Development of high-performance solid-state lithium metal batteries

Abstract

Solid-state lithium metal batteries (SSLMBs) that use a lithium metal anode and nonflammable solid-state electrolyte (SSE) promise to offer considerably higher energy density and safety than state-of-the-art lithium-ion batteries do, thus showing great potential for next-generation energy storage applications. However, the development of SSLMBs is hindered by several critical issues, including poor interfacial contact between SSEs and electrodes, low ionic conductivities, large thicknesses, and narrow electrochemical windows of SSEs, which result in poor rate performance and short cycle life. This thesis aims to address these core challenges and create high-performance SSLMBs. We begin with overcoming the poor contact between the lithium metal anode and SSE by magnetron sputtering a SnO₂ nanolayer onto the garnet-type Li_6.4La₃Zr_1.4Ta_0.6O₁₂ (LLZTO). A highly Li⁺-conducting interlayer is automatically formed between the garnet and lithium metal after lithiation, thus significantly reducing the interfacial resistance from 1019 to 153 Ω cm². To reduce the production cost, we further develop a facile, scalable, and cost-effective spinning coating strategy to form a silica nanofilm on the LLZTO surface, which reduces the interfacial resistance to be as low as 49 Ω cm². To enhance the mechanical strength of SSEs, we design and fabricate a composite solid electrolyte that incorporates a 3D perovskite Li_0.33La_0.557TiO₃ (LLTO) nanofiber network and polyethylene oxide (PEO)-LiTFSI matrix, which inherits both the high shear modulus of perovskite and the flexibility of PEO polymer. A thin PEO layer is meticulously prepared on either side of the PEO-perovskite composite to simultaneously improve the electrolyte/electrodes interfaces. This novel sandwich design enables a full Li/LiFePO₄ battery to deliver a high capacity of 135.0 mAh g^-1 at 2 C at 60 ℃ with a high capacity retention of 79.0% after 300 cycles. To improve the ionic conductivities of composite electrolytes, we further develop a vertically-aligned LLTO framework by the ice-templating method, which provides fast, continuous, and the shortest pathways for Li⁺ transport, thus boosting the ionic conductivity from 0.038 to 0.13 mS cm^-1. As a result, a Li/LiFePO₄ full battery assembled with the developed electrolyte delivers a specific discharge capacity of 144.6 mAh g^-1 at 1 C at 60 ℃ with a high capacity retention of 96.0% after 100 cycles. To broaden the electrochemical windows of SSEs and promote the pairing of lithium metal anodes with high-voltage cathodes to achieve high energy density SSLMBs, a poly(acrylonitrile) (PAN)-LiClO₄-boron nitride nanoflake (BNNF) composite electrolyte modified by a BNNF layer (PBCEB) is developed. It is demonstrated that the PAN-LiClO₄-BNNF composite can sustain an oxidation voltage up to 4.5 V vs. Li/Li⁺, while the BNNF modifying layer prevents the PAN-LiClO₄-BNNF from reduction reaction with lithium metal anode. Thus, a Li/LiNi_0.8Co_0.1Mn_0.1O₂ full battery can deliver a high capacity of 173.6 mAh g^-1 at 0.2 C and be stably cycled for over 350 cycles at 1 C with a capacity retention of 68.1%. Finally, we rationally design and fabricate a Janus-faced, 3D perovskite LLTO nanofiber framework reinforced composite electrolyte (JPCE) to simultaneously achieve a wide electrochemical window (0 ~ 4.5 V vs. Li/Li⁺), high ionic conductivity (0.1 mS cm^{- 1}), and small thickness (24 μm). Excitingly, when paired with a lithium metal anode and a LiNi_0.8Co_0.1Mn_0.1O₂ cathode, the SSLMB delivers a reversible discharge capacity of as high as 176.1 mAh g^-1 at 0.2 C at room temperature, displaying the great promise of the JPCE for high-voltage SSLMBs applications. Keywords: Solid-state electrolyte; solid-state lithium metal battery; surface modification; interfacial resistance; ionic conductivity; electrochemical window; high-voltage cathode.

Date of Award	2020
Original language	English
Awarding Institution	The Hong Kong University of Science and Technology
Supervisor	Tianshou ZHAO (Supervisor)