Appropriate FeF2 enhancing interface stability of lithium battery with solid-liquid hybrid electrolyte

Abstract

Solid-state electrolytes (SSEs) have attracted much attention due to their high safety and cycling stability for lithium-ion batteries. However, the high interface impedance between the electrode and the solid-state electrolyte hinders their practical application. In this work, the solid-liquid hybrid electrolyte S-Li1.3Al0.3Ti1.7(PO4)3-LE05(S-LATP-LE05) (LATP: Li1.5Al0.5Ti1.5 (PO4)3) sheet is prepared by dropping liquid electrolyte (LE) with appropriate FeF2 into spark plasma sintering S-LATP (solid-liquid hybrid electrolyte), which shows high-density and high-ionic-conductivity (5.78×10−4 S/cm). When the amount of FeF2 is 0.5 wt% , the interfacial properties between the anode and electrolyte are improved, and the S-LATP is well protected by LiF-rich (solid electrolyte interface) (SEI) interface in cycling process. The Li|S-LATP-LE05|Li symmetric battery and full battery show better electrochemical performance and stability relatively. The overpotential of the Li|S-LATP-LE05|Li symmetric battery is smaller and shows more stable electrochemical performance after cycling for 350 h, revealing good compatibility with a lithium metal anode and can inhibit the growth of lithium dendrites effectively. The Li|S-LATP-LE05|LiFePO4 full battery delivers a specific discharge capacity of 160 mA·h/g at 0.2C for 50 cycles. The corresponding coulombic efficiency is about 99.9% and displays better rate performance compared with the battery without FeF2 LE.

1. Introduction

Lithium metal has an extremely high theoretical specific capacity (3860 mA·h/g), low electrochemical potential (−3.04 V vs standard hydrogen electrode) and low density (0.53 g/cm³ at room temperature), considered to be an ideal anode material for lithium-ion batteries [1, 2]. However, in traditional liquid electrolyte-based systems [3−5], the undesired reaction between Li metal and organic liquid electrolytes (LEs), and uneven deposition behavior limit the practical application [6−9]. Due to the advantages of low flammability, excellent electrochemical stability, thermal stability, high safety and energy density, solid-state electrolyte battery is one of the leading candidates to alleviate the problems mentioned above [10−13].

Solid-state electrolytes (SSEs) can be divided into three main categories: polymer solid electrolytes (SEs), inorganic SEs, and hybrid SEs. Polymer electrolytes have high flexibility and excellent processability. In polymer SEs, poly (ethylene oxide) (PEO) [14, 15], poly vinylidene fluoride (PVDF) [16, 17], and polyacrylonitrile (PAN) [18] have been used in solid-state battery whose application is influenced by the low ionic conductivity, inferior electrochemical stability and poor mechanical strength. Inorganic SEs mainly include sulfide-based electrolytes and oxide-based electrolytes. Sulfide-based SEs contain the binary systems such as Li₃PS₄ [19], Li₇P₃S₁₁ [20], and the ternary systems, such as Li₁₀GeP₂S₁₂ (LGPS) [21], which are sensitive to water and easy to react with steam in the air to produce H₂S gas, resulting in poor chemical stability. The main types of inorganic oxide-based electrolytes are perovskite Li_0.33La_0.55TiO₃ (LLTO) [22], garnet Li₇La₃Zr₂O₁₂ (LLZO) [23] and NASICON such as Li_1.5Al_0.5Ti_1.5(PO₄)₃ (LATP) and Li_1.5Al_0.5Ge_1.5(PO₄)₃ (LAGP) [24]. Although inorganic SEs have advantages of high ionic conductivity, good thermal stability and excellent mechanical properties [25], their brittleness and stiffness affect the interface performance. Most of the hybrid SEs are composed of polymers and inorganic materials [26−28]. Hybrid SEs have the advantages of both polymer SEs and inorganic SEs, but it is still a challenge to construct continuous Li⁺ transport channels and a stable interface between Li metal and hybrid SEs [29].

LATP with NASICON structure is a kind of SSEs with excellent performance due to its large electrochemical window, and outstanding thermal stability [30, 31]. However, there are some problems that need to be solved for application. Firstly, the ionic conductivity of LATP is lower than most traditional LEs [32]. Lots of approaches have been carried out to increase the ion conductivity. For example, introducing sintering aids facili...