| Sodium-ion batteries（SIBs）,as promising candidates for next-generation secondary battery systems,have attracted much attention in the energy storage market due to their cost-effectiveness,low cost and similar properties to lithium in storage devices.The cathode material is one of the most pivotal roles in batteries,which limits the cost and performance of the batteries.In the classification of cathode materials for SIBs,layered oxide materials have garnered significant attention because of their high ionic conductivity,fast diffusion rate,and high specific capacity.However,the layered oxides have a variety of structures,including P2,O3 and P3 phase structures,which have inherent defects in the charge and discharge process,so it is difficult for the cathode materials with a single structure to achieve excellent electrochemical performance to meet the practical requirements.Aiming at the limitations of single-phase structures,this paper designs multiphase structures to overcome the disadvantages of each structure,benefiting from their advantages and synergy effects.The research is aimed at the lack of clarity on the mechanism of function and formation of multiphase structure.By adjusting the calcination conditions,undoped triple-phase cathode materials are prepared to explore the mechanism of the independent effect of multi-phase structure on battery performance.Based on multiphase structure,high-entropy is introduced into the material and a general method for designing multiphase structures is summarized by preparing a series of multiphase high-entropy cathodes.The mechanism of the combination of high entropy and multiphase is proposed.Finally,the study explores the factors that affect the content of the multiphase structure and quantitatively analyzes the influence of the phase content on the battery performance.The details of the study are as follows:（1）P2,P3 and O3 phases have their own advantages and challenges.Taking into account the complementarity of each single-phase structure,constructing the composite structures serves as an efficient pathway to stabilize the structure and improve the electrochemical performance.Herein,three composite cathode materials owning the phase structures of P3/P2,P2/O3 and P3/P2/O3 are prepared by changing the calcinating conditions without introducing any doping element.Among them,the composite of P3/P2/O3shows the best capacity retention of 80 m Ah g-1 at 30 m A g-1after 200 cycles and the highest rate performance of 97.3 m Ah g-1 at a current density of 750 m A g-1.The improved electrochemical performance can be attributed to the structural constraint effect.The staggered arrangement of different phase structures and the gradient Na-extraction/intercalation voltages of different phases are the main reasons.In the multiphase materials,the slip of the transition metal layer is subjected to the constraint of the adjacent phase structure,thus inhibiting the phase transformation for capacity fading.This work provides an easy way for the preparation of composite cathode structures and brings a clear case for understanding the advantage of composite structures on electrochemical performance.（2）Cycling stability is the biggest challenge for cathodes of sodium-ion batteries（SIBs）.In order to further improve the cycling stability of multiphase cathode materials,high-entropy（HE）configuration is introduced based on Ni-Mn based oxides,designing a series of Ni-Mn-Cu-Ti-Sn five-component multiphase cathode materials.The weighted average ionic radius（WAIR）of all transition metals is demonstrated critical for the formation of the phase structure in HE composites.Through comparing a series of HE and multiphase cathodes,an empirical range of WAIR is obtained,which shows guidance for the design of other cathode materials.The combination of HE and multiphase structure is demonstrated helpful for maintaining the structure and improving the cycling stability.In the HE multiphase cathodes,the multiple elements at transition metal sites can enlarge the lattice and stabilize the structure simultaneously without causing obvious capacity drop,achieving the synergistic effect of multi-cations.In the HE cathodes consisting P2 and O3 phases,the harmful phase transition in high-voltage is suppressed and the cycling performance is improved.A capacity of 92.8 m Ah g-1（80.9%capacity retention rate）at 100m A g-1 after 200 cycles is delivered,and an improved rate performance of 88.7m Ah g-1 at 750 m A g-1 is observed,better than that of the low-entropy multiphase cathode（P2 and O3）and the HE oxide single O3-phase cathode.In addition,the air stability,long cycling structural stability,safety and stress distribution of the HE multiphase cathode have been significantly improved.（3）Based on the previous research,we further explore the factors that affect the phase content in HE multiphase materials and quantify the role of the multiphase structure in battery performance and structural evolution.It is found that both the size of WAIR and the sodium content can be realized to regulate the phase content.And the effects of phase content on structural parameters,morphology,local structure and structural evolution are summarized.The initial efficiency,cycling and rate performance can be optimized by rationally regulating the ratio of P2 and O3 phases in the multiphase materials.In the HE multiphase material of Ni/Mn/Cu/Fe/Zn system,the P2 phase can remain unchanged during the charging and discharging process,while O3 phase undergoes a more complex phase transition of O3-P3-O3’.The damage to the material structure can be minimized by adjusting the ratio of P2 and O3 phases.The increase of O3 phase can improve the capacity,but due to the poor reversibility in the high voltage region,when the content is too high,it may not be conducive to the improvement of cycling performance.By comparison,it is found that when the ratio of P2 and O3 phases in the multiphase material is29.45%:70.55%,the highest residual capacity after 200 cycles is 81.9 m Ah g-1.When the ratio of P2 and O3 phases is 67.31%:32.69%,the optimal multiplicative performance is 96.3 m Ah g-1 discharged at 750 m A g-1 high current density.These findings will draw the attention of researchers to the phase content and provide a new perspective for designing high performance multiphase cathode materials. |
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