Materials Science and Engineering B 209 (2016) 30–36 Contents lists available at ScienceDirect Materials Science and Engineering B jo ur nal home p age: www.elsevier.com/locate/mseb Influence of Ti and Zr dopants on the electrochemical performance of LiCoO 2 film cathodes prepared by rf-magnetron sputtering K. Sivajee-Ganesh a , B. Purusottam-Reddy a , O.M. Hussain a , A. Mauger b , C.M. Julien c,∗ a Thin Film Laboratory, Department of Physics, Sri Venkateswara University, Tirupati 517 502, India b Sorbonne Universités, UPMC Univ Paris 06, Institut de Minéralogie, de Physique des Matèriaux et de Cosmochimie (IMPMC), 75005 Paris France c Sorbonne Universités, UPMC Univ Paris 06, Laboratoire Physicochimie des Electrolytes et Nanosystèmes Interfaciaux (PHENIX), UMR 8234, 75005 Paris France a r t i c l e i n f o Article history: Received 18 December 2015 Received in revised form 5 March 2016 Accepted 5 March 2016 Available online 23 March 2016 Keywords: Lithium batteries Thin film cathodes Layered compound Doping effect a b s t r a c t In an attempt to enhance the microstructural and electrochemical properties, LiCoO 2 thin films were doped with titanium or zirconium. RF magnetron sputtering technique has been employed for the deposition of films on Au/Ti/SiO 2 /Si substrates from lithium-rich LiCoO 2 target with mosaic configu- ration. The as-deposited and Ti- and Zr-doped LiCoO 2 thin films at lower doping concentration exhibited the -NaFeO 2 structure with R ¯ 3m symmetry as confirmed from X-ray diffraction and Raman studies. The cyclic voltammogram of micro-electrodes in aqueous electrolyte exhibited perfect redox peaks with good reversibility. The chronopotentiometry studies revealed that the discharge capacity of pure LiCoO 2 was 64 Ah cm -2 m -1 , while 2% Ti- and Zr-doped films showed enhanced capacities 69 and 68 Ah cm -2 m -1 (248 mC cm -2 m -1 , 245 mC cm -2 m -1 ) respectively. The Zr-doped films exhibited good structural stability even after 25 cycles with the capacity retention of 95%. © 2016 Elsevier B.V. All rights reserved. 1. Introduction The reduction in size and miniaturization of electronic devices has prompted the development of all solid-state integrable micro-power sources with high energy and power density. The development of such micro-power sources is originated for the identification of ideal binder-free film cathode material with good electrochemical performance [1–3]. In this context, thin films of lithium transition-metal oxides (LiMO 2 with M = Co, Mn, Ni, etc.) have received considerable attention as high voltage positive elec- trode materials. Among these, LiCoO 2 is still considered to be the best choice as industrially viable cathode material owing to its high theoretical capacity, high operating voltage, high energy density and long cycling stability versus lithium [4–7]. LiCoO 2 crystallizes in the -NaFeO 2 structure (R ¯ 3m space group), which is a distorted rocksalt network, where the cations order in alternative (111) planes. The CoO 6 octahedra are shared with edges to form CoO 2 sheets and Li ions can intercalate in two dimensional (2-D) direc- tions between the CoO 2 slabs [8,9]. Thus, the Li//LiCoO 2 system has high theoretical specific capacity of 274 mAh g -1 and energy den- sity of 1070 W h kg -1 [10]. However, the achieved practical capacity ∗ Corresponding author. Tel.: +33 144273534; fax: +33 144273856. E-mail address: christian.julien@upmc.fr (C.M. Julien). of Li x CoO 2 is only half of the theoretical capacity. The major limita- tion is that the delithiation of LiCoO 2 is restricted to x = 0.5, which corresponds to 4.2 V vs. Li 0 /Li + (a capacity value of 140 mAh g -1 ). LiCoO 2 undergoes performance degradation or failure when over charged during long term cycling process [11,12]. One reason is that cobalt may be dissolved in the electrolyte when the electrode is delithiated during charging. Another reason is that the CoO 2 layer formed after full delithiation shears from the electrode surface and the layered structure collapses. In addition, the lattice parameter changes by the insertion of more Li ions in LiCoO 2 and can lead to micro cracking of the cathode particles [13–15]. Intensive investigations were performed to enhance the capac- ity and cycle performance of LiCoO 2 beyond 4.2 V, so as to increase the energy density. The alternates are: (i) doping, i.e. partial substi- tution of bare transition metal (Co) by a different metal elements (Ti, Cr, Mn, Fe, Ni, Cu, Zn, Zr, Nb, Rh, Ta, W) [15–23], (ii) surface modification of LiCoO 2 film by an insert metal oxide (MgO, Al 2 O 3 , TiO 2 , etc.) [24–27], (iii) surface modification of positive electrode materials for lithium-ion batteries development of nanocrystalline LiCoO 2 by properly calibrating deposition parameters and perform- ing a post-deposited annealing treatment [28,29] and (iv) growth of LiCoO 2 film on a textured substrate surface to alter the microstruc- ture [30]. Among these strategies, doping with bi- or tri-valent metal ions has met little success for reasons reported in [31]. On another hand, doping with a tetravalent ion, like Zr 4+ [23] or Ti 4+ http://dx.doi.org/10.1016/j.mseb.2016.03.003 0921-5107/© 2016 Elsevier B.V. All rights reserved.