On the Manganese Dissolution Process from LiMn2O4 Cathode Materials

Yonas Tesfamhret,Reza Younesi,Haidong Liu,Zhigang Chai,Erik Berg

doi:10.1002/celc.202001496

Abstract

AbstractTransition metal (TM) dissolution is a process experienced by most cathode materials based on lithium transition metal oxides. Spinel LiMn2O4 (LMO) is the best‐known cathode material that suffers from TM dissolution. Therefore, LMO is selected here to understand the dissolution process and derive an inductively coupled plasma optical emission spectroscopy (ICP‐OES) method for quantifying dissolved metal ions. Furthermore, the LMO powder is coated with thin Al2O3 films of different thicknesses using atomic layer deposition (ALD) in an attempt to suppress the dissolution of Mn. Two different types of counter electrodes, lithium iron phosphate (LFP) and Li‐metal, were used to investigate the role of the counter electrode on Mn dissolution. HF is identified as the lead cause of Mn dissolution, through comparisons of cells containing LiPF6 or LiClO4 based electrolytes. The results show that Li‐metal counter electrode effectively minimizes the dissolution process via likely consuming HF and H2O impurity. In contradiction to the purpose of the protective Al2O3 thin film coating, surface coated LMO showed higher dissolution of Mn compared to pristine LMO, both in LFP||LMO and Li||LMO configurations. Al2O3 is proposed to generate H2O when reacts with HF. H2O could have the possibility to migrate back in the electrolyte and participate in the hydrolysis of LiPF6, resulting in more HF and thereby more Mn dissolution.

Highlights

Since the introduction of LiCoO2 into the battery market, cathode materials based on transition metal oxides and their derivatives have attracted a substantial interest in research and commercial applications.[1,2] The transition metals (TMs) in these cathode materials play an important role in aging and safety of lithium ion batteries
Thickness of the Al2O3 coating can be estimated based on the amount of Al and 1.85 m2/g BET surface area measured of the Pristine LMO (PLMO) powder (Table S1 and S2)
A comparative quantification of Mn dissolution from LMO cathode materials based on inductively coupled plasma optical emission spectroscopy (ICP-OES) measurements is discussed in this work

Summary

Introduction

Tetrahedral sites.[7,8] Electrochemical intercalation and deintercalation of Li-ions occurs in the potential range of 3.5–4.3 V vs. Li+ /Li, offering a theoretical specific capacity of 148 mAh/g. Dissolution of Mn from LMO has been shown to be a result of (i) Jahn-Teller distortion and (ii) subsequent corrosion due to increased electrolyte acidity.[9,10,11,12,13,14] Jahn-Teller effect, which is induced by Mn3+, causes a symmetry distortion of the octahedra complex and transforms the spinel into a tetragonal symmetry.[8,15,16,17] This mechanism is more pronounced at high current rates on account of Li-diffusion in the electrolyte being much faster than in the LMO structure, resulting in Li+ concentrating more at the surface of LMO particles and forming a Mn3+ rich region.[16,18] It has widely been proposed that disproportionation reaction of the Mn3+ leads to Mn2+ dissolving into the electrolyte This identified behavior of aging mechanism due to TM dissolution makes LMO a model cathode material in this study for qualitative and quantitative investigation in understanding the underlying mechanisms of TM dissolution process and the related capacity fading. To understand the influence of counter electrode and cross-talk on the amount of Mn dissolution from the working electrode, two different types of counter electrodes, Li-metal or lithium iron phosphate (LFP), is used.[49]

Results and Discussion

LiMn2O4 ðsÞ þ 4 Hþ ðlÞ !

Conclusion

Conflict of Interest