Advanced strategies for the synthesis and application of Li3InCl6 in solid-state lithium-ion batteries

Solid-state lithium-ion batteries (SSLBs) have the advantages of high energy density and safety and are considered to be one of the most promising energy storage devices, but their marketization still faces obstacles and their practicality has also been questioned. Because most of the solid-state electrolytes (SSEs) at the core of SSLBs are very hard and cannot form a good interface contact with components such as electrodes under traditional packaging conditions, additional high pressure needs to be applied for SSLB to work properly, which limits the application of SSLB at this stage. In addition, the synthesis of SSEs is also relatively difficult, and most of the time they need to be obtained through high-energy and time-consuming solid phase methods such as sintering or ball milling. Halide solid electrolytes are an emerging class of SSEs, among which Li3InCl6 (LIC) has received the most attention due to its good comprehensive performance. First, we introduced a method for synthesizing LIC by a modified mechanochemical method, aiming to reduce the synthesis time and energy consumption, thereby improving the overall synthesis efficiency of LIC SSE powder. The new method successfully reduced the synthesis time from 20 h to 8 h, and the ball milling speed only needed to be increased from 500 rpm to 700 rpm. Although the particle size of the LIC powder obtained by the new method is larger than that of previous studies, it does not affect its performance. The obtained LIC has a higher ionic conductivity than the LIC synthesized by the traditional method, which increases from 1.21 mS cm-1 to 1.26 mS cm-1. At the same time, it has a lower electronic conductivity, which can effectively slow down the occurrence of short circuit caused by lithium dendrite growth, which is also confirmed by its better performance in symmetrical Li-Li cells. Finally, considering the importance of mechanical properties in practical applications and the fact that no one has done similar tests before, we measured the elastic modulus and hardness of LIC powder to fill the gap in research. Next is the application of LIC in SSB. In previous studies, LIC was still used in SSB by pressing powder into pellets, and a pressure of more than 100 MPa was required to ensure good interlayer contact during operation. This means that both the large-scale preparation and use of LIC are very difficult. To get rid of the harsh process, we used LIC, ethyl cellulose, etc. as raw materials and acetonitrile as solvent to successfully batch synthesize a self-supporting and flexible LIC-ACE SSE film through the liquid phase method. The resulting film has good flexibility and can be easily bent without breaking, and has better moisture resistance than LIC pellets. The ion conductivity of LIC-ACE film at room temperature can reach 0.76 mS cm-1, and it can be stably cycled for more than 900 h in symmetrical Li-Li cells without a buffer layer. Most importantly, after being assembled into SSB, it can be cycled at a pressure of less than 1 MPa, and the capacity retention rate is still 84.8% after 50 cycles at 0.2 C. This study innovatively synthesized a flexible LIC-ACE SSE film, which greatly reduced the pressure required during operation and the difficulty of synthesis, and reduced the proportion of powder tableting steps in the SSB production process, promoting the practical application of LIC SSE. Considering that the pressure applied by LIC-ACE in the previous study still has room for improvement, and inspired by the great contribution of polymer components in traditional composite SSE to the normal pressure working of SSB, we further introduced different proportions of PEO and lithium salt into LIC, and prepared a series of flexible composite LIC-PEO SSE films that can be used in coin cells by slurry casting. Among them, the LIC-PEO SSE film with the best performance has an ionic conductivity of 1.19 mS cm-1 at 35 °C and the Li+ transference number of 0.405. The linear sweep voltammetry test shows that its electrochemical stability window exceeds 5 V. Moreover, it has stronger flexibility and ductility than LIC-ACE, so it can work directly in the coin cell. In the symmetrical Li-Li cells test, it can work stably for more than 620 h, far exceeding pure LIC. At 35 °C, its capacity retention rate is 84.2% after 300 cycles at 0.2 C. Under the same conditions, after 150 cycles at 1.2 C, the capacity retention rate was 80.6%, and it can also perform well at higher current density. LIC-PEO SSE film combines the advantages of polymers and halides, not only showing good performance, but also easy to prepare on a large scale, so it has great application potential.