Prussian blue analogues (PBAs)-based anode materials (oxides, sulfides, selenides, phosphides, borides, and carbides) have been extensively investigated in the field of energy conversion and storage. This is due to PBAs’ unique properties, including high theoretical specific capacity, environmental friendly, and low cost.
Oxide derivatives with hollow nanostructures based on Prussian blue (PB) and Prussian blue analogue (PBA) were investigated as anode materials for varous kind of rechargeable batteries system [1][2][3][4][5][6][7][8]. For example, PB cube can be derived as Fe2O3 [3]. These Fe2O3 microboxes have distinct physical architectures. The physical architectures of these Fe2O3 microboxes are unique. Their porous or hollow architectures may improve electrolyte diffusion and Li+ ions transport in the electrode, and this nano structure can effectively avoid volume change during the charging and discharging process [9]. At the same time, high crystallinity and hollow Fe2O3 microboxes provide structural stability during the long terms of charging and discharging process.
In addition to iron oxide, structure-related lithium storage properties can also be found in cobalt oxides derived from PBA. Co3O4 has been used in Li-ion battery because of its easy synthesis and high specific capacity. Because of the charge storage provided by the Faraday processes, cobalt oxide has attracted a lot of attention and has been studied for a long time as an energy storage material. Various forms of cobalt oxides are easy to synthesize and have very high theoretical capacitance. Co3O4 can be converted into CoOOH in an alkaline electrolyte, and then Co3O4 can be converted into CoO2 [10]. Although its theoretical capacitance is high, its practical capacitance is lower due to conductivity reduction and parasitic species formation during the long terms of charging and discharging process. Furthermore, it is noted that the expansion/contraction phenomenon limits the cycling life of cobalt oxide-based electrode. The main way to overcome these shortcomings is to design various high surface area morphologies.
Hu et al. successfully demonstrated Co3O4 derived from Co3[Co(CN)6]2 PBA and obtained the porous Co3O4 nanocages [11]. Yi et al. found that porous Co3O4 nanocages had a better electrochemical performance as the anode of lithium solid cage. This nanostructure also has a high capacity for lithium storage . As an energy storage material, it also shows many advantages: (1) the porous shell facilitates Li+ diffusion; (2) the bimodal pore size distribution and larger surface area reduce electrolyte resistance by shortening Li diffusion length, increasing electrolyte/electrode area of contact, and increasing electrolyte contact area; (3) coating amorphous carbon can adapt to cyclic-induced strain; (4) surface atoms and structural strain are enriched by small nanoparticles. These characteristics enable PBA-derived porous Co3O4 nanocages to provide significant benefits in the application of Lithium-ion batteries (LIBs), which may be exploited to create high-performance LIBs. All of these discoveries demonstrate that PBA oxide derivatives perform greatly as energy storage materials, and that they may be used to enhance the performance of LIBs in the future.In order to pursue higher performance energy storage materials, researchers have carried out extensive exploration to further improve the new materials with a high capacity, stable cycle, and fast charging capacity. Because of their ease of availability, low cost, and high theoretical specific capacity, transition metal oxides are regarded as good energy storage materials [1]. The use of PBA-derived oxide nanomaterials will improve the optimization and design of the nanostructured electrode materials, improve ion diffusion ability and structural stability, and enable the production of high-performance energy storage materials. For instance, Zhang et al. [3] have explored the effect of Fe2O3 box secondary cells made of PB cubes at varying temperatures as anode materials for LIBs. Porous Fe2O3 micro particles composed of Fe2O3 nano particles can be easily synthesized by the oxidation and decomposition of PB nano cubes at high temperatures. When used as anode material of LIBs, the porous Fe2O3 nano cube obtained also shows the excellent cycle performance and high specific capacity (∼800 mA h g−1 at 200 mA g−1). In another experiment, researchers discovered that the electrochemical properties of Fe2O3 microboxes are also affected by the shell complexity [2]. The expandable synthesis of anisotropic hollow structures with numerous shell structures was demonstrated by the oxidative decomposition of PB microcube and crystal growth of iron oxide shell. The cycle stability of Fe2O3 microboxes with multiple shells is dramatically higher than that of Fe2O3 microboxes with single and double shells. When it is used as the anode material of LIBs, Fe2O3 has a well-defined hollow structure and microboxes with a layered shell, showing high specific capacity (∼950 mAh g−1 at 200 mA g−1) and outstanding cycle performance.
