富锂锰基三元材料Li_(1.208)Ni_(0.333)Co_(0.042)Mn_(0.417)O_(2)的电子结构和缺陷性质

Electronic structures and defect properties of lithium-rich manganese-based ternary material Li_(1.208)Ni_(0.333)Co_(0.042)Mn_(0.417)O_(2)

采用基于密度泛函理论的第一性原理方法,计算了R3m相的富锂锰基三元正极材料Li_(1.208)Ni_(0.333)Co_(0.042)Mn_(0.417)O_(2)的晶体结构、电子结构以及缺陷性质.结果表明,Li_(1.208)Ni_(0.333)Co_(0.042)Mn_(0.417)O_(2)是一种具有直接带隙的磁性半导体材料,其价带顶是O的p_(x),p_(y),p_(z)轨道与Ni的d_(xy),d_(yz),d_(xz)原子轨道之间的杂化,导带底除了有价带顶的特征外,还有部分的Ni-3d_(x^(2)-y^(2))和Mn-3d_(x^(2)-y^(2)),Mn-3d_(yz)轨道参与杂化.差分电荷密度图显示,金属原子与O原子之间的成键方式是共价键和离子键的混合.本文还计算了脱去单个过渡金属原子的空位形成能.脱去Mn原子时的缺陷体系的体积变化最大,而脱去Co原子的缺陷体系的体积则几乎不变.得到的金属空位形成能的大小依次为E_(f)(Mn-空位)>E_(f)(Co-空位)>E_(f)(Ni-空位).差分电荷密度图还显示,空位的产生仅强烈影响了空位附近的部分O原子,体现了金属空位影响的局域性特征.

Lithium-rich manganese-based ternary cathode material for lithium-ion batteries,Li_(1.208)Ni_(0.333)Co_(0.042)Mn_(0.417)O_(2),has excellent structural stability and electrochemical stability due to its high Ni content.In order to understand the physical properties of this lithium-rich material,its crystal structure,electronic structure and defect properties are calculated by employing the first-principles method based on the density functional theory.The obtained electronic structure shows that Li_(1.208)Ni_(0.333)Co_(0.042)Mn_(0.417)O_(2) is a magnetic semiconductor with a direct band gap of 0.47 eV.The analysis of the electronic state suggests that the electronic state at the valence band maximum(VBM)is the hybridization of p_(x),p_(y),p_(z) orbitals of oxygen and the d_(xy),d_(yz),d_(xz) orbitals of Niatom.The electronic state at the conduction band minimum has similar characteristics to those at the VBM,however,part of Ni-3 d_(x^(2)-y^(2)) and Mn-3d_(x^(2)-y^(2)),and Mn-3 dyz also contribute to the electronic hybridizations.The charge density difference calculations indicate that the bonding between O and transition metal atoms are through the mixture of covalent bond with ionic bond.The vacancy formation of a single metal atom is also calculated.The results show that the volumes of the defect systems containing metal vacancies are all reduced in comparison with the volume of perfect lattice.The volume change is the largest for the formation of Mnvacancy,while the volume is almost unchanged with Co atoms extracted.The vacancy formation energies of the metals are E_(f)(Mn)>E_(f)(Co)>E_(f)(Ni),and the vacancy formation energy of Mn is significantly higher than those of Ni and Co,indicating that the presence of Mn provides a major structural stability for the material.The calculated charge density differences also show that the formation of metal vacancies influences only the charge distribution of the oxygen atoms around the vacancy,showing the local character of the vacancy effect.Since the formation of metal vacancy breaks the bonding between the metal and the surrounding oxygen atoms,the O-2 p states near the Fermi surface are significantly increased as shown in the calculated electronic density of states.Such a picture suggests that the electrons on oxygen atoms in vicinity of the metal vacancies become freer.