High-Performance Olivine for Lithium Batteries: Effects of Ni/Co Doping on the Properties of LiFeaNibCocPO4 Cathodes

Nowadays, the rapid development in portable electronics, load leveling/peak shaving for the power grid and electric automotive requires significant progress in high-voltage and high-capacity storage systems [1, 2]. Lithium batteries are, to date, the most promising systems that can sustain this demand [3]; they have high specific energy, high efficiency and a long lifespan [4]. Lithium cobalt oxide (LiCoO₂) based cathode materials currently dominate the market [5] but, due to a low working potential (3.0 – 4.0 V vs. Li) and a high cost and toxicity, there is a broad scope for the development of new cathodic materials [6]. Lithium-transition metal-phosphates (LiMPO₄, M=Co, Fe, Mn or Ni) show very good performance: their olivine structure with a 2D framework of crossed tunnels allows the insertion and de-insertion of lithium ions during the discharge/charge of the battery [7]. Lithium iron phosphate (LiFePO₄) affords the highest specific capacity, albeit at a low potential. On the other hand, lithium cobalt phosphate (LiCoPO₄) or lithium nickel phosphate (LiNiPO₄) give rise to higher working potentials, but the lifespan and the specific capacity become very low [8-10].

In this work we describe the synthesis and the characterization of a new family of high-voltage cathodic materials based on lithium-transition metal mixture-phosphates of iron, nickel and cobalt, in order to best take advantage of all the positive characteristics of each element present in the structure (high voltage and high capacity) [11]. Five materials have been produced, varying the Ni/Co molar ratio; the effect of different degrees of Co and Ni doping on structure, morphology and electrochemical properties have been thoroughly studied. The stoichiometry is evaluated using Inductively Coupled Plasma Atomic Emission Spectroscopy (ICP-AES), the thermal stability is investigated by High Resolution – Thermo Gravimetric Analyses (HR-TGA), morphology and size distribution are characterized by Field Emission - Scanning Electron Microscopy (FE-SEM) and High-Resolution Transmission Electron Microscopy (HR-TEM) (see Figure 1); the structure is examined by powder X-Ray Diffraction (XRD) as well as a variety of IR spectroscopy techniques. Electrochemical characterization is carried out by Cyclic Voltammetry (CV) and charge/discharge tests (see Figure 2).

Indeed, the proposed materials are good cathodic candidates for the development of high-voltage lithium batteries: the best of our materials, LFNCP_0.61, showed a specific capacity and a specific energy of 125 mAh∙g^-1 and 560 mWh∙g^-1, respectively.

Acknowledgements

The authors thank: (a) the strategic project “From Materials for membrane electrode Assemblies to electric Energy conversion and SToRAge devices” (MAESTRA) of the University of Padova for funding this study; and (b) the “Centro Studi di Economia e Tecnica dell’Energia Giorgio Levi Cases” for grants to G.P. and E.N.

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