Lithium metal is regarded as the most promising anode material for the next-generation lithium batteries due to its high theoretical specific capacity (3860 mAh/g) and lowest electrochemical potential (-3.04 V vs SHE). However, dendrite growth and volume changes in Li metal anodes during battery cycles hinder the industrialization of Li metal anodes severely. Recent research progress has shown that introducing 3D host in Li metal can not only suppress dendrite growth, but also relieve volume changes of Li anode, thus improving cycle performance and safety of lithium metal batteries. Therefore, designing 3D host/Li metal composite anodes is regarded as an emerging strategy that can solve the problem of Li metal anodes effectively. This review summaries the recent progress on 3D host/Li metal composite anodes prepared by thermal infusion strategy. We firstly discuss prelithiation methods of 3D host and analyze influencing factors of host lithiophilicity in thermal molten infusing. Afterwards, different 3D host framework and their features are discussed followed by the improved strategies. Finally, we summarize existing problems of 3D host/Li metal composite anodes and give their future prospects.
Contents
1 Introduction
2 Thermal infusion method for preparing Li composite anodes
2.1 The impact factors of Li wettability of 3D frameworks
Lithium-ion batteries are widely used in electric vehicles, communication equipment, electronic equipment and energy storage devices because of their high specific energy, no memory effect and long service life[1⇓⇓~4]. However, the current energy density of commercial lithium batteries has gradually failed to meet the needs of emerging applications. The improvement of battery energy density mainly depends on electrode materials[5⇓⇓~8]. As far as the anode is concerned, the theoretical specific capacity of the most mature commercial graphite anode for lithium-ion batteries is only 372 mAh/G, which can not meet the needs of the new generation of high specific energy batteries, so it is urgent to develop high-capacity anode materials. Among many potential anode materials, lithium metal anode is considered to be the most ideal anode material because of its high theoretical specific capacity (3860 mAh/G) and low redox potential (-3.040 V vs. SHE)[9⇓~11]. However, due to the characteristics of lithium metal anode, a series of phenomena such as lithium dendrite growth, volume change and unstable reaction interface structure will occur during the cycle process, which significantly reduces the electrochemical performance of lithium metal anode. What is more serious is that lithium dendrites are easy to pierce the battery diaphragm and contact with the positive electrode, resulting in short circuit in the battery and inducing safety problems such as combustion and explosion. These problems are the key factors that make lithium metal anode unable to be commercialized so far[12⇓⇓⇓⇓~17].
In order to solve the above problems, researchers at home and abroad have proposed a variety of solutions, such as electrolyte modification, solid electrolyte replacement, artificial solid electrolyte interface (SEI) film and skeleton supported composite lithium metal[18⇓⇓⇓⇓⇓~24]. As we all know, the nucleation and deposition morphology of lithium ions on the surface of lithium metal and traditional copper foil current collectors is generally uneven, especially at high rates, the large local current density promotes the formation of lithium dendrites and "dead lithium" layers, resulting in poor cycle performance and potential safety hazards of batteries. In contrast, the introduction of three-dimensional (3D) framework with spatial structure into lithium metal can play a role in dispersing the local current density on the electrode surface, thus effectively reducing the trend of lithium dendrite growth at the electrode reaction interface[25⇓⇓⇓⇓⇓~31]. At the same time, as the host of lithium metal reaction, the 3D framework can alleviate the volume change of the electrode during charging and discharging, thus stabilizing the structure and chemical composition of the reaction interface. In addition, lithiophilic modification on the framework surface can regulate the deposition/dissolution behavior of lithium on the framework in situ, and affect the nucleation kinetics and deposition morphology of lithium at the molecular level. Therefore, the introduction of 3D framework into metallic lithium is considered to be one of the effective strategies to achieve zero-overpotential, dendrite-free and high-safety metallic lithium anode[32⇓⇓⇓⇓⇓~38].
At present, the reported skeleton-supported composite metal lithium anodes are mainly prepared by the following three methods: electrochemical deposition, mechanical contact and hot melt infusion. The electrochemical deposition method is to assemble the prepared 3D framework and metal lithium sheets into a half-cell, and apply a small current density to continuously deposit metal lithium on the surface of the framework to obtain a composite electrode. The mechanical contact method is to compound the 3D skeleton with the metal lithium block by mechanical extrusion, so as to obtain the metal lithium composite electrode. The hot melt impregnation method refers to heating metal lithium to a molten state (the melting point of metal lithium: 180 ° C) in an inert atmosphere, then impregnating the liquid lithium into the 3D framework material, and cooling to form the composite metal lithium anode, as shown in Figure 1[39]. Compared with the electrodeposition method and the mechanical contact method, the hot melt infusion method has the following advantages: (1) the lithium pre-storage process is not controlled by an electric field and pressure, and metal lithium is uniformly filled in the vertical direction of the composite electrode structure; (2) the liquid lithium is in close contact with the 3D framework, and the filling density of the metal lithium in the electrode is high, so that the electrode has excellent unit energy density; (3) the introduction of impurities can be avoided through the process of first melting and then solidification; And (4) that efficiency of the prestore lithium is high, and the method can be applied to the preparation of the composite electrode with high lithium content. However, the hot melt infusion method is not applicable to any framework material, which requires that the framework must have high physical and chemical stability and mechanical properties under high temperature conditions, and that the framework must have good Li wettability[40,41]. In this paper, the research progress of 3D framework/metal lithium composite electrodes prepared by hot melt infusion method in recent years is reviewed, and the factors affecting lithium wettability are discussed in detail. At the same time, a series of current research work on composite electrodes and the modification strategies of lithium wettability are comprehensively summarized.
