The escalating global energy and environmental crises have intensified the need for sustainable energy solutions. In response, numerous policies have been implemented, with a clear focus on achieving peak carbon dioxide emissions by 2030 and carbon neutrality by 2060. Within this framework, the development of new energy technologies has become a paramount task. Energy storage is indispensable for any clean energy system, and batteries play a pivotal role in this endeavor. Lithium-ion batteries have been extensively researched and applied due to their cycle life and high operating voltage. However, conventional lithium-ion batteries, which utilize liquid organic electrolytes, face limitations in their operating environments and efficiency.
In recent years, solid state batteries have garnered increasing attention. Their high rate performance, long cycle life, high energy density, and broad operating conditions enable them to fundamentally address the issues plaguing traditional lithium batteries. This article explores the composition, advantages, research progress, and challenges of solid state batteries, with a particular focus on electrolyte development and interfacial issues.
The composition of solid state batteries is similar to that of traditional lithium-ion batteries, with the key difference being the electrolyte. In solid state batteries, the electrolyte is solid, replacing the conventional liquid electrolyte and separator, while the electrodes remain largely unchanged. This configuration maintains the “rocking-chair” battery mechanism. The solid-state electrolyte, often referred to as a fast ion conductor, is the core material. It exists in a solid state, exhibits excellent electronic insulation, and possesses high ionic conductivity, typically several orders of magnitude higher than its electronic conductivity. The solid-state electrolyte itself has certain mechanical strength and lithium-ion conduction capability, effectively serving as the medium for ion transport.
The working principle of a solid state battery involves lithium ions de-intercalating from the cathode material during charging, migrating through the solid-state electrolyte, and embedding into the anode material. The reverse occurs during discharge. Ion transport within the electrolyte is driven by the electrochemical potential gradient. The Nernst-Einstein relation, combined with Fick’s diffusion laws, can describe the ion conduction process in solid-state electrolytes. The ionic flux, J, can be expressed using Fick’s first law:
$$ J = -D \frac{\partial c}{\partial x} $$
where D is the diffusion coefficient, c is the ion concentration, and x is the spatial coordinate. The Nernst-Einstein relation connects the diffusion coefficient to the ionic mobility, μ:
$$ D = \frac{\mu k_B T}{q} $$
where k_B is Boltzmann’s constant, T is the absolute temperature, and q is the charge of the ion. These relationships are crucial for understanding ion transport in solid state batteries.
Solid state batteries offer several significant advantages over their liquid electrolyte counterparts, including high energy density, enhanced safety, and flexibility. Firstly, the high energy density stems from the replacement of liquid electrolytes and separators with solid electrolytes. In traditional batteries, the liquid electrolyte accounts for approximately 25% of the mass and 40% of the volume. Solid state batteries can reduce the distance between electrodes to mere micrometers, drastically improving volumetric energy density. Furthermore, the use of lithium metal anodes instead of graphite can significantly increase capacity. Lithium metal has the lowest electrochemical potential in nature, allowing for pairing with a wider range of high-specific-energy cathode materials. These factors contribute to higher energy density at the battery pack level after system integration, reducing assembly costs and improving efficiency. Secondly, the high safety of solid state batteries is a critical advantage. Traditional liquid electrolytes are organic and can generate substantial heat during prolonged operation. Moreover, lithium dendrite formation on the anode is a common issue. Liquid electrolyte leakage, combined with high temperatures, can lead to combustion and explosion. Solid-state electrolytes are non-volatile, non-corrosive, and non-flammable, eliminating leakage risks. They also suppress lithium dendrite growth, inherently ensuring operational safety. Thirdly, the flexibility of solid state batteries is enabled by their high energy density and the potential for thin electrolyte layers. When the solid electrolyte layer is reduced to sub-millimeter thickness, it can achieve pliability and bendability. With appropriate encapsulation techniques, this allows for the development of flexible batteries and wearable devices.
