1. Introduction

Amid the ongoing transformation of the global energy landscape and the implementation of carbon neutrality strategies, developing high-safety electrochemical energy storage technologies has become a core issue for the large-scale adoption of renewable energy. For energy storage in extreme environments, the development of low-temperature solid-state lithium-ion batteries holds significant theoretical value and engineering potential. Breakthroughs in this technology play a key role in advancing both military-civil fusion strategies and the construction of modern technological systems [1].As activities such as high-altitude exploration and polar expeditions expand, and as emerging fields like new energy vehicles and space navigation equipment continue to evolve, the performance stability of energy storage devices under low-temperature conditions has become a major bottleneck for their practical application. This challenge mainly arises because, at low temperatures, the electrolyte tends to solidify, which dramatically increases charge transfer impedance and slows down interfacial kinetics, resulting in degraded battery performance. Research into solid-state electrolyte systems presents an important technological pathway to overcome these problems.

Currently, this field faces three main technological bottlenecks. First, failure at the solid–solid interface disrupts charge transport pathways, requiring interface modification strategies to construct three-dimensional conductive channels. Second, while sulfide-based electrolytes offer high ionic conductivity, they suffer from a trade-off between chemical stability and manufacturing feasibility, which hinders their industrialization. Existing element doping approaches have not fully resolved the inherent material defects. Third, oxide-based electrolytes need to overcome high grain boundary resistance, while polymer-based materials face challenges in achieving a balance between mechanical strength and ionic conductivity. These challenges demonstrate that despite promising research progress, the practical application of solid-state electrolytes still faces significant hurdles.

Solid-state electrolytes are the core component of all-solid-state batteries and are generally categorized into four main types: oxides, sulfides, polymers, and composite electrolytes. Perovskite-type electrolytes, such as LLTO, offer relatively high ionic conductivity (~10⁻⁴ S/cm), but suffer from severe interfacial side reactions with metallic sodium. Garnet-type electrolytes, like LLZO, have excellent chemical stability but require sintering at temperatures above 1200 °C, which leads to high grain boundary resistance. Sulfide-based electrolytes, such as Na₃PS₄, exhibit ionic conductivities up to 10⁻³ S/cm at room temperature, but are extremely sensitive to moisture and lack thermal stability [2]. In contrast, NASICON-type sodium superionic conductors have attracted significant attention due to their three-dimensional interconnected pore structures, wide electrochemical window, and excellent thermal stability [3].

In this paper, we systematically summarize several key aspects of NASICON-type electrolytes—as shown in Figure 1—including their crystal structures, ion transport mechanisms, practical applications, and doping modification strategies. We aim to provide a comprehensive overview of the current challenges facing NASICON materials and offer forward-looking perspectives and potential solutions.

2. Crystal Structure and Ion Transport Mechanism of Nasicon-type Lithium-ion Electrolytes

2.1. Crystal Structure Characteristics

NASICON-type solid electrolytes generally adopt the formula AM'M''P₃O₁₂, where A is a monovalent alkali metal cation (e.g., Li⁺, Na⁺, or K⁺), and M', M'' are multivalent metal cations such as Ti⁴⁺, Zr⁴⁺, or Ge⁴⁺. When A is lithium, the compound becomes a NASICON-type lithium-ion electrolyte, typically with the formula Li(A₂P₃O₁₂). The structure comprises a three-dimensional framework of interconnected [AO₆] octahedra and [PO₄] tetrahedra, creating open ion-conducting channels. Lithium ions mainly occupy the M1 sites, which are octahedrally coordinated and provide low-energy migration pathways, while M2 sites in interstitial spaces assist in ion diffusion. Doping with ions like Ti⁴⁺ at M2 sites can distort the lattice and widen migration channels, improving ionic conductivity. Despite these favorable structural features, performance is still limited by factors such as material densification and the concentration of mobile lithium ions [4].

2.2. Ion Transport Mechanism

Li⁺ ions in NASICON-type structures migrate through a three-dimensional network via two main pathways: straight channels along the c-axis and zigzag routes involving hopping between M1 and M2 sites. This migration is influenced by local energy barriers caused by distortions in the [MO₆] octahedra. Enhancing ionic conductivity depends on minimizing these distortions and improving channel connectivity [5].For instance, doping with Sc³⁺ reduces activation energy to around 0.2 eV, enabling conductivities up to 10⁻³ S/cm at room temperature. However, excessive Li⁺ concentration (>50%) can lead to electrostatic repulsion and hinder diffusion. Thus, optimizing the M-site environment through doping (e.g., with Sc³⁺ or Al³⁺) is critical for improving ion transport performance in NASICON electrolytes.

