To date, the finding of novel cathode materials for Li-ion microbatteries with the superior performances in terms of safety, cycle life, power and energy densities is still a challenge in order to develop small-scale electrical power sources for modern microelectronic devices ¹. Recently, electrodes composed of Cu have been investigated as cathode materials for Li-ion batteries such as LiCuPO₄ (LCP) ², Cu₃(PO₄)₂ ³, Li_3-xCu_xPO₄ ⁴_, CuF₂ ⁵ etc. Besides low cost and environmentally benign, they could deliver a high theoretical gravimetric capacity due to its ability to intercalate more than one Li ion per formula unit ⁶.

In this work, we report the remarkable electrochemical properties of LiCuPO₄ thin-film deposited by radio frequency (r.f.) sputtering without heating treatments as cathode material. Fig. 1a shows the XRD patterns of the initial target, crystalline LCP and as-deposited LCP_. The initial target and the slurry obtained from a crystalline powder show the same peaks corresponding to well-crystallized LCP phases. However, after deposition at room temperature, the XRD pattern of the as-deposited LCP thin-film has no visible diffraction peak, indicating that the film is amorphous. Fig. 1b shows the SEM images of the as-deposited LCP thin-films on Si substrate.

Fig. 1. XRD patterns of the initial target, crystalline LCP and as-deposited LCP a). Corresponding SEM images of as-deposited LCP film from the top view b) and cross-sectional view (inset in figure 1b).

Fig. 2 shows the cycle performance of amorphous LCP film at different kinetics. The capacity fading observed for the initial cycles can be attributed to the loss of the active material. Indeed, dissolution of Cu into electrolyte is supposed to form intermediate Cu⁺ compound leading to the increasingly ineffective reconversion process during recharging ⁷. The battery delivers a stable capacity of 70 mAh.g^-1 (22 μAh.cm^-2) at C/10. The capacity can be recovered after cycling at fast C-rate over 160 cycles, revealing that the reversibility of the conversion reactions could be improved with thin-films by sputtering.

During this work, the electrochemical performance of both amorphous LCP thin-film and crystalline LCP composite electrode will be discussed.

Fig. 2. Discharge capacity of the amorphous LCP film at multi C-rates

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