2 Preparation of metal lithium composite electrode by hot melt infusion method
2.1 Factors affecting the wettability of three-dimensional framework lithium
In the hot melt infusion method, the 3D framework with high lithium wettability can be quickly infiltrated by liquid lithium, while the 3D framework with low or no lithium wettability is not compatible with liquid lithium all the time, that is, liquid lithium can not effectively diffuse into the internal structure of the 3D framework to form a composite electrode. Therefore, the wettability of lithium is one of the important conditions to judge whether the 3D skeleton can be prepared by hot melt infusion method. It is found that the lithium wettability of 3D framework is related to many factors, such as the melt temperature, the surface chemistry of 3D framework, the type and thickness of the lithium wetting modification layer, and the surface morphology of the framework.
2.1.1 Hot Melt Temperature
The hot melt temperature refers to the temperature at which metal lithium is pre-stored in the 3D skeleton during the hot melt infusion process. Generally, the melting point of metal lithium is 180 ℃ under the protection of inert atmosphere, but due to its high activity, the surface of metal lithium is always wrapped by a layer of lithium compounds (Li2CO3, Li2O, Li3N, LiOH, etc.), which hinders the conduction of hot melting temperature to a certain extent, so the hot melting temperature is generally higher than 250 ℃. In addition, even in an inert environment, the newly formed liquid lithium surface is prone to react with trace amounts of oxygen and water, which affects its mobility and is not conducive to the wetting behavior of the 3D framework. At a higher melting temperature, the liquid surface of liquid lithium fluctuates and accelerates, which is conducive to increasing the contact area between 3D skeleton and liquid lithium, thereby improving the recombination efficiency. On the other hand, with the increase of melting temperature, the reaction entropy of the system increases, resulting in the decrease of interfacial tension and overall viscosity of liquid lithium, which is conducive to improving the compatibility between 3D skeleton and liquid lithium and boosting the infiltration efficiency of liquid lithium[42,43]. Wang et al. Have systematically established the relationship between the melting temperature and the wettability of lithium[44]. As shown in fig. 2a and fig. 2b, when the initial temperature is 215 ℃, the metal lithium on the surface of the substrate changes from a block to a molten state, and shrinks into metal liquid globules due to the effect of surface tension, and the morphology is similar to that of metal mercury at room temperature. In addition, at the initial temperature, the contact angles of liquid lithium on stainless steel, titanium, molybdenum, nickel, lithium fluoride and carbon substrates are all greater than 120 °, indicating that these materials have low lithium wettability. However, with the increase of temperature (to 350 ℃), the contact angles of liquid lithium on different substrates all show a downward trend, reflecting the effect of increasing temperature on improving the wettability of lithium. Among them, the nickel substrate has the smallest liquid lithium contact angle at 350 ° C, implying relatively excellent lithium wettability. To sum up, the lithium wettability of 3D framework increases with the increase of hot melt temperature to a certain extent.
2.1.2 Surface properties of the skeleton
The thermodynamic driving force corresponding to the Gibbs free energy change (ΔG) of the reaction between liquid lithium and 3D framework or its surface modification layer is the key factor to improve the wettability of lithium or the efficiency of liquid lithium infiltration. From this point of view, the improvement of lithium wettability is attributed to the formation of new chemical bonds between liquid lithium and 3D framework medium, which reduces the surface energy of liquid lithium to improve their compatibility. In general, when designing a 3D skeleton or surface modification layer, it is judged whether the 3D skeleton will react with liquid lithium at a proposed hot melt temperature. When there is a spontaneous reaction tendency (ΔG < 0) and the mechanical properties of 3D framework change little, it can be considered that chemical bonds can be spontaneously formed between them to reduce the contact angle of liquid lithium (lithium wettability is improved), which promotes the rapid flow of liquid lithium on the surface of the framework, thus providing a stronger driving force for the diffusion of liquid lithium in the 3D framework structure[45,46]. Therefore, the trend of 3D framework lithium wettability can be judged by the ΔG of the reaction.
