Electronic Thesis and Dissertation Repository

Thesis Format

Integrated Article

Degree

Doctor of Philosophy

Program

Mechanical and Materials Engineering

Supervisor

Sun, Xueliang

Abstract

All-solid-state lithium batteries (ASSLBs) have become increasingly attractive due to the demand of high-energy-density and high-safety lithium-ion batteries for electric vehicles (EVs). As the core component of ASSLBs, solid-state electrolytes (SSEs) are regarded as essential to determine the electrochemical performance of ASSLBs. The inorganic SSEs is one of the most important categories in all developed SSEs, representing the advance of superionic lithium conductors as well as the cornerstone to construct flexible polymer/inorganic composite SSEs. The sulfide-based inorganic SSE is one of the most promising SSEs that is receiving a lot of attentions, because only sulfide SSEs can show ultrahigh ionic conductivity (up to 10-2 S cm-1) at room temperature (RT) that can be comparable to conventional liquid electrolytes. However, sulfide SSEs are suffering interfacial instability at both anode and cathode sides, as well as poor air stability. These drawbacks are hindering the commercialization of ASSLBs using sulfide as the SSE.

In this thesis, first, from the point view of electrolyte synthesis, strategies of element substitution are rationally developed to realize good Li anode compatibility or (and) air stability. It is noted that these strategies are on the premise to achieve decent or improved ionic conductivity for the parent sulfide SSEs. Specifically, replacing Cl partially with F in the Argyrodite sulfide Li6PS5Cl SSEs can trigger to generate LiF-rich Li anode interface, which can realize ultrastable Li plating and stripping. Additionally, Sn-substituted Argyrodite Li6PS5I and Li3PS4 glass-ceramic sulfide SSEs are developed, respectively. The versatile Sn is verified to improve the air stability, ionic conductivity, and Li anode compatibility, simultaneously. The mechanism of multi-functionality obtained from Sn substitution has been well explored. Overall, these novel sulfide SSEs can be viewed as new choices for developing all-solid-state Li metal batteries with high energy densities. Second, atomic layer deposition is used to design new lithium zirconium oxides (LZO) as the interfacial buffer layer to alleviate the cathode interface problems (slow Li+ transport and side reaction) between Li6PS5Cl sulfide SSEs and LiCoO2 cathode materials. These works focusing on the synthesis of new sulfide SSEs and the interface engineering pave the way to achieve high-performance sulfide-based ASSLBs.

Summary for Lay Audience

Conventional Li-ion batteries using liquid electrolytes (LEs) are suffering from insufficient energy density and safety issues when used for the flourishing market of electric vehicles (EVs). Replacing LEs with solid-state electrolytes (SSEs) to fabricate all-solid-state lithium batteries (ASSLBs) has been regarded as an essential route to improve energy density and safety. Inorganic SSEs, as one kind of the most popular SSEs, are attracting increasing attention. A quantified SSE requires high ionic conductivity, air/moisture stability, and electrode compatibility to enable high-performance ASSLBs. Currently, sulfide SSEs become attractive due to their high ionic conductivity that can be comparable to the liquid electrolyte. However, sulfide SSEs are suffering poor electrode compatibility (anode and cathode) and air sensitivity, hindering their applications in practical ASSLBs.

In this thesis, first, from the standing point of synthesizing sulfide SSEs, the strategy of element doping (fluorine and tin) is developed to increase the Li metal compatibility and air stability of sulfide SSEs, as well as ionic conductivity for some defective sulfide SSEs. All-solid-state lithium metal batteries (ASSLMBs) using these newly developed sulfide SSEs exhibit promising electrochemical performance at room temperature (RT). These element substitution strategies help to alleviate the problem of existing sulfide SSEs essentially. Second, proceeding from the interface modification, new lithium zirconium oxides (LZO) is developed as cathode coating layers by the advanced nanofabrication technique of atomic layer deposition (ALD). The LZO buffer layer is Li-ion conducting but electron insulating, preventing the direct contact between cathode particles and sulfide SSEs, so that excellent cathode interface is achieved to enable good electrochemical performance at RT. Overall, all findings presented in the thesis would make contributions to the development of qualified superionic conductors and high-performance sulfide-based ASSLBs.

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