L. Y. Beaulieu

Reaction of Li with Alloy Thin Films Studied by In Situ AFM

Many intermetallic materials deliver poor capacity retention when cycled vs. Li. Many authors have attributed this poor capacity retention to large volume expansions of the active material. Here we report the volume changes of continuous and patterned films of crystalline Al, crystalline Sn, amorphous Si (a-Si), and a-Si 0 . 6 4 Sn 0 . 3 6 as they reversibly react with Li measured by in situ atomic force microscopy (AFM). Although these materials all undergo large volume expansions, the amorphous phases undergo reversible shape and volume changes. The crystalline materials do not. We attribute this difference to the homogeneous expansion and contraction that occurs in the amorphous materials. Inhomogeneous expansion occurs in the crystalline materials due to the presence of coexisting phases with different Li concentrations. Thin films of a-Si and a-Si 0 . 6 4 Sn 0 . 3 6 show good capacity retention with cycle number.

The electrochemical reaction of Li with amorphous Si-Sn alloys

Si12x Snx samples for 0 , x , 0.5 were prepared by magnetron sputtering using a combinatorial materials science approach. The room-temperature resistivity and X-ray diffraction ~XRD! patterns of the samples were used to select materials having both an amorphous structure and good conductivity for further study. The reaction of lithium with amorphous Si0.66Sn0.34 was then studied by electrochemical methods and by in situ XRD. The electrode material apparently remains amorphous throughout all portions of the charge and discharge profile, in the range 0 , x , 4.4 in LixSi0.66Sn0.34 . No crystalline phases are formed, unlike the situation when lithium reacts with tin. Using the Debye scattering formalism, we show that the XRD patterns of the a-Si0.66Sn0.34 starting material and a-Li4.4Si0.66Sn0.34 can be explained by the same local atomic arrangements as found in crystalline Si and Li4.4 Si or Li4.4 Sn, respectively. In fact, the in situ XRD patterns of a-LixSi0.66Sn0.34 , for any x, can be well approximated by a linear combination of the patterns for x 5 0 and x 5 4.4. This suggests that predominantly only two local environments for Si and Sn are found at any value of x in a-LixSi0.66Sn0.44 . However, based on differential capacity vs. potential results for Li/a-Si 0.66Sn0.34 there is no evidence for two-phase regions during the charge and discharge profile. Thus, the two local environments must appear at random throughout the particles. We speculate that the charge-discharge hysteresis in the voltage-capacity profile of Li/ a-LixSi0.66Sn0.34 cells is caused by the energy dissipated during the changes in the local atomic environment around the host atoms.

The Electrochemical Reaction of Lithium with Tin Studied By In Situ AFM

During the electrochemical reaction of lithium with tin, there is sometimes anomalous high-voltage irreversible capacity characterized by an irreversible plateau at about 1.6 V vs. lithium. This has been attributed to electrolyte decomposition catalyzed by pure tin surfaces. Here, we show evidence for this catalytic reaction by in situ atomic force (AFM) and optical microscopy. During the early stages of the anomalous high-voltage plateau we have observed the formation of a film of decomposition products by both in situ AFM and optical microscopy. If lithium is supplied to the tin electrode at a rate greater than the catalytic process can support, then the cell potential decreases and Li-Sn alloys can form. Therefore, by subjecting the Li/Sn cell to a rapid initial discharge to 0.8 V the formation of this film can be suppressed, as we prove with AFM experiments. This rapid discharge to 0.8 V forms the first Li-Sn phase (Li 2 Sn 5 ) which apparently has no catalytic surfaces that decompose electrolyte. Since little film is formed using this method we then monitored the changes in volume and morphology of patterned Sn electrodes as they reacted reversibly with lithium. The volume of the patterned tin electrodes grows with cycle number due to substantial morphology changes, unlike electrodes made from amorphous Si or amorphous Si-Sn.

Reaction of Li with Grain‐Boundary Atoms in Nanostructured Compounds

Intermetallic compounds react with Li to produce high capacity negative electrodes for lithium-ion batteries. Because of the violent reactions occurring during the alloying process between lithium atoms and the active alloy, the cycle life of these materials is generally poor. In this paper we show that nanostructured SnMn 3 C, which has a low affinity for lithium, behaves differently from any intermetallic system reported to date. Using in situ X-ray diffraction, in situ 119 Sn Mossbauer spectroscopy, and electrochemical experiments on mechanically alloyed samples of nanostructured SnMn 3 C, we show that the grain boundaries apparently act as channels to allow Li to enter the particles. The lithium atoms then reversibly react with Sn atoms at and within the grain boundaries to deliver a working capacity of approximately 150 mAh/g with no capacity loss with cycle number.

