articles analyze the electrochemical mechanism of manganese oxides in AZIBs and also discuss the effects of electrolytes with different pH values on the battery capacity. The mechanism of improving the electrochemical performance of manganese oxides is not described in detail [[29], [30], [31]]. This review focuses on the interlayer spacing regulation of layered Mn/V oxide cathode materials through guest pre-intercalation. However, it does not provide a detailed discussion on the pre-intercalation of other crystalline structures of MnO2-based oxide cathode materials, such as tunnel-type and 3D structures [32]. While the article offers a thorough overview of common enhancement tactics for manganese-based cathode materials for AZIBs, it does not involve the intrinsic mechanisms of major changes in electronic structure behind their modification [33]. These reviews conclude the structural features and energy storage mechanisms of various cathode materials, recalling their existing problems and corresponding solutions, but does not focus on the strategy of guest pre-intercalation manganese-based materials [[34], [35], [36]]. The article only discusses the cation pre-intercalation engineering of manganese-based electrode materials for rechargeable metal-ion batteries, which is rather one-sided. It does not provide a summary and analysis of strategies for pre-intercalating anions or organic molecules [37]. Obviously, no thorough and critical evaluation has been done on the application of guest pre-intercalation strategy to enhance the electrochemical performance of manganese-based cathode materials.

Consequently, this review concentrates on the most recent research advances in guest pre-intercalation strategies for enhancing the performance of Mn-based oxide cathode materials. First, the problems of Mn-based oxide cathode materials and the respective in-depth reasons are briefly introduced, including the dissolution of Mn ions, unstable crystal structure, and poor electrical conductivity. Secondly, the performance improvement mechanisms and electronic structure change mechanisms of guest pre-intercalation in various Mn-based oxide materials are discussed, including α-MnO2, β-MnO2, ε-MnO2, δ-MnO2 and γ-MnO2. At last, some perspectives on the challenges, future developments and application prospects of the guest pre-intercalation strategy in the modification of Mn-based cathode materials are put forward. This review serves as a reference for the future development of Mn-based oxide cathode materials for high-performance AZIBs, while also facilitating the practical application of AZIBs (Fig. 2).