Elsevier

Journal of Power Sources

Volume 342, 28 February 2017, Pages 231-240
Journal of Power Sources

Nanoparticle shapes of LiMnPO4, Li+ diffusion orientation and diffusion coefficients for high volumetric energy Li+ ion cathodes

https://doi.org/10.1016/j.jpowsour.2016.11.111Get rights and content

Highlights

  • Rod-shaped, elongated and cubic nanoparticles of LiMnPO4 were synthesized.

  • The shape of LiMnPO4 altered the 1D diffusion direction of Li+.

  • Particles with the shortest dimension along the b-axis possessed the highest DLi+.

  • The shape of LiMnPO4 affected its tap density.

  • The volumetric capacity of LiMnPO4 increased via the formation of dense composite.

Abstract

Nanoparticles of LiMnPO4 were fabricated in rod, elongated as well as cubic shapes. The 1D Li+ preferred diffusion direction for each shape was determined via electron diffraction spot patterns. The shape of nano-LiMnPO4 varied the diffusion coefficient of Li+ because the Li+ diffusion direction and the path length were different. The particles with the shortest dimension along the b-axis provided the highest diffusion coefficient, resulting in the highest gravimetric capacity of 135, 100 and 60 mAh g1 at 0.05C, 1C and 10C, respectively. Using ball-milling, a higher loading of nano-LiMnPO4 in the electrode was achieved, increasing the volumetric capacity to 263 mAh cm−3, which is ca. 3.5 times higher than the one obtained by hand-mixing of electrode materials. Thus, the electrochemical performance is governed by both the diffusion coefficient of Li+, which is dependent on the shape of LiMnPO4 nanoparticles and the secondary composite structure.

Introduction

Energy storage systems are necessary when the electrical energy is produced excessively from renewable energy devices and to provide energy whenever needed. One of the promising methods to store energy is by using electrochemically rechargeable system such as Li-ion batteries.

Olivine structured materials, LiMPO4 (M = Fe, Mn, Co, Ni) are well known to possess an excellent structural stability versus Li+ insertion/extraction. The covalent P-O bond strengthens the framework such that it does not collapse upon redox processes occurring at the transition metal ion M2+/3+ [1]. This structural property of olivines provides the outstanding cyclability and thermal stability in the application of lithium ion batteries [2], [3], [4], [5], [6].

Among olivine materials, LiFePO4 has been commercialized due to its facile synthesis and its higher electronic conductivity compared to other olivine materials. The drawback of LiFePO4 is a low potential (3.6 V vs. Li+/Li), leading to a maximum energy density of 578 Wh kg−1 (170 mAh g−1 × 3.6 V). In contrast, LiMnPO4 has a higher potential (4.1 V vs. Li+/Li) with 171 mAh g−1 theoretical capacity, thus providing a potentially 20% higher energy density of 701 Wh kg−1 (170 mAh g−1 × 4.1 V) than that of LiFePO4. LiCoPO4 and LiNiPO4 have even higher cell potentials of 4.8 and 5.1 vs. Li+/Li, respectively, but a lack of stable electrolytes at such high potentials, preventing their commercial application [7], [8], [9]. Thus, LiMnPO4 is a strong candidate as cathode material for high energy Li-ion batteries.

To be used in a battery, LiMnPO4 is also required to have good rate capabilities such as > 70% of theoretical capacity at 1C. A two-pronged approach in reducing the particle size of LiMnPO4 on one hand and having a carbon coating on the surface of LiMnPO4 particles on the other hand are most promising ways to obtain the high capacity due to the improvement of both ionic and electronic conductivity. Recently, Hatta et al. reported a high rate capability using pyrolytic carbon and Li3PO4 coating on rod shaped LiMnPO4 nanoparticles, providing 160 and 145 mAh g−1 at 0.1C and 1C, respectively [10]. Platelet shaped LiMnPO4 was reported to have a discharge capacity of 100 and 113 mAh g−1 at 1C at room temperature and 50 °C, respectively [4].

Furthermore, the Li+ diffusion has to be controlled in the olivine materials. Olivines typically consist of slightly distorted LiO6 and MO6 octahedra and PO4 tetrahedra in the crystal structure [1], [11], [12]. For LiMnPO4 with space group Pnma, the Li+ ion diffusion occurs in a zigzag-ed path along the b axis via edge sharing LiO6 octahedra [13], [14]. One way to improve the rate capability is thus to shorten the [010] diffusion path for Li+ ion release and insertion. In addition, the exposure of the (010) facet on the surface of LiMnPO4 particles can accelerate the kinetics of Li+ in the Pnma system [15], [16], [17]. Controlling the morphology and crystal size is thus vital for the battery performance as these parameters determine the preferred direction of the Li+ diffusion channel and the shape of the particles [18], [19].