The research progress and challenges in solid state batteries primarily revolve around the solid-state electrolyte and interfacial issues. The solid-state electrolyte serves the dual role of internal ion transport and separator. Its properties directly influence the electrochemical performance of the solid state battery. An ideal solid-state electrolyte should meet several criteria: high ionic conductivity, high ionic transference number, low activation energy for reaction, good chemical compatibility, excellent mechanical properties, low cost, environmental friendliness, ease of fabrication, wide electrochemical stability window, and good chemical stability. The development of solid-state electrolytes began with Faraday’s discovery of ionic conductivity in Ag₂S in the 1830s, but the true starting point was the application of alumina in high-temperature sodium-sulfur batteries. The discovery of polymer polyethylene oxide (PEO) in the 1980s broke the boundary of inorganic materials. Currently, research focuses on three main types: polymer solid-state electrolytes, sulfide solid-state electrolytes, and oxide solid-state electrolytes.
| Electrolyte Type | Ionic Conductivity (S/cm) | Advantages | Disadvantages | Common Materials/Modifications |
|---|---|---|---|---|
| Polymer | 10⁻⁸ – 10⁻⁹ (base), improved with fillers | Flexible, easy processing, good interfacial contact | Low intrinsic conductivity, limited thermal stability | PEO-based, PVDF-based; fillers: Al₂O₃, SiO₂ (inert), LLZO (active) |
| Sulfide | Up to 2.5 × 10⁻² | Very high ionic conductivity, soft grain boundaries | Air sensitivity, narrow electrochemical window, interfacial reactions | LSPSCI, LGPS; doping (e.g., Sb), element substitution |
| Oxide | 10⁻⁴ – 10⁻³ | Good chemical stability, wide electrochemical window | Brittle, high grain boundary resistance, high sintering temperatures | Garnet (LLZO), Perovskite (LLTO), NASICON; doping (Ta, Ga), surface modification |
Polymer solid-state electrolytes consist of a polymer matrix and lithium salt. They can be prepared by solution casting, extrusion, or hot pressing. Common polymer matrices include polyethylene oxide (PEO), polyvinylidene fluoride (PVDF), polyacrylonitrile (PAN), and others. Research on PEO-based solid-state electrolytes often focuses on modification by adding inorganic fillers. Fillers can be inert (e.g., Al₂O₃, SiO₂, TiO₂, CuO, ZrO₂, MgO) or active (e.g., highly conductive inorganic electrolytes like LLZO). Inert fillers enhance ionic conductivity, reduce PEO crystallinity, and improve mechanical properties. Active fillers, being highly conductive themselves, similarly reduce crystallinity and interact with the polymer to enhance conductivity and strength. The ion transport mechanism in PEO-based electrolytes involves lithium ions migrating through the amorphous regions of the polymer, facilitated by segmental motion of the polymer chains. The diffusion can be described by various modes: intra-chain ion hopping between coordination sites, inter-chain hopping between ion clusters, and hopping between coordination sites on single chains or between chains. The coulombic interaction between the lone pair electrons on the oxygen atoms of PEO chains and lithium ions enables coordination and dissociation of lithium salt ions, facilitating lithium-ion conduction.
Sulfide solid-state electrolytes are derived from oxide electrolytes through element substitution. The weaker bonding between sulfur atoms and ions results in more freely moving lithium ions. The larger atomic radius of sulfur provides larger ion migration channels, leading to higher ionic conductivity. To further improve the comprehensive performance of sulfide electrolytes, extensive research has been conducted on modifications. For instance, a collaboration developed a crystal structure with a three-dimensional ion transport pathway, LSPSCI, achieving an ionic conductivity of 2.5 × 10⁻² S/cm, along with good chemical stability and higher air stability. Doping strategies, such as incorporating Sb into LGPS electrolytes, form strong covalent bonds, enhancing stability even in environments with 3% humidity.
