追踪锂金属负极的压力与形貌变化

Tracking Pressure Changes and Morphology Evolution of Lithium Metal Anodes

采用高载量氧化物正极(>4mAh·cm^(-2))和超薄锂金属负极(<50μm)可以构建高比能锂金属二次电池。然而,该类电池的循环寿命和安全性受到锂金属不可控沉积的严重制约。高比表面积的锂枝晶和锂“苔藓”导致了较低的库伦效率,前者有一定可能穿刺隔膜,造成电池内短路,是亟待解决的安全隐患。因此,提升锂金属二次电池的循环寿命和安全性的关键在于实现锂金属的致密沉积。文献中已有多种化学方法可达到这样的效果。由于锂金属较软,受力容易发生形变,对锂金属电池施加机械压力是另一种促进锂金属致密沉积和提高循环性能的方法。然而,机械压力、锂金属形态的演变、和循环性能之间的关系尚未被完全理解。本文报道了一种基于薄膜压力传感器的电池压力测量装置,可以实时跟踪纽扣型锂金属电池内部的压力变化,并且探究外加机械压力对电池循环性能的影响。研究发现,在纽扣电池和高比能的软包电池(5 Ah,>380 Wh·kg^(-1))中,一定程度的压力可以促进锂金属的致密沉积,改善电池循环性能;而过大的压力则会导致锂金属向负极内部沉积,造成负极变形和电池性能恶化。我们的研究结果凸显了压力控制对于锂金属沉积行为的重要作用,为进一步改善高比能锂金属电池的循环性能提供了指导。

High-energy rechargeable lithium metal batteries(LMBs)have attracted significant attention recently.These batteries can be bulit using high areal-capacity(>4 m Ah·cm^(-2))layered oxide cathodes and thin lithium(Li)metal anodes(<50μm in thickness),whose cycle performance are severely limited by the unregulated growth of Li particles having high surface areas,including dendrites and mossy Li.To improve the cycle performance of LMBs,many approaches have been developed in recent years to promote dendrite-free and dense Li electrodeposition,such as electrolyte engineering(for liquid cells),Li anode surface modification,three-dimensional current collector design,and using solid-state electrolytes.In addition to these heavily researched chemical-based approaches,applying external pressure to LMBs can also strongly impact the morphology of the electrochemically deposited Li particles due to the malleable nature of metallic Li and has been shown to improve the cycle performance.However,the relationship between the applied pressure,morphological evolution of the Li anode and the cycle performance has not been fully understood,especially in coin cells,which are widely used for LMB research.Here we report a custom-designed pressure applying/measurement device based on thin-film pressure sensors to realize real-time tracking of the pressure evolution in LMB coin cells.Our results show that moderate pressure is conducive to dense Li deposition and increases the cycle life,whereas excessive pressure causes Li inward-growth and the deformation of Li anode,which will impare the electrochemical performance of LMBs.Although these observations are made in coin cells,they could have important implications for pouch cells and solid-state batteries,both of which are commonly tested under pressure.The cycle performance of LMBs is significantly improved in both coin cells(under actual relevant conditions)and large pouch cells.A 5 Ah pouch-type LMB with a high energy density exceeding 380 Wh·kg^(-1)could achieve stable cycling over 50 cycles under a stack pressure of~1.2 MPa.It was also confirmed that the cell holders or clamps commonly used for coin cells can only exert a small amount of pressure,which is unlikely to exaggerate the cycle performance of the LMB coin cells.However,we do suggest that the electrochemical performance of LMBs should be reported along with the information on the applied pressure.This research practice will improve the consistency and quality of the reported data in the LMB research community and help unite the efforts to further improve the high energy density LMBs.