Laha S, Natarajan S, Gopalakrishnan J, Morán E, Sáez-Puche R, Alario-Franco M Á, Dos Santos-Garcia A J, Pérez-Flores J C, Kuhn A, García-Alvarado F
Solid State and Structural Chemistry Unit, Indian Institute of Science, Bangalore 560012, India.
Phys Chem Chem Phys. 2015 Feb 7;17(5):3749-60. doi: 10.1039/c4cp05052e. Epub 2015 Jan 5.
We describe the synthesis, crystal structure and lithium deinsertion-insertion electrochemistry of two new lithium-rich layered oxides, Li3MRuO5 (M = Mn, Fe), related to rock salt based Li2MnO3 and LiCoO2. The Li3MnRuO5 oxide adopts a structure related to Li2MnO3 (C2/m) where Li and (Li0.2Mn0.4Ru0.4) layers alternate along the c-axis, while the Li3FeRuO5 oxide adopts a near-perfect LiCoO2 (R3[combining macron]m) structure where Li and (Li0.2Fe0.4Ru0.4) layers are stacked alternately. Magnetic measurements indicate for Li3MnRuO5 the presence of Mn(3+) and low spin configuration for Ru(4+) where the itinerant electrons occupy a π*-band. The onset of a net maximum in the χ vs. T plot at 9.5 K and the negative value of the Weiss constant (θ) of -31.4 K indicate the presence of antiferromagnetic superexchange interactions according to different pathways. Lithium electrochemistry shows a similar behaviour for both oxides and related to the typical behaviour of Li-rich layered oxides where participation of oxide ions in the electrochemical processes is usually found. A long first charge process with capacities of 240 mA h g(-1) (2.3 Li per f.u.) and 144 mA h g(-1) (1.38 Li per f.u.) is observed for Li3MnRuO5 and Li3FeRuO5, respectively. An initial sloping region (OCV to ca. 4.1 V) is followed by a long plateau (ca. 4.3 V). Further discharge-charge cycling points to partial reversibility (ca. 160 mA h g(-1) and 45 mA h g(-1) for Mn and Fe, respectively). Nevertheless, just after a few cycles, cell failure is observed. X-ray photoelectron spectroscopy (XPS) characterisation of both pristine and electrochemically oxidized Li3MRuO5 reveals that in the Li3MnRuO5 oxide, Mn(3+) and Ru(4+) are partially oxidized to Mn(4+) and Ru(5+) in the sloping region at low voltage, while in the long plateau, O(2-) is also oxidized. Oxygen release likely occurs which may be the cause for failure of cells upon cycling. Interestingly, some other Li-rich layered oxides have been reported to cycle acceptably even with the participation of the O(2-) ligand in the reversible redox processes. In the Li3FeRuO5 oxide, the oxidation process appears to affect only Ru (4+ to 5+ in the sloping region) and O(2-) (plateau) while Fe seems to retain its 3+ state.
我们描述了两种新型富锂层状氧化物Li3MRuO5（M = Mn，Fe）的合成、晶体结构及锂脱嵌-嵌入电化学性质，它们与基于岩盐结构的Li2MnO3和LiCoO2相关。Li3MnRuO5氧化物采用与Li2MnO3（C2/m）相关的结构，其中Li层和（Li0.2Mn0.4Ru0.4）层沿c轴交替排列，而Li3FeRuO5氧化物采用近乎完美的LiCoO2（R3[combining macron]m）结构，其中Li层和（Li0.2Fe0.4Ru0.4）层交替堆叠。磁性测量表明，对于Li3MnRuO5，存在Mn(3+)且Ru(4+)为低自旋构型，巡游电子占据一个π*带。χ对T曲线在9.5 K处出现净最大值的起始点以及-31.4 K的魏斯常数（θ）负值表明，根据不同途径存在反铁磁超交换相互作用。锂电化学性质表明，这两种氧化物表现出相似的行为，且与典型的富锂层状氧化物行为相关，在典型行为中通常会发现氧化物离子参与电化学过程。对于Li3MnRuO5和Li3FeRuO5，分别观察到较长的首次充电过程，容量分别为240 mA h g(-1)（每化学式单元2.3个Li）和144 mA h g(-1)（每化学式单元1.38个Li）。初始倾斜区域（开路电压至约4.1 V）之后是一个长平台期（约4.3 V）。进一步的充放电循环表明部分可逆性（Mn和Fe分别约为160 mA h g(-1)和45 mA h g(-1)）。然而，仅经过几个循环后，就观察到电池失效。对原始的和电化学氧化后的Li3MRuO5进行X射线光电子能谱（XPS）表征发现，在Li3MnRuO5氧化物中，在低电压倾斜区域，Mn(3+)和Ru(4+)部分氧化为Mn(4+)和Ru(5+)，而在长平台期，O(2-)也被氧化。可能发生了氧释放，这可能是循环过程中电池失效的原因。有趣的是，据报道，一些其他富锂层状氧化物即使在可逆氧化还原过程中有O(2-)配体参与，也能实现可接受的循环。在Li3FeRuO5氧化物中，氧化过程似乎仅影响Ru（在倾斜区域从4+变为5+）和O(2-)（平台期），而Fe似乎保持其3+状态。
