Ehrenberg, Helmut

Esmezjan, Lars

Indris, Sylvio

Kaus, Maximilian

Trouillet, Vanessa

Yavuz, Murat

KITopen

KIT – University of the State of Baden-Wuerttemberg and National Research Center of the Helmholtz Association Nano-structured lithium manganese phosphate via solvothermal reaction and anion substitution Lars Esmezjan[a], Sylvio Indris[a], Maximilian Kaus[a], Vanessa Trouillet[a], Murat Yavuz[a, b], Helmut Ehrenberg[a, b] [a]Karlsruhe Institute of Technology (KIT), Institute for Applied Materials (IAM)-Energy Storage Systems (ESS) [b]Helmholtz Institute of Ulm (HIU) Electrochemical Energy Storage, Karlsruhe Institute of Technology (KIT),  Hermann-von-Helmholtz-Platz 1, 76344 Eggenstein-Leopoldshafen, Germany Experimental Details Synthesis:  Nano-structured LiMnPO4 and doped LiMn(PO4)0.9(MO4)0.1 with M = V, Ti, Si were synthesized in diethylene glycol via solvothermal reaction at 200 °C (Fig.2). As precursors Mn(CH3COO)2, Li(CH3COO) and H3PO4 were used. For substitution of the phosphate ion V2O5, Ti(OiPr)4 or Li4SiO4 was added during synthesis. The as-prepared materials were then coated with carbon and annealed at 650 °C for 4 h in an Ar/H2-atmosphere.  LiAc + MnAc2 + (1-x) H3PO4 + x MO4-precursor           LiMn(PO4)1-x(MO4)x   Fig. 2: Synthesis of doped LiMnPO4.  References [1] J. W. Fergus, J. Power Sources 2010, 195, 939-954. [2] D. Jugovic, D. Uskokovic, J. Power Sources 2009, 190, 538-544. [3]  Z. Dai, L. Wang, X. He, F. Ye, C. Huang, J. Li, J. Gao, J. Wang, G. Tian, M. Ouyang, Electrochimica Acta 2013, 112, 144-148.    [4]  S. K. Martha, B. Markovsky, J. Grinblat, Y. Gofer, O. Haik, E. Zinigrad, D. Aurbach,      T. Drezen, D. Wang, G. Deghenghi, I. Exnarb, J. Electrochem Soc. 2009, 156,  A541-A552. [5]  B. Scrosati, J. Garche, J. Power Sources 2010, 195, 2419-2430. [6] J. Kim, G. Cheruvally, J. Ahn, J. Solid State Electrochem. 2008, 12, 799–805. [7] Y. Zhanga, Q. Huo, P. Du, L. Wang, A. Zhang, Y. Song, Y. Lv, G. Li, Synth. Met. 2012, 162, 1315-1326. Conclusion  phase-pure products with orthorhombic structure, space group Pmnb  homogeneous, nanosized particles (100-300 nm)  doping of V on Mn-site (octahedral)   cation substitution doping of Ti/Si on P-site (tetrahedral)   anion substitution Introduction Lithium-ion batteries are commonly built with LiCoO2 as cathode material. However, there are still some issues that need to be improved like toxicity and safety [1]. Thus, new cathode materials were developed. Among them, the olivine-type orthophosphates LiMPO4 (with M = Fe, Mn, Co, Ni) attract much attention. They have an orthorhombic structure with space group Pmnb. Tetrahedral vacancies are occupied by phosphorus and octahedral vacancies by lithium and the transition metal [2]. The lithium diffusion takes place in 1D-diffusion paths along the a-axis (Fig. 1). Depending on the used metal M the material has operation voltages of 3.5 V (LiFePO4), 4.1 V (LiMnPO4), 4.8 V (LiCoPO4) and 5.1 V (LiNiPO4) vs Li/Li+ [3]. Nevertheless, LiMnPO4 shows the highest value with regard to the stability window of the commercial used electrolytes. Further advantages of this material are the abundance of the elements in the earth’s crust, better safety characteristics and lower toxicity. The greatest disadvantage of LiMnPO4 is the very low ionic and electronic conductivity, even worse compared to the iron compound. This leads to a sluggish kinetic and poor rate capability [4]. To overcome these drawbacks several approaches have been established in literature: • synthesis of nanoparticles to get shorter diffusion paths [5], • carbon coating of the active material to enhance the conductivity [6], • doping of the material with different metals [7].  Energy Storage Systems (ESS) Results: XPS measurements Results: Electrochemical investigations Fig. 1: Olivine structure (Pmnb) of LiMnPO4 with MnO6-octahedron, PO4-tetrahedron and lithium diffusion path;              atom colours: Li (green), Mn (purple), P (blue) and O (red).   ΔT, p  Results: Powder  characterization  510515520525normalized intensity [a.u.] Binding energy [eV] V 2p3/2 4+ 5+ 638640642644646648650652normalized intensity [a.u.] Binding energy [eV] LiMn(PO4)(VO4)  sample Mn (II)   LiMnPO4 reference  Mn (II) MnPO4 reference  Mn (III) Mn 2p3/2  Characterization:  XRD experiments were carried out with a STOE Stadi P diffractometer (Mo-Kα1-radiation, λ = 0.70926 Å) in Debye-Scherrer mode. The morphology of the particles was studied with LEO 1530 SEM. XPS measurements were performed with a Thermo Fischer Scientific K-Alpha Spectrometer with micro-focused monochromatic-AlKα X-ray source. Charge compensation was applied with low energy electrons and Ar+-ions. Electrochemical Experiments:  Electrodes were prepared by mixing the active material with Super P carbon (Timcal) and polyvinylidene fluoride (PVdF) binder (Solvay) in a weight ratio of 8:1:1. After adding 1-Methyl-2-pyrrolidinone (NMP) the slurries were coated on Al-foil and dried. The electrodes were electrochemical measured with Li metal as counter electrode on a VMP3 potentiostat (with 0.02 C, between 2,5 V and 4,6 V).  XRD pattern of as- prepared LiMnPO4: • impurities are marked with asterisk, • variation of synthesis conditions led to phase-pure samples (bottom). doped LiMnPO4  undoped LiMnPO4  SEM images show homogeneous and nanosized particles with small size distribution impurities XPS spectra of doped LiMnPO4: • oxidation states of the elements LiMn+2(P+5O4)1-x(MO4)x with M = V+4/+5, Ti+4, Si+4 • content x of the doped metals V+4/+5 = 0.13 Ti+4  = 0.09 Si+4  = 0.23 charge/discharge cycles of undoped LiMnPO4: • non-coated samples vs. carbon coated samples  discharge capacities of doped LiMnPO4: • non-coated samples vs. carbon coated samples CV curves of doped LiMnPO4: • no shift of redoxpotential  (current 0,1 mV/s) high values for the first cycles undoped LiMnPO4  carbon coating doped LiMnPO4  carbon coating doped  carbon coating partial reduction  of vanadium during synthesis  with carbon coating better cycling stability  doping enhanced discharge  capacity   (from 30 mAh/g up to 100 mAh/g)  Si-doping also improved cycling stability  (97 % capacity retention after 10 cycles)  only Mn+2/Mn+3 is electrochemically active 130132134136138normalized intensity [a.u.] Binding energy [eV] P 2p 

Nano-structured lithium manganese phosphate via solvothermal reaction and anion substitution

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