Binary V, Mn Oxide/Hydroxides rGO As High Sulfur Content Hostmaterials for Lithium-Sulfur Battery

Abstract

Lithium-sulfur battery (Li-S) owing to the high theoretical energy density (2600 Wh kg-1) and high theoretical capacity (1675 mA h g-1) is one of the most promising candidates to replace Lithium-ion batteries (LIBs). However, contrary to conventional Li-ion batteries, Li/S cells transport charge via a series of complex chemical reactions involving solid and soluble sulfur species, which can cause severe morphological changes of the positive electrode and induce the shuttle mechanism. In order to enables the reversible electrochemical reaction of Li-S battery especially at high current rates, not only confined sulfur in highly conductive host materials is a key factor, but also the cathode materials have to provide enough polarity in order physically bonded LiPSs and mitigate shuttle mechanism. Among different metal oxides, Vanadium and Manganese oxides due to special metal-insulator transition, high adsorption ability toward LiPSs in addition to electrocatalytic effect of Mn oxide in the reduction of soluble LiPSs to Li2S2 and Li2S, exhibit part of the requirements of sulfur cathode material1,2.Here in, we report synthesis of bimetallic nanorod V/Mn hydroxides/oxides on the surface of reduced graphene oxide nanosheets (rGO) by a simple one step hydrothermal method and followed by thermal calcination in the N2 atmosphere. Indeed, rGO which benefits from high electrical conductivity and also high surface area, provide a well-distributed pattern of transition metal-based structures. The above-mentioned composite is able not only to alleviate the LiPSs shuttle by anchoring the LiPSs species but also electrocatalytically improve the LiPSs conversion process in the discharge state of the Li-S battery. The synergetic effect of V and Mn in different oxidation state was evaluated using several ratios V/Mn. The behaviors of the electrode, composed by the V/Mn-rGO/S composite with 80% of S, in term of capacity, cyclability and also electrocatalytic effect in Li-S battery were investigated.Figure 1. shows the well-dispersed VxMnyNrGO( x,y represent the molar raio and N represent N-doped rGO after calcination) on the rGO nanosheets and the performances and cyclability of the different cells at 0.2C. The sulfur loading of all the cathodes are 1.9 mg/cm2 and the E/S ratio is about 12µL per mg of Sulfur.Figure 1 a) SEM image of V,Mn oxide on the surface of rGO, b) Decharge capacity vs cycle number (1.9mgS/cm2) of different materials.The best result with a stable capacity of 700 mAh/g up to 45 cycles was obtained for the V1Mn1rGO cathode (1:1 molar ratio V and Mn without calcination). This very promising results have to be optimized in order to improve the performances at high C-rate, indeed because of high percentage of Sulfur in the V/Mn-rGO based composite, cycling batteries in higher C rates facing obstacles.(1) Song, Y.; Zhao, W.; Zhu, X.; Zhang, L.; Li, Q.; Ding, F.; Liu, Z.; Sun, J. Vanadium Dioxide-Graphene Composite with Ultrafast Anchoring Behavior of Polysulfides for Lithium–Sulfur Batteries. ACS Appl. Mater. Interfaces 2018, 10 (18), 15733–15741. https://doi.org/10.1021/acsami.8b02920.(2) He, J.; Hartmann, G.; Lee, M.; Hwang, G. S.; Chen, Y.; Manthiram, A. Freestanding 1T MoS 2 /Graphene Heterostructures as a Highly Efficient Electrocatalyst for Lithium Polysulfides in Li–S Batteries. Energy Environ. Sci. 2019, 12 (1), 344–350. https://doi.org/10.1039/C8EE03252A. Figure 1