Improving the electrochemical performance of lithium vanadium fluorophosphate cathode material: Focus on interfacial stability

Highlights

• Li₃PO₄ coating builds stable LiVPO₄F/electrolyte interface for the first time.

• LiVPO₄F coated with Li₃PO₄ shows enhanced cycle performance at 55 °C.

• Li₃PO₄ is proposed to act as a physical barrier as well as a Li⁺ transfer media.

Abstract

To improve the stability of LiVPO₄F electrode/electrolyte interface, Li₃PO₄ is used to modify LiVPO₄F composite (P-LVPF) for the first time. Morphological characterization shows that LiVPO₄F particles are wrapped by amorphous carbon and lithium ionic conductor Li₃PO₄ as the interlayer and outer layer, respectively. Compared to the pristine sample, the resultant P-LVPF exhibits greatly improved rate capability and elevated-temperature cycle performance when applied as the cathode material for lithium ion batteries. Specifically, the Li₃PO₄ modified sample specific capacity maintains 77.6% at 1 C after 100 cycles under 55 °C. Such improvement is attributed to the fact that the Li₃PO₄ coating layer not only acts as a good ionic conductor for LiVPO₄F, but also serves as a physical barrier between electrode and electrolyte which can build a stable interface.

Introduction

Lithium-ion batteries (LIBs) are regarded as one of the most promising power sources for transport applications [1], [2], [3], [4], [5]. Materials based on the phosphate polyanions have been extensively investigated as safe and durable cathodes for LIBs [6]. Among these, tavorite LiVPO₄F has been proposed as one of favorable alternatives for conventional oxide-based materials primarily because of its relatively high operating voltage (4.2 V) and excellent thermal stability [7], [8], [9], [10], [11], [12], [13]. However, LiVPO₄F suffers from poor cycling performance caused by its relatively low conductivity and unstable electrode/electrolyte interface [14], [15], [16]. Up to now, great efforts have been devoted to addressing this issue including the doping with metallic elements (Na [17], Mn [15], Al [18], [19], Ti [20], etc.) and coating with the conductive materials (graphene [21], carbon nanotube [22], polyaniline [23], and Ag [24], etc.). Nevertheless, to the best of our knowledge, few attentions are directly paid to the chemistry and stability of LiVPO₄F/electrolyte interface [16], [25], [26], which is closely related to the cycling performance of high-voltage materials especially at elevated temperature. Therefore, it is of great importance to improve the interface stability of LiVPO₄F so that its high-temperature cycling performance can be enhanced.

Lithium phosphate (Li₃PO₄), especially in disordered phase, is an excellent and stable lithium ionic conductor [27], [28]. Since Li₃PO₄ is chemically inert to the electrolyte in a wide voltage and temperature range, this material has been proven to be an effective coating agent to improve electrochemical performance of LiFePO₄ [29], [30], LiNi_xCo_yMn_1-x-yO₂ [31], [32] and Li-rich layered oxides [33], [34], [35], etc. In this work, we report for the first time the use of amorphous Li₃PO₄ to modify LiVPO₄F/C composite. With a skillful and mild aqueous solution method, Li₃PO₄ can be effectively coated on the surface of the LiVPO₄F nanoparticles. Considering Li₃PO₄ is a super ionic conductor, it would help to enhance the lithium ion diffusion of LiVPO₄F on the surface. More importantly, given the excellent chemical stability of Li₃PO₄ at high-voltage and elevated-temperature, the interface stability of Li₃PO₄-coated LiVPO₄F is also expected to be improved.