Transition-metal cathodes of lithium-ion batteries

The charge and discharge process of a 3d transition-metal-based intercalation cathode involves the evolution of both atomic and electronic structures.^{Reference Mizushima, Jones, Wiseman and Goodenough6–Reference Thackeray, Johnson, Depicciotto, Bruce and Goodenough8} This is where XAS, in both the soft and hard x-ray regime, becomes important.^{Reference Yoon, Chung, Oh and Kim9–Reference Liu, Liu, Qiao, Yu, Li, Suo, Hu, Chuang, Shu, Chou, Weng, Nordlund, Sokaras, Wang, Lin, Barbiellini, Bansil, Song, Liu, Yan, Liu, Qiao, Richardson, Prendergast, Hussain, de Groot and Yang11} XAS has been employed to investigate cathode materials and has provided valuable information, including the oxidation states of metal ions, site symmetries, and covalent bond strength.

This is illustrated by a comprehensive study on LiCo_1/3Ni_1/3Mn_1/3O₂.^{Reference Yoon, Balasubramanian, Chung, Yang, McBreen, Grey and Fischer12}Figure 2a–c shows the XAS spectra at Mn, Co, and Ni K-edges. In the pre-edge region, as magnified in the inset in each panel, the small hump is the dipole-forbidden 1s to 3d transition, gaining intensity from the metal 3d–4p mixing or metal-3d–ligand-2p mixing under one-electron orbital approximation. The main absorption peak is the dipole-allowed 1s to 4p transition, and a shoulder on the lower energy side is attributed to the ligand-to-metal charge transfer. The entire edge shift is normally considered as an indicator for the change of the oxidation state of the metal ions. In Figure 2c, it is clear that Ni increases the oxidation state during battery charging, because the edge shifts to a higher energy. No such clear shift is observed for Mn and Co.

Figure 2.

(a–c) Metal (Mn, Co, Ni) K-edge x-ray absorption near-edge structure (XANES), (d–f) metal K-edge extended x-ray absorption fine structure (EXAFS), (g–i) metal L-edge near-edge x-ray absorption fine structure (NEXAFS) spectra of Li_1–xCo_1/3Ni_1/3Mn_1/3O₂. The spectral changes in Ni K-edge XANES and L-edge NEXAFS and the decrease in Ni–O bond length in EXAFS indicate that Ni²⁺ increases to Ni⁴⁺ during battery charging, while Mn and Co remain mostly unchanged as Mn⁴⁺ and Co²⁺. Reproduced with permission from Reference Reference Yoon, Balasubramanian, Chung, Yang, McBreen, Grey and Fischer12. © 2005 American Chemical Society.

It is impossible to determine the change in the oxidation state only by the edge feature, since many other factors would also affect it. In this respect, metal L-edge XAS in the soft x-ray region has an advantage, because it directly probes the unoccupied 3d states through the dipole-allowed 2p-to-3d transition. As shown in Figure 2g–i, the L-edge spectra consist of sharp peaks with high intensity in two regions (L_III and L_II), resulting from the core–hole spin–orbital coupling. The multiplet structure in the L_III-edge is a fingerprint of different oxidation states of metal ions. It is obvious that Mn and Co remain mostly unchanged in Mn⁴⁺ and Co²⁺ states at different states of charge, while Ni²⁺ increases to Ni⁴⁺ with delithiation.

This is further confirmed by the metal K-edge EXAFS measurements from the point of view of atomic structure, as shown in Figure 2d–f. The EXAFS peak features are sensitive to the variation of atoms in different coordination shells. The significant change in the first peak intensity in the Ni spectra indicates a decrease in the Ni–O bond length and the occurrence of charge compensation at the Ni sites. All other bond lengths can be calculated from quantitative analysis of EXAFS measurements. Many other important properties of cathodes, such as phase transformation, metal-d–ligand-p hybridization, and local structure distortion, have also been studied with XAS.^{Reference Liu, Liu, Qiao, Yu, Li, Suo, Hu, Chuang, Shu, Chou, Weng, Nordlund, Sokaras, Wang, Lin, Barbiellini, Bansil, Song, Liu, Yan, Liu, Qiao, Richardson, Prendergast, Hussain, de Groot and Yang11,Reference Yang, Liu, Qiao, Olalde-Velasco, Spear, Roseguo, Pepper, Chuang, Denlinger and Hussain13–Reference Zhou, Ma, Hu, Yu, Gu, Nam, Chen, Wang and Yang15}

