| Emerging technologies of portable electronics and electric vehicles have stimulated the ever-increasing demand for high energy density lithium-ion batteries(LIBs)in the field of energy storage.To meet the development of high energy density LIBs,new anode materials are required to replace currently existing graphite anodes with low capacity.Silicon(Si)is well known to be a possible alternative for graphite anodes due to its higher capacity(4200m Ah·g-1)which is over tenfold greater than graphite(370 m Ah·g-1).However,there are three challenges which prevent the Si anodes from being commercialized.These are volume-expansion induced capacity fade,poor conductivity and low initial coulombic efficiency(ICE).In order to overcome these challenges,there are three strategies:buffering action of MgO,carbon coating and pre-lithiation,which have been adopted to modify the Si-based material for achieving better electrochemical performance.In addition,electrode design has been carried out to investigate the influence of binder selection and thick film electrode on the electrochemical properties,with the aim to develop high performance Si-based electrode for next generation LIBs with high energy density and cycling stability.Starting with micrometer grade SiO and magnesium powder(Mg)raw materials,high energy ball milling(HEBM)technique can been applied to initiate chemical reaction of these two materials during the mechanochemical process.A composite structure of Si-MgO(denoted as SM)can be synthesized via an in-situ solid state reaction of Mg+SiO?Si+MgO under conditions:at 500rpm after 1h pre-milling for particle size reduction of Si O,and then mixed with Mg for 5h milling.In order to improve the conductivity of Si-MgO composite reaction product,the addition of graphite(denoted as G)can constitute a multi-phase mixture of Si-MgO-G(denoted as SMG)after 1h ball-milling.Carbon coating can be carried out on the multi-phase mixture by high temperature calcining of low cost asphalt,resulting in a core-shell structure(Si-MgO-G)@C(denoted as SMG@C)for both better accommodation of volume expansion and enhancement of electrical conductivity.In addition,the effects of binder selection and thick electrode forming technique on the cycling stability and area specific capacity have been explored by electrode design of the aforementioned composite materials.Combined with materials characterization and electrochemical testing methods for multi-phase microstructure and electrochemical property analysis,the research results obtained in this work are stated as follows:(1)Anode composite material:Raw material SiO went through high-energy ball milling(HEBM)process for 1 hour pre-milling,and then was mixed with Mg during different blend milling times(3,5 and 7 h)for mechanochemical reaction.It has been found that the reaction degree of Mg and SiO is poor after(1+3)h ball milling,but increases after(1+5)h and(1+7)h ball milling.The XRD and SEM results showed that the composite structure Si-Mg O(SM)was obtained with milling times greater than 5 h,but there existed particle agglomeration in the SM powder samples after(1+7)h ball milling.As a result,the SM powder obtained after(1+5)h milling was chosen to mix with graphite after 1h milling used for our SMG anode composite material.The charge-discharge cyclic performance of SMG anode composite material is greatly improved compared with SiO.For the SiO as a reference,the reversible charge capacity(deintercalation lithium capacity)and reversible discharge capacity(intercalation lithium capacity)fade quickly at a current density of 100 m A·g-1.In contrast,for the SMG anode composite material,the charge capacity retention rate is 71.5%in the 20th cycle,which is33.5%higher than Si O.Even after 177 cycles,the charge capacity retention rate is 54.8%,and the discharge capacity retention rate is 52.8%.Based on the fact that the charge-and discharge-capacity of the SMG anode composite material only return to 49%and 44%of the achievable reversible capacity respectively at the initial current density of 100 m A·g-1,its charge-discharge rate and cycling stability need to be further improved.(2)Carbon-coated anode composite material:The SMG composite material acting as a core was coated with a thin layer of carbon shell by high temperature asphalt carbonization Such core-shell structure was confirmed by the XRD and TEM results.The core is composed of Si,Mg O,graphite,and the shell is a thin of amorphous carbon with an average thickness of 35nm.The cycling stability and rate performance of the carbon coated SMG(SMG@C)composite material were carried out together with SMG for comparison.After carbon coating on the SMG,the SMG@C anode composite material showed the superior performance in both cycling stability and rate capacity.At a current density of 100 m A·g-1,the cycling performance of SMG@C anode composite material was relatively stable.This was demonstrated by its high capacity retention rate,the value of the reversible charge capacity and discharge capacity of the70thcycle relative to the 10th cycle being 95.2%and 94.9%respectively.In contrast,the non-carbon coated SMG anode composite material showed a continuous capacity fade trend during the whole cycling.Its 10thcycle involved 76.2%and 74.9%respectively of reversible charge capacity and discharge capacity.In addition,the long-cycle testing of SMG@C anode composite material at a current density of 200 m A·g-1displayed a very good capacity retention rate of 87.8%after 440 cycles.Also its rate capacity is the highest of three anode materials at different current densities.The Warburg coefficients of Si O,SMG,and SMG@C active materials were measured to be 7.25?