LiFePO4 BASED COMPOSITE ELECTRODES FOR Li-ION BATTERY APPLICATION

Abstract

The climate change due to excessive emission of greenhouse gases (GHG) in the last two to three decades has adversely affected ecological life due to ocean pollution and global warming. It is an urgent requirement to find an alternative energy sources to replace conventional fossil fuel energy sources. However, most of the renewable energy sources like solar energy, wind energy and hydro energy are dependent on the natural conditions, e.g. solar energy depends on sun light and wind energy has dependence on wind flow. Therefore, to ensure the continuous energy supply, this has become mandatory to have energy storage system with high safety and affordable cost. Among all available energy storage technologies, electrochemical energy storage is the most beneficial technology which stores energy in batteries, fuel cells, and electrochemical capacitors or supercapacitors. However, batteries have advantages over other electrochemical systems due to large area of applications from portable electronics to hybrid electric vehicles (HEVs) and grid level storage. In past several decades, many battery systems such as lead-acid, Ni-Metal Hydride, Ni-Cd, Li-metal, and Li-ion battery were investigated depending on the applications. However, among various rechargeable batteries, lithium-ion batteries have high gravimetric and volumetric energy densities, long cycle life and very low self-discharge. The basic principle of Li-ion battery is to convert chemical energy into electrical energy through exothermic reaction. A Li-ion battery cell consists of several components such as anode, cathode, electrolyte, separator and current collectors. However, the performance of Li-ion battery mainly depends on the thermodynamics and kinetics of anode, cathode, electrolyte and their compatibility among each other. The energy of Li-ion battery is also affected by internal resistances develop inside the cell which is due to polarization in the cell. The output voltage of a battery cell depends on the voltage window of electrolyte (energy gap between the lowest unoccupied molecular orbital and highest occupied molecular orbital) and intrinsic potential of electrodes. The designing of electrodes of high capacity and their electrochemical potential lying within the voltage window of the electrolyte is a main challenge. For the safety and life of a Liion cell, electrodes must be chemically stable with electrolyte. Most of the electrodes (anode/cathode) used for practical application have host structure into/from which Li+ ions are inserted/extracted repeatedly during charging and discharging of the battery. Although, elemental lithium metal (Li) can be considered as an ideal anode for Li-ion battery but chemical potential of lithium lies outside of the presently available electrolytes. The mismatch in the energy of lithium metal anode and electrolyte can cause of short-circuits in Liion battery. The graphitic carbon anode is found much safer as compared to lithium metal. However, the redox potential plays an important role in development of the new cathode materials. The position of the M(n+1)+/Mn+ redox couple (for example, M(n+1)+/Mn+ = Co3+/Co2+, Fe3+/Fe2+ etc.) relative to the Li/Li+ couple of pure lithium metal defines the cell voltage. The redox potential values of Fe, Mn, Co and Ni are 3.45 V, 4.1 V, 4.8 V and 5.1 V, respectively. The liquid organic carbonate-based electrolytes in Li-ion battery are decomposed above 4.5 V, which limits the investigation of high voltage cathodes. There are various electrode materials introduced for the Li-ion battery till date but only a few of them are successfully commercialized. Since voltage of the battery cell is decided by cathode materials, therefore rigorous research has been done on cathode materials and various materials were proposed, so far for the cathode. The most successful cathode materials are LiCoO2, LiMn2O4 and LiFePO4, and other cathode materials LiNi1/3Co1/3Mn1/3O2, V2O5 and LiV3O8 are also proposed. The chapter-2 provides the detailed overview of selection criteria of LiFePO4 over the other types of cathode materials along with various models explaining charge-discharge mechanism, synthesis process and various techniques to overcome the limitations of LiFePO4. LiFePO4 