Simulation of Hybrid All-Solid-State Battery Performance Under Consideration of Ceramic-Polymer Phase Boundaries Using a Physicochemical Modelling Approach
Authors: Scheller Maximilian , Durdel Axel , Kriegler Johannes , Frank Alexander , Jossen Andreas 
Published: September 2023

With the growing interest in all-solid-state battery (ASSB) technology for high-energy and high-power applications, the electrochemical performance of cell components and production-related characteristics must be improved to achieve reliable and cost-effective scale-up of laboratory cell concepts [1]. Combining inorganic ceramic and polymer solid electrolytes (SEs) serves to tune the ionic transport and the mechanical properties of composite cathodes [2, 3]. A polymer electrolyte share in the composite cathode is expected to improve the mechanical contact between the cathode active material (CAM) and the SE, resulting in facilitated charge transfer and improved cell performance. Furthermore, polymer SEs serve to overcome the challenges of co-sintering dense composites of CAM and inorganic SE [4, 5].

In hybrid cell concepts with polymer- and inorganic ceramic SE, the ionic transport path crosses a ceramic-polymer phase boundary, which leads to additional polarization through charge transfer and ohmic resistance. State-of-the-art literature discusses the influence of ceramic particles in polymer electrolytes, but falls short on the impact of ceramic-polymer phase boundaries on the total cell performance of ASSBs [6, 7, 8].

To allow for the simulation of hybrid full cells, a pseudo-two-dimensional (p2D) physicochemical model is introduced within this work. The model is based on the Newman approach, modified with a description for the ceramic-polymer phase boundary [9]. In equilibrium state, the potential drop across the ceramic-polymer boundary was modeled with the Donnan-potential condition as recently proposed by Kim et al. [10]. Under current flow, additional polarization occurs at the interface due to electrochemical charge transfer, which was modeled by the Butler-Volmer equation, and ohmic contact resistance. The conservation of mass and charge over the phase boundary was secured with a current, flux, and a potential boundary condition. A model cell was defined (see figure 1) and parametrized using material-specific values for the ionic and electronic transport characteristics as well as the interface and charge transfer properties. Since the focus of this study was on the ceramic-polymer phase boundary, ideal plating and stripping behavior at the Li-metal anode were assumed neglecting irregular lithium deposition, e.g., lithium dendrite formation. As separator material, the well-known ceramic LLZO SE was modeled. A composite cathode with NMC-622 as CAM and PEO/LiTFSI as polymer electrolyte was assumed. The lower part of figure 1 shows the reduced 1D-model geometry and the ion transport mechanisms in each model domain.

The established physicochemical model was applied to identify performance-limiting effects in hybrid ASSBs to conclude on cell designs achieving high energy and power densities.

To quantify and localize the polarization contributions in each domain, arising from SE ionic conductivity, diffusion or charge-transfer processes, as well as phase boundaries, the method proposed by Nyman et al. was used [11]. Figure 2 a) shows the results of polarization analysis when simulating a 0.1C charge, while figure 2 b) depicts the results for a 1C charge. Diffusion limitation in the polymer electrolyte led to high concentration gradients in the polymer-phase of the composite cathode, resulting in high diffusion polarization at elevated charging rates as shown in figure 2 b). This determined a critical current density at cell level, which was caused by large Li-ion concentration gradients and a possible depletion of Li-ions near the ceramic separator.

The overall cell polarization was further enhanced by the ceramic-polymer phase boundary. For the contact case of LLZO versus PEO/LiTFSI considered here, the equilibrium potential between the phases was calculated according to the theory of Donnan to 31 mV.

Since a wide range of values for the contact resistance at the ceramic-polymer interface is reported in the literature [6, 7], ohmic polarization could be important and was therefore evaluated as a function of different contact resistances. A critical contact resistance was determined to achieve the requirements for future battery technologies regarding energy and power density.

Figure 1