Electrochemical properties of sandwich-structured solid electrolyte for all-solid-state Li battery

Abstract

To improve the solid–solid interface performance of all solid-state lithium batteries (ASSLBs), a novel sandwich-structured solid electrolyte (SSE, total thickness of 0.7 mm) was investigated. It comprises a central layer of perovskite-type Li0.37Sr0.44Zr0.25Ta0.75O3 (LSZT) electrolyte (thickness of 0.5 mm) sandwiched between two layers of composite solid polymer electrolyte (CSPE, each with a thickness of 0.1 mm). The thin CSPE interlayer not only effectively reduces interfacial resistance between LSZT and electrodes, but also suppresses Li-induced reduction degradation of LSZT while ensuring uniform current density distribution across the interface. The SSE demonstrates an ionic conductivity of 8.76 × 10−5 S·cm−1 at 30°C, increasing to 1.13 × 10−3 S·cm−1 at 100°C, with an activation energy of 0.36 eV. In addition, SSE is stable for Li metal and achieves electrochemical stability up to 4.58 V vs. Li+/Li. SSE shows outstanding electrode/electrolyte interfacial compatibility and significant suppression of the growth of Li dendrite. Ascribing to these merits, Li | SSE | Li symmetric cell maintained stable operation for 500 h at a current density of 0.3 mA·cm−2 without short circuit, confirming robust inter-facial compatibility between SSE and Li electrode. The all-solid-state LiFePO4 | Li battery with SSE has an initial reversible discharge capacity of 109.8 mAh·g−1 and a reversible capacity of 118.1 mAh·g−1 after 50 cycles at a charge/discharge rate of 0.1C (30°C), demonstrating good cycling performance.

1. Introduction

All solid-state lithium (Li) batteries (ASSLBs) employing Li-ion conducting solid electrolyte (SE) and Li anode are considered promising candidates for enhancing battery safety and energy density to meet the escalating demands of electric vehicles [1–3]. ASSLBs with thin Li+-conductive solid electrolytes couple with high-voltage/capacity cathodes demonstrate superior safety and higher energy densities compared to traditional rechargeable Li-ion batteries using liquid organic electrolytes [4–6]. SEs, as critical components of ASSLB, are generally categorized into oxides, sulfides, and polymer SEs [7–8]. To achieve practical implementation, SEs should have high Li-ion conductivity, high mechanical strength, and good electrochemical stability [9–10]. Nevertheless, two critical challenges persist: (i) Insufficient electrochemical stability of inorganic SEs against Li metal anodes; (ii) Excessive interfacial resistance at solid–solid electrode/electrolyte interfaces [4,11–12].

The high solid–solid interfacial resistance could stem from factors such as insufficient contact area between electrolyte and electrode, electrolyte thickness limitations, and hindered Li-ion migration at the interface [13]. These factors collectively elevate local current density at the interface, which may induce rapid battery short-circuiting [14]. To address this, researchers have introduced trace amounts of liquid electrolyte at the solid–solid interface to enhance wettability and reduce impedance, subsequently integrating this hybrid electrolytes into Li batteries [15–19]. Furthermore, SEs are recognized for their high compactness and low electronic conductivity properties that effectively inhibit Li dendrite growth. Although this strategy substantially decreases liquid electrolyte content relative to conventional designs, the residual liquid phase still introduces non-negligible safety concerns. Consequently, such hybrid systems should be viewed as transitional technologies rather than ultimate solutions [20]. Zhao et al. [21] proposed a novel organic acid treatment (OAT) strategy to reduce interface impedance, which constructed an interface layer with both electron-blocking and lithiophilic properties through the spontaneous reaction of formic acid (HCOOH) with Li2CO3 on the surface of the garnet electrolyte. After treatment, Li symmetric cells exhibit low interfacial impedance (3 Ω·cm2) and high critical current density (1.7 mA·cm−2) at room temperature.

In contrast to inorganic SEs, the flexible polymer SEs composed of Li salt and high-molecular polymer, which exhibit superior interfacial contact and stability with Li metal anodes, effectively reducing solid–solid interfacial resistance in ASSLB [22–26]. Liu et al. [27] reported an ultra-thin composite solid-state electrolyte membrane (SPE/SiO2–SO3Li) with a thickness of only 3 μm. It exhibits good electrochemical properties and robust mechanical properties. The electrolyte shows no thermal shrinkage at 180°C and effectively suppresses lithium dendrite growth, demonstrating its high safety and practical potential. Wei et al. [28] designed a novel gel polymer electrolyte (UPP-5), which is composed of ionic liquid-incorporated metal and organic framework nanoparticles (IL@MOFs). It exhibited stable lithium stripping/plating behavior (1000 h without short-circuiting) with low interfacial resistance in symmetric cells, due to the