The Effect of Lithium Polysulphides on the Cycling Performance of a Lithium Electrode in 1M LiClO4 in Sulfolane

The effect of lithium polysulphides (Li₂S_n) on the electrochemical behaviour of lithium electrodes attracts great interest due to the development of batteries with high energy density based on the lithium-sulphur electrochemical system. A feature of lithium-sulphur batteries is the formation of soluble lithium polysulphides, generated during the electrochemical discharge (equations 1-2) and charge (equations 3-5) [1-3].

Discharge:        

(1) nS⁰ + 2e^- + 2Li⁺ → Li₂S_n

(2) Li₂S_n + 2(n-1)e^– +   2(n-1)Li⁺ → nLi₂S↓

Charge:

(3) xLi₂S_k → yLi₂S_n   + 2(x-y)e^- + 2(x-y)Li⁺

(where x·k = y·n)

(4) yLi₂S_n + zLi₂S →   xLi₂S_k,

(where y·n+z = x·k)

(5) Li₂S_n → nS^o + 2e^–   + 2Li⁺

During the cycling of lithium-sulphur batteries lithium polysulphides diffuse and/or migrate to the surface of the lithium electrode. Lithium sulphide and shortened lithium polysulphides are formed by the interfacial reaction between lithium and the lithium polysulphides dissolved in the electrolyte:

(6) 2Li⁰ + Li₂S_n → Li₂S↓ + Li₂S_(n-1)

Lithium sulphide is insoluble in electrolyte systems based on aprotic dipolar solvents and thus deposits on the lithium to form a surface film - Solid-Electrolyte Interface (SEI).

Lithium sulphide can react with “long-chain” lithium polysulphides forming shortened lithium polysulphides, which are soluble in the electrolyte:

(7) Li₂S + Li₂S_n → Li₂S_m   + Li₂S_k,

(where m+k = n+1)

“Short”- and “medium-chain” lithium polysulphides can form low solubility solvate complexes that will also deposit on the lithium metal surface. 

It is known, that the pattern of electrochemical behaviour of the lithium electrode, including its high cycleability, are determined by the properties of SEI [4]. Therefore, the presence of lithium sulphide and solvate complexes of lithium polysulphides in the surface film should have an effect on the electrochemical behaviour of lithium electrode.

The aim of the present work is to assess the effect of dissolved lithium polysulphides on the pattern of electrochemical behaviour of lithium electrodes.

A study of the lithium electrode’s cycling behaviour was carried out in Swagelok type cell. Working and auxiliary electrodes were made from lithium metal foil (99.9 %). The combination of 2 layers of porous polypropylene Celgard^®3501 with 2 layers of non-woven polypropylene between them was used as a separator. 1M LiClO₄ in sulfolane and 0.4M Li₂S₆ in 1M LiClO₄ in sulfolane were used as electrolytes. Electrolyte volume in the cells was 0.02 ml/cm². 

Lithium polysulphides solutions were prepared by dissolving the specified amount of lithium sulphide (Li₂S) and sulphur (S) in 1M LiClO₄ in sulfolane.

Dissolution/deposition efficiency testing of the lithium was carried out galvanostatically with a PG12-100 potentiostat between ±0.5 V at +30 ^oC and +85 ^oC. The dissolution/deposition current density was 0.2 mA/cm², the dissolution/deposition capacity of lithium was 0.5 mAh/cm².

It has been shown that the cycle life of lithium electrodes is significantly increased in the presence of lithium polysulphides, especially at +85 ^oC (Fig. 1).

It is probable that an interface “sulphide” film with good protective properties and high ionic conductivity is formed in electrolyte systems containing lithium polysulphides and that this film is preserved during extended cycling [5-6]. This film inhibits reactions between lithium and the components of the electrolyte solution, in particular with sulfolane molecules.

The main limiting factor for lithium electrode cycling is decomposition rate of solvent and anions of supporting salts as a result of their reduction by freshly formed metallic lithium. The surface of cathodicaly deposited lithium, in the presence of lithium polysulphides, is quickly passivated by lithium sulphide (reaction 6), as the result the rate of reaction of metallic lithium with electrolyte (solvent and lithium salts anions) is decreased. That is why the lithium electrode in the presence of lithium polysulphides is cycled longer.

We believe the increase in temperature leads to the increase in the rate of reaction of metallic lithium with lithium polysulphides. Therefore the increase in temperature leads to increase in the cycle life of lithium electrode.

References

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6. M.W. Wagner, C. Liebenow, J.O. Besenhard, J. Power Sources, 1997, 68, 328-332.