Solid-electrolyte interface of lithium-ion batteries

The solid-electrolyte interface (SEI) is a Li⁺-conducting but electron-insulating layer between an electrode and the electrolyte in lithium-ion batteries. It forms mostly during the first several cycles, resulting from the degradation of a small amount of organic electrolyte triggered by electrolyte reduction/oxidation at the cathode/anode, respectively. The SEI acts as a passivation layer to prevent continuous electrolyte degradation and lithium consumption during cycling. Although the SEI has been explored for almost 40 years, it remains a not well-understood phenomenon. A better understanding of the mechanism of formation is critical for optimizing the combination of electrolyte–electrode and improving the efficiency, stability, and lifetime of lithium-ion batteries.^{Reference Gauthier, Carney, Grimaud, Giordano, Pour, Chang, Fenning, Lux, Paschos, Bauer, Maglia, Lupart, Lamp and Shao-Horn16}

The SEI contains a variety of decomposition products from the electrolyte and dissolution or contamination products from electrodes, which consist of 3d transition-metal and low-Z elements (where Z is the atomic number), such as C, Si, N, O, and F. The absorption edges of all of these “light” atoms are within the energy regime of soft x-rays. Balasubramanian et al.^{Reference Balasubramanian, Lee, Sun, Yang, Moodenbaugh, McBreen, Fischer and Fu17} studied the formation of the SEI at different cycling temperatures on Li(Ni_xCo_1–x)O₂ cathode surfaces by XAS at L-edges of Ni and Co, and at K-edges of F and O. They found that the SEI mainly contained poly(vinylidene fluoride) (PVDF) and LiF. By fitting the F K-edge XAS spectrum of the SEI with linear combinations of standard PVDF and LiF spectra, they could further estimate the percentage of each component in the SEI under different cycling conditions. While the PVDF was the electrode binder and remained intact during cycling, LiF was considered the decomposition product of the LiPF₆ electrolyte. Delacourt et al.^{Reference Delacourt, Kwong, Liu, Qiao, Yang, Lu, Harris and Srinivasan18} investigated the effect of manganese contamination on the SEI properties on a model anode surface. The Mn L-edge XAS indicated the presence of Mn²⁺ ions in the SEI films. A multiphase transformation process was proposed, where Mn²⁺ from the electrolyte was first reduced to Mn⁰ at the electrode surface and further reoxidized back to Mn²⁺ by reacting with solvent molecules.

Beyond Li-ion batteries

The capacity of Li-ion batteries is still not high enough to satisfy the current requirements for electrical transportation and stationary electricity storage. Novel electrochemistry and materials beyond Li-ion batteries are urgently demanded;^{Reference Bruce, Freunberger, Hardwick and Tarascon19} in particular, Li-air and Li-S batteries have attracted extensive research interest.^{Reference Cui, Abouimrane, Lu, Bolin, Ren, Weng, Sun, Maroni, Heald and Amine20–Reference Wujcik, Velasco-Velez, Wu, Pascal, Teran, Marcus, Cabana, Guo, Prendergast, Salmerón and Balsara22} One important issue in Li-air batteries is the catalyst for the oxygen-reduction (ORR) and oxygen-evolution reactions (OER) taking place at the interface of the electrodes and the aqueous/nonaqueous electrolyte. The applications of XAS in this field are discussed along with catalysts for CO₂ reduction and water oxidation reactions in the next section.

The biggest obstacle to the Li-S battery involves high dissolution of polysulfides and its shuttling to the anode during the discharge process. To overcome this, various materials, such as polymers and C/S nanocomposites, have been used to immobilize S and Li polysulfides. Ji et al. synthesized a nanocomposite of graphene oxide with a sulfur-coating (GO-S) as the cathode material for Li-S batteries.^{Reference Ji, Rao, Zheng, Zhang, Li, Duan, Guo, Cairns and Zhang23} With a low-temperature heat treatment, some functional groups, such as epoxy and hydroxyl, still remained on graphene oxide (GO) (Figure 3a). These functional groups enhanced the binding of S in the composite cathode and improved the reversibility, capacity stability, and rate capability of the Li-S battery.

Figure 3.

(a) Representative pattern of graphene oxide (GO) immobilizing S. The hydroxyl enhances the binding of S to the C=C bond. Yellow, red, and white balls denote S, O, and H atoms, respectively. Others are C atoms. (b) C K-edge x-ray absorption spectroscopy (XAS) spectra of GO and GO with a sulfur coating (GO-S) nanocomposites after heat treatment in Ar at 155°C for 12 h. The enhanced transition to the π* state and the excitonic state in GO-S indicates that the incorporated S improves the ordering of the sp ²-hybridized carbon structure. Adapted with permission from Reference Reference Kuhl, Cave, Abram and Jaramillo25. © 2011 American Chemical Society.

