Combination and High Rate Capability of Fe-Mn Based Li-Rich Cathode Materials with Conducting-Polymer Polypyrrole Nanowires
Authors: Zhao Yujuan , Lv Zhi , Wang Yang , Zhao Zongkui , Ji Changwei 
Published: February 2020

The Lithium-rich solid solutions between layer-structured Li2MnO3 and LiMO2 (M=Ni、Co、Fe) are attractive cathode materials for lithium ion batteries with high specific energy densities. Among them, the Fe-Mn based Li-rich cathode materials Li1+x(FeyMn1-y)1-xO2 are the most potential candidates with high theoretical capacity [1] and low cost, because Fe element is much more inexpensive and abundant compared to Ni and Co. However, there are several drawbacks in the Fe-Mn based materials, such as the disorder of Li and Fe atoms impedes the migration of Li+ and worsen the electrochemical performance [2], and the Li2MnO3 component with worse conductivity leads to poor rate property. To overcome these shortcomings, nano crystalline Fe-Mn based Li-rich materials are suggested, and conductive polymer webs also are introduced to combine with the nano particles of active materials, which could improve their rate capability by reducing the lithium ion diffusion length [3]and conductive network formation.

Polypyrrole (PPy), a typical soft conducting polymer with good mechanical exibility and chemical stability during the electrochemical process, has been attracted much attention. PPy also can act as a host material for Li+ ion insertion/extraction in the voltage range of 2.0-4.5 V versus Li/Li+, with a theoretical capacity of 72 mAh g-1 [4, 5]. Therefore, polypyrrole not only can improve conductivity between Li1.26Fe0.22Mn0.52O2 particles, but also exhibit extra electrochemical activity as cathode material.

In this work, polypyrrole nanowires used as a conductive network were synthesized by a modified oxidative template assembly route, as shown in Fig. 1 (a). Li-rich layered oxide cathode Li1.26Fe0.22Mn0.52O2
(LFMO) is synthesized via a low temperature molten salt method. The composite of LFMO with PPy nanowires content of 10wt% was prepared as following: LFMO powder was added into ethanol and distilled water (Volume=1:1) in an ultrasonic bath for 30min to obtain the suspension A, then as-prepared PPy nanowires were dispersed in ethanol in an ultrasonic bath for 20min to obtain the suspension B. After an additional 1h holding period in the ultrasonic bath, suspension A and B were mixed under continuous stirring for 24h. The final composite powders were filtered, washed and dried at 60°C in a vacuum oven, then heated at 100°C for 2h. The obtained Li1.26Fe0.22Mn0.52O2sample is combined with PPy nanowires about 10% mass ratio, labelled as LFMO/PPy.

As shown in Fig.1, the morphologies of PPy nanowires and LFMO/PPy composite are characterized by Transmission electron microscope (TEM). The fiber diameter of PPy nanowires used as three-dimensional network is approximate 90 nm, seen from Fig. 1 (a). The Fig. 1 (b) demonstrates that nano LFMO particles are indeed adhereing to the PPy nanowires, and the PPy matrix can reduce the particle-to-particle contact resistance, improving the electrical conductivity of the composite. The rate performance of LFMO/PPy is shown in Fig. 1(c) and (d). It is clearly that the cycle performance with different rate ratios of LFMO is greatly enhanced by combining with PPy nanowires. The capacities of LFMO/PPy composite is stable at 132.2mAh g-1 at 1C and 98 mAh g-1 at 3C after 50cycles respectively, which is about 50-30 mAh g-1higher than those of the pristine. The excellent rate capability is mainly due to greatly increasing the electrode conductivity via the conducting-network of PPy nanowires, which also ensured higher discharge plateau voltage even at 3C. And more test results show that the existing of PPy conductive network improves the initial coulombic efficiency and the cycling stability of LFMO material.

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Figure 1