Abstract
The degradation mechanism in a sodium cell of a layered Na0.48 Al0.03 Co0.18 Ni0.18 Mn0.47 O2 (NCAM) cathode with P3/P2 structure is investigated by revealing the changes in microstructure and composition upon cycling. The work aims to rationalize the gradual performance decay and the alteration of the electrochemical response in terms of polarization, voltage signature, and capacity loss. Spatial reconstructions of the electrode by X-ray computed tomography at the nanoscale supported by quantitative and qualitative analyses show fractures and deformations in the cycled layered metal-oxide particles, as well as inorganic side compounds deposited on the material. These irreversible morphological modifications reflect structural heterogeneities across the cathode particles due to formation of various domains with different Na+ intercalation degrees. Besides, X-ray photoelectron spectroscopy data suggest that the latter inorganic species in the cycled electrode are mainly composed of NaF, Na2 O, and NaCO3 formed by parasitic electrolyte decomposition. The precipitation of these insulating compounds at the electrode/electrolyte interphase and the related structural stresses induced in the material lead to a decrease in cathode particle size and partial loss of electrochemical activity. The retention of the NCAM phase after cycling suggests that electrolyte upgrade may improve the performance of the cathode to achieve practical application for sustainable energy storage.
Highlights
Sodium-ion batteries are currently gaining a great deal of attention as viable alternatives to conventional lithium-ion systems,[1,2] in view of outstanding results recently obtained at the laboratory scale.[3,4] The investigation of sodium batteries began alongside pioneering studies demonstrating the electrochemical intercalation of alkali ions in chalcogenides,[5] which were driven by the high theoretical energy density of lithium, sodium, and potassium, having a redox potential versus standard hydrogen electrode (SHE) of −3.04, −2.71, and −2.93 V, respectively, as well as a relatively low weight.[6]
Sodium cells employing the NCAM cathode may undergo a gradual deterioration during cycling, which may be reflected as capacity decay along with alteration of the characteristic voltage profile.[23]
The performance of the Na/NCAM cell reported in Figure 1a evidences a decrease in reversible capacity from 170 to 100 mAh g−1 after 100 cycles, that is, about a 40% loss, whilst the coulombic efficiency values range from 97.5% to 99.6%, suggesting a parasitic electrolyte decomposition.[2]
Summary
Sodium-ion batteries are currently gaining a great deal of attention as viable alternatives to conventional lithium-ion systems,[1,2] in view of outstanding results recently obtained at the laboratory scale.[3,4] The investigation of sodium batteries began alongside pioneering studies demonstrating the electrochemical intercalation of alkali ions in chalcogenides,[5] which were driven by the high theoretical energy density of lithium, sodium, and potassium, having a redox potential versus standard hydrogen electrode (SHE) of −3.04, −2.71, and −2.93 V, respectively, as well as a relatively low weight.[6]. Na0.48Al0.03Co0.18Ni0.18Mn0.47O2 (NCAM) layered cathode reversibly reacting in sodium cells by a single-phase, solid-solution mechanism with smooth potential curve and rather constant slope within the wide range from 1.4 V versus Na+/Na to 4.6 V versus Na+/Na, delivering a reversible capacity of about 175 mAh g−1.[23] Such a structurally optimized multi-metal formulation ensured a very promising performance and high reversibility upon the initial charge/discharge cycles, avoiding phase transitions upon Na+ (de)intercalation and mitigating the Jahn-Teller distortion on Mn3+.[23] the inclusion of Co and Ni may improve the electrode operation in the cell above 3 V versus Na+/Na,[21] whilst aluminum has proven to stabilize the structure at high voltage, despite being electrochemically inactive.[22] the capacity decay over cycling was not fully mitigated.[23] we aim in this work to investigate fully the degradation mechanisms of NCAM during cycling in the sodium cell by a comprehensive approach principally based on the 3D reconstructions of the cathode before and after cycling by X-ray computed tomography (CT) This alternative approach enables us to display the evolution of the NCAM particles at the nanoscale and the changes in electrode microstructure. Our study sheds further light on various phenomena driving the performance of layered oxide cathodes and suggests potential strategies to achieve sodium-ion batteries with enhanced cycle life and practical interest
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