Meanwhile, the combination of metal oxides and carbon materials is a beneficial way to enhance the performance of LIBs. Using two PB/graphene foam precursors, Shao et al. [12] successfully prepared Fe2O3/graphene foam composites. The prepared Fe2O3/graphene foam material was used as a LIB’s independent electrode, combining the advantages of lead-derived metal oxides and graphene foam. It exhibited better lithium storage performance than pure Fe2O3 and graphene foam due to the synergistic effect of the two components. These research findings will help to develop and improve electrode materials for the storage of energy.
In addition, Hu et al. [11] suggested a new and simple method for creating porous Co3O4 nanocages based on the Kirkendall effect, which includes the thermal decomposition of PBA Co3[Co(CN)6]2 truncated nano cubes at 400 °C. These nanocages overcame the inherent disadvantage of Co3O4 anodes by exhibiting stability, high capacity, and excellent cycle efficiency. When applied to the electrode material of LIBs, the prepared Co3O4 porous nanocage showed excellent battery performance. At a current density of 300 mA g−1, it still had a capacity of 1465 mA h g−1 after 50 cycles. Metal oxides have a huge storage capacity; however, they suffer from significant volume fluctuations and have low conductivity when charged and discharged. Zhu et al. overcame this challenge by calcining preformed PBA Zn3[Co(CN)6]2 nanospheres to create self-assembled ZnO/Co3O4 nanocomposites [13]. These ZnO/Co3O4 nanocomposites have good cycling and lithium storing capacity. The rational design of the formed cluster structure, together with the synergistic impact of the bicomponent functional nanoparticle system, considerably improves the electrochemical performance of the electrode material generated from it. This special nanostructure has superior lithium storage capacity, which provides a new direction for further optimization of LIBs.
Metal sulfides have been shown to have higher conductivity, mechanical stability, thermal stability, and electrochemical activity than equivalent metal oxides in recent years [14][15][16]. Metal sulfides offer higher conductivity than comparable metal oxides, as well as a rich valence state, mechanical and thermal stability, all of which are beneficial to electrochemical performance [17]. Mixed metal sulfide is gaining popularity as a viable electrode material for electrochemical energy storage and conversion systems [18]. When compared to a single metal sulfide, mixed metal sulfide performs significantly better electrochemically [19][20], owing to better electronic conductivity and more diverse redox processes. Metal sulfide has emerged as a potential electrode material due to its extensive redox chemistry, high conductivity, and the synergistic action of two metal ions. Because of their better performance and widespread use in recent years, lithium sulfur batteries are a potential new energy storage technology that can fulfill the rising energy demand. A typical LIB is made up of a negative electrode, a positive electrode, an electrolyte, and a separator sandwiched between parallel electrodes [21][22][23]. The working principle of both devices is voltage-driven cation migration (Na+, Li+, H+, K+, etc.) or anions (OH−, etc.) through electrolyte towards electrodes for reversible electrochemical reactions. At the same time, electrons flow via the external circuit to keep the charge balanced. These lithium batteries require appropriate and effective materials, particularly as matrix, sandwich, and laminated metal organic skeleton as a novel porous material. The chemical characteristics and physical architectures of electrode materials must be appropriate for an efficient energy storage system [24][25]. Metal–organic framework composites and metal–organic framework (MOF) derivatives have a better performance, reducing the shortcomings of pure MOFs [26][27].
