Influence of Using Additives in Spray Drying Process on Structural and Electrochemical Properties of LiNi0.5Mn1.5O4 cathode Material

As one of the most promising candidates for the next generation lithium ion batteries, LiNi_0.5Mn_1.5O₄ (LNMO) has attracted much attention since its discovery in 1997.^1,2 With a theoretical capacity of 146.7 mAh g^-1 and a high average operational voltage of 4.7 V from Ni²⁺/Ni⁴⁺ redox couple, it offers a larger energy density compared to the common cathode materials like LiCoO₂, LiFePO₄ or LiMn₂O₄.³ The LNMO provides three dimensional diffusion channels for Li⁺-ion insertion/extraction, leading to a good rate capability and good safety characteristics. In addition, it is more economical and less toxic than the materials employing cobalt.

The spinel structure of LNMO is reported to exist in two polymorphs, depending on synthesis conditions: an ordered structure (P4₃32), where the Mn and the Ni ions are ordered, occupying different lattice sites (12d and 4a, respectively) and a disordered structure (Fd-3m), where the transition metal ions are randomly distributed on 16d sites. These polymorphs are reported to possess different properties for instance in terms of ion conductivity or manganese dissolution.

Besides of the obtained phase after synthesis process, the morphology of primary particles and the way of their agglomeration into secondary particles plays a crucial rule on the electrochemical performance.

In this work, LiNi_0.5Mn_1.5O₄ spinel material was synthesized by a spray drying process using different additives as well as different heating devices. The additives used were urea, citric acid and polyvinylpyrrolidon (PVP). For the heat treatment, a conventional muffle furnace and a microwave assisted muffle furnace were employed. The microwave assisted muffle furnace offers advantages over conventional heating in terms of higher heating, more homogeneous temperature profiles in the sample or a reduction of activation energies for reactions during calcination, leading to reduced energy costs and/or purer products.

The effect of the additives and heating method on the structure and electrochemical performance of LiNi_0.5Mn_1.5O₄ is studied in detail.

First results show an influence of the additive on the primary and secondary particle morphology. Adding Polyvinylpyrrolidon (PVP) in the spray drying process leads to smaller particle of precursor and the formation of flake-like secondary LNMO particles after high temperature thermal treatment. Also the cycling stability and the rate capability could be increased when using PVP. A further improvement in terms of cycling stability and rate capability, as being showed in figure 2, can be achieved when heating the precursor rapidly by using the microwave assisted muffle furnace. From the SEM image in figure 1, this can be attributed to the formation of thermodynamically unfavorable lattice planes like {110} or {100} in which the Li⁺ ion diffusion rate is higher than for the more stable {111}.^4,5

References

1 K. Amine, H. Tukamoto, and Y. Fujita, J. Electrochem. Soc., 1996, 143, 1607-1613.

2 Q. M. Zhong, A. Bonakdarpour, M. J. Zhang, Y. Gao and J. R. Dahn, J. Electrochem. Soc., 1997, 144, 205-213.

3 G. B. Zhong, Y. Y. Wang, Z. C. Zhang, and C. H. Chen, Electrochim. Acta, 2011, 56, 6554-6561.

4 J. Mao, K. H. Dai, and Y. C. Zhai, Electrochim. Acta, 2012, 63, 381-390.

5 M. Hirayama, H. Ido,K. Kim, W.Cho, K. Tamura, J. Mizuki, R. Kanno, J. Am. Chem. Soc., 2010, 132, 15268–15276