To control the morphology and surface structure of LiMnPO4 particles, synthetic parameters such as precursors, surfactants and solvents play an important role [4], [20], [21], [22], [23], [24]. Obtaining LiMnPO4 nanoparticles is however more difficult than for LiFePO4. When the same synthesis method is applied to generate both, the particle size of LiFePO4 is smaller than the one of LiMnPO4 [25]. Various synthesis routes have so far produced morphologies of LiMnPO4 with spherical [26], plate-shaped [4], [27], [28], rods [25], [29], [30], wires [31], [32], microporous [33] and flower-like structures [34], [35]. Their electrochemical properties were affected by different morphologies of LiMnPO4 [36], which may have different orientations of Li+ diffusion.

While one advantage of using nanoparticles is to improve rate capabilities [2], [4], [23], [37], [38], [39], however, as their surface area is higher than the one of micron-sized particles, the volume of a nanomaterial composite is extremely high due to the empty space between the particles, resulting in a low tap density. This feature decreases the loading of nanomaterial on the specific area of the current collector and limits the volumetric energy density of nanomaterials based batteries. In case of LiFePO4, the formation of micron-sized secondary particles increases the volumetric energy density compared to nanoparticles, increasing thus the tap density of the composite. Liu et al. increased the tap densities of LiMn0.4Fe0.6PO4 and carbon by the formation of a secondary microsphere composite prepared by spray drying [26]. For a micron-sized composite consisting of nano-LiFePO4 and carbon, the volumetric discharge capacity increased was shown to increase by a factor of 2.5 compared to the one of nano-LiFePO4 alone [40].

Therefore, there are a few challenges remaining for LiMnPO4; 1) generating a desired shape of nano-LiMnPO4 in a controlled way in order to facilitate Li+ mobility, 2) improving the volumetric energy density of LiMnPO4 nanomaterial without losing the high performance property of LiMnPO4 and 3) optimising the specific capacity of this nano-LiMnPO4, which is still a challenge probably due to the inhomogeneous mixing between agglomerated nanoparticles of LiMnPO4 and carbon [35].

Although the particle shapes of LiMnPO4 and their electrochemical performances were reported, Li+ diffusion coefficients combined with the diffusion orientations were not further investigated in different shapes of particles. We present here a synthesis method based on thermal decomposition which allows us to obtain various sizes and shapes of LiMnPO4 nanoparticles. The direction of the ionic diffusion (b-axis) in particles of various shapes was investigated using TEM electron diffraction in combination with the Li+ diffusion coefficients (D˜Li) on single particles. We furthermore examined the tap densities of various morphologies of nano-LiMnPO4 and its composites with carbon. The volumetric and gravimetric capacities were further investigated with those materials.

Section snippets

Chemicals

Manganese nitrate hexahydrate (Mn(NO3)2·6H2O), manganese acetate tetrahydrate (Mn(CH3COO)2·4H2O, >99%), manganese chloride tetrahydrate (MnCl2·4H2O, >98%), lithium hydroxide monohydrate (LiOH·H2O, >98%), oleic acid (C8H15COOH, >99%), oleylamine (C9H17NH2, >70%), citric acid (C6H8O7, >99%) and solvents (benzyl ether, ethanol, hexane and N-methyl-2-pyrrolidone) were purchased from Sigma-Aldrich and Acros and used as received. Ketjenblack carbon, polyvinylidene fluoride (PVDF, -(C2H2F2)n-) and

Controlling size and shape of LiMnPO4 nanoparticles

The thermal decomposition method was used to synthesize different shapes and sizes of LiMnPO4 nanoparticles. We found that the concentration of oleic acid plays a crucial role for the general particle shape, while the reaction temperature influences the thickness and the length of the particles, and hence their aspect ratio. We also studied the influence of different kinds of precursors and their concentrations. Manganese acetate (Mn(CH3COO)2·4H2O) formed polyhedral nanoparticles with a mean

Conclusion

Nanoparticles of LiMnPO4 were synthesized in an elongated spherical, thin nano-rod, thick nano-rod, cubic and platelet shapes by thermal decomposition method. The shapes of LiMnPO4 nanoparticles affected the tap densities, the path and the direction of Li+ diffusion, determined by electron diffraction spot patterns and electrochemical tools. The tap density of carbon-contained composite increased 3 times higher by the formation of dense agglomeration via ball milling. The increased tap density

Acknowledgements

This work was supported by Swiss National Research Program (NRP 64; Project number 406440_141604) of ‘opportunities and risks of nanomaterials’, and the Swiss Competence Center for Energy Research (SCCER) Heat and Energy Storage.

Dr. Kwon leaded the project and designed the experiments and electrochemical analyses. Yin and Vavrova synthesized LiMnPO4 and prepared electrodes, respectively. Jonathan H-W Lim determined Li+ diffusion coefficients using EPS. Prof. Grobéty and Dr. Kwon did electron

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