Area Oversizing of Lithium Metal Electrodes in Solid-State Batteries: Relevance for Overvoltage and thus Performance?

Invited for this month's cover is the group of Dr. Johannes Kasnatscheew from the Research Center Jülich GmbH. The image shows how area oversizing of lithium can affect the overall power of batteries, in particular at lower temperature. The Full Paper itself is available at 10.1002/cssc.202100213.

Area Oversizing of Lithium Metal Electrodes in Solid-State Batteries: Relevance for Overvoltage and thus Performance?

Systematic and systemic research and development of solid electrolytes for lithium batteries requires a reliable and reproducible benchmark cell system. Therefore, factors relevant for performance, such as temperature, voltage operation range, or specific current, should be defined and reported. However, performance can also be sensitive to apparently inconspicuous and overlooked factors, such as area oversizing of the lithium electrode and the solid electrolyte membrane (relative to the cathode area). In this study, area oversizing is found to diminish polarization and improves the performance in LiNi0.6 Mn0.2 Co0.2 O2 (NMC622)||Li cells, with a more pronounced effect under kinetically harsh conditions (e. g., low temperature and/or high current density). For validity reasons, the polarization behavior is also investigated in Li||Li symmetric cells. Given the mathematical conformity of the characteristic overvoltage behavior with the Sand's equation, the beneficial effect is attributed to lower depletion of Li ions at the electrode/electrolyte interface. In this regard, the highest possible effect of area oversizing on the performance is discussed, that is when the accompanied decrease in current density and overvoltage overcomes the Sand's threshold limit. This scenario entirely prevents the capacity decay attributable to Li+ depletion and is in line with the mathematically predicted values.

Kinetical threshold limits in solid-state lithium batteries: Data on practical relevance of sand equation.

The here shown data support the article "The Sand Equation and its Enormous Practical Relevance for Solid-State Lithium Metal Batteries". [1] In this data set, all cells include the poly (ethylene oxide)-based solid polymer electrolyte (PEO-based SPE). The behaviour in symmetric Li||Li cells are provided in a three-electrode cell setup, thus with the use of a reference electrode. Moreover, the Sand behaviour is reported for varied negative electrodes with the focus on polarization onset, defined as transition time. The data of the electrochemical response after the variation of additional parameter, i.e. SPE thicknesses, are shown, as well. The theoretical Sand equation is linked with practically obtained values also for varied Li salt concentration. Finally, the discharge behaviour is provided including further charge/discharge cycles with the use of LiNi0.6Mn0.2Co0.2O2 (NMC622) as active material for positive electrodes.

Conventional Electrolyte and Inactive Electrode Materials in Lithium-Ion Batteries: Determining Cumulative Impact of Oxidative Decomposition at High Voltage.

High-voltage electrodes based on, for example, LiNi0.5 Mn1.5 04 (LNMO) active material require oxidative stability of inactive materials up to 4.95 V vs. Li|Li+ . Referring to literature, they are frequently supposed to be unstable, though conclusions are still controversial and clearly depend on the used investigation method. For example, the galvanostatic method, as a common method in battery research, points to the opposite, thus to a stability of the inactive materials, which can be derived from, for example, the high decomposition plateau at 5.56 V vs. Li|Li+ and stable performance of the LNMO charge/discharge cycling. This work aims to unravel this apparent contradiction of the galvanostatic method with the literature by a thorough investigation of possible trace oxidation reactions in cumulative manner, that is, over many charge/discharge cycles. Indeed, the cumulated irreversible specific capacity amounts to ≈10 mAh g-1 during the initial 50 charge/discharge cycles, which is determined by imitating extreme LNMO high-voltage conditions using electrodes solely consisting of inactive materials. This can explain the ambiguities in stability interpretations of the galvanostatic method and the literature, as the respective irreversible specific capacity is obviously too low for distinct detection in conventional galvanostatic approaches and can be only detected at extreme high-voltage conditions. In this regard, the technique of chronoamperometry is shown to be an effective and proper complementary tool for electrochemical stability research in a qualitative and quantitative manner.

Elimination of "Voltage Noise" of Poly (Ethylene Oxide)-Based Solid Electrolytes in High-Voltage Lithium Batteries: Linear versus Network Polymers.

Frequently, poly(ethylene oxide) (PEO)-based solid polymer electrolytes (SPEs) reveal a failure with high-voltage electrodes, e.g. LiNi0.6Mn0.2Co0.2O2 in lithium metal batteries, which can be monitored as an arbitrary appearance of a "voltage noise" during charge and can be attributed to Li dendrite-induced cell micro short circuits. This failure behavior disappears when incorporating linear PEO-based SPE in a semi-interpenetrating network (s-IPN) and even enables an adequate charge/discharge cycling performance at 40°C. An impact of any electrolyte oxidation reactions on the performance difference can be excluded, as both SPEs reveal similar (high) bulk oxidation onset potentials of ≈4.6 V versus Li|Li+. Instead, improved mechanical properties of the SPE, as revealed by compression tests, are assumed to be determining, as they mechanically better withstand Li dendrite penetration and better maintain the distance of the two electrodes, both rendering cell shorts less likely.

Poly(Ethylene Oxide)-based Electrolyte for Solid-State-Lithium-Batteries with High Voltage Positive Electrodes: Evaluating the Role of Electrolyte Oxidation in Rapid Cell Failure.

Polyethylene oxide (PEO)-based solid polymer electrolytes (SPEs) typically reveal a sudden failure in Li metal cells particularly with high energy density/voltage positive electrodes, e.g. LiNi0.6Mn0.2Co0.2O2 (NMC622), which is visible in an arbitrary, time - and voltage independent, "voltage noise" during charge. A relation with SPE oxidation was evaluated, for validity reasons on different active materials in potentiodynamic and galvanostatic experiments. The results indicate an exponential current increase and a potential plateau at 4.6 V vs. Li|Li+, respectively, demonstrating that the main oxidation onset of the SPE is above the used working potential of NMC622 being < 4.3 V vs. Li|Li+. Obviously, the SPE│NMC622 interface is unlikely to be the primary source of the observed sudden failure indicated by the "voltage noise". Instead, our experiments indicate that the Li | SPE interface, and in particular, Li dendrite formation and penetration through the SPE membrane is the main source. This could be simply proven by increasing the SPE membrane thickness or by exchanging the Li metal negative electrode by graphite, which both revealed "voltage noise"-free operation. The effect of membrane thickness is also valid with LiFePO4 electrodes. In summary, it is the cell set-up (PEO thickness, negative electrode), which is crucial for the voltage-noise associated failure, and counterintuitively not a high potential of the positive electrode.