**1. Introduction**

Today, lithium-ion batteries (LIBs) are omnipresent in everyday life. After their commercial introduction, LIBs have largely superseded other battery technologies due to their superior properties. They are used in nearly all portable devices such as mobile phones, smartwatches, cameras, laptops, e-cigarettes, and power tools. Furthermore, LIBs are increasingly penetrating the automotive market, where the battery is a crucial component for mobility. The battery cells of an electric car still account for the largest share of vehicle costs, and therefore, the lifetime and reliability of this component is essential. In almost every application, LIBs are integrated into housings and must be mechanically clamped. Previous work indicates that the design of a battery module consisting of several battery cells in terms of mechanical bracing is the crucial element of lifetime and reliability. Cannarella et al. showed that too much initial pressure applied to LIBs decreases their lifetime due to closed pores in the separator, which causes undesirable degradation effects [1].

Further work by the same research group shows that these effects can include local lithium deposition and dendrite growth, affecting electrochemical kinetics [2–5]. Wünsch et al. show that not only the initial pressure is an essential factor for the lifetime of LIBs, but also the type of bracing. Usually, a rigid bracing is used in the construction of LIB modules. Since LIBs with a graphite anode expand during the charging processes, under thermal stress and over a lifetime, the pressure on the cells in the module increases over time [6,7]. According to Wünsch et al., the cycle life can be significantly increased with a quasi-constant force bracing in contrast to the usually applied fixed bracing [7]. From this, it can be deduced that the application of constant pressure can significantly improve the lifetime of LIB modules.

While the work discussed in the previous paragraph analyzed the influence of homogenous pressure in LIB, also inhomogeneous pressure must be considered for real-life applications as the mechanical bracing used in battery modules does not always ensure that the pressure homogeneously distributed. The work of Cannarella and Liu shows that local mechanical deformations of the separator can cause local lithium plating due to inhomogeneous ion flux distributions [4,5]. Furthermore, the work of Tang et al. has shown that lithium plating occurs preferentially at electrodes edges of LIBs due to geometrics effects, which generates overpotential at the edges of the electrodes and leads to conditions which favor lithium plating [8]. Other investigations by Rahe et al. using Nano X-Ray tomography show particle cracks and current-collector corrosion on the cathode side [9], which also leads to a local pressure increase and strongly influences the porosity and thus the tortuosity of the active, cathode and anode, as well as the passive, separator, materials. Another effect observed by Waldmann et al. is that mechanical deformations occur at the jelly roll of a round cell at the end of lifetime. This mechanical deformation induces local inhomogeneous pressure on the active materials [10], which leads to inhomogeneous ion flux distributions and causes lithium plating. Bach et al. linked the sudden degradation effect at the end-of-life of round cells triggered by the appearance of lithium plating confined to small characteristic areas at the jelly roll, generated by heterogeneous compression, and mention the importance of cell and pack design considering a well mechanical bracing without inducing unwanted effects [11].

As presented in the previous paragraph, mechanical defects of the separator are linked to lithium plating. For lithium plating to occur, the local anode potential must be below 0 V vs. Li/Li+ [8,12]. Only under this condition, it is energetically more favorable for the lithium-ions to bond with each other and form metallic lithium. Especially for graphite anodes, lithium dendrites will continue to grow under this condition. Furthermore, as the lithium metal is exposed to the electrolyte, electrolyte degradation products (EDPs) will be produced, so that the lithium dendrites are covered with a thick lithium–electrolyte interface (LEI) [13]. This degradation process directly leads to capacity loss and gas formation. Also, localized defects can lead to local lithium deposition and dendrite growth, and the plating of lithium could trigger a short circuit by the penetration of a thin separator. In the worst case, dendrites can lead to catastrophic failure due to associate these internal short circuits and lead the LIB into the thermal runaway, as it happens in case of the Samsung Galaxy Note 7 [12,14].

Most of the previous work concerning local deformation of the separator and the resulting lithium plating was only carried out on half-cell experiments in the laboratory. The separator was locally deformed before installation in the half-cell causing locally closed separator pores, which lead to uneven ion currents [2,4,5]. There is no work available that has investigated local induced inhomogeneities in commercial pouch cells. Therefore, in this work, local pressure is applied to a commercial high energy pouch cell manufactured by Kokam SLPB526495, and a local deformation applied to the LIB. A stainless-steel sphere used as a force transmitter, which leads to radial inhomogeneous pressure distribution. The LIB cycled under this mechanical load. A post-mortem analysis (PMA) of the cell stack and the active and passive materials was then performed to analyze inhomogeneities in state of charge, surface layer formation, and particle morphology.

#### **2. Materials and Methods**

#### *2.1. Investigated Lithium-Ion Battery*

For the experiment, a LIB of the manufacturer Kokam SLPB526495 with a capacity of 3.3 Ah is used. The LIB is specified for a charging current of 2C and a voltage range of 2.7 V to 4.2 V between 0 to 45 ◦C. According to the manufacturer, the cell is made of a graphite anode, a Li(NiCo)O2 cathode, and an EC/EMC mixture with LiPFO6 as an electrolyte. Figure 1 shows the mechanical structure of the LIB which consists of two electrode stacks connected in parallel. The first stack consists of six double-sided coated anodes, five double-sided coated cathodes, and one one-sided coated cathode. The second stack consists of one additional double-sided cathode. Each stack individually wrapped in a Z-shaped separator.

