Transparent Thin Film Solid-State Lithium Ion Batteries.

Transparent electrochemical energy storage devices have attracted extensive attention for the power supply of next-generation transparent electronics. In this paper, semitransparent thin film batteries (TFBs) with a grid-structured design have been fabricated on glass substrates using specific photolithography and etching processes to achieve LiCoO2/LiPON/Si structures below human eye resolution. UV-vis transmittances up to 60% have been measured for the obtained TFBs. A discharge capacity as high as 0.15 mAh has been recorded upon galvanostatic cycling at the C/2 rate within 4.2-3 V voltage range for the highest transmittances. The capacity variation trend exhibits an initial phase of a gradual decrease with an average capacity loss of 0.15% per cycle and thereafter a second phase with almost stable capacity. Particular attention has been given to the effects of architecture parameters on the TFB optical and electrochemical properties. To the best of our knowledge, this work is the first demonstration of transparent, all inorganic, thin film lithium ion batteries. While reported studies are limited to battery structures involving liquid or polymer materials, our devices will contribute to improve form factor freedom, extend operating ranges, enhance long-term stability, and will be relevant to the integration into various optoelectronic devices.

Elucidating the Phase Transformation of Li4Ti5O12 Lithiation at the Nanoscale.

This work provides insight regarding the fundamental lithiation and delithiation mechanism of the popular lithium ion battery anode material, Li4Ti5O12 (LTO). Our results quantify the extent of reaction between Li4Ti5O12 and Li7Ti5O12 at the nanoscale, during the first cycle. Lithium titanate's discharge (lithiation) and charge (delithiation) reactions are notoriously difficult to characterize due to the zero-strain transition occurring between the end members Li4Ti5O12 and Li7Ti5O12. Interestingly, however, the latter compound is electronically conductive, while the former is an insulator. We take advantage of this critical property difference by using conductive atomic force microscopy (c-AFM) to locally monitor the phase transition between the two structures at various states of charge. To do so, we perform ex situ characterization on electrochemically cycled LTO thin-films that are never exposed to air. We provide direct confirmation of the manner in which the reaction occurs, which proceeds via percolation channels within single grains. We complement scanning probe analyses with an X-ray photoelectron spectroscopy (XPS) study that identifies and explains changes in the LTO surface structure and composition. In addition, we provide a computational analysis to describe the unique electronic differences between LTO and its lithiated form.

Probing battery chemistry with liquid cell electron energy loss spectroscopy.

We demonstrate the ability to apply electron energy loss spectroscopy (EELS) to follow the chemistry and oxidation states of LiMn2O4 and Li4Ti5O12 battery electrodes within a battery solvent. This is significant as the use and importance of in situ electrochemical cells coupled with a scanning/transmission electron microscope (S/TEM) has expanded and been applied to follow changes in battery chemistry during electrochemical cycling. We discuss experimental parameters that influence measurement sensitivity and provide a framework to apply this important analytical method to future in situ electrochemical studies.

Understanding the Role of NH4F and Al2O3 Surface Co-modification on Lithium-Excess Layered Oxide Li1.2Ni0.2Mn0.6O2.

In this work we prepared Li1.2Ni0.2Mn0.6O2 (LNMO) using a hydroxide co-precipitation method and investigated the effect of co-modification with NH4F and Al2O3. After surface co-modification, the first cycle Coulombic efficiency of Li1.2Ni0.2Mn0.6O2 improved from 82.7% to 87.5%, and the reversible discharge capacity improved from 253 to 287 mAh g(-1) at C/20. Moreover, the rate capability also increased significantly. A combination of neutron diffraction (ND), high-resolution transmission electron microscopy (HRTEM), aberration-corrected scanning transmission electron microscopy (a-STEM)/electron energy loss spectroscopy (EELS), and X-ray photoelectron spectroscopy (XPS) revealed the changes of surface structure and chemistry after NH4F and Al2O3 surface co-modification while the bulk properties showed relatively no changes. These complex changes on the material's surface include the formation of an amorphous Al2O3 coating, the transformation of layered material to a spinel-like phase on the surface, the formation of nanoislands of active material, and the partial chemical reduction of surface Mn(4+). Such enhanced discharge capacity of the modified material can be primarily assigned to three aspects: decreased irreversible oxygen loss, the activation of cathode material facilitated with preactivated Mn(3+) on the surface, and stabilization of the Ni-redox pair. These insights will provide guidance for the surface modification in high-voltage-cathode battery materials of the future.

Type I Clathrates as Novel Silicon Anodes: An Electrochemical and Structural Investigation.

