Effect of Lithium Ion in Block Copolymer Electrolytes Phase Separation Process by using 4D-STEM

Polymers that dissolve lithium ions are promising electrolyte materials for the next generation of rechargeable batteries. Both high mechanical stability and high ionic conductivity can be achieved through microphase separated block copolymer electrolytes by using one phase to conduct ions while the other provides rigidity[1]. The ion transport properties of such composite materials are not easily predicted solely from ion transport properties of the conducting domain alone [2], as the microphase and morphology play an important role in electrochemical performance [3, 4]. In previous research, heavy element staining bright field imaging and Energy-filtered transmission electron microscopy spectrum-imaging (EFTEM-SI) have been used to study PS-<i>b</i>-PEO [1, 2, 5]. Here, we utilize cryo-4D-STEM (four-dimensional scanning transmission electron microscopy) to investigate both the polymer morphology as well as the crystallinity as a function of Li concentration. 4D-STEM is a technique by which a nearly parallel electron nanoprobe is rastered across a sample (two dimensions in real space), recording the diffraction pattern (two dimensions in reciprocal space) at each scan point.[6] The diffraction patterns acquired form a 4D dataset provide real space information about the phase distribution and orientation of the polymer materials. It is a powerful tool to unveil the microphase of PS-<i>b</i>-PEO and provide further information about how the structure effects electrochemical performance.[2, 3]<br/><br/>Adding lithium salt into PS-<i>b</i>-PEO is a crucial step for real applications. Here we used mixtures of poly (styrene-block-ethylene oxide) copolymers (SEO) and bis(trifluoromethane)sulfonimide lithium salt (LiTFSI). By utilizing an air-free glove box and a cryo-transfer holder, we have established a workflow that avoids air exposure to these air-sensitive samples. 4D-STEM was later applied to show how the phase and DPs change when lithium concentrations change from r = 0 to 0.1. We calculated and carefully controlled the beam damage and successfully produced 4D-STEM results in cryo-condition. Compared with traditional RuO₄ staining 4D-STEM bright field imaging, 4D-STEM is shown to produce similar phase information without using toxic chemicals. Furthermore, 4D-STEM can provide additional phase distribution and orientation information inside the crystalline phase. The relation between the electrochemical performance and phase distribution, in relation to the crystallinity with and without the Li salt will also discussed.<br/><br/>1. Enrique D. Gomez, Ashoutosh Panday, Edward H. Feng, Vincent Chen, Gregory M. Stone, Andrew M. Minor, Christian Kisielowski, Kenneth H. Downing, Oleg Borodin, Grant D. Smith, and Nitash P. Balsara*, Effect of Ion Distribution on Conductivity of Block Copolymer Electrolytes. Nano letters, 2009.<br/>2. Maslyn, J.A., et al., Limiting Current in Nanostructured Block Copolymer Electrolytes. Macromolecules, 2021. 54(9): p. 4010-4022.<br/>3. Galluzzo, M.D., et al., Measurement of Three Transport Coefficients and the Thermodynamic Factor in Block Copolymer Electrolytes with Different Morphologies. J Phys Chem B, 2020. 124(5): p. 921-935.<br/>4. Loo, W.S., et al., Phase Behavior of Mixtures of Block Copolymers and a Lithium Salt. J Phys Chem B, 2018. 122(33): p. 8065-8074.<br/>5. Allen, F.I., et al., Deciphering the three-dimensional morphology of free-standing block copolymer thin films by transmission electron microscopy. Micron, 2013. 44: p. 442-50.<br/>6. Bustillo, K.C., et al., 4D-STEM of Beam-Sensitive Materials. Acc Chem Res, 2021. 54(11): p. 2543-2551.