Decoupled Ion Transport in Protein-Based Solid Electrolyte through Ab Initio Calculations and Experiments

Abstract

Decoupling the ion motion and segmental relaxation is significant for developing advanced solid polymer electrolytes with high ionic conductivity and high mechanical properties. Our previous work proposed a decoupled ion transport in a novel protein-based solid electrolyte. Herein, we investigate the detailed ion interaction/transport mechanisms through first-principles density functional theory (DFT) calculations in a vacuum space. Specifically, we study the important roles of charged amino acids from proteins. Our results show that the charged amino acids (i.e., Arg and Lys) can strongly lock anions (ClO4 –). When locked at a proper position (determined from the molecular structure of amino acids), the anions can provide additional hopping sites and facilitate Li+ transport. The findings are supported from our experiments of two protein solid electrolytes, in which the soy protein (with plenty of charged amino acids) electrolyte shows much higher ionic conductivity and lower activation energy in comparison to the zein (lack of charged amino acids) electrolyte.

Effects of Anions and Protein Structures on Protein‐Based Solid Electrolytes

“…Clearly, adding Arg amino acid is effective in increasing the lithium ionic conductivity by decreasing the lithium activation energy. In addition, when compared with zein – LiClO 4 based electrolyte, [ 18 ] the ionic conductivity has an average of 1 order of magnitude increase throughout the temperature range, which we believe is due to the lower lithium binding energy in protein – LiPF 6 system from our DFT calculations.…”

“…e ., F1···H1 and F2···H2. The bonding distances are 1.75 and 1.74 Å respectively, which are a little bit larger than O···H (1.68 and 1.67 Å), [ 18 ] this is because the fluorine atoms are less negatively charged (≈‐1 |e|) than the oxygen atoms in $$ClO_4^ - $$ (‐2 |e|). The detailed charge distribution of the anion and the guanidino group is calculated by Bader charge analysis method [ 28 ] and listed in Table 1 .…”

“…Lithium‐ion transport mechanism in protein‐based electrolyte has been explored in our previous work. [ 18 ] Anions locked by the positively charged amino acids were found to be able to provide intermediate hopping site for lithium‐ion transport along the peptide chain, and significantly reduce the energy barrier. However, the appropriate locking positions for anions play an important role to lithium‐ion transport, but have not yet been investigated.…”

“…Hence, through manipulation of the protein structure, we can potentially improve the ionic conductivity and enhance electronic performance. Further ab initio calculations were performed, [ 18 ] the interaction energy between the anion ($$ClO_4^ - $$) and the side group of Lys and Arg were determined to be 2.73 and 2.58 eV, respectively. Moreover, the locked anion with a proper distance to the backbone chain was found to be able to provide intermediate hopping sites for lithium ions migration, leading to a lower energy barrier of 0.37 eV compared to the 0.74 eV without the assistance of anion.…”

Robust Artificial Interphases Constructed by a Versatile Protein‐Based Binder for High‐Voltage Na‐Ion Battery Cathodes

“…Previous study on protein-based solid-state electrolyte has discovered that positively charged side groups of specific amino acids (e.g., arginine, histidine, and lysine) can adsorb ClO 4 − anion in the battery electrolyte by electrostatic interactions and thus affects lithium-ion diffusion. [38,39] Inspired by this finding, three peptide chains using arginine, histidine, and lysine as the building blocks were constructed by dehydration condensation to have a clue of sodium-ion diffusion in sericin protein (Figure S12, Supporting Information). DFT fully relaxed constructed models consisting of one peptide chain and one free ClO 4 − anion close to the positively charged group in a vacuum space are shown in Figure S13 (Supporting Information).…”