稀有金属(英文版) 2020,39(04),332-334

Electrochemical mechanism of high Na-content P2-type layered oxides for sodium-ion batteries

Ying Yang Wei-Feng Wei

State Key Laboratory of Powder Metallurgy,Central South University

作者简介：*Wei-Feng Wei e-mail:weifengwei@csu.edu.cn;

Electrochemical mechanism of high Na-content P2-type layered oxides for sodium-ion batteries

Ying Yang Wei-Feng Wei

State Key Laboratory of Powder Metallurgy,Central South University

The development on capacity and structure issues of P2-type cathode has so far focused on ion-doping/substitution strategy.In a recent report published in Journal of the American Chemical Society,Hu and colleagues demonstrated that high Na-content P2-type layered oxides exhibit higher capacities as well as great structural stability.

P2-type layered oxides with the formula of Na_xTMO₂(TM:transition metal),which enable the fast Na⁺migration process due to the direct Na⁺transport pathways between the TMO₂ slabs and provide the opportunity to achieve high cycle/rate capabilities,have been gaining much attention as the most potential electrodes for sodiumion batteries (SIBs) [ 1] .However,the high active voltage of TM redox couples leads to low capacity under 4.0 V,and the unfavorable phase transitions (P2 to O2 or OP4/‘Z’phases) during charge/discharge contribute to cycling instability,which have become major obstacles to their practical applications [ 2, 3] .

Numerous efforts have been devoted to enhancing the electrochemical properties of P2-type materials.Chemical doping with Mg²⁺,Ti⁴⁺,Cu²⁺and Li⁺were reported to alleviate but not completely suppress the structural instability [ 4, 5, 6] .In addition,the TM³⁺-based P2-type oxides have been proved effective to increase charge capacity,but often suffer from structural transitions and air sensitivity [ 7] .Developing high Na-content P2-type materials is also regarded as a promising strategy to address above issues fundamentally.High Na-content in P2 host ensures that the electrostatic repulsions between the TMO₂ slabs can be alleviated to avoid the structural transition from the P-to O-type stackings [ 8] .Furthermore,high Na-content materials are expected to show higher Na storage capacity under lower cutoff voltage,which is ascribed to the lower average oxidation state and the raised 3d eg*level of TMs [ 8] .

In a recent report published in Journal of the American Chemical Society,Hu and colleagues at the University of Chinese Academy of Sciences reported that a high-Na P2-type oxide with a chemical composition of Na_45/54Li_4/54Ni_16/54Mn_34/54O₂ was successfully synthesized by Li⁺substitution [ 8] .X-ray diffraction (XRD) pattern and highresolution transmission electron microscopy (HRTEM)image display that the as-prepared material is isostructural with typical P2-type layered structure with P63/mmc space group.When tested in 2.0-4.0 V voltage range,this high Na-content P2-type cathode delivers higher reversible capacity of～102.5 mAh·g^-1 (Fig.1b)than～82 mAh·g^-1 from pristine Na_2/3Ni_1/3Mn_2/3O₂(Fig.1a).Interestingly,the higher capacity exhibited by this material originates from smaller amount of Ni²⁺(16/54 mol) in contrast to Na_2/3Ni_1/3Mn_2/3O₂ material (1/3 mol),demonstrating that higher Na-content can promote the electron oxidation of Ni²⁺/Ni³⁺redox couple.When the cutoff voltage was increased to 4.6 V,a charge capacity of～150 mAh·g^-1 is obtained (Fig.1c) in this material,revealing that almost～0.58 mol Na⁺has been deintercalated.Usually,a high cutoff voltage promotes more Na⁺to be extracted,but results in structural degradation and electrolyte decomposition [ 9] .However,there is no significant capacity decay observed in the material.

The authors conducted the first-principles density functional theory (DFT) calculations to obtain more insights into the redox activity under different cutoff voltages.It is indicated from the calculated magnetization of Mn ions that Mn⁴⁺is inactive during the electrochemical process [ 8] .As shown in Fig.1d,e,upon early desodiation(x=0-3),Ni²⁺shows an oxidation tendency to Ni³⁺,and O^2-maintains a relative constant redox state.With further desodiation to 4.0 V (x=5),all Ni²⁺has been oxidized to Ni³⁺and partial Ni³⁺is further oxidized to Ni⁴⁺to provide additional capacity.At a cutoff voltage of 4.6 V (x=7),the Ni⁴⁺content decreases and the oxidation of O^2-is triggered,which limits a higher capacity.These charge compensation mechanisms are also reflected by O 2p and Ni 3d orbitals in partial density of states (pDOS) and O(2p)-TM(3d-eg*)-hybridized state in soft X-ray absorption spectroscopy (XAS) analyses [ 8] .

