The influence of external pressure on dendrite morphology in lithium-metal batteries is a critical area of research for improving battery safety and performance. Lithium-metal anodes offer high theoretical capacity but face challenges due to dendritic growth, which can lead to short circuits and thermal runaway. Applying controlled external pressure has been shown to alter dendrite formation, promoting more uniform lithium deposition. This article examines the physics behind pressure-induced uniform deposition, reviews in-situ microscopy observations, and discusses stack pressure optimization in pouch cells alongside the challenges of maintaining consistent pressure over cycles.

Lithium dendrites form due to non-uniform lithium-ion flux during plating, creating needle-like structures that grow toward the cathode. Without intervention, these dendrites penetrate the separator, causing internal short circuits. External pressure influences dendrite morphology by altering the mechanical stress distribution at the lithium-electrolyte interface. Studies indicate that pressures between 5 and 50 psi can suppress dendritic growth by promoting planar deposition. The mechanism involves stress-induced redistribution of lithium ions, reducing localized hotspots where dendrites typically initiate.

In-situ microscopy studies have provided direct evidence of how pressure affects lithium deposition. At low pressures (below 5 psi), lithium tends to form mossy or dendritic structures due to uneven surface energy distribution. As pressure increases to 10–20 psi, the deposition becomes more columnar, with fewer branching structures. Above 30 psi, the lithium layer exhibits a dense, smooth morphology with minimal porosity. These observations align with models suggesting that applied pressure reduces the thermodynamic driving force for dendritic growth by homogenizing the lithium-ion flux.

The physics of pressure-induced uniform deposition can be explained through mechanical and electrochemical interactions. Under pressure, the lithium metal experiences plastic deformation, which flattens surface irregularities and reduces localized current density spikes. Additionally, pressure enhances the contact between the lithium anode and the separator, minimizing voids where dendrites could nucleate. Electrochemical impedance spectroscopy data supports this, showing reduced interfacial resistance at higher pressures, indicating better electrode-electrolyte contact.

Stack pressure optimization in pouch cells is crucial for maintaining uniform lithium deposition across large-area electrodes. Pouch cells typically require pressures in the range of 10–30 psi to achieve stable cycling. Below this range, uneven pressure distribution leads to localized lithium plating and stripping, accelerating degradation. Excessive pressure (above 50 psi) can damage cell components, such as the separator, or induce lithium extrusion through the electrode edges. Engineering solutions such as spring-loaded fixtures or compressible gaskets help maintain optimal pressure throughout cycling.

Maintaining consistent stack pressure over multiple cycles presents several challenges. Lithium-metal anodes undergo significant volume changes during cycling, with thickness variations exceeding 20% in some cases. This dynamic behavior causes pressure fluctuations, leading to mechanical fatigue in cell components. Over time, repeated expansion and contraction can cause pressure relaxation, reducing the effectiveness of dendrite suppression. Advanced pressure management systems, including self-adjusting mechanisms and pressure-sensitive materials, are being explored to address these issues.

Another challenge is the spatial variation of pressure across large-format cells. Edge regions often experience lower pressure due to non-uniform clamping forces, creating preferential sites for dendrite formation. Finite element analysis has shown that pressure gradients as small as 5 psi across a cell can lead to heterogeneous lithium deposition. Strategies to mitigate this include optimized cell stacking designs and pressure-distributing materials that ensure uniform force application.

The relationship between pressure and cycling stability has been quantified in several studies. Cells cycled at 20 psi demonstrate up to 50% longer cycle life compared to those at 5 psi, with coulombic efficiency improvements of 5–10%. However, excessive pressure beyond 40 psi can accelerate electrolyte decomposition or separator damage, negating these benefits. The optimal pressure range depends on cell chemistry, with sulfur-based electrolytes requiring lower pressures than conventional liquid electrolytes due to their mechanical properties.

In-situ pressure monitoring techniques have been developed to study real-time pressure evolution during cycling. Fiber-optic sensors and piezoelectric films embedded within cells reveal that pressure fluctuates dynamically during charge and discharge. These measurements show that pressure drops during lithium stripping and rebounds during plating, highlighting the need for adaptive pressure control systems. Without such systems, the initial stack pressure may not remain effective after hundreds of cycles.

Material selection also plays a role in pressure management. Compliant separators with elastic properties can accommodate volume changes while maintaining interfacial contact. Ceramic-reinforced separators exhibit higher mechanical strength, resisting dendrite penetration even under moderate pressures. Electrolyte additives that form stable solid-electrolyte interphases further enhance pressure-driven uniformity by reducing side reactions that exacerbate dendrite growth.

Future research directions include the development of smart pressure-adaptive cells that dynamically adjust clamping forces in response to state-of-charge and temperature. Machine learning models are being explored to predict pressure requirements based on cycling history and operational conditions. Additionally, advanced manufacturing techniques aim to produce cells with built-in pressure regulation, eliminating reliance on external fixtures.

The interplay between pressure and dendrite suppression underscores the importance of mechanical factors in battery design. While electrochemical solutions such as electrolyte engineering remain critical, mechanical pressure provides a complementary approach to achieving stable lithium-metal batteries. By optimizing stack pressure and addressing its challenges, researchers can unlock the full potential of lithium-metal anodes for next-generation energy storage.