We have prepared intermetallic phases and mixtures of such phases in the Sn‐Fe‐C Gibbs' triangle by mechanical alloying methods or by direct melting of elemental powders. This first paper in a three‐part series focuses on the materials which fall on the two‐phase line connecting Sn2normalFe and C. Using in situ X‐ray diffraction, Mössbauer spectroscopy, and electrochemical methods, we show that Sn2normalFe reacts with Li in normalLi/Sn2normalFe cells to form lithium‐tin alloys and very small metallic iron grains. The experimental capacity for this reaction is about 800 mAh/g, as expected. During the first charge of such cells about 650 mAh/g of Li can be extracted up to 1.5 V vs. Li. The density of these materials is near 7 normalg/cm3 , so first‐cycle volumetric capacities near 4500 Ah/L have been attained. It was our hope that the formed iron would act as an electrically conductive, inactive matrix to support the Li‐Sn alloy grains and that good cycling behavior would result. However, the extended cycling life of these materials between 1.5 and 0.0 V is poor. On the other hand, reasonable cycle life is obtained if the cycling range is restricted to between 0.0 and 0.55 V, but in this case, the irreversible capacity is about 600 mAh/g and the reversible capacity only about 200 mAh/g. We show strategies to overcome these difficulties in the next papers in this series. © 1999 The Electrochemical Society. All rights reserved.

The primary objective of research activities in Na-ion battery development is the implementation of inexpensive, sustainable and environmentally friendly battery materials. Here we report an investigation of Na x Fe x Mn 1-x O 2 electrode materials which contain elements commonly found in the earth's crust, have very low toxicity and cost, and can be prepared in open air atmosphere using simple procedures. Samples with x > 0.65 were found to have the same structure as O3 α-NaFeO 2 under selected synthesis conditions. Samples with x < 0.65 did not have the P2 or O3 structures, but retained a layered structure in which staging likely occurs. Samples having x = 0.5 were found to cycle reversibly at a capacity over 170 mAh/g (corresponding to a theoretical energy density of approximately 460 Wh/kg) with 85% capacity retention after 20 cycles. When the capacity is restricted to 135 mAh/g (340 Wh/kg), capacity retention approaches 95% after 20 cycles.High energy density batteries made from inexpensive sustainable resources would be attractive for a number of applications, including automotive and grid storage. Such applications require batteries capable of excellent coulombic efficiency (>99.99%) and structural stability. Na-ion batteries are a possible candidate technology for these applications. Major research in Na-ion batteries began in the 1980's, but research interest decreased as Li-ion batteries gained prominence. Because of heightened environmental concerns and new markets for secondary batteries in consumer electronics, grid storage and automotive applications, there has been a renewed research interest in Na-ion batteries in recent years, especially those which comprise electrodes made from sustainable inexpensive materials.Iron based cathode materials are of particular interest because of the abundance and low cost of iron. While Fe 3+ and Li + cations tend to mix in lithium iron oxides, 1 this is not the case for Na. NaFeO 2 has two polymorphs. β-NaFeO 2 has an orthorhombic structure and has not been reported to be electrochemically active. α-NaFeO 2 is the prototype structure for the O3 type layered metal oxide phases. Na deintercalation from α-NaFeO 2 was first reported by Kikkawa et al. in 1985, 2 where Na 0.9 FeO 2 was achieved using bromine as an oxidizing agent. The O3 structure was preserved during this process. The first electrochemical study of α-NaFeO 2 was reported in 1994 by Takeda et al., where α-NaFeO 2 was incorporated into Li half cells using LiClO 4 as a lithium salt and a lithium foil counter electrode. 3 A new monoclinic phase of Na 0.5 FeO 2 was reported after electrochemical deintercalation of Na (corresponding to a capacity of 120 mAh/g), with the monoclinic distortion appearing in a two phase reaction during sodium removal. It was argued that the distortion was too small to be due to Jahn-Teller effects, but was more likely caused by structural distortion in response to Na-vacancies. Li could not be effectively intercalated into the structure to form isostructural or other type Li ...

The electrochemical reaction of lithium with ␣-LiFeO 2 , ␤-Li 5 FeO 4 , and CoO is studied by in situ X-ray diffraction and in situ Mo ¨ssbauer measurements. The results of the measurements show that these metal oxides are immediately decomposed during discharge to form lithia and the reduced metal. This reaction proceeds through a single intermediate or surface phase. The reaction products are nanometer-sized, but are not amorphous as was suggested previously. During charge the metal displaces the lithium in lithium oxide to form a metal oxide and lithium. In the case of CoO, the original lithium oxide oxygen lattice is preserved and the reaction resembles an ion exchange process. This also appears to be the case for the iron oxides. Upon discharge, the reverse occurs and the lithium replaces the metal in the metal oxide, once again forming lithium oxide and reduced metal on the bottom of discharge. Further cycling proceeds via oxidation/reduction of the metal by these displacement reactions with lithium.

