Alkaline sulfur liquid battery (SLIQ) is a liquid battery which consists of only one rechargeable liquid and a technology which can be used for grid storage.
Battery chemistry and active material
One of the most promising possibilities of enhancing battery energy storage is to use sulphur as the positive electrode. Lithium-sulphur batteries are a tempting solution due to sulphur having a high theoretical capacity (1675 mAh g-1), as well as being non-toxic, abundant, and very low in cost. The discharge reaction in a lithium-sulphur cell, when using elemental sulphur as the positive electrode, can be written in its simplified form below:
S8+16 Li →8Li2S
In reality, the reactions taking place are much more complicated and occur through several intermediates, collectively known as polysulphides. For the charging or discharging reactions to take place, carbon must be present as a catalytic current collector, allowing electrons to pass through. Carbon is used as a three-phase interphase, giving a site where sulphur or polysulphides, lithium ions and electrons can all interact in one step. This greatly increases the rate of reaction, making lithium-sulphur cells a practical possibility not just a theoretical one. For a long time, the exact nature of the intermediate reactions during the charging or discharging of the cell were left up to speculation, due to the highly reactive nature of lithium polysulphides. This high reactivity makes it almost impossible to perform any type of analysis on the substrates, as they inevitably degrade within seconds. SLIQ battery closely resembles the voltage profile and the gravimetric capacity of a lithium sulfur battery cell. SLIQ battery has four discrete stages during discharge. The capacity Vs voltage plot of a SLIQ battery is shown below which clearly shows these four discrete stages during discharging.
The first stage occurs between the voltages of 2.4 V and 2.1 V, with the sharp drop in voltage against capacity between 2.3 V and 2.1 V representing the second stage. The third stage is gently sloping plateau between 2.1 V and 2.05 V before the second sharp drop that is the fourth stage, occurring between 2.05 V – 1.5 V. Reactions taking place in each stage of the discharge are listed below;
Primary stage of reactions are:
S8 + 2e− → S82-
S8 + 4e− → S72- + S2-
S8 + 4e− → S62- + S22-
S8 + 4e− → S52- + S32-
S8 + 4e− → 2S42-
Secondary stage of reactions are;
S82- + 2e− → S62- + S22-
S82- + 2e− → S52- + S32-
S82- + 2e−→ 2S42-
S72- + 2e− → S52- + S32-
S62- + 2e− → 2S32-
S52- + 2e− → S32- + S22-
S42- + 2e− → 2S22-
The third stage of reactions are;
S32- + 2e− → S22- + S2-
The fourth stage of reactions are;
S22- + 2e− → 2S2-
Longer chain polysulphides (Li2Sx, where 4 ≤x≤ 8) are mainly generated in the first stage with a smaller contribution being made by the second stage. This is important to note, as longer chain polysulphides are responsible for one of the main issues in lithium sulphur cells: polysulphide shuttling. SLIQ has been developed by solving the polysulphide shuttling issue using chemical, electrical and mechanical methods.
The third and fourth stages of the discharge are where S2- is produced. This molecule is known as an insulating product (Li2S), and the formation of it can cause a passivation layer on the electrode, causing the sulphur to be under-utilised. The passivation layer conducts electrons even more poorly than elemental sulphur, a known insulator, so the sulphur must migrate to the current collector surface in order to react. The passivation layer further interferes with this process as Li2S does not easily conduct elemental sulphur (S8), making the reaction kinetics very sluggish. SLIQ technology has been developed providing practical solutions to solve these known problems.