THESIS
2022
1 online resource (xx, 164 pages) : illustrations (some color)
Abstract
Lithium-ion batteries (LIBs) are the most widely used power source for electric vehicles (EVs),
owing to their high energy and power densities compared with other commercial batteries.
The energy density of LIBs, however, is still much lower than that of gasoline, limiting the
driving range of EVs. The New Energy Vehicle Industry Development Plan (2021-2035)
issued by the State Council of China sets an energy density target of 500 Wh kg
-1 by 2030,
which clearly cannot be achieved by current materials for LIBs. To understand the upper limit
of energy densities of current LIBs, a systematic screening of material systems that can achieve
an energy density of 500 Wh kg
-1 is conducted in this thesis. The energy densities of
conventional lithium-ion and lithium metal batteries are calculated...[
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Lithium-ion batteries (LIBs) are the most widely used power source for electric vehicles (EVs),
owing to their high energy and power densities compared with other commercial batteries.
The energy density of LIBs, however, is still much lower than that of gasoline, limiting the
driving range of EVs. The New Energy Vehicle Industry Development Plan (2021-2035)
issued by the State Council of China sets an energy density target of 500 Wh kg
-1 by 2030,
which clearly cannot be achieved by current materials for LIBs. To understand the upper limit
of energy densities of current LIBs, a systematic screening of material systems that can achieve
an energy density of 500 Wh kg
-1 is conducted in this thesis. The energy densities of
conventional lithium-ion and lithium metal batteries are calculated precisely. The analysis
indicates that the gravimetric energy density of the batteries using three cathode materials,
including Ni-rich layered oxide, high-voltage lithium cobalt oxide, and sulfur, can exceed 500
Wh kg
-1 only when lithium metal is used as the anode. Therefore, the development of lithium
anode and high-energy cathode has become critical.
The solid-state electrolyte is believed to be able to alleviate safety concerns and promote the
direct use of lithium metals. In this work, I develop a solid-state electrolyte composite
including a porous framework based on ultrahigh molecular weight polyethylene (UHMWPE)
providing support and mechanical strength, and lithium-ion conductors (PEO-LiTFSI) filling
the voids of the porous framework. The strategy proposed in this work points to a way to
resolve the conflicts between mechanical strength and ionic conductivity of polymer
electrolytes. However, the operating temperature of polymer solid-state batteries based on this
electrolyte still needs to be higher than 50 °C, which is difficult to meet the needs of practical
applications. Lowering the operating temperature is crucial, which faces issues like severe
lithium dendrite growth and low ionic conductivity. In my next work, an in-situ polymerization
technology is adopted to optimize the interfacial contact and enhance mass transport. A
fluoride-rich layer is constructed between the lithium metal and the electrolyte film to protect the lithium metal anode. With a systematical optimization, a quasi-solid-state lithium metal
battery is assembled which can be operated well at room temperature and exhibited excellent
performance (> 1500 cycles at 1 C, > 2500 cycles at 5 C).
In addition to the employment of lithium metal anode, high-energy cathode materials are
crucial to achieving the goal of 500 Wh kg
-1. The final work develops a Ni-rich cathode
material (HP-NMC811) with a superb rate capability (> 165 mAh g
-1 at 10 C) which is also
suitable for long-term cycling (> 95% after 300 cycles at 1 C). HP-NMC811 possesses a
unique structure with large primary particles arranged radially from the center to the surface
of the secondary particle, alleviating the influence of the volume expansion and contraction.
These works are expected to be helpful and accelerate the progress of achieving the goal of
500 Wh kg
-1.
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