PEPTIDE-ASSISTED SYNTHESIS AND ASSEMBLY OF CATHODE MATERIALS FOR LITHIUM ION BATTERIES

Abstract

Biotemplating presents a unique approach for the synthesis and organization of technologically-relevant materials at the nanoscale. Among existing biotemplates, solid-binding peptides (SBPs) identified for the material of interest provide a high binding affinity and selectivity due to their unique sequence of amino acids with diverse functionalities. Applications of nanomaterials assembled and synthesized with SBPs range from drug delivery to catalysis and energy storage, to name a few. This dissertations is focused on developing a peptide-templated approach for the assembly and synthesis of cathode materials for Li-ion batteries in an attempt to address their existing limitations. Generally, the multiple charge and discharge cycles of a battery results in a loss of connectivity between electroactive and conductive materials, and thus, contributes to decreased battery performance. We hypothesize that dual-affinity SBPs would aid in the assembly of those heterofunctional materials resulting in a close proximity and improved connectivity between them. In this work, SBPs were identified and characterized for high voltage cathode materials LiNi0.5Mn1.5O4, LiMn2O4, and Li2MnSiO4 via phage display procedure optimized for these targets. The binding affinities of the selected peptides were confirmed by performing phage binding assays using the peptide expressed on a minor phage coat protein (pIII), followed by characterization of the binding affinity of individual peptides with isothermal titration calorimetry and fluorescence assays. Bifunctional peptides were designed by conjugation of two binding domains, for electroactive material and conductive multiwalled carbon nanotubes (MWCNTs). As demonstrated in this study, the binding affinity of the resulting bifunctional peptides for a target material was not only retained but also improved by the addition of the MWCNTs binding domain. Electrochemical characterization of bio-tethered electrodes indicated a general decrease in charge transfer resistance, which corresponds to improved movement of electrons in the system. Moreover, electrodes assembled with bifunctional peptides showed stable rate capability and cyclability with a current rate of 1C for over 60 cycles. This demonstrates that bifunctional peptides can serve as functional binders in electrode preparation using conventional organic solvents, and upon further optimization, these peptides can also be utilized as water-soluble binders for aqueous electrode assembly. Furthermore, the capabilities of SBPs to facilitate the synthesis of cathode materials with improved properties were explored for high energy density, earth abundant, and low-cost lithium orthosilicates. Peptide-templated Li2MnSiO4 displayed orthorhombic crystal structure with the Pmnb space group and minimal impurities when compared to the control material with the Pmn21 space group. Both polymorphs displayed irreversible structural changes upon cycling, yet peptide-templated materials exhibited higher capacity during the initial cycle. Potential optimization steps for the peptide-templated synthesis of Li2MnSiO4 are outlined in the final chapter of this work.