(Invited) Understanding Fast Charge Behavior and Limitations of Anodes and Cathodes for Hybrid Ion Capacitors and Libs

Abstract

Sustained fast charge behavior is essential for many next-generation LIB applications as well as for emerging hybrid ion capacitors (HICs) that offer energies up to four times higher that EDLC systems. Fast charging anodes for both LIBs and HICs are often based on advanced disordered carbons where orderly ion intercalation is minimized while ion adsorption at bulk and surface sites is enhanced. This presentation covers several recent embodiments of such microstructures including crumpled activated graphene and low surface area nanoboxes. It is demonstrated that as long as the volume changes associated with charging are minimized, the structures are relatively stable over tens of thousands and even hundreds of thousands of fast charge cycles. A generally similar carbon structure design approach is effective for lithium ion capacitors (LICs), sodium ion capacitors (NICs) and potassium ion capacitors (KICs). With all three ions, a solid electrolyte interphase (SEI) can be stable as long as the carbons don't exfoliate or are otherwise structurally damaged. Attention then turns to the cathode, where the materials options are more limited. High surface area heteroatom-rich surface-adsorption carbons do work but offer very limited reversible capacity at high voltages. Their triangular discharge profile also substantially cuts into the device's energy, making it in effect a supercapacitor but with sub-par cyclability. To achieve reasonable fast charge capacity and voltage characteristics, carbon-coated lithium iron phosphate (LiFePO4) remains one of the best choices. However, the energy of LFP degrades with extended cycling, with the capacity-voltage fade mechanism not being understood. This presentation provides the first atomic-scale description of the fade process as well as the role of pyrrole coating in mitigating it. Cycling causes near-surface (∼ 30 nm) amorphization of the Olivine crystal structure, with isolated amorphous regions also being present deeper in the bulk crystal. Within this amorphous shell, some of the Fe2+ is transformed into Fe3+. Simulations predict that amorphization significantly impedes ion diffusion in LiFePO4 and even more severely in FePO4. A pyrrole coating suppresses the dissolution of Fe and allows for extended retention of the Olivine structure. It also reduces the level of crossover of iron to the metal anode and stabilizes its solid electrolyte interphase, thus also contributing to the half-cell cycling stability.