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The lithium-ion battery has been widely employed in mobile applications because of its advantages, such as high energy density, insignificant self-discharge rate, and reasonable price. While it is also the most promising candidate power source for advanced automobiles, it needs further development to meet the specifications of electric vehicles1,2. For example, the driving distance of an electric vehicle on a single charge is becoming more and more important, which requires batteries with ever higher energy density. One of the main strategies to enhance the energy density is to raise the average potential and specific capacity of the cathode materials3. For this purpose, Ni rich lithium transition metal oxides have been developed for promising cathode materials4,5. Nevertheless, there are several limitations that need to be surmounted before the mass production of high energy density batteries containing Ni rich lithium transition metal oxides. In particular, significant capacity fading has been observed with lithium ion battery using Ni rich lithium transition metal oxides6,7. In this study, we found that cathode (Ni-rich LiNi0.8Co0.1Mn0.1O2) surface modification – phase transformation from a layered structure to a cubic phase – exerted a significant influence on battery performance. Such cubic phase hindered ionic and electronic conductivity of cathode particles. The kind of the cubic phase was different by changing the composition of the electrolyte. As a result, the resistance of the cylindrical cells after 300 cycles using different electrolyte showed a large gap, especially on cathode side. [1] B. Scrosati, J. Garche, J. Power Sources, 2010, 195, 2419 [2] M. Armand, J.-M. Tarascon, Nature, 2008, 451, 652 [3] J. B. Goodenough, Y. Kim, Chem. Mater., 2010, 22, 587 [4] W. Liu et al, Angew. Chem. Int. Ed. Engl. 2015, 54, 4440 [5] A. Manthiram et al, Adv. Energy Mater, 2016, 6, 1501010 [6] D.P. Abraham et al, Electrochem. Comm., 2002, 4, 620 [7] S. Watanabe et al, J. Power Sources, 2014, 258, 210

Lithium-rich oxides LixNi0.25Co0.10Mn0.65O(3.4+x)/2 (x=1.6, 1.4, 1.2, 1.0, 0.8) were synthesized and characterized for their structural, morphological, and performance as cathode materials in Li-ion batteries. The baseline composition x=1.4 had been previously determined as having the best electrochemical performance due to its small primary particle size and homogeneous distribution of nano-domains consisting of (C2/m) and (R3¯m) phases. In this study, the Rietveld refinement results indicate the presence of two phases at high lithium levels (x=1.6 and 1.4): Li2MnO3 (C2/m) and Li M O2 ( M = Ni, Co, Mn) (R3¯m); the latter contains Ni2+ and Ni3+. At low lithium levels (x=1.2, 1.0, and 0.8) an additional spinel phase Li M 2O4 (Fd3¯m) emerges, which is known to affect the electrochemical performance of the oxide. Structural analysis reveals that the spinel phase contains mixed transition metals Ni, Co, and Mn as [Li+,Co2+][Ni2+,Co3+,Mn4+]2O4. A low lithium level is found to induce primary particle growth, as well as Co and Ni segregation within the secondary particles. These results are expected to contribute to material optimization and commercialization of lithium-rich oxide cathodes.