DELTA.G of the reaction and the relationship between the interfacial energy of the contact surface and the contact angle of liquid lithium in hot melt infusion can be described by Equation (1-1):[47,48]
Where γlv is the liquid-vapor interfacial energy, Δγsl is the change in γsl (solid-liquid interfacial energy) due to the change in the physicochemical properties of the interface, θ0 is the contact angle when there is no reaction between liquid lithium and the 3D framework, θ1 is the contact angle formed after the reaction, and ΔG is the Gibbs free energy change of the reaction. It can be seen that the decrease of γsl (Δγsl<0) and the change of Gibbs free energy less than zero (ΔG < 0) can improve the lithium wettability of the framework. In addition, directly from the formula (1-1), it can be seen that the more negative the ΔG of the reaction is, the larger the value of the corresponding cosθ1 is, that is, the smaller the contact angle (θ1) is, indicating that the faster the 3D framework is wetted by liquid lithium. To sum up, the spontaneous reaction between liquid lithium and the framework surface or the modified layer can trigger an effective lithium infiltration process. However, because this reaction is accompanied by phase change and stress change, in order to maintain the spatial structure and stability of the 3D framework, it should be noted that the framework itself is an inert (ΔG > 0) material, while the surface or modification layer of the framework is an active (ΔG < 0) material, such as Au, Ag, Si, Al, Zn, CuO, ZnO, SnO2, SiOx, etc. (Fig. 3).
2.1.3 Thickness of the lithium wetting modification layer
Most of the oxides as the lithium wetting modification layer can effectively accelerate the wetting rate of liquid lithium, which is mainly due to the spontaneous reduction reaction of liquid lithium on oxides. However, the oxide modification layer leads to the formation of inert Li2O in the metal lithium phase, and accumulates on the surface of liquid lithium under tension, forming a more dense passive film of Li2O, thus mechanically isolating the reaction interface. This phenomenon greatly hinders the further reaction of liquid lithium with the framework surface, thus reducing the infiltration efficiency. On the other hand, the Li2O passivation film encapsulating liquid lithium reduces the mobility of liquid lithium and promotes its diffusion in the 3D framework to be greatly hindered by kinetics. In addition, when the Li2O content in the composite electrode is high, it will also have a serious impact on the electronic conductivity of the electrode, resulting in poor electrochemical performance[44]. Therefore, the thickness of the oxide modification layer cannot be ignored. On the other hand, when the thickness of the oxide modification layer on the surface of the 3D framework is thin, liquid lithium gradually spreads on the surface of the framework under the chemical force, and the surface Li2O passive film is broken by tension. At this time, due to the low amount of Li2O formed on the reaction interface, the broken Li2O film structure on the surface of liquid lithium can not be quickly replenished, so that the effective reaction interface increases, the contact angle of liquid lithium decreases, and the lithium wetting process can be carried out normally. In addition to the oxide modification layer material, other types of modification layers, such as elemental metals, alloys, etc., should also consider the impact of thickness. Even on the premise of effectively improving the wettability of 3D framework lithium, the mass per unit area of composite 3D framework is also one of the important reference standards for commercial metal lithium electrodes.
2.1.4 Skeleton surface morphology
In addition to the melting temperature, chemical reaction thermodynamics, the thickness of the modified layer and other factors, the surface morphology of the 3D framework also affects the infiltration process of liquid lithium to a certain extent. It is believed that liquid lithium is more likely to diffuse along smooth surfaces, while rough surfaces hinder the migration of lithium. In addition, during the hot melt infusion process, there is gas in the rough structure or pores of the skeleton surface, which further hinders the diffusion of liquid lithium. Here, the relationship between the surface roughness and the liquid lithium contact angle can be expressed by Cassie's equation (1-2):[49]
Where θs is the contact angle of a droplet on a smooth surface, θr is the contact angle of a droplet on a rough surface, and fs is the surface area fraction occupied by a solid per unit area. Thus, a rougher surface corresponds to a smaller fs. According to the equation, the smaller the cosθr, the larger the θr, and thus the poorer the wettability of liquid lithium. However, the chemical properties of the wetted surface are not defined in the above equation, and the relationship between the surface roughness of the 3D framework and the wettability of lithium may change when the surface is chemically reacted.
