Pressure Effects in Lithium-Metal Batteries: Optimizing Battery Performance through Fixture Design

Academic Background

With the rapid development of electric vehicles (EVs) and renewable energy, the demand for high-energy-density batteries is increasing. Lithium-metal batteries are considered strong candidates for next-generation battery technology due to their high theoretical capacity (3860 mAh/g) and low electrode potential (-3.04 V vs. SHE). However, the commercialization of lithium-metal batteries faces multiple challenges, including lithium dendrite growth, non-uniform formation of the solid electrolyte interphase (SEI), and electrolyte consumption. These issues are particularly prominent in large-scale batteries, leading to reduced cycle life and safety performance.

To address these problems, researchers have begun exploring the effects of external pressure on lithium-metal battery performance. External pressure can improve the uniformity of lithium deposition/stripping, reduce dendrite growth, and enhance electrolyte wetting. However, the specific impact of different pressure fixture designs on battery performance has not been systematically studied. This study investigates the influence of different pressures and fixture designs on lithium-metal battery performance through various external pressure fixtures, revealing the failure mechanisms.

Source of the Paper

This paper is a collaborative effort by Corey M. Efaw, Zihan Wang (王子涵), Hongxing Zhang (张红星), and several other authors from renowned institutions such as Idaho National Laboratory, Brookhaven National Laboratory, and the University of Connecticut. The paper was published on April 18, 2025, in the journal Device, with the DOI 10.1016/j.device.2024.100660.

Research Process and Methods

1. Research Design

This study focuses on single-layer lithium-metal/nickel-manganese-cobalt oxide (Li-NMC811) pouch cells to explore the effects of different initial pressures (2 psi, 10 psi, 30 psi) and fixture designs on battery performance. The fixture designs primarily include Constant Gap (CG) and Constant Pressure (CP), with the addition of flexible foam as an interface material in the CG design.

2. Experimental Methods

• Battery Assembly: Single-sided NMC811 is used as the cathode, 50 µm thick lithium foil as the anode, and Celgard 2325 as the separator. The batteries are assembled in a glovebox and filled with a localized high-concentration electrolyte (LHCE) composed of LiFSI, DME, and TTE.
• Fixture Design:
  • CG fixtures: Fixed with screws to ensure a constant gap between the upper and lower plates.
  • CP fixtures: Use springs to maintain pressure, allowing gap adjustment during battery expansion.
  • CG+Foam fixtures: Add a layer of flexible foam to the CG design to distribute pressure uniformly.
• Electrochemical Testing: The batteries undergo cycling tests in an environment at 25°C, with a voltage range of 2.8-4.4 V. The tests include two formation cycles followed by aging cycles, recording capacity, voltage, and pressure changes.
• Pressure Monitoring: Real-time pressure changes are monitored using pressure sensors installed on the batteries.
• Postmortem Analysis: Techniques such as scanning electron microscopy (SEM), X-ray photoelectron spectroscopy (XPS), and synchrotron X-ray diffraction (XRD) are used to analyze the morphology and composition of the electrodes.

3. Data Analysis

Electrochemical analysis (e.g., differential capacity dq/dv and differential pressure dp/dv curves) and finite element analysis (FEA) simulations are used to study stress distribution and failure mechanisms under different fixture designs.

Main Results

1. Comparison of Constant Gap (CG) and Constant Pressure (CP) Fixtures

• Cycling Performance: At an initial pressure of 10 psi, batteries with CG fixtures exhibit higher capacity retention, while CP fixture batteries show rapid capacity decay. CG fixture batteries maintain over 80% capacity after 250 cycles, whereas CP fixture batteries experience significant capacity loss after 100 cycles.
• Pressure Changes: CG fixtures show a broader range of pressure changes, reflecting variations in lithium anode thickness. CP fixtures exhibit smaller pressure changes but allow significant battery expansion.
• Failure Mechanisms: CG fixture batteries fail primarily due to increased internal resistance, while CP fixture batteries fail due to rapid dendrite growth and non-uniform SEI formation.

2. Introduction of Flexible Foam

• Low Pressure (10 psi): CG+Foam fixtures significantly improve battery cycle life, exceeding 200 cycles. The flexible foam effectively alleviates local stress, enhancing uniform lithium deposition.
• High Pressure (30 psi): CG+Foam fixtures perform worse than CG fixtures, with cycle life dropping below 100 cycles. Under high pressure, the flexible foam creates local hotspots, accelerating electrolyte depletion and cathode particle damage.

3. Stress Distribution Simulation

Finite element analysis reveals that CG+Foam fixtures create localized high-stress regions under high pressure. These regions become hotspots for dendrite growth and lead to electrolyte depletion. In contrast, under low pressure, the flexible foam distributes stress evenly, prolonging battery life.

Conclusions and Outlook

This study systematically explores the effects of different fixture designs and external pressures on lithium-metal battery performance, leading to the following conclusions:

1. CG Fixtures Outperform CP Fixtures: Under high pressure, they effectively suppress dendrite growth and extend battery life.
2. Applicability of Flexible Foam: It improves battery performance under low pressure but causes local hotspots and accelerates failure under high pressure.
3. Pressure Dependence: Fixture design selection should be adjusted based on the pressure range in practical applications to optimize battery performance.

This research provides important guidance for fixture design in high-pressure lithium-metal batteries and serves as a reference for optimizing other high-volume-change electrodes (e.g., silicon anodes). Future research will further explore the impact of interlayer materials in multilayer pouch cells to advance the commercialization of high-energy-density batteries.

Research Highlights

1. Systematic Study: The first to systematically investigate the effects of different fixture designs and pressures on lithium-metal battery performance, filling a gap in the field.
2. Multidisciplinary Approach: Combines electrochemical analysis, pressure monitoring, and postmortem techniques to comprehensively reveal battery failure mechanisms.
3. Practical Application Value: Provides critical guidance for fixture design in lithium-metal batteries, facilitating their large-scale application.

Through this in-depth study, the future development of more efficient and safer lithium-metal batteries is anticipated, offering strong technical support for the fields of electric vehicles and renewable energy.