The mechanism of anchoring S is revealed by C K-edge XAS, as shown in Figure 3b. Two variations are observed in the spectra of GO and GO-S nanocomposites. The first is the peaks at A and D that correspond to the transition of the C 1s core electron to the π* state and the excitonic state, respectively, which become sharper in GO-S. This indicates that the incorporated S improves the ordering of the sp ²-hybridized carbon structure. The other change is that the peak C weakened significantly along with the appearance of a new peak B after S was doped into GO. It is evident that the C–O bond in functional groups on the GO surface is reduced, and a strong chemical interaction between incorporated S species with GO forms C–S bonds. XAS spectroscopy with verification by theoretical simulations thus provides valuable information for better understanding variations in material properties and the mechanism of interactions in complicated composites.

Electrocatalytic CO₂ reduction by Cu/Au(111) monolayer

Anthropogenic CO₂ poses a serious threat to humanity as a greenhouse gas. It could become a major resource if an economically feasible technology is found for its conversion to liquid fuels and feedstock chemicals. The existing technologies^{Reference Centi and Perathoner24} for this are energy intensive. A suitable catalyst can reduce the energetic barrier for CO₂ reduction to an organic. Thus, it is important to understand the interaction between CO₂ molecules and the catalyst from the viewpoint of the electronic structure surrounding the catalytic center.

Compared to metal electrodes that require large overpotentials (∼1 V) for the conversion of CO₂ to a hydrocarbon without selectivity over H₂ evolution,^{Reference Kuhl, Cave, Abram and Jaramillo25} Cu/Au alloys were reported to have improved faradaic efficiencies for CO₂ reduction. Friebel et al.^{Reference Friebel, Mbuga, Rajasekaran, Miller, Ogasawara, Alonso-Mori, Sokaras, Nordlund, Weng and Nilsson26} investigated the electronic structures of a Cu monolayer (ML) and of Cu multilayers on Au(111) substrates in alkaline solution using in situ high-energy-resolution fluorescence-detection x-ray absorption spectroscopy (HERFD XAS,^{Reference Safonova, Tromp, van Bokhoven, de Groot, Evans and Glatzel27,Reference Glatzel, de Groot, Manoilova, Grandjean, Weckhuysen, Bergmann and Barrea28} see Figure 4c for setup scheme). One can apply the “bond-length-with-a-ruler” concept^{Reference Sette, Stohr and Hitchcock29,Reference Stohr, Sette and Johnson30} to rectify the scattering phase difference between 1 ML Cu/Au(111) and Cu/“Cu” (bulk Cu), resulting from different lattice constants of the substrates (d _Au = 2.88 Å versus d _Cu = 2.56 Å). The in situ HERFD XAS spectrum of 1 ML Cu/Au(111) with rescaled energy axis aligns profoundly with that of bulk Cu (Figure 4b). This indicates the Cu ML on the Au(111) substrate is in the metallic state.

Figure 4.

(a) In situ high-energy-resolution fluorescence-detection x-ray absorption spectroscopy (HERFD XAS) of Cu/Au(111). (b) Comparison of XAS for Cu foil and one monolayer (ML) Cu/Au(111) onto the scaled axis based on the “bond-length-with-a-ruler” concept. (c) HERFD XAS setup. The Cu ML on the Au(111) substrate is in the metallic state. Reproduced with permission from Reference Reference Kuehn, Hormes, Matoussevitch, Boennemann and Glatzel33. © 2014 American Chemical Society.

The significant tensile strain of a single Cu ML on Au alters the redox behavior of Cu. This provides another parameter dimension in designing new catalysts by “tuning” the interaction between the strained Cu surface and the reaction to control the product selectivity.

