Solid-state lithium metal batteries (SSLMBs) are promising candidates for next-generation energy storage systems due to their inherent safety and high energy density. However, they still have the problem of poor interface stability, which results in high interface resistance and short cycle life.
Here, Professor Gu Meng of Southern University of Science and Technology, Professor Yang Xuming of Shenzhen University and others jointly developed a new polymer electrolyte (PPE) based on poly(vinylidene fluoride-hexafluoropropylene) (PVDF-HFP), and studied the atomic resolution interface structure between Li and PPE. PPE (PPE-BF) containing LiBF4 and PC plasticizer was selected through a comparative study of PPE containing various salts and additives.
Figure 1. Li|| PPE-BF|| Li、Li|| PPE-BF|| Electrochemical performance of LFP
The study found that the CE value of symmetrical lithium batteries containing PPE-BF was as high as 91.3%, and it could stably cycle for more than 1000 hours under the condition of 0.2 mA cm-2, with an average capacity of 0.2 mAh cm-2. The LFP battery has excellent cycling and rate performance: after 100 cycles at 0.3 C, the capacity retention reaches 78.7%, and the reversible capacity at 5 C is up to 97.5 mAh g-1.
In addition, interfacial features were studied using cryo-transmission electron microscopy (Cryo-TEM) and Li || was found PPE-TFSI SEI is amorphous, with a homogeneous distribution of Li oxides and carbonates, while Li || PPE-BF SEI is a bilayer structure - an amorphous outer layer plus a crystalline inner layer dominated by Li2O, which can effectively transfer Li+ and help prevent continuous side reactions between Li and PPE.
Overall, this work not only highlights the importance of improving interface compatibility for polymer-based solid-state batteries, but also demonstrates the feasibility of obtaining detailed interface structures between lithium metal and polymer-based electrolytes by cryo-EM technology. In the future, Cryo-TEM will be used more to reveal the interface structure of solid-state batteries.
Figure 2. Li|| PPE-TFSI interface characterization