Design of Composite Electrodes with Anion-Absorbing Active Materials

Abstract

Organic radical polymers (ORP) and conjugated polymers provide exceptional rate capability as cathode materials for lithium-ion batteries, albeit low volumetric energy density. To optimize overall power and energy density, we consider a composite of ORP with oxide cathode. Upon charge (oxidation), ORP absorb anions from the electrolyte, which causes the salt content in the cell to decrease during charge. Even if the ORP is only 10 volume% of the cathode, the cell's salt content will decrease by >0.6 M in practical cell designs. Furthermore, the conductivity of typical electrolytes varies strongly with concentration. We use electrochemical modeling of a composite positive electrode containing both lithium-releasing and anion-absorbing active materials to explore the tradeoffs and design implications among salt concentration, porosity, and volume fraction of ORP. Organic-radical polymers (ORP) have demonstrated exceptionally high rate capability for lithium batteries, accepting over 80% of capacity at charge rates exceeding 1000 C.1,2 The high rate capability is achieved because the polymers form a gel with liquid electrolytes, which means they have high interfacial area with the electrolyte and polymer-phase transport lengths on the order of tens of nanometers. Diffusion therefore occurs predominantly in the liquid electrolyte, whose diffusivity is several orders of magnitude higher than the solidphase diffusivity which limits the capability of conventional oxide cathode materials.Many important applications, such as electric vehicles and portable power, highly value volumetric energy density and fast charge capability. While ORPs have been discovered with good gravimetric capacity (up to 150 mAh/g), they suffer from low density (∼1 g/cm 3 ). In contrast, oxide cathodes have high density (4.6 to 5.0 g/cm 3 ), but charging and discharging rates are limited by diffusion resistance. Therefore we consider a composite electrode comprised of ORP with an oxide cathode, e.g. LiCoO 2 (LCO). The goal of the composite is to combine the high energy density of LCO with the high rate capability of ORP to maximize the overall volumetric and gravimetric power and energy density. 4,5The reaction mechanism of a prototypical ORP, 2,2,6,6-tetramethylpiperidin-1-oxyl (PTMA), is shown in Figure 1. During charge, the nitroxide radical is oxidized and the charge is balanced by absorption of an anion, e.g. PF 6− , from the electrolyte. In contrast, traditional inorganic cathode materials release lithium cations into the electrolyte during charge. All known negative electrode materials for lithium-ion batteries absorb lithium cations during charge. If a lithium-absorbing cathode is paired with a lithium-releasing anode, then there is no net change in the salt content of the cell (although there will be gradients in salt composition as lithium ions are transported from the anode to the cathode). However, the salt content will decrease during charge of a cell with cation-absorbing negative electrode and anion-absorbing positive electrode. T...

Electrochemically Active Polymer Components in Next-Generation LiFePO4 Cathodes: Can Small Things Make a Big Difference?

“…Gravimetric capacities found for best polymers are in the same range as for traditional cathode materials for Li-ion batteries: as high as 223 mAh/g is reported for a phenazine-based polymer [28] and 230 mAh/g is reported for lithium emeraldine [29]. It is worth to note that while these materials are often proposed as the main active materials in Li-ion cells, they can properly function only in excessive electrolyte conditions because of the anion type of doping [30]. However, considering these materials as auxiliary LIB cathode components with the loading of a few wt% is viable even in industry-relevant cells that contain limited amount of electrolyte.…”

Assessing n‐type organic materials for lithium batteries: A techno‐economic review

“…Typically, n‐type materials have a lower average voltage, slower kinetics, and higher specific capacity compared with p‐type materials. The p‐type materials also behave differently from typical lithium‐ion battery electrodes due to the fundamental role of the electrolyte as a source of anions in the redox reaction, hence they are similar to lead‐acid battery electrodes 33–35 …”

“…The p-type materials also behave differently from typical lithium-ion battery electrodes due to the fundamental role of the electrolyte as a source of anions in the redox reaction, hence they are similar to lead-acid battery electrodes. [33][34][35] This review focuses on n-type materials, which have a redox mechanism analogous to that of lithium-ion cathodes and anodes, allowing for a more meaningful comparison. The n-type materials have the potential to offer an economical and sustainable solution for energy storage applications.…”