Degradation Mechanism of Li₂TiSiO₅ Anode Material—Direct Observation of Gradational Pulverization via Cracking

Lithium-ion batteries (LIBs) have rapidly gotten a hold on the rechargeable secondary battery market from small and portable electronic devices to large-scale devices such as electric vehicles and power tools that require fast charge/discharge, large capacity, and high stability.¹ Among various electrode materials, Li₄Ti₅O₁₂ is a commercialized anode material with a high potential above 1.5 V vs. Li preventing the formation of dendritic lithium.^2,3 This Ti-based material shows high reversibility, excellent structural and chemical stabilities due to zero-strain characteristics during electrochemical reactions. However, the pristine Li₄Ti₅O₁₂ has an inferior energy density resulting from low capacity (175 mAh g^-1), high operating voltage (~1.5 V vs. Li/Li⁺), and poor electronic/ionic conductivities. Although many researchers reported the effects of heterogeneous metal doping such as Cr, Mg, and Mn on electronic conductivities, dopant atoms can cause structural deformation and lose zero-strain characteristics.⁴
Here, we report on the direct observation of electrochemical kinetics of Li₂TiSiO₅ anode material including phase transformation using in situ transmission electron microscopy (TEM) technique. In addition, we clearly demonstrate the degradation of electrodes caused by amorphization and pulverization along cracks. Li₂TiSiO₅ anode material shows higher energy than Ti-based Li₄Ti₅O₁₂, but capacity fades and degraded performance under prolonged cycles were observed. In order to unveil the origin of electrochemical degradation, phase transformation was characterized during the lithiation process via in situ TEM and selected area electron diffraction (SAED) technique. Li₂TiSiO₅ showed an area expansion of 2.35 % with expanded lattice parameters via the solid solution process during the intercalation process. However, the subsequent volume expansion and phase transformation introduced the formation/propagation of major cracks inside electrodes after electrochemical cycles. In addition, the pulverization and amorphization with nanostructure are accelerated by additional fine cracks at longer cycles, causing the degradation of electrochemical performance. Our findings provide not only invaluable insights into the understanding of fundamental mechanisms in Li₂TiSiO₅ materials but also a clue for various approaches to address the mechanical degradation and improve electrochemical performance.
[1] J. Am. Chem. Soc. 2013, 135, 1167-1176.
[2] Chem. Mater. 2004, 16, 5721-5725.
[3] Nano Lett. 2014, 14, 2597-2603.
[4] Energy Environ. Sci. 2012, 5, 9903-9913.