| Lithium-air battery, with a high energy density of 11425 Wh/kg(excluding oxygen), is considered the biggest energy storage system compared to lithium ion batteries, fuel cells and air batteries, thus attracting unprecedentedly broad attention. Research and development of high-energy lithium air batteries, is expected to solve series of problems, such as energy shortage, environment pollution, high fuel prices and energy resources competition, and to meet the energy demand of long distance transport in electric vehicle industry.In the lithium air batteries, cathode structure plays a vital role for that it is in charge of providing space for discharged products accommodation and free path for oxygen, e- and Li+ transport. However, pore blockage, cathode passivation all result in low discharge capacity and poor cycling capability. To get rid of these predicaments, a novel air cathode composed of carbon nano-tubes(CNT) and KB is used to construct a lithium-air battery. By changing the mass ratio of CNT and KB, a series of porous carbon cathode are prepared. The effects of electrochemical performance on the cycling performance of lithium-air batteries, as well as the charge/discharge products and reaction mechanism are explored.1. CNT, N-CNT and KB, as highly conductive porous materials, all have a high electrical conductivity, a large total pore volume and an average pore diameter around 3.5 nm. KB porous carbon materials contain lots of meso-porous; large total pore volume is 2.79 m3/g and high specific surface area of 1589.92 m2/g. Carbon nanotubes(CNT), with a one-dimensional hollow tube structure, can greatly increase the active reaction sites and transferring efficiency of oxygen and lithium ions. Mixing KB with carbon nano-tubes(CNT) with appropriate proportion can get an ideal air cathode of larger specific surface area and pore volume. It can render more storage space for discharged products and better diffusion paths for gas and electrolyte, thus synergistically to further enhance its electrochemistry property and cycling performance.2. The battery possesses highest energy efficiency and cycle performance for the composite cathode constituted of identical weight CNT and KB. With the proportion of 1:1, the composite cathode can fully show their advantages and adapt to the lithium air batteries by providing enough storage space for discharge products and fast diffusion paths for gas and electrolyte. Therefore, it greatly improves the electrochemical stability and cycle performance of the battery. Finally, a highly reversible lithium air battery is successfully obtained which can run 186 cycles with a fixed specific capacity of 1000 m Ah/g(0.4 m Ah) at a high current density of 0.3 m A/cm2.3. For 1 M Li TFSI/Sulfolane electrolyte system, all lithium air batteries in this paper have exhibited excellent cycle performance. Most of the cells obtain hundreds of cycles at a current density of 0.3m A/cm2, even for a higher case of 0.5m A/cm2. Such favorable cycling performance is a strong proof to demonstrate that sulfolane system is a kind of relatively stable electrolyte, which possibly resists high battery voltage and thus establishes reversible cycles. It is concluded that sulfolane electrolyte is suitable for lithium air battery.4. After discharge, lamellar lithium peroxide covers the cathode surface. After the corresponding charging process, the coverage almost disappears. Because of poor conductivity of lithium peroxide and high decomposition voltage, the discharged products do not completely decompose and even suffer decomposition to some degree. Circle by circle, as the discharge products accumulate, the cathode channel is blocked,resulting in cathode passivation and battery breakdown. So, for a deep research into the cathode, constructing appropriate porous structure and reducing air cathode polarization are important methods to improve the performance of lithium air batteries. |
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