Silicon clathrates contain cage-like structures that can encapsulate various guest atoms or molecules. An electrochemical evaluation of type I silicon clathrates based on Ba8Al y Si46-y as the anode material for lithium-ion batteries is presented here. Postcycling characterization with nuclear magnetic resonance and X-ray diffraction shows no discernible structural or volume changes even after electrochemical insertion of 44 Li (≈1 Li/Si) into the clathrate structure. The observed properties are in stark contrast with lithiation of other silicon anodes, which become amorphous and suffer from large volume changes. The electrochemical reactions are proposed to occur as single phase reactions at approximately 0.2 and 0.4 V versus Li/Li+ during lithiation and delithiation, respectively, distinct from diamond cubic or amorphous silicon anodes. Reversible capacities as high as 499 mAh g-1 at a 5 mA g-1 rate were observed for silicon clathrate with composition Ba8Al8.54Si37.46, corresponding to ≈1.18 Li/Si. These results show that silicon clathrates could be promising durable anodes for lithium-ion batteries.

Effect of morphology and manganese valence on the voltage fade and capacity retention of Li[Li2/12Ni3/12Mn7/12]O2.

We have determined the electrochemical characteristics of the high voltage, high capacity Li-ion battery cathode material Li[Li2/12Ni3/12Mn7/12]O2 prepared using three different synthesis routes: sol-gel, hydroxide coprecipitation, and carbonate coprecipitation. Each route leads to distinct morphologies and surface areas while maintaining the same crystal structures. X-ray photoelectron spectroscopy (XPS) measurements reveal differences in their surface chemistries upon cycling, which correlate with voltage fading. Indeed, we observe the valence state of Mn on the surface to decrease upon lithiation, and this reduction is specifically correlated to discharging below 3.6 V. Furthermore, the data shows a correlation of the formation of Li2CO3 with the Mn oxidation state from the decomposition of electrolyte. These phenomena are related to each material's electrochemistry in order to expand upon the reaction mechanisms taking place-specifically in terms of the particle morphology produced by each synthetic approach.

In situ determination of the liquid/solid interface thickness and composition for the Li ion cathode LiMn(1.5)Ni(0.5)O4.

Using neutron reflectometry, we have determined the thickness and scattering length density profile of the electrode-electrolyte interface for the high-voltage cathode LiMn(1.5)Ni(0.5)O4 in situ at open circuit voltage and fully delithiated. Upon exposure to a liquid electrolyte, a thin 3.3 nm Li-rich interface forms due to the ordering of the electrolyte on the cathode surface. This interface changes in composition, as evident by an increase in the scattering length density of the new layer, with charging as the condensed layer evolves from being lithium rich to one containing a much higher concentration of F from the LiPF6 salt. These results show the surface chemistry evolves as a function of the potential.

The reaction mechanism of FeSb(2) as anode for sodium-ion batteries.

The electrochemical reaction of FeSb2 with Na is reported for the first time. The first discharge (sodiation) potential profile of FeSb2 is characterized by a gentle slope centered at 0.25 V. During charge (Na removal) and the subsequent discharge, the main reaction takes place near 0.7 V and 0.4 V, respectively. The reversible storage capacity amounts to 360 mA h g(-1), which is smaller than the theoretical value of 537 mA h g(-1). The reaction, studied by ex situ and in situ X-ray diffraction, is found to proceed by the consumption of crystalline FeSb2 to form an amorphous phase. Upon further sodiation, the formation of nanocrystalline Na3Sb domains is evidenced. During desodiation, Na3Sb domains convert into an amorphous phase. The chemical environment of Fe, probed by (57)Fe Mössbauer spectroscopy, undergoes significant changes during the reaction. During sodiation, the well-resolved doublet of FeSb2 with an isomer shift around 0.45 mm s(-1) and a quadrupole splitting of 1.26 mm s(-1) is gradually converted into a doublet line centered at about 0.15 mm s(-1) along with a singlet line around 0 mm s(-1). The former signal results from the formation of a Fe-rich FexSb alloy with an estimated composition of 'Fe4Sb' while the latter signal corresponds to superparamagnetic Fe due to the formation of nanosized pure Fe domains. Interestingly the signal of 'Fe4Sb' remains unaltered during desodiation. This mechanism is substantially different than that observed during the reaction with Li. The irreversible formation of a Fe-rich 'Fe4Sb' alloy and the absence of full desodiation of Sb domains explain the lower than theoretical practical storage capacity.

Unraveling manganese dissolution/deposition mechanisms on the negative electrode in lithium ion batteries.

The structure, chemistry, and spatial distribution of Mn-bearing nanoparticles dissolved from the Li1.05Mn2O4 cathode during accelerated electrochemical cycling tests at 55 °C and deposited within the solid electrolyte interphase (SEI) are directly characterized through HRTEM imaging and XPS. Here we use air protection and vacuum transfer systems to transport cycled electrodes for imaging and analytical characterization. From HRTEM imaging, we find that a band of individual metallic Mn nanoparticles forms locally at the SEI/graphite interface while the internal and outermost layer of the SEI contains a mixture of LiF and MnF2 nanoparticles, which is confirmed with XPS. Based on our experimental findings we propose a new interpretation of how Mn is reduced from the cathode and how metallic Mn and Mn-bearing nanoparticles form within the SEI during electrochemical cycling.

