碳批覆之α-Si3N4, β-Si3N4, 以及β-Si3N4@Si負極應用於鋰電池之研究;Carbon-coated α-Si3N4, β-Si3N4, and β-Si3N4@Si anodes for lithium-ion batteries

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Title:	碳批覆之α-Si3N4, β-Si3N4, 以及β-Si3N4@Si負極應用於鋰電池之研究;Carbon-coated α-Si3N4, β-Si3N4, and β-Si3N4@Si anodes for lithium-ion batteries
Authors:	秋茬;Trang, Le Thi Thu
Contributors:	材料科學與工程研究所
Keywords:	α-Si3N4;β-Si3N4;β-Si3N4@Si;α-Si3N4;β-Si3N4;β-Si3N4@Si
Date:	2020-03-27
Issue Date:	2020-06-05 17:12:58 (UTC+8)
Publisher:	國立中央大學
Abstract:	近些年，由於Si材料具有非常高的理論電容值(Li3.75Si有3579 mAh/g，Li4.4Si具有4200 mAh/g)，在地球上的資源非常豐富，經濟環保，又非常的安全等優點，其作為鋰電池的負極被越來越多的學者所關注。儘管有上述諸多優點，但是Si這個材料在鋰化以及去鋰化的過程中，體積會劇烈膨脹(膨脹300-400%)，因此會導致其電容值快速下降，造成循環壽命的衰退。因此，學者們開始考慮使用Si3N4，因為其具有較為優秀的機械性能(比如：高強度，高硬度，高的斷裂韌性)，可以承受在充放電循環過程中的體積變化以及機械應力。基於以上的原因，本研究將通過比較不同相的Si3N4 (α-Si3N4以及β-Si3N4)的結構、物理性質以及電化學性質，從而選擇出較為合適的相，再與Si通過球磨法進行混合，形成混合物。隨後，在混合物表面批覆一層非晶碳層，因為碳在鋰化以及去鋰化的過程中體積變化較小並且具有較高的導電性。經過實驗後，β-Si3N4試片在第一圈充放電中展現了92.7 mAh/g的可逆電容值，並且在高速下，可以保持維持率為32.4%，遠高於α- Si3N4所展示出來的84.4 mAh/g的電容值以及30.6%的維持率。而經過非晶碳層批覆之β-Si3N4/C在50 mA/g速度下，在循環100圈之後展示了92.9%的電容值維持率以及7.4 x 10-14鋰離子擴散系數(D-Li+)。隨後，本實驗將β-Si3N4與Si均勻混合後進行非晶碳層批覆，成功形成β-Si3N4@Si/C。其在500 mA/g下循環100圈之後展示了高達22%的電容值可逆性，而傳統的Si只有0.4%。-Si3N4@Si/C在高速5000 mA/g下也有高達14.1%的維持率以及較高的鋰離子擴散系數(D-Li+) 9.4 x 10-14。而之所以會有如此優秀的循環壽命以及高速下電容維持都要歸功於β-Si3N4優異的機械性能以及表面非晶碳層的批覆。;Utilizing silicon as an anode material in the rechargeable Li-ion batteries (LIBs) has received much attention during the past decades due to its superior theoretical capacity (3579 mAh/g for Li3.75Si or 4200 mAh/g for Li4.4Si), as well as its abundant natural resources, economic affordability, and safety. Nevertheless, Si undergoes severe volume expansion (300-400%) and mechanical vulnerability during lithiation/de-lithiation, causing fast capacity fading and poor cyclability. Then, stoichiometric silicon nitride (Si3N4) was taken into consideration since it has excellent mechanical properties (e.g., high strength, high hardness, and high fracture toughness) tolerating volume changes and mechanical pressure upon cycling. Based on the above properties, in this work, the structural, physical properties and electrochemical behavior of Si3N4 phases (α-Si3N4 and β-Si3N4) were compared to find the better phase, which was then combined with Si to produce the silicon nitride-based composite by the ball-milling method to establish a synergistic relationship. Furthermore, the exploitation of amorphous carbon protective coatings was approached, since carbon has small volume change during the electrochemical lithiation/de-lithiation processes and high electrical conductivity. As expected, the β-Si3N4 sample which obtained 92.7 mAh/g for the first reversible capacity, 32.4 % for the high-rate reversible capacity retention, was proved to be more efficient than the α-Si3N4 sample which attained 84.4 mAh/g and 30.6 %, for the first reversible capacity and high-rate reversible capacity retention, respectively. The β-Si3N4 sample with the amorphous carbon shell (β-Si3N4/C) exhibited 92.9 % of reversible capacity retention after 100 cycles at 50 mA/g and 7.4 x 10-14 of lithium diffusion coefficient (D-Li+), further demonstrating its potential capability. Subsequently, the β-Si3N4@Si composite and the β-Si3N4@Si composite with the amorphous carbon shell (β-Si3N4@Si/C) were successfully synthesized and evaluated. Moreover, the amorphous carbon-coated Si sample was also compared in this study. After 100 cycles at 500 mA/g, the β-Si3N4@Si/C composite showed the highest reversible capacity retention of 22.0 %, whereas for Si only 0.4 % initial capacity was retained. The β-Si3N4@Si/C composite also demonstrated its better high-rate performance with 14.1 % of capacity retention at 5000 mA/g and its much higher ion-diffusion acceleration with 9.4 x 10-14 of D-Li+. The enhanced cyclability and high-rate performance of silicon-based anodes owe to the high mechanical durability and excellent adhesive properties of β-Si3N4 along with the protective function of the amorphous carbon.
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