On the other hand, some studies have shown that the physical force between liquid lithium and 3D skeleton during hot melt implantation can be considered as a capillary phenomenon. According to the theory of fluid mechanics, the capillary effect can be described by equations (1-3):[50]
Where H is the rise height of the liquid in the capillary, γ is the surface tension of the liquid, θ is the contact angle, ρ is the density of the liquid, G is the acceleration of gravity, and R is the radius of the capillary. It can be seen that when the contact angle decreases and H increases, the capillary action increases, corresponding to the rapid lithium infiltration process. At the same time, according to the Wenzel model, increasing the roughness will make the hydrophobic surface more hydrophobic and the hydrophilic surface more hydrophilic. Therefore, when the Wenzel model is applied to the lithium wetting process, it can be considered that when the 3D framework of the surface modified lithiophilic layer is used, the higher the roughness of the modified layer is, the better the lithiophilicity is, thus producing a larger capillary force, which shows an improved lithium wettability. In our research group, ZnO was used as the research medium, and the scientific mechanism of the surface roughness of the lithiophilic modified layer on the improvement of lithium wettability was studied by comparing the effects of ZnO modified layers with different morphologies on the wettability of carbon-based framework lithium[51]. Firstly, ZnO can spontaneously react with liquid lithium at 300 ℃ to form Zn and lithium oxide, and Zn can be further alloyed with liquid lithium (LiZn) to reduce the interfacial energy of liquid lithium and increase the wetting rate. Therefore, ZnO is a kind of lithiophilic substance. When the morphology of ZnO is changed to increase the specific surface area of the modified layer, the corresponding lithium wetting efficiency can be theoretically improved according to formulas (1-2) and (1-3). By comparing the hot melt impregnation process, it is found that the spinous ZnO modified layer with relatively high surface roughness has the most obvious ability to improve the lithium wettability of the carbon-based framework, while the ZnO modified layer with low surface roughness can not effectively improve the lithium wettability rate of the framework (Fig. 4). To sum up, when the 3D framework has a lithiophilic surface or is covered with a lithiophilic modification layer, increasing the surface roughness can further promote the diffusion kinetics of liquid lithium in the 3D framework.
2.2 Metal and carbon-based 3D frameworks
In the work on 3D framework/metal lithium composite anode, researchers have reported a large number of various framework structures for pre-storage of metal lithium, and these frameworks also have the optimization function of regulating lithium deposition/dissolution behavior to achieve non-dendritic, small volume change of metal lithium anode. By summarizing, the current skeleton materials can be divided into metal-based skeleton and carbon-based skeleton. In the metal skeleton, the most common is the three-dimensional structure of nickel foam, copper foam and metal fiber mesh. In addition, the porous metal sponge obtained by dealloying is also regarded as a high-order skeleton structure. There are many choices of carbon-based frameworks, which can be divided into woven frameworks (carbon fiber cloth, carbon nanotube paper, graphitized carbon felt, etc.) And stacked frameworks (graphene sponge, graphene paper, mesoporous carbon, hollow carbon spheres, etc.). Compared with the carbon-based framework, the metal-based framework has the advantages of higher mechanical strength and framework continuity, and excellent physical and chemical stability and thermal stability, thereby having better electronic conductivity.
2.2.1 Metal-based 3D skeleton
Chi et al. prepared a foam nickel/metal lithium composite electrode (Li-Ni) by hot melt infusion, as shown in Figure 5A[52]. Thanks to the continuous 3D framework structure and low surface energy of nickel foam, liquid lithium can climb along the framework surface and gradually diffuse into the spatial structure of the framework. At the same time, the melting point of nickel foam (1400 ℃) is much higher than the melting temperature of lithium, which ensures that the skeleton does not decompose during the hot melting process. In addition, they found that the porous structure of nickel foam can provide a large active specific surface area for lithium deposition/dissolution reactions. Moreover, the interconnected 3D structure can provide a smooth ion/electron channel for electrochemical reactions, which is conducive to reducing polarization and interface resistance. In order to further improve the lithium wettability of nickel foam framework, Zhang et al. Assembled CuO with thorn-like structure on the surface of nickel foam[53]. They first deposited a dense layer of copper on the surface of the original nickel foam by magnetron sputtering, and then obtained a CuO modified layer with thorn-like array structure by oxidation heat treatment. In order to maintain the stability of CuO in the process of lithium deposition/dissolution, they used hydrogen plasma technology to reduce the surface of CuO nano-thorns into ultra-thin Cu layers, which also improved the electronic conductivity of the framework. The hot melt infusion test found (Fig. 5B) that liquid lithium can rapidly infiltrate the composite framework (Cu-CuO-Ni), and the driving force of infiltration is derived from the chemical reaction between part of CuO and liquid lithium. Moreover, the large specific surface area of Cu-CuO-Ni framework can also provide a strong capillary force for the diffusion of liquid lithium. In order to improve the lithium wettability of the original nickel foam, we designed and synthesized a nickel foam skeleton (CONF) modified by thorny CoO fibers[54]. Fig. 5C shows that the lithium implantation process efficiency of the CONF framework is significantly higher than that of the pristine nickel foam. Under the capillary action, the process of liquid lithium penetrating into the CONF structure only takes 5 s, and the formed CONF-Li electrode has a distinct multi-level structure. SEM characterization showed that the CoO nanofibers still maintained the thorn-like morphology after lithium implantation and were covered by the metal lithium film. This prevents the skeletal synapses from penetrating the septum. In addition, the large specific surface area of the porous layer can disperse the local current density on the surface of the electrode, thereby reducing the reaction polarization and avoiding the formation of large-size lithium dendrites.
