Capacitive vs Faradaic Energy Storage in a Hybrid Cell with LiFePO₄/RGO Positive Electrode and Nanocarbon Negative Electrode

Abstract

We report an advanced device based on a Nitrogen-doped Carbon Nanopipes (N-CNP) negative electrode and a lithium iron phosphate (LiFePO 4 ) positive electrode. We carefully balanced the cell composition (charge balance) and suppressed the initial irreversible capacity of the anode in the round of few cycles. We demonstrated an optimal performance in terms of specific capacity 170 mAh/g of LiFePO 4 with energy density of about 203 Wh kg −1 and a stable operation for over 100 charge−discharge cycles. Lithium ion batteries (LIBs) are presently struggling to meet very demanding standards in terms of cost, charge/discharge rate, power and energy densities, and safety in order to enter new emerging markets such as those of electric vehicles and the storage of renewable electrical energy. 1Batteries exhibit relatively high energy densities as a result of faradaic reactions in the bulk of active particles, but are rate-limited. A remarkable example of a well-known material that undergoes a redox process during the battery charge and discharge process is Lithium iron phosphate (LiFePO 4 ). After two decades of the seminal work by Goodenough and col., 2 LiFePO 4 has been established as one of the most widely used positive electrode materials for LIBs thanks to its low cost, abundant raw materials, safety, low toxicity, structural stability and excellent electrochemical properties. The active material can be reversibly charged and discharged with a stable voltage of 3.45 V vs Li + /Li with a very small change in unit cell parameters during the LiFePO 4 /FePO 4 phase transition. On the other hand, for the development of high power batteries based on this material, it is essential to understand and overcome the factors limiting lithium transport through the electrode. Indeed, despite its relatively high theoretical specific capacity (170 mAh/g) and long cycling life, the high-rate performance of pure LiFePO 4 is restricted by its poor electronic conductivity (10 −9 S/cm) and slow lithium diffusion. 3 Many different approaches involving surface coating have been tried to improve the capacity and rate performance of LiFePO 4 as cathode for LIBs. Increasing the conductivity by coating the LiFePO 4 surface with carbon 4,5 or conducting polymers 6,7 have been two of the most popular. In comparison to these carbon materials, graphene can offer an improved interfacial contact because of its superior conductivity, flexible two-dimensional structure, and high surface area. 8 However, due to the thermodynamic instable structure of graphene, it tends to aggregate. Among various graphene derivatives, reduced Graphene Oxide is more polar and hydrophilic than graphene and better conducting that Graphene Oxide and therefore constitutes an optimal choice among graphenes. A critical issue of LIBs technology is the relatively low theoretical specific capacity of conventional graphite anodes, limited to 372 mAh/g. 9 * Electrochemical Society Student Member. z E-mail: pedro.gomez@icn2.cat Electrochemical capacitors 10 can deliver high power at ...

Polypyrrole Nanopipes as a Promising Cathode Material for Li‐ion Batteries and Li‐ion Capacitors: Two‐in‐One Approach

“…The present value of reversible capacity is quite comparable to the recently reported capacities for carbon-based anodes. [37,38,39] In addition, the N-CNPipes electrode exhibited good cycling stability over 500 cycles measured at 0.1 A/g ( Figure S6).…”