K-Birnessite Cathode For Potassium-Ion Battery:Preparation Technology And K Storage Mechanism

Novel and low-cost rechargeable batter:ies are of considerable interest for application in large-scale energy storage systems.In this context,the K-Bimessite is synthesized using a facile solid-state reaction as a promising cathode for potassium-ion batteries.The phase-pure Bimessite sanple is prepared with the nominal K/Mn ratio of 0.7 at 850?(denoted as KBir),which is in consistence with the standard mineral of K0.27(Mn0.98O2)·0.53(H2O)(JCPDS:97-005-5408).The first charge and discharge capacities of KBir electrode are 50 and 90 mAh g-1,respectively.The capacities of KBir electrode at 20th and 50th cycles are 65 and 58 mAh g-1,which are?72%and 64%of its initial discharge capacity.Furthermore,the ion exchange protocols,including solution-based(s-KBir)and high-temperature(h-KBir)method,are employed to activate and/or increase the K+ion content in KBir electrode.The s-KBir electrode demonstrates the better K-storage performance with repsect to the h-KBir.The first charge and discharge capacities of s-KBir electrode are 75 and 125 mAh g-1 and the capacities at 20th and 50th cycles are 97 and 80 mAh g-1,the capacity rentention of which are?78%and 64%.The inductively coupled plasma-mass(ICP)analysis shows that the K/Mn ratios in the samples of KBir,s-KBir and h-KBir are?0.21,0.31 and 0.25 respectively,whose results evidence clearly the incorporation of excessive K+ions into the pristine sample upon ion exchange.Consequently,the ion exchange benefits on not only enhancing the K contents in the pristine KBir,but also reducing the grain size to shorten the K+ion diffusion trajectory,thus to facilitate the faster kinetics,which has a prevailing influence on the material's design and optimization.In-situ X-ray diffraction reveals that the material undergoes.a reversible phase transition upon K extraction and insertion.The structural stability is then checked using first-principles calculations.It is found that the Mn ions are diffcult to migrate into the K layer.Of special interest is that the concerted ionic diffusion mechanism on K+ion diffusion is found to be rational with a much lower activation energy of 0.12 eV in P2-K0.3MnO2 electrode,which explains the fast K+diffusion during charge and discharge.These new findings provide new insights into the comprehension of electrode process kinetics,and lay a solid foundation as well on material design and optimization of potassium-ion battery for large-scale energy storage.