Iron Phosphate Coated Flexible Carbon Nanotube Fabric as a Multifunctional Cathode for Na‐Ion Batteries

Abstract

Conventional slurry casted electrodes cannot stand high loads or be repeatedly flexed or bent without being fractured, which inhibits their use in flexible batteries. Carbon nanotube (CNT) fabric exhibits a paramount mechanical stability and, due to its porosity, can additionally accommodate an active material within its structure. While solution-based protocols cannot achieve conformal coatings of active materials, chemical vapor deposition (CVD) gives a unique opportunity to control and vary the thickness and homogeneity of the coating. Herein, a conformal CVD coating of amorphous iron (III) phosphate (a-FePO , FP) on flexible CNT fabric and its ability to reversibly accommodate large radius Na ions is reported. The freestanding binder-free CNT@FP electrodes exhibit high-rate capabilities and exceptional cycle stabilities with 98% of retention of initial capacity after 100 cycles. Such electrodes additionally demonstrate high mechanical stability under high loads, remarkable bending characteristics, and modulus of toughness (12 MJ m ) exceeding that of Al. The presented concept of flexible CNT@FePO electrodes with high load-bearing characteristics opens new perspectives toward the formation of light-weight, flexible, multifunctional Na-ion battery electrodes based on abundant materials.

Flexible Nanofiber‐Reinforced Solid Polymer Lithium‐Ion Battery

“…At the higher sweep rates, some degree of polarization is seen that is expected for the amorphous FePO 4 , unlike typical redox peaks of crystalline forms of LiFePO 4 . In addition, the amorphous nature of FePO 4 is likely to be responsible for the broad appearance of the redox peaks, which was discussed earlier in the work . However, one may assume a possible contribution of the presence of the interfacial barrier separating PEO‐Li + and CNT@FP phases to the peak broadening .…”

“…Although battery current collectors such as copper and aluminum exhibit relatively large ultimate strength, they suffer from significant fatigue upon repeated mechanical deformation . Conversely, CNT‐based electrodes can exhibit good electrical conductivity and serve as a mechanical support for the deposition of the conformal layers of various cathode materials capable of Li‐ and Na‐ion intercalations, which do not suffer from mechanical fatigue under strain …”

“…The conformal deposition of iron (III) phosphate (FePO 4 , FP) coatings over mechanically strong and conductive CNT fabric was used in this work as the nanofiber‐based cathode for subsequent infiltration of the polymer electrolyte. The infiltration of the PEO‐LiTFSI electrolyte into the CNT@FP fabric was conducted by means of the vacuum infiltration technique.…”

“…For many demanding applications, energy storage devices should ideally be able to adjust to a variety of shapes, forms, and functions, while exhibiting excellent mechanical and electrochemical properties . However, the currently practically achievable levels of energy density (≈600 Wh L −1 ) and specific energy (≈220 Wh kg −1 ) in the conventional pouch or prismatic LIBs decrease dramatically when a battery cell is manufactured to be flexed or bent or to carry a mechanical load . Excellent visualization of specific energy of batteries as a function of their mechanical properties such as specific strength or stiffness can be seen in the work of Thomas and Qidwai .…”

“…Our recent discovery of an ultra‐low‐cost route to produce strong and stiff Al 2 O 3 nanowires has allowed the fabrication of a flexible, mechanically strong, and thermally resistant 100% ceramic separator. In addition, the utilization of the conducting carbon nanotube (CNT) support with the deposited conformal layers of cathode or anode materials has opened the path to the mechanically strong (up to ≈100 MPa or above) and tough (strain up to ≈30%) LIB electrode fabrics to sustain considerable mechanical loads …”