Direct measurement of the chemical reactivity of silicon electrodes with LiPF6-based battery electrolytes.

We report the first direct measurement of the extent of the spontaneous non-electrochemically driven reaction between a lithium ion battery electrode surface (Si) and a liquid electrolyte (1.2 M LiPF6-3 : 7 wt% ethylene carbonate : dimethyl carbonate). This layer is estimated to be 35 Å thick with a SLD of ∼ 4 × 10(-6) Å(-2) and likely originates from the consumption of the silicon surface.

Probing the electrode/electrolyte interface in the lithium excess layered oxide Li1.2Ni0.2Mn0.6O2.

A detailed surface investigation of the lithium-excess nickel manganese layered oxide Li1.2Ni0.2Mn0.6O2 structure was carried out using X-ray photoelectron spectroscopy (XPS), total electron yield and transmission X-ray absorption spectroscopy (XAS), and electron energy loss spectroscopy (EELS) during the first two electrochemical cycles. All spectroscopy techniques consistently showed the presence of Mn(4+) in the pristine material and a surprising reduction of Mn at the voltage plateau during the first charge. The Mn reduction is accompanied by the oxygen loss revealed using EELS. Upon the first discharge, the Mn at the surface never fully recovers back to Mn(4+). The electrode/electrolyte interface of this compound consists of the reduced Mn at the crystalline defect-spinel inner layer and an oxidized Mn species simultaneously with the presence of a superoxide species in the amorphous outer layer. This proposed model signifies that oxygen vacancy formation and lithium removal result in electrolyte decomposition and superoxide formation, leading to Mn activation/dissolution and surface layer-spinel phase transformation. The results also indicate that the role of oxygen is complex and significant in contributing to the extra capacity of this class of high energy density cathode materials.

Predictions of particle size and lattice diffusion pathway requirements for sodium-ion anodes using η-Cu6Sn5 thin films as a model system.

Geometrically well-defined Cu6Sn5 thin films were used as a model system to estimate the diffusion depth and diffusion pathway requirements of Na ions in alloy anodes. Cu6Sn5 anodes have an initial reversible capacity towards Li of 545 mA h g(-1) (Li3.96Sn or 19.8 Li/Cu6Sn5), close to the theoretical 586 mA h g(-1) (Li4.26Sn), and a very low initial irreversible capacity of 1.6 Li/Cu6Sn5 (Li0.32Sn). In contrast, the reaction with Na is limited with a reversible capacity of 160 mA h g(-1) compared to the expected 516 mA h g(-1) (Na3.75Sn). X-ray diffraction and (119)Sn-Mössbauer spectroscopy measurements show that this limited capacity likely results from the restricted diffusion of Na into the anode nanoparticles and not the formation of a low Na-content phase. Moreover, our results suggest that the η-Cu6Sn5 alloy should have optimized particle sizes of nearly 10 nm diameter to increase the Na capacity significantly. An alternative system consisting of a two-phase mixture of Cu6Sn5 and Sn of nominal composition 'Cu6Sn10' has been studied and is able to deliver a larger initial reversible storage capacity of up to 400 mA h g(-1). Finally, we have demonstrated that the presence of Cu in Cu6Sn5 and 'Cu6Sn10' suppresses the anomalous electrolyte decomposition normally observed for pure Sn.

In situ ambient pressure X-ray photoelectron spectroscopy studies of lithium-oxygen redox reactions.

The lack of fundamental understanding of the oxygen reduction and oxygen evolution in nonaqueous electrolytes significantly hinders the development of rechargeable lithium-air batteries. Here we employ a solid-state Li(4+x)Ti(5)O(12)/LiPON/Li(x)V(2)O(5) cell and examine in situ the chemistry of Li-O(2) reaction products on Li(x)V(2)O(5) as a function of applied voltage under ultra high vacuum (UHV) and at 500 mtorr of oxygen pressure using ambient pressure X-ray photoelectron spectroscopy (APXPS). Under UHV, lithium intercalated into Li(x)V(2)O(5) while molecular oxygen was reduced to form lithium peroxide on Li(x)V(2)O(5) in the presence of oxygen upon discharge. Interestingly, the oxidation of Li(2)O(2) began at much lower overpotentials (~240 mV) than the charge overpotentials of conventional Li-O(2) cells with aprotic electrolytes (~1000 mV). Our study provides the first evidence of reversible lithium peroxide formation and decomposition in situ on an oxide surface using a solid-state cell, and new insights into the reaction mechanism of Li-O(2) chemistry.