Copper foam has a 3D spatial structure similar to nickel foam. However, due to its relatively poor lithium wettability, copper foam can not be directly used to prepare composite metal lithium anode by hot melt infusion method, so its surface needs to be modified by lithium wettability. Zhu et al. improved the liquid lithium infiltration efficiency of copper foam framework by loading graphene/ZnO[55]. Upon alloying (fig. 6a), ZnO greatly improves the lithium wettability of the lithiophobic framework. In the formed composite electrode (CuFG @ Li) structure, metal lithium can not only adhere to the framework, but also fill the framework pores, forming a dense electrode structure. In addition, the secondarily stacked graphene network not only has stable physical and chemical properties, but also can be used as a framework for carrying metal lithium to provide more pore characteristics for the interior of the electrode, thereby being beneficial to confining the metal lithium in a small area and avoiding the formation of dendrites. Similarly, Deng et al. modified the copper foam skeleton with spinous Cu2O nanowires (Cu2O NWs/CF) (Figure 6 B), and verified the role of this modified layer in improving the electrochemical performance of the electrode[56]. In the process of loading lithium, a rapid lithium infiltration process occurs only after the edge of the Cu2O NWs/CF skeleton contacts the liquid lithium. In addition, the Li-Cu alloy formed after lithium loading has high lithiophilicity, which has a certain regulatory effect on the nucleation and deposition behavior of lithium. In addition to the foam-type framework, the woven metal nanowire mesh can also be used as a 3D framework for storing metallic lithium. As shown in fig. 6C, Ma et al. proposed a Cu-Ni core-shell nanowire network framework (Cu @ Ni NWs) with high lithium wettability and high stability to reduce the volume change of lithium and reduce the local current density on the electrode surface[57]. After the copper nanowires are coated with a metal nickel layer, the lithium wettability of the 3D framework is improved due to the reduction of the surface energy, so the unit time for liquid lithium to completely infiltrate the 3D framework is also reduced to a few seconds. The Li-Cu @ Ni composite anode not only has a large specific surface area, but also effectively inhibits the stress fluctuation in the reaction process due to the spatial network formed by the winding of copper nanowires. In the carbonate electrolyte, the Li-Cu @ Ni anode shows excellent electrochemical performance and can be stably cycled more than 500 times at a high current density of 3 mA/cm2.
The surface of molten liquid lithium is always covered with a passivation layer during the hot melt infusion process, which leads to the decrease of the surface energy of liquid lithium. The decomposition temperature of the inert layer is much higher than the melting temperature of metal lithium, so that a two-phase structure of a passive film "skin" and a liquid lithium "filling" is formed. In response to this phenomenon, our research group first proposed a reverse hot melt infusion method (Fig. 7), and prepared an ultra-thin metal lithium film anode supported by 2D metal mesh[58]. The Mo net/Li metal film (MLF) electrode has a typical asymmetric structure with an inert and dense lithium oxide layer on the side in contact with the separator and a 2D Mo net on the opposite side. In this structure, the oxide film on the surface of lithium metal has the function of stabilizing the electrode/separator interface, while the molybdenum mesh on the reverse side can support the lithium metal film as a skeleton, and the 2D woven structure can also reduce the local current density of the electrode and avoid the regional deposition of lithium ions. The COMSOL simulation results show that the interface between lithium film and molybdenum mesh has higher reactivity than that of lithium oxide due to the high electronic conductivity of 2D molybdenum mesh, which realizes a unique "top-down" lithium deposition/dissolution mode and effectively inhibits the growth of lithium dendrites and the formation of dead lithium at the electrode/separator interface.
2.2.2 Carbon-based 3D framework
The metal skeleton itself has a large mass, which reduces the mass ratio of metal lithium in the composite electrode, so the actual unit capacity density of the electrode is low. According to statistics, in most of the reported composite anodes with metal skeleton (hot melt infusion method), the loading of metal lithium per unit area is generally 20 wt% ~ 50 wt%. To some extent, this weakens the capacity advantage of lithium metal as a high-capacity anode material. Therefore, less dense and lighter carbon-based skeletons have attracted more and more attention. In addition, it is a hot spot and demand in the field of current research and industrial application to increase the metal lithium loading in the composite electrode to more than 90 wt% while fulfilling the 3D skeleton to regulate the lithium deposition behavior.
