Solid State Sciences 4 (2002) 1487–1493 www.elsevier.com/locate/ssscie Influence of carbon coating on the performance of a LiMn 0.5 Ni 0.5 O 2 cathode Brian L. Cushing a , John B. Goodenough b,∗ a Advanced Materials Research Institute, University of New Orleans, New Orleans, LA 70148-2820, USA b Texas Materials Institute, ETC 9.102, The University of Texas at Austin 1 University Station, C2200, Austin, TX 78712-1063, USA Received 24 July 2002; accepted 30 September 2002 Dedicated to Professor Neil Bartlett on the occasion of his 70th birthday Abstract The effects of coating α-NaFeO 2 -structured LiMn 0.5 Ni 0.5 O 2 cathode materials with carbon particles has been investigated. The coated cathodes exhibit average discharge capacities of 108, 101 and 87 mAh/g at current densities of 0.25, 0.50 and 1.0 mA/cm 2 , respectively, when cycled between 2.75 and 4.25 V, versus capacities of 88, 82 and 69 mAh/g for the uncoated cathodes cycled under the same conditions. The carbon coatings are shown to suppress the capacity fade observed in the uncoated cells by increasing the electronic conductivity and reducing cell polarization, thus preventing the evolution of oxygen from the cathodes.  2002 Éditions scientifiques et médicales Elsevier SAS. All rights reserved. Keywords: Lithium-ion battery; Cathodes; Insertion compounds; Manganese–nickel oxides; Carbon coating 1. Introduction LiMn 0.5 Ni 0.5 O 2 (LMNO) has recently been proposed as an alternative cathode material to LiCoO 2 in rechargeable lithium-ion batteries [1]. LMNO, while exhibiting a dis- charge capacity as high as 150 mAh/g at a current density of 0.1 mA/cm 2 , does not exhibit good electronic conductiv- ity, which dramatically reduces the specific capacities that can be achieved at even moderate current densities. The problem of low electronic conductivity has plagued numerous otherwise promising cathode materials. Olivine- structured LiFePO 4 , for instance, exhibits a loss of 18% of its 170 mAh/g theoretical capacity even when cycled at a very low current density of 0.05 mA/cm 2 , and it loses ca- pacity precipitously as current is increased [2]. To circum- vent this problem, Armand et al. applied carbon coatings to LiFePO 4 particles [3]. The carbon allows Li + ions to tun- nel to or from the underlying LiFePO 4 and electrons to tun- nel between the carbon and the two-phase front in the parti- cles. Borrowing from the method of Armand, we report here the effects of carbon coatings on the cathode performance of LiMn 0.5 Ni 0.5 O 2 . * Corresponding author. E-mail address: jgoodenough@mail.utexas.edu (J.B. Goodenough). LiMn 0.5 Ni 0.5 O 2 has been reported to be isostructural with α-NaFeO 2 , frequently referred to as the hexagonal O3 struc- ture (O = octahedral coordination of Li + ;3 = number of ∞ [MO 2 ] n layers in the unit cell) common to the well-known LiCoO 2 and LiNiO 2 cathode materials [4,5]. The struc- ture consists of alternating infinite layers of edge-sharing (Mn,Ni)O 6 and LiO 6 octahedra. The layers arrange in an approximately fcc arrangement to give an ABCABC oxygen packing sequence along the c-axis (Fig. 1). This structure has proven particularly well-suited to application as a cath- ode material in secondary batteries. The alternating-layer arrangement provides a very suitable two-dimensional con- duction pathway for the Li + ions, and the 90 ◦ M–O–M bond angles allow reasonable π -bond electronic conduction in the transition metal layers, but only poor σ -bond electronic con- duction. 2. Experimental LiMn 0.5 Ni 0.5 O 2 was prepared by direct reaction between LiOH (Fisher, reagent grade), Ni metal (Aldrich, 99.9+%) and Mn 2 O 3 (Cerac, 99.9%). The reactants were ground in an agate mortar and pestle and heated to 800 ◦ C overnight in air. The mixtures were then reground, compacted into 2.5- cm-diameter pellets and heated to 900 ◦ C for 20 h in air. 1293-2558/02/$ – see front matter  2002 Éditions scientifiques et médicales Elsevier SAS. All rights reserved. PII:S1293-2558(02)00044-4