A research team led by WU Zhongshuai and BAO Xinhe from the Dalian Institute of Chemical Physics of the Chinese Academy of Sciences, in collaboration with HE Yanbing from Graduate School at Shenzhen of Tsinghua University, reported a new-type all-solid-state flexible planar lithium ion micro-capacitors (LIMCs). 

With features of landmark volumetric energy density, superior thermal stability and safety at high temperature, outstanding mechanical flexibility, and exceptional integration of bipolar cells, these LIMCs were based on interdigital patterned films of carbon-coated lithium titanate (LTO) nanospheres as anode, activated graphene (AG) as cathode, and high-voltage ionogel as electrolyte. 

The emerging interdigital micro-supercapacitors (MSCs) are one highly-competitive miniaturized energy source. However, most reported MSCs still suffer from low volumetric energy density that can’t compete with lithium thin-film batteries. Also, MSCs are limited by poor high-temperature stability, unsatisfying flexibility and limited integration.

Lithium ion capacitors (LICs) represent promisingly competitive power sources. However, all the LICs reported have been constructed in a nonplanar device geometry, by sandwiching a separator or solid-state electrolyte between two electrode-based substrates.

All-solid-state flexible planar Lithium Ion Micro-Capacitors (Imaged by ZHENG Shuanghao) 

To solve the above problems, scientists developed the interdigital LIMCs, which were fabricated via layer-by-layer mask-assisted deposition of graphene (EG) and electrode materials, without metal current collector, additive and polymer binder. The resulting patterns of sandwich-like electrode structures showed outstanding uniformity, mechanical flexibility, and high electrical conductivity.

The fabricated all-solid-state LTO//AG-LIMCs delivered higher ultrahigh volumetric energy density than those of AG//AG-MSCs and lithium thin-film batteries, and superior to the reported MSCs so far.

Furthermore, LTO//AG-LIMCs exhibited superior cycling stability with 98.9% of capacitance retention after 6000 cycles, high-temperature electrochemical stability at 80 °C. Moreover, LTO//AG-LIMCs showed outstanding mechanical flexibility without performance degradation under various bending and twisting.

These finding are highly scalable for mass production of multiple flexible LTO//AG-LIMCs to modulate the output voltage and capacitance for integrated circuits. Therefore, they hold great potential for future flexible and wearable electronics.

The study was published online in Energy & Environmental Science. It was supported by the National Natural Science Foundation of China, National Key R&D Program of China, Natural Science Foundation of Liaoning Province, and Recruitment Program of Global Expert Thousand.