۱- Introduction

۲- Experimental

۳- Results and discussion

۴- Conclusions

References

ABSTRACT

Organic-based electrode materials for lithium-ion batteries (LIBs) are promising due to their high theoretical capacity, structure versatility and environmental benignity. However, the poor intrinsic electric conductivity of most polymers results in slow reaction kinetics and hinders their application as electrode materials for LIBs. A binder-free self-supporting organic electrode with excellent redox kinetics is herein demonstrated via in situ polymerization of a uniform thin polyimide (PI) layer on a porous and highly conductive carbonized nanofiber (CNF) framework. The PI active material in the porous PI@CNF film has large physical contact area with both the CNF and the electrolyte thus obtains superior electronic and ionic conduction. As a result, the PI@CNF cathode exhibits a discharge capacity of 170 mAh·g−۱ at 1 C (175 mA·g−۱ ), remarkable rate-performance (70.5% of 0.5 C capacity can be obtained at a 100 C discharge rate), and superior cycling stability with 81.3% capacity retention after 1,000 cycles at 1 C. Last but not least, a four-electron transfer redox process of the PI polymer was realized for the first time thanks to the excellent redox kinetics of the PI@CNF electrode, showing a discharge capacity exceeding 300 mAh·g−۱ at a current of 175 mA·g−۱ .

Introduction

Lithium-ion batteries (LIBs) have played an essential role on energy storage for portable electronic devices in recent decades. However, the ever-growing energy storage demands, especially those driven by electrical vehicles and grid-scale storage of renewable electricity, have generated a great concern not only over the energy density and power density, but also the safety, cost, and sustainability of LIBs [1, 2]. Considering the elaborate preparation process, limited resource and environmental and safety issues of the conventional inorganic battery electrode materials, organic materials are intriguing alternatives for sustainable batteries [3, 4] because of the potentially low-cost, high safety, recyclablility, design diversity of molecular structures, and high theoretical capacities due to low formula weight and capability of multiple electron transfer [5–۸]. The development of organic compounds as electrode material actually started as early as that of the inorganic materials [9], but the progress is lagging behind due to the tremendous success of inorganic intercalation compounds in the late 1980’s [10]. The main problem of most organic materials is their electronic insulating nature. This means that a large amount of conductive carbon (usually more than 30 wt.%) is normally required for the preparation of an organic electrode using the typical slurry coating method [11]. When the mass of the conductive carbon, the polymer binder and the Al current collector is considered, the practical energy density of organic electrodes is quite low [7]. Besides, uniform distribution and good physical contact of organic active materials and the conductive additive in the electrode is often difficult to achieve with the slurry coating method, resulting in low utilization of the active material and poor rate performance [12]. Polyimide (PI), an important engineering plastic with excellent mechanical strength and outstanding thermal stability, belongs to the category of organic carbonyl compounds, which bear high theoretical capacity for storage of lithium ion [13, 14]. Li ions reversibly that absorb or desorb on oxygen atoms as the carbonyl groups are reduced or oxidized [15]. This process is also known as enolation [15, 16]. Theoretically, PI polymers with 1 mole of repeating units can react to 4 moles of Li ions, resulting in a theoretical capacity of ~ 400 mAh·g−۱٫ However, redox processes beyond two-electron transfer reaction have not been reported for PI electrodes in LIBs. It was predicated that four-electron redox process is nearly impossible to be realized reversibly because deep enolation would damage the molecular structure of PI materials [12, 15]. Furthermore, reaction kinetics of PI electrodes needs to be improved for practical considerations [17].