Alloying-type anode materials for advanced lithium and potassium-ion batteries

This thesis describes the synthesis and application of high capacity alloying-anode materials for next-generation lithium and potassium-ion batteries; specifically, Li-alloying high loading silicon nanowires (Si NWs) and K-alloying antimony (Sb), bismuth (Bi) and effect of electrolyte additives on the electrochemical performance. The core chapters are arranged as research articles with introductory summaries at the beginning of each chapter.

Si NWs have great promise as an anode material for lithium‐ion batteries (LIBs) due to their very high specific capacity of 3579 mAh.g–1 . Achieving adequate mass loadings for binder–free Si NWs have been restricted by low surface area, mechanically unstable and poorly conductive current collectors (CCs), as well as complicated and expensive fabrication routes. Therefore, a highly effective and scalable fabrication route for the high mass loading and dense growth of Si NWs on a mechanical robust and highly conductive substrate is desirable for practical LIBs. In case of potassium-ion batteries (PIBs), alloying anode materials such as Sb and Bi are of huge interest due to their high capacity 660 mAh.g–1 (K3Sb), and 385 mAh.g–1 (K3Bi), respectively, and moderate working potential. Realizing stable electrochemical performance for these materials in PIBs is hindered by the enormous volume variation (~400%) that occurs during cycling, causing a significant loss of the active material and disconnection from conventional CCs. In addition, this severe volume expansion during charging/discharging leads to unstable solid electrolyte interface (SEI) on the anode surface. The SEI layer is under constant repair, causing significant electrolyte consumption and irreversible reactions with the anode material, leading to continued capacity fade. A suitable film forming electrolyte additives is a vital strategy to alleviate this problem.

Chapter 3 describes a tunable mass loading and dense Si NW growth on a highly conductive, flexible, fire-resistant and mechanically robust interwoven stainless steel fiber cloth (SSFC) using a simple glassware setup. The SSFC CC facilitates dense growth of Si NWs where its open structure allows a buffer space for expansion/contraction during Li–cycling. Galvanostatic cycling of the Si NWs@SSFC anode with a mass loading of 1.32 mg.cm–2 achieves a stable areal capacity of ~2 mAh.cm–2 at 0.2C after 200 cycles. Notably, large–scale fabrication of robust and flexible binder–free Si NWs@SSFC architectures is also demonstrated. Chapter 4 describes the direct growth of a highly dense copper silicide (Cu15Si4) nanowire (NW) array from a Cu mesh substrate to form a 3D CC that is used for the direct deposition of high capacity Sb to fabricate an anode for PIBs in a core-shell arrangement (Sb@Cu15Si4 NWs). The 3D Cu15Si4 NW array provide a strong anchoring effect for Sb, while the spaces between the NWs act as a buffer zone for Sb expansion/contraction during K–cycling. The binder-free Sb @Cu15Si4 anode displays stable capacity of 250.2 mAh.g–1 at 200 mA.g–1 for over 1250 cycles with a capacity drop of ≈0.028 % per cycle. Furthermore, ex-situ electron microscopy reveal that the stable performance is due to the complete restructuring of the Sb shell into a porous interconnected network of mechanically robust ligaments. Chapter 5 explores the 3D Cu NWs CC with the porous architecture for direct deposition of Bi and compare its electrochemical performance with the planar Cu foil as anode in PIBs. The Cu NWs improve the reaction kinetics of the Bi, leading to improved capacity retention. Chapter 6 investigates the effect of electrolyte additive such as vinylene carbonate (VC) and fluoroethylene carbonate (FEC) on the cycling performance of the Bi anode, and the compositional and electrochemical properties of the SEI layer in PIBs. Finally, chapter 7 presents the conclusion, recommendation for further studies and outlook for the future development of alloying type anode materials for LIBs and PIBs.