The miniaturization of portable electronic devices and urgent demand for long-range electric vehicles (EVs) drive researchers to seek for advanced lithium ion batteries (LIBs) as power sources with high energy densities. Traditional cathode materials (LiCoO₂, LiMn₂O₄, LiFePO₄ and their derivatives) with specific capacities less than 200 mA h g^-1 are not able to fulfil the challenge. As one of the most promising alternatives, lithium-rich layered oxides (LRLOs) with exceptionally high capacities exceeding 250 mA h g^-1 have been attracting extensive research interests for high energy density LIBs during the last decade.^1-3 However, intrinsic disadvantages of voltage decay and capacity fade resulting from structural instability hinder practical use of LRLOs. Strategies such as ionic doping and surface engineering have been developed to improve electrochemical performance of LRLOs, but with limited understanding of the origins of their high capacity, voltage decay and capacity fade.

Ru-based LRLOs stand out their counterparts for the good rate performance benefiting from low resistivity of Ru-based compounds. As the major component of Ru-based LRLOs, although Li₂RuO₃ exhibits large specific capacity and good rate capability, it suffers severely from capacity fade and voltage decay upon prolonged cycling. Herein in this work, for the first time, Cr has been employed as a dopant in Li₂RuO₃ compound and the correlation between Cr doping and electrochemical performance of Li₂RuO₃ is investigated. Elemental Cr is selected as dopant for plenty of reasons. Firstly, LiCrO₂ possesses a layered structure with R-3m symmetry, which has similar oxygen stacking framework with Li₂RuO₃ layered structure. Besides, the ionic radius of Cr³⁺ (0.615 Å) is close to that of Ru⁴⁺ (0.62 Å). The similarity in crystal structure and ionic radius guarantees that the structure of Li₂RuO₃ is not destroyed by Cr doping. Secondly, Cr³⁺/Cr⁶⁺ three-electron redox reaction is electrochemically active which is expected to favor the capacity improvement of Li₂RuO₃ compound, as lots of literatures reported that Cr-doped Mn-based LRLOs showed enhanced reversible capacity because of the introduction of three-electron redox reaction of Cr³⁺/Cr⁶⁺. Thirdly, Singh and colleagues reported that Cr in the Cr-doped composite Li₂MnO₃-LiMn_0.5Ni_0.5O₂ acts as a catalyst for the activation of Li₂MnO₃ component in the electrochemical reaction,⁴ which makes it reasonable to deduce that Cr activates anionic electrochemistry and thus contributes to capacity improvement. Based on the above considerations, we synthesized Cr-doped Li₂RuO₃ with different doping degrees (Li₂RuO₃, Li₂Ru_0.98Cr_0.02O₃, Li₂Ru_0.95Cr_0.05O₃ and Li₂Ru_0.9Cr_0.1O₃, denoted as LRO, LRO-Cr0.02, LRO-Cr0.05 and LRO-Cr0.1, respectively) by high-temperature sintering method. It is shown that with appropriate amount of Cr doping, the capacity, rate performance and cycling stability can be significantly improved for Li₂RuO₃ compound.