Synthesisofc-lifepo4 composite by solid state reaction method

In this research, the enhancement of LiFePO4 conductivity was conducted by doping method with carbon materials. Carbon-based materials were obtained from the mixture of sucrose, and the precursor of LiH2PO4 and α-Fe2O3 was synthesized by solid state reaction. Sintering temperature was varied at 700°C, 800°C, 900°C and 1,000°C. The result showed that C-LiFePO4 could be synthesized by using solid state reaction method. Based on the XRD and FTIR spectrums, C-LiFePO4 can be identified as the type of crystal, characterized by the appearance of sharp signal on (011), (211) and typical peak of LiFePO4 materials. The result of conductivity measurement from C-LiFePO4 at sintering temperature of 900°C and 1,000°C was 2×10-4 S/cm and 4×10-4S/cm, respectively. The conductivity value at sintering temperature of 700°C and 800°C was very small (<10-6 S/cm), which cannot be measured by the existing equipment.


Introduction
LiFePO 4 has been known as lithium battery cathode material that has a high theoretical capacity (170 mAh/g), a stable potential voltage of 3.5 V vsLi│Li+, and environmental friendliness. However, LiFePO 4 have weaknesses, those are a slow kinetic diffusion of lithium ion and a low electronic intrinsic conductivity. Therefore, it needs a modification to improve its electrochemical performance [1,2].
One of the attempts to increase the low kinetic diffusion of lithium ion is coating the surface of LiFePO 4 with carbon, copper, and silver [3]. Enhancing electronic conductivity can be conducted by minimizing the particle size with modification in the sintering process. One of the methods for synthesizing LiFePO 4 is solid state reaction. The solid state reaction is more effective than sol-gel method. In addition, doping process with carbon can be conducted simultaneously in the solid state reaction. Therefore, with solid state reaction [3,4], the results of synthesizing LiFePO 4 are expected to produce a material with a fairly high conductivity.
The purpose of this study was to synthesize C-LiFePO 4 composite by solid state reaction method.The present method used carbon for enhancing and getting excellent LiFePO 4 material. Carbon was selected because carbon is easily produced. [5]
The initial step for synthesizing C-LiFePO 4 was preparing the precursor from the material LiH 2 PO 4 , α-Fe 2 O 3 and sucrose. Those three precursors were made by the molar ratio Li: Fe: P: C= 1:1:1:1,2. After weighing those three precursors and mixed it into mortar containing aquades, the mixture was crushed by ball mill for 2 hours. After that, drying process was conductedat temperature 80 o C, using oven. After the three precursors were dried, it was heated at temperature of 520 o C for six hours in furnace without flowing N 2 gas, then the powder was crushed again in mortar for 2 hours. The powder which had subtle was heated with temperature variation of 700 o C, 800 o C, 900 o C and 1,000 o C in the furnace for 16 hours with flowing N 2 gas. The physical characteristics of cathode materials were analysed by using the x-ray diffractometer(XRD), infrared spectrophotometer (FTIR). The production of Film C-LiFePO 4 was conducted by mixing C-LiFePO 4 , and PVDF as an adhesive with weight composition of 92%:8% in total weight of 2 gram. The mixture was put in 5 mL of NMP solvent while stirred by magnetic stirrer for 16 hours until the homogenous LiFePO 4 liquid were produced. The production of LiFePO 4 film is conducted by printing thehomogenous liquid of LiFePO 4 on glass substrates with size of 2cm x 2cm.The film of LiFePO 4 was dried in the oven at temperature of 80ºC for 4 hours. Then, each thickness and conductivity of the dried film was measured by four line probe method.

Result and Discussion
During the calcination process, areduction inthe mass ofthe precursorfrom28.8992gram to23.4556gram was observed. This is caused by the releasing of water on calcinations process which occurred at temperature of 100 o C until 300 o C, chemical reaction which occurred on calcinations process was written in equation (1) and (2) (2) During the sintering, there were threestages of microstructural processes which contained expansion on the initial condition, there was no reaction yet, and particle composition did not change. During theinitialstage of sintering, rearrangement occurred, there was a little moves or particle rotation to enhance the number of contacts between the particles andthe formation oflinkagesbetween the particles, C-LiFePO 4 composite materialswere stillred, it indicated that there was no reaction of formation a perfect C-LiFePO 4 composite.
In second stage, size of linkages between particles grew and its porosity decreased due to the particles which come closer each other. On this stage, grain growth started, formed the pore, the distance between the particles became smaller, and shrinkage occurred as shown in Figure 1(b),and (c), C-LiFePO 4 composite begun to form and characterized by changing into blackish, it indicated that reaction stage was at intermediate stage.
On this stage, the pore became a round pore eventually, size of particle increased and the rate of shrinkage pores became smaller as shown inFigure 1(d),C-LiFePO 4 compositematerialhad beenformed then compaction and hardening occurred. It showed that effect of the higher sintering temperature would change form structures material of C-LiFePO 4 composite became more solid and the color turn into blakish. The reaction on the sintering process Li 2 H 2 P 2 O 7 +2FeO (Fe 3 O 4 +Fe)→2LiFePO 4 +H 2 O [2]. It occurred due to the heating of the sintering temperature, the higher sintering temperature, then the more formation of the perfect C-LiFePO 4 would be formed, otherwise at the temperature of 700-800 o C, C-LiFePO 4 had not formed completely. It showed that the higher sintering temperature, then the more perfect C-LiFePO 4 would be formed. Each thickness and conductivity of dried film would be measured by four line probe method. Table  1 showed that increasing of conductivity occurred along with the enhancing of the sintering temperature.It occurred because the enhancing of the sintering temperature then the crystal structure had been formed, which the bonding between the powder particles bind each other in these conditions. The conductivity of C-LiFePO 4 composite materials at temperature of 700-800 o C was not detected on the equipment, the ability of the equipment was only capable of reading up to 10 -6 S/cm.   Figure 3 shows that x-ray diffractogram pattern from the variation of sintering temperature. All the products were identified as the type of olivine crystal LiFePO 4 , marked by the appearance of sharp signal on (020), (011), (120), (101), (111) and (211) but the relativeintensitiesofthe peaknumberswere different amongsamples, due to the effecton the size ofthe crystal. There were alsosignals thatdid not match and it was possibility of the existence of impurities.  To determine the functional groups contained in the C-LiFePO 4 , the characterization by FTIR. Figure 4.(a) is an FTIR spectrum measurement results of C-LiFePO 4 sintering 700 o C. On the spectrum, formed in wave numbers 3419,7 cm -1 , 2349,1 cm -1 , 1039,4 cm -1 , and 650,9 cm -1 that each identifies -OH stretch vibration, Un-deprotonated P-OH , and shows PO 4 3functional groups. Figure  5(b) is an FTIR spectrum measurement results of C-LiFePO 4 sintering 800 o C. On the spectrum, formed in wave numbers 2344,4 cm -1 , 1039,4 cm -1 , and 579,1 cm -1 identifies each functional group PO 4 3-. At 800 o C sintering visible loss of OH and P-OH, the high temperature heating leads to loss of impurities such as OH. From the results of FTIR, two samples identified as PO 4 3functional groups.