Understanding Why Silicon Anodes of Lithium-Ion Batteries Are Fast to Discharge but Slow to Charge

Silicon anodes for lithium-ion batteries are capable of quickly delivering high power but charge at a much lower rate. High-power and high-rate performance of batteries is determined by the intrinsic electrochemical reaction rates. The forward and backward reaction rates for reversible electrochemical reactions are not necessarily identical. This work demonstrates different charging and discharging rates (also known as C-rates, which are defined as rate of discharge [or charge] as compared to the capacity of the battery) of lithium-ion battery electrodes, and explores the scientific origin for this phenomenon.

In the model system consisting of dense 70-nanometer-thick silicon thin films, under an ultra-high current density of 420 A/g about 72% of the total available charge capacity can be delivered during discharge, while only 1% capacity can be delivered during charging. This asymmetric rate performance originates from the potential-concentration profile and ohmic voltage shift. Furthermore, chemical diffusivities for Li in Si are larger for discharging and smaller for charging, which affects the charging and discharging rates further. This work clarifies that the charge and discharge rates of both anodes and cathodes in batteries should be evaluated separately in order to identify the bottleneck component for high-power performance. 

 

Juchuan Li, Nancy J. Dudney, Xingcheng Xiao, Yang-Tse Cheng, Chengdu Liang, and Mark W. Verbrugge, “Asymmetric Rate Behaviors of Si Anodes for Lithium-Ion Batteries: Ultrafast De-Lithiation vs. Sluggish Lithiation at High Current Densities,” Advanced Energy Materials  http://dx.doi.org/10.1002/aenm.201401627.   

 

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