DESIGN AND DEVELOPMENT OF NEXT-GENERATION BINDER-FREE LITHIUM-ION BATTERY

Abstract

Rechargeable lithium-ion batteries (LIBs) are being used extensively to power various portable electronic appliances such as laptop computers, mobile phones, etc. Research is being carried out towards improving parameters like energy/power density, long-term cyclability, safety and reducing the cost so that LIBs can be used in electric vehicles and stationary backup power supplies. Many of the above parameters significantly depend upon the intrinsic and extrinsic nature of the electrode materials, and it is essential to investigate new electrode materials that exhibit high energy density, high rate performance, long lifespan, and cost-effectiveness. Most of the commercial LIBs employ LiCoO2 as the cathode (positive electrode), non-aqueous Li-ion electrolyte, and graphite as the anode (negative electrode). But graphitic carbon exhibits poor experimental capacity (330 mAh g−1), low insertion potential (∼0.2 vs. Li/Li+), poor tap density, and safety issues at a high current rate, which are the main hurdles for its implementation in the high-end applications of LIBs. Therefore, continuous efforts are being made to develop alternative anode materials for next-generation LIBs. In this context, ternary metal oxide, specifically manganese (Mn) based oxides, are recognized as prospective anode materials due to their high theoretical capacity, nontoxic properties, rich resources, lower cost, greater number of oxidation states of Mn, lower working potential, and environmentally acceptable nature. However, this class of materials suffers from rapid capacity decay and low cyclability due to excessive volume fluctuation during lithiation/delithiation, sluggish ion diffusion kinetics, large irreversible capacity loss, and poor electrical conductivity, which hinder their commercialization. To overcome these drawbacks and improve the Li-storage performance of Mn-based oxides, several strategies have been utilized. A common technique to prevent unit-cell volume variation during cycling is the incorporation of an electrochemically inactive matrix component (such as MgO) into the Mn-based oxides. Further, nanostructuring of active electrode materials has also shown a constructive impact on the Li-storage properties. Particularly, the 1D nanofibric morphology of active materials can offer a high aspect ratio and surface area that can provide an efficient transport pathway for electrons/ions, leading to enhanced Li-ion kinetics. Moreover, incorporating oxygen vacancies into Mn-based oxides is another effective strategy to enhance the Li-storage properties. The oxygen vacancies constructed an internal electric field, enhanced the conductivity, and increased the adsorption energy with Li-ion, thus improving the electrochemical performance of the as-prepared electrodes. Another viable strategy for achieving maximum capacity for Mn-based oxides is to utilize compounds that contain metal ions capable of undergoing Li-cycling via both conversion and alloying-de-alloying reactions. Furthermore fabricating electrodes without binder can also improve LIBs' overall energy density. The electrodes without binders provide a well-connected path between active materials and current collectors, ensuring efficient electronic and ionic movement within the electrode materials. In the present thesis, all the above-mentioned strategies are systematically employed in the ternary Mn-based oxide (MgMn2O4; MMO) nanofibers (NFs) fabricated via electrospinning technique and investigated their Li-storage properties. Firstly, the binder-free electrodes of MMO NFs are fabricated via electrophoretic deposition (EPD) technique and then analysed the effect of EPD voltage on the surface morphology of the fabricated electrodes and their electrochemical performance. The electrochemical investigations confirm that the deposition voltage of the EPD considerably modifies the surface microstructures of the deposited electrodes and, hence, efficiently controls their Li-storage properties. At a particular EPD voltage (100 V), the MMO NFs electrode attained the optimum gaps/cracks on its surface with a 3D porous morphology. The porous structure allows fast Li-ion diffusion within the electrode materials during lithiation/delithiation process. Further, the properties (electrical, Li-storage, and surface) of MMO NFs have been tuned by creating oxygen defects in them. The oxygen defects are generated by heat treatment at different temperatures (450, 600, and 750 °C). XPS and EPR analysis revealed the presence of a higher concentration of oxygen defects in MMO-600 NFs compared to their counterparts. These defects lead to facile Li-kinetics during the cycling process at high current density and, thus, provide good rate performance (228 mAh g−1 at 5 A g−1) and notable cyclability (817 mAh g−1 at 1 A g−1 after 400 cycles) in MMO-600 electrode. Further, the viable strategy of employing the compounds that contain metal ions which can undergo the Li-cycling via both conversion and alloying-de-alloying reactions to achieve maximum and stable capacity of a given compound has been proven for Mn-based oxide system, such as ZnxMg1−xMn2O4 (x = 0.1, 0.25, and 0.5). As an anode for LIB, MMO NFs with 0.5 mol of Zn exhibits good cyclic stability (947 mAh g−1 at 50 mA g−1 after 100 cycles and 186 mAh g−1 at 2.5 A g−1 after 2000 cycles) and improved electrochemical potential (1.27 V vs Li/Li+). For the commercial viability of MMO and Zn-doped MMO NFs anode, full cell performances were investigated against the binder-free LiNi1/3Mn1/3Co1/3O2 (NMC) cathode. The energy densities for the NMC//MMO and NMC//Zn-doped MMO are calculated to be 166 and 180 Wh Kg−1 (as per total active mass of anode and cathode), respectively. These obtained energy densities are found to be better than many reported full cells in the literature and are comparable to the commercial products. Hence, in this study, we have observed that all the strategies coupled with each other enable the high-performance in Mn-based oxides as binder-free anode for next-generation LIBs.