One-Step Low-Temperature Route for the Preparation of Electrochemically Active LiMnPO 4 Powders C. Delacourt, P. Poizot, M. Morcrette, J.-M. Tarascon, and C. Masquelier* Laboratoire de Re ´ activite ´ et de Chimie des Solides, CNRS UMR 6007, Universite ´ de Picardie Jules Verne, 33 Rue St. Leu, 80039 Amiens Cedex 9, France Received July 30, 2003. Revised Manuscript Received October 23, 2003 Pure and well-crystallized LiMnPO 4 powders were obtained by a direct precipitation route in an aqueous medium at 373 K. A thermodynamic study of the system Li + /Mn(II)/phosphate/ H 2 O identified a pH range in which LiMnPO 4 precipitation was the most probable. From these theoretical considerations, the olivine-type compound precipitation turned out to be straightforward, i.e., no further thermal treatment was needed. A systematic study of the influence of synthesis parameters enabled tailoring of the size of the as-obtained particles. The smallest precipitated particles (∼100 nm) led to a global improvement of the electrochemical behavior when used in lithium batteries, consisting in lowering the polarization and enhancement of the reversible capacities (∼70 mAh/g at C/20). A two-phase Li extraction/insertion mechanism was identified, using the potential intermittent titration technique (PITT) and in situ X-ray diffraction, between LiMnPO 4 and the delithiated phase MnPO 4 , indexed in the space group Pmnb with unit cell parameters a ) 5.93(2) Å, b ) 9.69(6) Å, and c ) 4.78(1) Å. 1. Introduction Among the known Li insertion compounds, the lay- ered rock salt oxides LiCoO 2 1 and LiNiO 2 , 2 and the spinel LiMn 2 O 4 3,4 are now commercially used as 4-V positive electrode materials in rechargeable lithium batteries. However, such materials (namely the layered structures) suffer from poor chemical/electrochemical stability in their highly oxidized state, 5 and LiMn 2 O 4 suffers from dissolution in the electrolyte leading to a rapid capacity fading under high-temperature storage. 6 Phosphate-based electrode materials, especially the phospho-olivines LiMPO 4 (where M ) Fe, Mn, Co, Ni), are now recognized as attractive alternatives. 7 Beyond their high theoretical capacities (∼170 mAh‚g -1 ), the strong P-O covalent bonds drive the potential of the M 3+ /M 2+ redox couple to values greater than 3.5 V vs Li + /Li. Several groups succeeded in tailoring LiFePO 4 /C composite electrodes approaching theoretical capacities at high rates of charge and discharge. 8-12 Therefore, because the gravimetric density (∼3.5 g.cm -3 ) of LiFe- PO 4 is lower than that of LiCoO 2 , its performance is still lower than that of the latter. To counterbalance such an effect the LiMnPO 4 phase is quite attractive owing to the potential of the Mn 3+ /Mn 2+ redox couple located at 4.1 V vs Li + /Li, and which is compatible with the electrolytes presently used in Li-ion batteries. Besides the original work of Padhi, 7 only limited literature is available on LiMnPO 4 so far. 13,14 Padhi et al. noticed that extraction of lithium from Li[Mn y Fe 1-y ]- PO 4 compositions occurred at 3.4 V vs Li + /Li and 4.1 V vs Li + /Li for the Fe 3+ /Fe 2+ and Mn 3+ /Mn 2+ couples, respectively. The relative capacities associated with each of these redox couples are directly linked to the Fe and Mn contents in Li[Mn y Fe 1-y ]PO 4 . Therefore, for y > 0.75, the experimental capacity was found to be far below the expected one. More recently, Yamada et al. corroborated these results and tried to explain the large increase in the polarization overshoot to oxidize Mn 2+ for y > 0.75 as nested in the elastic tolerance limit of the delithiated form [Mn y Fe 1-y ]PO 4 (y > 0.75). Indeed, strong electron (Mn 3+ : 3d 4 ) lattice interactions (Jahn- Teller effect) induce large anisotropic distortion in the Fe 3+ PO 4 -Mn 3+ PO 4 binary system, and thus may limit the resulting solubility to y < 0.8. 13 Controversial results were communicated by another group from the same * To whom correspondence should be addressed via e-mail at christian.masquelier@u-picardie.fr. (1) Mizushima, K.; Jones, P. C.; Wiseman, P. C.; Goodenough, J. B. Mater. Res. Bull. 1980, 15, 783. (2) Thomas, M. G. S. R.; David, W. I. F.; Goodenough, J. B.; Groves, P. Mater. Res. Bull. 1985, 20, 1137. (3) Thackeray, M. M.; David, W. I. F.; Bruce, P. G.; Goodenough, J. B. Mater. Res. Bull. 1983, 18, 461. (4) Thackeray, M. M.; Johnson, P. J.; de Picciotto, L. A.; Bruce, P. G.; Goodenough, J. B. Mater. Res. Bull. 1984, 19, 179. (5) Dahn, J. R.; Fuller, E. W.; Obrovac, M.; Von Sacken, U. Solid State Ionics 1994, 69, 26. (6) Blyr, A.; Sigala, C.; Amatucci, G.; Guyomard, D.; Chabre, Y.; Tarascon, J. M. J. Electrochem. Soc. 1998, 145, 194. (7) Padhi, A. K.; Nanjundaswamy, K. S.; Goodenough, J. B. J. Electrochem. Soc. 1997, 144 (4), 1188. (8) Morcrette, M.; Wurm, C.; Gaubicher, J.; Masquelier, C. Electrode Materials Meeting, Bordeaux Arcachon, 27 May-1 June 2001. (9) Ravet, N.; Goodenough, J. B.; Besner, S.; Simoneau, M.; Hovington, P.; Armand, M. Joint International Meeting, Hawaii, 17 Oct.-22 Oct. 1999. (10) Yamada, A.; Chung, S. C.; Hinokuma, K. J. Electrochem. Soc. 2001, 148 (3), A224-A229. (11) Huang, H.; Yin, S.-C.; Nazar, L. F. Electrochem. Solid State Lett. 2001, 4 (10), A170-A172. (12) Franger, S.; Le Cras, F.; Bourbon, C.; Rouault, H. Electrochem. Solid State Lett. 2002, 5 (10), A231-A233. (13) Yamada, A.; Chung, S.-C. J. Electrochem. Soc. 2001, 148 (8), A960. (14) Li, G.; Azuma, H.; Tohda, M. Electrochem. Solid State Lett. 2002, 5 (6), 135. 10.1021/cm030347b CCC: $27.50 © xxxx American Chemical Society PAGE EST: 7 Published on Web 00/00/0000