Increasing the Pressure‐Free Stripping Capacity of the Lithium Metal Anode in Solid‐State‐Batteries by Carbon Nanotubes

Abstract

Lithium metal is the key anode material for solid‐state‐batteries as its successful implementation will drastically increase their energy and power densities. However, anode contact loss during stripping leads to dendrites upon plating and subsequent cell failure. Design strategies to mitigate these issues are crucial to enable the use of lithium metal anodes. This paper reports the dissolution kinetics of composite anodes made of lithium metal and carbon nanotubes (CNTs) with a garnet‐type solid electrolyte (SE). In addition to an enhancement of the effective diffusion within the anode, its dissolution is fundamentally changed from being 2D to 3D. By maintaining contact with the SE, the CNTs facilitate lithium transport to the interface, which yields more than 20 mAh cm−2 discharge capacity at 100 µA cm−2 without the application of external stack pressure (>1 MPa). Conclusions drawn from electrochemical data on the anode microstructure are validated using cryo‐focused‐ion‐beam scanning electron microscopy and correlated with the mechanical properties. Micro‐indentation, acoustic analysis, and stress–strain testing show that mechanical properties of the anode, like yield strength and hardness, are adjustable. Overall, it is shown that the mechanical and electrochemical properties of Li–CNT composite electrodes can be tailored to suit the requirements of a practical cell.

3D Impedance Modeling of Metal Anodes in Solid-State Batteries–Incompatibility of Pore Formation and Constriction Effect in Physical-Based 1D Circuit Models

“…15,17 This indicates an ordering of time constants according to τ Bulk < τ GB < τ Cstr . In this sense, the qualitative results of the corresponding theoretical case study are comparable with the experimental observations, e.g., during the dissolution (stripping) or deposition (plating) of the lithium metal anode with LLZO as SE 15,40,41 The results for τ Bulk < τ Cstr < τ GB are also of great relevance, since the constriction effect strongly depends on the interface morphology, especially on the pore depth (distribution). Here, the effect of deep pores can occur at quite high frequencies, which may lead to an influence on the macroscopic GB impedance contribution, e.g., due to an overlap of the corresponding impedance signals.…”

“…Therefore, the macroscopic transport quantities R GB and C GB are independent of the contact area A contact as depicted in the lower part of Figure a). Such a behavior is observed experimentally, e.g., during the dissolution (stripping) or deposition (plating) of the lithium metal anode with LLZO as SE. ,, …”

“…However, the qualitative trends derived in the respective case studies can also be observed for more realistic values of the individual time constants (i.e., overlapping impedance contributions). Typical experimental impedance data of reversible metal anodes consists of three (well-) separated contributions. , These are usually assigned to bulk, GB and interface with decreasing frequency, respectively. The origin of “ the interface impedance ” is often not entirely clear in experiments.…”

“…Such a behavior is observed experimentally, e.g., during the dissolution (stripping) or deposition (plating) of the lithium metal anode with LLZO as SE. 15,40,41 The transport behavior is different when the order of the time constants of constriction and GB is reversed in case 2, i.e., τ Cstr < τ GB , as can be seen in Figure 3b). In this case, the GB signal lies in a frequency range where dynamic constriction is present at the interface, i.e., the pores are insulating.…”

“…Typical experimental impedance data of reversible metal anodes consists of three (well-) separated contributions. 15,41 These are usually assigned to bulk, GB and interface with decreasing frequency, respectively. The origin of "the interface impedance" is often not entirely clear in experiments.…”