Journal of Power Sources 136 (2004) 150–153 Short communication LiNiPO 4 –LiCoPO 4 solid solutions as cathodes J. Wolfenstine ∗ , J. Allen Army Research Laboratory, AMSRD-ARL-SE-DC, 2800 Powder Mill Road, Adelphi, MD 20783-1197, USA Received 12 April 2004; accepted 12 May 2004 Available online 8 July 2004 Abstract LiNiPO 4 –LiCoPO 4 solid solutions were investigated as potential cathodes for use in Li-ion batteries. Cyclic voltammetry and discharge curves reveal that the discharge capacity of the LiNiPO 4 –LiCoPO 4 solid solutions is associated entirely with the Co 3+ /Co 2+ redox couple at a potential of 4.7–4.8 V. The effect of Ni is to reduce the length of the 4.7–4.8 V plateau. © 2004 Elsevier B.V. All rights reserved. Keywords: Cathode; Li-ion batteries; Olivine; Phosphates; Solid solution 1. Introduction Recently, there has been interest in the use of lithium transition metal phosphates with an ordered olivine struc- ture, LiMPO 4 (M = Fe, Mn, and Co), as potential cathodes for Li-ion batteries [1–9]. The main problem with the above phosphates is their poor rate capability [1–3,6,8–10]. This is attributed to low Li-ion diffusion and/or low electronic con- ductivity [1–10]. Previous attempts to increase the rate have primarily focused on decreasing particle size [8,9], carbon coating of the particle surface [7,11] and lattice doping with aliovalent cations [10]. There is another possible method to improve rate, which has received less attention, which entails increasing Li-ion diffusion [12]. This involves the forma- tion of a solid solution between two lithium transition metal phosphates (i.e., LiMnPO 4 –LiFePO 4 ) where the conductiv- ity (Li-ion diffusion) of one end member (i.e., LiFePO 4 ) is higher than that of the other end member (i.e., LiMnPO 4 ) so that the resulting solid solution (i.e., LiMnPO 4 –LiFEPO 4 ) will have enhanced conductivity compared to the lower con- ductivity end member [2,12]. The potential of the M 3+ /M 2+ redox couple versus Li of the above materials is as follows; 3.5 V for LiFePO 4 , 4.1 V for LiMnPO 4 and 4.8 V for LiCoPO 4 [1–9]. In addition, there is another member of the LiMPO 4 series, LiNiPO 4 , which has been postulated to have a higher redox potential (≈5.2–5.4 V [1,4,13]) than LiCoPO 4 . Previous studies on ∗ Corresponding author. Tel.: +1 301 394 0317; fax: +1 301 394 0273. E-mail address: jwolfenstine@arl.army.mil (J. Wolfenstine). LiNiPO 4 have shown that no Li can be discharged when it was charged to 5.2 V [1,3]. It is the purpose of this short paper to investigate the ef- fects of: (1) adding LiCoPO 4 and (2) charging to higher po- tentials (>5.2 V) than previously investigated [1,3], on the discharge behavior of LiNiPO 4 . LiCoPO 4 was chosen for the following reasons: (1) it has been recently shown that the electrical conductivity of a LiNi 0.5 Co 0.5 PO 4 solid solution was an order of magnitude higher than that for LiNiPO 4 [12] and (2) of the lithium transition metal phosphates; LiFePO 4 , LiMnPO 4 and LiCoPO 4 , LiCoPO 4 has the highest redox po- tential, thus a LiNiPO 4 –LiCoPO 4 solid solution will have the highest energy density of the three possible solid solu- tions (i.e., LiNiPO 4 –LiFePO 4 ) since, the theoretical capac- ity for LiFePO 4 , LiMnPO 4 and LiCoPO 4 is about the same, ≈170 mAh/g [1–9]. 2. Experimental The materials investigated in this study were: (1) LiNiPO 4 , (2) LiNi 0.8 Co 0.2 PO 4 , (3) LiNi 0.5 Co 0.5 PO 4 , (4) LiNi 0.2 Co 0.8 PO 4 and (5) LiCoPO 4 . Powders of these ma- terials were obtained using a two-step solid-state reaction method. In the first step stoichiometric amounts of NiO, CoO and LiH 2 PO 4 were mixed in a jar mill for two hours and then heated at 375 ◦ C for 20h in air. The powders were then crushed and ground and pressed into a pellet. The pellet was fired in air at 775 ◦ C for 48 h. After which the powders were crushed and ground and sieved to less than 45 m. The powders were characterized by X-ray diffrac- tion using Cu K radiation. 0378-7753/$ – see front matter © 2004 Elsevier B.V. All rights reserved. doi:10.1016/j.jpowsour.2004.05.017