Gradient distribution of functional components in PAN-based electrolytes to endow solid-state lithium sulfur batteries with long cycle life

Abstract

Solid-state lithium–sulfur batteries (SSLSBs) face the difficulty of slow S₈ (solid) ↔ Li₂S (solid) conversion kinetics. Such solid reactions are highly ion-transmission-demanding and only take place at/near electrode/electrolyte interfaces. Increasing the reactant concentration near the interfaces or deliberately creating ample reaction-favored areas other than the obvious electrode/electrolyte interface would certainly contribute the most to the overall energy-storage performance. Herein, polyacrylonitrile (PAN)-based composite polymer electrolytes (CPEs) where the functional components showed a gradient distribution were successfully prepared by phase inversion and named Grad-CPE p-PPmCMoLi. Viewing along the anode-to-cathode direction, the functional components inside Grad-CPE p-PPmCMoLi appear gradually, including polymethyl acrylate (PMMA), which is beneficial for uniform Li⁺ deposition, the MoP/MoS₂@C heterojunctions, which specialize in S species adsorption as well as catalytic S₈ ↔ Li₂S conversion, and LiTFSI salt, for Li⁺ supply. As a consequence, near the cathode/Grad-CPE interfaces, the S species accumulation ability is strengthened by 300%, so that these enriched S species in situ meet the Li⁺ reactants provided by the surrounding LiTFSI, not only catalytically enabling the charging/discharging related reactions but also effectively alleviating the problem of Li⁺ shortage. Compared to a homogeneous electrolyte, the gradient distribution of the functional components makes the transference number of Li⁺ distinctly increase from 0.17 to 0.65. Besides, the concentrated PMMA at the anode/Grad-CPE interfaces helps the Li anode exhibit a uniform morphology, which is beneficial to extending the cycle life of Li|p-PPmCMoLi|Li to more than 1000 h. Given these merits, Grad-CPE p-PPmCMoLi improves the kinetics of the related SSLSB and maintains satisfactory stability (450 cycles at 0.5 mA cm⁻² with a decay rate of 0.079% per cycle).