Previous research found that interstitial water promoted the insertion and extraction of multivalent ions. Prussian blue analogues (PBAs) have excellent electrochemical performance because water weakens the electrostatic repulsion between ions, lowering the activation energy of ion diffusion and interface transfer [28]. Lithium ion is relatively easy to embed in PBAs for ion diffusion, whereas sodium or potassium are more difficult to embed due to defects that inhibit sodium/potassium diffusion and coordination water. The vacancy and water content in PBA will be significantly reduced through a well controlled crystallization process, so that the cycle capacity and capacity of sulfur ion battery (SIB) and LIB can be improved [29]. PBA and its sulfide derivatives play an important role in the application of energy storage materials because they provide sufficient gaps for the transfer of ions and electrolytes, as well as many active sites for the insertion of ions.
2.2. Application of Sulfide as Energy Storage Material
PB and its sulfide derivatives are favored by many researchers because of their excellent performance, so PB and its sulfide derivatives have many applications in energy storage materials.
A graded iron sulfide nano cube was synthesized using PB as the starting material in a two-step in situ transformation process, and it was coated with several layers of graphene (Fe1-xS@C/rGO). When used as the anode of SIBs, the graded nano cube shows excellent rate capability of 323 mAh g−1 at a current density of 10 A g−1 [30]. For 150 cycles, the iron base sodium ion battery has a classification of Fe1-xS@C/rGO anode and the capacity of PB cathode is 323 mAh g−1. The Fe1-xS@C/rGO nano cube has good sodium ion storage performance due to the stable layered building structure and highly graphitized carbon obtained during the conversion process. The nano cube structure of graphene coating prevents the amalgamation of iron sulfide carbon core–shell nanoparticles and also adapts to the large volume expansion during the cycle. Carbon’s high crystallization degree conferred great electronic conductivity, enabled sodium ion accessibility, and enhanced mechanical durability on Fe1-xS@C/rGO nano cubes.
Zeng et al. created 3D hollow CoS2 nanoframes (HCSN) using partial anion replacement of Co-based PB nano cubes [31]. Because of its uniform broken hollow structure and excellent dispersion, the synthesized three-dimensional (3D) HCSN has a large specific surface area, leading to good electrochemical performance, such as the high specific capacitance of 568 F g−1 at 0.5 A g−1 and the excellent cycle stability of 88.3% specific capacitance retention after 5000 cycles. The HCSN supercapacitor has a great energy density of 32.3 Wh kg−1 and a power density of 4 kW kg−1. The HCSN also exhibit excellent structural stability and low density, indicating a potential application in energy storage materials.
The development of these new devices and their high performance show that PB sulfide derivatives play a significant role in the application of energy storage materials, as well as offering new ideas for the further development and utilization of sulfide in the future.
PBA selenide derivatives can be used to build porous or hollow nanostructures with a high specific surface area and a unique shape that promotes electron transmission while avoiding significant volume expansion and thereby improving conductivity [36][37][38]. At the same time, by combining nanomaterials with different band gaps, it can promote the diffusion rate of Li+/Na+ and accelerate the surface reaction kinetics, which can form the design of bimetallic selenide architecture with unique interface effects. PBA selenide derivatives have a wide range of applications in energy storage materials. In addition, PBAs, as a MOF composed of metal centers and organic connectors, are available as self templates for the synthesis of porous materials [39]. MOF can limit and generate small size metal particles in situ, which can then be used to design metal selenides. Due to their excellent properties, many researchers are drawn to develop and use them.
