Low Temperature Synthesis of Highly Crystallined Spinel Oxides: LiNi1/2Mn3/2O4

Introduction

Spinel-type LiNi_1/2Mn_3/2O₄ is one of the promising cathode materials for high-voltage lithium ion batteries.  The biggest challenge of this material is the capacity degregation due to the decomposition of the electrolyte.  However a significant improvement of the cycle stability was acheved by the highliy crystallined LiNi_1/2Mn_3/2O₄cathode [1].

We also successfully synthesized the highly crystallined LiNi_1/2Mn_3/2O₄ cathode and demonstrated > 95 % of capacity retention after 100 cycles at 60ºC.  However, the formation mechanism of highly crystallined spinel oxides is not well understood yet.  Therefore, we have been investigated the crystal growth process of this material and sucseeded the morphology controled LiNi_1/2Mn_3/2O₄with small particle by a low temperature synthesis.

In this study, the crystal growth mechanism of the LiNi_1/2Mn_3/2O₄ spinel and the influences of the crystallinity of the cathodes to the electrochemical properties were discussed.

 

Experimental

Highly crystallined LiNi_1/2Mn_3/2O₄ was synthesized by a conventional solid-state method.  A nickel manganese double hydroxide (Ni : Mn = 0.99 : 3.01) was annealed at 500ºC for 8 hours.  The obtained complex oxide was mixed with stoichiometric amount of LiOH•H₂O and calcined at 1000ºC for 10 h in air and subsequently Re-oxidized at 700ºC for 12 h, 650ºC for 12 h and 600ºC for 24 h.

The HT-XRD measurements were periodically taken during the calcination process.  Furthermore, weight change was measured by TG/DTA measurement and the morphology of the samples during the heating process was observed using scanning electron microscope (SEM).

Result & discussion

The HT-XRD measurement revealed the formation of spinel phase started at 400ºC and then single phase of LiNi_1/2Mn_3/2O₄spinel was observed at 700ºC.  Above 900ºC, the spinel phase gradually transformed to a NiO-like rock salt phase.  During the cooling step, the spinel phase was reformed once the temperature reached at 700ºC. 

Thermogravimetric analysis proved the correlation between the reversible phase transitions and the oxygen loss process at high temperature.  We suspect reduction of the transition metals initiated by the oxygen loss, stimulated the diffusion of the transition metals resulting in the phase transition to the rock salt phase. 

The structural rearrangement associated with the diffusion of the transition metals also accelerated the particle growth to minimize the surface energy of the particle.  A schematic representative of the particle growth process of the LiNi_1/2Mn_3/2O₄spinel is shown in Fig. 1. 

Since the reversible phase transition was strongly associated with the oxygen loss process, the temperature of the phase transition should be affected by oxygen partial pressure.  By changing the synthesis atmosphere, we successfully controlled the particle size of the highliy crystallined LiNi_1/2Mn_3/2O₄ in the range of 1 - 3 seen in Fig. 2.  The detailed crystal growth mechanism of the LiNi_1/2Mn_3/2O₄spinel and the influences of the crystallinity of the cathodes to the electrochemical properties will be discussed in the meeting.

References

[1] K. Ariyoshi, Y. Maeda, T. Kawai, T. Ohzuku, Journal of the Electrochemical Society, 158 (2011) A281.