Distinguishing sites in (Ni,Fe)OOH for water electrolysis

A water-oxidation catalyst with low overpotential for the OER is desirable for use as the anode in electrochemical and photoelectrochemical cells. To rectify the performance loss resulting from the high overpotential of existing OER catalysts, one would have to use high-bandgap photoabsorbers or increase catalyst loading to match the catalytic current density^{Reference Walter, Warren, McKone, Boettcher, Mi, Santori and Lewis31} and geometric photocurrent. Among mixed-metal oxides composed of earth-abundant elements, a mixed Ni-Fe compound is the most promising OER catalyst in an alkaline electrolyte. The addition of Fe to NiOOH results in a 500-fold higher OER activity,^{Reference Friebel, Louie, Bajdich, Sanwald, Cai, Wise, Cheng, Sokaras, Weng, Alonso-Mori, Davis, Bargar, Norskov, Nilsson and Bell32} compared to pure Ni and Fe oxyhydroxide parent compounds. One can use in situ HERFD XAS to probe the local electronic structure at Ni and Fe sites, over the full range of the Fe/Ni ratio at potentials below and during OER conditions, to understand the correlation between the catalytic activity and the composition of Ni and Fe sites within the catalysts.

EXAFS data^{Reference Friebel, Louie, Bajdich, Sanwald, Cai, Wise, Cheng, Sokaras, Weng, Alonso-Mori, Davis, Bargar, Norskov, Nilsson and Bell32} show changes both in the Fe–O and Ni–O bond lengths with increasing applied potential. There are two types of Ni sites, characterized as α-Ni(OH)₂ and γ-NiOOH. A strong correlation found between nearest metal−metal distances in Ni and Fe K-edge EXAFS data indicated that Fe substitutes for Ni in both Ni sites. In addition, Fe and Ni K-edge EXAFS show collinear Fe-Ni-Ni, Ni-Fe-Ni, and Ni-Ni-Ni arrangements, indicating that Fe is not intercalated between the hexagonal [NiO₂] sheets, but instead substitutes for Ni within the sheets.

Identifying the active site in spinel Co₃O₄ for water oxidation

XAS is an averaging technique, which means the collected spectrum is a superposition of x-ray absorption of all probed elements. If there is more than one site in a catalyst, one can either use distinct spectral features between the sites, resulting from differences in charge, spin,^{Reference Kuehn, Hormes, Matoussevitch, Boennemann and Glatzel33,Reference Grush, Christou, Hamalainen and Cramer34} or geometry, or substitute a specific site with a different inert element to collect site-selective XAS. Accordingly, the active site of the catalyst can be deduced and identified from site-selective XAS combining with the corresponding catalytic activities.

Spinel cobalt oxide (Co₃O₄) is an efficient OER catalyst,^{Reference Tueysuez, Hwang, Khan, Asiri and Yang35–Reference Jiao and Frei37} with activity competitive with and comparable to noble metal-based catalysts. There are two geometrical sites in Co₃O₄ with different oxidation states: one divalent ion at the tetrahedral site (Co²⁺_Td, green balls in Figure 5a) and two trivalent ions at the octahedral site (Co³⁺_Oh, blue balls in Figure 5a). H.M. Chen and Liu et al.^{Reference Wang, Hung, Chen, Chan, Chen and Liu38} used catalytically inactive Zn²⁺ and Al³⁺ to replace Co²⁺_Td and Co³⁺_Oh, respectively, to disclose the identity of the OER active site. In situ EXAFS data (Figure 5b) showed that both substituted cobalt oxides, ZnCo₂O₄ and CoAl₂O₄, maintain the cubic-spinel structure as that of the pure Co₃O₄.

Figure 5.

(a) Illustration of Co₃O₄ with divalent tetrahedral (Td) sites in green and trivalent octahedral (Oh) sites in blue. (b) Fourier transform (FT) of extended x-ray absorption fine structure (EXAFS) spectra and (c) x-ray absorption near-edge structure (XANES) spectra of substituted Co₃O₄ as a function of the applied bias potentials. The changes in Co–O bond distance and absorption intensity under oxygen evolution reaction (OER) conditions indicate that Co²⁺_(Td) is the active species in Co₃O₄ for OER. Reproduced with permission from Reference Reference Wang, Hung, Chen, Chan, Chen and Liu38. © 2016 American Chemical Society.

Throughout electrolysis, the principal absorption maximum (Figure 5c) of the Co K-edge XANES remained constant for CoAl₂O₄, but kept increasing for ZnCo₂O₄ as the applied positive bias increased. This indicates that positive charge accumulates on cobalt ions in CoAl₂O₄ under an applied bias, promoting the affinity to oxygen ions on the catalyst surfaces to form cobalt oxyhydroxide (CoOOH).

These findings from the x-ray spectroscopy study^{Reference Wang, Hung, Chen, Chan, Chen and Liu38} disclose that Co²⁺_Td is the active species in Co₃O₄ for OER, releasing electrons under applied positive bias.