What is the Role of Relative Humidity on Conductivity in Polymer Electrolytes?

Area of Science:

• Materials Science
• Electrochemistry
• Computational Chemistry

Background:

• Ion transport in polymer electrolytes is crucial for applications like batteries.
• The low hydration regime (0-80% relative humidity) remains poorly understood at the molecular level.
• Existing studies focus on dry or highly water-swollen conditions, neglecting intermediate hydration.

Purpose of the Study:

• To investigate the molecular mechanisms of ion transport in LiTFSI-doped polyethers at low hydration levels.
• To elucidate the impact of varying water content on lithium-ion (Li+) and TFSI- anion mobilities.
• To understand how polymer hydrophilicity and salt concentration influence ion dynamics in the low hydration regime.

Main Methods:

• Atomistic molecular dynamics simulations were employed.
• Simulations focused on LiTFSI-doped polyethers with low water content (<10% by volume).
• Analysis centered on ion mobilities and conductivities under varying hydration conditions.

Main Results:

• Li+ ion mobility exhibits two distinct regimes with increasing water content.
• At very low hydration, Li+ ions are weakly hydrated, showing limited mobility increase.
• At higher low hydration, Li+ ions form hydration shells in water-rich domains, significantly enhancing mobility.
• TFSI- anion mobility increases monotonically with water content due to weak polymer/water interactions.
• Increased polymer hydrophilicity and salt concentration promote larger water-rich domains and faster Li+ mobility.

Conclusions:

• Hydration plays a critical, complex role in ion transport within polymer electrolytes at low water content.
• The formation of water-rich domains is a key factor in enhancing Li+ mobility.
• Findings provide molecular-level insights into ionic conductivity relevant for advanced battery electrolytes and separation technologies.