Lithium metal anode has high theoretical capacity and low electrode potential. Compared with traditional lithium ion batteries, lithium metal batteries have higher energy density and a wider choice of cathode materials, which can be matched with traditional lithium-containing polyanion frameworks and layered oxide materials, or with emerging ones. Lithium-free fluoride material compound with theoretical energy density. Generally, lithium metal batteries use electrolyte as the lithium ion transmission medium, and the main components are lithium salts and organic solvents. However, due to the many side reactions of the liquid medium and the flammability of organics, this type of battery has certain safety risks. Replacing the electrolyte with a solid electrolyte as a lithium ion transport conductor can improve the safety and stability of the battery and expand the temperature range of lithium metal batteries. Among them, the ceramic-based Garnet-type solid electrolyte is a good choice. In recent years, the doped lithium lanthanum zirconium oxide (Li7La3Zr2O12, LLZO) solid electrolyte has high room temperature ion conductivity, simple synthesis process, and electrochemical stability. The advantages of wide window and no redox active elements are the main candidates for ceramic-based solid electrolytes. However, the LLZO solid electrolyte faces problems such as unstable air, easy passivation of the surface, and poor contact with the lithium metal interface, resulting in a large impedance at the electrolyte/lithium negative electrode interface, which hinders the interface transmission of lithium ions, and has a limited interface Contact easily causes uneven deposition of lithium ions, leading to the growth of lithium dendrites and affecting the service life of the battery. Therefore, the elimination of passivation or improvement of the lithium affinity of the LLZO solid electrolyte/lithium metal anode interface is an urgent problem to be solved.
Recently, the team of Li Chilin, a researcher at the Shanghai Institute of Ceramics, Chinese Academy of Sciences, made a series of progress in the interface modification of ceramic-based solid-state batteries and the activation of the lithium-fluorine conversion reaction.
The team proposed the idea of ​​"eutectic alloy induced solid-solid convection" to modify the LLZO/Li interface, which achieved a high degree of healing of the solid-solid interface in the electrochemical process, and on this basis, successfully driven the conversion reaction Highly reversible cycle of type iron trifluoride (FeF3) cathode in ceramic-based solid-state battery system. The sodium element and the lithium element belong to the same main group, with similar chemical properties, and the metallic sodium is soft and easy to handle. The lithium-sodium eutectic alloy can form a good interface contact with LLZO. It can be seen from the lithium-sodium binary phase diagram that lithium and sodium can form a eutectic alloy in almost any ratio, so there is no need to specifically adjust the lithium-sodium ratio, which is simpler and more flexible than other alloy modification methods reported in the literature. Due to the different concentration gradients of the sodium and lithium crystal domains, solid-solid convection is prone to occur between the two, so that the electrolyte/negative electrode interface can always maintain a relatively stable homogeneous alloy state, thereby maintaining a tight alloy-ceramic interface contact. The symmetric battery modified by the eutectic alloy can be cycled stably for more than 3500 hours, and the interface resistance and overpotential are only 18.98 Ω·cm2 and 10.8mV at 60℃. The excellent interface durability promotes the successful operation of Li-Na/LLZO/FeF3 solid-state batteries, which show good cycle and rate performance, 100, 150, 200, 300, 400 and 500 μA·cm- at 60℃ The capacity of 507.3, 422.0, 383.4, 350.6, 297.6 and 275.1 mAh·g-1 can be released respectively at the current density of 2, and the capacity at 500 μA·cm-2 still exceeds the theoretical capacity of traditional LiFePO4 (175 mAh·g- 1) It shows the advantages of conversion FeF3 cathode material, and also provides the possibility for the future application of ceramic-based lithium-fluorine conversion solid-state batteries. Related results were published in ACS Energy Letters.
The team proposed a "candle soot grilled ceramic" mode to modify the LLZO/Li interface strategy, which significantly thinned the passivation layer on the ceramic surface and realized the ultra-long reversible cycle of the "conversion type" lithium fluoride solid-state battery . After LLZO is in contact with air, it easily reacts with water and carbon dioxide in the air, forming a passivation layer containing LiOH and Li2CO3 on the surface. This passivation layer severely affects the contact between Li and LLZO and blocks the lithium at the interface The ion transmission channel causes the interface impedance of the battery to be too large, and the battery performance is severely limited. Therefore, the removal of the passivation layer is one of the important directions of current research on the modification of LLZO/Li interface. In this context, the team proposed a simple candle flame vapor deposition method. In the high temperature environment generated by candle burning, the Li2CO3 passivation layer on the surface of LLZO can be reduced by the candle soot deposited on the surface, which has a polycrystalline graphitization structure. The soot carbon black layer can generate LiC6 crystal domains after lithiation, which has ion/electron mixed conductivity, which is conducive to the high flux transmission of lithium ion flow in the middle layer of the interface. This interface-modified Li/CS-LLZO/FeF3 solid-state battery exhibits excellent long cycle and rate performance. Its initial reversible capacity can reach 500 mAh·g-1, and its cycle life can reach at least 1500 cycles at 200 μA. ·The reversible capacity after 700 cycles at cm-2 current density remains at 201.0 mAh·g-1. The cycle performance of the ceramic-based solid Li-FeF3 battery can even exceed the liquid Li-FeF3 system reported in the literature. Related results were published in ACS Applied Materials & Interfaces.
The solid-state battery framework can produce a better interface confinement effect on the positive extreme conversion reaction products, and can effectively inhibit the dissolution of active materials in the electrolyte. In addition, the lithiated negative terminal interface interlayer has excellent mixed conductivity and interface wettability, which can effectively inhibit the growth of lithium metal dendrites. These ensure the long-cycle performance of ceramic-based lithium-fluorine conversion solid-state batteries, and the ceramic-based solid electrolyte expands the future development direction of fluorine-based batteries.
The first author of the related results is Zhang Yang, a PhD student in Shanghai Institute of Ceramics. The research is funded and supported by the National Key Research and Development Program, the National Natural Science Foundation of China, and enterprise cooperative research and development projects.
Li-Na/LLZO/FeF3 solid-state battery structure, schematic diagram of solid-solid convection of eutectic alloy, rate and cycle performance of Li-Na/LLZO/FeF3 battery
Schematic diagram of candle flame vapor deposition ceramic surface, long-cycle performance of Li/CS-LLZO/FeF3 solid-state battery
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