The physics and chemistry of high capacity carbonaceous materials for lithium-ion batteries.

摘要

The mechanism of lithium insertion depends on the carbon type. The carbonaceous materials of commercial relevance as lithium-ion anodes are divided into three groups: (1) graphitic carbons, (2) hydrogen containing carbons heated to between 500{dollar}spcirc{dollar}C and 800{dollar}spcirc{dollar}C, and (3) some hard carbons heated to about 1000{dollar}spcirc{dollar}C.; The probability P of turbostratic disorder (random shifts or rotations between adjacent carbon layers) in graphitic carbons determines the capacity Q for lithium intercalation, i.e., {dollar}Q=372(1-P){dollar} mAh/g. This suggests that no lithium can be inserted between adjacent parallel layers which are turbostratically misaligned. The effect of turbostratic disorder on staging phase transitions which occur during the intercalation of lithium in graphitic carbons was carefully studied by in-situ X-ray diffraction and electrochemical methods. A staging phase diagram was then developed in the P-x plane, where x is the lithium concentration in intercalated graphitic carbons.; Many organic precursors heated between 500 and 800{dollar}spcirc{dollar}C contain substantial amounts of hydrogen. They all have similar voltage profiles for lithium insertion, with very large capacity, approaching 900 mAh/g, but also have large hysteresis between charge and discharge. We demonstrated for the first time that this high capacity exhibiting large hysteresis is proportional to the hydrogen content of the carbons. We have carefully studied the electrochemical insertion of lithium in these hydrogen-containing carbons using a variety of charge-discharge rates and cycling temperatures. These measurements allow the hysteresis to be quantified. It is believed that the lithium atoms may bind on hydrogen-terminated edges of hexagonal carbon fragments causing a change in the bond from sp{dollar}sp2{dollar} to sp{dollar}sp3.{dollar} A simple model has been developed to understand the hysteresis. Achieving high capacity carbons needs a coupling of fundamental research (understanding) and applied research (to apply concepts in synthesis). Good understanding will lead to high quality samples.; A number of disordered hard carbons were prepared from phenolic resins. These materials have reversible capacities which exceed 500 mAh/g, a low voltage plateau near zero volts, and little hysteresis in their voltage profiles. These hard carbons with high capacity were found to be made up with a large fraction of single layers, stacked like a "house of cards". We believe that lithium can be adsorbed onto both sides of the carbon sheets, leading to a large capacity. Using small-angle scattering techniques, we showed that the micropore sizes in these carbons are on the order of 10 to 15 A in diameter. The understanding of high capacity of hard carbons makes it possible to prepare high capacity carbons with low cost and good performance.; More recently, we have been studying ways to make such hard carbons with high capacity from coal. Coal has a large carbon content and can have a highly aromatic condensed structure, leading high product yields after heat treatment. We thought this might be a cheap way to make carbon electrode material for lithium-ion batteries. The physical properties of the pyrolyzed coals varies from sample to sample because of the varied chemistries of the initial coals. Nevertheless, the amount of lithium that the pyrolyzed coals can accommodate is largest for coals with large fraction of single layers and many nanoscopic pores. This gives us further confidence that our model for the reversible capacity of coals is correct.