Oxide solid-state electrolytes can be classified into amorphous and crystalline types. Crystalline oxides primarily include garnet-type, perovskite-type, and NASICON-type structures. Amorphous oxides are mainly represented by LiPON. Garnet-type electrolytes, with a general formula A₃B₂(XO₄)₃ where X is Li⁺, have been widely studied. Li₇La₃Zr₂O₁₂ (LLZO) is a prominent example, existing in cubic and tetragonal phases. The cubic phase exhibits superior ionic conductivity, reaching up to 3 × 10⁻⁴ S/cm at room temperature, along with excellent chemical and thermal stability. Doping with elements like Ta can yield ionic conductivities as high as 8.7 × 10⁻⁴ S/cm with an activation energy of 0.22 eV, while Ga doping can achieve 14.6 × 10⁻⁴ S/cm. A major challenge is the compatibility at the interface with electrodes; surface residues like Li₂CO₃ severely hinder ion transport. Converting Li₂CO₃ to LiCoO₂ through reaction with Co₃O₄ has been proposed to ensure tight contact between solid electrolyte particles and cathode materials. Perovskite-type oxides, such as LLTO, possess a crystal structure composed of TiO₆ octahedra. LLTO can achieve ionic conductivities of 1 × 10⁻³ S/cm, but grain boundary resistance often reduces the overall conductivity. Designing two-dimensional layered films with ordered crystal arrangements and no grain boundary defects has been reported to improve performance and provide excellent mechanical properties. NASICON-type electrolytes, with the general formula LiM₂(PO₄)₃ where M is Zr, Ge, Ti, etc., are another important class. Substituting transition metal M with Al³⁺ to form NASICON-type electrolytes is a research hotspot. Increasing the heat treatment crystallization temperature has been shown to improve ionic conductivity by three orders of magnitude. To reduce interfacial side reactions and resistance, adding protective layers can enhance interfacial affinity, improve ionic conduction, and suppress lithium dendrite growth.
The interfacial issues between electrodes and electrolytes are critical for the performance of solid state batteries. Research on sulfide cathodes paired with sulfide electrolytes is prevalent due to the moderate voltage and theoretical specific capacity of metal sulfide cathode materials. One approach involves compositing active materials with mixed conductors to create electrodes that provide both electronic and ionic transport channels, transforming three-phase interfaces into two-phase interfaces and significantly enhancing charge transfer, thereby improving rate performance and stability. Another method involves constructing tight interfaces between metal sulfide cathodes and solid electrolytes. However, volume changes during charge and discharge pose a challenge. Utilizing hydrothermal methods to prepare electrode materials as carbon nanotubes has been shown to further enhance rate and cycling performance. Layered oxide cathodes, such as lithium cobalt oxide and nickel-rich NMC materials, offer high specific energy and operating voltage but face compatibility issues with sulfide electrolytes due to the narrow electrochemical window of sulfides. Sulfide electrolytes can be oxidized to sulfites, phosphates, and highly oxidized P₂Sₓ compounds during cycling, forming interfacial phases that impede ion transport. This is often attributed to excess lithium compounds on the surface of layered oxides. Introducing a monolayer of electron-insulating oxide between the layered oxide cathode and sulfide electrolyte can effectively suppress the space charge layer effect. Using single-crystal layered oxides, which lack internal grain boundaries, can provide better lithium-ion diffusion channels.
In summary, sulfide electrolytes represent a promising solution for next-generation high-energy-density and high-safety battery technologies. However, compatibility issues with layered oxide cathodes and anodes limit their application. Constructing stable carrier transport interfaces within the ion transport path is key to addressing large interfacial impedance and lithium dendrite growth. Coating composite cathode materials can also effectively mitigate side reactions. To accelerate the industrialization of all-solid-state batteries, several areas require focus: Firstly, developing efficient binders suitable for solid-state electrolytes is necessary to further enhance energy density. Secondly, comprehensive efforts to reduce the synthesis cost of electrolytes and identify large-scale preparation methods are crucial. Thirdly, leveraging advanced and in-situ characterization techniques, combined with AI and machine learning, will enable a deeper understanding of interfacial behaviors. The future development of solid state batteries is imminent, and continued research will unlock their full potential.
The performance of solid state batteries can be modeled using various electrochemical equations. For instance, the ionic conductivity σ can be related to the concentration of charge carriers n, their charge q, and mobility μ:
$$ \sigma = n q \mu $$
The activation energy E_a for ion conduction can be derived from the Arrhenius equation:
$$ \sigma T = A \exp\left(-\frac{E_a}{k_B T}\right) $$
where A is the pre-exponential factor. Understanding these parameters is essential for optimizing solid-state electrolytes. Furthermore, the interfacial resistance R_int plays a significant role in the overall cell impedance. The total cell resistance R_cell can be approximated as:
$$ R_{\text{cell}} = R_{\text{bulk}} + R_{\text{int}} $$
where R_bulk is the bulk resistance of the electrolyte and electrodes. Minimizing R_int is critical for achieving high performance in solid state batteries. Research continues to focus on material design and interface engineering to overcome these challenges and realize the full potential of solid state battery technology.