Solid state lithium sulfur batteries combine low cost, high specific energy, and high safety, and are a key research direction for the next generation of high specific energy power batteries internationally. However, the electronic insulation properties of sulfur positive electrode materials and the enormous volume expansion during electrochemical processes make it difficult for mass and charge to achieve stable transfer at changing solid-state interfaces, hindering the further development and application of solid-state lithium sulfur batteries.
In response to this challenge, Professor Ping Liu's research group at the University of California, San Diego proposed a sulfur crystal structure control strategy, designed a crystal structure with iodine molecules embedded in sulfur vacancies, and synthesized a new S9.3I positive electrode material with high electronic conductivity and solid-state interface in-situ repair function, breaking through the bottleneck of solid-state lithium sulfur batteries and achieving high rate and long cycle stability of solid-state lithium sulfur battery devices.
The related research results are titled "Healable and conductive sulfur iodide for solid state Li-S batteries" and were published in the journal Nature on March 6, 2024. The corresponding authors of the paper are Professor Liu Ping and Professor Shyue Ping Ong, with Dr. Zhou Jianbin as the first author and Dr. Chandrapa as the co first author.
The research group can obtain S9.3I crystal material through simple grinding, melting at 80 ℃, and cooling steps. DSC and in-situ heating XRD confirmed that S9.3I has a low melting point of 65 ℃. Small angle XRD and X-ray PDF structural analysis confirmed the crystal structure of iodine molecules embedded in sulfur vacancies (Figure 1a-b). The author further demonstrated through theoretical calculations that the structure is thermodynamically stable (Figure 1c), and the embedded iodine molecules effectively reduce the interaction between S8 molecules in the sulfur crystal, thereby lowering the melting point of the sulfur material. X-ray absorption spectroscopy (XAS) analysis confirmed that iodine and sulfur did not form a bond (Figure 1d). In addition, electrochemical testing analysis shows that S9.3I has a semiconductor level electronic conductivity (5.9 × 10-7 S cm-1, Figure 1e), which is about 11 orders of magnitude higher than the conductivity of sulfur. Density of states analysis proves that the embedded iodine molecules introduce a semiconductor bandgap structure into the wide bandgap electronic band structure of sulfur, thereby enhancing electronic conductivity.
Figure 1: Structural analysis of S9.3I material. Image source: Nature
The electrochemical performance test analysis of solid-state lithium sulfur batteries shows that during the electrochemical cycling process, the S9.3I positive electrode exhibits higher specific capacity, lower overpotential, and better redox reaction kinetics compared to the elemental S positive electrode. The electrochemical working curves also demonstrate different redox reaction mechanisms (Figure 2a-b). In terms of long cycle testing, by using certain heating and cooling strategies to repair the solid interface of the positive electrode, the S9.3I positive electrode can still achieve a capacity retention rate of 87% after 400 cycles at room temperature.
Figure 2: Electrochemical performance of S9.3I and element S positive electrode material in solid-state batteries. Image source: Nature
Figure 3: Working mechanism of S9.3I positive electrode in solid-state battery. Image source: Nature
作The working mechanism of S9.3I cathode material during electrochemical charge and discharge processes was studied using non in situ XAS (Figure 3a) and X-ray photoelectron spectroscopy (XPS, Figure 3b) systems. Research has found that during the discharge process, the sulfur element in S9.3I first transforms from zero valent sulfur to lithium polysulfides. As the discharge progresses, it ultimately transforms into lithium sulfide and long-chain lithium polysulfides. This mechanism is highly reversible during the charging process. However, the positive electrode of element S does not involve lithium polysulfide in the electrochemical process, which is consistent with the traditional understanding of the working mechanism of solid-state lithium sulfur batteries that directly convert elemental sulfur into lithium sulfide. Therefore, the introduction of iodine has also changed the reaction mechanism of traditional solid-state lithium sulfur batteries, and the polysulfide lithium in the electrochemical process is crucial for improving the performance of solid-state lithium sulfur batteries. The cryo electron microscopy results of S9.3I material after lithiation (Figure 3c) demonstrate that sulfur, iodine, and lithium elements did not undergo phase separation after lithiation.
Figure 4: Solid state interface repair mechanism of S9.3I positive electrode. Image source: Nature
The author further investigated the changes in the solid-state interface of the S9.3I positive electrode during electrochemical cycling. In the original state, the interface between the positive electrode and the solid electrolyte is in good contact (Figure 4a). After 50 cycles at room temperature (Figure 4b), a gap is formed at the interface, which hinders the transfer of mass and charge at the interface and increases the interface impedance. However, after heating (Figure 4c), the interface was repaired again, and electrochemical impedance analysis also confirmed the interface repair process (Figure 4d). DSC thermal analysis (Figure 4e) further demonstrated that the S9.3I positive electrode can achieve in-situ interface repair through heating liquefaction.
In summary, the research team has developed a novel sulfur crystal structure control strategy based on the embedding mechanism of lattice vacancy molecules (Figure 2a), and designed and synthesized a conductive, solid-state interface repairable S9.3I cathode material, effectively solving the key scientific challenge of solid-state lithium sulfur batteries. In addition, the introduced iodine element also promotes the generation of lithium polysulfides in the electrochemical process, effectively improving the electrochemical performance of solid-state lithium sulfur batteries. The discovery of the new S9.3I positive electrode material provides a new approach for the development of high specific energy solid-state lithium sulfur batteries.
Editor: Sichuan Jinzhongde Science and Technology Research Institute
Source: Today's New Materials
