The Size and Shape Effect of LiMnPO4 Nanoparticles on the Lithium Ion Diffusion

Advanced lithium ion batteries require higher safety, lower cost, longer durability and lower toxicity to apply larger applications [1].

LiMnPO₄ can be an alternative cathode material due to its stable structure, low material cost, lower toxicity, high theoretical capacity (170 mAh/g), high operating voltage (4.1 V vs. Li) and good capacity retention. However, it suffers from poor electronic and ionic conductivity [2, 3]. Its poor ionic conductivity can be overcome by employing nano-particles in order to shorten Li-ion path lengths [4, 5]. Enhancement in electron transport is achieved by carbon coated nanocomposite cathode material. Most high-performing LiMnPO₄ materials were so far achieved by adding a large amount of carbon (15 – 30 wt%) in order to increase the electronic conductivity [6-9]. Recently, we reported < 30 nm sized nano-LiMnPO₄ reached 97 % of theoretical capacity with 10 wt% of carbon additive in total in the electrodes [10]. It shows that controlling the particle size of LiMnPO₄ and the construction of nanocomposite of nano-LiMnPO₄ and carbon are critical to maximize the electrochemical properties. Therefore, we investigated further to address the following questions; i) which direction is favorable for lithium ions in different shapes of nano-LiMnPO₄? ii) What is the desired composite structure to improve the electrochemical properties ?

Since olivine LiMnPO₄ materials have a preferred direction of lithium ion diffusion in the lattice, it can be different depending on the shapes and sizes of nano-LiMnPO₄ shown in Fig. 1. Chemically exfoliated graphene from graphite flake was applied to nano-LiMnPO₄, forming a thin coating on the surface of the active material shown in Fig. 2.

We determined the lithium ion diffusion coefficients in terms of shapes and sizes of LiMnPO₄ nanomaterials. We also studied the influence of nanocomposite structures, which affected significantly the electrochemical behavior of LiMnPO₄ cathode. Several processes of making nanocomposites have been investigated to understand the kinetics of diffusivity in LiMnPO₄cathode.

References:

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Fig. 1. TEM images show the various shapes and sizes of LiMnPO₄nanoparticles.

Fig. 2. TEM image shows the nanocomposite containing nano-LiMnPO₄ in dark color attached on the surface of graphene sheet in light color.