# Constructing Enhanced Composite Solid-State Electrolytes with Sb/Nb Co-Doped LLZO and PVDF-HFP
## 1 Introduction  
Since lithium-ion batteries (LIBs) were successfully commercialized by Sony in 1991 for the first time, they have been widely used in cell phones, notebook computers, and portable electronic products due to their high energy density, high voltage, low self-discharge, and long cycle life [1; 2; 3]. Today, the application field of LIBs has also expanded from small digital electronic devices to electric vehicles (EVs), energy storage power plants, and large-scale scenarios [4]. However, the current commercial lithium batteries use organic material as the electrolyte, which not only requires high environmental standards for the production and packing processes but also poses risks of potential safety hazards such as liquid leakage, fire, and explosions [5; 6]. As a good substitute for organic electrolytes, solid electrolytes have received much attention because they may solve safety issues and enhance the energy density of lithium-ion batteries. Solid-state electrolytes (SSEs) were usually divided into solid polymer electrolytes (SPEs), inorganic solid-state electrolytes (ISEs), and composite solid electrolytes (CSEs) [7; 8]. ISEs include oxides [9; 10], sulfides [11; 12], nitrides [13], and hydrides [14]. Oxide electrolytes show good commercial value because of their high ionic conductivity, wide electrochemical window, high strength, and good thermal stability [15; 16]. Among them, the common oxide solid electrolytes are lithium superionic conductors (LISICON), sodium superionic conductors (NASICON), perovskite Li\({}_{3}\)La\({}_{2/3-x}\)TiO\({}_{3}\) (LLTO), and garnet Li\({}_{7}\)La\({}_{3}\)Zr\({}_{2}\)O\({}_{12}\) (LLZO) [17]. Specifically, since garnet-type LLZO was first reported by [PERSON] in 2007 [18], it has been widely noticed and studied because of its potential advantages in energy density, thermal stability, and safety performance. However, for LLZO, there are two crystal phases present at room temperature: the tetragonal phase and the cubicphase. Although the tetragonal phase is more stable, its ionic conductivity is two orders of magnitude lower than that of the cubic phase. LLZO also has some shortcomings that need to be improved [19], such as cubic phase LLZO with high ionic conductivity requiring a high sintering temperature; it was unstable at room temperature and easy to decompose or convert into tetragonal-phase LLZO with low ionic conductivity during the cooling process. From the reported results, it was observed that doping is an effective strategy for LLZO to overcome this challenge [20; 21; 22]. Additionally, the application of inorganic ceramic electrolytes (ICEs) still faces some challenges, such as poor contact with the electrode and high interfacial impedance. In contrast, SPEs have better interfacial contact properties and a simpler preparation process than ICEs [23] but lower conductivity and bad thermal stability. The composite solid-state electrolytes (CSEs), composed of polymers and ceramics, not only combine the advantages of inorganic and polymer solid electrolytes but also overcome their disadvantages and have become research hotspots. Adding inorganic fillers to the polymer matrix may improve the ionic conductivity of the CSEs [24]. [PERSON] et al. [25] reported that composite electrolytes might achieve enhanced ionic conductivity and stability, resulting from the inorganic component and synergistic interactions between the polymer and inorganic phase. Since the amorphous phase in a polymer is more favorable for ion transport, combining inorganic fillers with the polymer electrolytes is an effective way to decrease glass transition temperature and suppress the crystallization of polymers. [PERSON] et al. [26] found that polymer electrolytes with dissolved Li-salts interact with the ceramic fillers through surface interactions, which increase the amorphous phase zone, facilitate Li+ migration, and decrease the activation energy for Li+ transport. And, the stronger the binding between TFSI- and inorganic filler surface, the weaker its binding with Li+ will be.