Substituent Effects on Redox Potential of Organic Electrode Materials in Li/Na Rechargeable Batteries: Electronic Effects Vs. Cation-Substituents Interaction
Authors: Lee Sechan , Hong Jihyun , Kown Ji Eon , Park Soo Young , Kang Kisuk 
Published: February 2020

1. Introduction

Conventional Li rechargeable battery system contains transition metal based electrodes. These conventional transition metal based electrodes have severe problems such as scarcity of resources, potentially high cost, and toxicity. With respect to the high sustainability for the next-generation energy storage system, another candidate of electrode materials is needed. Due to high sustainability and chemical tunability, organic battery system holds a great potential in next generation energy storage system. In order to develop these new battery system, understanding factors affecting a performance should have priority than others. It is well-known that the LUMO level of organic compounds has a linear correlation with redox voltage in rechargeable batteries (Figure 2). 6 However, it has been recently reported additional factors affecting redox voltage of organic electrode materials. 4,5

This research is focusing on categorization of factors affecting a voltage, especially, on cation-substituents interaction. With many references and our experiment results, it is convinced that voltage of organic materials is determined by several factors. Additional substituent effect and cation species effect should be considered for predicting an exact voltage behavior with molecular tuning.

2. Results and Discussion

The target species based on terephthalate are shown in Figure 3. Terephthalate is one of the simplest form in organic electrode materials. Therefore, another effect can be reduced to study cation-substituent effect. The plot of LUMO level vs. discharge voltage of target species is shown in Figure 4. Four types of cathode materials have same redox center based on terephthalate already reported by J.-M. Tarascon group.7 Tendency of almost species follows conventional rules which of correlation between LUMO level and discharge voltage, but some species having special substituent shows different behavior.
Table 1. LUMO level vs. Experimental voltage

Species

Na2H2TP

Na2TP

Na2DMTP

Na2MeO2TP

Li cell

(vs. Li/Li+)

0.93

0.84

0.81

0.9

Na cell

(vs. Na/Na+)

0.55

0.59

0.54

0.38

LUMO

(eV)

-1.58

-1.41

-1.4

-1.34

3. Concluding remarks

Firstly, some special substituent gives a big effect to determine a voltage. Methoxy group (-OCH3) is an electron-donating substituent. With LUMO level, voltage should be lower than others. But, in Li cell system, it does not follow. Interaction between lithium and oxygen from methoxy group make a stable discharge form. On the other hand, in Na cell system, sodium ion has a bigger size than lithium ion. So, interaction between oxygen cannot be occurred because of steric strain. Secondly, with LUMO level, voltage of Na2H­2TP should be higher than Na2TP. But, in Na cell system, it is not. Steric strain affect to the voltage of Na2H2TP. In our conclusion, lithium ion size is negligible affecting to the voltage but sodium ion size starts to affect the lowering of voltage in Na2H2TP electrode material. It is expected that competition between factors affecting a voltage of organic electrode materials. In summary, cation-substituent interaction should be considered to predict an exact voltage. It is important for tuning an organic electrode material to make an appropriate voltage in our use. More additional factors may exist and it is needed to categorize them. The more understanding of factors affecting to a voltage gathered, then more accurate prediction and molecular tuning can be possible.

4. References

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Figure 1