Highly Stable Three Lithium Insertion in Thin V₂O₅ Shells on Vertically Aligned Carbon Nanofiber Arrays for Ultrahigh‐Capacity Lithium Ion Battery Cathodes

Abstract

factor limiting the energy storage capability of LIBs. [5] For example, in the most common commercial LIBs, a near theoretical specific capacity of 372 mAh g −1 can be achieved with the graphite anode while only ≈140 mAh g −1 specific capacity with LiCoO 2 cathode. [5] Hence, the current LIB research has put an extensive emphasis on improving the charge storage capacity and power delivery capability of the cathode materials. [6,7] Vanadium pentoxide (V 2 O 5 ) is regarded as a promising cathode material owing to its high theoretical capacity due to multiple Li + insertion/extraction reactions in addition to the low cost, nontoxic chemical properties, high electrode potential in lithium-extracted state (up to 4.0 V), and easily accessible layered structure for Li + ion insertion. [8] Theoretically, crystalline V 2 O 5 can achieve a reversible specific capacity of 294 mAh g −1 (for a two Li + / V 2 O 5 insertion process) and an irreversible capacity of 441 mAh g −1 (for a 3 Li + /V 2 O 5 insertion process), [9] which are significantly higher than today's commercial cathode materials such as LiCoO 2 (140 mAh g −1 ), LiMn 2 O 4 (148 mAh g −1 ), and LiFePO 4 (170 mAh g −1 ) [10] that are limited to only one Li + insertion/extraction. A few reports have even suggested that amorphous V 2 O 5 in supercritically dried xerogels and aerogels can achieve extremely high capacities up to 560 and 650 mAh g −1 , respectively (equivalent to 4 and 5.8 Li + insertion/extraction per V 2 O 5 ), owing to their highly porous network, high surface area, and short Li + diffusion paths. [11,12] Unfortunately, these materials suffer from poor mechanical/ chemical stability and rapid capacity fading, making it difficult to realize the predicted high capacity in practical LIBs.Achieving reliable capacity with V 2 O 5 cathodes close to the theoretical value has been a challenge due to the small Li + ion diffusion coefficients (≈10 −15 to 10 −12 cm 2 s −1 ) and low electrical conductivities (≈10 −3 to 10 −5 S cm −1 ) in this material, which ultimately hinder the performance of V 2 O 5 cathodes in practical EES devices. [13,14] Also, it is well known that the Li + intercalation process in crystalline V 2 O 5 is accompanied by multiple phase transitions. Trace amounts of Li + intercalation result in α-Li x V 2 O 5 (x < 0.01) structure, which is transformed into ε-Li x V 2 O 5 (0.35 < x < 0.7) after further lithiation. Insertion of exactly one Li + leads to the formation of δ-phase Li x V 2 O 5 (x = 1). Further lithiation converts the δ-phase to γ-Li x V 2 O 5 (1 < x < 2).Here the authors demonstrate an approach to achieving stable 3 Li + insertion into vanadium pentoxide (V 2 O 5 ) by implementing a 3D core-shell structure consisting of coaxial V 2 O 5 shells sputter-coated on vertically aligned carbon nanofiber cores. The hydrated amorphous microporous structure in the "asdeposited" V 2 O 5 shells and the particulated nanocrystalline V 2 O 5 structure formed by thermal annealing are compared. The former provides remarkably high capacities of 36...

Disordered Bilayered V2O5 ⋅ nH2O Shells Deposited on Vertically Aligned Carbon Nanofiber Arrays as Stable High‐Capacity Sodium Ion Battery Cathodes ⋅

“…This assumption is consistent with a report by Marx et al that described Raman peaks in the range 750–950 cm −1 corresponding to short‐range order vibrations of metastable V 2 O 5 . It should be noted that the hydration peak significantly decreases in intensity in the annealed structure with no other significant differences . The water content was not quantified in this work, however, the 11.0 Å d‐space indicates that it likely contains no more than one mole of H 2 O per mole of V 2 O 5 , i. e. with a chemical formula V 2 O 5 ⋅ n H 2 O (n ≤ 1) ,.…”

“…The nominal thickness of V 2 O 5 deposition was measured to be 425 nm using a flat Si substrate as reference (see Figure S3) . From this, the mass of V 2 O 5 deposited on each 2.40 cm 2 VACNF disk electrode can be estimated to be 0.343 mg based on the density of bulk V 2 O 5 (3.36 g cm −3 ) (see detailed procedure in SI following the same methods in our previous papers) ,,. To better assess the materials properties and compare them with the theoretical values, only the mass of V 2 O 5 is used for gravimetric capacity calculations in this study.…”

“…VACNFs offer a distinctive advantage over other carbon allotropes owing to their abundancy of graphitic edges along the sidewall, which provide an excellent electrochemical interface leading to strong interactions with shell materials . Previously we have reported that the as‐deposited amorphous V 2 O 5 shells have the ability to provide a stable capacity of 547 mAh g −1 as a LIB cathode in traditionally irreversible 3 Li + /V 2 O 5 processes, which is about 25 % higher than the theoretical value of crystalline V 2 O 5 cathodes . In addition, the core‐shell structure provided remarkable power rates and cycling stability.…”

Macroscopic‐Scale Three‐Dimensional Carbon Nanofiber Architectures for Electrochemical Energy Storage Devices

“…During charging, the process is reversed. [7] Because of its advantages such as a high energy density, a long lifespan, little self-discharge and no memory effects, [214,224,225] the LIB is currently recognized as one of the most popular power sources for portable electronic devices and electric vehicles. [226][227][228][229][230] It is believed that the 3D CNF nanostructures not only facilitate the rapid transport of electrons along the interconnected carbon network, but also provide multidimensional buffer space for relieving the volume changes during the lithiation/ delithiation process, thereby further enhancing Li + storage capacities and improving the life span.…”

Extended Limits of Reversible Electrochemical Lithiation of Crystalline V2O5

“…This phase can be cycled in 1.9–4.0 V range, however, few insertion/extraction cycles lead to complete amorphization. However, using of nanostructured and disordered materials (e. g. core‐shell structures with carbon fiber core, or V 2 O 5 anchored to carbon nanotubes), has recently enabled reasonably sustainable cycling of such electrodes in 1.5–4.0 V range.…”