Spinel LiMn2O4 Thin Films for 3 V Operating All-Solid-State Lithium Microbatteries

Nowadays, all-solid-state microbatteries have become a choice of alternative power source for microelectronic devices such as secured smart cards, RFID tags, stand-alone sensors, etc. In this context, low potential lithium-based systems suitable for microelectronic requirements have to be developed. Here, spinel LiMn₂O₄material, known for operating in the 4 V region, was investigated in the 3 V region.

Thin films were prepared using physical vapor deposition (RF-magnetron sputtering). SEM micrographs reveal a dense microstructure and a typical columnar growth. The as-deposited material appears to crystallize in a cubic spinel structure according to X-ray diffractograms. Atomic absorption spectroscopy shows a slight over-lithiation of the samples. Process parameters such as Ar/O₂ratio and total pressure were optimized to improve electrochemical performances of the material.

First results were obtained in a coin cell configuration with an organic liquid electrolyte. Low crystallinity thin films elaborated with optimized conditions led to a good specific capacity of about 60 µAh.cm^-².µm^-1. A large 3 V plateau, which corresponds to the structural transformation from cubic spinel to tetragonal phase, can be observed after a low temperature post deposition heat treatment at 300°C in air (Fig. 1a, red curve).

LiMn₂O₄ thin films were then integrated in all-solid-state microbatteries using LiPON as solid electrolyte (Fig. 1b). The microbatteries integrating LiMn₂O₄thin films exhibit a good cycle performance.

The authors acknowledge ST-Microelectronics for the financial support.