is one of the most promising cathode materials for Li-ion batteries due to its commendable life cycle, high energy density, high thermal stability, low cost of production and environment friendly in nature. It has olivine structure with Pnma (62) space group and quite flat charge-discharge plateau at 3.45 V vs. Li/Li+ in half-cell. Moreover, the LiFePO4 has high specific theoretical capacity of 170 mAh g-1 which gives a high energy density (3.45 × 170 = 586 Wh kg-1). However, LiFePO4 suffers from poor electronic conductivity (< 10-9 S cm-1) which leads to high impedance, and low rate of Li+ ion diffusion (10-14 -10-16 cm2 s-1) at LiFePO4/FePO4 boundary region. There are various methods to improve conductivity and Li+ ion diffusion of LiFePO4 viz. by reducing particle size in nano range, reducing antisite defects in LiFePO4, coating with conductive agents, and doping with supervalent ions etc. On the basis of the literature review, it may be concluded that there are two ways to improve the performance of Li-ion battery either by improving the materials chemistry or electrode engineering. Therefore, both techniques are applied in the thesis to improve the electrical and ionic conductivities of LiFePO4 followed by electrochemical performance of battery viz. reducing antisite defects, coating with carbon and conducting polymer, using conducting polymer as binder and doping with transition metals having high redox potential. The chapter-3 describes the synthesis process of bare, carbon coated LiFePO4, and various characterization techniques used in the studies. The LiFePO4 samples were synthesized by sol-gel process in optimized conditions under argon gas atmosphere using metal acetate, metal nitrate, phosphate, oxalic and citric acids as starting materials. The prepared materials were characterized by various techniques. X-ray diffraction (XRD) technique was used to determine the phase(s) purity. Field emission-scanning electron microscopy (FESEM) and Transmission electron microscopy (TEM) were used to determine particle size and morphology of the synthesized powders. Energy dispersive X-ray spectroscopy (EDX) was used for elemental composition and mapping analysis. Fourier transform infrared spectroscopy (FTIR) was used for determining chemical information about the functional group. Electrochemical impedance spectroscopy (EIS) was used to measure charge transfer resistance, Li-ions diffusion. While, Xray photoelectron spectroscopy (XPS) was used for surface chemistry analysis and oxidation state of elements. Raman spectroscopy was performed to find carbon structure. Thermogravimetric analysis (TGA) was used to determine the weight loss of precursor materials during heat treatment, Inductive coupled plasma mass spectroscopy (ICP-MS) was used to determine metal concentrations. Four probe method was used to determine resistivity of electrode sheet. Electrochemical measurements like galvanostatic charge-discharge and cyclic voltammetry were performed to determine the charge-discharge capacities, cycle life and polarization of the battery. In the chapter-4, study of atomic defects in LiFePO4 generated during synthesis process is done. Several LiFePO4 samples are synthesized by sol-gel process with varying calcination time and argon flow rate to observe the effect on defect formation in LiFePO4. Quantitative analysis of XRD patterns through Rietveld refinements is carried out to get the best profile peak fitting by assigning Li and Fe atoms at different sites as well as considering vacancies in the samples. The Rietveld refinement analysis confirms the 2% antisite defects in the LiFePO4 samples when calcination time as well as argon gas flow rate used are less than the required. To examine the effect of antisite defects on Li-ion diffusion and electrochemical performance (capacity and cycle life) of LiFePO4, two samples, one of antisite defects free and second with 2% antisite defects were further studied by electrochemical analysis and electrochemical impedance spectroscopy. Sample with 2% antisite defects has shown lower discharge capacity 138 mAh g-1 as compared to 150 mAh g-1 for defect free sample at 0.1C (1C = 170 mA g-1) rate. The Li-ion diffusion coefficient was determined on prepared electrodes before and after completion of 200 charge-discharge cycles. Sample