A High Voltage Olivine Cathode for Application in Lithium- Ion Batteries Daniele Di Lecce, [a] Rosaria Brescia, [b] Alice Scarpellini, [b] Mirko Prato, [b] and Jusef Hassoun* [a, c] Introduction Since the report of Padhi et al., [1] polyanionic materials with the olivine structure as cathodes for lithium-ion batteries (LIBs) have been extensively studied. [2] Among them, LiFePO 4 is the most investigated material owing to its remarkable thermal stability, [3] attributed to the strong covalent P O bond within the polyanionic structure, and a theoretical reversible capacity of 170 mA h g 1 delivered at about 3.45 V versus Li + /Li. [4] More- over, LiFePO 4 is an environmentally-friendly and low-cost mate- rial, compared to common Co-based layered cathodes, [5] and reversibly exchanges lithium ions through a two-phase reac- tion, which evolves by following a flat and stable voltage pla- teau. [2] Carbon coating and particle size reduction successfully solved kinetic problems related to the low intrinsic electronic and ionic conductivity values of LiFePO 4 olivine, allowing for its commercial exploitation. [6] However, the low working volt- age of LiFePO 4 , which leads to relatively low energy density, has triggered increasing interest of the scientific community towards the substitution of iron by other transition metals, such as Mn, Co, and Ni, within the olivine framework. [7, 8] In par- ticular, LiMnPO 4 and LiCoPO 4 , with operating voltages of about 4.1 V and 4.8 V versus Li + /Li, respectively, received great atten- tion. [9–19] Indeed, the electrochemical reaction of LiNiPO 4 is lim- ited by the operating voltage (above 5.1 V vs. Li + /Li), [20] which is a high value well beyond the electrochemical stability window of common carbonate-based electrolytes. [21] Despite the expected advantages of Mn and Co substitution, LiMnPO 4 has sluggish kinetics, mainly owing to its insulator character and Jahn–Teller deformation around Mn 3 + , [22, 23] whereas LiCoPO 4 suffers from capacity fading [24] and high irre- versible capacity owing to electrolyte decomposition at high voltage, [8] as well as from severe self-discharge. [25] Therefore, an effective strategy to increase the operating voltage of olivine cathodes is the use of mixed compositions, including various metals, such as the case of LiFe x Mn 1x PO 4 solid solution that was widely investigated with promising results. [26–31] Indeed, iron incorporation within the olivine framework enhances both electronic and ionic conductivity of LiMnPO 4 [32, 33] and reduces Jahn–Teller deformation. [34] Furthermore, the study of mixed compositions may provide better understanding of the electro- chemical behavior of transition metal couples within the oli- vine structure, aimed at further improvement of bare materials. Following this trend, Chen et al. firstly reported the synthesis of LiFe 1/3 Mn 1/3 Co 1/3 PO 4 using a hydrothermal method. [35] A sub- sequent report [36] showed its electrochemical reaction in a lithi- um cell through a one-phase process with three different volt- age steps, owing to the Fe 3 + /Fe 2 + , Mn 3 + /Mn 2 + , and Co 3 + /Co 2 + redox couples, and underlined the sluggish kinetics within the manganese region. A homogeneous distribution of transition metals within the olivine framework has proven to enhance the electrochemical activity of Mn. [37] Furthermore, the working voltages related to Fe 3 + /Fe 2 + and Co 3 + /Co 2 + in LiFe 1/3 Mn 1/3 Co 1/3 PO 4 may shift to higher and lower values com- pared to bare LiFePO 4 and LiCoPO 4 . [36–38] Optimization of the A new olivine composition (i.e., LiFe 0.25 Mn 0.5 Co 0.25 PO 4 ) is pro- posed as electrode material with increased energy density for application in lithium-ion batteries. The new formulation in- creases the working voltage and induces different electro- chemical behavior with respect to bare olivine materials based on Fe. The study provides deep insight into the features of the Fe 3 + /Fe 2 + , Mn 3 + /Mn 2 + , and Co 3 + /Co 2 + redox couples within the olivine lattice in terms of electrochemical activity, Li + trans- port properties, and Li-cell behavior. The electrochemical char- acterization clearly reveals the voltage signatures correspond- ing to the various metals; however, the Mn 3 + /Mn 2 + process has higher intrinsic polarization with respect to Fe 3 + /Fe 2 + and Co 3 + /Co 2 + . This issue is efficiently mitigated by carbon coating the material, resulting in enhanced electrochemical performan- ces. [a] D. Di Lecce, Prof.Dr. J. Hassoun Sapienza University of Rome Chemistry Department P.le Aldo Moro 5, 00185 Roma (Italy) E-mail : jusef.hassoun@unife.it [b] Dr. R. Brescia, A. Scarpellini, Dr. M. Prato Department of Nanochemistry Istituto Italiano di Tecnologia (IIT) Via Morego 30, 16163 Genova (Italy) [c] Prof.Dr. J. Hassoun Department of Chemical and Pharmaceutical Sciences University of Ferrara Via Fossato di Mortara, 17, 44121, Ferrara (Italy) Supporting Information and ORCID(s) from the author(s) for this article are available on the WWW under http://dx.doi.org/10.1002/ cssc.201501330. ChemSusChem 2016, 9, 223 – 230 # 2016 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim 223 Full Papers DOI: 10.1002/cssc.201501330