Structural Design and Electrochemical Performance of Pitch-Based Anode Materials for Lithium and Potassium-Ion Batteries

Author:Liu Chang

Supervisor:qiu jie shan


Degree Year:2019





So far,lithium-ion batteries(LIBs)are the most promising energy storage devices,which have been widely used in portable electronic devices and electric vehicles.However,the development of lithium-ion batteries still faces two bottlenecks:1)the structural characteristics of commercial graphite anode materials hinder the further improvement of rate performance and cycle stability of lithium-ion batteries;The shortage of lithium resources limits the application of LIBs in large-scale energy storage.For the former,the fabrication of novel carbon-based anode materials with novel structure and controllable component to replace the commercial graphite anode is the key to improving battery performance.For the latter,the development of a new secondary battery system based on high-reservative potassium is an effective supplement to the large-scale energy storage application of lithium-ion batteries.Creating anode materials for large-radius potassium ion storage is the key to realize the application of potassium ion batteries(PIBs).In short,the core of realizing the above academic ideas lies in the establishment and development of new technical strategies for the structure and composition of carbon anode materials,and to explore the structure-effect relationship material structure and its electrochemical properties.In this paper,pitch and quinoline from coal tar were used as raw materials.Based on physical and chemical methods,the chemical composition and molecular structure of raw materials are designed and controlled.Combined with electrospinning,template replication and vaporization-deposition-transformation technology,new structure of carbon-based anode materials with controllable structure and composition are fabricated.The details are as follows:Hard/soft composite carbon nanofibers(CNFs)with abundant nanocarbon clusters distributed in amorphous carbon matrix were fabricated by electrospinning of polyacrylonitrile(PAN)and coal tar pitch,followed by high temperature carbonization.The amorphous area provides fast lithium ion transport.Nano carbon clusters can efficiently improve the carbon crystallinity,and further facilitates electron transfer.The relatively fast ion and electron guarantee the composite CNFs high rate performance.At 5 A g-1,the reversible capacity reaches to 296 mAh g-1,with a high capacity retention of 53.7%,which is more than twice of that of PAN-based CNFs.After 1000 cycles at 1 A g-1,the capacity still remains 323 mAh g-1 with a capacity retention of 99.7%and an average Columbic efficiency of more than 99.8%,The nitrogen-rich pitch(9.7 wt.%N content)was synthesized by AlCl3 catalytic cross-linking of quinoline,a main component of coal tar.After co-electrospinning with PAN solution and carbonization,the composite CNFs with high nitrogen content were fabricated.Compared with the CNFs composited with coal tar pitch mentioned above,the relatively higher nitrogen content can effectively increase the lithium storage capacity.In addition,the introduction ofnitrogen-rich pitch can improve the carbon crystallinity and reduce the defect density,which is beneficial to the formation of a stable SEI film,and reducing the charge transfer resistance of lithium ions at the electrode interface,especially at large current densities.The reversible capacity is 264 and 240 mAh g-1 at 5 and 10 A g-1,respectively,It presents a stable cycle performance with 88.0%retention at 2 A g-1 even after 1000 cycles,which is higher than that of PAN-700(61.5%).Further study shows that carbon crystallinity difference is the main reason that affects the properties of SEI films,thus causing the difference of cycle performance.For PIBs,aiming at the relatively lower carbon layer distance and low potassium storage capacity of pitch-based carbon materials,nitrogen-doped soft carbon frameworks built of well-interconnected nanocapsules have been fabricated,by using nitrogen-rich pitch as the precursor and nano-MgO as template.The in-situ nitrogen doping and sp3 hybridized carbon partially converted from sp2 hybridized carbon during the cross-linking process would effectively reduceπ-π interaction between hydrocarbon molecules during carbonization.As a result,the interlayer distance expands from 0.347 nm to 0.356 nm.Both extended interlayer distance and relatively ordered carbon arrangement play vital roles in facilitating potassium ion transfer.The relatively ordered carbon arrangement of soft carbon and enriched quaternary nitrogen are beneficial for electron conductivity,yielding a high rate performance.The reversible capacity increases from 245 mAh g-1 to 293 mAh g-1 at a current density of 0.05 A g-1.It can still retain 151 mAh g-1 at 5 A g-1,with a high capacity retention of 51.5%,which is about 1.5 times of that for coal tar pitch-based soft carbon material,and even 5 times of that for nitrogen-doped hard carbon.In order to further improve the energy density of potassium and lithium storage for pitch-based carbon materials,the activated porous carbon material with nano-box structure was constructed by using coal tar pitch as carbon source through vaporization-adsorption-conversion strategy.The influence of carbon microstructure and mass loading of red phosphorus on the storage properties of potassium/lithium ion were investigated,establishing the synthetic strategy of building high-performance pitch-based carbon composites.The three-dimensional porous carbon skeleton not only provides abundant storage sites for red phosphorus,but also effectively alleviates the volume expansion during charge and discharge.In addition,the cross-linked conductive network can facilitate the charge transfer.Red phosphorus in ultra-fine amorphous phase nanoparticles were anchored into micropores of carbon matrix,which is benefit for stable cycle performance.When used as PIBs anode,the it shows a high reversible capacity of 682 mAh g-1 at 0.05 A g-1.And it presents a stable cycle performance with 84.4%capacity retention at 0.5 A g-1 after 500 cycles.As LIBs anode,it shows high capacity of 968 and 593 mAh g-1 at 0.1 and 5 A g-1,respectively.