Cobalt Free Structurally Integrated Positive Electrode Materials for Lithium-Ion Batteries

Abstract

Introduction Rechargeable Lithium-ion batteries (LIBs) having high energy density have been widely used as energy storage device for electric vehicles (EVs) and hybrid-electric vehicles (HEVs). It is necessary to increase the energy density of LIBs in order to meet future energy requirements. Since energy density depends mainly on positive electrodes, exploration of new positive electrode materials with high specific capacities, good rate capabilities and cycling stabilities become crucial.In this context, positive electrodes, which are composites of Li-rich layered materials (high specific capacity) and spinel materials (high rate capability) attracted intense attention. Li-rich layered positive electrode materials deliver a capacity of ~250 mAh g-1 when charged above 4.5 V[1]. A large irreversible initial capacity is observed, which is attributed to the complete electrochemical activation of Li2MnO3 with the simultaneous removal of oxygen and lithium (“Li2O”) from the crystal lattice[1]. Further, this class of materials exhibits poor rate capability[2]. Spinel-type positive electrodes are marked by its high operating voltage (4.7 V), good cycling stability as well as high rate capability due to the fast Li+ diffusion through its three-dimensional structure, even though it offers a low discharge capacity (about 140 mAh g-1)[4]. Additionally, Fe substitution to the spinel positive electrode materials was found to be effective in improving thermal stability, structural stability as well as electrochemical performance[5]. Further, Fe is cost-effective and is earth abundant[3]. Results and Discussion In the present work, Fe–containing cobalt free structurally integrated (at molecular level) Li-rich layered-spinel positive electrode material was synthesized by sol-gel method. Four different annealing temperatures were selected to get final products. The structure and morphology of the materials were characterized by using X-ray diffraction (XRD) and Field emission scanning electron microscopy (FESEM). X-ray diffraction (XRD) studies showed that obtained composite materials contain spinel (space group Fd-3m) as well as Li-rich layered phases (space group C2/m). Mössbauer spectroscopy experiments revealed the coordination environment and oxidation state of the Fe present in the composite. Furthermore, the electrochemical performance was investigated within the voltage range 2.00 - 4.95 V vs. Li+/Li. The obtained cyclic voltammograms further confirmed the coexistence of Li-rich layered and spinel components in the composites. Selected materials delivered high discharge capacity, close to 200 mAh g-1 and ˃ 90% capacity retention after 50 cycles and will be discussed in detail. Acknowledgement: A.Bhaskar and H. Enale acknowledge the financial support from funding agency, Science and Engineering Board (SERB), New Delhi, India through ECR/2017/002556 program. D. Dixon acknowledges the financial support from funding agency, Science and Engineering Board (SERB), New Delhi, India through Ramanujan Fellowship, under the grant number SB/S2/ RJN-162/2017. A. Surendran is grateful to UGC New Delhi (Ref. no.:63/CSIR-UGC NET DEC. 2017) for the UGC SRF grant and Academy of Scientific and Innovative Research (AcSIR), Ghaziabad- 201002, India. The CIF, CSIR-CECRI is acknowledged for technical support with material characterization. A. Thottungal acknowledges the Council of Scientific and Industrial Research (CSIR) for a senior research fellowship.