Electrochemical Property of LiCoMnO4 Cathode Thin Film Prepared By PLD and Oxygen Post Annealing
Authors: Ishigaki Norikazu , Kuwata Naoaki , Kawamura Junichi 
Published: February 2020

Introduction

Lithium ion batteries used in electric vehicles and high power/energy storage systems are required to improve their high voltage and capacity. Spinel structured lithium manganese cobalt oxide (LiMnCoO4) is a potential cathode material to solve these problems. Since, its operating potential (about 5V) is very high compared to other cathode materials. [1]. Moreover, its theoretical discharge capacity is 740 Wh/kg, which is 1.3 times higher than LiCoO2: the most popular cathode material at present.
However, previous reports on cyclic voltammetry studies on LiCoMnO4 revealed an unexpected peak around 4V, in addition to the main peak at 5V corresponding to the Co3+/4+ redox couple. This electrochemical reaction at 4V reduces the capacity of LiCoMnO4 above 5V. [2]. This peak at 4V is associated with the Mn3+/4+ redox couple in the spinel structure. Pasero et al. reported that Mn3+/4+ redox peak is due to the loss of oxygen content in the LiCoMn1-2δ
+4Mn2δ
+3O4-δ
structure where the oxygen loss increases the Mn3+ content [3]. They reported that the control of oxygen content is effective to increase the capacity above 5V. In this study, we tried to control the loss of oxygen content in LiCoMnO4thin films by post annealing process and to improve its capacity around 5V.

Experiment

LiCoMnO4 was prepared by solid-state reaction method using mixtures of Li2CO3, Co3O4 and MnCO3. All starting materials were ground thoroughly in an gate mortar, treated in a muffle furnace at 850℃ for 24 hour at a rate of 100℃/h. Following this, the samples were ball milled in a planetary mill for 2 hours and calcined at 600℃ for 12 h. The process was repeated thrice. A PLD target of LiCoMnO4was prepared by hydrostatic pressing at 75 MPa for 15 min and 90 MPa for 15 min followed by sintering at 600℃ for 24 h.
LiCoMnO4 thin-films were grown by pulsed laser deposition (PLD) at room temperature with an oxygen partial pressure of 0.1Pa. After preparing the films were treated by post annealing condition to study the difference in structural change and electrochemical property by post annealing condition. After preparing the thin films, the samples were heated in atmospheric pressure of O2 at 500, 600, and 700℃ for 1 ~ 72 h at a rate of 800℃/h in a tube furnace and then cooled rapidly to room temperature. An electrochemical cell was assembled with the LiCoMnO4 thin film as positive electrode on a Pt/Cr/SiO2 glass substrate, Li metal as the negative electrode and reference electrode, and 1 mol/L LiPF6 in ethylene carbonate (EC:DMC, 1:1 volume,) as the electrolyte. Cyclic voltammetry (CV) was performed on the cell in the potential range of 3.0 - 5.3 V vs. Li/Li+at a scan rate of 0.5 mV/s.

Results & Discussion

The results of post annealing of LiCoMnO4 suggests that the optimum temperature is 600℃. Fig.1 shows the XRD peak profile for LiCoMnO4/Pt/Cr/SiO2 heated at 600℃ for 0h~72h. The as-prepared sample shows no peak corresponding to LiCoMnO4, confirming its amorphous structure. Upon annealing a peak appeared at 2θ value of 18° which is attributed to the (111) Bragg reflection of LiCoMnO4. Similarly, the XRD peaks of (311), (400), (333) and (400) are observed after post-annealing, which shows the formation of spinel structure of LiCoMnO4.
Fig.2 shows the enlarged XRD spectra in the (311) reflection region as shown in Fig.1. The peaks were fitted with voigt profiles. All peaks are found to shift toward lower 2θ against the LiCoMnO4 target peak, which is shown the black line in Fig.2. These shifts are caused by oxygen deficiency in LiCoMnO4 thin-films. Oxygen deficiency is related to the crystal spacing and hence XRD peak shift. This result confirms that the post-anneal time strongly affects the oxygen deficiency amount in the LiCoMnO4 thin films. The CV of LiCoMnO4 ­thin films are shown in Fig.3. This result confirm that the post-annealing increases the capacity of LiCoMnO4 at 5V region due to the decrease in oxygen loss. However, the 4V region increases if the annealing time is longer than 6h. Thus, this study shows that the optimum condition for post-annealing is found to be at 600℃ for 6 h in 1 atm O2.

References

[1] H.Kawai, M.Nagata, H.Kageyama, H.Tukamoto, and A.R.West, Electrochimica Acta, 45 (1999) 315－327.
[2] D.P asero S.de.Souza, N.Reeves and A.R.West, Journal of Materials Chemistry, 15 (41) (2005) 4435－4440.
[3] N.Kuwata, S.Kudo, Y.Matsuda, and J.Kwamura, Solid State Ionics, 262 (2014) 165－169.

Figure 1