Study on low-temperature performances of Nb₁₆W₅O₅₅ anode for lithium-ion batteries

Highlights

• The performances of Nb₁₆W₅O₅₅ anode at various temperatures are studied.

• The charge-transfer impedance and Li⁺ diffusion coefficient are the key factors.

• The cyclic stability and structure are almost unaffected by operation temperatures.

Abstract

Niobium tungsten oxide Nb₁₆W₅O₅₅ is a competitive anode for lithium-ion batteries, while seldom research on low-temperature electrochemical properties. In this paper, Nb₁₆W₅O₅₅ is synthesized by solid phase reaction and the effects of operation temperature (25, 0 and −20 °C) on the coulombic efficiency, cycling stability, rate capacity and crystal structure are studied in detail. At 25 °C, Nb₁₆W₅O₅₅ electrode can give good cycling stability of 184 mAh g⁻¹ after 404 cycles and rate capacity of 101 mAh g⁻¹ at 4000 mA g⁻¹. By contrast, the electrochemical properties decrease obviously at subambient temperatures. Electrochemical impedance spectroscopy reveal that the sharp increase of the charge-transfer impedance and the decrease of the solid-phase Li⁺ diffusion coefficient are the essential factors leading to the deterioration of low temperature performances. However, the low temperature mainly affects specific capacities and rate performances, but has little effect on cyclic stability and crystal structure of Nb₁₆W₅O₅₅ electrode, indicated by ex-situ X-ray diffraction analysis. These results provide some new insights for the practical application of Nb₁₆W₅O₅₅ anode.

Introduction

At present, graphite is the main anode for commercial lithium-ion batteries (LIBs). The lithium intercalation potential of graphite, however, is similar to that of lithium metal plating, which may lead to the formation of lithium dendrites and safety concerns. Additionally, the low diffusion coefficient of Li⁺ in graphite inhibits the rapid Li⁺ migration under high current charging/discharging, resulting in unsatisfactory rate performances [[1], [2], [3], [4]]. As a strong competitor, Li₄Ti₅O₁₂ anode with high intercalation potential (~1.55 V) can effectively avoid the formation of lithium dendrites and greatly improve battery safety. Moreover, its spinel structure can ensure the efficient Li⁺ transportation, and has zero-strain characteristics, thus obtaining excellent cycle stability and rate performance. Nevertheless, the lower theoretical capacity (175 mAh g⁻¹) restricts the large-scale application of Li₄Ti₅O₁₂ anode [[5], [6], [7], [8], [9]]. In order to solve these problems, the development of new anode materials is necessary. Recently, tungsten niobium oxides, such as Nb₁₄W₃O₄₄ [10], Nb₄W₇O₃₁ [11], Nb₈W₉O₄₇ [12], Nb₆₀WO₁₅₃ [13], Nb₁₆W₅O₅₅ [14], Nb₁₈W₁₆O₉₃ [15], Nb₁₂WO₃₃ [16] and Nb₁₂W₁₁O₆₃ [17], have attracted extensive attention. Niobium tungsten oxides deliver a high intercalation potential (1.57– 1.70 V) similar to Li₄Ti₅O₁₂ and more redox electron pairs (Nb⁵⁺/Nb⁴⁺, Nb⁴⁺/Nb³⁺, W⁶⁺/W⁵⁺ and W⁵⁺/W⁴⁺) during the cycles, demonstrating an improved safety and higher specific capacity.

Among the niobium tungsten oxide anode materials, Nb₁₆W₅O₅₅ reported by Grey’ group exhibits outstanding performances with high energy density, enhanced structural stability and convenient synthesis [14]. Nb₁₆W₅O₅₅ belongs to monoclinic structure, consisting of corner-shared NbO₆ octahedron and WO₆ octahedron arrange into ReO₃ configuration, and Nb/W atoms in the center of octahedron are arranged randomly. Each layer plane of Nb₁₆W₅O₅₅ is composed of a large number of shear plane units, which contains 4 (wide) × 5 (long) MO₆ (M = Nb, W) octahedrons. The layered structure formed between the shear planes is beneficial to lithium ion storage and transportation. During the lithium intercalation, Nb₁₆W₅O₅₅ can produce four redox reactions with theoretical capacity of 343 mAh g⁻¹ and structural stability of only 5.5% expansion. As the voltage range is 1.0– 2.5 V, the initial reversible capacity and coulomb efficiency is 226 mAh g⁻¹ and 98%, respectively. Even the current density increases to 6860 mA g⁻¹, the reversible capacity can be maintained 85 mAh g⁻¹, which clearly surpasses that of Li₄Ti₅O₁₂ [[7], [8], [9]]. Though Nb₁₆W₅O₅₅ is inferior to graphite anode in terms of gravimetric capacity and voltage platform, the characteristics of high tap density prompt Nb₁₆W₅O₅₅ (2.61 g cm⁻³ vs 0.72 g cm⁻³ of graphite [18]) to exhibit an overwhelming volumetric energy density at high current density, indicating a broad prospect for Nb₁₆W₅O₅₅ in the development of fast-charging power LIBs.

In addition to the excellent electrochemical performances of Nb₁₆W₅O₅₅ anode at room temperature, its low temperature performance should also perform well, however, which has received relatively little attention. Herein, Nb₁₆W₅O₅₅ anode is prepared by solid phase reaction and the characteristic properties are analyzed by X-ray diffraction (XRD), Raman spectrum, scanning electron microscopy (SEM), X-ray photoelectron spectroscopy (XPS), and inductively coupled plasma (ICP-AES). Nb₁₆W₅O₅₅ anode exhibits high initial coulombic efficiency (92.6%), improved cycling stability (184 mAh g⁻¹ after 404 cycles) and rate performance (101 mAh g⁻¹ at 4000 mA g⁻¹) at 25 °C. Moreover, the low-temperature (0 and − 20 °C) behaviors of Nb₁₆W₅O₅₅ anode are studied in detail. It delivers the disappointing performances with the specific capacities of 38 and 18 mAh g⁻¹ at 4000 mA g⁻¹ under 0 and − 20 °C, respectively, which is mainly due to the high charge-transfer impedance and low Li⁺ diffusion coefficient. Except for the lower capacity, however, Nb₁₆W₅O₅₅ anode displays consistent cyclic (retains 92.8% capacity after 404 cycles at 0 °C) and structural stability (no phase changed) even at −20 °C. These results need to be considered in the practical application of Nb₁₆W₅O₅₅ anode for LIBs.