Solid-state batteries represent a transformative advancement in energy storage technology, offering enhanced safety and higher energy density compared to conventional liquid electrolyte systems. The core component, solid-state electrolytes (SSEs), eliminates flammable organic solvents, mitigating risks of leakage and thermal runaway. However, the performance of solid-state batteries, particularly lithium metal batteries, degrades significantly at low temperatures (≤0 °C) due to reduced ionic conductivity and increased interfacial resistance. This comprehensive review examines the fundamental challenges and recent progress in developing SSEs for low-temperature applications, focusing on material design, ion transport mechanisms, and interfacial engineering strategies. We explore inorganic, polymer, and composite SSEs, highlighting innovative approaches to overcome kinetic limitations in extreme environments. The integration of advanced characterization techniques and theoretical modeling provides insights into ion dynamics and degradation pathways, guiding the rational design of next-generation solid-state batteries.
The evolution of solid-state batteries has been driven by the demand for safer and more efficient energy storage solutions. In solid-state batteries, SSEs serve as the ion-conducting medium, replacing volatile liquid electrolytes. Key advantages include a wide electrochemical stability window, suppression of lithium dendrite growth, and compatibility with high-capacity electrodes. Despite these benefits, the implementation of solid-state batteries in low-temperature conditions remains a formidable challenge. Ionic conductivity in SSEs follows thermally activated processes, as described by the Arrhenius equation for crystalline materials:
$$ \sigma = \frac{A}{T} \exp\left(-\frac{E_a}{RT}\right) $$
where $\sigma$ is the ionic conductivity, $A$ is the pre-exponential factor, $T$ is the absolute temperature, $E_a$ is the activation energy, and $R$ is the gas constant. For amorphous materials like polymers, the Vogel-Tammann-Fulcher (VTF) equation is more appropriate:
$$ \sigma = \sigma_0 T^{-1/2} \exp\left(-\frac{B}{T – T_0}\right) $$
Here, $\sigma_0$ is the pre-exponential factor, $B$ is the fitting activation energy, and $T_0$ is the reference temperature, typically 50 K below the glass transition temperature ($T_g$). At low temperatures, ion mobility decreases exponentially, leading to poor performance in solid-state batteries. This review delves into the material-level strategies to enhance low-temperature functionality, emphasizing the role of SSEs in enabling reliable solid-state batteries for aerospace, military, and consumer applications.
Low-Temperature Chemical Characteristics and Ion Transport Mechanisms
Understanding ion transport in SSEs is crucial for optimizing solid-state batteries under low-temperature conditions. In inorganic SSEs, ion conduction occurs via vacancies, interstitials, or concerted migration mechanisms within crystalline lattices. For instance, garnet-type Li$_7$La$_3$Zr$_2$O$_{12}$ (LLZO) and NASICON-type Li$_{1+x}$Al$_x$Ti$_{2-x}$(PO$_4$)$_3$ (LATP) exhibit three-dimensional ion diffusion pathways, but their conductivity drops sharply below 0 °C due to increased activation barriers. The ionic conductivity $\sigma$ can be expressed as:
$$ \sigma = n q \mu $$
where $n$ is the charge carrier concentration, $q$ is the charge, and $\mu$ is the mobility. At low temperatures, $n$ and $\mu$ decrease, as defect formation and ion hopping are thermally suppressed. In polymer SSEs, such as poly(ethylene oxide) (PEO)-based systems, ion transport relies on segmental motion of polymer chains. Below $T_g$, chain dynamics freeze, reducing conductivity by orders of magnitude. The VTF equation captures this behavior, where $T_0$ correlates with $T_g$. Composite SSEs combine inorganic fillers with polymers to create hybrid ion conduction pathways, but interfacial resistance between phases often limits low-temperature performance.
