Superior Doped Olivine Cathodes for Fast Charge/Discharge Lithium Batteries

Abstract

In recent years, the lack of high-performing electrical energy storage systems is seriously bottlenecking the operation of appliances in several fields, that include portable electronics, electric automotive, and stationary devices [1]. Until now, lithium batteries (LiBs) are the highest-performing systems that could address satisfactorily all the requirements of such appliances [2]. Indeed, LiBs show a high specific capacity, a high efficiency, and a long lifespan [3]. However, improvements are still needed in the operating potential (that influences the specific energy and power of the LiBs) and in the rate capability properties of the cathodes. Indeed, electric vehicles need high power during acceleration, and a large amount of stored energy to achieve a sufficient mileage. Herein we describe the synthesis and the study of an innovative family of high-energy and high-rate cathodes for application in LiBs. The structure of a high-performing olivine material of the type LiMPO4 (M = Fe, Ni, and Co) [4] has been doped with high-valence transition metal ions (V(V), Nb(IV), or Ta(IV)). In a first step, the synthesis procedure has been optimized. Five different materials have been obtained, as follows: (i) LFNCVP is a vanadium-doped olivine obtained under oxidizing conditions; (ii) LFNCVP/C is a vanadium-doped olivine obtained under partially reducing conditions (graphite is added during the pyrolysis); (iii) LFNCVP/H is a vanadium-doped olivine obtained under reducing conditions (pyrolysis is carried out under a hydrogen atmosphere); (iv) LFNCNP is a niobium-doped olivine obtained under oxidizing conditions; and (v) LFNCTP is a tantalum-doped olivine obtained under oxidizing conditions. The effects of the different synthesis protocols and ion insertion on the structural, morphological, and electrochemical properties of the cathodes have been thoroughly investigated. The composition of the cathodes has been evaluated by means of Inductively Coupled Plasma Atomic Emission Spectroscopy (ICP-AES), the morphology and size distribution have been gauged by Scanning Electron Microscopy (SEM) and High-Resolution Transmission Electron Microscopy (HR-TEM); the structure has been revealed through powder X-Ray Diffraction (XRD) and IR spectroscopy techniques. The electrochemical performance has been investigated by Cyclic Voltammetry (CV), Electrochemical Impedance Spectroscopy (EIS), and coin battery charge/discharge tests at different current rates. As expected (see Figure 1), the proposed materials reveal a high performance in terms of working potential (4.0÷5.0 V vs. Li/Li+), specific capacity (149 mAh∙g-1), and specific energy (656 mWh∙g-1). Moreover, the insertion of high-valence transition metals is able to boost the rate capability of the cathodes, allowing for fast charge and discharge processes without depleting the cathodes.