Journal of Power Sources 132 (2004) 172–180 A simple mechano-thermal coating process for improved lithium battery cathode materials George Ting-Kuo Fey a,∗ , Hao-Zhong Yang a , T. Prem Kumar a,1 , Sajo P. Naik a , Anthony S.T. Chiang a , Dzu-Chi Lee b , Jinn-Ren Lin b a Department of Chemical and Materials Engineering, National Central University, Chung-Li 32054, Taiwan, ROC b Industrial Technology Research Institute 195, Section 4, Chung Hsing Road, Chu Tung, Hsin-chu 310, Taiwan, ROC Received 22 September 2003; received in revised form 7 January 2004; accepted 16 January 2004 Abstract A simple, economical and convenient mechano-thermal coating procedure for the production of LiCoO 2 with improved cycling per- formance is described. The coating material was pre-formed nanoparticulate fumed silica. TEM studies with a 1.0 wt.% silica-coated cathode suggested that the silica species partially diffused into the bulk of the cathode material. XRD studies showed a diminished lattice parameter c upon coating, indicating that a substitutional compound of the LiSi y Co 1-y O 2+0.5y type might have formed upon calcination. SEM images, R-factor values from XRD studies and electrochemical studies showed that a coating level of 1.0wt.% gave an optimal performance in capacity and cyclability. SEM images showed that above this level, the excess silica formed spherules, which got glued to the coated cathode particles. Galvanostatic cycling studies showed that at a coating level of 1.0wt.%, cyclability improved three and nine times for two commercial LiCoO 2 samples. © 2004 Elsevier B.V. All rights reserved. Keywords: Coated cathodes; Mechano-thermal coating; Coated LiCoO 2 ; Fumed silica; Lithium-ion battery 1. Introduction Lithiated transition metal oxides of the general formula LiMO 2 , where M is a transition metal like V, Cr, Fe, Co or Ni, adapt the hexagonal -NaFeO 2 -type structure, in which the transition metal ions reside in the octahedral interstitial sites in a cubic closely-packed array of oxygen atoms in such a way that the MO 2 layers are formed by edge-sharing [MO 6 ] octahedra. The lithium ions are present in octahe- dral [LiO 6 ] coordination between these MO 2 layers. The high electronegativity of oxygen results in repulsive inter- actions between adjacent layers, which are compensated by the cations residing between the layers [1,2]. Thus, complete delithiation of these oxides results in thermody- namically unstable MO 2 compositions, which assume the layered CdCl 2 -type structure. Among these iso-structural oxides, only LiCoO 2 , LiNiO 2 and their solid solutions, LiNi 1-y Co y O 2 , have attained industrial importance as ∗ Corresponding author. Tel.: +886-3-425-7325/422-7151x4206; fax: +886-3-425-7325. E-mail address: gfey@cc.ncu.edu.tw (G.T.-K. Fey). 1 On deputation from: Central Electrochemical Research Institute, Karaikudi 630006, TN, India. cathode-active materials in lithium batteries. LiCoO 2 is by far the most exploited because of its features like high voltages versus lithium, a theoretical specific capacity of 274 mAh/g, and good cycling features. Furthermore, it can be easily prepared by a variety of synthetic approaches. The deintercalation of lithium from LiCoO 2 is accom- panied by an expansion of the hexagonal lattice in the c-direction, as a result of the increased electrostatic repul- sion between adjacent oxygen layers [3,4], and a contraction in the Co–Co distance [5,6]. This anisotropic volume change during repeated cycling causes a structural degradation of the host material [7] which results in large capacity fades [8,9]. Based on their ab initio calculations, Ceder et al. [10] suggested that doping LiCoO 2 with non-transition metal ions could improve the structural characteristics of the compound. Accordingly, several studies have been made on the effect of doping of LiCoO 2 with such dopants as Al 3+ [11–13], Mg 2+ [14,15] and B 3+ [16]. However, in some cases, the structural stability degraded upon cycling, while in others, any improvement in structural stability was ob- tained at the expense of the deliverable capacity. In recent years, an alternative approach has been adopted to enhance the cyclability of cathode materials, wherein the cathode particles are coated with a thin film of oxide materials such 0378-7753/$ – see front matter © 2004 Elsevier B.V. All rights reserved. doi:10.1016/j.jpowsour.2004.01.033