The Preparation Of Silicon-based Anode For Lithium-Ion Battery And Study On Its Loading And Electrochemical Performance Improvement

High-performance,high-energy-density lithium-ion battery is currently one of the most important cutting-edge technologies.At present,the mainstream graphite anode of lithium ion battery is close to its theoretical specific capacity(372 mAh g-l),which makes it difficult to further improve the energy density of the battery.Employing high specific capacity battery anode materials to improve the energy density is one of the most important solutions.Among all the promising anode materials,silicon(Si)stands out due to its highest theoretical capacity of 4200 mAh g-1(ten times more than conventional graphite anode),low operation voltage of 0.4 V versus Li/Li+,nontoxicity and worldwide abundance.Nevertheless,large volume change(about 300%)in Si occurs during the lithiation and delithiation process.This may cause severe pulverization of Si particles,continuous formation and reformation of the solid electrolyte interfaces(SEI)and loss of electrical contact.These will further lead to capacity fading,cycling instability and finally failure of the battery.Meanwhile,low electrical and ionic conductivity of Si might impede the loading of Si and its rate performance.The objective of the current dissertation is to design and prepare a novel block polymer,poly(acryli cacid)-polystyrene-poly(methyl acrylate)-polystyrene(PAA-b-PSt-b-PMA-b-PSt,ASMAS),which is used as binder to form a cross-linked network to maintain the stability of electrode and improve ion transportation.Metal nanowires are used as conductive additives to improve electron transportation.A laminated conductive structure is built by spraying layer upon layer to further enhance the electron transportation for high mass loading anode.Finally,Si anode with high mass loading,high capacity retention and large rate capacity is achieved.Main conclusions and results are as follow.(1)Four block copolymer ASMAS with microphase separation structure was designed and controllably synthesized via RAFT emulsion polymerization.A physically cross-linked polymer network is expected in the electrode.The hard blocks of PS at the two ends of the polymer chain do not adsorb electrolyte.Instead,they form physical cross-linked points and maintain the stability of the polymer binder network.The PAA block can "grasp" the silicon nanoparticles by strong interactions between-COOH and Si-OH.The third block poly(methyl acrylate)(PMA)can swell with adequate liquid electrolyte to provide ion transportation pathway and the required elasticity for the electrode.This stable polymer binder network can accommodate the huge volume change of Nano-Si particles to retain the integrity of electrode during the process of lithiation and delithiation.At a low mass loading(0.7 mg cm-2),the initial specific capacity of the nano-silicon anode with 7.5 wt%ASMAS is 4140 mAh g-1.After 100 cycles,the specific capacity can still be maintained at 2200 mAh g-1.At the same time,the nano-silicon anode still has a reversible specific capacity of 1070 mAh g-1 after 600 cycles.Meanwhile,the rate performance of ASMAS-containing battery is significantly higher than that without ASMAS.At the current density of 1 C(4200 mA g-1),the rate capacity is increased from 1620 mAh g-1 to 2750 mAh g-1.(2)Metal nanowires are employed as conductive additives,with the support of carbon black,to improve the electron transportation.Meanwhile,a laminated conductive structure is designed and built by spraying layer upon layer to further enhance the electron transportation for high Si mass loading anode.Such design enables nano-Si particles to stay in the conductive network more sufficiently even upon huge volume changes.As a result,capacity of Si anode can be reserved at a higher level after hundreds of cycles.At a relatively high mass loading of 1.69 mg cm-2,Si anode with such laminated conductive structure has a capacity of 1146 mAh g-1 at a current density of 2.0 C(8400 mA g-1),which is the best rate performance among current research results.Meanwhile,Based on the ASMAS binder and the laminated conductive structure containing AgNWs,a high mass loading,high performance nano-Si anode with a loading of 5.31 mg cm-2 was successfully prepared.After 100 cycles,the reversible specific capacity is 1609 mAh g-1 and the area capacity is 8.49 mAh cm-2,which is the highest area capacity obtained among all current silicon-based anode studies.At a current density of 1.0 C(4200 mA g-1),the rate capacity is 2050 mAh g-1.For high Si mass loading electrodes,a capacity limitation strategy was uesd to receive a stable cycle performance.When the capacity was limited to 1200 mAh g-1,an extremely stable discharge capacity is obtained even after 200 cycles,and the average coulombic efficiency was as high as 99.29%.Combined with the simulation calculation,the energy density of the high mass loading composite silicon anode can reach 400 Wh kg-1.(3)Using ASMAS as the binder,copper nanowires(CuNWs)with lower cost and high conductivity as conductive additives,a high performance SiO/C silicon anode with industrial application prospects was successfully prepared.When the mass loading of SiO/C anode was 10.26 mg cm-2,a high specific capacity retention of 606 mAh g-1 after 200 cycles can be achieved.At 1.0 C(1255 mA g-1),the reversible rate capacity was 620 mAh g-1.These result is one of the best performances currently obtained for the SiO/C anode research.