Solid Electrolyte Interphase Engineering for Improving Performances of Lithium and Sodium Batteries

Abstract

With the ever-increasing energy demands from the fast development of electric vehicles and large-scale energy storage systems, batteries with high capacities, high energy densities, and high power densities are urgently demanded. The current commercial graphite anode for lithium ion batteries (LIBs) cannot meet the requirement due to its low theoretical capacity (372 mAh g^-1). Silicon suboxide (SiO_x, 0 ≤ x ≤ 2) is considered to be an alternative anode for LIBs due to its alleviated volume change during cycling and high theoretical specific capacity. However, typical solid electrolyte interphases (SEIs) are not robust enough to protect SiO_x upon cycling, which results in structure destruction of the SiO_x structure and thus leads to poor cycling stability. Additionally, the typical SEIs, derived from side reactions between electrolytes and electrodes, are rich in organic content. It is noted that the organic-rich SEIs are lithiophilic, which can lead to uneven Li deposition during lithiation/de-lithiation. The heterogeneous Li deposition results in severe lithium-dendrite growth in a process of Li plating/stripping, causing performance degradation of lithium metal anodes (LMAs). The typical SEIs cannot suppress formatting Li-dendrite and piercing of separators which leads to circuit short and causes safety issues. Similarly, the dendrite problem that cannot be confined by the typical SEIs also plagues sodium metal anodes (SMAs). Therefore, the SEIs play an essential role in enhancing battery performance. This thesis mainly focuses on engineering the SEIs to improve the performance of SiO_x anode, LMAs, and SMAs for lithium/sodium batteries.

Chapter 1 of this thesis introduces the background of the SiO_x anodes, LMAs, and SMAs. Strategies including in-situ/ex-situ SEIs engineering and other current methodologies for improving these anodes are summarized here. Additionally, covalent organic frameworks (COF) and their application to the SiO_x anodes, LMAs, and SMAs are discussed.

Chapter 2 reports a new pre-coating approach for fabricating a conformally protective LiF layer on both anode and cathode materials (SiO_x and LiNi_0.8Co_0.1Mn_0.1O₂ (NCM811) as examples). Combining density functional theory (DFT) calculations with SEM and XPS characterizations, it was found for the first time that the pre-coating LiF layer has strong preferential adsorption of LiPF₆ molecules on the surface, leading to the formation of inorganic-rich SEI during cycling, then enabling fast Li-ion uptake/removal process and improving electrode integrity. In addition, the full cell assembled with the prepared anode (SiOG@LiF3) and cathode (NCM811@LiF3) shows outstanding cycling stability. This work not only sheds light on the working mechanism of the LiF layer, it also demonstrates a new method for engineering functional SEIs on electrodes.

Chapter 3 part of this thesis reports an electrochemically inert and catalytic sp²c covalent organic framework (sp²c-COF) separator which can induce an in-situ LiF-rich SEI to improve the electrochemical performance of the LMAs. Based on electrochemical analysis and ab-initio molecule dynamics (AIMD) calculation, it is found that nano-pores of sp²c-COF enable homogenous Li⁺ flux. Electrochemical inertness of the sp²c-COF also averts side reactions. XPS analyses show that cyano-groups on sp²c-COF are critical for generating an inorganic-rich SEI. In-situ observations and SEM images show that the Li-dendrite is greatly suppressed. Therefore, Li symmetrical cells demonstrate excellent Li plating/stripping behaviors for 3300 and 1100 h under 0.5 and 40 mA cm^-2, respectively. Additionally, full batteries assembled with LiCoO₂ and LiNi_0.8Co_0.1Mn_0.1O₂ cathodes (178 and 174.6 mg cm⁻² for LCO and NCM811 loadings) demonstrate impressive areal capacity of 18.5 and 35.0 mAh cm⁻², respectively. This work demonstrates an important strategy toward the stabilization of LMA for practical use in rechargeable batteries.

Given the good performance of the COF separator in the LMAs (Chapter 3), the same sp²c-COF separator was also applied for inducing a robust SEI to suppress Na dendrite growth considerably and enhance Na⁺ kinetics effectively (Chapter 4). XPS analyses and theoretical calculations demonstrate that the SEI is rich in NaF because the structure of NaPF₆ is unstable due to influences from the COF separator. In-situ observations show that the Na dendrite is effectively suppressed even at a high current density of 20 mA cm^-2. Satisfactorily, the COF separator exhibits a high transference number of 0.78, achieving a fast Na plating/stripping process. Therefore, a symmetric Na|COF|Na cell exhibits a stable lifespan of 1500 h even at a high current density of 20 mA cm⁻². In addition, a full cell of Na|COF|NaTi₂(PO₄)₃ (NTP) presents good rate performance (50 C) and long lifespan, obtaining capacity retention of 72% after 5000 cycles at 10 C. This work applies the multi-functional COFs to the SMAs and achieves good electrochemical performance, shedding light on an efficient strategy for the development of the SMAs.

Chapter 5 gives an overall conclusion of all the above studies mentioned and also discusses the future development of the SiO_x, LMAs, and SMAs anodes.

Date of Award	10 Aug 2023
Original language	English
Awarding Institution	City University of Hong Kong
Supervisor	Chun Sing LEE (Supervisor)