elemental-sensitive-detection-chemistry-batteries-through-soft-x-ray

Elemental-sensitive Detection of the Chemistry in Batteries through Soft X-ray Absorption Spectroscopy and Resonant Inelastic X-ray Scattering

Energy storage has become more and more a limiting factor of today's sustainable energy applications, including electric vehicles and green electric grid ...

Lawrence Berkeley National Laboratory

Peking University Shenzhen Graduate School

Transition metal oxide materials Li(NixMnyCoz)O2 (NMC) based on layered structures are expected to replace LiFePO4 in automotive Li-ion batteries because of their higher specific capacity and operating potential. However, the actual usable capacity is much lower than the promised theoretical value [Uchaker, E.; Cao, G. Nano Today 2014, 9, 499-524; Tarascon, J.-M.; Armand, M. Nature 2001, 414, 359-367], in addition to the often poor cycling performance and the first-cycle Coulombic efficiency, for which Mn(II)-dissolution, its immobilization in solid electrolyte interface (SEI), oxidation of electrolytes by Ni, and other parasitic process thereat have been held responsible [Zhan, C., et al. Nat. Commun. 2013, 4, 2437; Wang, L., et al. J. Solid State Electrochem. 2009, 13, 1157-1164; Lin, F., et al. Nat. Commun. 2014, 5, 4529]. Previously, we reported a composite Li(Ni0.5Mn0.3Co0.2)O2 (NMC532) depolarized by the embedded carbon nanotube (CNT) and achieved capacity close to the theoretical limit [Wu, Z., et al. Nano. Lett. 2014, 14, 4700-4706]; unfortunately, this high capacity failed to be maintained in long-term cycling due to the degrading contacts between the active ingredient and CNT network. On the basis of that NMC532/CNT composite, the present work proposes a unique "prelithiation process", which brought the cathode to low potentials before regular cycling and led to an interphase that is normally formed only on anode surfaces. The complete coverage of cathode surface by this ∼40 nm thick interphase effectively prevented Mn(II) dissolution and minimized the side reactions of Ni, Co, and Mn at the NMC interface during the subsequent cycling process. More importantly, such a "prelithiation" process activated a structure containing two Li layers near the surface of NMC532 particles, as verified by XRD and first principle calculation. Hence, a new cathode material of both high capacity with depolarized structure and excellent cycling performance was generated. This new structure can be incorporated in essentially all the NMC-based layered cathode materials, providing us with an effective tool to tailor-design future new cathode materials for lithium batteries.

Prelithiation Activates Li(Ni0.5Mn0.3Co0.2)O2 for High Capacity and Excellent Cycling Stability.

LiFePO4 has long been held as one of the most promising battery cathode for its high energy storage capacity. Meanwhile, although extensive studies have been conducted on the interfacial chemistries in Li-ion batteries,1-3 little is known on the atomic level about the solid-liquid interface of LiFePO4/electrolyte. Here, we report battery cathode consisted with nanosized LiFePO4 particles in aqueous electrolyte with an high charging and discharging rate of 600 C (3600/600 = 6 s charge time, 1 C = 170 mAh g(-1)) reaching 72 mAh g(-1) energy storage (42% of the theoretical capacity). By contrast, the accessible capacity sharply decreases to 20 mAh g(-1) at 200 C in organic electrolyte. After a comprehensive electrochemistry tests and ab initio calculations of the LiFePO4-H2O and LiFePO4-EC (ethylene carbonate) systems, we identified the transient formation of a Janus hydrated interface in the LiFePO4-H2O system, where the truncated symmetry of solid LiFePO4 surface is compensated by the chemisorbed H2O molecules, forming a half-solid (LiFePO4) and half-liquid (H2O) amphiphilic coordination environment that eases the Li desolvation process near the surface, which makes a fast Li-ion transport across the solid/liquid interfaces possible. 

Janus Solid-Liquid Interface Enabling Ultrahigh Charging and Discharging Rate for Advanced Lithium-Ion Batteries.

High-voltage (>4.3 V) operation of LiNi(x)Mn(y)Co(z)O2 (NMC; 0 ≤ x, y, z < 1) for high capacity has become a new challenge for next-generation lithium-ion batteries because of the rapid capacity degradation over cycling. In this work, we investigate the performance of LiNi(0.5)Mn(0.3)Co(0.2)O2 (NMC532) electrodes with and without an atomic-layer-deposited (ALD) Al2O3 layer for charging/discharging in the range from 3.0 to 4.5 V (high voltage). The results of the electrochemical measurements show that the cells with ALD Al2O3-coated NMC532 electrodes have much enhanced cycling stability. The mechanism was investigated by using X-ray photoelectron spectroscopy, X-ray absorption spectroscopy, and electrochemical methods. We find that the ultrathin ALD Al2O3 film can reduce the interface resistance of lithium-ion diffusion and enhance the surface stability of NMC532 by retarding the reactions at NMC532/electrolyte interfaces for preventing the formation of a new microstructure rock-salt phase NiO around the NMC532 surfaces.

