The preparation of the pyridine-SO₃ complexes of formula (I) is known to the person skilled in the art. The pyridine-SO₃ complexes of formula (I) may for example be prepared by reaction of the respective of the respective pyridine compound with SO₃.

The concentration of the pyridine-SO₃ complexes of formula (I) in the electrolyte composition according to the present invention is usually at least 0.01 wt.-%, based on the total weight of the electrolyte composition. The upper limit of the concentration of the pyridine-SO₃ complexes of formula (I) in the electrolyte composition is usually 5 wt.-%. Preferably, the concentration is in the range of 0.01 to 5 wt.-%, more preferred in the range of 0.01 to 2 wt.-%, and most preferred in the range of 0.05 to 1 wt.-% based on the total weight of the electrolyte composition.

A further object of the present invention is the use of pyridine-SO₃ complexes of formula (I) in electrolyte compositions, e.g. as additive. Preferably pyridine-SO₃ complexes of formula (I) are used as film forming additive in electrolyte compositions. They may be used as anode film forming additive and as cathode film forming additive. Their use leads to better cycling properties at elevated temperatures even at high cycling voltages.

Pyridine-SO₃ complexes of formula (I) are usually used by adding the desired amount of the pyridine-SO₃ complex to the electrolyte composition. Pyridine-SO₃ complexes of formula (I) are usually used in the electrolyte composition in the concentrations described above and as described as preferred.