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

锂离子二次电池由于具有较高的电密度，被广泛用作手机、个人电脑等消费类电子设备。具有橄榄石相关结构的磷酸盐体系LiMPO_4(M=Fe，Ni，Co)是近年来被广泛研究的正极候选材料。在橄榄石型磷酸盐中，人们普遍认为PO4多阴离子单元在远离费米能级的地方形成价带和导带，费米能级是电子交换主要发生的地方，因为它的闭合壳层电子构型。因此，过渡金属价电子倾向于与多阴离子的价电子隔离，从而导致锂的去除/吸收引起的电子交换主要发生在过渡金属离子上。隔离的过渡金属电子结构将是观察到的高电压特性的原因，同时它也导致低的电子电导，导致工艺…L在电池制造方面遇到了更多困难。因此，了解它们独特的电子结构对于应用于可充电锂离子电池是非常重要的。利用同步辐射在束线BL-7C、BL-11B和BL-11A上进行了Co、P和0 K边XAS测量。在电化学法制备的LiCoPO4样品在阈值能量附近的Co K边XANES中，LiCoPO4的阈值能量接近于二价参照物CoCO3的阈值能量，Co离子在LiCoPO4中几乎是二价的。随着电荷反应的进行，阈值能量逐渐向更高的能量移动，表明Co离子被氧化以补偿整个电荷反应区域(从x=0.0到1.0)。随着电化学除锂的进行，P-K边XANES的吸收峰逐渐移动，在x=1.0时产生0.2 eV的漂移。这表明，磷酸根离子也有助于电子结构的变化，并随着电荷的增加而变得不那么负。Li_&lt；1-x&GT；CoPO_4的0K边XANES结果表明，SSARP吸收峰出现在较低能区。由于它们是由于Co3d和02p轨道之间向杂化未占据轨道的跃迁引起的，因此A峰被归因于与Co3d-02p杂化轨道有关的吸收。在较小的x值时，峰A逐渐向较低的能量方向移动(约为0 5x)。0.5)。A峰的强度随x的增加而增大，表明氧位上的空穴增多。也就是说，观察到的峰移动和A峰强度的增加表明从过渡金属离子到杂化轨道的d电子给予减少。此外，在x[大约等于]0.5的区域内，在0 K边XAS处只观察到很小的变化，这与在Co K边XANES中观察到的结果一一对应。Li_&lt；1-x&gt；CoPO_4的XANES分析表明，两种效应对PO4多阴离子的电子结构有影响，(1)Co3d与02p轨道的杂化效应，(2)Li离子的强离子特性引起的极化效应。Co-、P-和0K边XANES的变化揭示了Li-O-P序列中Co与O离子的杂化和极化的重要作用。较少

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