LATP Electrolyte Degradation Mechanisms

• LATP degradation is characterized by interfacial reactions where unstable SEI layers form and decompose, resulting in increased impedance and lithium plating.
• Ionic and electronic transport bottlenecks at grain boundaries trigger localized phase changes and lithium precipitation under high overpotentials.
• Mechanical stresses from intercalation-induced expansion lead to fractures and contact loss, undermining ionic conductivity and cell durability.

LATP solid electrolyte degradation encompasses a range of interconnected physicochemical, electrochemical, and mechanical processes that limit the performance and durability of Li-ion and all-solid-state batteries employing Li₁₊ₓAlₓTi₂₋ₓ(PO₄)₃ or related NASICON-type ceramics. Degradation phenomena arise from thermodynamic instability at the interface with metallic lithium, kinetically controlled reactions, mechanical fracture induced by intercalation strain, deleterious phase evolution, electronic transport bottlenecks, and interfacial contact loss. The complex nature of these mechanisms requires multi-scale investigation combining atomistic modeling, continuum simulation, and advanced characterization. Below, key aspects of LATP degradation are presented in detail.

1. Thermodynamic and Kinetic Instabilities at Li/LATP Interfaces

Contact between LATP and lithium metal subjects the solid electrolyte to highly reducing conditions, frequently triggering decomposition. Initial interfacial layers formed during cycling may contain SEI constituents such as lithium carbonate (Li₂CO₃) and lithium ethylene dicarbonate (LEDC). While these species are electrochemically inert, DFT studies show they are thermodynamically unstable when excess Li is present, decomposing rapidly via reactions such as:

$$\mathrm{Li_2CO_3} + 4\,\mathrm{Li} \rightarrow 3\,\mathrm{Li_2O} + \mathrm{C}$$

The Arrhenius rate of decomposition for SEI components is governed by

$$k = k_0 \exp\left(-\frac{\Delta E^*}{k_B T}\right),$$

where $\Delta E^*$ is the kinetic barrier. For LEDC, barriers as low as 0.22 eV yield decomposition on battery time scales, while Li₂CO₃ reactions are also exothermic with similarly accessible barriers. Such transformations result in Li₂O formation, which is stable both thermodynamically and kinetically under these conditions (Leung et al., 2016). Uncontrolled Li₂O layer formation, accompanied by loss of organic SEI, leads to increased interfacial impedance and possible facilitation of further lithium plating, which accelerates LATP degradation.

2. Electronic and Ionic Transport Bottlenecks and Internal Phase Formation

Microstructural features such as grain boundaries act as ionic transport bottlenecks inside LATP. The passage of ionic (Li⁺) and electronic currents through these regions induces local chemical potential jumps. The redox equilibrium is expressed as:

$\mathrm{Li}^+ + e^- \rightleftharpoons \mathrm{Li}$

Large local jumps in chemical potential, dictated by conductivity minima at grain boundaries, trigger unwanted internal phase formation, such as Li metal islands, particularly under charging conditions. The governing relation for the chemical potential jump is:

$\Delta \mu_\mathrm{Li} = -4\Delta \mu_e,$

with the gradient given by

$$- \frac{d\mu_{\text{Li}}}{dx} = \frac{4eL J_{\mathrm{Li}^+}}{\sigma_{\mathrm{Li}^+} + \sigma_{e+h}}$$

where $J_{\mathrm{Li}^+}$ is the Li-ion flux and $\sigma$ terms are ionic and electronic conductivities. This phenomenon is polarity dependent: under high overpotential (charging), local chemical potential at bottlenecks can exceed the boundary values, leading to Li precipitation and microstructural instability (Dong et al., 2018).

3. Mechanical Degradation: Fracture, Contact Loss, and Interfacial Stress

Mechanical stresses originating from the volumetric expansion of lithiated active particles induce tensile and shear stresses in the LATP matrix. Finite element models incorporating nonlinear kinematics and cohesive zone laws demonstrate that mechanically compliant LATP (lower Young's modulus, $E_\mathrm{SE} \sim 15$ GPa) is more susceptible to micro-cracking than stiffer electrolytes:

$$G_c = \int_0^{\delta_\text{cr}} T(\delta)\, d\delta$$

where $G_c$ is the fracture energy, $T(\delta)$ is the traction-separation law, and $\delta_\text{cr}$ marks complete decohesion. Fracture is triggered when a dimensionless elastic-to-fracture energy ratio

$$\mathcal{G} = 0.5 \, k_\mathrm{SE} (3\beta_\mathrm{AM} A_\mathrm{AM})^2 / (H G_c)$$

exceeds a limiting value ($\mathcal{G} < 1000$ for intactness) (Bucci et al., 2017). Interfacial fractures between LATP and electrodes degrade ionic and electronic contact, suppressing charge transfer and increasing impedance. Advanced discontinuous finite element frameworks permit rigorous simulation of sharp interfacial fractures, including stress-dependent kinetics and arbitrarily complex particle geometries (Zhang et al., 2023).

