Nanocarbon-structured Electrode Materials Prepared By Polymeric Microgel And Their Electrochemical Study

The kinetics of electrode process are determined by the migration rates of ions and electrons for the electrochemical reaction based on Li-intercalation. People always coat the surface of nanoparticles with carbon layer to promote migration of Li ions as well as electrons. Nevertheless, it is very difficult to coat uniformly the surface of fabricate electrode of Li-ion batteries includes two steps:(1) casting the slurry, in which electro active materials was mixed with conductive agents, on the metallic current collector(Al, or Cu foils);(2) drying the casted electrode at a high temperature. Thus, spherical electrode materials are favor of fluently/casting. Consequently, it is significant to embed the nanoparticles in the porous carbon matrix forming the nanocarbon-structured materials with three-dimensional(3D) electron-ion conductive network.Polymeric microgels are a kind of cross-linked polymer microspheres with 3D network pores. Metallic ions could interact with polar groups in the macromolecules through electrostatic force thus are trapped within the polymer microgels evenly. The hydroxide nanoparticles would form in situ and filled in the 3D network pores if the value of pH charged. As a result, a hybrid inorganic-organic material yielded. After the hybrid material was fired at a high temperature in an inert atmosphere, nanocarbon-structured electrode material would be obtained. As one kind of cathode materials of commercial LIBs, spinel LiMn2O4 have the advantages such as good rate capability, environmental friendliness and abundant storage. Unfortunately, it suffers from serious capacity degradation because of John-Teller effect and dissolution of Mn2+ ions, especially at an elevated temperature. To inhibit the capacity degradation, the main methodologies are surface coating and morphology control. If the spinel LiMn2O4 nanoparticles were embedded uniformly in a porous carbon matrix, they would not only decrease their dissolution into the electrolyte by reducing the direct contraction area with the electrolyte, but also buffer their volume charge during charge and discharge. As a result, the electrochemical properties of them could be improved greatly. In compassion to commercial cathode materials, V2O5 possesses a theoretic capacity of 441 mAh g-1(three electrons per mole V2O5) abased on a multielectron reaction. However, low conductivities of electron and ion limit its rate capability. Furthermore, its dissolution into the electrolyte results in a bad cycling performance. Carbon coating could improve its rate capability and cycliability. To reach the above-mentioned aims, we have carried out the following research works.1. Preparation of P(AM-co-AA) template, determination of carbonization condition, and its electrochemical property study.The process to synthesize the cross-linked P(AM-co-AA) microgels was optimized. Porous carbon microspheres were prepared by the following steps:(1) filling the KCl and SiO2 into the microgels;(2) carbonization at a high temperature;(3) removing the filled KCl and SiO2. The carbon microspheres from KCl filling here the pore size distribution of <2 nm and 3.7 nm. The pore volume is 0.22 cm3 g-1 for the former and 0.13 cm3 g-1 for the latter. The former has a specific surface area of 420 m2 g-1 while the latter has a specific surface area of 136 m2 g-1. Those carbon microspheres from SiO2 filling display the particle size of 10~20 μm, and the pore size distribution of 3.8 nm and 7.8 nm. The former possesses a pore volume of 0.05 cm3 g-1 and a specific surface area of 62.1 m2 g-1 while the latter has a pore volume of 0.289 cm3 g-1 and a specific surface area of 118 m2 g-1. At a current density of 0.5 A g-1, the assembled capacitors could deliver a specific capacity of 159 F g-1 for the carbon microspheres from KCl filling and 110 F g-1 for the carbon microspheres from SiO2 filling when using the electrolyte of 6 mol L-1 KOH in water. Compare with those from KCl filling, the carbon microspheres from SiO2 filling with hirechirical porous structure exhibited higher utilization of surface area.2. P(AM-co-AA) template to prepare the LMO@C composite material and its electrochemical properties study.Mn2+ ions were absorbed within the P(AM-co-AA) microgels through an electrostatic force at first, hydrolyzed in situ by virtue of change of pH value, and filled in the 3D network pores for the hydrid Mn(OH)2@P(AM-co-AA) precursor. The obtained precursor was treated at a high temperature to form the nanocarbon surrounding LiMn2O4 material(LMO@C). The LMO@C material appear as spheres with particle size of 1~2 μm, and is constructed with LiMn2O4 nanospheres, which embedded in a porous carbon matrix. The LMO@C could deliver the specific capacities of 142, 137, 126, 107 and 91 mAh g-1 at the rates of 0.1, 1, 5, 10 and 20C(1C=148 mA g-1), respectively, exhibiting an excellent rate capability. When cycled at the 1C rate, it could maintain 80% of the initial capacity after 1000 cycles at room temperature and 86% of the initial capacity after 200 cycles at 60 oC, displaying a superior cycling performance. The results suggest that the LMO@C spheres are very promising to be used as cathode material for high-power lithium-ion batteries.3. PMMA template to synthesize the V2O5@C composite material and its electrochemical properties study.Porous V2O5@C microspheres were prepared by using a cheap PMMA microgels template. They are formed by many primary particles with an average diameter of 200 nm. The primary particles are constructed with V2O5 nanoparticles(30 nm) that embedded in the carbon matrix. The BET surface area and the BJH pore size are 23.5 m2 g-1 and 3.8 nm, respectively. The assembled Li/V2O5@C cells could deliver the initial capacities of 291, 263, 239, 192 and 166 mAh g-1 at the rates of 0.1, 0.5, 2, 5 and 10 C after 300 cycles, respectively. The capacity ratios are 90%, 77%, and 63% when cycled at the rates of 0.5, 5, 10 C, respectively. The results suggest the as prepared V2O5@C microspheres have excellent rate capability and cycling performance.In one word, polymeric microgels can be used to prepare not only the porous carbon microspheres directly, but also the porous LiMn2O4@C and V2O5@C composite materials as the template. The obtained porous carbon microspheres and nanocarbon-structure electrode materials revealed excellent electrochemical properties.