Electrochemical Performance of Nanosized Disordered LiVOPO₄

Abstract

ε-LiVOPO 4 is a promising multielectron cathode material for Li-ion batteries that can accommodate two electrons per vanadium, leading to higher energy densities. However, poor electronic conductivity and low lithium ion diffusivity currently result in low rate capability and poor cycle life. To enhance the electrochemical performance of ε-LiVOPO 4 , in this work, we optimized its solid-state synthesis route using in situ synchrotron X-ray diffraction and applied a combination of high-energy ball-milling with electronically and ionically conductive coatings aiming to improve bulk and surface Li diffusion. We show that high-energy ball-milling, while reducing the particle size also introduces structural disorder, as evidenced by 7 Li and 31 P NMR and X-ray absorption spectroscopy. We also show that a combination of electronically and ionically conductive coatings helps to utilize close to theoretical capacity for ε-LiVOPO 4 at C/50 (1 C = 153 mA h g –1 ) and to enhance rate performance and capacity retention. The optimized ε-LiVOPO 4 /Li 3 VO 4 /acetylene black composite yields the high cycling capacity of 250 mA h g –1 at C/5 for over 70 cycles.

Vanadyl Phosphates AxVOPO4 (A = Li, Na, K) as Multielectron Cathodes for Alkali‐Ion Batteries

“…[ 10a ] It should be noticed that both Bianchini's and Lin's works used LiVOPO 4 synthesized by solid‐state method and subjected to high‐energy ball‐milling to reduce particle size, which introduces structural disorder as will be discussed later. [ 20 ] Wangoh et al., using a combination of hard and soft X‐ray photoelectron and absorption spectroscopy techniques, have shown that uniform Li distribution in ε‐LiVOPO 4 is a prerequisite to the formation of Li 1.5 and Li 1.75 VOPO 4 phases. Also, it ensures uniform second Li intercalation to form the Li 2 VOPO 4 .…”

“…There are several methods for synthesizing LiVOPO 4 , the most commonly used ones being the solvothermal (ST), [ 7b,18c,31 ] sol–gel (SG), [ 18b,26,32 ] solid‐state (SS) [ 10a,19,20,30,33 ] methods, or by using a (Li)VOPO 4 ⋅2H 2 O precursor. [ 7a,17,34 ] We placed general information on advantages of each of these approaches in the Supporting Information, and will focus here on the application of these methods to LiVOPO 4 synthesis.…”

“…A comparison of the unit cell volumes of each LiVOPO 4 polymorph from various studies. [ 7a,b,17,18b,c,20,31a,d,32d,e,33a,34 ] …”

“…[ 26a ] They show that the high‐crystalline β‐LiVOPO 4 has better electrochemical performance than the low‐crystalline β‐LiVOPO 4 , improving the capacity from 75 to 95 mAh g −1 (C/50) within the voltage range of 3.0–4.5 V. This suggests that crystallinity plays an important factor in the electrochemical performance of LiVOPO 4 . This is important because HEBM reduces the particle size while also reducing the crystallinity, [ 20,32d ] and its implications will be discussed in Section 5.4. In a later publication, Azmi et al.…”

“…provides the mechanisms involved in the SS method for synthesizing LiVOPO 4 . [ 20 ] Through in situ XRD with heating in Ar, they show that LiVOPO 4 does not form until ≈650 °C, with good crystallinity and purity not being achieved until 750–800 °C. This temperature is above the β → ε transition temperature, meaning the only phase possible using the SS method is ε‐LiVOPO 4 .…”