Synthesis, growth mechanism, and morphology control of LiFe 1/3 Mn 1/3 Co 1/3 PO 4 via a microwave-assisted hydrothermal

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Hans-Peter Meyer Institute of Earth Sciences, Heidelberg University, Heidelberg D-69120, Germany

Rüdiger Klingeler Kirchhoff Institute for Physics, Heidelberg University, Heidelberg D-69120, Germany; and Centre for Advanced Materials, Heidelberg University, Heidelberg D-69120, Germany (Received 25 November 2014; accepted 23 February 2015)

LiFe1/3Mn1/3Co1/3PO4 (LFMC) has been synthesized by a microwave-assisted hydrothermal technique. During the crystal growth, two evolutionary routes coexist and compete with each other after the nuclei have been stably formed. One of them is the continuous growth of single particles and the other one is agglomeration. The size and morphology of the products are determined by the interplay of the two competing routes. The growth morphology is quantitatively analyzed from first principle calculations. A phase diagram is constructed, which guides to control the morphology by adjusting CM and pH. Static magnetic properties imply long range antiferromagnetic order below TN 5 39 K and a paramagnetic Curie–Weiss-like behavior with h 5 75 K and peff 5 5.51 lB at high temperatures. Cyclic voltammetry shows two distinct peaks corresponding to the Fe21/Fe31 and Co21/Co31 redox couples, respectively, whereas the Mn21/Mn31 redox couple is not observed due to its sluggish kinetics induced by the Jahn–Teller effect of Mn31.

I. INTRODUCTION

As one of the most efficient portable energy storage systems at the moment, rechargeable lithium-ion batteries based on electrochemical intercalation materials are crucial to realize electric vehicles which are able to reduce the CO2 emissions by petroleum-fueled internal combustion engine vehicles.1 To meet the challenging requirements, extensive research has been underway on cathode materials since they are the bottleneck, hindering improvements with regard to energy density, cell voltage, capacity, cyclic performance, lifetime, etc.2–4 Olivine-structured LiMPO4 (M 5 Fe, Mn, Co, and Ni) with a theoretical specific capacity of around 170 mAh g1 has recently attracted increasing interest as a promising cathode material for rechargeable lithium-ion batteries due to as well low cost as safety benefits.5–8 However, the electrochemical performance of pristine phosphates still needs to be improved, even though there has been huge progress in the case of LiFePO4.5 Here, poor rate capabilities caused by low electrical conductivity and low ionic diffusivity have been overcome by nanosizing

Contributing Editor: Michael E. McHenry a) Address all correspondence to this author. e-mail: [email protected] DOI: 10.1557/jmr.2015.63 914

J. Mater. Res., Vol. 30, No. 7, Apr 14, 2015

http://journals.cambridge.org

Downloaded: 17 Apr 2015

and carbon coating.9,10 In contrast, the other olivinestructured materials with a single transition metal ion (M 5 Mn, Co, and Ni) have not reached commercial applicability so far. For example, there are severe issues of capacity fading for LiMnPO4 because of the Jahn–Teller effect of Mn31 ions.6,11,12 Besides other issues, t

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