As the demand for higher energy density and enhanced safety in energy storage systems grows, solid-state batteries have emerged as a promising alternative to conventional lithium-ion batteries. Traditional liquid electrolytes face limitations such as instability at high voltages, leakage risks, and dendrite formation, which can lead to short circuits. In contrast, solid-state batteries utilize solid electrolytes, offering improved safety and compatibility with high-voltage cathodes and lithium metal anodes. However, challenges like ion transport barriers across phases, interfacial impedance, and inefficient ion/electron transport in thick electrodes hinder their commercialization. To address these issues, we propose “lithium-ion transport throughput” as a comprehensive metric to evaluate the performance of solid-state batteries. This review examines strategies to enhance lithium-ion transport throughput by focusing on bulk ion transport in solid electrolytes, electrode/electrolyte interface design, and synergistic ion/electron networks within electrodes. By integrating material innovations and structural optimizations, we aim to pave the way for high-performance solid-state batteries.
The lithium-ion transport throughput, denoted as $\Phi_{Li^+}$, quantifies the moles of lithium ions passing through the electrode/electrolyte interface per unit area per hour during charge/discharge cycles. It is defined by the equation:
$$\Phi_{Li^+} = \frac{10000 \cdot C_{area}}{C_{Li} \cdot M_{Li} \cdot t}$$
where $C_{area}$ is the areal capacity in mA·h·cm⁻², $C_{Li}$ is the theoretical specific capacity of lithium metal (3860 mA·h·g⁻¹), $M_{Li}$ is the molar mass of lithium (6.941 g·mol⁻¹), and $t$ is the charge or discharge time in hours. This metric integrates areal capacity and current density, providing a holistic view of the electrochemical processes within solid-state batteries. Compared to external parameters like current density, $\Phi_{Li^+}$ better reflects internal ion transport efficiency, which is critical for optimizing solid-state battery performance.
Recent studies have demonstrated significant progress in enhancing $\Phi_{Li^+}$ for various solid-state battery systems. For instance, sulfide-based solid-state batteries have achieved $\Phi_{Li^+}$ values up to 1.977 mol·m⁻²·h⁻¹ under high-rate conditions, while polymer-based systems reach 0.485 mol·m⁻²·h⁻¹. Despite these advances, solid-state batteries still lag behind liquid electrolyte systems, which can achieve $\Phi_{Li^+}$ values exceeding 2.799 mol·m⁻²·h⁻¹. To bridge this gap, we explore three key areas: improving bulk ion transport in solid electrolytes, optimizing interfacial ion transport, and enhancing ion/electron conduction within electrodes.
Enhancing Bulk Ion Transport in Solid Electrolytes
The ionic conductivity of solid electrolytes is a fundamental factor influencing lithium-ion transport throughput in solid-state batteries. Inorganic solid electrolytes, such as oxides, sulfides, and halides, typically exhibit ionic conductivities ranging from 10⁻⁴ to 10⁻² S·cm⁻¹, but they often fall short of liquid electrolytes (∼10⁻² S·cm⁻¹). Ion transport in these materials relies on lithium ions hopping between lattice sites, influenced by factors like anion frameworks, cation arrangements, and vacancy concentrations. Strategies to enhance ionic conductivity include introducing high-entropy structures, amorphous phases, and optimized vacancy densities.
High-entropy doping has proven effective in creating local disorder, which broadens the energy distribution of adjacent sites and facilitates ion hopping. For example, high-entropy sulfide electrolytes like Li₉.₅₄[Si₀.₆Ge₀.₄]₁.₇₄P₁.₄₄S₁₁.₁Br₀.₃O₀.₆ achieve room-temperature ionic conductivities of 32 mS·cm⁻¹, enabling $\Phi_{Li^+}$ values of 0.21 mol·m⁻²·h⁻¹ in thick electrodes. Similarly, halide-based solid electrolytes, such as Li₂.₇₅Y₀.₁₆Er₀.₁₆Yb₀.₁₆In₀.₂₅Zr₀.₂₅Cl₆, exhibit enhanced conductivity and stability due to multi-cation incorporation. Amorphous solid electrolytes, like Li–Ta–Cl systems, offer isotropic ion transport pathways with conductivities up to 7.16 mS·cm⁻¹, though their disordered structures pose characterization challenges.