with antisite defects is found to have lower diffusion coefficients of 1.198×10-13 cm2 s-1 and 0.436×10-13 cm2 s-1 in both cases, respectively as compared to 1.669×10-13 cm2 s-1 and 0.968×10-13 cm2 s-1 for sample without defects. Therefore, antisite defects slow down the Li-ion diffusion in LiFePO4. In the chapter-5, bare LiFePO4 (B-LFP) and carbon coated LiFePO4 (LFP) are synthesized by sol-gel process. LFP samples are prepared with three different stoichiometric ratios of metal ions and citric acid namely 1:0.5, 1:1 and 1:2 and samples are designated as LFP (1:0.5), LFP (1:1) and LFP (1:2). The results show that LFP (1:1) with an optimum thickness of 4.2 nm and higher graphitic carbon coating has highest discharge capacity of 148.2 mAh g-1 at 0.1C (1C = 170 mA g-1) rate and 113.1 mAh g-1 at a high rate of 5C among all the four samples prepared. The sample LFP (1:1) shows 96% capacity retention after 300 charge-discharge cycles at 1C rate. Whereas, B-LFP shows lowest discharge capacity of 111.1 mAh g-1 at 0.1C and capacity was decreased very fast and its electrochemical action was sustained only upto 147 cycles. Moreover, cyclic voltammetry has also revealed the lowest polarization of 0.19 V for LFP (1:1) and highest 0.4 V for B-LFP. Furthermore, the influence of poly (3,4-ethylene dioxythiophene): poly (styrene sulfonate) (PEDOT:PSS) conducting polymer coating with varying its content on electrochemical performance of B-LFP and carbon coated LFP (1:1) was also studied and presented in the chapter. The weight ratios of active material (B-LFP or LFP) and polymer were taken 100:0, 95:5 and 90:10. The electrical conductivity of PEDOT:PSS coated samples is increased by many folds (104-108) as compared to B-LFP and LFP (1:1). The study reveals that different optimum amount of PEDOT:PSS namely 10 wt.% and 5 wt.% are required for best electrochemical performance of B-LFP and LFP (1:1) respectively, and samples are designated as B-LFP-10P and LFP-5P . The sample B-LFP-10P has shown discharge capacity of 140.8 mAh g-1 whereas LFP-5P has 154.6 mAh g-1 at current rate of 0.1C. The same samples have shown highest capacity retention of 92% and 96% respectively after 200 cycles. Present study reveals that PEDOT:PSS conducting polymer has positive impact on capacity and cycle life of the battery. Therefore, further study of PEDOT:PSS as binder in the electrode is carried out and presented in the next chapter. The aim of the study presented in the chapter-6 is to replace or minimize the nonconducting/ low-conducting materials used in LiFePO4 based electrode with high conducting materials. Non-conducting PVDF binder is replaced with PEDOT:PSS conducting binder, and low conducting carbon black is replaced with CNTs (SWCNT/MWCNT). The present chapter reports a detailed study on synergy in PEDOT:PSS and CNTs with LiFePO4 (LFP) cathode material. For the study, composites of LFP and CNTs (LFP/SWCNT or LFP/MWCNT) are prepared by probe sonication. The work is divided into two parts. In the first part, work is discussed on the electrodes prepared by LFP and LFP/SWCNT composite using PEDOT:PSS binder. Whereas, second part presents the study on the electrodes prepared by LFP/MWCNT composite using PEDOT:PSS binder. The comparative study is done with electrodes prepared using PVDF binder.The study has revealed that PEDOT:PSS binder alone cannot fulfill the requirements of high power application of Li-ion battery when LFP based electrode is prepared without conductive additive. The electrodes prepared by LFP/CNT composite using PEDOT:PSS binder have provided very high electrochemical performance and lower cell resistance as compared to the electrodes prepared with PVDF binder. Electrodes having 9 wt.