Self-Standing 3D Thin Film Cathodes for Micobatteries
Authors: Xia Hui , Xia Qiuying , Meng Ying Shirley 
Published: February 2020

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.

Figure 1