The influence of mechanical stack pressure on battery power density is a critical factor in the design and performance optimization of pouch and prismatic cells. Power density, defined as the rate of energy delivery per unit mass or volume, is highly sensitive to interfacial contact resistance between cell components, which is modulated by applied stack pressure. This article examines the relationship between controlled mechanical pressure and power output, detailing experimental methodologies, observed electrochemical effects, and optimal pressure ranges for various battery chemistries.

Experimental setups for studying stack pressure effects require precise control mechanisms to apply uniform pressure across cell assemblies. A common approach involves using hydraulic or pneumatic systems with pressure plates to exert force on the cell surface, while load cells monitor the applied pressure in real time. Pouch cells, with their flexible packaging, are typically sandwiched between rigid plates, whereas prismatic cells, housed in fixed containers, may require internal pressure application through spring-loaded mechanisms. Environmental chambers maintain temperature stability during testing to isolate pressure effects from thermal influences.

Interfacial resistance between electrodes and separators is a key parameter affected by stack pressure. Insufficient pressure leads to poor contact, increasing ionic resistance and reducing power capability. Excessive pressure, however, can compress porous electrode structures, impede electrolyte wetting, and increase tortuosity for ion transport. Studies on lithium-ion pouch cells reveal that interfacial resistance decreases with increasing pressure up to an optimal range, typically between 0.5 and 2.0 MPa for graphite anodes and nickel-manganese-cobalt (NMC) cathodes. Beyond this range, diminishing returns or performance degradation occur.

Different chemistries exhibit distinct pressure dependencies. Lithium iron phosphate (LFP) cells, for instance, tolerate higher stack pressures due to their robust electrode structures, with optimal performance observed between 1.5 and 3.0 MPa. In contrast, high-energy NMC chemistries with silicon-blended anodes require lower pressures, around 0.3 to 1.5 MPa, to prevent particle cracking and solid-electrolyte interphase (SEI) damage. Solid-state batteries, which rely on intimate solid-solid contact, demand significantly higher pressures, often exceeding 5.0 MPa, to maintain adequate ionic conduction across interfaces.

Zero-pressure scenarios, while rarely practical in commercial cells, provide a baseline for understanding pressure effects. Without external compression, pouch cells exhibit higher polarization losses, particularly under high-current discharge, due to increased contact resistance. Prismatic cells, which often incorporate internal compression mechanisms, show less severe degradation but still suffer from power fade when interfacial pressure is insufficient. Comparative studies indicate that power density in lithium-ion pouch cells can improve by 15-25% under optimal stack pressure relative to zero-pressure conditions.

The impact of pressure on power density is also influenced by cycling conditions. Cells subjected to dynamic loading during charge-discharge cycles experience varying interfacial contact, which can accelerate degradation if pressure is not maintained within a stable range. Experimental data shows that constant pressure application enhances cycle life and power retention compared to uncontrolled or fluctuating pressure scenarios. For example, NMC-based prismatic cells maintained at 1.2 MPa demonstrate 10-15% higher power retention after 500 cycles compared to cells without pressure regulation.

Optimal pressure ranges must account for cell format and operational requirements. Pouch cells, lacking rigid external casings, benefit from moderate pressures that prevent delamination without inducing mechanical stress on sealing edges. Prismatic cells, with their rigid enclosures, can sustain higher pressures but require careful distribution to avoid localized stress concentrations. Pressure uniformity is critical; non-uniform application leads to uneven current distribution, exacerbating power loss and aging effects.

In summary, mechanical stack pressure plays a pivotal role in determining battery power density by modulating interfacial resistance and electrode-electrolyte interactions. Controlled experiments demonstrate that each chemistry and cell format has a distinct optimal pressure range, balancing contact improvement against structural integrity. These findings inform design strategies for high-power applications, emphasizing the need for precise pressure management in advanced battery systems. Future research may explore adaptive pressure control systems that dynamically adjust to operational conditions, further optimizing power delivery and longevity.