FABRICATION AND ELECTROCHEMICAL CHARECTARIZATION OF THE CR2032 COIN CELLS USING THE DEVELOPED PURE AND CARBON COATED

Transcription

1 FABRICATION AND ELECTROCHEMICAL CHARECTARIZATION OF THE CR2032 COIN CELLS USING THE DEVELOPED PURE AND CARBON COATED LiMPO 4 (M= Mn, Co & Ni) NANOPARTICLES CHAPTER VI 181

2 CHAPTER - VI FABRICATION AND ELECTROCHEMICAL CHARACTERIZATION OF THE CR2032 COIN CELLS USING THE DEVELOPED PURE AND CARBON COATED LiMPO 4 (M= Mn, Co & Ni) NANOPARTICLES 6.1 General introduction 6.2 Fabrication of CR2032 coin cells 6.3 Electrochemical characterization LiMnPO 4 /Li and carbon coated LiMnPO 4 / Li cells LiCoPO 4 /Li and carbon coated LiCoPO 4 / Li cells LiNiPO 4 /Li and carbon coated LiNiPO 4 / Li cells 6.4 Conclusions References 182

3 6.1 General Introduction The international electrotechemical commission (IEC) is developing standards for the designation, marking, electrical testing, and safety testing of Li-ion cells and batteries. A proposed designation and marking system for Li-ion cells utilize five figures in the case of cylindrical cells and six figures in the case of prismatic cells. For example, the common round cell that uses the C/LiCoO 2 cell chemistry is designated ICR2032. The first letter I designates an intercalation negative electrode. The second letter designates the type of positive electrode employed, such as C for a cobalt type, N for Nickel type, M for Manganese type or V for a Vanadium type, etc... The third letter will designate the shape of the cell, R for round. The next two figures will designate the diameter in millimeters and then the next three figures the height of the cell, in tenths of millimeters, as they are 20 mm in diameter and 3.2 mm in height. Battery researchers will use to call ICR2032 cell as CR2032. In this chapter, The CR2032 coin cells were assembled with a metallic Li anode to evaluate the pure and carbon coated LiMPO 4 (M=Mn, Co & Ni) and evaluate their electrochemical properties. There are two types of processes to evaluate the electrochemical properties of material. The first is rate capability test, here the cell is charged at slow rate with a constant voltage and then the fully charged cell is discharged at various current rates to measure the capacity obtained from each discharge rate. Another one is capacity retention test, in this method; the cell is charged at specific rate and is discharged at the same rate and measure how long the cell sustains the initial capacity without significant degradation of the capacity. The effect of carbon coating 183

4 over LiMPO 4 (M= Mn, Co & Ni) on the electrochemical properties of carbon coated LiMPO 4 / Li batteries have been investigated and presented in this chapter. 6.2 Fabrication of CR2032 coin cells The cathode materials are coated on aluminum foil and metallic lithium is used as anode. The aluminum foil acts as a current collector for conducting the current in and out of the cell. Both of cathode and anode materials are delivered to the factory in the form of black powder and to the untrained eye and these are almost indistinguishable from each other. Since contamination between the anode and cathode materials will ruin the battery, great care must be taken to prevent these materials from coming into contact with each other. For this reason, the anodes and cathodes are usually processed in different rooms. The flowchart for the general electrode processing is shown in figure 6.1. The electrochemical performance of CR2032 coin-type cells made up of pure and carbon coated LiMPO 4 (M= Mn, Co & Ni) nanoparticles were evaluated. The cathodes were prepared by the following process. First, the synthesized samples of pure and carbon coated LiMPO 4 (M= Mn, Co & Ni) were ground to slurry with carbon black (C65, Timcal cooperation, USA) and PVDF (Polyvinylidene fluoride; Sigma Aldrich) binder in N-Methyl-2-pyrrolidone (NMP) solvent. The formulation of electrode was 80 (active material): 15(carbon black): 5(binder) in weight percentage. Cathodes were prepared by coating of cathode materials slurries on aluminum foil and then dried at 80 o C for 5 h under vacuum. Lithium metal (Sigma Aldrich) was used as an anode, and a micro porous plastic film (Cellgard 2400, Cellgard Co., USA) was used as separator and the electrolyte solution used comprised of 1.5 M LiPF 6 in a 1:1:1 mixture of ethylene carbonate (EC), 184

