Efficient and economical energy conversion and storage have become important for addressing current environmental issues. Reversible energy storage devices, such as Li-ion batteries (LIB) and supercapacitors, have attracted significant attention in recent years. Since the original work on LiFePO4 (LFP), olivine phosphates LiMPO4 (M=Fe, Co, Ni, Mn) have been considered as some of the most promising cathode materials for LIBs to be used in hybrid electric vehicles (HEVs), electric vehicles (EVs)

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... nd modern catenary-free trams. In the family of olivine phosphates, LFP has attracted a lot of interest due to its excellent electrochemical properties, low cost, nontoxicity, excellent thermal stability and environmental compatibility. However, its application is limited by poor electronic conductivity and ionic diffusivity. Although a number of efforts have been made to improve the electronic conductivity and ionic diffusivity, experimental and theoretical studies on the electronic structure of LiMPO4 are limited and the treatments of LiMPO4 in theoretical calculations are still under debate. In this study, the electronic structure of LiMPO4 has been investigated both theoretically and experimentally. All the theoretical calculations have been carried out in the framework of density-functional theory (DFT). The results obtained using different functionals have been compared and validated by experimental measurements. Surface Li-depletion has been detected on carefully as-prepared LiMPO4 particles using surface sensitive characterization techniques and its effect on the electronic structure has been discussed. The presence of surface Li-depletion has been proven to be associated with optical absorption peaks at energies lower than the main edge and with tails in optical absorbance spectra or their corresponding Tauc plots. With improved understanding on the electronic band structure of the pristine materials and the influence of surface Li-depletion, doping has been conducted for rate performance improvement in this research. Doped LFP samples with the dopants reported beneficial for rate capability enhancement in the literature have been synthesized and well characterized. More pronounced surface Li-depletion of the iv Study on Electronic Structure and Rate Performance of Olivine Phosphate Cathode Materials doped LFP has been noticed compared with the undoped LFP, indicating that the dopant ends up predominantly on the surface. The rate performance has also been improved by blending LFP particles with two different particle sizes and Li insertion mechanisms. The optimization of the particle size distribution offers better packing density and contact of the active material particles. The improvement of packing results in a better pathway for electronic transport and lower contact resistance. Simultaneously, the smaller LFP particles with a single-phase Li insertion mechanism reduce the polarization of the cell at high Crate by acting as easily accessible reservoirs of Li ions. Compared with the pristine LFP samples, improved initial capacity at C/10 and superior rate capability have been obtained with a mixing ratio of 50:50 wt.% of particles with average diameters 50 and 350 nm. In addition, the nanoparticles also displayed the characteristic surface Lidepletion. Since smaller particles have a larger fraction of surface, it can be inferred that the surface Li-depletion has had a positive contribution to battery performance. Surface delithiation on battery material powders appears related to the stoichiometric "point compound" nature of the materials within the phase diagrams. When the exact targeted composition is not met during synthesis, which is likely to occur statistically, the deviation in stoichiometry appears to accumulate on the surface. This phenomenon is similar to what happens during sintering of ceramics, where slight deviations from the stoichiometry end up as thin boundary film layers across grain boundaries during sintering. Because in batteries we deal with powders, it is not as easily identified or connected to their processing. Preliminary characterization of lithium nickel manganese cobalt oxide (NMC) cathode materials also indicates the presence of surface Li-depleted layers compared to the cores. The phenomenon thus appears to be more general than just for phosphates. This work highlights new perspectives and approaches that need to be adopted for improved understanding of battery material formulations and optimisation of their performance. The surfaces provide the interfaces and control the interaction with the electrolytes, their thorough understanding and design are therefore essential ingredients in optimization of performance and cyclability of the batteries.