Table 1 summarizes the ionic conductivity and activation energy of representative SSEs at room temperature and -20 °C, illustrating the challenges in maintaining conductivity at low temperatures.
| Electrolyte Type | Material | Ionic Conductivity at 25 °C (S/cm) | Ionic Conductivity at -20 °C (S/cm) | Activation Energy, $E_a$ (eV) |
|---|---|---|---|---|
| Inorganic | Li$_{10}$GeP$_2$S$_{12}$ (LGPS) | 1.2 × 10$^{-2}$ | 4.0 × 10$^{-4}$ | 0.25 |
| Inorganic | Li$_7$La$_3$Zr$_2$O$_{12}$ (LLZO) | 1.0 × 10$^{-4}$ | 5.0 × 10$^{-7}$ | 0.40 |
| Polymer | PEO-LiTFSI | 1.0 × 10$^{-6}$ | 1.0 × 10$^{-9}$ | 0.80 |
| Composite | PEO-LLZTO | 1.0 × 10$^{-4}$ | 1.0 × 10$^{-6}$ | 0.35 |
The degradation of solid-state batteries at low temperatures is exacerbated by interfacial phenomena. The solid-electrolyte interphase (SEI) and cathode-electrolyte interphase (CEI) become more resistive, impeding charge transfer. Lithium ion transference number ($t_+$), defined as:
$$ t_+ = \frac{\sigma_+}{\sigma_+ + \sigma_-} $$
where $\sigma_+$ and $\sigma_-$ are the cationic and anionic conductivities, respectively, often decreases at low temperatures due to ion pairing and reduced dissociation. In solid-state batteries, $t_+$ values below 0.5 lead to concentration polarization and capacity fade. Thus, enhancing $t_+$ through material design is critical for low-temperature operation.
Failure Mechanisms in Low-Temperature Solid-State Batteries
Solid-state batteries face multiple failure modes at low temperatures, originating from bulk and interfacial processes. First, the ionic conductivity of SSEs declines exponentially, as per the Arrhenius relationship. For example, in sulfide-based SSEs like Li$_6$PS$_5$Cl, conductivity drops from 10$^{-2}$ S/cm at 25 °C to 10$^{-4}$ S/cm at -30 °C, increasing internal resistance and reducing power density. Second, electrode-electrolyte interfaces become unstable, leading to high charge-transfer resistance. The charge-transfer resistance $R_{ct}$ follows:
$$ R_{ct} = \frac{RT}{nF i_0} $$
where $n$ is the number of electrons, $F$ is Faraday’s constant, and $i_0$ is the exchange current density. At low temperatures, $i_0$ decreases due to sluggish kinetics, raising $R_{ct}$ and causing voltage hysteresis.
Third, lithium metal anodes exhibit irregular plating and stripping, forming dendrites and “dead lithium.” The critical current density (CCD) for dendrite initiation decreases with temperature, as described by:
$$ \text{CCD} = \frac{\sigma RT}{nF L} \ln\left(\frac{\sigma}{\sigma_0}\right) $$
where $L$ is the electrolyte thickness. At -20 °C, CCD values for many SSEs fall below 1 mA/cm$^2$, leading to short circuits. Fourth, composite electrodes in solid-state batteries suffer from poor interfacial contact, increasing impedance. For instance, in LiNi$_{0.8}$Co$_{0.1}$Mn$_{0.1}$O$_2$ (NCM811) cathodes, volume changes during cycling cause delamination from SSEs at low temperatures, accelerating capacity fade.
Table 2 outlines the primary failure mechanisms and their impact on solid-state battery performance at low temperatures.
