Structural and magnetic properties of Lix(MnyFe1−y)PO4 electrode materials for Li-ion batteries
Structural and magnetic properties of Lix(MnyFe1−y)PO4 electrode materials for Li-ion batteries
复制标题
DOI：
10.1016/j.jpowsour.2008.12.096
复制
发表时间：
2009-04
影响因子：
9.2
通讯作者：
M. Kope;A. Yamada;G. Kobayashi;S. Nishimura;R. Kanno;A. Mauger;F. Gendron;C. Julien
中科院分区：
文献类型：
--
作者：
M. Kope;A. Yamada;G. Kobayashi;S. Nishimura;R. Kanno;A. Mauger;F. Gendron;C. Julien
在0≤y≤1范围内制备了一系列LiMnyFe 1 − yPO 4样品。化学脱锂可以在0≤y≤0.8的范围内得到MnyFe 1 − yPO 4，保持相同的晶相（橄榄石结构，空间群Pnma）。组成y=0.8是脱锂相仍然结晶的极限，但突然遭受分子尺度的应变，这一点由光谱和X射线衍射证明。磁性分析表明，在所有样品中，杂质的浓度可以忽略不计。极化子的浓度，无论是与Li空位在LiMnyFe 1 − yPO 4或电子相关的Li留在MnyFe 1 − yPO 4的矩阵，被发现是小的（≤1%）在所有的样品。当y≤0.6时，所有的Mn 3+离子MnyFe 1 − yPO 4都处于高自旋态（S=2）。然而，在更大的锰浓度下，超过临界浓度yc=0.6的Mn 3+离子经历到低自旋状态（S=1）的转变。因此，与以前的工作相比，我们发现Mn0.8Fe0.2PO4具有小得多的磁相互作用，并且在该化合物中没有检测到反铁磁有序，至少在20 K以上。到目前为止报道的MnyFe 1 − yPO 4在大y组成下的反铁磁有序可能来自不完全脱锂。Mn 3+浓度（y-yc）到低自旋状态的自旋转变是在分子尺度上的应变场的起源，对于y>0.6，应变场随着y增加，并且对于y>0.8，最终阻止完全脱锂。这一结果揭示了富锰的杂多酸-紫铁矿系列化合物阴极性能下降的原因，而电化学性能在y≤0.6范围内良好，但仅在缓慢速率下，这是由于小极化子的跳跃迁移率非常小。
A series of LiMnyFe1−yPO4samples have been prepared in the whole range 0≤y≤1. Chemical delithiation could be achieved to obtain MnyFe1−yPO4in the range 0≤y≤0.8, keeping the same crystal phase (olivine structure, space group Pnma). The composition y=0.8 is the limit where the delithiated phase is still crystallized, but abruptly suffers strains at the molecular scale evidenced by both optical spectroscopy and X-ray diffraction. The analysis of the magnetic properties shows that in all the samples the concentration of impurities is negligible. The concentration of polarons, either holes associated to Li vacancies in LiMnyFe1−yPO4or electrons associated to the existence of Li left in the matrix of MnyFe1−yPO4, is found to be small (≤1%) in all the samples. For y≤0.6, all the Mn3+ions MnyFe1−yPO4are in the high-spin state (S=2). At larger manganese concentration, however, the Mn3+ions in excess of the critical concentration yc=0.6 undergo a transition to the low-spin state (S=1). As a consequence, and in contrast with prior works, we find that Mn0.8Fe0.2PO4has magnetic interactions that are much smaller, and no antiferromagnetic ordering in this compound is detected, at least above 20K. Antiferromagnetic ordering that had been reported so far for MnyFe1−yPO4at large y-composition might come from incomplete delithiation. The spin-transition of Mn3+in concentration (y–yc) to the low-spin state is at the origin of the strain fields at the molecular scale that increase with y for y>0.6, and ultimately prevents the full delithiation for y>0.8. This result sheds light on the reason for the degradation of cathode properties in Mn-rich compounds of the heterosite–purpurite series, while the electrochemical properties are good in the range y≤0.6 but only at slow rates, due to the very small hopping mobility of the small polaron.