The solid-electrolyte interface (SEI) in lithium-ion batteries systems has crucial influence on the cell performance and its lifetime. The composition and properties of passivation layer which is formed at this interface depend on many factors like electrode material and electrolyte composition, the morphology of the electrode, potential range in which electrode is working or current densities. Therefore, appropriate selection of all of those parameters is essential to ensure the optimal properties of final battery.

Lithium-manganese spinel LiMn₂O₄ (LMO) is relatively well known and successfully used cathode material in some commercial application (18% of whole cathode materials market according to Avicenne ENERGY reports from 2013). Despite the fact that LMO is used to power electric tools, medical devices or powertrains, its applicability is greatly limited due to the LMO’s poor capacity retention. This restricted cyclability is related to material’s structural lability and its limited stability versus organic solvents used in electrolytes for Li-ion batteries. LMO at room temperatures can undergo reversible phase transition from cubic (Fd-3m) to orthorhombic (Fddd) structure [1]. This transition which is related to the Jahn-Teller distortion of high-spin Mn³⁺ ions causes irreversible loss of capacity during subsequent charging/discharging cycles [1-6]. Stabilization of LMO is possible by distorting long-range order of crystalline structure. This can be achieved by partial substitution in cationic or anionic sublattice. Substitution of manganese ions with other transition metals (Co and Ni are most often used) leads to good stabilization but in the same time have negative impact on material cost. Izoelectronic partial substitution of oxygen atoms in spinel structure with sulfur seems to be better approach because it not only inhibit undesirable phase transition, which improves columbic efficiency of electrochemical reaction and cycle life of battery, but also significantly increase the capacity of the material [7]. Moreover, electrochemical impedance spectroscopy studies of the sulfur doped LMO (LiMn₂O_4-yS_y) indicate that the surface of such modified material is more stable than pristine LMO.

The goal of this studies is to analyze the influence of sulfur presence in the cathode material’s structure on the composition and stability of solid-electrolyte interface in working Li-ion battery cell.

The stability and lifetime of working spinel electrode is strongly dependent on the properties of the material’s passivation layer. The composition of this interface and its stability in the electrolyte in the potential range of 3-4.5 V at different current densities were analyzed during electrochemical reaction for pristine LMO and sulfur doped samples.

ACKNOWLEDGMENT

This work is supported by National Centre for Research and Development, Poland under research grant No. LIDER/463/L-6/14/NCBR/2015.

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