The effect of calendering and adding graphites of different particle sizes to composite electrodes comprising Si alloy particles was evaluated. It was found that calendered alloy coatings containing graphite results in increased cycling performance, reduced volume expansion and increased energy density compared to a pure alloy coating. Such high energy density, low volume expansion coatings are expected to be practical for implementation in high energy density Li-ion cells.The development of high energy Li-ion cells is of great technological importance. One method to increase Li-ion cell energy is to use active alloys in the negative electrode. Si-based alloys are promising candidates for high energy density anode materials due to the low average voltage and high volumetric capacity 2194 Ah/L (corresponding to Li 15 Si 4 ) of Si. 1 However, the lithiation of Si is associated with a large volume expansion of up to 280%. 2 This expansion can lead to structural degradation of the electrode and the disconnection of active regions from each other and the current collector. The corresponding capacity loss is a major detriment to the application of pure Si as an anode material in Li-ion cells. One way to circumvent this problem is the use of active/inactive alloys. 3 The design principles of active/inactive alloys have been described in Reference 2. The idea of this approach is to dilute the expansion of the active phase during lithiation with an inactive matrix. In this way alloys with lower volume expansion and with good cycle life have been achieved. 4 The performance of composite coatings containing alloy particles is also highly dependent on binder chemistry and electrode processing conditions. Although alloys generally cycle poorly in poly(vinylidene fluoride) (PVDF) binder, Li et al. reported that composite silicon coatings with PVDF binder have significantly better cycle life after a heat-treatment above the PVDF melting point, such that the PVDF forms a continuous film on the Si surface. 5 Better cycling performance can be achieved when the binder forms strong bonds with the alloy particle surface in addition to forming a continuous coating. Hochgatterer et al. investigated Si composite coatings with sodium carboxymethyl cellulose (CMC) binder and concluded that the carboxylic acid sites formation of a covalent bond between the carboxylic acid sites of CMC and Si surface effectively improved the cycling stability. 6 Cycling can be further improved with binders that contain a higher concentration of carboxylic acid sites than CMC, such as lithium polyacrylate (LiPAA), 7 polyacrilic acid (PAA) 8 or alginate. 9 Komaba et al. showed that in addition to forming strong bonds at the alloy surface, PAA binder continuously coats the alloy surface as well. 10 These studies and others 11,12 indicate that good binders for Si alloys should be capable of forming a continuous coating on alloy particles that binds strongly to the alloy surface.After coating, composite electrodes are highly porous. In order to achieve a high vo...

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Lithium metal phosphates are amongst the most promising cathode materials for high capacity lithium-ion batteries. Owing to their inherently low electronic conductivity, it is essential to optimize their properties to minimize defect concentration and crystallite size (down to the submicron level), control morphology, and to decorate the crystallite surfaces with conductive nanostructures that act as conduits to deliver electrons to the bulk lattice. Here, we discuss factors relating to doping and defects in olivine phosphates LiMPO4 (M = Fe, Mn, Co, Ni) and describe methods by which in situ nanophase composites with conductivities ranging from 10(-4)-10(-2) S cm(-1) can be prepared. These utilize surface reactivity to produce intergranular nitrides, phosphides, and/or phosphocarbides at temperatures as low as 600 degrees C that maximize the accessibility of the bulk for Li de/insertion. Surface modification can only address the transport problem in part, however. A key issue in these materials is also to unravel the factors governing ion and electron transport within the lattice. Lithium de/insertion in the phosphates is accompanied by two-phase transitions owing to poor solubility of the single phase compositions, where low mobility of the phase boundary limits the rate characteristics. Here we discuss concerted mobility of the charge carriers. Using Mössbauer spectroscopy to pinpoint the temperature at which the solid solution forms, we directly probe small polaron hopping in the solid solution Li(x)FePO4 phases formed at elevated temperature, and give evidence for a strong correlation between electron and lithium delocalization events that suggests they are coupled.

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 Mössbauer 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.

A study was made of LixNi1−yFeyO2 lithium‐ion cell cathode materials. Through a series of experiments including cyclic voltammetry, X‐ray diffractometry, Mössbauer spectroscopy, in situ X‐ray diffractometry, and in situ Mössbauer spectroscopy, it was determined that the phenomenon commonly referred to as “irreversible capacity” in LixNi1−yFeyO2 lithium‐ion cell cathode materials is a misnomer. Cells utilizing LixNi1−yFeyO2 with 0.05 ≤ y ≤ 0.10 as the cathode active material (vs. a lithium metal anode) have been reversibly cycled between 0.64 ≤ x ≤ 1.00 (100 mAh/g). The recovery of the “irreversible capacity” involved a discharge through a ∼2 V plateau during which it is believed that a surface layer of Li>1Ni1−yFeyO2 is present. Also, a new transition has been observed between two O3 phases (belonging to the space group Rtrue3¯m ) during charge beginning at the start of charge and reaching completion by x ≈ 0.82 (50 mAh/g). The cause of this transition as well as the formation of the Li>1Ni1−yFeyO2 surface layer during discharge are explained in terms of the mobility of lithium ions in the intercalation host being related to the availability of monovacancy and divacancy hopping paths. © 2000 The Electrochemical Society. All rights reserved.