With the advancement in electronic applications ranging from portable devices to large-scale ones, such as electric vehicles (EVs), hybrid electric vehicles (HEVs) and stationary energy storage systems for smart grid community, lithium ion batteries (LIBs) with high energy density and power density are seriously required. To meet the requirement, a battery design capable of affording more active material loading on an electrically conductive network without other additives is therefore preferred. Regarding the above issue, fiber-type electrodes, namely using carbon fibers as current collectors, have been developed to improve the battery performance. [1,2] Due to its binder-free electrode design, lithium ions can directly intercalate into and deintercalate from the host material based on an effective conductive connection, thus leading to improved electrochemical performances. To combine the advantages of carbon fiber and high-voltage active material, we develop a binder-free fiber-type cathode composed of nickel-manganese-based (LMO/LNMO) active material and evaluate its electrode performance.

Experimental

To prepare the fiber-type cathode, a sequential synthesis process, including electrodeposition, hydrothermal reaction and post-calcination treatment, was applied. In the first step, a carbon fiber tow aligned in the same orientation was partly immersed into a solution with nickel and manganese nitrate to carry out the electrodeposition for the formation of precursor. Secondly, the fiber precursor was rinsed, dried and then moved to an autoclave with a LiOH solution, where the hydrothermal treatment was performed under 100ºC for 20 h. After rinsing and drying, the fiber tow was calcined at 400ºC for 1 h and then a fiber-type LMO/LNMO cathode can be eventually obtained. To analyze the morphology and crystalline structure of the active material, SEM and XRD are respectively applied. For its electrochemical evaluation, a coin cell (CR2032) was used, in which the fiber-type cathode and lithium foil were used as working and counter/reference electrodes, respectively. In addition, the electrolyte was 1 M LiPF₆ in a mixture of ethylene carbonate (EC) and diethyl carbonate (DEC) in a volume ration of 1:1.

Results and Discussion

Based on the SEM observation, the active materials, which are synthesized with different Mn/Ni ratio during the electrodeposition step, are found to differ in morphology. From the XRD analysis of the active materials, three crystalline phases of layered Li_1.88[Mn_xNi_(1-x)]_1.12O₃, spinel LiMn₂O₄ and LiNi_0.5Mn_1.5O₄ crystalline phases can be identified. As for the electrochemical performance, the fiber-type LMO/LNMO cathode shows an initial discharge capacity of ca. 140 mAh/g with two plateaus at 4.7 and 4.0 V in its discharge profile. In addition, for the cycling test, the capacity retention at the 50th cycle is 98% and the coulomb efficiency for each cycle is above 90%, exhibiting a stable electrochemical behavior in the voltage range 2.5‒4.9 V. Therefore, it is suggested that the fiber-type cathode is beneficial for the reversible ion intercalation/deintercalation into and from the host structure of the active material LMO/LNMO, thus contributing to the improved electrode performance under high voltage operation.

References

1. J. Yao, K. Nishimura, T. Mukai, T. Takasaki, K. Tsutsumi, Kondo-Francois Aguey-Zinsou, and T. Sakai, ECS Electrochem. Lett., 1, A83–A86 (2012).

2. Y. H. Liu, T. Takasaki, K. Nishimura, M. Yanagida, and T. Sakai, J. Power Sources, 290, 53–158 (2015).