Effect of High Voltage on the Structure and Electrochemistry of LiNi0.5Mn0.5O2: A Joint Experimental and Theoretical Study

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• 作者：Julien Bré ; ger ; Ying S. Meng ; Yoyo Hinuma ; Sundeep Kumar ; Kisuk Kang ; Yang Shao-Horn ; Gerbrand Ceder ; Clare P. Grey
• 刊名：Chemistry of Materials
• 出版年：2006
• 出版时间：October 3, 2006
• 年：2006
• 卷：18
• 期：20
• 页码：4768 - 4781
• 全文大小：647K
• 年卷期：v.18,no.20(October 3, 2006)
• ISSN：1520-5002

文摘

A combination of neutron diffraction (ND), ⁶Li magic-angle spinning NMR, electrochemistry, andfirst principles calculations have been used to determine and rationalize the structural changes that occurduring cycling of the layered material Li_x(Ni_0.5Mn_0.5)O₂ (x = 1), synthesized via the hydroxide route.ND and ⁶Li NMR experiments confirm that Li is lost from the transition metal (TM) layers, very earlyon in the charge process. On charging to higher voltages (above 4.5 V), the Li is lost from the tetrahedraland residual Li octahedral sites in the Li layers. This process is accompanied by a migration of morethan 75% of the Ni ions originally present in the Li layers into the TM layers, to occupy the sites vacatedby Li. Calculations suggest that (i) these Ni migrations occur via the tetrahedral sites, (ii) activationenergies for migration depend strongly on the original position of the Ni ions in the Li layers though thedriving force for migration is large (>1 eV), and (iii) because neither Ni³⁺ nor Ni⁴⁺ is stable in thetetrahedral site, migration will not occur once the Ni ions in the Li layers are oxidized to Ni³⁺ or Ni⁴⁺.Electrochemical measurements (open circuit voltage, OCV, and galvanostatic mode) are consistent witha high voltage process (approximately 4.6 V) associated with a large activation energy. The new Ni sitesin the TM layers are not necessarily stable, and on discharge, 60% of the ions return to the Li layers. Inparticular, Ni ions surrounded by six Mn⁴⁺ ions are found (in the calculations) to be the least stable.Because the Li ions originally in the TM layers in the as-synthesized sample are predominantly in thisenvironment, this is consistent with the Ni migration observed experimentally. Materials charged to 5.3V can be cycled reversibly with stable capacities of over 180 mAh g^-1.