| Failure Mechanism | Description | Impact on Performance |
|---|---|---|
| Reduced Ionic Conductivity | Exponential decrease in ion mobility | Lower capacity and power density |
| Increased Interfacial Resistance | Sluggish charge transfer at electrodes | Voltage polarization and efficiency loss |
| Lithium Dendrite Growth | Uncontrolled Li plating due to low CCD | Short circuits and safety hazards |
| Electrode Delamination | Loss of contact between electrode and SSE | Rapid capacity fade and cycle life reduction |
Optimization Strategies for Inorganic Solid-State Electrolytes
Inorganic SSEs, including oxides, sulfides, and halides, are promising for solid-state batteries due to their high intrinsic conductivity and stability. However, their performance at low temperatures requires careful material engineering. Oxide-based SSEs, such as LLZO and LATP, exhibit moderate room-temperature conductivity (10$^{-4}$ to 10$^{-3}$ S/cm) but suffer from grain boundary resistance and interfacial reactivity. Doping strategies, like Ta-substituted LLZO (Li$_6.4$La$_3$Zr$_{1.4}$Ta$_{0.6}$O$_{12}$), reduce activation energy and enhance low-temperature conductivity. The conductivity $\sigma$ can be modeled as:
$$ \sigma = \sigma_{\text{bulk}} + \sigma_{\text{gb}} $$
where $\sigma_{\text{bulk}}$ and $\sigma_{\text{gb}}$ are the bulk and grain boundary conductivities, respectively. At low temperatures, $\sigma_{\text{gb}}$ dominates, necessitating sintering aids and surface modifications to minimize resistance.
Sulfide SSEs, such as Li$_{10}$GeP$_2$S$_{12}$ (LGPS) and argyrodites (e.g., Li$_5.5$PS$_{4.5}$Cl$_{1.5}$), achieve high conductivity (>10$^{-2}$ S/cm) at room temperature, but their hygroscopic nature and narrow electrochemical window limit practicality. High-entropy design, as in Li$_9.54$[Si$_{0.6}$Ge$_{0.4}$]$_{1.74}$P$_{1.44}$S$_{11.1}$Br$_{0.3}$O$_{0.6}$, disrupts long-range order and lowers $E_a$, enabling conductivity of 9 mS/cm at -10 °C. The Arrhenius plot for such materials shows a linear relationship between $\ln(\sigma T)$ and $1/T$, with slopes proportional to $E_a$.
Halide SSEs, like Li$_3$InCl$_6$ (LIC), offer excellent oxidative stability and compatibility with oxide cathodes. Their layered structures facilitate 2D ion transport, but conductivity drops below 0 °C due to increased lattice stiffness. Composite approaches, such as blending LIC with carbon nanotubes, improve electronic percolation and reduce interfacial resistance. The effective conductivity $\sigma_{\text{eff}}$ in composites follows the Bruggeman model:
$$ \sigma_{\text{eff}} = \sigma_m \left(1 – \phi\right)^{3/2} $$
where $\sigma_m$ is the matrix conductivity and $\phi$ is the filler volume fraction. Optimizing $\phi$ enhances low-temperature performance in solid-state batteries.
Table 3 compares the low-temperature performance of various inorganic SSEs in solid-state batteries.
| Electrolyte | Type | Conductivity at -20 °C (S/cm) | Activation Energy (eV) | Application in Solid-State Batteries |
|---|---|---|---|---|
| Li$_{10}$GeP$_2$S$_{12}$ | Sulfide | 4.0 × 10$^{-4}$ | 0.25 | High-rate cycling at -30 °C |
| Li$_5.5$PS$_{4.5}$Cl$_{1.5}$ | Argyrodite | 1.0 × 10$^{-3}$ | 0.20 | Stable operation at -20 °C |
| Li$_3$InCl$_6$ | Halide | 1.6 × 10$^{-4}$ | 0.30 | Wide-temperature range (-30 to 60 °C) |
| LLZTO | Oxide | 5.0 × 10$^{-7}$ | 0.40 | Limited to above -10 °C |
Advances in Polymer and Composite Solid-State Electrolytes
Polymer SSEs, particularly PEO-based systems, are flexible and processable but exhibit low conductivity at low temperatures. Strategies to enhance performance include plasticization, cross-linking, and adding inorganic fillers. Plasticizers like succinonitrile (SN) lower $T_g$ and increase amorphous content, boosting conductivity. For example, PEO-SN electrolytes achieve 0.19 mS/cm at 25 °C and 0.1 mS/cm at 0 °C. The VTF equation parameters $\sigma_0$ and $B$ are optimized through molecular design, such as copolymerization with vinyl ethylene carbonate (VEC).
