The Interface Regulation And Mechanism Research Of The Tin-based Anode In Lithium-Ion Batteries

With rapid socio-economic developments and excessive consumption of non-renewable fossil fuels,human society’s demand for new energy sources is growing at an explosive pace.At the same time,the 75^th session of the United Nations General Assembly has pointed out that CO₂ emissions are striving to peak by 2030 and to achieve the goal of carbon neutrality by 2060.Therefore,it is urgent to develop a green energy source to replace the traditional energy structure.In recent years,the development of electrode materials with higher energy density and high-rate charge/discharge characteristics has become a top priority.It is well known that the SnO₂-based anode material has been widely studied in the field of lithium-ion batteries,this paper takes the development of lithium-ion batteries with fast（dis）charge capabilities as an important goal based on SnO₂ materials,designs and prepares the thermal-sticked SnO₂@NC/Cu electrodes,SnO₂@NC nanofiber membranes with nerve cell-like structure,and SnS_x@NC nanofiber membranes with ultra-small particle structure.The works in this paper are focusing on comparing the electrochemical reversibility at three kinds of reaction interfaces（active material to conductive additive,active material to coating,and current collector to active material/coating）under the integrated electrode structure,the effect of the oxygen or sulfur contents in the composite on the alloying reaction,and the investigation of the lithiation mechanism.The three kinds of electrode materials proposed in this paper are enlightening for the practical application of high-performance lithium-ion batteries,and the research results are achieved as followed:（1）Retarding electron conductor endows high reversibility and rate-capability for Li-ion battery.The elimination of interface resistances（electronic and ionic）within the battery electrode is generally believed as an efficient way to sustain high current density recharging.However,we find that a properly increased interface resistance between the current-collector and active materials can obviously improve high rate capability.The interface resistance is usually ignored by most researchers due to its small contribution（～1%）to the whole internal resistance.The uniform carbon shelled hollow SnO₂-Cu electrode of SnO₂@NC/Cu is obtained by the thermal-sticking and calcination from the PVP coated hollow SnO₂ nanospheres onto Cu foil at 600°C,which leads to a slight increase of the interface resistance between current-collector and active materials compared with the conventional electrode containing graphitized carbon conductor.Such an unusual electrode delivers a reversible capacity of 672.9 mAh g^-1 at 5 A g^-1versus 425.0 mAh g^-1 of SnO₂@NC and 97.3 mAh g^-1 of SnO₂ conventional electrodes along with reversible transfer of SnO₂ to Snand Li₂O,then to Li_xSn.The capacity still remains to 673.1 mAh g^-1 from the initial 1086 mAh g^-1 after 800 cycles at 2 A g^-1.It means the fading rate is as low as 0.047%capacity per cycle.In-depth investigations reveal that the slightly increased interface resistance between Cu foil and the thermal-sticked materials prevents the charge/discharge from mainly taking place on the interface but extends to the whole electrode with a uniform charge/discharge rate.（2）The Li₂O catalysis of（de）lithiation in a nerve-cell-like anode of Li-ion battery.We reveal that the oxygen atoms in Sn-based anode materials catalyze de/lithiation in form of Li₂O intermedium by using a nerve-cell-like anode in Li-ion battery.The nerve-cell-like anode structure was featured by conductive nitrogen-doped carbon nanofiber linked with carbon-shell-enclosed cages where the active Sn-based materials with tunable oxygen content were isolated for de/lithiation.The electrochemical test results show that Li-Snalloys with more Li₂O matrix can exhibit lower overpotential and higher current densities during the de-lithiation process.And the DFT results deliver that the Li₂O intermedium increased the Li adsorption energy in lithiation,and it amorphized the Li-Snalloy that accelerates delithiation.As a result,the high rate（dis）charge property with long-term cycle life is achieved in the nerve-cell-like anode,which delivers an outstanding rate capability of 691.9 mAh g^-1 at 7 A g^-1 and capability maintenance of 842.3 mAh g^-1 after 1000 cycles at 2 A g^-1.（3）The lithium storage mechanism under the ultra-small particle interface can be changed from intercalation-controlled to the surface-reaction mechanism,thus satisfying the structural properties which were required for high-rate LIBs electrode materials.We report ultra-small particles structured SnS_x anode,in which the active particles with a diameter of 2-5 nm are well encapsulated and in full contact with robust carbon nanofiber.The ultra-small particles decrease the Li⁺diffusion paths and weaken the volume changes ratio,while the pseudocapacitance of active particles was greatly enhanced in contrast.Based on the electrochemical test results the ultra-small particle structure may change the Li-storage mechanism from the Li-interaction mechanism to the surface reaction mechanism,which endows a fast（de）lithiation characteristic for every active particle.Besides,a catalytic effect of Li₂S intermedium was confirmed during the（de）lithiation process,that is,the Li₂S intermedium increased the Li adsorption energy in lithiation and accelerates the delithiation process of Li-Snalloy.As a result,the high rate（dis）charge property with long-term cycle life is achieved in the SnS_x@NC anode,which delivers an outstanding rate capability of 633.4 mAh g^-1 at 7 A g^-1 and capability maintenance of 785.2 mAh g^-1 after 1600 cycles at 2 A g^-1.