·S-1/2,2.84?·S-1/2,and 1.56?·S-1/2,respectively,indicating Li transport is more easily through the core-shell structure of SMG@C composite material.Overall,the SMG@C anode composite material has both higher conductivity ability and better electrochemical properties.(3)Buffering action mechanism of magnesium oxide(MgO):The co-existence of Si with MgO was confirmed by high-resolution TEM.The two adjacent phases consisted of crystalline Si domains surrounded by MgO nanodomains.Moreover,by using HRTEM lattice fringes,a special structural relationship between Si and MgO prepared by HEBM solid reaction was observed to have a coherent relationship,which is described as Si(220)//Mg O(200).This structural feature of Si adjoined with MgO as a buffering phase is helpful to alleviate the volume change of silicon,giving a better cycling performance in subsequent cycles.In addition,by analyzing the geometric mechanical model of Si attached to Mg O(as a substrate).we have derived a ratio of half-height(H)to half-length(L0)of Si attached to MgO,that is the geometric stability conditions(0<H/L0?0.61)for avoiding the occurrence of Si detachment from Mg O.The H/L0value of silicon was measured to be 0.39 approximately,which falls into the calculated geometric stability range,thus indicating that Si is difficult to separate from the MgO substrate.In other words,the inactive MgO with higher elastic modulus can suppress the active Si dimensional change to some degree.Therefore,the structural stability of Si attached to MgO is very important for the cycling stability improvement of the Si-based anode composite material.(4)Pre-lithiation additive:Lithium-silicide Li57Si43powder was synthesized by means of HEBM mechanochemical reaction.A mixture of pure Li chips and Si powder was put in a stainless-steel jar together with balls under Ar atmosphere,and ball-milled for 5 hours.The Li-Si alloy powder was mixed with graphite and porous carbon at a speed of 500 rpm for 2 hours,and coated with carbon via high temperature asphalt carbonization so that the pre-lithiation agent can be stably exposed to ambient air for the preparation of negative electrode.The Li-Si alloy pre-lithiation agent was added into the SMG@C anode composite material to investigate its influence on the initial coulombic efficiency.The ICE of SMG@C anode composite material electrode increased from 52.5%to 66.1%before and after the addition of 5 wt%pre-lithiation agent.This is attributed to additive pre-lithiation which can compensate the active Li+loss caused by the formation of solid electrolyte interphase and the lithiation reaction of residual Si O present in the composite material.Furthermore,the addition of the pre-lithiation agent results in the increase of discharge capacity retention from 86.1%to 91%in the 19thcycle,showing a good cycling stability.(5)Binder selection:Four kinds of binders including polyimide(PI),sodium alginate(SA),polyvinylidene fluoride(PVDF),and sodium carboxymethyl cellulose(CMC)were chosen to investigate their effects on the electrochemical properties of the SMG@C composite anode material during charge-discharge process.The action results of these different binders showed that the electrochemical performance of SMG@C-SA electrode was better than the other SMG@C-PI,SMG@C-PVDF and SMG@C-CMC electrodes.At a current density of 100m A·g-1,the SMG@C-SA electrode displayed the reversible charging capacity of 1207 m Ah·g-1after the 75thcycle.Its charging capacity retention rate(93.3%)was increased by 5.9%,34.8%and 35.1%respectively,compared with SMG@C-PI(87.4%),SMG@C-CMC(58.5%)and SMG@C-PVDF(58.2%).In addition,the combination of SA binder with SMG@C composite anode material allows the capacity to return to the initial value at a current density of 100 m A·g-1,after going through different current densities,showing a very good rate performance.The Warburg coefficients obtained from the SMG@C with different binders CMC,PVDF,PI and SA were 95.95?·S-1/2?23.85?·S-1/2?7.15?·S-1/2and 1.81?·S-1/2,respectively,indicating that the use of SA binder can most effectively improve electrical conductivity.Overall,the SA binder is the best candidate used for the Si-based composite material by the virtue of displaying lower impedance,better cycling stability and higher rate performance.(6)Thick electrodes:The thick electrode forming technique was adopted to investigate the influence of thick film structure electrode prepared by green and scalable kneading and open mixing method,on electrochemical performance of the composite anodes.The thickness and load of thick electrode are about 7.4 times and 6 times greater than thin electrode.The corresponding area's specific capacity is 7.04 m Ah·cm-2,and 6.2 times higher than the thin electrode's capacity(1.14 m Ah·cm-2).The initial charging capacity of thick electrode and thin electrode is 1347 m Ah·g-1and 1303 m Ah·g-1at a current density of 100 m A·g-1,respectively.After the 11thcycle,the charge/discharge capacity retention rate of thick electrode(82.5%/81.2%)increased by 5.8%and 6%compared with that of thin electrode(76.7%/75.2%),respectively.The Warburg coefficients of thick film electrode and thin film electrode were measured to be 4.54?·S-1/2and 95.9?·S-1/2,respectively,indicating much lower impedance to active Li transport through thick electrode. |
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