ORIGINAL PAPER Olivine-type nanoparticle for hybrid supercapacitors Rajaraman Vasanthi & Dharmalingan Kalpana & Nedumaram Gopalan Renganathan Received: 19 July 2007 / Revised: 10 September 2007 / Accepted: 27 September 2007 / Published online: 7 November 2007 # Springer-Verlag 2007 Abstract LiCoPO 4 nanoparticles were precipitated from polyethylene glycol solution of lithium acetate, cobalt acetate, and ammonium dihydrogen phosphate by refluxing at 250 °C for 35 h. The resultant powder samples were heated at 800 °C for different time periods of 2 and 4 h to study the effect of annealing time on the growth of samples. The X-ray diffraction pattern of the obtained samples exhibited olivine phase. The scanning electron microscopic images of dried powder sample and samples heated at 800 °C for 2 and 4 h showed a homogenous orthorhombic morphol- ogy with a particle size of few nanometers range. For the first time, orthorhombic olivine was introduced as positive electrode for a hybrid electrochemical supercapacitor cell with carbon nanofoam as negative electrode in 1 M LiClO 4 in ethylene carbonate and propylene carbonate (1:1 in volume) solution. A sloping voltage profile of 2 to 0 V is observed for all the three hybrid cells. From the impedance results, we inferred that LiCoPO 4 nanoparticles synthesized by polyol process offers less resistance than lithium titanium oxide. According to the results of electrochemical testing for the first time, maximum power density of 192 W/kg at 11 Wh/kg energy density was obtained for LiCoPO 4 nanoparticles annealed at 800 °C for 2 h. The dried sample and the sample heated at 800 °C for 2 and 4 h exhibited high capacitances of 5, 19, and 4 F/g, respectively, with an excellent rate capability over 1,000 cycles. Keywords LiCoPO 4 . Olivine . Hybrid supercapacitor . Carbon nanofoam . Energy Density . Power density Introduction The characteristics of high capacity values in farads range and low in resistance are devices required for application in high current loads. To meet this requirement, higher energy density (150–200 Wh/kg) realized by Li ion batteries [1] and higher power density (5,000 W/kg) by carbon–carbon double-layer capacitor [2] shall be combined together to yield hybrid supercapacitor. This essentially consists of an electrochemical double-layer capacitor (EDLC) electrode and a battery electrode, such as (AC)/Ni(OH) 2 aqueous cell [3] or AC/Li 4 Ti 5 O 12 non-aqueous cell [4]. Amatucci et. al. [5] and Pasquier et. al. [6] have explored non-aqueous hybrid devices with Li intercalation anode, Li 4 Ti 5 O 12 , and Wang et. al. [7, 8] worked on LiMn 2 O 4 , LiCoO 2 , and LiCo 1/3 Ni 1/3 Mn 1/3 O 2 in aqueous system. To our knowl- edge, this is the first attempt to use lithium metal phosphates with ordered-olivine structure for hybrid super- capacitor. To date, LiCoPO 4 has been prepared in different routes like sol-gel synthesis, solid-state reaction, hydrother- mal synthesis [9–11], etc., to yield a pure phase. A new method to obtain nanoparticles with pure phase had been achieved by polyol process [12]. Polyol process has been widely used to synthesize metal colloid in which polyol is J Solid State Electrochem (2008) 12:961–969 DOI 10.1007/s10008-007-0438-6 Contribution to ICMAT 2007, Symposium K: Nanostructured and bulk materials for electrochemical power sources, July 1–6, 2007, Singapore. R. Vasanthi (*) : D. Kalpana Electrodics and Electrocatalysis, Central ElectroChemical Research Institute, Karaikudi 630 006 Tamilnadu, India e-mail: vasanthi_rajaram@yahoo.com N. G. Renganathan Lithium Batteries Division, Central ElectroChemical Research Institute, Karaikudi 630 006 Tamilnadu, India