Pre-Doping Strategies of Graphite Negative Electrode in Lithium-Ion Capacitors

Abstract

Lithium-ion capacitors (LICs) are intermediate devices between lithium-ion batteries (LIBs) and electrical double-layer capacitors (EDLCs).1,2,3 Their design consists of an activated carbon positive electrode that stores charges by capacitive behavior, while lithium cations are inserted in the graphite negative electrode. Thanks to their high cell voltage as well as the capacity of their electrodes, they can address quite a wide range of applications. Indeed, their energy density approaches that of lead-acid batteries, and their power density is close to that of EDLCs. However, the interest in such hybrid design is mitigated by the need of an initial prelithiation step of the graphite anode which is mandatory to further operate the LIC device. This step is usually performed using a sacrificial metallic lithium foil.3 More recently, alternative strategies have been proposed such as taking advantage of specific charging profiles on the first cycle that provide lithium cations from the electrolyte4, or using lithiated compounds that serve on the first charging cycle as the lithium cations source5,6. All these strategies introduce in the device many drawbacks including potential hazards, difficult manufacturing process, or/and high CO2 footprint. In this communication, we will present strategies that take advantage of low CO2 footprint material. Lithiated organic molecules have been recently envisioned as electrode materials for all-organic Li-ion batteries.7 Some of these molecules do not show any cycling ability and only provide lithium cations in an irreversible manner during the first charge. This drawback can be turned into an advantage in LICs where sacrificial compounds used for the prelithiation must be the lithium cations source but only during the first charge. During the subsequent cycles, the delithiated compounds must remain electrochemically inactive. Subsequently, our strategy consists in the use of a lithiated organic material, namely 3,4–dihydroxybenzonitrile dilithium salt (Li2DHBN) that is mixed with activated carbon at the positive electrode of the LIC, and which can irreversibly provide lithium cations to the graphite electrode during an initial in-situ charging step without any negative effects with respect to further operation of the LIC.8 It paves the way toward simpler and safer LICs that are easily recyclable.