Synthesis and electrochemical study of tunable alloying-conversion hybrid anodes for rechargeable Li-ion batteries
Creators: name: YADAV, Poonam
Publisher: name: RMIT University
Year: 2024
Subjects: classificationCode: 340604
subject: Electrochemistry

description: Several key challenges remain in the development of anode materials for improved secondary lithium-ion batteries (LIBs), such as problems caused by volume expansion, low cycling stability, and high voltage hysteresis. These issues impede the performance of commercial batteries and need to be addressed in new materials that are considered as a replacement of pure carbon-based anodes. Conversion-alloying materials (CAMs) such as SnP, Sb2S3, CuSbS2 and SnFe2O4 have high theoretical capacity, low Li insertion potential and improved safety.
 
The development of nano-structured materials prepared as composites with appropriate conducting material is seen as a remedy to above mentioned limitations of anodes. These conducting additives can be different forms, either intercalation type carbon anodes or Li-inactive elements such as Cu and Fe. In non-carbonaceous anode materials, alloying with Li leads to a volume increase that affects practical applications, and increase in particle size, amorphization and reduced conductivity can all lead to loss of battery performance. Composites of conversion cum alloying materials are promising because they have high theoretical capacity and stability.

This thesis demonstrates that the union of an electrochemically efficient phase of Tin phosphide (SnP0.94) in the matrix of conducting carbon nano-fibers can be synthesized via an electrospinning and solid-state phosphorization route. The importance of the use of CNF (carbon-nano fiber) in a composite material has been confirmed through electrochemical and supporting characterization techniques, which have indicated a superior performance of the composite compared to the pure phosphide.  A capacity as high as 750 mAh g-1 has been achieved at a current density of 100 mA g-1 in the composite form with acceptable cyclic stability and rate performance.  The better performance of the composite (SnP0.94-CNF) compared to bare tin phosphide (SnP0.94) has been correlated to the superior mobility of Li ions in the former (diffusion coefficient = 1.86 x 10-11 cm2 s-1) compared to later (diffusion coefficient = 8.57 x 10-14 cm2 s-1). Metal phosphides combined with carbon nanofibres in nanoscale composite materials are thus confirmed as promising lithium battery anode materials, capable of addressing some limitations of pure carbon-based anodes. The facile preparation procedure, which combines electrospinning with solid-state phosphorization, has been shown to be a versatile synthetic strategy.

The stability of the capacity upon cycling is crucial for practical anode materials. Volume expansion, structural changes, amorphization, loss of contact from the current collector, and loss of conductivity are the main reasons for capacity fading with cycling. To address this issue of poor electrochemical performance, in this thesis we have added the conducting Cu diluent ion to binary Sb2S3 to produce the ternary CuSbS2, and directly compared the two materials. When the ternary material undergoes cycling, inactive (to Li) Cu provides conductivity with facile charge transfer and prevents the Sb particles coming in contact with each other due to a dilution effect, restricting the increase in the particle size which may affect performance. The ternary material was also shown to have superior intrinsic electrochemical characteristics that aided in its performance and stability. To further increase the capacity and stability, we have made composite of CuSbS2 with rGO and successfully demonstrated that this composite had a high capacity and stability up to 1000 cycles. The sheets of rGO helped in preventing loss of electrical contact from the current collector and restricting the agglomeration of particles which otherwise would have resulted in the inaccessibility of the inner part of the material for Li alloying and dealloying. This study may act as an impetus to incorporate Cu and rGO into other binary and ternary materials that show promise as anode materials but tend to agglomerate or lose capacity with cycling.
 
Also in this thesis, we present a low-cost waste polymer packaging derived carbon (PDC) and it¿s composite with easily synthesizable, low-cost SnFe2O4 (SFO) CAM as an anode for Li-ion batteries. This composite can address the low capacity issue of carbon anodes, and stability issues of CAMs. Precursor materials are abundantly available. Synthesis methods used are scalable for large scale industrial applications. Superior electrochemical performance was obtained in the composite due to desired union of robust as well as conducting carbon, and the high capacity of the SFO component. The first reversible capacity of 1133 mAh g-1 is obtained at 100 mA g-1 for the SFO-PDC composite which is higher than PDC (646 mAh g-1) and SFO (680 mAh g-1). The enhanced electrochemical performance in the case of the composite is associated with the lower charge transfer resistance of SFO-PDC in comparison to unaccompanied SFO. The high capacity of SFO is stabilized for longer cycles due to the presence of the robust carbon additive in SFO-PDC. As such, lower resistance, high capacity, and improved stability are obtained in the composite anode for Li storage with the added advantages of low cost and scalability.