Enhancing the High-Voltage Cycling Performance of LiNi(0.5)Mn(0.3)Co(0.2)O2 by Retarding Its Interfacial Reaction with an Electrolyte by Atomic-Layer-Deposited Al2O3.

In this work, the interfacial properties of a series of metal-free organic naphthodithienothiophene (NDTT)-based photosensitizers adsorbed on TiO2 surfaces were investigated by a combination of ab initio calculations and experimental measurements. The calculations and experiments reveal that because of the efficient charge transfer from the adsorbed dyes to TiO2 nanocrystal surface there is an upward shift for the energy levels of dyes and a downward shift for the conduction band of surface TiO2 and that the band gaps for both of them are also reduced. Such electronic level alignments at the interface would lead to increased light absorption range by adsorbed dyes and increased driving force for charge injection but reduced open-circuit potential (V(oc)). More interestingly, we found that molecule engineering of the donor group and introducing additional electron-withdrawing unit have little effect on the electronic level alignments at the interface (because band gaps of the dyes adsorbed on TiO2 surfaces become approximately identical when compared with those of the dyes measured in solution) but that they can affect the steric effect and the charge separation at the interface to tune V(oc) and the short-circuit current density (J(sc)) effectively. All these findings suggest that optimizing the interfacial properties of dyes adsorbed on TiO2 surfaces by synchronously modifying steric effects of dye molecules anchored on TiO2 and charge-transfer and separation properties at the interfaces is important to construct efficient dye-sensitized solar cells.

How to Optimize the Interface between Photosensitizers and TiO2 Nanocrystals with Molecular Engineering to Enhance Performances of Dye-Sensitized Solar Cells?

Lithium iron phosphate, a widely used cathode material, crystallizes typically in olivine-type phase, α-LiFePO4 (αLFP). However, the new phase β-LiFePO4 (βLFP), which can be transformed from αLFP under high temperature and pressure, is originally almost electrochemically inactive with no capacity for Li-ion battery, because the Li-ions are stored in the tetrahedral [LiO4] with very high activation barrier for migration and the one-dimensional (1D) migration channels for Li-ion diffusion in αLFP disappear, while the Fe ions in the β-phase are oriented similar to the 1D arrangement instead. In this work, using experimental studies combined with density functional theory calculations, we demonstrate that βLFP can be activated with creation of effective paths of Li-ion migration by optimized disordering. Thus, the new phase of βLFP cathode achieved a capacity of 128 mAh g(-1) at a rate of 0.1 C (1C = 170 mA g(-1)) with extraordinary cycling performance that 94.5% of the initial capacity retains after 1000 cycles at 1 C. The activation mechanism can be attributed to that the induced disorder (such as FeLiLiFe antisite defects, crystal distortion, and amorphous domains) creates new lithium migration passages, which free the captive stored lithium atoms and facilitate their intercalation/deintercalation from the cathode. Such materials activated by disorder are promising candidate cathodes for lithium batteries, and the related mechanism of storage and effective migration of Li-ions also provides new clues for future design of disordered-electrode materials with high capacity and high energy density.

Storage and Effective Migration of Li-Ion for Defected β-LiFePO4 Phase Nanocrystals.

An endstation with two high-efficiency soft x-ray spectrographs was developed at Beamline 8.0.1 of the Advanced Light Source, Lawrence Berkeley National Laboratory. The endstation is capable of performing soft x-ray absorption spectroscopy, emission spectroscopy, and, in particular, resonant inelastic soft x-ray scattering (RIXS). Two slit-less variable line-spacing grating spectrographs are installed at different detection geometries. The endstation covers the photon energy range from 80 to 1500 eV. For studying transition-metal oxides, the large detection energy window allows a simultaneous collection of x-ray emission spectra with energies ranging from the O K-edge to the Ni L-edge without moving any mechanical components. The record-high efficiency enables the recording of comprehensive two-dimensional RIXS maps with good statistics within a short acquisition time. By virtue of the large energy window and high throughput of the spectrographs, partial fluorescence yield and inverse partial fluorescence yield signals could be obtained for all transition metal L-edges including Mn. Moreover, the different geometries of these two spectrographs (parallel and perpendicular to the horizontal polarization of the beamline) provide contrasts in RIXS features with two different momentum transfers.