Polymer-based solid electrolytes, such as poly(ethylene oxide) (PEO) composites, provide flexibility and good interfacial contact but suffer from low room-temperature ionic conductivity (∼10⁻⁵ to 10⁻⁴ S·cm⁻¹). To address this, composite electrolytes incorporating inorganic fillers (e.g., LLZO or LATP) form percolation networks that enhance ion transport. For instance, PEO–LPSC composites with tailored organic–inorganic interfaces achieve conductivities of 2.47 × 10⁻⁴ S·cm⁻¹, leading to $\Phi_{Li^+}$ values of 0.063 mol·m⁻²·h⁻¹. Advanced characterization techniques, such as solid-state NMR and synchrotron diffraction, are crucial for understanding ion transport mechanisms and guiding material design.
The following table summarizes key solid electrolytes and their impact on lithium-ion transport throughput:
| Electrolyte Type | Ionic Conductivity (S·cm⁻¹) | Key Strategy | $\Phi_{Li^+}$ (mol·m⁻²·h⁻¹) |
|---|---|---|---|
| High-Entropy Sulfide | 3.2 × 10⁻² | Multi-cation doping | 0.21 |
| Halide (HE-LIC) | 1.04 × 10⁻³ | Local disorder | 0.23 |
| Amorphous Halide | 7.16 × 10⁻³ | Glass-phase network | N/A |
| PEO–LLZO Composite | 2.26 × 10⁻⁴ | Percolation network | 0.042 |
| PVDF-Based Composite | 1.03 × 10⁻³ | Weak coordination environment | 0.48 |
Ionic conductivity ($\sigma$) can be modeled using the Nernst-Einstein equation:
$$\sigma = \frac{n q^2 D}{k_B T}$$
where $n$ is the carrier concentration, $q$ is the charge, $D$ is the diffusion coefficient, $k_B$ is Boltzmann’s constant, and $T$ is temperature. Enhancing $D$ through structural modifications is key to improving $\sigma$ and, consequently, $\Phi_{Li^+}$ in solid-state batteries.
Optimizing Electrode/Electrolyte Interfacial Ion Transport
Interfacial impedance between electrodes and solid electrolytes is a major bottleneck for lithium-ion transport throughput in solid-state batteries. Solid–solid contacts often lead to point-to-point interactions, increasing resistance and limiting ion flux. Strategies to mitigate this include designing porous interfacial layers, mixed conductive interlayers, and alloy-based coatings.
Porous structures, such as mixed ion–electron conductive (MIEC) layers in garnet electrolytes, distribute potential uniformly and reduce local hotspots, enabling critical current densities up to 100 mA·cm⁻² and $\Phi_{Li^+}$ values of 0.69 mol·m⁻²·h⁻¹. Similarly, polymer electrolytes with engineered pores inhibit crystallization and enhance interface stability. Mixed conductive interlayers, like Li₇N₂I–CNT or Li₇N₂I–Mg, combine high ionic conductivity with controlled lithiophobicity, guiding lithium deposition and achieving $\Phi_{Li^+}$ up to 0.44 mol·m⁻²·h⁻¹. Alloy-based interfaces (e.g., Li–Si or Li–Mg) form conductive buffers that improve wettability and cycle stability. For instance, Mg–Bi interlayers in LPSC-based solid-state batteries facilitate $\Phi_{Li^+}$ of 0.939 mol·m⁻²·h⁻¹ at elevated temperatures.
On the cathode side, coatings like LiI or LiNbO₃ suppress electrolyte decomposition and promote ion diffusion. High-voltage stability is achieved through halide or oxide layers that mitigate space-charge effects. The table below highlights interfacial strategies and their outcomes:
| Interface Type | Key Material/Design | Function | $\Phi_{Li^+}$ (mol·m⁻²·h⁻¹) |
|---|---|---|---|
| Porous MIEC | Ta-LLZO with porous layers | Stress distribution | 0.69 |
| Mixed Conductive | Li₇N₂I–CNT | Guided Li deposition | 0.44 |
| Alloy-Based | Mg–Bi on LPSC | Enhanced wettability | 0.939 |
| Cathode Coating | LiNbO₃ on NCM | Decomposition suppression | 0.037 |
| Polymer Composite | PEO with dielectric fillers | Space-charge layer mitigation | 0.37 |
The interfacial ion transport rate can be described by the Butler-Volmer equation, modified for solid-state systems:
$$j = j_0 \left[ \exp\left(\frac{\alpha n F \eta}{RT}\right) – \exp\left(-\frac{(1-\alpha) n F \eta}{RT}\right) \right]$$
where $j$ is the current density, $j_0$ is the exchange current density, $\alpha$ is the transfer coefficient, $n$ is the number of electrons, $F$ is Faraday’s constant, $\eta$ is the overpotential, $R$ is the gas constant, and $T$ is temperature. Reducing $\eta$ through interface engineering is crucial for maximizing $\Phi_{Li^+}$ in solid-state batteries.
