Biphase-to-monophase structure evolution of Na_(0.766+x)Li_(x)Ni_(0.33-x)Mn_(0.5)Fe_(0.1)Ti_(0.07)O_(2) toward ultradurable Na-ion batteries

Layered composite oxide materials with O3/P2 biphasic crystallographic structure typically demonstrate a combination of high capacities of the O3 phase and high operation voltages of the P2 phase.However,their practical applications are seriously obstructed by difficulties in thermodynamic phase regulation,complicated electrochemical phase transition,and unsatisfactory cycling life.Herein,we propose an efficient structural evolution strategy from biphase to monophase of Na_(0.766+x)Li_(x)Ni_(0.33-x)Mn_(0.5)Fe_(0.1)Ti_(0.07)O_(2) through Li+substitution.The role of Li+substitution not only simplifies the unfavorable phase transition by altering the local coordination of transition metal(TM)cations but also stabilizes the cathode–electrolyte interphase to prevent the degradation of TM cations during battery cycling.As a result,the thermodynamically robust O_(3)-Na_(0.826)Li_(0.06)Ni_(0.27)Mn_(0.5)Fe_(0.1)Ti_(0.07)O_(2) cathode delivers a high capacity of 139.4 mAh g^(-1) at 0.1 C and shows prolonged cycling life at high rates,with capacity retention of 81.6%at 5 C over 500 cycles.This work establishes a solid relationship between the thermodynamic structure evolution and electrochemistry of layered cathode materials,contributing to the development of long-life sodium-ion batteries.

Elucidation of the sodium kinetics in layered P-type oxide cathodes

Sodium-ion intercalation oxides generally possess high compositional diversity according to their different stacking sequences.The sodium diffusion pathway in layered P-type materials used in sodium-ion batteries is open,which can increase their rate capability by directly transmitting Na+between adjacent triangular prismatic channels,rather than passing through an intermediate tetrahedral site in O-type structure.However,how the structure chemistry of the P-type oxides determines their electrochemical properties has not been fully understood yet.Herein,by comparing the crystalline structures,electrochemical behaviors,ion/electron transport dynamics of a couple of P-type intercalation cathodes,P2-Na_(2/3)Ni1/3Mn_(2/3)O_(2)and P3-Na_(2/3)Ni_(1/3)Mn_(2/3)O_(2)with the same compositions,we demonstrate experimentally and computationally that the P2 phase delivers better cycling stability and rate capability than the P3 counterpart due to the predominant contribution of the faster intrinsic Na diffusion kinetics in the P2 bulk.We also point out that it is the electronic conductivity that captures the key electrochemistry of layered P3-type materials and makes them possible to enhance the sodium storage performance.The results reveal that the correlation between stacking structure and functional properties in two typical layered P-type cathodes,providing new guidelines for preparing and designing alkali-metal layered oxide materials with improved battery performance.