Novel Cross-Linked Gel Polymer Electrolyte (GPE) with Long-Term Cycle Life for Li-Metal Batteries

Abstract

Lithium metal batteries (LMBs) are considered among the most promising energy storage technologies due to their very high energy density and lightweight. While liquid electrolytes (LEs) offer high ionic conductivities (10−3−10−2 S cm−1) and low interfacial resistance with electrodes, yet the risks of leakage and combustion of organic solvents, lithium (Li) dendrite formation, thermal runaway, and environmental concerns regarding using organic solvents hinder their full potential applications. Li dendrites are mainly induced by nonuniform distribution of charge or ions and formation of a fragile, unstable solid electrolyte interphase (SEI) layer on Li metal anode during electrochemical Li deposition. The dendritic morphology of electrochemically deposited Li results in inefficient lithiation/delithiation and the formation of inactive or dead Li and constant degradation of organic solvents during Li plating/stripping, leading to capacity decade and decline in cycle life. Furthermore, lithium dendrites can pose a safety issue by penetrating through a porous or mechanically soft separator and making a short-circuit. High voltage instability of conventional electrolytes is another issue restricting application of high potential cathodes to enhance the energy density of LMBs. Gel polymer electrolytes (GPEs) are considered a promising alternative for LEs because of their high ionic conductivities (10−3−10−2 S cm−1), prevention of liquid leakage, low flammability, chemical and electrochemical stability, high flexibility for portable and wearable electronics, suppression of dendrite formation, and ample chemistry and design [1-4].In this work, we have developed a novel, highly efficient PEO-based GPE by a simple, facile chemical method and using UV light-induced cross-linking. Cross-linking is essential to retain the structure of the polymer electrolyte and it is also shown to suppress Li dendrite growth by offering a high stiffness and rigidity (high shear modulus) toward Li dendrites. The designed GPE owns a very high ionic conductivity similar to commercial LEs, electrochemical stability up to 5 V, long-term cycle life and stability. While Li|Li symmetric cells made with 1 M LiPF6 in EC/DEC and 1 M LiTFSI in DOL/DME LEs showed an increase in overpotential during cycling (mainly due to the formation of dead Li and evolution of SEI) followed by a short-circuit (probably due to penetration of Li dendrites through Celgard separator), Li|Li cells made with GPE exhibit a long-term stability with a low voltage polarization of less than 0.1 V at a high current density of 0.5 mAh cm−2 for 500 cycles. Furthermore, Li|LFP full cells made with GPE display a long-term cycle life with discharge capacity of ~120 mAh g−1 at 0.5 C in ambient temperature with around 80% capacity retention after 500 cycles.In situ light microscopy which is a simple, versatile technique to study the nucleation and growth of Li particles during electrochemical platting/stripping was also used to further illustrate uniform deposition of Li by the developed GPEs. Hence, we have also performed light microscopy analysis to investigate Li deposition in Li|Li cells made of LEs and GPEs during electrochemical deposition process. It was observed that unlike LEs made of 1 M LiPF6 in EC/DEC and 1 M LiTFSI in DOL/DME which results in non-uniform, dendritic growth of Li, a uniform and non-dendritic deposition of Li was observed in case of GPE, which can be due to uniform regulation of Li through complexation with oxygen atoms of the PEO-based GPE.