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Impact of Cl Doping on Electrochemical Performance in Orthosilicate (Li2FeSiO4): A Density Functional Theory Supported Experimental Approach Shivani Singh,† Anish K Raj,† Raja Sen,‡ Priya Johari,*,‡ and Sagar Mitra*,† †
Electrochemical Energy Laboratory, Department of Energy Science and Engineering, Indian Institute of Technology Bombay, Powai, Mumbai 400076, India ‡ Department of Physics, School of Natural Sciences, Shiv Nadar University, Gautam Buddha Nagar, Greater Noida, Uttar Pradesh 201314, India S Supporting Information *
ABSTRACT: Safe and high-capacity cathode materials are a long quest for commercial lithium-ion battery development. Among various searched cathode materials, Li2FeSiO4 has taken the attention due to optimal working voltage, high elemental abundance, and low toxicity. However, as per our understanding and observation, the electrochemical performance of this material is significantly limited by the intrinsic low electronic conductivity and slow lithium-ion diffusion, which limits the practical capacity (a theoretical value of ∼330 mAh g−1). In this report, using first-principles density functional theory based approach, we demonstrate that chlorine doping on oxygen site can enhance the electronic conductivity of the electrode and concurrently improve the electrochemical performance. Experimentally, X-ray diffraction, X-ray photoelectron spectroscopy, and field-emission gun scanning electron microscopy elemental mapping confirms Cl doping in Li2−xFeSiO4−xClx/C (x ≤ 0.1), while electrochemical cycling performance demonstrated improved performance. The theoretical and experimental studies collectively predict that, via Cl doping, the lithium deinsertion voltage associated with the Fe2+/Fe3+ and Fe3+/Fe4+ redox couples can be reduced and electronic conductivity can be enhanced, which opens up the possibility of utilization of silicate-based cathode with carbonatebased commercial electrolyte. In view of potential and electronic conductivity benefits, our results indicate that Cl doping can be a promising low-cost method to improve the electrochemical performance of silicate-based cathode materials. KEYWORDS: Cl doping, conductivity, density of states, high-voltage cathode, lithium-ion batteries, Li2FeSiO4
1. INTRODUCTION In the recent past, Li2FeSiO4, a polyoxyanion-based cathode material, has received considerable amount of attention owing to its properties such as thermal stability, environmentally benignity, higher theoretical discharge capacity (333 mAh g−1), lower cost, and lesser toxicity.1−5 The possibility of exchange of two lithium ions per formula unit, resulting in higher theoretical capacity compared to that of commercially available alternatives, has also added a promising proposition to the study of this class of materials. However, lower electronic and ionic conductivity is an inherent disadvantage of this material, which impacts the electrochemical performance in various battery applications. Moreover, higher operating voltage corresponding to the second redox couple, i.e., Fe3+/Fe4+ ≈ 4.8 V, also limits the extraction of second Li from the host matrix, which further results in lower capacity. Various methods such as nanostructuring, the introduction of carbon conductive additive, supervalent cationic and anionic doping etc., have been employed to overcome these issues.6−10 However, these methods have been found to improve the overall electrode electronic conductivity, but anionic doping has been proven to © 2017 American Chemical Society
substantially change the inherent electronic properties of the material.11 For example, nitrogen (N), sulfur (S), fluorine (F), and chlorine (Cl) doping have been reported as effective dopants to improve the electrochemical performances of various electrode materials for lithium-ion batteries (LIBs).9,12−19 The anionic (N, S, and F) doping has been very well studied experimentally and theoretically in the case of LiFePO4 and Li3V2(PO4)3, whereas, in the case of Li2FeSiO4, it has only been investigated through simulations. However, the impact of Cl doping in the case of silicates has not been studied yet, while it has been demonstrated experimentally that, in the case of LiFePO4 and Li3V2(PO4)3, the electrochemical performance was enhanced with doping.15−18 It has been also observed that anionic doping can shift potential plateau voltage position and, thus, can help in the tailoring of material properties.9,11 Using density functional theory (DFT)-based calculations, Armand et al. have demonstrated the change in Received: May 26, 2017 Accepted: July 19, 2017 Published: July 19, 2017 26885
DOI: 10.1021/acsami.7b07502 ACS Appl. Mater. Interfaces 2017, 9, 26885−26896
Research Article
ACS Applied Materials & Interfaces potential plateau position by replacing O by N in Li2FeSiO4.12 The substitution of N for O results in decrease in the lithium deinsertion voltage because a less-ionic Fe−N bond (because N is less electronegative than O) would destabilize the antibonding Fe d orbitals, leading to voltage decrease.11,13,14 In this research work we have, therefore, attempted to analyze the effect on the intrinsic properties of Li2FeSiO4 due to anionic doping by replacing O with the relatively lesselectronegative Cl atom. For this, we modeled Li2FeSiO4−xClx structures with different concentrations (x) of Cl, where x is considered as 0.125, 0.25, 0.5, and 1.0 in the given formula unit. Using a first-principles DFT-based approach, the ground-state geometry of these doped structures have been determined, and corresponding electronic properties have been calculated. The density of states for doped Li2FeSiO4 clearly indicates the change in electronic properties of material with an increase in Cl concentration. To validate the proposed model for doped Li2 FeSiO 4 , pure and doped materials have also been synthesized. It is noteworthy to mention that the effect of chlorine doping in Li2FeSiO4 electrode results in decreasing the cathode voltage. Being less-electronegative, Cl loses electrons more likely than O; Cl substitution into orthosilicate cathodes will therefore influence the distribution of electrons between various ions, which would facilitate more efficient Li-ion insertion and extraction. The current work will demonstrate the validity of this hypothesis.
2.3. Electrochemical Characterization. For electrochemical characterization, slurry was prepared by the mixing of Li2FeSiO4− MWCNT active material (80%) with Super-P carbon powder (10%) and polyvinylidene fluoride binder (10%) in an excess of a solvent Nmethyl-2-pyrrolidone−acetone mixture. The homogeneous slurry was coated onto an Al-current collector. The foil was then dried overnight in a vacuum oven at 120 °C to remove solvents, and then a circular disk of 12 mm was cut to fabricate the cell. The typical active material loading was 1.6−1.3 mg cm−2. A Swagelok cell was used to observe the electrochemical performance of prepared cathode material and was assembled in an Ar-filled glovebox (Mbraun, Germany) with oxygen and water levels of