Graphene has the advantages of light weight, high electronic conductivity and large specific surface area, so the most representative carbon-based framework is the graphene network framework. Cui et al. Used layered graphene oxide paper as the host framework, and successfully predeposited metal lithium between the graphene sheets by hot melt infusion, and the resulting composite electrode exhibited a nanoscale metal lithium/graphene periodic stacking structure (Fig. 8A)[59]. In addition, when the liquid lithium is in contact with the graphene oxide framework, the oxygen-containing functional groups and residual moisture in the graphene structure are vaporized instantaneously at high temperature, and the resulting pressure difference drives the flow of the liquid lithium in the framework. On the other hand, the graphene stack structure forms a gap under the action of pressure release and regularly stores metal lithium in the stack structure. This effectively improves the utilization rate of the reaction interface and skeleton of the electrode, not only ensures a higher mass ratio of metal lithium, but also disperses the current density and inhibits the growth of lithium dendrites. In addition to graphene materials, Ji et al. Designed an ultra-thin and flexible self-supporting carbon nanotube paper 3D skeleton (CNT paper) by floating catalyst chemical vapor deposition method, and applied it to composite electrodes with high lithium content[60]. The CNT framework has a large specific surface area (118 m2/g), high electronic conductivity (1.1×105S/m) and good tensile strength (27 MPa), which can not only maintain the structural integrity in the hot melt infusion, but also efficiently disperse the local current. As shown in Fig. 8B, at a metal lithium loading of 81 wt%, the interlaced network of CNT skeleton elasticity can be accompanied by reversible spatial changes in the lithium deposition/extraction cycle, so that the metal lithium is always in close contact with the skeleton. Compared with 3D frameworks such as nickel foam, the elastic CNT paper effectively improves the effective contact area between the framework and metal lithium in the cycle, and plays a key role in stabilizing the electrode reaction interface. For the same purpose, our group prepared a hollow carbon-based 3D framework with relatively small specific surface area, and prestored lithium metal to form a high-capacity lithium metal composite electrode (OCCu-Li) by hot melt infusion method[61]. As shown in fig. 8C, soybean oil was used as the carbon source, and a layer of polymer pyrolytic carbon was successfully coated on the surface of the metal foam skeleton by a simple chemical vapor deposition method, and a hollow 3D carbon-based skeleton was obtained by etching. It was found that the OCCu-Li composite electrode contained 94 wt% lithium metal, corresponding to a high theoretical capacity of 3628 mAh/G.
On the other hand, Go et al. Used commercial carbon fiber cloth as the 3D framework of lithium metal from the application point of view, and improved the lithium wettability of the framework by introducing nano-defects[62]. They first heat treated the carbon fiber cloth at an ambient atmosphere of 500 ℃ for 4 H to remove impurities, and then directly soaked it in liquid lithium to form a CC/Li composite electrode (Fig. 9a). Compared with the original skeleton, the surface of the carbon fiber cloth after high temperature treatment is rich in a large number of nano-defects and microporous structures, so that proper capillary action is generated in the hot melt infusion process, and the infiltration efficiency of liquid lithium is improved. Zhang et al. introduced a SnO2 layer on the surface of the carbon felt skeleton by a simple liquid phase method to improve the poor lithium wettability of the carbon-based skeleton (Fig. 9b)[63]. Liquid lithium can react with the SnO2 modification layer during hot melt infusion, which removes the kinetic barrier to lithium infiltration. The metallic lithium loading in the composite electrode was about 18~26 mg/cm2, corresponding to 52% of the total electrode mass. In addition, the Li-Sn alloy formed on the surface of the framework is similar to other lithium-based alloys, which can reduce the nucleation potential of lithium and promote uniform lithium deposition behavior. Some unique biomass carbons can also be used in composite metal lithium electrodes. For the first time, Hu et al. used Wood as the source material to obtain a 3D carbon-based framework (C-Wood) with a porosity of 73% after high temperature carbonization, and its internal pores have the characteristics of vertical run-through ordered distribution (Fig. 9c)[64]. After ZnO modification, the infiltration process of liquid lithium has been significantly improved. In addition to limiting the volume change of lithium metal during the charge-discharge process, the through-type electrode structure of C-Wood can improve the stability of the electrode/separator interface, so that the lithium dendrite is not easy to pose a direct threat to the separator. To sum up, carbon-based 3D skeletons are rich in variety and have a wide range of research fields. However, compared with the advantages of lightweight texture, the poor lithium wettability and unsatisfactory mechanical properties of carbon-based skeleton have a profound impact on the preparation of composite electrodes. Therefore, the high-performance carbon-based 3D framework still needs to be further developed to match the commercial production requirements.
2.3 Method for improving wettability of 3D framework lithium
In the previous section, we introduced a series of 3D framework/metal lithium composite electrodes prepared by hot melt infusion method, in which the lithium wettability of 3D framework is the key factor that dominates the smooth diffusion of liquid lithium into the internal space of the framework. The skeleton with high lithium wettability is characterized in that the skeleton itself or a surface modification layer thereof can reduce the interfacial energy with liquid lithium in a physical or chemical manner,O as to improve the compatibility between the 3D framework and the 3D framework, promote the flow of the liquid lithium in the framework, and enable the 3D framework to be rapidly infiltrated by the liquid lithium to form a compact composite electrode. At present, the main methods to improve the wettability of 3D framework lithium are oxide modification, elemental modification and other surface modification.
2.3.1 Oxide modified
In order to improve the lithium wettability of the 3D framework with high mechanical properties, the preparation of a modified layer on the surface of the framework is considered to be an effective solution. At present, there are many kinds of modification layers that can improve the wettability of lithium, among which the oxide modification layers with nanostructure have attracted wide attention, such as SiO2, ZnO, CuO, Al2O3, SnO2 and Co3O4[57,65,66]. When in contact with liquid metal lithium, these oxide modified layers can be reduced by liquid lithium to form metal simple substances and Li2O, as shown in formula (1-4). Among them, part of the formed metal elementary substance can further carry out alloying reaction with liquid lithium (1-5) to provide driving force for lithium infiltration behavior[64,67].
