Rational design and performance study of solid-state lithium-oxygen batteries

Abstract

Lithium-oxygen (Li-O2) batteries have received significant attentions and research interests since the beginning of the twenty-first century due to their extremely high theoretical energy density. These batteries employ oxygen as the positive active substance, which can be obtained from air, resulting in a lighter and more affordable battery. Despite these advantages, lithium-oxygen batteries face several challenges, primarily due to the use of organic liquid electrolyte, which poses risks of volatilization, leakage, combustion, and explosive hazards. Additionally, the use of lithium metal anode generates lithium dendrites that can puncture the electrolyte layer, leading to short-circuit accidents. The cathode of lithium-oxygen batteries has an open-hole structure designed to absorb active material oxygen, which allows the seepage of carbon dioxide and water into the electrolyte, adversely impacting battery performance. To address these challenges, a viable approach is to replace the organic liquid electrolyte with an inorganic solid electrolyte.

However, all-solid-state lithium-oxygen batteries are still in the early phases of development, and several key scientific and technical hurdles need to be tackled. One of the significant challenges is the lack of solid electrolytes that exhibit excellent performance. Additionally, the problem of interface contact between the solid electrolyte and the lithium anode must be resolved. The electrolyte and cathode also experience high interfacial impedance, resulting in poor capacity retention and short cycle life.

This thesis has dedicated considerable efforts and endeavors to address the aforementioned challenges. The highlights of this study are presented below.

1. Study of solid-state lithium-oxygen batteries based on oxide solid-state electrolytes Li1.5+xAl0.5Ge1.5sSixP3-xO12 and Li6.4La3Zr1.4Ta0.6O12.
Firstly, the synthesis and investigation of two different systems of solid-state electrolytes-Li1.5+xAl0.5Ge1.5sSixP3-xO12 and Li6.4La3Zr1.4Ta0.6O12 electrolytes are performed. These electrolytes were assembled with lithium metal anodes and carbon cathodes to construct solid-state (quasi-solid-state) lithium-oxygen cells. The Si-doped LAGP-Si glass-ceramic materials with high grain conductivity were prepared using inexpensive raw materials, and the effect of Si doping on the electrolyte properties of LAGP glass-ceramic solids was investigated. The solid-state and semi-solid-state lithium-oxygen batteries assembled with LAGP-Si demonstrated a certain cyclable capability at low current densities. High-performance garnet-type solid electrolyte LLZTO was successfully synthesized, and the solid-state lithium-oxygen battery assembled with it also showed a certain cycling capability. However, the polarization voltage was large, and the interfacial impedance between it and the electrode was high. Further research is needed to develop cost-effective cathode catalysts, prepare high-performance solid-state electrolytes, and enhance the interface between solid-state electrolytes and electrode materials. This work sets the groundwork for the subsequent investigations along this topic.

2. Study of bilayer NASICON/polymer hybrid electrolyte for stable solid-state lithium-oxygen batteries.
The usage of lithium-oxygen batteries in practical applications is still hampered various factors i.e., the growth of lithium dendrites, the deployment of flammable and unstable organic liquid electrolytes, which could lead to safety hazards and poor cycling stability. To overcome these issues, a bilayer organic/inorganic hybrid solid-state electrolyte is proposed. The Si-doped NASICON-type electrolyte Li1.51Al0.5Ge1.5Si0.01P2.99O12 acts as an inorganic rigid backbone, ensuring high ionic conductivity and creating a barrier between active oxygen and lithium anode. The polymer buffer layer Poly(ethylene glycol) methyl ether methacrylate (PEGMEM) is used due to its compatibility with lithium. The hybrid electrolyte, obtained from the synergistic effect between LAGP-Si and PEGMEM, displays high ionic conductivity and stability against the lithium anode. As a result, the polarization of the Li symmetric cell is significantly decreased by substituting pure LAGP-Si with a bilayer hybrid electrolyte. The solid-state lithium-oxygen batteries using PEGMEM@LAGP-Si electrolyte have an enhanced initial discharge-charge capacity of 7.3 mA h cm-2 and improved cyclic performance for 39 cycles, with a limited capacity of 0.4 mA h cm-2.

3. Study of garnet-based integrated architecture for high-performance all-solid-state lithium-oxygen batteries.
In this part, an integrated architecture consisting of a garnet electrolyte (Li6.4La3Zr1.4Ta0.6O12, LLZTO) and a porous composite cathode is proposed to develop high-performance all-solid-state lithium-oxygen batteries. The exceptional internal structure of the battery effectively diminishes the interfacial impedance, offers a substantial number of active sites at the triple-phase boundaries, and enhances the electrochemical stability. Consequently, the resulting batteries display an excellent cyclic performance (86 cycles) and a superior full discharge capacity of 13.04 mA h cm-2. In addition, X-ray photoelectron spectroscopy (XPS), differential electrochemical mass spectrometry (DEMS), and theoretical calculations of density functional theory (DFT) authenticate the effectiveness of this design in improving the interfacial performance, electrochemical performance, and stability of the battery. These findings are anticipated to facilitate the practical implementation of all-solid-state lithium-oxygen batteries and even other metal-oxygen (air) battery systems. Although the last work solved the contact problem between lithium anode and electrolyte, the present solid-state lithium-oxygen batteries still encounter a significant challenge due to the high impedance at the cathode/electrolyte interface. Moreover, the deficiency of triple-phase boundaries comprising Li+, e-, and O2 substantially limits the active sites for the electrochemical reaction in the battery cathode.

The findings presented herein make a significant contribution to the ongoing development of all-solid-state lithium-oxygen batteries. The current investigation demonstrates the potential of utilizing all-solid-state lithium-oxygen (air) batteries as a highly promising energy storage solution for the future. Further research endeavors are warranted to concentrate on the refinement of the design and fabrication of the solid-state battery, as well as to enhance the efficacy of interfacial modifications within this system.

Date of Award	15 Jul 2023
Original language	English
Awarding Institution	Univerisity of Nottingham
Supervisor	Yong Sun (Supervisor), Xiayin Yao (Supervisor), Jun He (Supervisor) & George Zheng Chen (Supervisor)