High-efficiency in situ resonant inelastic x-ray scattering (iRIXS) endstation at the Advanced Light Source.

Ni/Li exchange (disordering) usually happens in layered Li(NiMnCo)O (NMC) materials and affects the performance of the material in lithium-ion batteries. Most of previous studies attributed this phenomenon to the similar size of Ni and Li, which implies that Ni should be more favorable than Ni to be located at Li 3b sites in the Li slab. However, this theory cannot explain why in Ni-rich NMC materials where most Ni cations are Ni, Ni/Li exchange happens even more frequently. Using extensive ab initio calculations combined with experiments, here we report that a superexchange interaction between transition metals plays a dominating role in tuning the Ni/Li disordering in NMC materials. Under this scheme, we also propose a new charge compensation mechanism that describes that after Ni/Li exchange the nearest Co transforms to Co in Ni-rich NMC materials. On the basis of this theory, the existence of Co in the initial Ni-rich NMC samples was predicted for the first time, which was further confirmed by our synchrotron-based soft X-ray absorption spectroscopy.

Role of Superexchange Interaction on Tuning of Ni/Li Disordering in Layered Li(NiMnCo)O.

A sodium-ion battery (SIB) solution is attractive for grid-scale electrical energy storage. Low-cost hexacyanometalate is a promising electrode material for SIBs because of its easy synthesis and open framework. Most hexacyanometalate-based SIBs work with aqueous electrolyte, and interstitial water in the material has been found to strongly affect the electrochemical profile, but the mechanism remains elusive. Here we provide a comparative study of the transition-metal redox in hexacyanometalate electrodes with and without interstitial water based on soft X-ray absorption spectroscopy and theoretical calculations. We found distinct transition-metal redox sequences in hydrated and anhydrated NaMnFe(CN)·zHO. The Fe and Mn redox in hydrated electrodes are separated and are at different potentials, leading to two voltage plateaus. On the contrary, mixed Fe and Mn redox in the same potential range is found in the anhydrated system. This work reveals for the first time how transition-metal redox in batteries is strongly affected by interstitial molecules that are seemingly spectators. The results suggest a fundamental mechanism based on three competing factors that determine the transition-metal redox potentials. Because most hexacyanometalate electrodes contain water, this work directly reveals the mechanism of how interstitial molecules could define the electrochemical profile, especially for electrodes based on transition-metal redox with well-defined spin states.

Modification of Transition-Metal Redox by Interstitial Water in Hexacyanometalate Electrodes for Sodium-Ion Batteries.

Solid-electrolyte interphase (SEI) is the key component that enables all advanced electrochemical devices, the best representative of which is Li-ion battery (LIB). It kinetically stabilizes electrolytes at potentials far beyond their thermodynamic stability limits, so that cell reactions could proceed reversibly. Its ad hoc chemistry and formation mechanism has been a topic under intensive investigation since the first commercialization of LIB 25 years ago. Traditionally SEI can only be formed in nonaqueous electrolytes. However, recent efforts successfully transplanted this concept into aqueous media, leading to significant expansion in the electrochemical stability window of aqueous electrolytes from 1.23 V to beyond 4.0 V. This not only made it possible to construct a series of high voltage/energy density aqueous LIBs with unprecedented safety, but also brought high flexibility and even "open configurations" that have been hitherto unavailable for any LIB chemistries. While this new class of aqueous electrolytes has been successfully demonstrated to support diversified battery chemistries, the chemistry and formation mechanism of the key component, an aqueous SEI, has remained virtually unknown. In this work, combining various spectroscopic, electrochemical and computational techniques, we rigorously examined this new interphase, and comprehensively characterized its chemical composition, microstructure and stability in battery environment. A dynamic picture obtained reveals how a dense and protective interphase forms on anode surface under competitive decompositions of salt anion, dissolved ambient gases and water molecule. By establishing basic laws governing the successful formation of an aqueous SEI, the in-depth understanding presented in this work will assist the efforts in tailor-designing better interphases that enable more energetic chemistries operating farther away from equilibria in aqueous media.

Zengqing Zhuo

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Lawrence Berkeley National Laboratory

Elemental-sensitive Detection of the Chemistry in Batteries through Soft X-ray Absorption Spectroscopy and Resonant Inelastic X-ray Scattering

Advanced Light Source