Layered-rocksalt intergrown cathode for high-capacity zero-strain battery operation

The dependence on lithium-ion batteries leads to a pressing demand for advanced cathode materials. We demonstrate a new concept of layered-rocksalt intergrown structure that harnesses the combined figures of merit from each phase, including high capacity of layered and rocksalt phases, good kinetics of layered oxide and structural advantage of rocksalt. Based on this concept, lithium nickel ruthenium oxide of a main layered structure (R\documentclass[12pt]{minimal} \usepackage{amsmath} \usepackage{wasysym} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{amsbsy} \usepackage{mathrsfs} \usepackage{upgreek} \setlength{\oddsidemargin}{-69pt} \begin{document}$$\bar{3}$$\end{document}3¯m) with intergrown rocksalt (Fm\documentclass[12pt]{minimal} \usepackage{amsmath} \usepackage{wasysym} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{amsbsy} \usepackage{mathrsfs} \usepackage{upgreek} \setlength{\oddsidemargin}{-69pt} \begin{document}$$\bar{3}$$\end{document}3¯m) is developed, which delivers a high capacity with good rate performance. The interwoven rocksalt structure successfully prevents the anisotropic structural change that is typical for layered oxide, enabling a nearly zero-strain operation upon high-capacity cycling. Furthermore, a design principle is successfully extrapolated and experimentally verified in a series of compositions. Here, we show the success of such layered-rocksalt intergrown structure exemplifies a new battery electrode design concept and opens up a vast space of compositions to develop high-performance intergrown cathode materials.


Supplementary Note 1.
The Rietveld refinement of the ND reflections of chemically delithiated Li0.5Ni0.4Ru0.4O2 indicates the preference of Li + extraction from rocksalt rather than layered phase during initial Li + extraction.
A specific constrain is created in the refinement process as described below: Firstly, a routine two-phase refinement process was conducted, including the optimization of parameters for cell parameter, thermal parameter, atomic position for O in layered phase, zero position, phase-fraction, scale, profile, and background. At this stage, the molar ratio between lithium and the transition metal ions were assumed to be the same in both layered and rock-salt phases. A weighted residue value (wRp) of 3.95% was obtained, indicating that the simulated and experimental patterns were in pretty good agreement. Then, a new constrain was created to allow a movement of lithium ions from the layered to rock-salt phase but their total quantity was kept constant. Based on our previous refinement analysis for the pristine phase that the molar ratio between layered and rock-salt was found to be 70:30, the constrain was set such as the change of the quantity of lithium ions in 3a in the layered phase (3a is the Li site; 3b is the TM site in conventional LiMO2 of R3 � m) times 6 was equal to the change of quantity of lithium ions in 4a (4a is the cation site; 4b is the O site) in the rock-salt phase times 7. Once this constrain was set, the occupancy parameters for the lithium ion in 3a position in the layered phase and the 4a position in the rock-salt phase were allowed to refine. Surprisingly, the weighted residue value was further reduced to 3.62%. The summary of the result indicates that lithium content was lower in the rocksalt phase than that in the layered phase.
To verify this result, two refinement scenarios were compared. In one case, we arbitrarily removed the lithium ion from the layered phase first, by maintaining the nominal composition of rock-salt same as Li0.5Ni0.4Ru0.4O2. The slightly large weighted residue value of 4.2% was obtained.
In a second case, we arbitrarily remove the lithium-ion from the rock-salt phase first, by maintaining the nominal composition of layered phase as Li0.5Ni0.4Ru0.4O2. A weighted residue of 3.63% was obtained. The results suggest that lithium ions have a preferential removal mechanism from the rock-salt phase than layered phase.
The chemical composition of Li0.5Ni0.4Ru0.4O2 is determined by inductively coupled plasma mass spectrometry (ICP-MS) analysis. Assuming full delithiation of rocksalt phase (30 mol%), the chemical composition of the layered component in Li0.5Ni0.4Ru0.4O2 is Li0.714Ni0.4Ru0.4O2. (d) monoclinic Li-rich layered oxide of C/2m; and three scenarios in layered oxide cathode including (e) intergrown layered-rocksalt phase proposed in present work, the rocksalt component in the as-proposed layered-rocksalt intergrown oxide is electrochemically active and the sole layered component can't account for such a high degree of Li + extraction/insertion and the intergrown phase exhibits an isotropic lattice change upon cycling.; (f) layered phase with Li/Ni intermixing, Li/Ni intermixing in the layered cathode can alleviate the phase transition, to some extent, but it can't suppress its intrinsic anisotropic lattice change; and (g) layered phase with surface rocksalt, the rocksalt phase that presents a thin layer (a couple nanometer) at the surface during synthesis or upon cycling is electrochemically inactive densified layer with low electronic and ionic conductivity. The green, purple, and red spheres represented Li, TM, and O, respectively.  (b) is 1 µm and 100 nm, respectively. The particle size of the as-produced Li1.2Ni0.4Ru0.4O2 is ~500 nm, which is directly used for electrochemical characterization without further nanoscaling.