5 eythyl methyl carbonate (EMC), and dimethyl carbonate (DMC) in weight percent procured from Sigma Aldrich. We have used DMC: DEC: EMC ratio as 1:1:1 for electrolytes of LiCoPO 4 and LiNiPO 4 cathode materials cells to avoid the oxidation polymerization of EC in to PEC. CR2032 coin cells were assembled for all electrochemical testing, purchased from Hohsen Crop., USA. Figure 6.2 shows the schematic diagram of CR2032 coin cell construction. Figure 6.3 shows the photographs of fabricated CR 2032 coin cells using the prepared LiMnPO 4 nanorods. Figure 6.1: Flowchart for the general electrode processing for the lithium ion batteries 185

6 Coin cell Figure 6.2: Schematic diagram of CR2032 coin cell construction Figure 6.3: Photographs of fabricated CR 2032 coin cells using the prepared LiMnPO 4 nanorods 186

7 6.3 Electrochemical performance LiMnPO 4 /Li and carbon coated LiMnPO 4 / Li cells The voltage profiles obtained using the measured voltage between 2.9 to 4.5 V at 1C rate for the 1 st and 30 th cycles of cells made up of the pure and carbon coated LiMnPO 4 nanorods are shown in Figure 6.4. The discharge curves of LiMnPO 4 /Li cells showed continuous sloping because of the electronic structure and nanosized properties of the material. Kim et. al, suggested that nano-size particles resulted in continuous sloping in discharge curves of the Li/LiMnPO 4 cell, because electronic structure and property of nano-size particle lies in between bulk crystalline material and individual molecular state and Jahn Teller distortions leading to capacity fading [1]. However, the carbon coated LiMnPO 4 nanorods showed better discharge capacity at 1C than the pure ones. The discharge capacity for the 1 st cycle of pure LiMnPO 4 nanorods was almost same as the 30 th cycle of the carbon coated LiMnPO 4 nanorods, indicating an improvement in the rate capability by coating of carbon over LiMnPO 4. From figure 6.4, the LiMnPO 4 /Li cells are delivered discharge capacity of 98 mah g -1 during the 1st cycle and 64 mah g -1 during 30th cycle at 1 C rate. Carbon coated LiMnPO 4 /Li cells are delivered 122 mah g -1 during the 1st cycle and 30 th cycle it was 97 mah g -1 at 1C rate. Figure 6.5 shows the capacity retention of the pure and carbon coated LiMnPO 4 nanorods at 1C rate. From figure 6.5, the cells made up of pure and carbon coated materials showed good capacity retention with cycling. The carbon coated LiMnPO 4 achieved 122 mah g -1 at 1C, which is higher than the capacity of the pure LiMnPO 4 (about 98 mah g -1 ). The improved capacity was attributed to a one dimensional electron transport pathway in LiMnPO 4 nanorods [2,3]. The significance of our pure LiMnPO 4 nanorods is its increased capacity at 1C compared 187

8 to recently reported nanocomposite LiMnPO 4 /C, [4] off-stoichiometric LiMnPO 4,[5] carbon coated LiMnPO 4 particles,[6] and also, multiwalled carbon nanotube coated LiMnPO 4 thumb shaped nanorods [7]. Figure 6.4: Discharge profile of the lithium batteries fabricated using pure and carbon coated LiMnPO 4 nanorods 188

9 Figure 6.5: Capacity retain plot of the lithium batteries fabricated using pure and carbon coated LiMnPO 4 nanorods 189

10 6.3.2 LiCoPO 4 /Li and carbon coated LiCoPO 4 / Li cells Figure 6.6 shows the charge-discharge curves of the cells made up of pure and carbon coated LiCoPO 4 samples at 0.1C rate. These cells were characterized between the voltages of V up to 20 cycles. Both pure and carbon coated LiCoPO 4 samples exhibited flat voltage plateau at 4.8 V, which is the characteristics of the lithium intercalation at 4.8 V (Vs. Li) of LiCoPO 4 /Li cell. Similarly, cells made up of carbon coated LiCoPO 4 sample delivered high capacity than pure LiCoPO 4 /Li cell. From figure 6.6a, the pure LiCoPO 4 /Li cells show the continuous discharge curve, which may be due to the capacity fading. The discharge capacity of LiCoPO 4 /Li cells at 0.1C rate during the 1st cycle was observed as 144 mah g -1, while at the 20th cycle it was 133 mah g -1. From figure 6.6b, The LiCoPO 4 /Li cell shows the flat lithium intercalation voltage curve at 4.8 V (Vs. Li). The discharge capacity of lithium-ion coin cells at 0.1C rate during the 1st cycle was observed as 180 mah g -1, while at the 20th cycle it was 162 mah g -1. The discharge capacity loss in the Li/carbon coated LiCoPO 4 cell was around 18% between the 1st and the 20 th cycles. Figure 6.7 shows the cycling performance of cells made up of pure and carbon coated LiCoPO 4 samples at 0.1C. The capacities of LiCoPO 4 /Li, carbon coated LiCoPO 4 /Li cells were found to be 144 mah g -1 and 180 mah g -1 respectively. The enhancement in electrochemical performance of carbon coated LiCoPO 4 /Li cells compared to pure LiCoPO 4 /Li may be due to the increased electrical conductivity through carbon coating. The capacity retentions of both pure and carbon coated LiCoPO 4 were almost 80% even after 20 cycles, which indicate that the pure and carbon coated LiCoPO 4 nanoparticle 190

