Binder-free hybrid Li4Ti5O12 anode for high performance lithium-ion batteries
Graphical abstract
Introduction
Rechargeable batteries have become indispensable in everyday life. Most portable electronic devices, electric vehicles, and medical devices use rechargeable batteries for energy storage. Among various rechargeable batteries, lithium-ion batteries are especially prominent because of their high working voltage, high energy density, and portability [[1], [2], [3], [4], [5], [6]]. Current Li-ion batteries use graphite, Sn, or Si as anode materials. However, graphite has poor performance with a highly irreversible reaction and forms lithium dendrite when cycled at high C-rate [7,8]. Meanwhile, Sn and Si undergo structural reorganization with huge volume changes during the charge/discharge process [[9], [10], [11]].
To solve these problems, many researchers have explored alternative anode materials. Recently, spinel Li4Ti5O12 (LTO) has been considered among the most promising anode materials. Although it has a relatively low capacity, there is active research to apply it to hybrid electric vehicle (HEV) batteries because of the possibility of realizing high power. Li4Ti5O12 shows a very flat voltage plateau in charge–discharge curves, due to the two-phase equilibrium junction. The LTO anode can intercalate up to three lithium ions per formula unit, which allows a theoretical capacity of 175 mA h g-1 with low volume change during electrochemical reaction. Furthermore, the operating voltage is high and the surface of the negative electrode is not formed by the electrolyte decomposition reaction. Therefore, the efficiency of the initial charge and discharge cycle is almost 100%. The LTO anode could exhibit stable charging and discharging characteristics, making it an attractive candidate for stable rechargeable lithium-ion batteries.
However, due to the slow diffusion rate of lithium ions and low electronic conductivity in its structure, LTO suffers from capacity loss and poor rate capability [[12], [13], [14], [15]]. To solve these problems, the synthesized LTO particles should be small (nanometers in size) and highly porous, in order to shorten the ion movement distance with carbon coating. These approaches have resulted in improved electrochemical properties during charge and discharge [[16], [17], [18], [19]].
Nevertheless, highly porous nanoparticles require a large amount of solvent to prepare the slurry when manufacturing the electrode plate, resulting in lower productivity. Porous and nano-sized particles are also sensitive to moisture. Upon exposure to air, excess moisture is adsorbed on the surface of the particles. Residual moisture in the electrode can decompose the material electrochemically into hydrogen in the cell and release a large amount of gas, thereby deteriorating the cell performance [20]. Coating the hydrogenated Li4Ti5O12 with carbon also presents a safety problem [[21], [22], [23]]. In addition, the usage of porous nanoparticles and carbon coating lowers the energy density of lithium-ion batteries. Therefore, we need a novel strategy to achieve both high energy density and high rate-capability for the LTO-based anodes.
In this study, poly (styrenesulfonate) (PSS) was composited with LTO without binder because poly (styrenesulfonate) can store the lithium ion and rapidly exchange the ions to improve rate-capability and energy density. PSS was coated on LTO surfaces by in-situ polymerization to facilitate the lithium ion migration. PSS contains sulfonyloxy radicals in the repeat unit structure, and it can quickly interact with the lithium cations and transfer them inside the Li4Ti5O12 particle. Moreover, replacing the carbon with PSS and Li-doped poly (3,4-ethylenedioxythiophene) (PEDOT) in the electrode as conductor enables the fabrication of binder-free electrodes. The conducting polymer can help bind the particles, and improves the energy density compared to when using non-conductive binders. As a result, excellent electrochemical performance was confirmed, together with increased capacity and high rate-capability.
Section snippets
Experimental
Li4Ti5O12 (LTO) powder was prepared by a solid-state reaction. Li2CO3 and TiO2 powders were mixed in a ball mill for 1 h at 3000 rpm. The mixed powders were sintered at 850 °C for 10 h. Then, the PSS-coated LTO anode material was obtained by in-situ polymerization of styrenesulfonate (SS). LTO was mixed with SS (Aldrich) with LiCl in water. After removal of the liquid phase by filtration, the product was dried at room temperature under high vacuum. To obtain the LTO–PSS composite by
Results and discussion
Fig. 1 is a schematic diagram of the LTO–PSS anode material, and the electrochemical mechanism of PSS is also shown. PSS was formed by in-situ polymerization of styrene sulfonate to cover the surface of LTO (Fig. S1). This electrode material displays a core-shell configuration. Commonly, redox reactions of electrode materials during charge/discharge occur by direct intercalation and deintercalation of lithium ions. However, the LTO–PSS electrode displays a hybrid mechanism with both ion
Conclusions
In summary, a new hybrid LTO–PSS anode material was synthesized by in-situ polymerization and showed promising electrochemical performance with high Coulombic efficiency, remarkable cycle stability, and outstanding rate capability. The initial discharge capacity was found to be 249 mAh g−1, and 239 mAh g−1 of it remained after 100 cycles at a current density of 0.1C. Even when cycled at 5C current density, the discharge capacity of LTO–PSS was maintained at 214.9 mAh g−1. These results indicate
Acknowledgements
This research was supported by the Basic Science Research Program through the National Research Foundation of Korea (NRF) funded by the Ministry of Education (2017R1D1A1B03031123 and 2017M1A2A2087577) and the Human Resources Program in Energy Technology of the Korea Institute of Energy Technology Evaluation and Planning (KETEP), which was granted financial resources from the Ministry of Trade, Industry & Energy, Republic of Korea (20174010201240). This research was partially supported by the
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