grids. Current growth rates in lithiumion battery (LIB) manufacturing are not sustainable given our limited lithium resources. In this context, alternative battery systems with low cost are sought, with sodium-and potassium-ion batteries (SIBs and PIBs, respectively) regarded as suitable replacements due to the high natural abundance of sodium and potassium and their similar working mechanism to LIBs. [1] Of these, PIBs are more promising due to the closeness of the K + /K redox potential (−2.9 V vs H + /H 2 O) to that of Li + /Li (−3.0 V vs H + /H 2 O), as compared to Na + /Na (−2.7 V vs H + /H 2 O), and the consequential ease of reversible K + intercalation into the most common LIB negative electrode, graphite. [2][3][4] The slow kinetics of the reversible insertion of K + in electrode materials as a result of its relatively large ionic radius of 1.38 Å is a major issue for the realization of high-performance PIBs. [5][6][7] For PIB technology to be successful, scientists need to find stable electrode materials capable of reversibly hosting K + relatively quickly. Transition metal layered oxides possess a 2D structure that can accommodate large ions and are attracting interest as potential PIB electrode materials. [8][9][10][11][12][13][14] Among these, manganese layered oxides are particularly promising due to the high natural abundance and nontoxicity of manganese. A range of layered K x MnO 2 structure types can be prepared [15,16] including Potassium-ion batteries (PIBs) are an emerging, affordable, and environmentally friendly alternative to lithium-ion batteries, with their further development driven by the need for suitably performing electrode materials capable of reversibly accommodating the relatively large K + . Layer-structured manganese oxides are attractive as electrodes for PIBs, but suffer from structural instability and sluggish kinetics of K + insertion/extraction, leading to poor rate capability. Herein, cobalt is successfully introduced at the manganese site in the K x MnO 2 layered oxide electrode material and it is shown that with only 5% Co, the reversible capacity increases by 30% at 22 mA g -1 and by 92% at 440 mA g -1 . In operando synchrotron X-ray diffraction reveals that Co suppresses Jahn-Teller distortion, leading to more isotropic migration pathways for K + in the interlayer, thus enhancing the ionic diffusion and consequently, rate capability. The detailed analysis reveals that additional phase transitions and larger volume change occur in the Co-doped material as a result of layer gliding, with these associated with faster capacity decay, despite the overall capacity remaining higher than the pristine material, even after 500 cycles. These results assert the importance of understanding the detailed structural evolution that underpins performance that will inform the strategic design of electrode materials for high-performance PIBs.
