2D MXenes based anode materials for all-solid-state Li-ion batteries

Description

The project will aim to a deeper understanding of 2D MXene materials commonly synthesized by top-down wet chemical approaches. This often results in residual surface terminal functional groups (OH, F, Cl, O) which are detrimental for the Li-ion adsorption and transport properties. Direct growth of 2D MXene with pristine surfaces by bottom-up synthesis, like chemical vapor deposition (CVD), can overcome this limitation. However, the growth mechanisms in these materials by CVD are not fully understood. Fundamental studies on initial growth stages with the assistance of in-situ (S)TEM and near in-situ XPS will be performed to gain a deeper understanding. When used as an anode material in ASSB, their metal-ion (de-)intercalation mechanisms are also not fully understood. Such fundamental knowledge is necessary for the development of MXenes based anode materials for ASSBs. T2D-SSB relies on interdisciplinary expertise within functional materials synthesis and the assessment of structureproperties relations. The PIs and senior researchers will secure a collaborative research environment to optimize and accelerate knowledge transfer. Competence, experimental facilities and best practices will be shared not only among senior researchers but also with young researchers in the research teams who will be involved as work package or task leaders. This will enable them to develop project management skills valuable for their future research career. Common internationally refereed publications, at least 7, in high-impact open access journals is expected to be the outcome of the jointly addressed research challenges. Popular publications will be prioritized to increase societal awareness of the scientific results. The 2D-SSB partners will also aim to future joint applications in Horizon Europe.

Summary of project results

The project aimed to address some fundamental issues and challenges in the field of 2D transition metal carbides (TMCs), the chemical etching synthesised counter parts of these materials are popularly known as MXenes, and their application in lithium-ion battery technology. Project aim was to establish the groundwork for the development of next-generation all-solid-state lithium-ion batteries incorporating 2D materials. 

This is further elaborated in the following: 

Addressing the absence of reported CVD growth methods for transition metal carbides, namely Mo2C, Ti2C, V2C, and Cr2C TMCs.  

At the time when the project started, only Mo2C has been successfully synthesized by CVD method, highlighting the necessity for further research into developing CVD growth methods for different transition metals and exploring their properties. The feasibility studies and development of chemical vapor deposition (CVD) methods for Ti2C, V2C, and Cr2C, had not yet been reported. The MXenes exhibit promising properties that could surpass those of graphite, the most common battery anode material. However, one major challenge lies in the presence of surface termination functional groups (-OH, -O-, -F, -Cl), which are unavoidable byproducts of the wet chemical synthesis processes dominating MXene research. Theoretical predictions showed that the presence of surface functional groups can impede the material''s performance. Therefore, there is a strong scientific and technological motivation to develop bottom-up synthesis routes for 2D MXenes with pristine surfaces free of functional groups.  

 Filling the knowledge gap on understanding the CVD-growth processes of these 2D TMCs, in the specific approach of using stacked metal foils, through in-depth studies using FIB/SEM, TEM, Raman spectroscopy and XPS techniques. 

The method to grow 2D crystals of these TMCs using the CVD approach of stacking desired transition metal foil under Cu metal foil is a very recent development in this research field. This project aimed to investigate the feasibility of this approach; firstly, in improving and achieving large area coverage growth in the case of Mo2C, i.e. the reported case, and in further extending it to other V, Ti, Cr transition metals. Within this project we aimed to explore and study these materials with different microstructural and analytical spectroscopic characterization techniques.  

 Developing novel methodologies for precise manipulation and integration of 2D TMD crystals, alongside characterizing their structural and electronic properties for potential battery applications. 

Characterizing the electrical properties of 2D-TMCs crystals through dedicated (S)TEM microelectromechanical systems (MEMS) chips, an area where experimental data were currently lacking. Integrating them into full micro-sized all-solid-state batteries (ASSBs) using FIB-SEM methodology and MEMS TEM chips for in operando investigations. 

The key research activities performed during

1.  Successful growth of α-Mo2C 2D crystals utilizing two different chemical vapor deposition (CVD) approaches, as part of research task 1-4, sub-task 1.  

1.  CVD growth experiments were also conducted to synthesize V2C, Ti2C and Cr2C 2D TMCs based on stacked metal foils approach, as part of research task 1-4, sub-tasks 2, 3, and 4. 

1. The growth experiments were followed by detailed characterization studies using Raman spectroscopy, SEM, S/TEM and XPS. 

