水系锌离子电池嵌入负极材料TiX2（X=S,Se）的储锌机制.pdf

水系锌离子电池(ZIBs)以其低成本、高安全性和环境友好的优点受到了研究者的广泛关注，成为大规模电化学储能系统的理想选择之一。然而锌金属负极在应用时面临着锌枝晶生长、腐蚀反应和副反应等难以克服的障碍，严重制约了水系锌离子电池的发展。探索可替代锌金属的储锌负极是应对上述问题的有效策略，因此研究者围绕过渡金属氧化物、硫化物和导电聚合物开展了深入研究。以TiX2(X=S,Se)为代表的二维过渡金属硫族化合物(TMDs)具有较大的层间距和快速的离子传输通道，可作为锌离子电池的负极，但其储锌反应机制尚未得到完整的揭示。在本文中，我们使用密度泛函理论(DFT)计算方法系统地研究锌离子在TiX2中的嵌入反应。首先我们采用群论去描述嵌锌TiX2的稳定层间构型的特点，定义了一个依赖于超胞并且只涉及平移旋转两种对称操作的群，其子群可以用来描述层间构型的对称性，而且用来描述最稳定构型的子群总是倾向于有最大的阶数。基于该计算得到的一系列对应于不同放电深度的TiX2的稳定结构，我们发现TiS2和TiSe2两种材料在锌嵌入/脱出过程中的开路电压(OCV)均低于0.5 V。态密度(DOS)的计算结果表明TiX2具有很好的电子导电性，而分波态密度(PDOS)的结果显示随着锌的嵌入闭壳层的Ti4+还原成开壳层的Ti3+，并且伴随着ZnX键的生成。Bader电荷分析的结果表明随着X的嵌入，X相比Ti得到了更多的负电荷，意味着X也参与了TiX2的氧化还原过程。爬坡弹性带方法(CINEB)计算的结果证实了Zn2+在TiX2中具有较低的扩散能垒(对于TiS2是0.333 eV，对于TiSe2是0.338 eV)。本文的研究结果不仅从本质上证明了TiX2适合作为锌离子电池的嵌锌负极材料，而且为其他高性能TMDs电池材料的DFT研究提供了新的见解。关键词：关键词：锌离子电池；TiX2负极；第一性原理计算；群论 中图分类号：中图分类号：O646 1 Introduction Among all kinds of aqueous batteries,aqueous Zn-ion battery(ZIBs)has attracted significant attentions towards its potential application in large-scale electrochemical energy storage systems due to its low cost,inherent safety,and environmental benignity 1,2.To date,tremendous endeavors have been devoted to exploring transition metal oxides as the cathodes(MnO2 and V2O5)for ZIBs 36.Despite the extremely high capacity achieved for these cathodes,the zinc metal anode still suffers from dendrites growth,corrosion,and side reactions which substantially hinder the development of ZIBs 7,8.Facing thestubborn challenges,alloying tactics,surface engineering and electrolyte optimization have been proposed as effect strategies to promote the electrochemical reversibility and cycle life of Zn metal anodes 911.However,the inherent shortcomings of zinc anode still exist,which necessitated the investigation of zinc-free anode for aqueous ZIBs 12,13.To date,several zinc-free anode materials for aqueous ZIBs has been reported,for instance,the Chevrel phase Mo6S8,hexagonal MoO3,and two-dimensional metal dichalcogenides(TMDs).Among them,TMDs especially TiX2(X=S,Se),are regarded as the most appealing candidates because of their large interlayer space and facile 2D ion-transport channels.Jiang and co-workers demonstrated that presodiated TiS2(Na0.14TiS2)could work as a long-cycling intercalated anode for aqueous ZIBs with a suitable potential of 0.3 V(vs.Zn2+/Zn)14.Gao et al.15 verified the interlayer spacing of 0.601 nm makes TiSe2 suitable for(de)intercalation of zinc ions.Those recent works consistently pointed out that TiX2 is a family of potential anode materials for ZIBs,whereas the reaction mechanism is still lack of fundamental research.Herein,we perform first-principle calculations to systematically investigate the Zn2+intercalation mechanisms of TiX2(X=S,Se)for the first time.The Zn2+intercalation site is determined,and group theory is used to depict the interlayer configuration of zinc-intercalated TiX2,which gives insight into the investigation of the structural properties of other TMDs electrode materials for aqueous ZIBs.Based on the methodology above,redox potentials,charge transfer properties and Zn2+diffusion barriers are comprehensively studied to illustrate the superiority of TiX2 as zinc-free anode materials for aqueous ZIBs.2 Computational details First-principle calculations based on density functional theory(DFT)were implemented in Vienna ab initio simulation package(VASP)code.The projector augmented wave(PAW)pseudopotential 16 was used and the outer-shell electron configurations were 3p63d24s2 for Ti,3d104s2 for Zn,3s23p4 for S and 4s24p4 for Se.We employed the Perdew-Burke-Ernzerh of 物理化学学报 Acta Phys.