Porous MXene Synthesis and Applications for Batteries

Abstract

Efficient electrochemical energy storage devices are of particular importance for accelerating the widespread adaption of renewable energy, and electrode materials are critical to achieve optimal system performance. MXene (transition metal carbides and/or nitrides) are recently discovered two-dimensional materials with desirable properties such as metallic conductivity and high hydrophilicity. They have shown promises in applications such as lithium ion, zinc ion and dual ion batteries, but limitations such as hazardous precursor, uncontrollable edge terminations and low surface area have restricted fulfillment of their potentials. Moreover, insights into the precise mechanism of MXene in composite formation and electrochemical reactions are still limited. To address the above issues, this thesis mainly focuses on developing novel strategy to synthesize high surface area porous MXene with uniform terminations, while probing their capability as high-performance electrodes for different types of battery systems. Particular focus is placed on investigating the influence of uniform Cl-termination and in-plane porosity on MXene when used as dual-ion battery anode. Afterwards, functions of the porous MXene as Prussian blue analogue (PBA) crystal growth inducing host for composite material and zinc ion redistribution protective layer for dendrite-free Zn anode are systematically investigated. Firstly, a novel one-step eutectic mixture etching method is developed to synthesize Cl-terminated MXene (Ti3C2Cl2) with abundant in-plane porosity through carefully controlling the phase transition of the etchants. Specifically, precise selection of etching parameters and salt mixture composition activates a mechanism that enables both pore generation and preservation through timely formation of solid salt particles within the material structure. The resulting in-plane porous MXene sheets exhibit homogeneously distributed mesoporosity and four-folds expansion in surface area to 85 m2 g−1. X-ray spectroscopy characterizations reveal predominately edge Cl-terminations with minimal oxidation on the as-synthesized samples, which ensure orderly crystal arrangement. Meanwhile, density functional theory calculations confirm lower diffusion barrier that are beneficial for ion diffusion. When evaluated as anode for dual ion batteries, the optimized porous Ti3C2Cl2 achieves a high specific capacity and excellent capacity retention. This work opens a green chemistry approach of incorporating in-plane mesoporosity to MXenes for electrochemical application and provides insights for environmental responsible development in the field of energy storage. Secondly, the porous MXene is utilized as multifunctional host material to manipulate the morphology and crystallinity of cobalt hexacyanoferrate (CoHCF). It is discovered that the negative charges on the MXene surface can induce formation of high surface area CoHCF with reduced defects, which improves the material resilience toward reversible Zn2+ intercalation /deintercalation. Combined with the high conductivity of porous MXene, the CoHCF/MXene composite achieves superior electrochemical performance as cathode material for zinc ion battery, delivering a high capacity of 197 mAh g−1 and robust capacity retention of 94.4% over 3000 cycles. This work unveiled the inducing effect of MXene when used as template to grow Prussian blue analogues, paving way for its broad composite chemistry. In the last section, the porous MXene is adapted in a surface engineering strategy as coating layer to regulate the electrodeposition of Zn2+ towards dendrite-free Zn anode. Its uniform surface charge serves to homogenize the surface electric field for even deposition, while its crystal structure resemblance with the Zn (0002) plane promotes growth of the specific crystal plane known to favor layer-by-layer deposition. Moreover, porous structure of the MXene nanosheet is also favorable for ensuring sufficient ion diffusion and electrolyte coverage during high-rate cycling. Given these advantages, the MXene coated zinc foil achieves long period cycling of over 2000 hours at a high current density of 10 mA cm-2, promoting the development of zinc anodes for large-scale energy storage. In summary, this work opens a green chemistry approach to synthesize in-plane porous MXenes, which resolves the long-standing issue of limited porosity and low surface area of MXene. Meanwhile, the proposed effect on inducing ordered CoHCF crystal growth provides insights for designing MXene-based composite material with well-controlled crystal structure. In terms of energy storage application, this thesis addresses the importance of MXene termination and porosity in two application scenarios, as DIBs anode material and as protective layers for Zn anode, offering deep understanding on the relations between the structural properties of MXene and electrochemical behavior.