Enhancing Ion/Electron Transport in Thick Electrodes
High-loading electrodes are essential for achieving high energy density in solid-state batteries, but they often suffer from inefficient ion and electron transport, leading to low active material utilization. To address this, researchers have developed integrated ion–electron conduction networks, aligned structures, and composite electrodes that reduce tortuosity and enhance transport kinetics.
For example, “solid–polymer–solid” elastic networks using La₂Zr₂O₇ nanofibers in PEO-based cathodes facilitate rapid ion transport, enabling $\Phi_{Li^+}$ of 0.28 mol·m⁻²·h⁻¹ at 3 C. Magnetic-field-aligned LLTO nanowires in LiFePO₄ electrodes create vertical ion percolation paths, achieving $\Phi_{Li^+}$ of 0.11 mol·m⁻²·h⁻¹ with areal capacities of 3 mA·h·cm⁻². Dual-conductive frameworks (DCFs) that combine ionic and electronic pathways reduce interfacial complexity and lower tortuosity to 1–4, compared to 1.5–10 in conventional electrodes. In lithium metal anodes, composite structures like Li–Mg–graphite improve diffusion rates and maintain stable interfaces, supporting $\Phi_{Li^+}$ of 0.10 mol·m⁻²·h⁻¹ at high stripping capacities.
Innovative designs, such as single-phase electrolyte–electrode systems (e.g., Li₃TiCl₆ or Li₃VCl₆), merge ion conduction and redox activity, simplifying interfaces and boosting capacity. These approaches highlight the importance of holistic electrode design for improving lithium-ion transport throughput in solid-state batteries.
The following table compares electrode optimization strategies:
| Electrode Type | Key Feature | Areal Capacity (mA·h·cm⁻²) | $\Phi_{Li^+}$ (mol·m⁻²·h⁻¹) |
|---|---|---|---|
| LZON–PEO Composite | Elastic ion network | ~2.37 | 0.28 |
| Aligned LLTO Nanowires | Vertical ion channels | 3.00 | 0.11 |
| Li–Mg–Graphite Anode | Enhanced Li diffusion | ~0.54 | 0.10 |
| Dual-Conductive Framework | Low tortuosity | N/A | N/A |
| Li₃VCl₆ Hybrid Cathode | Redox-active electrolyte | N/A | 0.062 |
The effective ion diffusion in porous electrodes can be modeled using Fick’s law with a tortuosity factor ($\tau$):
$$D_{eff} = \frac{D}{\tau^2}$$
where $D_{eff}$ is the effective diffusion coefficient and $D$ is the intrinsic diffusion coefficient. Minimizing $\tau$ through aligned structures or composite networks enhances $D_{eff}$ and $\Phi_{Li^+}$ in solid-state batteries.
Conclusion and Future Perspectives
Solid-state batteries represent a transformative technology for next-generation energy storage, but their performance hinges on overcoming ion transport limitations. The lithium-ion transport throughput ($\Phi_{Li^+}$) serves as a vital metric for evaluating and optimizing these systems. By advancing bulk ion transport in solid electrolytes, refining interfacial designs, and engineering efficient electrode networks, we can significantly enhance $\Phi_{Li^+}$ and bridge the gap with liquid electrolyte batteries.
Future research should focus on developing low-cost, environmentally friendly solid electrolytes with higher ionic conductivities, stable interfaces that withstand long-term cycling, and scalable electrode architectures that balance energy and power density. Interdisciplinary approaches, combining advanced characterization (e.g., in situ spectroscopy and NMR), theoretical modeling, and machine learning, will accelerate material discovery and system integration. As solid-state batteries evolve, they hold immense potential for applications in electric vehicles and grid storage, contributing to a sustainable energy future.
In summary, the pursuit of higher lithium-ion transport throughput in solid-state batteries requires a synergistic approach across multiple domains. Through continued innovation, we can unlock the full potential of solid-state batteries and achieve the safety and performance standards demanded by modern energy storage needs.