**Figure 1.** Dimensions (in mm) and internal construction of the Kokam SLP526495 cell.

## *2.2. Experimental Setup*

For examination and cycling of the lithium-ion battery, the cell test system Digatron MCFT 20-05-50ME with a current measurement precision of 0.2% is used. The measurement setup during cell cycling depicted in Figure 2a. In order to exert inhomogenous pressure, a stainless-steel sphere with a diameter of 4 mm placed on the center of the cell surface. A quasi-statically force is applied to the sphere via a hydraulic press so that the cell deforms—the applied force is measured with a load cell placed between the sphere and the hydraulic press. The applied pressure was chosen based on the analysis of Wang et al. and Tran et al. to apply as much force as possible without damaging the active material particles. Wang et al. suggests a pressure of 10 to 20 MPa as optimal pressure regarding the compression and contacting of graphite particles during the calendaring process of anodes after coating [15]. According to Tran et al., the pressure during calendaring of NCA cathodes is significantly higher and optimal at 500 to 692 MPa [16] . Therefore, a pressure of 20 MPa was chosen for the experiments.

The applied force by the press in order exert this pressure is calculate based on the formula for Hertzian pressure for a sphere-plane configuration. For this calculation, the Young's modulus of the LIB must be known. As this quantity varies with cell type and not stated in the cell's datasheet, different forces were applied to the cell, and the penetration depth was measured using a dial gauge. After this series of tests, a force of 1.4 kN with a penetration depth of 0.8 mm was determined to achieve a maximum pressure of 20 MPa in the center of the area under the sphere. Assuming the LIB to be an elastic body, Young's modulus can be approximated to be 210 MPa. Nevertheless, since the active materials in LIB are porous and consist of multi-particle components, the force will be distributed over the particles. Therefore, the maximum local pressure could be much higher due to the tiny contact areas of the complex porous particle system. By applying the pressure, defects induced in the materials beneath the sphere. As the pressure decreases towards the edge, Figure 2b–d shows a transition area created where the porosity increases and therefore creates an unevenly increasing ion flux.

Another essential effect of particles in LIBs is the swelling during the lithiation process, and this adds additional local pressure on the contact areas. Also, the deposition of lithium metal will lead to an increase in the local pressure due to thickness change and stress the local particles furthermore.

**Figure 2.** Setup and principle of the experiment (**a**) Li-cell with stainless-steel sphere and load cell (**b**) non-deformed state (homogeneous ionic current distribution yellow arrows) (**c**) deformed state due to inhomogeneous induced pressure distribution (**d**) deformed state with inhomogeneous flow field of ionic current density.

To analyze the cell aging under the above-described inhomogeneous mechanical pressure, the tested cell then cycled several times. Before cycling, the pressure adjusted to 1.3 kN at a cell voltage of 3.55 V and a temperature of 20 ◦C. After the application of the pressure, the cell relaxed for 30 min and was then charged with 1C to 4.15 V until the force increased to 1.4 kN. The force increases due to the swelling of the graphite anode particles during the charging process [17]. The LIB has then cycled in CC/CV mode three times with a current of 1C between 4.2 V and 2.7 V. The CV-phase takes 30 min in each cycle. During cycling under the hydraulic press, the force fluctuated by a maximum of 150 N. Finally, the cell was fully charged with a current of 1C to 4.2 V and then opened for a post-mortem analysis.

To evaluate the influence of inhomogeneous pressure during the operation of a commercial, two stacked pouch, cell one cycled, and one uncycled were analyzed in this experiment. Subsequently, the surface of carefully selected areas analyzed by scanning electron microscopy analysis (SEM). The electron microscope used was a Leo Supra 35 VP from Carl Zeiss AG with an INCA Energy 200 EDS detector from Oxford Instruments. Best images were recorded using an in-lens BSE detector in a working distance of 7 mm at a high vacuum (10 to 6 mbar) with an acceleration voltage of 5 kV. EDS spot measurements or mappings were carried out at 10 kV. The uncycled cell was opened in a pristine state and acted as a reference cell. Individual dry layers of the anode and cathodes were pressurized with a homogeneous pressure of 30 MPa to have a comparison to the inhomogeneous pressure effects on the electrodes of the LIB.

The cycled LIB was opened fully charged at 4.2 V under argon atmosphere in a glove box, as in the fully charged state, inhomogeneities within the cell can be better identified. A particular property of graphite is a color change depending on the lithiation, i.e., the state of charge. While completely delithiated graphite that corresponds to an SOC of 0% is black or gray and between a lithiation state 0% to 30% the color is very similar to that of the pristine graphite (gray-black), it discolors with increasing lithiation over dark blue (30 to 50% SOC) and red (50 to 90% SOC) to golden (90 to 100% SOC) [18,19]. This property makes it easy to determine the lithiation and thus SOC inhomogeneities of the opened cell.

The cathode electrode compositions are measured with Varian 725 induced coupled plasma-optical emission spectrometer (ICP-OES) (Agilent, Santa Clare, United States of America). For this, a disc of the double-coated electrode with a diameter of 20 mm is taken. This sample is washed with Dimethylcarbonat (Dimethylcarbonat Msynth plus, Merck KGaA, Darmstadt, Germany) before it is dissolved in aqua regia. The solution was filled with distilled water until a 100 ml solution was obtained. This solution was analyzed with the ICP-OES.