Fig.1 Galvanostatic charge-discharge curves of a Na_2/3Ni_1/3Mn_2/3O₂ and b Na_45/54Li_4/54Ni_16/54Mn_34/54O₂ electrodes at a rate of 0.15 C in the voltage range of 2.0-4.0 V versus Na⁺/Na;c galvanostatic charge-discharge curves of Na_45/54Li_4/54Ni_16/54Mn_34/54O2 at a rate of 0.15 C in the voltage range of 2.0-4.60 V;magnetization and oxidation state evolution during the desodiation process of d Ni and e O ions in intermediate phases from Na₁₀LiNi₃MngO₂₄ to Na₃LiNi₃Mn₈O₂₄;f in situ XRD patterns of Na_45/54Li_4/54Ni_16/54Mn_34/54O₂ electrode recorded at a rate of0.05 C charged to 4.60 V

In situ XRD experiments were carried out to investigate the structural evolution in Na_45/54Li_4/54Ni_16/54Mn_34/54O₂material.Until charged to 4.6 V,only two new P2 phases with similar cell parameters were formed and no diffractions corresponding to O2 or OP4/‘Z’phases were detected(Fig.1f),which is due to the fact that more remaining Na⁺in the interlayers lowers the electrostatic repulsion and suppresses the gliding of TMO₂ slabs.During Na⁺intercalation process,the three P2 phases recombine to a single P2 phase,and the electrode material still maintained the hexagonal space group P63/mmc even after 100 cycles,showing a high structural stability [ 8] .Importantly,accelerated aging measurements were also conducted to study the chemical stability,and the authors confirmed that this high Na-content material shows a high insensitivity to air and water,which greatly improves the stability and safety for assembled batteries.

Overall,the report by Hu et al.provides a feasible strategy of preparing high Na-content P2-type oxides.Detailed structural chemistry information reveals that high Na-content promotes the oxidization of Ni²⁺to gain more capacity and suppress the unfavorable phase evolution induced by interlayer electrostatic repulsion.Such a research provides a promising pathway for the development of high-capacity/stability P2-type cathode materials under low cutoff voltage.

参考文献

[1] Ma C,Alvarado J,Xu J,Clement RJ,Kodur M,Tong W,Grey CP,Meng YS.Exploring oxygen activity in the high energy P2-type Na_(0.78)Ni_(0.23)Mn_(0.69)O_2 cathode material for Na-ion batteries.J Am Chem Soc.2017;139(13):4835.

[2] Wang PF,You Y,Yin YX,Wang YS,Wan LJ,Gu L,Guo YG.Suppressing the P2-02 phase transition of Na_(0.67)Mn_(0.67)Ni_(0.33)O_2by magnesium substitution for improved sodium-ion batteries.Angew Chem Int Ed.2016;55(26):7445.

[3] Talaie E,Kim SY,Chen N,Nazar LF.Structural evolution and redox processes involved in the electrochemical cycling of P2-Na_(0.67)[Mn_(0.66)Fe_(0.20)Cu_(0.14)]O_2.Chem Mater.2017;29(16):6684.

[4] Xiao Y,Zhu YF,Yao HR,Wang PF,Zhang XD,Li H,Yang X,Gu L,Li YC,Wang T,Yin YX,Guo XD,Zhong BH,Guo YG.A stable layered oxide cathode material for high-performance sodium-ion battery.Adv Energy Mater.2019;9(19):1803978.

[5] Cao X,Li X,Qiao Y,Jia M,Qiu F,He Y,He P,Zhou H.Restraining oxygen loss and suppressing structural distortion in a newly Ti-substituted layered oxide P2-Na_(0.66)Li_(0.22)Ti_(0.15)Mn_(0.63)02.ACS Energy Lett.2019;4:2409.

[6] Huang Q,Liu J,Xu S,Wang P,Ivey DG,Huang B,Wei W.Roles of coherent interfaces on electrochemical performance of sodium layered oxide cathodes.Chem Mater.2018;30(14):4728.

[7] Yabuuchi N,Kajiyama M,Iwatate J,Nishikawa H,Hitomi S,Okuyama R,Usui R,Yamada Y,Komaba S.P2-type Na_((x))[Fe_((1/2))Mn_((1/2))]O_2 made from earth-abundant elements for rechargeable Na batteries.Nat Mater.2012;11(6):512.

[8] Zhao C,Yao Z,Wang Q,Li H,Wang J,Liu M,Ganapathy S,Lu Y,Cabana J,Li B,Bai X,Aspuru-Guzik A,Wagemaker M,Chen L,Hu YS.Revealing high Na-content P2-type layered oxides as advanced sodium-ion cathodes.J Am Chem Soc.2020.https://doi.org/10.1021/jacs.9b13572.

[9] Wang K,Yan PF,Sui ML.Phase transition induced cracking plaguing layered cathode for sodium-ion battery.Nano Energy.2018;54:148.