1. In-situ growth experiments were performed investigating growth processes in Mo2C using scanning/transmission electron microscopy (S/TEM) and Mo2C and V2C in X-ray photoelectron spectroscopy (XPS). It included activities related to FIB/SEM-based TEM sample preparation and use of a near in-situ XPS growth chamber, as part of research task 1-4. 

1. We performed FIB/SEM based TEM sample preparation for the micro-sized all-solid state battery lamella preparation and in-situ electrical biasing experiments in S/TEM as part of research task 5 

1. To augment our understanding of the atomistic-level processes governing the synthesis and performance of 2D transition metal carbides (TMCs), Density Functional Theory (DFT) simulations were performed.  

Outputs: 

1. Fundamental knowledge pertaining to the influence of some key CVD growth of Mo2C 2D crystals on stacked metal foils approach is gained. 

1. CVD growth and characterization studies on 2D transition metal carbides crystals of V-C, Ti-C and Cr-C material systems. Insights were gained into the challenges and limitations of the stacked metal foils CVD approach for realizing 2D M2C type carbides growth for V, Ti, and Cr transition metals. 

1. Development of FIB/SEM sample preparation methods for in-situ growth studies of Mo2C, on stacked Cu/Mo metal layers and performing in situ TEM growth experiments with TEM gas holder. 

1. Development of FIB/SEM sample preparation methods for assembling micro-sized all-solid state battery lamella from as grown Mo2C crystals within the project and performance of in situ electrical biasing S/TEM experiments with TEM electrical biasing holder. 

1. DFT simulation studies provided valuable insights and understanding of the initial growth mechanisms and morphology of Mo2C MXenes. 

Result 1: Large area coverage CVD growth of Mo2C: 

Outcome: A significant enhancement in synthesizing α-Mo2C TMCs via a modified CVD method is developed in this work. This work achieved up to 50% surface area coverage and large single-phase Mo2C flakes nearing 200 μm in lateral size. Optimal CVD conditions were identified for maximizing the yield. XPS.

reference spectra obtained on CVD-grown Mo2C as well as of other phases formed during the process assisted in understanding and optimizing the CVD growth processes.  

Impact: The findings of this result were successfully published in peer-reviewed scientific journal IOP-Nanotechnology.  

 Result 2: Investigation of the possibilities and challenges associated with CVD growth of carbides of V, Ti, Cr (V2C, Ti2C, Cr2C), using stacked metal foils approach: 

Outcome: Extensive experimental research efforts that were dedicated on synthesizing V2C, Ti2C, and Cr2C using the same approach that was successful for Mo2C growth, namely the stacked metal foils approach, showed that it more challenging to obtain these desired carbide phases. We experimentally observed that this because of several additional fundamental factors, like residual oxygen in the foils, in the case V, Ti and Cr compared to Mo. Near in-situ XPS experiments were performed to mimic the CVD-growth of V2C on Cu-Ag alloy.  

Impact: The experimental results obtained in this outcome are valuable in gaining the fundamental knowledge that stacked metal foils approach of CVD may not be adapted universally to synthesise M2C type carbides. The results obtained, particularly in the case of V-C CVD, has led to a manuscript currently under preparation. 

Result 3: In situ growth and battery experiments performed using (S)TEM and XPS: 

Outcome: State-of-the-art in situ and operando (S)TEM experiments were conducted at UiO by researchers from UiO and PORT. Experiments were conducted on FEI Titan G2 60-300 microscope and using the gas/heating in-situ (S)TEM holder (Protochips atmosphere). Furthermore, we conducted biasing studies on ASS Li-ion batteries lamella prepared by FIB on E-chips, using CVD grown Mo2C crystals as anodes at UiO. In-situ XPS and near in-situ MXene growth experiments exposed the challenges when established processes are adapted to new infrastructure, as well the potential of near in-situ XPS experiments.  

Impact: The methodology for performing ASS MXene-based battery biasing experiments is achieved. The conducted experiments are of high-complexity and novelty. 

Beneficiaries: 

Successful knowledge-transfer among partners benefited all the researchers in the project. We achieved methodological development by conducting for the first time challenging and quite novel approaches on E-chip sample preparation (PORT) and in situ/operando (S)TEM studies (UiO) and near in-situ XPS studies (SINTEF). The outcomes are expected to have impact on the broader scientific community in the field of materials science.  

Summary of bilateral results

Bilateral cooperation seems to be satisfactory. The report details the activity of each of the contractors, both on the Polish and foreign side. Mutual participation in promotional events is also positive. Partners have not submitted any future project application. However, during the project closing meeting that took place in 28th -29th Feb’ 2024 at Lukasiewicz – PORT, Wroclaw, the research partners expressed strong willingness to work together in future projects.