Alternatively, lithium-sulfur batteries based on multi-step redox conversion reaction provide a theoretical specific capacity of 1675 mAh/g. However LiS batteries also suffer from many problems, such as severe volumetric expansion/contraction during lithiation/de-lithiation, polysulfide (PS) dissolution and shuttling dendritic lithium growth on the anode, corrosion of aluminum current collector, low sulfur utilization and low current efficiency (CE).
In this manuscript, we demonstrate that specific types of sulfur allotropes react differently from standard orthorhombic sulfur used in state of art LiS batteries. Significant improvements are achieved by using sulfur allotropes as both cathode active mass and 3D structural additive. Dissolution of polysulfides is confined within the cathode by the use of a bi-functional additive capable of controlling cathode reaction and the first discharge plateau is extended to deliver a capacity of 408 mAh/g at 0.2C which is close to theoretical limit of 418 mAh/g for long chain PS (Fig.1). Instead of using various methods for controlling PS dissolution and migration which are mostly based on PS binding and interaction with host-catalyst structures, we used a bi-functional additive capable to control both reaction proceeding as well as its kinetics. This method allows PS dissolution but reactions are strictly limited into cathode compartment and cannot proceed outside the cathode. An important aspect of this approach is significantly reduced side-reaction between PS and electrolyte.
Fig.1: Comparison of discharge curves of standard electrode and electrode with bi-functional additive 0.7M LiTFSI + 0.25M LiNO3, DOL/DME 2:1 v/v, active mass S loading bi-additive 1.95 mg/cm2, standard 1.65 mg/cm2
This research has been carried by Graphene Batteries AS in close cooperation with the Centre for Research and Utilization of Renewable Energy (CVVOZE) as a part of collaboration project NPU I programme (project No. LO1210), (project No. FEKT-S-14-2293)