High Voltage Lithium Ion Batteries Based on carbon LiMPO by fby10358


									  High Voltage Lithium Ion Batteries Based on carbon/
         LiMPO4 (M=Mn,Co,Ni) Composites

          J. Moralesa, R. Trócolia, R. Saint-Martinb,
                 S. Frangerb, J. Santos Peñaa
      Department of Inorganic Chemistry, University of
                    Córdoba, 14071 Spain
        Laboratoire de Physico-Chimie de l’Etat Solide,
   ICMMO, Université Paris Sud, 91405 Orsay Cedex,

Since the discovery of Padhi et al. [1], lithium transition
metal phosphates with nominal formula LiMPO4 have
attracted much attention as positive electrodes for lithium
ion batteries. The most studied is LiFePO4, a cheap and
environmentally friendly material that develops a flat
plateau at 3.5V versus Li+/Li. The electrochemical
activity of other members of the family (LiCoPO4 and
LiMnPO4) has also been studied but in less detail as they
are insulators. Recently, measurements on single crystals
evidenced that for electrochemical purposes, electronic
conduction is a more limiting factor compared with
lithium ion diffusivity [2]. Nevertheless, the voltage
where divalent manganese or cobalt are oxidised is higher
than for iron and therefore, cells based on both phosphates
are attractive. This is specially important for LiNiPO4,
where Ni(II) oxidation takes place at 5.1V vs Li+/Li.

Recently, Graetzel et al. [3] proposed the ball-milling of
carbon with nanosized LiMnPO4 in order to obtain a
suitable electrode material. In this communication we
present our results on three different phospholivines,
LiMPO4 (M=Mn,Co,Ni). The three systems were
obtained by heating stoichiometric mixtures of the
corresponding divalent transition metal phosphate with
Li3PO4 and subsequently heating at 800°C. Two different
techniques were envisaged for the preparation of
carbon/phosphate composites. First, powders were ball
milled with almidon for 15h and subsequently heated
under argon atmosphere at 600°C for 30 min. Under these
conditions, the pyrolisis of almidon produced a carbon
coating onto the phosphate particles. This procedure
cannot be used for preparing C/LiNiPO4 composites as
Ni(II) is reduced to Ni(0). The second procedure involves
the ball-milling of super P carbon with the phosphates for
15 h.

The electrodic materials have been characterised by
different techniques, including XRD, HRTEM, FTIR, TG,
DSC and XPS. Electrochemical impedance has been
applied in order to know the total conductivity of the
systems and discriminate different interfacial process.
Electrochemical tests were conducted by using 1M LiPF6
in (PC,EC,DMC) electrolyte.

1. A. K. Padhi, K. S. Nanjundaswamy, and J. B.
Goodenough, J. Electrochem. Soc. 144 (4) (1997) 1188-
2. J. Santos-Peña, R. Saint-Martin, L. Pinsard-Gaudart,
and S. Franger, Solid State Sciences (submitted).
3. D. Wang, H. Buqa, M. Crouzet, G. Deghenghi, T.
Drezen, I. Exnar, N.-H. Kwon, J. H. Miners, L. Poletto,
and M. Graetzel, J. Power Sourc. 189 (2009) 624-628.

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