In situ polymerization is a powerful method to form conformal interfaces in solid-state batteries. Monomers like 1,3-dioxolane (DOL) are polymerized within cells, creating gel polymer electrolytes (GPEs) with integrated ion transport pathways. The ionic conductivity $\sigma$ of GPEs is given by:
$$ \sigma = \frac{1}{3} n q^2 \lambda^2 \nu \exp\left(-\frac{E_a}{RT}\right) $$
where $\lambda$ is the jump distance, and $\nu$ is the attempt frequency. At -30 °C, in situ-polymerized DOL electrolytes retain conductivities of 1.0 mS/cm, enabling solid-state batteries to operate in extreme conditions.
Composite SSEs combine polymers with inorganic fillers (e.g., LLZTO, SiO$_2$, MOFs) to create synergistic ion conduction. The Maxwell-Wagner model describes the effective permittivity and conductivity in composites:
$$ \sigma_{\text{eff}} = \sigma_p \frac{1 + 2\phi (\sigma_f – \sigma_p)/(\sigma_f + 2\sigma_p)}{1 – \phi (\sigma_f – \sigma_p)/(\sigma_f + 2\sigma_p)} $$
where $\sigma_p$ and $\sigma_f$ are the polymer and filler conductivities, respectively. Fillers like metal-organic frameworks (MOFs) provide ordered pores for rapid ion transport, while polymers ensure mechanical flexibility. For instance, UiO-66-based composites achieve conductivities of 2.07 × 10$^{-4}$ S/cm at 25 °C and 1.0 × 10$^{-5}$ S/cm at -20 °C, supporting solid-state batteries over a wide temperature range.
Table 4 summarizes the properties of polymer and composite SSEs for low-temperature applications.
| Electrolyte | Composition | Ionic Conductivity at -20 °C (S/cm) | Transference Number, $t_+$ | Notable Features |
|---|---|---|---|---|
| PEO-SN | PEO with succinonitrile | 1.0 × 10$^{-4}$ | 0.6 | Low $T_g$, stable interface |
| In situ DOL | Polymerized 1,3-dioxolane | 2.0 × 10$^{-3}$ | 0.7 | Conformal coating, fast kinetics |
| PEO-LLZTO | Composite with LLZTO filler | 1.0 × 10$^{-6}$ | 0.5 | Enhanced mechanical strength |
| MOF-Polymer | UiO-66 in polymer matrix | 1.0 × 10$^{-5}$ | 0.8 | Ordered ion channels, high $t_+$ |
Interfacial Engineering and Future Perspectives
The interface between SSEs and electrodes is a critical factor in low-temperature solid-state batteries. Engineering stable, low-resistance interfaces involves surface coatings, buffer layers, and functional additives. For example, LiNbO$_3$ coatings on NCM cathodes reduce interfacial resistance and prevent side reactions, enabling operation at -30 °C. The interfacial resistance $R_i$ can be expressed as:
$$ R_i = \frac{\delta}{\sigma_i} $$
where $\delta$ is the interface thickness and $\sigma_i$ is the interfacial conductivity. Minimizing $\delta$ and maximizing $\sigma_i$ are key goals.
Future research should focus on novel materials, such as high-entropy alloys and covalent organic frameworks (COFs), which offer tunable ion transport properties. In situ characterization techniques, like cryo-electron microscopy and solid-state NMR, will provide real-time insights into degradation mechanisms. Standardization of testing protocols for low-temperature solid-state batteries is essential for commercialization.
In conclusion, solid-state electrolytes hold immense potential for enabling high-performance lithium metal batteries in low-temperature environments. Through material innovation and interfacial design, solid-state batteries can overcome current limitations, paving the way for widespread adoption in demanding applications. The continuous advancement in solid-state battery technology will drive the next generation of energy storage systems.