The rechargeable lithium-ion batteries (LIBs) with organic electrolytes have become the alternative energy supply for portable equipments, consumer electronic devices and high power applications such as electric vehicles and power storage for renewable energy [1]. However, their safety issue and high cost limit their application for large scale energy storage because LIBs use the flammable and expensive organic electrolytes. An aqueous rechargeable lithium battery (ARLB) was first reported by Dahn et al., which has advantages such as safety and low cost [2]. However, its practical application was hindered due to its poor cycling and not good rate capability. Here, we have shown that surface modifications of LiFePO₄ showed a much better rate capability in aqueous electrolyte than pristine LiFePO₄.

2. Results and discussion

The influence of the surface coating on the electrochemical properties of LiFePO₄ cathode in aqueous electrolyte solution was investigated by systematic electrochemical and instrumental analyses. The electrochemical performance of pristine LiFePO₄ and coated LiFePO₄ is presented in Fig. 1. Despite the high ionic conductivity of the aqueous electrolyte (1 M Li₂SO₄ in water), the pristine LiFePO₄ did not show the high rate capabilities. In detail, the pristine LiFePO₄ and coated LiFePO₄ in aqueous electrolyte, which can deliver the initial discharge capacity of 127.5 and 132.1 mAh g⁻¹ at 1 C, respectively. The pristine LiFePO₄ and coated LiFePO₄ have still the discharge capacity of 108.9 and 122.7 mAh g⁻¹ after 100 cycles. Based on these results, it is confirmed that the resistance from the surface failure mode of LiFePO₄ could be a main hurdle for cycle decaying especially in the aqueous electrolyte. After the surface modification of the electrochemically stable inorganic species, the rate capability and the cycle life are further improved. This coating enhance the surface stability to the aqueous solution and presents very promising cycle and rate capabilities for the development of ARLBs. Furthermore, they are investigated in detail by using XPS, SEM and TEM analyses.

Acknowledgments

This research was supported by the Post-Doctor Research Program (2014-2015) through the Incheon National University (INU) and the Basic Science Research Program through the National Research Foundation of Korea (NRF), funded by the Ministry of Science, ICT & Future Planning (NRF-2014R1A1A1038248).

Reference

[1] M.R. Palacin, Recent advances in rechargeable battery materials: a chemist’s perspective, Chem. Soc. Rev. 38 (2009) 2565-2575.

[2] W. Li, J.R. Dahn, D.S. Wainwright, Rechargeable Lithium Batteries with Aqueous Electrolytes, Science 264 (1994) 1115-1118.

Fig. 1. The cycling performance of pristine LiFePO₄ and coated LiFePO₄ in aqueous solution.