Changes in electronic structure upon Li-ion extraction of LiMPO_4 (M=Fe,Co) using X-ray absorption spectroscopy and First Principle calculation

锂离子二次电池因其高电密度而被广泛应用于手机、个人电脑等消费类电子设备中。具有橄榄石相关结构的LiMPO_4 （M = Fe,Ni,Co）磷酸盐体系最近被广泛研究作为正极的候选材料。在橄榄石型磷酸盐中，PO_4聚阴离子单元的价带和导带形成于远离费米能级的地方，在费米能级，由于其闭壳电子构型，电子交换主要发生在费米能级。因此，过渡金属的价电子往往与多阴离子的价电子相隔离，导致Li的去除/吸收产生的电子交换主要发生在过渡金属离子上。孤立的过渡金属电子结构可能是观察到的高电压特性的原因，同时导致低电子导电性，给电池的制造带来技术上的困难。因此，了解其独特的电子结构对于应用于可充电锂离子电池非常重要。利用同步辐射在BL-7C、BL-11B和BL-11A束流线上进行Co、P和0 k边XAS测量。在Co k边XANES中，电化学制备的样品Li_<1-x>, coo_4， LiCoPO_4的阈值能接近二价参考样品CoCO_3的阈值能，在LiCoPO_4中Co离子几乎是二价的。随着电荷反应，阈值能量逐渐向更高的能量转移，表明Co离子在整个电荷反应区域（从x = 0.0到1.0）被氧化以补偿电荷。P - k边XANES的吸收峰随着电化学去除Li而逐渐移动，在成分x = 1.0时产生0.2 eV的位移。这表明磷酸盐离子也有助于电子结构的变化，并随着充电而减少负电荷。Li_<1-x>CoPO_4的0 k边XANES结果表明，Ssarp吸收峰出现在低能区。由于它们是由Co 3d和0 2p轨道之间的杂化未占据轨道跃迁引起的，峰值A归因于Co 3d-0 2p杂化轨道相关的吸收。峰值A在较小的x值处逐渐移向较低的能量（约为0 5_x）。0.5)。A峰的强度随着组分x的增加而增加，表明氧位点空穴的增加。也就是说，观察到的峰移和A峰强度的增加表明过渡金属离子给杂化轨道的d电子的减少。此外，在x【约等于】0.5区域，0 k边XAS观测到的变化很小，这与Co k边XANES观测到的变化是一一对应的。因此，氧化离子只在萃取锂的前期参与电荷补偿过程，而钴离子则在反应后期参与电荷补偿过程。对Li_<1-x>CoPO_4的XANES分析表明，PO_4聚阴离子的电子结构的改变主要有两种作用，一种是Co 3d轨道与0 2p轨道之间的杂化作用，另一种是Li离子的强离子特性引起的极化作用。Co-、p -和0 k边XANES的变化揭示了Co和0离子杂化和Li-O-P序列极化的重要作用。少

Li-ion secondary batteries have widely been utilized as consumers' electronic devices such as cellular phones, personal computers, etc. because of their high electrical density. A phosphate system, LiMPO_4 (M = Fe,Ni,Co) with the olivine-related structure, has recently been intensively studied as a candidate for positive electrodes. In the Olivine-type phosphate, it has been generally recognized that PO_4 polyanion units form their valence and conduction bands far from the Fermi level where an electronic exchange mainly occurs because of its closed shell electronic configuration. As a result, the transition-metal valence electrons tend to be isolated from those of polyanions, leading to the fact that the electronic exchange arising from Li removal/uptake mainly occurs at transition metal ions. The isolated transition-metal electronic structure would be the reason for the observed high voltage properties, and it simultaneously leads to the low electronic conductivities, causing technica … More l difficulties in fabrication of batteries. Thus, an understanding of their unique electronic structure is important to apply to rechargeable lithium-ion batteries.Co,P, and 0 K-edge XAS measurements were carried out using synchrotron radiation at beam lines BL-7C,BL-11B, and BL-11A. In Co K-edge XANES around the threshold energy for electrochemically prepared samples, Li_<1-x>,CoPO_4, the threshold energy of LiCoPO_4 is close to that of a divalent reference sample, CoCO_3, the Co ions could be almost divalent in LiCoPO_4. The threshold energy gradually shifted to higher energy with charge reaction, indicating that Co ions are oxidized for the charge compensation over an entire region of the charge reaction (from x = 0.0 to 1.0). A absorption peak in P K-edge XANES gradually shifted with electrochemical Li removal, resulting an 0.2 eV shift at a composition of x = 1.0. This indicates that phosphate ions also contribute to the variation of the electronic structure and become less negative with charging. The results of 0 K-edge XANES of Li_<1-x>CoPO_4 indicate Ssarp absorption peaks appeared at the lower energy region. Since they are caused by the transition to hybridized unoccupied orbitals between the Co 3d and 0 2p orbitals, peak A is attributed to an absorption associated with the Co 3d-0 2p hybridized orbital. Peak A gradually shifted to lower energy at small x values (0 5_x approx. 0.5). The intensity of peak A increased with composition x, indicating an increase of holes at oxygen sites. Namely, the observed peak shift and the intensity increase of peak A indicate a decrease in d electron donation from the transition metal ions to hybridized orbitals. In addition, only a little variation was observed at the 0 K-edge XAS in the region of x 【approximately equal】0.5 which is a one-to-one correspondence to that observed in the Co K-edge XANES. Therefore, the oxide ions only contribute to charge compensation process at an early stage of lithium extraction, while the cobalt ions contribute to it at a later stage of the reaction.The analysis of XANES for Li_<1-x>CoPO_4 shows that two effects would be responsible for the modification of the electronic structure of PO_4 polyanion, (1) the hybridization effect between the Co 3d and 0 2p orbitals, and (2) the polarization effect induced by strong ionic character of Li ions. The variation of the Co-, P-and 0 K-edge XANES reveals the important role of the hybridization between the Co and 0 ions and of the polarization in Li-O-P sequences. Less