11 cathode materials prepared by PVP assisted polyol process had the better cycling performance [8]. a b Figure 6.6: Discharge profile of the lithium batteries fabricated using a) pure and b) carbon coated LiCoPO 4 nanoparticles 191

12 Figure 6.7: Capacity retain plot of the lithium batteries fabricated using pure and carbon coated LiCoPO 4 nanoparticles 192

13 6.3.3 LiNiPO 4, carbon coated LiNiPO 4 / Li cells The discharge curves of the cells made up of pure and carbon coated LiNiPO 4 samples, characterized between V at 0.1 C shown in figure 6.8. All the cells exhibited voltage plateau at 5.1 V as a characteristic of the oxidation/reduction reaction of LiNiPO 4 with respect to lithium anode. From figure 6.8a, the discharge capacity of pure LiNiPO 4 /Li cell is 74 mah g -1 for 1 st cycle and 56 mah g -1 for 20 th cycles at 0.1C rate. From figure 6.8b, the carbon coated LiNiPO 4 /Li cell is showing enhanced discharge capacities like 98 mah g -1 for 1 st cycle and 66 mah g -1 for 20 th cycles at 0.1C rate. Figure 6.9 shows the cycling performance of the cells made up of pure and carbon coated LiNiPO 4 samples at 0.1C. The capacities of pure and carbon coated LiNiPO 4 samples were found to be 74 mah g -1 and 98 mah g -1 respectively. The improvement in discharge capacity for carbon coated sample might be due to the enhancement in electrical conductivity of material. The capacity retentions of both pure and carbon coated LiNiPO 4 were almost 80% even after 20 cycles, which indicated that LiNiPO 4 prepared by PVP assisted polyol process also has the good cycling performance and it can be used as a high voltage cathode in lithium batteries [9]. It is reported that the electrochemical performance of cathodes could be significantly improved by adding amount of conductive materials [10-15]. In this work, we have successfully synthesized pure and carbon coated Lithium transition metal phosphate nanoparticles with high electrochemical properties compared to earlier reported results. The discharge capacities of all prepared compounds along with previous work are listed in table

14 a b Figure 6 8: Discharge profile of lithium batteries fabricated using a) pure and b) carbon coated LiNiPO 4 nanoparticles 194

15 Figure 6.9: Capacity retain plot of lithium batteries fabricated using pure and carbon coated LiNiPO 4 nanoparticles Table 6.1: Discharge capacities of all pure and carbon coated LiMPO 4 (M= Mn, Co & Ni) nanoparticles along with reported work Compound Present obtained Reported capacity References capacity (mah g -1 ) (mah g -1 ) LiMnPO , 7 LiMnPO 4 /C , LiCoPO LiCoPO 4 /C LiNiPO LiNiPO 4 /C

16 6.4 Conclusion In this chapter, CR2032 coin cells fabricated by using synthesized pure and carbon coated LiMPO 4 (M= Mn, Co & Ni) cathode materials and their electrochemical properties were discussed. The discharge capacity of the carbon coated LiMPO 4 (M= Mn, Co & Ni) nanomaterial increased 20% compared to the pure ones. Furthermore, the pure and carbon coated LiMPO 4 (M= Mn, Co & Ni) material shows the good electrochemical performance, especially rate capability. This improvement is partly ascribed to small particle size, which acts as an inhibitor for grain growth because small particles improve bulk lithium transport. Carbon coating upon the LiMPO 4 (M= Mn, Co & Ni) materials will increase the electronic conductivity of the cathodes. Therefore, these electrode materials show the extremely high discharge capacity and good cycleability. Hence, newly developed carbon coated LiMnPO 4 nanorods, using modified polyol and resin coating processes will improve the lithium battery performance. 196

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