Applied Surface Science 394 (2017) 25–36 Contents lists available at ScienceDirect Applied Surface Science jou rn al h om ep age: www.elsevier.com/locate/apsusc Full length article Development of surface functionalized ZnO-doped LiFePO 4 /C composites as alternative cathode material for lithium ion batteries Rakesh Saroha a , Amrish K. Panwar a,∗ , Yogesh Sharma b , Pawan K. Tyagi a , Sudipto Ghosh c a Department of Applied Physics, Delhi Technological University, Delhi 110042, India b Department of Physics, IIT Roorkee, Roorkee, Uttarakhand 247667, India c Department of Metallurgical & Materials Engineering, IIT Kharagpur, West Bengal 721302, India a r t i c l e i n f o Article history: Received 27 May 2016 Received in revised form 12 September 2016 Accepted 21 September 2016 Available online 21 September 2016 Keywords: Olivine-cathode ZnO-doping Charge separation Surface force gradient Ragone plot a b s t r a c t Surface modified olivine-type LiFePO 4 /C-ZnO doped samples were synthesized using sol-gel assisted ball-milling route. In this work, the influence of ZnO-doping on the physiochemical, electrochemical and surface properties such as charge separation at solid-liquid interphase, surface force gradient, sur- face/ionic conductivity of pristine LiFePO 4 /C (LFP) has been investigated thoroughly. Synthesized samples were characterized using X-ray diffraction, scanning electron microscopy, atomic force microscopy, and transmission electron microscopy. All the synthesized samples were indexed to the orthorhombic phase with Pnma space group. Pristine LiFePO 4 retain its structure for higher ZnO concentrations (i.e. 2.5 and 5.0 wt.% of LFP). Surface topography and surface force gradient measurements by EFM revealed that the kinetics of charge carriers, e - /Li + is more in ZnO-doped LFP samples, which may be attributed to diffusion or conduction process of the charges present at the surface. Among all the synthesized samples LFP/C with 2.5 wt.% of ZnO (LFPZ2.5) displays the highest discharge capacity at all C-rates and exhibit excellent rate performance. LFPZ2.5 delivers a specific discharge capacity of 164 (±3) mAh g -1 at 0.1C rate. LFPZ2.5 shows best cycling performance as it provides a discharge capacity of 135 (±3) mAh g -1 at 1C rate and shows almost 95% capacity retention after 50 charge/discharge cycles. Energy density plot shows that LFPZ2.5 offers high energy and power density measured at high discharge rates (5C), proving its usability for hybrid vehicles application. © 2016 Published by Elsevier B.V. 1. Introduction Intense pressure on non-renewable sources of energy like coal, crude oil, etc. and problems associated with their use like air pollu- tion, greenhouse gasses ejection, global warming has forced the research community to look for their substitute energy sources which prove to be environmental friendly and less health haz- ardous. At present, rechargeable lithium ion battery coupled with renewable energy sources like solar power are playing a vital role in the direction of developing such kind of energy stor- age devices which meet requirements of safety, non-toxic, low cost, high energy and power density to power electric vehicles (EV) and hybrid electric vehicles (HEV) [1,2]. Olivine-type LiFePO 4 is one such kind of cathode material which proves to be most promising cathode material as far as structure stability, the flat ∗ Corresponding author. E-mail addresses: amrish.phy@dce.edu, panwaramar@gmail.com (A.K. Panwar). charge/discharge voltage plateau and excellent rate performance have been concerned. It has undergone extensive research since it was first reported by Padhi et al. in 1997 [3]. It satisfies most of the requirements needed for a cathode material like high the- oretical capacity (170 mAh g -1 ), low cost, structural stability with excellent capacity retention and environmentally benign due to its non-toxicity [4–14]. It possesses a stable voltage plateau at around 3.45 V vs. Li + /Li because of two-phase redox reaction corresponding to the transition metal Fe 2+ / 3+ couple. However, its low electronic conductivity (10 -9 –10 -11 S/cm) and sluggish lithium ion diffusion kinetics (∼10 -18 cm 2 /s) have restricted its widespread use in auto- motive industry and hybrid vehicles [15–17]. Various ways have been adopted so far to increase the surface/electronic conductivity of pristine LiFePO 4 cathode material which includes carbon coat- ing [18,19] and cation doping at the lithium (M1) or iron (M2) site [20,21]. Apart from this, enhancement in conductivity can be attained by coating/surface modification of LiFePO 4 material with electronically conductive materials like SnO 2 [1], CeO 2 [2], LaPO 4 http://dx.doi.org/10.1016/j.apsusc.2016.09.105 0169-4332/© 2016 Published by Elsevier B.V.