While the microelectronic industry is advancing at a rapid pace with smaller and smaller devices, the implementation of microelectro-mechanical systems (MEMS) on the market strongly depends on the availability of on-board power sources. However, traditional rechargeable batteries based on liquid electrolyte are not applicable due to the restrictions for on-chip design, size and inherent risk of leakage, which give rise to the development of all-solid-state thin film lithium-ion microbattery technology. As more energy is required in microelectronic devices, two-dimensional (2D) thin film microbatteries are no longer the ideal battery design because the limited footprint area results in a compromise between energy density and power density for the build-in power source. A move to all-solid-state three-dimensional (3D) architectures has been proposed as a promising approach to tackle this challenge and achieve both high energy and power densities within the footprint area [1].

  3D microbatteries generally require the fabrication of 3D nanoarchitectured electrodes with large capacity at high charge/discharge rates and long-term cycling capability. Without using binders or conductive additives, the direct fabrication of nanoarchitectures for the electrode materials on conductive substrates represents a new class of electrodes: 3D self-supported electrodes. Except as the key component for constructing 3D microbatteries, the 3D self-supported electrodes using flexible substrates (such as metal foil and carbon cloth) can be ideal electrodes for flexible lithium-ion batteries. Active research efforts have been devoted to the synthesis and characterization of various 3D self-supported electrodes on different conductive substrates. However, most of the previous works focused on the preparation of 3D self-supported nanoarchitectures for anode materials such as Si, Co₃O₄, and TiO₂ [2], while very limited work has been reported on the 3D self-supported nanoarchitectures for cathode materials. The difficulty lies in the fact that the synthesis of cathode materials such as LiCoO₂ and LiMn₂O₄ involves high temperature treatment, which is difficult for retaining the nanostructures after heat treatment.

  In this work, we present a new "hydrothermal lithiation" method to construct 3D LiCoO₂ and LiMn₂O₄ nanoarrays on metal substrates [3,4]. Mesoporous low-temperature LiCoO₂ nanowire arrays can be directly prepared by a two-step hydrothermal method and they can be easily converted into chain-like high-temperature LiCoO₂ nanowire arrays through further calcination. The layered LiCoO₂ nanowire arrays exhibit both high gravimetric capacity and areal capacity, while maintaining good cycling stability and rate capability. Similar method was used to prepare vertically aligned porous LiMn₂O₄ nanowall arrays, comprising highly crystallized spinel nanoparticles, on various conductive substrates without high temperature treatment. The "hydrothermal lithiation" can effectively convert Mn₃O₄ spinel nanowall arrays into LiMn₂O₄ spinel nanowall arrays without severe morphology change. The binder-free three-dimensional porous LiMn₂O₄ nanowall arrays exhibit high specific reversible capacity up to 131 mAh g^-1 (or 0.29 mAh cm^-2) as well as outstanding cycling stability and rate capability, making them promising as cathodes for both three-dimensional thin film lithium-ion microbatteries. The 3D cathode design and the effective low-temperature synthesis route will lead to new opportunities to develop high-performance microbatteries and flexible lithium-ion batteries. 

References

[1] Wen Xiong, Qiuying Xia, Hui Xia*, Three-dimensional self-supported metal oxides as cathodes for microbatteries, Functional Materials letters 7 (2014) 1430003

[2] Hui Xia*, Wen Xiong, Chew Keat Lim, Qiaofeng Yao, Yadong Wang* Jianping Xie*, Herarchical TiO₂-B nanowre@a-Fe₂O₃ nanothorn core-branch arrays as superior electrodes for lithium-ion microbteries, Nano Research  7 (2014) 1797.

[3] Hui Xia*, Yunhai Wan, Wilfried Assenmacher, Werner Mader, Guoliang Yuan, Li Lu*, Facile synthesis of chain-like LiCoO₂ nanowire arrays as three-dimensional cathode for microbatteries, NPG Asia Materials 6 (2014) e126.

[4] Hui Xia, Qiuying Xia, Binghui Lin, Junwu Zhu, JoonKyo Seo, Ying Shirley Meng, Self-standing porous LiMn₂O₄ nanowall arrays as promising cathodes for advanced 3D microbatteries and flexible lithium-ion batteries, Nano Energy (2016), in press.