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...[ Read more ]

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.