Elsevier

Journal of Power Sources

Volume 275, 1 February 2015, Pages 106-110
Journal of Power Sources

Improving electrochemical properties of porous iron substituted lithium manganese phosphate in additive addition electrolyte

https://doi.org/10.1016/j.jpowsour.2014.11.028Get rights and content

Highlights

  • Porous LiMn0.6Fe0.4PO4 (LMFP) was synthesized by a modified sol–gel process.

  • The discharge capacity and energy density are 152 mAh g−1 and 570 Wh kg−1, respectively.

  • To prevent Mn dissolution, TMSP was added into the electrolyte as additive.

  • TMSP additive in the electrolyte improved cycle stability and rate-capability of LMFP.

Abstract

Porous LiMn0.6Fe0.4PO4 (LMFP) is synthesized by a modified sol–gel process. Highly conductive LMFP due to uniform dispersion of carbon throughout LMFP particles are achieved by the addition of sucrose as an additional carbon source. The LMFP obtained has a high specific surface area with a uniform, porous, and web-like nano-sized carbon layer on the surface. The initial discharge capacity and energy density of the LMFP cathode is 152 mAh g−1 and 570 Wh kg−1, respectively, at 0.1C current rate. The combined effect of high porosity and high electrical conductivity lead to fast lithium ion diffusion and enhance initial capacity compared to materials prepared by the general sol–gel method. However, with conventional electrolyte (1M LiPF6 in EC/DMC) poor cycle performance is observed due to HF attack. To improve the cycle stability we add tris (trimethylsily) phosphite (TMSP) as an additive in the electrolyte which dramatically improves cycle stability and rate-capability.

Introduction

Lithium iron phosphate has become a promising cathode material for rechargeable lithium ion batteries, due to its low cost, environmental friendliness (non-toxic), appreciable theoretical capacities (∼170 mAh/g), and high thermal and electrochemical stabilities [1], [2]. The success of lithium iron phosphate inspired many research groups to further develop attractive phosphate alternatives. Compared to the cobalt-based cathode materials such as LiCoO2 and the layered LiNi1/3Mn1/3Co1/3O2 [3], [4], phosphate-based materials are more suitable for large scale applications, such as hybrid electric vehicles (HEVs) and backup power systems.

Recently, partial substitution of Mn and Co for Fe in LiFePO4 was employed to achieve high energy density and high stability lithium ion batteries. Indeed, LiMnPO4 and LiCoPO4 have potentials of 4.1 and 4.9 V vs. Li, respectively [5], [6], [7]. However, these materials have drawbacks that limit their use – the dissolution of substituted metal and decomposition of electrolyte above 4.5 V vs. Li. Moreover, LiMnPO4 and LiCoPO4 show poor discharge capacities and cycle stabilities [8]. Several groups have proposed solutions to mitigate these drawbacks. For instance, a coexistence of metal cations in LiFePO4 is a promising strategy for high energy density lithium ion batteries [9], [10], [11]. Yamada et al. studied the dependence of Mn content on the structure and electrochemical properties of LiMnyFe1−yPO4 and we have achieved good electrochemical properties with a LiMn0.4Fe0.6PO4 composite [12], [13]. However, sufficient enhancement has not been made. The objective of this paper is to introduce a new method to improve the cycle ability and rate-capability of LiMn0.6Fe0.4PO4 cathode material.

In this work, we have designed a porous LiMn0.6Fe0.4PO4 cathode prepared by modified sol–gel method to improve lithium-ion battery capacity. In the modified sol–gel method sucrose is used as additional carbon source with citric acid because the carbon prepared by sucrose show high electrical conductivity [14], [15]. Although the capacity and energy density have been significantly improved by high surface area and high electrical conductivity in the LiMn0.6Fe0.4PO4, poor cycle performance was shown with high Mn dissolution. To enhance the cycle ability tris (trimethylsily) phosphite (TMSP) was added as additive into electrolyte. The TMSP additive can form SEI layer on the surface of cathode and prevent Mn dissolution caused by the destruction of SEI with HF attack, resulting in enhancement of cycle ability and rate-capability.

Section snippets

Experimental

LiMn0.6Fe0.4PO4 was synthesized by the modified sol–gel method, described in a previous study for LiFePO4 [14]. Li2CO3, FeC2O4·2H2O, Mn(COOCH3)2·4H2O and NH4H2PO4 (All 99%, Aldrich) and citric acid (Shinyo Pure Chemicals, 99%) were used as starting materials to synthesize LiMn0.6Fe0.4PO4. Sucrose was added as an additional carbon source. All the components were dissolved in deionized water at room temperature and added to a citric acid solution. After homogenous mixing, the sol was dried by

Results and discussion

The chemical composition of LiMn0.6Fe0.4PO4 obtained from ICP analysis matched the theoretical molar ratio of Li: Mn: Fe: P as 1.01:0.61:0.39:0.99 (±0.02) and contained ∼10 wt.% carbon. The XRD pattern of LiMn0.6Fe0.4PO4 (LMFP) is shown in Fig. 1. The crystal structure of the LMFP is consistent with a standard ordered orthorhombic olivine structure of PDF card No. 40–1499, and no impurities were detected, which indicates that the high purity LMFP could be synthesized by a typical sol–gel

Conclusion

A highly porous LiMn0.6Fe0.4PO4 composite cathode material was prepared by the modified sol–gel process. The process with sucrose incorporation resulted in the synthesis of micro-sized particles of high specific surface area, surrounded by a uniform and porous web of nanometer sized carbon. The high porosity and homogenous carbon web can facilitate lithium ion diffusion and electron conduction. The LMFP cell showed a high discharge capacity and energy density of 157 mAh g−1 and 570 Wh kg−1 at

Acknowledgments

This work was supported by the Creativity and Innovation Project Fund (1,140009,01) of Ulsan National Institute of Science and Technology (UNIST) and US National Science Foundation under Grant No. 1335850.

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