Does Ni and/or S Doping of Lithium-Manganese Spinel Enhance Surface Stability of a Cathode?
Authors: Swietoslawski Michal , Bakierska Monika , Mech Krzysztof , Molenda Marcin 
Published: February 2020

Around 18% of all of the commercial Li-ion batteries is based on lithium-manganese spinel LiMn2O4 (LMO). This material stands out among other cathode systems by its price and environmental friendliness [1]. 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 columbic efficiency. This limitation in electrochemical performance is connected with LMO structural instability. LMO at room temperatures can undergo reversible phase transition (caused by Jahn-Teller distortion of high-spin Mn3+ ions) from cubic (Fd-3m) to orthorhombic (Fddd) structure which is electrochemically inactive [2]. Moreover, in working Li-ion battery cell, the LMO can degrade through the reaction with components of liquid electrolyte [3]. The spinel disproportionation reaction with electrolyte (El) oxidation is presented below.
LiMn2O4 + 3xLi+ + xEl → Li1+3xMn2-xO4 + xMn2+ + xEl+

To overcome the first of these problems and suppress the unfavorable phase transition in material the long range order in the spinel structure must be disordered. 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 has negative impact on material cost [4]. Izoelectronic partial substitution of oxygen atoms in the spinel structure with sulfur gives the same result but does not influence the electrochemistry of the material or its cost [5].
In this studies we are taking a closer look on the surface of modified LMO spinels and theirs stability in Li-ion battery cell. The goal of this research is to analyze the influence of dopant presence in the cathode material’s structure on the stability of its surface against liquid electrolytes as well as the composition and stability of solid-electrolyte interface which is formed during electrochemical intercalation/deintercalation of Li+.
Materials obtained via a sol-gel process with different compositions LiMn2-xNixO4-ySy, x = (0;0.5), y = (0;0.05) were studied in terms of materials corrosion and passivation in Li-ion half-cell. Besides the electrochemical studies (cyclic voltammetry, galvanostatic charge/discharge tests, electrochemical impedance spectroscopy), the analysis of passivation layer by X-ray photoelectron and infrared spectroscopy as well as transmission electron microscopy were conducted. The manganese dissolution in liquid electrolytes was evaluated using both electrochemical methods and atomic absorption/emission spectroscopy analysis.

ACKNOWLEDGMENT

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

References

[1] J.W. Fergus, Recent developments in cathode materials for lithium ion batteries, J. Power Sources 195 (2010) 939-954.
[2] K.Y. Chung, K.B. Kim, Investigations into capacity fading as a result of a Jahn–Teller distortion in 4 V LiMn2O4 thin film electrodes, Electrochim. Acta 49 (2004) 3327-3337.
[3] C.H. Lu, S.W. Lin, Dissolution kinetics of spinel lithium manganate and its relation to capacity fading in lithium ion batteries, J. Mater. Res. 17 (2002) 1476-1481.
[4] A.V. Churikov et al., Electrochemical Properties of LiMn2–yMeyO4 (Me = Cr, Co, Ni) Spinels as Cathodic Materials for Lithium-ion Batteries, Russ. J. Electrochem. 45 (2009) 175-182.
[5] M. Molenda et al., Structural and electrochemical characterization of sulphur-doped lithium manganese spinel cathode materials for lithium ion batteries, Solid State Ionics 272 (2015) 127-132.