Efficient energy storage systems are essential to fulfil the rapidly growing demand of energy with minimum degradation to the environment. The enormous impact of lithium ion batteries (LIBs) on the technological developments enabled by powering portable electronic devices, electric vehicles and smart grids is recognized globally with the 2019 Nobel Prize in chemistry. However, conventional LIBs may not satisfy the rising demand of next-generation rechargeable batteries in disparate sectors which calls for diversification of purpose-built batteries. Sodium has emerged as a potential alternative to lithium, thanks to the abundance of cheap sodium precursors. Hence, research on high capacity anodes for sodium ion and sodium metal batteries (SIBs, SMBs) has been resuscitated in the...[ Read more ]

Efficient energy storage systems are essential to fulfil the rapidly growing demand of energy with minimum degradation to the environment. The enormous impact of lithium ion batteries (LIBs) on the technological developments enabled by powering portable electronic devices, electric vehicles and smart grids is recognized globally with the 2019 Nobel Prize in chemistry. However, conventional LIBs may not satisfy the rising demand of next-generation rechargeable batteries in disparate sectors which calls for diversification of purpose-built batteries. Sodium has emerged as a potential alternative to lithium, thanks to the abundance of cheap sodium precursors. Hence, research on high capacity anodes for sodium ion and sodium metal batteries (SIBs, SMBs) has been resuscitated in the past decade. However, their practical realization is hindered by several critical issues including the complicated synthesis routes, the continuous increase in cell impedance, the poor kinetics of electrochemical reactions, and inadequate structural stability. Therefore, this thesis is dedicated to address these issues by facile and scalable fabrication of nanostructured active materials and their composites with functionalized carbon nanotubes (CNTs) and systematically investigating their Na storage performance.

The NiP₃ particles and their composites with CNTs are assembled by a high energy ball milling (HEBM) method. The generation of P–C and P–O–C bonds in the composites is revealed by the density functional theory (DFT) calculations and ab-initio molecular dynamic (AIMD) simulations. Benefitting from the nanosized features of NiP₃ particles chemically attached with the flexible and conductive CNTs, the NiP₃/CNT anode displays two orders of magnitude increase in Na-ion diffusion coefficient (D_Na+) over the pristine NiP₃ counterpart. The ex-situ X-ray photoelectron spectroscopy (XPS) study elucidates the presence of stable carbonates and phosphates in the solid electrolyte interface (SEI) layer formed on the composite electrode being responsible for reduced charge transfer resistance (R_ct). The NiP₃/CNT anodes demonstrate high reversible capacities of 853 and 493.6 mA h g^-1 at 0.2 and 3.2 A g^-1, respectively.

The sodium storage performance of two emerging metal chalcogenides, i.e. Sb₂Se₃ and Sb₂Te₃, and their composites with CNTs are also studied in both half and full cells. The in-situ transformation of Sb₂Se₃ particles from crystalline (c-Sb₂Se₃) to amorphous (a-Sb₂Se₃) phase is noticed to form a-Sb₂Se₃/CNT composite by ball-milling. The first-principles calculations substantiate the formation of Sb–O–C and Se–C bonds in the composite and energetically favorable Na storage in Sb₂Se₃. A thin SEI layer of ~35.7 nm is revealed on a-Sb₂Se₃/CNT anode by the advanced ex-situ cryogenic transmission electron microscopy (cryo-TEM), which corroborates its enhanced electrochemical kinetics and reduced R_ct. As a result, the a-Sb₂Se₃/CNT anodes exhibit the reversible capacity of 454 mA h g^-1 at a high rate of 12.8 A g^-1 and maintain 99% capacity after 120 cycles. As a proof of concept, the a-Sb₂Se₃/CNT║Na₃V₂(PO₄)₂F₃ full cells deliver a remarkable power density of ~5784 Wh kg^-1 at 80 C, signifying their ultrafast sodium storage capability.

The Sb₂Te₃/CNT composites are also fabricated by HEBM and the insights into their chemical environment are acquired by XPS analysis and first-principles calculations. The cryo-TEM examination unveils a thin SEI film of ~19.1 nm formed on the Sb₂Te₃/CNT anode and its phase evolution and volume expansion are probed by employing ex-situ/in-situ TEM. The Sb₂Te₃/CNT anodes present high reversible capacities of 422 and 318 mA h g^-1 at 0.1 and 6.4 A g^-1, respectively, and retain over 97% of capacities after 300 cycles, thanks to the fast electrochemical kinetics of the robust composite anode. The Sb₂Te₃/CNT║Na₃V₂(PO₄)₂F₃ full cells exhibit a notable energy density of ~229 Wh kg^-1 at 0.5 C and a high power density of 5384 W kg^-1 at 40 C. The pouch full cells demonstrate bendability and stable operation at different temperatures from 40 to -20˚C.

The Na plating/stripping performance of the Ti₂CT_x MXene coated Cu current collector (Ti₂CT_x/Cu) is explored for advanced SMBs. DFT calculations reveal the enhanced affinity of Ti₂CT_x MXene for Na in the presence of oxygen moieties, which is substantiated by the exceptional cyclic stability of half cells for 9000 and 5000 cycles at 2 and 3 mA cm^-2, respectively. The symmetric cells present remarkably stable operation for 10,000 hr at 1 mA cm^-2 and full cells show equally impressive stability for 1700 cycles. The cryo-TEM discloses a compact and ~68 nm thick SEI layer formed on the Na plated Ti₂CT_x/Cu anode, manifesting the low cell impedance.