3. Applications of Nasicon-type Solid Electrolytes

3.1. LiTi₂(PO₄)₃ (LTP)

LiTi₂(PO₄)₃ (LTP) is a representative NASICON-type solid electrolyte, demonstrating excellent battery performance with a discharge capacity of 80 mAh·g⁻¹ when used with a LiMn₂O₄ thin-film cathode, as shown by Dokko et al [6]. using a sol–gel coating method. The synthesis approach significantly affects LTP’s properties; for example, Sharma et al. enhanced grain boundary conductivity by forming a glass–ceramic composite through ball-milling LTP with ion-conducting glass ((Li₂SO₄)₃–(LiPO₃)₂), achieving an ionic conductivity of ~10⁻⁴ S/cm at room temperature.

3.2. LiGe2(PO4)3(LGP)

LiGe₂(PO₄)₃ (LGP) is a NASICON-type solid electrolyte with mobile Li⁺ ions and exhibits high ionic conductivity (~10⁻⁴ S/cm) when doped with Al³⁺. Unlike LiTi₂(PO₄)₃ (LTP), which suffers from Ti⁴⁺ undergoing redox reactions with lithium metal, compromising stability, LGP’s Ge⁴⁺ is more chemically stable. Additionally, LGP offers a wide electrochemical stability window (1.8–7 V), making it a promising candidate for high-voltage lithium-ion batteries [7].

3.3. LiZr2(PO4)3(LZP)

LiZr₂(PO₄)₃ (LZP) is a NASICON-type solid electrolyte with good compatibility with lithium metal and moderate ionic conductivity (~5 × 10⁻⁶ to 2 × 10⁻⁴ S/cm), attributed to Frenkel-type defect migration. Unlike LTP and LGP, LZP forms a stable solid electrolyte interphase (SEI) through redox reactions with lithium, enhancing interfacial stability [8]. It remains electrochemically stable up to 5.5 V and demonstrates good cycling performance, with discharge capacities of 140 and 120 mAh·g⁻¹ at different current densities. However, its lower conductivity compared to titanium-based materials and higher processing demands limit broader application [9].

4. Preparation Methods of Nasicon-type Lithium-ion Electrolytes

4.1. Conventional Synthesis Techniques

Conventional methods for preparing NASICON-type lithium-ion electrolytes include high-temperature solid-state reactions and the sol–gel method [10].The solid-state method involves mixing oxide precursors and calcining at >1000 °C, offering simplicity and low cost but suffering from issues like uneven particle size and high grain boundary resistance[11].In contrast, the sol–gel method forms nanoscale powders through hydrolysis and low-temperature treatment (600–800 °C), yielding higher surface area and improved ionic conductivity (up to 6.15 × 10⁻⁴ S/cm) [12].Though more complex and costly, the sol–gel method is favored for producing high-performance NASICON electrolytes [13].

4.2. Emerging Preparation Techniques

Emerging techniques for synthesizing NASICON electrolytes include co-precipitation, spark plasma sintering (SPS), and melt-quenching. Co-precipitation yields fine, high-purity powders with enhanced conductivity (up to 4.2 × 10⁻⁴ S/cm). SPS enables rapid densification at lower temperatures, significantly reducing grain boundary resistance. Melt-quenching forms glass-ceramics with tunable phase composition and high ionic mobility, though it requires precise process control[14]. These advanced methods improve performance and highlight future efforts to simplify processing, ensure uniformity, and enhance material purity.

5. Modification Strategies and Performance Optimization

Modification strategies for NASICON-type electrolytes focus on reducing impedance from grain boundaries and the bulk phase through optimized synthesis and doping. A-site doping—by introducing lower-valence ions like Ca²⁺ or higher-valence ions such as Sc³⁺, Y³⁺, or Cr³⁺—can adjust lattice parameters, create vacancies, and widen Li⁺ migration channels, significantly enhancing ionic conductivity. Co-doping (e.g., Y³⁺/Er³⁺) further improves performance. Though less studied, P-site substitution—replacing PO₄ groups with [SiO₄] or [VO₄]—modifies the crystal framework, lowers migration barriers, and boosts conductivity.

6. Challenges and Future Perspectives

Despite their advantages in ionic conductivity and stability, NASICON-type solid electrolytes face key challenges: limited ionic conductivity (typically ~10⁻³ S/cm), poor interfacial compatibility with lithium metal (e.g., Ti⁴⁺ reduction), and manufacturing difficulties due to high sintering temperatures and material costs. To overcome these issues, future research should explore broader doping strategies—including substitutions at M, P, or Li sites—and develop advanced composite materials to lower grain boundary resistance and enable efficient low-temperature Li⁺ transport, supporting the practical application of NASICON electrolytes.