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 NiP
3 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 NiP
3 particles chemically attached with
the flexible and conductive CNTs, the NiP
3/CNT anode displays two orders of magnitude
increase in Na-ion diffusion coefficient (D
Na+) over the pristine NiP
3 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 NiP
3/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. Sb
2Se
3 and Sb
2Te
3,
and their composites with CNTs are also studied in both half and full cells. The in-situ
transformation of Sb
2Se
3 particles from crystalline (c-Sb
2Se
3) to amorphous (a-Sb
2Se
3) phase
is noticed to form a-Sb
2Se
3/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 Sb
2Se
3. A thin SEI layer of ~35.7 nm is revealed on a-Sb
2Se
3/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-Sb
2Se
3/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-Sb
2Se
3/CNT║Na
3V
2(PO
4)
2F
3 full cells deliver a remarkable power density of ~5784 Wh kg
-1 at
80 C, signifying their ultrafast sodium storage capability.
The Sb
2Te
3/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 Sb
2Te
3/CNT anode and its phase
evolution and volume expansion are probed by employing ex-situ/in-situ TEM. The
Sb
2Te
3/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 Sb
2Te
3/CNT║Na
3V
2(PO
4)
2F
3 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 Ti
2CT
x MXene coated Cu current collector
(Ti
2CT
x/Cu) is explored for advanced SMBs. DFT calculations reveal the enhanced affinity of
Ti
2CT
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 Ti
2CT
x/Cu anode, manifesting the low cell impedance.
Post a Comment