Uncovering the Root Leading to Accelerated Capacity Fade of Li‐Ion Coin Cells in Fast Charging

Coin cells have been widely adopted for the evaluation of battery materials and chemistry. Herein, however, the coin cells are found unable to correctly reflect the fast‐charging capability of batteries. This is because, in the coin cells, the pressure between electrodes is provided by a wave spring while the wave spring cannot apply sufficient pressure to maintain the necessary electric contact for fast charging, leading to high polarization and resultant Li plating at the graphite anode. Testing on graphite/LiNi 0.80 Co 0.10 Mn 0.10 O 2 coin cells shows that replacing the wave spring with a flat O‐ring washer dramatically stabilizes capacity retention in fast charging. Alternating current‐impedance analyses on the cells before and after 200 5 C‐charging cycles indicate that the cell with the O‐ring washer initially has a larger surface layer resistance, however, after fast‐charging test retained much smaller overall resistance compared with the cell using the wave spring. Visual observation of the cycled cell components reveals that the improvements are due to significantly reduced Li plating at the graphite anode, which is attributed to the appropriate pressure and the uniform distribution of the pressure applied by the flat O‐ring washer.