Electrochimica Acta 56 (2011) 9168–9171 Contents lists available at ScienceDirect Electrochimica Acta j ourna l ho me pag e: www.elsevier.com/locate/electacta FTIR spectroscopy of a LiMnPO 4 composite cathode Nick S. Norberg, Robert Kostecki ∗,1 Environmental Energy Technologies Division, Lawrence Berkeley National Laboratory1 Cyclotron Road, Berkeley, CA 94720, USA a r t i c l e i n f o Article history: Received 2 April 2011 Received in revised form 26 July 2011 Accepted 26 July 2011 Available online 5 August 2011 Keywords: Li-ion batteries Cathode LiMnPO4 FTIR spectroscopy Jahn–Teller distortion a b s t r a c t A Li x MnPO 4 (x = 1.0–0.15) composite cathode was investigated by Fourier-transform infrared spec- troscopy at different states of charge. Significant spectral changes of the PO 4 3- vibrations, which are correlated with the Jahn–Teller distortion of Mn 3+ in MnPO 4 and the 3rd ionization potential of Mn, were observed upon electrochemical delithiation of LiMnPO 4 . The presence of two sets of peaks observed in the series of delithiated Li x MnPO 4 spectra is consistent with a two-phase process for delithiation. These results provide insight into the structural changes that occur during lithium extraction and insertion in LiMnPO 4 . © 2011 Elsevier Ltd. All rights reserved. 1. Introduction Lithium metal phosphates (LiMePO 4 , Me = Mn, Fe, Co, and Ni) are among the most promising cathode materials for lithium ion bat- teries [1–3]. LiFePO 4 first gained interest due to its relatively high redox potential (3.5 V vs. Li/Li + ), high capacity (170 mAh/g), thermal stability, and inexpensive and environmentally benign constituents [3]. Following the success of the LiFePO 4 in commercial Li-ion sys- tems, there has been an increased focus on LiMnPO 4 , because its higher operating potential (4.1 V vs. Li/Li + ) at an equivalent dis- charge capacity can offer a significantly higher energy density than LiFePO 4 [1,4–6]. There has been extensive research into fundamen- tally understanding the structural changes and phase transitions in LiFePO 4 during the charge–discharge processes [3,7–10]. Much less is known about the behavior of LiMnPO 4 , mainly because of the difficulty in preparing composite electrodes that exhibit close to theoretical capacity and high discharge rates. The poor elec- trochemical behavior of LiMnPO 4 has been attributed to its even lower electronic conductivity than LiFePO 4 [6,11]. The preparation of composite LiMnPO 4 cathodes with good electrochemical per- formance, with capacities approaching the theoretical maximum, has been achieved through careful nano-engineering of the active material particles, carbon coating and optimization of the compos- ite electrode [4,5,12,13]. The phosphate anion (PO 4 3- ) vibrational modes in LiMePO 4 olivines, which belong to the Pnma (D 2h 16 ) space group, are ∗ Corresponding author. Tel.: +1 510 486 6002; fax: +1 510 486 5454. E-mail address: r kostecki@lbl.gov (R. Kostecki). 1 ISE member. strongly influenced by the presence of Li + . The four oxygen atoms that are covalently bonded to the tetrahedrally coordinated P 5+ cation (C s site symmetry) occupy sites with two different bond- ing arrangements [14]. Two of these oxygen atoms bond to two Me 2+ and one Li + each, and the two other oxygen atoms bond to two Li + and one Me 2+ each. The phosphate group vibrational ener- gies are therefore influenced by the identity of the Me 2+ atoms, their oxidation states, and the presence of Li + coordinated to the phosphate oxygen atoms. For example, the splitting of the anti- symmetric stretching and bending vibrations of PO 4 3- have been correlated with the 2nd ionization potential of the Me in LiMePO 4 [14–16]. Fourier-transform infrared (FTIR) and Raman spectra of Li x FePO 4 display two sets of distinct bands which support the two- phase mechanism during lithium insertion/extraction in LiFePO 4 [8,17]. However, little has been reported on the vibrational behav- ior of Li x MnPO 4 mainly due to the difficulty of delithiating pure LiMnPO 4 either chemically or electrochemically. Chen and Richard- son observed a monotonic energy shift in FTIR peaks associated with the phosphate bending vibrations with varying Li content, revealing the presence of a small solid solution phase of Li x MnPO 4 near x = 0 [18]. Other reports have used Li x (Mn y Fe 1-y )PO 4 cathodes (0 < x ≤ 1, y ≤ 0.8) to investigate the effects of Mn on the vibra- tional structure of olivine phosphates, since delithiation occurs much more readily with the presence of Fe [19,20]. Burba and Frech as well as Kopec et al. observed only minor differences between the IR spectra of LiFePO 4 and Li(Mn y Fe 1-y )PO 4 , but the phosphate stretching and bending bands in the Mn-containing materials upon lithium extraction were much broader and less-resolved than for pure FePO 4 , particularly for y = 0.8 [19,20]. The broadening was attributed to the Jahn–Teller distortion of the Mn 3+ ion, which 0013-4686/$ – see front matter © 2011 Elsevier Ltd. All rights reserved. doi:10.1016/j.electacta.2011.07.116