Zhang et al. took full advantage of the porosity of PBA materials and the high capacity and high conversion of selenides, and used Zn-Fe PBA@polydopamine as a precursor to carbonization-selenization to design the microporous nanostructure ZnSe-Fe3Se4@NC successfully [45]. In addition, they investigated the electrochemical performance of N-doped Zn-Fe-Se heterostructures in LIBs. The ZnSeFe3Se4@NC anode exhibits excellent cycle stability and rate performance at 1 A g−1 after 723 cycles for LIBs (900 mA h g−1) because of the advantages of high conductivity carbon coating and porous structure. Previous research has shown that nanostructure fabrication and hybridization with carbonaceous materials are effective methods for mitigating mechanical strain caused by volume and structure changes during charge and discharge processes [40][41]. On the one hand, due to the short transmission path, reducing the electrochemical active materials to nano scale facilitates ions and electrons [42]. The carbonaceous reagent, on the other hand, can not only improve the transmittance of the electrode but also act as an effective buffer matrix to mitigate volume and structure changes during the cycle [43]. Wang et al. [44] successfully designed porous Mn-Fe-Se adhered/inserted with interlaced CNTs using a simple chemical precipitation approach and a one-step carbonization-selenization of the Mn-Fe PBA precursor process. Mn-Fe-Se/CNT exhibit remarkable electrochemical properties as well as high cycling and rate capabilities due to the synergistic effect of interlaced CNT. N-doped FeC@C box, which acts as the efficient polyselenide reservoir, is successfully created via a simple pyrolysis method using PB nano cubes as a precursor by Wang et al. [45]. Their results show that the PBA selenide derivative battery has high reversible specific capacity, better conduction rate, and cycle stability.
The above results indicate that PBA selenide is a promising electrode material for LIB. This result is expected to arouse people’s interest in energy storage and conversion of PBA selenide derivatives.
Transition metal phosphides are one of the candidates for LIBs polar materials, because they have good polarity, high catalytic activity, thermal stability, and chemical stability [46][47][48]. Therefore, it is critical to investigate the use of TMPs in Li-S batteries. Huang et al., using the density functional theory, showed that FeP can form a strengthening bond with polysulfides, promoting further redox conversion [49]. Cheng et al. [50] created double-layer Ni-Fe-P/N-doped carbon nanomaterials and demonstrated their stable cycle and excellent rate performance in LIBs. Hard carbon and soft carbon are the most common anode materials, but their capacities are limited (generally less than 300 mAh g−1)[51]. FeP has recently been considered as a potential candidate due to its good theoretical capacity (926 mAh g−1), medium operational potential, and environmental friendliness [52]. Nevertheless, major problems related to the non-negligible volume change of FeP particles and low inherent conductivity must be solved, which will greatly reduce the relationship between dynamics and cycle stability [53]. To address the previously mentioned tradeoff between specific capacity and cycle stability, particle size reduction can effectively decrease diffusion length while increasing sodium diffusion coefficient [54][55]. FeP nanoparticles (12nm) anchored and uniformly dispersed on nitrogen-doped carbon frames (FeP@NC). It has a greater reversible capacity of 253.9 mAh g−1 under 2 A g−1 [56]. Considering the uniform distribution of Fe in the PB framework, it is expected that the phosphating/carbonization of PB will lead to the uniformly distributed of FeP nanoparticles in the carbon matrix.
The excellent properties of PBA phosphide derivatives in energy storage materials make them favorable to many researchers, and many researchers have developed and applied them.
Song et al. [57] used Fe-Ni-P@nitrogen-doped carbon (Fe-Ni-P@NC) derived from Fe-Ni PBA, which is used as the efficient sulfur host of Li-S batteries. Fe-Ni-P particles not only effectively promote the conversion of LiPS, but also improve the adsorption of LiPS. Furthermore, the CN- of PBAs is easily converted into nitrogen-doped carbon during pyrolysis, which can improve the composite’s conductivity. Because of these benefits, LIBs/SIBs with S@Fe-Ni-P@NC composite cathodes demonstrated excellent electrochemical performance, excellent rate capability, and a stable cycle of 500 cycles at 1 ℃, and the capacity decay rate of each cycle is low, 0.08%.
By combining the tubular PB cathode with its derived phosphating anode, Jiang et al. demonstrated an excellent sodium ion battery [58]. Because of the tubular configuration, the PB has fast reaction kinetics and, thus, a high specific capacity of 94.4 mAh g−1 even at a high rate of 5.0 A g−1. PB-derived phosphides are characterized by encapsulating FeP nanoparticles in a conductive carbon matrix, thereby achieving excellent sodium energy storage. The as-assembled sodium-ion full cell has a record capacity of 105.3 mAh g−1 at 2.0 A g−1 and a long cycling lifetime. Their research proposes a homologous design strategy for outstanding sodium-ion full cells based on PB and its derivatives.