-Chim.Sin.2023,39(8),2212037(3 of 11)(PBE)functional of generalized gradient approximation(GGA)17 to describe the exchange-correlation potential and the cutoff energy was set to 500 eV.In order to describe the strong electron-correlation effect on the Ti 3d orbitals,the Hubbard GGA+U model was used with an effective Ueff value of 2.1 eV 18 for calculations of TiS2 and 3.7 eV 19 for calculations of TiSe2.A gamma-centered k-mesh of 12 12 7 was used to sample the Brillouin zone of the TiX2 unit cell.The energy convergence criteria were set to 105 eV and the force convergence criteria was 0.1 eVnm1 DFT-D3 method 20 was employed for van der Waals correction.A supercell of 4 4 1 with one zinc ion intercalated was used to investigate the zinc ion intercalation site and the diffusion barrier of zinc ion.Different supercells of 4 4 1,3 3 1 and 2 2 1 were used to investigate the interlayer configuration of zinc-intercalated TiX2.The climbing image nudged elastic band(CINEB)method 21 was employed to calculate the diffusion barriers of zinc ions.3 Results and discussion 3.1 Zinc ion intercalation sites and TiX2 structure TiX2 composed of infinite X/Ti/X sandwiched monolayers combined by van der Waals force,has a trigonal phase with space group P3m1 18,19,TiX2 is in 1T phase,as shown in Fig.1a.The calculated lattice parameters of TiS2 are a=b=0.3437 nm and c=0.5780 nm,and the calculated lattice parameters of TiSe2 are a=b=0.3606 nm and c=0.6153 nm,which are in accordance with experimental results 22,23.In order to determine the optimal zinc ion intercalation site,we employed a 4 4 1 supercell with one Zn2+ion intercalation for both TiS2 and TiSe2 to calculate their formation energy using the following formula,Ef=EZn(TiX2)4 E(TiX2)4 EZn (1)where EZn(TiX2)4 and E(TiX2)4 are energies of the TiX2 supercells with and without one Zn2+ion intercalation,and EZn is the energy of zinc metal per atom.According to the formula,the optimal intercalation site has the most negative value of Ef.The results show that there are only two kinds of intercalation sites which are octahedral site(O-site)and tetrahedral site(T-site)because Zn2+ion would move to one of these two sites after structural optimization if we put the Zn2+ion somewhere else in the POSCAR file,as shown in Fig.1b,c.The calculated formation energies demonstrate that O-site is the optimal Zn2+intercalation site for both TiS2 and TiSe2,as shown in Table 1.These results are consistent with the intercalation behaviors of Li+and Na+ions into interlayers of 1T phase TMDs reported before 2426.3.2 Configurations of zinc-intercalated TiX2 It is still tough to investigate the structural changes of TMDs during the metal ion(de)intercalation because many possible configurations need to be taken into consideration which is extremely computationally expensive.For example,in order to investigate the structural changes of TiS2 during the process of Na+intercalation,Li et al.27 chose two supercells(2 2 3 and 6 1 2)which were not large.However,1038 configurations had been taken into consideration even if a method was proposed to reduce the number of configurations.Hence,in order to make larger supercells,more complex configurations,and more accurate calculations available,a more effective method is needed to greatly reduce the number of configurations.It is worth noting that a specific supercell intercalated by a specific number of metal ions has many configurations but only one configuration with the lowest energy.Moreover,Ran et al.28 have proposed a group-subgroup transformation method to predict the ordered ground states of systems.So,it is possible to characterize those configurations by group theory.Based on these works,we defined a group to characterize the configurations with the lowest energy and greatly reduced the number of configurations that need to be considered for a specific supercell of TX2 