  Externally applied pressure impacts the performance of batteries particularly
in those undergoing large volume changes, such as lithium metal batteries. In
particular, the Li$^+$ electroplating process in large format pouch cells
occurs at a larger dimension compared to those in smaller lab-scale cells. A
fundamental linkage between external pressure and large format electroplating
of Li$^+$ remains missing but yet critically needed to understand the
electrochemical behavior of Li$^+$ in practical batteries. Herein, this work
utilizes 350 Wh/kg lithium metal pouch cell as a model system to study the
electroplating of Li$^+$ ions and the impact of external pressure. The
vertically applied uniaxial pressure on the batteries using liquid electrolyte
profoundly affects the electroplating process of Li$^+$ which is well reflected
by the self-generated pressures in the cell and can be correlated to battery
cycling stability. Taking advantage of both constant gap and pressure
application, all Li metal pouch cells demonstrated minimum swelling of 6-8%
after 300 cycles, comparable to that of state-of-the-art Li-ion batteries.
Along the horizontal directions, the pressure distributed across the surface of
Li metal pouch cell reveals a unique phenomenon of Li$^+$ migration during the
electroplating (charge) process driven by an uneven distribution of external
pressure across the large electrode area, leading to a preferred Li plating in
the center area of Li metal anode. This work addresses a longstanding question
and provides new fundamental insights on large format electrochemical plating
of Li which will inspire more innovations and lead to homogeneous deposition of
Li to advance rechargeable lithium metal battery technology.