Study of the reaction of lithium with isostructural A{sub 2}B and various Al{sub x}B alloys

The electrochemical alloying reaction of Li with isostructural A{sub 2}B and Al-based alloys has been investigated. The binary A{sub 2}B alloys the authors selected (Sb{sub 2}Ti, Sb{sub 2}V, Sn{sub 2}Co, Sn{sub 2}Mn, Sn{sub 2}Fe, Al{sub 2}Cu, and Ge{sub 2}Fe) are isostructural (Al{sub 2}Cu type) and comprise an active element (A) that alloys with lithium, and an inactive one (B) that does not. These compounds were prepared by mechanical alloying and have small grain size (10--20 nm). With the exception of Al{sub 2}Cu, the authors observed a full reaction of A with lithium (A{sub 2}B + 2xLi {yields} B + 2Li{sub x}A, where the theoretical values of x are 1 for Al, 3 for Sb, and 4.4 for Si, Ge, and Sn). Extremely slow electrochemical cycling at 55 C and potentiostatic tests at lithium potential proved the total inactivity of the Al{sub 2}Cu vs. lithium. However, thermodynamic considerations predict that the reaction of Al{sub 2}Cu with Li should occur and that the formation of LiAl should be observed. Other Al-transition metal intermetallics were studied and were also found to be inert toward Li, suggesting that the Al-transition metal bond has unique features.

Electrochemistry of InSb as a Li insertion host problems and prospects

Ballmilling of In and Sb has been used to produce InSb for use in electrochemical and in situ X-ray diffraction studies (XRD) of Li/l MLiPF 6 ethylene carbonate:diethyl carbonate/InSb cells. The cell capacity decays rapidly when cycled between 0 and 1.3 V, while the capacity reduction is less pronounced when cycling is restricted to the 0.65-1.4 V range. In situ XRD studies reveal that Li 3 Sb and in are termed during the tirst plateau labove 0.65 V). according to the reaction 3Li 4 InSb - Li 3 Sb + In. The indium product subsequendy reacts with Li forming the InLi x phases InLi and In 4 Li 7 in sequence. When cells are cycle in the absence of InLi x formation) capacity retention improves significantly, remaining relatively constant near 250 mAh/g. Detailed in situ XRD studies of these cells suggest that 0.27 Li atoms per InSb may be intercalated during a sharp drop in the cell potential, acconling to the reaction xLi + InSb. LiInSb (v max - 0.27). This intercalation accounts for only a small (about. 30 mAh/g) fraction of the overall capacity of 680 mAh/g. Consequently, it appears that the reactivity of In and Sb with Li. not the structure type, detemines the reaction path. Therefore. InSb is not an attractive intercalation host for Li, in contrast to the claims made in the literature.

The Reaction of Lithium with Sn‐Mn‐C Intermetallics Prepared by Mechanical Alloying

Intermetallic phases and mixtures of intermetallic phases in the Sn-Mn-C ternary system were prepared by mechanical alloying. The reaction of lithium with these phases was studied using in situ X-ray diffraction and electrochemical methods. Studies concentrated on Sn 2 Mn, SnMn 1,77 , SnMn 3 , and SnMn 3 C. Nanoscale two-phase mixtures of Sn 2 Mn/SnMn 1.77 and Sn 2 Mn/SnMn 3 C were also prepared and studied. When Sn 2 Mn is mixed with sufficient amounts of a second phase, electrodes capable of delivering over 200 mAh/g for more than 50 cycles can be prepared. Grain boundary tin atoms in nanostructured SnMn 3 C appear to be able to react reversibly with lithium for hundreds of cycles.

Cantilever-based sensing: the origin of surface stress and optimization strategies

Many interactions drive the adsorption of molecules on surfaces, all of which can result in a measurable change in surface stress. This article compares the contributions of various possible interactions to the overall induced surface stress for cantilever-based sensing applications. The surface stress resulting from adsorption-induced changes in the electronic density of the underlying surface is up to 2-4 orders of magnitude larger than that resulting from intermolecular electrostatic or Lennard-Jones interactions. We reveal that the surface stress associated with the formation of high quality alkanethiol self-assembled monolayers on gold surfaces is independent of the molecular chain length, supporting our theoretical findings. This provides a foundation for the development of new strategies for increasing the sensitivity of cantilever-based sensors for various applications.

A system for performing simultaneous in situ atomic force microscopy/optical microscopy measurements on electrode materials for lithium-ion batteries

An atomic force microscope (AFM) equipped with an optical charge coupled device camera has been placed in an Ar filled glovebox for the purpose of studying the change in morphology of electrode materials as they react with lithium. In order to minimize noise induced by vibration, the AFM is mounted on granite blocks suspended from the ceiling of the glovebox by a combination of flexible rubber cords and metal springs. The AFM, which is equipped with an environmental chamber surrounding the sample, is then enclosed in a specially constructed draft shield that allows the circulation of Ar gas by the purification system during imaging. A special electrochemical cell was constructed to hold the working electrode under study. Repeated imaging with little drift is possible while electrodes are reacted with lithium for periods of many days. Examples of measurements made by this device will be given for the case of lithium alloying with sputter-deposited Si–Sn thin films. The optical and AFM images obtained as a function of lithium content in the films are assembled into time-lapsed “movies” showing the evolution of the morphology of the sample along with the corresponding electrochemistry. These movies are available for download through the Electronic Physics Auxiliary Publication Service (EPAPS).

Calibrating laser beam deflection systems for use in atomic force microscopes and cantilever sensors

Most atomic force microscopes and cantilever-based sensors use an optical laser beam detection system to monitor cantilever deflections. We have developed a working model that accurately describes the way in which a position sensitive photodetector interprets the deflection of a cantilever in these instruments. This model exactly predicts the numerical relationship between the measured photodetector signal and the actual cantilever deflection. In addition, the model is used to optimize the geometry of such laser deflection systems, which greatly simplifies the use of any cantilever-based instrument that uses a laser beam detection system.