4. Lithium Dendrite Penetration and Void Formation

Void formation at electrode/SSE interfaces precedes lithium filament (dendrite) growth, a major limitation in LATP cells containing Li metal. COMSOL-based electrostatic models show that current density at void edges is amplified:

$j_\text{edge} \approx 10^4 \cdot j_\text{avg}$

Consequently, even modest average current densities can drive local filament nucleation once interfacial voids are present. Alloys with high lithium solubility (e.g., Al) exacerbate vacancy-induced void formation. Interlayers with low lithium solubility (e.g., W) delay void growth and promote more uniform current distribution, mitigating dendritic degradation (Raj et al., 2020).

5. Interfacial Contact Loss and Electrical Impedance Scaling

LATP degradation is strongly correlated with the evolution of real (recoverable) and unrecoverable contact area at interfaces. Experimental and simulation studies reveal the interfacial resistance $R_\text{int}$ follows power law scaling with pressure $P$ and contact area $A_\text{real}$:

$$R_\text{int} \sim a P^n, \quad n \approx -0.5\ \text{to}\ -0.67$$

under various contact-loss scenarios. The constriction resistance for limited contact, relevant at degraded interfaces, obeys:

$R_c = \frac{1}{2d\sigma_b}$

Distributed contact geometries yield lower impedance, as current crowding and local voltage drops are reduced. Uniform contact distribution is a key mitigation strategy (Limon et al., 31 Dec 2024).

6. Phase Evolution, Grain Boundary Effects, and Compositional Transformations

Grain boundaries in LATP are major sites for resistive phase formation and ionic transport bottlenecks. Addition of glassy additives (e.g., Li₂.₉B₀.₉S₀.₁O₃.₁) improves densification, decomposes secondary phases, and replaces resistive grain boundary layers with lithium conducting phases (LiTiPO₅, Li₄P₂O₇):

Material/Phase	Effect on Grain Boundaries	Resulting Conductivity
Berlinite, amorphous aluminophosphate	Increases resistance	Low
LiTiPO₅, Li₄P₂O₇, Li₃PO₄	Promotes conduction	High

Optimized sintering (e.g., 800°C for LATP–0.1LBSO) yields $\sigma_\text{tot} = 1.5 \times 10^{-4}$ S cm⁻¹ (Kwatek et al., 2020). Microstructure, composition, and sintering protocols must be tightly controlled to minimize grain boundary resistance and suppress emergence of resistive interphases.

7. Atomistic Mechanisms: Al/Ti Ratio, Li Migration Energy Landscape, and Computational Stability Windows

DFT studies indicate that the Al/Ti ratio in LATP not only alters average migration barriers but introduces heterogeneous energy profiles along Li migration pathways. Local Al occupancy near migration transition states reduces barriers; excessive Al content leads to structural contraction and possible bottlenecks, impeding Li transport and promoting localized degradation (Pfalzgraf et al., 2020). Computational assessments of LATP's electrochemical stability window via stoichiometry, HOMO–LUMO, and phase stability methods reveal:

• Stoichiometry window (PBE): $\sim$0.66 V to 3.13 V vs Li, width $\sim$2.47 V
• Phase decomposition window often larger, but in LATP, stoichiometry changes occur at lower potentials than thermodynamic bulk phase decomposition.

This suggests the onset of degradation in LATP may be kinetically controlled by local atomic environment and can occur before gross phase changes (Binninger et al., 2019).

8. Practical Modeling and Mitigation Approaches

Reduced-order models, such as the Single Particle Model with electrolyte and side reactions, incorporate SEI growth, lithium plating, and mechanical degradation. These frameworks include additional kinetic terms for side reactions, interfacial degradation, and link mechanical stress to time-dependent conductivity. Mitigation strategies encompass compositional tuning, addition of sintering aids, optimized pressure protocols, and use of low-solubility interlayers (Planella et al., 2022, Kwatek et al., 2020, Raj et al., 2020).

Summary

LATP solid electrolyte degradation comprises interfacial chemical reactions, electronic and ionic transport challenges, mechanical damage, microstructural evolution, and phase transformations. Thermodynamic stability criteria must be reconciled with kinetic barriers and local atomic structure; the development of crack-resistant microstructures, uniform interfacial contacts, and compositions that suppress resistive phase formation are essential for realizing durable LATP-based solid-state batteries. Multi-scale modeling and advanced characterization remain critical for elucidating and mitigating these intertwined degradation pathways.