Investigation to electrolytes of lithium-ion batteries for high safety

Abstract

Lithium-ion batteries (LIBs) with the advantages of high energy and cycle stability have occupied an absolutely dominant position in the field of energy storage applications, such as portable devices and electric vehicles. However, the safety hazard associated with traditional flammable electrolytes and graphite anodes that are easy to grow dendrites has always been the focus of attention. Though a lot of materials and methods have been proposed to ensure the safety of the battery, more smart and efficient technologies are needed to achieve battery safety while ensuring the cycling stability of the battery. We begin with a high conductivity and stability gel electrolyte composed of PVDF HFP matrix Pyr₁₃FSI and LiFSI. The presence of low viscosity Pyr₁₃FSI and small size LiFSI in the electrolyte reduces the crystallinity of PVDF-HFP polymer matrix, increases the ion conductivity (3.3 mS/cm) of the electrolyte, and greatly improves the electrode-electrolyte interface contact, which enables the battery to exhibit a specific capacity of 123 mAh/g at the current of 1 C at room temperature. In addition, benefit from the superior properties of ionic liquids, such as non-flammability and negligible vapor pressure, and the highly stable and safe LFP/LTO system, the final battery maintains 80% of its initial capacity after 2000 cycles and high safety when exposed to high temperature or fire. To develop a more smart and safe battery, we develop a thermos-responsive separators prepared through in-situ polymerization on the hydrophilic separator and use this separator in an LMO/C-LTP aqueous lithium-ion battery. The thermos-responsive separator blocks the lithium ion transport channels at high temperatures and reopens when the battery cools down; more importantly, this transition is reversible. The influence of lithium salts on the thermos-responsive behaviors of the hydrogels was investigated. Then suitable lithium salt (LiNO₃) and concentration (1 M) was selected in the electrolyte to achieve self-protection without sacrificing battery performance. The shut-off temperature can be tuned from 30 to 80 °C by adjusting the hydrophilic and hydrophobic moiety ratio in the hydrogel for targeted applications. This self-protecting LMO/C-LTP battery shows promise for smart energy storage devices with high safety and extended lifespan in case of high operating temperatures. Finally, to improve the cycling stability and safety of the aqueous lithium-ion battery, we further develop a hybrid electrolyte with a wide electrochemical window (2.15 V) and a low freezing point (-60 °C) by using 1-ethyl-3-methylimidazolium diethyl phosphate (EMIMDep) as a novel additive. The hydrophobic EMIM+ accumulates on the negatively charged electrode and repels the water molecules, thus suppressing the water splitting. Meanwhile, the hydrophilic Dep- forms strong hydrogen bonds with water, thereby reducing the freezing point of the electrolyte. In addition, the hybrid 1 M LiNO₃ in EMIMDep₂₀-H₂O₈₀ electrolytes exhibits high safety and stability due to the non-flammability, non-volatility, and low toxicity of the EMIMDep compared with other organic additives. Owing to the advantages of the water/EMIMDep electrolyte, the full battery with LiTi₂(PO₄)₃ anode and LiMn₂O₄ cathode delivers an average voltage of 1.6 V and a specific capacity of 120 mAh/g with a capacity retention of 80% after 500 cycles at 1 C. In addition, the full battery working at -35 °C delivers 60% specific capacity of that at room temperature.

Date of Award	2022
Original language	English
Awarding Institution	The Hong Kong University of Science and Technology
Supervisor	Baoling HUANG (Supervisor)