intercalated by specific number of zinc ions.Obviously,the interlayer O-sites and the TiX2 monolayer have the same two-dimensional Bravais lattice,and every single O-site in the same interlayer can be abstracted into a lattice point of the two-dimensional Bravais lattice,in other words,there is a one-to-one relationship between O-sites and Ti atoms.As shown in Fig.2a.As the O-sites in the same interlayers and the TiX2 monolayer has the same symmetry,we used the two-dimensional Bravais lattice to discuss the issue and used configuration to represent the interlayer configuration of zinc-intercalated TiX2 below.For a specific supercell of TiX2 intercalated by specific number of zinc ions,for example,a 3 3 1supercell inter-calated by one zinc ion,different configurations mean the zinc ion corresponds to different 9 lattices points in the supercell,so in this case,the total number of different configurations is C91=9,as shown in Fig.2b.If the 3 3 1supercell is intercalated by two or three zinc ions,the total numbers of different configurations are C92=36 and C93=84 respectively.So,if an M N 1 supercell is intercalated by P zinc ions(M,N,P are Table 1 Formation energy(eV)of one Zn into TiS2 and TiSe2.System O-site T-site TiS2 0.900 0.749 TiSe2 0.188 0.007 Fig.1 (a)Side and top views of 1-T phase TiX2 where the blue balls denote Ti atoms and the black balls denote X atoms.(b)Side and top views of O-site where the red balls denote zinc atoms.(c)Side and top views of T-site.物理化学学报 Acta Phys.-Chim.Sin.2023,39(8),2212037(4 of 11)all positive integers and MN P),the total number of different configurations is CMNP.Obviously,there are over one hundred different configurations even if the small 3 3 1supercell is intercalated by one,two and three zinc ions.In such simple case,a great number of different configurations still need to be taken into consideration that is why the method of reducing the number of configurations is necessary.It is easy to find that all the 9 configurations of the 3 3 1supercell intercalated by one zinc ion are equivalent,in other words,the nine different configurations represent the same structure,and these configurations can coincide with each other by symmetric operations of translation and rotation on the Bravais lattice.In more general cases,for example,an M N 1 supercell intercalated by P zinc ions,there are also many equivalent configurations which can coincide with each other by similar symmetric operations of rotation and translation.These operations are related to the symmetry of the two-dimensional Bravais lattice and specific supercell,which can be accurately described by group theory,and many equivalent configurations can be reduced to one configuration.A cyclic group of order 6(order means the number of elements of a group)was chosen to describe the rotational symmetry of the Bravais lattice,which can be denoted as R=r,r2,r3,r4,r5,e=r6.The axis of rotation is perpendicular to the plane of the Bravais lattice and can intersect the plane at any lattice points,and r,r2,r3,r4,r5,e=r6 denote operations of rotation by/3,2/3,4/3,5/3,2(or 0)respectively.If the rotation axis is determined,the Bravais lattice will coincide with itself after any operation of group R.Considering the group that describes the translation symmetry of the Bravais lattice is infinite group,it is more convenient to define a new finite group to describe the translation symmetry for a specific supercell than to use the infinite group.Herein,The 3 3 1supercell was used as an example to define the new finite group.Firstly,considering the translation of the Bravais lattice along the direction of basis vector a,a cyclic group of order 3 was chosen to describe the translation symmetry of the Bravais lattice