(M:Si、Zn、Sn、Al、Co、Cu、Ni…)
(M:Si、Zn、Sn、Al…)
Because the above reaction is spontaneous in the hot melt infusion process of liquid lithium, the corresponding Gibbs free energy change is negative, so that the interfacial energy and tension of the contact surface are reduced by chemical driving, thereby improving the infiltration efficiency of liquid lithium on the 3D framework. Table 1 summarizes the schemes of related oxides to improve the wettability of the framework lithium. In addition, the lithium-based alloy formed on the surface of the framework can also play a role in regulating the nucleation of lithium and inhibiting the growth of large-size spinous lithium dendrites. However, the Li2O formed during the hot melt infusion process will reduce the electronic conductivity of the electrode, which will also affect the wettability of lithium. Therefore, the thickness of the oxide modification layer should be strictly controlled.
Li et al. theoretically calculated the Gibbs free energy of the reaction between Co3O4 and liquid lithium to be -197.5 kcal/mol (Fig. 10a), indicating that Co3O4 has the possibility of improving the wettability of framework lithium[65]. After hot melt infusion, they found that the Co nanofibers formed by reduction not only dispersed the lithium ion flow on the surface of the electrode, but also increased the effective contact area between the electrode and the electrolyte. This effectively suppresses the formation of lithium dendrites and reduces the polarization of the electrochemical reaction. Rate measurements show that the Co-Cs/Li symmetrical battery can be cycled stably for 120 cycles at a current density of 20 mA/cm2, and the deposition potential of lithium is only 280 mV, which is much lower than that of the symmetrical battery with lithium sheet anode (800 mV).
Our group prepared a CCOF-Li composite electrode with 3D framework by hot melt infusion method to inhibit the growth of lithium dendrite and the formation of dead lithium layer (Fig. 10B)[78]. The preparation method of the CCOF framework is relatively simple, and a cuprite layer with a thickness of about 100 nm can be formed on the surface of the copper foam framework by a simple heat treatment, so that the lithium wettability of the framework can be improved by a chemical means. In addition, the micron-sized copper hexagonal sheets formed by reduction are uniformly distributed in the CCOF-Li composite electrode, which plays a role in dispersing the local current density and realizing the homogenization of the reaction interface. In addition to using an independent 3D skeleton, the 3D alloy skeleton of the composite electrode can also be constructed by hot-melt oxide powder. Luo et al. mixed SiO2 powder directly with liquid lithium to form a Li22Si5 alloy block containing metallic lithium as a composite electrode (fig. 10C)[66]. This study found that if the grain size in the composite electrode is smaller than the critical size, the stress due to the volume fluctuation can be accommodated by the electrode, and the active particle cracking phenomenon does not occur. Therefore, in the BNL electrode formed by secondary melting, not only the alloy grain size is smaller, but also the spatial network formed by the interconnection of particles enhances the mechanical strength and fatigue resistance of the electrode, thus preventing the electrode from crushing during cycling. In addition, the diffusion barrier of lithium atoms on the surface of Li22Si5 alloy is 0. 11 eV, which is smaller than that of lithium atoms on the surface of lithium metal (0. 39 eV), so the Li22Si5 framework in BNL has good lithiophilicity.
2.3.2 Simple substance modification
Compared with oxide, the advantage of simple substance as lithium wettability modification layer is that it does not produce Li2O phase in hot melt implantation. Wang et al. collated the Gibbs free energies of reactions of different elementary substances with liquid lithium, as shown in Fig. 11[80]. It is found that both green and yellow elements can be alloyed with liquid lithium above 200 ℃, which indicates that they can be used as a modification layer to improve the wettability of framework lithium. This work provides a detailed summary of the scientific mechanism of 3D framework lithium wettability, which provides an important theoretical reference for the development of composite metal lithium anode.
Table 2 summarizes the currently reported schemes for improving the lithium wettability of 3D frameworks with relevant elemental modification layers. Representatively, Cui et al. loaded a Si film with a thickness of 20 nm on carbon fiber paper by chemical vapor deposition to improve the lithium wettability of the framework[39]. As shown in fig. 12A, liquid lithium can be rapidly filled in the spatial structure of the carbon fiber paper and is uniformly dispersed thanks to the chemical driving force initiated by the formation of the Li-Si chemical bond by the alloying reaction. At the same time, the Li-Si alloy formed on the surface of the framework has high lithiophilicity, which inhibits the formation of dendrites. Zhang et al. improved the lithium wettability of carbon fiber cloth by loading Ag nanoparticles, taking advantage of the fact that Ag and liquid lithium can form an alloy. As shown in fig. 12b, the carbon fiber cloth with the coral-like Ag modified layer can be rapidly wetted by liquid lithium while forming a Li-Ag alloy on the surface of the skeleton[81]. Compared with the binding energy of Li — C bond (-1. 14 eV), the binding energy of Li-LiAg bond is larger (-2. 07 eV), indicating that the nucleation of lithium ions on the surface of LiAg alloy is less hindered. Therefore, the surface morphology of the CF/Ag-Li composite electrode is more uniform after repeated charge-discharge without obvious dendrite formation.
