The abundance, cost-effectiveness, and greater experimental capacity of manganese-based oxides make them a promising anode material for next-generation lithium-ion batteries (LIBs). However, the rapid capacity fading of these LIBs and the low throughput capacity due to volume fluctuations during the cycle hamper their practical applications.
Study: Design of binderless conversion-based manganese oxide nanofibers as a highly stable and throughput-capable anode for next-generation Li-Ion batteries. Image Credit: Immersion Imaging/Shutterstock.com.
In a paper recently published in the journal ACS Applied Energy Materials, the authors reported the fabrication of one-dimensional (1D) magnesium manganite (MgMn2O4; MMO) nanofibers with a high aspect ratio, which had morphological deficiencies and can be applied as binderless negative electrodes for LIBs.
Anode materials in LIBs
Global warming can be mitigated by using renewable energy sources in the energy storage system. To this end, electrochemical batteries present themselves as promising energy storage devices. LIBs with longer cyclability, high energy density, lower self-discharge, no memory effect and easy storage mechanisms are recognized as the most efficient energy storage technology.
LIBs currently in use require four to five times the power density to meet emerging industrial demand. Therefore, there is a need for a new anode material with properties such as high specific capacity, good working potential, long life, high rate performance, environmental friendliness and the profitability.
Although carbon is used as the anode material due to the high operating voltage, its high-end application is limited due to its low insertion potential, lower experimental capacity and lower safety. Thus, there is an emerging need for alternative anode materials with improved reversible capability, good cyclability, and high-rate performance.
In the present work, the authors synthesized 1D MMO nanofibers with morphological voids as binderless anode materials for LIBs. The anode material was synthesized by the electrophoretic deposition (EPD) technique.
The as-prepared anode material provides a well-designed network between the current collector and the active materials, which improves the energy density of the LIBs. Additionally, the spaces between the inactive magnesium oxide (MgO) matrix and the individual MMO nanofibers act as buffer spaces to accommodate the volume variation of lithiation and delithiation.
The three-dimensional (3D) microstructure of the synthesized electrode and the 1D porous morphology of the MMO are the main reasons for the excellent electrochemical performance of the fabricated anode.
Field Emission Scanning Electron Microscope (FESEM) images of MMO nanofibers showed the uniform and smooth surface of nanofibers with diameters between 400 and 500 nanometers without bead formation. Thermal stability studies were used to investigate the change in the composition of MMO nanofibers using thermogravimetric analysis (TGA). The initial weight loss of 8.9% between 24 and 280 degrees Celsius was associated with the loss of absorbed moisture, residual solvents and water captured in the MMO nanofibers. Additionally, 25.8% at 365 degrees Celsius is associated with the decomposition of metal precursors and polyvinylpyrrolidone (PVP).
The extreme weight loss at 365 to 400 degrees Celsius represents the removal of the polymer backbone from the MMO nanofibers. The weight loss observed between 400 and 800 degrees Celsius corroborated the removal of residues from the as-prepared nanofibers. Additionally, the authors were able to obtain phase-pure MMO at 500 degrees Celsius.
The results of energy dispersive X-ray spectroscopy (EDX) revealed a good proportion of stoichiometry and a good distribution of precursors like magnesium (Mg), manganese (Mn) and oxygen (O) in the MMO nanofibers. Moreover, the X-ray diffraction (XRD) pattern of the MMO nanofiber revealed the characteristic peaks corresponding to the tetragonal MMO.
The porous surface of the as-prepared MMO nanofibers was studied using the nitrogen adsorption-desorption isotherms of the BET method at 77 kelvin. The authors confirmed that the MMO nanofiber isotherm is analogous to a type IV isotherm. The isothermal curve showed nitrogen absorption at low pressure, which corroborated the microporous nature of the nanofibers, and the power spectral density (PSD) curve validated the porosity of the MMO nanofibers. The pores are centered at 1.82 nanometers, confirming the microporosity of the nanofiber.
Storage properties of lithium (Li)
The extraordinary Li-storage properties of MMO nanofibers have been explained by 1D MMO nanofibers with high aspect ratio exhibiting morphological discrepancies between individual nanoparticles. Voids/spaces reduce the possibility of aggregation of the active material. The authors avoided the electrochemical inactive polymer binder in the synthesis of anode materials to favor charge transfer kinetics.
Additionally, the gaps on the surface of the active electrode material enhance Li-ion diffusion at the junction of electrode and electrolyte. Additionally, a small number of multi-walled carbon nanotubes (MWCNTs) introduced into the electrode material improved adhesion and electrical conductivity.
In summary, the authors fabricated 1D, porous, aligned MMO nanofibers with a high aspect ratio using an electrospinning technique. Additionally, they fabricated binder-free MMO nanofiber-based electrodes for LIBs via an environmentally friendly EPD approach.
The EPD deposition voltage changes the microstructure of the coated electrode and helps improve the Li storage properties. At an EPD voltage of 100 volts, the as-synthesized MMO nanofiber achieved a 3D porous morphology with voids/spaces, which allows the diffusion of Li-ion into the electrode materials during the lithiation and delithiation process.
Tandon, A., Rani, S. and Sharma, Y. (2022). Design of binderless conversion-based manganese oxide nanofibers as a highly stable and throughput-capable anode for next-generation Li-Ion batteries. ACS Applied Energy Materials. https://pubs.acs.org/doi/10.1021/acsaem.2c00487