Thin-film Li-ion battery (TFLIB) anodes that alloy with Li, including Si, Ge, Sn, and Al have specific capacities that significantly exceed that of carbon-based intercalation anodes. However, the large volume expansion/contraction that accompanies charging/discharging processes lead to prominent mechanical stresses that ultimately induce loss in capacity and failure of the anodes.
Here, we combine real-time scanning electron microscopy under ultra-high vacuum conditions with electrochemical cycling to quantify the dynamic degradation of the Al anode upon charging/discharging of a TFLIB with a N-doped LiPO (LiPON) electrolyte and a LiCoO cathode. Our approach allows us to precisely control the lithiation rate, record the voltage, and to correlate these parameters with specific changes in the electrode morphology; providing a quantitative and real-time analysis of the anode degradation. Surprisingly, we find that significant changes in the Al film morphology occur at very low lithiation level, at ≈ 1.0 % Li in Al. A capacity of 20 μAh/cm is reached on the first charge cycle, which is equivalent to 94% of theoretical cathode capacity and 20% of anode capacity. With increasing number of cycles the smooth surface of the Al anode film is significantly roughened and covered with Fd3m AlLi mounds.
The battery degrades accordingly, losing ≈90 % of its capacity after 100 cycles. The origin of the discharge capacity fade is directly related to the Li being trapped in the mounds, which is due to the blockage of Li and Al diffusion pathways necessary for the decomposition of LiAl at room temperature. This process is a direct consequence of the extremely low diffusivity of Li within Al, which will be discussed in details. The spatially resolved and in situ measurements of Li+ diffusion during lithiation/de-lithiation in an operating TFLIB model system represents an important step towards understanding and engineering the surface of metal anodes to improve capacity in rechargeable batteries.