2.3.3 Other modification methods
Functional groups (— OH, —SO3H, —NH2) can react with molten lithium to form new bonds, thereby improving the lithium wettability of the 3D framework. Huang et al. prepared an N-doped graphene framework with 3D nanoporous structure by nanoporous nickel-based chemical vapor deposition[86]. The introduction of N element causes the formation of more nano-holes in the graphene layered structure, so the liquid lithium rapidly diffuses into the 3D framework under the capillary effect to form a composite electrode (Fig. 13), and the metal lithium loading is as high as 95.3 wt%, which is equivalent to the theoretical capacity of 3678 mAh/G. Because the N-doped graphene framework has more excellent lithiophilicity, mass transfer channel and conductivity, the full cell of the NG-Li anode shows significantly improved rate performance and voltage hysteresis. See Table 3 for other related work.
2.4 Existing Problems of Composite Electrodes Prepared by Hot Melt Infusion
Lithium metal composite anode prepared by hot melt infusion method has outstanding advantages, which not only has high lithium storage efficiency and lithium metal loading, but also can promote the uniform and dense distribution of lithium metal in the skeleton 3D structure without reaction activity gradient. In addition, in addition to inhibiting the volume change, the highly lithiophilic 3D framework can effectively reduce the local current density on the electrode surface, improve the deposition/dissolution behavior of lithium, and disperse the lithium ion flow at the reaction interface to inhibit the formation and growth of lithium dendrites in local areas. However, the method still has the following problems to be solved.
(1) The introduction of 3D framework significantly increases the effective reaction interface of the composite anode, which leads to the formation of more SEI although the local current density is reduced. Therefore, the aggravation of side reactions consumes more electrolyte and causes the electrolyte to dry up rapidly, leading to battery failure. It can be seen that the performance of 3D framework/metal lithium composite anode will be seriously degraded under the condition of poor electrolyte. Therefore, the stability of the reaction interface structure and the physical and chemical properties of the SEI film are very important for the development and application of the composite electrode.
(2) In commercial lithium-ion batteries, the highest capacity per unit area of the commonly used anode is about 6 mAh/cm2, while the capacity of the currently reported 3D framework/metallic lithium composite anode is much higher than this, resulting in low actual utilization of metallic lithium. Therefore, besides studying the growth mechanism of lithium dendrite, we should pay more attention to the utilization of lithium metal. At the same time, the composite electrode with reasonable thickness has better practical value, and the thickness of the composite electrode prepared by the hot melt infusion method mainly depends on the thickness of the 3D skeleton. Therefore, the preparation of an independent 3D skeleton with high mechanical properties and a thickness of 20 ~ 50 μm is the main difficulty to be overcome in this field.
(3) In all-solid-state batteries, the interface instability caused by the structural evolution of the lithium metal anode/solid-state electrolyte interface is one of the key issues restricting the development of solid-state batteries. However, the introduction of the 3D skeleton into the metallic lithium anode may aggravate the instability of the interface[89]. The key point is that the exposed 3D pore structure leads to the decrease of the contact area of metal lithium/solid electrolyte after the lithium removal from the anode, which affects the interfacial ion transport.
3 Conclusion and prospect
The design and development of high-performance lithium metal anode is one of the keys to the development of next-generation lithium battery technology. The 3D framework composite metal lithium anode has attracted much attention due to its ability to effectively improve the lithium deposition behavior and inhibit the formation and growth of lithium dendrites. In this paper, recent studies on the fabrication of 3D framework composite anodes by hot melt impregnation are summarized, and the main factors affecting the lithium wettability during the hot melt impregnation process are analyzed in detail, including the melting temperature, the surface properties of the framework, the lithium wettability modification layer, and the framework morphology. In short, the Gibbs free energy of the reaction between 3D framework and molten lithium in the key step of hot melt infusion is the basis for judging the excellent wettability of lithium. In addition, the lithium wettability of the framework can be improved by increasing the melting temperature and the lithiophilic surface roughness. By summarizing the current research work on metal-based and carbon-based framework composite anodes prepared by hot melt infusion method, the effects of different types of surface modification layers on improving the lithium wettability of framework and inhibiting the growth of lithium dendrite were compared. It must be admitted that the introduction of the 3D host framework in metallic lithium alone is not once and for all. Although this can inhibit the volume change of lithium dendrite and metal lithium, it will also cause more complex electrode/electrolyte interface reactions, which will affect the electrochemical performance of the battery. Therefore, in future research, it is necessary to combine with other improvement strategies (electrolyte modification, solid-state electrolyte, artificial SEI film, etc.) To improve the electrochemical performance and safety of metal lithium anode more effectively, so as to realize the practical application of metal lithium batteries as soon as possible.
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