利用第一原理研究鎵/鋁摻雜對鋰鑭鋯氧化合物(LLZO)結構穩定性以及鋰離子擴散機制的影響

Abstract

固態電解質(SSEs)對於開發更安全、更穩定的鋰離子電池至關重要。本研究運用DFT模擬、AIMD和NEB，研究了石榴石型固態電解質Li7La3Zr2O12 (LLZO)在不同摻雜條件下的結構穩定性和鋰離子擴散機制。我們通過詳細分析四面體位置(T-site)占據率、空缺分布、平均平方位移和能障，來探討了鎵(Ga)和鋁(Al)摻雜對相變和離子電導率的影響。藉由能量分析，我們發現在0K下的立方相和四方晶相存在著顯著的四面體位置占據率，立方相具有較高的四面體位置占據率而後者的四面體位置占據率則較低，利用參雜，可以藉由改變參雜附近的原子排列來達到更高的四面體位置占據率，並使得立方相在參雜到一定濃度後具有較穩定的能量，所以參雜對立方相的LLZO具有穩定效果，值得注意的一點是，鎵(Ga)和鋁(Al)其實對於穩定立方相LLZO有類似的效果。

Ia-3d(230)到I-43d(220)最大的差異在於鋰所在之四面體位置有是否可以分辨，如果可以分辨成12a和12b，說明此系統為220空間群，若無法分辨為12a和12b，則統一利用24d進行描述，是為230空間群，在進行AIMD時，我們發現參雜系統相較於純系統具有主導的四面體位置(12b)，而純系統的四面體位置會隨著時間的推移出現不同的主導四面體位置，說明純系統的四面體其實並不可分辨，而參雜系統中，隨著參雜濃度的提升，其主導的四面體位置佔據率越高，所以可以知道隨著較高的參雜濃度，LLZO系統也是越趨近於220空間群的。

我們的研究揭示了摻雜濃度、溫度和離子電導率之間的複雜關係。在低溫條件下(1000K以下)，少量摻雜通過維持空間群I-43d(220)的特性來提高電導率。較高的摻雜濃度具有更明顯空間群I-43d(220)的特徵，但是因為活性空缺減少和阻塞效應增加反而會降低電導率。在純LLZO系統中，會出現有序區和無序區，前者有明顯的主導四面體位置而後者沒有，而在無序區中會出現穩定的T-site配對來困住鋰空缺，致使純系統在低溫下表現出較低的電導率，但在高溫下(1000K以上)因為沒有摻雜物引起的阻塞效應而表現出相對優異的性能。藉由觀察AIMD的擴散行為，我們發現了在220空間群中有兩個主要的擴散行為，分別為(1.)12b位置到八面體位置以及(2.)八面體位置到八面體位置兩種，對於擴散行為來說，後者為速率決定步驟，而我們也發現鎵(Ga)系統具有較小的能障，而這和AIMD的結果具有一致性。這些發現為優化下一代電池應用中的LLZO基固態電解質提供了新的見解。

Solid-state electrolytes (SSEs) are crucial for developing safer and more stable lithium-ion batteries. This study employs DFT simulations on static calculation, AIMD, and NEB to investigate the structural stability and Li-ion diffusion mechanisms in garnet-type Li7La3Zr2O12 (LLZO) under different doping conditions. We examine how Ga and Al doping affects the phase transitions and ionic conductivity through detailed analysis of tetrahedral site (T-site) occupancy, vacancy distribution, mean square displacement, and energy barriers.

Through energy analysis, we find that at 0K, both cubic and tetragonal phases exhibit distinct tetrahedral site occupancies, with the cubic phase showing higher occupancy than the tetragonal phase. Through doping, we can achieve higher tetrahedral site occupancy by modifying the atomic arrangement near the dopants, leading to enhanced stability of the cubic phase at sufficient doping concentrations. Notably, both Ga and Al demonstrate similar stabilizing effects on cubic LLZO.

The key difference between Ia-3d(230) and I-43d(220) lies in the distinguishability of Li tetrahedral sites. If the sites can be differentiated into 12a and 12b, the system belongs to space group 220; if indistinguishable, they are described uniformly as 24d sites in space group 230. Our AIMD simulations reveal that doped systems exhibit a dominant tetrahedral site (12b), while in pure systems, the dominant tetrahedral site changes over time, indicating indistinguishable tetrahedral sites. As doping concentration increases, the occupancy of the dominant tetrahedral site increases, suggesting the system increasingly adopts space group 220 characteristics.

Our research reveals complex relationships between doping concentration, temperature, and ionic conductivity. At low temperatures (below 1000K), low-level doping enhances conductivity by maintaining space group I-43d(220) characteristics. Higher doping concentrations also maintain space group I-43d(220) but decrease conductivity due to reduced active vacancies and increased blocking effects. Pure LLZO exhibits ordered and disordered regions, with the former showing dominant tetrahedral sites while the latter lacks them. Stable T-site pairs in disordered regions trap Li vacancies, resulting in lower conductivity at low temperatures, but superior performance at high temperatures (above 1000K) due to the absence of dopant-induced blocking effects. Through AIMD diffusion behavior observations, we identify two primary diffusion mechanisms in space group 220: (1) 12b site-to-O-site migration and (2) O-site-to-O-site exchange. The latter serves as the rate-determining step, with Ga-doped systems showing lower energy barriers, consistent with AIMD results. These findings provide new insights for optimizing LLZO-based solid electrolytes in next-generation battery applications.