External Pressure in Polymer-Based Lithium Metal Batteries: An Often-Neglected Criterion When Evaluating Cycling Performance?

Solid-state batteries based on lithium metal anodes, solid electrolytes, and composite cathodes constitute a promising battery concept for achieving high energy density. Charge carrier transport within the cells is governed by solid–solid contacts, emphasizing the importance of well-designed interfaces. A key parameter for enhancing the interfacial contacts among electrode active materials and electrolytes comprises externally applied pressure onto the cell stack, particularly in the case of ceramic electrolytes. Reports exploring the impact of external pressure on polymer-based cells are, however, scarce due to overall better wetting behavior. In this work, the consequences of externally applied pressure in view of key performance indicators, including cell longevity, rate capability, and limiting current density in single-layer pouch-type NMC622||Li cells, are evaluated employing cross-linked poly(ethylene oxide), xPEO, and cross-linked cyclodextrin grafted poly(caprolactone), xGCD-PCL. Notably, externally applied pressure substantially changes the cell's electrochemical cycling performance, strongly depending on the mechanical properties of the considered polymers. Higher external pressure potentially enhances electrode–electrolyte interfaces, thereby boosting the rate capability of pouch-type cells, despite the fact that the cell longevity may be reduced upon plastic deformation of the polymer electrolytes when passing beyond intrinsic thresholds of compressive stress. For the softer xGCD-PCL membrane, cycling of cells is only feasible in the absence of external pressure, whereas in the case of xPEO, a trade-off between enhanced rate capability and minimal membrane deformation is achieved at cell pressures of ≤0.43 MPa, which is considerably lower and more practical compared to cells employing ceramic electrolytes with ≥5 MPa external pressure.


Derivation of the Cell Stack Pressure in Pouch cells
A Tekscan OEM Development Kit with a FlexiForce square-shaped (50 mm*50 mm) sensor was utilized to determine the applied external pressure in case of the single-layer pouch-type cells.The device was connected to a computer and controlled by FlexiForce Microview software.A threepoint-calibration was performed prior to accessing the applied pressures.For calibration, three different known weights (Table S1) were put on the sensor and the digits (a.u.) were count.Thus, the to be determined pressure applied to the cells is related to a specific mass in kilograms.
According to equation (1) the pressure p can be obtained from: with m being the mass of the weight, g being the gravity acceleration (g = 9.81 m s -2 ) and A the overall area onto which the pressure is applied (A = 1600 mm 2 ).The Tekscan OEM development kit derived the mass as a mean value of 100 data points.To minimize the impact of various metal plates and exact position of the pouch cell sandwiched in between the metal plates, three different plates were used and the masses were obtained from the mean of three different test series for each tightening torque.A series of eight tightening torques were applied on the sensor and the pressure was calculated according to equation (1).The results are displayed in Table S2.Note that the standard deviation increases for higher tightening torques as the corresponding weights fall significantly outside the calibration region while for lower tightening torques, a higher accuracy can be assumed.The calculated pressure was then plotted against the tightening torque (Figure S2) and the curve was fitted by the exponential function with a = 3.25, b = 3.06 and c = 0.69.Expression (2) was invoked as fit function since it represents the data points very well with a coefficient of determination R 2 of 0.996.With equation (2), the pressure of each torque can be calculated.For comparison of the impact of pressure on the achievable battery performance, three different externally applied pressures were set (Table S3).The selected values are adequately spaced apart to prevent overlapping regions, considering the increased standard deviations observed at higher tightening torques.Notably, a pressure of 0.43 MPa, which results from a torque of 0.21 Nm, is similar to established pressures in coin cell setups, which is described in more detail in the following section.In addition, pouchtype cells were operated without metal plates, labelled 'No external pressure', where only the vacuum-sealing ensures contact between electrodes and electrolyte.

Calculation of the Cell Stack Pressure in Coin Cells
According to expression (3) the pressure p can be calculated as: where F is the force and A is the area onto which the force is applied.The force can be determined from the spring constant D (D = 100 N mm -1 ) and the deflection of the spring ∆L.D was measured invoking a ZwickRoell spring testing machine.Thus, a force of F = 50 N was applied and the corresponding deflection ∆L of the spring was measured.Here, a deflection of ∆L = 0.5 mm resulted as an average from ten measurements.According to equation (4), the respective spring constant was obtained as D = 100 N mm -1 .
the thickness of the individual cell components (Table S4) from the thickness of the crimped cells, being hcell = 3.2 mm for CR2032-type cells.
Table S4: Thickness of the cell components required to calculate the deflection of the spring when the cell is crimped.In case of the considered NMC622||Li cells, the determined resistances reflecting the semi-circles amount to 28.2 Ω cm 2 (yellow), 52.3 Ω cm 2 (blue) and 85.8 Ω cm 2 (green), respectively, accounting for the electrolyte resistance, SEI resistance and charge transfer resistance.Note that

Figure S1 :
Figure S1: Pouch cell setup.a) cell stack before vacuum sealing, b) vacuum-sealed pouch cell and

Figure S2 :
Figure S2: Pressure against the torque used to apply external pressure onto the pouch-type cells.
the cell stack pressure can be varied by using different lower or upper spacers.Herein and according to equation (3), when using a lower spacer with L = 1.0 mm, an upper spacer with L = 0.75 mm and an area of A = 113 mm 2 , corresponding cell stack pressures of p = 0.43 MPa were established.

Table S1 :
Mass of the different weights of the three-point-calibration.

Table S2 :
Parameter for calculating the external pressures, which are applied onto the pouch-type cells by tightening the screws with a specific torque.

Table S3 :
Externally applied pressures resulting from different tightening torques.

Table S5 :
Fit parameters of the invoked elements of the equivalent circuit model displayed in FigureS3d).