% PEDOT:PSS binder with LFP/SWCNT (LFP/SC-9P) in one case and LFP/MWCNT (LFP/MC-9P) in another case composite materials show lowest charge transfer resistance and highest lithium ion diffusion values among their groups prepared. In addition, samples of same above compositions show the highest capacities and stable cycle life. However, sample LFP/SC-9P shows discharge capacity of 166.6 mAh g-1 at 0.1C (1C = 170 mA g-1), and 88% capacity retention after 2500 cycles at 10C and also sustains very high current rate upto 100C which is the highest performance in all tested samples. These superior results make combination of PEDOT:PSS and CNTs with LiFePO4 an excellent composite electrode of the next generation Li-ion battery for electric vehicles. The aim of chapter-7 is to incorporate higher concentration of transition metals having high redox potential into olivine structure of LiFePO4 and their effects on electrochemical performance of LiFePO4. A series of LiFe0.7(MnxCoxNix)3x-yVyPO4 (y = 0, 0.01, 0.02, 0.03, 0.04, 0.05, 0.06, 0.08 and 0.10) compositions doped with high concentration (30 mol%) of transition metals (together) having higher redox potential are synthesized by sol-gel process. Any amount of substitution with vanadium (V) is assumed to be equally divided among Mn, Co and Ni, and thereby changes in structural and electrochemical characteristics of the compound are systematically investigated. The X-ray diffraction analysis confirms that V is doped successfully into host lattice with 0.0 ≤ y ≤ 0.04, whereas a second impurity phase Li3V2(PO4)3 is observed for substitution range 0.05 ≤ y ≤ 0.10. The Rietveld refinement performed on XRD data shows continuous change in lattice parameters and cell volume with increasing y. X-ray photoelectron spectroscopy study confirms the oxidation state of Fe, Mn, Co and Ni in +2, whereas V in +4 state. The electrochemical characteristics show the second phase Li3V2(PO4)3 has favorable contribution in capacity as well as cycle life among doped samples prepared. However, nondoped pristine LiFePO4 shows superior electrochemical performance among all such samples. Therefore, the electrochemical results of doped samples infer that high concentration of multielements doping is not feasible in terms of capacity and cell voltage with presently used carbonate-based electrolyte. The chapter-8 reports the study on two phase (LiFePO4/FePO4) transformation mechanism of LiFePO4 during charge-discharge. The study is conducted using ex-situ and in situ X-ray diffraction (XRD), and X-ray photoelectron spectroscopy (XPS) techniques. The Rietveld refinements are performed on ex-situ XRD data of electrode materials collected at various stages of first charge-discharge cycle. The results reveal the existence of solid solution at room temperature. Moreover, similar study is also repeated by in-situ XRD and ex-situ XPS techniques which also confirm the transformation of LiFePO4 into FePO4 phase during charging and vice versa during discharging. Furthermore, the study also conducted on fully charged state (at different current rates from 0.1C to 5C) of various electrodes by ex-situ XRD found that phase transformation (LiFePO4 to FePO4) depends on the charging current rate. At the lower current, complete transformation of LiFePO4 phase into FePO4 is observed whereas at higher current rate, a trace amount of residual phase LiFePO4 along with FePO4 is found in fully charged state. Therefore, the finding of study reveals that time dependent lithium ion diffusion phenomenon in LiFePO4 causing lower capacity at higher current rate. The overall conclusion and future scope of current research is presented in chapter-9. The thesis reports synthesis of LiFePO4 by sol-gel process at the optimum condition and study of various factors which influence the electronic, ionic conductivities as well as electrochemical performance of LiFePO4. An optimum amount of carbon and polymer are required for the best electrochemical performance of the electrode material. The electrode with conductive binder has shown better performance as compared to non-conductive binder. The synergistic effect of conducting binder and CNTs in LiFePO4 based electrode has shown the electrochemical performance at the level of high-power application of Li-ion battery. However, substitution with high content of multi-transition metals in LiFePO4 is found not feasible in terms of capacity and cell voltage with carbonate-based electrolyte. At the end, thesis reports two phase (LiFePO4/FePO4) transformation mechanism of LiFePO4 during charge-discharge cycle. The lithium iron phosphate is a very suitable cathode material for high power application of the Li-ion battery. However, electronic and ionic conductivities of LiFePO4 are not achieved upto the mark that can make pristine LiFePO4 ready to use in electric vehicles. Therefore, the improvement of the engineering of LiFePO4 based is required.