Lithium-Sulfur Batteries

6 Lithium Sulfide

Sylwia Waluś

OXIS Energy, Culham Science Centre, Abingdon, Oxfordshire, OX14 3DB, UK

6.1 Introduction

Lithium sulfide (Li₂S) plays multiple roles in lithium–sulfur (Li–S) cells, with the important one being the ultimate solid discharge product, formed during the reduction of elemental sulfur (S₈) according to the general reaction [1,2] S₈ + 16 Li⁺ + 16 e⁻ → 8 Li₂S. It is an insulating, both electronically and ionically [3] (σ ∼ 10⁻¹³ S cm⁻¹ at 25 °C) solid compound, poorly or non‐soluble in most of the organic solvents, with Li⁺ diffusivity [4] being as low as 10⁻¹⁵ cm² s⁻¹. Li₂S is considered by many as a Li–S cell enemy, being the main cause of capacity fade during cycling [5,6] and limitation in reaching full discharge capacity [7–9]. However, its formation is indispensable to extend the discharge capacity, as transformation of S₈ to the final reduced form of Li₂S involves 16e⁻. On the other hand, the advantage of Li₂S being a lithiated counterpart of sulfur is that it can be used as an active material during cathode formulation. Li₂S‐based cathodes give the opportunity of being paired with metallic Li‐free anodes (such as silicon, tin, hard carbons), eliminating the problems faced when using pure Li metal as anode, such as poor coulombic efficiency and mossy or dendritic growth leading to safety issues [10,11]. The predicted specific energy of a Li₂S/Si system (1550 Wh kg⁻¹ [12]) exceeds that with existing Li ion technology by even four times. ...