Comparison of the structural evolution and mechanical stability of NCM622, NM64 and Co-NM64. (a1–c1) In-situ XRD characterizations of NCM622 (a1), NM64 (b1) and Co-NM64 (c1) before cycling. (a2–c2) Cross-sectional images of NCM622 (a2), NM64 (b2) and Co-NM64 (c2) charged to 4.5 V after 100 cycles in the 2.7–4.5 V (vs. Li/Li⁺) voltage range. (a3–c3) HAADF-STEM image of NCM622 (a3), NM64 (b3) and Co-NM64 (c3) at a secondary-particle surface discharged to 2.7 V (vs. Li/Li⁺) after 100 cycles. (a4–c4) EELS line scan of NCM622 (a4), NM64 (b4) and Co-NM64 (c4) at different depths after 100 cycles. Scale bars, 4 μm (a2–c2), 2 nm (a3–c3).